Cell phone electronics basics
an summary or tutorial about the basics of cell phone electronics and what
is contained within a mobile phone.
The mobile phone or cell phone as it is often called is
equally important to the network in the operation of the complete cellular
telecommunications network. Despite the huge numbers that are made, they still
cost a significant amount to manufacture, discounts being offered to users as
incentives to use a particular network. Their cost is a reflection of the
complexity of the mobile phone electronics. They comprise several different
areas of electronics, from radio frequency (RF) to signal processing, and
general processing.
The design of a cell phone is particularly challenging. They
need to offer high levels of performance, while being able to fit into a very
small space, and in addition tot his the electronics circuitry needs to consume
very little power so that the life between charges can be maintained.
Mobile phone contents
Mobile phones contain a large amount of circuitry, each of
which is carefully designed to optimise its performance. The cell phone
comprises analogue electronics as well as digital circuits ranging from
processors to display and keypad electronics. A mobile phone typically consists
of a single board, but within this there are a number of distinct functional
areas, but designed to integrate to become a complete mobile phone:
- Radio frequency - receiver and transmitter
- Digital signal processing
- Analogue / digital conversion
- Control processor
- SIM or USIM card
- Power control and battery
Radio frequency elements
The radio frequency section of the mobile phone is one of the
crucial areas of the cell phone design. This area of the mobile phone contains
all the transmitter and receiver circuits. Normally direct conversion techniques
are generally used in the design for the mobile 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 the IQ 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 battery consumption to a minimum. For GSM this is not too
much of a problem. The modulation used is Gaussian Minimum Shift Keying. This
form of signal does not incorporate amplitude variations and accordingly it does
not need linear amplifiers. This is a distinct advantage 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 linear RF amplifier. As linear amplifiers consume
considerably more current this is a distinct disadvantage. To overcome this
problem the design for the mobile phone is organised so that phase information
is added to the signal at an early stage of the transmitter chain, 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 the signals between analogue and digital formats that
are used in different areas. The radio frequency sections of the design use
analogue techniques, whereas the processing is all digital.
The digital / analogue conversion circuitry enables the voice
to be converted either from analogue or to digital a digital format for the send
path, but also between digital and analogue for the receive path. It also
provides functions such as providing analogue voltages to steer the VCO in the
synthesizer as well as monitoring of the battery voltage, 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 digital signal
processing functions.
Another function that may sometimes be included in this area
of the mobile phone design or within the DSP is that of the voice codecs. As the
voice data needs to be compressed to enable it to be contained within the
maximum allowable data rate, the signal needs to be digitally compressed. This
is undertaken using what is termed a codec.
There are a number of codec schemes that can be used, all of
which are generally supported 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 scheme known as AMR (Adaptive Multi-Rate) is now widely used as
it enables the data rate to be further 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.
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 lower frequencies 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 processors enable the signal processing to be undertaken in a far
more power effective manner than if analogue circuits are used.
Control processor
The control processor is at the heart of the design of the
phone. It controls all the processes occurring in the phone from the MMI (Man
machine interface) which monitors the keypad presses and arranging for the
information to be displayed on the screen. It also looks after all the other
elements of the MMI including all the menus that can be found on the phone.
Another function of the control processor is to manage the
interface with the mobile network base station. The software required for this
is known as the protocol stack and it enables the phone to register, make and
receive calls, terminate them and also handle the handovers that are needed when
the phone moves from one cell to the next. Additionally the software formats the
data to be transmitted into the correct format with error correction codes
included. Accordingly the load on this processor can be quite high, especially
when there are interactions with the network.
The protocols used to interact with the network are becoming
increasingly complicated with the progression from 2G to 3G. Along with the
increasing number of handset applications the load on the processor is
increasing. To combat this, the design for this area of the phone circuitry
often uses ARM processors. This enables high levels of processing 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 the sophisticated monitoring and control required to ensure
that the battery is properly charged and the user can be informed about the
level of charge left, this is an important 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 cells were 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 single charge, the capacity
of the battery is very important, and all the time the performance of these
cells is being improved.
Summary
Although mobile phones are one of the msot commonplace pieces
of electronics equipment these days, they are nevertheless complicated inside.
An understanding of the mobile phone basics can often be useful when looking at
the way a cellular network and cellular technology in general works.
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