The main purpose of cellular communication/telephones is to provide communications between two mobile/moving units, or between a moving unit and nonmoving unit. To achieve this, phone companies create cells with a radius ranging from 1-12 miles relating to the population density of the area (smaller radius cells for more densely populated areas, and larger radius cells for large geographical areas with small populations). Cells are typically thought of as hexagons. Each cell contains a base station that is used essentially as a big antenna for all the mobile units to communicate with. The base station is connected to a mobile switching center. The mobile switching center facilitates and directs communications from all the base stations located within the cells and the Public switched telephone network. Because only a small number of radio channel frequencies are available for mobile communication systems phone companies need to find a way to reuse the same frequencies to carry more than one conversation at a time. This is made possible by combining cells into clusters, and combining clusters to form a honeycomb like structure. Each cell in a cluster is assigned a set of frequencies that is unique to that cell and different from neighboring cells. Because cell phones and base stations use low-power transmitters, the same frequencies can be reused in nonadjacent cells, basically as long as the cells are far enough away from each other, there will be no interference, and the same frequencies can be reused. Using this cellular design, a problem arose when a moblile caller would roam from one cell into another during a call. Since cells located next to one another do not use the same radio frequencies the call has to be transferred from one base station to another base station on another range of frequencies, or it needs to be dropped. Obviously dropping a call would be unacceptable, so the signal needs to be handed off to another base station. The term handoff describes the process of when a call is switched from one base station to another. During a call, two parties are on one voice channel. As a mobile phone moves out of the cells coverage area the signal strength begins to become weak, as the signal continues to weaken the base station requests a handoff. After the handoff request is received the signal is switched to the base station with the stronger signal frequency. Modern cellular systems use soft handoff so that the caller is unaware of the any change in signal frequency.
Current cellular networks use a culmination of 3G, and 4G technology. The G in Cellular simply means generation. It’s really just a way of saying how they are improving on already existing technology by increasing data rates. Current trend is in building 4 G networks. In order for phone service providers to build these 4 G networks they need spectrum. Besides the money required for building infrastructure, spectrum is the most vital resource for cellular service providers. To phone companies spectrum is a very limited resource. Spectrum is the range of radio frequencies that government agencies auction to carriers, and in each area there’s only so much spectrum to go around. Service providers compete with one another of the available spectrum in order to provide the best reception to their customers.