Additional interference types

Another fading phenomenon, Ricean fading, occurs when the signals are sometimes partially obstructed and the subscriber or mobile devices are mostly nomadic. This type of fade typically is found in computer-based wireless devices that are stationary for long periods, but the receiver can be moved when the signal is weak or when fading occurs. Ricean fades are moderate — in the range of 6-9 dB for 95% confidence — and are long-lived. But, because the receiver can move out of the fade, these values should be regarded as fluctuating; consequently, a fade margin of 6-7 dB is recommended. If antenna spatial diversity is used at the base-station site or at the subscriber end, the fade margin value can be lowered by 3-6 dB. Again, MIMO also can reduce the effects of Ricean fading.

Raleigh fading is the worst-case RF signal-deterioration scenario and is found when the subscriber is moving at fast speeds, sometimes up to 125 miles per hour. At an average speed of 60 miles per hour, the fade can range from 6 dB to 30 dB. However, the subscriber often moves so quickly through the fade that the effects are minimized to about 10 dB for 95% confidence. In no cases should this value be less than 10 dB, but there may be cases — for instance, when data packets are sent outbound, with or without forward error correction — when the fade-margin value used in radio coverage and interference predictions may exceed 10 dB.

Carrier to co-channel interference (C/I) indicates 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 they can be in the range of 10 to 12 dB. Note that the smaller the C/I 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."

When the C/N value of 7.6 dB is added to a fade margin for an average moving subscriber, it is termed "faded C/(I+N)." The roaming subscriber is taken as an average case of a fast-moving object using a Raleigh fade margin of 10.1 dB for 95% confidence. This results in a faded C/(I+N) of 17.7 dB, which further equates to digital audio quality, or DAQ, of 3.4 (out of 4).

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.

Carrier to co-channel interference, adjacent-channel interference and noise, or C/(I+A+N), represents all of the interference types that affect the targeted receiver

Understanding these values, as well as the different types of interference, lets system designers assess the ability of a radio site and the subscriber units that operate on that site to perform properly. These values can be placed into radio coverage and interference software tools to visualize cause and effect. The number of radio sites required to cover a targeted area can be greatly affected when interference is simulated and detected. Mitigation techniques — including the moving of sites, or the adding of antenna spatial diversity or other antenna optimization techniques like MIMO — will minimize the impact on valuable resources and allow for an efficient design.