This contribution provides a very high data rate UWB system design based on direct sequence spreading. In order to attain > 500Mbps for short range communication, two classical solutions are feasible: either use very short spreading sequences or very large bandwidth. These classical strategies increase intersymbol interference. In contrast, our approach consists in allocating multiple long codes to each user. Long codes allow to cope with channel impairments while the use of multiple codes maintains very high bit rates. With such high data rates the receiver obviously has to be simple by requiring a low complexity digital processing. This paper shows that the proposed strategy is compatible with simple receivers. In fact, even if the amount of interference increases, joint usage of very long codes and simple linear receiver can result in high data rate, high performance UWB system. This assertion is substantiated through comparisons with existing UWB solutions.

Direct sequence ultra wide band (DS-UWB) is a strong candidate for high data rate (HDR) wireless personal area networks (WPAN) IEEE 802.15.3a standard for achieving up to 480 Mbps. This paper presents a new system design methodology based on a multicode approach where the spreading sequences have a duration longer than the channel impulse response (CIR) with the goal of achieving higher data rates up to 1.6 Gbps. Multiple codes are allocated to one user and transmitted synchronously in a TDMA access scheme. All codes are modulated by the same QAM constellation, the best combination of number of codes and QAM order for achieving the largest range of coverage is illustrated for 4 target rates: 10, 110, 480 and 1600 Mbps. The selection methodology is based on a theoretical SNIR derivation at the output of a linear minimum mean square error equalizer using classical WPAN channels as specified by IEEE 802.15.3a. Compared to the DS-UWB proposal for IEEE 802.15.3a, the proposed system grants around 3 times range increase

This contribution proposes an iterative channel impulse response (CIR) estimation scheme in the context of the pseudo-random-postfix OFDM (PRP-OFDM) modulation. While conventional techniques reduce noise and interference of OFDM data symbols on the CUR estimates by simple mean value calculation, the new proposal uses soft decoder outputs in order to perform iterative OFDM data symbol interference cancellation. In a typical example for 64QAM constellations, the mean-square-error (MSE) of the CIR estimates is improved by approx. 12 dB after three iterations. Based on PRP-OFDM postfixes only, it is thus possible to perform channel estimation for higher order constellations (64QAM and higher) with an initial CIR estimation over a small observation window (for 64QAM typically 30 to 40 OFDM symbols). Any loss in throughput due to additional redundancy for CIR estimation purposes, e.g. pilot tones, learning symbols, etc. is avoided at the cost of an increase in decoding complexity.

Impulse radio (IR) ultra wide band systems (UWB) by nature require an accurate timing. This contribution proposes three digital algorithms for compensating the time jitter resulting from the oscillator driving the receiver sampler. The main idea is to estimate jointly useful data and a fraction of the time jitter by using coloration properties of the oscillator phase noise. A maximum likelihood (ML) estimation based algorithm, a suboptimal but with lower complexity one and corresponding adaptive versions are detailed.

This contribution extends the recently proposed pseudo-random-postfix OFDM (PRP-OFDM) transceiver to the multiple transmit multiple receive (MTMR) antennae context. In this paper a new generic MTMR modulator is proposed that grants a low complexity semi-blind identification of all channels between any transmit and receive antenna based on order-one statistics. A new channel tracking strategy is detailed based on a moving average (MA) Doppler modeling that preserves the Jakes channel coefficients correlation properties. Moreover two transceiver architectures are presented with an Alamouti based apace-time (ST) block code: equalization schemes with or without postfix interference cancellation and related modulators. Performance is illustrated by simulations in the context of the 5.2 GHz OFDM WLAN IEEE 802.11a standard.

In this paper the capacity of a specific PPM (pulse position modulation) ultra wide band (UWB) system using a semi-digital demodulator architecture is evaluated. Then the impact of time desynchronization between the local pulse templates and the received signal on the capacity of the link is assessed, the goal being to quantify the jitter effect and delay estimation error on this capacity and issue design recommendations for real system construction.

Recent results on the asymptotic empirical eigenvalue distribution of random matrices have enabled the study of the asymptotic limits of linear precoded orthogonal frequency-division multiplexing systems with minimum mean-square error (MMSE) equalization. In this letter, we extend these results to the MMSE successive interference cancellation detector and quantify the nonlinearity gain for certain type of precoding matrices.

The paper details a new OFDM modulator based on the use of a pseudo random postfix (PRP-OFDM) and low complexity equalization architectures. The main advantage of this new modulation scheme is the ability to estimate and track channel variations blindly using order one statistics of the received signal. This scheme is thus very well suited in the presence of large Doppler spreads where channel tracking becomes essential. Moreover, the proposal of various equalization structures, derived from the zero padded transmission schemes, allows implementations ranging from low-complexity/medium performance to increased-complexity/high performance.

Recent results on the asymptotic empirical eigenvalue distribution of the sum or product of matrices, based on the free probability theory, have enabled the study of the asymptotic limits of linear precoded OFDM systems with MMSE equalization for ergodic and non-ergodic channels. We extend these results to the MMSE successive interference cancellation detector with realistic channels and give an explicit answer on the impact of the choice of certain types of spreading codes.

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.

This contribution aims at analyzing the performance of large linear precoded OFDM transmissions over fading wireless channels with limited diversity and MMSE equalization. Linear Precoding consists in multiplying by a N ◊ K isometric matrix a K dimensional vector obtained by serial to parallel conversion of a symbol sequence to be transmitted. Based on the Free Probability Theory, asymptotical analysis (N tends to infinity, K tends to infinity and K/N >tends to a constant) of the SINR is conducted to understand the different parameters involved in Linear Precoded OFDM schemes. The theoretical results are confirmed by numerical simulations when considering convolutional coding with finite memory.

In this paper, a new model and compensation scheme of quadrature imbalance in analog I/Q OFDM transceivers is presented. Classically, the effect of I/Q imbalance is modeled by a crosstalk between pairs of symmetrical OFDM sub-carriers. We show that this model is not valid when the carrier frequency offset is compensated after the source of I/Q mismatch. Assuming a perfect digital compensation of the frequency offset in baseband, the I/Q imbalance generates interference between all sub-carriers and not only symmetrical ones. When analog components, multipath propagation and frequency offsets are taken into account, the complete matrix analysis has to make use of mathematical results such as the diagonalization of pseudo-circulant matrices. The existing models happen to be a sub-case of this new more general result. A new compensation scheme applying to every case is proposed as well as a receiver autocalibration procedure. It is shown that system-specific assumptions can simplify the implementation of the compensation. These assumptions are validated by simulation in the framework of a 5 GHz WLAN transceiver design with realistic production spread of the analog components.

This article extends turbo demodulation to the zero-padded OFDM (ZP-OFDM) system by accounting for the noise color and the symbols estimates correlation introduced by equalization. Resorting to realistic simulations, we show that turbo demodulation used with set partitioning labeling can significantly outperform noniterative decoding with Gray labeling. We also show that it increases the performance gap between ZP-OFDM and OFDM with cyclic prefix (CP-OFDM) relative to noniterative decoding, because it amplifies the performance gain due to the guaranteed symbol recovery of ZP-OFDM.

Zero padding (ZP) of multicarrier transmissions has been proposed as an appealing alternative to the traditional cyclic prefix (CP) orthogonal frequency-division multiplexing (OFDM) to ensure symbol recovery regardless of the channel zero locations. In this paper, both systems are studied to delineate their relative merits in wireless systems where channel knowledge is not available at the transmitter. Two novel equalizers are developed for ZP-OFDM to tradeoff performance with implementation complexity. Both CP-OFDM and ZP-OFDM are then compared in terms of transmitter nonlinearities and required power backoff. Next, both systems are tested in terms of channel estimation and tracking capabilities. Simulations tailored to the realistic context of the standard for wireless local area network HIPERLAN/2 illustrate the pertinent tradeoffs.

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.

Linear precoding consists in multiplying by a N/spl times/K matrix a K-dimensional vector obtained by serial to parallel conversion of a symbol sequence to be transmitted. We analyse the performance of MMSE receivers for certain large random isometric precoded systems on fading channels. Using new tools, borrowed from the so-called free probability theory, it can be shown that the signal to interference plus noise ratio 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 optimally balance the redundancy introduced between linear precoding and classical convolutional coding, while preserving a simple MMSE equalization scheme at the receiver.

The widespread application of OFDM for local area mobile wireless broadband systems is strongly motivated by the simple equalization it affords. OFDM's bottleneck of channel-dependent performance was only previously addressed by replacing the cyclic prefix (CP) with trailing zeros (TZ). Such zero-padded OFDM transmissions enable FIR channel-irrespective symbol recovery which results in significant BER gains. The price paid is increased receiver complexity compared to the classical CP-OFDM. To reduce complexity, this paper proposes two novel equalizers for TZ-OFDM. One is based on an overlap and add approach and exhibits exactly the same performance and complexity as CP-OFDM. The second offers BER performance close to the TZ-OFDM MMSE equalizer and incurs a moderate complexity increase. An extension of an existing CP-OFDM pilot-based channel estimation method is also derived for the proposed equalization schemes. Simulations are conducted in a realistic HiperLAN/2 scenario comparing the various OFDM transceivers.

This paper proposes a simple equalizer for a multicarrier block transmission scheme which pads zeros (as opposed to a cyclic prefix) in each transmitted block. In the absence of high-power amplifier induced nonlinear effects, it is shown that the resulting scheme is simply the dual of the classical cyclic prefix OFDM transceiver. Comparison between the two systems that takes into account nonlinear distortions introduced by the clipper is also performed in the practical context of the wireless broadband 5 GHz HiperLAN/2 system. Both the classical pilot-based as well as novel (semi-) blind subspace algorithms are applied for channel estimation. The advantage of subspace methods in lowering the variation of channel tracking is corroborated with simulations which verify practically accurate channel estimates along the bursts.

Two new blind channel identification methods suited to multicarrier system (OFDM) exploiting the redundancy introduced by the adjunction of a cyclic prefix at the emitter and relying on the evaluation of the received signal autocorrelation matrix are presented. The proposed algorithms are able to identify any channel without any constraint on their zeroes location (including non-minimum phase channels) and are robust to the addition of white noise. Moreover a further enhancement of the estimation accuracy can easily be achieved by taking advantage of already present training symbols in current systems operating in a semi-blind context. Furthermore one of the two identification strategies has a very low arithmetical complexity which makes it particularly attractive in practice. Notice that these methods are not restricted to classical DFT-type modulators and still apply with any perfect reconstruction modulator.

Classical transform-domain algorithms adapt the filter coefficients (in each "frequency bin") by minimizing a criterion depending on a full-band time-domain error. This paper proposes an algorithm which updates each portion of the frequency response of the adaptive filter according to the error in the same subband. For this purpose, a multirate adaptive filter is used where a subband decomposition of the error is performed using critically sampled lossless perfect reconstruction filter banks. This new algorithm is based on the minimization of a weighted criterion by a stochastic gradient algorithm and leads to improvements in the convergence rate compared to both LMS and classical frequency domain algorithms.