Oussama Souihli

Joint feedback and scheduling scheme for service-differentiated multiuser MIMO systems

In this correspondence, we consider the problem of scheduling, in closed-loop multiuser MIMO systems, a large set of users with different Quality of Service (QoS) requirements, supplied by a single base station (BS) by means of Zero-Forcing Beamforming (ZFBF). We provide a power-and-bandwidth efficient feedback scheme in which only ZFBF-optimal users feedback their CSI, thereby reducing the number of required feedbacks and the computational burden of exhaustive search for best users at the BS. Afterwards, we show that conventional sum-capacity maximizing scheduling policies fall short to meet the requisites of delay-sensitive applications, and we provide appropriate scheduling scheme for such-constrained users. Simulation results demonstrate that the proposed scheduling approach successfully meets both average and instantaneous delay constraints of delay-sensitive applications.

Benefits of Rich Scattering in MIMO Channels: A Graph-Theoretical Perspective

In this letter, the impact of scattering on a multiple-input multiple-output (MIMO) propagation environment is studied based on a recently-proposed graph-theoretical channel model. An old result that rich scattering in MIMO propagation environments is a requisite for the channel matrix to have full rank (equivalently for the channel capacity to scale linearly with the number of antennas) is verified. Then, novel insights on the importance of scattering in MIMO channels from a graph-theoretical perspective are provided. By showing that the adjacency matrix of the whole propagation channel has same spectrum (eigenvalues) as the sub-graph inter-connecting the scatterers, it is inferred that all graph-theoretical properties of the propagation graph are contained in the scattering fraction of the channel. In particular, it is inferred that the channel model can be almost-surely reconstructed from the sole knowledge of the spectrum of the sub-graph interconnecting the scatterers. These results may find application in MIMO channel modeling in settings where full pilot-based channel estimation is infeasible or impractical, as in high-mobility scenarios.

Joint Feedback and Scheduling Scheme for Service-Differentiated Multiuser MIMO Systems

In this correspondence, we consider the problem of scheduling, in closed-loop multiuser MIMO systems, a large set of users with different Quality of Service (QoS) requirements, supplied by a single base station (BS) by means of Zero-Forcing Beamforming (ZFBF). We provide a power-and-bandwidth ef- ficient feedback scheme in which only ZFBF-optimal users feedback their CSI, thereby reducing the number of required feedbacks and the computational burden of exhaustive search for best users at the BS. Afterwards, we show that conventional sum- capacity maximizing scheduling policies fall short to meet the req- uisites of delay-sensitive applications, and we provide appropriate scheduling scheme for such-constrained users. Simulation results demonstrate that the proposed scheduling approach successfully meets both average and instantaneous delay constraints of delay- sensitive applications.

A case where noise increases the secrecy capacity of wiretap channels

In physical-layer security, communication secrecy is commonly achieved by providing the legitimate receiver a physical-layer advantage over the eavesdropper or, equivalently, by making the eavesdroppers channel more degraded than the legitimate receivers, a setting traditionally known as the degraded wiretap channel. There may be situations, however, where such advantages cannot be guaranteed. One example is the wireless channel where the legitimate receivers channel may be more degraded than the eavesdroppers owing to a deep fade (outage), additive interference at the edge of a network cell, etc. To our knowledge, such scenarios have not been remedied in the related literature. Thus, we propose in this work the first physical-layer security scheme that achieves communication in a setting where the eavesdropper has the physical-layer advantage over the legitimate receiver, a scenario that we call the reversely-degraded wiretap channel. Precisely, we show that by constraining the transmitters input to be discrete uniformly distributed (such as an on-off keying modulation) and by using nonlinear threshold-based signal detection, the legitimate receiver is guaranteed to have a positive secrecy capacity even though the eavesdropper has a better signal-to-noise ratio (SNR). The former requirement on the input distribution is to limit the capacity gains of the eavesdropper at high SNR. The latter requirement allows the communication channel to exhibit supra-threshold stochastic resonance, a phenomenon whereby noise improves signal detection and increases the mutual information. A numerical example is provided to confirm our claims.

Transparent Inband Feedback for Training-Based MIMO Systems

Recently, Echo-MIMO, a delay-free feedback scheme has been proposed for Closed-Loop MIMO systems, where the receiver echoes the received signal on the fly to the transmitter without any processing. While this reduced feedback latency allows for more use of the channel’s coherence time for data transmission, it comes at high power-and-bandwidth costs, as two MIMO transmissions are required in the feedback phase. In this paper, we present a feedback scheme that preserves the advantages of Echo-MIMO while requiring only one feedback transmission. The echoed signals are judiciously combined with the receiver’s signals such that their separation at the transmitter be lossless, and that no extra transmit power nor bandwidth be required. In addition, we highlight the estimation accuracy degradation in Echo-MIMO owing to the echoed noise, and analytically confirm the intuition that removing the noise prior to echoing the received signal provides better estimation than echoing the noisy received signal as is and later account for the noise effect upon echo reception. Simulation results show that the proposed scheme outperforms Echo-MIMO in terms of channel estimation accuracy and achievable capacity.

Typical set cognitive sensing

In cognitive radio (CR) cooperative sensing schemes, wireless sensor nodes deployed in the network sense the licensed spectrum and send their local sensing decisions to a fusion center (FC) that makes a global decision on whether to allow the unlicensed user transmit on the licensed spectrum, based on a decision (fusion) rule. k-out-of-N is widely used in the literature owing to its practical simplicity. Regrettably, it exhibits a tradeoff between the achievable probabilities of false alarm and miss detection, which could have consequent effects on the performance of CR. In this paper, based on the notion of typical sequences, we propose a novel fusion rule in which the false alarmandmiss detection probabilities can be simultaneously made as small as desired (asymptotically zero as the number of sensors goes to infinity).

Network MIMO in the Presence of Transmit Desynchronization

In this paper, we study the performance of Network MIMO in the presence of transmit desynchronization between the base stations (BS), in Rayleigh fading radio environments. We model the transmit desynchronization using lower shift matrices and we explain how a receiver can reliably detect space-time codewords while mitigating synchronization errors. Subsequently, we derive a closed form of the system capacity and bounds on the capacity scaling. Interestingly, we find out that maximum capacity scaling can be preserved even in the presence of desynchronization, provided that the latter be uniformly distributed over [0,T-1], where T is the codeblock length. Thus, our work suggests that, rather than synchronizing all BSs to a common clock, it would be more beneficial, from a capacity scaling perspective, to have their signal clocks independent from each other.

A Novel Capacity Outer Bound for the MIMO Gaussian Interference Channel and Application to Relay-Assisted Communications

A novel Genie-aided capacity outer bound is provided for MIMO Gaussian interference channels. Rather than requiring each side-informed receiver to cancel the interference from its received signal, it is suggested that each receiver views the interference as the useful signal undergoing a different channel, which we call an equivalent channel representation. Reflections on the existence and capacity enhancement of such equivalent channel representation are made. Subsequently, an application to relay-assisted communications is provided for which an achievable capacity region is determined. Finally, numerical examples are provided to demonstrate an increase of capacity owing to the proposed interference processing method.