Susceptibility to interference and noise

6.4 Susceptibility to interference and noise

Adjacent cross-polarized channel interference (ACI) may become one of the main factors in the design of radio systems. The value of ACI is dependent upon antenna cross-polar discrimination (XPD). As the XPD degrades during fading, ACI may become crucial in noise budget calculations of the radio system. In order to minimize the detrimental effects of ACI on 64-QAM and other multi-state modulation methods, improved antenna XPD or a smaller roll-off factor pulse shaping filter may be necessary.

The factor, W, also gives a measure of the received carrier power level required to maintain a given level of degradation from fixed broadband interference, as the symbol rate is varied. However, when interference is between a number of similar transmissions, all with symbol rates, occupied bandwidth and relative spectral positions reduced in the same ratio, the degrading effects can only be maintained at a constant level by introducing some means of compensatory interference reduction – for example, improved antenna discrimination (at network nodes), improved XPD, or larger RF channel spacings (on a route). If this last expedient is adopted, improvement in spectrum utilization efficiency on a route is no longer inversely proportional to symbol rate. The magnitude of the interference reduction required is proportional to the change in the value of W and to the ratio of symbol rates.

Various interference factors introducing ground scattering become more significant in multi-state modulation systems (see Recommendation ITU‑R P.530).

The outage probability due to thermal noise and interference is given by (see Recommen­dation ITU‑R P.530):

where:

_s: spatial coefficient between two antennas.

Suppose that the outage probabilities due to waveform distortion and interference noise are represented as P_d and P_n, respectively. The total outage probability (P₀) may be evaluated from the composite fade margin concept, for example:

Where digital fixed wireless systems share the same frequency band with FDM-FM systems in dense networks, FM co-channel interference may be excessive without the use of interference cancellers.