Physical Layer Network Coding In Two-Way Relay Channels

Cooperation communication (CC) is one of the indispensable key techniques formodern wireless/mobile communication system. As a new generation of CC, two-wayrelaying (TWR) has become a research hotspot for the wireless/mobile communicationsystem. With the help of physical layer network coding (PLNC), TWR is shown to beable to overcome half-duplex constraint and increase the spectral efciency signif-cantly. In this thesis, we conduct extensive research on the PLNC of TWR channels.Firstly, analysis and comparison of existing PLNC schemes are given under the samecriteria. We fnd the merits and demerits of these PLNC schemes and the gaps betweenthem and optimal bound. Then, we propose a new PLNC scheme which can approachthe optimal bound. Approximation capability is also analyzed. Moreover, we considerimplementation issues of the PLNC and propose an universal decoding frame at therelay based on adaptive PLNC. Lastly, we present a TWR transmission protocol withjoint full decoding and partial decoding design though introducing a suferable delay atthe relay bufer. The performance obtained by this protocol breaks through the existingoptimal bound of TWR system. Concretely, our work can be divided into four aspectsas follows.1. Based on the existing results, we still do not know among these TWR schemeswhich one can produce the best performance, and there is lake of a fair yardstick forwe to compare these TWR schemes. So, we study several practical relaying strategiesover TWR Gaussian channel under the same criteria. Bitwise XOR-based decode-and-forward (DF), symbol-level superposition-based DF, and amplify-and-forward (AF)are considered. We show that achievable rate pair of each TWR scheme is governed byasetoflinearinequalities. Asaresult, weobtainexplicit, analyticalexpressionsaswell as geometrical shapes of the rate regions for each scheme. Correspondingly, we alsoobtain their maximum sum-rates with or without proportional fairness constraint. Ouranalytical results provide a convenient and efcient way to evaluate the performance ofTWRatanychannelconditions. Numericalexamplesarealsopresentedtodemonstrateand compare the rate regions and maximum sum-rates of all considered schemes atdiferent relay locations.2. Since neither of existing TWR schemes can approach the capacity bound whenthe channels in the broadcast phase are asymmetric, we present a new PLNC scheme,called superimposed XOR. The new scheme advances the existing schemes by specif-ically taking into account the channel asymmetry as well as information asymmetryin the broadcast phase. We obtain its achievable rate regions over Gaussian channelswhen integrated with two known time control protocols in TWR. We also demonstratetheiraveragemaximumsum-ratesandservicedelayperformancesoverfadingchannel-s. Numerical results show that the proposed superimposed XOR achieves a larger rateregion than both XOR and superposition and performs much better over fading chan-nels. We further deduce the boundary of its achievable rate region of the broadcastphase in an explicit and analytical expression. Based on these results, we then showthat the gap to the capacity bound approaches zero at high signal-to-noise ratio.3. Due to that existing partial decoding realizations are designed for Gaussianchannels and with a static PLNC, we propose a new solution for joint network codingand channel decoding at the relay, called pairwise check decoding (PCD), for low-density parity-check (LDPC) coded TWR system over block fading channels. Theproposed PCD algorithm is universal for any adaptive PLNC mapping The main ideais to form a check relationship table (check-relation-tab) for the superimposed LDPCcoded packet pair in the multiple-access (MA) phase in conjunction with an adaptivePLNC mapping in the broadcast (BC) phase. Using PCD, we then present a partialdecoding method, two-stage closest-neighbor clustering with PCD (TS-CNC-PCD),with the aim of minimizing the worst pairwise error probability. Moreover, we proposethe minimum correlation optimization (MCO) for selecting the better check-relation-tabs. Simulation results confrm that the proposed TS-CNC-PCD ofers a sizable gainover the conventional XOR with belief propagation (BP) in fading channels. 4. Given the traditional TWR protocols with immediate forwarding, the transmis-sion rates in both directions are known to be limited by the hop with lower capacity,i.e., the min operations. To cancel the min operations in the TWR fading channels,we propose a new transmission protocol, named as alternative awaiting and broadcast(AAB).Theprocessfow,averageexchangesum-rateandaveragedelayoftheproposedAAB protocol are analyzed. Moreover, we derive an achievable ergodic sum-rate (ES-R) and the corresponding average delay of the AAB protocol base on the well-knownlattice codes. Compared with the delay of bit service in the source bufer, the delayin the relay bufer induced by the proposed AAB protocol is very small and negligi-ble. Numerical results show that1) the average delay of the proposed AAB protocol isbounded and less than about100time units, if the diference between two upload trans-mission rates is less than about97%of that between two download transmission rates,2) the proposed AAB protocol with lattice codes signifcantly improves the achievableESR with an average delay of only some dozen seconds at the relay node compared tothe traditional non-delay TWR protocols,3) the proposed AAB protocol with latticecodes approaches the new upper bound of the ergodic sum-capacity at asymptoticallylarge SNRs.