JPL's Wireless Communication Reference Website Chapter: Analog and Digital Transmission Section: CDMA

CDMA Array Processing

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.

• Spread Spectrum Bandwidth
  The chip rate of the spread spectrum signal is inversely proportional to the chip period. A number of different chip rates have been proposed for such systems, but a chip period of approximately 800 ns (chip rate 1.25 MHz) will be assumed for here. Such a system is often called narrowband CDMA, because the baseband bandwidth is much smaller than the 900 - 1800 MHz RF carrier frequency, and 3rd generation PCS systems will be wideband, i.e. with chip rates of 5 - 20 MHz.
• Multipath Diversity
  In urban areas, multipath propagation is common, whereby the receiver observes a number of copies of the transmitted signal, each with a different time delay. This provides a form of multipath diversity, can be exploited by using a RAKE receiver to combine the the outputs of multiple code correlators.
• Asynchronous Operation
  The reverse link of a CDMA system is usually asynchronous, in the sense that the arrival times for each user's code are different. This means that the receiver for each user will observe interference from all other users in the system, as the transmitted codes will not be orthogonal. Hence, the number of users that can be simultaneously accommodated in one cell is interference-limited and, in general the reverse link thus has less user capacity than the synchronous forward link.
• Power Control
  It has always been known that to operate effectively spread spectrum systems must experience similar received user power levels as the discrimination between users relies on the correlation properties of the (PN) spreading codes. In cellular systems power control is thus essential on the reverse link, to minimize multiple access or multi-user interference. Otherwise, mobile transmitters far away from the cell's base station will be swamped by interference generated by larger signal level users closer to the receiver. If all signals arrive with the same received power level, the receiver's tolerance to CDMA interference is proportional to the processing gain W = T_s / T_c, where T_s is the symbol period for the slow traffic data bits and T_c is the chip time. The obvious way to increase the capacity of a CDMA system is to reduce the levels of multiple-access interference, for instance by adaptively controlling transmit power

Further Reading

• Why use arrays?, Uplink versus downlink
• Channel modeling
• Antenna processing principles
• Smart antenna receivers
• Simulations
• Performance
• Literature

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.

Slides

• Slides from the "1997 Distinguished Lecturer" presentation in PDF
• Additional slides in PDF/PS
• CDMA Array Processing paper in PDF/PS

JPL's Wireless Communication Reference Website © 1999. Peter Grant (Author) and Jean-Paul Linnartz (Ed.)

Acknowledgments

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.