Subspace Tracking for Mobile Communications

In recent years space-division multiple access (SDMA) has been suggested in order to increase the capacity of a cellular mobile communication system and to simultaneously combat the losses due to multipath and interference. This is achieved by using spatial diversity introduced by an antenna array and therefore SDMA can easily be implemented as an additional component into existing systems that use timeand/or frequency-division multiple access. An important characteristic of an SDMA system is the direction of arrival of the di erent wavefronts impinging on the antenna array which can be obtained from well known high resolution methods that exploit knowledge of the signal or noise subspace. As the scenario is time-varying the subspaces will also change with time and therefore they have to be tracked to avoid repetitive computation of them at each time step. This subspace tracking has been the topic of many research papers over the last years but all of them only deal with snapshot vectors. On the other hand, many existing mobile communication systems have a TDMA component so that a whole burst of data is transmitted during one periodically recurring time slot. Unfortunately, the existing algorithms for subspace tracking do not work with burst-wise data. Therefore we introduce three algorithms that are based on known tracking algorithms for snapshot vectors which are extended to work with bursty data. Another important problem is the estimation of the current number of waveforms impinging on the antenna array because only exact knowledge of that number ensures correct estimation of the directions of arrival. In a mobile communication system, however, this number may rapidly change because of the varying number of re ections of the signal. We therefore propose an e cient algorithm for detecting changes in the number of signals that works with burst-wise data again. Simulation results demonstrate the good performance of all proposed algorithms in a time-varying mobile communication scenario. Email: [email protected]