On RAKE allocation for CDMA receiver employing antenna arrays

Utilization of antenna arrays allows capacity enhancement in cellular CDMA networks. For wideband receivers employing antenna arrays a 2D-RAKE receiver is required and a search for best RAKE taps has to be performed in spatio-temporal domain. This simulation study focuses on 2D-RAKE allocation in the mean energy sense assuming a digital beamforming approach. The scheme permits also a straightforward way to downlink beamforming. In the current method a 2D-channel impulse response is evaluated first and then a two-dimensional search gives the RAKE taps with most energy. A wideband spatio-temporal channel model for urban macro cells is employed. The simulations produce the relative signal-to-interference-and-noise ratio (SINR) as a function of the number of allocated RAKE taps and the beamwidth of the antenna array. The results show that SINR gains depend strongly on the angular spread of the desired signal. The enhancement in SINR varied approximately between 6-9 dB for a 10-element antenna when four 2D-RAKE taps were exploited. This improvement can be interpreted as an approximate increase in capacity. Increasing the system bandwidth from 1.25 MHz to 5 MHz only 1-3 additional 2D-RAKE taps were required for maintaining the desired SINR level. In the downlink the performance was about 1-3 dB worse than in the uplink