 far down as 1 km in radius. Cochannel interference tends to increase with cell splitting. Cell sectorization can reduce the interference level. Figure 18.15 shows a three- and a six-sector plan. Sector- ization breaks a cell into three or six parts each with a directional antenna. With a standard cell (using an omnidirectional antenna), cochannel interference enters from six directions. A six-sector plan can essentially reduce the interference to just one direction. A separate channel frequency set is allocated to each sector. The three-sector plan is often used with a seven-cell repeating pattern (Figure 18.16) resulting in an overall requirement for 21 channel sets. The six-sector plan with its improved cochannel performance and rejection of secondary interferers allows a four- cell repeat plan (Figure 18.2) to be employed. This results in an overall 24-channel set requirement. Sectorization entails a larger number of channel sets and fewer channels per sector. Outwardly it appears that there is less capacity with this approach; however, the ability to use much smaller cells results in a higher capacity operation. 18.8 PERSONAL COMMUNICATIONS SERVICES (PCS) 18.8.1 Defining Personal Communications Personal communications services (PCS) are wireless. This simply means that they are radio based. The user requires no tether. The conventional telephone is connected by a wire pair through to the local serving switch. The wire pair is a tether. We can only walk as far with that telephone handset as the "tether" allows. Both of the systems we have dealt with in the previous sections of this chapter can be classified as PCS. Cellular radio, particularly with the hand-held terminal, gives the user tetherless telephone communication. Paging systems provided the mobile/ambulatory user a means of being alerted that someone wishes to talk to that person on the telephone or of receiving a short message. The cordless telephone is certainly another example that has extremely wide use around the world with a rough estimate of 6