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AD6421AST-AD6422AST
GSM/DCS1800/PCS1900 Baseband Processing Chipset
REV.0
GSM/DCS1800/PCS1900
Baseband Processing Chipset
SYSTEM ARCHITECTURE
FEATURES
Complete Baseband Processing Chipset Performs:
Speech Coding/Decoding (GSM 06.XX)
Channel Coding/Decoding (GSM 05.03)
Equalization with 16-State Viterbi, Soft Decision
All ADC and DAC Interface Functions
Includes All Radio, Auxiliary and Voice Interfaces
Support for GSM Data Services
Embedded 16-Bit Microcontroller
Embedded 16-Bit DSP
Integrated SIM and Keyboard Interface
Interface to AD6430 GSM RF Chipset
Interface to EFR Coprocessor
JTAG Boundary Scan
Layer 1 Software Provided with Chipset
Software Compatible with AD20msp410
Full Phase 2 Protocol Stack Software Available
Full Reference Design Available for
Baseband Section and Radio Section
Ultralow Power Design
2.7 V to 3.3 V Operating Voltage
Intelligent Power Management Features
XXX mW Power Dissipation in Talk Mode
XX mW Power Dissipation in Standby Mode
Two TQFP Devices, Occupying Less than 7.5 cm2
APPLICATIONS
GSM/DCS1800/PCS1900 Mobile Radios
GENERAL DESCRIPTIONThe Analog Devices GSM Baseband Processing Chipset provides
a competitive solution for GSM based Mobile Radio Systems.
It is designed to be fully integrated, easy to use, and compat-
ible with a wide range of product solutions. Examples are
GSM900, DCS1800, PCS1900 handsets and PCMCIA data
cards. The AD20msp415 is the higher integrated successor of
the AD20msp410 chipset, which passed European GSM Type
Approval in June, 1996.
The chipset consists of two highly integrated, sub-micron, low
power CMOS components that perform the entire baseband
signal processing of the GSM handset. The system architecture
is designed to be easily integrated into current designs and form
the basis for next generation designs.
The chipset uses an operating supply voltage of 2.7V to 3.3V
which, coupled with the extensive power management features,
significantly reduces the drain on battery power and extends the
handsets talk time and standby time.
CHIPSET COMPONENTS
GSM Processor (GSMP)The AD6422 combines application specific hardware, an em-
bedded 16-bit DSP and an embedded 16-bit microcontroller
(Hitachi H8/300H). It performs channel coding and decoding
and executes the protocol stack and user software. The DSP
implements full rate speech transcoding according to GSM
specifications, including Discontinuous Transmission (DTX)
and Comfort Noise Insertion (CNI). A high performance soft-
decision Viterbi equalizer is also implemented in software em-
bedded in the DSP. The embedded microcontroller executes
the Layer 1, 2, 3 and user MMI software. The required Layer 1
software is supplied with the chipset. To ensure minimum power
consumption, the GSMP has been designed to control all the
power-down functions of the other components in the handset.
Voiceband/Baseband Converter (VBC)The AD6421 performs the voiceband and baseband analog-to-
digital and digital-to-analog conversions, interfacing the digital
sections of the chipset to the microphone, loudspeaker and radio
section. In addition, the VBC contains all the auxiliary convert-
ers for burst-ramping, AFC, AGC, battery and temperature
monitoring. The chipset interfaces directly with the radio and
supplies all the synthesizer and timing control signals required
to support two synthesizers and a variety of radio architectures
including the AD6450 GSM RF-Chipset.
SoftwareThe required Layer 1 software is supplied with the chipset. In
addition, an object code license is available for Layers 2 and 3 of
the protocol stack.
AD20msp415
ARCHITECTURE OVERVIEWAnalog Devices and The Technology Partnership (TTP) pro-
vide a cost effective and proven method of attaining the baseband
processing subsystem and protocol stack software. This data
sheet includes functional descriptions of the baseband process-
ing subsystem and the Protocol Stack Layer 1. The Technology
Partnership can provide licenses to software and reference de-
signs in all areas of a GSM handportable terminal.
For detailed information about the individual chipset compo-
nents, please refer to the AD6421 (VBC) and AD6422 (GSMP)
data sheets for electrical characteristics and timing information.
FUNCTIONAL DESCRIPTIONFigure 1 is a functional block diagram of the GSM baseband
processing chipset. The chipset can be viewed as a functional
block that contains a number of discrete functional units. The
electrical and functional interfaces to the rest of the system are
briefly described at the end of this section and described in
detail in the individual data sheets for each component.
Figure 1.Functional Block Diagram
UplinkThe uplink baseband processing functions include the following
operations:
Analog-to-Digital Voice Conversion (VBC)A conventional microphone, connected directly to the VBC,
provides an analog input signal to the ADC. The analog voice
signal is sampled at 8 kHz, producing 13-bit linear values corre-
sponding to the magnitude of the input. The ADC includes all
required filtering to meet the GSM specifications. The sampled
voice data is passed to the GSMP through a dedicated serial
port.
Speech Encoding (GSMP)The GSMP receives the voice data stream from the VBC and
encodes the data from 104 kb/s to 13 kb/s. The algorithm used is
Regular Pulse Excitation, with Long Term Prediction (RPE-
LTP) as specified in the 06-series of GSM Recommendations.
Channel Coding (GSMP)The information received from the speech coder contains param-
eters that have different levels of priority. These are protected to
different levels within the channel coding. The encode protec-
tion process incorporates block coding and convolutional
encoding. In addition to the normal speech traffic channels, the
channel coding function also supports data transmission at full
rate and half rate. After the interleave process (if necessary) the
data is encrypted using the required A5/1 or A5/2 encryption
algorithm. Data is then formatted into bursts, with the required
timing and training sequences, and sent to the VBC through a
dedicated serial port.
GMSK Modulation and D/A Conversion (VBC)The VBC receives data at 270 kb/s. The VBC uses an on-chip
lookup table to perform GMSK modulation. A pair of 10-bit
matched differential DACs convert the modulated data and pass
I and Q analog data to the transmit section of the radio subsystem.
DownlinkThe downlink baseband processing functions include the follow-
ing operations:
Analog-to-Digital Conversion (VBC)The receiver I and Q signals are sampled by a pair of ADCs at
270 kHz. The I and Q samples are transferred to the GSMP
through a dedicated receive path serial port.
Equalization (GSMP)The equalizer recovers and demodulates the received signal and
establishes local timing and frequency references for the mobile
unit as well as RSSI calculation. The equalization algorithm is a
version of the Maximum Likelihood Sequence Estimation (MLSE)
using the Viterbi algorithm. Two confidence bits per symbol
provide additional information about the accuracy of each deci-
sion to the channel codec’s convolutional decoder. The equal-
izer outputs a sequence of bits including the confidence bits.
Channel Decoding (GSMP)Data is decrypted as required, using the A5/1 or A5/2 decryption
algorithm prior to the deinterleave process. The deinterleave
process is an exact inversion of the interleave process used by
the transmit section. The decode function then performs convo-
lutional decoding and parity check. The convolutional decoder
uses a Viterbi algorithm, with two soft decision confidence bits
supplied by the equalizer. Error control mechanisms are used to
ensure adequate bad frame indication.
Speech Decoding (GSMP)Encoded speech data is transferred at 20 ms intervals in blocks
of 260 bits plus the Bad Frame Indicator (BFI). The speech
decoder supports a Comfort Noise Insertion (CNI) function
that inserts a predefined silence descriptor into the decoding
process. The GSMP also implements control of talker side-tone
and short term echo cancellation. The resulting data, at 104 kb/s,
is transferred to the VBC through a dedicated serial path.
Voice Digital-to-Analog Conversion (VBC)The Voice DAC function of the VBC operates at 8 kHz and
includes all the needed filtering. The analog signal can be con-
trolled in volume and directly drive a small earpiece as well as a
separate auxiliary output.
AUXILIARY SYSTEM FUNCTIONSThe GSMP and the VBC perform a number of auxiliary func-
tions that are essential to build a complete mobile radio.
A general radio section constitutes the three functions of trans-
mitter, receiver and synthesizer. Figure 2 shows how the baseband
chipset interfaces to a typical radio architecture. The transmitter
is fed with baseband analog I and Q signals from the VBC and
upconverted to 900 MHz for GSM applications and 1800 MHz
for PCN applications.
A dedicated power amplifier increases the RF signal to the
required level. The receiver amplifies the antenna signal, down-
converts it to an intermediate frequency (IF) and amplifies it
there again. After second conversion to baseband the I and Q
components of the signal are fed into the VBC.
The three auxiliary functions, AGC, AFC and Power Ramping
are included to interface to the radio section.
Power Ramp EnvelopeTo meet the spectral and time-domain specifications of the
transmitted output signal, the burst has to follow a specified
power envelope. The envelope for the power profile originates in
the GSMP as a set of coefficients, down-loaded and stored in
the VBC. This envelope profile is fed to the RAMP DAC on the
VBC with each burst. The analog output is fed into the RF
power amplifier, controlling the power profile and absolute level
of the transmitted data. The power control loop of the power
amplifier can also feedback an error control signal that indicates
whether the output functions are out of specification and the
radio can be switched off accordingly.
Automatic Gain Control (AGC)The mobile radio has to cope with a wide range of input signal
levels. The major part of the overall gain is provided in the IF
amplifier. The incoming signal level is analyzed in the GSMP
and a digital gain control signal is sent to the VBC. The AGC
DAC generates the appropriate analog control signal for the IF
amplifier.
Automatic Frequency Control (AFC)The mobile radio tracks the master clock provided by the base
station to compensate for temperature/frequency drifts in the
crystal oscillator. Drift of the crystal oscillator over time and
temperature has to be compensated as well as frequency shifts
due to the Doppler effect in the case of a moving mobile radio.
The received signal is analyzed in the GSMP and a digital con-
trol signal is generated. This signal is sent to the AFC DAC in
the VBC to control the voltage controlled, temperature compen-
sated crystal oscillator (VCTCXO).
Synthesizer ControlThe GSMP and the respective parts of the Layer 1 software
control the overall timing and frequency generation of the radio
subsystem. This includes control signals for two synthesizers,
power-down control signals and power amplifier monitor sig-
nals. Detailed information can be found in the AD6422 data
sheet.
Figure 2.Control of Radio Subsystem
AD20msp415
CAUTIONESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000V readily
accumulate on the human body and test equipment and can discharge without detection.
Generation of Auxiliary Audio SignalsUnder control of Layer 1, the GSMP can generate a variety of
fixed and user-programmable tones. This includes all standard
DTMF and Call Progress tones as well as user defined tones.
The tone structure can consist of up to four frequency compo-
nents with individual durations. The GSMP also generates
Talker Sidetone as specified in the GSM recommendations. In
comparison to traditional hardware implementations, this soft-
ware implementation provides manufacturing flexibility over a
wide range of speaker/microphone sensitivities.
Figure 3.Audio/Auxiliary Section of the AD6421
Figure 3 shows the audio section and the auxiliary ADC of the
VBC. Input signals can come from either a directly connected
microphone or from a remote microphone in a car kit. Input
gain can be set to 0 dB or +26 dB. The output signal can be
directly connected to a small earpiece and, for further amplifica-
tion, to an external car kit. The output PGA can be programmed
for –15 dB or +6 dB.
DATA SERVICESData Services is considered to be an essential feature for GSM
terminals and the AD20msp415 chipset is designed to provide
flexible and low cost implementation of Data Services supported
via the GSM air interface.
The selected system architecture shown in Figure 4 provides for
minimum terminal Bill of Materials, the lowest possible number
of interconnection points and the lowest power consumption
when running speech traffic only. The GSMP performs full
channel coding and decoding for TCH/F9.6, TCH/F4.8 and
TCH/F2.4 data rates. The interface to the chipset is a user-
configurable, 3-wire synchronous or asynchronous serial inter-
face supplying V110´ data packets as defined in GSM 05.03,
combined with protocol information and control to the Applica-
tion Layer.
External to the terminal is the Data Terminal Adapter (DTA)
that runs the Data Services Software. Included in the DTA are
the rate adaptation functions and the Data Services application.
The Command Interpreter resident on the mobile supports a
serial interface protocol with the DTA by which both traffic data
and control information are communicated.
EXTERNAL
DATA TERMINAL
ADAPTER
HANDSETFigure 4.Implementation of Data Services
SOFTWARE IMPLEMENTATIONSA full implementation of the GSM Layer 1 functionality is sup-
plied as an object code module, for execution on the controller,
embedded in the GSMP. Functions performed by this software
include:Initial scan of GSM/E-GSM/PCN band and selection of
strongest thirty channels as required by 03.22 and 05.08.Mobile oscillator adjustment, timing synchronization and
BCCH decoding from serving cell (camping-on).Base station frequency and timing measurements and BSIC
extraction from neighbor cells under control of Layer 3.Frequency hopping according to 05.02.Full implementation of discontinuous reception (DRX) and
transmission (DTX).Reporting of received level and signal quality.Full engineering and test mode support.Support for all phase 1 and phase 2 handover modes.SIM Interface driver.Message interfacing to Layer 3 (Radio Resources Manager)
and Layer 2 (data link layer, both signaling and data).External functions for AGC, AFC and synthesizer setting are
called by Layer 1. These allow the user to configure the sys-
tem for a wide range of radio architectures.
The higher layers of the protocol stack also reside on this em-
bedded processor.
POWER DISSIPATION CONSIDERATIONSIn mobile applications, minimizing the power consumption of
all devices is critical to achieve longer standby and talk times. In
a GSM handset the baseband subsystem dominates the current
consumption of the phone in standby. The design of the GSMP
and VBC includes extensive features to reduce power consump-
tion and give standby times of up to 100 hours.
Both devices are specifically designed to operate from 2.7 V to
3.3 V, to enable three or four cell battery designs.
The GSMP incorporates intelligent power management, permit-
ting automatic control of power consumption in the Channel
Codec part of the GSMP and the peripheral circuitry. Data
processing modules are switched on only when they process
data, otherwise they are powered down. Additional control
signals are provided that enable the Layer 1 software to control
the external subsystems, such as the VBC, the radio and
memory components, so that their power is intelligently switched
by the GSMP.
In the VBC, the power-down functions are split separately
among receive, transmit and auxiliary circuits. This provides
optimal analog power performance when operating in different
modes.
INTERFACESThe chipset has eleven external interfaces (see Figure 5) that
have to be considered in the design of the complete mobile
radio.Analog Voice Interface to VBCRadio Interface to VBC and GSMPDigital SIM Card Interface to GSMPDigital Interface to the KeypadDigital Bus Interface from GSMP to Memory and DisplayDigital Audio Interface (DAI)Digital Interface to GSMP for Data ServicesDigital Interface from the GSMP to the EEPROMDigital Interface from GSMP to AccessoriesDigital Test InterfaceDigital Interface from GSMP to Optional EFR Coprocessor
Analog Voice Interface to VBCThe analog voice interface to the VBC is specified in the AD6421
data sheet. Several design examples are given for single-ended
or differential inputs or outputs. A voltage reference for biasing
the microphone signal is provided on the VBC. The analog
output of the VBC is capable of directly driving an earpiece with
an impedance of 32 Ω. For optional use of a separate external
microphone and power amplifier, a set of auxiliary input/output
signals are provided on the VBC.
Radio Interface to VBC and GSMPThe analog interface between the VBC and the radio subsystem
is specified in detail in the AD6421 data sheet. The digital inter-
face between the GSMP and the radio subsystem is specified in
detail in the AD6422 data sheet.
Digital SIM Card Interface to GSMPThe GSMP is designed to interface directly to the SIM. How-
Digital Interface to KeypadKeypad interface logic for up to 30 keys is provided on the
GSMP. This interface provides keyboard scan for six rows
and fourcolumns. Additionally, an extra column can be imple-
mented by using the “ghost column” method.
Digital Bus Interface to Memory and DisplayExternal RAM and ROM, as well as the display controller,
interface directly to the 21-bit address bus and 16-bit data bus
of the GSMP.
Interface to FLASH MemoryThe large FLASH memory can contain all programs for the
embedded Control Processor of the GSMP. This includes the
complete GSM protocol software as well as the User Interface
Software. A total size of 8 Mbits is suggested, assuming a typical
size of User Interface and a GSM Phase 2 Software. Enhanced
features requiring larger memories are easily supported by the
large address space of the embedded Control Processor.
To support FLASH memory, the GSMP provides embedded
code to download the software into the FLASH memory via its
standard serial port.
Interface to SRAMIn addition to the FLASH memory, the Control Processor also
supports static RAM to store user-defined variables, typically
those used by the Protocol Stack or Application Layer. Standard
SRAMs interface directly to the address and data bus of the
GSMP.
Interface to Display ControllerThis interface is achieved through the address and data buses
and associated read and write strobes, as well as a specific en-
able signal. One backlight pin with PWM control is provided by
the GSMP to control brightness of backlight.
Digital Audio Interface (DAI)As required by the GSM specifications, a digital audio interface
is provided to allow certain tests of the audio section during type
approval. This interface is activated in one of the test modes. A
fully functional “DAI box” needed for the FTA process may be
obtained from Analog Devices upon request.
Digital Interface to GSMP for Data ServicesThe chipset uses a serial interface that is connected to an exter-
nal data terminal adapter as described in the AD6422 data
sheet.
Digital Interface from the GSMP to the EEPROMThe GSMP provides separate pins to interface directly to an
external serial EEPROM via a serial bus. This EEPROM is
typically used for storage of calibration or user variable param-
eters like:Handset Identifier (IMEI)LanguageKeypad LockDTMF ON/OFFRadio Calibration Parameters
A typical size of the EEPROM is 2K × 8 bits, but this depends
on the individual design of the handset.
AD20msp415
Digital Interface from the GSMP to AccessoriesTo allow proper control of external accessories like a car kit, the
AD6422 provides a 10-pin accessory interface comprised of
eight general purpose I/O channels, one chip select and one
power control signal.
Digital Test InterfaceThe AD6421 and the AD6422 both support advanced test
methodologies by providing JTAG Boundary Scan. Addition-
ally, the AD6422 provides Test/Mode pins, which select differ-
ent test and operating modes.
Digital Interface from GSMP to Optional EFR-CoprocessorThe AD6422 provides a digital interface to an external EFR
Coprocessor (DSP). This coprocessor is required to handle the
Enhanced Full Rate speech codec in PCS1900 mobile radios.
Baseband Processing Key Parts ListTable I lists the major hardware components necessary to com-
plete the GSM baseband processing subsystem.
Figure 5.Chipset Interfaces
Table I.List of Key ComponentsNOTESThese components comprise the AD20msp415
chipset.The EFR-Coprocessor is required only in systems
requiring support of the Enhanced Full Rate speech
codec.A size of 4 Mbits is recommended to allow storage of
all GSM Layer (1, 2, 3) programs for GSM Phase 2
as well as a typical User Interface (MMI).Can be omitted if parameters are stored in FLASH
memory.
