256-QAM subcarrier transmission for broadband distribution networks

Broadband distribution networks are presented combining digital microwave radio with optics using subcarrier multiplexing (SCM) transmission. In the first implementation of a high-capacity transmission, the authors present the prospect of fiber-optic transmission of microwave 256 quadrature amplitude modulation (QAM) signals at 10-Mb/s and 50-Mb/s transmission rates. Bit error rate (BER) performance results are shown as a function of received optical power and optical modulation index. It is shown that the results of theoretical BER performance calculations are in agreement with that of preliminary experiments for two types of 256-QAM signals. Error correction coding using a BCH code is very effective in improving the system performance. Finally, the choice of the total optical modulation index for multichannel transmission is discussed