OPTIMIZATION OF DISCRETE MULTITONE TO MAINTAIN SPECTRUM COMPATIBILITYWITH OTHER TRANSMISSION-SYSTEMS ON TWISTED COPPER PAIRS

The growing demand to transmit high-speed digital data in many local a rea networks (LAN's) and digital subscriber lines (DSL's) has resulted in a wide variety of transmission systems that have to co-exist on tw isted wire copper pairs. In this paper, we address the problem of main taining spectrum compatibility between various services that may use d ifferent transmission technologies, by shaping in an optimal manner, t he power spectral density (PSD) of the transmit signal. A multitone mo dulation scheme such as discrete multitone (DMT) has the flexibility o f optimizing the power spectrum over more than one (disjoint) frequenc y band, and is suitable for twisted pair subscriber loops, and other t ransmission media, where the optimized transmit spectrum is likely to occupy more than one frequency bands. DMT has been selected by the Ame rican National Standards Institute (ANSI) T1E1.4 Standards Committee a s the standard modulation scheme for asymmetric DSL (ADSL). The result s presented in this paper are for the specific application of DMT to t ransport ADSL payloads of over 6 Mb/s from the network to the customer . We consider spectral compatibility between ADSL, the T1 repeater sys tem, high bit-rate DSL (HDSL), and integrated services digital network s (ISDN) basic rate access (BRA) systems. The simulation results show that: 1) One can customize the transmit PSD to achieve optimum ADSL pe rformance in a specified noise environment; 2) this optimum performanc e 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 m aximum transmit power is still consistent with the limit set by the T1 E1.4 committee. Further work is required to support the simulation res ults 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 practica lity of implementing the proposed optimization routine requires furthe r investigation.