This article discusses the mobile cellular network aspect of teletraffic measurements. Mobile radio networks have traffic issues that do not arise in connection with the fixed line PSTN. Important aspects of cellular traffic include: quality of service targets, traffic capacity and cell size, spectral efficiency and sectorization, traffic capacity versus coverage, and channel holding time analysis.
Teletraffic engineering in telecommunications network planning ensures that network costs are minimised without compromising the quality of service (QoS) delivered to the user of the network. This field of engineering is based on probability theory and can be used to analyse mobile radio networks, as well as other telecommunications networks.
A mobile handset which is moving in a cell will record a signal strength that varies. Signal strength is subject to slow fading, fast fading and interference from other signals, resulting in degradation of the carrier-to-interference ratio (C/I). A high C/I ratio yields quality communication. A good C/I ratio is achieved in cellular systems by using optimum power levels through the power control of most links. When carrier power is too high, excessive interference is created, degrading the C/I ratio for other traffic and reducing the traffic capacity of the radio subsystem. When carrier power is too low, C/I is too low and QoS targets are not met.
Traffic load and cell size
The more traffic generated, the more base stations will be needed to service the customers. The number of base stations for a simple cellular network is equal to the number of
cells. The traffic engineer can achieve the goal of satisfying the increasing population of customers by increasing the number of
cells in the area concerned, so this will also increases the number of base stations. This method is called cell splitting (and combined with sectorization)
is the only way of providing services to a burgeoning population. This simply works by dividing the cells already present into smaller sizes
hence increasing the traffic capacity.
Reduction of the cell radius enables the cell to accommodate extra traffic.
In TDMA/FDMA cellular radio systems, Fixed Channel Allocation (FCA) is used to allocate channels to customers. In FCA the
number of channels in the cell remains constant irrespective of the number of customers in that cell. This results in
traffic congestion and some calls being lost when traffic gets heavy.
A better way of channel allocation in cellular
systems is Dynamic Channel Allocation (DCA) which is supported by the GSM, DCS
and other systems. DCA is a
better way not only for handling bursty cell traffic but also in efficiently using the cellular radio resources.
DCA allows the number of channels in a cell to vary with the traffic load, hence increasing channel capacity with little
costs. This formula shows that as the coverage area A<sub>c</sub> is increased, the
channel density decreases.
Channel holding time
Important parameters like the carrier-to-interference ratio (C/I), spectral efficiency and reuse distance determine the quality of service of a cellular network. Channel Holding Time is another parameter that can affect the quality of service in a cellular network, hence it is considered when planning the network. Calculating the channel holding time, however is not easy. (This is the time a Mobile Station (MS) remains in the same cell during
a call).