NEAR FAR RESISTANCE OF MULTIUSER DETECTORS IN ASYNCHRONOUS CHANNELS

We consider an asynchronous code-division multiple-access environment in which the receiver has knowledge of the signature waveforms of all the users. Under the assumption of white Gaussian background noise, we compare detectors by their worst case bit error rate in a low background noise near-far environment where the received energies of the users are unknown to the receiver and are not necessarily similar. Conventional single-user detection in a multiuser channel is not near-far resistant, while the substantially higher performance of the optimum multiuser detector requires exponential complexity in the number of users. We explore suboptimal demodulation schemes which exhibit a low order of complexity while not exhibiting the impairment of the conventional single-user detector. Attention is focused on linear detectors, and it is shown that there exists a linear detector whose bit-error-rate is independent of the energy of the interfering users. Moreover it is shown that the near-far resistance of optimum multiuser detection can be achieved by a linear detector. The optimum linear detector for worst-case energies is found, along with existence conditions, which are always satisfied in the models of practical interest. © 1990 IEEE