Design and implementation of an asynchronous transmission process by code division multiple access

Abstract

Figure 1

We want to establish unidirectional links between N transmitters Ei and N receivers Rj by means of a common channel, an optical line for example. This channel has the property to transmit a binary alphabet and delivers the logic OR results of binary informations presented by the N transmitters.

The problem is to find a means to code the information presented by the Ei so that Rj can recover the corresponding information without error and without extern synchronisation, except the fact this transmitter has the elementary rhythm Δ.

The author proposes a solution where each transmitter can be in two states : the low state, where it transmits 0 and the high state, (accessible from the low state). The duration of each state is a multiple of a βi period. Only in the high state can the transmitter Ei transmit his own sequence Si, (composed of Mi elementary moments δ), on the line. The sequence length Li-Mi. δ is inferior to βi. The associated receiver Rj is equipped with a coincidence detector, called decoder, which verifies the existence of all "1" pulses corresponding to the Si sequence in those Mi elementary moments which compose it. A formating circuit resets the initial information by the common clock of periode δ and a programmed counter.

The N sequences are designed in such way that, in absence of the transmission of Si, no other combination of sequences could give the set of "1" required.

This process is similar to an asynchronous transmission by code division multiple access.