Communications and Network, 2013, 5, 308-311
http://dx.doi.org/10.4236/cn.2013.53B2057 Published Online September 2013 (http://www.scirp.org/journal/cn)
Multiuser Receiver Scheme with SIC for PUCCH in High
Speed Train Envir onment*
Wei Wang, Guangliang Ren
The State Key Laboratory of ISN, Xidian University, Xi’an, China
Email: weiwang1013@gmail.com
Received July, 2013
ABSTRACT
A multiuser receiver scheme with successive interference cancellation (SIC) is proposed to suppress multiuser interfer-
ence for physical uplink control channel (PUCCH) in high speed train (HST) environment. In the proposed algorithm,
each user’s signal is detected iteratively in a descending order according to the signal strength at eNB. During each it-
eration, the strongest signal of all users’ is detected and regenerated, and then is subtracted from the composite signal
before decoding the next user. Simulation results show that the proposed scheme obtains remarkable gains, e.g. 2 dB for
PUCCH format 2 with 3 users in HST scenario 1. The improvement is more pronounced in the case of increasing num-
ber of users, e .g. 3.3 dB with 6 users.
Keywords: PUCCH; Successive Interference Cancellation; High Speed Train; Multiuser
1. Introduction
PUCCH is an important narrow-band channel, which
carries the uplink control information, such as acknowl-
edge/non-acknowledge (ACK/NACK) bits, downlink
channel quality indicator (CQI), scheduling request, pre-
coding matrix indicator (PMI), rank indication (RI) and
HARQ-ACK feedback information, and it requires much
higher transmission quality than the oth er channels. With
the aid of orthogonal CDMA [1], multiple users share the
same time-frequency resources, i.e., a resource block
(RB) in the same sub-frame duration in the PUCCH, and
the multiuser interference has been the main factor that
limits the transmission quality of the control information.
Recently, several literatures have been focused on the
scheme of the PUCCH multiuser receiver in the low mo-
bility scenario. Literature [2] proposed a parallel-receiver
scheme for the PUCCH multiuser receiver with low ref-
erence signals (RSs) density. By using the maximum
likelihood (ML) mechanism, literature [3] presented a
semi-blind receiver with a high performance. However,
the two schemes will be greatly degraded in the HST
scenario, because there exists a large Doppler shift,
which leads to loss of code orthogonality and also intro-
duces inter-carrier interference (ICI) within a single SC-
FDMA symbol [4]. Both the two schemes above can not
work with so large a Doppler shift without carrier fre-
quency offset (CFO) estimation and correction. Besides,
the near far resistance is high in HST environment due to
the fast variation of users’ location. The received signal
strength at eNB from near users is higher than that from
the far ones, and users’ power differs greatly, which
makes the multiuser interference severe. In this case, the
performance of the parallel-receiver is degraded due to
the poor performance of the farther users. And it is lim-
ited by the number of users while the user density in HST
is very high. Semi-blind algorithm has higher computa-
tional complexity with increasing number of users and
CQI bits. Consequently, the available receiver schemes
can not directly apply to such a special scenario. A re-
ceiver, which can overcome the influence of frequency
offset and power differences among users, is required to
maintain the link quality for PUCCH in HST environ-
ment. Hence, we propose a multiuser receiver scheme
with SIC for PUCCH. The proposed scheme can greatly
suppress the interf erence of the strong on th e weak and is
robust to the number of users, increasing the system ca-
pacity. Moreover, it has lower complexity and is suitab le
for implementation.
The following sections are organized as: In section 2,
system model is described. In section 3, the multiuser
receiver scheme with SIC is proposed and in section 4
simulation results are presented and analyzed. Finally,
basic conclusion is drawn in section 5.
2. System Model
*This work was supported in part by the State Natural Science Founda-
tion of China, Grant No.61072102 and National Major Specialized
Project of Science and Technology, Grant No.2011ZX03001-0 0 7 -01.A PUCCH subframe consists of two 0.5ms slots, each of
C
opyright © 2013 SciRes. CN
W. WANG, G. L. REN 309
which contains seven SC-FDMA symbols and occupies
12 subcarriers distributing at band edges. PUCCH sup-
ports seven formats, namely formats 1/1a/1b, format
2/2a/2b and format 3. The receiver we proposed applies
to all PUCCH formats. In the following, we mainly talk
about format 2 for the transmission of CQI bits. The time
frequency structure of PUCCH format 2 is shown in
Figure 1. CQI information bits from different users are
coded into 20 bits by a RM code [5], and then these bits
are QPSK modulated, resulting in a block of com-
plex-valued modulation symbols 01 9
. Those
data symbols occupy 10 SC-FDMA symbols of a sub-
frame. The rest symbols of a PUCCH format 2 subframe
are used to transmit RSs. Each symbol shall be spreaded
by a cyclically shifted length 12 sequence. The sequence
of different users in a cell, generated by phase-rotated
basic sequence, is orthogonal to each other. The rotation
value of user k can be expressed as lk
,,,dd d

, where
l
varies with symbols and k
varies with users [6].
The user’s modulated symbols in a subframe,
spreading sequence of the symbol and spreaded
symbol can be described respectively as
kth
llth
,0 ,1,13
,0,1,2,3,4
,5 ,6,7,8 ,9
[,,, ]
,1,, ,,1, ,
,1, ,, ,1,
kkk k
kkkkk
kkkkk
ss s
ddddd
ddddd
s 
(1)
,, ,,
[(0),(1),,( 1)]
R
B
klklklkl SC
cc cNc T
(2)
,,klkl kl
ss
,
c (3)
where . K is the total number of users. l
denotes symbol index within a subframe and
. .
0,1 ,1kK
, ,13
 0,1,
0,1l, 1
RB
SC
iN 
R
B
SC
N is the number
of subcarriers in a RB. ,kl
s is mapped to a group of lo-
calized subcarriers allocated for the user. By using
the N-point inverse fast Fourier transform, an SC-FDMA
symbol of the user can be obtained as follows
kth
kth
Figure 1. Time-frequency structure of PUCCH format 2.
1
,,,
0
1()exp 2()
0,1, ,1
N
kl kllm
kmn
sa mj
NN
xn
nN

 
(4)
where is mapped by the length-12 sequence ,
and ,kl
a
kl
,kl
c
,, ,kl kl
[(0),,( 1)aaNa]
 .
In HST channel model, the LOS component is con-
stant and frequency offset exists caused by the Doppler
shift. The scattered component in the uplink also has
frequency offset half that of LOS component [4]. The
received complex baseband symbol in time domain can
be descri b ed as
,
1
0
,
()(exp 2
)
)
exp( )(
Kgs
k
k
gs sll
l
kk
kl
rn NlNn
xn jc
N
NlNn
jhn
N




wn


 



(5)
where
N is the length of the cyclic prefix (CP).
s
N
indicates the samples in a SC-FDMA symbol, and
sg
NNN
. and , represent the coefficient
of LOS component and NLOS component respectively.
k
c()
s
kl
hn
is normalized CFO of the LOS component.
is the AWGN term. ()wn
l
By removing CP, FFT process and subcarrier demap-
ping at eNB, the received signal on the subcarrier
during the SC-FDMA symbol can be represented as
mth
lth
1
0
,, ,
,,
2
() ()exp()
exp2( )
()() ()
N
ll
n
gs
kklkl
kl kl l
jnm
Rm rnN
NlN
jsa
N
ImUmWm





 
kl
mH
(6)
1
,,
0
1exp( )
exp 2
Ngs s
klk kl
n
k
NlNn
H
cj h
NN
n
jN


n








(7)
where , denotes the ICI of the user. ,
is the interference of the other users to the user and
l is noise in frequency domain. ,kl
()
kl
Im
)
kth ()
kl
Um
kth
(Wm
H
is an at-
tenuation coefficient caused by channel and residual car-
rier frequency of fset.
3. Proposed Scheme
In this section, the proposed SIC-receiver is described in
detail. In the scheme, the symbol of each user is detected
in a descending order based on the received power levels.
The strongest signal of all users’ in each iteration is de-
tected and regenerated, and then the contribution of the
strongest signal is cancelled from the composite signal
Copyright © 2013 SciRes. CN
W. WANG, G. L. REN
310
before the detection of the next user, from which the
multiuser interference is progressively reduced. Symbol
detection of the each user is performed by single user
detection (SUD) method. Firstly, s ome of the SUD com-
bining methods considered in this paper are reported.
,kl
H
is estimated by the scheme proposed in [2], and
the carrier frequency offset is estimated by a iteration
method in [7]. Let ,
ˆkl
H
denote the estimation of ,kl
H
,
and ˆk
the estimation of k
.
Define
as the iteration number. Initialize l
to . The SIC algorithm for PUCCH in HST sce-
nario is described as follows.
0()Ri
()
l
Ri
Step 1, despreading and power calculation. The
symbol of user k is expressed as
lth
,
1
,kl
Hl
kl RB
SC
RN
cR
(8)
where .
11 11
01 11
[(),(),,()]
T
ll ll
RmRm Rm
 
R 
 i
m
is the useful subcarrier index.
Let k
p
be the power of user k in the th
iteration,
find the user of max-power, and let

12
argmax,,,K
kPP P

 .
Step 2, data symbol detection of the s user.
th
*
,,
ˆ
ˆ
ˆ{exp( 2)}
ll
sQj HR


 ,l

(9)
where is a quantization operation, and it assigns
an element of the data symbol alphabet to each value of
Q{}
*,,
ˆ
ˆ
exp( 2)ll
jHR


.
Step 3, signal regeneration. The SC-FDMA
symbol of the user is regenerated as
lth
th
,, ,,
1
0
1ˆ
ˆ()exp 2
ˆ
e
ˆ(
x2
)
p
N
llll
m
gs
mn
sa mHj
N
Nl
r
N
j
n
n
N









N
(10)
Step 4, subtraction of the regenerated max-power sig-
nal. Let ,
ˆ()
l
Ri
be the transformation of ,
ˆ()
l
rn
in
frequency domain. Subtract ,
ˆ()
l
Ri
from the composite
signal, and we have
1,
()() ˆ()
ll
Ri RiiR


l
(11)
Step 5, the power of user is calculated again and is
denoted as
P
. If PP
4. Simulation Results
In this section, the performances of the proposed receiver
scheme and the parallel-receiver scheme are investigated.
The simulation parameters are listed in Table 1.
Block error ratio (BLER) of CQI in HST scenario 1
and 2 is shown. Consider a worse case that the SNR of
the strong is higher than that of the weak by 9.5dB, and
when the number of users is three, one has higher power;
the number is 2 while there are 6 users. The noise at-
tenuation varies with the average power of all users’ to
keep SNR constant. The simulation results are presented
as follows.
Figure 2 presents the average BLER of SIC-receiver
and parallel-receiver with 8 CQI bits for PUCCH format
2 in HST scenario 1. The proposed algorithm provides a
gain of around 2dB and 3.3 dB over the parallel-receiver
for 3 users and 6 users respectively. The proposed
scheme is robust to number of users, whereas the per-
formance of parallel-receiver decreases with the increas-
ing number of users .
Table 1. Simulation parameters.
Parameters Description
System bandwidth 10M at 2.3GHz
Number of FFT/I FFT points 1024
PUCCH formats Format 2
Channel model HST
# Users 3 Users, 6 Users
shifts [0, 1, 2], [0, 1, 2, 3, 4, 5]
Velocity of the train 350 km/h
1
R
() ()
ll
Ri i

, the next iteration follows
on. Otherwise, set , and the power of the
user zero in the following iteration. Then start the
next iteration. The iteration will not stop until the weak-
est user is detected. Moreover, the data symbols detec-
tion of a user may be performed repeatedly due to the
iteration process. The one when the user’s power is
maximum is chosen as the final result.
th
Figure 2. SIC-receiver vs. parallel -receiver, scenario 1.
Copyright © 2013 SciRes. CN
W. WANG, G. L. REN
Copyright © 2013 SciRes. CN
311
PUCCH receiving. The improvement is more pro-
nounced with increasing number of users. The proposed
system is robust to multiuser interference and near-far
resistance, which translates to significant increase in sys-
tem capacity.
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5. Conclusions
In this paper, a multiuser receiver scheme with SIC for
PUCCH in LTE system is discussed. An iteration method
is adopted to reduce multiuser interference. During each
iteration, the max-power user is detected, regenerated
and then subtracted from the composite signal before the
detection and cancellation of the next user. Simulation
results show that it improves the BLER performance for
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