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FREQUENCY REUSE 777

to 62 CDMA channels (plus one pilot channel and one synchronization channel)

or some 50% capacity increase. The Qualcomm system also operates in the 1.7-

to 1.8-GHz band [1].

Consult Ref. 25 for further reading on CDMA.

8 FREQUENCY REUSE

Because of the limited bandwidth allocated in the 800-MHz band for cellular

radio communications, frequency reuse is crucial for its successful operation. A

certain level of interference has to be tolerated. A major source of interference is

co-channel interference from a nearby cell using the same frequency group as

the cell of interest. For the 30-kHz bandwidth AMPS system, Ref. 5 suggests that

C/Ibe at least 18 dB. The primary isolation derives from the distance between

the two cells with the same frequency group. In Figure 18.2 there is only one

cell diameter for protection.

Refer to Figure 18.22 for the definition of the parameters R and D, where D

is the distance between cell centers of repeating frequency groups, and R is the

radius of a cell. We let

a = D/R

TheD/Rratio is a basic frequency reuse planning parameter. If we keep theD/R

ratio large enough, co-channel interference can be kept to an acceptable level.

Lee [6] calls a the co-channel reduction factor and relates path loss from theinterference source to R4.

A typical cell in question has six co-channel interferers, one on each side of

the hexagon. So there are six equidistant co-channel interference sources. The

goal is C/I18 dB or a numeric of 63.1. So

C/I = C/I = C/6I = R4/6D4 = a4/6 = 63.1, then a = 4.4

This means that D must be 4.4 times the value ofR. IfR is 6 miles (9.6 km),

then D = 4.4 6 = 26.4 miles (42.25 km).

Lee [6] reports that co-channel interference can be reduced by other meanssuch as directional antennas, tilted beam antennas, lowered antenna height, and

an appropriately selected site.

Figure 18.22. Definition of R and D.

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778 WIRELESS AND CELLULAR/MOBILE RADIO

If we consider a 26.4-mile path, what is the height of earth curvature at mid-

path? From Chapter 7, h = 0.667(d/2)2/1.33 = 87.3 ft (26.9 m). Provided that

the cellular base station antennas are kept under 87 ft, the 40-dB/decade rule of

Lee holds. Of course, we are trying to keep below line-of-sight conditions.

The total available (one-way) bandwidth is split up into N sets of channelgroups. The channels are then allocated to cells, one channel set per cell on a

regular pattern, which repeats to fill the number of cells required. As N increases,

the distance between channel sets (D) increases, reducing the level of interfer-

ence. As the number of channel sets (N) increases, the number of channels per

cell decreases, reducing the system capacity. Selecting the optimum number of

channel sets is a compromise between capacity and quality. Note that only certain

values ofN lead to regular repeat patterns without gaps. These are N= 3, 4, 7,

9, and 12, and then multiples thereof. Figure 18.23 shows a repeating 7-pattern

for frequency reuse. This means that N = 7 or there are 7 different frequency

sets for cell assignment.

Cell splitting can take place especially in urban areas in some point in time

because the present cell structure cannot support the busy hour traffic load. Cell

splitting, in effect, provides more frequency slots for a given area. Macario in [5]

reports that cells can be split as far down as a 1-km radius.

Co-channel interference tends to increase with cell splitting. Cell sectorization

can cut down the interference level. Figure 18.24 shows a three- and six-sector

plan. Sectorization breaks a cell into three or six parts each with a directional

antenna. With a standard cell, co-channel interference enters from six directions.

A six-sector plan can essentially reduce the interference to just one direction.A separate channel set is allocated to each sector.

The three-sector plan is often used with a seven-cell repeating pattern result-

ing in an overall requirement for 21 channel sets. The six-sector plan with

4

7

6

1

2

5

3

4

6

7

2

1

5

4

3

7

6

2

5

1

2

1

5

4

3

3

4

7

2

6

Figure 18.23. A cell layout based on N= 7.

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PAGING SYSTEMS 779

Figure 18.24. Breaking a cell up into three sectors (left) and six sectors (right).

its improved co-channel performance and the 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 requires 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 actually a much higher capacity operation [5].

9 PAGING SYSTEMS

9.1 What Are Paging Systems?

Paging is a one-way radio alerting system. The direction of transmission is from

a fixed paging transmitter to an individual. It is a simple extension of the PSTN.Certainly, paging can be classified as one of the first PCS (personal communi-

cation system) operations. The paging receiver is a small box, usually carried

on a persons belt. As a minimum, a pager alerts the user that someone wishes

to reach him/her by telephone. The person so alerted goes to the nearest tele-

phone and calls a prescribed number. Some pagers have a digital readout which

provides the calling number, whereas others give the number and a short mes-

sage.

Most paging systems now operate in the VHF and UHF bands with a 3-kHz

bandwidth. Transmitters have 1 to 5 watts output, and paging receivers have

sensitivities in the range of 10 V/m to 100 V/m.

Source : Ref. 5.

9.2 Radio-Frequency Bands for Pagers

All three ITU regions have some or all of the following frequency bands allocated

to mobile services:

26.1 50 MHz 68 88 MHz

146 174 MHz 450 470 MHz

806 960 MHz