JPL's Wireless Communication Reference Website

Chapter: Analog and Digital Transmission. Section: Adaptive antennas

Adaptive antennas

 The growing spread of wireless communication necessitates technical measures to enhance spectral efficiency and subscriber capacity. Adaptive antennas at the base station estimate the location of the mobile terminal and receive and transmit only from and to this desired direction. This

Research is being conducted into such systems, meanwhile the first products are entering the market. Ericsson and Mannesmann conducted tests in Germany in 1997. Their system uses a 6 by 8 matrix of patch antennas. Typically an horizontal element spacing of l/2 is used. The technology of using such "smart antennas" is often called Space Division Multiple Access (SDMA). SDMA is mainly being researched for (future extensions of) GSM and IS95.

Adaptive power control appears necessary to achieve the best performance gains.

Combining signals from different antenna branches

In a noise-limited environment, the best diversity combining scheme is maximum ratio combining. If interference is present, Interference Rejection Combining optimises the C/I at the output. Improvements of more than 10 dB appear feasible in the uplink. For the downlink, such improvements are more difficult to achieve. Typically a 6 dB gain appears practical, except in bad urban multipath environments with small cell sizes.

Interview


Sound file.
1 min 53 sec.
At the Technical University of Vienna, Prof. Ernst Bonek studied adaptive antenna for capacity increase of GSM-like systems. Smart antennas may allow a frequency reuse cluster factor of 1. This is substantially better than the cluster size of 9. Due to practical limitations with non-hexagonal cells, an effective cluster size of 15 is sometimes for GSM. References:
Cellular reuse, Cell planning GSM, Diversity

SDMA systems with dense frequency reuse may require many intracell handovers, to keep the angles of arrival well balanced. In particular signals arriving from almost the same direction, and signals coming from side lobes of main beams should be avoided.

The complexity of the base station software increases to accommodate SDMA. The software of existing base stations has to be modified. New synchronisation words may be needed. The existing equalizer training sequence in the GSM technical specification appears not suitable for training SDMA receivers and tracking multiple co-channel (interfering) users. Particularly, the cross correlation properties are not very appropriate for signal separation.

The angle of signal arrival is estimated in the uplink (mobile-to-base). These measurements are used to steer signals in the downlink as well. Calibration of the antenna, to eliminate the effect of antenna imperfections, is needed to avoid incorrect steering of downlink signals. According to simulations, for a 30 dB C/I improvement, the accuracy of of adapting the signals to different antennas has to be better than 3 degree phase offset and it has to exhibit less than 0.5 dB amplitude imperfections. Particularly outside the main lobe of each antenna element, this poses a challenge.



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