Improved Joint Network-Channel Coding for the Multiple-Access Relay Channel

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1 2012 7th International ICST Conference on Communications and Networking in China (CHINACOM) Improved Joint Network-Channel Coding for the Multiple-Access Relay Channel Wei Fang, Chunjing Hu, Zhuo Sun, Shibo Hou Wireless Signal Processing and Network Lab Key Laboratory of Universal Wireless Communications, Ministry of Education Beijing University of Posts & Telecommunications, Beijing China Abstract This paper proposes a novel joint network- channel coding (JNCC) system based on improved distributed turbo coded (DTC) scheme for multiple-access relay channel, using decode-andforward protocol. The diversity of improved DTC scheme is that the two parity streams of the two RSC encoders of UE are converted into one stream by puncturing. In addition, we adopt the punctured parity streams of Convolutional Turbo Code (CTC) at the relay and BS, which takes advantage of the feature of two inputs for the decoded information bits from two mobile stations. Here, CTC acts as a network coding as well as channel coding between relay and BS. The performance of this communication system depends on the twice decoding at the BS according to the side information and CTC punctured parity information. The simulation results show that performance of proposed scheme overperforms the reference network coding scheme which separates channel decoding from network coding, and has 0.8dB improvement at a bit error rate (BER) of 10-3 in AWGN channel than the reference scheme. Keywords-component; joint network- channel coding (JNCC); distributed turbo coded (DTC); convolutional turbo code (CTC); multiple-access relay channel (MARC). I. INTRODUCTION Since the introduction of network coding in [1], it has recent received much attention because of its potential for improving network throughput and robustness. A physical layer network coding (PNC) approach is introduced recently from theoretical view [2], it made use of the additive nature of simultaneously arriving electromagnetic (EM) waves for network coding to further increase network throughput. Similar to channel coding decoding scheme of single channel, the scheme [3] based on PNC and MAC takes advantage of linearity of Turbo code and network coding to directly estimate network-coding codeword, and thus is easier to implement. In [4] joint channel-network coding is proposed for the multiple access half duplex relay channel. Rooted in the theory of network coding, XOR, architecture for wireless mesh networks was proposed [5]. According to that the destination nodes may have received packets intended for other nodes in previous transmission, Feng Xue and Sumeet Sandhu [6] proposed a simple adaptive coding scheme. A distributed Low-Density Parity-Check (LDPC) code [7] can be applied as a joint network-channel code for the MAC. Network coding for the time-division two-way relay channel (TWRC) [8] is studied as This work was supported by National Science and Technology Major Projects under grant 2012ZX and 2012ZX , and the BUPT Research Innovation Project under grant 2011RC0111. well. While existing network coding schemes for the TWRC perform hard network coding, Andreas Winkelbauer and Gerald Matz proposed a novel soft-information-based joint network-channel coding scheme. For the time-division twoway relay communication based on distributed turbo coding, where two users communicate to each other via a relay, relay conducts XOR and transmits the combined information to two users [9]. This scheme [10] adopts a multi-hop decode-andforward relay protocol such that two canonical subnetworks are relevant, namely broadcast channel with receiver side information (BC-RSI) and orthogonal multiple access channel with correlated sources and receiver side information (MAC- CS-RSI). A joint network and channel coding (JNCC) strategy is proposed by exploiting ARQ, RSI, and correlated sources. A joint network and channel coding (JNCC) strategy [11] with retransmissions is proposed for two canonical network elements, broadcast channel with receiver side information (BC-SI) and orthogonal multiple access channel with correlated sources (MAC-CS). A scheme [12] of joint network coding and channel coding for cooperative relay communication system is proposed. With the proposed joint network coding and channel coding, the soft probability information can be combined with the redundancy contained in the relay transmission by the operation relationship in network coding and diversity gain can be obtained. Alongside, Zhao and Valenti proposed a distributed turbo coded (DTC) system in [13], [14] for a 2-hop relay network to get additional interleaving gain. A distributed turbo code is formed rather than doing maximum ratio combining at the destination with the information obtained from the source and the relay. In DTC technique, the source encodes the information by a single recursive systematic convolutional (RSC) encoder and broadcasts the coded information which is received by both the relay and the destination. It has been shown that distributed turbo coding (DTC) can approach the capacity of a wireless relay network. The proposed scheme in [15] calculates and forwards the corresponding soft information instead of making a decision at the relay. Two improved DTC schemes [16], which executed superposition and puncture at the users, increase the decoding success rate at the relay. In this paper, we explain how to use JNCC based on turbo codes and CTC for the MARC. The novel scheme, due to the unique encoding structure at the user and relay, exploits the /12/$ IEEE

2 additional redundancy which is contained in the transmission of the relay to enhance the overall system performance. Simulation results reveal the proposed schemes outperform the existing DTC scheme [14]. To further demonstrate our scheme, the performance of this proposed is compared with the single link and conventional DTC scheme [16]. The rest of the paper is organized as follows. In section II, the system model is presented. Section III describes the improved JNCC scheme. Simulation results are presents in section IV and finally, Section V concludes the paper. II. SYSTEM MODEL A. System Setup We consider a typical multiple access relay channel with network coding depicted in figure 1.There are two mobile stations MS1 and MS2, one relay station and one base station. The relay station cooperates with the mobile stations to deliver messages. Firstly the mobile stations MS1 and MS2 broadcast their messages u1 and u2 to the relay station and base station.the relay station R receives and decodes the message from MS1 and MS2 and encodes them using the CTC encoder to a stream u3 and forwards it to the base station. III. THE PROPOSED JNCC FOR THE MARC In this section, we explain how the proposed scheme of JNCC can be performed with a turbo coding and CTC for the mobile station and relay, respectively. A. MS Channel Coding The punctured channel coding scheme [16] is adopted at the mobile station. The systematic information bits x L and the parity bits x P, which is composed by the punctured streams from RSC 1 and RSC 2 with 1/2 code rate, is shown in Fig. 2. Puncture procedure, which pattern is [1 0; 0 1], effectively improves the system code rate. These bits would be mapped to a transmitted symbol sequence using QPSK constellation and delivered to the relay and base station. For simplicity, we assume that all nodes use the same constellation of QPSK modulation. Fig. 2 channel coding at the mobile station Fig. 1. Block diagram of a two-hop multiple-access relay system B. Channel Model All the channels in this system are assumed to be additive white Gaussian noise (AWGN) channels; the received samples can be moded as yi ai xi ni (1) Here, the channel factor a i is assumed to be 1.Noise n i of the four channels is zero mean complex Gaussian random variables with two sided power spectral density of N0/2 per dimension. The signal-to-noise ratio (SNR) is given by γ MB where we assume that both mobile stations have the same distance to the base station. And the SNR of mobile station relay links is given byγ MR =γ MB +15dB. We assume that the relay is close to the base station with SNRγ RB = γ MB +7dB. B. Relay Network coding At the relay the two received sequences are successively sent to the iterative turbo decoder as the decoding procedure shown in Fig. 3. The outputs of de-puncturing are passed into the turbo decoders. In addition, the two decoders at the relay iteratively exchange information between each other until the final decision is made by decoder 1. Fig. 3 channel decoding at the relay The decoded bits from two users will be re-encoded with the following CTC encoder as shown in Fig.4. To better improve the system code rate, we discard the systematic 723

3 information bits of CTC and puncture the parity bits with puncturing pattern [1 0; 1 0; 0 1; 0 1]. For instance, BS receives symbol s 1 from MS. y h s n (2) MB MB 1 MB The LLR [22] of Symbol s1 is calculated by 2 ( ymb hmbs ) MB 2 LLR ( s ) log P( y s ) 2 (3) Fig. 4 CTC as the network and channel coding Here is the structure of constituent encoder in Fig.5. where h 2 MB is the estimation of the channel state, is the variance of the noise, s is all the possible value of s 1, for QPSK, i=1~4. Then the log bit likelihood ratio (LLR) of bit j in s 1 is calculated by LLRb ( j) log P( ymb bj 1) / p( ymb bj 0) P( y s ) exp( LLR ( s )) log MB i i si ; bj 1 si ; bj 1 log P( y s ) exp( LLR ( s )) MB k k sk ; bj 0 sk ; bj 0 For a QPSK symbol, j, k=1~2. The series of LLRs are inputted to the CTC decoder. (4) Fig. 5 CTC Consituent encoder C. BS Joint Network and channel Decoding Since the first decoding procedures at BS work in a similar manner at relay, we simplify the introduction of this procedure and focus on the CTC JNCC. Fig. 7 CTC Decoding at the BS Fig. 6 Turbo Decoding at the BS In case of successful decoding at the first decoding, the following decoding is not necessary to continue. If not, the decoded systematic bits of the output of turbo decoding obtained in the side information are delivered to CTC decoding as A and B of CTC coder. Y1, W1, Y2, and W2 are generated from the output of de-puncturing of y RB. Moreover, to accomplish the cooperate decoding, we employ soft-input, soft-output MAP detector and demapper, which exports soft information which is presented as log-likelihood ratio (LLR) to CTC decoder. The possible gain offered by this method in the JNCC and MARC scenario will be presented and discussed in the next section. IV. SIMULATION RESULTS We provide simulation results comparisons from various DTC schemes in the same MARC scenario. All simulations are performed for the QPSK modulation and a frame size of 240 symbols for 1000 frames. There are 2 iterations in turbo decoder and 8 iterations in CTC decoder. As described in the section II, the SNR on MS-BS link isγ MB. The SNR per information of MS-RELAY link is set γ MR =γ MB +15dB,and that of RELAY-BS isγ RB =γ MB +7dB. 724

4 We compare the bit error rate (BER) between the source bits and decoded bits for all simulated systems. The first reference system is the improved DTC scheme [16] on MARC and the second reference system is single link. Fig.8 depicts the bit error rate (BER) of these schemes over the signal to noise ratio (SNR) in db. From the slope of the error rates curves we can see the diversity which is provided by the use of our proposed schemes outperforming other schemes. At the BER of 10-3 the proposed scheme achieve around 0.8 db SNR gain over the improved DTC scheme. Fig. 8 BER performance comparison of the improved DTC scheme, proposed scheme and single link. The above scheme is under the same overall system code rate. Furthermore, if the parity information bits are retained, and namely the relay transmits the complete CTC parity bits, the better performance by around 3dB superior to the first reference system can be attained as Fig.9. Fig. 9 BER performance comparison of the improved DTC scheme,unpunct proposed scheme and single link. V. CONCLUSION In this paper, we have presented a new joint channelnetwork coding scheme for MARC network based on the distributed turbo coding. Such a system could be used for the cooperative uplink for two mobile stations to a base station with the help of a relay. Simulation results confirm that the diversity gain can be achieved more efficiently on exploiting the redundancy by adopting the proposed scheme, offering about 0.8dB gain compared to the conventional one. REFERENCES [1] R. Ahlswede, N. Cai, S.-Y. R. Li, and R. W. Yeung, Network information flow, IEEE Trans. Inf. Theory, vol. IT-46, pp , [2] S. Zhang, S. Liew, and P. Lam, Physical layer network coding, in Proc. ACM MobiCom 06, pp , LA, USA, [3] A. Zhan and C. He, Joint design of channel coding and physical network coding for wireless networks", Proc. Int. Conf. Neural Netw. Signal Process., pp [4] A. Hatefi, R. Visoz and A. Berthet, "Joint channel-network coding for the semi-orthogonal Multiple Access Relay Channel," Proc. IEEE VTCFall' 10, Ottawa, Canada, Sept [5] S. Katti, H. Rahul, W. Hu, D. Katabi, M. Medard, and J. Crowcroft, XORs in the air: Practical wireless network coding, IEEE/ACM Trans. Networking, vol. 16, no. 3, pp , Jun [6] Xue F, Sandhu S, PHY-layer Network Coding for Broadcast Channel with Side Information, Proceedings of IEEE Information Theory Worksh op (ITW 07), Sep 2-6, 2007, pp: [7] C. Hausl, F. Schreckenbach, I. Oikonomidis, G. Bauch, Iterative Network and Channel Decoding on a Tanner Graph, In Allerton Conf. on Communication, Control, and Computing, Sept [8] Winkelbauer, A.Matz, G, "Soft-Information-Based Joint Network- Channel Coding for the Two-Way Relay Channel," In Network Coding (NetCod), 2011 International Symposium on,pp: 1-5, July [9] J. Hou, C. Hausl, and R. Koetter. Distributed Turbo Coding Schemes for Asymmetric Two-Way Relay Communication. In International Symposium on Turbo Codes and Related Topics, Sept [10] Qiang Li; See Ho Ting; Chin Keong Ho,"A Joint Network and Channel Coding Strategy for Wireless Decode-and-Forward Relay Networks", In Communications, IEEE Transactions on, pp: , January 2011 [11] Q. Li, S. H. Ting, and C. K. Ho, "Joint network and channel coding for wireless networks", IEEE Conf. Sensor, Mesh and Ad Hoc Communications and Networks, pp. 1--6, June [12] R. Yu, T. Wu, "Joint network coding and channel coding for cooperative relay communication system," Wireless Communications and Signal Processing (WCSP), pp. 1-4, Oct [13] B. Zhao and M. C. Valenti, Distributed turbo codes: towards the capacity of the relay channel, in Proc. IEEE VTC 03, fall, vol.1, Oct. 2003, pp [14] B. Zhao and M.C. Valenti, Distributed turbo coded diversity for relay channel, Electron.Lett., vol. 39, no. 10, pp , May [15] Y. Li, B. Vucetic, T. F. Wong, and M. Dohler, "Distributed turbo coding with soft information relaying in multihop relay networks," IEEE Journal on Selected Areas in Communications, vol. 24, no. 11, [16] M. A. Karim, J. Yuan, and Z. Chen, "Improved distributed turbo code for relay channels, " in Proc. IEEE VTC'09, Fall, Anchorage, AK, Sep [17] C. Hausl, and P. Dupraz, Joint network-channel coding for the multiple-access relay channel, 3rd Annual IEEE Communications Society onsensor and Ad Hoc Communications and Networks, Virginia,U.S.A.,vol. 3, pp , Sept [18] S. Zhang, Y. Zhu, S. C. Liew, and K. B. Letaief, Joint design of network coding and channel decoding for wireless networks, Proc. IEEE WCNC 2007, Hongkong, pp , Mar [19] C. Hausl and J. Hagenauer, Iterative network and channel decoding for two-way relay channel, in Proc. IEEE Int. Conf. Commun., June [20] C. Hausl and P. Dupraz, Joint network-channel coding for multipleaccess relay channel, in Proc. of International Workshop on Wireless 725

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