A real time channel evaluation technique for medium-speed digital skywave communications
Abstract
An investigation is reported of real-time channel evaluation (RTCE), for the transmission of medium-speed digital data over skywave circuits. RTCE involves an estimation of the quality of the signal in each allocated channel. Suitable RTCE techniques are investigated for the so-called parallel-tone medium speed modems. Typically data is transmitted at a 150 bits/s rate of each of 16 data tones (overall rate 2400 bits/s), the modulation employed being 4-level differential phase-shift keying. The application of the proposed RTCE scheme is to a communications network dedicated to the continuous transmission of operational traffic. The approach adopted is to determine whether propagation measurements made on low-power transmissions (at frequencies close to the carrier but just outside the communications channel) are related to the bit-error rate and therefore appropriate as a basis for RTCE systems. Parameters measured include Doppler frequency spread, time delay spread and signal-to-(noise + interference) ratio. Of the parameters examined, the BER is most strongly dependent on S/(N+I) level when the latter is less than 30 dB. At higher levels (up to at least 60 dB), the BER dependence of the S/(N+I) level, Doppler and time-delay spreads is comparable, and measurement of at least one of the latter two parameters as well as the S/(N+I) level is advisable for assessment of channel quality.
- Publication:
-
3rd Conference on the Ionosphere and Radio Wave Propagation
- Pub Date:
- 1985
- Bibcode:
- 1985irwp.confR....C
- Keywords:
-
- Bit Error Rate;
- Propagation Modes;
- Pulse Communication;
- Radio Waves;
- Real Time Operation;
- Sky Waves;
- Channels (Data Transmission);
- Data Flow Analysis;
- Signal To Noise Ratios;
- Communications and Radar