Adaptive DFE for GMSK in indoor radio channels

Abstract

We simulate the performance of an equalized Gaussian minimum shift keying (GMSK) signal in an indoor radio environment with fading, noise, imperfect carrier recovery, co-channel interference (CCI), and intersymbol interference (ISI). We show that data rates of 20 Mb/s at bit error rates (BER) ≤ 10-4 are possible with root mean square (rms) delay spreads up to 25 ns using a simple limiter-discriminator-integrator (LDI) receiver and a (6,4) decision feedback equalizer (DFE). In environments with larger rms delay spreads, coherent detection is required for the same performance. We show that using a decision-directed second-order digital carrier synchronizer with time varying loop filters, frequency offsets up to 200 kHz can be corrected with negligible performance degradation. This paper utilizes a DFE structure which compensates for both modulator and channel ISI, and yet requires no power-intensive multiplication operations in the feedback section. A DFE(8,8) with two-level switched (selection) diversity is shown to allow 20 Mb/s data transfer at BER ≤ 10-4 for rms delay spreads under 150 ns, with CCI. A light BCH(26,31) code allows error-free reception of over 90% of packets with rms delay spreads under 150 ns, and up to 70% of packets with rms delays of 150 ns.