Analysis and Dynamic Range Enhancement of the Analog-to-Digital Interface in Multimode Radio Receivers

Abstract

The rapidly developing wireless market has spawned a multitude of

different standards for cellular, PCS, and wireless data. To allow users

the ability to access services conforming to disparate standards,

multimode handsets capable of software reconfiguration are needed.

These "software radios" are distinguished from their traditional

counterparts by their strong reliance on digital channel filtering and

demodulation which may be reprogrammed to receive different

standards. In these radios, higher dynamic range is required from the

analog portion, most notably, the analog-to-digital converter (ADC).

This research examines through analysis and simulation the

performance requirements of analog-to-digital converters for use in

radios which are conformant to the AMPS, IS-54, GSM, and IS-95

cellular standards. Simulations reveal the degradation in performance

under conditions of off-channel interference, fading, and converter

nonlinearities. Included in this analysis is the design of automatic gain

control (AGC) for narrowband and IS-95 spread spectrum systems to

optimize quantization noise and distortion due to A/D overload. Lastly,

methods for improving the dynamic range of the analog-to-digital

interface such as nonuniform quantization, companding, and dither are

presented. The development of a novel A/D using a direct-sequence

pseudo-noise (DSPN) technique in conjunction with an asymmetrical

quantizer is presented and compared with standard dither techniques.

Advantages of this technique compared to ordinary ADC's include an

almost one bit improvement in resolution, quantization noise whitening,

elimination of A/D offsets, and the ability to simultaneously digitize

multiple analog signals with a single quantizer. The technique requires

no synchronization and is easily implemented.