On the next generation of high-speed voiceband modems

The ITU-T Recommendation V.90 for voiceband modems allows for a maximum downstream (from the central office to the user) transmission rate of 53.3 kbps and maximum upstream (from the user to the central office) transmission rate of 33.6 kbps when the remote modem at the Internet service provider (ISP) has a direct digital connection to the central office (CO) and the client modem is connected to the telephone network through an analog subscriber loop. In order to increase the upstream data rate beyond 33.6 kbps, it is necessary to considerably reduce the quantization noise due to the analog-to-digital converter (ADC) in the line-card at the central office. In this paper, we propose a method to achieve this by using a single linear pre-equalizer arrangement in the transmitting modem along with spectral shaping of the input to compensate for the channel response before the ADC. We solve for the coefficients of the optimum pre-equalizer subject to the average transmitted voiceband power constraint imposed by the FCC. We demonstrate that it is possible to achieve data rates above 50 kbps upstream at a bit-error-rate (BER) less than 10^-6 over typical telephone channels assuming that we have a fairly accurate estimate of the ADC slicing levels and sampling times