Generation of GMSK using direct digital synthesis

The advent of digital mobile communications has led to a requirement for constant envelope modulation schemes whose spectrum is contained within a narrowband. A modulation scheme that meets these requirements is Gaussian filtered minimum shift keying (GMSK). Generation of GMSK using direct digital synthesiser (DDS) techniques can produce very accurate I and Q signals. However, this accuracy is offset by the errors inherent in quadrature modulation (QM). The affect of QM errors on the envelope and phase trajectory have been presented and it has been shown that the spectral shaping of the GMSK signal is unaffected. Finally, a technique has been described for QM error correction that employs amplitude modulation feedback to predistort the I and Q signals