In this paper, we propose buffer-aided physical-layer network coding (PLNC) techniques for improving data transmission over cooperative networks. In particular, we develop buffer-aided PLNC schemes and relay pair selection algorithms for direct-sequence code-division multiple access (DS-CDMA) systems. We devise PLNC techniques based on optimal linear network coding matrices according to the maximum likelihood and minimum mean-square error design criteria in order to generate the network coded symbols that are sent to the destination. In the proposed buffer-aided PLNC schemes, relay pair selection algorithms are developed to obtain the relay pair and the packets in the buffer entries with the best performance and the associated link combinations are used for the data transmission. An analysis of the computational complexity of the proposed techniques along with their sum-rate analysis is carried out. Simulation results show that the proposed techniques significantly outperform previously reported approaches.

In this work, we propose a buffer-aided distributed spacetime coding (DSTC) scheme for cooperative direct-sequence codedivision multiple access systems. We first devise a relay selection algorithm that can automatically select the optimum set of relays among both the source-relay phase and the relay-destination phase for DSTC transmission according to the signal-to-interference-plus-noise ratio (SINR) criterion. Multiple relays equipped with buffers are introduced in the network, which allows the relays to store data received from the sources and wait until the most appropriate time for transmission. Simulation results show that the proposed buffer-aided DSTC scheme and algorithm outperform prior art.

In this work, we propose a cross-layer design strategy based on the parallel interference cancellation (PIC) detection technique and a multi-relay selection algorithm for the uplink of cooperative direct-sequence code-division multiple access (DS-CDMA) systems. We devise a low-cost greedy list-based PIC (GL-PIC) strategy with RAKE receivers as the front-end that can approach the maximum likelihood detector performance. We also present a low-complexity multi-relay selection algorithm based on greedy techniques that can approach the performance of an exhaustive search. Simulations show an excellent bit error rate performance of the proposed detection and relay selection algorithms as compared to existing techniques.