S5N8944B Datasheet, PDF (12/21 Pages) Samsung semiconductor – G.Lite ADSL Transceiver for CO and CPE

S5N8944B
G.Lite ADSL Transceiver for CO and CPE
drivers and hybrid components to connect the PSTN. The ADSL Transceiver provides all the
digital functions as depicted in Figure 4.
The input bit stream is divided into bit slices and they are fed into the QAM which are allocated
to 128 subchannels according to the bit loading table. The bit slices are then converted to
frequency-domain complex samples by the QAM encoders. The 256 complex samples are
changed to 256 time-domain samples by IFFT. The Tx filter performs band separation and
interpolation functions.
The received signals are attenuated and distorted in terms of both phase and amplitude. PLL
fixes the phase errors within 4 samples using the 276kHz pilot tone transmitted from the CO
side. The ones over 4 samples are fixed by the sync recovery algorithm using a known
synchronization symbol. The TEQ is a filter that adaptively alters the channel so that the
impulse response is reduced to the length of the cyclic prefix which will be removed prior to FFT.
The FEQ is a one tap complex adaptive filter for each subchannel, which adjusts the gains and
phases of the received signals. The equalizers are adaptively updated due to the transmission
channel environment.
In FDM-based DMT (Discrete MultiTone) modulation, the frequency band, 0 to 552kHz, is
divided into 128 equi-spaced subchannels, of which 26kHz (#6) to 134kHz (#31) is allocated for
the upstream, and 142kHz (#33) to 548kHz (#127) for the downstream. The Nyquist rate,
therefore, should be 1.104MHz(276kHz) .
DMT inherently transmits an optimized time-variable spectrum. This spectrum is adjusted
according to the desired data rate and the transmission characteristics (transfer function and
noise spectrum) on each and every subchannel. For this, CO and CPE transmit 128 4kHz-wide
tone downstream and upstream respectively to each other during initialization. They measure
the quality of each of these received tones and then decide whether a tone has sufficient quality
to be used for further transmission and, if so, how much data this tone should carry relative to
the other tones that are used. They inform the bit loading result to each other.
ATM
or
Non-ATM
ATM
Framer
FEC
Codec
Host
DSP
QAM
Codec
FEQ
IFFT
256/64
FFT
256/64
TEQ
TX
Filter
RX
Filter
Figure 4: Functional Block Diagram of the S5N8944B
DAC
ADC
CONFIDENTIAL
12
Preliminary Information (Rev.2.1 )