On the use of 15 KHz tertiary channels in 150 MHz IMTS

Abstract

The growth of the Land Mobile Radio Services has been so rapid that it has resulted in an ever increasing demand for additional channels. A channel split is an approach that nearly doubles the number of channels available. This paper presents the known factors that need to be considered in order to use channels spaced 15 kHz apart in IMTS systems where the current channel spacing is 30 kHz. One approach in using 15 kHz channels would be to replace all existing land and mobile equipment with new radios having narrower bandwidth and operating with lower frequency deviation. The approach considered here is economically more attractive. It makes use of current equipment with minor modifications, and retains the bandwidth and deviation in current use. The 15 kHz channels could be assigned to adjacent systems that could be located closer than co-channel systems, thereby providing some relief. Even greater relief is possible if the interstitial 15 KHz channels are operated at the same location as the current 30 KHz channels. The results presented here address this cosited case, but are applicable to adjacent systems as well. Consider first the base-to-mobile direction. In IMTS systems, a mobile unit scans through all equipped channels until it locks on the channel marked idle with a 2 kHz tone at full deviation (i.e., 5 kHz). The possibility exists that while scanning, the mobile unit may lock on the wrong channel by detecting the idle tone of an adjacent interstitial channel. It will be shown that the deviation of signaling tones may be reduced down to 2 kHz with no loss in signaling reliability. This lower deviation on idle tone, together with the current practice of transmitting an unmodulated low power carrier on channels not in use, essentially eliminates the false lock problem. The other problem in the base-to-mobile direction is the interference on a voice circuit produced by signaling tones or voice modulating the adjacent interstitial channel. It will be shown that the interfering signal must be 20 dB greater than the desired in order to seriously degrade a commercial call. By locating both transmitters together (not necessarily on the same antenna), the average power levels of the desired and interfering channels received at the mobile will be nearly equal regardless of the mobile's location. Thus, the interference to voice circuits in the base-to-mobile direction is not a serious problem. In the mobile-to-base direction, it is possible that the disconnect signal transmitted when a mobile terminates a call may be detected in a busy adjacent interstitial channel and falsely terminate the call on that channel. As with the "false lock" problem, this situation is alleviated by reducing the signaling deviation in the mobile. Since there may be many mobiles whose signaling deviations have not been reduced, say roamers, a modification can be made in the control terminal so that a test is made of the absence of RF carrier after detecting the disconnect signal before terminating a call. The most serious problem with the use of interstitial channels is the possible interference on voice circuits in the mobile-to-base direction. As given before, an interfering signal 20 dB or more above the desired signal could result in serious degradation. (Note that the effect of Rayleigh fading is included in the protection ratio of 20 dB.) Since there is no effective way of controlling the relative power levels received at a base station from mobiles on adjacent channels, a solution is to equip base receivers with an "off-channel" squelch circuit with a 20 dB threshold setting. There must be enough receiver sites such that all sections of the service area are covered by at least two receivers. With this arrangement, the occurrence of strong interference will cause the weaker signal to be switched to an alternate receiver. Thus, cosited interstitial channel operation is possible if the signaling deviation is reduced to approximately 2 kHz, base receivers are equipped with an "off-channel" squelch circuit, and a minor modification is made in the control terminal. It will be shown that the same modifications would be necessary to implement adjacent systems if the separation between systems is to be significantly less than the co-channel distance.