cell phone electronics basics

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February 01, 2011

Cell phone electronics basics :: Radio-Electronics.Com

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- an summary or tutorial about the basics of cell phoneelectronics and what is contained within a mobile phone.

This basic cellular concepts tutorial tutorial is split into several pages each of whichaddress different aspects of cellular telecommunications technology:

[1] Basic cellular concepts[2] Cellular multiple access schemes[3] TDD and FDD duplex schemes[4] Celphone electronics basics[5] Cellular network architecture basics[6] Cellphone registration[7] Cellphone handover / handoff

The mobile phone or cell phone as it is often called is equally important to the network inthe operation of the complete cellular telecommunications network. Despite the hugenumbers that are made, they still cost a significant amount to manufacture, discountsbeing offered to users as incentives to use a particular network. Their cost is areflection of the complexity of the mobile phone electronics. They comprise several

different areas of electronics, from radio frequency (RF) to signal processing, andgeneral processing.

The design of a cell phone is particularly challenging. They need to offer high levels ofperformance, while being able to fit into a very small space, and in addition tot his theelectronics circuitry needs to consume very little power so that the life betweencharges can be maintained.

Mobile phone contents

Mobile phones contain a large amount of circuitry, each of which is carefully designedto optimise its performance. The cell phone comprises analogue electronics as well asdigital circuits ranging from processors to display and keypad electronics. A mobilephone typically consists of a single board, but within this there are a number of distinctfunctional areas, but designed to integrate to become a complete mobile phone:

Radio frequency - receiver and transmitterDigital signal processingAnalogue / digital conversionControl processorSIM or USIM cardPower control and battery
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Radio frequency elements

The radio frequency section of the mobile phone is one of the crucial areas of the cellphone design. This area of the mobile phone contains all the transmitter and receivercircuits. Normally direct conversion techniques are generally used in the design for themobile phone receiver.

The signal output from the receiver is applied to what is termed an IQ demodulator.Here the data in the form of "In-phase" and "Quadrature" components is applied to theIQ demodulator and the raw data extracted for further processing by the phone.

On the transmit side one of the key elements of the circuit design is to keep the batteryconsumption to a minimum. For GSM this is not too much of a problem. The modulationused is Gaussian Minimum Shift Keying. This form of signal does not incorporateamplitude variations and accordingly it does not need linear amplifiers. This is a distinctadvantage because non linear RF amplifiers are more efficient than linear RF amplifiers.

Unfortunately EDGE uses eight point phase shift keying (8PSK) and this requires a linearRF amplifier. As linear amplifiers consume considerably more current this is a distinctdisadvantage. To overcome this problem the design for the mobile phone is organisedso that phase information is added to the signal at an early stage of the transmitterchain, and the amplitude information is added at the final amplifier.

Analogue to Digital Conversion

Another crucial area of any mobile phone design is the circuitry that converts thesignals between analogue and digital formats that are used in different areas. The radiofrequency sections of the design use analogue techniques, whereas the processing isall digital.

The digital / analogue conversion circuitry enables the voice to be converted either fromanalogue or to digital a digital format for the send path, but also between digital andanalogue for the receive path. It also provides functions such as providing analoguevoltages to steer the VCO in the synthesizer as well as monitoring of the batteryvoltage, especially during charging. It also provides the conversion for the audio signals

to and from the microphone and earpiece so that they can interface with the digitalsignal processing functions.

Another function that may sometimes be included in this area of the mobile phonedesign or within the DSP is that of the voice codecs. As the voice data needs to becompressed to enable it to be contained within the maximum allowable data rate, thesignal needs to be digitally compressed. This is undertaken using what is termed acodec.

There are a number of codec schemes that can be used, all of which are generallysupported by the base stations. The first one to be used in GSM was known as LPC-

RPE (Linear Prediction Coding - Regular Pulse Excitation). However another schemeknown as AMR (Adaptive Multi-Rate) is now widely used as it enables the data rate to befurther reduced when conditions permit without impairing the speech quality too much.By reducing the speech data rate, further capacity is freed up on the network.


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Digital Signal Processing

The DSP components of the mobile phone design undertake all the signal processing.Processes such as the radio frequency filtering and signal conditioning at the lowerfrequencies are undertaken by this circuitry. In addition to this, equalisation and

correction for multipath effects is undertaken in this area of the design.

Although these processors are traditionally current hungry, the current processorsenable the signal processing to be undertaken in a far more power effective mannerthan if analogue circuits are used.

Control processor

The control processor is at the heart of the design of the phone. It controls all theprocesses occurring in the phone from the MMI (Man machine interface) whichmonitors the keypad presses and arranging for the information to be displayed on thescreen. It also looks after all the other elements of the MMI including all the menus thatcan be found on the phone.

Another function of the control processor is to manage the interface with the mobilenetwork base station. The software required for this is known as the protocol stackand it enables the phone to register, make and receive calls, terminate them and alsohandle the handovers that are needed when the phone moves from one cell to thenext. Additionally the software formats the data to be transmitted into the correct

format with error correction codes included. Accordingly the load on this processorcan be quite high, especially when there are interactions with the network.

The protocols used to interact with the network are becoming increasingly complicatedwith the progression from 2G to 3G. Along with the increasing number of handsetapplications the load on the processor is increasing. To combat this, the design for thisarea of the phone circuitry often uses ARM processors. This enables high levels ofprocessing to be achieved for relatively low levels of current drain.

A further application handled by this area of the design of the mobile phone is the

monitoring the state pf the battery and control of the charging. In view of thesophisticated monitoring and control required to ensure that the battery is properlycharged and the user can be informed about the level of charge left, this is animportant area of the design.

Battery

Battery design and technology has moved on considerably in the last few years. This

has enabled mobile phones to operate for much longer. Initially nickel cadmium cellswere used, but these migrated to nickel-metal-hydride cells and then to lithium ion cells.With phones becoming smaller and requiring to operate for longer from a singlecharge, the capacity of the battery is very important, and all the time the performanceof these cells is being improved.


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