Noise detected, fractionally spaced equalization for analog cellular data modems

In the analog cellular environment there is a need for a faster more robust data modem. This paper presents the findings of an investigation into the feasibility of using a fractionally spaced equalizer with a burst noise detector as a means of achieving the above goal. The equalizer input is a discriminator detected radio signal whose output is the QAM baseband signal plus noise that has been further distorted by a telephone circuit. The performance of a T/4 and a T/2 spaced equalizer is compared with the performance of a symbol spared equalizer for low to medium signal to noise ratios (SNRs) commonly encountered in the analog cellular regime. Performance criteria are minimum mean square error, timing sensitivity, SNR after equalization, signal to interference ratio (SIR) after equalization and peak distortion (PD) after equalization. By means of simulation the convergence aspects of the T/2 spaced equalizer were investigated under differing burst and correlated noise scenarios. A noise detection plus equalizer control scheme is proposed and evaluated to maintain operation of the equalizer under burst noise conditions. A standard equalizer without burst noise detection and control fails under normal analog cellular operating conditions. However, the noise detected equalizer maintains operation through the burst at a level of error performance that is suitable for error control procedures to work well