Prabhat K. Upadhyay

Performance Evaluation and Optimization for Two-Way Relaying With Multi-antenna Sources

In this paper, we investigate the performance of an analog network coding (ANC)-based two-way relay system that employs beamforming at the multi-antenna sources. Specifically, we analyze the overall system performance by deriving an exact expression for the overall symbol error rate (OSER) and an upper bound expression for the ergodic sum-rate (ESR) over independent and non-identically distributed Nakagami- m fading channels. We also derive closed-form representations of these expressions in the asymptotic high signal-to-noise ratio (SNR) regime to provide useful insights into the system behavior, and the optimal power allocation and relay location. Moreover, we address the joint optimization problem of power allocation and relay location to minimize the OSER and to maximize the ESR. Our results highlight the influence of key system/channel parameters on the overall system performance.

Max-Max User-Relay Selection Scheme in Multiuser and Multirelay Hybrid Satellite-Terrestrial Relay Systems

We consider a hybrid satellite-terrestrial relay system that employs a multiantenna satellite to communicate with multiple users via multiple amplify-and-forward relays. We employ a max-max user-relay selection scheme to minimize the outage probability of the system. Considering both variable and fixed gain relaying protocols, we derive exact closed-form expressions of outage probability and their asymptotic behavior over independent and identically distributed fading channels for each hop. Numerical and simulation results validate our analysis and highlight the performance gains of the proposed scheme.

Outage Analysis of Cognitive Opportunistic Relay Networks With Direct Link in Nakagami-

This letter presents a unified framework for the outage performance analysis of cognitive opportunistic amplify- and-forward relay networks with a direct link using maximum ratio combining (MRC) and selection combining (SC) schemes in the presence of multiple primary receivers. Specifically, we derive unified closed-form expressions of outage probability and asymptotic outage behavior for both the schemes under Nakagami-m fading. We manifest analytically that the primary network affects only the coding gains of these schemes, and not the diversity gains that are found to be same in both cases. Moreover, we provide an analytical high signal-to-noise ratio (SNR) gap expression between the two schemes, which is shown to be independent of the primary network parameters. Numerical and simulation results validate our theoretical analysis.

m

In this study, the authors consider a cooperative spectrum sharing network with multiple primary users (PUs) and multiple secondary users (SUs). Hereby, one end-PU communicates bidirectionally with one of the best selected other end-PUs with the cooperation of the best selected SU using time division broadcast protocol. Based on an overlay approach, the selected SU allocates its partial power to relay the PUs’ signals and utilises the remaining power to transmit its own signal. The authors devise a low-complexity distributed PU–SU selection strategy to minimise the overall outage probability of the primary system. For Nakagami-m fading, the authors derive tight expressions of outage probability for both primary and secondary systems of the considered scheme. Further, by deriving their asymptotic behaviour at high signal-to-noise ratio, the authors highlight the performance gain accrued by cooperative and multiuser diversity along with that offered by the channels under Nakagami-m fading. Besides, the authors demonstrate the effective locations for the SUs to access the spectrum. Above all, the authors illustrate that the increase of the number of SUs is more desirable than the number of PUs to explore the spectrum sharing opportunities. Numerical and simulation results substantiate the effectiveness of the proposed scheme and the theoretical analysis.

Fading

In this paper, we investigate the impact of outdated channel estimates on the overall performance of a two-way multirelay system that employs a three-phase (3P) analog network coding (ANC)-based opportunistic relay selection (ORS) scheme under Rayleigh fading. Specifically, considering the imperfect estimation and the time variation of the fading channels, we formulate the 3P ANC-based ORS strategy for the best relay among multiple relays in terms of both outdated and imperfect channel state information (CSI). With such a practical CSI modeling, we derive a tight lower bound expression for the overall outage probability (OOP) and an upper bound expression for the ergodic sum-rate (ESR) of the considered scheme. In addition, by deriving an asymptotic outage behavior in the high signal-to-noise ratio (SNR) regime, we highlight the impact of outdated and imperfect CSI on the achievable diversity order. Moreover, based on the asymptotic analysis, we investigate three optimization problems, namely, optimal relay power allocation for fixed relay location, optimal relay location for fixed power allocation, and jointly optimal relay power allocation and location to minimize the OOP and to maximize the ESR of the considered system. Our results reveal that the optimal relay power allocation has a more significant impact on the performance when the CSI is imperfect, whereas the optimal relay location has a more noticeable impact when the CSI is outdated and unbalanced over the two hops. Furthermore, we show that the imperfect CSI can degrade system performance more severely than the outdated CSI, and it can even reduce the system diversity order to zero. The numerical and simulation results corroborate our theoretical analysis and findings and demonstrate that the 3P ANC-based ORS scheme can achieve better performance than its two-phase counterpart, depending upon the quality of the direct link, particularly in the medium-to-high-SNR regime.

Cooperative spectrum sharing in two-way multi-user multi-relay networks

We consider a two-way relay system where multi-antenna sources employ beamforming to communicate bidirectionally via a single-antenna relay with analog network coding over Nakagami-m fading channels. We address the problem of both separate and joint optimization of power allocation and relay location in order to minimize the overall system outage probability. Our results demonstrate that separate optimization of relay location is more significant than that of power allocation to improve the overall system outage performance. We further emphasize that the joint optimization of power allocation and relay location can provide noticeable outage performance improvement over unbalanced per-hop fading conditions and/or nonidentical antenna configurations.

Impact of Outdated Channel Estimates on Opportunistic Two-Way ANC-Based Relaying With Three-Phase Transmissions

In this letter, we analyze the performance of cognitive amplify-and-forward multirelay networks with active direct link in the presence of relay transceiver hardware impairments. Considering distortion noises on both interference and main data links, we derive tight closed-form outage probability expressions and their asymptotic behavior for partial relay selection (PRS) and opportunistic relay selection (ORS) schemes over Rayleigh fading channels. Both PRS and ORS schemes are found to attain full diversity for low target rates only, otherwise a relay cooperation ceiling (RCC) phenomenon occurs. We further illustrate that a perfect direct link can partially compensate for this RCC effect over high data rates for the two considered schemes. Numerical and simulation results validate our theoretical analysis.

Joint power and location optimization for analog network coding with multi-antenna sources

In this letter, we investigate the performance of a two-way multi-relay system employing three-phase analog network coding (ANC) with best relay selection over independent and non-identically distributed Nakagami-m fading channels. Specifically, we derive a tight closed-form expression for the overall system outage probability. We also derive an asymptotic outage expression in high signal-to-noise ratio (SNR) regime, from which the diversity order and diversity-multiplexing tradeoff are examined. In addition, we present an optimal relay power allocation to minimize the overall system outage. Numerical and simulation results validate the theoretical analysis and demonstrate the advantages offered by the considered scheme.

Cognitive Relaying With Transceiver Hardware Impairments Under Interference Constraints

In this paper, we investigate the performance of two-way amplify-and-forward (AF) relaying scheme over independent but not necessarily identically distributed (i.n.i.d.) cascaded generalized-K fading channels. We derive expressions for both upper and lower bounds on overall outage probability (OOP) for the considered two-way AF relaying system. Further, we derive tight closed-form approximate expressions for the OOP as well as for the average symbol error probability (ASEP) for the considered relaying system. We present simulation and numerical results to validate the proposed theoretical analysis, and to demonstrate the effects of various channel parameters on the overall system performance. Our results reveal that the overall system performance improves with increase in shadowing and/or fading parameters, as well as with decrease in number of cascading channels over two hops.

Performance of Three-Phase Analog Network Coding with Relay Selection in Nakagami-m Fading

We consider an end-to-end antenna selection (E2E-AS) scheme for a multiple-input multiple-output (MIMO) two-way relaying system with amplify-and-forward (AF) based relay terminal between two source terminals, each having multiple antennas. The proposed E2E-AS strategy involves selecting a single best antenna at each terminal of the system. Under a Rayleigh flat-fading environment, we present exact and asymptotic expressions of the overall system outage probability. We show that with single-antenna selection at each terminal, the underlying strategy achieves the same diversity order as the scheme using all the available antennas. We also highlight the impact of relay location and power allocation on the overall system outage performance. Finally, we provide numerical and simulation results to confirm the theoretical analysis and the performance of the proposed scheme.