Chadi Abou-Rjeily

Joint distributed synchronization and positioning in UWB ad hoc networks using TOA

In this paper, we describe a global distributed solution that enables the simultaneous performance of time synchronization and positioning in ultra-wideband (UWB) ad hoc networks. On the one hand, the proposed synchronization scheme basically relies on cooperative two-way-ranging/time-of-arrival transactions and a diffusion algorithm that ensures the convergence of clock parameters to average reference values in each node. Although the described solution is generic at first sight, its sensitivity to time-of-arrival accuracy imposes the choice of an impulse-radio ultra-wideband physical layer in the very context. On the other hand, a distributed algorithm coupled with this synchronization scheme mitigates the impact of non-line-of-sight ranging errors on positioning accuracy without any additional protocol hook. More particularly, the realistic UWB ranging error models we use take into account UWB channel effects, as well as detection noises and relative clock drifts. Then, it is demonstrated that a cooperative and distributed maximization of the log-likelihood of range estimates can reduce the uncertainty on estimated positions in comparison with classical distributed weighted least squares approaches. Finally, the proposed distributed maximum log-likelihood algorithm proves to preserve a reasonable level of complexity in each node by approximating asynchronously the positive gradient direction of the log-likelihood function. For both distributed synchronization and positioning algorithms, simulation results are provided to illustrate the relevance of such a solution.

Cooperative Diversity for Free-Space Optical Communications: Transceiver Design and Performance Analysis

In this paper, we investigate the cooperative diversity technique as a candidate solution for combating turbulence-induced fading over Free-Space Optical (FSO) links. In particular, a one-relay cooperative diversity scheme is proposed and analyzed for non-coherent FSO communications with intensity modulation and direct detection (IM/DD). The error performance is derived in semi-analytical and closed-form expressions in the presence and absence of background radiation, respectively. Results show the enhanced diversity orders that can be achieved over both Rayleigh and lognormal fading models.

Cooperative FSO Systems: Performance Analysis and Optimal Power Allocation

In this paper, we investigate the cooperative diversity technique as a candidate solution for combating turbulence-induced fading over free-space optical (FSO) links. In particular, we propose a novel cooperation strategy that is suitable for quantum-limited FSO systems with any number of relays and we derive closed-form expressions for the error performance of this strategy. In scenarios where the channel-state-information (CSI) is available at the different nodes, we propose an optimal power allocation strategy that satisfies the Karush-Kuhn-Tucker (KKT) conditions and that further boosts the performance of FSO networks. It turned out that this closed-form optimal solution corresponds to transmitting the entire optical power along the “strongest link” between the source and the destination nodes. A simple procedure is proposed for selecting this link and for distributing the power among its different hops.

UWB SIMO channel measurements and simulations

Ultra-wideband (UWB) technology has recently attracted the entire wireless community with its great potential for short-range and high data-rate communications. On the other hand, multiantenna (or multiple-input multiple-output) techniques are considered as an effective way to improve the link reliability and transmission rate. In this paper, both aspects are studied through a complete UWB single-input multiple-output (SIMO) channel sounding campaign. The time-domain channel sounder with its antenna array is firstly detailed. Secondly, measurement results will be used to describe interesting antenna array effects and channel behavior (like correlation results and underlying spatial diversity). Finally, in order to show the utility of spatial diversity in UWB systems, the performance of a SIMO system with a Rake receiver will be presented through bit error rate simulations based on measurement results.

Space–Time Coding for Multiuser Ultra-Wideband Communications

In this paper, we present the construction of full rate, fully diverse, and totally real space-time (ST) codes for ultra-wideband (UWB) transmissions. In particular, we construct two families of codes adapted to real carrierless UWB communications that employ pulse position modulation, pulse amplitude modulation, or a combination of the two. The first family encodes adjacent symbols and is constructed from totally real cyclic division algebras. The second family encodes the pulses used to convey one information symbol, and permits achieving high performance levels with reduced complexity. The first family of codes achieves only a fraction of the coding gain of the second one. Moreover, these coding gains are independent from the size of the transmitted constellation. For time-hopping multiple-access channels, the amplitude spreading code associated with the second family of codes is taken to be user-specific. In this case, a simple design criterion is proposed, and spreading matrices constructed according to this criterion permit reducing the level of multiple-access interference (MAI). Simulations performed over realistic indoor UWB channels verify the theoretical claims and show high performance levels and better immunity against MAI

On the Optimality of the Selection Transmit Diversity for MIMO-FSO Links with Feedback

We propose an optimal power allocation strategy for Multiple-Input-Multiple-Output (MIMO) Free-Space Optical (FSO) links with Intensity Modulation (IM) and Direct Detection (DD). The optimization is performed for shot noise limited systems in the presence of complete feedback. The derived analytical solution turns out to be the same as the selection transmit diversity scheme proposed for MISO-FSO systems corrupted by Gaussian noise. We also propose and analyze a novel transmission strategy for the limited-feedback case.

Achievable Diversity Orders of Decode-and-Forward Cooperative Protocols over Gamma-Gamma Fading FSO Links

In this paper, we consider the problem of relay-assisted free-space optical (FSO) transmissions with any number of relays. At a first time, we consider the quantum limited scenario and analyze the simple Decode-and-Forward (SDF) protocol where all symbols received at a certain relay are retransmitted to the destination. We also propose a novel Selective-DF (SelDF) protocol that can be implemented without requiring any kind of channel state information (CSI) neither at the transmitter nor at the receiver sides. We derive the diversity orders that can be achieved by SDF and SelDF over gamma-gamma fading channels and highlight the superiority of SelDF. We also prove that for some network configurations, SDF might achieve the same diversity order as SelDF. On the other hand, for most of the network configurations, the SDF protocol, that was studied extensively in the context of N_r-relays radio-frequency (RF) systems and one-relay FSO systems, is not suitable for FSO systems with more than one relay since it results in reduced diversity orders. At a second time, the above protocols are extended to FSO systems that are corrupted by background radiation. For SelDF, we propose a novel metric that captures the fidelity with which a certain message is detected at the relay thus determining whether this relay will participate in the cooperation effort or not.

Space-Time Codes for MIMO Ultra-Wideband Communications and MIMO Free-Space Optical Communications with PPM

In this paper, we consider the problem of space-time (ST) coding with pulse position modulation (PPM). While all the existing ST block codes necessitate rotating the phase or amplifying the amplitude of the transmitted symbols, the proposed scheme can be associated with unipolar PPM constellations without introducing any additional constellation extension. In other words, full transmit diversity can be achieved while conveying the information only through the time delays of the modulated signals transmitted from the different antennas. The absence of phase rotations renders the proposed scheme convenient for low- cost carrier-less multiple-input-multiple-output (MIMO) time- hopping ultra-wideband (TH-UWB) systems and for MIMO free-space optical (FSO) communications with direct detection. In particular, we propose two families of minimal-delay ST block codes that achieve a full transmit diversity order with PPM. Designate by n the number of transmit antennas and by M the number of modulation positions. For a given set of values of (n, m), the first family of codes achieves a rate of 1 symbol per channel use (PCU) which is the highest possible achievable rate when no constellation extensions are introduced. The second family of codes can be applied with a wider range of (n, m) at the expense of a reduced rate given by: R=1/n+n-1/n log2(M-1)/n log2(M).

On the Decode-and-Forward Cooperative Diversity with Coherent and Non-Coherent UWB Systems

In this paper, we extend the decode-and-forward cooperative diversity scheme to the context of impulse radio ultra-wideband (IR-UWB). We develop coherent and non-coherent schemes that exploit the spatial diversity in a distributed manner among the different terminals of a wireless network. These schemes are specific to IR-UWB and they take advantage of the pulse repetitions used to convey each information symbol. If Nf is the number of pulses per symbol, the coherent scheme is based on joint symbol and pulse coding and it achieves full diversity with any number of relays for Nf>1. The non-coherent scheme is based on pulse coding and it achieves full diversity with a maximum number of Nf-1 relays

Performance Analysis of Selective Relaying in Cooperative Free-Space Optical Systems

In this paper, we evaluate the performance of a selective-relaying (SR) protocol that is suitable for cooperative free-space optical (FSO) communications with any number of relays. The SR strategy is based on transmitting all information symbols either along the direct link between the source and the destination or along one of the indirect links via the relays. We derive closed-form expressions of upper-bounds on the average error probability that can be achieved over Rayleigh and lognormal fading channels. We also provide an asymptotic analysis for quantifying the diversity gain and reduction in fading variance that can be realized by the cooperative scheme. The performance analysis shows the superiority of the SR scheme over the strategy that corresponds to activating all available links.