2012 International Conference on Computer Technology and Science (ICCTS 2012)
IPCSIT vol. 47 (2012) © (2012) IACSIT Press, Singapore
DOI: 10.7763/IPCSIT.2012.V47.77
A New Design of Receiver for LTE PUCCH FORMAT 2
Fatang Chen+, Taotao Liang and Shaoxu Wu
School of Communication and Information Engineering,
ChongQing University of Posts and Telecommunications, Chongqing, China
Abstract. In this article, a new method to design the receiver for PUCCH Format 2 is proposed for
performance optimization in a LTE system. This method based on an LS estimator and linear interpolation is
blind decoding procedure.
And it adopts metric comparison to compare errors from each branch and selects optimal metric. Through the
simulation, we find that the performance of new receiver for PUCCH Format 2 is better than that of conventional
receiver.
Key words: Receiver; LTE; PUCCH; blind decoding
1. Introduction
Long Term Evolution (LTE) launched by 3GPP (3rd.Generation Partnership Project) is satisfied with requirement for
4G. Link adaptation in LTE adjusts the transmitted information data rate (modulation scheme and channel coding rate)
dynamically to match the prevailing radio channel capacity for each user [1]. Channel quality indicator (CQI) is important
parameter for link adaptation and representing the recommended modulation scheme and coding rate that should, preferably,
be used for the downlink transmission. In uplink, periodic channel quality indicator reports are delivered using the PUCCH
format 2 by UE.
When transmitting a channel quality indicator at the same time as a hybrid-ARQ acknowledgement (ACK), for normal
cyclic prefix, the second reference symbol in each slot is modulated by ACK/NAK symbols. However, the ACK/NAK
symbols on the reference signal are unknown to eNodeB[2].In [3], the conventional receiver for PUCCH Format 2 consists of
a Least Square(LS) and linear interpolation, and decoding the acknowledgement bit(s) modulated onto the second reference
symbol using the first reference symbol for channel estimation [4].The design for conventional receiver is simple and
beneficial to realization of engineering[9]. This approach works well for low to medium Doppler frequencies; however, for
higher Doppler frequency, the performance of the conventional receiver is declining very serious.
In this paper we propose a new receiver to reduce the ICI of LTE PUCCH Format 2 in advance using Rayleigh fading
channel. Simulation results demonstrated that the receiver based on blind decoding is suitable for high speed environment.
The ergodic theory is adopted in receiver algorithm which considers all the HARQ information of PUCCH Format 2.In the
end, it adopts metric comparison to compare errors from each branch and selects optimal metric. However, this method
increases the computational complexity. So, in the future, we should investigate the new receiver with Low Complexity.
The paper is organized as follows. In Section II, the system model of PUCCH LTE Format 2 is described. In Section III,
the receiver algorithm is detailed. Simulations results and analysis are provided in section IV and the paper is concluded in
section V
+
Corresponding author.
E-mail address: chenfatang@cqcyit.com.
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2. System Model
2.1.
PUCCH Format 2
Each radio frame of uplink LTE consists of 10 subframes, and two consecutive slots of which is 0.5ms and
includes 7 SC-FDMA symbols are available in a subframe. The mapping is in principle done such that PUCCH
format 2 (channel-status reports) is transmitted on the edges of the uplink cell bandwidth and only one resource
block is available for PUCCH. For normal cyclic prefix, each slot in PUCCH format 2 has two SC-FDMA
symbols used for reference signals. When information of CQI and ACK/NAK are transmitted simultaneously and
separate coding, the CQI bits are converted into 10 SC-FDMA symbols by Special QPSK modulation. ACK/NAK
bits are modulated into two symbols which indices 6 for Slot 1 and 13 for Slot 2. Each of CQI symbol sequences
are phase rotations of the same cell-specific sequence, and phase-rotated sequences are orthogonal, this structure
can provide accuracy of information transfer [5]. The time-frequency transmission structure is depicted in Fig.1
Figure 1. Time-frequency transmission structure for PUCCH Format 2
2.2.
Signal Model of PUCCH
Figure 2. Signal model of PUCCH transceiver system
A Signal model of PUCCH transceiver system is illustrated in Fig. 2.We consider an LTE uplink control
channel with NT transmit antennas and N R receive antennas, and we consider multi-user of which has one
antenna. Transmit signal x(t ) is obtained after the add of CP, and The received signal r (t ) in Rayleigh fading
channel is given by
r (t ) = x(t ) ∗ h(t ) + z (t )
(1)
Where h(t ) is the continuous-time impulse response of the channel, * represents the convolution operation，
z (t ) is the additive noise. The noise is with zero mean and variance σ z2 ，Assuming that x(t ) is band-limited to
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⎡ 1 1 ⎤
, ⎥ ，the continuous-time signal x(t ) can be sampled at sampling rate TS such that the Nyquist criterion
⎢
⎣ 2TS 2TS ⎦
is satisfied.
Frequency domain signal X k [m] can be expressed as
X k [ m] =
1
⎡
N
n⎤
∑ x [n]exp ⎢⎣−2 jπ m N ⎥⎦
N
k
(2)
n =1
By applying the Fourier transform, the equivalent received signal in the frequency domain can be obtained,
⎡Xk[0] 0
⎡ Rk[0] ⎤ ⎢
⎢ # ⎥ = ⎢ 0 XK[1]
⎢
⎥ ⎢ #
#
⎢⎣Rk[N −1]⎥⎦ ⎢
0
0
⎣
⎤
0 ⎥⎥
"
%
# ⎥
⎥
" X[N −1]⎦
"
0
⎡ H[0] ⎤
⎢ H[1] ⎥ ⎡ Zk[0] ⎤
⎢
⎥ +⎢ # ⎥
⎥
⎢ # ⎥ ⎢
⎢
⎥ ⎢⎣Zk[N −1]⎥⎦
⎣H[N −1]⎦
(3)
In this paper, we restrict our analysis on the link that no spatial correlation exists, so each receive antenna is
independent. So (3) can be written as
Rk [m] = X k [m] • H [m] + Z k [m]
(4)
So in each of the receive antennas, there are N channel frequency parameters to be calculated, it is simple to
achieve channel estimate [1].
3. Receiver Processing
In high speed environment, the conventional receiver is still applicable. However, due to channel variations
within a slot, performance of the ACK/NAK detection degrades badly. Consequently, channel estimates of CQI
data symbols are obtained by reference signals embedded with ACK/NAK bits, which impacts the CQI
performance. In the following, first, a blind receiver structure for PUCCH format 2 is presented for high speed
environment, and then the block “Decode ACK/NAK and CQI” is described in detail.
3.1.
Receiver Structure
Figure 3. Blind Receiver Structure for PUCCH Format 2
A Blind Receiver Structure for PUCCH Format 2 is illustrated detailedly in Fig. 3.The algorithm can be
summarized as follows:
step 1） The received signal is processed by the “subcarriers DeMap” block to obtain the CQI RS and CQI Data
symbols.
step 2） CQI RS,CQI Data symbols and assumed ACK/NAK symbol Rm are three inputs of block for “Decode
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ACK/NAK and CQI”, Possible Decoded ACK/NAK,CQI bits and associated Metric can be obtained by
block for “Decode ACK/NAK and CQI”.
We consider m with 2 or 4 that is separately corresponding to PUCCH Format 2a or 2b.
step 3） Associated Metric I m is an important block in the receiver and calculated according to
M1
M2
I = ∑ Qm − H mWm + ∑ Qm − H mWm
2
m =1
2
(5)
m =1
Where Qm is the m-th received CQI data symbol, H m is the obtained channel estimate for the m-th CQI data
symbol and Wm is the m-th re-CQI data symbol. We define M1 and M2 as the number of all the CQI data
symbols for antenna 1 and 2.
step 4） Optimal metric will be selected by block of “Metric comparison”, and corresponding CQI and
ACK/NAK bits will be delivered to the high layer before the process of descramble.
3.2.
Decode ACK/NAK and CQI
Block “Decode ACK/NAK and CQI” is illustrated detailedly in Fig. 4.Block “Decode ACK/NAK and CQI”
represent the blind decoding procedure. CQI reference signals Y 2 embedded with ACK/NAK bits and Assumed
ACK/NAK symbol Rn are two inputs of LS estimator [6] from which we can get impulse response of the
channel H2, where
H 2 = Rn −1Y 2
(6)
The channel estimates for all CQI data symbols H can be obtained by linearly interpolating [7] the channel
estimates on the CQI RS H2 and H6.where H can be obtained by
H (k , l ) =
l6 − l
l − l2
× H ( k , l2 ) +
× H ( k , l6 )
l6 − l 2
l6 − l2
(7)
In (8), l is position of CQI data symbol, and l2 , l6 denote the position of CQI RS symbols. Subsequently, CQI
Data symbols are obtained by Signal detection with input of CQI Channel Estimates H and CQI Data symbols Y.
CQI Data symbols are demodulated to CQI bits. and ACK bits are demodulated according to TABLE I.
TABLE I.
CHART OF PUCCH ACK DEMODULATION
PUCCH Modulated symbol Demodulated
format
bits
1
0
2a
1
−1
00
1
−j
01
2b
j
10
11
−1
The demodulate CQI bits are re-modulated and re-mapped to resource grid. The re-modulated CQI data
symbols are compared to the received CQI data symbol to obtain a metric, which shows the reliability of the
corresponding blind decoding branch. Finally，The block “ACK/NAK and CQI Metric Comparison” selects the
branch with the optimal metric, and outputs the corresponding ACK/NAK bits and CQI bits. In this paper,
We consider that the optimal result is the minimum of Metric I.
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Figure 4. The block “Decode ACK/NAK and CQI.”
4. Computer Simulations
To demonstrate the effectiveness of our proposals, computer simulations will be provided in this section. The
simulation is under the extended typical urban model (ETU) which has the large multi-path delay and from LTE
specifications [8]. The simulation parameters are listed in Table II, obviously, there are totally 25 RBs in the
uplink and we consider two Doppler frequency cases with Fd = 20 0, 300HZ, respectively. Furthermore, the
performance of conventional receiver and blind receiver are both simulated. In our simulations, length of CQI and
ACK/NAK are 12 and 2 bits. The performance of Receiver are measured by BLER, we denote x label as SNR and
y label as BLER.
TABLE II.
SIMULATION PARAMETERS
Parameters
Carrier frequency
Bandwidth
FFT size
Cyclic prefix
MIMO configuration
Modulation
Channel model
Mobile speed
Length of CQI/ACK bits
Number of simulation
Values
2GHz
5MHz
2048
Normal
1T2R
QPSK
ETU
105/160 Km/h
12/2
5000
In Figure 5 and 6, we evaluate the performance of blind receiver and conventional receiver under the Doppler
frequency with 200HZ and 300HZ. Performance Curve of ACK/NAK and CQI have the similar trend, because
the recovery of CQI data symbols is based on reference signals embedded with ACK/NAK bits. Along with the
signal-to-noise ratio increases, Performance of curve CQI and ACK performance will be approached. The number
of CQI bits is more than that of ACK; as a result, Performance of CQI bits is more serious than that of ACK. In
Fig 5, as the signal-to-noise ratio increases, the performance of conventional receiver become more badly than
that of blind receiver. The performance difference becomes substantial by comparing Fig 5 with Fig 6.
Consequently, through the comparison, we find that blind receiver for PUCCH Format 2 is more suitable for high
speed environment. Each of CQI and ACK/NAK symbols is modulated to 12 subcarriers by frequency domain
spreading. Consequently, blind receiver still achieves better performance under low SNR.
5. Conclusions
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In this paper, a new design of the receiver for PUCCH Format 2 is proposed, which improves the performance
of conventional receiver. The method of blind decoding and Metric Comparison can minimize the ICI caused by
Rayleigh fading channel. Our simulation results indicate that the performance of the blind receiver is better than
that of conventional receiver [3], and more suitable for high speed environment.
6. Acknowledgment
This work is supported by Major Project of National Science and Technology in Development of TD-LTE
Wireless Integrated Test Instrument, and under Grant no.2009ZX03002-009.
7. References
[1] Stefania Sesia,Issam Toufik and Matthew Baker, “LTE-The UMTS Long Term Evolution:From Theory to Practice”,
2009 John Wiley & Sons,Ltd.ISBN:978-0-470-69716-0.
[2] Erik Dahlman, Stefan Parkvall, Johan Skold and Per Beming,” 3G evolution : HSPA and LTE for Mobile Broadband”,
2nd ed.2008 Elsevier Ltd. ISBN: 978-0-12-374538-5.
[3] Texas Instruments, ‘R1-080190: Embedding ACK/NAK in CQI Reference Signals and Receiver Structures’,
www.3gpp.org, 3GPP TSG RAN WG1, meeting 51bis, Sevilla, Spain, January 2008.
[4] LI Xiao-wen,PAN Di, “Implementation of LTE-TDD uplink channel estimation based on DSP ”,Journal of Chongqing
Univerity of Posts and Telecommunications(Natural Science Edition),vol.22,no.1,pp.14-18,Feb.2010.
[5] 3GPP TS 36.211 V9.0.0(2009-12), “Evolved Universal Terrestrial Radio Access (E-UTRA)Physical Channels and
Modulation”.
[6] Sinem Coleri, Mustafa Ergen, Anuj Puri,and Ahmad Bahai, “Channel Estimation Techniques Based on Pilot
Arrangement in OFDM Systems,” IEEE Transactions on Broadcasting, vol.48,no.3, pp. 223-229,Sept 2002.
[7] M.Hsieh and C.Wei, “Channel estimation for OFDM systems based on comb-type pilot arrangement in frequency
selective fading channels,”IEEE Trans. Consumer Electron, vol. 44, no. 1, Feb. 1998.
[8] 3GPP TS 36.104 V9.3.0(2010-03), “Evolved Universal Terrestrial Radio Access (E-UTRA) Base Station (BS) radio
transmission and reception.”
[9] SHEN Bi-chuan,Shen Min,ZHENG Jian-hong, “Analysis and study of channel estimation on performannce effect of
linear zero forcing receiver”, Journal of Chongqing Univerity of Posts and Telecommunications(Natural Science
Edition),vol.19,no.5,pp.1-4,Oct.2007
0
10
ACK/NAK BLER,Conv
CQI BLER,Conv
ACK/NAK BLER,Blind
CQI BLER,Blind
-1
BLER
10
-2
10
-3
10
-4
10
-16
-14
-12
-10
SNR
-8
-6
-4
Figure 5. Comparison of the blind receiver with the conventional receiver, for Doppler frequency 200HZ
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0
10
ACK/NAK BLER,Conv
CQI BLER,Conv
ACK/NAK BLER,Blind
CQI BLER,Blind
-1
BLER
10
-2
10
-3
10
-4
10
-16
-14
-12
-10
-8
-6
-4
-2
SNR
Figure 6. Comparison of the blind receiver with the conventional receiver, for Doppler frequency 300HZ
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