The "Cell Phone"!

Almost everywhere in the world, we see cell phones or mobile phones used widely. They are important devices in the daily lives of people. Even people in rural villages of Africa, India and other countries who do not know English have mobile phones. They just use basic phones to serve their communication purpose. There are many people in these rural areas that do not even know numbers and just identify them as symbols… Most users of these phones may not even wonder how they work. Well, it seems that mobile phones have very complicated mechanism but we do not know the truth. Let’s just explore…

Cell phones are actually sophisticated radios by which both parties can talk at the same time. It is hence a full duplex radio in which the two ends of the phone call can talk at the same time. Cell phones work using different frequency channels of radio waves. In the cellular system, a city is divided into a mesh of hexagonal cells. This allows extensive frequency reuse across the city enabling thousands of people to use cell phones simultaneously. Each cell has a range within which a cell phone is connected to. Each cell has a base station made of a cell tower and radio equipment.

Notice the three important terms – radio waves, frequencies and channels. Knowing of these terms is required to understand the working of a cell phone. Well, radio waves can crudely be defined as a range in the electromagnetic spectrum having a wavelength between 3x10⁴ and 3x10^-4 m or frequency between 10⁴ and 10¹² Hz (λ=c/f). Frequency can be defined as the number of cycles occurring per unit time. Different calls are transmitted at different frequencies provided by a cell mast. This is done so that no two calls interfere with each other. Full-duplex systems are controlled by two frequencies. One frequency is for talking channel and the other for listening channel.

A channel is a small bandwidth (range of frequencies) around a carrier frequency (the centre frequency) and is given a specific number known as the channel number. For example, On North American FM radio dial, frequency 88.5 is channel 203 and each radio station (channel) has a bandwidth of 200kHz. Similarly, GSM-900 band uses two frequency ranges – one for uplink and the other for downlink. One uplink frequency combined with one downlink frequency acts as one channel for one phone call. There are 124 RF (radio frequency) channels spaced at 200kHz in these bands.

There are limited radio wave channels available (about 900) for each cell phone mast/tower. All the users connected to a particular cell phone tower share the same channels. Hence areas densely populated have phone masts closer to each other to reuse the same frequencies more times. Out of the Ultra High Frequency (UHF) band used by television, Wi-Fi and Bluetooth, specific frequencies are allotted for cellular purposes. Of these, a specific range is used by a specific service provider (like Reliance, Docomo etc.) and each mast of the service provider has the same set of frequency channels. Of these frequencies, there are some allotted for control purposes (about 42). The rest of the frequencies are provided for talking purpose. This implies that 800 channels are available for calling purpose and 800 calls could use up all the available channels. This is the reason that in densely populated areas, masts are packed closely to provide more channels for communication.

Mobile phones are very interesting devices because of the way they function. They convert sound waves to different types of energy several times. When we are calling somebody in another town, a mobile phone does several functions within milliseconds. It converts our voice in the form of sound waves into electric signals or pulses which it transmits in the form of radio waves. When transmitting, the radio waves may change several masts or go via landlines cables in between cities. Here is how it works.

When a phone is powered on, it identifies its home system (carrier) through a unique identifier code. When a call is initiated, the phone connects to base station via reserved control channels. Then a registration request is transmitted to the MTSO (Mobile Telephone Switching Office) which tracks the phone location and the connected cell tower via database. The MTSO provides a frequency pair (channel) to the initiated call. When the cell phone is moving, the signal is diminishing in the cell the phone is connected to, while it is increasing in a nearby approaching cell. At a certain point both the cells coordinate to hand off the phone conversation between them for uninterrupted conversation.

Mobile phones have really small antennae (well, we cannot even see them these days), which is why they only emit weak signals. But, these signals are large enough to be caught and be transmitted by the cell phone masts. The signal is picked up by a mast near our house and is sent via landline cables to the mast the person we are talking is connected to! Then, the signal is changed back to voice at the other phone.

I feel that usage of cell phones would increase further because rural areas are adopting them more and more. In my opinion, the cell phone is successful just because the number of advantages of using a cell phone far outweighs the disadvantages.

The main disadvantage of cell phones is the amount of electromagnetic radiation they emit during calls. They emit more radiation when searching for tower while moving from the range of one tower to another. This radiation is hazardous for our health. During long calls, the continuous small radiation builds up to a lot of exposure.

The usage of cell phones has now started to grow widespread with people of all types and communities not leaving their cell phones behind. From market vendors to farmers in African tribes – all of them own mobile phones. As cell phone towers or masts are being set up almost everywhere, even in the most remote areas of the world like deserts, mobile coverage is available. This is now possible due to steps taken to improve cell phone coverage. People staying in their tribe in some isolated area of Africa can also access to cell phone coverage easily. It is relatively easier to provide phone coverage via radio waves compared to copper cable based coverage in remote rural areas because the cost of laying a copper cable across thousands of kilometres will be prohibitive. Mobile phones have started connecting modern technology with the poorest regions of the world.

The cell phone technology has been available for several years now and hence old second generation phones are being sold at very low rates as demand for the cell phones is growing. Mobile phones are being purchased just to connect the modern technological world with the tribal world and the tribal world with themselves. Poor people are able to see themselves into the technological future. The new generation smart phones are also helping increase in the usage of cell phones because of the additional functionality they provide other than calling.

-Suhas

29/1/14