The Design and Development of a Spread Spectrum Digital Cordless Telephone for the Consumer Market

Abstract

The design and development of a low-cost spread spectrum consumer cordless telephone is described. The telephone operates in the 902–928 MHz band with a peak RF output power of 1 watt. The telephone combines a simple, direct-conversion receiver architecture with DSP techniques for spreading, coding, demodulation, correlation, and speech compression. Most DSP functions, all glue logic, the controlling state machine, and all circuitry for time division duplex (TDD) operation are implemented in a single ASIC. A telecom codec with ADPCM compression are utilized in the speech path, resulting in high (toll) speech quality. The transceiver subsystem, ASIC, and controlling firmware are described. Some performance benchmarks achieved by the product: digital security through three independent mechanisms; dropout-free operation; and 1 mile line-of-sight operation. The telephone has received FCC approval under Part 15.247 and is currently in production. Adaptive transmit power control and the multipath mitigation technique RSS (recombinant spread spectrum) are employed to give dropout-free performance while minimizing the possibility of interference to and from other systems. Some of the problems solved in the course of development are described, including digital-to-RF self-interference and isolation of sensitive, analog and audio circuits from the relatively high field-strength transmit signal.