P. Loubton

Bio: P. Loubton is an academic researcher. The author has contributed to research in topics: Spread spectrum & Spectral efficiency. The author has an hindex of 1, co-authored 1 publications receiving 32 citations.

Spread OFDM performance with MMSE equalization

Abstract: Upper-bounds on the bit error probability of an OFDM/CDMA system in a single user context are derived for a linear minimum mean-square-error (MMSE) receiver structure. All the calculus are performed in presence of a frequency-selective Rayleigh fading channel both in a coded (convolutive forward error correcting scheme) and uncoded scenario. Surprisingly, we show that coded SOFDM outperforms COFDM if the signal to noise ratio is greater than a given threshold which only depends on the code rate and not on the code structure. These theoretical results provide tools to optimally combine coding and spreading for increasing the spectral efficiency of OFDM/CDMA schemes.

MMSE analysis of certain large isometric random precoded systems

Abstract: Linear precoding consists in multiplying by an N/spl times/K matrix a K-dimensional vector obtained by serial-to-parallel conversion of a symbol sequence to be transmitted. In this paper, new tools, borrowed from the so-called free probability theory, are introduced for the purpose of analyzing the performance of minimum mean-square error (MMSE) receivers for certain large random isometric precoded systems on fading channels. The isometric condition represents the case of precoding matrices with orthonormal columns. It is shown in this contribution that the signal-to-interference-plus-noise ratio (SINR) at the equalizer output converges almost surely to a deterministic value depending on the probability distribution of the channel coefficients when N/spl rarr/+/spl infin/ and K/N/spl rarr//spl alpha//spl les/1. These asymptotic results are used to analyze the impact of orthogonal spreading as well as to optimally balance the redundancy introduced between linear precoding versus classical convolutional coding, while preserving a simple MMSE equalization scheme at the receiver.

High-throughput, high-performance OFDM via pseudo-orthogonal carrier interferometry spreading codes

Abstract: The paper introduces to orthogonal frequency-division multiplexing (OFDM) systems a novel pseudo-orthogonal carrier interferometry spreading code which spreads each parallel data stream over all the OFDM carriers. Pseudo-orthogonal carrier interferometry (PO-CI) spreading codes are carefully selected to introduce the following benefits to OFDM: up to 2N parallel data streams can be coded onto N carriers, with little degradation in performance; when rate 1/2 channel coding is applied in addition to PO-CI spreading codes, the resulting binary phase-shift keying OFDM systems demonstrate the performance of coded OFDM and the throughput of uncoded OFDM; PO-CI codes are carefully selected to spread in a manner which eliminates the peak-to-average power ratio problems characteristic of traditional OFDM.

Analysis and design of short block OFDM spreading matrices for use on multipath fading channels

Abstract: We consider the use of block spreading in a multicarrier system to gain diversity advantage when employed over multipath fading channels. The main idea is to split the full set of subcarriers into smaller blocks and spread the data symbols across these blocks via unitary spreading matrices in order to gain multipath diversity across each block at the receiver. We pose the problem of designing the spreading matrix as a finite dimensional optimization problem in which the asymptotic error is minimized. This formulation allows us to find explicit solutions for the optimal spreading matrices. The performance is validated for the uncoded channel as well as for the coded channel employing turbo-iterative decoding. We further demonstrate that suboptimal linear complexity equalization strategies for spread orthogonal frequency division multiplexing (OFDM) do not gain any diversity advantage over traditional diagonal OFDM.

The elimination of peak-to-average power ratio concerns in OFDM via carrier interferometry spreading codes: a multiple constellation analysis

Abstract: Orthogonal frequency division multiplexing (OFDM) demonstrates symbol-by-symbol fluctuations in peak-to-average power ratio (PAPR), a direct consequence of independently modulated carriers. This, in turn, leads to inefficient operation of the transmit power amplifier, and/or in-band and out-of-band distortion due to power amplifier saturation. This paper extends our previous work, which explains how carrier interferometry (CI) spreading codes may be applied to OFDM (creating CI/OFDM) to eliminate PAPR fluctuations. Specifically, we analyze the PAPR benefits of CI codes in higher-order constellation OFDM systems (QPSK, 16-QAM, and 64-QAM OFDM). This work confirms that the proposed technique (of spreading the data symbols onto all carriers) ensures the elimination of high peaks in the signal envelope (thereby eliminating the PAPR problem): It is further shown that the choice of constellation size does little to change the PAPR benefits of the CI spreading technique.

High-performance MIMO-OFDM via carrier interferometry

Abstract: This paper introduces a novel merger of OFDM systems, MIMO systems, and carrier interferometry (CI) spreading codes. Specifically, previous works have illustrated the performance benefits of (1) applying CI spreading codes to OFDM and (2) merging MIMO and OFDM systems. In this paper, OFDM systems are combined with both a MIMO system and CI spreading codes, resulting in the so-called MIMO-CI/OFDM system, capable of very reliable communication at SNRs previously possible only with channel coding.