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4.2 Parameters

4.2.3 Clusters Spreads and Shadow Fading

Here, a similar approach given in [12] is followed, but different values for the parameters so as to adopt them to the FM Band are introduced. The standard deviation of the log-normally distributed shadow fading in dB scale is given by 6 dB for 2 GHz, while in the 145 MHz measurements in [19] and [6], the standard deviation is given by 6.25 dB. Hence, it is apparent that the log-normal shadow fading random variable is not too dependant on frequency of operation as long as there is no change in the environment type, which is HTR in those studies. Therefore, it is expected that the standard deviation of the shadow fading random variable for the FM Band will not be far away from 6 dB. For the delay spread, it was clearly stated in [38] that the median delay spread increases as the distance from the transmitter increases. This is very acceptable in the outdoor environments for the high frequencies of operation where only one cluster is received by the receiver in most of the cases according to Table II in [12]. Receiving only one

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cluster indicates that the delay spread of the cluster is the same as total delay spread of the channel. Therefore, the total channel delay spread increases as the distance from the transmitter increases. However, as discussed earlier, for the case of low frequency of operation, the number of clusters decreases on average as the distance from the trans- mitter increases, and the number of received clusters is generally greater than one in the vicinity of the transmitter. The total channel delay spread is not the same as the cluster delay spread in case more than one cluster exists. Therefore, the average increase in the cluster delay spread as the distance for the transmitter increases does not have to imply a corresponding increase in the total channel delay spread. In fact, it is the opposite of that for the given measurements at 145 MHz [6]. This means that the arrival of extra clusters at relatively small distances from the transmitter compensates the increment in the delay spread of the cluster as the distance from the transmitter increases. This is because the clusters mostly arrive separately in time. To sum it up all, according to the given measurements in [6], the cluster delay spread increases as the distance from the transmitter increases but the total channel delay spread decreases because the extra time taken by the following clusters to arrive is typically larger than the increase in the cluster delay spread due to the increase of the distance from the transmitter. Therefore in the modelling, the same cluster delay spread formula presented in [12] is used here but with a median cluster delay spread for 1000 meter separation between the transmitter and the receiver of 5 µsec according to the given measurements in [6].

The measurements performed at 145 MHz do not provide any information concerning the angular spreads [19], [6]. To get the required parameters for the modelling of the angular spreads, a review of the available angular measurements performed at different frequencies of operation is required. As reported in Table IX in [12], the suggested median of the log-normally distributed azimuth angular spread for the urban areas was 10o with a standard deviation of 3 dB. This means that the mean is approximately 10.2o. This was validated at 2 GHz [12]. In [13], the study of the angular spread at 300 MHz was on two basis: whether it is an LOS case or not, and whether the angular spread is at the transmitter or the receiver. However, according to the given values, there seems not to be a significant difference between the transmitter and the receiver case, although the angular spread at the receiver is greater. On the other side, there is a considerable difference in the means of the angular spread between the LOS case and the NLOS case. Among all the different cases, the lowest given mean was 14.6o, which is

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much larger than the value reported in [12]. According to this, it is clear that there is an inverse proportionality between the frequency of operation and the angular spread. By averaging the LOS and the NLOS case, it is expected that the median angular spread for the 300 MHz band in HTR will be around 8o. It is also expected to increase even more for the FM Band case. Thus, the same suggested formula in [12] is used in this study with a median angular azimuth spread at the BS of 9o. For the rest of the angular spread values, the same expressions in [12] are used with the same parameters.

Regardless of the increase in the cluster’s angular spread, the relatively high number of received clusters in the vicinity of the transmitter increases the overall channel angular spread. Therefore, the total channel angular spread is expected to increase in the FM Band case compared to the higher frequency bands.

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