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AD6426

Enhanced GSM Processor

Analog Devices 

Preliminary Technical Information

A feature of the GSM system is the application, as part of the

TRANSMIT process, of data encryption for the purpose of link

security. After the INTERLEAVE module the data may be

encrypted using the prescribed A5/1 or A5/2 encryption

algorithm.

The RECEIVE function requires unmodulated baseband data

from the equalizer. As necessary the data is decrypted and

written to the DEINTERLEAVE module. This is conducted at

TDMA frame rate, although precise timing is not necessary at

this stage.

The DECODING process reads data from the

DEINTERLEAVE module, inverting the interleave algorithm

and decodes the error control codes, correcting and flagging

errors as appropriate. The data also includes a measure of

confidence expressed as two additional bits per received

symbol. These are used in the convolution decoder to improve

the error decoding performance. The resultant data is then

presented to the original sources as determined by the control

programming. The Channel Codec interfaces with the speech

transcoder for speech traffic data and with an equalizer for

recovered receive data. In the AD6426 the equalizer and

speech transcoder are implemented in the DSP.

Processor Sub-System

The Processor Sub-System consists of a high performance 16-

bit microcontroller together with a selection of peripheral

elements. The processor is a version of the Hitachi H8/300H

that has been developed to support GSM applications and

which is well suited to support the Protocol Stack and

Application Layer software.

DSP Sub-System

The DSP Sub-System consists of a high performance 16-bit

digital signal processor (DSP) with integrated RAM and ROM

memories. The DSP performs two major tasks: speech

transcoding and channel equalization. Additionally several

support functions are performed by the DSP. The instruction

code, which advises the DSP to perform these tasks, is stored

in the internal ROM. The DSP sub-system is completely self-

contained, no external memory or user-programming is

necessary.

Speech Transcoding

In Full Rate mode the DSP receives the speech data stream

from the EVBC and encodes the data from 104 kbit/s to 13

kbit/s. The algorithm used is Regular Pulse Excitation, with

Long Term Prediction (RPE-LTP) as specified in the 06-series

GSM Recommendations.

In Enhanced Full Rate mode, the DSP encodes the 104 kbit/s

speech data into 12.2 kbit/s (speech) +0.8 kbit/s (CRC and

repetition bits) as additionally specified in the Phase 2 version

of the 06-series GSM Recommendations. In both modes, the

DSP also performs the appropriate voice activity detection and

discontinuous transmission (VAD/DTX) functions.

AD6426

Alternatively the DSP receives encoded speech data from the

channel codec sub-system including 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 resulting data, at 104 kbit/s, is

transferred to the EVBC.

Equalization

The Equalizer recovers and demodulates the received signal

and establishes local timing and frequency references for the

mobile terminal 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 decision to the channel codec’s

convolutional decoder. The equalizer outputs a sequence of

bits including the confidence bits to the channel codec sub-

system.

Audio Control

The DSP subsystem is also responsible for the control of the

audio path. The EVBC provides two audio inputs and two

audio outputs, as well as a separate buzzer output, which are

switched and controlled by the DSP. Furthermore the EVBC

provides for variable gain and sensitivity which is also

controlled by the DSP under command of the Layer 1

software.

Tone Generation

All alert signals are generated by the DSP and output to the

EVBC. These alerts can be used for the buzzer or for the

earpiece. The tones used for alert signals can be fully defined

by the user by means of a description which provides all the

parameters required such as frequency content and duration of

components of the tone. The tone descriptions are provided by

the Layer 1 software.

Automatic Frequency Control (AFC)

The detection of the frequency correction burst provides the

frequency offset between the mobile terminal and the received

signal. This measure is supplied to the Layer 1 software which

then requests a correction of the master clock oscillator

frequency via the AFC-DAC in the EVBC. In order to do so

the Layer 1 software includes a transfer function for the

oscillator frequency against the voltage applied. The DSP

provides the measurements for the AFC.

Automatic Gain Control (AGC)

The DSP is also responsible for making measurements of the

power in the received signal. This is used for a number of

functions including RSSI measurement, adjacent channel

monitoring and AGC. The Layer 1 software passes the

requested gain level to the DSP, which then analyzes the

received signal and generates an AGC control signal.

Depending on the radio architecture, this control signal will be

used in digital form or, converted by the AD6425 in analog

form.

This Information applies to a product under development. Its characteristics and specifications are subject to change without notice. Analog Devices assumes no

obligation regarding future manufacture unless otherwise agreed to in writing. No responsibility is assumed by Analog Devices for its use; nor for any

infringements of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent

rights of Analog Devices.

Revision Preliminary 2.3 (June 9, ´98)

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Confidential Information

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