Welcome to the World of CDMA
Multiple Access Wireless Communications
The goals of multiple access communications systems, meaning cellular and PCS, are:
Regulatory agencies have allocated limited bandwidth to these
services, so that the solutions must achieve high spectral
efficiency, measured in Erlangs
per unit service area, per MHz. Cellular operators have 25
MHz each, split between the two directions of communications.
The PCS service in the United States has three 30 MHz and
three 10 MHz allocations, also split.
Frequency ReuseCentral to the cellular concept is the concept of frequency reuse. Although there are hundreds of channels available, if each frequency were assigned to only one cell, total system capacity would equal to the total number of channels, adjusted for the Erlang blocking probability: only a few thousand subscribers per system. By reusing channels in multiple cells the system can grow without geographical limits.
Reuse is critically dependent upon the fact that the electromagnetic field attenuation in the cellular bands tends to be more rapid with distance than it is in free space. Measurements have shown repeatedly that typically the field intensity decays like R-n, with 3 < n < 5. In free space n = 2. In fact, it is easily shown that the cellular concept fails completely due to interference that grows without bound if the propagation is exactly free space.
Typical cellular reuse (pre-CDMA, that is!) is easily rationalized by considering an idealized system. If we assume that propagation is uniformly R-n, and that cell boundaries are at the equisignal points, then a planar service area is optimally covered by the classical hexagonal array of cells ...
No similarly colored cells are adjacent, and therefore there
are no adjacent cells using the same channel. While real systems
do not ever look like these idealized hexagonal tilings of
a plane, the seven-way reuse is typical of that achieved
Antenna SectorizationThe pictures above assume that the cells are using omnidirectional antennas. It might be expected that system capacity could be increased by antenna sectorization. Sites are in fact sectorized by the operators, usually three-ways. That is, each site is equipped with three sets of directional antennas, with their azimuths separated by 120°. Unfortunately the sectorization does not in practice lead to an increase in capacity. The reason is that the sector-to-sector isolation, often no more than a few dB, is insufficient to guarantee acceptably low interference. Only in part is this due to the poor front-to-back ratio of the antennas. The vagaries of electromagnetic propagation in the real world also conspire to mix signals between sectors. The practical result of sectorization is only an increase in coverage because of the increased forward gain of the directional antenna. Nothing is gained in reuse. The same seven-way cell reuse pattern applies in sectored cells as in omnidirectional cells. Viewed from the standpoint of sectors, the reuse is K = 7 *3 = 21, not 7.
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