Quiz 21: Cellular Radio
Before the cell phones and radio telephones came into existence the entire communication system used to rely on a single antenna tower which used to serve an entire city. This made the phones to use a large antenna for transmission and reception purposes. In addition there were a number of limitations regarding usage the given channels. To overcome this potential problem, the large geographical area is allowed to be split into smaller areas. Each area is assigned a separate antenna tower. Although limited channels allotted by the tower, there was no overlapping. This made the same frequency to be utilized in each one, which made the mobile communication service providers to accommodate more number of users. This concept is called as frequency reuse. IMTS was the Improved Telephone Service which into existence in mid 1980s. This service was capable of automatically assigning the channels. This is done by a tone being transmitted from the base station on unoccupied channels. The disadvantage with IMTS was the lack of option for frequency reuse. Any attempt made to reuse the frequency ended up in causing interference. The reason being available bandwidth for one repeater is extended for up to 60 kms or more. AMPS - Advanced Mobile Phone Service which was later released later used the concept of frequency reuse. By reducing the output radio power the frequency reuse concept was maximized by the AMPS, which made the connectivity and mobility among different service providers simple for all the subscribers.
PSTN is the Public Switched Telephone Network. Today entire world is running on digital format. But in the beginning it is the circuit switching telephone networks that played their dominance. PSTN is a collection of the globally interconnected public telephone networks. For proper cellular communications, each cell area is assigned an individual base station. In a particular area (or) region, all the cell sites which are there are connected by usually fiber optics, microwave links or the traditional copper cables to an office called as the central office. This central office has a name MSC - Mobile Switching Center. These MSCs are in turn interconnected with each other, so that all the mobile phones connected comes into its vicinity. All the base stations are connected to the MSC and these MSCs are connected to the Public Switched Telephone System - PSTN. Therefore, the cellular telephone networks are connected to the PSTN.
Radio signals can be broadcasted using amplitude modulation (or) frequency modulation. AM does the work by the varying the amplitude of the carrier signal with respect to the message signal (input signal) keeping the frequency constant. While FM on the other hand the information is transmitted by changing the frequency of the given signal keeping its amplitude constant. For cellular radio, FM is preferred than AM in spite of its wider bandwidth because it is less prone to interference. Also it offers better sound quality because of its higher bandwidth. AM has poor sound quality when compared to FM and also it is highly susceptible to noise.