MTC20154

8/21

Analog TX/RX Signals

The reference impedance for all power calculations is 100

Ω.

DMT Signal

A DMT signal is basically the sum of N independently QAM modulated signals, each carried over a distinct

carrier. The frequency separation of each carrier is 4.3125 KHz with a total number of 256 carriers (ANSI).

For large N, the signal can be modelled by a gaussian process with a certain amplitude probability density

function. Since the maximum amplitude is expected to arise very rarely, the signal is clipped to trade-off

the resulting SNR loss against AD/DA dynamic range.

A clipping factor (Vpeak/Vrms = ”crest factor”) of 5 is used resulting in a maximum SNR of 75 dB. ADSL

DMT signals are nominally sent at -40 dBm/Hz +/- 3 dB (-3.65 dBm/carrier) with a maximal power of 100

mW for downlink transmitter and 4.5 mW for uplink transmitter.

The minimum SNR+D needed for DMT carrier demodulation is about (3*N+20) dB with a minimum of 38

dB where N is the constellation size of a carrier (in bits).

Table 7. Signal Levels (on the line)

Table 8. Total Signal Level (on the line)

LT side

NT side

Description

RX

TX

RX

TX

Max level

839 mVpdif

15.8 Vpdif

3.95 Vpdif

3.4 Vpdif

Max RMS level

168 Vrms

3.16 Vrms

791 mVrms

671 mVrms

Min level

54 mVpdif

3.95 Vpdif

42 mVpdif

839 mVpdif

Min RMS level

11 mVrms

791 mVrms

8 mVrms

168 mVrms

LT side

NT side

Description

RX

RX

Max level for receiver

4 Vpdif (Long line)

4.2 Vpdif (Short line)