A mobile phone is an electronic device used for mobile telecommunications over a cellular network of specialized base stations known as cell sites. A cell phone offers full Duplex Communication and transfer the link when the user moves from one cell to another. As the phone user moves from one cell area to another, the system automatically commands the mobile phone and a cell site with a stronger signal, to switch on to a new frequency in order to keep the link.
Mobile phone is primarily designed for Voice communication. In addition to the standard voice function, new generation mobile phones support many additional services, and accessories, such as SMS for text messaging, email, packet switching for access to the Internet, gaming, Bluetooth, camera with video recorder and MMS for sending and receiving photos and video, MP3 player, radio and GPS.
Signal Frequency in Cell Phone
The cellular system is the division of an area into small cells. This allows extensive frequency reuse across that area, so that many people can use cell phones simultaneously. Cellular networks has a number of advantages like increased capacity, reduced power usage, larger coverage area, reduced interference from other signals etc.
FDMA and CDMA Systems
Frequency Division Multiple Access (FDMA) and Code Division Multiple Access (CDMA) were developed to distinguish signals from several different transmitters. In FDMA, the transmitting and receiving frequencies used in each cell are different from the frequencies used in the neighboring cells. The principle of CDMA is more complex and the distributed transceivers can select one cell and listen to it. Other methods include Polarization Division Multiple Access (PDMA) and Time Division Multiple Access (TDMA). Time division multiple access is used in combination with either FDMA or CDMA to give multiple channels within the coverage area of a single cell.
Codes in the Mobile Phone
Mobile phones have special codes associated with them. These include
1. Electronic Serial Number (ESN) -Unique 32-bit number programmed in the phone
2. Mobile Identification Number (MIN) – 10 digit number derived from the phone’s number.
3. System Identification Code (SID) – unique 5 digit number that is assigned to each carrier by the FCC.
ESN is a permanent part of the phone while MIN and SID codes are programmed in the phone when a service plan is selected and activated.
Mobile phone is a Duplex device. When we use one frequency for talking, a second separate frequency is used for listening. So that both the people on the call can talk at once. The Mobile phone can communicate on 1,664 channels or more. The Mobile phones operate within the cells, so that it is easy to switch on to different cells as they move around. A person using a cell phone can drive hundreds of kilometers and can maintain a conversation during the entire time because of the cellular approach.
Activation of SIM Card
SIM card (Subscriber Identification Module (SIM) ) is a type of Smart card used in mobile phone. The SIM is a detachable smart card containing the user’s subscription information and phone book. This allows the user to retain his or her information even after switching off the handset. Alternatively, the user can also change service providers while retaining the handset simply by changing the SIM.SIM card Securely stores the service subscriber key having 15 digits. The digits of the key are :
1.First 3 digits – Mobile country code
2.Second 2 digits – Mobile network code
3.Third 10 digits – Mobile station identification number
When the Mobile phone is used for the first time, it sends a number called International Mobile Subscriber Identity – IMSI present in the SIM card to the network, which looks it up in a database to ensure the card is registered. If the IMSI is recognized, the network creates another number called a Temporary Mobile Subscriber Identity (TMSI), which is encrypted and sent back to the phone. In all subsequent calls, the phone identifies itself by broadcasting the TMSI.
What happens when we make a call?
1. When we switch on the mobile phone, it tries for an SID on the Control channel. The Control channel is a special frequency that the phone and base station use to talk to one another. If the Mobile phone finds difficulty to get link with the control channel, it displays a “no service” message.
2. If the Mobile phone gets the SID, it compares the SID with the SID programmed in the phone. If both SID match, the phone identifies that the cell it is communicating is the part of its home system.
3. The phone also transmits a registration request along with the SID and the MTSO keeps track of your phone’s location in a database. MTSO knows in which cell you are when it wants to ring the phone.
4. The MTSO then gets the signal, it tries to find the phone. The MTSO looks in its database to find the cell in which the phone is present. The MTSO then picks a frequency pair to take the call.
5. The MTSO communicates with the Mobile phone over the control channel to tell it what frequencies to use. Once the Mobile phone and the tower switch on those frequencies, the call is connected.
6. When the Mobile phone move toward the edge of the cell, the cell’s base station will note that the signal strength is diminishing. At the same time, the base station in the cell in which the phone is moving will be able to see the phone’s signal strength increasing.
7. The two base stations coordinate themselves through the MTSO. At some point, the Mobile phone gets a signal on a control channel and directs it to change frequencies. This will switch the phone to the new cell.
The GSM System
Global System for Mobile Communications is the standard for mobile telephone systems in the world. In GSM, the signaling and speech channels are digital, therefore GSM is considered a 2G (Second Generation) system. This helps wide-spread implementation of data communication applications. There are five different cell sizes in a GSM network These are macro, micro, pico, femto and umbrella cells. Macro cells are cells where the base station antenna is installed on a mast above average roof top level. Micro cells are cells whose antenna height is under average roof top level. Pico cells are small cells whose coverage diameter is a few dozen metres. These are mainly used in indoors applications. Femto cells are cells designed for use in residential or small business environments and connect to the service provider’s network via a broadband internet connection. Umbrella cells are used to cover shadowed regions of smaller cells and fill in gaps in coverage between those cells.Horizontal radius of the cell varies depending on the antenna height, antenna gain and propagation conditions. Maximum distance the GSM supports is 35 kilometers. Most 2G GSM networks operate in the 900 MHz or 1800 MHz bands while 3G GSM in the 2100 MHz frequency band.
Time Division Multiplexing technique is used to share eight full-rate or sixteen half-rate speech channels per radio frequency channel. There are eight radio time slots grouped into a TDMA frame.
Mobile phone converts voice, text, multi-media messages or data calls into Radio Frequencies (RF). Mobile phone base stations transmit and receive these RF signals and connect callers to other phones and other networks. Mobile phone network is divided into thousands of overlapping, individual geographic areas or ‘cells’, each with a base station. The size of a cell depends on the area of coverage and the number of calls that are made in that area. The smallest cells are in crowded urban areas with large buildings and heavy population density, while the biggest cells are in rural areas, where people are dispersed.
There are two types of channels used in GSM . These are Control channels and Traffic channels.
These are responsible for housekeeping tasks such as telling the mobile when a call is coming in and which frequency to use. To ensure this handover works, the phone constantly monitors the broadcast control channel of up to 16 neighboring cells. In normal operation, phones continually adjust the power of the radio waves they send out to be the minimum needed for the base station to receive a clear signal. If a phone moves far away from its base station and if the signal is weak, the network consults the list and triggers a handover to a neighboring cell with best signal.
It is used to carry calls or other data from the mobile phone to the base station and vice versa. In the Traffic channel, voice or text data is carried in bursts. Each burst comprises two consecutive strings of bits (a series of signals representing 1s and 0s), each 57 bits long.
The range within which mobile devices can connect is not a fixed figure. It depends on a number of factors like the frequency of signal in use, the transmitter’s rated power, the transmitter’s size etc.
Inside the Mobile phone
Mobile phone is a sophisticated device using SMD components, Microprocessor, Flash memory etc. In addition to the Circuit board, Mobile phone also has Antenna, Liquid Crystal Display(LCD) ,Keyboard , Microphone , Speaker and Battery.
The circuit board is the heart of the Mobile phone. It has chips like Analog-to-Digital and Digital-to-Analog conversion chips that translate the outgoing audio signal from analog to digital and the incoming signal from digital back to analog.Following are the Chips present in Mobile phone.
1. Digital signal processor
It is generally rated as having 40 MIPS (millions of instructions per second) to conduct calculations of signal manipulation at high speed. This chip deals with both compression and decompression of the signals.
It handles all the housekeeping tasks for the keyboard and display. It also deals with command and control signaling with the base station, and coordinates the rest of the functions on the board.
3.The Flash memory and ROM Chips of the Mobile phone act as a storage location for the phone. These chips store the customizable options of the cell phone, as well as the entire operating system. The power and radio frequency sections of the phone, phone recharging and power management etc are controlled by this chip. It also controls several hundred FM channels. The RF amplifiers focus on signals that go in and out of the phone’s antennae.
Mobile Phone Maintenance
Mobile phone is a delicate device and needs care for its proper functioning. Here are the common measures to keep the mobile phone in good condition.
1. Do not keep Mobile phone in wet area or use it with wet hands. Moisture can cause non-repairable internal corrosion of parts.
2. Do not drop the phone or damage the connection points.
3. Do not over stress the phone. It may damage the display.
4. Do not keep the phone near heat generating devices. High temperature in a car can damage its electronics.
5. Do not over charge the battery. Charge the battery only its charge status goes below 50 percent.
6. Prevent Cloning.
A phone is “Cloned” when someone steals its ID number and is able to make fraudulent calls on the owner’s account. When the phone makes a call, it transmits the ESN and MIN- a unique tag- for your phone to the network at the beginning of the call. When the phone transmits its MIN/ESN pair, it is possible to capture the ESN-MIN pair. Using a Scanner device it is easy to modify another phone so that it contains your MIN-ESN tags. This allows the person to make calls on your account. Cloning may also occurs when the phone is repaired in an unauthorized service centre. It is possible to copy the data including images and video present in the phone.