EGPRS Test: Meeting the Challenge of 8PSK Modulation

Gaussian minimum shift keying (GMSK) is the modulation format chosen by the European Telecommunications Standards Institute (ETSI) for GSM and GPRS systems. More recently, ETSI adopted an 8PSK modulation format for use in EDGE.

This application note starts with a review of the 8PSK modulation format. The reasons for adopting 8PSK with 3π/8 rotation in the EDGE system are presented. The relationship between GMSK and 8PSK is reviewed. The challenges of 8PSK power measurements are then considered. The dependence of burst power on modulating data is discussed and quantified. In this context, the trade-off between burst power measurement accuracy and speed is studied. The estimated carrier power concept is introduced as a technique for achieving the required measurement accuracy without having to compromise measurement speed.

From power measurements, we move on to consider the 8PSK modulation accuracy measurements. The manner in which modulation accuracy is specified in the standards reflects the noise-like nature of this measurement. A statistical model is derived to help understand and quantify the rms EVM parameter.

The problem of IQ modulator tuning is addressed. A method for tuning IQ modulators while transmitting normally formatted 8PSK bursts is presented.

The application note concludes with a review of the test modes defined by the standards to support EGPRS mobile test.