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GSM Mobile Phone

This tutorial section on GSM basics covers following sub topics:
Main Page  Frame  Burst types  PHY  gsm stack  speech processing  ARFCN  Channel types  GSM Security  Network Entry  Mobile basics 

This article covers basics of mobile phone internal modules.It gives emphasis to GSM mobile phone. Typically Mobile phone will have display (LCD, touch screen), keypad, microphone, speaker, SIM card, battery, USB port, antenna, memory unit(RAM,ROM), camera, CODEC, RF part, DAC/ADC, baseband part (L1/Layer1/physical layer) running on DSP, Application/protocol layers running on CPU, ON/OFF switch and Bluetooth/GPS features. For Mobile Phone which works on GSM network baseband part (Layer 1) and protocol stack running on CPU differs, which will be based on GSM standard. For CDMA Mobile phone the same Layer 1 and protocol stack will be based on CDMA standard and so on for LTE, HSPA standard based mobile phones. Mobile phone provides connectivity with laptop/other devices using WLAN, Bluetooth and GPS. All these features are based on specific standard specifications designed. GSM Mobile phone is in use earlier days for only voice applications. Now-a-days it has become more popular for SMS/MMS and internet applications due to GPRS feature. After the introduction of smartphone many applications such as facebook,Orkut,Twitter, various games comes built-in with the phone. Now mobile phone has slowly taken the place of laptop for many of the applications.

Following figure depicts components in a generic mobile phone irrespective to the technology on which they need to work such as GSM, CDMA, LTE and so on. We will understand Mobile Phone with respect to GSM standard here.

gsm mobile phone block diagram

Fig.1 gsm mobile phone block diagram

RF Part
As shown in the figure, every mobile phone will have RF part which consists of RF frequency up converter and rf frequency down converter. For GSM system, up converter converts modulated baseband signal(I and Q) either at zero IF(Intermediate frequency) or some IF to RF frequency(890-915 MHz). RF down converter converts RF signal(935 to 960 MHz) to baseband signal(I and Q). For GSM, GMSK modulation is used. There are two approaches employed in GSM Mobile phone receiver, i.e. heterodyne or homodyne. The basic component used for frequency conversion is RF mixer.To know more read our page on heterodyne vs homodyne. To know more on design of RF frequency converter one can refer our page in the articles section.

Refer table mentioning common intermediate frequencies used for different applications.

Antenna and Tx/Rx Switch
Antenna is the metallic object which converts electro-magnetic signal to electric signal and vice versa. Commonly used antennas in the mobile phone are of various types such as helix type, planar inverted F type, whip or patch type. Microstrip based patch type of antennas are popular among mobile phones due to its size, easy integration on the PCB and multi frequency band of operation. Todays mobile phones support various GSM bands and also various technologies such as CDMA,LTE,WiMAX and also WLAN,Bluetooth and so on. In these scenario this type of patch antennas do the job. To know more on antenna refer antenna tutorial.

Tx/Rx Switch
As there is only one antenna used for both transmit and receive at different times, Tx/Rx Switch is used to connect both Tx path and Rx path with antenna at different times. Tx/Rx Switch is controlled automatically by DSP based on GSM frame structure with respect to the physical slot allocated for that particular GSM mobile phone in both downlink and uplink. For FDD systems diplexer is used in place of switch which acts as filter to separate various frequency bands. To know RF switch basics and manufacturers read page on RF switch in terminology section.

Baseband Part
This part basically converts voice/data to be carried over GSM air interface to I/Q baseband signal. This is the core part which changes modem to modem for various air interface standards viz.CDMA,Wimax, LTE,HSPA and more. It is often named as physical layer or Layer 1 or L1. It is ported usually on DSP(Digital Signal Processor) to meet latency and power requirements of mobile phone. For Speech/audio, codec is used to compress and decompress the signal to match the data rate to the frame it has to fit in. CODEC converts speech at 8 KHz sampling rate to 13 kbps rate for full rate speech traffic channel. To do this RELP (Residually Excited Linear Predictive coder) speech coder is used which packs 260 bits in 20 ms duration to achieve 13 kbps rate. The baseband or physical layer will add redundant bits to enable error detection as well as error correction. Error detection is obtained with CRC and error correction with forward error correction techniques such as convolutional encoder(used at transmit part) and viterbi decoder(used at receive part). Other than this interleaving is done for the data of one burst which helps in spreading the error over the time hence helps receiver de-interleave and decode the frame(consecutively data burst) correctly.For more refer our page on GSM Physical Layer.

ADC(Analog to Digital Converter) and DAC(Digital to Analog Converter) is used to convert analog speech signal to digital signal and vice versa in the mobile handset. At Transmit path, ADC converted digital signal is given to speech coder. There are various ADCs available, among them popular one is sigma delta type. AGC(Automatic Gain Control) and AFC(Automatic Frequency Control) is used in the receiver path to control gain and frequency. AGC helps maintain working of DAC satisfactorily, as it keeps signal within the dynamic range of DAC.AFC keeps frequency error within limit to achieve better receiver performance.

Protocol stack
Other than physical layer there are other layers involved in GSM mobile phone to make it work with GSM network/base station. To know more on the protocol stack used in mobile refer GSM Protocol Stack. The entire protocol stack is ported on CPU of ARM or of any other type of processors.

Application layer
It also runs on CPU. various applications run in GSM mobile phone. It include audio,video and image/graphics applications. It supports various audio formats such as MP3,MP4,WAV,rm. JPEG image formats are usually available. It supports video formats e.g.MPEG-1 to MPEG-5. Mobile phone supports CIF, QCIF video standard resolutions.

Operating system
various operating systems are supported in mobile phone such as Symbian,java,android,RT-Linux,Palm. It runs on CPU of different manufacturers. For time critical applications RTOS (real-time operating system) is used.

It is the only major source of power to make/to keep mobile phone functional. There are various types of batteries made of Nickel Cadmium(NiCd),Nickel Metal Hydride(NiMH), based on lithium,Li-ion and so on. The major factors for designers is to reduce battery size, last for more talk time,increase battery life. Battery comes usually with 3.6 or 3.7 voltage and 600mAh or 960 mAh ratings. Battery Charger is usually provided with mobile phone to charge the mobile phone battery. Battery charger is AC to DC converter.

Connectivity (WLAN, Bluetooth, USB, GPS)
To make data transfer fast enough between mobile phone and other computing devices(laptop,desktop,tablet) or between mobile and mobile various technologies are evolved which include WLAN,Bluetooth,USB. GPS(global positioning system) is used for location assistance and will enable google map to work efficiently.

Microphone or mic converts air pressure variations(result of our speech) to electrical signal to couple on the PCB for further processing. Usually in mobile phone mic of types condenser,dynamic,carbon or ribbon is used.

It converts electrical signal to audible signal(pressure vibrations) for human being to hear. This is often coupled with audio amplifier to get required amplification of audio signal. It also tied with volume control circuit to change(increase or decrease) the amplitude of the audio signal.

Now-a-days with almost all the mobile phone camera feature is available for one to click pictures at various occasions. It is the major specifications in increasing cost of mobile phone. There are various mega pixel camera for mobile phones are available such as 12 mega pixel, 14 mega pixel and even 41 mega pixel available in smartphones.This has become evident because of advancement in sensor technology.If one wants to buy low cost mobile phone, they usually go for non camera mobile phone.

There are various display devices used in mobile phone such as LCD(liquid crystal display), TFT(Thin-film transistor) screen,OLED(organic light emitting diode),TFD(thin film diode), touch screen of capacitive and resistive type etc.

Earlier days keypad was simple matrix type keypad which contains numeric digits( 0 to 9), alphabets( a to z),special characters and specific function keys. These has been designed for various applications such as accepting call,rejecting call,cursor movement(left,right,top,down) dialling number, typing name/sms/mms and so on. Now-a-days keypad has been removed from the phone design and it has become part of mobile phone software. It pops on the display screen itself which can be operated by user using touch of a finger tip.


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