Getting ahead of the curve in wireless communications / Être à l’avant-garde en matière de communications sans fil

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    Optimisation of power allocation for asymmetric relay placement in multi-hop relay systems
    (Institution of Engineering and Technology, 2013-04) Dhaka, Kalpana; Mallik, Ranjan K.; Schober, Robert
    In this study, schemes for optimisation of power allocation (OPA) for asymmetric relay placement are presented for multi-hop communication in a Rayleigh-fading environment. For a decode-and-forward (DF) multi-hop communication system, expressions are derived for optimised power allocation based on symbol error probability (SEP) and global channel state information (GCSI). The analysis for OPA based on GCSI is extended to a hybrid combination of amplify-and-forward (AF) and DF relays. Analysis is done for two kinds of modulation schemes: M-ary phase-shift keying with coherent detection and orthogonal M-ary frequency-shift keying with non-coherent detection. Simulation results show that for a multi-hop system with asymmetric relay placement, power optimisation schemes perform better than the conventional equal power allocation scheme. In addition, power optimisation based on GCSI shows substantially improved performance compared with power allocation based on end-to-end SEP. Further, performance comparison is shown for increase in number of relay nodes in an AF and DF multi-hop system with and without power allocation. The performance of a DF system improves with increase in number of relay nodes whereas performance of an AF system degrades. Hybrid relaying provides an option to exercise switching between DF and AF so as to extract the maximum advantage of the two relaying schemes.
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    Joint power allocation and relay selection in energy harvesting AF relay systems
    (IEEE, 2013-04) Ahmed, Imtiaz; Ikhlef, Aissa; Schober, Robert; Mallik, Ranjan K.
    In this letter, we propose joint relay selection and power allocation schemes for maximization of the throughput of an amplify-and-forward (AF) cooperative communication system where the source and the relays are energy harvesting (EH) nodes. We formulate an offline optimization problem which can be solved optimally by the generalized Bender's decomposition. For real-time implementation with low computational complexity, we propose two suboptimal online power allocation schemes. The performance of the proposed schemes is evaluated via simulations.
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    Beamforming and combining in two-way AF MIMO relay networks
    (IEEE, 2013-07) Arti, M.K.; Mallik, Ranjan K.; Schober, Robert
    In this letter, we propose a beamforming and combining scheme for a two-way amplify-and-forward (AF) multiple-input multiple-output (MIMO) relay network. An approximate expression for the symbol error rate of the scheme in Rayleigh fading for M-ary phase-shift keying is obtained based on the moment generating function of the instantaneous received signal-to-noise ratio. The diversity order is also analytically derived. It is shown by simulations that the proposed scheme outperforms existing schemes for beamforming and combining in two-way AF MIMO relay systems. We also discuss beamforming and combining in a two-way AF MIMO relay system with estimated channel gains.
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    WLAN service providers' price competition with uncertainty in user demand
    (IEEE, 2013-03) Kumar, Abhinav; Mallik, Ranjan K.; Schober, Robert
    In this letter, uncertainty in users' network selection is modeled by a Markov chain. In the presence of such uncertainty, the price competition game of wireless local area network (WLAN) service providers (SPs) is analyzed and the existence of Bayesian Nash equilibrium is proved. Using the price of anarchy as a metric of efficiency for social welfare maximization, it is shown that an increase in competition does not result in significant losses in efficiency. Compared to a monopoly, an unregulated duopoly of WLAN SPs is recommended as it results in a more equitable distribution of surplus amongst the SPs and the users.
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    Sensing time and power optimization in MIMO Cognitive Radio Networks
    (IEEE, 2012-09) Moghimi, Farzad; Mallik, Ranjan K.; Schober, Robert
    In this paper, we investigate the sensing-throughput tradeoff in multi-antenna cognitive radio (CR) systems. Specifically, we optimize the sensing threshold, sensing time, and transmit power of a multi-input multi-output (MIMO) CR system for maximization of the opportunistic system throughput under transmit power, probability of false alarm, and probability of missed detection constraints. To this end, we propose a new transmission protocol which allows the CR user to simultaneously perform data transmission and spectrum sensing on different spatial subchannels. We formulate non-convex optimization problems for the optimal choice of the sensing threshold, sensing times, and transmit powers in each spatial subchannel for both single-band and multi-band MIMO CR systems. Since finding the global optimal solution of these problems entails a very high complexity, we develop two iterative algorithms that are based on the concept of alternating optimization and solve only convex subproblems in each iteration. Thus, the complexity of these algorithms is low, and we prove their convergence to a fixed point analytically. Simulation results show that the developed algorithms closely approach the global optimal performance and achieve significant performance gains compared to baseline schemes employing equal powers or equal sensing times in all subchannels.
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    Network coded multi-source cooperative communication in BICM-OFDM Networks
    (IEEE, 2012-09) Islam, Toufiqul; Nasri, Amir; Schober, Robert; Mallik, Ranjan K.; Bhargava, Vijay K.
    In this paper, we study a cooperative diversity scheme for wireless systems employing network coding and the combination of bit-interleaved coded modulation (BICM) and orthogonal frequency division multiplexing (OFDM). The considered system comprises multiple sources, one relay, and one destination. The relay decodes the signal received from all sources and performs network coding before forwarding the signal to the destination. We propose a simple cooperative maximum-ratio combining scheme for the destination which can successfully exploit the full spatial and frequency diversity offered by the channel for arbitrary numbers of sources and arbitrary linear modulation schemes. Furthermore, we propose techniques to reduce the signaling overhead and the decoding complexity at the destination. To gain insight for system design, we derive a closed-form upper bound for the asymptotic worst-case pairwise error probability and the diversity gain of the considered network coded cooperative BICM-OFDM system. These analytical results reveal the influence of various system parameters, including the number of sources, the free distance of the code, and the frequency diversity of the involved links, on performance. Based on the derived analytical results, we develop schemes for optimal relay placement and power allocation. Simulation results corroborate the derived analytical results and confirm the effectiveness of the developed optimization framework.
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    Relay subset selection and fair power allocation for best and partial relay selection in generic noise and interference
    (IEEE, 2012-05) Ahmed, Imtiaz; Nasri, Amir; Michalopoulos, Diomidis S.; Schober, Robert; Mallik, Ranjan K.
    Best relay selection (BRS) has received considerable attention in the literature as it makes efficient use of the system resources and achieves full diversity. Partial relay selection (PRS) is an alternative to BRS and performs relay selection based on local channel state information (CSI) only at the expense of a loss in diversity. Although, in practice, relays may be deployed in unfavorable environments exposing them to non-Gaussian impairments, existing analyses and design guidelines for BRS and PRS are limited to additive white Gaussian noise channels. In this paper, we analyze the error rate of BRS and PRS in the asymptotic regime of high signal-to-noise ratio (SNR) for amplify-and-forward (AF) relays and impairment by generic noise and interference. The derived analytical results are valid for Gaussian and non-Gaussian noises with finite moments, independent and non-identically distributed Rayleigh fading, and arbitrary linear modulation schemes. In order to reduce the signaling overhead required for CSI acquisition, we propose a relay subset selection scheme for BRS and PRS. Furthermore, to guarantee fairness in energy resource usage across the relays, we introduce a fair and flexible power allocation scheme with energy consumption constraints which does not affect the achievable diversity gain. The proposed relay subset selection and power allocation schemes only require knowledge of the average CSI of the links and certain moments of the noises impairing the relays and the destination.
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    Asymptotic performance of generalized selection combining in generic noise and fading
    (IEEE, 2012-04) Ahmed, Imtiaz; Nasri, Amir; Schober, Robert; Mallik, Ranjan K.
    In this letter, we investigate the performance of generalized selection combining in channels impaired by generalized fading and generic noise with finite moments. In particular, we derive accurate high signal-to-noise ratio approximations for the bit and symbol error rates of linear modulation schemes, which offer significant insight into the impact of the number of selected diversity branches, the type of fading, and the type of noise. Our analysis is applicable to all practically relevant types of fading including Rayleigh, Rician, Nakagami-q, and Nakagami-m fading, all types of noise with finite moments including Gaussian noise, Gaussian mixture noise, generalized Gaussian noise, and co-channel interference, and non-identically distributed fading and noise across the diversity branches. Our results reveal that while performance always improves with increasing number of selected branches for identically distributed noise, this is not necessarily true for non-identically distributed noise.
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    Performance analysis of decode-and-forward multi-hop communication : a difference equation approach
    (IEEE, 2012-02) Dhaka, Kalpana; Mallik, Ranjan K.; Schober, Robert
    The presence of multiple users between signal transmission source and destination enables division of long links into multiple shorter links; this reduces signal attenuation, extends battery life, and provides broader and cheaper coverage along with high spectral efficiency. Cooperation among multiple users communicating in chronological sequence enables the realization of high data rates as users act as both information source and relay. This paper examines benefits of multi-hop transmission and concludes it has significant advantages over direct signal transmission.
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    Asymptotic Performance of Lp-Norm MIMO Detection
    (2010) Ahmed, I.; Schober, R; Mallik, R K
    Full search L1-norm (FS-L1) and sphere decoding L∞-norm (SD-L∞) detectors have been previously proposed for multiple-input multiple-output (MIMO) systems as they entail a lower receiver complexity than the optimal L2-norm detector. However, the performance loss caused by application of these suboptimal detectors in additive white Gaussian noise (AWGN) has not been well investigated yet. In this paper, we analyze the asymptotic bit error rate of general FS-Lp and SD-Lp detectors in independent identically distributed Rayleigh fading channels. Our results are valid for all types of noise with finite moments including AWGN. We show that both FS-Lp and SD-Lp detectors achieve a diversity gain equal to the number of receive antennas independent of the metric parameter p and the type of noise. However, except for the conventional L2 case, the performances of FS-Lp and SD-Lp detectors are not identical and the performance differences may be several dB for large numbers of receive antennas. Also, our results show that p=2 is not optimal in non-Gaussian noise and L1 and L∞ detectors may result in large performance gains or degradations compared to L2 detectors depending on the type of noise.
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    Cooperative BICM-OFDM: Sub-carrier and power allocation
    (2010) Islam, T; Schober, R; Mallik, R K; Bhargava, V K
    In this paper, we consider amplify-and-forward cooperative diversity for wireless systems employing bit-interleaved coded modulation (BICM) and orthogonal frequency division multiplexing (OFDM). In the second hop, multiple relays transmit concurrently over disjoint sets of sub-carriers. We propose uniform and non-uniform sub-carrier allocation schemes, which guarantee full diversity if all involved channels have identical lengths. The non-uniform allocation scheme also guarantees full diversity if the involved channels have different lengths. In addition, we formulate a power allocation problem for minimization of an upper bound on the worst-case pairwise error probability of the considered cooperative diversity scheme and provide an efficient solution using geometric programming. Simulation results confirm the effectiveness of the proposed sub- carrier and power allocation schemes.
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    Network Coded Cooperative BICM-OFDM
    (2011) Islam, T; Nasri, A; Schober, R; Mallik, R K; Bhargava, V K
    In this paper, we study a cooperative diversity scheme for wireless systems employing network coding and the combination of bit-interleaved coded modulation (BICM) and orthogonal frequency division multiplexing (OFDM). The considered system comprises multiple sources, one relay, and one destination. The relay decodes the signal received from all sources and performs network coding before forwarding the signal to the destination. We propose a simple cooperative maximum-ratio combining scheme for decoding at the destination which can successfully exploit the full spatial and frequency diversity offered by the channel for arbitrary number of sources and arbitrary modulation schemes. We derive a closed-form upper bound for the asymptotic worst-case pairwise error probability and the diversity gain of the considered network coded cooperative BICM-OFDM scheme. Simulation results corroborate the derived diversity gain and show the effectiveness of a low complexity decoding scheme.
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    Analysis and relay placement for DF cooperative BICM-OFDM systems
    (2011) Islam, T; Nasri, A; Schober, R; Mallik, R K
    In this paper, we study a decode-and-forward cooperative diversity scheme for wireless systems using the combination of bit-interleaved coded modulation (BICM) and orthogonal frequency division multiplexing (OFDM). We propose a simple cooperative maximum-ratio combining scheme for the destination which can successfully exploit the full spatial and frequency diversity offered by the channel. Taking possible decision errors at the relay into account, we develop a closed- form upper bound on the asymptotic worst-case pairwise error probability (PEP) of the considered cooperative diversity system which provides insight into the influence of various code and channel parameters on performance. The PEP expression is then exploited for optimal relay placement. Simulation results corroborate the derived analytical results regarding the diversity gain of the system and confirm the effectiveness of the proposed relay placement scheme.
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    Amplify-and-Forward Cooperative Diversity for BICM-OFDM Systems
    (2010) Islam, T; Schober, R; Mallik, R K; Bhargava, V K
    In this paper, we propose a novel amplify-and- forward cooperative diversity scheme for wireless systems using the combination of bit-interleaved coded modulation (BICM) and orthogonal frequency division multiplexing (OFDM). In the proposed scheme, multiple relays transmit concurrently over disjoint sets of sub-carriers. A careful joint design of the sub- carrier allocation at the relays and the interleaver at the source ensures that the proposed scheme can exploit the maximum spatial and frequency diversity offered by the wireless channel. An upper bound on the asymptotic worst-case pairwise error probability of the proposed cooperative diversity scheme is developed which provides insight into to the influence of various code and channel parameters on performance. Simulation results corroborate the analytical results regarding the diversity gain of the system and confirm the effectiveness of a newly developed relay selection criterion.
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    Hybrid Coherent/Energy Detection for Cognitive Radio Networks
    (2010) Moghimi, F; Schober, R; Mallik, R K
    Cognitive radio (CR) networks require reliable spectrum sensing techniques in order to avoid interference to the primary users of the spectrum. Most existing spectrum sensing techniques are based on simple energy detection. However, in practice, the signals transmitted by primary users usually also contain known symbol such as pilots and preambles for synchronization and channel estimation purposes. Coherent, correlation based spectrum sensing techniques can exploit these known symbols but waste the energy contained in the data symbols. In this paper, we propose a hybrid spectrum sensing scheme which exploits both the pilot and the data symbols transmitted by the primary user. For the practically relevant low signal-to-noise ratio regime, we derive a locally optimal hybrid detection metric, which turns out to be a linear combination of an energy detection metric and a correlation metric. Simulation and analytical results confirm that the hybrid metric outperforms both energy detection and coherent detection even if the pilot positions are not known a priori and have to be estimated by the CR receiver.
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    SEP of a scaled selection combining scheme with decode and forward relaying for MPSK signaling
    (2010-07) Selvaraj, M D; Mallik, R K
    The performance analysis of a cooperative diversity system with decode and forward relaying has drawn considerable attention recently. The use of conventional maximal-ratio combiners does not suit well for a cooperative diversity scenario and the assumption of no detection errors at the relay is also practically hard to achieve. Hence, we propose a scaled selection combining scheme, where we scale the relay-to-destination links by a deterministic positive scale factor to incorporate the effect of the source-to-relay links. At the destination, we choose a link which has maximum scaled instantaneous signal-to-noise ratio (SNR), from a set which consists of instantaneous SNR of the source-to-destination link and the scaled instantaneous SNRs of the relay-to-destination links. For a flat Rayleigh fading environment with statistically independent links, the end-to-end symbol error probability (SEP) of this scheme for M-ary phase-shift keying is derived in closed form. We also give a method of obtaining the optimum scale factor that minimizes the end-to-end SEP. The analysis is validated through extensive simulations.
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    Single-Relay Cooperative Diversity with Scaled Selection Combining
    (2011) Selvaraj, M D; Mallik, R K
    For a single-relay cooperative diversity system employing the decode-and-forward protocol in Rayleigh fading, a scaled signal-to-noise ratio-based selection combining scheme, which uses a deterministic scale factor for link selection to incorporate the effect of the source-to-relay link, is presented. Its error probability for binary phase-shift keying is derived in closed-form.
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    Scaled Selection Combining Based Cooperative Diversity System With Decode and Forward Relaying
    (2010) Selvaraj, M D; Mallik, R K
    Consider a cooperative diversity network that consists of a source, cooperating relays, and a destination. The relays cooperate with the source through the decode and forward protocol to transmit a data symbol from the source to the destination. In a conventional signal-to-noise ratio based selection combining scheme at the destination, the effect of the source-to-relay links is not accounted for when choosing one link from the set consisting of the source-to-destination and relay-to-destination links. To overcome this drawback, we modify the conventional scheme by including, in the selection process, a deterministic scale factor that incorporates the effect of the source-to-relay links. For statistically independent links that undergo flat Rayleigh fading, we derive, in closed form, the exact end-to-end symbol error probability (SEP) of this modified scheme, which we call a scaled selection combining scheme for M-ary phase-shift keying constellation. We also give a method of obtaining the optimum scale factor that minimizes the end-to-end SEP. A system designer should select the optimum scale factor to achieve the best performance of the scheme.
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    Hybrid Coherent/Energy Detection for Cognitive Radio Networks
    (2011) Moghimi, F; Schober, R; Mallik, R K
    Cognitive radio (CR) networks require reliable spectrum sensing in order to avoid interference to the primary users of the spectrum. Most existing spectrum sensing techniques are based on simple energy detection. However, in practice, the signals transmitted by primary users often also contain known pilot symbols for synchronization and channel estimation purposes. Coherent correlation based spectrum sensing techniques can exploit these known symbols but fail to utilize the energy contained in the data symbols. In this paper, we propose a hybrid coherent/energy detection scheme for spectrum sensing which exploits both the pilot and the data symbols transmitted by the primary user. Since the complexity of the globally optimal hybrid detection metric is very high, we develop a simple locally optimal hybrid metric, which turns out to be a linear combination of an energy detection metric and a correlation metric. We derive the probabilities of false alarm and missed detection of the proposed hybrid detector, and investigate the asymptotic behavior of both quantities for low signal-to-noise ratio and large sample size assuming that the CR network is optimized using a Neyman-Pearson framework. Simulation and analytical results confirm that the hybrid metric outperforms both energy detection and coherent detection even if the positions of the pilot symbols are not known a priori.
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    Performance Analysis of a Multi-Relay Cooperative Diversity Network with Decode and Forward Relaying
    (IEEE, 2010-01) Selvaraj, M D; Mallik, R K
    Consider a multi-relay cooperative diversity network consisting of a source, N relays, and a destination. For the transmission of a data symbol from the source to the destination, the re- lays cooperate with the source through the decode and forward protocol. All the links are statisti- cally independent and undergo flat Rayleigh fad- ing. Conventional instantaneous signal-to-noise ratio based selection combining at the destina- tion chooses one link from the set consisting of the source-to-destination link and the N relay-to- destination links. However, this scheme does not account for the effect of the source-to-relay links in the error performance. To overcome this draw- back, we present here a scaled selection combining scheme, which accounts for this effect through a deterministic scale factor. The end-to-end symbol error probability (SEP) of this scheme with binary phase-shift keying is derived in closed form. We also give a method of obtaining the optimum scale factor that minimizes the end-to-end SEP.