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)] 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 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). 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 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 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 is estimated by the scheme proposed in [2], and the carrier frequency offset is estimated by a iteration method in [7]. Let , ˆkl denote the estimation of ,kl , 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. REFERENCES [1] 3rd Generation Partnership Project (3GPP), “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Rel-11),” Available: http://www.3gpp.org/ftp/specs/archive/36_series/36.211/. htm, Mar. 2013. [2] D. Wang, S. Z. Yang, Y. Liao and Y. Liu, “Efficient Re- ceiver Scheme for LTE PUCCH,” IEEE Communications Letters, Vol. 16, 2012, pp. 352-355. doi:10.1109/LCOMM.2012.012412.112251 Figure 3. SIC-parallel-receiver vs. parallel-receiver, scenario 2. [3] H. Yang, D. Astely, R. Baldemair and S. Falahati, “Semi-blind Multi-user Detection for LTE PUCCH,” Proceedings of Wireless Communications and Network- ing Conference, Budapest, Hungary, 2009, pp. 1-5. We consider a SIC-parallel-receiver in HST scenario 2. BLER of CQI of the SIC-parallel-receiver comparing with the parallel-receiver is shown in Figure 3. The par- allel-receiver with SIC performs much better over the parallel-receiver, providing a gain of around 1.3 dB and 3.4 dB when BLER achieves for 3 users and 6 us- ers respectively. The improvement is more pronounced with 6 users. It is clear that the gaps tend to be more sig- nificant for higher signal to noise ratio (SNR) values. This is because the limiting factor of performance at lower SNR is noise, whereas it is multiuser interference at higher SNR. The parallel-receiver with SIC in HST scenario 2 is also robust to the number of users. 3 10- [4] L. H. Yang, G. L. Ren, B. K. Yang and Z. L. Qiu, “Fast Time-Varying Channel Estimation Technique for LTE Uplink in HST Environment,” IEEE Transactions on Ve- hicular Technology, Vol. 61, No. 9, 2012, pp. 4009-4019. doi:10.1109/TVT.2012.2214409 [5] 3rd Generation Partnership Project (3GPP), “Evolved Universal Terrestrial Radio Access (E-UTRA); Multi- plexing and channel coding (Rel-11).” Available: http://www.3gpp.org/ftp/specs/archive/36_series/36.212/. htm, Feb. 2013. [6] M. Zhou, B. Jiang, W. Zhong and X. Gao, “Efficient Channel Estimation for LTE Uplink,” Proceedings of Wireless Communications & Signal Processing Confer- ence, Nanjing, China, 2009, pp. 1-5. 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 [7] X. N. Ren, G. L. Ren and C. H. Le, “Carrier Frequency Offset Estimation for PUCCH in High Speed Train Envi- ronment,” Proceedings of the 12th International Confer- ence on Electronic Packaging Technology and High Den- sity Packag in g, Shanghai, China, 2011, pp. 1-4.
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