Fabien Buda

An Analysis of OFDM Peak Power Reduction Techniques for WiMAX Systems

The main drawback of Orthogonal Frequency Division Multiplexing (OFDM) systems is the high peak-to-average power ratio (PAPR), which significantly reduces the efficiency of the transmit high power amplifier (H PA). Several methods have been proposed in the literature to reduce the peak power of OFDM signals and substantial gains were reported. In this paper, the effectiveness of some recently proposed methods is evaluated for WiMAX systems. Using typical HPA models and spectral masks, these PAPR reduction methods are evaluated in terms of the total system degradation.

Downlink Performance Analysis of Full-Rate STCs in 2x2 MIMO WiMAX Systems

MIMO techniques play a key role in the current wireless specifications including IEEE 802.11n, IEEE 802.16e, and LTE. Indeed, spatial multiplexing (SM) and Alamouti code are already in use in WiMAX systems based on IEEE 802.16e specifications (1). But more efficient full-diversity full-rate codes, such as Golden code (2), are still under discussion and have been proposed for future evolutions of the standards. However, the maximum likelihood (ML) or ML-like decoding of these codes is still not feasible for practical implementation. Existing papers include performance comparisons of such complex codes limited to simple scenarios and their performances in a real WiMAX environment have not been discussed in detail. In this paper, we present the performances of the Golden code, which is known to be the best full-rate full-diversity 2×2 STC, on a 2×2 downlink MIMO transmission and compare them with those of SM. The results show that the well-known performance advantage of Golden code over SM diminishes when real system scenarios are considered using powerful channel codes. Keywords—Multiple-input multiple-output (MIMO), space-time codes (STCs), WiMAX systems.

Description of a complete multi-carrier spread spectrum transmission chain for robust and discrete tactical communications

Multi-carrier spread spectrum (MC-SS) techniques have been widely studied for the last few years. This paper describes a complete transmission chain based on an MC-SS technique for tactical communications. The constraints on the waveform are discreteness and robustness to frequency selective channels and jammers. Therefore, polyphase filtering is used in order to meet these requirements. Besides, the system must be able to provide a high data rate for different tactical configurations. The paper presents the features of these configurations (filtering, modulation, coding, spreading), and the algorithms used for time synchronization and for the phase and frequency estimations. Then performance obtained over an AWGN channel (with-and without a power amplifier), over a Rummler channel or with jammers is presented. Finally, complexity estimation reveals that the implementation of such a modem is technically achievable. The study shows that the modem is able to resist to multipath channels, and that encoding and filtering allows it to resist a powerful jammer.

New advances in multi-carrier spread spectrum techniques for tactical communications

Spread spectrum techniques are classically used in tactical communications, in order to mitigate interference due to jamming, while providing a low probability of intercept. This paper addresses a new SS technique, referred to as multi-carrier spread spectrum (MC-SS). An original MC-SS scheme is proposed using polyphase filtering. As for tactical communications, dramatic performance improvements are achieved, especially for partial-band jammers. Moreover, classical problems of synchronization encountered in multi-carrier modulation systems (MCM) are resolved by the proposed waveform. This provides an extremely good safety level. The MC-SS technique consists in a combination of standard DS/SS with a MCM scheme. Such a combination is motivated by the advantages of the MCM, which provides robustness in the case of frequency selective channels, as well as an efficient digital implementation in its well known OFDM form (implemented in DAB, DVB, ...). The paper highlights that this new MC-SS method fits tactical communications constraints. It shows that this new modem proposes an active and reliable method to combat agile jammers. Special attention is paid to synchronization issues. For tactical communications, this leads to a new protection feature.

A comparison of SISO algorithms for iterative decoding of multidimensional product codes

In this paper, we consider iterative decoding of multidimensional product codes (MPC). Such codes, also called iterated codes, were first introduced by Elias (1954). After a formal definition of MPG, we compute their low-weight distribution. Using this distribution, we derive an improved asymptotic union bound on maximum likelihood (ML) decoding of such codes over a Gaussian channel. The main part of the paper is devoted to a comparison of efficient SISO algorithms, used as basic tools in the iterative decoding process. Simulation results show that MPC may be more suitable than any other schemes for packet wireless transmission involving packet sizes of less than 1000 information bits and requiring very low packet error rates (PER).

A novel approach to channel coding without bandwidth expansion

This paper introduces a new block coding technique without bandwidth expansion. The proposed technique derives from the multiple access concept recently introduced to increase the number of users on a multiple access channel. The basic principle consists of time spreading the parity-check symbols to the block length using Walsh-Hadamard (WH) sequences and superimposing the resulting signals to the block of information symbols. The interference between the two groups of symbols is handled using an iterative detection/decoding technique whose operation resembles that of turbo decoders. Several code constructions are presented using very simple block codes. Performance of the proposed coding technique is evaluated by means of computer simulations. After observing that correlation of the intermediate decision errors in this iterative detection/decoding process limits the achievable performance, a block interleaver is introduced and is shown to substantially reduce the performance degradation due to interference between the serially transmitted symbols and the WH-spread symbols.