Specifications

Sierra Wireless, Inc. CDPD Primer
2130006 Rev 1.0 Page 5
• the subscriber moves so that a different cell site provides better coverage, when the MTSO
de-assigns the radio channel from the old cell site and assigns a new radio channel from the
new cell site—one which provides better signal quality. This scenario is known as a handoff
(see section 2.2.7). The user might notice a handoff as a very brief (about 1/10 second) gap in
a voice call during the switch to a new channel.
The designers of AMPS chose a hexagonal shape for cells because hexagons can be tiled together
indefinitely over any reasonably flat terrain (see Figure 2-3), and so can adapt to a city of any size.
Since radio frequencies in the 824-894 MHz range travel line-of-sight, and because transmissions
from 50 W antennas fall off rapidly, cell sites need to be evenly spaced. They can be more or less
densely packed depending on the number of subscribers in the area, and on the number and type of
obstacles nearby. Cells are likely to be more densely packed, and the hexagons smaller, in
downtown cores than in outer suburbs, for instance.
2.2.6. Reusing Frequencies
Cellular radio engineers have improved system throughput and capacity by dividing each cell into
three sectors (although some systems support up to six sectors per cell). The basic cell reuse
pattern consists of seven cells in which only 21 individual radio channels are required to provide
radio coverage over any desired geographic area.
Figure 2-3: Cellular channel reuse using three sectors per cell
In this example, the seven cell sites are labeled A though G, and the three sectors per cell site are
labeled 1, 2, and 3. Thus, the 21 sectors per reuse pattern are labeled A1, A2, A3, B1, B2, and so
on through G1, G2, and G3. The cellular switch assigns a unique radio channel in each of these
sectors for use in that sector only.
So, for instance, the three radio channels at cell site F are not reused until approximately four cell
radii away. Such an arrangement avoids interference and crosstalk between separate cellular
telephone calls. The seven-cell pattern can also be tiled like puzzle pieces, so no two cells sharing
radio channels are close enough to cause interference problems.