EVERYTHING YOU WANTED TO KNOW ABOUT DOUBLE DIFFERENTIAL ENCODERS BUT WERE AFRAID TO ASK

Abstract

The existing offset quadrature phase shift keying (OQPSK) differential encoder in IRIG-106 is a curious scheme with a rather mysterious origin. In this paper, an alternative scheme known as double differential encoding is proposed. In many aspects, the proposed scheme has equivalent performance to the existing scheme: it successfully resolves the 4-phase ambiguity introduced by most carrier phase tracking loops and it also produces two decoded bit errors for each detection error. However, the proposed scheme has a number of conceptual advantages: it can be derived easily from first principles, it decouples the operations of even-bit/odd-bit demultiplexing and differential encoding, and it greatly simplifies the overly-complicated binary-to-ternary symbol mapping for OQPSK. It is also demonstrated to have tangible benefits, such as improved performance in systems with error control coding.