Analog and Digital Transmission
The most complex and expensive part of the radio link in personal cellular systems is the base station. As a result, manufacturers have been developing methods which have high efficiency in terms of the bandwidth occupied and the number of users which can be accommodated per base station. Considerable system capacity gains are available from exploiting the different spatial locations of cellular users [3,4,5]. This gives rise to the concept of the smart or intelligent antenna. There are a number of methods to achieve this, from simple fixed beam sectorisation schemes to complex adaptive antenna array techniques. Adaptive antenna have previously been a heavily researched area in the 1960s and 1970s. Here we consider antenna arrays for the mobile-to-base station or reverse link of a CDMA cellular system such as the IS-95 standard.
Antenna arrays have been introduced to reduce cellular interference levels and improve coverage. Some researchers suggest that using M antennas can multiply the reverse link capacity by a factor of roughly M. Channel modeling aspects need to be considered in such evaluation: in urban areas, several channel taps are often resolvable. The significant channel taps or signals, observed at antenna arrays, may be modeled as the summation of array steering vectors. In urban areas, it is common for each vector entry to fade according to the Rayleigh distribution. The total angular spread at the base station receiver is small for rural and urban environments but, in dense urban and indoor scenarios then the spread can exceed 180 degrees.
The 2D RAKE filter (or space-time processor) is a promising and thoroughly investigated approach to releasing an effective CDMA antenna array receiver. Several algorithms can be used.
All the algorithms are known to degrade in the presence of high Doppler frequency signals, particularly when the angular spread from the users is large. This limits the performance or data rate for highly mobile users but, in indoor environments, or for walking pace dense urban users, then these algorithms do give the increase in performance to provide higher data rate services to these users. The eigenfilter approach performs consistently well.
As the angular width of the channel response increases, the receiver is able to exploit more spatial diversity.
This work was sponsored by the UK Engineering & Physical Sciences Research Council and Nortel under their Smart Antennas international University collaboration program. The assistance of Dr J.S. Thompson in conducting the simulations is gratefully acknowledged.