Analysis Of Transmission Protocols For Two-Way Relaying Systems

Recently, two-way relaying has received increasing attentions for its ca-pability of providing high spectral efficiency, where two sources exchange in-formation with the help of a relay. There are mainly four existing protocols for amplify-and-forward-based two-way relay networks, namely, analog net-work coding protocol, time division broadcast protocol, opportunistic source selection protocol and adaptive relay-assisted/direct transmission protocol. For ANC, it is always supporting simultaneous bidirectional traffic even if the chan-nels are not strong enough for two flows. Therefore, the reliability of the sys-tems is degraded when channel conditions are bad, and only diversity order of one can be achieved since the disuse of the direct link. For TDBC, it exploits the direct link while needs three time slots to complete the transmission.The main contributions of this paper are summarized as follows:Firstly, an adaptive mode selection (AMS) protocol is pro-posed. We consider two modes to assist efficient transmission. These modes include i) two-way transmission mode and ii) one-way transmission mode. The proposed AMS protocol has the ability to adapt to the variation of the channels and simul-taneously improves the outage performance and the sum-rate of the considered network. The performance of the proposed protocol is theoretically derived. Particularly, a tight lower bound and an asymptotic expression in high SNR regime of the outage probability are derived. Based on these theoretical results, the proposed protocol is shown to achieve the diversity order of two. Simula-tion results are provided to verify the theo-retical derivations. The simulation results also show that the proposed protocol outperforms the existing protocols.Secondly, a low-complexity two-step source-relay selection protocol is proposed. Specifically, the best source is first selected based on the channel quality of the direct links, then the best relay is chosen by an Max-Min crite-rion. In this way, the implementation complexity of the proposed scheme is reduced, and only local CSI in the destination and relays is required instead of global CSI. The performance of the proposed scheme is theo-retically derived. Specifically, a tight closed-form lower boundis derived for the outage proba-bility of the considered system, from which asymptotic analysis is performed to characterize the outage behavior at high signal-to-noise ratio (SNR). Based on these analysis, it is shown that a diversity order of M+N can be achieved in the considered scenarios, where M and N are the number of source nodes and relay nodes, respectively. Our scheme achieves the maximal diversity order of M+N, while the traditional schemes can only achieve a diversity order ofN. Simulation results validate the theoretical derivations.Thirdly,we propose a new full-duplex (FD) scheme based on Alamouti space-time code for the two-way AF relay networks. For this scheme, we consider the systems that both the two sources and the relay operate in the full-duplex mode. And the residual self-interferences (RSI) of all the three nodes have been taken into consideration. The proposed scheme exploits the direct link between the two sources instead of neglecting it or considering it as interference. As a result, the proposed scheme has a much better outage performance than the conventional full-duplex two-way relay schemes. In ad-dition, we characterize the instantaneous capacity of the proposed Alamouti based scheme. In order to further improve the performance of the FD two-way relay networks, three hybrid half/full duplex two-way relay schemes are also proposed in this paper. These hybrid schemes eliminate the platform phe-nomenon of the outage probability caused by the residual self-interference and outperform all the previous HD and FD schemes. The theoretical analyses are given and the numerical results are provided to prove the superiority of the three proposed schemes.