One of the physical measures of RF channel quality is the carrier-to-interference or C/I ratio2.18 . This ratio is logarithmically proportional to the signal quality enjoyed by the receiver of the signal.2.19 The larger the C/I ratio, the better the channel quality.
C/I ratios of 17 dB are ideally used to determine the edge of coverage for a cell. If the measured C/I falls below this level, the mobile should be in the coverage region of another cell and a cell handoff should be performed. The interior of the cell should provide C/I ratios which exceed 17 dB, unless the mobile is located in an RF coverage "hole."
There are two kinds of RF interference possible: cochannel and adjacent channel. Either of these forms of interference can occur if the cellular frequency reuse scheme is inadequate (i.e., the "n" in n-cell reuse is too small for the geography available and power level in use).
Cochannel interference results when two transmitters within range of a common receiver use the same channel (frequency) simultaneously. The receiver will receive a combination of the two signals and will be unable to make any sense of the combined signal.2.20 In this case, the two channels can be said to have interfered with one another.
Adjacent channel interference occurs when two transmitters within range of a common receiver use adjacent channels (i.e., neighboring frequencies) simultaneously. Because the physical characteristics of the RF channel causes some spill-over of the signal into neighboring frequencies, adjacent channel transmissions could interfere with one another.
Because of the interference caused by the simultaneous transmission on cochannels or adjacent channels within a cell, only one can transmit at a time. The earliest mobile phone systems used the same channel for the network and the mobile devices. This half-duplex mode of operation greatly hindered the efficiency of these early mobile RF systems.
Current cellular systems are full-duplex. This is accomplished by using different physical channels in the forward channel direction (i.e., network to mobile) and the reverse channel direction (i.e., mobile to network). The channels used in each direction are sufficiently separated in the frequency domain (they are separated by 45 MHz in current AMPS systems) so as to prevent interference. Of course, full-duplex operation means that the mobile and the base station must each have two RF transceivers (i.e., one transmitter, one receiver) simultaneously engaged.2.21
Even if no other transmitters are causing interference, a received RF signal can be garbled due to a phenomenon known as multipath orRayleigh fading. This form of self-interference occurs when multiple out-of-phase copies of the same signal destructively interfere with one another due to reflections of the signal off of natural or man-made surfaces.2.22 In a fading situation, the reflected signal is delayed sufficiently that it is out-of-phase enough to interfere with the direct line-of-sight path. Multipath fading can occur when the mobile is stationary or in motion.