COHERENCE MULTIPLEXED SUBCARRIER MULTIPLEXING (CM SCM) LIGHTWAVE SYSTEM FOR MICROCELLULER MOBILE COMMUNICATIONS/

Subcarrier multiplexing (SCM) transmission over optical fibers for mic rocelluler mobile communication systems has been actively studied. How ever, as increasing the number of channels, intermodulation distortion (IMD) becomes the intrinsic problem. On the other hand, coherence mul tiplexed (CM) system for broad-band access communications has been dev eloped and may find wide spread application because of its simplicity and flexibility. However the interference noise produced by the mixing of different optical channels at a photodetector is the serious probl em. Therefore, on the down link for fiber optic microcell systems, it is attractive to consider the combination of SCM and CM: coherence mul tiplexed/subcarrier multiplexing (CM/SCM). In this hybrid scheme, sinc e each optical channel is modulated by M microwave subcarriers, the ba ndwidth allocation is very flexible. In addition, the same microwave s ubcarrier frequency may be used among the different optical channels, because the optical channels are uncorrelated each other. We derive th e received carrier-to-noise ratio (CNR) characteristics of CM/SCM syst em with optical amplifier as preamplifier or in-line amplifier for the optical down link from central station to base stations. The system p erformance of CM/SCM system is compared with those of coherent SCM (C- SCM) and CM systems from the viewpoint of the maximum number of base s tations to be admitted. It is shown that the performance of CM/SCM sys tem is superior to those of C-SCM and CM systems when optical power at the input to in-line amplifier is limited. CM system has good perform ance when preamplifier gain and optical power are large, and C-SCM sys tem is the best when in-line amplifier is used.