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In this paper, we study the achievable diversity-multiplexing gain (D-MG) tradeoffs over the IEEE 802.15.3a channel model. In particular, we give an exact expression of the D-MG tradeoff for single antenna systems. This expression is based on a statistical model of the integrated energy of the considered channels. At a second time, we evaluate lower and upper bounds on the D-MG tradeoffs of multi-antenna systems. This formulation gives us more insights on the real potential of multi-antenna techniques with UWB systems. It is shown that even though the asymptotical diversity gain has an infinite value, the multi-antenna techniques can be very beneficial in the practical range of signal-to-noise ratios.

In this paper, we examine the impact of spatial diversity on ultra-wideband (UWB) systems. This diversity is exploited by deploying two antennas at the UWB terminals. For single-antenna systems, spatial diversity is exploited in a distributed manner where a neighboring node can help the source in delivering its message to a destination. In particular, two space-time (ST) coding schemes are proposed. The first one is optimal among all totally-real codes having a constant transmitted energy per antenna. Unlike the classical approach of constructing ST codes over infinite fields, the second construction takes the structure of 2-dimensional constellations into consideration in order to propose an appropriate modulation-specific code. These schemes are associated with multi-antenna arrays and with virtual arrays based on the amplify-and-forward (AF) strategy.

In this paper, the advantage of applying space-time (ST) coding on ultra wideband (UWB) systems is discussed. In order to achieve very high data rates and important performance gains combined pulse position modulation (PPM) and pulse amplitude modulation (PAM) constellations along with full diversity and full rate ST codes are used at the transmitter side. The proposed coding-decoding scheme takes into account the nature of the highly frequency selective channel and the multi-dimensional constellation. Furthermore, the coding gain is optimized over different constellations and the possibility of compromising performance to lower peak-to-average power ratios (PAPR) is discussed. Moreover, for simplified receiver structures, V-BLAST decoding scheme was adapted to multi-dimensional signals. Results show considerable performance gains and better immunity against inter symbol interference (ISI) and multi user interference (MUI).

The performance prediction and simulation of new communication and positioning systems based on UWB technology require an important effort to reach a deep knowledge of the physical channel. The multipath nature of the indoor channel is the limiting factor for positioning accuracy and communication capabilities of UWB systems in real environments, in particular ISI for high data rate applications. Most of the measurement performed to characterise the UWB channel have been performed in the time-domain by using a sampling oscilloscope with resolution greater than 1 ns. This paper describes a frequency approach based on a vector network analyser with a subnanosecond resolution in accordance with usually UWB pulse width. After an introduction the measurement technique is described and the performances of the sounding chain evaluated. Then an experimental methodology is proposed in order to assess the channel characteristics at different scale levels (small, medium, large). Finally the statistical post-processing for the future measurements is described.

LETI, a laboratory of the French Atomic Energy Commission, has built several direct sequence spread spectrum systems using SAW devices, the binary data rate of these systems goes from 256 kbit/s up to 8 Mbits/s and the frequency range is large (from 900 MHz up to 60 GHz). Taking into account its knowledge of this particular field of radio communications in multipath environments, LETI has designed a powerful programmable single chip that is able to perform as the matched filter of a direct spread spectrum receiver, the delay line of a differential demodulator, the demodulation, the data generation and the clock recovery.

Presents recent radio communications research work carried out by LETI (Laboratoire d'Electronique, de Technologie et d'Instrumentation), which is a laboratory belonging to the French Atomic Energy Commission. At the outset, a team from the LETI was involved in a programme to develop robot intervention in hostile environments. One of the first problems to be solved was to find a reliable and robust radio modulation technique for multi-path environments. In this type of situation, conventional radio solutions are unsuccessful and provide only extremely low data rates. The performance of wireless transmissions is much poorer than conventional communications using cables and they have to cope with a transmission channel that is subject to much more interference. In particular, multi-path transmission of radio waves leads to interference phenomena such as widening of the channel impulse response, fading and Doppler noise, which reduce the quality of the transmission. A novel reception structure, implementing direct-sequence spread-spectrum techniques combined with a diversity system, actually benefits from the multi-path phenomena which usually damage conventional radio transmissions. The first receivers developed were based on a surface acoustic wave filter and were subsequently replaced by dedicated digital circuits that perform all the base band processing. The resulting flexibility has made the design of CDMA and high-data-rate wireless systems possible. This research has potential applications in a wide variety of areas, e.g. video surveillance and track-to-train communications in underground systems and factory communications for machine synchronisation.

The direct sequence spread spectrum (DSSS) modulation technique has been widely used for a number of years. For point-to-point applications, LETI has proposed the concept of a matched filter receiver able to take into account all the propagation path signals implementing a diversity combining method. The proven interest, in different kinds of environments, of this original architecture had led to the development of a family of flexible ASICs. First this paper describes two ASICs (ICARE and VICASSO), developed in 1999. Then, special attention will be given to the implementation of an asynchronous code division multiple access (CDMA) receiver. The advanced CDMA architecture developed is based on co-channel interference cancellation (IC) schemes, which meanwhile have better performance than conventional detectors and are particularly attractive for hardware implementation. Finally, this article will focus on a synchronous CDMA receiver architecture.

In this paper, we present two space-time schemes for exploiting diversity in multi-antenna impulse radio ultra wide-band (IR-UWB) systems when neither the transmitter nor the receiver has access to channel state information. The first scheme encodes the pulses of the same data symbol by a combination of differential phase shift keying and permutation codes and can be associated with any number of transmit and receive antennas. The second scheme encodes different symbols with two transmit antennas. These schemes are associated with Rake receivers and achieve full spatial and multi-path diversity with no data rate loss for constant-modulus multi-dimensional constellations. Adaptive versions of these schemes are also presented and shown to approach the performance of coherent receivers in stationary indoor channels.