Radio-telephone with automatic channel evaluation
High Frequency radio was used for decades to provide long distance telecommunications via ionospheric modes of propagation. However, the variability of the ionosphere and high levels of background radio noise made HF radio less than satisfactory as an adjunct of a telephone system. Experience in northern Canada has shown that skilled operators are required to achieve circuit reliability anywhere near 70 percent. The ionosphere is dynamic in nature, requiring the operator to effect frequency changes to accommodate the predictable components of ionospheric variation (diurnal and seasonal) as well as the unpredictable variations due to solar activity. In areas where skilled operators are not available, the level of success may be less than 70 percent. Overall, equipment reliability has been less than satisfactory. This has resulted in a general abandonment of efforts to improve HF radio performance in favour of newer (and costlier) communications techniques, such as satellite relay.
The objective of this project was to produce an improved HF radiotelephone system that could be directly and automatically interconnected with the domestic telephone system.
In the RACE system, circuit availability is maximized through the proven concept of real-time channel evaluation. This is achieved through the use of frequency agile transceivers coupled to wide band antennas, and rapid testing of the radio circuit on each of the available frequencies to ensure that the best one is chosen for the subsequent call. Microprocessor control of the channel evaluation process permits fully automatic operation, thereby eliminating the need for an operator and facilitating automatic interconnection with a telephone system. Real-time channel evaluation represents a step beyond frequency prediction programs because prediction cannot account for factors such as ionospheric disturbances and interference from distant stations. These factors have a strong influence on the selection of a frequency channel. Also, noise and speech quality, a common problem with HF radio, is meant to be improved by the additional use of the Syncompex system.
Research began in 1975. Trials began in 1978-1979. A RACE system was installed in Labrador from 1985-89. Key personnel were Sherman Chow, G.W. Irvine, B.D. McLarnon, L. Hatton
Research into the design of this system grew out of the Radio Systems Research and Development Branch's work on an HF trail radio system for use in northern Canada. Preliminary tests of the experimental system were carried out using sites at Ottawa, Ontario; Low, Quebec, and Hammond Plains, Nova Scotia. These sites were chosen because they included both a short skywave path of about 60 kilometres and a medium range path of about 1,000 kilometres. Eight frequencies between 3 and 21 MHz were chosen as a suitable complement of frequencies.
Over one thousand calls were completed and it became apparent that the objective of 90 percent or better availability was achievable. In order to gather more accurate figures on system performance, a lengthy period of continuous operation was begun. This operation took place from April through June 1980. Since it was not feasible to man the stations for this length of time, printers were installed at the remote stations to log the data on the quality of the channel sounding transmissions received from the master station.
In analysing the channel quality, two levels of performance were distinguished. The first level is achieved when the bit error rate is sufficiently low that virtually all digital messages are received without errors. Such a channel offers very good voice communications. Level 2 is when approximately one third or more of the message is received without error. This roughly corresponds to the lower limit at which voice communication can be carried on without major difficulties. Several dialling attempts would be required when the channel quality is near the lower limit.
Also, the most difficult time for HF propagation is usually in the early morning hours prior to sunrise, which coincides with a period of very low traffic density. The results of the one-air tests clearly demonstrated the value of real-time channel evaluation. Many instances of propagation well above the predicted maximum useable frequency (MUF) were encountered. Interference from distant stations sharing the same channel, another unpredictable element in HF communications, was also found to be a very important factor in determining the best channel to use.
RACE was a spin-off of work being done at CRC on an HF trail radio system. RACE is related to research on Syncompex.
Chow, S.M., Irvine, G.W. and McLarnon, B.D. "RACE: An Automatic High Frequency Radio Telephone System for Communications in Remote Areas." CRC Report No. 1338. Ottawa; Department of Communications, 1980.
Bhaneja, B., Lyrette, J., Davies, T.W. and Dohoo, R.M. "Technology Transfer by Department of Communications: A Study of Eight Innovations." MOSST Background Paper. Ottawa; Supply and Services, 1980.
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Last updated on Octoberr 2, 1997 by Cynthia Boyko
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