U.S. patent application number 12/809888 was filed with the patent office on 2010-10-28 for communication system and method.
This patent application is currently assigned to Samsung Electronics Co., Ltd.. Invention is credited to Kyeongin Jeong, Gert-Jan Van Lieshout, Himke Van Der Velde.
Application Number | 20100273490 12/809888 |
Document ID | / |
Family ID | 39048424 |
Filed Date | 2010-10-28 |
United States Patent
Application |
20100273490 |
Kind Code |
A1 |
Velde; Himke Van Der ; et
al. |
October 28, 2010 |
COMMUNICATION SYSTEM AND METHOD
Abstract
A communication system comprises: mobile user equipment; and a
communication network adapted to communicate with the user
equipment, the communication network comprising: a first base
station adapted to communicate with the user equipment using radio
signals and according to a first defined radio frame format
comprising a first sequence of radio frames each having the same
length and each being allocated a respective first system frame
number (SFN) such that the radio frames of the first sequence are
sequentially numbered; and a second base station adapted to
communicate with the user equipment using radio signals and
according to a second defined radio frame format comprising a
second sequence of radio frames each having said same length and
each being allocated a respective second SFN such that the radio
frames of the second sequence are sequentially numbered, the user
equipment being adapted to receive radio signals from the first
base station and to use said received signals to synchronise with
the first sequence of numbered radio frames, such that the user
equipment can transmit radio signals to the first base station in
accordance with the first defined radio frame format, and determine
the first SFN of a frame of the first sequence at a particular
time, the communication network being further adapted to transmit a
radio signal to the user equipment, the radio signal comprising
first information indicative of a difference between the first and
second SFNs at a particular time and second information usable with
said first information to determine unambiguously at least a least
significant bit of the SFN in binary form of a frame of the second
sequence at a particular time from a knowledge of the SFN of a
frame of the first sequence at a particular time, the user
equipment being further adapted to receive said radio signal and,
when synchronised with said first sequence, to use the first and
second information to synchronise with the second sequence, such
that the user equipment can transmit radio signals to the second
base station in accordance with the second defined radio frame
format. A corresponding communication method is provided.
Inventors: |
Velde; Himke Van Der;
(Dutch, NL) ; Van Lieshout; Gert-Jan; (Dutch,
NL) ; Jeong; Kyeongin; (Gyeonggi-do, KR) |
Correspondence
Address: |
THE FARRELL LAW FIRM, LLP
290 Broadhollow Road, Suite 210E
Melville
NY
11747
US
|
Assignee: |
Samsung Electronics Co.,
Ltd.
Gyeonggi-do
KR
|
Family ID: |
39048424 |
Appl. No.: |
12/809888 |
Filed: |
December 12, 2008 |
PCT Filed: |
December 12, 2008 |
PCT NO: |
PCT/KR08/07384 |
371 Date: |
June 21, 2010 |
Current U.S.
Class: |
455/436 |
Current CPC
Class: |
H04W 92/10 20130101;
H04W 56/00 20130101; H04W 36/165 20130101; H04W 36/0072 20130101;
H04W 36/14 20130101; H04W 48/10 20130101 |
Class at
Publication: |
455/436 |
International
Class: |
H04W 36/00 20090101
H04W036/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 20, 2007 |
GB |
0724833.9 |
Claims
1-30. (canceled)
31. A communication method in a communication system comprising
mobile user equipment and a communication network adapted to
communicate with the user equipment, the communication network
comprising: a first base station adapted to communicate with the
user equipment using radio signals and according to a first defined
radio frame format comprising a first sequence of radio frames each
having the same length and each being allocated a respective first
system frame number (SFN) such that the radio frames of the first
sequence are sequentially numbered; and a second base station
adapted to communicate with the user equipment using radio signals
and according to a second defined radio frame format comprising a
second sequence of radio frames each having the same length and
each being allocated a respective second SFN such that the radio
frames of the second sequence are sequentially numbered, the length
of the radio frames of one of the first and second formats being an
integer multiple of the length of the radio frames of the other
format, the method comprising: receiving radio signals from the
first base station at the user equipment and using said received
signals to synchronise the user equipment with the first sequence
of numbered radio frames, such that the user equipment can transmit
radio signals to the first base station in accordance with the
first defined radio frame format, and to determine the first SFN of
a frame of the first sequence at a particular time; transmitting a
radio signal to the user equipment from the communication network,
the radio signal comprising first information indicative of a
difference between the first and second SFNs at a particular time
and second information usable with said first information to
determine unambiguously at least a least significant bit of the SFN
in binary form of a frame of the second sequence at a particular
time from a knowledge of the SFN of a frame of the first sequence
at a particular time; and receiving said radio signal at the user
equipment when synchronised with said first sequence, and using the
first and second information to synchronise the user equipment with
the second sequence, such that the user equipment can transmit
radio signals to the second base station in accordance with the
second defined radio frame format.
32. The communication method in accordance with claim 31, further
comprising transmitting a radio signal from the user equipment to
the second base station in accordance with the second defined radio
frame format.
33. The communication method in accordance with claim 32, wherein
the second base station is an enhanced node B (eNB), and the step
of transmitting a radio signal from the user equipment to the
second base station comprises transmitting a RACH signal.
34. The communication method in accordance with claim 32,
comprising continuing communication between the second base station
and user equipment, after transmitting said radio signal from the
UE to the second base station, by exchanging radio signals in
accordance with the second defined format.
35. The communication method in accordance with claim 34,
comprising ceasing communication between the first base station and
user equipment after transmitting said radio signal from the UE to
the second base station.
36. The communication method in accordance with claim 31, further
comprising determining said first information in the communication
network from signals received from the first and second base
stations.
37. The communication method in accordance with claim 31, further
comprising determining a time difference between radio frame
boundaries of the first and second sequence, and using said time
difference together with an estimate of an accuracy of said time
difference to determine said second information.
38. The communication method in accordance with claim 37,
comprising determining said second information according to the
relative magnitudes of said time difference and said estimate of
accuracy.
39. The communication method in accordance with claim 38,
comprising determining said second information to indicate that
said difference between SFNs applies between a particular frame of
the first sequence and the frame of the second sequence having a
start time closest to that of said particular frame in the event
that the magnitude of said time difference is less than the
magnitude of said estimate of accuracy.
40. The communication method in accordance with claim 38,
comprising determining said second information to indicate that
said difference between SFNs applies between a particular frame of
the first sequence and the frame of the second sequence having a
start time within said particular frame in the event that the
magnitude of said time difference is greater than the magnitude of
said estimate of accuracy.
41. The communication method in accordance with claim 37, wherein
determining said time difference comprises using the NTP.
42. The communication method in accordance with claim 37, further
comprising the step of determining said estimate of accuracy.
43. The communication method in accordance with claim 31, wherein
said second information is usable with said first information to
determine unambiguously at least two least significant bits of the
SFN in binary form of a frame of the second sequence at a
particular time from a knowledge of the SFN of a frame of the first
sequence at a particular time.
44. The communication method in accordance with claim 31, wherein
said second information is usable with said first information to
determine unambiguously the SFN of a frame of the second sequence
at a particular time from a knowledge of the SFN of a frame of the
first sequence at a particular time.
45. The communication method in accordance with claim 31, wherein
said step of transmitting a radio signal to the user equipment from
the communication network comprises transmitting the radio signal
from the first base station.
46. The communication method in accordance with claim 31, wherein
the radio signal comprising the first and second information
further comprises additional information indicative of frame
structure in said second defined radio frame format.
47. A communication system comprising: mobile user equipment; and a
communication network adapted to communicate with the user
equipment, the communication network comprising: a first base
station adapted to communicate with the user equipment using radio
signals and according to a first defined radio frame format
comprising a first sequence of radio frames each having the same
length and each being allocated a respective first system frame
number (SFN) such that the radio frames of the first sequence are
sequentially numbered; and a second base station adapted to
communicate with the user equipment using radio signals and
according to a second defined radio frame format comprising a
second sequence of radio frames each having the same length and
each being allocated a respective second SFN such that the radio
frames of the second sequence are sequentially numbered, the length
of the radio frames of one of the first and second formats being an
integer multiple of the length of the radio frames of the other
format, the user equipment being adapted to receive radio signals
from the first base station and to use said received signals to
synchronise with the first sequence of numbered radio frames, such
that the user equipment can transmit radio signals to the first
base station in accordance with the first defined radio frame
format, and determine the first SFN of a frame of the first
sequence at a particular time, the communication network being
further adapted to transmit a radio signal to the user equipment,
the radio signal comprising first information indicative of a
difference between the first and second SFNs at a particular time
and second information usable with said first information to
determine unambiguously at least a least significant bit of the SFN
in binary form of a frame of the second sequence at a particular
time from a knowledge of the SFN of a frame of the first sequence
at a particular time, the user equipment being further adapted to
receive said radio signal and, when synchronised with said first
sequence, to use the first and second information to synchronise
with the second sequence, such that the user equipment can transmit
radio signals to the second base station in accordance with the
second defined radio frame format.
48. The communication system in accordance with claim 47, wherein
the second base station is an eNB.
49. The communication system in accordance with claim 48, wherein
the user equipment is further adapted to transmit a RACH signal to
the eNB in accordance with the second defined radio frame format
when synchronised with said second sequence
50. The communication system in accordance with claim 48, wherein
the communication network is an E-UTRA network, and the first and
second base stations are first and second eNBs respectively, and
the user equipment is further adapted to transmit a measurement
report to the first eNB, the first eNB is responsive to the
measurement report to send a handover request to the second eNB,
the second eNB is responsive to the handover request to send a
handover request acknowledgement to the first eNB comprising said
first and second information, the first eNB is responsive to the
handover request acknowledgement to transmit said radio signal
comprising first and second information to the user equipment, and
the user equipment is then arranged to send a RACH message to the
second eNB.
51. The communication system in accordance with claim 47, wherein
the communication network is arranged to determine a time
difference between radio frame boundaries of the first and second
sequences, and to determine said second information according to
said time difference and an estimate of an accuracy of said time
difference.
52. The communication system in accordance with claim 51, wherein
the communication network is adapted to determine said second
information according to the relative magnitudes of said time
difference and said estimate of accuracy.
53. The communication system in accordance with claim 51, wherein
the communication network is arranged to determine said time
difference using the NTP.
54. The communication system in accordance with claim 51, wherein
the communication network is further adapted to determining said
estimate of accuracy using the NTP.
55. The communication system in accordance with claim 47, wherein
said second information is usable with said first information to
determine unambiguously at least two least significant bits of the
SFN in binary form of a frame of the second sequence at a
particular time from a knowledge of the SFN of a frame of the first
sequence at a particular time.
56. The communication system in accordance with claim 47, wherein
said second information is usable with said first information to
determine unambiguously the SFN of a frame of the second sequence
at a particular time from a knowledge of the SFN of a frame of the
first sequence at a particular time.
57. The communication system in accordance with claim 57, wherein
the communication network is adapted to transmit said radio signal
to the user equipment from the first base station.
Description
TECHNICAL FIELD
[0001] The present invention relates to communication systems and
methods in which mobile user equipment (such as a mobile telephone,
personal digital assistant (PDA) or other device) communicates with
base stations of a communication network using radio signals in
accordance with defined radio frame formats. Certain embodiments of
the invention are concerned in particular with communication
methods and systems in which user equipment communicates with a
Universal Terrestrial Radio Access (UTRA) Network (also known as
UTRAN, or UMTS Terrestrial Radio Access Network) or an Evolved
Universal Terrestrial Radio Access (E-UTRA) Network (or
E-UTRAN).
BACKGROUND ART
[0002] A variety of communication networks comprising a plurality
of base stations with which mobile, portable user equipment can
communicate, using radio signals, are known. They include, for
example, the GSM network, and the 3rd Generation (3G) network
(UTRAN). These networks are typically arranged to provide
connectivity between the mobile user equipment (UE) and a core
network, such that the user equipment can communicate with other
user equipment or indeed other devices at different locations.
These communication networks may also be referred to as cellular
networks, with different base stations being arranged to provide
network coverage (i.e. provide radio communication with the UE) in
different areas, known as cells. The coverage of adjacent base
stations is typically arranged to overlap, so that there is no loss
in network connection as UE moves from one area to another.
Clearly, as UE moves within the area covered by such a network it
may become necessary for communication that was previously with one
base station to be handed over to another base station. The term
"base station" in this specification is being used in a broad
sense; it is not intended to be limited to the radio transceivers
of any particular communications network. It simply refers to a
station (or device) of the network which is arranged to transmit
radio signals to, and receive radio signals from the UE and so
provide connection between the UE and the networks. The base
station may also be referred to as a radio transceiver, and base
stations include, for example, the base stations of the type used
in GSM systems, and the base stations of the UTRA and E-UTRA
networks, which are commonly referred to as node Bs (NBs) and
enhanced-node Bs (eNBs) respectively.
[0003] In certain communications systems, for example those using
UTRA or E-UTRA networks, the communication between the user
equipment and the base station in a particular area or cell is by
radio signals in (i.e. according to) a predefined radio frame
format, that format comprising a sequence of radio frames, each
having the same length, and the frames of the sequence being
numbered sequentially with a System Frame Number (SFN). The frames
typically have a defined structure, and this structure may vary
with SFN. In such systems, for the UE to be able to communicate
with a particular base station it needs to synchronise with the
radio frame format being used by that base station, so that, for
example, when the UE sends a radio signal to the base station it
does so in an appropriate time slot according to the frame format.
Typically, the UE will perform this synchronisation using signals
received from the base station; it can detect frame
edges/boundaries, and will read SFN numbering information from
signals received from the base station, together with reading other
information required for communication with the base station, such
as information regarding the particular frame format (including
frame structure--i.e. structure at the sub-frame level). In certain
systems, and again for example those using the UTRA and E-UTRA
networks, the radio frame formats of different base stations are,
in general, not synchronised with each other. Furthermore, although
the frame lengths (i.e. durations) are typically the same, the
frame structures of different base stations may be different (with
structure varying with SFN in one format, and remaining the same,
or varying in a different way with SFN in the different format of
an adjacent base station or cell). The SFN may also be required by
the UE for other purposes (i.e. other than to determine frame
format) in order for the UE to communicate with the base station
(for example by sending a RACH transmission). For example, a
resource or resources allocated to the UE may be expressed in
relation to the SFN (i.e. the resource may apply in some of the
radio frames, but not others). In certain radio frames, the UE may
not be required to receive any signals, and so it may go into sleep
mode to conserve battery power. Thus, for a variety of reasons, the
UE may require a knowledge of at least one or more least
significant bits of the SFN (when expressed in binary form), or
indeed the whole SFN, in order to operate appropriately when in
communication with a base station.
[0004] Thus, in general, in a system using the UTRA or E-UTRA
networks, at a particular time the current frames of two different
base stations will have different SFNs, and the frame boundaries
will be occurring at different times (i.e. the beginning of a frame
in one format will occur at a different time to that of a frame in
the second format). Clearly, this lack of synchronisation between
base station's frame formats poses problems for handover of
communications; the UE will be synchronised with the frame format
of the base station with which it is currently communicating (the
source base station), but cannot communicate with the target base
station until it can synchronise with the frame format of that
target.
[0005] In more detail, before handover, the UE normally measures
the target cell. If this target cell becomes a good candidate, or
indeed the best candidate, the UE reports this to the network (via
the source cell, i.e. the current cell in which the UE is
communicating with a respective base station). As an initial step,
preceding the actual measurements, the UE searches for and detects
the candidate/target cell using specific physical layer channels
defined for this purpose (and known as SCH). As part of this search
process, the UE is able to determine the boundaries of the radio
frames in the target cell (i.e. the boundaries of the frames of the
sequence of frames in the defined format for the base station of
the target cell). A UE typically starts communication in an E-UTRA
cell using the random access procedure, which involves an initial
transmission on the Random Access Channel (RACH). E-UTRA uses a
radio frame format in which radio frames have a duration of 10 ms,
and are numbered by means of the System Frame Number (SFN). This
SFN is indicated on the primary broadcast channel (P-BCH) (or
simply the broadcast channel BCH), i.e. the SFN information of the
particular base station is contained in signals transmitted from
the base station on the P-BCH/BCH. To perform the initial
transmission on the RACH, the UE needs to be aware of the SFN
timing of the concerned cell (the target base station). This is
needed for the following reasons:
[0006] Firstly, the UE needs the target SFN timing information to
find the RACH slots (i.e. determine the time slots in the target
base station's frame format in which the UE may transmit its RACH
signal; if it does not transmit in the correct slot or slots,
communication with the base station will not be set up). For the
frequency division duplex (FDD) mode of operation, the interval
between RACH slots in UTRA/E-UTRA formats can be as follows: 1, 2,
5, 10, or 20 ms. If the target cell is arranged to apply an
interval of 20 ms (which means that a RACH slot occurs not in every
frame, but in every other frame) the UE needs to know the least
significant bit of the SFN to be able to find the RACH. In other
words, if the UE knows from other frame format information provided
to it in a handover signal, for example, that RACH slots occur only
in evenly numbered frames, the UE then must be able to determine
the least significant bit of the SFN in binary form of a particular
frame in order to transmit a RACH signal at an appropriate
time.
[0007] Secondly, the UE needs the target SFN timing information to
know the time frequency resources when hopping is used for RACH. In
certain systems frequency hopping techniques are used such that the
RACH signal frequency hops in time according to a particular
pattern. In UTRA/E-UTRA systems the RACH preamble frequency hopping
period can either be 10 or 40 ms. Thus, in the former case the RACH
frequency changes every frame, whilst in the latter case, the RACH
frequency changes every four frames. If the target cell applies a
40 ms hopping period, the UE therefore needs to know the two least
significant bits of the SFN to be able to determine the frames in
which RACH frequency changes, so that it can correctly access the
RACH (i.e. send a RACH signal, using the appropriate frequency, at
the appropriate time).
[0008] The SFN may be needed for other reasons as well, or
alternatively (e.g. where resource allocation is SFN dependent) as
discussed above.
[0009] Thus, when a handover from one base station to another is
required (or when communication between the UE and target base
station is required for some other purpose) the UE, synchronised
with the source base station, needs the SFN and timing information
of the target so that it can initiate communication with the
target.
[0010] One mechanism by which the UE can determine the SFN and
timing information of the target cell is for the UE to read the
PBCH or BCH of the target cell (base station). The UE can then
implicitly determine the two least significant bits of the SFN from
decoding the BCH, and can detect frame edges. The BCH is repeated
every 40 ms, meaning that it takes on average 20 ms to receive the
BCH, i.e. on average it will take 20 ms before the UE can determine
the target SFN timing information it requires to begin
communication with the target (by sending a RACH signal). BCH
reading delay therefore increases the handover interruption time.
Currently there are no other reasons for the UE to receive the BCH
prior to accessing the target cell's RACH. All other information
the UE requires to access the target cell is assumed to be
semi-static, so it can be provided to the UE in the handover
command that the target eNB generates and transfers to the UE via
the source eNB, i.e. prior to the actual handover. This way, there
is no need for the UE to read system information from the target
cell.
[0011] It is an object of certain embodiments of the present
invention to provide communication systems and methods which
obviate or mitigate at least one of the problems associated with
the prior art. It is an object of certain embodiments to provide
communication systems and methods offering improved handover (e.g.
faster and/or to reduce service interruption).
DISCLOSURE OF INVENTION
Technical Solution
[0012] According to a first aspect of the invention there is
provided a communication method in a communication system
comprising mobile user equipment and a communication network
adapted to communicate with the user equipment, the communication
network comprising: a first base station adapted to communicate
with the user equipment using radio signals and according to a
first defined radio frame format comprising a first sequence of
radio frames each having the same length and each being allocated a
respective first system frame number (SFN) such that the radio
frames of the first sequence are sequentially numbered; and a
second base station adapted to communicate with the user equipment
using radio signals and according to a second defined radio frame
format comprising a second sequence of radio frames each having the
same length and each being allocated a respective second SFN such
that the radio frames of the second sequence are sequentially
numbered, the length of the radio frames of one of the first and
second formats being an integer multiple of the length of the radio
frames of the other format, the method comprising:
[0013] receiving radio signals from the first base station at the
user equipment and using said received signals to synchronise the
user equipment with the first sequence of numbered radio frames,
such that the user equipment can transmit radio signals to the
first base station in accordance with the first defined radio frame
format, and to determine the first SFN of a frame of the first
sequence at a particular time,
[0014] transmitting a radio signal to the user equipment from the
communication network, the radio signal comprising first
information indicative of a difference between the first and second
SFNs at a particular time and second information usable with said
first information to determine unambiguously at least a least
significant bit of the SFN in binary form of a frame of the second
sequence at a particular time from a knowledge of the SFN of a
frame of the first sequence at a particular time,
[0015] receiving said radio signal at the user equipment when
synchronised with said first sequence, and using the first and
second information to synchronise the user equipment with the
second sequence, such that the user equipment can transmit radio
signals to the second base station in accordance with the second
defined radio frame format.
[0016] In certain embodiments the "integer" of the integer multiple
is one, such that the frames of the first format have the same
length as those of the second format. In alternative embodiments,
the integer may be greater than one, with the first format frame
length being greater than that of the second format, or vice
versa.
[0017] The radio signal in certain embodiments is transmitted in
accordance with the first format, from the first base station, to
the synchronised UE (i.e. synchronised with the first format).
[0018] Thus, the communication network itself provides information
to the UE which enables the UE to synchronise with the second radio
frame format (in the sense that it is able to transmit one or more
signals, for example a RACH signal, in accordance with that format)
without the UE first having to read a signal such as a P-BCH signal
from the second (i.e. target) base station. This saves time and/or
reduces interruption in service; the UE can synchronise with the
target base station from the received information and transmit a
signal to the target base station in the next available slot, the
UE having been able to determine the position of that next
available slot from (using) the received information.
[0019] As will be appreciated, the radio frame formats of the first
(source) and second (target) base stations will in general not be
synchronised (their edges will not occur at precisely the same
time), and so the first information alone is insufficient to
identify unambiguously the SFN of a frame of the second sequence at
a particular time from the SFN of a frame of the first sequence at
a particular time; the instantaneous difference in frame numbers
will alternate between two values with time. However, the second
information is arranged to enable this ambiguity to be removed. The
second information can be regarded as providing information on how
the first information (indicative of a simple numerical difference
in frame numbers) is to be applied; in effect, it tells the UE
between which of the source and target frames that difference is to
be applied.
[0020] In certain embodiments, the method further comprises
transmitting a radio signal from the user equipment to the second
base station in accordance with the second defined radio frame
format (i.e. after the UE has synchronised with the second base
station). For example, the communication network may be a UTRA or
an E-UTRA network, or a hybrid network comprising UTRA and/or
E-UTRA and/or other network devices, and the step of transmitting a
radio signal from the user equipment to the second base station may
comprise transmitting a RACH signal to the second base station to
begin communication with that station. This may form part of a
handover procedure, from the source to the target base station, and
the method may then comprise continuing communication between the
target base station and user equipment, after transmitting the
radio signal from the UE to the target base station, by exchanging
radio signals in accordance with the second defined format. In
certain embodiments the second base station is an enhanced node B
(eNB). The first base station may also be an eNB, or may be another
form of base station. In other words, in certain embodiments the
SFN information discussed above (and in the following description)
is relevant for when a UE performs handover to an E-UTRA cell. This
is irrespective of whether the source cell is using the E-UTRA
radio technology or another radio access technology (RAT) e.g. GSM.
If the SFN information is provided by the target eNB, this enables
handover to be performed in the inter-RAT case (where the source
and target cells use different RATs) provided the network
synchronisation solution is able to cover the particular source
cell RAT and provided also that the source and target RATs employ
radio frame formats with frames of the same RF duration, e.g. 10
ms, or durations which have a defined relationship to one another
(with one being an integer multiple of the other).
[0021] Thus, in certain embodiments the target cell is an E-UTRA
cell, since for this type of cell we currently clearly need the SFN
information. Embodiments of the invention may involve other RATs,
for example where the other RATs act as source cells. Thus, the
first base station may be an E-UTRA device or a non-E-UTRA device,
according to the particular embodiment.
[0022] After establishing communication with the second base
station the method may then comprise ceasing communication between
the first base station and user equipment. This would constitute a
complete handover. In alternative embodiments, however, it may be
desirable to maintain communication with the first base station,
with the UE synchronised with both the first and second formats, in
the sense that it is able to send signals at appropriate times to
either base station. As further information, if the UE is capable
of dual operation (i.e. can carry out communication with both the
source and target at the same time) the UE would normally be able
to receive the BCH of the target cell prior to performing the
handover, i.e. the measurements normally take sufficient time. For
such cases, there would normally be no reduction in service
interruption. In other words, embodiments of the invention are
particularly applicable to single transceiver operation.
[0023] In certain embodiments, the method further comprises
determining said first information in the communication network
from signals received from the first and second base stations. For
example, the first and second base stations may be eNBs of a UTRAN,
that UTRAN also comprising a radio network controller (RNC)
connected (wirelessly or otherwise) to the eNBs and able to
communicate with them. The RNC may be arranged to determine a SFN
difference (which may also be referred to as an SFN offset) from
signals received from the two eNBS. In alternative embodiments, one
of the base stations may be able to determine the SFN offset value
from its own radio frame format and from a signal received from the
other base station.
[0024] In certain embodiments the method further comprises
determining a time difference between radio frame boundaries of the
first and second sequence, and using the time difference together
with an estimate of an accuracy of the time difference to determine
said second information. The step of determining a time difference
may also be described as estimating a time difference, for example
from system measurements. In certain embodiments the time
difference referred to above is an indication of the difference
between the time at which a frame of one of the frame formats
begins and the time at which the next frame in the other format
begins. This estimated time difference may also be referred to as a
frame edge offset, or a radio frame transmission timing
difference.
[0025] In certain embodiments, the method further comprises
determining the second information according to the relative
magnitudes of the time difference and the estimate of accuracy.
[0026] For example, in certain embodiments the method comprises
determining the second information to indicate that the difference
between SFNs applies between a particular frame of the first
sequence and the frame of the second sequence having a start time
closest to that of the particular frame (whether that start time of
the frame of the second sequence occurs before of after the start
time of that particular frame of the first sequence) in the event
that the magnitude of the time difference is less than the
magnitude of the estimate of accuracy.
[0027] The method may also comprise determining the second
information to indicate that the difference between SFNs applies
between a particular frame of the first sequence and the frame of
the second sequence having a start time within that particular
frame, in the event that the magnitude of the time difference is
greater than the magnitude of the estimate of accuracy.
[0028] In certain embodiments, determining the time difference
comprises using a timing/synchronisation protocol. Certain
embodiments use the Network Time Protocol (NTP) to
determine/estimate the time difference between the frame edges in
the different formats.
[0029] In certain embodiments, the estimate of accuracy is a fixed,
predetermined value (for example selected on the basis of certain
assumptions). In other embodiments, however, the method comprises
the step of determining the estimate of accuracy, for example from
system signals and/or system measurements. For example, the
inaccuracy in a frame timing difference value obtained by using a
protocol like NTP will largely depend on the jitter in the
transport network between the two eNBs. It should be possible for
an operator to deploy the infrastructure to the eNBs with
relatively limited jitter (in an operator control network
environment), i.e. it should be possible for an operator to realise
a network deployment with sufficiently low jitter so that e.g. NTP
would result in sufficiently accurate results. As to whether a
protocol like NTP would itself be able to make some estimate of the
inaccuracy, to some extent this should be possible; an accurate
time difference estimate is obtained by longer term averaging of
the measured results. If the node performing the measurement
detects a lot of difference between the different individually
measured results, it may conclude that the jitter is quite high and
thus the accuracy of the estimate is not good. Thus, NTP may be
used in embodiments of the invention to provide an estimated time
difference, and may additionally be used to provide an indication
of the accuracy of the estimated value.
[0030] In certain embodiments the second information is usable with
the first information to determine unambiguously at least two least
significant bits of the SFN in binary form of a frame of the second
sequence at a particular time from a knowledge of the SFN of a
frame of the first sequence at a particular time.
[0031] In certain embodiments the second information is usable with
the first information to determine unambiguously the full SFN of a
frame of the second sequence at a particular time from a knowledge
of the SFN of a frame of the first sequence at a particular
time.
[0032] In certain embodiments, the step of transmitting a radio
signal (comprising the first and second information) to the user
equipment from the communication network comprises transmitting the
radio signal from the first base station. Thus, the transmitted
signal will be in the first defined frame format (with which the UE
is synchronised). The UE can thus quickly use the information on
the received signal to synchronise with the second format and begin
sending signals for reception by the second base station.
[0033] In certain embodiments the radio signal comprising the first
and second information further comprises additional information
indicative of frame structure in said second defined radio frame
format. This can be used by the UE in conjunction with the first
and second information to enable the UE to transmit signals in
appropriate time slots to the second base station, and for example
at the appropriate frequencies (the additional information in
general may comprise other information that the UE requires to be
able to carry out communication with the second base station, such
as frequencies, hopping intervals, configuration information
etc.).
[0034] A second aspect of the invention provides a communication
system comprising:
[0035] mobile user equipment; and
[0036] a communication network adapted to communicate with the user
equipment,
[0037] the communication network comprising:
[0038] a first base station adapted to communicate with the user
equipment using radio signals and according to a first defined
radio frame format comprising a first sequence of radio frames each
having the same length and each being allocated a respective first
system frame number (SFN) such that the radio frames of the first
sequence are sequentially numbered; and
[0039] a second base station adapted to communicate with the user
equipment using radio signals and according to a second defined
radio frame format comprising a second sequence of radio frames
each having the same length and each being allocated a respective
second SFN such that the radio frames of the second sequence are
sequentially numbered, the length of the radio frames of one of the
first and second formats being an integer multiple of the length of
the radio frames of the other format,
[0040] the user equipment being adapted to receive radio signals
from the first base station and to use said received signals to
synchronise with the first sequence of numbered radio frames, such
that the user equipment can transmit radio signals to the first
base station in accordance with the first defined radio frame
format, and determine the first SFN of a frame of the first
sequence at a particular time,
[0041] the communication network being further adapted to transmit
a radio signal to the user equipment, the radio signal comprising
first information indicative of a difference between the first and
second SFNs at a particular time and second information usable with
said first information to determine unambiguously at least a least
significant bit of the SFN in binary form of a frame of the second
sequence at a particular time from a knowledge of the SFN of a
frame of the first sequence at a particular time,
[0042] the user equipment being further adapted to receive said
radio signal and, when synchronised with said first sequence, to
use the first and second information to synchronise with the second
sequence, such that the user equipment can transmit radio signals
to the second base station in accordance with the second defined
radio frame format.
[0043] Again, the integer in certain embodiments is one, but in
alternative embodiments may be greater than one.
[0044] Advantages associated with this second aspect will be
apparent from the above discussion of the first aspect.
[0045] In certain embodiments the second base station is an
eNB.
[0046] In certain embodiments the user equipment is further adapted
to transmit a RACH signal to the eNB in accordance with the second
defined radio frame format when synchronised with said second
sequence
[0047] In certain embodiments the first and second base stations
are first and second eNBs respectively (e.g. forming part of an
E-UTRA network) and the user equipment is further adapted to
transmit a measurement report to the first eNB, the first eNB is
responsive to the measurement report to send a handover request to
the second eNB, the second eNB is responsive to the handover
request to send a handover request acknowledgement to the first eNB
comprising said first and second information, the first eNB is
responsive to the handover request acknowledgement to transmit said
radio signal comprising first and second information to the user
equipment, and the user equipment is then arranged to send a RACH
message to the second eNB. Thus, the system may be arranged to
perform a handover.
[0048] In certain embodiments the communication network is arranged
to determine a time difference between radio frame boundaries of
the first and second sequence, and to determine said second
information according to said time difference together and an
estimate of an accuracy of said time difference. For example, in
certain embodiments the communication network is adapted to
determine said second information according to the relative
magnitudes of said time difference and said estimate of
accuracy.
[0049] In certain embodiments the communication network is arranged
to determine said time difference using the NTP, and may be further
adapted to determine said estimate of accuracy using the NTP.
[0050] In certain embodiments the second information is usable with
said first information to determine unambiguously at least two
least significant bits of the SFN in binary form of a frame of the
second sequence at a particular time from a knowledge of the SFN of
a frame of the first sequence at a particular time, and in certain
embodiments may be usable to determine the full SFN.
[0051] In certain embodiments the communication network is adapted
to transmit said radio signal to the user equipment from the first
base station.
[0052] Another aspect of the invention provides user equipment
adapted for operation in a communication system in accordance with
the second aspect.
[0053] Another aspect provides a communication network adapted for
operation in a communication system in accordance with the first
aspect.
BRIEF DESCRIPTION OF THE DRAWINGS
[0054] Embodiments of the invention will now be described with
reference to the accompanying drawings, of which:
[0055] FIG. 1 is a diagram illustrating the timing and SFN
differences between radio frame formats of two different base
stations in an embodiment of the invention;
[0056] FIG. 2 is another diagram illustrating the timing and SFN
differences between radio frame formats of two different base
stations in an embodiment of the invention;
[0057] FIG. 3 is yet another diagram illustrating the timing and
SFN differences between radio frame formats of two different base
stations in an embodiment of the invention;
[0058] FIG. 4 is a diagram illustrating a handover message sequence
in an embodiment of the invention;
[0059] FIG. 5 is a flow diagram illustrating the operation of a
base station (eNB) in a communication system and method embodying
the invention;
[0060] FIG. 6 is a flow diagram illustrating the operation of User
Equipment (UE) in a communication system and method embodying the
invention;
[0061] FIG. 7 is a block diagram illustrating the structure of an
eNB usable in a communication system embodying the invention;
[0062] FIG. 8 is a block diagram illustrating the structure of User
Equipment (UE) usable in a communication system embodying the
invention;
[0063] FIG. 9 is a schematic diagram of a communication system
embodying the invention; and
[0064] FIG. 10 is a schematic diagram of another communication
system embodying the invention.
MODE FOR THE INVENTION
[0065] From the following description it will be appreciated that
methods embodying the invention can generally be described as
providing SFN offset signalling. Certain embodiments are applicable
to Universal Terrestrial Radio Access (UTRA), and Evolved Universal
Terrestrial Radio Access (E-UTRA) Networks, and avoid the need for
the UE to read the P-BCH or BCH prior to accessing a neighbouring
cell, which, for UEs in connected mode, reduces the handover
interruption (for example by .about.15 ms).
[0066] Certain embodiments of the invention use a signalling
mechanism by which the network can indicate to the UE the SFN
applicable for a radio frame in a target neighbouring cell by
reference to a radio frame in the source cell. According to this
signalling mechanism employed by embodiments, the network provides
the UE with first information (a number) indicating a difference in
SFN between the source and target cell (in other words, between the
first base station's frame format and the second base station's
frame format), i.e. an SFN offset. The network also provides second
information which indicates a method that the UE shall apply to
determine between which radio frames in the source and target cell
the SFN offset applies.
[0067] The following features of embodiments of the invention will
become apparent from the description below: the methods the UE
shall use to determine between which radio frames in the source and
target cell the SFN offset applies; the rules by which the E-UTRAN
shall determine which method is appropriate; the associated
signalling.
[0068] In certain embodiments of the invention the communication
system comprises a E-UTRAN which is arranged to indicate, to a UE,
an offset between the SFN used in a serving cell (i.e. in a source
cell, covered by a first base station using a first frame format)
and a target cell (covered by a second base station using a second
frame format). The advantage of this approach is that the UE is not
required to receive a P-BCH or BCH signal from the target base
station/cell before accessing that target cell. However, it should
be noted that in general E-UTRA cells are not synchronised, and it
is not sufficient for the network to provide just an offset value
to the UE; that information alone does not enable the UE to
correctly synchronise with the target cell's frame format. Further
timing information is required, and embodiments of the invention
obtain that information and provide it to the UE as follows.
[0069] There are a number of methods or protocols usable to provide
the network timing information required in embodiments of the
invention. One of these protocols is the Network Timing Protocol
(NTP). NTP can be used to enable a client A, which has no perfect
clock, to tune to a reference clock running at Server B (see for
example http://www.ntp.org/ntpfaq/NTP-s-algo.htm). Using NTP it is
possible to estimate time differences with an accuracy of a few ms
in an operator controlled environment, as used in cellular
communication networks.
[0070] Thus, certain embodiments of the invention use a network
timing solution, e.g. NTP, to provide a source base station (e.g. a
first eNB) with the SFN timing of the target cell's eNB, or vice
versa, with some inaccuracy. A reasonable assumption of this
inaccuracy is +/-2 ms, and this value will be used in certain
examples discussed below. However, it will be appreciated that
other estimated values may be appropriate in other circumstances.
Furthermore, in certain embodiments the network may be able to
obtain a measure of this inaccuracy, and use the measured value
instead of an assumed one.
[0071] Again, It should be noted that the network nodes in
embodiments of the invention are assumed not to be synchronised
i.e. there is a time difference that is drifting slowly. Note also
that the network timing solution is a continuously ongoing process
i.e. there is no need, in certain embodiments, to perform timing
related actions upon handover preparation/execution.
[0072] In the description below the following terms are used:
[0073] Estimated RF time difference (ETD): time difference between
the radio frame boundaries in the source and target cell (i.e. in
the source and target radio frame formats), estimated based on a
network timing solution e.g. NTP
[0074] Estimated RF time difference inaccuracy (ETDi): inaccuracy
in the ETD. Clearly, this value is a "worst case" figure, and the
actual time difference will in general be within the ETDi of the
ETD.
[0075] This following description addresses the question how the
SFN timing/offset information should be conveyed to the UE. The SFN
timing/offset information in certain embodiments comprises two
elements:
[0076] 1) first information: an SFN offset i.e the number that the
UE should add to the SFN detected in a radio frame in the source
cell to obtain the SFN applicable for a radio frame in the target
cell
[0077] 2) second information: a Reference Frames Identification
(RFI) method that the UE should apply to determine between which
radio frames in the source and target cell the SFN offset
applies.
[0078] In the following we will show that it is not possible for
the UE to apply a single RFI method. In fact, the UE has to apply
one of the following methods to identify the reference radio
frames:
[0079] a) the SFN offset is applicable between the radio frame in
the source and the radio frame in the target which start is closest
to the start of radio frame in the source
[0080] b) the SFN offset is applicable between the radio frame in
the source and the radio frame in the target which start occurs
within the radio frame in the source
[0081] Which RFI method applies depends on the estimated RF time
difference as well as the inaccuracy in this. Since only the
E-UTRAN is aware of this (not the UE), the E-UTRAN has to signal to
the UE which RFI method it should apply. In the following the two
RFI methods are explained in more detail, as well as the E-UTRAN
rule regarding when to apply which signalling option.
[0082] In the following description of certain embodiments, it is
assumed that the target eNB provides the SFN timing information,
because it is the target eNB that compiles the entire handover
message in these examples. However, the mechanism could equally
well be applied such that the source eNB provides the SFN
timing/offset information to the UE in other embodiments.
[0083] As mentioned above, under certain circumstances the second
information will indicate that the SFN offset is applicable between
radio frames whose start times are closest. This signalling option
is explained by means of an example:
[0084] Consider the situation where Estimated RF time difference
(ETD)=1 ms (target cell radio frame starts 1 ms later than source
cell)
[0085] Consider ETD inaccuracy (ETDi)=.+-.2 ms
[0086] This means that the radio frame in the target cell could
start 1 ms before that of source cell (case 1), it could start 3 ms
after that of the source cell (case 2) or could start anywhere
(i.e. any time) in-between.
[0087] FIG. 1 illustrates the example, showing the target cell
timing for the two extreme cases, referred to as case 1 & case
2 in the previous paragraph. In this figure, 4a represents the
sequence of numbered frames in time according to the defined radio
frame format of the source cell, i.e. the radio frame format with
which the user equipment can communicate with the base station for
that cell. This sequence 4a comprises a series of radio frames 40a,
which themselves have structure and are divided into time slots
41a. The frames 40a are sequentially numbered with system frame
numbers (SFN1, SFN2 etc). In this particular format there are no
gaps in the frame sequence, with the frame boundaries marking the
end of one frame and the beginning of the next frame being
indicated by reference number 410a. Also in the figure the
sequences of frames for the target cell (i.e. in accordance with
the defined radio frame format of the base station for the target
cell) are denoted by reference number 4b. As will be seen, the
frames of the source and target cell all have the same approximate
length (i.e. duration) but the source and target cells are not
synchronised so that there is an offset in the positions of the
frame boundaries 410a and 410b.
[0088] In this example, the hatched box marks the range of possible
target cell timings. All these timing options have one
characteristic in common: the start of the radio frame (RF) with
SFN 11 in the target cell is closest to the start of the radio
frame with SFN 1 in the source cell
[0089] In other words, the SFN difference between the radio frames
in the target and source whose start times are closest equals +10
(i.e. SFN-target is 10 higher than SFN-source)
[0090] Hence, in this case the E-UTRAN would indicate:
[0091] SFN offset=+10 (SFN of radio frame in target is 10 higher
than in source cell)
[0092] RFI method=A) i.e. the SFN offset applies between the radio
frames whose start times are closest.
[0093] Consider now the offset applicable for the target RF
starting within the source RF. This signalling option is again
explained by means of an example:
[0094] Consider the case where Estimated RF time difference (ETD)=4
ms (target cell radio frame starts 1 ms later than source cell)
[0095] Consider ETD inaccuracy (ETDi)=.+-.2 ms
[0096] This means that the radio frame in the target cell could
start 2 ms after that of the source cell (case 1), it could start 6
ms after that of the source cell (case 2) or could start anywhere
in-between.
[0097] FIG. 2 illustrates the example, showing the target cell
timing for the two extreme cases, referred to as case 1 & case
2 in the previous paragraph. In this example, the hatched box marks
the range of possible target cell timings.
[0098] All these timing options have one characteristic in common:
the start of the radio frame with SFN 11 in the target cell is
within the radio frame with SFN 1 in the source cell.
[0099] In other words, the start of the radio frame with SFN 11 in
the target cell is after the start of the radio frame with SFN 1 in
the source cell.
[0100] Hence, in this case the E-UTRAN would indicate:
[0101] SFN offset=+10 (SFN of radio frame in target is 10 higher
than in source cell)
[0102] RFI method=B) i.e. the SFN offset applies between the radio
frame in the source cell and the radio frame of the target cell
whose start occurs within the source cell's radio frame starts (or
in other words, the SFN offset applies between the radio frame in
the source cell and the radio frame of the target cell whose start
immediately follows the start of the source cell's radio
frame).
[0103] In embodiments of the invention the network needs to decide
which of the above methods should be signalled to the UE to enable
correct synchronisation with the target base station/cell to be
achieved. In embodiments utilising E-UTRAN, it is up to E-UTRAN to
signal the correct SFN offset as well as the correct method (of
applying that numerical offset) to the UE. Which method to apply
depends on the Estimated RF time difference (ETD) and the
inaccuracy in this estimate i.e. the ETDi.
[0104] Due to the 10 ms duration of a RF in UTRAN/E-UTRAN systems,
the value range of ETD is from -5 ms to +5 ms.
[0105] The value range of the ETDi depends on the network timing
mechanism that is implemented in the network. Let us describe the
value range of ETDi=[-.epsilon., +.epsilon.]. It should be noted
that the network based solution can only work if .epsilon. is
smaller than 5 ms.
[0106] FIG. 3 illustrates that if ETD<.epsilon., the start of
the radio frame in the target cell may be prior to the start of the
radio frame in the source cell. In such a case, method B (as
referred to above) will not provide correct/consistent results.
[0107] Hence, the E-UTRAN should select the RFI method as
follows:
[0108] If ETD.ltoreq..epsilon. (i.e. ETD-.epsilon. (the start of
the target cell frame) might fall in the source radio frame, or the
preceding source frame): method A should be used.
[0109] Else (ETD>.epsilon.): method B should be used.
[0110] In other words, where the relative magnitudes of estimated
time difference and inaccuracy are such that there is uncertainty
as to whether a target frame will begin in a particular source
frame or in the preceding source frame, method A needs to be used.
Alternatively, where the relative magnitudes indicate that the
target frame will start somewhere within the source frame, method B
should be used. Following these rules, ambiguity is removed and the
UE can synchronise with the target cell using the first and second
information provided by the network.
[0111] For completeness, we consider the possibility of the network
being able to synchronise the network nodes (eNBs) in a manner such
that the E-UTRAN would only need to provide the SFN offset to the
UE in order for it to synchronise with the target cell/node. In
such a hypothetical situation, there could be a timing difference
between adjacent cells, but that timing difference would not be
drifting. However, there would be some jitter (i.e. there would
still be an ETDi). Further to this, the network synchronisation
solution would have to ensure that there were no cells having an
ETD that would require method A above.
[0112] Suppose that there is a small network based timing
inaccuracy, say less than 1 ms. In such a case, E-UTRAN would have
to ensure that the timing of all adjacent cells is at least 1 ms
apart. This would require a network synchronisation
master/coordinator to manage the timing of the cells. Clearly, this
would represent a relatively complicated solution, requiring high
accuracy, and hence is a solution that most networks would like to
avoid.
[0113] Embodiments of the invention, by providing the first and
second information, enable the UE to synchronise with the target
base station even when timing differences and offsets are changing,
so avoiding the need for accurate inter-node synchronisation and
the associated costs/complexity.
[0114] It will be appreciated that embodiments of the invention
provide a network based SFN solution which provides the advantage
that the handover interruption is reduced significantly (without
this solution, using for example the reading of the P-BCH method
described in the background to the invention, the average
interruption may double from .about.15 to .about.30 ms).
[0115] In order to achieve these advantages, in certain embodiments
the (asynchronous) network needs to implement a network based
timing mechanism. The signalling mechanism used in certain
embodiments indicates an `RFI-method` in addition to an offset
value, to enable synchronisation to be achieved.
[0116] Referring now to FIG. 4, this is a message sequence diagram
showing the sequence of messages in a communication method (in
particular a handover method) embodying the invention). The steps
in this sequence are as follows:
[0117] 0) A network based timing solution is used e.g. NTP. As a
result of this network based timing solution the source and target
eNB are always aware of the Estimated Time Difference between the
two cells, as well as the inaccuracy, i.e. no specific timing
actions need to be performed upon handover
preparation/execution.
[0118] 1) The UE 1 provides a measurement report which may suggest
that it is desirable that the network triggers a handover to
another cell, the target cell. This step is optional i.e. the
source may also initiate handover without having received a
measurement report (i.e. blind handover)
[0119] 2) The source eNB 21a initiates handover by requesting the
target eNB 21b to prepare for the handover e.g. to allocate radio
resources.
[0120] 3) The target cell compiles a handover command message that
is to be sent to the UE via the source eNB. The handover command
includes the SFN offset and the RFI method
[0121] 4) The source eNB forwards the handover command received
from the target cell to the UE
[0122] 5) Upon receiving the handover command the UE initiates
handover. The UE applies the SFN offset and the RFI method to
determine the SFN of the radio frames in the target cell. Upon
receiving the handover command, the UE initiates random access in
the target cell. This random access is performed using the firstly
occurring RACH slot in the target cell. The UE applies the SFN
offset and the RFI method to find this RACH slots (in case the
target cell applies one slot every 20 ms) and/or the time frequency
resources used for this RACH slot (in case the target cell applies
a hopping period of 40 ms)
[0123] As indicated in the above (see step 5), the SFN offset and
the RFI method are needed depending on the RACH configuration in
the target cell i.e. the information is optional to include in the
concerned messages. Even if needed, the network could omit the
information e.g. to avoid the complexity associated with the
network based timing solution. The handover would still succeed,
but the service interruption will be larger.
[0124] In the above example, the SFN offset information was
provided by the target cell (i.e. by the target base station).
However, it will be appreciated that in alternative embodiments the
SFN offset information may be provided by the source cell (source
base station).
[0125] Referring now to FIGS. 5 and 6, these shows flow diagrams
illustrating operation of an eNB and a UE in certain embodiments of
the invention. In the method illustrated in FIG. 5, the eNB (which
may be the base station of the target cell or the source cell) in
step S1 determines if the UE will need SFN information in order to
find the RACH in the target cell (SFN information will of course
not be required if there is a RACH slot defined in each frame, but
at least the least significant bit of the SFN will be required if
RACH slots occur only in alternate frames, etc). In step S2 a
decision is taken according to whether SFN information is needed.
If that information is not needed the method proceeds to step S7 in
which handover command preparations are performed. If SFN
information is needed the method instead proceeds to step S3 in
which the base station determines which RFI method it should use,
based on the relative magnitudes of the ETD and ETDI. In step S4,
if method B is appropriate then the flow is directed to step S6 in
which the SFN offset value and information indicative that RFI
method B should be used are included in the handover command being
prepared by the base station. Alternatively, if it has been decided
that method A is appropriate, then step S5 is performed in which
the SFN offset and information indicating that RFI method A should
be used are inserted in the handover command. Referring to FIG. 6,
this shows the method steps performed by the UE on receipt of the
handover message from the network (e.g. from the source base
station or the target base station). In step S8 the UE receives the
handover command which includes information on the RACH
configuration of the target cell (base station). In step S9 the UE
determines if SFN information is required in order to find a RACH
slot (and perhaps additionally the RACH frequency). In step 10, if
SFN information is not required the method is directed to step S14
in which handover execution is actioned. Alternatively if SFN
information is needed then the flow proceeds to step S11 in which
it is determined whether or not the SFN information has been
provided. If appropriate SFN timing information has not been
provided then the method proceeds to step S12 in which the UE
acquires the P-BCH or BCH signal from the target cell to determine
the SFN and timing information from the signals received on that
channel. Alternatively, if step S11 determines that the appropriate
SFN timing information has been provided, flow proceeds to step
S13. In that step, the received SFN offset information and the
other information (e.g. the RFI method information) which tells the
UE how to use the SFN offset, are used to synchronise with the
radio frame format of the target cell (in other words, to find a
RACH slot in which the UE can send a signal to the target base
station to initiate communication as part of the handover process).
After step S13 the method then proceeds to step 14 in which other
handover execution actions are performed.
[0126] Some further details on the operations indicated in the flow
diagrams are as follows:
[0127] Determining if SFN information is needed to find RACH (eNB,
UE): the SFN is needed in the event that the RACH configuration in
the target cell meets the following criteria: one RACH slot is
configured every 20 ms; or RACH hopping is used with a period of 40
ms.
[0128] RFI method selection (eNB). is selected as follows: select
RFI method B IF ETD>.epsilon.; select RFI method A
otherwise.
[0129] Applying the RFI methods to determine the SFN in the target
(UE): if using RFI method A, the SFN offset applies between the
radio frames whose start times are closest; if using RFI method B,
the SFN offset applies between the radio frame in the source cell
and the radio frame of the target cell whose start occurs within
the source cell's radio frame.
[0130] A handover message/command (RRC Connection Reconfiguration)
used in embodiments of the invention may contain the following
information (this message is used by eUTRAN to modify or release an
RRC connection):
TABLE-US-00001 TABLE 1 Table 1: Handover command contents
Information Element/Group name Need Description Message Type MP RRC
transaction identifier MP Measurement configuration OP Mobility
control information OP >Target cell identity MP >Carrier
frequency OC >Additional spectrum emission OC requirement
>Semi-static common channel OP configuration information >End
time dedicated preamble OP >SFN information OP >>SFN
offset MP Number reflecting the difference in SFN timing between
target and source >>RFI method MP Method A or B NAS dedicated
information OP Radio resource configuration OP Security
configuration OP UE related information OP Idle mode mobility
control FFS Optionally present in the information message used to
perform connection release. It is FFS if a message is introduced
specific for connection release
[0131] In the above table only the further details of the Mobility
control information are shown since the other information elements
are not essential for the purpose of understanding the present
invention. The abbreviations used in the above table are as
follows:
TABLE-US-00002 TABLE 2 Abbreviation Meaning MP Mandatory present An
information element that always needs to be signalled. If the
transfer syntax allows absence (e.g. because the information
concerns an extension), the UE shall consider this to be protocol
error OP Optional An information element that is optional to
signal. The UE behaviour that applies in case the IE is absent is
specified in the procedural specification OC Optional, Continue An
information element that is optional to signal. In case the
information element is absent, the UE shall continue to use the
existing value (and the associated functionality) FFS For Further
Study
[0132] Referring now to FIGS. 7 and 8, these are general block
diagrams of an eNB and a UE respectively which may be used in
communication systems and methods embodying the present invention.
In general, the functionality required for these elements to
operate in communication methods and systems embodying the
invention is provided by appropriate programming of the transceiver
controllers 602 and 502, the hardware being otherwise as in prior
art systems. The eNB 21 comprises a controller module 600, a
transceiver module 610 and a memory module 620. The controller 600
comprises a measurement controller 601, a transceiver controller
602 and a scheduler 603. The transceiver 610 comprises a control
information transceiver 611 and a data transceiver 612. The UE1
comprises a controller module 500, a transceiver module 510 and a
memory module 520. The controller 500 comprises a measurement
controller 501 and a transceiver controller 502. The transceiver
510 comprises a control information transceiver 511 and a data
transceiver 512.
[0133] The eNB 21 is adapted to communicate with user equipment
using radio signals according to a defined radio format comprising
a sequence of radio frames each having the same length and each
being allocated a system frame number. This eNB 21 is further
adapted to prepare and transmit a radio signal to user equipment in
accordance with the defined radio frame format, that signal
comprising first information indicative of a difference between the
SFN at a particular time in the eNB's frame format and the SFN of
another eNB's frame format at that time, together with second
information which can be used by user equipment receiving the
signal in conjunction with the first information to enable that UE
to synchronise with the other, target base station. The eNB is
arranged to determine the first and second information by
communication within its network, e.g. by communicating with
another eNB to determine the SFN offset information and the frame
boundary timing information required to generate the second
information.
[0134] Similarly, the UE 1 in FIG. 8 is adapted to be able to
synchronise with a particular base station using signals received
from that base station in its particular radio frame format. The UE
is also adapted so that when it is synchronised with a particular
base station format it can receive an appropriate signal containing
first and second information (as described above), indicating an
SFN offset value and how to use it, and can then synchronise with
the radio frame format of another base station (of a target cell).
When synchronised, it can send a communication-initiating signal
(such as a RACH signal) at a time appropriate in the target cell
frame format.
[0135] Referring now to FIG. 9, this shows a communication system
embodying the invention. The system comprises mobile user equipment
1 in the form of a mobile phone, a cellular communications network
2, and a core network 3. In this example the cellular
communications network 2 is a UTRAN network comprising a plurality
of node Bs 21a, b, c, and d, and a plurality of radio network
controllers RNC 22. Although in this example the network is a UTRAN
network, in alternative embodiments the network may be a E-UTRAN
network, in which case the base stations 21 would be ENBs. Thus,
the UTRAN 2 is arranged to provide connectivity between the UE 1
and the core network, and so enables the UE to be connected to
other UEs (for example at remote locations) and to other devices
and systems. The RNCs 22 provide control functions for one or more
of the node Bs 21. The logical interface between the RNCs and the
NBs, by which these devices communicate with one another, is
referred to as the IuB. There are four interfaces connecting the
UTRAN elements internally and externally to other entities, these
four interfaces being known as Iu, Uu, Iub and Iur. The Iu
interface is an external interface that connects the RNC to the
core network 3. The Uu is also external, connecting the particular
node B with the user equipment 1. The Iub is an internal interface
connecting the RNC with the node B, and finally there is the Iur
interface which is an internal interface most of the time, but can
in certain conditions be an external interface. This Iur connects
two RNCs with each other. In the figure, the coverage area or cell
corresponding to a first node B 21a is denoted by 210a. This is the
region within which the UE can communicate with the first node B
21a using radio signals according to the radio frame format
appropriate to that base station 21a and its corresponding cell
210a. Similarly, the cell of a second node B 21b is indicated by
reference number 210b. As can be seen, the UE1 is shown located in
an area of overlap between the two coverage areas. The UE1 may
synchronise with the first cell 210a (i.e. synchronise with the
radio frame format of its node B 21a). Using techniques described
above, the network 2 is arranged to send a radio signal to the UE1
containing first information indicative of a SFN offset value
between the first cell 210a and second cell 210b and second
information which can be used in conjunction with the first
information by the UE1 to synchronise with the second cell 210b.
Having received that information, the UE1 is then able to initiate
communication with the second base station 21b and after that it
may then drop communication with the first node B 21a as it moves
from the overlap area into the area covered by the second base
station 21b alone.
[0136] Referring now to FIG. 10, this shows another communication
system embodying the invention. The system comprises a target cell
210b which is an E-UTRA cell in an E-UTRAN architecture, and a
source cell 210a of a different (i.e. non-E-UTRAN) RAT. The source
cell comprises a source base station 21a communicating with the
user equipment 1, which is located in the overlap region of the
source and target cells. The source cell 210a is connected to the
source core network (a non-E-UTRAN RAT) 121 and this
connection/interface is denoted generally by reference number 102.
The E-UTRAN consists of eNBs, providing the E-UTRA user plane and
control plane protocol terminations towards the UE 1. The eNBs are
interconnected with each other by means of the X2 interface. The
eNBs are also connected by means of the S1 interface to the EPC
(Evolved Packet Core), more specifically to the MME (Mobility
Management Entity) by means of the S1-MME and to the Serving
Gateway (S-GW) by means of the S1-U. The S1 interface supports a
many-to-many relation between MMEs/Serving Gateways and eNBs. As
mentioned above, in this example the source cell 210a is of another
RAT, and there is a connection between the source and target cells
at the level of the Core Networks i.e. the communication between
the source RAT and the target cell in the U-UTRAN is via the EPC.
The connection between the source core network 121 and the E-UTRAN
core network is indicated by reference number 101. Thus, in certain
embodiments the source base station (e.g. a node B, if the source
RAT is UTRAN) is not directly connected to the EPC nodes but via
another node in its own core network. It will also be appreciated
that in certain embodiments there are many eNBs connected to an MME
& to an SAE Gw.
ABBREVIATIONS
[0137] AS Access Stratum. All that extends from below NAS to the
actual physical layer.
[0138] CN Core Network
[0139] eNB Enhanced Node B
[0140] ETD Estimated Time Difference
[0141] ETDi Estimated Time Difference inaccuracy
[0142] GMM GPRS Mobility Management
[0143] LA Location Area
[0144] LTE Long Term Evolution
[0145] MME Mobility Management Entity
[0146] NAS Non-Access Stratum. Commonly understood to extend above
AS right up to the interface with the application level.
Specifically it is what is given in 3GPP TS 24.008 encompassing the
MM, GMM, CC, SMS, SM, SS.
[0147] RA Routing Area
[0148] RFI Reference frames identification
[0149] SAE System Architecture Evolution
[0150] SFN System Frame Number
[0151] SM Session management
[0152] TA Tracking Area
[0153] UMTS Universal Mobile Telecommunications System
[0154] E-UTRA Evolved Universal Terrestrial Radio Access
[0155] E-UTRAN Evolved Universal Terrestrial Radio Access
Network
[0156] UPE User Plane Entity
INDUSTRIAL APPLICABILITY
[0157] With regard to applicability, it will be appreciated that
certain methods embodying the invention are particularly applicable
to systems comprising E-UTRA base stations and UEs in the form of
mobile phones. The invention helps reduce service interruption for
intra LTE handover. However, the use of a methods embodying the
invention for other handover scenarios is not precluded.
Furthermore, the present invention is applicable to other
communication systems (it is not limited to UTRA or E-UTRA
systems). Methods embodying the invention may be applicable in
other networks where a mobile device needs to know the radio frame
timing of a cell prior to cell access.
[0158] For the purpose of ease of elaboration and also for readers
unfamiliar with terms and abbreviations within the 3GPP, some of
the abbreviations and terms used in this document are provided
here. It must be clearly noted and understood by readers that
whilst every attempt has been made to have the terms and
abbreviations used in this paper to be an exact match with those
terms and abbreviations used in 3GPP, the terms and abbreviations
here listed are strictly only for the purpose of use relating to
this document.
* * * * *
References