Besma Smida

Near Shannon Limit and Low Peak to Average Power Ratio Turbo Block Coded OFDM

In this paper, we present an advanced solution for the long standing problem of large peak to average power ratio (PAPR) in orthogonal frequency division multiplexing (OFDM) systems. Although the design of low PAPR codewords has been extensively studied and the existence of asymptotically good codes with low PAPR is also proven, still no code has been constructed to satisfy all requirements. The main goal of the paper is to develop a coding scheme that not only generates low PAPR codewords, but it also performs relatively close to the Shannon limit. We achieve this goal by implementing a time-frequency turbo block coded OFDM. In this scheme, we design the frequency domain component to have a tightly bounded PAPR. The time domain component code is designed to obtain good performance while the decoding algorithm has reasonable complexity. Through comparative performance evaluation we show that utilizing the proposed method, we achieve considerable improvement in terms of PAPR while we slightly loose the performance compared to capacity achieving codes with similar overall block length.

Analysis of Network Coded HARQ for Multiple Unicast Flows

In this paper, we consider network coded (NCed) Hybrid-ARQ (HARQ) for multiple unicast flows. The main contribution of the paper is the derivation of a throughput expression for NCed HARQ with arbitrary number of users in i.i.d. channels. We apply the result to Rayleigh fading channels for incremental redundancy (IR) and chase combining (CC) based NCed HARQ. We verify the analytical approach with simulations and observe substantial SNR (or energy) improvements over regular ARQ with network coding as well as classical (H)ARQ. The SNR gains in the high and low throughput regimes are mainly due to the network coding and HARQ aspects, respectively. For low SNRs, NCed HARQ with IR surpass the CC performance.

MC-CDMA performance evaluation over a multipath fading channel using the characteristic function method

This letter presents an efficient and accurate performance evaluation method for binary MC-CDMA systems with deterministic signature sequences. This method is based on a formulation of the characteristic function, and does not resort to any assumption on the statistical or spectral behavior of the interference. The accuracy of the Gaussian approximation method is also considered.

Exact BER performance of asynchronous MC-DS-CDMA over fading channels [Transactions Letters]

In this contribution an accurate average Bit Error Rate (BER) formula is derived for MC-DS-CDMA in the context of asynchronous transmissions and random spreading sequences. We consider a flat Nakagami-m fading channel for each subcarrier. Our analysis is based on the Characteristic Function (CF) and does not rely on any assumption concerning the statistical behavior of the interference. We develop a new closed-form expression for the conditional CF of the inter-carrier interference and provide a procedure for calculating the exact BER expressed in the form of a single numerical integration. The accuracy of the Standard Gaussian Approximation (SGA) technique is also evaluated. Link-level results confirm the accuracy of the SGA for most practical conditions.

Capacity bounds on multi-pair two-way communication with a base-station aided by a relay

The multi-pair bi-directional relay network under consideration consists of one base-station, multiple (say m) terminal nodes and one relay, all of which are half-duplex, in which, contrary to prior work, each node has a direct link with every other node. Each of the m terminal nodes exchanges messages with the base-station in a bi-directional fashion, leading to 2m total messages to be communicated with the (possible) help of the relay. Our contributions are: 1) the introduction of three new temporal protocols which fully exploit the two-way nature of the data, over-heard side-information through network coding, random binning, and compress-and-forward terminal node cooperation, 2) derivations of achievable rate regions and 3) cut-set based outer bounds for the multi-pair network, and 4) a numerical evaluation of the derived regions in Gaussian noise which illustrate the performance of the proposed protocols.

A Spectrum-Efficient Multicarrier CDMA Array-Receiver with Diversity-Based Enhanced Time and Frequency Synchronization

This paper proposes a spectrum-efficient spatio-temporal array-receiver for multi-carrier CDMA systems named MC-STAR. First, we derive a new post-correlation model for MC-CDMA that characterizes the structure of the channel in space, time and frequency. Based on this model, we introduce a new multi-carrier array-receiver with rapid and accurate joint synchronization in time and frequency. There, we exploit jointly the spatial, temporal and frequency diversities as well as the intrinsic inter-carrier correlation (termed hereafter frequency gain) to improve the channel identification and the synchronization operations. In addition, based on a new link/system-level performance analysis, with a band-limited chip waveform assumption, we provide a comparative performance study of MC-STAR over two multi-carrier CDMA air-interface configurations, namely MT-CDMA and MC-DS-CDMA, in the most realistic operating conditions. Link/system-level results confirm the advantages of MT-CDMA in increasing throughput and bandwidth efficiency. The current trend is to design radio air-interfaces with flat fading subcarriers. In contrast, with MC-STAR we show that the positive effects of multipath diversity and frequency gain over large strongly-overlapping subcarriers is more significant than the negative effects of multipath and multi-carrier interference.

A Multicarrier-CDMA Space–Time Receiver With Full-Interference-Suppression Capabilities

This paper proposes a low-complexity multicarrier-code-division-multiple-access (MC-CDMA) space-time receiver with full-interference-suppression capabilities. First, we derive a complete model of the interference which takes into account multiple-access, intersymbol, and intercarrier interferences. Based on this model, we introduce a new multicarrier-interference-subspace-rejection (MC-ISR) receiver and analyze its performance in an unknown time-varying Rayleigh channel with multipath, carrier offset, and cross correlation between subcarrier channels. We also propose a realistic implementation of this receiver, which includes an efficient strategy for carrier-offset recovery in a multicarrier- and multiuser-detection scheme. In addition, based on the Gaussian assumption, we derive a link-/system-level performance analysis of the MC-ISR over the two MC-CDMA air-interface configurations, which are the multitone CDMA and the multicarrier direct-sequence CDMA, and validate it by simulations. The gains in the throughput, which are attainable by the MC-ISR, are significant and evaluated in this paper as functions of the air-interface configuration, the number of subcarriers, and the modulation order.

Multicarrier-CDMA STAR with time and frequency synchronization

This paper proposes a spectrum-efficient spatio-temporal array-receiver (STAR) for multi-carrier CDMA systems named MC-STAR. First, we derive a new post-correlation model for MC-CDMA that supports both the MT-CDMA and MC-DS-CDMA air-interfaces. Based on this model, we introduce a new multi-carrier receiver with rapid and accurate joint synchronization in time and frequency. We also exploit the intrinsic subcarrier correlation to improve the channel identification and the synchronization operations. We analyze the performance of MC-STAR in an unknown time-varying Rayleigh channel with multipath, carrier offset and cross-correlation between subcarrier channels. Simulation results confirm the accuracy of the joint time/frequency synchronization. They also confirm that for each MC-STAR configuration there exists an optimum number of subcarriers which results in maximum throughput. A higher number of subcarriers increases the inter-carrier interference while a lower number of subcarriers reduces the frequency gain. With four receiving antennas and five MT-CDMA subcarriers in 5 MHz bandwidth, MC-STAR provides about 1.2 bps/Hz at low mobility for DBPSK, i.e., an increase of 30% in spectrum efficiency over DS-CDMA.

Joint time-delay and frequency offset synchronization for CDMA array-receivers

This paper addresses the problem of joint time-delay and frequency synchronization in the CDMA receiver STAR. We observe that its space/time structural approach enables us to decouple time and frequency offsets. We thus introduce a new procedure based on space/time separation that implements a simple linear regression approach for both time-delay and frequency synchronization. We analyze the performance of this procedure in an unknown time-varying channel with multipath and carrier offset. Simulations show that the modified STAR provides significant performance gain over its previous version. The link-level gain is more than 3 dB for BPSK (128 kbit/s) when the frequency offset equals 0.1 ppm (i.e., 200 Hz). This gain increases at higher frequency offsets and small-constellation modulations.

Full-duplex wireless communication based on backscatter amplifier

In this paper, we propose a full-duplex wireless communication system by means of reflected power. Our concept is based on backscatter modulation (BM), where electromagnetic waves are modulated, amplified and reflected by the same antenna that receives them. We consider two nodes that wish to exchange data over wireless additive white Gaussian noise with Rayleigh fading channel. Within this framework the end-user receives self-interference free signal. The proposed system includes negative resistances to guarantee sufficient power for the demodulator while exhibiting maximal difference in the backscattered field. We analyze the performance of the proposed system and evaluate its ergodic capacity.