U.S. patent application number 12/058144 was filed with the patent office on 2008-10-30 for system and method for self-optimization of interference coordination in communication systems.
Invention is credited to Leping Huang, Kodo Shu.
Application Number | 20080268833 12/058144 |
Document ID | / |
Family ID | 39735569 |
Filed Date | 2008-10-30 |
United States Patent
Application |
20080268833 |
Kind Code |
A1 |
Huang; Leping ; et
al. |
October 30, 2008 |
System and Method for Self-Optimization of Interference
Coordination in Communication Systems
Abstract
In accordance with aspects of the present invention, a method,
apparatus and system are provided for self-optimization of
interference coordination in communication systems. In one
embodiment, the apparatus includes a measurement and reporting
subsystem configured to perform measurements regarding a status of
the apparatus and to provide a reliability report to a base station
as a function thereof. The apparatus is configured to notify a
strongest interferer about the status via the reliability report.
In another embodiment, the apparatus includes a measurement report
handler configured to process a reliability report received from a
user equipment and an interference coordination subsystem
configured to adjust interference coordination parameters based on
the reliability report received from the user equipment.
Inventors: |
Huang; Leping; (Tokyo,
JP) ; Shu; Kodo; (Kawasaki, JP) |
Correspondence
Address: |
SLATER & MATSIL, L.L.P.
17950 PRESTON RD, SUITE 1000
DALLAS
TX
75252-5793
US
|
Family ID: |
39735569 |
Appl. No.: |
12/058144 |
Filed: |
March 28, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60921247 |
Mar 30, 2007 |
|
|
|
Current U.S.
Class: |
455/425 ;
455/438; 455/501; 455/67.13 |
Current CPC
Class: |
H04L 1/1867 20130101;
H04W 72/082 20130101; H04B 7/024 20130101; H04W 72/0413
20130101 |
Class at
Publication: |
455/425 ;
455/67.13; 455/501; 455/438 |
International
Class: |
H04Q 7/34 20060101
H04Q007/34; H04B 17/00 20060101 H04B017/00; H04Q 7/20 20060101
H04Q007/20; H04B 15/00 20060101 H04B015/00 |
Claims
1. An apparatus comprising a measurement and reporting subsystem
configured to perform measurements regarding a status of said
apparatus and to provide a reliability report to a base station as
a function thereof, said apparatus configured to notify a strongest
interferer about said status via said reliability report.
2. The apparatus as recited in claim 1 wherein said base station is
a source base station in a cell serving said apparatus.
3. The apparatus as recited in claim 1 wherein said base station is
a target base station in a target cell following a handover event
thereto.
4. The apparatus as recited in claim 1 wherein said reliability
report is sent as part of a radio resource control message to said
base station.
5. The apparatus as recited in claim 1 wherein said strongest
interferer is configured to adjust an interference coordination
parameter based on said reliability report, said strongest
interferer being configured to reduce transmission power at a cell
center sub-band thereof when said reliability report indicates poor
results and increase transmission power at a cell center sub-band
thereof when said reliability report indicates good results.
6. The apparatus as recited in claim 1 wherein said strongest
interferer is a target base station.
7. The apparatus as recited in claim 1 wherein said reliability
report includes information selected from the group consisting of:
intra-frequency handover measurements to detect said strongest
interferer for said apparatus, a channel quality indicator report
of a downlink of said base station, a radio link failure, a number
of automatic retransmit requests/hybrid automatic retransmit
requests of a handover command, and an average number of automatic
retransmit requests/hybrid automatic retransmit requests when said
apparatus is in a edge of an area served by said base station.
8. An apparatus, comprising: means for performing measurements
regarding a status of said apparatus; means for providing a
reliability report to a base station as a function thereof; and
means for notifying a strongest interferer about said status via
said reliability report.
9. The apparatus as recited in claim 8 wherein said base station is
a source base station in a cell serving said apparatus or a target
base station in a target cell following a handover event
thereto.
10. The apparatus as recited in claim 8 wherein said strongest
interferer is configured to adjust an interference coordination
parameter based on said reliability report, said strongest
interferer being configured to reduce transmission power at a cell
center sub-band thereof when said reliability report indicates poor
results and increase transmission power at a cell center sub-band
thereof when said reliability report indicates good results.
11. A computer program product comprising program code stored in a
computer readable medium configured to perform measurements
regarding a status of a user equipment, provide a reliability
report to a base station as a function thereof, and notify a
strongest interferer about said status via said reliability
report.
12. The computer program product as recited in claim 11 wherein
said base station is a source base station in a cell serving said
apparatus or a target base station in a target cell following a
handover event thereto.
13. The computer program product as recited in claim 11 wherein
said strongest interferer is configured to adjust an interference
coordination parameter based on said reliability report, said
strongest interferer being configured to reduce transmission power
at a cell center sub-band thereof when said reliability report
indicates poor results and increase transmission power at a cell
center sub-band thereof when said reliability report indicates good
results.
14. A method, comprising: performing measurements regarding a
status of a user equipment; providing a reliability report to a
base station as a function thereof; and notifying a strongest
interferer about said status via said reliability report.
15. The method as recited in claim 14 wherein said base station is
a source base station in a cell serving said user equipment.
16. The method as recited in claim 14 wherein said base station is
a target base station in a target cell following a handover event
thereto.
17. The method as recited in claim 14 wherein said reliability
report is sent as part of a radio resource control message to said
base station.
18. The method as recited in claim 14 wherein said strongest
interferer is configured to adjust an interference coordination
parameter based on said reliability report, said strongest
interferer being configured to reduce transmission power at a cell
center sub-band thereof when said reliability report indicates poor
results and increase transmission power at a cell center sub-band
thereof when said reliability report indicates good results.
19. The method as recited in claim 14 wherein said strongest
interferer is a target base station.
20. The method as recited in claim 14 wherein said reliability
report includes information selected from the group consisting of:
intra-frequency handover measurements to detect said strongest
interferer for said user equipment, a channel quality indicator
report of a downlink of said base station, a radio link failure, a
number of automatic retransmit requests/hybrid automatic retransmit
requests of a handover command, and an average number of automatic
retransmit requests/hybrid automatic retransmit requests when said
user equipment is in a edge of an area served by said base
station.
21. An apparatus, comprising: a measurement report handler
configured to process a reliability report received from a user
equipment; and an interference coordination subsystem configured to
adjust interference coordination parameters based on said
reliability report received from said user equipment.
22. The apparatus as recited in claim 21 further comprising a
packet scheduler configured to allocate uplink and downlink
resources for said apparatus in response to said reliability report
received from said user equipment.
23. The apparatus as recited in claim 21 wherein said interference
coordination subsystem is configured to selectively allocate
relative transmitter power levels in each of a plurality of
sub-bands of an area served by said apparatus.
24. The apparatus as recited in claim 21 wherein said interference
coordination subsystem is configured to adjust interference
coordination parameters in response to a reliability report from a
user equipment operating in a served area of another apparatus.
25. The apparatus as recited in claim 21 further comprising a
handover subsystem configured to perform a handover decision
sequence for said user equipment as a function of said reliability
report.
26. The apparatus as recited in claim 21 wherein said reliability
report includes information selected from the group consisting of:
intra-frequency handover measurements to detect a strongest
interferer for said user equipment, a channel quality indicator
report of a downlink of said apparatus, a radio link failure, a
number of automatic retransmit requests/hybrid automatic retransmit
requests of a handover command, and an average number of automatic
retransmit requests/hybrid automatic retransmit requests when said
user equipment is in an edge of an area served by said
apparatus.
27. The apparatus as recited in claim 21 wherein said interference
coordination parameters are selected from the group consisting of:
an antenna pattern, a transmission power, a transmission frequency,
a transmission time, and a level of traffic in a center sub-band of
an area served by said apparatus.
28. An apparatus, comprising: means for processing a reliability
report received from a user equipment; and means for adjusting
interference coordination parameters based on said reliability
report received from said user equipment.
29. The apparatus as recited in claim 28 further comprising means
for allocating uplink and downlink resources for said apparatus in
response to said reliability report received from said user
equipment.
30. The apparatus as recited in claim 28 further comprising means
for performing a handover decision sequence for said user equipment
as a function of said reliability report.
31. A computer program product comprising program code stored in a
computer readable medium configured to process a reliability report
received from a user equipment and adjust interference coordination
parameters based on said reliability report received from said user
equipment.
32. The computer program product as recited in claim 31 further
comprising program code stored in a computer readable medium
configured to allocate uplink and downlink resources for said
apparatus in response to said reliability report received from said
user equipment.
33. The computer program product as recited in claim 31 further
comprising program code stored in a computer readable medium
configured to perform a handover decision sequence for said user
equipment as a function of said reliability report.
34. A method, comprising: processing a reliability report received
from a user equipment; and adjusting interference coordination
parameters for a base station serving said user equipment based on
said reliability report received from said user equipment.
35. The method as recited in claim 34 further comprising allocating
uplink and downlink resources for said base station in response to
said reliability report received from said user equipment.
36. The method as recited in claim 34 wherein said adjusting
comprises selectively allocating relative transmitter power levels
in each of a plurality of sub-bands of an area served by said base
station.
37. The method as recited in claim 34 wherein said adjusting
comprises adjusting interference coordination parameters in
response to a reliability report from a user equipment operating in
a served area of another base station.
38. The method as recited in claim 34 further comprising performing
a handover decision sequence for said user equipment as a function
of said reliability report.
39. The method as recited in claim 34 wherein said reliability
report includes information selected from the group consisting of:
intra-frequency handover measurements to detect a strongest
interferer for said user equipment, a channel quality indicator
report of a downlink of said base station, a radio link failure, a
number of automatic retransmit requests/hybrid automatic retransmit
requests of a handover command, and an average number of automatic
retransmit requests/hybrid automatic retransmit requests when said
user equipment is in an edge of an area served by said base
station.
40. The method as recited in claim 34 wherein said interference
coordination parameters are selected from the group consisting of:
an antenna pattern, a transmission power, a transmission frequency,
a transmission time, and a level of traffic in a center sub-band of
an area served by said base station.
41. A communication system, comprising a user equipment, including:
a measurement and reporting subsystem configured to perform
measurements regarding a status of said user equipment and to
provide a reliability report to a base station as a function
thereof, and a base station, including: a measurement report
handler configured to process said reliability report received from
said user equipment; and an interference coordination subsystem
configured to adjust interference coordination parameters based on
said reliability report received from said user equipment.
42. The communication system as recited in claim 41 wherein said
base station is a source base station in a cell serving said user
equipment or a target base station in a target cell following a
handover event thereto.
43. The communication system as recited in claim 41 wherein said
base station further includes a packet scheduler configured to
allocate uplink and downlink resources for said base station in
response to said reliability report received from said user
equipment.
44. The communication system as recited in claim 41 wherein said
interference coordination subsystem is configured to selectively
allocate relative transmitter power levels in each of a plurality
of sub-bands of an area served by base station.
45. The communication system as recited in claim 41 wherein said
base station further includes a handover subsystem configured to
perform a handover decision sequence for said user equipment as a
function of said reliability report.
Description
[0001] This application claims the benefit of U.S. Provisional
Application No. 60/921,247 entitled "System and Method for
Self-Optimization of Interference Coordination in Communication
Systems," filed on Mar. 30, 2007, which is incorporated herein by
reference.
TECHNICAL FIELD
[0002] The present invention is directed, in general, to
communication systems and, more particularly, to a system and
method for self-optimization of interference coordination in
communication systems.
BACKGROUND
[0003] As wireless communication systems such as cellular
telephone, satellite, and microwave communication systems become
widely deployed and continue to attract a growing number of users,
there is a pressing need to accommodate a large and variable number
of communication devices transmitting a growing volume of data with
fixed resources. Traditional communication system designs employing
a fixed resource (e.g., a fixed data rate for each communication
device) have become challenged to provide high, but flexible, data
transmission rates in view of the rapidly growing customer base and
expanding levels of service.
[0004] The Third Generation Partnership Project Long Term Evolution
("3GPP LTE") is the name generally used to describe an ongoing
effort across the industry to improve the universal mobile
telecommunications system ("UMTS") for mobile communications to
cope with continuing new requirements and the growing base of
users. The goals of this broadly based project include improving
communication efficiency, lowering costs, improving services,
making use of new spectrum opportunities, and achieving better
integration with other open standards. The 3GPP LTE project is not
itself a standard-generating effort, but will result in new
recommendations for standards for the UMTS.
[0005] One of the areas addressed by the ongoing efforts is
interference between cells in a wireless communication system.
Interference coordination (also referred to as interference
mitigation or interference control) is a generic term for any kind
of system-level radio resource management-based interference
mitigation scheme such as interference coordination and avoidance,
inter-cell interference randomization, and inter-cell interference
cancellation. Soft frequency reuse refers to one type of
interference coordination scheme for evolved UMTS terrestrial radio
access network ("e-UTRAN") communication systems, which provides
frequency coordination (or time-frequency coordination for
synchronous systems) by applying power restrictions (reduced power)
in certain time-frequency domains to achieve power
coordination.
[0006] Within various interference coordination schemes in the 3GPP
LTE project, power-sequence-based interference control, which is a
soft frequency reuse interference coordination scheme, is the most
commonly studied class of interference coordination schemes. In a
soft frequency reuse-based interference coordination scheme,
frequency (or time) resources are divided into different power
level groups and an allocation procedure is utilized to divide the
resources. In the allocation procedure, each sector is assigned
portions of the frequency sub-bands according to a communication
system plan, wherein different power constraints are assigned to
different portions of frequency bands. Flexibility for interference
reduction is achieved by varying power levels in the bands. Soft
frequency reuse schemes can alleviate interference between cells
because a fraction of the bandwidth in each cell is not used
(affecting frequency reuse) and/or a penalty is not incurred
because the fraction of the bandwidth in each cell is partially
overlapped (as seen from the fractional frequency reuse
schemes).
[0007] Proposals have been presented in the 3GPP LTE project for
signaling to support adaptive interference control schemes that
have focused on both a raw physical layer measurements point of
view, such as the exchange of user equipment channel quality
indicator measurements or pilot measurements between cells and
radio resource management, and a quality-of-service point of view.
While the proposals have addressed the interference coordination,
there is still an opportunity for improvement therefor. Thus,
considering the current limitations of communication systems
including cellular networks, a system and method to manage
interference between elements of a communication system to provide
a higher level of communication reliability, particularly at the
edge of the coverage area of a base station, that does not add
substantially to signaling overhead and that is operable in
real-time would be beneficial.
[0008] Accordingly, what is needed in the art is a system and
method to self-optimize a level of interference between elements of
a communication system in an interference coordination scheme.
Advantageously, the reliability of handover signaling would thereby
be increased. Otherwise, communication systems such as 3GPP
LTE-compatible communication systems employing handover signaling
may lose a competitive edge against other communication systems.
Additionally, the system and method to self-optimize a level of
interference between elements of a communication system to increase
reliable handover should avoid inducing significant architectural
changes to the existing communication systems.
SUMMARY OF THE INVENTION
[0009] These and other problems are generally solved or
circumvented, and technical advantages are generally achieved, by
embodiments of the present invention, which include a method,
apparatus, and system to adaptively manage interference between
elements of a communication system in a self-optimization process.
In one embodiment, the apparatus (e.g., user equipment) includes a
measurement and reporting subsystem configured to perform
measurements regarding a status of the apparatus and to provide a
reliability report to a base station as a function thereof. The
user equipment is configured to notify a strongest interferer about
the status via the reliability report. In another embodiment, the
apparatus (e.g., a base station) includes a measurement report
handler configured to process a reliability report received from a
user equipment and an interference coordination subsystem
configured to adjust interference coordination parameters based on
the reliability report received from the user equipment.
[0010] In another aspect, the present invention provides a
communication system including user equipment and a base station.
In one embodiment, the user equipment includes a measurement and
reporting subsystem configured to perform measurements regarding a
status of the user equipment and to provide a reliability report to
a base station as a function thereof. The base station includes a
measurement report handler configured to process the reliability
report received from the user equipment, and an interference
coordination subsystem configured to adjust interference
coordination parameters based on the reliability report received
from the user equipment.
[0011] The foregoing has outlined rather broadly the features and
technical advantages of the present invention in order that the
detailed description of the invention that follows may be better
understood. Additional features and advantages of the invention
will be described hereinafter which form the subject of the claims
of the invention. It should be appreciated by those skilled in the
art that the conception and specific embodiment disclosed may be
readily utilized as a basis for modifying or designing other
structures or processes for carrying out the same purposes of the
present invention. It should also be realized by those skilled in
the art that such equivalent constructions do not depart from the
spirit and scope of the invention as set forth in the appended
claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] For a more complete understanding of the invention, and the
advantages thereof, reference is now made to the following
descriptions taken in conjunction with the accompanying drawing, in
which:
[0013] FIGS. 1 and 2 illustrate system level diagrams of a
communication system including a wireless communication system that
provides an environment for the application of the principles of
the present invention;
[0014] FIGS. 3 and 4 illustrate system level diagrams of a wireless
communication system that provides an environment for the
application of the principles of the present invention;
[0015] FIGS. 5 and 6 illustrate block diagrams of embodiments of
user equipment and a base station of a communication system
demonstrating, respectively, downlink and uplink communication
between the user equipment and the base station constructed in
accordance with the principles of the present invention;
[0016] FIG. 7 illustrates a block diagram of an embodiment of user
equipment and a base station that provides the exemplary functional
layers thereof constructed in accordance with the principles of the
present invention;
[0017] FIG. 8 illustrates a block diagram of an embodiment of user
equipment and base stations of a communication system constructed
in accordance with the principles of the present invention;
[0018] FIG. 9 illustrates a signaling diagram of an embodiment of
performing self-optimization according to the principles of the
present invention;
[0019] FIGS. 10 and 11 illustrate signaling diagrams of an
embodiment of a handover sequence according to the principles of
the present invention; and
[0020] FIG. 12 illustrates a diagram of an embodiment for
performing a handover sequence according to the principles of the
present invention.
DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
[0021] The making and using of the presently preferred embodiments
are discussed in detail below. It should be appreciated, however,
that the present invention provides many applicable inventive
concepts that can be embodied in a wide variety of specific
contexts. The specific embodiments discussed are merely
illustrative of specific ways to make and use the invention, and do
not limit the scope of the invention.
[0022] Proposals have been presented in the 3GPP LTE project for
signaling to support adaptive interference control schemes. For
example, as provided in 3GPP TR 25.814, referred to as "3rd
Generation Partnership Project; Technical Specification Group Radio
Access Network; Physical layer aspects for evolved Universal
Terrestrial Radio Access (UTRA), (Release 7)," V7.0.0 (2006-06),
which is incorporated herein by reference, three approaches to
inter-cell interference mitigation are currently being considered,
namely, inter-cell interference randomization, inter-cell
interference cancellation, and inter-cell interference coordination
and avoidance. (See, e.g., Section 7.2.1.6.) The different
approaches may, at least to some extent, complement one another and
are not necessarily mutually exclusive.
[0023] Section 7.1.2.6.1 of 3GPP TR 25.814 deals with inter-cell
interference randomization, while section 7.1.2.6.2 deals with
inter-cell interference cancellation. The latter section provides
that the user equipment is signaled whether it can perform a
cancellation of the received inter-cell interference and inter-cell
signaling, and interfering signal configurations (e.g., interleaver
pattern identification, modulation scheme, forward error correction
scheme and coding rate) are also signaled to the user
equipment.
[0024] In section 7.1.2.6.3 of 3GPP TR 25.814, a common theme of
inter-cell interference coordination and avoidance is to apply
restrictions to the downlink resource management (configuration for
the common channels and scheduling for the non-common channels) in
a coordinated way between cells. These restrictions can be in the
form of restrictions on what time and frequency resources are
available to the resource manager, or restrictions on the transmit
power that can be applied to certain time and frequency resources.
The restrictions within a cell provide the possibility for
improvement in a signal-to-noise plus interference ratio and cell
edge data rates and coverage on the corresponding time and
frequency resources in a neighbor cell.
[0025] It is further noted that different assumptions can be made
regarding user equipment measurement and reporting to support
downlink interference coordination. In a first alternative, no
additional user equipment measurement and reporting is needed
beyond the channel quality indictor reports employed to support
channel-dependent scheduling and link adaptation. In a second
alternative, additional user equipment measurement and reporting of
average path loss (including shadowing) to current and neighboring
cells are needed. In a third alternative and in addition to the
measurements and reporting of the second alternative, additional
measurement and reporting are made on the average interference for
frequency reuse sets.
[0026] It is further noted that inter-cell interference
coordination employs communication between different communication
system nodes (e.g., base stations) to set and reconfigure the above
mentioned scheduler restrictions. Two cases are considered, namely,
static interference coordination, wherein reconfiguration of the
restrictions is completed on a time scale corresponding to days and
the inter-node communication is very limited (basically with a rate
of the order of days), and semi-static interference coordination,
wherein reconfiguration of the restrictions is performed on a time
scale corresponding to seconds or longer.
[0027] The inter-node communication corresponds to information
employed to decide on reconfiguration of the scheduler
restrictions. Examples of communicated information are given as
traffic distribution within the different cells, and the downlink
interference contribution from a source to a target cell, for
instance. The inter-node communication also applies to the actual
reconfiguration decisions. Most of the current semi-static
interference coordination schemes require a base station to
exchange such information with a lot of neighboring base stations
frequently. This increases the complexity of the communication
system and causes a large amount of signaling overhead thereon.
[0028] It should also be understood as described in 3GPP TS 36.300,
referred to as the "3rd Generation Partnership Project; Technical
Specification Group Radio Access Network; Evolved Universal
Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial
Radio Access Network (E-UTRAN); Overall Description; Stage 2
(Release 8)," V1.0.0 (2007-03), which is incorporated herein by
reference, a self-configuration process is defined as a process
wherein newly deployed nodes are configured by automatic
installation procedures to set the necessary basic configuration
for system operation. (See, e.g., Section 22.1.) This process works
in a pre-operational state. The pre-operational state is understood
as the state from when the base station is powered up and has
backbone connectivity before the radio frequency transmitter is
switched on. The functions handled in the pre-operational state
include basic setup and initial radio configuration.
[0029] In view of the foregoing, the present invention will be
described with respect to exemplary embodiments in a specific
context of adaptively managing interference between elements of a
communication system in a self-optimization process. In general,
embodiments of the invention may be applied to any form of
communication system and network such as a wireless communication
system and network. An exemplary embodiment of the present
invention will now be described. This is merely to illustrate one
way of implementing the invention, without limiting the scope or
coverage of what is described elsewhere in this application.
[0030] The possibility to perform inter-cell interference
cancellation at the user equipment is considered irrespective of
the interference mitigation scheme adopted at the transmitter. The
radio interface description should facilitate the acquisition of
channel parameters of a limited number of (strongest) interfering
cells (e.g., through orthogonal reference signals).
[0031] Referring initially to FIG. 1, illustrated is a system level
diagram of a communication system including a wireless
communication system that provides an environment for the
application of the principles of the present invention. The
wireless communication system may be configured to provide evolved
UMTS terrestrial radio access network ("e-UTRAN") universal mobile
telecommunications services. A mobile management entity
("MME")/system architecture evolution gateway ("SAE GW") provides
control functionality for an e-UTRAN node B (designated "eNB," an
"evolved node B," also referred to as a "base station") via an SI
communication link. The base stations communicate via an X2
communication link. The various communication links are typically
fiber, microwave, or other high-frequency metallic communication
paths such as coaxial links, or combinations thereof.
[0032] The base stations communicate with user equipment
(designated "UE"), which is typically a mobile transceiver carried
by a user. Thus, communication links (designated "Uu" communication
links) coupling the base stations to the user equipment are air
links employing a wireless communication signal such as a 1.8
gigahertz ("GHz") orthogonal frequency division multiplex ("OFDM")
signal.
[0033] Turning now to FIG. 2, illustrated is a system level diagram
of a communication system including a wireless communication system
that provides an environment for the application of the principles
of the present invention. The wireless communication system
provides an e-UTRAN architecture including base stations
(designated "eNBs") providing e-UTRA user plane (packet data
convergence protocol/radio link control/media access
control/physical) and control plane (radio resource control)
protocol terminations towards user equipment (designated "UE"). The
base stations are interconnected with an X2 interface or
communication link. The base stations are also connected by an SI
interface or communication link to an evolved packet core ("EPC")
including a mobile management entity ("MME")/system architecture
evolution gateway ("SAE GW"). The SI interface supports a multiple
entity relationship between the mobile management entity/system
architecture evolution gateway and the base stations. For
applications supporting inter-public land mobile handover,
inter-eNB active mode mobility is supported by the mobile
management entity/system architecture evolution gateway relocation
via the SI interface.
[0034] The base stations may host functions such as radio resource
management (e.g., internet protocol ("IP") header compression and
encryption of user data streams, ciphering of user data streams,
radio bearer control, radio admission control, connection mobility
control, dynamic allocation of resources to user equipment in both
the uplink and the downlink), selection of a mobility management
entity at the user equipment attachment, routing of user plane data
towards the user plane entity, scheduling and transmission of
paging messages (originated from the mobility management entity),
scheduling and transmission of broadcast information (originated
from the mobility management entity or operations and maintenance),
and measurement and reporting configuration for mobility and
scheduling. The mobile management entity/system architecture
evolution gateway may host functions such as distribution of paging
messages to the base stations, security control, termination of
U-plane packets for paging reasons, switching of U-plane for
support of the user equipment mobility, idle state mobility control
and system architecture evolution bearer control. The user
equipment receives an allocation of a group of information blocks
from the base stations.
[0035] Turning now to FIG. 3, illustrated is a system level diagram
of a wireless communication system that provides an environment for
the application of the principles of the present invention. In the
illustrated embodiment, the wireless communication system is a
cellular communication system that includes first and second base
stations eNB_A, eNB_B and user equipment UE. As illustrated herein,
each base station eNB_A, eNB_B covers a cell designated as Cell_A
for the first base station eNB_A and Cell_B for the second base
station eNB_B. In a multicell environment, the user equipment UE
may receive a signal from one or more neighboring cells. The
wireless communication system as illustrated in FIG. 3 is
frequently employed to provide high data-rate communication
services such as video transmissions as well as large quantities of
raw data from user-generated files. Those skilled in the art should
also understand that the user equipment may take the form of a
mobile station including a cellular telephone, a personal digital
assistant ("PDA") having wireless communication capabilities, a
portable computer having wireless communication capabilities, an
image capture device such as a digital camera having wireless
communication capabilities, a gaming device having wireless
communication capabilities, a music storage and playback appliance
having wireless communication capabilities, an internet appliance
permitting wireless internet access and browsing, as well as a
portable unit or terminal that incorporates combinations of such
functions.
[0036] During a handover ("HO") event, a first base station eNB_A
may be considered a source base station (i.e., the base station (or
"cell") to which the user equipment UE is currently connected and
communicating in an associated serving cell). The second base
station eNB_B may be considered a target base station (i.e., the
base station to which the user equipment UE is to be connected and
communicating with in the target cell after the handover procedure
is completed). The serving cell and the target cell may at least
partially overlap one another in served area.
[0037] Handover refers generally to the process of transferring an
ongoing communication session from one radio channel connected to a
communication system to another. Handover may be performed between
channels within a single cell (intra-base station handover), or
between channels in different cells (inter-base station handover).
Handover generally occurs when user equipment moves away from the
area covered by a serving cell into the area covered by a target
cell. The ongoing communication session is transferred to the
target cell to avoid termination thereof when the user equipment
moves outside a range of the serving cell. A handover may also
occur when the capacity for accommodating new communication
sessions in a serving cell is used up, and an existing or newly
originated communication session from user equipment that is
located in an area overlapping a target cell is transferred thereto
to free up capacity in the serving cell for other users connected
to the serving cell. A handover may also be performed to transfer a
communication session from a rapidly moving user equipment such as
user equipment in a moving vehicle connected to a large,
umbrella-type cell, when the user equipment stops, thereby allowing
the user equipment to be transferred to a smaller macrocell or even
to a microcell to free up capacity in the umbrella cell for other
fast-traveling user equipment.
[0038] Turning now to FIG. 4, illustrated is a system level diagram
of a wireless communication system that provides an environment for
the application of the principles of the present invention. The
wireless communication system includes user equipment (designated
"UE") adapted to communication with a source base station
(designated "eNB.sub.SRC") in a serving cell and a target base
station (designated "eNB.sub.TRC") in a target cell, which are
coupled to a network control element (designated "NCE") such as an
access gateway. The serving cell and the target cell may at least
partially overlap, not shown. Prior to a handover event, the user
equipment is connected to and communicates with the source base
station. After the handover event, however, the user equipment is
connected to and communicates with the target base station.
[0039] The user equipment includes a data processor (designated
"DP"), a memory (designated "MEM") that stores programs (designated
"PRGM"), a timer (designated "TIMER") and a radio frequency
transceiver (designated "TRC") for bidirectional wireless
communications with the base stations. The base stations include a
data processor (also designated "DP"), a memory (also designated
"MEM") that stores programs (also designated "PRGM"), and a radio
frequency transceiver (also designated "TRC") for bidirectional
wireless communications with the user equipment. Additionally, the
network control element includes a data processor (also designated
"DP"), and a memory (also designated "MEM") that stores programs
(also designated "PRGM"). In general, the base stations provide the
e-UTRA user plane (e.g., radio link control/media access
control/physical) and control plane (e.g., radio resource control)
protocol terminations toward the user equipment. The base stations
communicate with the network control element via an SI interface or
communication link, and are interconnected via X2 interface or
communication link.
[0040] The memory as introduced above may be of any type suitable
to the local technical environment and may be implemented using any
suitable data storage technology such as semiconductor-based memory
devices, magnetic memory devices and systems, optical memory
devices and systems, fixed memory and removable memory. The data
processors may be of any type suitable to the local technical
environment, and may include one or more of general purpose
computers, special purpose computers, microprocessors, digital
signal processors ("DSPs") and processors based on a multi-core
processor architecture, as non-limiting examples. The programs
include program instructions that, when executed by the associated
data processor, enable the electronic device to perform tasks as
described herein. Exemplary embodiments of the system, subsystems
and modules as described herein may be implemented at least in part
by computer software executable by the data processors of the user
equipment and base stations, or by hardware, or combinations
thereof.
[0041] Turning now to FIG. 5, illustrated is a block diagram of an
embodiment of user equipment and a base station of a communication
system constructed in accordance with the principles of the present
invention. The illustrated communication system demonstrates
downlink communication from the base station to the user equipment.
The user equipment (designated "UE") and base station (designated
"Node B") are compatible with 3GPP TS 36.300, as introduced
above.
[0042] The base station receives transport information (designated
"transport blocks") 510 for the user equipment and provides the
transport information to a cyclic redundancy check module
(designated "CRC") 515 for error detection. The transport
information is thereafter provided to a coding module (designated
"Coding+RM") 520 for performing coding and retransmission functions
for the transport information.
[0043] An interleaving module 525 of the base station interleaves
other information within the transport information. The base
station also includes a data modulation module 530 for modulating
the transport information using, for instance, quadrature phase
shift keying or quadrature amplitude modulation as directed by a
media access control ("MAC") scheduler 535. A resource mapping
module 540 of the base station thereafter allocates resources for
the user equipment in accordance with the transport information as
directed by the media access control scheduler 535. An antenna
mapping module 545 of the base station thereafter maps the
transport information for antenna ports of an antenna 550 as
directed by the media access control scheduler 535 to transmit the
transport information over a wireless communication link to the
user equipment. A hybrid automatic repeat request module
(designated "HARQ") 555 cooperates with the coding module 520 to
accommodate retransmissions.
[0044] The user equipment receives the transport information via an
antenna 557 and an antenna demapping module 560 demaps the
transport information from antenna ports of the antenna 557. A
resource demapping module 565 demaps the resources allocated to the
user equipment from the base station. A data demodulation module
570 demodulates the transport information and a deinterleaving
module 575 of the user equipment deinterleaves the transport
information transmitted from the base station. A decoding module
(designated "Decoding+RM") 580 decodes the transport information
using an opposite decoding process to the coding module 520 of the
base station and supports retransmits from a hybrid automatic
repeat request module (designated "HARQ") 585. The transport
information is thereafter provided to a cyclic redundancy check
module (designated "CRC") 590 for error detection.
[0045] Functionality performed by the base station and the user
equipment may be organized and modeled as a stack of physical,
transport, and control layers in accordance with the open system
interconnection seven-layer model. Among the layers therein is the
media access control (also referred to as "MAC") layer and other
layers located above the media access control layer such as
networking and transport layers. The media access control layer
offers certain services to the higher layers including services
that are related to the operation of, for instance, an uplink and a
downlink (i.e., channels for transmissions from user equipment to
the base station, and vice versa). The media access control layer
includes an implementation of a media access control protocol. The
media access control protocol provides procedures followed by the
user equipment and the base station to transmit and receive
information therebetween.
[0046] A physical layer is located below the media access control
layer. The media access control layer requests certain services
from the physical layer. These services are related to the physical
transmission of packets to the base station. The media access
control layer receives one or more flows from the higher layers. A
flow is a stream of data, typically corresponding to a specific
application such as a voice over internet protocol ("VoIP")
communication session, videotelephony, gaming, or the like.
[0047] Turning now to FIG. 6, illustrated is a block diagram of an
embodiment of user equipment and a base station of a communication
system constructed in accordance with the principles of the present
invention, illustrating uplink communication from the user
equipment to the base station. The elements illustrated in FIG. 6
including the general function of uplink transmission control
provide functions corresponding to those illustrated in FIG. 5 for
uplink communication from user equipment to a base station, and
will not be redescribed in the interest of brevity.
[0048] In order to communicate the physical layer packet format to
a user equipment, a compatible physical layer signaling or media
access control channel signaling is generally employed. Each media
access control layer packet section may include one or more media
access control layer payloads in accordance with the media access
control layer multi-user packet format. In accordance with the
foregoing, FIG. 7 illustrates a block diagram of an embodiment of
user equipment and a base station that provides the exemplary
functional layers thereof constructed in accordance with the
principles of the present invention and compatible with 3GPP TS
36.300, as introduced above.
[0049] Turning now to FIG. 8, illustrated is a block diagram of an
embodiment of user equipment and base stations of a communication
system constructed in accordance with the principles of the present
invention. A source base station (designated "eNB_a") may perform a
handover of user equipment (designated "UE") to a target base
station (designated "eNB b"). The base stations are also connected
to a core network including a mobile management entity
("MME")/system architecture evolution gateway ("SAE GW") with an
interference coordination subsystem (designated "IC S/S") 810.
Among other things, the base stations include a measurement report
handler (designated "MRH") 815, packet scheduler (designated "PS")
820, interference coordination subsystem (designated "IC S/S") 825,
handover subsystem (designated "HO S/S") 830 and memory (designated
"MEM") 835. The measurement report handler 815 in the base stations
processes uplink data received from the user equipment such as a
reliability report including the channel quality. The packet
scheduler 820 allocates uplink and downlink resources within the
base stations in accordance with, for instance, a channel quality
indicator ("CQI") report from the user equipment. The handover
subsystem 830 performs a handover decision sequence based on input
from, for instance, the measurement report handler 815, and uses
resource and communication performance information to execute the
handover of the user equipment to a target base station as
described below. The interference coordination subsystem 825 in the
base stations adjusts interference coordination parameters in a
self-optimization process based on a reliability report (also
referred to as or including a channel quality report) from a user
equipment as described herein. The memory 835 may be nonvolatile
memory associated with data processing in the source base
station.
[0050] A self-optimization process is a process wherein user
equipment and base station measurements and performance
measurements are used to auto-tune the communication system. A
self-optimization process works in an operational state. An
operational state is understood as the state wherein the radio
frequency interface is switched on. The functions handled in the
operational state include optimization and adaptation.
[0051] The user equipment includes, among other things, a
measurement and reporting subsystem (designated "M&R S/S") 850
that performs measurements regarding a status of the user equipment
and provides reports to the base stations as a function thereof.
For instance, the measurement and reporting subsystem 850 may
detect a cell status thereof, employ intra-frequency handover
measurements to detect its strongest interferers at a cell edge
thereof, and compose a reliability report such as a downlink
channel quality indicator at the cell edge for the base stations.
The user equipment may then notify the strongest interferer (e.g.,
a target base station in a handover situation) about the
measurement results via the reliability report transmitted to the
source base station or a reliability report transmitted to the
target base station after a handover event. It should also be
understood that the user equipment is constructed analogously to
the base stations, and that the measurement and reporting subsystem
850 generally resides within the radio resource control module. It
should further be understood that the systems, subsystems and
modules associated with the system of the present invention may
reside in a single module as illustrated, or separated into
distributed subsystems, and multiple modules of like functionality
may form the systems, subsystems and modules as described
herein.
[0052] The interference coordination subsystem 810 in the mobile
management entity is responsible for an initialization and static
configuration of interference coordination functionalities in each
base station. For example, in case soft frequency reuse is used as
interference coordination in the system, before the communication
system begins to operate, the interference coordination subsystem
810 in the mobile management entity configures soft frequency reuse
parameters in the base stations with the same power mask, or with
different power masks based on experimental results from a drive
test. After the communication system starts to operate, the
quantity of user equipment communicating with each base station may
vary because of, for instance, user equipment mobility. As a
result, each base station will have a different and changing
interference level. The interference coordination subsystem 825 in
the base station is responsible for adjusting the interference
coordination parameters in each base station independently as set
forth herein to accommodate the dynamics in the communication
system such as unbalanced load therein, and user equipment unevenly
distributed in the communication system, to name a few.
[0053] Regarding the handover sequence, intra-frequency handover is
typically triggered when user equipment is in a cell edge of a
serving cell associated with a source base station. The need for a
handover may also be triggered, for example, by interfering user
equipment, by a large number of user equipment loading system
resources, or by detection of a failed system element. In a
frequency reuse-one communication system wherein adjacent cells
reuse the same frequency sub-bands such as employed with a 3GPP LTE
compatible communication system, the user equipment may experience
strong interference and a low signal-to-noise plus interference
ratio [referred to as "SNIR," which equals signal power/(noise
power+interference power), e.g., less than -5 db] on the downlink
at a cell's edge of the serving cell. Given such a low
signal-to-noise plus interference ratio, the block error rate
("BLER") of a message after an automatic retransmit request ("ARQ")
or hybrid automatic retransmit request ("HARQ") may also be very
high in some scenarios. This raises the concern whether handover
execution signaling (mainly, handover command in the downlink) can
be reliably communicated between a base station and user equipment.
In other words, this raises the concern whether there is a coverage
problem for handover execution signaling in a 3GPP LTE compatible
communication system. Furthermore, it also raises the concern
whether there is a coverage problem in a cell edge of a base
station.
[0054] There may be a higher probability of handover when the load
on a serving cell is high. If the transmission for the handover
command fails, the user equipment typically transitions to an idle
state, and starts cell reselection. The procedure due to the
failure of the handover command typically causes a long
interruption time (e.g., greater than 500 milliseconds). Frequent
radio link failure is not desirable, especially for voice over
internet protocol ("VoIP") applications.
[0055] Communication systems such as the universal mobile
telecommunications system ("UMTS") are a frequency reuse-one
system. In the universal mobile telecommunications system, soft
handover is used to avoid or solve the handover signaling error
problem. It has been proposed to employ forward handover signaling
(user equipment-based mobility in active state) as a backup (or
alternative) to current backward handover signaling. Under such a
handover signaling system, the user equipment searches for a new
cell if the user equipment did not receive a requested handover
command within some period of time (e.g., timeout period). The soft
handover and forward handover signaling systems, however, cause
significant architectural changes to the 3GPP LTE radio access
network ("RAN"), which is not preferable.
[0056] Soft handover generally refers to a handover in which the
channel in a source cell is retained and used in parallel with a
channel in a target cell. The connection to the target cell is
established before the connection to the source cell is broken
(i.e., it is a make-before-break arrangement). A soft handover may
involve connections to more than two cells such that connections to
three, four, or more cells can be simultaneously maintained with
user equipment, and the best of the used channels can be utilized
for the communication session at a given moment, or all the signals
can be combined to produce a clearer copy of signal. Alternatively,
a hard handover is one in which the channel to the user equipment
is released before the channel in the target cell is engaged.
[0057] According to a system associated with the principles of the
present invention as described in the environment of an exemplary
embodiment, the communication system employs self-optimization of
an interference coordination scheme. As described above, a base
station communicates with a plurality of user equipment within its
coverage area by transmitting signals in downlinks in an allocated
frequency band to the user equipment. It is noted that the
frequency band is typically divided into multiple (e.g., three or
five) sub-bands, and a particular user equipment is assigned to one
of the multiple sub-bands. Within each sub-band, the base station
communicates at a particular transmitter power level to the user
equipment assigned thereto over independent channels, the
independent channels formed by orthogonal pseudorandom noise
sequences. Thus, each user equipment assigned to a particular
sub-band receives signals from the base station at a common
downlink power level. The overall transmitter power level of the
base station is constrained so that the sum of the transmitter
power levels in each of the sub-bands is a constant.
[0058] A resource allocation process within the base station
selectively allocates the relative transmitter power level in each
of the sub-bands in view of the communication needs of the user
equipment. For example, user equipment within a central portion of
the coverage area (i.e., those in an area near the base station or
cell center) are assigned to a common sub-band, and the base
station communicates in that sub-band at a low power level because
the communication distances are relatively small. User equipment at
the edge of the coverage area (e.g., cell edge) is assigned to a
different sub-band, and the base station communicates with the user
equipment at the edge at a high power level.
[0059] The frequency (or time) resources are divided into different
power-level groups. The frequency bandwidth and the power levels
are predefined during the communication system planning phase. The
relative power levels of the sub-bands are generally referred to as
a "power mask" (e.g., a soft frequency reuse parameter, a type of
interference coordination parameter) and is expressed as a sequence
of numbers indicating the power levels in lower power sub-bands
relative to the power level in the highest power sub-band. For
example, the power mask sequence (0 dB, -4 dB, -4 dB) indicates
that the power levels of the second and third sub-bands are each 4
dB below the power level of the first sub-band.
[0060] Typically, the communication system allocates radio
resources in a high power sub-band to user equipment at the cell
edge, and allocates radio resources in a low power sub-band to user
equipment at the cell center. By increasing the power difference
between high power and low power sub-bands, the communication
system can increase the throughput of user equipment at the cell
edge at the cost of reducing throughput of user equipment at the
cell center. By decreasing the power difference between high power
and low power sub-bands, the communication system can increase the
throughput of user equipment at the cell center at the cost of
reducing the throughput of user equipment at the cell edge.
[0061] It should be understood that the system and method of the
present invention is applicable to various interference
coordination schemes including soft frequency reuse. In other
interference coordination schemes, self-optimization is realized by
adjusting other interference coordination parameters such as,
without limitation, an antenna pattern, a transmission power, a
transmission frequency, and a transmission time. In addition, the
use of beamforming antenna solutions at the base station is a
general method that can be employed for downlink inter-cell
interference mitigation.
[0062] In a soft frequency reuse and a handover situation, the
handover command is typically sent from a source base station's
high power sub-band (a cell edge sub-band), and interfered by the
target base station's low power sub-band (a cell center sub-band).
Also, the target base station is often the strongest interferer to
a reception of the handover command. By reducing a transmission
power of the target base station's low power sub-band, the user
equipment can get higher signal-to-noise plus interference ratio
for the handover command and data control at the source base
station. As a result, the user equipment can achieve higher cell
edge throughput and can receive a handover command more reliably.
On the other hand, if the handover command is reliable and/or cell
edge throughput is high, the power of the low power sub-band of the
target base station can be increased, which will increase the
target base station's throughput.
[0063] Based thereon, the user equipment may report the channel
quality that the user equipment experienced within the last
handover procedure to the target base station after handover. This
improves both the reliability of the handover command and user
equipment's throughput at the cell edge. Alternatively, the user
equipment may send the reliability report of a handover command
(e.g., in the format of radio link failure or the number of
automatic retransmit request events) to the target base station,
even if a handover is unsuccessful.
[0064] Regarding cell interference, FIG. 9 illustrates a signaling
diagram of an embodiment of performing self-optimization according
to the principles of the present invention. After receiving packet
data from a source base station in a serving cell, a user equipment
(designated "UE") detects a cell status thereof (cell edge or cell
status) via various approaches such as via a dedicated radio
resource control signal, broadcast signal, information from a
packet scheduler, or a special event (e.g., handover, radio link
failure). The user equipment employs intra-frequency handover
measurements to detect its strongest interferers at a cell edge
thereof. The user equipment composes a reliability report such as a
downlink channel quality indicator at the cell edge. This
measurement indicates whether a current configuration of
interference coordination is proper, too aggressive, or too
conservative. The reliability report may include at least one of,
without limitation, a channel quality indicator report of source
base station's (from the serving cell) downlink, a radio link
failure, a number of automatic retransmit requests/hybrid automatic
retransmit requests of a handover command, and an average number of
automatic retransmit requests/hybrid automatic retransmit requests
when the user equipment is in a cell edge thereof.
[0065] The user equipment then notifies the strongest interferer
about the measurement results via, without limitation, a
reliability report transmitted to the source base station in the
serving cell or a reliability report to a target base station in a
target cell after a handover event. In the second option, the
message can be sent as a separate radio resource control message or
attached to a radio resource control message exchanged in a
handover process such as handover confirm. The strongest interferer
then adjusts an interference coordination parameter based on the
reliability report. In a handover situation, the strongest
interferer is usually the target base station.
[0066] Again, the strongest interferer uses the reliability report
to adjust an interference coordination parameter (e.g., a power
mask if soft frequency reuse scheme is used). When the report
indicates poor or bad results, the strongest interferer can reduce
transmission power or reduce the traffic at its cell center
sub-band, which increases the source base station's cell edge
throughput. In the case of handover, this can increase the
reliability of the handover command. When the report indicates good
results, the strongest interferer can increase transmission power
at its cell center sub-band. As a result, this can increase the
strongest interferer's cell center throughput. In a handover
situation, this can improve cell center throughput in the target
cell.
[0067] The system and method as described herein may be implemented
as set forth below. Within the handover procedure, the user
equipment may collect at least one of the following statistics,
without limitation, the downlink channel quality indicator of
source base station's downlink at the time of handover command
reception, a radio link failure, and the number of times for an
automatic retransmit request/hybrid automatic retransmit request
for retransmission of the handover command. The user equipment
reports the statistics in the form of a reliability report to the
target base station along with a handover confirmation (when the
handover succeeds) or a re-establishment request (or similar
message after handover failure).
[0068] After receiving the report, the target base station adjusts
the interference coordination parameters. As an example, if one of
the following conditions is true, namely, the signal-to-noise plus
interference ratio of the handover command is less than a
lower-bound of a signal-to-noise plus interference ratio threshold,
a radio link failure is observed, or the number of automatic
retransmit request/hybrid automatic retransmit request
retransmissions is greater than a higher-bound retransmission
threshold, then the target base station reduces power at the low
power sub-band (cell center sub-band) of the target cell [e.g., (0,
-4 db, -4 db) to (0, -6 db, -6 db)]. Another approach is to select
the type of packet scheduler procedure and/or the parameters in the
packet scheduling procedure to allow a lower level of traffic in
the cell center sub-band. This improves the signal-to-noise plus
interference ratio of the handover command and data channel for
user equipment at the source base station's cell edge, at the
potential cost of reducing throughput for the user equipment at the
target base station's cell center.
[0069] As another example, if one of the following conditions is
true, namely, that the signal-to-noise plus interference ratio of
the handover command is less than a higher-bound signal-to-noise
plus interference ratio threshold, or the number of automatic
retransmit request/hybrid automatic retransmit request
retransmissions is less than a lower-bound retransmission
threshold, the target base station can increase power at low power
sub-band of the target cell [e.g., (0, -4 db, -4 db) to (0, -2 db,
-2 db)]. This increases throughput for user equipment at the
target's cell center, at the potential cost of reducing
signal-to-noise plus interference ratio of the handover command and
data channel for user equipment within the source base station's
cell edge. Since the report from the user equipment indicates good
channel quality, it will not seriously affect the reliability of
the handover command in the source base station. Some time domain
sliding window techniques can also be applied to improve
statistical reliability.
[0070] Turning now to FIGS. 10 and 11, illustrated are signaling
diagrams of an embodiment of a handover sequence according to the
principles of the present invention. More specifically, FIGS. 10
and 11 demonstrate successful and failed handover sequences,
respectively. The handover sequence begins with transmitting a
downlink allocation from a source base station (designated "Source
eNB") to user equipment (designated "UE"). The user equipment
transmits a reliability report to the source base station. The
source base station then determines if a target base station
(designated "Target eNB") has sufficient resources to accommodate
the user equipment and, if so, transmits a handover command from
the source base station to the user equipment. The target base
station allocates resources in an uplink shared channel for use by
the user equipment to transmit access bursts.
[0071] The user equipment then transmits a handover confirmation
message (see FIG. 10) or a re-establishment message (see FIG. 11)
with a reliability report to the target base station. Note that in
FIG. 11, the user equipment still selects the target cell to
connect after radio link failure. It is also possible that the user
equipment may select a different new cell than the target cell in
accordance with the principles of the present invention. Under such
circumstances, the user equipment can transmit the reliability
report to the target cell via the new cell. The target base station
then adjusts an interference coordination ("IC") parameter based on
the reliability report. If the reliability report indicates poor
results, the target base station reduces transmission power or
reduces the traffic at a cell center sub-band thereof. If the
reliability report indicates good results, the target base station
can increase transmission power at the cell center sub-band
thereof. The target base station thereafter completes the handover
procedure with the source base station to the target base station
and access bursts are transmitted from the user equipment to the
target base station. It should be understood that the reliability
report includes any one of or combinations of the statistics
described above.
[0072] With respect to the user equipment, there are several
mechanisms that may be employed to capture a status thereof. For
instance, the user equipment may be aware of its cell status (cell
edge or cell status) via various approaches such as dedicated radio
resource control ("RRC") signaling, broadcast signaling,
information from a packet scheduler, a special event such as
handover, and radio link failure. The user equipment may also use
intra-frequency handover measurement to detect its strongest
interferers when at a cell's edge. (See, e.g., 3GPP TSG RAN WG1
#47bis meeting contribution R1-070390, entitled Open Loop DL
Transmit Diversity for Common Control Channels," by Nokia, Jan.
15-19, 2007, which is incorporated herein by reference).
[0073] The user equipment can also use a channel quality indicator
measurement to know the user equipment's downlink channel quality
at the cell edge. This measurement indicates whether the current
configuration of interference coordination is proper, too
aggressive, or too conservative. The user equipment can also notify
the strongest interferers about the measurement results via a
reliability report via a current serving cell, and a reliability
report to a target cell after a handover. In the second option, the
message can be sent as a separate radio resource control message,
or attached to part of the radio resource control message exchanged
in a handover process such as handover confirmed.
[0074] Turning now to FIG. 12, illustrated is a diagram of an
embodiment for performing a handover sequence according to the
principles of the present invention. As illustrated herein, when
the user equipment reliability report indicates poor channel
quality, the target base station changes a power mask for power
levels P3, P1 and P2 for sub-bands 3, 1 and 2, respectively, from
(-4 db, 0, -4 db) to (-6 db, 0, -6 db). This reduces the
interference on the source base station's cell edge sub-band. If
another user equipment moves from the source base station to the
target base station at a later time, it will experience better
signal-to-noise plus interference ratio before and during the
handover. On the other hand, another good effect is that the user
equipment in source base station's cell edge sub-band experiences
better throughput and signal-to-noise plus interference ratio. When
the user equipment's reliability report indicates good channel
quality, the target base station changes a power mask for power
levels P3, P1 and P2 for sub-bands 3, 1 and 2, respectively, from
(-4 db, 0, -4 db) to (-2 db, 0, -2 db). This increases target
cell's throughput at cell center sub-band, but potentially
increases the interference on source base station's cell edge
sub-band. Since user equipment's reliability report indicates good
channel quality, this will not seriously affect the reliability of
handover command in source base station.
[0075] The system as described herein is related to PCT Patent
Application No. PCT/IB2008/000264, entitled "Method and Apparatus
for providing Interference Coordination in Handover Signaling," by
Huang, filed Feb. 6, 2008, claiming priority to U.S. Patent
Application Ser. No. 60/888,516, which is incorporated herein by
reference. Huang describes a method that includes transmitting a
report to a source base station, specifying a target base station
as an interference source. The source base station coordinates a
handover procedure with the target base station to reduce
interference stemming from the target base station.
[0076] The handover system as described herein increases a
reliability of the handover command and throughput of data channel
at a cell's edge. It is typically a reactive approach, which means
that the target base station takes action after radio link failure
or a poor handover command reception is identified. The handover
system avoids complicated inter-cell interference coordination
functionality (distributed or centralized operations and
maintenance server) to manage interference mapping (which base
station is the strongest interferer to which user equipment), and
avoids extra signaling to update and maintain the interferer, and
signaling of interference coordination in existing inter-cell
interference coordination schemes. The handover system also may
also avoid signaling on the X2 interface as described above.
Instead, the user equipment may report its performance at cell edge
along with the information such as the measured channel quality
indicator or the events of a successful/failed handover.
[0077] Reliability of handover signaling may become a bottleneck to
a 3GPP LTE compatible communication system. In comparison to
existing solutions such as forward handover or soft handover, the
handover system according to the principles of the present
invention employs current system architectures with a similar level
of gain regarding coverage and reliability of handover
signaling.
[0078] Thus, a system and method have been introduced to perform
base station optimization in a communication system. In one
embodiment, after receiving packet data from a source base station
in a serving cell, a user equipment detects a cell status thereof,
employs intra-frequency handover measurements to detect its
strongest interferers at a cell edge thereof, and composes a
reliability report including a downlink channel quality report at
the cell edge (e.g., by a measurement and reporting subsystem in
the user equipment). The user equipment then notifies the strongest
interferer (e.g., a target base station in a handover situation)
about the measurement results via a reliability report transmitted
to the source base station or a reliability report transmitted to
the target base station after a handover event (e.g., by the
measurement and reporting subsystem in the user equipment and
received by the measurement report handler in the base stations).
The strongest interferer then adjusts interference coordination
parameters based on the reliability report (e.g., by a interference
coordination subsystem). When the report indicates poor or bad
results, the strongest interferer can reduce transmission power or
reduce the traffic at its cell center sub-band, which increases the
source base station's cell edge throughput (e.g., in the case of
handover, this can increase the reliability of the handover
command). When the report indicates good results, the strongest
interferer can increase transmission power at its cell center
sub-band, which increases the strongest interferer's cell center
throughput (e.g., in a handover situation, this can improve cell
center throughput in the target cell).
[0079] In another embodiment, a system and method are introduced
for use in a communication system to coordinate allocation of
resources for user equipment to be handed over from a source base
station to a target base station. The method includes transmitting
an uplink allocation from the source base station to the user
equipment (e.g., by a packet scheduler) and transmitting a
measurement report from the user equipment to the source base
station (e.g., by a measurement and reporting subsystem in the user
equipment). The method also includes composing a reliability report
from a user equipment (by the measurement and reporting subsystem
in the user equipment), determining if the target base station has
sufficient resources to accommodate the user equipment (e.g., by a
packet scheduler), transmitting a handover command from the source
base station to the user equipment (e.g., by a handover subsystem),
allocating resources in an uplink shared channel in the target base
station for use by the user equipment to transmit access bursts
(e.g., by a packet scheduler) and transmitting a handover
confirmation message or a re-establishment message with a
reliability report (e.g., a channel quality indicator report) from
the user equipment to the target base station (by the measurement
and reporting subsystem in the user equipment). The method
continues by adjusting an interference coordination parameter based
on the reliability report (e.g., by a interference coordination
subsystem). If the reliability report indicates poor results, the
target base station reduces transmission power or reduces the
traffic at a cell center sub-band thereof. If the reliability
report indicates good results, the target base station can increase
transmission power at the cell center sub-band thereof. The method
concludes by completing the handover procedure from the source base
station to the target base station (e.g., by handover subsystems),
and transmitting access bursts from the user equipment to the
target base station.
[0080] As described above, the exemplary embodiment provides both a
method and corresponding apparatus consisting of various modules
providing functionality for performing the steps of the method. The
modules may be implemented as hardware (including an integrated
circuit such as an application specific integrated circuit), or may
be implemented as software or firmware for execution by a computer
processor. In particular, in the case of firmware or software, the
exemplary embodiment can be provided as a computer program product
including a computer readable storage structure embodying computer
program code (i.e., software or firmware) thereon for execution by
the computer processor.
[0081] Although the present invention and its advantages have been
described in detail, it should be understood that various changes,
substitutions and alterations can be made herein without departing
from the spirit and scope of the invention as defined by the
appended claims. For example, many of the processes discussed above
can be implemented in different methodologies and replaced by other
processes, or a combination thereof, to advantageously coordinate
allocation of resources for user equipment to be handed over from a
source base station to a target base station without contention and
without a need for sharing timing information therebetween, as
described herein. Moreover, the scope of the present application is
not intended to be limited to the particular embodiments of the
process, machine, manufacture, composition of matter, means,
methods and steps described in the specification. As one of
ordinary skill in the art will readily appreciate from the
disclosure of the present invention, processes, machines,
manufacture, compositions of matter, means, methods, or steps,
presently existing or later to be developed, that perform
substantially the same function or achieve substantially the same
result as the corresponding embodiments described herein may be
utilized according to the present invention. Accordingly, the
appended claims are intended to include within their scope such
processes, machines, manufacture, compositions of matter, means,
methods, or steps.
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