A network-coded cooperative relaying aided free-space optical (FSO) transmission scheme is designed. The resultant multiple-source cooperation diversity is exploited by the relay to mitigate the strong turbulence-induced fading experienced in FSO channels. At the destination, an iterative multiple source detection algorithm is proposed in conjunction with a chip-level soft network decoding method. Our performance evaluation results using simulation analysis demonstrate that the proposed FSO multiple source detection is capable of approaching the single-user-bound for transmission over Gamma–Gamma turbulence channels. Also, the network-coded cooperative FSO scheme can achieve a significant BER improvement in comparison with conventional noncooperation schemes. Copyright © 2012 John Wiley & Sons, Ltd.

Network-coded multiple-source cooperation aided relaying for free-space optical transmission

Physical layer network coding (PNC) improves the throughput in wireless networks by enabling two nodes to exchange information using a minimum number of time slots. The PNC technique is proposed for two-way relay channel free space optical (TWR-FSO) communications with the aim of maximizing the utilization of network resources. The multipair TWR-FSO is considered in this paper, where a single antenna on each pair seeks to communicate via a common receiver aperture at the relay. Therefore, chip interleaving is adopted as a technique to separate the different transmitted signals at the relay node to perform PNC mapping. Accordingly, this scheme relies on the iterative multiuser technique for detection of users at the receiver. The bit error rate (BER) performance of the proposed system is examined under the combined influences of atmospheric loss, turbulence-induced channel fading, and pointing errors (PEs). By adopting the joint PNC mapping with interleaving and multiuser detection techniques, the BER results show that the proposed scheme can achieve a significant performance improvement against the degrading effects of turbulences and PEs. It is also demonstrated that a larger number of simultaneous users can be supported with this new scheme in establishing a communication link between multiple pairs of nodes in two time slots, thereby improving the channel capacity.

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Investigation on iterative multiuser detection physical layer network coding in two-way relay free-space optical links with turbulences and pointing errors

Diversity combining schemes have been extensively used in wireless and free space optical communication systems. The present invention of a system and method for free space optical communications where multiple gain combiner (MGC) provides a better performance as compared to selection combining (SC) and equal gain combining (EGC) and can also be used for wireless applications. It also gives a cost effective solution with a minimal degradation in quality when maximal ratio combining (MRC) is compared in terms of hardware complexity and implementation cost. There is no need for present channel state information. Also, Adaptive weight adjustment is not required in present invention. Therefore, the MGC of system and method disclosed in the present invention is a simple, cost effective and high performance diversity combiner.

System and a method for free space optical communications

We have investigated the performance and reliability improvement of cooperative free-space optical (FSO) communication over single input single output (SISO) system in this paper. The bit error rate (BER) analysis with quadrature phase shift keying (QPSK), gamma---gamma channel model and pointing error has been demonstrated for SISO and cooperative system. The performance improvement with different combining techniques in cooperative system for different channel environments has been shown in the paper. Markov models for reliability analysis of FSO systems in SISO and cooperative communication have been developed. We have obtained significant increase in availability and mean time between failures with cooperative communication over SISO model.

Bit error rate and reliability analysis of cooperative communication in free-space optical systems

Congested and expensive radio spectrum coupled with an insatiable demand for higher throughput in the last mile makes optical wireless an attractive alternative. The limitation of coverage area of optical wireless links is compensated by their ultrahigh bandwidth and unregulated spectrum. Cooperative relay strategies can further enhance the capacity and availability of optical wireless. A number of researchers have studied how the capacity of an optical relay channel is effected by fading processes. However, the capacity of an optical wireless relay channel (OWRC) has received little attention. This paper attempts to delineate the achievable capacity region of a Gaussian OWRC. Decode and forward, compress and forward, and amplify and forward relay strategies are used to determine the capacity bounds that constitute the achievable region. High and low signal asymptotes of the bounds are developed. The probability of outage under fading induced by turbulence and pointing error is also discussed. The results presented in this paper will facilitate the design of optical wireless relay networks in terms of choice of relaying strategy and input distribution.

Achievable capacity region of a Gaussian optical wireless relay channel

In this paper, we illustrate our attempt in an effective method of free space optical (FSO) communications. Firstly, a pulse-position modulated (PPM) optical interleave-division multiple-access (OIDMA) system is designed. Secondly, to alleviate the degrading effects of atmospheric turbulence, cooperative relay technique is adopted by OIDMA for achieving spatial diversity and robust transmission performance. Thirdly, to study the convergence behavior of the proposed relay-assisted optical PPM-OIDMA system, extrinsic information transfer (EXIT) charts are analyzed. By deriving the trajectories of the exchanged extrinsic information from EXIT charts, the time of iterations required for convergence can be easily predicted. The effects of scintillation, thermal noise, background noise and multi-user interference have been taken into consideration.

Jun Liu

Papers

Design and evaluation of an IDMA cooperative relay free-space optical system