AD20msp415

–2–

REV. 0

ARCHITECTURE OVERVIEW

Analog 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 DESCRIPTION

Figure 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.

VOICE

ADC

SPEECH

ENCODE

CHANNEL

ENCODE

INTER-

LEAVE

ENCRYPT

VOICE

DAC

SPEECH

DECODE

CHANNEL

DECODE

DEINTER-

LEAVE

DECRYPT

EQUALIZER

BASEBAND

ADC

CONTROL + MMI + I/O

VBC

GSMP

BASEBAND

DAC

Figure 1. Functional Block Diagram

Uplink

The 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.

Downlink

The 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.