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Mobile Wireless Communications in .NET Encoder Code 128 Code Set A in .NET Mobile Wireless Communications




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Mobile Wireless Communications using none toadd none for asp.net web,windows application SQL server +20 +10 0 10 20 none for none 30 40 50 60 70 80 90 100 .1 A f = 836 MHz, ht = 150 m Philadelphia B f = 900 MHz, ht = 137 m New York C f = 922 MHz, ht = 140 m Tokyo C: A: B: Free space transmission. Signal power relative to OdBm free space at 1 km Distance from base none none station antenna (km). Figure 2.7 Received signal power as function of distance (from Jakes, 1974: Fig. 2.

2 10). Hence the ground di none none stance r between the transmitting (base station) and receiving (mobile terminal) systems may be used in place of the distance d in determining average received power. We shall, in fact, always refer to the ground distances between systems. Now consider some measurements roughly validating these two-ray results.

Figure 2.7 shows received signal power as a function of distance from the transmitters in New York, Philadelphia, and Tokyo at 900 MHz using relatively high antennas (Jakes, 1974). These measurements are compared with the free-space transmission model of (2.

1) in which the power drops off as 1/d2 . Figure 2.8 shows the effect of base station antenna height (indicated by the symbol hb in the gure) on received signal strength measured in Tokyo at a frequency of 922 MHz, again taken as a function of distance from the base station.

Note that these curves agree roughly with the simple expression (2.13a), obtained using the two-ray model. They indicate that the propagation loss does fall off more rapidly than that in the free-space case; they also show that higher base station antennas increase the average received signal power, as predicted by (2.

13a). Multipath fading: Rayleigh/Ricean models Recall that we indicated earlier that we would be discussing in this section three significant propagation effects due to the scattering, diffraction, and re ection of radio waves that appear in wireless systems. All three effects appear in the simple received power equation (2.

4). One effect was the 1/dn dependence of average power on distance. We have just provided the commonly used n = 4 case, developed from a two-ray propagation model.

. Mobile radio environment propagation phenomena 0 10 Signal power none for none relative to OdBm free space at 1 km 20 30 40 50 60 70 80 90 100 110 120 0.6 1 2 4 6 10 Distance (km) 20 40 60 100. f = 922 MHz hm = 3 m hb = 45 m hb = hb = 140 m 220 m hb = 820 m Free space Figure 2.8 Effect o none none f base station antenna height on received power (from Jakes, 1974: Fig. 2.

2 11). We discussed prior to this log-normal or shadow fading, used to represent large-scale random variations about the average power. We now move on to the third phenomenon, that of small-scale multipath fading, which we shall show can be modeled by using Rayleigh/Ricean statistics. As noted earlier, amplitude and power variations occur over distances of wavelengths, hence the reference to small-scale fading.

Speci cally, with Rayleigh statistics, the probability density function f ( ), 0, of the random variable in (2.4) is given by f ( ) = 2 /2 r2 e r2 0 (2.15).

with r2 an adjusta none for none ble Rayleigh parameter. It is readily shown that the second moment 2 of the Rayleigh distribution is E( 2 ) = 2 r . The Rayleigh function is shown sketched in Fig.

2.9. Ricean statistics will be discussed later.

In this case of modeling the effect of 2 received power by (2.4) it is readily shown that we must set r = 1/2. To demonstrate this value of r2 = 1/2 in this case, it suf ces to return to the basic propagation equation of (2.

4). We have indicated earlier, a number of times, that the average received power is just PT g(d)G T GR . This implies taking the expectation of the instantaneous received signal power PR with respect to the two random variables x and .

Doing so, we nd that E( 2 ) = 1. Hence we must set r2 = 1/2, as indicated. An alternative approach is to take expectations with respect to x and in the dB received power expression of (2.

7). It is clear then that E(x) = 0 and E( 2 ) = 1. We shall also show that the instantaneous local-mean received power PR is, in this case, an exponentially distributed random variable.

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