A 2.1 kBd speech transmission system for Rayleigh-fading channels

In the global microcellular mobile network of the near future extremely high traffic densities have to be carried. The well-known GSM system is capable of handling a traffic density of typically 10 erlang/MHz/km/sup 2/ among adverse channel conditions in the large, maximum 35 km diameter traffic cells. In the future global microcellular mobile network the initiated traffic densities can be as much as three orders of magnitude higher, whence extreme spectral efficiency is required. Also, the frequency reuse distances have to be reduced to enable handling of the initiated traffic. This results into a microcellular structure with favourable propagation characteristics. In this situation the channel can generally be described by a Rician model, although in the worst-case scenario the flat Rayleigh-fading model applies. As a consequence, low bit rate speech codecs with moderate error correction coding and multilevel modulation schemes without channel equalisers can be deployed to achieve the required bandwidth efficiency.<
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