U.S. patent application number 14/762546 was filed with the patent office on 2015-12-24 for radio communication system, radio communication method, radio base station and user terminal.
The applicant listed for this patent is NTT DOCOMO, INC.. Invention is credited to Hiroyuki Ishii, Yoshihisa Kishiyama, Satoshi Nagata, Shimpei Yasukawa.
Application Number | 20150373654 14/762546 |
Document ID | / |
Family ID | 51227259 |
Filed Date | 2015-12-24 |
United States Patent
Application |
20150373654 |
Kind Code |
A1 |
Yasukawa; Shimpei ; et
al. |
December 24, 2015 |
RADIO COMMUNICATION SYSTEM, RADIO COMMUNICATION METHOD, RADIO BASE
STATION AND USER TERMINAL
Abstract
The present invention is designed to establish synchronization
between transmission points when downlink signals are transmitted
from a plurality of transmission points to a user terminal. A radio
communication system has a first radio base station that forms a
first cell, a second radio base station that forms a second cell,
which is placed on the area of the first cell in an overlapping
manner, and a user terminal that is capable of carrying out radio
communication with the first radio base station and the second
radio base station, and the second radio base station has a
receiving section that receives synchronization correction
information, which is for establishing synchronization with a
synchronization target, from the user terminal, and a
synchronization correction section that corrects synchronization
based on the synchronization correction information.
Inventors: |
Yasukawa; Shimpei; (Tokyo,
JP) ; Nagata; Satoshi; (Tokyo, JP) ;
Kishiyama; Yoshihisa; (Tokyo, JP) ; Ishii;
Hiroyuki; (Palo Alto, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NTT DOCOMO, INC. |
Chiyoda-ku, Tokyo |
|
JP |
|
|
Family ID: |
51227259 |
Appl. No.: |
14/762546 |
Filed: |
December 20, 2013 |
PCT Filed: |
December 20, 2013 |
PCT NO: |
PCT/JP2013/084277 |
371 Date: |
July 22, 2015 |
Current U.S.
Class: |
370/338 |
Current CPC
Class: |
H04W 56/00 20130101;
H04W 74/0891 20130101; H04W 56/0055 20130101; H04W 56/001
20130101 |
International
Class: |
H04W 56/00 20060101
H04W056/00; H04W 74/08 20060101 H04W074/08 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 24, 2013 |
JP |
2013-011456 |
Claims
1. A radio communication system comprising a first radio base
station that forms a first cell, a second radio base station that
forms a second cell, which is placed on an area of the first cell
in an overlapping manner, and a user terminal that is capable of
carrying out radio communication with the first radio base station
and the second radio base station, wherein the second radio base
station comprises: a receiving section that receives
synchronization correction information, which is for establishing
synchronization with a synchronization target, from the user
terminal; and a synchronization correction section that corrects
synchronization based on the synchronization correction
information.
2. The radio communication system according to claim 1, wherein:
the second radio base station further comprises: a generating
section that generates a synchronization signal, which is used to
estimate information about desynchronization as the synchronization
correction information in the user terminal; and a transmitting
section that transmits the synchronization signal to the user
terminal; and the user terminal comprises: a receiving section that
receives the synchronization signal; and an estimation section that
estimates desynchronization between the second cell and the
synchronization target based on the synchronization signal
received.
3. The radio communication system according to claim 1, wherein:
the user terminal comprises a generating section that generates the
synchronization correction information; and the second radio base
station further comprises an estimation section that estimates
information about desynchronization with the synchronization target
in accordance with the synchronization correction information.
4. The radio communication system according to claim 3, wherein the
synchronization correction information is a RACH (Random Access
Channel) signal.
5. The radio communication system according to claim 2, wherein the
synchronization target is the first cell or a GPS (Global
Positioning System) clock.
6. The radio communication system according to claim 3, wherein the
synchronization target is the first cell or a GPS (Global
Positioning System) clock.
7. The radio communication system according to claim 2, wherein the
synchronization target is another second cell that is placed near
the second cell where the second radio base station is placed.
8. (canceled)
9. A radio base station in a radio communication system comprising
a first radio base station that forms a first cell, a second radio
base station that forms a second cell, which is placed on an area
of the first cell in an overlapping manner, and a user terminal
that is capable of carrying out radio communication with the first
radio base station and the second radio base station, the radio
base station comprising: a receiving section that receives
synchronization correction information, which is for establishing
synchronization with a synchronization target, from the user
terminal; a synchronization correction section that corrects
synchronization based on the synchronization correction
information; a generating section that generates a synchronization
signal, which is used to estimate information about
desynchronization as the synchronization correction information in
the user terminal; and a transmitting section that transmits the
synchronization signal to the user terminal.
10. A user terminal in a radio communication system comprising a
first radio base station that forms a first cell, a second radio
base station that forms a second cell, which is placed on an area
of the first cell in an overlapping manner, and the user terminal
that is capable of carrying out radio communication with the first
radio base station and the second radio base station, the user
terminal comprising: a receiving section that receives a
synchronization signal; and an estimation section that estimates
desynchronization between the second cell and a synchronization
target based on the synchronization signal received.
Description
TECHNICAL FIELD
[0001] The present invention relates to a radio communication
system, a radio communication method, a radio base station and a
user terminal that are applicable to cellular systems and so
on.
BACKGROUND ART
[0002] In a UMTS (Universal Mobile Telecommunications System)
network, attempts are made to optimize features of the system,
which are based on W-CDMA (Wideband Code Division Multiple Access),
by adopting HSDPA (High Speed Downlink Packet Access) and HSUPA
(High Speed Uplink Packet Access), for the purposes of improving
spectral efficiency and improving the data rates. With this UMTS
network, LTE (Long-Term Evolution) is under study for the purposes
of further increasing high-speed data rates, providing low delay
and so on (non-patent literature 1).
[0003] In a third-generation system, it is possible to achieve a
transmission rate of maximum approximately 2 Mbps on the downlink
by using a fixed band of approximately 5 MHz. Meanwhile, in an LTE
system, it is possible to achieve a transmission rate of about
maximum 300 Mbps on the downlink and about 75 Mbps on the uplink by
using a variable band which ranges from 1.4 MHz to 20 MHz.
Furthermore, with the UMTS network, successor systems of LTE are
also under study for the purpose of achieving further
broadbandization and higher speed (for example, LTE-advanced
("LTE-A")). The system band of an LTE-A system includes at least
one component carrier (CC), where the system band of the LTE system
is one unit. Achieving broadbandization by gathering a plurality of
components carriers (cells) in this way is referred to as "carrier
aggregation" (CA).
[0004] Now, as a promising technique for further improving the
system performance of the LTE system, there is inter-cell
orthogonalization. For example, in the LTE-A system, intra-cell
orthogonalization is made possible by orthogonal multiple access on
both the uplink and the downlink. That is to say, on the downlink,
orthogonality is established between user terminals UE (User
Equipment) in the frequency domain. Meanwhile, between cells, like
in W-CDMA, interference randomization by one-cell frequency re-use
is fundamental.
[0005] So, in the 3GPP (3rd Generation Partnership Project),
coordinated multi-point transmission/reception (CoMP) techniques
are under study as techniques to realize inter-cell
orthogonalization. In this CoMP transmission/reception, a plurality
of cells coordinate and perform the process for transmitting and
receiving signals, for one user terminal UE or for a plurality of
user terminals UE. For example, on the downlink, simultaneous
transmission by multiple cells by employing precoding, coordinated
scheduling/beamforming and so on are under study. By employing
these CoMP transmission/reception techniques, improvement of
throughput performance is expected, especially with respect to user
terminals UE located on cell edges.
CITATION LIST
Non-Patent Literature
[0006] Non-Patent Literature 1: 3GPP, TR25.912 (V7.1.0),
"Feasibility Study for Evolved UTRA and UTRAN," September 2006
SUMMARY OF INVENTION
Technical Problem
[0007] Up to LTE Rel. 10, a user terminal UE had only to carry out
the receiving process on the assumption that downlink signals were
transmitted from a single radio base station. However, from Rel. 11
onward, following the introduction of the above-noted CoMP
techniques and/or the like, transmission modes to transmit downlink
signals from a plurality of transmission points to a user terminal
UE have also been assumed.
[0008] When downlink signals are transmitted from a plurality of
transmission points (radio base stations), cases might occur where,
depending on the position relationship between a user terminal UE
and each transmission point, and so on, every downlink signal shows
different characteristics (received time, frequency offset and so
on). In such cases, if the user terminal UE carries out the
synchronization process on the assumption that the downlink signals
are transmitted from a single radio base station as has been the
case heretofore, there is a threat that the time synchronization
and frequency synchronization of the downlink signals cannot be
established, and the reliability of reception decreases.
[0009] Consequently, when downlink signals are transmitted from a
plurality of transmission points to a user terminal UE,
synchronization needs to be established between the transmission
points.
[0010] The present invention has been made in view of the above,
and it is therefore an object of the present invention to provide a
radio communication system, a radio communication method, a radio
base station and a user terminal, whereby, when downlink signals
are transmitted from a plurality of transmission points to a user
terminal, synchronization can be established between the
transmission points.
Solution to Problem
[0011] The radio communication system according to the present
invention is a radio communication system to have a first radio
base station that forms a first cell, a second radio base station
that forms a second cell, which is placed on an area of the first
cell in an overlapping manner, and a user terminal that is capable
of carrying out radio communication with the first radio base
station and the second radio base station, and, in this radio
communication system, the second radio base station has a receiving
section that receives synchronization correction information, which
is for establishing synchronization with a synchronization target,
from the user terminal, and a synchronization correction section
that corrects synchronization based on the synchronization
correction information.
Advantageous Effects of Invention
[0012] According to the present invention, when downlink signals
are transmitted from a plurality of transmission points to a user
terminal, it is possible to establish synchronization between the
transmission points.
BRIEF DESCRIPTION OF DRAWINGS
[0013] FIG. 1 provides diagrams to explain coordinated
multiple-point transmission;
[0014] FIG. 2 is a diagram to explain a heterogeneous network
structure;
[0015] FIG. 3 provides diagrams to show radio communication systems
according to the present embodiment;
[0016] FIG. 4 is an example sequence diagram to show a radio
communication method according to the present embodiment;
[0017] FIG. 5 is an example sequence diagram to show a radio
communication method according to the present embodiment;
[0018] FIG. 6 is an example sequence diagram to show a radio
communication method according to the present embodiment;
[0019] FIG. 7 is an example sequence diagram to show a radio
communication method according to the present embodiment;
[0020] FIG. 8 is an example sequence diagram to show a radio
communication method according to the present embodiment;
[0021] FIG. 9 is an example sequence diagram to show a radio
communication method according to the present embodiment;
[0022] FIG. 10 is a diagram to explain a system structure of a
radio communication system;
[0023] FIG. 11 is a diagram to explain an overall structure of a
radio base station;
[0024] FIG. 12 is a functional block diagram that corresponds to a
baseband processing section in a radio base station;
[0025] FIG. 13 is a functional block diagram that corresponds to a
baseband processing section in a radio base station;
[0026] FIG. 14 is a diagram to explain an overall structure of a
user terminal; and
[0027] FIG. 15 is a functional block diagram that corresponds to a
baseband processing section in a user terminal.
DESCRIPTION OF EMBODIMENTS
[0028] Now, an embodiment of the present invention will be
described below in detail with reference to the accompanying
drawings.
[0029] First, coordinated multiple point (CoMP) transmission on the
downlink will be described with reference to FIG. 1. Downlink CoMP
transmission includes coordinated scheduling/coordinated
beamforming (CS/CB), and joint processing. CS/CB refers to a method
of transmitting a shared data channel (PDSCH: Physical Downlink
Shared Channel) from only one transmission/reception point (or
radio base station, cell, etc.) to one user terminal UE, and, as
shown in FIG. 1A, allocates radio resources in the frequency/space
domain, taking into account interference from other
transmission/reception points, interference against other
transmission/reception points, and so on.
[0030] Meanwhile, joint processing refers to a method of
transmitting a shared data channel from a plurality of
transmission/reception points simultaneously by employing
precoding, and includes joint transmission (JT) to transmit a
shared data channel from a plurality of transmission/reception
points to one user terminal UE as shown in FIG. 1B, and dynamic
point selection (DPS) to select one transmission/reception point
dynamically and transmit a shared data channel as shown in FIG. 1C.
There is also a transmission mode referred to as "dynamic point
blanking (DPB)," which stops data transmission in a certain region
with respect to a transmission/reception point that causes
interference.
[0031] CoMP transmission is employed to improve the throughput of
user terminals UE located on cell edges. Consequently, CoMP
transmission is controlled to be applied when there is a user
terminal UE located on a cell edge. In this case, a radio base
station apparatus finds differences between the quality information
which the user terminal UE generates on a per cell basis (for
example, the RSRP (Reference Signal Received Power)), the RSRQ
(Reference Signal Received Quality), the SINR (Signal Interference
plus Noise Ratio) and so on, and, when such differences equal or
fall below a threshold--that is, when there is little difference in
quality between the cells--decides that the user terminal UE is
located on a cell edge, and applies CoMP transmission.
[0032] As for the environment to employ CoMP
transmission/reception, there are, for example, a structure to
include a plurality of remote radio equipment (RRE) that are
connected to a radio base station (radio base station eNB) via
optical fiber and/or the like (RRE structure-based centralized
control), and a structure of a radio base station (radio base
station eNB) (independent base station structure-based autonomous
distributed control).
[0033] When CoMP transmission/reception is employed, downlink
signals (downlink control signals, downlink data signals,
synchronization signals, reference signals and so on) are
transmitted from a plurality of transmission points or from a
specific transmission point, to a user terminal UE. Upon receiving
the downlink signals, the user terminal UE performs the receiving
process by using, for example, the reference signals (cell-specific
reference signals (CRSS), user-specific demodulation reference
signals (DM-RSs), channel state measurement reference signals
(CSI-RSs) and so on). The receiving process carried out by the user
terminal UE includes, for example, signal processing such as
channel estimation, a synchronization process, a demodulation
process, a feedback information (CSI) generation process and so
on.
[0034] Now, as a radio network structure to employ LTE-A, as shown
in FIG. 2, a heterogeneous network structure to arrange many small
cells S on the area of a macro cell M is under study. For example,
in a heterogeneous network, on the area of a macro cell M using
conventional frequencies (for example, 2 GHz and 800 MHz), small
cells S to use different frequencies (for example, 3.5 GHz) from
those of the macro cell M are overlaid. In LTE Rel. 12, a study is
in progress to increase the density of such small cells S even more
(SCE: Small Cell Enhancement). For example, a study is in progress
to arrange several hundreds of small cells S for a single macro
cell M.
[0035] As shown in FIG. 2, in a network where small cells S are
densely placed on a macro cell M area, there is a possibility that
CoMP transmission is conducted between the small cells S with
respect to a user terminal UE. In this case, the user terminal UE
can achieve high throughput constantly, by using the plurality of
small cells S seamlessly.
[0036] However, although a plurality of small cells S can be
synchronized if the small cells S are controlled in a centralized
manner by means of optical configuration, the general premise is
that a plurality of small cells S are not synchronized. If small
cells S stay unsynchronized, CoMP transmission between the small
cells S is difficult. Consequently, to realize CoMP transmission
between such small cells S, it is necessary to establish time
synchronization and frequency synchronization between these small
cells S.
[0037] The present inventor has noticed that, in a network
structure in which small cells S are placed densely on a macro cell
M area, it is possible to establish synchronization between the
small cells S, by allowing the small cells S to estimate the
desynchronization (out-of-synchronization) between the small cells
S by transmitting and receiving information related to the
synchronization that is acquired from radio signals, by using one
of the macro cell M, nearby small cells S and user terminals UE,
and, furthermore, correct the synchronization in steps or all at
once based on information about the estimated desynchronization,
and thereupon arrived at the present invention. That is, the
present invention is designed to estimate the desynchronization
between small cells S by transmitting and receiving information
related to synchronization that is acquired from radio signals, by
using one of a macro cell M, nearby small cells S or user terminals
UE, and correcting the synchronization between the small cells S
based on information about the estimated desynchronization.
[0038] Generally, the synchronization process includes a
"synchronization acquisition process," which is the process of
establishing a synchronous state at the beginning of communication,
and a "synchronization tracking process," which is the process of
continuing monitoring after synchronization is established so that
the synchronous state is not lost upon modulation, due to the
condition of noise, and so on. "Synchronization" as used herein
refers to one or both of "synchronization acquisition" and
"synchronization tracking," unless explained otherwise, and the
"synchronization process" as used herein refers to one or both of
the "synchronization acquisition process" and the "synchronization
tracking process."
[0039] In the examples shown below, the macro cell M can acquire
absolute synchronization (hereinafter also referred to as "GPS
synchronization"), by using a GPS clock that is extracted from
electromagnetic waves from a GPS (Global Positioning System)
satellite. Furthermore, the macro cell M can collect the
desynchronization (out-of-synchronaization) information, and report
the amount of correction of synchronization to each small cell
S.
[0040] Part of the small cells S can acquire absolute
synchronization by using a GPS clock. Also, part of the small cells
S can collect the desynchronization information as a
representative, and report the amount of correction of
synchronization to each small cell S. Furthermore, the small cells
S can assume user terminal UE mode and execute part or all of the
functions of the user terminals UE.
[0041] Part or all of the user terminals UE can acquire absolute
synchronization by using a GPS clock. Also, the user terminals UE
may be able to connect with the small cells S, or may be able to
connect with the macro cell M. Alternatively, the user terminals UE
may be able to connect with both the small cells S and the macro
cell M.
First Example
[0042] In a first example, also referred to as "macro-assisted," a
macro cell M and a group of small cells S (S #1 to S #n) are
provided as shown in FIG. 3A, where the small cells S receive radio
signals transmitted from the macro cell M, and establish
synchronization between the small cells S based on these
signals.
[0043] Now, with reference to FIG. 4, a case will be described
below, as a "macro-assisted" example, in which small cells S
receive radio signals transmitted from a macro cell M, and
establish synchronization between the small cells S based on these
signals.
[0044] First, the macro cell M transmits synchronization signals,
which are radio signals, to the small cells S (step S101). Then,
the small cells S correct their synchronization in accordance with
the synchronization signals received (step S102). By executing step
S102, the small cells S synchronize with the macro cell M. By
executing steps S101 and S102 with respect to each small cell S (S
#1 to S #n), synchronization is established between the small cells
S.
[0045] When the small cells S do not know the information that is
required to receive the synchronization signals from the macro cell
M (for example, the cell ID), the small cells S can send a request
for the information to the macro cell Min advance, and acquire a
priori information. In this case, for example, a structure may be
employed in which the small cells S discover the cell ID of the
macro cell M by performing a cell search. By this means, it is
possible to receive the synchronization signals without errors.
[0046] Alternatively, it is also possible to report a priori
information from the macro cell M to the small cells S in advance.
In this case, for example, a structure to send the a priori
information by using a backhaul link may be employed. By this
means, it is possible to receive the synchronization signals
without errors.
[0047] As for the synchronization signals, it is possible to use
conventional signals such as the PSS/SSS (PSS: Primary
Synchronization Signal, and SSS: Secondary Synchronization Signal),
the CRS, the CSI-RS, the DM-RS, the PRS (Positioning Reference
Signals), the SRS (Sounding Reference Signal) and so on, or use
signals that are newly defined. The newly defined signals may
include, for example, a signal in which a conventional signal is
multiplexed in arbitrary subframe intervals, a discovery signal and
so on.
[0048] The discovery signals refers to a signal that is defined on
the downlink in the radio communication scheme for the local areas,
and is a detection signal which the user terminals UE use to detect
the small cells S. Note that the discovery signal may be referred
to as, for example, the "PDCH (Physical Discovery Channel)," the
"BS (Beacon Signal)," the "DPS (Discovery Pilot Signal)" and so
on.
[0049] Note that signals having the following characteristics may
be used as the discovery signal. The discovery signal may be formed
with one of the signals (a) to (e) shown below, or may be formed by
combining the signals (a) to (e) in an arbitrary manner.
[0050] (a) The synchronization signals (PSS and SSS) defined in LTE
(Rel. 8) may be used.
[0051] (b) Signals that use the same sequences as the
synchronization signals defined in LTE (Rel. 8), and multiplex
these sequences in different locations along the time/frequency
direction may be used. For example, signals to multiplex the PSS
and the SSS in different slots may be used.
[0052] (c) Discovery signals that are defined anew for small cell
selection may be used. For example, signals that have
characteristics of having a long transmission cycle and having a
large amount of radio resources per transmission unit compared to
the synchronization signals (PSS and SSS) defined in LTE (Rel. 8)
may be used.
[0053] (d) Conventional reference signals (the CSI-RS, the CRS, the
DM-RS, the PRS and the SRS) that are defined in LTE-A (Rel. 10) may
be used. Also, part of the conventional reference signals (for
example, a signal to transmit the CRS of one port in a 5-msec
cycle) may be used.
[0054] (e) Signals that are multiplexed in the same multiplexing
locations as conventional reference signals defined in LTE-A (Rel.
10) (the CSI-RS, the CRS, the DM-RS, the PRS and the SRS), and that
nevertheless use different signal generating methods for scrambling
sequences and so on may be used.
[0055] When radio signals from the macro cell M or synchronization
information of the macro cell M is used in the correction of
synchronization in the small cells S, there is a possibility that
desynchronization in time is produced between the small cells S due
to the influence of propagation delay. However, in a network
structure in which small cells S are densely placed on a macro cell
M area as shown in FIG. 2, the propagation delay with respect to
the macro cell M gives similar values between neighboring small
cells S, so that it is possible to reduce the influence of
propagation delay. For example, approximately 0.33 [.mu.s] of
propagation delay may be produced over 100 m. When CoMP is
conducted between small cells S, the coordinated cells are assumed
to neighbor each other, so that the propagation delay has little
influence from the perspective of CoMP.
[0056] The radio communication system according to the first
example may be structured to further have a synchronization
management server. The synchronization management server can be
accessed from the macro cell M or from the small cells S, and can
collect desynchronization information and report the amount of
correction of synchronization to each small cell S.
[0057] Now, with reference to FIG. 5, a case will be described
below, as a "macro-assisted" example, where a structure to have a
synchronization management server is employed, and where small
cells S receive radio signals transmitted from a macro cell M and
establish synchronization between the small cells S based on these
signals.
[0058] First, the macro cell M transmits synchronization signals,
which are radio signals, to the small cells S (step S111). The
small cells S estimate their desynchronization from the
synchronization signals that are received (step S112), and report
desynchronization information to the synchronization management
server by using a wired link (step S113). The synchronization
management server determines the amount of correction of
synchronization from the desynchronization information that is
reported (step S114), and reports the amount of correction of
synchronization to the small cells S by using a wired link (step
S115). Then, the small cells S correct their synchronization based
on the amount of correction of synchronization that is received
(step S116). By executing step S116, the small cells S synchronize
with the macro cell M.
[0059] By providing a synchronization management server, it is
possible to collect the desynchronization information in the
synchronization management server and manage the state of
synchronization.
[0060] In this way, with the synchronization method for small cells
according to the first example, the small cells S receive radio
signals transmitted from the macro cell M, and establish
synchronization between the small cells S based on these signals.
By this means, it becomes possible to establish time
synchronization and frequency synchronization between the small
cells S, which is for realizing CoMP transmission between the small
cells S with respect to user terminals UE.
Second Example
[0061] In a second example, also referred to as "UE-assisted
autonomous," a macro cell M, a group of small cells S (S #1 to S
#n) and a user terminal UE are provided as shown in FIG. 3B, and
the user terminal UE assists the synchronization between the small
cells S in an autonomous distributed manner, so that
synchronization is established between the small cells S.
[0062] Now, with reference to FIG. 6, a case will be described
below, as a "UE-assisted autonomous" example, where the user
terminal UE reports desynchronization (out-of-synchronaization) by
using the synchronization signals from the small cells S.
[0063] First, the small cells S transmit synchronization signals to
the user terminal UE (step S121). As for the synchronization
signals, the same synchronization signals as those used in the
first example can be used.
[0064] Note that, before executing step S121, it is also possible
to transmit a signal to designate the target cells to estimate the
desynchronization in step S122, from the macro cell M or the small
cells S to the user terminal UE, by using higher layer signaling
and so on. By this means, it is possible to designate the target
cells to estimate the desynchronization.
[0065] Also, in step S121, it is also possible to transmit
information as to whether or not GPS synchronization is established
between the small cells S, from the macro cell M or the small cells
S to the user terminal UE, by using higher layer signaling and so
on. By this means, it becomes possible to report information about
the small cells S where absolute synchronization is established, to
the user terminal UE.
[0066] Following this, the user terminal UE, having received the
synchronization signals, estimates the deviation
(desynchronization) in time and frequency with respect to nearby
small cells S (step S122). The target cells to estimate the
deviation in time and frequency may be all the small cells S from
which the user terminal UE can receive the synchronization signals,
or may be small cells S (group) that are designated in advance.
Furthermore, cells that rank high in the received quality of the
synchronization signals may be arbitrarily selected on the user
terminal UE side as target cells.
[0067] Following this, the user terminal UE reports
desynchronization information to the connecting small cell S or the
macro cell M (step S123a and S123b). The small cell S having
received the desynchronization information from the user terminal
UE can transfer part or all of this received desynchronization
information to other small cells S that have the cell ID contained
in the desynchronization information. Also, it is equally possible
that the macro cell M having received the desynchronization
information from the user terminal UE determine the amount of
correction of synchronization from this reported desynchronization
information, and transmit the amount of correction of
synchronization to each small cell S.
[0068] Then, the small cells S having received the
desynchronization information or the amount of correction of
synchronization correct their synchronization based on this
information (step S124). By executing above steps S122 and S123a
(S123b) in each user terminal UE, synchronization is established
between the small cells S.
[0069] Note that the radio communication system according to the
second example may be structured to further have a synchronization
management server. In this case, in steps S123a and S123b, the user
terminal UE may report the desynchronization information to the
synchronization management server. Then, the synchronization
management server, having received the report, may be structured to
determine the amount of correction of synchronization from the
desynchronization information reported, and transmit the amount of
correction of synchronization to each small cell S.
[0070] Now, with reference to FIG. 7, a case will be described
below as a "UE-assisted autonomous" example, where user terminals
UE transmit RACH (Random Access Channel) signals to the small cells
S, and the small cells S correct the desynchronization based on
this RACH information.
[0071] First, user terminals UE that are synchronized with the
macro cell M transmit RACH signals to the small cells S based on
the time of the macro cell M (step S131). Alternatively, user
terminals UE that are GPS-synchronized transmit the RACH signals
based on the GPS clock (step S131).
[0072] Note that, in order to prevent the RACHs from colliding, it
is possible to report the preamble indices to use for the RACHs
from the macro cell M or the small cells S to the user terminals
UE. Alternatively, the user terminals UE may select the preamble
indices in advance.
[0073] Also, when the user terminals UE are GPS-synchronized, it is
equally possible to send a report to the small cells S to the
effect that GPS synchronization is established.
[0074] Then, the small cells S correct their desynchronization
based on the RACH information received (step S132). The small cells
S can estimate the average desynchronization by using, for example,
RACH information that is transmitted from a plurality of user
terminals UE. Alternatively, the small cells S can locate user
terminals UE that are present near the small cells S based on the
times the RACHs are received, the received quality of the RACHs,
and so on, and estimate the desynchronization based on the RACH
information from these user terminals UE. By executing above step
S132 in each small cell S, synchronization is established between
the small cells S.
[0075] Now, with reference to FIG. 8, a case will be described
below, as a "UE-assisted autonomous" example, where user terminals
UE that are synchronized with the macro cell M or that are
GPS-synchronized report desynchronization (out-of-synchronaization)
by using the synchronization signals from the small cells S.
[0076] First, the small cells S transmit synchronization signals to
the user terminals UE (step S141).
[0077] Following this, user terminals UE that are synchronized with
the macro cell M estimate the desynchronization between the macro
cell M and the small cells S by using the synchronization signals
received (step S142). Alternatively, user terminals UE that are
GPS-synchronized estimate the desynchronization between the macro
cell M and the small cells S by using the synchronization signals
received (step S142).
[0078] The user terminals UE to estimate the desynchronization may
be all the user terminals UE that can receive the synchronization
signals, or may be user terminals UE (group) that are designated in
advance. Alternatively, it is equally possible to arbitrarily
select user terminals UE that rank high in the received quality of
the synchronization signals on the small cell S side as user
terminals UE to estimate the desynchronization, and send notice to
these user terminals UE. Note that the small cells S can estimate
the received quality in the user terminals UE by using uplink
reference signals from the user terminals UE. Also, it is equally
possible that the user terminals UE report the received quality in
the user terminals UE to the small cells S.
[0079] Following this, the user terminals UE report
desynchronization information to the connecting small cells S (step
S143a). Alternatively, the user terminals UE report the
desynchronization information to the small cells S via the macro
cell M (step S143b).
[0080] Then, the small cells S having received the
desynchronization information correct their synchronization based
on this information (step S144). By executing above steps S141 and
S144 in each small cell S, synchronization is established between
the small cells S via the synchronization between the user
terminals UE and the macro cell M, or via the absolute
synchronization of the user terminals UE with a GPS clock.
[0081] Note that, in above steps S143a and S143b, the user
terminals UE may report the desynchronization information to the
synchronization management server. Then, the synchronization
management server, having received the report, may be structured to
determine the amount of correction of synchronization from the
desynchronization information that is reported, and transmit the
amount of correction of synchronization to each small cell S.
[0082] In this way, with the synchronization method for small cells
according to the second example, user terminals UE assist the
synchronization between the small cells S in an autonomous
distributed manner, so that synchronization is established between
the small cells S. By this means, it becomes possible to establish
time synchronization and frequency synchronization between the
small cells S, which is for realizing CoMP transmission between the
small cells S with respect to user terminals UE.
Third Example
[0083] In a third example, also referred to as "small cell
cooperation," a group of small cells S (S #1 to S #n) are provided
as shown in FIG. 3C, and the synchronization between the small
cells S is established by transmitting and receiving radio signals
between the small cells S.
[0084] Now, with reference to FIG. 9, a case will be described
below as an example of "small cell cooperation," where the
synchronization between the small cells S is established by
transmitting and receiving radio signals between the small cells
S.
[0085] First, small cell S #1 transmits synchronization signals,
which are radio signals, to neighboring small cell S #2 (step
S151). As for the synchronization signals, the same synchronization
signals as those used in the first example can be used.
[0086] Referring to step S151, from which small cells S, at what
times and using which resources the synchronization signals are
transmitted can be determined in each small cell S. Alternatively,
the macro cell M or the synchronization management server may
determine the small cells S to transmit the synchronization
signals, and send notice to these small cell S.
[0087] Also, the synchronization signals may be transmitted on a
regular basis. Alternatively, a small cell S may directly make a
request to other small cells S so that the synchronization signals
are transmitted from these other small cells S. In addition, a
small cell S may make a request to the macro cell M or the
synchronization management server so that the synchronization
signals are transmitted from other small cells S.
[0088] Following this, the small cells S having received the
synchronization signals measure the desynchronization with respect
to nearby small cells S by using the synchronization signals (step
S152).
[0089] Then, based on the desynchronization measured, the small
cells S having received the synchronization signals correct their
synchronization so that the desynchronizations measured between the
small cells become smaller (step S153). The correction of
synchronization may be conducted by estimating the average
desynchronization by using, for example, the synchronization
signals transmitted from a plurality of small cells S.
Alternatively, it is equally possible to locate nearby small cells
S of the transmission point and correct the synchronization based
on the synchronization signals from these small cells S, or correct
the synchronization by using only the synchronization signals
transmitted from the macro cell M, other small cells S or small
cells S (group) that are designated in advance.
[0090] Also, to reduce the likelihood that a plurality of small
cells S correct their synchronization all at the same time, it is
possible to carry out the correction of synchronization at random
times or at times that are designated in advance on a per cell
basis.
[0091] Furthermore, instead of determining the amount of correction
of synchronization in small cells S that have received the
synchronization signals, it is equally possible to employ a
structure in which desynchronization information is reported to a
representative small cell 5, the macro cell M or the
synchronization management server, the amount of correction of
synchronization is determined where the desynchronization
information is reported to, and then reported to each small cell S,
and each small cell S corrects the synchronization based on the
amount of correction of synchronization that is reported.
[0092] By repeating above steps S151 to S153 between small cells S,
synchronization is established between the small cells S.
[0093] With the third example, a small cell S may operate as a user
terminal UE (user terminal UE mode) and realize the UE-assisted
autonomous example that has been described with the second example,
without involving user terminals UE. Also, by operating as a user
terminal UE (user terminal UE mode), a small cell S may perform a
terminal discovery process (discovery process) by using the
above-described discovery signals in terminal-to-terminal
communication (D2D communication), and establish synchronization
with other small cells S that operate as user terminals UE.
[0094] Also, with the third example, the synchronization signals
may be transmitted not only to the small cells S, but may also be
transmitted to the user terminals UE as well. By this means, it is
possible to establish synchronization between the small cells S by
combining the example of small cell cooperation, which has been
described with the third example, and the UE-assisted autonomous
example, which has been described with the second example.
[0095] In this way, with the synchronization method for small cells
according to the third example, the synchronization between the
small cells S is established by transmitting and receiving radio
signals between the small cells S. By this means, it becomes
possible to establish time synchronization and frequency
synchronization between the small cells S, which is for realizing
CoMP transmission between the small cells S with respect to user
terminals UE.
[0096] The desynchronization information that is reported from the
small cells S to the synchronization management server in the first
example (step S113), or the desynchronization information that is
reported from the user terminals UE to the macro cell M and the
small cells S in the second example (step S123a and 123b or steps
S143a and 143b) is one of the information (a) to (h) shown below,
or may be formed by combining the following information (a) to (h)
in an arbitrary manner.
[0097] (a) Identification information (ID) of the user terminals UE
that estimate the desynchronization or the target small cells S may
be used.
[0098] (b) Information as to whether or not the user terminals UE
that estimate the desynchronization or the target small cells S are
in absolute synchronization such as GPS synchronization may be
used.
[0099] (c) Identification information (ID) of the user terminals UE
or the small cells S being the source from which the
synchronization signals used in the estimation of desynchronization
are transmitted may be used.
[0100] (d) Information as to whether the user terminals UE or the
small cells S being the source from which the synchronization
signals used in the estimation of desynchronization are transmitted
are in absolute synchronization such as GPS synchronization may be
used.
[0101] (e) Information about the radio quality of the
synchronization signal such as the RSRQ and the SINR may be
used.
[0102] (f) As desynchronization estimation results, identification
information to represent an absolute clock and a reference clock
may be used. For example, it is possible to use indicators that can
identify whether or not GPS is used, identify the macro cell M,
identify the cell IDs of the small cells S having a reference
clock, or identify arbitrary combinations of these.
[0103] (g) As desynchronization estimation results, information
about an absolute clock such as a GPS clock may be used.
[0104] (h) As desynchronization estimation results, information
about the desynchronization with respect to a reference clock may
be used. Note that the reference clock here refers to, for example,
the clock of one of a GPS, the macro cell M, a specific small cell
S, and the recipient of synchronization information. Furthermore,
the desynchronization here includes one or both of
desynchronization in time and desynchronization in frequency.
[0105] Also, in order to reduce the amount of signaling when
reporting the above desynchronization information and achieve
improved reliability, the small cells S or the user terminals UE
can execute one of the controls (a) to (g) shown below, or execute
control combining (a) to (g) in an arbitrary manner.
[0106] (a) Control to reduce the number of reporting bits when
reporting desynchronization information may be executed. For
example, if rough synchronization is the premise (macro
synchronization or rough synchronization between small cells), it
is possible to reduce the number of bits to use for the reporting.
Note that rough synchronization refers to synchronization at
several hundreds of Hz in frequency, and refers to synchronization
on the subframe or the frame level in time.
[0107] (b) Control to reduce the frequency of reporting
desynchronization information may be executed. For example, a small
cell S, once synchronized, is unlikely to go completely out of
synchronization in short time, so that it is possible to reduce the
frequency of reporting by using the magnitude of the
desynchronization that is detected.
[0108] (c) Control to designate the user terminals UE or the small
cells S to report desynchronization information in advance may be
executed.
[0109] (d) Control to designate the user terminals UE or the small
cells S to report desynchronization information from the macro cell
M or a small cell S may be executed.
[0110] (e) Control to designate the user terminals UE or the small
cells S to report desynchronization information from a small cell S
or a user terminal UE that is near the user terminals UE or the
small cells S to which the desynchronization information is
reported may be executed. Note that whether or not a user terminal
UE or a small cell S is near the user terminals UE or the small
cells S to which the desynchronization information is reported can
be judged based on radio quality or the time of reception.
[0111] (f) Control to designate the user terminals UE or the small
cells S to report desynchronization information from a user
terminal UE or a small cell S of high radio quality may be
executed.
[0112] (g) Control to designate the user terminals UE or the small
cells S to report desynchronization information from a user
terminal UE or a small cell S where the desynchronization is equal
to or less than a predetermined range may be executed.
[0113] According to the first example to the third example, the
synchronization management server, to which desynchronization
information is reported, determines the amount of correction of
synchronization based on that desynchronization information. When
this takes place, in order to improve the reliability of, and for
the simplification of, synchronization, the synchronization
management server can execute one of the controls (a) to (c) shown
below, or execute control to combine (a) to (c) in an arbitrary
manner.
[0114] (a) When a plurality of reports are received in a certain
period of time with respect to the same small cell S, it is
possible to determine the amount of correction of synchronization
by selecting one of a report to indicate high radio quality, a
report to indicate little desynchronization, the average value of
the reported values, and a report to indicate that absolute
synchronization (such as GPS synchronization) is established, or by
selecting an arbitrary combination of these.
[0115] (b) By finding the average desynchronization over a certain
period time, it is possible to determine the amount of correction
of synchronization.
[0116] (c) By collecting desynchronization information in a
representative small cell S, the macro cell M, or the
synchronization management server, it is possible to determine the
amount of correction of synchronization for each small cell S.
[0117] According to the first example to the third example, each
small cell S can estimate the reliability of the synchronization
that is established between the small cells S as a result of
synchronization. By this means, even when synchronization is not
established in all the small cells S with required reliability, it
is still possible to carry out CoMP only between the small cells S
where the reliability of synchronization meets is equal to or
higher than a certain level.
[0118] The reliability of synchronization between small cells S may
be estimated by using one of the methods (a) to (f) shown below, or
by combining the methods (a) to (f) in an arbitrary manner.
[0119] (a) The method of estimating the reliability of
synchronization from the reliability of synchronization that can be
achieved at a minimum by the synchronization method used, may be
used.
[0120] (b) The method of estimating the reliability of
synchronization from the number of user terminals UE or small cells
S that report synchronization information in a certain period of
time may be used when synchronization is established by using radio
signals.
[0121] (c) The method of estimating the reliability of
synchronization from the frequency of correcting synchronization
(for example, the time that has passed since last synchronization)
may be used when synchronization is established by using radio
signals.
[0122] (d) The method of estimating the reliability of
synchronization from the variation of synchronization correction
values over time (for example, the distribution over time) may be
used when synchronization is established by using radio
signals.
[0123] (e) The method of estimating the reliability of
synchronization from the magnitude of the synchronization
correction value may be used when synchronization is established by
using radio signals.
[0124] (f) The method of estimating the reliability of
synchronization from the radio quality of the synchronization
signals may be used when synchronization is established by using
radio signals.
[0125] (Radio Communication System)
[0126] Now, the radio communication system according to the present
embodiment will be described in detail below. FIG. 10 is a diagram
to show a schematic configuration of the radio communication system
according to the present embodiment. Note that the radio
communication system shown in FIG. 10 is a system to accommodate,
for example, an LTE system or SUPER 3G. This radio communication
system adopts carrier aggregation to group a plurality of
fundamental frequency blocks (component carriers) into one, where
the system band of the LTE system constitutes one unit. Also, this
radio communication system may be referred to as "IMT-advanced," or
may be referred to as "4G" or "FRA (Future Radio Access)."
[0127] The radio communication system 1 illustrated in FIG. 10
includes a radio base station 21 that forms a macro cell C1 as a
first cell, and radio base stations 22a and 22b that form small
cells C2 as second cells, which are placed inside the macro cell C1
and which are narrower than the macro cell C1. Also, in the macro
cell C1 and in each small cell C2, user terminals 10 are placed.
The user terminals 10 are structured to be able to perform radio
communication with both the radio base station 21 and the radio
base stations 22.
[0128] Communication between the user terminals 10 and the radio
base station 21 is carried out using a carrier of a relatively low
frequency band (for example, 2 GHz) and a wide bandwidth (referred
to as a "legacy carrier" and so on). Meanwhile, between the user
terminals 10 and the radio base stations 22, a carrier of a
relatively high frequency band (for example, 3.5 GHz) and a narrow
bandwidth may be used, or the same carrier as that used in the
radio base station 21 may be used. The radio base station 21 and
each radio base station 22 are connected by wire connection or by
wireless connection.
[0129] The radio base station 21 and the radio base stations 22 are
each connected with a higher station apparatus 30, and are
connected with a core network 40 via the higher station apparatus
30. Note that the higher station apparatus 30 may be, for example,
an access gateway apparatus, a radio network controller (RNC), a
mobility management entity (MME) and so on, but is by no means
limited to these. Also, each radio base station 22 may be connected
with the higher station apparatus via the radio base station
21.
[0130] Note that the radio base station 21 is a radio base station
having a relatively wide coverage, and may be referred to as an
"eNodeB," a "radio base station," a "transmission point" and so on.
Also, the radio base stations 22 are radio base stations having
local coverages, and may be referred to as "pico base stations,"
"femto base stations," "Home eNodeBs," "RRHs (Remote Radio Heads),"
"micro base stations," "transmission points" and so on. The radio
base stations 21 and 22 will be hereinafter collectively referred
to as "radio base station 20," unless distinction needs to be drawn
otherwise. The user terminals 10 are terminals to support various
communication schemes such as LTE and LTE-A (for example, UEs of
Rel. 11 and earlier versions and UEs of Rel. 12 and later
versions), and may include mobile communication terminals as well
as fixed communication terminals.
[0131] In the radio communication system, as radio access schemes,
OFDMA (Orthogonal Frequency Division Multiple Access) is applied to
the downlink, and SC-FDMA (Single-Carrier Frequency Division
Multiple Access) is applied to the uplink. OFDMA is a multi-carrier
transmission scheme to perform communication by dividing a
frequency band into a plurality of narrow frequency bands
(subcarriers) and mapping data to each subcarrier. SC-FDMA is a
single-carrier transmission scheme to reduce interference between
terminals by dividing the system band into bands formed with one or
continuous resource blocks, per terminal, and allowing a plurality
of terminals to use mutually different bands.
[0132] Now, communication channels to be used in the radio
communication system shown in FIG. 10 will be described. Downlink
communication channels include a PDSCH (Physical Downlink Shared
Channel), which is used by each user terminal 10 on a shared basis,
and downlink L1/L2 control channels (a PDCCH, a PCFICH, a PHICH and
an EPDCCH). User data and higher control information are
transmitted by the PDSCH. Scheduling information for the PDSCH and
the PUSCH and so on are transmitted by the PDCCH (Physical Downlink
Control CHannel). The number of OFDM symbols to use for the PDCCH
is transmitted by the PCFICH (Physical Control Format Indicator
Channel). HARQ ACKs and NACKs for the PUSCH are transmitted by the
PHICH (Physical Hybrid-ARQ Indicator CHannel). Also, scheduling
information for the PDSCH and the PUSCH and so on may be
transmitted by the EPDCCH (Enhanced PDCCH) as well. This EPDCCH can
be arranged to be frequency-division-multiplexed with the
PDSCH.
[0133] Uplink communication channels include a PUSCH (Physical
Uplink Shared CHannel), which is used by each user terminal 10 on a
shared basis as an uplink data channel, and a PUCCH (Physical
Uplink Control CHannel), which is an uplink control channel. User
data and higher control information are transmitted by this PUSCH.
Also, downlink radio quality information (CQI: Channel Quality
Indicator), ACKs/NACKs and so on are transmitted by the PUCCH.
[0134] Next, with reference to FIG. 11, an overall structure of a
radio base station 20 (which may be all of 21, 22a and 22b)
according to the present embodiment will be described.
[0135] The radio base station 20 has transmitting/receiving
antennas 201, amplifying sections 202, transmitting/receiving
sections (transmitting section/receiving section) 203, a baseband
signal processing section 204 (214), a call processing section 205
and a transmission path interface 206. Transmission data to be
transmitted from the radio base station 20 to user terminals 10 on
the downlink is input from the higher station apparatus 30 to the
baseband signal processing section 204 (214), via the transmission
path interface 206.
[0136] In the baseband signal processing section 204 (214), a
downlink data channel signal is subjected to a PDCP layer process,
division and coupling of transmission data, an RLC (Radio Link
Control) layer transmission process such as an RLC retransmission
control transmission process, MAC (Medium Access Control)
retransmission control, including, for example, an HARQ
transmission process, scheduling, transport format selection,
channel coding, an inverse fast Fourier transform (IFFT) process
and a precoding process. Furthermore, the signal of a physical
downlink control channel, which is a downlink control channel, is
also subjected to transmission processes such as channel coding and
an inverse fast Fourier transform.
[0137] Also, the baseband signal processing section 204 (214)
reports control information for allowing each user terminal 10 to
perform radio communication with the radio base station 20, to the
user terminals 10 connected to the same cell, by a broadcast
channel. The information for allowing communication in the cell
includes, for example, the uplink or downlink system bandwidth,
root sequence identification information (root sequence indices)
for generating random access preamble signals in the PRACH and so
on.
[0138] The transmitting/receiving sections 203 convert baseband
signals output from the baseband signal processing section 204
(214) into a radio frequency band. The amplifying sections 202
amplify the radio frequency signals having been subjected to
frequency conversion, and transmit the results through the
transmitting/receiving antennas 201. Note that the
transmitting/receiving sections 203 function as a receiving section
that receives synchronization correction information, which is for
establishing synchronization between the targets of
synchronization, from each user terminal 10, and function as a
transmitting section that transmits the synchronization signals to
each user terminal 10.
[0139] On the other hand, as for signals to be transmitted from the
user terminals 10 to the radio base station 20 on the uplink, radio
frequency signals that are received in the transmitting/receiving
antennas 201 are each amplified in the amplifying sections 202,
converted into baseband signals through frequency conversion in the
transmitting/receiving sections 203, and input in the baseband
signal processing section 204 (214).
[0140] In the baseband signal processing section 204 (214), the
transmission data that is included in the baseband signals received
on the uplink is subjected to an FFT (Fast Fourier Transform)
process, an IDFT (Inverse Discrete Fourier Transform) process,
error correction decoding, a MAC retransmission control receiving
process, and RLC layer and PDCP layer receiving processes. The
decoded signals are transferred to the higher station apparatus 30
via the transmission path interface 206.
[0141] The call processing section 205 performs call processing
such as setting up and releasing communication channels, manages
the state of the radio base station 20 and manages the radio
resources.
[0142] FIG. 12 is a block diagram to show the structure of the
baseband signal processing section provided in the radio base
station 21 shown in FIG. 11. The baseband signal processing section
204 is primarily formed with a layer 1 processing section 2041, a
MAC processing section 2042, an RLC processing section 2043 and a
synchronization signal generating section 2044.
[0143] The layer 1 processing section 2041 primarily performs
processes pertaining to the physical layer. For example, the layer
1 processing section 2041 applies processes to signals that are
received on the uplink, including channel decoding, a discrete
Fourier transform (DFT), frequency demapping, an inverse fast
Fourier transform (IFFT), data demodulation and so on. Also, the
layer 1 processing section 2041 applies processes to signals to
transmit on the downlink, including channel coding, data
modulation, frequency mapping and an inverse fast Fourier transform
(IFFT).
[0144] The MAC processing section 2042 performs processes for the
signals that are received on the uplink, including MAC layer
retransmission control, scheduling of the uplink/downlink,
transport format selection for the PUSCH/PDSCH, resource block
selection for the PUSCH/PDSCH, and so on. The RLC processing
section 2043 performs, for packets that are received on the
uplink/packets to transmit on the downlink, division of the
packets, coupling of the packets, RLC layer retransmission control
and so on.
[0145] The synchronization signal generating section 2044 generates
the synchronization signals shown earlier with the first example.
That is, the synchronization signal generating section 2044
generates synchronization signals that serve as a basis when the
small cells synchronize.
[0146] FIG. 13 is a block diagram to show the structure of the
baseband signal processing section in the radio base stations 22a
and 22b shown in FIG. 11. The baseband signal processing section
214 is primarily formed with a layer 1 processing section 2141, a
MAC processing section 2142, an RLC processing section 2143, a
synchronization signal generating section 2144, a synchronization
correction section 2145 and a synchronization estimation section
2146.
[0147] The layer 1 processing section 2141, the MAC processing
section 2142 and the RLC processing section 2143 perform the same
processes as those by the layer 1 processing section 2041, the MAC
processing section 2042 and the RLC processing section 2043 shown
in FIG. 12.
[0148] The synchronization signal generating section 2144 generates
the synchronization signals shown earlier with the second example
and the third example. That is, the synchronization signal
generating section 2144 generates synchronization signals that are
used to estimate information about desynchronization as information
for the correction of synchronization in the user terminals. The
synchronization correction section 2145 corrects the
synchronization based on the synchronization correction information
that is received. The synchronization estimation section 2146
estimates information about the desynchronization with respect to
the synchronization target (for example, the macro cell, GPS clock
and so on) in accordance with the synchronization correction
information.
[0149] Next, an overall structure of a user terminal according to
the present embodiment will be described with reference to FIG. 11.
An LTE terminal and an LTE-A terminal have the same hardware
structures in principle parts, and therefore will be described
without drawing distinction between them. A user terminal 10 has
transmitting/receiving antennas 101, amplifying sections 102,
transmitting/receiving sections (transmitting section/receiving
section) 103, a baseband signal processing section 104 and an
application section 105.
[0150] As for downlink data, radio frequency signals that are
received in the transmitting/receiving antennas 101 are amplified
in the amplifying sections 102, and converted into baseband signals
through frequency conversion in the transmitting/receiving sections
103. These baseband signals are subjected to an FFT process, error
correction decoding, a retransmission control receiving process and
so on, in the baseband signal processing section 104. In this
downlink data, downlink transmission data is transferred to the
application section 105. The application section 105 performs
processes related to higher layers above the physical layer and the
MAC layer, and so on. Also, in the downlink data, broadcast
information is also transferred to the application section 105.
[0151] Meanwhile, uplink transmission data is input from the
application section 105 into the baseband signal processing section
104. The baseband signal processing section 104 performs a mapping
process, a retransmission control (HARQ) transmission process,
channel coding, a DFT process and an IFFT process. The baseband
signals that are output from the baseband signal processing section
104 are converted into a radio frequency band in the
transmitting/receiving sections 103. After that, the amplifying
sections 102 amplify the radio frequency signals having been
subjected to frequency conversion, and transmit the results from
the transmitting/receiving antennas 101.
[0152] Note that the transmitting/receiving sections 103 function
as a receiving section to receive the synchronization signals.
[0153] FIG. 15 is a block diagram to show a structure of the
baseband signal processing section in the user terminal shown in
FIG. 14. The baseband signal processing section 104 is primarily
formed with a layer 1 processing section 1041, a MAC processing
section 1042, an RLC processing section 1043, a synchronization
estimation section 1044 and a synchronization correction
information generating section 1045.
[0154] The layer 1 processing section 1081 mainly performs
processes related to the physical layer. The layer 1 processing
section 1081, for example, applies processes such as channel
decoding, a discrete Fourier transform (DFT), frequency demapping,
an inverse Fourier transform (IFFT) and data demodulation, to a
signal received on the downlink. Also, the layer 1 processing
section 1081 performs processes for a signal to transmit on the
uplink, including channel coding, data modulation, frequency
mapping and an inverse fast Fourier transform (IFFT).
[0155] The MAC processing section 1042 performs, for the signal
received on the downlink, MAC layer retransmission control (hybrid
ARQ), an analysis of downlink scheduling information (specifying
the PDSCH transport format and specifying the PDSCH resource
blocks) and so on. Also, the MAC processing section 1082 performs,
for the signal to transmit on the uplink, MAC retransmission
control, an analysis of uplink scheduling information (specifying
the PUSCH transport format and specifying the PUSCH resource
blocks) and so on.
[0156] The RLC processing section 1043 performs, for the packets
received on the downlink/the packets to transmit on the uplink,
division of the packets, coupling of the packets, RLC layer
retransmission control and so on.
[0157] The synchronization estimation section 1044 estimates the
desynchronization between the small cells and the synchronization
target based on the synchronization signals that are received. The
synchronization correction information generating section 1045
generates synchronization correction information. The
synchronization correction information is formed by including, for
example, the desynchronization information estimated in the
synchronization estimation section 1044, the RACH signals shown
earlier with the second example, and so on.
[0158] Note that the radio base stations 21, 22a and 22b may have
the functions of a synchronization management server. That is, the
radio base stations 21, 22a and 22b may have synchronization
information management functions for collecting desynchronization
information and reporting the amount of correction of
synchronization.
[0159] Note that the present invention is by no means limited to
the above embodiment and can be carried out with various changes.
With the above embodiment, the size, shape and so on shown in the
accompanying drawings are by no means limiting, and can be changed
as appropriate within the range in which the effect of the present
invention is kept optimal. Besides, the present invention can
employ various changes and be implemented without departing the
scope of the object of the present invention.
[0160] The disclosure of Japanese Patent Application No.
2013-011456, filed on Jan. 24 2013, including the specification,
drawings and abstract, is incorporated herein by reference in its
entirety.
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