Improved spectral efficiency for CDMA systems

Code Division Multiple Access (CDMA) · largely on frequency reuse and voice activity gating; based on spread-spectrum techniques finds increasing for. data transmission within a single cell environment,· use in commercial communications. Although CDMA the capacity of the current generation of CDMA sysoffers important advantages for wireless communicatems does ·not approach the capacity of Time or Fretions, current CDMA systems are less spectrally effi. quency Division Multiple Access. Furthermore, current . cient than ·systems employing orthogonal multiple acCDMA systems depend heavily on accurate power concess techniques in a single cell environment, and are detrol to avoid the near-far problem which arises when pendent on accurate power control. We survey research an interfering signal is received with higher power than into a variety of techniques which will increase the specthe desired signal. Even with an elaborate combinatral efficiency and robustness of subsequent generations tion of open~ loop and closed-loop power control, field of CDMA systems. These techriiques include multiuser trials indicate received signal powers will have a variand single user interference rejection techniques, the deance of 1-2 dB about their desired values in a mobile ployment of smart antenna technology, and the develenvironment[4]. This small variation can reduce the opment of advanced error correction coding techniques. capacity of by 15-30%~from the ideal case when conWe show how these techniques can be applied to inventional receivers· are employed[5]. crease the capacity of CDMA systems. New processing techniques· on the horizon will enable