MTC20455

12/23

Functional Description

Fig.4 shows the global block diagram of the MTC20455. The functions can be grouped into the following:

DMT Modem Description

The following section essentially describes the sequence of actions for the receive direction, correspond-

ing functions for the transmit direction are readily derived.

DSP Front-End

The DSP Front-End contains 4 parts in the receive direction: the Input Selector, the Analog Front-End In-

terface, the Decimator and the Time Equaliser. The input selector is used internally to enable test loop-

backs inside the chip. The Analog Front-End Interface transfers 1 6-bits word, multiplexed on 4 input/

output signals. As a result, 4 clock cycles are needed to transfer 1 word. The Decimator receives the 16-

bits samples at 8.8 MHz (as sent by the Analog Front-End chip) and reduces this rate to 2.2 MHz. The

Time Equaliser (TEQ) module is an FIR filter with programmable coefficients. Its main purpose is to reduce

the effect of Inter-Symbol Interference (ISI) by shortening the channel impulse response. Both the Deci-

mator and TEQ can be bypassed. In the transmit direction, the DSP Front-End includes: sidelobe filtering,

clipping, delay equalisation and interpolation. The sidelobe filtering and delay equalisation are implement-

ed by IIR filters, reducing the effect of echo in FDM systems. Clipping is a statistical process limiting the

amplitude of the output signal, optimising the dynamic range of the AFE. The interpolator receives data at

2.2 MHz and generates samples at a rate of 8.8 MHz.

Figure 4. DSP Front-End

DMT Modem

This computational module is a programmable DSP unit. Its instruction set enables functions like FFT, IF-

FT, Scaling, Rotor and Frequency Equali-sation (FEQ). This block implements the core of the DMT algo-

rithm as specified in ANSI T1.413. In the RX path, the 512-point FFT transforms the time-domain DMT

symbol into a frequency domain representation which can be further decoded by the subsequent de-map-

ping stages. After the first stage time / domain equalisation and FFT block – an essentially ICI (InterCarrier

Interference) – free carrier information stream has been obtained.

This stream is still affected by carrier-specific channel distortion resulting in an attenuation of the signal

Bypass

IN

select

Analog

Interface

To DMT

modem

AFE

I/F

DEC

TEQ