Summary: Joint Channel and Echo Impule Response Shortening on Digital Subscriber Lines
Naofal AlDhahir \Lambda
, Member IEEE
EDICS Category 2:8:1 (DSP Applications in Communications)
A new scheme for joint shortening of two long impulse responses using a single FIR equalizer is presented.
The optimum (in the mean square error sense) settings of the equalizer and the two (unit--tap constrained)
shortened impulse responses are derived. The main application of interest is joint shortening of the channel
and echo impulse responses for high--speed transmission on digital subscriber lines.
The feasibility of high--speed reliable transmission over unloaded digital subscriber lines has been recently
demonstrated . In one transmission scheme, called high--bit--rate digital subscriber lines (HDSL), two
twisted pairs are used to transport an unrepeatered bi--directional T1--rate data within a range of 2 miles.
Impedance mismatches at the four--wire to two--wire hybrid of an HDSL transceiver result in echo noise.
Therefore, an effective echo canceler is needed to separate the two directions of transmission. Due to the
high sampling rate of HDSL transceivers (400 kHz is typical), the overall channel and echo impulse response
lengths are very long. This in turn increases the complexity of the echo canceler significantly (128--tap
FIR echo canceler is commonly used ). The dispersive nature of the channel necessitates an effective
ISI--mitigating receiver structure such as decision feedback equalizers, multicarrier transceivers, or maximum
likelihood sequence estimation using the Viterbi algorithm. Whatever receiver structure is employed, its