Search

Ultraviolet (UV) communication enables non-line-of-sight (NLOS) outdoor wireless connectivity and is particularly desirable to relax or eliminate pointing and tracking requirements of infrared links. In comparison to infrared counterparts, UV links are subject to relatively higher path loss as well as intersymbol interference resulting from frequency selective nature of the channel. In this paper, we propose the powerful combination of relay-assisted transmission and multi-carrier architecture based on orthogonal frequency division multiplexing (OFDM). Specifically, we consider a cooperative diversity system with orthogonal cooperation protocol and use DC-biased optical OFDM as the underlying physical layer. We consider both amplify-and-forward (AF) and detect-and-forward (DF) relaying. We investigate the error rate performance of the proposed relay-assisted OFDM UV system under consideration and demonstrate performance gains over point-to-point OFDM UV systems. We further determine optimal power allocation schemes to improve the performance. We also propose a variable-rate UV OFDM system and improve system throughput via bit loading.

Due to copyright restrictions, the access to the full text of this article is only available via subscription. Free space optical (FSO) links provide an efficient point-to-point wireless transmission solution. They are particularly useful for filling the connectivity gap between the radio frequency (RF) access network and the fiber optic-based backbone network. To address this combined use, there has been a growing interest in so-called mixed RF/FSO systems where RF transmission is used at one hop and FSO transmission at the other in a dual-hop configuration. In this paper, we investigate the performance of mixed RF/FSO systems along with a direct RF link where the FSO link and RF links, respectively, experience double generalized gamma turbulence with pointing error and Nakagami-m fading. Moreover, the effect of cochannel interference is considered at both relay and destination. Under the assumption of amplify-and-forward relaying and selection combining at the destination, we derive closed form expressions for outage probability and bit error probability in terms of Meijer's-G and two-variate Fox-H functions. Our analysis provides a generalized framework for several existing results, since our adopted turbulence and fading statistical models are the most general ones. Based on the high signal-to-noise-ratio analysis, we further provide a diversity gain analysis and discuss the bottlenecks in the mixed RF/FSO systems. Iran National Science Foundation (INSF)

Due to copyright restrictions, the access to the full text of this article is only available via subscription. This paper presents a comprehensive study on the problem of decoding complexity in a Multi-Input Multi-Output (MIMO) Hybrid Automatic Repeat reQuest (HARQ) system based on Space-Time Block Codes (STBCs). We show that there exist two classes of fast-decodable MIMO HARQ systems: independent and dependent STBC structures. For the independent class, two types of protocols namely, fixed and adaptive threshold-based are presented and their effectiveness in both computational complexity reduction and spectral efficiency preservation are discussed. For the dependent class, a fast Sphere Decoder (SD) algorithm with a low computational complexity is proposed for decoding process of HARQ rounds. Two new concepts are introduced to leverage the fast-decodable notion in MIMO HARQ systems. Simulation results show that the proposed fast-decodable MIMO HARQ protocols in both classes provide a significant reduction in the decoding complexity as compared with the original MIMO HARQ method.

Due to copyright restrictions, the access to the full text of this article is only available via subscription. In this paper, we propose high-rate distributed space-time-frequency codes (DSTFCs) to exploit maximum achievable diversity gains over frequency-selective fading channels. The proposed designs achieve full-rate for any number of cooperative nodes, and allow channel variations over multiple OFDM blocks within one DSTFC codeword. We analyze diversity gains of DSTFCs through both conditional and average pairwise error probability (PEP), and we proposes better design criteria based on one-side channel conditional PEP. We show that the difference between the frequency-selective channel orders of source-to-relay and relay-to-destination links may provide extra diversity advantages, thus additional performance gains. Through Monte-Carlo simulations, we demonstrate that proposed high-rate DSTFCs provide notable diversity advantages over existing designs. National Natural Science Foundation of China