A++ Random Access for Two-way Relaying in Wireless Networks

Two way relaying is a promising technique to improve the performance of wireless networks because it dramatically enhances the spectral efficiency comparing to traditional one way relaying. This technique is flourished by seminal works [1] [2] [3] [4], which introduce different two-way relaying schemes. Generally, these schemes can be divided into two categories: three-step schemes and two-step schemes. In three-step scheme also known as decode-and-forward (DF) relay, two end nodes in two way relay channel transmit their packets sequentially in the first two time slots and the relay node codes two received packets by applying bitwise XOR operation and broadcasts the coded packet to two end nodes in next slot. In two-step schemes also known as amplify-and-forward (AF) relay or denoise-and-forward (DNF) relay depending on the operation applied in the relay node, two end node send packets concurrently to the relay node in the first time slot and the relay node directly broadcasts the received waveform (AF relay) or perform physical layer network coding (PLNC) on received signals and broadcast the coded packet (DNF relay) to two end nodes in the second time slot. As shown in [3], two-step schemes are more efficient than three-step scheme especially in high SNR regime. Hence in this paper we only take two-step two way relaying schemes into consideration. Motivated by enhancing the performance of the network, many research papers investigate how to apply the two way relaying technique in wireless networks. Most of these papers focus on networks with regular topologies, such as star topology [5], layered topology [6], two-tier topology [7], a routing path in multihop networks [8], source-relays-source topology [9, 10] and etc. For general ad hoc networks, literature [11] provides a solution for applying two-way relaying in such networks based on transmission scheduling. However the optimal scheduling in this paper turns out to be NP-hard, and similar conclusion is further confirmed in [12]. Hence the scheduling based solutions for applying two-way relaying are more than complicated in general ad hoc network, and are not practical in realistic systems. Therefore to apply two way relaying in general networks in a more practical and more scalable way, random access schemes should be taken into consideration. Recently, a few papers deal with topics related to the design of random access MAC protocol to support two-way relaying. Majid Khabbazian [13] presents a MAC design for analog network coding (similar with AF relay). However the solution is on theoretical level and far from a practical design. In addition, Shiqiang Wang [14] proposes a distributed MAC protocol to enable the application of PLNC in the network. However, the protocol suffers from some defects. First, the protocol has no mechanism to guarantee the performance gain when there are no bi-direction data flows, namely when not both two end nodes have packets to each other the throughput would degrade to the level when no two way relaying is applied. This limits the usage of this protocol. Also, in the protocol the transmission is initiated by the relay node instead of transmitters of data. This requires that nodes have to provide their queue information to neighbor nodes, and this operation would result in the degradation of the performance due to the overhead and the increase of mean packet delay. To our best knowledge, no better solution schemes are published up to now. Hence a more practical and more efficient random access MAC protocol which supports two way relaying is high needed. To support random access MAC protocol, physical layer implementation of two way relaying should be revised. Some requirements of two way relaying such as the symbol synchronization and frame alignment are difficult to meet in random access protocol, and hence should be removed. Some previous works [2] [15] [16] [17] [18] [19] remove or relax these requirements and make the two way relaying technique more practical. However, these solutions have some imperfections respectively and are insufficient to serve as powerful physical layer schemes to support random access MAC protocol. In [2], analog network coding scheme is proposed without any synchronization requirement. However, the scheme is designed only for MSK modulation and cannot support QAM modulation which is more widely used. The OFDM based