Receiver performance of frequency division multiple access using spectrally overlapping signals

The data rate of a digital communication system depends on the bandwidth available to the system. Nyquist´s theorem states that it is possible to transmit without intersymbol interference (ISI) at 2B symbols/s, given a bandwidth of B Hz. At present, no commercially available system achieves this Nyquist efficiency. It is well known that under certain orthogonality conditions, data transmission using multiple, spectrally overlapping signals can achieve the Nyquist efficiency. In practice, orthogonality is difficult because distortion in a physical channel, generally unknown at the receiver, spoils orthogonality; and even for an ideal, distortionless channel, orthogonality requires strict synchronisation of symbol time, frequency and phase in the system. The performance of a spectral overlap system in point-to-multipoint (PMP) networks is investigated. The orthogonality conditions are partly satisfied to minimise interferences, but conditions that are difficult to achieve are relaxed: therefore interference is present. Joint receivers are proposed to combat this interference. The optimum maximum likelihood sequence estimation (MLSE) receiver is derived, and it is shown that orthogonal system performance can be achieved. Also, simpler, sub-optimum receivers are proposed, and it is shown by adjusting system parameters that they too can achieve near orthogonal performance at a fraction of the cost of the MLSE receiver.