Outage-optimal opportunistic scheduling with analog network coding in multiuser two-way relay networks

EURASIP Journal on Wireless Communications and Networking, Dec 2011

This paper investigates the performance of an outage-optimal opportunistic scheduling scheme for a multiuser two-way relay network, wherein an analog network coding-based relay serves multiple pairs of users. Under a Rayleigh flat-fading environment, we derive an exact expression for cumulative distribution function (CDF) of the minimum of the two end-to-end instantaneous signal-to-noise ratios (SNRs) and utilize this to obtain an exact expression for the outage probability of such a greedy scheduling scheme. We then develop a modified scheduler that ensures fairness among user pairs of the considered system. By using a high SNR approximation of derived CDF, we present a simple closed-form expression for outage probability of the overall system and establish that a multiuser diversity of order equal to the number of user pairs is harnessed by the scheme. We also present an efficient power allocation strategy between sources and relay, subject to a total power constraint, that minimizes the outage probability of the overall system. Further, by deriving both upper and lower bound expressions for the average sum-rate of the proposed scheme, we demonstrate that an average sum-rate gain can also be achieved by increasing the number of user pairs in the system. Numerical and simulation results are presented to validate the performance of the proposed scheme.

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Outage-optimal opportunistic scheduling with analog network coding in multiuser two-way relay networks

Prabhat K Upadhyay 0 Shankar Prakriya 0 0 Department of Electrical Engineering, Indian Institute of Technology Delhi , New Delhi, 110016, India This paper investigates the performance of an outage-optimal opportunistic scheduling scheme for a multiuser two-way relay network, wherein an analog network coding-based relay serves multiple pairs of users. Under a Rayleigh flat-fading environment, we derive an exact expression for cumulative distribution function (CDF) of the minimum of the two end-to-end instantaneous signal-to-noise ratios (SNRs) and utilize this to obtain an exact expression for the outage probability of such a greedy scheduling scheme. We then develop a modified scheduler that ensures fairness among user pairs of the considered system. By using a high SNR approximation of derived CDF, we present a simple closed-form expression for outage probability of the overall system and establish that a multiuser diversity of order equal to the number of user pairs is harnessed by the scheme. We also present an efficient power allocation strategy between sources and relay, subject to a total power constraint, that minimizes the outage probability of the overall system. Further, by deriving both upper and lower bound expressions for the average sum-rate of the proposed scheme, we demonstrate that an average sum-rate gain can also be achieved by increasing the number of user pairs in the system. Numerical and simulation results are presented to validate the performance of the proposed scheme. 1 Introduction Cooperative relaying techniques have recently gained great research interest because of their potential in enhancing the throughput or reliability of wireless networks. Several schemes have been extensively studied in literature to achieve cooperative diversity utilizing the one-way relaying protocol [1]. However, the half-duplex constraint at the relays incurs a spectral efficiency loss in such schemes. Recent research has shown that such a loss can be effectively mitigated by exploiting the idea of network coding [2] in bidirectional communication scenarios [3-7]. Bidirectional cooperative relaying strategies facilitate information exchange between two users in either four, three, or two time phases via a half-duplex relay. The four-phase protocol follows the conventional approach by requiring two separate time phases for data flow in each direction and hence is spectrally inefficient. However, the bidirectional communication has been shown to be accomplished in even three phases in [3-7]. In the three-phase protocol (called physical layer network coding (PNC) [6] or time division broadcast (TDBC) [7]), the two users transmit successively in first and second phases, the relay then decodes both the data, applies network coding, and forwards the combined data to both users in the third phase. After canceling the self-interferences (as they are known by the respective users), the intended message can be received at each of the user terminals. Recently, a two-way relaying protocol [8,9] has emerged as a promising technique to mitigate the spectral efficiency loss of conventional half-duplex relaying systems. In this scheme, the two users communicate bidirectionally (in the absence of a reliable direct link) in just two time phases, namely the multiple access channel (MAC) phase and the broadcast Table 1 Selected values of channel variances over the two hops for i.ni.d. user pairs a2,k kK=1 0.2 0.8 0.4 0.9 0.4 0.2 0.9 0.1 0.7 0.2 0.3 0.6 0.8 0.5 0.2 0.4 channel (BC) phase. In the first phase (MAC), both users transmit their data simultaneously to the relay, and the relay broadcasts the processed signal to both users in the second phase (BC). When an amplify-andforward (AF) processing is applied on the superimposed signal received during the MAC phase at the relay, such a scheme is usually termed as analog network coding (ANC) [10-13]. The two-phase two-way relaying protocol has also been generalized to a multiuser scenario in which multiple pairs of users communicate bidirection-ally via one or more relays [8,14-18]. The authors in [8,14] considered several relays or antennas that orthogonalize multiple pairs by a distributed zero-forcing technique. A spread spectrum based interference management scheme wherein each pair shares a common spreading signature, and the relay uses a jointly demodulate-andXOR forward strategy is proposed in [15]. The information theoretic capacity for such a scheme is studied in [16] and [17] by considering a deterministic channel model and a Gaussian two-pair two-way full-duplex relay network, respectively. To combat interference at each user of such a system, the authors in [18] proposed different beamforming schemes with amplify-and-forward (AF) and quantize-and-forward (QF) strategies at the relay. However, to the best of our knowledge, a performance analysis exploiting multiuser diversity for this system has not been reported so far. Although the twophase two-way relay (...truncated)


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Prabhat K. Upadhyay, Shankar Prakriya. Outage-optimal opportunistic scheduling with analog network coding in multiuser two-way relay networks, EURASIP Journal on Wireless Communications and Networking, 2011, pp. 194, Volume 2011, Issue 1, DOI: 10.1186/1687-1499-2011-194