Optimization of discrete multitone to maintain spectrum compatibility with other transmission systems on twisted copper pairs

摘要

The growing demand to transmit high-speed digital data in many local area networks (LANs) and digital subscriber lines (DSLs) has resulted in a wide variety of transmission systems that have to co-exist on twisted wire copper pairs. In this paper, we address the problem of maintaining spectrum compatibility between various services that may use different transmission technologies, by shaping in an optimal manner, the power spectral density (PSD) of the transmit signal. A multitone modulation scheme such as discrete multitone (DMT) has the flexibility of optimizing the power spectrum over more than one (disjoint) frequency band, and is suitable for twisted pair subscriber loops, and other transmission media, where the optimized transmit spectrum is likely to occupy more than one frequency band. DMT has been selected by the American National Standards Institute (ANSI) T1E1.4 Standards Committee as the standard modulation scheme for asymmetric DSL (ADSL). The results presented in this paper are for the specific application of DMT to transport ADSL payloads of over 6 Mb/s from the network to the customer. We consider spectral compatibility between ADSL, the T1 repeater system, high bit-rate DSL (HDSL), and integrated services digital networks (ISDN) basic rate access (BRA) systems. The simulation results show that: 1) one can customize the transmit PSD to achieve optimum ADSL performance in a specified noise environment; 2) this optimum performance can result in as much as approximately 6 dB improvement in signal-to-noise ratio (SNR) when compared to the nonoptimized PSD chosen by the T1E1.4 committee; 3) in achieving the above improvements, the total maximum transmit power is still consistent with the limit set by the T1E1.4 committee. Further work is required to support the simulation results with measured data. The mathematical analysis is based on the use of Lagrange multipliers to solve the constrained optimization problem, and is easily extended to other asymmetric and full-duplex wireline transmission systems operating at much higher data rates. The practicality of implementing the proposed optimization routine requires further investigation.