Variable Gain Relay Research Articles

Performance analysis of mixed RF/FSO systems with STBC users

Abstract In consideration of either fixed or variable gain relay scheme, end-to-end performance analysis of a dual-hop relay system over the asymmetric links composed of both radio-frequency (RF) and free-space optical (FSO) links with space time block coding (STBC) and two transmit antennas for the RF users is provided. In the analysis the RF links are modeled as the Rayleigh fading channel and the FSO link as the Gamma–Gamma fading channel with pointing errors. Closed-form expressions for the end-to-end outage probability (OP) and average symbol error probability (ASEP) in terms of the Meijer’s G functions are obtained. Based on the asymptotic expansion of the Meijer’s G function at high-SNR region, the asymptotic OP performance of the mixed RF/FSO systems with the fixed gain relay and misalignment FSO channels is discussed. Numerical simulations demonstrate that the system performance deteriorates when channel conditions become severe, and the RF users using STBC can achieve better communication quality than that of the regular RF users. In addition, the system with the fixed gain relay is better than that with the variable gain relay under the same average SNR in both the RF links and the FSO links.

Performance of Mixed RF/FSO With Variable Gain over Generalized Atmospheric Turbulence Channels

In this paper, we consider a free-space optical (FSO) communication scheme with the help of a variable gain relay where the links from the source to the relay are radio-frequency (RF) links while the links between the relay and the destination are FSO links with $\mathcal{M}$ -distribution. To mitigate the effects of multipath fading and atmospheric turbulence, we apply transmit diversity at the source and selection combining (SC) at the destination, while the relay is only equipped with single RF receive antenna for receiving signal and one aperture for relaying the information. With this setup, we first derive expressions for the outage probability, bit-error rate, and the average capacity. We further investigate the effect of pointing errors on the system performance.

Performance Analysis of Mixed Nakagami- $m$ and Gamma–Gamma Dual-Hop FSO Transmission Systems

In this paper, we carry out a unified performance analysis of a dual-hop relay system over the asymmetric links composed of both radio-frequency (RF) and unified free-space optical (FSO) links under the effect of pointing errors. Both fixed and variable gain relay systems are studied. The RF link is modeled by the Nakagami-m fading channel and the FSO link by the Gamma-Gamma fading channel subject to both types of detection techniques (i.e., heterodyne detection and intensity modulation with direct detection). In particular, we derive new unified closed-form expressions for the cumulative distribution function, the probability density function, the moment generating function (MGF), and the moments of the end-to-end signal-to-noise ratio (SNR) of these systems in terms of the Meijer's G function. Based on these formulas, we offer exact closed-form expressions for the outage probability (OP), the higher order amount of fading, and the average bit error rate (BER) of a variety of binary modulations in terms of the Meijer's G function. Furthermore, an exact closed-form expression of the end-to-end ergodic capacity is derived in terms of the bivariate G function. Additionally, by using the asymptotic expansion of the Meijer's G function at the high-SNR regime, we derive new asymptotic results for the OP, the MGF, and the average BER in terms of simple elementary functions.

Performance Analysis of Relay-Assisted All-Optical FSO Networks Over Strong Atmospheric Turbulence Channels With Pointing Errors

In this study, we consider a relay-assisted free-space optical communication scheme over strong atmospheric turbulence channels with misalignment-induced pointing errors. The links from the source to the destination are assumed to be all-optical links. Assuming a variable gain relay with amplify-and-forward protocol, the electrical signal at the source is forwarded to the destination with the help of this relay through all-optical links. More specifically, we first present a cumulative density function (CDF) analysis for the end-to-end signal-to-noise ratio. Based on this CDF, the outage probability, bit-error rate, and average capacity of our proposed system are derived. Results show that the system diversity order is related to the minimum value of the channel parameters.

高空平台多跳光链路可变增益中继性能分析

高空平台多跳光链路可变增益中继性能分析

Closed-Form Error Analysis of Variable-Gain Multihop Systems in Nakagami-m Fading Channels

In this paper, infinite integrals involving the product of Bessel functions of different arguments are solved in closed-form. The obtained solutions form a framework for the error probability analysis of wireless amplify and forward (AF) systems with an arbitrary number of variable-gain relays operating over independent but not necessarily identical Nakagami-m fading channels. Here we show that the error probability can be described by generalized hypergeometric functions, namely, Gauss's and Lauricella's multivariate hypergeometric functions. This work represents a significant improvement over previous contributions and extends previous formulas pertaining to dual-hop transmissions over identical Nakagami-m fading channels. Numerical examples show an excellent match between simulation and theoretical results.

Impact of Opportunistic Scheduling on Cooperative Dual-Hop Relay Networks

This letter advocates the performance of a multiuser relay network (MRN) equipped with a single amplify-and-forward (AaF) relay over Rayleigh fading environments. We derive new expressions for the cumulative distribution function (CDF) of the highest instantaneous end-to-end signal-to-noise ratio (SNR) taking into consideration the two cases of fixed gain relays and variable gain relays. Relying on these statistical results, we derive new expressions for the outage probability and symbol error rate (SER), both of which are obtained in exact closed form. Furthermore, we derive simple asymptotic outage probability and SER. Our asymptotic results confirm that opportunistic scheduling has no impact on the diversity order. We further prove that the array gain is what determines the SNR advantage of opportunistic scheduling over the single user scenario.

Convergence and Performance of Distributed Power Control Algorithms for Cooperative Relaying in Cellular Uplink Networks

We investigate an application of distributed power control (DPC) and opportunistic power control (OPC), respectively, to cooperative relaying systems that consist of multiple sources, multiple relays, and a single destination. Two types of relaying gains are considered for half-duplex amplify-and-forward (AF) relays: fixed and variable gains, respectively. Using half-duplex AF relays, remote users benefit by making the path loss smaller by two shorter paths. However, they lose bandwidth and suffer from additional interference due to amplification of the noise at the relays and the unnecessary signal from the relays, where power control plays an important role. We begin with redefining and extending the existing power control algorithms to be fit for the relaying environments and then, using standard techniques, construct the convergence, except when the number of variable-gain relays is even. With numerical investigation, we show the advantages and disadvantages of the power-controlled systems with relays in terms of the outage probability, the average transmit power consumption, and the transmission capacity. Simulation results show that DPC with relays achieves significant improvement in the outage performance and power consumption. On the other hand, in OPC, some negative effects arise by using the relays in the capacity since the relays increase the interference that severely affects the opportunistic capacity.

Exact Evaluation of Ergodic Capacity for Multihop Variable-Gain Relay Networks: A Unified Framework for Generalized Fading Channels

The closed-form expressions of the probability density function (pdf) and the moment-generating function for the exact end-to-end signal-to-noise ratio (SNR) of variable-gain relay networks are unknown for generalized fading models; thus, the exact evaluation for the ergodic capacity of these systems is cumbersome. In this paper, we develop a new unified framework for the exact ergodic capacity of multihop networks equipped with variable-gain relays, assuming that channel-state information (CSI) is available only at the receiving terminals. The resulting expression (which is based on the exact end-to-end SNR) is in the form of a single truncated infinite series and is valid for an arbitrary number of hops and for various fading models that are typically encountered in realistic scenarios. Furthermore, we show that the exact ergodic capacity of dual-hop variable-gain relay networks can be written in terms of the ergodic capacities of two equivalent single-input-single-output (SISO) channels and one single-input-multiple-output channel (SIMO), the closed-form expressions of which are already available for many commonly used fading/shadowing models. Finally, we show that there is an exact match between the Monte Carlo simulation and theoretical results over a whole range of per-hop average SNR.

The Role of Phase Compensation in Preventing Channel Dispersion in Variable-Gain Relay OFDM Systems

We show that the length of a composite channel is regarded up to the whole symbol duration for variable-gain relay (VGR), which generates the received power constant instantaneously. The enlarged channel length causes system burden, which is due to long-channel estimation, i.e., an increase in pilot overhead, a decrease in bandwidth efficiency, and a degradation of system performance. To prevent this effect and minimize the length of the composite channel, phase compensation of the source-relay (S-R) link channel should be added to the VGR, although this addition increases complexity. A system that adopts VGR with phase compensation performs better and has higher bandwidth efficiency.

Effect of Channel-Estimation Error on BER Performance in Cooperative Transmission

In this paper, a framework for evaluating the bit-error-rate (BER) performance of amplify-and-forward (AF) relay-assisted cooperative transmission is provided in the presence of imperfect channel estimation. First, the AF relaying is classified into two cases - a variable gain and a fixed gain. For each case, the closed-form average BER is derived by using the moment-generating function (MGF) of the effective output SNR, which quantifies the SNR penalty due to the noisy channel estimate. Moreover, the asymptotic BER bound is also provided at a high SNR regime for the insight of our approach. In particular, the effect of the feedforward delay on informing the source-relay channel estimate as to the destination is presented in the variable-gain relay. Numerical investigation shows that the analytic result makes an exact match with the simulation, and the comparative superiority between the fixed- and variable-gain relay is discussed.

On the Ergodic Capacity of Wireless Relaying Systems over Rayleigh Fading Channels

The ergodic capacities in Rayleigh fading of various wireless relaying systems with an arbitrary number of half duplex relays are analyzed, assuming channel state information is only known at the receivers. Closed-form analytical expressions for calculation of the ergodic capacities of these systems are derived. It is shown that systems with nonregenerative fixed gain relays achieve higher ergodic capacities than the corresponding systems with nonregenerative variable gain relays. A modified fixed gain relay, which incorporates the power constraint at the relays, is proposed. It is shown that systems with modified fixed gain relays slightly outperform the corresponding systems with nonregenerative variable gain relays and fixed gain relays at small signal-to-noise ratios, but attain almost the same ergodic capacities as systems with nonregenerative variable gain relays as the signal-to-noise ratio increases. In addition, the ergodic capacity of a hybrid system with both regenerative and nonregenerative relays is studied. Systems with regenerative relays employing decode-and-forward relaying offer higher ergodic capacities than the corresponding systems with any classes of nonregenerative relays or hybrid relays.