Both techniques use the antenna array to locate mobile users. Mobile positioning can also be used for advanced user hand-off schemes, improving spectral efficiency, link quality, and battery life. In a synchronous CDMA model, all mobile radio signals arriving at the basestation are synchronized to within a fraction of a chip time interval, which is typically 200 ns. The use of orthogonal codewords can greatly enhance performance. For large cells with large multipath delays, synchronization may be extremely difficult, and a better channel model is asynchronous.
Conventional approaches use training sequences which are sent periodically.
Although subspace-based algorithms can eliminate training sequences, they only apply when the basestation is lightly loaded or when a few users are active. We develop a blind subspace-based algorithm that works for overloaded systems, where the number of users exceeds the spreading factor.