If it is possible to control the relative timing of the transmitted signals, like on the down-link, the transmitted signals can be made perfectly orthogonal and, if the channel only adds white Gaussian noise, matched filter receivers are optimal for extracting a signal from the superposition of waveforms. If the channel is dispersive because of multi-path the signals arriving at the receiver will be no longer orthogonal and will introduce some multiple-access interference, i.e., signal components from other signals which are not rejected by the matched filter.
On the up-link extremely tight synchronization to within a fraction of a chip period, which is defined as the inverse of the spreading rate, is generally not possible and measures to control the impact of multiple-access interference must be taken. Otherwise, the near-far problem, i.e., the problem of very strong undesired users' signals overwhelming the weaker signal of the desired user, can severely decrease performance. Two approaches are proposed to overcome the near-far problem: power control with soft handovers and multi-user detection.
Power control attempts to ensure that signals from all mobiles in a cell arrive at the base station with approximately equal power levels. To be effective power control must be accurate to within about 1 dB and fast enough to compensate for channel fading. For a mobile moving at 55 mph and transmitting at 1 GHz, the Doppler bandwidth is approximately 100 Hz. Hence, the channel changes its characteristic drastically about 100 times per second and on the order of 1000 bit/s must be sent from base station to mobile for power control purposes. As different mobiles may be subject to vastly different fading and shadowing conditions a large dynamic range of about 80 dB must be covered by power control. Notice, that power control on the down-link is really only necessary for mobiles which are about equidistant from two base stations, and even then neither the update rate nor the dynamic range of the up-link is required.
The interference problem that arises at the cell boundaries where mobiles are within range of two or more base stations can be turned into an advantage through the idea of soft handover. On the down-link, all base stations within range can transmit to the mobile which in turn can combine the received signals to achieve some gain from the antenna diversity. On the up-link a similar effect can be obtained by selecting the strongest received signal from all base stations which received a user's signal. The base station which receives the strongest signal will also issue power control commands to minimize the transmit power of the mobile. Note, however, that soft handover requires fairly tight synchronization between base stations and one of the advantages of CDMA over TDMA is lost.
Multi-user detection is still an emerging technique. It is probably best used in conjunction with power control. The fundamental idea behind this technique is to model multiple-access interference explicitly and devise receivers which reject or cancel the undesired signals. A variety of techniques have been proposed ranging from optimum maximum-likelihood sequence estimation via multi-stage schemes, reminiscent of decision feedback algorithms, to linear decorrelating receivers. An excellent survey of the theory and practice of multi-user detection was given by Verdu [Verdu, 1992].
