PI/4 DQPSK modulation with coarse mapper precession and fine filter precession

Abstract

Abstract The amount of memory needed for a ROM-based π/4 DQPSK filter is reduced by using a mapper in an IQ modulator (inverse mapper in a demodulator) that incorporates a 90° phase shift for every other symbol. During the intervening symbols no additional phase shift is incorporated by the mapper. This produces a coarse precession of 90° for every other symbol. The 90° of coarse precession may be produced by using alternate DQPSK mappers for alternate symbols. Both mappers share a common repertoire, so no new modulation state symbols are required. During the alternate 0° symbols the filter inserts a 45° phase shift, but does not insert such a 45° phase shift during those intervening times when the mapper (or inverse mapper) is inserting a 90° phase shift. This produces a fine precession. The result is to produce a uniform precession of 45° between each modulation state symbol. The technique described involves allocating part of the precession to the mapper (coarse precession) and a remaining part (fine precession) to the filter. In a preferred case: the amount of overall precession is half the nominal symbol-to-symbol phase period; the coarse precession from the mapper equals the nominal symbol-to-symbol phase period and ocurrs every other symbol; and, the fine precession from the filter ocurrs on intervening symbols and can be represented by a single bit. Other amounts of coarse precession from the mapper and fine precession from the filter are concievable, however, which may involve more than one bit of description for the fine precession.

Inventors

• 👤 Brian P. Fetz

Bibliographic Details