The advantages of close spacing
The Carey curves still are used to this day for predicting the range of radio systems and for determining interference from co-channel users. But given the extremely crowded radio spectrum and the still growing demand for more radio transmitters, RF engineers are using newer models and formulas to determine short-spacing opportunities. With electronic geophysical maps available today, more accurate prediction systems that are extremely refined are being used to pinpoint areas where coverage issues exist. By fine-tuning the signal levels in specific directions, transmitters may be added to a new or existing system in locations that otherwise would not have been allowed.
It's important to note that frequency coordinators do an excellent job of keeping new systems and new transmitters from interfering with existing systems. If a new system is proposed in an area where co-channel or adjacent-channel interference would prevent the new system from being constructed, the new system can be engineered with advanced techniques mentioned earlier and by using lower power, directional antennas, lower antenna heights, tower shadowing, and other methods that will keep the interference at a minimum level.
Meanwhile, regional planning committees in the narrowband portion of the 700 MHz and 800 MHz bands are commissioned by the FCC to manage that spectrum and determine interference and short-spacing. The RPCs are allowed to use methods other than the Carey curves and HAAT rules to determine optimum short-spacing and frequency re-use.
With the new propagation software on the market today, the techniques described can be added to the prediction programs so that the coordinators can see that the interference levels will be acceptable for the new system, while still protecting the incumbents on the channels. This will allow new systems to be added with closer spacing that the older prediction programs and charts do not deem acceptable.
The RF tools use two methods to determine whether co-channel or adjacent-channel interference is a potential problem for short-spacing opportunities. From last month's LMR 200 article on interference:
Carrier to co-channel interference, or C/I, represents the ability of the receiver to distinguish between one signal and another on the same channel, i.e., to lock onto the desired signal and reject unwanted signals on the same channel. C/I values vary by type of radio and manufacturer, but can be in the range of 10 to 12 dB. Note that the smaller the number, the better the receiver's ability to distinguish between signals. In analog simulcast systems, and in some other wireless technologies, this capability is termed "capture."
Carrier to adjacent channel interference, or C/A, represents the amount of desired signal that must be above an adjacent channel interferer's effect on the target receiver. This value is typically 65 to 70 dB and is a radio manufacturer specification. C/A is a good approximation of the receiver's ability to avoid desense. Any signals closer than the adjacent channel can be considered "on-frequency" or "on-channel." This is due to the inherent filtering found either in the front end of the receiver, before the IF stages, or in an external filter or waveguide.