HB Blog 73: Inside The Mobile Phone.

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.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.

Codes in the Mobile Phone

Mobile phones have special codes associated with them. These include:

• Electronic Serial Number (ESN) -Unique 32-bit number programmed in the phone
• Mobile Identification Number (MIN) - 10 digit number derived from the phone’s number.
• 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.

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 :

• First 3 digits – Mobile country code
• Second 2 digits – Mobile network code
• Third 10 digits – Mobile station identification number

Subscriber Identification Module SIM

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.

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. 

2. Microprocessor

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 

They 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.