Spatial-Modulated Physical-Layer Network Coding Based on Block Markov Superposition Transmission for Maritime Relay Communications

As an alternative to satellite communications,multi-hop relay networks can be deployed for maritime long-distance communications.Distinct from terrestrial environment,marine radio signals are affected by many factors,e.g.,weather conditions,evaporation ducting,and ship rocking caused by waves.To ensure the data transmission reliability,the block Markov superposition transmission(BMST)codes,which are easily configurable and have predictable performance,are applied in this study.Meanwhile,the physical-layer network coding(PNC)scheme with spatial modulation(SM)is adopted to improve the spectrum utilization.For the BMST-SMPNC system,we propose an iterative algorithm,which utilizes the channel observations and the a priori information from BMST decoder,to compute the soft information corresponding to the XORed bits constructed by the relay node.The results indicate that the proposed scheme outperforms the convolutional coded SM-PNC over fast-fading Rician channels.Especially,the performance can be easily improved in high spatial correlation maritime channel by increasing the memory m.

Log Likelihood Ratio-Based Relaying for Distributed Turbo Codes

A distributed turbo codes（ DTC） scheme with log likelihood ratio（ LLR）-based threshold at the relay for a two-hop relay networks is proposed. Different from traditional DTC schemes,the retransmission scheme at the relay,where imperfect decoding occurs,is considered in the proposed scheme. By employing a LLR-based threshold at the relay in the proposed scheme,the reliability of decoder-LLRs can be measured. As a result,only reliable symbols will be forwarded to the destination and a maximum ratio combiner（ MRC） is used to combine signals received from both the source and the relay. In order to obtain the optimal threshold at the relay,an equivalent model of decoderLLRs is investigated,so as to derive the expression of the bit error probability（ BEP） of the proposed scheme under binary phase shift keying（ BPSK） modulation. Simulation results demonstrate that the proposed scheme can effectively mitigate error propagation at the relay and also outperforms other existing methods.

Online search for UAV relay placement for free-space optical communication under shadowing

Unmanned aerial vehicle(UAV)relaying is promising to overcome the challenge of signal blockage in free-space optical(FSO)communications for users in dense urban area.Existing works on UAV relay placement are mostly based on simplified line-of-sight(LOS)channel models or probabilistic channel models,and thus fail to capture the actual LOS status of the optical communication link.By contrast,this paper studies three-dimensional(3D)online placement for a UAV to construct relay links to two ground users in deep shadow with LOS guarantees.By analyzing the properties of the UAV relay placement problem,it is found that searching on a plane that approximates the equipotential surface can achieve a good performance and complexity trade-off for a good placement of the UAV relay in 3D.Based on these insights,a two-stage online search algorithm on an equipotential plane(TOSEP)is developed for a special case where the equipotential surface turns out to be an equipotential plane.For the general case,a strategy called gradient projected online search algorithm on an approximated equipotential plane(GOSAEP)is developed,which approximates the equipotential surface with a perpendicular plane using the gradient projection method.Numerical experiments are conducted over a real-world city topology,and it is shown that the GOSAEP achieves over 95%of the performance of the exhaustive 3D search scheme within a 300-m search length.

Tighter bounds of multihop relayed communications in Nakagami-m fading

Closed-form bounds for the end-to-end performance of multihop communications with non-regenerative relays over Nakagami-m fading channels are investigated.Upper and lower bounds of the end-to-end signal-to-noise ratio(SNR)are first developed by using the monotonicity.Then,the probability density functions(PDFs),the cumulative distribution functions,and the momentgenerating functions(MGFs)of the bounds are derived.Using these results,the bounds for the outage and average bit error probability(ABEP)are obtained.Numerical and simulation results are executed to validate the tightness of the proposed bounds.

Asynchronous diversity receiving scheme and analysis on cooperative relay networks

The transmission antennas of cooperative systems are spatially distributed on multiple nodes, so the received signal can be asynchronous due to propagation delays. A receiving scheme for cooperative relay networks is proposed, multiple asynchronous signals are reconstructed at the receiver by forward and backward interference cancellation, which can obtain gains of cooperative transmission diversity with obvious delay and with no requiring timing synchronization or orthogonal channelization between relays. Analysis and simulation show that the bit error rate（BER） of the proposed scheme is similar to Alamouti code, and the scheme has the diversity order of orthogonal transmission scheme accompanied by minimal BER losses. It is demonstrated that the performance can be further improved by adding an error correcting code（ECC）.