Thesis in Electronics Engineering (DU) on ADSL system with DMT modulation in respect of the Standard ANSI T1.413

Basic implementation

Figure 3 shows a block diagram of a basic DMT transmitter.




Figure 3: DMT -- basic block diagram (Transmission Path)

The key element of the DMT implementation is the FFT/IFFT. IFFT is an elegant and efficient way to create a sum of N carriers each modulated by its own phase and amplitude. Since we implement a baseband line code, we artificially duplicate each carrier with its conjugate counterpart at a high frequency to generate an IFFT output that is real only, holding 2N time domain samples. This vector is applied using a digital to analog converter to the line. The input to the IFFT module is a vector of QAM constellation points - N complex numbers, defining the amplitude and phase of each carrier. Figure 4 shows an example of 2 and 3 bit constellation according to the ADSL standard.




Figure 4: 3-bit QAM Constellation



At the receiver the complement operation is done by FFT. The resulting N carriers are transformed back to their amplitude and phase information and then decoded back to bits.

Figure 5 shows a detailed description of a DMT modem.



Figure 5: DMT Modem - detailed block diagram




Encoder/ decoder

As was described before the encoder takes the data bit stream and encodes it into N QAM constellation points. This encoding is done according to the bit loading table which defines the number of bits carried by each tone. Clearly high SNR (Signal to Noise Ratio) carriers can carry more bits than low SNR carriers so the bit loading table reflects the variation of the SNR over frequency. Figure 6 shows typical example of SNR and bit loading.

The bit loading table is calculated during startup according to the actual measured SNR to allow optimal use of channel capacity. Loading is limited by the ADSL standard to 2-15 bits per tone. When we want to serve the customer with a specific bit rate, we allocate the bits to the carriers in such a way that the sum of all the bits on all carriers matches the desired rate, and the probability of error on each carrier is about the same. When we want to give each customer the maximal available bit rate, we allocate to each carrier the maximum number of bits that we can transmit without errors, based on the measured SNR of that carrier. This latter mode is usually referred as the rate-adaptive mode. This way a customer living close to the exchange will have a high SNR and high data rate, whereas a customer living far from the exchange will suffer from more line attenuation and less bits shall be allocated on each of the carriers.




Figure 6: Relation between SNR (Signal-to-Noise Ratio) and bit-loading


Gain

The gain stage implements the following functions:

- Normalizing all constellations to a constant unit energy. Note that high loading constellation has higher energy, see figure 6.

- Compensation of analog front end (AFE) frequency response.

- Fine equalization of BER among the different channels by gain adjustment.

IFFT / FFT

This module was previously described.

Cyclic Prefix add/drop

Each symbol has a cyclic prefix that is 1/16 symbols length. This CP separates the symbols in time in order to decrease intersymbol interference (ISI). As is well known, the signal going through the line is linearly convolved with the impulse response of the line. If the impulse response is shorter than the duration of the cyclic prefix, each symbol can be processed separately, and there is no intersymbol interference (ISI). Also, in this case the receiver views the incoming signal as if it has gone through a cyclic convolution. This matches the FFT processing very well and ensures orthogonality between carriers.

Echo cancellation

This module is described below.

Time equalizer -- TEQ

Time equalizer is a linear filter designed to minimize the intersymbol interference (ISI) and interchannel interference (ICI). This is done by shrinking the total impulse response of line to the length of the cyclic prefix -- see figure 7. In that way one symbol can not interfere with the next one so ISI is eliminated.



Impulse response
without TEQ








Impulse response
with TEQ

Figure 7: TEQ Influence over Impulse Response

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