Multi-Carrier GMSK Modulator

Abstract

In conventional base station solutions, the transmitted carriers are combined after the power amplifier (PA), as shown in Figure 11.1. This chapter describes an architecture where a multi-carrier GMSK modulated IF signal is up converted to RF by two mixers and bandpass filters (BPFs) as shown in Figure 13.1. This saves a huge number of analog components, many of which require production tuning. Consequently, an expensive and tedious part of the manufacturing will be eliminated. The proposed multi-carrier GMSK modulator does not use an analog I/Q modulator, therefore the difficulties of adjusting the dc offset, the phasing and the amplitude levels between the in-phase and quadrature phase signal paths are avoided. A single linearized multi-carrier power amplifier replaces a bank of individual amplifiers whose high-power outputs are conventionally combined by using selective cavities. Hence, power losses inherent to cavity-filter combiners are avoided which results in space and cost savings as well as greater reliability. The GMSK modulation method used in the GSM 900 and the DCS 1800 is a constant envelope modulation scheme. As a number of these GMSK carriers are combined to produce a multi-carrier signal, the beneficial properties are lost. Because of the strongly varying envelope of the composite signal, very stringent linearity requirements are imposed on the wideband D/A converter, up conversion mixers and the PA.