Reference Oscillator
Timer
Card Reader
Power Management
Converter
Digital Signal Processor
Microprocessor
ASICs
What is in a GSM Phone
Compared to first generation mobile devices such as C-Netz phones, a GSM cell phone was significantly more complex. Until now, sending and receiving was analog and required little monitoring. With digital transmission, extensive digital operations must be carried out constantly. It requires special digital technology and a digital signal processor (DSP).
The illustration above shows how a “simple” GSM telephone was build in the early 1990s.
Reference Oscillator
A core element of the cell phone is the reference oscillator. For certain reasons, this runs exactly at 26 MHz . The requirements regarding accuracy are extremely high. The oscillator must be precisely tuned to the frequency of a base station in order to receive digital data. For this purpose, special frequency correction bursts were defined as discussed.
The oscillator must be voltage controlled (VCO = Voltage Controlled Oscillator) to be adjusted. The correction voltage is usually generated by a small digital-to-analog converter which is controlled by the mobile phone’s controller.
As described in previous chapters, a crystal oscillator is very stable but still temperature dependent. For this reason, the temperature of the oscillator is also regulated. Such oscillators are called TC-VCXO, Temperature Compensated Voltage Controlled Chrystal Oscillator.
The reference oscillator forms the basic clock for all other units, especially the frequency synthesizer which generates the frequencies for the transmitter and receiver.
Timer
A TDMA system like GSM has complex timing sequences that must be set precisely. The exact trigger signals must be generated. E.g. to start the sampling of a new receive time slot or to send a pre-assembled transmit time slot. Even before these periods occur, the transmitter or receiver must be switched on in time, so that they run stable when the sampling begins. A special module is therefore required in which a large number of counters can be programmed and operated. This module is called the timer.
Card Reader
To read and write the SIM card, a separate module was initially required, which often even contained its own microprocessor. It controlled all processes related to the SIM card including its power supply. The control of such a card had very strict requirements. E.g. such a card reader must also be able to detect if the SIM card is removed. It was required, that the mobile phone would immediately stop transmitting, if a SIM card was removed.
Power Management
A power management device has several tasks. First, it monitors the voltage of the battery to ensure safe operation of the mobile phone. If the voltage is too low, it gives a warning to stop operation. Second, it is managing the charging the battery. Finally it provides different voltages to the different devices. Typically a digital device required a low volage and an analog RF device higher voltage. For this purpose so called DC/DC or linear converters where used, that could create lower voltage from a higher voltage. The power amplifier was often directly connected to the battery since it required the highest voltage to operate efficiently.
Converter
Analogue/digital converters are required for the speech signals and for the radio signals. For speech, the usual sampling frequency of 8 kHz is used.
On the radio transmit side the 270 kbit data stream is used to drive an I/Q modulator. For transmit, the incoming I/Q signals where sampled with up to 12 bits for further processing.
In addition to these converters speech and radio converter, there were also a number of other more simple converters to generate voltages, e.g. to drive the reference oscillator.
Digital Signal Processor
The Digital Signal Processor (DSP) was mainly responsible for speech encoding and speech decoding. However, it could also be used for tasks in channel encoding and decoding and/or channel equalization.
Only 16 bit processors were used, for example standard DSPs from Texas Instruments or AT&T. Later, special DSPs were developed only for the use in cell phones.
Microprocessor
The central core of every cell phone was the microprocessor. Due to the diversity and complexity of the GSM standard, the tasks became so demanding that a simple 8-bit processor, as was still used for analog cell phones, was no longer sufficient. This meant that powerful 16-bit processors were required, such as the 68000 from Motorola. This was a processor like the one used for personal computers.
The processor had to regulate all time-critical processes. The so-called lowest layer (layer 1 or physical layer) was particularly critical. All critical TDMA functions had to be controlled by setting different registers. Continuous operation hat to be guarantied at all times, especially during critical handover scenarios. All incoming and outgoing control information had to be processed.
A strict protocol as defined by the standard had to be followed to communicate with the network. The associated Software is called the protocol stack and it became an important piece of development that continuously had to be expanded and improved.
In addition, the microprocessor was responsible for the user interface. This was initially a fairly simple task. Keyboard entries had to be processed and the display had to be operated. The display was meanwhile a dot matrix display so that text could also be displayed. These were available as LED or LCD displays, with LCD displays being the most popular because of their lower power consumption. The microprocessor could also create a personal phone book and manage different modes of the phone. Later also the processing of SMS (Short Message Service) was managed.
ASICs
Not all functions of a GSM phone could be managed by „standard ICs“. DSPs and Microcontroller were standard ICs but certain high frequency components or special circuits like the timer were not available in the form that was required. Therefore, special ICs for these applications had to be created by the semiconductors industry. These ICs were called ASICs (Application Specific Integrated Circuits). Some companies such as Siemens, Philips, Motorola had their own semiconductor production facilities to create ASICs. Other companies such as Ericsson and Nokia entered into close collaborations with semiconductor companies to build ASICs.