Performance Of Asynchronous Physical Layer Network Coding Based On BP Algorithm

Physical-layer network coding(PNC) is a natural extension of network coding,which is an important field of network information theory, in the physical-layer ofwireless network. PNC takes full advantage of electromagnetic addition property.As a result, PNC can significantly improve the throughput of the wholecommunication system. With the release of wireless relay in the4G standard, relayis no longer confined to the traditional amplification or forwarding. Therefore, thefuture development prospect of PNC is quite impressive in wirelesscommunication. This paper studies asynchronous problems from signalstransmitted by multiple transmitters in PNC and focuses on the performance withsymbol offset and carrier phase offset at the receiver.In this paper, the research of asynchronous physical-layer networkcoding(APNC) is based on two-way wireless relay channel model. This paperstudies a general framework of APNC based on BP decoding scheme. Theframework can effectively respond to symbol and phase asynchrony in PNC.Unchannel-coded PNC makes full use of the samples processed by over-sampling receiver and achieves the decoding process based on BP algorithm at therelay. Simulation results show that BP algorithm can significantly reduce theadverse effects caused by asynchronous. Especially in BP-UPNC with QPSKmodulation, when there is half symbol offset, BER performance penalty can bereduced from about7dB down to less than1dB with different phase offset. Thepresence of symbol asynchrony only cause limited BER degradation due to phaseasynchrony in BP-UPNC.With the introduction of channel coding, relay decoding can also be achievedby channel-coded PNC(CPNC) based on BP algorithm. The channel coding, whichintegrate with PNC, is LDPC code in CCSDS standards for deep spaceapplications. Balancing system performance and implementation complexity, thispaper focuses on XOR-CD scheme, which is a disjoint channel-decoding andnetwork-coding scheme. Similar conclusion can be obtained by simulation results,symbol asynchrony can improve BER degradation casued by phase asynchrony.Also with the correlation of the channel coding, system performance is lesssensitive for symbol and phase asynchrony compared to the case without channelcoding. Both BP-UPNC and BP-CPNC in XOR-CD scheme, if symbol deviation canbe perfect controlled, it would have better robustness for phase asynchrony byintroducing appropriate symbol deviation in PNC. Simulation results of this papercan be well explained by diversity and certainty propagation.