Note: Descriptions are shown in the official language in which they were submitted.
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METHOD AND SYSTEM FOR SUBBAND INDICATOR SIGNALLING
Field of the Invention
[0001] The present invention relates to the field of communication
technologies, and
particularly to a method and system for subband indicator signalling.
Background of the Invention
[0002] In 3GPP Evolved UTRA (E-UTRA) frequency domain scheduling is an
important
feature. To enable such scheduling for downlink in case of frequency domain
duplex,
frequency dependent channel quality information needs to be transmitted from a
mobile
terminal to a base station (Node B) in some form. In E-UTRA a channel quality
indicator
(CQI) is transmitted to report the channel quality. The whole frequency band
can be divided
into subbands wherein the channel quality (CQI) then is estimated separately
for each
subband.
[0003] Furthermore, in E-UTRA using MIMO precoding, a preferred precoding
vector
index (PVI) is transmitted from the receiver to the Node B to report the
preferred MIMO
precoding vector. Due to the frequency selectivity of the channel, the whole
bandwidth need
also in this case be divided into subbands, and the PVI is estimated
separately for each
subband.
[0004] The most straightforward way to transmit channel quality or precoding
vector
information is to transmit the PVIs and CQIs for all subbands. However, such a
scheme may
cause a considerable amount of signalling. Therefore, several alternatives
have been proposed
to reduce the amount of signalling. Such alternatives, of which only the first
will be
elaborated further here, include, among others: to only feedback information
about the Q
subbands which have the best CQI, to apply differential feedback information
in time or
frequency, to use bitmap techniques indicating which subband or subbands
reflect a reported
CQI/PVI value, to use a hierarchical tree structure, and to use a set of
orthogonal functions to
approximate a frequency selective fading profile, see, for example, 3GPP TR
25.814 v.1Ø1
"Physical Layer Aspects for Evolved UTRA," December 2005.
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[00051 Regarding the first alternative, this can be further be elaborated by
the following
method: send one CQI value representative for the best Q subbands, send the
indices of these
subbands, and send a CQI representative for the whole frequency band. Such a
method has
been shown to give only a small degradation in throughput compared to a method
wherein
CQI values are transmitted for each and every subband, see 3GPP TSG RAN, R1-
060228,
Huawei, "Sensitivity of DL/UL Performance to CQI-Compression with Text
Proposal,"
January 2006.
[00061 An important issue in this method, however, is how to transmit the
indices of the Q
subbands for CQI reporting (and PVI reporting in case of precoded MIMO) with a
minimum
amount of signalling. If there are N subbands, indexed 1, 2, ..., N, thus
constituting a set of N
subbands, a sequence of N bits can be transmitted, wherein the corresponding
bit positions of
the indices of the particular Q subbands may, e.g., be set as ones, while the
remaining bit
positions are set as zero. This method, however, requires rather extensive
signalling.
[00071 One example of a prior art compression technique for bitmaps in general
is run-
length encoding, see A. Bookstein and S. T. Klein, "Construction of Optimal
Graphs for Bit-
Vector Compression," Proc. 13th annual international ACM SIGIR conference on
Research
and development in information retrieval, pp 327-342, 1990, Brussels, Belgium,
wherein a set
of consecutive ones/zeros is represented by a flag indicating one/zero and the
number of
ones/zeros. However, such a compression technique cannot be guaranteed to
always compress
the input, in particularly not when, e.g., 5 subbands of 24 are indicated, in
which case selected
subbands may be arbitrarily distributed among the 24 possibilities.
[00081 A more efficient method relies on the fact that there are (N) Q ways to
select Q
subbands out of N subbands, where (n) k is the binomial coefficient:
(n) n! k!- 1, k = 0 (1}
k k!(n-k)! 1.2.3===k, k>0
[00091 Thus, to convey the indices of Q out of N subbands, at least loge N Q
bits are
needed compared to N bits if a simple bitmap representation is used. F x]
denotes the smallest
integer greater than or equal to x. For example, if N=24, and Q=5, 16 bits are
needed instead
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of 24 bits for the bitmap. In this way, it is possible to obtain a significant
data signalling
reduction when signalling the indices of the subbands.
[0010] A straightforward method to map the possible subsets of Q subbands
selected from
the N subbands onto the minimum number of bits required is to tabulate them.
The possible
subsets of Q subbands raise fast with increasing Q. For example, in the case
of N=24, and
Q=5, there exist more than 40000 different subsets. However, arranging a table
of possible
subsets in a way that enables sufficiently fast mapping and de-mapping
presents a problem.
[0011] Consequently, there exists a need for an improved method for signalling
subband
identities.
Summary of the Invention
[0012] It is an object of the present invention to provide a system and a
method for
signalling subband identities from a first transceiver to a second transceiver
in a wireless data
communication system, which provides a more efficient way of obtaining a
representation of
a particular subset of subbands.
[0013] According to the present invention, groups of subsets are established,
wherein each
subset in a group contains the identities of the same number of subbands, and
wherein subsets
within a group are ordered, and identities of a subset of a set of subbands
are signalled from
the first transceiver to the second transceiver by signalling a representation
of the identity of
the group of the subset and a representation of the position of the subset in
the group. For
example, the subset of the subbands may be represented by a number r, wherein
r is
determined by the identity of the group of the subset and the position of the
subset in the
group.
[0014] This has the advantage that the first transceiver, e.g., a mobile
terminal, in a simple
and efficient manner can obtain a representation of the subset for
transmission to the second
transceiver, e.g. a base station. This further has the advantage that the
representation may be
obtained faster and using fewer resources in the mobile station. The
signalling of a subset of
subbands may be used, e.g., in signalling of CQI and/or PVI and/or any other
subband
specific parameters.
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[0015] The identity of the group may be determined by a group specific offset.
This has the
advantage that the identity can be obtained in a simple manner.
[0016] The method may further comprise the steps, wherein subbands are indexed
(1, ...,
N), of
a) ordering subsets ml,..., mg, in a group such that the subset wherein the
lowest index
ml has the highest possible value n is arranged first in the group, followed
by all subsets
wherein the lowest index is n-1 and so on until the value of the lowest index
is 1,
b) setting i = 1,
c) ordering subsets having an equal lowest index m1, or, if i>1, equal lowest
indices
ml,..., m;, such that the first subset is the one in which the lowest of the
remaining indices
mi+1 has the highest possible value, then all subsets such that the lowest of
the remaining
indices has the next to highest possible value, and so on until the lowest of
the remaining
indices is mi+l,
d) setting i = i+1 and repeating step c) until i = Q-1.
[0017] This has the advantage that the representation of a specific subset may
be obtained
even faster. Further, since these steps may be performed using mathematical
calculations
instead of a table look-up, considerable memory savings in a mobile terminal
may be made.
[0018] The present invention also relates to a system and a communication
system.
[0019] The invention will be explained more fully below with reference to the
appended
drawings.
Brief Description of the Drawings
[0020] Fig. 1 shows an exemplary wireless data communication system in which
the present
invention advantageously may be utilised.
[0021] Fig. 2 shows an example of possible communication resources in a
communication
system according to Figure 1.
Detailed Description of Embodiments
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[00221 Figure 1 shows a communication system 1 wherein the present invention
may be
advantageously utilized. The figure shows a base station antenna 2, having
capabilities to
communicate with one or more mobile terminals 3. The communication resources
consist of
at least one frequency band, which is divided into subbands. This is disclosed
in detail in
Figure 2, in which a communication resource scheme suitable for use with the
present
invention is shown. As is shown in the figure, the frequency spectrum of the
communication
system is divided into 24 subbands (n1, ..., nN), for example constituting
equal portions of the
frequency spectrum, as equal frequency subbands are preferred to facilitate
resource
management (for example, it is easier to allocate the available resources).
However, division
into non-equal frequency subbands is, of course, also possible.
(00231 The communication resources may be divided into time-slots (not shown)
in the time
domain, typically having a certain length, e.g. a number of OFDM symbols, and
wherein a
user may be allocated all or part of one or more time-slots in one or more
subbands.
Alternatively, a user may be allotted all or part of a subband, wherein one or
more users
continuously may be communicating at the same subband.
[0024] Only one base station is shown in the communication system in Figurel.
As is
apparent to a person skilled in the art, however, the communication system 1
may comprise a
plurality of base stations, each providing coverage in part of the
communication system.
Further, the coverage area of one base station may be divided into sectors.
[00251 The channel quality for a specific mobile station may vary
substantially among the
various subbands, even between closely located subbands, e.g., between n1 and
n2 or between
ni and n3 in Figure 2. Consequently, in order to utilise communication
resources as efficiently
as possible, it is necessary that the mobile station communicates channel
parameters, such as
channel quality (CQI) and a preferred precoding vector index (PVI) to the base
station. As the
parameters may vary substantially from subband to subband, this information
needs to be
transmitted to the base station. However, as stated above, signalling channel
parameters for all
subbands require extensive signalling, and a way of reducing this signalling
is to only
transmit the identities of those subbands that exhibit the most favourable
properties.
Alternatively, if all but a few channels provide satisfactory properties,
subband identities of
those subbands providing poor channel properties could be signalled instead,
as this would
offer the base station enhanced flexibility when allocating channel resources.
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[0026] Still, as it is inherent in a wireless communication system to utilise
available
bandwidth as efficiently as possible, also the amount of data needed to
transmit identities of a
limited number of subbands should be signalled to the base station as
efficient as possible, i.e.,
if each subset of Q subbands is considered as a bitmap of N bits, Q of them
being ones, the
problem to be solved is how to transmit indices of Q subbands out of a total
of N subbands
with a minimum number of bits and in a way that allows fast mapping from the
subset of
indices to the sequence of bits as well as fast de-mapping.
[0027] The mapping can be viewed as a compression from a bitmap of N bits,
where the
original bitmap has the restriction that there are exactly Q ones. The problem
to be solved is
then to devise an efficient compression of the bitmap.
As has been disclosed above, to convey the indices of Q out of N subbands
(N)]
N = 1092 Q (2)
[0028] bits are needed to signal the indices to the base station (node B) from
the mobile
station. Although this is a substantial improvement as compared to using a
simple bitmap, as
described above, it is still very important that the mobile station in a short
period of time and
with a minimum load on processing resources is capable of determining the
particular
representation that represents Q subbands out of N subbands.
[0029] For example, if, as in Figure 1, there are 24 subbands, i.e. N=24, and
five particular
ones of these (Q=5) are to be transmitted to the base station, there are more
than 40 000
possible combinations. The number N may, however, be even larger, usually
depending on
the bandwidth. For a bandwidth of 20 MHz, N may be 48. Further, it is common
to allow
transmission of identities (indices) of an arbitrary number of subbands up to
a given
maximum. Using the same example, if any number of subbands up to 5 is allowed,
there are
more than 55000 possibilities. If each of these possibilities is represented
by a number, it is
difficult to obtain the particular number to transmit to the base station
(e.g., in binary form) in
a sufficiently quick and efficient manner.
[0030] The present invention provides a method for mapping a subset of subband
indices
selected from a total set of N subbands on a minimum number of bits in a way
that enables
fast mapping and de-mapping.
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[00311 In the general case, the allowed numbers Q of subbands in the subset is
given by the
set M = {m1, m2 , ... , m, } of unique non-negative integers where 0< mi -< N,
j = 1,2,... , J.
[0032] The number of bits required to map all possible subsets is then
[1092 IN (3)
kÃM k
[0033) The degree of reduction in signaling due to this mapping depends on N
and the set M.
For the above example, where there are in total N = 24 subbands and 0 to 5
subbands are in
the subset of best subbands, i.e. M={0,1,2,3,4,5}, 0, 1, 2, 3, 4 or 5 subbands
can be indexed
using only
5
(24))] z x15.7591=16 bits (4)
N = [1092
1
k=0
instead of N=24 bits.
[0034] According to the invention, each subset of indices is labelled by a
number r. E.g., a
subset may comprise the indices 1,7,12,15,21. The subset of indices is mapped
on, e.g., a
sequence of bits as a binary representation of r. However, it is clear to
anyone skilled in the
art that the number r can also be mapped on a sequence of symbols from another
alphabet, for
example, as a ternary representation or a decimal representation.
[0035] In the case when r is mapped on a binary sequence of N bits, the range
of r must be
within [0, 2 N -I].
[0036] According to the present invention the range of r is divided into J
intervals such that
all values of r within interval j represent subsets of m; indices. The
smallest r within interval j
is called the jth offset Oj. Each interval must be large enough to contain all
subsets with m;
indices, i.e.
N N
O+1>- m +0;, j=1,2,...,J-1 and 0,S2N-1- m (5)
r
must be valid. The minimum offsets possible are given by
j-, N
k=1 Mk
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100371 The invention will be further exemplified using an example wherein N=5,
M={m1=0,
m2=1, m3=2}, and 9=4. In this example, the offsets are 01=0, 02=1, 03=6.
Consequently, r=0
for the empty subset, 1 < r < 5 for subsets containing one index, and 6 < r:5
15 for subsets of
two indices, as is shown in Table 1.
Table 1
R Number of
indices, Q
0 0
1 1
2
5
6 2
7
[0038] Accordingly, the number r labelling a subset of Q=m; indices can be
obtained as the
sum of the offset Oj plus the position of the subset in a group of subsets.
The above definition
10 of Oj is exemplary and may be defined in numerous other ways. For example,
O; may be
calculated as the last position in a group of subsets, wherein the number r is
obtained as Oj
subtracted by a certain number. Using the above method of dividing the range
of r into
intervals, or groups, a considerably quicker "look-up" may be obtained, i.e.,
the particular r to
be transmitted is identified by the mobile station more efficient as compared
to the prior art.
15 In this way, the base station will receive a more accurate measurement, in
particular when the
mobile station is moving fast or the propagation properties are subject to
frequent and sudden
changes. Further, the present invention also has the advantage that the
processor load in the
mobile terminal is reduced.
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[0039] However, the above method can be enhanced even further by introducing
an
additional step, which will be described in the following.
[0040] In the alternative embodiment, the additional step consists of sorting
the subsets in
each group in a particularly efficient manner.
[0041] First, the indices of a subset are sorted. The sorted subset of Q
indices out of the total
number of indices N, wherein the N indices ranges from I to N may be expressed
as
1_<s <N
{sk }Q o , wherein k (7)
Sk < Sk+l
[0042] Next, the subsets in a group are ordered in the following way: The
first subset in the
group is the one with sO=N-Q+1, followed by all subsets with so=N-Q and so on
until so=1.
[0043] In a next step all subsets of indices wherein so=x, the subsets of
indices are ordered
such that one first finds all subsets such that s1=N-Q+2, then all subsets
such that s1=N-Q+1,
and so on until s1=x+l. The same procedure to order the subsets of indices is
repeated for
each index k in the subset. For Q"--O, only a single set exists and no
ordering is needed. r is
hence given by the offset Oj plus sum of the number of all subsets with Q
indices larger than
so and the number of all subsets with Q-1 indices larger than Si and so on.
10044] The mapping for the previous example N=5, M={m1=0, m2=1, m3=2}, and
offsets
01=0, 02=1, 03=6 is illustrated in Table 2.
Table 2 Mapping example
r Subset of
selected
indices, {sk}
0 {}
1 {5}
2 {4}
3 {3}
4 {2}
5 {1}
6 {4,5}
7 {3,5}
8 {3,4}
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9 {2,5}
{2,4}
11 {2,31
12 {1,5}
13 {1,4}
14 {1,3}
{1,2}
[0045] The above mapping (ordering) of subsets allows for fast mapping and de-
mapping,
even without need of a full table, as will be shown below. Further, it is, of
course, equally
possible to arrange the list such that one first finds so=1, followed by all
subsets with so=1+1
5 and so on until so=N-Q+1, whereupon the following equations will be changed
correspondingly.
[0046] Continuing with the arrangement as disclosed in table 2, if sk=x, then
it comes after
all subsets with the same indices sj, j<k, and with x < sk < N in the list.
The number of such
subsets equals N - x since there are Q-k indices (l=k,k+1,...,Q-1) in the
range from x+1 to
(Q-k)
10 N. In case Sk has its highest possible value, i.e. sk =N-Q+k+l, there are
of course no subsets
with higher values of sk. In that case N - Sk = Q - k -1 but [QQk 1 not
mathematically
- k
defined. Therefore, the notation may be simplified by defining the extended
binomial
N
coefficient
(k):
N = k N >- k . (8)
k 0 N<k
15 [0047] It is now easy to calculate r for a given subset of mj indices by
evaluating and
summing the numbers of all possible subsets with higher values of the indices
and by adding
the offset O. The mapper takes the subset {sk }k!o' as input and creates a
number r in the
range 0 <- r 2 N 1 using the following equation:
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mj-1 N - S
r = I k _~Oj (9)
k=o mj - k
where Oj is obtained by (6). The number r can now be transferred to the Node B
using N
bits.
[0048] Accordingly, the present invention has the substantial advantage that r
can be
calculated using a mathematical expression without a need for storing or
producing a table
each time a subset of subband indices is to be signalled to a base station.
[0049] The present invention further has the advantage that it is equally, or
substantially
equally easy to extract r from the received sequence of bits in the receiver
(base station), and
this de-mapping algorithm will now be described. The task of the de-mapper is
to extract r
from the received sequence of bits, detect j and then regenerate the subset of
indices {sk }k_'o, .
This is performed in two steps, according to the following:
Step
[0050] j is given as the largest integer such that Oj :- r. Find j and
calculate r':
r'=r-Oj.
Step2
[0051] Given j and the number r' from Step 1 set Q-mj and the indices {sk }Q '
will now be
found by executing the following algorithm, given in a generic software code
form:
for k=0 to Q-1
Find the largest integer xJV Q+1 +k such that m = N - x < r'
Q-k
Sk x
r' = r'-m
end
[0052] An example of a more detailed algorithm is shown below:
xrln =1
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for k=0 to Q-1
x=xnrin
N-x
m=
Q-k
while in > r'
x=x+1
N-x
m
Q-k
end
Sk = X
xnvn = Sk +I
r' = r'-m
end
[00531 The mapper and de-mapper need the binomial coefficients k for n =
0,..., N -1
and k = 0,1, ..., max {mj } to perform the above operations. These
coefficients can be pre-
calculated and stored in a table.
[00541 As has been disclosed above, the present invention provides an
efficient method for
finding a representation of a particular subset of subband indices to be
transmitted to a base
station. The present invention further provides an efficient method for
retrieving r in a
receiver.
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