Prediction methods for digital mobile communication

The P. 1546 method is recommended for use for frequencies in the range 30–3000 MHz and distances of 1–1000 km. The curves provided by P. 1546 predict the field strength at a height of 10 m above the ground. This cor- responds fairly well to the height of a roof of a typical two-storey building. However, mobile terminals are often at street level. This means that not only are they at a lower height than that for which the curves predict, but also they are often in the shadow of buildings. The P. 1546 method does include a height gain factor to consider such situations but designers of mobile net- works usually rely on other different propagation models specifically designed for such purposes. Designers do not need a propagation model that is valid for distances up to 1000 km and much of the value of P. 1546 would be negated. Additionally, speed of computation is crucial because many coverage calculations would be needed in order to give details of the field strength produced over a large area.

We shall consider two such models here:  Okumura–Hata

 Walfisch–Ikegami

2.6.1 The Okumura–Hata model

This model, developed in1980by Hata [1] and based on measurements reported by Okumura et al. [2] in1968, can be simplified when used for a particular frequency such as 900 MHz and a typical mobile antenna height of 1.5 metres to give

loss¼ 146:8
13:82 log h þ ð44:9
6:55 log hÞlog d dB, ð2:15Þ where d is the distance from the base-station in kilometres and h is the height of the base station antenna in metres. It is claimed to be valid for ranges of distance from 1 km to 20 km and for ranges of base-station height from 30 metres to 200 metres.

When requiring a quick check on coverage, an engineer may well assume a particular base-station antenna height such as 30 metres.

Then the equation simplifies to

loss¼ 126:4 þ 35:2 log d: ð2:16Þ A pan-European ‘COST’ project (COST action 231) entitled ‘Digital mobile radio towards future generation systems’ further developed each of the two above-mentioned models. Since GSM was beginning to occupy frequencies around 1800 MHz, development of the models to be applicable in the range 1500–2000 MHz was a key goal. This goal was achieved. When planning a network, a planning engineer will need to know the most appropriate model for a particular frequency band. For a typical frequency of 1800 MHz and a standard assumed height of a mobile antenna of 1.5 metres, the model put forward as suitable for an urban environment can be written as

loss¼ 157:3
13:82 log h þ 44:9
6:55 log hð Þlog d, ð2:17Þ where d is the distance from the base station in kilometres and h is the height of the base-station antenna in metres. It is claimed to be valid for ranges of distance from 1 km to 20 km and for ranges of base station height from 30 metres to 200 metres.

When requiring a quick check on coverage, an engineer may again assume a typical base-station antenna height such as 30 metres. Then the equation simplifies to

loss¼ 136:9 þ 35:2 log d: ð2:18Þ The minimum signal required by a GSM mobile is approximately −105 dBm.

A 50-watt (47-dBm) transmitter with an antenna gain of 16 dBi (typical for a sectored antenna) would have a transmit EIRP of 63 dBm if there were no feeder losses. Feeder losses would reduce this to approximately 60 dBm and therefore a loss of 165 dB could be tolerated. If an allowance for building penetration of 20 dB is provided for, then a maximum loss of 145 dB must be targeted. The equation must be

transposed to determine the maximum range,

d¼ 10loss
136:9=35:2

¼ 10145
136:9_=35:2

¼ 1:7 km, ð2:19Þ

suggesting a coverage range of 1.7 km for a typical, high-power GSM base station operating at 1800 MHz with a high-gain sectored antenna at a height of 30 metres. In a later section we will see that an extra margin for error needs to be added to the system losses in order to give us confidence that coverage will be obtained. This entails adding in a margin of about 5 dB, thus reducing the range by a factor of 105=35:2_{¼ 1:4. This reduces}

the expected range from a base station from 1.7 km to 1.2 km.

A further model, which is a combination of two separate models by Walfisch and Ikegami (and hence known as the Walfisch–Ikegami model), is more complex than the Okumura–Hata model. It requires knowledge of the street width and orientation of the street relative to the direction of the base station. Such data might not be readily available.

The starting point of the Walfisch–Ikegami method is an equation similar in style to the Okumura–Hata equation, with the exception that terms accounting for ‘rooftop-to-street diffraction’ and ‘multi-screen diffraction’ are included. Determining suitable values for these extra terms requires knowledge about roof heights and street widths. A further correction for street orientation is added.

The Walfisch–Ikegami method requires more information to be available. It is also slower to compute. Its advantages are that it has validity (although with reduced accuracy) even when the transmitting antenna is below the surrounding roof height. It is generally felt to be superior to the Okumura–Hata method when predicting signal strength over short distances in an urban environment (the minimum range of validity of the Okumura–Hata method is 1 km; the Walfisch–Ikegami method can be used over distances as short as 20 metres).

2.6.2 Comparison between propagation at 900 MHz and propagation at 1800 MHz

For coverage estimates at a frequency of 900 MHz we have been using the following approximate equation: loss¼ 126:4 þ 35:2 log d. The equation

for free-space loss has a 20 log f element that suggests that, if the frequency is doubled to 1800 MHz, then the loss will increase by 20 log 2¼ 6 dB. Indeed, we have seen that a suitable approximation for estimating path loss at 1800 MHz is loss¼ 136:9 þ 35:2 log d. The loss at 1800 MHz is approximately 10 dB greater than that at 900 MHz. The free-space loss is 6 dB greater at 1800 MHz but, additionally, the radio waves will not propagate as well around buildings at 1800 MHz as they do at 900 MHz. When designing antennas, great attention is paid to the shape of the beam required. Because the shape of the beam and its beamwidth are directly related to the antenna gain, the gains of antennas used in digital mobile communications systems will be approximately the same at 900 MHz as they are at 1800 MHz. The only advantage that using 1800 MHz has is that the antennas will be physically smaller. If antennas of the same gain are used, the range of a 900-MHz antenna will be significantly greater than that of a 1800-MHz antenna. The path-loss difference of 10.5 dB translates into a difference in range of
1010=35:2 ¼ 1:92: We have previously estimated the range from a base station when providing indoor coverage in an urban environment to be 1.2 km at 1800 MHz. The range at 900 MHz would be estimated to be 2.3 km.