The bandwidth-distance product limitations of multimode fiber caused by intermodal dispersion may be overcome by using subcarrier multiplexing. Since there are deep nulls in the frequency response within the frequency band of interest, it is necessary either to avoid these frequencies adaptively or to use an appropriate coding technique. We consider the case of using a training sequence to identify the subcarriers to be discarded, and also diversity coding in which the information may be recovered in the presence of the loss of a fixed number of subcarriers. Simulation results indicate that diversity coding, which is less complex than the use of training sequences, provides very good performance over a wide range of channel conditions.

For a binary fiber-optic communication system in which an optical preamplifier is used, and for which the amplifier bandwidth (B) is large compared to the bit rate (1/T), it is shown that the minimum number of photons per bit required to achieve an error probability P/sub e/=10/sup -9/ is given by 36+6 square root BT. This generalizes a result of P.S. Henry (1989) who showed that if the amplifier bandwidth is matched to the signal bandwidth, the sensitivity is 38 photons/bit.>

Quantized feedback, a DC restoration technique utilizing decision feedback equalization, can be used to compensate for unavoidable low-frequency cutoff in digital regenerators. An analysis is provided of the error probability and sensitivity to timing offsets of quantized feedback for a single-pole high-pass filter regenerator characteristic. Error probability is expressed in terms of the moments of the intersymbol interference (ISI), and these moments are calculated using the Markov chain properties of the ISI. To keep degradations small, the low-frequency cutoff should be no greater than 15% of the bit rate.>

The following design considerations for long-haul lightwave systems are reviewed: outage and error rate requirements, terminal architecture, regenerator span design and performance specification, capacity upgrade alternatives, and broad-gauge economic factors.