STLC60135 datasheet(9/25 Pages) STMICROELECTRONICS

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This stream is still affected by carrier specific

channel distortion resulting in an attenuation of

the signal amplitude and a rotation of the signal

phase. To compensate, a Frequency domain

equalizer (FEQ) and a Rotor (phase shifter) are

implemented. The frequency domain equalisation

performs an operation on the received vector in

order to match it with the associated point in the

constellation. The coefficient used to perform the

equalisation are floating point, and may be up-

dated by hardware or software, using a mecha-

nism of active and inactive table to avoid DMT

synchro problems.

In the transmit path, the IFFT reverses the DMT

symbol from frequency domain to time domain.

The IFFT block is preceded by Fine Tune Gain

(FTG) and Rotor stages, allowing for a compen-

sation of the possible frequency mismatch be-

tween the master clock frequency and the trans-

mitter clock frequency (which may be locked to

another reference).

The Inverse Fast Fourier Transform process is

used to transform from frequency domain to time

domain ( transmit path). On ATU-C side, 512 fre-

quencies are processed, giving 1024 samples in

the time domain. On ATU-R side, 256 positive

frequencies are processed, giving 512 samples in

the time domain.

The FFT module is a slave DSP engine controlled

by the firmware running on an external controller.

It works off line and communicates with other

blocks through buffers controlled by the “Data

Symbol Timing Unit”. The DSP executes a pro-

gram stored in a RAM area, which constitutes a

flexible element that allows for future system en-

hancements.

DPLL

The Digital PLL module receives a metric for the

phase error of the pilot tone. In general, the clock

frequencies at the ends (transmitter and receiver)

do not match exactly. The phase error is filtered

and integrated by a low pass filter, yielding an es-

timation of the frequency offset. Various proc-

esses can use this estimate to deal with the fre-

quency mismatch.

In particular, small accumulated phase error can

be compensated in the frequency domain by a ro-

tation of the received code constellation (Rotor).

Larger errors are compensated in the time do-

main by inserting or deleting clock cycles in the

sample input sequence.

Eventually that leads to achieve less than 2ppm

between the two ends.

Mapper/Demapper, Monitor, Trellis Coding,

FEQ Update

The Demapper converts the constellation points

computed by the FFT to a block of bits. This

means to identify a point in a 2D QAM constella-

tion plane. The Demapper supports Trellis coded

demodulation and provides a Viterbi maximum

likelihood estimator. When the Trellis is active,

the Demapper receives an indication for the most

likely constellation subset to be used.

In the transmit direction, the mapper receives a

bit stream from the Trellis encoder and modulates

the bit stream on a set of carriers (up to 256). It

generate coordinates for 2n QAM constellation,

where n < 15 for all carriers.

The Mapper performs the inverse operation,

mapping a block of bits into one constellation

point (in a complex x+jy representation) which is

passed to the IFFT block. The Trellis Encoder

generates redundant bits to improve the robust-

ness of the transmission, using a 4-Dimensional

Trellis Coded Modulation scheme. This feature

can be disabled.

The Monitor computes error parameters for carri-

ers specified in the Demapper process. Those

parameters can be used for updates of adaptive

filters coefficients, clock phase adjustments, error

detection,etc. A series of values is constantly

monitored, such as signal power, pilot phase de-

viations, symbol erasures generation, loss of

frame,etc.

Generic TC Layer Functions

These functions relate to byte oriented data

streams. They are completely described in ANSI

T 1.4 13. Additions described in the Issue 2 of

PMD

SCRAMBLER

DESCRAMBLER

DATA

PATX

MERGER

CODING

DECODING

INTERLEAVER

DE-INTERLEAVER

ATM

TO/FROM

DEMAPPER

D98TL317A

INDICATION BITS

AOC EOC

PMD

SCRAMBLER

DESCRAMBLER

FRAMER

DEFRAMER

FAST

Figure 6. Generic TC Layer Functions

STLC60135

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