Step-Size Transmitting Differential Coders for Mobile Telephony

Recent developments in speech digitization 1 have prompted an examination of digital coding as a possibility for mobile radio telephony that conventionally employs analog techniques for speech transmission. Conceivably, much of the signaling supervision and "bookkeeping" in a mobile radio link can be digital; in this case, if the speech were handled digitally as well, it would be simple to interleave the voice bits with the control bits for transmission. Digital coding also 1557 offers the possibilities of inexpensive coder-decoder implementation, straightforward speech encryption (by bit scrambling), and efficient signal regeneration. Perhaps the greatest incentive for the use of digital speech, however, is the thought that a properly designed digital code may be more resistant than analog systems to the multipath fading t h a t characterizes mobile radio. Figure 1 shows the envelope of a Rayleigh-fading signal t h a t is typical in mobile telephony. 2 An important parameter is the fading rate, which is approximately the ratio of vehicle speed V to the carrier wavelength A. For the example in Fig. 1, this ratio is about 15 Hz. Note also that the 5 m represent a total travel time of about 1 s at the indicated vehicle speed, and that the fading is slow or correlated in the sense that a given fade (signal strength below a specified threshold) can last for several tens of milliseconds (which will represent several hundred speech bits for the codes of this paper). The probability of a fade can be decreased by an order of magnitude by the use of diversity reception (two-branch, equal-gain or switched diversity, for example).