Constrained mean-square-error space-time pre-equalizer for the downlink channel of UMTS-TDD

This paper deals with space-time pre-equalization for UMTS-TDD systems. The motivation of this proposal lies in the fact that in TDD the up and downlink channels can be highly correlated. If this is the case, the equalization functions can be moved to the base-station (BS), therefore simplifying the mobile unit receiver design, where computational requirements and power consumption issues impose severe restrictions. A design technique, which explores both the frequency redundancy of DS-SS signals and the spatial diversity achieved through the use of an array of antennas at the BS, is derived. The used criterion intends to perform the minimization of the transmitted energy per bit, subject to the condition that signal error power spectral density (PSD) at the decision point is below a given threshold, ensuring the desired signal to distortion ratio. After the mathematical background, simulation results, obtained in typical scenarios, are presented and compared with conventional emitter and receiver based schemes using the RAKE combining principle.