Ahmad Suhail Salim

An asynchronous two-way relay system with full delay diversity in time-varying multipath environments

We consider design of asynchronous OFDM-based diamond two-way-relay (DTWR) systems in time-varying frequency-selective (doubly-selective) fading channels such as underwater acoustic (UWA) channels. In a DTWR channel, two users exchange their messages with the help of two relays. Most of the existing work on asynchronous DTWR systems assume only small relative propagation delays between the received signals at each node. However, in practical systems, significant delays may take place. Our proposed system is able to tolerate the delay even if it exceeds the length of the OFDM block which is almost inevitable in UWA channels. We provide analytical and numerical results to verify the advantages of the proposed scheme in mitigating large delays in different fading conditions.

A Delay-Tolerant Asynchronous Two-Way-Relay System over Doubly-Selective Fading Channels

We consider design of asynchronous orthogonal frequency division multiplexing (OFDM) based diamond two-way-relay (DTWR) systems in a time-varying frequency-selective (doubly-selective) fading channel. In a DTWR system, two users exchange their messages with the help of two relays. Most of the existing works on asynchronous DTWR systems assume only small relative propagation delays between the received signals at each node that do not exceed the length of the cyclic-prefix (CP). However, in certain practical communication systems, significant differences in delays may take place, and hence existing solutions requiring excessively long CPs may be highly inefficient. In this paper, we propose a delay-independent CP insertion mechanism in which the CP length depends only on the number of subcarriers and the maximum delay spread of the corresponding channels. We also propose a symbol detection algorithm that is able to tolerate very long relative delays, that even exceed the length of the OFDM block itself, without a large increase in complexity. The proposed system is shown to significantly outperform other alternatives in the literature through a number of specific examples.

Performance of Layered Steered Space---Time Codes in Wireless Systems

Layered Steered Space–Time Codes (LSSTC) is a recently proposed multiple-input multiple-output (MIMO) system that combines the benefits of vertical Bell Labs space–time (VBLAST) scheme, space–time block codes (STBC) and beamforming. In this paper, we derive the error performance and capacity of a single-user LSSTC system. The analysis is general enough to any layer ordering and modulation schemes used. In addition, the derived analysis is general for any LSSTC structure in which layers may have different number of antenna arrays and may be assigned power according to any power allocation. Furthermore, we analytically investigate the tradeoff between the main parameters of the LSSTC system, i.e., diversity, multiplexing and beamforming. Our results give recursive expressions for the probability of error for LSSTC which showed nearly perfect match to the simulation results. Results have also revealed the possibility of designing an adaptive system in which it was shown that combining beamforming, STBC, and VBLAST has better performance than VBLAST at high SNR range.

Modeling the interference of pulsed radar signals in OFDM-based communications systems

Spectrum coexistence between radar and communications systems has received considerable attention recently as it has presented itself as one solution to the increasing demand for higher data rates in communications systems. This paper aims to model the interfering radar signal at the receiver of a communications system that uses orthogonal frequency-division multiplexing (OFDM). As an example, a rectangular-shaped pulsed radar signal is considered, for which the probability mass functions (PMFs) are derived for the amplitude and phase of the radar interference. The derived PMFs are shown to accurately model the radar interference generated via Monte Carlo simulations.

Differential modulation for asynchronous two-way relay systems over frequency-selective fading channels

We propose two schemes for asynchronous multi-relay two-way relay MR-TWR systems in which neither the users nor the relays know the channel state information. In an MR-TWR system, two users exchange their messages with the help of NR relays. Most of the existing works on MR-TWR systems based on differential modulation assume perfect symbol-level synchronization between all communicating nodes. However, this assumption is not valid in many practical systems, which makes the design of differentially modulated schemes more challenging. Therefore, we design differential modulation schemes that can tolerate timing misalignment under frequency-selective fading. We investigate the performance of the proposed schemes in terms of either probability of bit error or pairwise error probability. Through numerical examples, we show that the proposed schemes outperform existing competing solutions in the literature, especially for high signal-to-noise ratio values. Copyright

Performance and tradeoff analysis of Layered Steered Space-Time Codes (LSSTC)

In this work we study a recently proposed multiple-input multiple-output (MIMO) system called the Layered Steered Space-Time Codes (LSSTC) that combines the benefits of vertical Bell Labs space-time (VBLAST) scheme, space-time block codes (STBC) and beamforming. The aim of this research is to investigate the analytical error performance of single user LSSTC. In addition, the tradeoff between several parameters of LSSTC is analyzed.

Exchange of Correlated Binary Sources in Two-Way Relay Networks Using LDPC Codes

We consider the problem of exchanging messages in two-way relay (TWR) systems when the sources are correlated binary sequences. In a TWR system, two users communicate simultaneously in both directions to exchange their messages with the help of a relay. Harnessing the fact that the users have access to their own non-compressed messages as side information, each user can compress its message according to the Slepian-Wolf coding strategy by using low-density parity-check codes, particularly, the syndrome approach. Through numerical examples, we show that the proposed scheme offers significant improvements in compression rates compared to the existing solutions in the literature.

ARQ-based scheme for coded wireless cooperative communications

Cooperative communications have received an increasing attention in the last few years due to the spread of wireless devices. While multi-antenna systems can improve the system performance greatly, it is difficult to implement antenna arrays on hand-held devices due to size, cost and hardware limitation. Cooperation allows the users to share their single-antenna devices to create a virtual multi-antenna system that benefits from the transmit diversity. On the other hand, low density parity check (LDPC) code is a class of linear block codes that can theoretically approach the Shannon limit. In this work, we propose an automatic-repeat-request (ARQ)-based LDPC coded cooperative system that provides an improvement in both error rate and throughput.

Optimized Power Allocation for Layered-Steered Space-Time Codes

Layered Steered Space-Time Codes (LSSTC) is a recently proposed multiple-input multiple-output system that combines the benefits of vertical Bell Labs space-time (VBLAST) scheme, space-time block codes and beamforming. We suggest a new downlink scheme employing LSSTC with asymmetric power allocation, by assuming that the user feeds the BS with the average signal-to-noise ratio per VBLAST layer through the uplink feedback channel. The motivation behind proposing such a system is to enhance the error performance by assigning power to the layers in an optimal manner. We refer to the system proposed as the optimal power allocation LSSTC (OPA-LSSTC). Our analysis is general such that it includes asymmetric layered systems in which each layer may have different number of antennas and also the power can be assigned to layers asymmetrically.

A Layered-Steered Space-Time Coded system with optimal power allocation

Layered Steered Space-Time Codes (LSSTC) is a recently proposed multiple-input multiple-output (MIMO) system that combines the benefits of vertical Bell Labs space-time (VBLAST) scheme, space-time block codes (STBC) and beamforming. We suggest a new downlink scheme employing LSSTC with optimal power allocation, by assuming that the user feeds the BS with the average SNR per VBLAST layer through the uplink feedback channel. The motivation behind proposing such a system is to enhance the error performance by assigning power to the layers in an optimum manner. We refer to the system proposed as the optimum power allocation LSSTC (OPA-LSSTC). We will show that the OPA-LSSTC can provide about a 3 dB gain compared to the equal power allocation case.