Required signal‐to‐interference ratios for shortwave broadcasting

Abstract

Abstract. The required signal-to-Imerference (RSI) ratio for a specified grade of HF radio service is the hourly median warned signal power at the input of the receiver needed relative to the sum of the hourly median unwanted signal power and the hourly median radio noise power in the RF bandwidth of the receiver, adjusted so that the hourly median ratio will not fall below the RSI ratio more than a certain percentage of the time due to minute-to-minute fading within the hour. Shortwave listeners are well aware of the deleterious effects of cochannel and adjacem channel imerference. This type of interference is especially prevalent in the overcrowded international broadcast bands where it is manifested by cross talk and a beat note produced in the receiver by the carrier of the unwanted signal. Yet little agreemere exists as to the magnitude of the amplitudemodulated, double sideband (AM-DSB) interfering signal that can be tolerated by the listener. Numerous protection ratios have been proposed in the literature, as well as by elemems of the International Telecommunication Union. These values tend to range from 17 dB [International Frequency Registration Board, 1989] to as high as 50 dB for "good commercial quality, " offset in carrier frequency of 500 Hz and 10 dB short-term fade protection [CCIR, 1970]. In this paper, several significam experiments are reviewed for the purpose of normalizing their findings to a common set of parameters. The parameters relate to articulation scoring, type of noise (if used), fading of warned and unwanted signals, type of interference, listener skill, bandwidth of the receiver, carrier frequency offset, etc. From this compilation of normalized data, RSI values are recommended as they relate to the desired broadcast quality and the signal-to-noise ratio of the warned signal. The RSI ratios are compatible for use in HF sky wave prediction programs that comain appropriate RF noise and imerference combining subroutines. The recommended minimum acceptable for cochannel imerference is 36 dB, in order to maimain a fair grade of service where voice reception is understandable but with difficulty for at least 90 % of the hour. In order to provide good service ( i.e., the interference is perceived not to be annoying) to 90% of the listeners, the RSI should be 48 dB. The RSI needed for "no perceptible" interference is 53 dB.