Synchronous multiplexing of digital signals using a combination of time- and code- division multiplexing (T.D.M. and C.D.M.)

The paper describes some arrangements for multiplexing and demultiplexing digital signals in element synchronism. The multiplexed signals are transmitted over a common channel, from a single transmitter to a single receiver, and the demultiplexing of the signals is achieved in the detection process at the receiver. A combination of time division and code division multiplexing (t.d.m. and c.d.m.) is used, in which the t.d.m. signal-elements are orthogonal, as are the c.d.m. elements, but simultaneously transmitted t.d.m. and c.d.m. signalelements are not orthogonal. Arrangements using a linear combination of the two sets of orthogonal signals are studied, with various iterative detection processes, and their performances are compared with that of a system using a non-linear combination of the two orthogonal sets. It is shown that the latter system achieves a considerable advantage over the others and is particularly well suited to applications where the number of multiplexed signals is typically a little greater than the maximum number orthogonally multiplexed. Computer simulation has been used to measure the performances of the different systems.