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Optimum Diversity Combining in Multi-User Digital Mobile Telephone Systems

Claes Tidestav

Master Thesis, Report UPTEC 93094E, 63 pp, December 1993.

The basis for cellular telephone systems is the division of geographical areas into smaller areas called cells. By using low antennas, the spectrum in one cell can also be used in another cell. Is is possible for multiple users in the same cell to share the same spectrum? By using multiple antenna elements at the base stations, it is!

This thesis addresses the problem of letting two mobile telephone customers use the same frequency and/or time slot simultaneously. It presents an algorithm, which performs multivariable equalizing and detection, using a multivariable Decision Feedback Equalizer (DFE). The DFE presented can be used for arbitrary numbers of receivers and transmitters.

This algorithm is utilized on two different multivariable channel models, one deterministic and one stochastic. These models map characteristics of the antenna system onto channel characteristics. In the deterministic model, the phase difference of the carrier wave at the two antennas is the parameter, and for the stochastic model the envelope correlation of the signals at the two antennas is used. Monte Carlo simulations have been carried out on a simple FIR channel from a purely discrete-time viewpoint.

The results obtained from the deterministic model are very good, but rest on specific assumptions. The stochastic model also produces good results. Although it potentially can double the system capacity, already for moderately correlated antennas, the algorithm produces a BER, which is lower than a single antenna configuration succeeds with. A comparison has also been made with conventional antenna diversity methods, and also with a multivariable DFE with two antennas and one message. For the latter case, the achieved BER is superior for any antenna correlation.

Thesis Advisors:
Anders Ahlén and Mikael Sternad

Related publications:
PhD Thesis by Claes Tidestav.
Conference Paper (IEEE PIMRC'95) , also with an application to DS-CDMA multi-user detection.
Paper (IEEE Trans. IT 1990) on the design of scalar DFE's, based on IIR channel and noise models.

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