Design and implementation of a generalized parametrizable modulator for a reconfigurable radio

The present paper deals with design and development of a generalized parametrizable modulator (GPM) that can perform Gaussian minimum shift keying (GMSK) modulation and quadrature phase shift keying (QPSK) modulation in a reconfigurable baseband modulator for software defined radio (SDR) architecture. GMSK is the underlying modulation scheme for the global system for mobile (GSM) standard, while QPSK technique is the basic modulation scheme for code division multiple access (CDMA) standard. Although GMSK is an inherently non-linear modulation technique, the present work uses a linearly approximated GMSK technique to take the advantage of the simplicity of the filter structure. The normalised error in amplitude as a function of modulation index and bandwidth time product has been simulated. The proposed GPM is capable of working at a maximum data rate of 13.75 Mbps, thus satisfying the requirements of almost all 2G and 3G air interface standards. The GPM can perform QPSK using four pulse shaping filters, two raised cosine (RC) filters and two root raised cosine (RRC) filters. The proposed GPM architecture is implemented on a Spartan 3E starter kit.