Cooperative communication system is an active field of research as it promises extended coverage in weak reception areas. Furthermore, with physical-layer network coding (PNC) in a two-way relaying (TWR) setting, the transmission rate of the system can be restored to unity,like in point-to-point transmission. This dissertation addresses the issue of signal detection in a two-phase TWR system that employs pilot-less orthogonal modulation and PNC operating in a time-selective Rayleigh fading environment. We first introduce a partial-coherent receiver for detecting (at the relay) the sum symbol of the uplink pilot-less orthogonal modulations. Through exploitation of the orthogonal property of the modulation, this receiver is 3 dB more power efficient over the standard non-coherent detector in the absence of pilot symbols. To further increase the receiver performance, we propose a decision feedback (DFB) receiver built upon the partial-coherent detector. The proposed DFB receiver provides another 6 dB improvement in power efficiency over the already impressive partial-coherent detector and attains a performance close to that of the coherent detector. It exploits the fact that when the uplink symbols from the users are different, the fading gains affecting these symbols can be separated and individually tracked at the relay. In essence, the proposed DFB receiver performs random channel sounding even though no actual pilots are transmitted. The channel estimates obtained this way can be used subsequently in a coherent detector to improve the reliability of the relay’s detected data. To further demonstrate the usefulness of the proposed DFB receiver, we compare it against a similar 2P-TWR system that employs differential PSK in the uplink and decision-feedback multiple-symbol differential detection at the relay. We found that the proposed orthogonal modulation system can attain a significantly lower bit-error-rate (BER) than its DPSK counterpart. For static fading and a BER of 0.001 , the signal-to-noise ratio gap between the two approaches is 1 dB in the binary case, and 8 dB in the quaternary case. These gaps increase further with time-selective fading. Based on the result obtained from the proposed DFB receiver, the decision feedback methodology has strong potential for applications in other similar systems, and it worth being studied further.
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Thesis advisor: Ho, Paul
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