On Channel Identification and Evaluation of Adaptive AntennasJonas Strandell
Licentiate Thesis, January 1998.
Paper copies of the whole thesis can be obtained from Ylva Johansson, Signals and Systems Group, Uppsala University, Box 528, SE-75120 Uppsala, Sweden.
Considerable research is also done on how to utilize the spatial dimension. An obvious approach is of course to decrease the cell size. This is however associated with great costs. The spatial dimension can also be exploited by means of adaptive antennas. The most common approach is then to place the "smart" antenna at the base station site.
The modeling is illustrated with the pulse shaping function used in D-AMPS (IS-54). The method can be used for single user channel estimation, but is especially useful for multi-user channel estimation since it is economic with respect to the number of parameters to be estimated. This is shown in a multi-user channel estimation example.
The channel estimate obtained utilizing the pulse shaping information can be further improved by projection onto a spatially parameterized subset. The projection is performed in a spectrum norm sense, and has been investigated by means of simulations. By utilizing the channel estimation method in a multidimensional MLSE (maximum likelihood sequence estimator) detector, a significantly improved bit-error rate can be obtained.
Measurements of an adaptive DCS-1800 (Digital Communication System) base station antenna, used in uplink only, are then presented and analyzed. Both laboratory measurements and outdoor field trials have been performed. The antenna is improving the C/I (carrier-to-interference) ratio by more than 30 dB. Calculations based on measured data, assuming downlink performance equal to that of the uplink, predict six times improved spectral efficiency gain as compared to present base station systems.
The performance degradation of adaptive antennas due to inaccuracies in weights and quantization in A/D converters is also investigated. An analytical expression for the variance of the weight quantization error is derived for the log-phase-amplitude weighting technique and is used to calculate the output SINR (signal-to-interference-plus-noise-ratio) of the adaptive antenna. The impact of spatial correlation is also examined. The theoretical expressions are verified by means of laboratory measurements and outdoor field trials, using an adaptive base station antenna for the DCS-1800 system.
A major concern for adaptive antennas is the calibration prior to startup. Two methods of performing auto-calibration of an adaptive antenna for mobile telecommunications are proposed. The weighting is assumed to be implemented in hardware. The first method proposed is a closed-loop, LMS-like algorithm, whereas the second takes on a tracking approach. Simulations show that the two methods are able to maintain a good performance for realistic temperature drifts.
Conference paper (Antenn'97) Design and Evaluation of a Fully Adaptive Antenna.
Master Thesis by J Strandell and M Wennström, 1996.
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