ess telephony engineer can use the base-station antennas to do spot engi- neering. If a particular tunnel needs coverage, then the appropriate antenna can be se- lected and installed to reach subscribers there. Or service can be improved by aiming a base-station antenna down into a valley. If the spot engineering problem is too much for an antenna to fix, then we can employ microcells and repeaters (discussed in Sec. 4.6). 4.1.1 Smart antennas and phased arrays Often radiation patterns are created by using multiple antennas in a phased array. By broadcasting the same signal with the appropriate radio wave phase angle delays from several closely spaced antennas, a signal beam can be created with a very specific ra- diation pattern. The same can be done by using time-delay filters on the receiver. Smart antenna technology electronically manipulates the phased array so that it can even be used on a call-by-call basis to focus each radio beam very narrowly. Consider the array of six antennas shown in Fig. 4.2. If we want to send a radio wave toward the right side of the page, as is shown in the figure, then we can transmit the radio wave with a 0-degree phase shift at the left two antennas, a 45-degree phase shift at the middle two antennas, and a 90-degree phase shift at the right two antennas. The combination of these six radio sources coherently adds all six antenna outputs in a nar- rowly focused radio beam to the right. If we want to send a radio wave up and to the right, as is also shown in the figure, then we can transmit the radio wave with a 0-degree phase shift at the lower left two antennas, a 45-degree phase shift at the middle two antennas, and a 90-degree phase shift at the two upper right antennas. These six sources coherently add up to a nar- rowly focused radio beam to the upper right. primary source of interference in CDMA is traffic in the same sector, but after that, traffic in the other sectors in the same cell tend to contribute more interference than traffic in other cells. Downloaded from Digi