Michael O. Polley

PAR reduction for DMT systems with unloaded subchannels

The inverse discrete Fourier transform used by discrete multitone modulation produces transmitted signals with a large peak-to-average ratio. Peak-to-average ratio reduction methods allow the transmitter to reduce the probability of clipping, or maintain the same probability of clipping while reducing the dynamic range requirements of the analog front end. In this paper we propose a method of peak-to-average ratio reduction that uses unloaded subchannels to prevent clipping in the transmitted signal. Considerations such as efficient adaptation methods and extensions to compensate for upsampling and filtering are addressed.

Current and future Internet transmission methods: technical challenges and practical solutions for widespread acceptance of ADSL

The high data rates required to properly support emerging multimedia internet applications far exceed the capabilities of voiceband modems. For example, seamless real-time delivery of digital video clips might require connections up to 100 times faster. Asymmetric digital subscriber line (ADSL) modems provide residential internet users with a much-needed solution to this remote access bandwidth shortage. ADSL modems connected directly to the internet are installed in the telephone company central office, allowing remote access over the copper twisted pair telephone line by remote transceivers in the homes of residential customers. Because the internet data does not have to pass through the telephone switching network, the ADSL link can provide data rates in the Mbit/sec range. However, for ADSL modems to gain broad acceptance an displace their low rate voiceband counterparts, low cost, ease of installation, and high quality of service must be achieved.

Digital Television Broadcasting: Nondisruptive Improvement over Time

The Federal Communications Commission (FCC) decided, at an early date, that it was important for the digital broadcasting system to be upgradeable over time. Therefore, it established, as an important desideratum for the system to be chosen, that it could be improved in a nondisruptive manner i.e., without making the original equipment obsolete. An effective way to do this is to formulate a single baseline system that produces standard-definition imagery, and to carry out any later upgrading exclusively by adding one or more enhancement signals. Enhanced receivers would extract all data streams to create improved imagery. This approach to upgrading permits the manufacture of digital receivers (and set-top converters for NTSC receivers) of the lowest possible cost since high-definition decoding and signal-processing capability is unnecessary If the enhancement signals are transmitted by means of nonuniform constellation, then they will appear to be random noise to baseline receivers, guaranteeing that such receivers will continue to be usable regardless of the exact nature of the enhancement signal(s). While it would be preferable for the baseline system to be progressively scanned, it would be possible to permit both interlaced (I) and progressive (P) baseline formats to be used and yet to design enhanced receivers in such a way that they would operate properly with both. The source coding required in such a system is already well known by such names as pyramid coding, layered coding, or multiresolution coding. Channel-coding methods are also described that permit receivers to extract data from the transmitted signal in accordance with the local SNR and their signal-processing capabilities.