U.S. patent application number 11/548825 was filed with the patent office on 2007-05-31 for systems, methods and computer program products for mobility management in hybrid satellite/terrestrial wireless communications systems.
This patent application is currently assigned to ATC Technologies, LLC. Invention is credited to Christian Calamarte, Santanu Dutta, Michel Mouly, Jerome Tronc.
Application Number | 20070123252 11/548825 |
Document ID | / |
Family ID | 37728175 |
Filed Date | 2007-05-31 |
United States Patent
Application |
20070123252 |
Kind Code |
A1 |
Tronc; Jerome ; et
al. |
May 31, 2007 |
Systems, methods and computer program products for mobility
management in hybrid satellite/terrestrial wireless communications
systems
Abstract
Apparatus, methods and computer program products that support
inter-PLMN coordination in registration and handover operations are
provided. Hysteresis is introduced in registration of
radioterminals in a hybrid terrestrial/satellite mobile
communications environment. Inter-PLMN handover techniques are
provided, including techniques for coordination of communication of
timing information and traffic channel controls.
Inventors: |
Tronc; Jerome; (Sant Jean,
FR) ; Calamarte; Christian; (Toulouse, FR) ;
Mouly; Michel; (Paris, FR) ; Dutta; Santanu;
(Vienna, NC) |
Correspondence
Address: |
MYERS BIGEL SIBLEY & SAJOVEC
PO BOX 37428
RALEIGH
NC
27627
US
|
Assignee: |
ATC Technologies, LLC
|
Family ID: |
37728175 |
Appl. No.: |
11/548825 |
Filed: |
October 12, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60725813 |
Oct 12, 2005 |
|
|
|
Current U.S.
Class: |
455/427 |
Current CPC
Class: |
H04W 36/14 20130101;
H04B 7/18591 20130101; H04W 88/06 20130101; H04B 7/18563 20130101;
H04B 7/2125 20130101; H04W 84/06 20130101 |
Class at
Publication: |
455/427 |
International
Class: |
H04Q 7/20 20060101
H04Q007/20 |
Claims
1. A method of coordinating communications of a radioterminal with
a satellite wireless communications network and a terrestrial
wireless communications network, the method comprising: detecting a
potential degradation of coverage for the radioterminal while
registered with a first one of the satellite wireless
communications network and the terrestrial wireless communications
network; determining a duration of the degradation of coverage
responsive to detecting the potential degradation of coverage while
remaining registered with the first one of the satellite wireless
communications network and the terrestrial wireless communications
network; and determining whether to register the radioterminal with
a second one of the satellite wireless communications network and
the terrestrial wireless communications network responsive to
whether the determined duration meets a predetermined
criterion.
2. The method of claim 1, wherein determining a duration of the
degradation of coverage responsive to detecting the potential
degradation of coverage while remaining registered with the first
one of the satellite wireless communications network and the
terrestrial wireless communications network comprises determining a
signal strength for communications between the radioterminal and
the first one of the satellite wireless communications network and
the terrestrial wireless communications network, and wherein
determining whether to register the radioterminal with a second one
of the satellite wireless communications network and the
terrestrial wireless communications network responsive to whether
the determined duration meets a predetermined criterion comprises
registering the radioterminal with the second one of the satellite
wireless communications network and the terrestrial wireless
communications network responsive to the signal strength meeting a
predetermined level criterion for a predetermined time
interval.
3. The method of claim 1: wherein detecting a potential degradation
of coverage for the radioterminal while registered with a first one
of the satellite wireless communications network and the
terrestrial wireless communications network comprises detecting a
potential degradation of coverage for the radioterminal while
registered with the terrestrial wireless communications network;
wherein determining a duration of the degradation of coverage
responsive to detecting the potential degradation of coverage while
remaining registered with the first one of the satellite wireless
communications network and the terrestrial wireless communications
network comprises determining a duration of the degradation of
coverage responsive to detecting the potential degradation of
coverage while remaining registered with the terrestrial wireless
communications network; and wherein determining whether to register
the radioterminal with a second one of the satellite wireless
communications network and the terrestrial wireless communications
network responsive to whether the determined duration meets a
predetermined criterion comprises registering the radioterminal
with the satellite wireless communications network responsive to
the signal strength meeting a first predetermined level criterion
for a first predetermined time interval; and wherein the method
further comprises: detecting a potential improvement of coverage by
the terrestrial wireless communications network for the
radioterminal while registered with the satellite wireless
communications network; determining a duration of the potential
improvement of coverage responsive to detecting the potential
improvement of coverage while remaining registered with the
satellite wireless communications network; and registering the
radioterminal with the terrestrial wireless communications network
responsive to the determined duration of the potential improvement
of coverage meeting a second predetermined level criterion for a
second predetermined time interval.
4. The method of claim 3, wherein the first and second
predetermined time intervals are different.
5. The method of claim 4, wherein the second predetermined time
interval is less than the first predetermined time interval.
6. The method of claim 1, further comprising foregoing registration
of the radioterminal with the second one of the satellite wireless
communications network and the terrestrial wireless communications
network responsive to the radioterminal transiting through a
coverage hole of the first one of the satellite wireless
communications network and the terrestrial wireless communications
network before expiration of a predetermined time interval.
7. An apparatus for coordinating communications of a radioterminal
with a satellite wireless communications network and a terrestrial
wireless communications network, the apparatus comprising: a
handover controller configured to detect a potential degradation of
coverage for the radioterminal while registered with a first one of
the satellite wireless communications network and the terrestrial
wireless communications network, to determine a duration of the
degradation of coverage responsive to detecting the potential
degradation of coverage while remaining registered with the first
one of the satellite wireless communications network and the
terrestrial wireless communications network and to determine
whether to register the radioterminal with a second one of the
satellite wireless communications network and the terrestrial
wireless communications network responsive to whether the
determined duration meets a predetermined criterion.
8. The apparatus of claim 7, wherein the handover controller is
configured to determine a signal strength for communications
between the radioterminal and the first one of the satellite
wireless communications network and the terrestrial wireless
communications network and to register the radioterminal with the
second one of the satellite wireless communications network and the
terrestrial wireless communications network responsive to the
signal strength meeting a predetermined level criterion for a
predetermined time interval.
9. The apparatus of claim 7, wherein the handover controller is
configured to detect a potential degradation of coverage for the
radioterminal while registered with the terrestrial wireless
communications network, to determine a duration of the degradation
of coverage responsive to detecting the potential degradation of
coverage while remaining registered with the terrestrial wireless
communications network and to register the radioterminal with the
satellite wireless communications network responsive to the signal
strength meeting a first predetermined level criterion for a first
predetermined time interval, and wherein the handover controller is
further configured to detect a potential improvement of coverage by
the terrestrial wireless communications network for the
radioterminal while registered with the satellite wireless
communications network, to determine a duration of the potential
improvement of coverage responsive to detecting the potential
improvement of coverage while remaining registered with the
satellite wireless communications network and to register the
radioterminal with the terrestrial wireless communications network
responsive to the determined duration of the potential improvement
of coverage meeting a second predetermined level criterion for a
second predetermined time interval.
10. The apparatus of claim 9, wherein the first and second
predetermined time intervals are different.
11. The apparatus of claim 10, wherein the second predetermined
time interval is less than the first predetermined time
interval.
12. The apparatus of claim 7, wherein the handover controller is
further configured to forego registration of the radioterminal with
the second one of the satellite wireless communications network and
the terrestrial wireless communications network responsive to the
radioterminal transiting through a coverage hole of the first one
of the satellite wireless communications network and the
terrestrial wireless communications network before expiration of a
predetermined time interval.
13. A system comprising: a satellite wireless communications
network; a terrestrial wireless communications network; and a
handover controller configured to detect a potential degradation of
coverage for the radioterminal while registered with a first one of
the satellite wireless communications network and the terrestrial
wireless communications network, to determine a duration of the
degradation of coverage responsive to detecting the potential
degradation of coverage while remaining registered with the first
one of the satellite wireless communications network and the
terrestrial wireless communications network and to determine
whether to register the radioterminal with a second one of the
satellite wireless communications network and the terrestrial
wireless communications network responsive to whether the
determined duration meets a predetermined criterion.
14. The system of claim 13, wherein the handover controller is
configured to determine a signal strength for communications
between the radioterminal and the first one of the satellite
wireless communications network and the terrestrial wireless
communications network and to register the radioterminal with the
second one of the satellite wireless communications network and the
terrestrial wireless communications network responsive to the
signal strength meeting a predetermined level criterion for a
predetermined time interval.
15. The system of claim 13, wherein the handover controller is
configured to detect a potential degradation of coverage for the
radioterminal while registered with the terrestrial wireless
communications network, to determine a duration of the degradation
of coverage responsive to detecting the potential degradation of
coverage while remaining registered with the terrestrial wireless
communications network and to register the radioterminal with the
satellite wireless communications network responsive to the signal
strength meeting a first predetermined level criterion for a first
predetermined time interval, and wherein the handover controller is
further configured to detect a potential improvement of coverage by
the terrestrial wireless communications network for the
radioterminal while registered with the satellite wireless
communications network, to determine a duration of the potential
improvement of coverage responsive to detecting the potential
improvement of coverage while remaining registered with the
satellite wireless communications network and to register the
radioterminal with the terrestrial wireless communications network
responsive to the determined duration of the potential improvement
of coverage meeting a second predetermined level criterion for a
second predetermined time interval.
16. The system of claim 15, wherein the first and second
predetermined time intervals are different.
17. The system of claim 16, wherein the second predetermined time
interval is less than the first predetermined time interval.
18. A computer program product for coordinating communications of a
radioterminal with a satellite wireless communications network and
a terrestrial wireless communications network, the computer program
product comprising computer program code embodied in a computer
readable medium, the computer program code comprising: program code
configured to detect a potential degradation of coverage for the
radioterminal while registered with a first one of the satellite
wireless communications network and the terrestrial wireless
communications network, to determine a duration of the degradation
of coverage responsive to detecting the potential degradation of
coverage while remaining registered with the first one of the
satellite wireless communications network and the terrestrial
wireless communications network and to determine whether to
register the radioterminal with a second one of the satellite
wireless communications network and the terrestrial wireless
communications network responsive to whether the determined
duration meets a predetermined criterion.
19. The computer program product of claim 18, wherein the computer
program code comprises program code configured to determine a
signal strength for communications between the radioterminal and
the first one of the satellite wireless communications network and
the terrestrial wireless communications network and to register the
radioterminal with the second one of the satellite wireless
communications network and the terrestrial wireless communications
network responsive to the signal strength meeting a predetermined
level criterion for a predetermined time interval.
20. The computer program product of claim 18, wherein the computer
program code comprises program code configured to detect a
potential degradation of coverage for the radioterminal while
registered with the terrestrial wireless communications network, to
determine a duration of the degradation of coverage responsive to
detecting the potential degradation of coverage while remaining
registered with the terrestrial wireless communications network and
to register the radioterminal with the satellite wireless
communications network responsive to the signal strength meeting a
first predetermined level criterion for a first predetermined time
interval; and wherein the computer program code further comprises
program code configured to detect a potential improvement of
coverage by the terrestrial wireless communications network for the
radioterminal while registered with the satellite wireless
communications network, to determine a duration of the potential
improvement of coverage responsive to detecting the potential
improvement of coverage while remaining registered with the
satellite wireless communications network and to register the
radioterminal with the terrestrial wireless communications network
responsive to the determined duration of the potential improvement
of coverage meeting a second predetermined level criterion for a
second predetermined time interval.
21. The computer program product of claim 20, wherein the first and
second predetermined time intervals are different
22. The computer program product of claim 21, wherein the second
predetermined time interval is less than the first predetermined
time interval.
23. The computer program product of claim 18, wherein the computer
program code further comprises program code configured to forego
registration of the radioterminal with the second one of the
satellite wireless communications network and the terrestrial
wireless communications network responsive to the radioterminal
transiting through a coverage hole of the first one of the
satellite wireless communications network and the terrestrial
wireless communications network before expiration of a
predetermined time interval.
24. A method of operating first and second public land mobile
networks (PLMNs), the method comprising: conducting a call between
a radioterminal and the first PLMN; during the call, detecting
transit of the radioterminal into a coverage area of the second
PLMN; and handing over the radioterminal to the second PLMN while
maintaining the call.
25. The method of claim 24, further comprising conducting a
location update of the radioterminal in the second PLMN responsive
to termination of the call.
26. The method of claim 24, wherein initiation of the call is
preceded by exchanging subscriber-registration-related information
between the first and second PLMNs, and wherein handing over
comprises handing over using the exchanged
subscriber-registration-related information.
27. The method of claim 26, wherein handing over comprises handing
over without an authentication communication between the
radioterminal and the second PLMN based on a trust relationship
between the first and second PLMNs.
28. The method of claim 27, further comprising the first PLMN
denying handover of a second radioterminal to the second PLMN based
on the trust relationship.
29. The method of claim 24, wherein handing over is preceded by
providing PLMN-identifying information for candidate handover
frequencies in the second PLMN to the radioterminal.
30. The method of claim 24, further comprising generating
respective records of charges in the first and second PLMNs for the
call.
31. The method of claim 30, wherein the first PLMN comprises a
terrestrial PLMN and wherein the second PLMN comprises a satellite
PLMN and wherein generating respective records of charges in the
first and second PLMNs for the call comprises: initiating a charge
record for the call in the first PLMN responsive to initiation of
the call in the first PLMN; responsive to detecting handover of the
radioterminal to the second PLMN, identifying the radioterminal as
a subscriber to the second PLMN; and initiating a charge record for
the call in the second PLMN responsive to identifying the
radioterminal as a subscriber to the second PLMN.
32. The method of claim 31, wherein identifying the radioterminal
as a subscriber to the second PLMN comprises querying the
radioterminal for a subscriber identifier.
33. An apparatus for supporting interoperation of first and second
public land mobile networks (PLMNs), the apparatus comprising: a
inter-PLMN handover controller configured to support a call between
a radioterminal and the first PLMN, to detecting transit of the
radioterminal into a coverage area of the second PLMN during the
call, and to hand over the radioterminal to the second PLMN while
maintaining the call.
34. The apparatus of claim 33, wherein the inter-PLMN handover
controller is further configured to conduct a location update of
the radioterminal in the second PLMN responsive to termination of
the call.
35. A system comprising: first and second public land mobile
networks (PLMNs); and a inter-PLMN handover controller configured
to support a call between a radioterminal and the first PLMN, to
detect transit of the radioterminal into a coverage area of the
second PLMN during the call, and to hand over the radioterminal to
the second PLMN while maintaining the call.
36. The system of claim 35, wherein the inter-PLMN handover
controller is further configured to conduct a location update of
the radioterminal in the second PLMN responsive to termination of
the call.
37. The system of claim 35, wherein the inter-PLMN handover
controller is configured to hand over the call without an
authentication communication between the radioterminal and the
second PLMN based on a trust relationship between the first and
second PLMNs.
38. The system of claim 37, wherein the first PLMN is configured to
deny handover of a second radioterminal to the second PLMN based on
the trust relationship.
39. The system of claim 35, wherein the first PLMN is configured to
provide PLMN-identifying information for candidate handover
frequencies in the second PLMN to the radioterminal.
40. The system of claim 35, further comprising means for generating
respective records of charges in the first and second PLMNs for the
call.
41. A computer program product for coordinating operations of first
and second public land mobile networks (PLMNs), the computer
program product comprising computer program code embodied in a
computer readable medium, the computer program code comprising:
program code configured to support a call between a radioterminal
and the first PLMN, to detect transit of the radioterminal into a
coverage area of the second PLMN during the call, and to hand over
the radioterminal to the second PLMN while maintaining the
call.
42. The computer program product of claim 41, wherein the computer
program code further comprises program code configured to conduct a
location update of the radioterminal in the second PLMN responsive
to termination of the call.
43. The computer program product of claim 41, wherein the computer
program code comprises program code configured to hand over the
radioterminal without an authentication communication between the
radioterminal and the second PLMN based on a trust relationship
between the first and second PLMNs.
44. The computer program product of claim 43, wherein the computer
program code further comprises program code configured to generate
respective records of charges in the first and second PLMNs for the
call.
45. A method of conducting wireless communications, the method
comprising: conducting a communications session between a
radioterminal and a terrestrial wireless communications system;
detecting a condition for handover of the session from the
terrestrial wireless communications system to a satellite wireless
communications system; responsive to detection of the condition for
handover, communicating timing information for communications with
the satellite wireless communications system from the terrestrial
wireless communication system to the radioterminal; and responsive
to the communication of the timing information, conducting a
communications session between the radioterminal and the satellite
wireless communications system using the communicated timing
information.
46. The method of claim 45, wherein the timing information
comprises information for alignment with a timing epoch
47. The method of claim 45, wherein the timing information
comprises a timing advance.
48. The method of claim 45, further comprising determining the
timing information from information about relative positioning of
components the terrestrial wireless communications system and the
satellite wireless communications system.
49. The method of claim 47, wherein the components comprise a base
station antenna and a satellite.
50. The method of claim 47, wherein the components comprise the
radioterminal and a satellite.
51. An apparatus, comprising: a handover controller configured to
support a communications session between a radioterminal and a
terrestrial wireless communications system, to detect a condition
for handover of the session from the terrestrial wireless
communications system to a satellite wireless communications
system, to communicate timing information for communications with
the satellite wireless communications system from the terrestrial
wireless communication system to the radioterminal responsive to
detection of the condition for handover, and to conduct a
communications session between the radioterminal and the satellite
wireless communications system using the communicated timing
information responsive to the communication of the timing
information.
52. The apparatus of claim 51, wherein the timing information
comprises information for alignment with a timing epoch.
53. The apparatus of claim 51, wherein the timing information
comprises a timing advance.
54. The apparatus of claim 51, wherein the controller is further
configured to determine the timing information from information
about relative positioning of components the terrestrial wireless
communications system and the satellite wireless communications
system.
55. The apparatus of claim 54, wherein the components comprise a
base station antenna and a satellite.
56. The apparatus of claim 54, wherein the components comprise the
radioterminal and a satellite.
57. A computer program product for supporting wireless
communications, the computer program product comprising computer
program code embodied in a computer readable medium, the computer
program code comprising: program code configured to support a
communications session between a radioterminal and a terrestrial
wireless communications system, to detect a condition for handover
of the session from the terrestrial wireless communications system
to a satellite wireless communications system, to communicate
timing information for communications with the satellite wireless
communications system from the terrestrial wireless communication
system to the radioterminal responsive to detection of the
condition for handover and to conduct a communications session
between the radioterminal and the satellite wireless communications
system using the communicated timing information responsive to the
communication of the timing information.
58. The computer program product of claim 57, wherein the timing
information comprises information for alignment with a timing
epoch
59. The computer program product of claim 57, wherein the timing
information comprises a timing advance.
60. The computer program product of claim 45, wherein the computer
program code further comprises program code configured to determine
the timing information from information about relative positioning
of components the terrestrial wireless communications system and
the satellite wireless communications system.
61. The computer program product of claim 60, wherein the
components comprise a base station antenna and a satellite.
62. The computer program product of claim 60, wherein the
components comprise the radioterminal and a satellite.
63. A method of handing over communications of a radioterminal from
a terrestrial wireless communications system to a satellite
wireless communications system, the method comprising:
communicating a handover command message from the terrestrial
wireless communications system to the radioterminal; and responsive
to receipt of the handover command message at the radioterminal,
waiting a time interval before terminating a traffic channel
between the radioterminal and the terrestrial wireless
communications system
64. The method of claim 63, wherein the time interval is sufficient
to determine and communicate timing information for the satellite
wireless communications system to the radioterminal.
65. The method of claim 64, wherein the timing information
comprises a timing advance.
66. The method of claim 63, further comprising determining the time
interval responsive to receipt of a physical information message
from the satellite wireless communications system at the
radioterminal.
67. The method of claim 63, wherein terminating a traffic channel
comprises communicating a handover command acknowledgment from the
radioterminal to the terrestrial wireless communications
system.
68. A radioterminal comprising; a handover controller configured to
receive a handover command message from a terrestrial wireless
communications system for handover to a satellite wireless
communications system and to wait a time interval before
terminating a traffic channel between the radioterminal and the
terrestrial wireless communications system responsive to receipt of
the handover command message at the radioterminal.
69. The radioterminal of claim 68, wherein the time interval is
sufficient to determine and communicate timing information for the
satellite wireless communications system to the radioterminal.
70. The radioterminal of claim 69, wherein the timing information
comprises a timing advance.
71. A computer program product for controlling handover of a
radioterminal from a terrestrial wireless communications system to
a satellite wireless communications system, the computer program
product comprising computer program code embodied in a computer
readable medium, the computer program code comprising: program code
configured to receive a handover command message from the
terrestrial wireless communications system and to cause the
radioterminal to wait a time interval before terminating a traffic
channel between the radioterminal and the terrestrial wireless
communications system responsive to receipt of the handover command
message at the radioterminal.
72. The computer program product of claim 71, wherein the time
interval is sufficient to determine and communicate timing
information for the satellite wireless communications system to the
radioterminal.
73. The computer program product of claim 72, wherein the timing
information comprises a timing advance.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims the priority of U.S. Provisional
Application Ser. No. 60/725,813; filed Oct. 12, 2005, incorporated
by reference herein in its entirety.
FIELD OF THE INVENTION
[0002] This invention relates to wireless communications systems
and methods, and more particularly to hybrid satellite and
terrestrial wireless communications systems and methods.
BACKGROUND OF THE INVENTION
[0003] Satellite radiotelephone communications systems and methods
are widely used for radiotelephone communications. Satellite
radiotelephone communications systems and methods generally employ
at least one space-based component, such as one or more satellites,
that is/are configured to wirelessly communicate with a plurality
of satellite radiotelephones.
[0004] A satellite radiotelephone communications system or method
may utilize a single satellite antenna pattern (beam or cell)
covering an entire service region served by the system.
Alternatively, or in combination with the above, in cellular
satellite radiotelephone communications systems and methods,
multiple satellite antenna patterns (beams or cells) are provided,
each of which can serve a substantially distinct service region in
an overall service region, to collectively provide service to the
overall service region. Thus, a cellular architecture that is
similar to that used in conventional terrestrial cellular
radiotelephone systems and methods can be implemented in cellular
satellite-based systems and methods. The satellite typically
communicates with radiotelephones over a bidirectional
communications pathway, with radiotelephone communications signals
being communicated from the satellite to the radiotelephone over a
downlink or forward link (also referred to as forward service
link), and from the radiotelephone to the satellite over an uplink
or return link (also referred to as return service link). In some
cases, such as, for example, in broadcasting, the satellite may
communicate information to one or more radioterminals
unidirectionally.
[0005] The overall design and operation of cellular satellite
radiotelephone systems and methods are well known to those having
skill in the art, and need not be described further herein.
Moreover, as used herein, the term "radiotelephone" includes
cellular and/or satellite radiotelephones with or without a
multi-line display; Personal Communications System (PCS) terminals
that may combine a radiotelephone with data processing, facsimile
and/or data communications capabilities; Personal Digital
Assistants (PDA) that can include a radio frequency transceiver
and/or a pager, Internet/Intranet access, Web browser, organizer,
calendar and/or a global positioning system (GPS) receiver; and/or
conventional laptop and/or palmtop computers or other appliances,
which include a radio frequency transceiver. A radiotelephone also
may be referred to herein as a "radioterminal," a "mobile
terminal," a "user device," or simply as a "terminal". As used
herein, the term(s) "radioterminal, " "radiotelephone," mobile
terminal," "user device" and/or "terminal" also include(s) any
other radiating user device, equipment and/or source that may have
time-varying or fixed geographic coordinates and/or may be
portable, transportable, installed in a vehicle (aeronautical,
maritime, or land-based) and/or situated and/or configured to
operate locally and/or in a distributed fashion over one or more
terrestrial and/or extra-terrestrial location(s). Furthermore, as
used herein, the term "space-based component" or "space-based
system" includes one or more satellites at any orbit
(geostationary, substantially geostationary, medium earth orbit,
low earth orbit, etc.) and/or one or more other objects and/or
platforms (e. g., airplanes, balloons, unmanned vehicles, space
crafts, missiles, etc.) that has/have a trajectory above the earth
at any altitude.
[0006] Terrestrial networks can enhance cellular satellite
radiotelephone system availability, efficiency and/or economic
viability by terrestrially using/reusing at least some of the
frequencies that are allocated to cellular satellite radiotelephone
systems. In particular, it is known that it may be difficult for
cellular satellite radiotelephone systems to reliably serve densely
populated areas, because satellite signals may be blocked by
high-rise structures and/or may not penetrate into buildings. As a
result, satellite spectrum may be underutilized or unutilized in
such areas. The terrestrial use/reuse of at least some of the
satellite system frequencies can reduce or eliminate this potential
problem.
[0007] Moreover, the capacity of an overall hybrid system,
including space-based (i.e., satellite) and terrestrial
communications capability, may be increased by the introduction of
terrestrial frequency use/reuse of frequencies authorized for use
by the space-based component, since terrestrial frequency use/reuse
may be much denser than that of a satellite-only system. In fact,
capacity may be enhanced where it may be mostly needed, i.e., in
densely populated urban/industrial/commercial areas. As a result,
the overall system may become more economically viable, as it may
be able to serve more effectively and reliably a larger subscriber
base.
[0008] One example of terrestrial reuse of satellite frequencies is
described in U.S. Pat. No. 5,937,332 to inventor Karabinis entitled
Satellite Telecommunications Repeaters and Retransmission Methods,
the disclosure of which is hereby incorporated herein by reference
in its entirety as if set forth fully herein. As described therein,
satellite telecommunications repeaters are provided which receive,
amplify, and locally retransmit the downlink/uplink signal received
from a satellite/radioterminal thereby increasing an effective
downlink/uplink margin in the vicinity of the satellite
telecommunications repeater and allowing an increase in the
penetration of uplink and downlink signals into buildings, foliage,
transportation vehicles, and other objects which can reduce link
margin. Both portable and non-portable repeaters are provided. See
the abstract of U.S. Pat. No. 5,937,332. Satellite radiotelephones
for a satellite radiotelephone system or method having a
terrestrial communications capability by terrestrially
using/reusing at least some frequencies of a satellite frequency
band and using substantially the same air interface for both
terrestrial and satellite communications may be more cost effective
and/or aesthetically appealing compared to other alternatives.
Conventional dual band/dual mode radiotelephone alternatives, such
as the well known Thuraya, Iridium and/or Globalstar dual mode
satellite/terrestrial radiotelephones, duplicate some components
(as a result of the different frequency bands and/or air interface
protocols between satellite and terrestrial communications), which
leads to increased cost, size and/or weight of the radiotelephone.
See U.S. Pat. No. 6,052,560 to inventor Karabinis, entitled
Satellite System Utilizing a Plurality of Air Interface Standards
and Method Employing Same.
[0009] Satellite radioterminal communications systems and methods
that may employ terrestrial use and/or reuse of satellite
frequencies by an Ancillary Terrestrial Network (ATN) including at
least one Ancillary Terrestrial Component (ATC) are also described
in U.S. Pat. No. 6,684,057 to Karabinis, entitled Systems and
Methods for Terrestrial Reuse of Cellular Satellite Frequency
Spectrum; U.S. Pat. No. 6,785,543 to Karabinis, entitled Filters
for Combined Radiotelephone/GPS Terminals; U.S. Pat. No. 6,856,787
to Karabinis, entitled Wireless Communications Systems and Methods
Using Satellite-Linked Remote Terminal Interface Subsystems; U.S.
Pat. No. 6,859,652 to Karabinis et al., entitled Integrated or
Autonomous System and Method of Satellite-Terrestrial Frequency
Reuse Using Signal Attenuation and/or Blockage, Dynamic Assignment
of Frequencies and/or Hysteresis; and U.S. Pat. No. 6,879,829 to
Dutta et al., entitled Systems and Methods for Handover Between
Space Based and Terrestrial Radioterminal Communications, and for
Monitoring Terrestrially Reused Satellite Frequencies At a
Radioterminal to Reduce Potential Interference, and in U.S. Pat.
Nos. 6,892,068, 6,937,857, 6,999,720 and 7,006,789; and Published
U.S. Patent Application Nos. US2003/0054761 to Karabinis, entitled
Spatial Guardbands for Terrestrial Reuse of Satellite Frequencies;
US 2003/0054814 to Karabinis et al., entitled Systems and Methods
for Monitoring Terrestrially Reused Satellite Frequencies to Reduce
Potential Interference; US 2003/0073436 to Karabinis et al.,
entitled Additional Systems and Methods for Monitoring
Terrestrially Reused Satellite Frequencies to Reduce Potential
Interference; US 2003/0054762 to Karabinis, entitled
Multi-Band/Multi-Mode Satellite Radiotelephone Communications
Systems and Methods; US 2002/0041575 to Karabinis et al., entitled
Coordinated Satellite-Terrestrial Frequency Reuse; US 2003/0068978
to Karabinis et al., entitled Space-Based Network Architectures for
Satellite Radiotelephone Systems; US 2003/0153308 to Karabinis,
entitled Staggered Sectorization for Terrestrial Reuse of Satellite
Frequencies; and US 2003/0054815 to Karabinis, entitled Methods and
Systems for Modifying Satellite Antenna Cell Patterns In Response
to Terrestrial Reuse of Satellite Frequencies, and in Published
U.S. Patent Application Nos. 2004/0121727, 2004/0142660,
2004/0192395, 2004/0192200, 2004/0192293, 2004/0203742,
2004/0240525, 2005/0026606, 2005/0037749, 2005/0041619,
2005/0064813, 2005/0079816, 2005/0090256, 2005/0118948,
2005/0136836, 2005/0164700, 2005/0164701, 2005/0170834,
2005/0181786, 2005/0201449, 2005/0208890, 2005/0221757,
2005/0227618, 2005/0239457, 2005/0239403, 2005/0239404,
2005/0239399, 2005/0245192, 2005/0260984, 2005/0260947,
2005/0265273, 2005/00272369, 2005/0282542, 2005/0288011,
2006/0040613, 2006/040657 and 2006/0040659; all of which are
assigned to the assignee of the present invention, the disclosures
of all of which are hereby incorporated herein by reference in
their entirety as if set forth fully herein.
[0010] Owing to the greater capacity typical of terrestrial
wireless networks, the potential exists for the number of terminals
registered in a hybrid network including a satellite subnetwork
component and an ancillary terrestrial component (ATC) subnetwork
component, to far exceed the capacity of the satellite subnetwork,
It is also known that the coverage of terrestrial networks has
"coverage holes", i.e. pockets of areas surrounded by covered
regions, where there is insufficient signal strength from any base
station to execute either an incoming or outgoing call, or both. An
exemplary context of this invention is a hybrid wireless network,
including a satellite subnetwork and a terrestrial subnetwork with
roaming and handover allowed between the two subnetworks and where
the coverage of the satellite subnetwork blankets that of the
terrestrial subnetwork. In such a hybrid network, unless mitigating
measures are taken, a significant number of mobile terminals
passing through the coverage hole in the idle mode (when a call is
not in progress) will roam to the satellite subnetwork on entering
the coverage hole and roam back to the terrestrial network on
leaving the coverage hole, assuming that terrestrial access is
preferred to satellite access for quality-of-service and cost
reasons.
[0011] If the two subnetworks are considered distinct Location
Areas (LA's), this may involve an LA update. An LA update typically
involves a registration in which the mobile terminal communicates
with the subnetwork. This communication with the satellite
subnetwork, from a large number of mobile terminals passing through
a terrestrial coverage hole during a busy hour, may cause a
substantial load to the satellite subnetwork, which is typically
dimensioned to handing much less traffic, especially from a single
spotbeam. This overload may occur even when most of the mobile
terminals passing through the coverage hole may not actually engage
in any traffic communication with the satellite subnetwork. On
leaving the coverage hole, when the mobile terminal senses that
terrestrial coverage is again available, it is generally desirable
for the mobile terminal to deregister from the satellite
subnetwork, which may involve further communication with the
satellite subnetwork, and thereby add farther load to the latter.
For example, in the GSM protocol and its derivatives, such
deregistration may include an IMSI Detach procedure. U.S. Patent
Publication No. 2005-0090256, published Apr. 28, 2005, entitled
"SYSTEMS AND METHODS FOR MOBILITY MANAGEMENT IN OVERLAID MOBILE
COMMUNICATIONS SYSTEMS, and incorporated by reference herein,
describes implicit and explicit registration techniques that can
reduce such load on the satellite subnetwork.
[0012] Traditionally, handovers between Public Land Mobile Networks
(PLMNs) have not been performed by the commercial cellular
communications industry. Inter-PLMN idle mode roaming is common in
conventional systems, but such roaming typically involves a time
consuming authentication process designed to ensure that the
visiting mobile terminal has the right credentials to receive
service from the visited PLMN. Therefore, it may be undesirable to
make the full idle mode roaming procedure a component of a handover
procedure in a hybrid satellite/terrestrial system.
[0013] In GSM, call handover between one base transceiver station
(BTS) to another BTS (regardless of whether they belong to the same
LA or MSC), say from BTS-A to BTS-B, involves two levels of
synchronization. At a first level, the mobile terminal is
synchronized in frequency and phase to the forward control channel
of BTS-B. This may be achieved by "sniffing" the forward control
channels of adjacent cells periodically during idle periods in the
TDMA frame. This may be done before the mobile terminal has made
the transition to BTS-B and is referred to as pre-synchronization.
At a second level, after the handover, the mobile terminal may
re-adjust its TDMA frame timing advance to a new value that matches
the propagation delay to BTS-B, which, in general, will be
different from the propagation delay to BTS-A. This is referred to
as post-handover synchronization, or simply post-synchronization,
and may be performed synchronously or asynchronously.
[0014] In synchronous post-handover synchronization, the new timing
advance is known to the mobile terminal a priori. A variety of
techniques are used/allowed in legacy GSM systems with respect to
acquiring this a priori information, as described, for example, in
Michel Mouly and Marie-Bernadette Pautet, The GSM System for Mobile
Communications, Cell & Sys, 1992, ISBN 2-9507190-0-7, pp.
347-349. The handshaking process in synchronous post-handover
synchronization involves the MT sending a small number of access
probes (referred to in GSM as RIL3-RR Handover Access messages) to
the new BTS (BTS-B), which then activates the new channel, with the
new timing advance, in both directions.
[0015] In asynchronous post-handover synchronization, no a priori
information about the correct timing advance for communication
between the MT and BTS-B is used. The correct timing advance is
assessed by access probes sent by the MT to BTS-B, and the MT
conventionally is forbidden from transmitting on the new channel
until the new timing advance is unequivocally established, although
reception may be allowed. Handshaking involved in such a process is
discussed in Asha Mehrotra, GSI System Engineering, Artech House,
1997, ISBN 0-89006-860-7, pp. 147-148.
SUMMARY OF THE INVENTION
[0016] Some embodiments of the present invention provide methods of
coordinating communications of a radioterminal with a satellite
wireless communications network and a terrestrial wireless
communications network. A potential degradation of coverage for the
radioterminal while registered with a first one of the satellite
wireless communications network and the terrestrial wireless
communications network is detected. A duration of the degradation
of coverage is determined responsive to detecting the potential
degradation of coverage while remaining registered with the first
one of the satellite wireless communications network and the
terrestrial wireless communications network. It is determined
whether to register the radioterminal with a second one of the
satellite wireless communications network and the terrestrial
wireless communications network responsive to whether the
determined duration meets a predetermined criterion. For example,
registration of the radioterminal with the second one of the
satellite wireless communications network and the terrestrial
wireless communications network may be foregone responsive to the
radioterminal transiting through a coverage hole of the first one
of the satellite wireless communications network and the
terrestrial wireless communications network before expiration of a
predetermined time interval.
[0017] In some embodiments, determining a duration of the
degradation of coverage may include determining a signal strength
for communications between the radioterminal and the first one of
the satellite wireless communications network and the terrestrial
wireless communications network. Determining whether to register
the radioterminal with a second one of the satellite wireless
communications network and the terrestrial wireless communications
network responsive to whether the determined duration meets a
predetermined criterion may include registering the radioterminal
with the second one of the satellite wireless communications
network and the terrestrial wireless communications network
responsive to the signal strength meeting a predetermined level
criterion for a predetermined time interval.
[0018] In some embodiments, a potential degradation of coverage for
the radioterminal while registered with the terrestrial wireless
communications network is detected. A duration of the degradation
of coverage is determined responsive to detecting the potential
degradation of coverage while remaining registered with the
terrestrial wireless communications network. The radioterminal may
be registered with the satellite wireless communications network
responsive to the signal strength meeting a first predetermined
level criterion for a first predetermined time interval.
Subsequently, a potential improvement of coverage by the
terrestrial wireless communications network for the radioterminal
is detected while registered with the satellite wireless
communications network. A duration of the potential improvement of
coverage is determined. The radioterminal is registered with the
terrestrial wireless communications network responsive to the
determined duration of the potential improvement of coverage
meeting a second predetermined level criterion for a second
predetermined time interval, The first and second predetermined
time intervals may be different, e.g., the second predetermined
time interval may be less than the first predetermined time
interval.
[0019] According to further embodiments of the present invention,
an apparatus for coordinating communications of a radioterminal
with a satellite wireless communications network and a terrestrial
wireless communications network includes a handover controller
configured to detect a potential degradation of coverage for the
radioterminal while registered with a first one of the satellite
wireless communications network and the terrestrial wireless
communications network, to determine a duration of the degradation
of coverage responsive to detecting the potential degradation of
coverage while remaining registered with the first one of the
satellite wireless communications network and the terrestrial
wireless communications network and to determine whether to
register the radioterminal with a second one of the satellite
wireless communications network and the terrestrial wireless
communications network responsive to whether the determined
duration meets a predetermined criterion. The handover controller
may be implemented, for example, in one or more components of the
terrestrial network, in one or more components of the satellite
network and/or in hardware coupled to the terrestrial and satellite
network, such as an interconnecting network.
[0020] Additional embodiments provide a system including a
satellite wireless communications network, a terrestrial wireless
communications network and a handover controller configured to
detect a potential degradation of coverage for the radioterminal
while registered with a first one of the satellite wireless
communications network and the terrestrial wireless communications
network, to determine a duration of the degradation of coverage
responsive to detecting the potential degradation of coverage while
remaining registered with the first one of the satellite wireless
communications network and the terrestrial wireless communications
network and to determine whether to register the radioterminal with
a second one of the satellite wireless communications network and
the terrestrial wireless communications network responsive to
whether the determined duration meets a predetermined criterion.
Still further embodiments include a computer program product
including program code configured to detect a potential degradation
of coverage for the radioterminal while registered with a first one
of the satellite wireless communications network and the
terrestrial wireless communications network, to determine a
duration of the degradation of coverage responsive to detecting the
potential degradation of coverage while remaining registered with
the first one of the satellite wireless communications network and
the terrestrial wireless communications network and to determine
whether to register the radioterminal with a second one of the
satellite wireless communications network and the terrestrial
wireless communications network responsive to whether the
determined duration meets a predetermined criterion.
[0021] According to further aspects of the present invention,
methods are provided for operating first and second public land
mobile networks (PLMNs). A call between a radioterminal and the
first PLMN is conducted. During the call, transit of the
radioterminal into a coverage area of the second PLMN is detected.
The radioterminal is handed over to the second PLMN while
maintaining the call. A location update of the radioterminal in the
second PLMN may be conducted responsive to termination of the
call.
[0022] In some embodiments, initiation of the call is preceded by
exchanging subscriber-registration-related information between the
first and second PLMNs, and handing over includes handing over
using the exchanged subscriber-registration-related information.
The handover may occur without an authentication communication
between the radioterminal and the second PLMN based on a trust
relationship between the first and second PLMNs. Handover of a
second radioterminal to the second PLMN may be denied based on the
trust relationship. In some embodiments, handing over is preceded
by providing PLMN-identifying information for candidate handover
frequencies in the second PLMN to the radioterminal. Methods may
also include generating respective records of charges in the first
and second PLMNs for the call.
[0023] Additional embodiments provide apparatus for supporting
interoperation of first and second public land mobile networks
(PLMNs). An inter-PLMN handover controller is configured to support
a call between a radioterminal and the first PLMN, to detect
transit of the radioterminal into a coverage area of the second
PLMN during the call, and to hand over the radioterminal to the
second PLMN while maintaining the call. A system may include first
and second public land mobile networks (PLMNs) and an inter-PLMN
handover controller configured to support a call between a
radioterminal and the first PLMN, to detect transit of the
radioterminal into a coverage area of the second PLMN during the
call, and to hand over the radioterminal to the second PLMN while
maintaining the call. A computer program product for coordinating
operations of first and second public land mobile networks (PLMNs)
includes program code configured to support a call between a
radioterminal and the first PLMN, to detect transit of the
radioterminal into a coverage area of the second PLMN during the
call, and to hand over the radioterminal to the second PLMN while
maintaining the call.
[0024] Additional embodiments of the present invention provide
methods of conducting wireless communications. A communications
session between a radioterminal and a terrestrial wireless
communications system is conducted. A condition for handover of the
session from the terrestrial wireless communications system to a
satellite wireless communications system is detected. Responsive to
detection of the condition for handover, timing information for
communications with the satellite wireless communications system is
communicated from the terrestrial wireless communication system to
the radioterminal. Responsive to the communication of the timing
information, a communications session between the radioterminal and
the satellite wireless communications system is conducted using the
communicated timing information. The timing information may
include, for example, information for alignment with a timing
epoch. For example, the timing information may include a timing
advance. In additional embodiments, the timing information may be
determined from information about relative positioning of
components the terrestrial wireless communications system and the
satellite wireless communications system. For example, the
components may include a satellite and a base station antenna.
[0025] Further embodiments provide apparatus including a handover
controller configured to support a communications session between a
radioterminal and a terrestrial wireless communications system, to
detect a condition for handover of the session from the terrestrial
wireless communications system to a satellite wireless
communications system, to communicate timing information for
communications with the satellite wireless communications system
from the terrestrial wireless communication system to the
radioterminal responsive to detection of the condition for
handover, and to conduct a communications session between the
radioterminal and the satellite wireless communications system
using the communicated timing information responsive to the
communication of the timing information. A computer program product
for supporting wireless communications may include program code
configured to support a communications session between a
radioterminal and a terrestrial wireless communications system, to
detect a condition for handover of the session from the terrestrial
wireless communications system to a satellite wireless
communications system, to communicate timing information for
communications with the satellite wireless communications system
from the terrestrial wireless communication system to the
radioterminal responsive to detection of the condition for handover
and to conduct a communications session between the radioterminal
and the satellite wireless communications system using the
communicated timing information responsive to the communication of
the timing information.
[0026] Some embodiments of the present invention provide methods of
handing over communications of a radioterminal from a terrestrial
wireless communications system to a satellite wireless
communications system. A handover command message is communicated
from the terrestrial wireless communications system to the
radioterminal. Responsive to receipt of the handover command
message at the radioterminal, a time interval is provided before
terminating a traffic channel between the radioterminal and the
terrestrial wireless communications system, e.g., by delaying
transmission of a handover command acknowledgment by the
radioterminal. The time interval may be sufficient to determine and
communicate timing information for the satellite wireless
communications system to the radioterminal. The timing interval may
be fixed and/or variable. For example, in some embodiments, the
time interval may be determined responsive to receipt of a physical
information message from the satellite wireless communications
system at the radioterminal.
[0027] Further embodiments provide a radioterminal including a
handover controller configured to receive a handover command
message from a terrestrial wireless communications system for
handover to a satellite wireless communications system and to wait
a time interval before terminating a traffic channel between the
radioterminal and the terrestrial wireless communications system
responsive to receipt of the handover command message at the
radioterminal. A computer program product for controlling handover
of a radioterminal from a terrestrial wireless communications
system to a satellite wireless communications system may include
program code configured to receive a handover command message from
the terrestrial wireless communications system and to cause the
radioterminal to wait a time interval before terminating a traffic
channel between the radioterminal and the terrestrial wireless
communications system responsive to receipt of the handover command
message at the radioterminal.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] FIG. 1 illustrates a hybrid terrestrial/wireless
communications environment according to some embodiments of the
present invention.
[0029] FIG. 2 is a flowchart illustrating handover operations
according to some embodiments of the present invention.
[0030] FIGS. 3 and 4 are schematic diagrams illustrating inter-PLMN
handover signaling according to some embodiments of the present
invention.
[0031] FIG. 5 illustrates operations for terrestrial/satellite
handover according to some embodiments of the present
invention.
[0032] FIG. 6 illustrates a radioterminal according to some
embodiments of the present invention.
DETAILED DESCRIPTION
[0033] Specific exemplary embodiments of the invention now will be
described with reference to the accompanying drawing. This
invention may, however, be embodied in many different forms and
should not be construed as limited to the embodiments set forth
herein. Rather, these embodiments are provided so that this
disclosure will be thorough and complete, and will fully convey the
scope of the invention to those skilled in the art. It will be
understood that when an element is referred to as being "connected"
or "coupled" to another element, it can be directly connected or
coupled to the other element or intervening elements may be
present. Furthermore, "connected" or "coupled" as used herein may
include wirelessly connected or coupled.
[0034] The terminology used herein is for the purpose of describing
particular embodiments only and is not intended to be limiting of
the invention. As used herein, the singular forms "a", "an" and
"the" are intended to include the plural forms as well, unless
expressly stated otherwise. It will be further understood that the
terms "includes," "comprises," "including" and/or "comprising,"
when used in this specification, specify the presence of stated
features, integers, steps, operations, elements, and/or components,
but do not preclude the presence or addition of one or more other
features, integers, steps, operations, elements, components, and/or
groups thereof.
[0035] Unless otherwise defined, all terms (including technical and
scientific terms) used herein have the same meaning as commonly
understood by one of ordinary skill in the art to which this
invention belongs. It will be further understood that terms, such
as those defined in commonly used dictionaries, should be
interpreted as having a meaning that is consistent with their
meaning in the context of the relevant art and the present
disclosure, and will not be interpreted in an idealized or overly
formal sense unless expressly so defined herein.
[0036] It will be understood that although the terms first and
second are used herein to describe various elements, these elements
should not be limited by these terms. These terms are only used to
distinguish one element from another element. Thus, a first
radioterminal below could be termed a second radioterminal, and
similarly, a second radioterminal may be termed a first
radioterminal without departing from the teachings of the present
invention. As used herein, the term "and/or" includes any and all
combinations of one or more of the associated listed items. The
symbol "/" is also used as a shorthand notation for "and/or".
[0037] The present invention is described below with reference to
block diagrams, message flow diagrams and/or flowchart
illustrations of methods, apparatus (systems and/or devices) and/or
computer program products according to embodiments of the
invention. It is understood that a block of the block diagrams
and/or flowchart illustrations, and combinations of blocks in the
block diagrams and/or flowchart illustrations, can be implemented
by computer program instructions, These computer program
instructions may be provided to a processor of a general purpose
computer, special purpose computer, and/or other programmable data
processing apparatus to produce a machine, such that the
instructions, which execute via the processor of the computer
and/or other programmable data processing apparatus, create means
(functionality) and/or structure for implementing the
functions/acts specified in the block diagrams and/or flowchart
block or blocks.
[0038] These computer program instructions may also be stored in a
computer-readable memory that can direct a computer or other
programmable data processing apparatus to function in a particular
manner, such that the instructions stored in the computer-readable
memory produce an article of manufacture including instructions
which implement the function/act specified in the block diagrams
and/or flowchart block or blocks.
[0039] The computer program instructions may also be loaded onto a
computer or other programmable data processing apparatus to cause a
series of operational steps to be performed on the computer or
other programmable apparatus to produce a computer-implemented
process such that the instructions which execute on the computer or
other programmable apparatus provide steps for implementing the
functions/acts specified in the block diagrams and/or flowchart
block or blocks.
[0040] Accordingly, the present invention may be embodied in
hardware and/or in software (including firmware, resident software,
micro-code, etc.). Furthermore, the present invention may take the
form of a computer program product on a computer-usable or
computer-readable storage medium having computer-usable or
computer-readable program code embodied in the medium for use by or
in connection with an instruction execution system. In the context
of this document, a computer-usable or computer-readable medium may
be any medium that can contain, store, communicate, propagate, or
transport the program for use by or in connection with the
instruction execution system, apparatus, or device.
[0041] The computer-usable or computer-readable medium may be, for
example but not limited to, an electronic, magnetic, optical,
electromagnetic, infrared, or semiconductor system, apparatus,
device, or propagation medium. More specific examples (a
non-exhaustive list) of the computer-readable medium would include
the following: an electrical connection having one or more wires, a
portable computer diskette, a random access memory (RAM), a
read-only memory (ROM), an erasable programmable read-only memory
(EPROM or Flash memory), an optical fiber, and a portable compact
disc read-only memory (CD-ROM). Note that the computer-usable or
computer-readable medium could even be paper or another suitable
medium upon which the program is printed, as the program can be
electronically captured, via, for instance, optical scanning of the
paper or other medium, then compiled, interpreted, or otherwise
processed in a suitable manner, if necessary, and then stored in a
computer memory.
[0042] It should also be noted that in some alternate
implementations, the functions/acts illustrated may occur out of
the illustrated order. For example, two blocks or message flows
shown in succession may in fact be executed substantially
concurrently or may sometimes be executed in the reverse order,
depending upon the functionality/acts involved. Moreover, the
functionality of a given block of a flowcharts, block diagrams
and/or signal flow diagram may be separated into multiple blocks
and/or the functionality of two or more blocks of the flowcharts
and/or block diagrams may be at least partially integrated.
[0043] FIG. 1 illustrates a hybrid terrestrial/satellite wireless
communications environment according to some embodiments of the
present invention. In particular, FIG. 1 illustrates a terrestrial
PLMN 110, which includes infrastructure including a plurality of
geographically-distributed terrestrial base stations 112 (also
referred to base transceiver stations (BTSs) that serve respective
coverage areas 116 (e.g., cells), and other PLMN infrastructure 114
(e.g., base station controllers (BSCs), a mobile switching center
(MSC) and functional equivalents thereof) that supports call
routing, mobility management, billing and other operations in
concert with the base stations 112 and provides connectivity with
an interconnecting network 130, such as a public switched telephone
network (PSTN) and/or packet network. It will be appreciated that
the infrastructure of the terrestrial PLMN 110 may be arranged in a
number of different ways depending, for example, on the cellular
communications standard (e.g., GSM, CDMA, W-CDMA, etc.) supported
by the PLMN 110.
[0044] As further illustrated in FIG. 1, a satellite PLMN 120 has a
coverage area 126. For purposes of illustration, the satellite
coverage area 126 is shown overlapping the coverage areas 116 of
the terrestrial PLMN 110, but it will be appreciated that, in
various embodiments of the present invention, a coverage area of a
satellite PLMN that provides service in conjunction with a
terrestrial PLMN may completely overlap, partially overlap and/or
complement the coverage area of the terrestrial PLMN.
[0045] Still referring to FIG. 1, the satellite PLMN 120 includes
one or more satellites 122 configured to send/receive signals from
radioterminals in the coverage area 216, and supporting
ground-based infrastructure 124 that provides call management,
billing, mobility management and other functions, and that is
configured to provide connectivity to the interconnecting network
130. It will be appreciated that the one or more satellites 122 and
infrastructure 124 may, for example, support multiple spot beams
that support communication in respective subareas (e.g., cells) of
the coverage area 216, and that the infrastructure 124 may include,
for example, equipment that supports mobility management and other
functions to support communication of a radioterminal as it moves
about these various subareas.
[0046] Radioterminals 10 may be configured to communicate with both
the terrestrial PLMN 110 and the satellite PLMN 120 such that, for
example, the satellite PLMN 120 provides coverage when a
radioterminal is not within a coverage area of the terrestrial PLMN
110. The terrestrial PLMN 110 may also provide service to a
radioterminal 10 when in an area of high user density, to prevent
overload of the satellite PLMN 120. It will be appreciated the
PLMNs 110, 120 may be operated by the same network operator or by
separate operators.
[0047] As farther illustrated in FIG. 1, according to some
embodiments of the present invention, a handover controller 140 may
be provided to support handover between the terrestrial PLMN 110
and the satellite PLMN 120. According to various embodiments of the
present invention, the handover controller 140 may control various
aspects, such as conditions, timing and charging, associated with
transferring a call or other communications session of a
radioterminal 10 between the PLMNs. As illustrated in FIG. 1, the
handover controller 140 may be implemented in components of the
terrestrial PLMN and/or the satellite PLMN, e.g., in BSCs and MSCs
of the terrestrial PLMN 110 and/or functionally equivalent
components of the satellite PLMN 120. It will be understood that,
generally, functions of the handover controller 140 may be
implemented in a device or devices used to couple the PLMNs (egg.,
in the interconnecting network 130) and/or may be implemented in
various components of the PLMNs, and that such apparatus may
provide handover control operations independent of and/or in
concert with operations, such as signal monitoring, conducted by
the radioterminal 10.
[0048] In some embodiments of the present invention, a registration
with a satellite network, e.g., a Location Area (LA) update, such
as one conducted along the lines described in the aforementioned
U.S. patent application Ser. No. 10/948,606, is delayed by a
predetermined time period on entering a terrestrial coverage hole.
The delaying may involve, for example: [0049] (a) the mobile
terminal (MT) may start a timing counter when it senses that the
received signal strength, for example, a average signal strength
T.sub.avg over a predetermined period, exceeds a threshold value
T.sub.th; [0050] (b) if T.sub.avg remains continuously below
T.sub.th for a counted period, say C, then the MT registers to the
satellite subnetwork; [0051] (c) if T.sub.avg exceeds T.sub.th
before the count reaches C, the counter is reset and the MT stays
registered in the terrestrial subnetwork. Variations of the
above-described delaying procedures may be implemented in further
embodiments of the invention, and generally may be viewed as
providing hysteresis in the subnetwork switching process.
[0052] The above procedure may ensure that mobiles that are in a
coverage hole for a very brief period will not add to the capacity
load of the satellite subnetwork. This may eliminate a substantial
subset of MTs that pass through a coverage hole from tile global
set of mobiles that would otherwise immediately want to register on
to the satellite subnetwork. However, it is recognized that, during
this delay period, the MTs are denied communications service by any
subnetwork. Therefore, this technique may be thought as a means to
achieve a trade-off or compromise, between user experience and
network complexity and capacity.
[0053] A similar process may be followed by the MT for
deregistering from the satellite network and switching to the
terrestrial subnetwork. In other words, the MT may wait a first
predetermined period of time, during which the terrestrial signal,
averaged over a second predetermined period of time, continuously
exceeds a predetermined threshold value, before a subnetwork switch
is initiated. Given that the terrestrial subnetwork may be
preferred, the delay period in the switch from the satellite to the
terrestrial subnetwork may be shorter than its opposite
counterpart.
[0054] This approach may also be used to add hysteresis to
satellite-to-terrestrial mode transitions, regardless of which
method (e.g., one based on delay as described in the present
application, or any of the methods based on explicit or explicit
registration taught in U.S. patent application Ser. No.
10/948,606), is used to control the terrestrial-to-satellite mode
transition.
[0055] FIG. 2 illustrates exemplary operations for handover between
terrestrial and satellite networks according to some embodiments of
the present invention. A communications session, e.g., a call, of a
radioterminal is established with the terrestrial network (block
205). A degradation of coverage is detected (block 210). For
example, passage of the radioterminal into a coverage hole of the
terrestrial network may be detected using signal strength, bit
error and/or other measurements. If the degradation does not
persist for greater then a first threshold time period, the session
is continued with the terrestrial network (blocks 215, 220). If,
however, the degradation persists beyond the first threshold, the
session is handed over to the satellite network (blocks 215, 225),
and the session continues using the satellite infrastructure. An
improvement of coverage by the terrestrial network is subsequently
detected (block 230). For example, the radioterminal may pass out
of the coverage hole of the terrestrial network and back into a
terrestrial coverage area, which may be detected using signal
strength, bit error and/or other measurements. If the improvement
does not persist for greater than a second threshold time period,
the session is continued with the satellite network (blocks 235,
240). If the improvement persists for a sufficient time, however,
the session may be handed over to the terrestrial network (block
245).
[0056] In conventional wireless protocols, such as GSM, LA update
is typically performed in the idle mode (i.e., when a call is not
in progress), so that, when a call (either incoming or outgoing)
occurs, the MT registration is up to date. An exception to this may
occur if an LA change occurs during a call, the LA update may be
performed after the call is completed. However, in conventional
systems, this may apply only to intra-PLMN handovers. It is
noteworthy that, in intra-PLMN handover, there typically is no risk
of unauthorized access (as the mobiles credentials have already
been checked), which is an issue that can be addressed by the
techniques described in the aforementioned U.S. patent application
Ser. No. 10/948,606. In some embodiments of the present invention,
an alternative to these techniques includes a post-call location
update used in intra-PLMN handover adapted and extended to
inter-PLMN handovers.
[0057] In some embodiments, this may be complemented by MT
credentials checking at the source. Referring to FIG. 3, according
to some embodiments of the invention, substantially all handovers
from a source Mobile Switching Center (MSC), MSC-A, of a first
network PLMN-A to a destination MSC MSC-B of a second network
PLMN-B may be accepted as valid, where the source MSC MSC-A acts as
a "filter" to bar MTs not having access rights to the destination
satellite network PLMN-B from being handed over. A trust
arrangement may be set up between the two networks PLMN-A, PLMN-B.
For example, if the destination MSC MSC-B corresponds to a
satellite network PLMN, and the source MSC MSC-A belongs to a
terrestrial PCS PLMN, the source MSC MSC-A could check the MT's
subscription to see if it had the right to roam to the satellite
network before handing it over to the satellite network.
[0058] Besides MT credentials checking, all other information
exchange between the source MSC MSC-A and the destination MSC MSC-B
may be performed in a manner similar to conventional
intra-PLMN/inter-MSC handover, with the addition of appropriate
messages between the two MSCs to implement the exchange of
information not required in legacy GSM. Exemplary operations may
include: exchange of cell-related information between the two
PLMNs, e.g., through operations and maintenance centers (OMCs);
implementation of routing Signaling System 7 (SS7) tables allowing
the MSCs MSC-A, MSC-B to route to each other (an existing optional
function in GSM); and keeping a record of the call time spent in
the different networks PLMN-A, PLMN-B in the same call to determine
how to apportion the air time bill to the two networks PLMN-A,
PLMN-B.
[0059] The current GSM standard does not support inter-PLMN
handovers. In a hybrid satellite/terrestrial system according to
some embodiments of the present invention, a first possible
approach to inter-PLMN handover is to adopt handover principles
similar to those used in GSM, i.e., the handover decision is taken
by the function in charge of the connection prior to the handover
(e.g., the Base Station Controller (BSC) or Service Rating
Application Protocol (SRAP)), and the MT has essentially a slave
role during the whole process. In the following exemplary
embodiments described with reference to FIG. 3, a source BSC BSC-A
is the BSC in charge of the MT prior the handover, a source MSC
MSC-A is the MSC handling the call (the anchor MSC), a destination
BSC-B is the BSC (or SRAP) in charge of the target cell and a
destination MSC MSC-B (or satellite MSC) in charge of destination
BSC BSC-B.
[0060] It is assumed that the inter-MSC intra-PLMN handover
functionality is supported in both networks. The inter-PLMN case
will be analyzed through its differences with inter-MSC intra-PLMN
handovers. Generally, no substantial modifications are required in
the MT or in the radio interface procedures, inter-MSC procedures
directly related to calls can be used as they are currently
specified in GSM, and modifications may be made in configurations
(such as setting routing tables) and in call charging.
[0061] The existing GSM handover process does not require the MT to
be aware of which PLMN cells its measurements belong to. For a MT,
measurements are done on frequencies that it assumes carry a
Broadcast Control Channel (BCCH), taken from a list provided by the
source BSC BSC-A prior to the handover. Typically, no information
is provided concerning the PLMN to which those frequencies belong,
and the MT does not have to fetch or process information that
depends on those PLMNs.
[0062] In some embodiments of the present invention, the source BSC
BSC-A is aware of these frequencies so that it can send them to the
MTs. To support inter-PLMN, the source BSC BSC-A is made aware of
the frequencies of the cells the MTs might be handed over to.
According to some embodiments of the present invention, the first
network PLMN-A may know the frequencies of the relevant cells in
the second network PLMN-B, and may distribute them to the relevant
BSCs. This may require some geographical information on the cells.
Other cell-related information may also be required.
[0063] The exchange of such configuration data typically is not
supported by conventional standardized protocols. In some
embodiments of the present invention, a BSC receives information
pertaining to cells in other BSCs from an OMC. The implementation
of the modification described above could be done by connecting
OMCs. It should be noted that the information may change over time.
If the update time is short, automatic means might be used, rather
than manually entered data.
[0064] The MT may report power measurements and the Base Station
Identity Code (BSIC) of synchronized-on cells, where one of the
cells could be a satellite spotbeam. The BSIC allows solution of
ambiguities between cells having the same BCCH frequencies. Such an
ambiguity could prevent handing-over to the correct MSC. PLMN A
then needs to know the BSIC of the relevant cells in PLMN B, at
least in the ambiguous cases. This information is part of the
cell-related data, and can be handled as described below.
[0065] A handover may be decided by the source BSC BSC-A on the
basis of measurements reports from the MT, of measurements done by
radio functions and/or on configuration parameters such as
threshold values and weighting factors. Some of these parameters
are related to the target cells, and are part of the cell-related
information. In the case of two terrestrial networks, some other
parameters may require comparisons between the originating cell and
the target cell (e.g., to establish the boundary). Setting such
parameters might require closer collaboration than just sending
cell specific parameters.
[0066] Once the handover is decided, a new route may be established
from the source MSC MSC-A to destination MSC MSC-B to destination
BSC BSC-B. The source MSC MSC-A determines destination MSC MSC-B,
e.g., source MSC MSC-A is aware of a SS7 address of destination MSC
MSC-B. On the other hand, source BSC BSC-A may know the target cell
by a pair of parameters (e.g., frequency, BSIC). A first step may
be a translation by the source BSC BSC-A of the parameters (e.g.,
frequency, BSIC) to a cell identity.
[0067] The SS7 may route on the basis of the cell identity, e.g.,
on the Location Area Identity (LAI) part of it, and modification
may be needed in the source MSC MSC-A for this phase. The LAI is
unique worldwide because it is made up of Mobile Country Code
(MCC), Mobile Network Code (MNC) and Local Area Code (LAC), which
clearly identify the network and the MSC/VLR in charge of the
subscriber. This routing is possible if the SS7 networks are
connected (this is the case usually, since this typically is needed
for roaming between the two networks) and if SS7 nodes have in
their routing tables the data needed to reach the MSCs in the other
PLMN. The latter is not necessarily the case in conventional
systems. It may be necessary to augment SS7 routing tables to route
signaling between MSCs of the two PLMNs. As mentioned, this exists
already as an optional function described in GSM specifications.
The routing to destination BSC BSC-B may be done in a conventional
manner, as the fact that the MT will come from a different PLMN has
no impact on this function. The message back from destination MSC
MSC-B to source MSC MSC-A allows the establishment of a PSTN
connection, assuming the PSTNs are connected.
[0068] Still referring to FIG. 3, a RIL3 RR HANDOVER COMMAND
message may be generated by the source BSC-B and carried over the
different interfaces in different envelopes. For example, as shown
in FIG. 3, between the source BSC BSC-B and the source MSC MSC-B,
it is encapsulated in a BSSMAP HANDOVER REQUEST ACK, between
destination MSC MSC-B and source MSC MSC-A in a MAP/E PREPARE
HANDOVER ACK and between source MSC MSC-A and source BSC BSC-A in a
BSSMAP HANDOVER COMMAND.
[0069] When handovers are restricted to the same PLMN, the source
MSC (at the start of the call and managing the call from start to
end) may establish a call record covering all radio usage. With
inter-PLMN handovers this may no longer be the case, and it may be
necessary to maintain some record of the radio usage on a per-PLMN
basis. If modifications on the source side are to be limited, one
approach is to add a call record in destination MSC MSC-B. However,
with the GSM inter-MSC handover procedure, destination MSC MSC-B
conventionally would have limited access to subscriber data, and
then might have no knowledge of the charging center where to send
such a call record. According to some embodiments of the present
invention, there are ways to provide this functionality. For
instance, if the network PLMN-B (e.g., satellite network) has a
specific Home Location Register (HLR) keeping track of all
subscribers (including from other networks) allowed to access
network PLMN-B, then missing information may be limited to the
international mobile subscriber identity (IMSI), and this can be
obtained from the MT itself.
[0070] With modifications on the terrestrial side, call records may
be augmented to keep track of the time spent in each LAI in the
call. For existing GSM-based architectures, this may require
modifications of source MSC MSC-A, of the OMA node handling call
records and of the exchange protocol between the two. This tracking
of LAI might already be implemented for other reasons than
inter-PLMN handovers.
[0071] In the above-described embodiments, modifications to support
inter-PLMN handovers between two networks that already support
inter-MSC handovers include: exchange of cell-related information
between the two PLMNs, e.g., through OMCs;
[0072] implementation of the routing SS7 tables allowing MSCs to
route to each other, an already existing optional function; and
recording of call time spent in different PLMNs in the same
call.
[0073] A possible reason for not substantially following GSM
practices is to minimize the impact on a PCS network in a handover
from PCS to satellite. In such a case, the PCS is on the
terrestrial side as described above with reference to FIG. 3. A way
to reduce impact on the terrestrial side would be to remove the
need to handle configuration data pertaining to the satellite
network. One way is to have MT-triggered handovers. If this
simplifies the terrestrial side for the handover decision, the
impacts resulting from allowing the transfer, the re-routing, from
the source MSC-A to the destination MSC MSC-B may be very
significant. Conventionally, GSM supports a form of MT-triggered
handover (call re-establishment), but only inter-BSC, not
inter-MSC. One reason is because of the complexity of the
re-routing between two MSCs.
[0074] The timing advance resolution required in a conventional GSM
TDMA system is 3.7 microseconds and is the inverse of the channel
data rate (270 kbps) of GSM. At the speed of light, this
corresponds to a propagation distance of 1.11 km. Further, the GSM
frame structure allows approximately 8 channel bits for guard time
in the traffic channel bursts. This means that, if the range to the
satellite from the base station is known to approximately this
resolution, and if the distance of the MT from the base station is
also known to this resolution, the combined error will be well
within the specified guard time of 8 channel bits.
[0075] According to some embodiments of the present invention, the
following approach may be used for establishing a priori
information about the timing advance. GSM is used as an example but
the principles could be applied to other air interfaces containing
a TDM component, or any kind of time-framing component, as is
present in CDMA2000 (including EVDO), WCDMA and WiMax, and
satellite adaptations of the above air interfaces, where time
epochs of waveforms transmitted from a plurality of mobile
satellite terminals distributed over a large geographic area may be
aligned.
[0076] The location of the base station tower may be surveyed and
determined to a requisite accuracy. During a
terrestrial-to-satellite handover, the location of the base station
tower may be indicated to the satellite network, which, in turn,
may tell the terrestrial BSC/BTS the current range of the
satellite. While satellites in geostationary orbit nominally
maintain a fixed orbital location relative to the Earth, in
practice, their orbital location describes a figure-of-eight
pattern about the nominal location, with a periodic North-South and
East-West variation. These variations are slow compared to the time
epochs in an air interface, such as GSM, and can be predicted quite
accurately as a function of absolute time. Thus, predicting the
range to the satellite from the BTS tower is feasible. Based on the
latter, the terrestrial BSC may, in the handover command to the MT,
advise it of the timing advance to use on the satellite link.
[0077] According to some embodiments of the present invention, the
base station tower may be used as an approximation for the MT
position. Owing to the requirements of E911 and location based
services, this information is often available in the terrestrial
wireless communication system, either at the MT or at the BTS/MSC
or both. If the MT position information is available, it may be
used instead of the base station tower location with a
corresponding increase in the handover performance. The MT position
information may be obtained in a variety of ways. For example,
determination of MT position may encompass the use of Global
Positioning Satellites (GPS) and/or triangulation from multiple
base stations. All methods of MT geolocation are within the scope
of the present invention.
[0078] FIG. 4 shows a handshaking protocol that may be used to
implement synchronous handover using a priori information as
described above according to some embodiments of the present
invention. The initial phase of the exchange is "call in progress"
between the MT and the terrestrial infrastructure, comprising the
terrestrial base transceiver station (BTS) BTS-A, terrestrial BSC
BSC-A and terrestrial MSC MSC-A. Based on signal strength quality
measurements performed by the MT and terrestrial BTS BTS-A, and
possibly other criteria, a decision is made by terrestrial BSC
BSC-A to handover the call to the satellite network. A "RSM Channel
Activation" message is sent to the satellite BTS BTS-B via the
hybrid network's core, which includes both the terrestrial core
network and the satellite core network (including the satellite MSC
MSC-B and the satellite BSC BSC-B). According to some embodiments
of the present invention, the cell identity may be included in this
message. The information may be included, for example, as the
latitude and longitude of the base station tower or as a base
station tower ID which points to similar information in a database
carried by the satellite gateway.
[0079] On receipt of the RSM Channel Activation message, the
satellite network allocates radio resources (power, frequency and
bandwidth) to the new call and responds with a "RSM Channel
Activation Acknowledgement" message from satellite BTS-B back to
the terrestrial infrastructure (e.g., BTS-A/BSC-A/MSC-A). The
current satellite range from the satellite gateway may be included
in this message. Knowing its own location and the location of the
satellite gateway, the terrestrial BSC BSC-A may calculate the
range to the satellite from the base station tower. Based on this
calculation, BSC-A may also calculate the timing advance that the
MT should use on the satellite channel. This information may be
communicated to the MT as a part of a modified "RIL3-RR Handover
Command" message. In the above discussion, the satellite may be
viewed as an equivalent of the BTS tower in a handover context. For
timing advance and handover functions, the time reference may be
established at the satellite, not at the satellite gateway,
although the latter is where the signal demodulation and modulation
may be performed. This is because differential time delays with
respect to different mobile terminals exist only with respect to
the satellite--the transit delay between the satellite and the
gateway is identical for all mobile terminals.
[0080] As an alternative to the techniques described above, in
order to reduce changes to terrestrial infrastructure, the timing
advance may be calculated by the satellite BSC BSC-B using
knowledge of the cell tower's location supplied in the "RSM Channel
Activation" message and the timing advance information supplied to
terrestrial BSC BSC-A in the "RSM Channel Activation
Acknowledgement." Other variations of how the computation of timing
advance is distributed may be implemented in other embodiments of
the invention.
[0081] On receipt of the handover command with the timing advance,
the MT proceeds to send access probes, called "RIL3-RR Handover
Access," on the satellite channel. These probes can be used to
confirm to the satellite BTS BTS-B that the correct MT is using the
channel and that the correct timing advance is being used. These
probes are optional. After sending a predetermined number of
probes, the MT may send a "RR Handover Complete" message to the
satellite BSC BSC-B and resume traffic communication on the
satellite channel. Ancillary messages between the satellite and
terrestrial subnetworks, reflecting GSM practice, are also shown in
FIG. 4. These may switch the call on the core network side in an
orderly manner.
[0082] When the destination network involves a long-delay channel,
such as a satellite channel, the extent of handshaking in legacy
protocols, such as GSM, may introduce an excessively long delay
between the onset of handover and the time when the timing advance
is known to the MT. During this time, no communication path may be
available from the MT to the core networks of either the
terrestrial or satellite subnetworks. This may be perceived by the
user as a temporary loss of signal. Such delay could be 750 ms or
greater, which would tend to produce a negative user experience. By
anticipating this delay and delaying the switchover of the traffic
channel from the originating to the destination channel, the
problem may be mitigated.
[0083] According to some embodiments of the invention, a
predetermined delay is introduced in the communication of an
acknowledgement signal from a MT to a terrestrial BSC responsive to
a handover command message from the BSC. For example, in some
exemplary embodiments of the invention, a 500 ms delay is provided
in sending an ACK message from the MT in response to a RR Handover
Command message from a terrestrial BSC. This ACK causes the
terrestrial path to be shut down. The delay may be set at the
minimum possible time that would be required to determine and
communicate the new timing advance to the MT, e.g., a 250 ms
propagation delay for the first "RR Handover Access" message from
the MT to the Satellite BSC and another 250 ms propagation delay
from the Satellite BSC to the MT. Even if the MT started
transmitting traffic immediately on the satellite channel on
receipt of RR Physical Info, there could be another 250 ms
propagation delay before the traffic signals would reach the
satellite core network, resulting in the above mentioned net delay
of 750 ms.
[0084] According to some embodiments of the present invention, when
a handover occurs from terrestrial network to satellite network or
within satellite networks, the MT is already pre-synchronized with
the target cell or spotbeam. The example shown below is for a
hybrid satellite/terrestrial system employing geostationary
satellite(s). It will be appreciated that the specific times
discussed herein are provided for exemplary purposes, and that the
present invention encompasses systems and methods using other
times.
[0085] For emission from a MT, a timing advance may be used. A
coarse timing advance (CTA) may be independent from the MT
position, and can be sent in a HANDOVER COMMAND message. The MT may
determine a Fine Timing Advance (FTA) before starting emission.
[0086] Two possible solutions for determining FTA include:
[0087] 1) The satellite gateway BSC computes the Fine Timing
Advance only on the basis of the Terrestrial ATN or PCS Cell Id.
The Fine Timing Advance can then be provided to the MT by the
satellite gateway BSC in the HANDOVER COMMAND message between
satellite gateway and terrestrial network (PCS and ATN). This may
entail that each network will have to provide to the satellite
system operators a detailed radio coverage map. Furthermore,
terrestrial cell may need to be small enough to take into account
the delay variations within a terrestrial cell (FTA computed on
cell center position).
[0088] 2) An S-RACH message is sent to the satellite gateway in
order to compute the Fine Timing Advance of the terminal. This
solution is explained below and takes place after the reception of
RR HANDOVER COMMAND message.
[0089] When a network control center (NCC) detects a random access
transmission from the MT, it may determine the delay advance of the
MT signal relative to a signal timing that would be expected from a
MT relative to the coarse timing advance. The delay may be assessed
in such a way that the assessment error (due to noise and
interference as well as all timing uncertainties in the RF/IF and
signaling equipment paths within the NCC) is less than 1/4 return
bit periods for stationary MT and for MT moving at speeds up to 100
km/h. The gateway may derive a FTA parameter equivalent to the
delay.
[0090] When the MT accesses the radio resource(s) of the new base
station subsystem (BSS) with a HANDOVER ACCESS burst which contains
the received handover reference number: [0091] 1) the new BSS may
check the handover reference number to ensure that it is the same
as expected, and hence that there is a high probability that the
correct MT has been captured (if the handover reference is not as
expected then the new BSS shall wait for an access by the correct
MS); [0092] 2) if the handover reference number is as expected, the
new BSS may send a HANDOVER DETECT message to the MSC; [0093] 3)
when the MT is successfully in communication with the network,
e.g., the RR message HANDOVER COMPLETE has been received from the
MT, then the new BSS may immediately send a BSSMAP message HANDOVER
COMPLETE to the MSC; and [0094] 4) the SEND END SIGNAL may be sent
to the anchor MSC in order to terminate the procedure.
[0095] A time interval (e.g., t least 500 ms) may be necessary for
the FTA allocation (e.g., best case when the first HANDOVER ACCESS
message is handled by the satellite gateway). But only uplink (from
MT to gateway) may be impacted by the post synchronization issue.
As a stream of RR HANDOVER ACCESS messages is sent by the MT to the
new BSC, a short interval (e.g., a few milliseconds) may be needed
to complete the procedure if the first message is not answered
(worst case). A longer interval (e.g., 250 ms) may be needed
between MT transmission and gateway reception after handover.
[0096] Even though a delay (e.g., 750 ms) can be a constraint to a
seamless handover for post-synchronization purposes, according to
some embodiments of the present invention, existing post handover
synchronization procedures used in GSM may be used in a hybrid
system by modifying these procedures to include such a delay. In
order to provide a seamless handover from terrestrial to satellite
mode, the proposed post synchronization phase may require
modifications in relation to GSM procedures. The time necessary to
complete post handover synchronization takes into account long
satellite transmission delay.
[0097] In post handover synchronization phase in a conventional GSM
environment, an MT receives and transmits speech/data with a first
BSC until the reception of the RR HANDOVER COMMAND message. Then
the MT waits for the RR PHYSICAL INFO of a second BSC in order to
re-start speech/data reception/transmission on the target cell. In
a conventional terrestrial GSM environment, only few milliseconds
of suspended transmission/reception typically are necessary and
this generally has no impact on user perception of call
continuity.
[0098] In a hybrid terrestrial-to-satellite handover using such an
approach, a MT may:
[0099] 1) send a RR HANDOVER ACCESS to the satellite BSC (e.g.,
about 250 ms);
[0100] 2) suspend transmission and reception with the terrestrial
system and wait for RR PHYSICAL INFO (e.g., about 250 ms); and
[0101] 3) start reception/transmission with the satellite network
(e.g., about 250 ms between transmission from MT or gateway and
reception on the other side).
[0102] As a consequence, more than 750 ms may be used to complete
post synchronization. It may be noted that Layer 2 exchanges
(SABM-UA) may happen during communication phase after the MT is in
the satellite-connected mode. As a result, SABM and UA messages for
the satellite delay do not need to be taken into account.
[0103] According to some embodiments of the present invention
illustrated in the message flow diagram of FIG. 5, in order to
reduce the time in which transmission/reception are suspended,
shutdown of the traffic channel with the terrestrial system is
delayed for a time sufficient to allow all or most of
synchronization operations needed to initiate communications with
the satellite system while allowing the MT to continue transmission
reception with the terrestrial system, In some embodiments, for
example, a timer (eg., a 500 ms timer) is started when a MT
receives from terrestrial network BSC BSC-A the RR HANDOVER
COMMAND. The MT doesn't immediately send an ACK message to the
terrestrial BSC BSC-A. Instead, the MT switches to the satellite
frequency and transmits the RR HANDOVER ACCESS message to the
satellite BSC USC-B. Then, within terrestrial network, the MT may
continue radio transmission and reception, which is possible
because the ACK message has not been received by the terrestrial
BSC BSC-A. This is not a conventional GSM procedure. When timer
expires, the ACK message is sent to the terrestrial BSC BSC-B and
the call is handed over to the satellite BSC BSC-B.
[0104] The approach described above may be modified by making the
delay variable rather than fixed. For example, for the example of
FIG. 5, instead of waiting for expiration of a predetermined time
interval, the delay may be terminated by the receipt of a RR
Physical Info message from the Satellite BSC USC-B. If the first RR
Handover Access message is not received correctly by the satellite
BSC BSC-B, the ACK may be delayed by additional rounds of handshake
delays until the new path is truly open. Whether a fixed or
variable delay is used may, for example, depend on the tolerance of
the terrestrial infrastructure for departures from legacy
practices. Some embodiments of the invention may combine both
approaches.
[0105] FIG. 6 illustrates a radioterminal 600 according to some
embodiments of the present invention. The radioterminal 600
includes a radio transceiver 610 operatively associated with a
processor 620 configured to support radio communications with
satellite and terrestrial networks via the transceiver 610 as
discussed above with reference to FIGS. 1-5. In particular, the
processor 610, e.g., a microprocessor, microcontroller, digital
signal processor (DSP) or the like, is configured to implement a
handover control process 625 that may implement handover control
functions, such as signal strength measurements, handover candidate
channel identification, handover message interpretation and
generation, and other functions along the lines discussed above
with reference to FIGS. 1-5. As shown, the processor 620 may also
be configured to interoperate with user interface circuitry 630,
such as a display, keypad, microphone, speaker, etc.
[0106] It will be appreciated that the apparatus and operations
described above are illustrative examples, and that other
architectures and operations fall within the scope of the present
invention. More generally, in the drawings and specification, there
have been disclosed exemplary embodiments of the invention.
Although specific terms are employed, they are used in a generic
and descriptive sense only and not for purposes of limitation, the
scope of the invention being defined by the following claims.
* * * * *