Wireless transmissions in multipath environments benefit from direct sequence spread spectrum techniques

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.