Transhybrid Loss with RC Balance Circuits for Primary-Rate ISDN Transmission Systems

In this paner, we report on a method for the reduction of the near-end echo in the subscriber loop environment for primary-rate ISDN full-duplex data transmission. The achieved echo reduction results in a decrease in the requirements and cost of the analog-to-digital (A/D) converter that has to be used in the echo-cancellation based transceiver. It also reduces the amount of echo cancellation that must be achieved by a digitally implemented echo canceller. The proposed technique uses an RC network to function as the matching input line impedance at the transmitter to reduce the near-end echo. Data rates ranging from the ISDN basic rate with (2B + D) channels, to primary rate with (23B + D) channels, are covered in the study. The results presented indicate that a single basic configuration for the balancing line impedance may be adapted to keep the transhybrid loss in the range of about 22 dB in the worst case over the entire frequency range of interest. This is the maximum of the minimum (maximin) loss over the band. With loops having no bridged taps, the maximin loss can be held above 40 dB. When the spectral band is reduced, the maximin transhybrid loss will be increased to between 40 and 50 dB, even with loops including bridged taps. A certain amount of adaptability is essential for the impedance of the balancing circuit to match the loop and its bridged tap configuration. The balancing procedure for the hybrid takes place in a training sequence during which the component values of the RC matching circuit are adjusted from their initial values by an iterative adaptation procedure. Two techniques for optimizing the component values (least squares and maximin) have been verified. Our study is limited to selected loops that meet the carrier serving area (CSA) loop requirements. The band of spectral interest lies between 20-300 kHz and is suitable for 64-point QAM clusters for primary-rate ISDN data transmission. For all cases examined, including worst-case line configurations, the components of the RC matching circuit have converged to final values, for which the minimum transhybrid loss had a maximum above 20 dB.