Digital Cellular Systems (CDMA)

Mobile Radio Environment

The propagation of a narrow-band carrier signal is a conventional means of communication. However, in a CDMA system, using the ideas that were discussed in the previous section, the propagation of a wide-band carrier signal is used. Therefore in order to design CDMA systems, we have to study both, propagation of the narrow-band wave and then propagation of the wide-band wave.

A. Narrow-Band (NB) Wave Propagation.

Let us consider the case when a signal is transmitted from the cell-site and received by either a mobile or portable unit. Since the antenna height of the receptor unit is close to ground, three nondesirable effects are observed:

The signal received is not only from the direct path but also from the strong reflected path due to the fact that the antennas of the mobile units are close to ground.

Because of the short height of the receiving antennas, the human-made structures surrounding them would generate the multipath fading on the received signal. This effect is known as Rayleigh Fading .

A time delay spread phenomenon exists due to the time dispersive medium

Due of these phenomenona, it can be shown that if the transmission rate, Rb, is higher than 10K Samples/s, there is a need to use an equalizer in both, FDMA and TDMA techniques. On the other hand, CDMA does not need an equalizer but a simpler device called correlator.

B. Wide-Band Wave Propagation.

A couple of phenomenona should be consider in the system design for this case, specifically: Path Loss and Multipath Fading Characteristic on Wide-Band.

Path Loss problem, if the effective power received is Pr, when a signal is transmitted with a power Pt, it can be shown, that Pr is equal to:

---------(1)

Where k is a constant factor, c is the speed of light, Gt and Gr are the gains of the transmitting and receiving antennas, respectively. The above equation is a general formula. For a narrow-band signal, B<

(Narrow-band)---------(2)

On the other hand, from (1) we may find the B/fo ratio for the case of 1-dB difference in path loss between narrow-band and wide-band. That means, to solve the denominator of (1) as follows:

---------(3)

Solving, we finally obtain B = 0.66fo. Fortunately, in most wide-band applications, B will not be wider than fo/2. Thus, the narrow-band propagation path loss should be applied to the wide-band propagation path loss.

Multipath Fading Characteristic, It can be shown that the wider the bandwidth, the less the fading. The fading effect is strongly affected by the effective number of diversity branches, M. The M factor depends on the number of buildings surrounding the transmission area. M is larger in the urban area than in the suburban area.