28973-DSH-001-A Datasheet, PDF (33/119 Pages) M/A-COM Technology Solutions, Inc. – Single-Chip SDSL/HDSL Transceiver

RS8973
Single-Chip SDSL/HDSL Transceiver
2.0 Functional Description
2.2 Receive Section
2.2.5 Echo Canceller
2.2.5.1 Linear Echo
Canceller
2.2.5.2 Nonlinear Echo
Canceller (NEC)
The echo canceller (EC) removes images of the transmitted symbols from the
received signal and consists of two blocks: a linear and nonlinear echo canceller.
The organization of the blocks is displayed in Figure 2-3, Receiver Digital Signal
Processing.
The linear echo canceller (LEC) is a conventional LMS, finite impulse response
(FIR) filter, which removes linear images of the transmitted symbols from the
received signal. It consists of a 120-tap FIR filter with 32-bit adapted
coefficients.
When LEC is enabled, the last data tap of the echo canceller is treated
specially; the data value is set to a constant 1. This serves to cancel any DC offset
that may be present.
These modes, used less often, can also be enabled through the MCI:
• A freeze coefficient mode disables the coefficient updates.
• A special mode zeros all of the coefficients.
• An additional mode zeros the output of the FIR with no effect on the
coefficients.
• Individual EC coefficients can be read and written through the MCI.
The nonlinear echo canceller (NEC) reduces the residual echo power in the echo
canceller output caused by nonlinear effects in the transmitter, receiver, analog
hybrid circuitry, or line cables.
The delay of the transmit-symbol input to the NEC can be specified through
the MCI, Nonlinear Echo Canceller Mode Register [nonlinear_ec_modes; 0x09].
This allows the NEC to operate on the peak of the echo regardless of differing
delays in the echo path.
These modes, used less often, can also be enabled through the MCI:
• A freeze coefficient mode disables the coefficient updates.
• A special mode zeros all of the coefficients.
• An additional mode zeros the output of the look-up table with no effect on
the coefficients.
• The 64 14-bit, individual NEC coefficients can be read and written
through the MCI.
2.2.6 Equalizer
2.2.6.1 Digital
Automatic Gain Control
Four LMS filters are used in the equalizer to process the echo canceller output so
that received symbols can be reliably recovered. The filters are a digital automatic
gain controller, a feed forward equalizer, an error predictor, and a decision
feedback equalizer. Their interconnections are shown in Figure 2-3, Receiver
Digital Signal Processing.
The digital automatic gain control (DAGC) scales the echo-free signal to the
optimum magnitude for subsequent processing.
Two other modes, used less often, can also be enabled through the MCI:
• A freeze coefficient mode disables the coefficient update.
• The DAGC gain coefficient can be read or written through the MCI.
N8973DSD
Conexant
2-9
Preliminary Information