METHOD AND APPARATUS FOR ACHIEVING 180 DEG PHASE INVARIANT TRANSMISSION IN A PCM MODEM SYSTEM

摘要

In a PCM modem system in which equivalence classes are used to communicate information from a transmitter to a receiver, a method is provided to solve the problem of 180 DEG phase reversals in the communications channel which result in a garbled transmission. This is accomplished by remapping the equivalence classes into a form that can be differentially encoded and decoded such that equivalence class identity is not lost during a phase reversal of the channel. Referring to Figure 2, an incoming bit stream (10) is applied to a modulus converter (12) which is in turn coupled to a mapping block (14) which maps the output of the modulus converter to the type of structure shown in Figure 1 in which each equivalence class is provided with a label that includes a sign bit and magnitude bits.