Interference and the noise floor

Apart from the basic thermal noise, other forms of noise exist. At fre- quencies below about 200 MHz, man-made noise (from electronic and industrial sources) and galactic noise add considerably to thermal noise and become the major consideration when determining the required signal level. Above a few hundred megahertz thermal noise dominates and radio planners will use an estimate for this in determining the required signal level on the wanted service. Should the level of man-made noise and interference increase above frequencies of a few hundred megahertz then this will affect the performance of mobile telecommunication systems. This is not unlikely given the increasing speed and number of personal computers and also the increase in the number of portable radio terminals for which no licence is required. If the interference increases the effective noise floor by 1 dB, the range of a base station will decrease by a factor of approximately 101=35¼ 1:07. The area covered by a base station will decrease by a factor of 1:072_{¼ 1:14. Figure2.13 shows a comparison of coverage areas for a}

difference of 2 dB in path loss. In this case the coverage range would be reduced by a factor of 1.14 and the coverage area of each base station by a factor of 1.30. The reduction in coverage range would mean that the operator will have to deploy 30% more base stations to provide the same coverage. The situation is even more serious if the rise in the noise floor

Original coverage area

Reduced coverage area

Figure 2.13 The reduction in coverage area of a sector antenna caused by a 2-dB increase in the noise floor.

occurs after the network infrastructure has been installed. The operator’s customers will experience a reduction in the quality of service that will be difficult to rectify. If interference causes the increase in the effective noise floor, installing high-quality receivers will do hardly anything to improve the situation. If the operator has already installed such receivers, they will feel that the investment has been wasted.

2.10 Summary

When considering point-to-area communication it is very common to use the term ‘field strength’ rather than power density or received power. There is often only a single terminal and antenna (the base station) to consider rather than one at each end of a link. The base station produces a field strength at a particular distance. The received power then depends on the characteristics of the receiving antenna. Predicting the field strength at a distance can be undertaken using a variety of models, most notably ITU-R P. 1546. Using this recommendation, it is possible to predict that the field strength produced by a certain transmitted power does not depend greatly on frequency at short distances, but, on longer paths, particularly as the path becomes trans-horizon, the field strength produced reduces with increasing frequency. For point-to-area services, such as digital mobile communications, for which base stations serve a relatively small area, alternative prediction models that generally require less computation are used. One example of this is the Okumura–Hata model. Using this model, it is seen that link loss from the base station to the mobile terminal increases with frequency. One impact of this is that significantly more base stations will be required to provide coverage on a network at 1800 MHz than would be the case if a frequency of 900 MHz were used.

The base-station antenna is usually formed of an array of smaller elements. Two major categories of base-station antenna are ‘sectored’ and ‘omni-directional’ antennas. Sectored antennas have a reflector that directs the power in a particular direction. This provides more gain than would an equivalent omni-directional antenna. When using sectored antennas, it is common to find three antennas on a base station providing

complete panoramic coverage between them. Mobile-station antennas are omni-directional because requiring the user to point the antenna in a particular direction would be seen as highly inconvenient.

All predictions of coverage are made assuming a certain receiver sensitivity. This in turn assumes a level of background noise. If inter- ference increases the total level of unwanted power at the receiver, this will reduce the coverage from a base station.

Predicting the strength of a radio signal in the shadow of an obstacle is a vital function for propagation engineers. The mechanism by which a wave enters into the shadow of an obstacle is known as diffraction. Even the simplest of practical obstacles pose severe mathematical challenges. More easily solved approximations are adopted in order to estimate the strength of diffracted signals. The starting point for diffraction problems is the case where a receiver is in the shadow of a perfectly absorbing ‘knife-edge’ obstacle. This is then extended to encompass the situation where there are several such obstacles on the path. Many approximate multiple-knife-edge prediction methods exist and the most commonly used are analysed and compared. More accurate ‘near-exact’ methods are discussed. Although these methods usually make better predictions of the signal strength in the shadow of obstacles, they require sig- nificantly more computing time as well as being significantly more complicated to implement. Once an understanding of the properties of a diffracted signal has been obtained, it is possible to derive clearance requirements for a point-to-point path so that diffraction effects may be safely ignored. The insights gained by investigating the mechanism of diffraction into the shadow of an obstacle can be used to analyse two related phenomena: reflection from a finite surface and the formation of the radiation pattern of an aperture antenna.