The symbol rate of an M-ary DPSK signal, caused largely by inherent high irreducible error in the delay detection system, will be discussed for the case of applying the diversity-combining reception under high-speed Rayleigh fading in land mobile radio systems. A s for the predetection combining methods of the Lbranch, which comprise selective combining iso-gain combining and the maximum ratio combining, we will demonstrate anew that both error rates, caused by the envelope fade and random-FM can be represented entirely by L-square of the error rate in case of no diversity combiner. Particularly, our analysis centers on improvement of random-FM. We conclude that the L-square improvement could be applied to the random-FM similar to the envelope fade and, further, the optimum combining system which minimizes the mndom-FM is also maximum ratio combiner. This report will be helpful in doing comparisons among systems, because it shows that the error rates of M-ary DPSK with the diversity reception characteristics could be represented i n a thorough and understandable form. 42ISSN0424-8368/80/000 1-0042$7.50/0 0 1981 Scripta Publishing Co.

With the recent development of the office automation, portable in‐house terminals for data communication have attracted attention. For this purpose, wireless transmission by optical link is considered promising, since it does not require the installation of signal lines. The past discussions and experiments, however, considered only the one‐to‐one or one‐to‐many communications, but many‐to‐many, i.e., multiple connections were not considered. This paper aims at the application of the spread‐spectrum system to the wireless optical communication link to realize the multiple connections, and derives the basic theoretical expressions for the receiver characteristics. More precisely, the modulation scheme is selected taking the restriction into consideration, which is inherent to the inhouse wireless optical link. Then, the receiver error rate and the multiple connection characteristics of the proposed modulation scheme have been theoretically derived, providing the basis for the system design. Finally, a qualitative discussion is made on the construction and operation of the synchronous system for the proposed modulation scheme.