U.S. patent application number 13/194654 was filed with the patent office on 2011-12-01 for method for transmitting reference signals.
This patent application is currently assigned to Huawei Technologies Co., Ltd. Invention is credited to Jianghua Liu, Branislav Popovic.
Application Number | 20110293037 13/194654 |
Document ID | / |
Family ID | 42395095 |
Filed Date | 2011-12-01 |
United States Patent
Application |
20110293037 |
Kind Code |
A1 |
Liu; Jianghua ; et
al. |
December 1, 2011 |
METHOD FOR TRANSMITTING REFERENCE SIGNALS
Abstract
A method and apparatus for transmitting reference signals in a
wireless communication system is disclosed. The method and
apparatus includes reference signal, RS, transmission in resource
blocks supporting multiple antenna port transmission, at least one
broadcast channel being provided in resource blocks belonging to a
first set of resource blocks, a first number of reference signals
being transmitted in at least one resource block supporting
multiple antenna port transmission and a second number of reference
signals being transmitted in at least one resource block belonging
to a second set of resource blocks. Preferably, different resource
elements, REs, are used for each reference signal in resource
blocks. An example embodiment of reference signal support of, e.g.,
eight antenna ports is provided.
Inventors: |
Liu; Jianghua; (Beijing,
CN) ; Popovic; Branislav; (Kista, SE) |
Assignee: |
Huawei Technologies Co.,
Ltd
Shenzhen
CN
|
Family ID: |
42395095 |
Appl. No.: |
13/194654 |
Filed: |
July 29, 2011 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
PCT/CN2009/070329 |
Feb 1, 2009 |
|
|
|
13194654 |
|
|
|
|
Current U.S.
Class: |
375/295 |
Current CPC
Class: |
H04L 5/001 20130101;
H04W 48/08 20130101; H04L 27/261 20130101; H04L 5/005 20130101;
H04L 5/0023 20130101; H04L 5/0053 20130101; H04L 27/2613 20130101;
H04L 5/0051 20130101; H04L 1/0026 20130101; H04W 72/04
20130101 |
Class at
Publication: |
375/295 |
International
Class: |
H04L 27/00 20060101
H04L027/00 |
Claims
1. Method of reference signal transmission in resource blocks, RBs,
supporting multiple antenna port transmission in a wireless
communication system comprising: determining first and second
mutually exclusive sets of RBs, at least one broadcast channel,
BCH, being provided in RBs belonging to said first set of RBs;
transmitting a first number of reference signals, RSes, in at least
one resource block, RB, supporting multiple antenna port
transmission; and transmitting a second number of RSes in at least
one RB belonging to said second set of RBs.
2. The method according to claim 1, wherein at least one
synchronisation signal, SS, is provided in RBs belonging to said
first set of RBs, and said at least one SS is a primary
synchronisation signal, P-SS, or a secondary synchronisation
signal, S-SS.
3. The method according to claim 1, wherein said wireless
communication system is a cellular wireless communication system
further comprising, a first set of receive nodes, RNs, and a second
set of receive nodes, RNs, and said first number of RSes are common
to all RNs in a cell, and said second number of RSs are common to
all RNs belonging to said second set of RNs, and wherein said first
and second number of RSs are specific for said cell, and used for
measurement, or used for measurement and demodulation.
4. The method according to claim 1, wherein said RBs supporting
multiple antenna port transmission are transmitted in SFs
supporting multiple antenna port transmission, and wherein said SFs
supporting multiple antenna port transmission are divided into a
first and a second exclusive sets of SFs and said first set of RBs
belongs to said first set of SFs, and wherein said first number of
RSs are transmitted in at least one SF supporting multiple antenna
port transmission, and said second number of RSs are transmitted in
at least one SF belonging to said second set of SFs.
5. The method according to claim 4, wherein said second number of
RSes are transmitted in every Nth SF, where N is an integer and
N>1.
6. The method according to claim 4, wherein said second number of
RSes are transmitted in the second slot of said at least one SF
belonging to said second set of SFs.
7. The method according to claim 6, wherein said second number of
RSes are transmitted in every one, or every Mth RB in said second
slot, where M is an integer and M>1.
8. The method according to claim 1, wherein an RB belonging to said
second set of RBs comprises one of a physical downlink shared
channel, PDSCH, region; and a physical downlink shared channel,
PDSCH region and a control region for at least a physical downlink
control channel, PDCCH, a physical control format indicator
channel, PCFICH, or a physical hybrid ARQ indicator channel, PHICH;
wherein said second number of reference signals RSes are
transmitted in the PDSCH region in at least one RBs belonging to
said second set of RBs.
9. The method according to claim 1, wherein said at least one BCH
is a physical broadcast channel, PBCH.
10. The method according to claim 1, wherein said respective first
and second numbers of RSes equals 4.
11. The method according to claim 1, wherein said wireless
communication system is an LTE-A system and said RSes are
orthogonal to each other and provided in downlink, DL,
transmission.
12. The method in a receive node for receiving reference signals in
a wireless communication system, in which a reference signal, RS,
defines an antenna port, AP, for a transmit node, TN, and said TN
is arranged to transmit a multiple of RSes in resource blocks, RBs,
supporting multiple antenna port transmission, wherein said RBs
supporting multiple antenna port transmission are divided into a
first and a second exclusive sets of RBs, and at least one
broadcast channel, BCH, is provided in RBs belonging to said first
set of RBs, said method comprising: receiving a first number of
RSes in at least one RB supporting multiple antenna port
transmission; and receiving a second number of RSes in at least one
RB belonging to said second set of resource blocks RBs.
13. An apparatus for transmitting reference signals in a wireless
communication system, in which a reference signal, RS, defines an
antenna port, AP, for a transmit node, TN, and said TN is arranged
to transmit a multiple of RSes in resource blocks, RBs, supporting
multiple antenna port transmission, for which RBs supporting
multiple antenna port transmission first and second exclusive sets
of RBs, are determined and at least one broadcast channel, BCH, is
provided in RBs belonging to said first set of RBs, said transmit
node comprising transmit circuitry, and processing circuitry
arranged for transmission of a first number of RSes in at least one
RB supporting multiple antenna port transmission; and transmission
of a second number of RSes in at least one RB belonging to said
second set of RBs.
14. Equipment for receiving one or more reference signals in a
wireless communication system, in which a reference signal, RS,
defines an antenna port, AP, for a transmit node, TN, and said TN
is arranged to transmit a multiple of RSes in resource blocks, RBs,
supporting multiple antenna port transmission, for which resource
blocks, RBs, supporting multiple antenna port transmission first
and second exclusive sets of RBs, are determined and at least one
broadcast channel, BCH, is provided in RBs belonging to said first
set of RBs, said receive node, RN, being comprising receive
circuitry, and processing circuitry arranged for reception of a
first number of RSes in at least one RB supporting multiple antenna
port transmission; and reception of a second number of RSes in at
least one RB belonging to said second set of RBs.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of International
Application No. PCT/CN2009/070329, filed on Feb. 1, 2009, which is
hereby incorporated by reference in its entirety.
TECHNICAL FIELD
[0002] The present invention relates to a method for transmitting
reference signals in a wireless communication system. Furthermore,
a method in a transmit node, a method in a receive node, a transmit
node apparatus and a receive node apparatus relating to the method
above are disclosed.
BACKGROUND
[0003] In wireless communication system, one or a multiple of
common Downlink (DL) Reference Signals (RSs) may be used for
coherent demodulation and channel measurement for each mobile
terminal (also called a User Equipment (UE) in some systems) in a
given cell. In case of multi-antenna transmission, an antenna is
identified by a RS transmitted on that antenna. Each RS defines a
so-called antenna port at a transmitter in a given cell. If
multiple antennas use the same RS, they will belong to the same
antenna port. RSs of different antenna ports should be orthogonal
to each other in order to allow interference-free identification of
each corresponding propagation channel coefficients at a receiver.
The RSs are usually cell-specific to minimize interference between
RSs belonging to different cells in a wireless communication
system. The RSs are transmitted on exclusively reserved resources
of a cell, such as on time and frequency Resource Elements (REs),
codes, etc. To avoid interference, data is not transmitted on
reserved resources allocated for RSs.
[0004] RSs are used for measurement of the radio channel and
demodulation. For instance, a UE can determine Channel Quality
Indicator (CQI), Precoding Matrix Indicator (PMI) and Rank
Indicator (RI) by measuring received RSs, and feedback measurement
results including CQI/PMI/RI to a base station (such as a Node B or
a eNB) for scheduling; or the UE can estimate the channel using the
RSs, and use the estimated channel to demodulate data. The RSs used
for measurement are usually common to all UEs in a cell, and
cell-specific; the RSs used for demodulation can be common to all
UEs or dedicated for a specific UE, and hence RSs used for
demodulation can be cell-specific or UE-specific. RSs that are
common to all UEs in a cell will be denoted Common RSs (CRSs).
[0005] In the Long Term Evolution (LTE) standard, measurement RSs
and demodulation RSs share the same RSs, i.e. the same RSs are used
for both measurement and demodulation, and are common to all UEs in
cell-specific manner. In wireless communication systems with
scheduling functionality, a base station first needs to know radio
channel information for each UE, and then schedule the UEs based on
the radio channel information. In order to obtain the radio channel
information for each UE, the base station must transmit CRSs for
all the UEs to measure the channel. Therefore CRSs are necessary in
cellular wireless communication systems of this kind.
[0006] In the LTE standard, three types of cell-specific RSs are
supported; defining one, two and four antenna ports (3GPP TS 36.211
v8.5.0). FIG. 1 illustrates how REs are used for transmission of
RSs on each antenna port. It can be observed that resources for RSs
of different antenna ports are orthogonal to each other through
using different RE for each RS in Resource Blocks (RBs). Here a RB
is defined as Nsymb consecutive OFDM symbols in the time domain and
Nsc consecutive subcarriers in the frequency domain, but generally
relates to radio resources in the frequency/time domain.
[0007] In order to be able to use CRSs properly in the DL, the key
information which a UE needs is how many antenna ports that are
used for DL transmission and the position of the RS on each antenna
port. In the LTE standard, information about the number of antenna
ports is embedded in a signal transmitted on a Physical Broadcast
Channel (PBCH), and the position of the RS on each antenna port is
associated with cell Identity (ID), which is conveyed in the
Primary/Secondary Synchronization Signal (P/S-SS).
[0008] After successful cell search procedure, the UE will obtain
time and frequency synchronization with a specific cell, as well as
the cell ID for that cell. Based on the cell ID, the UE will know
the RS on each antenna port in that cell. However, the UE will
still not have information about the exact number of used antenna
ports. Since this information is embedded in the PBCH signal, the
UE has to make blind detection of that information, which means
that it has to check all possible variants of the information and
select the variant that is most probable conditioned on the
received PBCH signal. The transmission structure for PBCH and
P/S-SS according to the LTE standard is illustrated in FIG. 2 (note
that the RSs are not shown in FIG. 2).
[0009] The first T OFDM symbols in each Sub-Frame (SF) are used for
transmission of control information, such as Physical Downlink
Control Channel (PDCCH), Physical Control Format Indicator Channel
(PCFICH) and Physical Hybrid ARQ Indicator Channel (PHICH), where
T=1, 2, 3, or 4. The area containing control information in a slot
is called a control region, and the remaining resources in each
slot belong to a non-control region. In the non-control region, all
REs, except for the ones used for PBCH, P/S-SS and RSs, belong to
the Physical Downlink Shared Channel (PDSCH) region.
[0010] In FIG. 2, P-SS and S-SS are transmitted on the last two
OFDM symbols of slot0 (S0) and slot10 (S10), respectively, and are
located at the central frequency part (centre six RBs) of the
system bandwidth, while PBCH is transmitted in the central six RBs
of slot1 (S1). In the RBs for transmission of PBCH information,
data is mapped to time-frequency resources on the first four OFDM
symbols as if maximum number of antenna ports is used (i.e. four in
the LTE standard), i.e. as if all possible RSs in a cell are
transmitted. In other words, the REs for all RSs in the first four
OFDM symbols are reserved in a cell even if not all RSs are
actually transmitted. An example is given in FIG. 3 for the
illustration of PBCH resources corresponding to different number of
antenna ports. For convenience, in the example, only the PBCH
resources in one of the RBs used by the PBCH are shown.
[0011] In this way, the used resources for PBCH transmission are
constant and independent of the number of used antenna ports,
allowing for a single PBCH modulation and coding scheme, and
consequently stable PBCH channel quality is independent of the
number of used antenna ports. This is a key feature of PBCH channel
that allows for blind detection of the embedded information
relating to actually used antenna ports. That is, the information
about the antenna port configuration is embedded into the PBCH by
employing different Cyclic Redundancy Check (CRC) masks to indicate
the number of antenna ports used.
[0012] In embedding the number of antenna ports used by a
transmitter into PBCH, firstly, the entire PBCH transport block
a.sub.0, a.sub.1, . . . a.sub.A-1 is used to calculate the CRC
parity bits p.sub.0, p.sub.1, . . . p.sub.L-1, where A is the size
of the transport block, i.e. the number of information bits, and L
is the number of CRC parity bits which is set to 16 in the LTE
standard. Secondly, according to the antenna port configuration of
a specific cell, the CRC parity bits are scrambled by a sequence
x.sub.0.sup.n, x.sub.1.sup.n, . . . x.sub.15.sup.n with length 16
corresponding to a certain number of antenna ports n, where n=1, 2
or 4. After scrambling, the masked CRC parity bits are c.sub.0,
c.sub.1, . . . c.sub.15, where c.sub.i=(p.sub.i+x.sub.i.sup.n) mod
2, i=0, 1, . . . , 15. Then, the masked CRC parity bits are
attached to the transport block of PBCH to obtain the information
bits as a.sub.0, a.sub.1, . . . a.sub.A-1, c.sub.0, c.sub.1, . . .
, c.sub.15. The mapping relation between the three scrambling
sequences and the number of antenna ports is shown in Table 1.
TABLE-US-00001 TABLE 1 CRC mask for PBCH in LTE Number of antenna
ports CRC mask sequence 1 <0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0> 2
<1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1> 4
<0,1,0,1,0,1,0,1,0,1,0,1,0,1,0,1>
[0013] Finally, a set of operations including channel coding, rate
matching, modulation and resources mapping are performed on the
information bits. If there is only one antenna port used, the
modulation symbols are directly mapped to the reserved resources on
antenna port 0; in the case of two antenna ports used, Space
Frequency Block Coding (SFBC) is performed on the modulation
symbols, and then the output of SFBC is mapped to the reserved
resources on antenna port 0 and 1, respectively; and in the case of
four antenna ports used, SFBC and Frequency Switching Transmit
Diversity (FSTD) is performed on the modulation symbols, and then
the output of SFBC+FSTD is mapped to the reserved resources on
antenna port 0, 1, 2 and 3, respectively.
[0014] At the receiver side, corresponding inverse operations
including resource de-mapping, decoding (SFBC or SFBC+FSTD),
demodulation, channel decoding, CRC mask removal and CRC detection
are performed by e.g. a UE accessing a c ell. During the detection
of PBCH, there are three hypothesises (one, two or four antenna
ports) to be blindly detected by the receiver. Given one
hypothesis, if the final CRC detection is correct, then the PBCH
information bits and the information about the number of antenna
ports will be obtained.
[0015] The LTE-Advanced (LTE-A) system is a wireless communication
system intended to be an extension of the LTE system in which eight
antenna ports defined by RSs may be supported to further increase
system performance such as: peak data rate, cell average spectrum
efficiency, etc (3GPP TR 36.913 0.1.1). However, in order to fulfil
LTE-A backward compatibility requirements, it should be possible
for a system to serve both LTE UEs and LTE-A UEs in a LTE-A cell,
comprising up to eight RSs defining the same number of DL antenna
ports, where LTE UEs are UEs configured according to the LTE system
functionality and LTE-A UEs are UEs configured according to the
LTE-A system functionality.
SUMMARY
[0016] An object of an embodiment of the invention is to support
transmission of reference signals in resource blocks carrying more
than one reference signal.
[0017] Also, an object of an embodiment of the invention is to
support transmission of reference signals in resource blocks
carrying one or more control channels.
[0018] In an aspect of the invention, broadcast channel
requirements remain unaffected of transmission of added reference
signals.
[0019] According to a preferred method of transmitting reference
signals in a wireless communication system, e.g. UEs of LTE systems
can be served in a developed LTE-A wireless communication system
operating in accordance with the invention.
[0020] In an example system according to the invention, channel
measurement performance for UEs supporting an additional numbers of
antenna ports, such as LTE-A UEs, will not be impacted.
[0021] The invention provides a method and system of transmission
of reference signals in disjoint sets of resource blocks as
described in detail below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] The appended drawings are intended to clarify and explain
the present invention where:
[0023] FIG. 1 shows the mapping of DL RSs defined according to the
LTE standard;
[0024] FIG. 2 shows the transmission structures for PBCH and P/S-SS
in LTE;
[0025] FIG. 3 shows PBCH resources in relation to resource for
different number of antenna ports;
[0026] FIG. 4 shows how PBCH REs are punctured by RSs for
additional antenna ports in a solution according to prior art;
[0027] FIG. 5 shows how PBCH REs are punctured by RSs for
additional antenna ports in a solution according to prior art;
[0028] FIG. 6 shows mapping of RSs for four additional antenna
ports on reserved PRBs in a solution according to prior art;
[0029] FIG. 7 shows CRSs for eight antenna ports for LTE-A UEs;
[0030] FIG. 8 shows the relation between virtual antenna ports and
antenna ports;
[0031] FIG. 9 shows four virtual antenna ports;
[0032] FIG. 10 shows CRSs for eight antenna ports after virtual
antenna port mapping;
[0033] FIG. 11 shows CRSs for additional four antenna ports which
are not transmitted in RBs having PBCH;
[0034] FIG. 12 shows CRSs for additional antenna ports which are
not transmitted in RBs having PBCH or P/S-SS;
[0035] FIG. 13 shows transmission of CRSs for additional antenna
ports;
[0036] FIG. 14 shows how RSs for eight antenna ports are
transmitted on the fourth symbol of even-numbered slots and the
sixth symbol of odd-numbered slots;
[0037] FIG. 15 shows how RSs for eight antenna ports are
transmitted on the third and fourth symbol of even-numbered and
odd-numbered slots;
[0038] FIG. 16 shows transmission of CRSs for additional four
antenna ports;
[0039] FIG. 17 shows transmission of CRSs for additional antenna
ports in one sub-frame; and
[0040] FIG. 18 shows transmission of CRSs for additional four
antenna ports in the second slot of a sub-frame.
[0041] FIG. 19 shows an example transmit node according to the
invention.
[0042] FIG. 20 shows an example receive node according to the
invention.
DETAILED DESCRIPTION
[0043] As discussed, the LTE-A system is supposed to be an
extension of the LTE system where up to eight antenna ports will be
supported to further increase system performance. Since an antenna
port is defined by a RS, the RSs for more than four antenna ports
should be designed in LTE-A. In LTE, four antenna ports are already
defined, and in order to enable LTE UEs to work in LTE-A system,
i.e. backward compatibility, the four antenna ports as defined in
LTE should be reused in the LTE-A system. Hence, one problem is the
design of the RSs for the additional four antenna ports.
[0044] According to a first proposed prior art solution, RSs for
the additional four antenna ports are transmitted on the third OFDM
symbol of every slot; according to a second proposed prior art
solution, RSs for the additional four antenna ports are transmitted
on the third and the fourth OFDM symbol of every slot; and
according to a third proposed prior art solution, RSs for the
additional four antenna ports are transmitted in some reserved
Physical RBs (PRBs) in a given sub-frame (SF), which is illustrated
in FIG. 6.
[0045] It has been observed that in the methods according to the
first, second and third proposed prior art solution above, some REs
allocated for the PBCH will be punctured, since these REs are
allocated for additional four antenna ports (RSs), and the
performance for the PBCH will therefore be degraded in terms of
detection performance. In order to preserve the detection
performance for PBCH and also for P/S-SS, the REs for these
signals/channels should not be used for other purposes, i.e. the
PBCH and P/S-SS information should not be punctured arbitrarily as
correct detection of PBCH and P/S-SS is very important for
obtaining RS information for one cell; otherwise the UE will not be
able to correctly communicate with a base station for that
cell.
[0046] In the first prior art solution above, in slot 1, some of
the REs allocated for the PBCH will be punctured since these REs
are allocated for the additional four antenna ports, which is
illustrated in FIG. 4; in the second prior art solution above, some
of the REs allocated for the PBCH will be punctured since these REs
are allocated for the additional four antenna ports, which is shown
in FIG. 5; and in the third prior art solution above, if the
reserved PRBs collide with the PRBs in which the PBCH is
transmitted, all the REs allocated for the PBCH will be punctured
by the RSs for the additional four antenna ports, and performance
will substantially be degraded.
[0047] According to a fourth proposed prior art solution, RSs for
the additional four antenna ports are only located in the PDSCH
region. In RBs, in which the PBCH is transmitted in, the remaining
REs except for ones allocated for the PBCH and RSs all belong to
the PDSCH region. If the RSs for the additional four antenna ports
are located in the PDSCH region within the above mentioned RBs,
LTE-A UEs would identify eight antenna ports, including the four
antenna ports defined in the LTE standard, and the additional four
antenna ports proposed for the LTE-A system. However, LTE UEs can
only identify the RSs for the four LTE antenna ports and need to
receive the PBCH using the four identified antenna ports. In order
to avoid power imbalance between different antenna ports, not only
the first four antenna ports (for LTE) can be used. To uphold power
balance, virtual antenna port mapping is needed for the four LTE
antenna ports, but with virtual antenna mapping LTE-A UEs will not
accurately identify the first four antenna ports as defined in the
LTE standard.
[0048] In the LTE-A system, LTE-A UEs needs to measure the channel
(e.g. CQI, PMI and RI) by using CRSs for eight antenna ports, which
is shown in FIG. 7. Based on the CRSs for the eight antenna ports,
the LTE-A UE can measure the channel for each antenna port, and use
the estimated channel for each antenna port to e.g. calculate CQI,
and select PMI and RI. The operation of LTE-A UE measurement is
similar to the operation performed by LTE UE, and the only
difference is the number of measured antenna ports.
[0049] If there is data for a LTE UE transmitted on a RB having
CRSs defining eight antenna ports as shown in FIG. 7, the LTE UE
can only identify and use the four antenna ports defined in the LTE
standard to demodulate the data. A straight forward solution could
be to only use antenna ports 0.about.3 and refraining from
transmitting on the remaining antenna ports 4.about.7, which can be
seen as antenna selection. A major drawback with this solution is
the power imbalance between the different antenna ports, which is
very undesirable in power amplifiers. Given that the total
transmission power for all the eight antenna ports is constant, if
antenna ports 4.about.7 are not used, then the transmission power
for antenna ports 0.about.3 will be doubled. As the dynamic range
of a power amplifier is the same among the eight antenna ports,
with double transmission power the dynamic range of the power
amplifier will easily be exceeded. If transmission power exceeds
the dynamic range of the power amplifier, interference will be
introduced to adjacent channels. To avoid power imbalance between
different antenna ports, the data may be transmitted on four
virtual antenna ports which are mapped onto all eight antenna ports
through virtual antenna mapping as shown in FIG. 8. The RSs for
four virtual antenna ports are the same as the four antenna ports
defined in the LTE standard. The four virtual antenna ports
identified by a LTE UE, and the CRSs for eight antenna ports after
virtual antenna port mapping identified by LTE-A UE is illustrated
in FIG. 9 and FIG. 10, respectively.
[0050] It can be observed that the provided virtual antenna ports
are the same as the four antenna ports defined in LTE, i.e. the RSs
for the virtual antenna ports are the same as the four antenna
ports defined in LTE. Actually, one virtual antenna port is the
combination of two antenna ports in this example, e.g. virtual
antenna port 0 is obtained by adding antenna port 0 and antenna
port 4, and so on. The shown mapping is only an example, and a
virtual antenna port may be a combination of different antenna
ports, which depends on the virtual antenna mapping function of
which one example is shown in FIG. 8.
[0051] With virtual antenna mapping LTE UEs can work as usual, but
LTE-A UEs can only accurately measure the channel for antenna ports
4.about.7 by using the RSs for antenna ports 4.about.7,
respectively. When a LTE-A UE measures the channel for antenna
ports 0.about.3 by using the RSs for said ports 0.about.3, the
estimated channel is actually the estimated channel for a
combination of two antenna ports channels, and consequently the
LTE-A UE can not accurately measure the channel for antenna ports
0.about.3. Therefore, the CRSs for additional antenna ports can not
be transmitted in RBs, in which a physical channel that LTE UEs
need to detect for proper operation, is transmitted in.
[0052] Before accessing a given cell, a UE has to firstly detect
the PBCH for this particular cell after successful cell search
procedure. The allocated resources for the physical channel are
predefined regardless of the cell-specific information. In LTE, the
mapping resources for PBCH are defined, and LTE UEs receive the
physical channel according to the current definition. When the LTE
UE access a LTE-A communication system, the LTE UE will still
assume that it operates in a LTE system. For the LTE UE to work
properly in this system, the PBCH in a LTE-A cell must be detected
by the LTE UE. Thus, the mapping resources for the PBCH in the LTE
system should be reused, or otherwise LTE UEs can not successfully
access the LTE-A communication system. Therefore, any new features
in a LTE-A system should not impact on the mapping resources of the
PBCH according to the LTE system.
[0053] In the LTE-A system, the PBCH for LTE should be reused.
Since PBCH conveys system information for assisting communication
both LTE UEs and LTE-A UEs will detect the same PBCH. In order to
fulfill the requirement of backward compatibility, and to preserve
performance for PBCH (i.e. to avoid puncturing of PBCH by CRSs for
additional four antenna ports), and not to impact on measurement
performance for LTE-A UEs as mentioned above; the CRSs for
additional antenna ports should not transmitted in the RBs in which
a broadcast channel, such as PBCH, is transmitted in. Therefore,
the CRS for the first four antenna ports 0.about.3 should reuse the
current RS structure for first four antenna ports (i.e. antenna
ports 0.about.3) as defined in the LTE standard, and the CRSs for
additional second antenna ports (antenna ports 4.about.7) should
not be transmitted in RBs in which a broadcast channel is
transmitted in.
[0054] Therefore, a method for transmitting RSs in RBs in a
wireless communication system according to the present invention is
proposed. RBs are numbered from 0 to NRB-1 in the frequency domain,
and the RBs on each antenna port have the same numbering in case of
multi-antenna port transmission. In one slot, if the RBs with the
same index i (0<=i<=NRB-1) on different antenna ports can be
identified, all the identified RBs are denoted RBi, where
0<=i<=NRB-1, and this RB supports multiple antenna port
transmission. When L RSs corresponding to L antenna ports are
transmitted in one RB supporting multiple antenna port
transmission, each RS is transmitted in the RB on its corresponding
antenna port. Among these RBs supporting multiple antenna port
transmission, some are used for transmission of broadcast channels
while other is not, i.e. the RBs supporting multiple antenna port
transmission can be seen to be divided into two distinct and
exclusive sets, wherein a first set comprises all RBs providing a
broadcast channel and a second set comprises all other RBs
supporting multiple antenna port transmission. The idea is to
transmit a first number of RSs, relating to the RSs defined in LTE,
in at least one RB or in each RB supporting multiple antenna port
transmission; and transmit a second number of RSs, relating to the
additional number of RSs defined in LTE-A, in at least one RB which
is an element in the second set of RBs as defined above. Hence, the
second number of RSs will not be transmitted in a RB in which a
broadcast channel is transmitted in.
[0055] Furthermore, the P/S-SS defined in LTE should also be reused
in a LTE-A system to fulfil the requirement of backward
compatibility. In order to preserve synchronization performance,
the P/S-SS should not be impacted by other physical signals or
channels in a LTE-A system. To avoid the situation that P/S-SS will
be punctured by the RSs for additional antenna ports, the RBs
containing P/S-SS can be included in the above mentioned first set
of RBs, i.e. the second number of RSs will not be transmitted in a
RB in which a P-SS or S-SS, and a broadcast channel are transmitted
in.
[0056] In order to enable LTE UEs to operate properly in a LTE-A
system, the control information (e.g. Physical Downlink Control
Channel (PDCCH), Physical Control Format Indicator Channel (PCFICH)
and Physical Hybrid ARQ Indicator Channel (PHICH)) as defined in
the LTE standard also need to be reused, or otherwise LTE-UEs will
not be able to detect the control channels. In LTE, the control
information is transmitted in the first T OFDM symbols of each SF,
where T=1, 2, 3 or 4. The area for transmission of control
information is defined as control region, and hence the remaining
REs except for PBCH, P/S-SS and RS belong to the PDSCH region.
Thus, in a LTE-A system, at least the control information for LTE
UEs should have the same transmission structure as in the LTE
system. As discussed above, to uphold measurement performance for
the eight antenna ports for LTE-A UEs, the CRSs for the additional
four antenna ports should not be transmitted in the control region
in which LTE UE needs to detect control signalling information.
Therefore, the CRSs for the additional four antenna ports could be
transmitted in the PDSCH region, except in RBs in which a broadcast
channel is transmitted in.
[0057] Since, the control region is always located in the first
slot of given SFs, it means that each RB in the first slot
comprises a control region and a PDSCH region, and each RB in the
second slot of given SFs only comprises a PDSCH region. If the RSs
for additional four antenna ports are transmitted in the PDSCH
region of RBs comprising both a control region and a PDSCH region,
LTE-A UE can only measure the channel of antenna ports 4.about.7
and antenna ports 0.about.1. The reason is that the RSs for antenna
ports 2.about.3 are only transmitted in the second OFDM symbol of
each RB, which are in the control region of RBs including both the
control region and the PDSCH region. Therefore, the RBs comprising
control regions should be included into the above mentioned first
set of RBs. Based on this observation, the second number of RSs
should be transmitted in the RBs within the second slot of given
SFs, except in the RBs in which a broadcast channel is transmitted
in, i.e. the second number of RSs are transmitted in the second
slot of given SFs.
[0058] Hence, according to different embodiments of the invention,
RBs used for the additional number of antenna ports (second number
of RSs) should not be used by a PBCH or, a synchronisation signal
such as P-SS or S-SS, or a control channel, such as PDCCH, PDCCH or
PHICH, due to the problem of performance degradation of these
channels/signals and channel measurement performance. Further,
PDSCH regions and control regions of RBs may also be taken into
consideration when transmitting the additional number of RSs.
[0059] Furthermore, CRSs are used by all UEs in a given cell, and
transmitted in reserved resources. In the time domain, the CRSs for
additional antenna ports can be transmitted in every one, or every
N.sub.1 SFs, where N.sub.1 is an integer and larger than 1; in the
frequency domain, the CRSs for additional antenna ports can be
transmitted in every one, or every N.sub.2 RB, where N.sub.2 is an
integer and larger than 1. Usually, the CRSs are evenly distributed
in the time/frequency resources to achieve the performance balance
of channel measurement/estimation. Since the CRSs for additional
antenna ports are not transmitted in RBs containing PBCH, it is
difficult to distribute the CRSs for the additional antenna ports
evenly in SFs containing PBCH. Therefore, in this case the CRSs for
additional antenna ports should not transmitted in the SFs with
PBCH. According to the above discussion, in a further embodiment of
the invention, the CRSs for the additional antenna ports should not
be transmitted in SFs containing a broadcast channel (e.g. PBCH) or
P/S-SS. It should be further noted that if a SF comprises at least
one RB supporting multiple antenna port transmission that SF is
considered as a SF supporting multiple antenna port
transmission.
[0060] Some exemplary implementations of different embodiments of
the present invention will be given in the following, wherein in
the following examples it is assumed that there are eight antenna
ports in the LTE-A system, and the number of additional antenna
ports is therefore four. However, as understood by the skilled
person, the present invention is not restricted to this number of
additional of antenna ports, but the method according to the
present invention may be employed in wireless communication systems
in which a first number of antenna ports and an additional number
of RSs are used. Also, in these examples, only the allocated REs
for the RSs defining the additional antenna ports within one RB are
illustrated.
[0061] Exemplary Implementation 1
[0062] In this example the CRSs for additional four antenna ports
are not transmitted in RBs containing PBCH, and the CRSs are used
for channel measurement, or for channel measurement and
demodulation. The central six RBs in slot1 (S1) of each radio frame
are used for the PBCH, and the CRSs for the additional four antenna
ports are not transmitted in the RBs containing the PBCH. This is
illustrated in FIG. 11. This figure shows that the antenna ports
(0.about.3) as defined in LTE are transmitted in all RBs supporting
multiple antenna port transmission, while the additional number of
antenna ports (4.about.7) are transmitted in RBs in which no
broadcast channel, such as PBCH, is transmitted in.
[0063] Exemplary Implementation 2
[0064] In order to avoid that the CRSs for the additional four
antenna ports puncture the REs allocated for P/S-SS, the CRSs for
the additional four antenna ports are not transmitted in the RBs
containing PBCH or P/S-SS, which is illustrated in FIG. 12. In this
figure, the transmission of the CRSs for the additional four
antenna ports in the first SF is shown. The CRSs are not
transmitted in a RB containing a broadcast channel (PBCH) or a
synchronisation channel (P/S-SS) to avoid puncturing of these
channels and the CRSs for the first four antenna ports are
transmitted in each RB supporting multiple antenna port
transmission.
[0065] Exemplary Implementation 3
[0066] The CRSs for the additional four antenna ports are not
transmitted in a SF containing PBCH. Instead, they are transmitted
in every five SF, in SF2 and SF7, of each radio frame as shown in
FIG. 13. In SF2 and SF7, the CRSs may be transmitted in RBs
according to the structure as shown in FIG. 14 and FIG. 15,
respectively.
[0067] Exemplary Implementation 4
[0068] The CRSs for the additional four antenna ports can be
transmitted in every five SF, in SF3 and SF8, respectively, of
every radio frame as shown in FIG. 16. In SF3 and SF8, the CRSs for
the additional antenna ports are transmitted in every other RB in
the frequency domain, and on the third and fourth OFDM symbol of
these RBs. This is shown in FIG. 17.
[0069] Exemplary Implementation 5
[0070] The CRSs for the additional four antenna ports are
transmitted in a PDSCH region, except in RBs containing a broadcast
channel (PBCH), i.e. they are not transmitted in the control region
of each RB, and not in RBs with a broadcast channel (PBCH).
[0071] Exemplary Implementation 6
[0072] Since control regions exists in the first slot of each SF,
the RSs for antenna ports 0.about.3 in the control regions can not
be used for measurement by a LTE-A UE. In order to accurately
measure the channel for eight antenna ports, the CRSs for the
additional four antenna ports are transmitted in RBs belonging to
the second slot of given SFs, which comprises CRSs for the
additional four antenna ports. If the CRSs for the additional four
antenna ports are transmitted in the second slot of the SF
comprising PBCH, then the CRSs will not be transmitted in RBs
having PBCH. This is illustrated in FIG. 18, and it is shown in
this figure that the CRSs for the additional four antenna ports are
transmitted in the second slot of the SF, and distributed in every
RBs of that slot.
[0073] FIG. 19 shows a schematic block diagram of example transmit
equipment, such as a base station device or corresponding
apparatus, according to the invention. The transmit equipment (190)
comprises transmit circuitry (191) and processing circuitry (192)
providing processing instructions and signals for transmission. The
transmit equipment preferably also comprises storage means (193)
for storage of processing instructions and data.
[0074] FIG. 20 shows a schematic block diagram of example receive
equipment, such as user equipment or a mobile terminal, according
to the invention. The receive equipment (200) comprises receive
circuitry (201) and processing circuitry (202) providing processing
instructions and controlling received signals. The transmit
equipment preferably also comprises storage means (203) for storage
of processing instructions and data.
[0075] Also, as understood by the person skilled in the art, the
method for transmitting reference signals (RSs) in a wireless
communication system according to the present invention may be
implemented in a computer program, having code means, which when
run in a computer causes the computer to execute the steps of the
method. The computer program is included in a computer readable
medium of a computer program product. The computer readable medium
may consist of essentially any memory, such as a ROM (Read-Only
Memory), a PROM (Programmable Read-Only Memory), an EPROM (Erasable
PROM), a Flash memory, an EEPROM (Electrically Erasable PROM), or a
hard disk drive.
* * * * *