Fangxiang Wang

Distributed spatial–temporal precoding with limited feedback in MIMO multi-relay systems

Precoding techniques can be introduced into multi-relay systems due to the similarity between cooperative communication systems and traditional multi-input–multi-output (MIMO) systems. In this paper, a channel state information (CSI) feedback scheme based on the zero-forcing (ZF) relaying protocol is proposed at first, where the information of relaying channel and noise related to each relay node can be compressed into two positive real parameters. Then, based on the proposed feedback scheme, the singular-vector-based local temporal precoder is presented at the source node through two continuous transmitted vectors, which is termed as distributed spatial–temporal precoding (DSTP). Moreover, various spatial data rates can be conveniently supported by DSTP. Based on the analysis on DSTP, it is better that the number of data streams is not larger than the number of antennas equipped at the source node. The unitary DSTP with the proposed feedback scheme outperforms not only the close-loop direct transmission but also the simple ZF relaying method. Copyright

Performance analysis on conditional error ratio in HARQ transmission

In hybrid automatic repeat request (HARQ) process, a retransmission occurs on the condition that the first transmission failed. Multiple received packets are combined for detection at the receiver. Equivalent noise after combining is not additive Gaussian white noise and not independent on the transmitted signal. Unfortunately, the assumption of zero-mean-Gaussian and independent noise after combining is widely used in the analysis of HARQ, as well as other similar techniques. In this paper, conditional error ratio of the retransmission is accurately analyzed by taking the failure of the first transmission into account. Furthermore, approximate expressions of conditional error ratio are also presented for flexibility of analysis and application. Numerical results demonstrate that estimation error of the traditional estimation is much visible when channel quality of retransmission is worse than that of the first transmission.

Permutation Optimization in QRD Based Multi-Relay Systems

QR decomposition (QRD) based relay protocols can achieve both the distributed array gain and the intra-node array gain in multi-relay systems. Because the QRD is not unique if column permutation is utilized, the traditional QRD based relay protocol can be further improved. Row permutation of the forward channel and column permutation of the backward channel are proposed in this paper. Exhaustive and iterative search algorithms are presented to solve the permutation optimization problem. The iterative search algorithm with modified QRD is a good trade-off between capacity performance and complexity.

Effects of outdated channel state information in partial relay selection systems with multiple antennas at the destination

Cooperative relays have been proposed as effective solutions for radio range extension in mobile and wireless broadband networks. As such, a cooperative system is studied in this paper that consists of one single-antenna source node, L single-antenna relay nodes, and one M-antenna destination node. The impact of feedback delay to partial relay selection scheme is investigated, where a best relay is selected according to the instantaneous source-relay channel state information (CSI). The outage probability and symbol error rate (SER) are derived in closed-form expressions, and further simplified in exponential expressions to explicitly characterize the diversity order and array gain. Monte Carlo simulations verify our analytical results, which indicate that when CSI is outdated, the diversity order is reduced to 1 regardless of the accuracy degree. Results also show that better performance can be achieved when relays are placed closer to the source relative to the destination.

Distributed temporal precoding with a novel power allocation scheme in cooperative systems

In cooperative systems, precoding techniques can be utilized through temporal continuous symbols transmitted by the source node, which is termed as distributed temporal precoding (DTP). In this paper, we propose a novel power allocation scheme combining with DTP and the adaptive modulation and coding technique, which is based on the greedy algorithm and designed with practical modulation and coding schemes. Simulation results demonstrate that the proposed power allocation scheme achieves higher throughput than equal or water-filling power allocation schemes