Data communication typically occurs as the transfer of information from one communication device to another. This is typically accomplished by the use of a modem located at each communication endpoint. In the past, the term modem denoted a piece of communication apparatus that performed a modulation and demodulation function, hence the term “modem”. Today, the term modem is typically used to denote any piece of communication apparatus that enables the transfer of data and voice information from one location to another. For example, modern communication systems use many different technologies to perform the transfer of information from one location to another. Digital subscriber line (DSL) technology is one vehicle for such transfer of information. DSL technology uses the widely available subscriber loop, the copper wire pair that extends from a telephone company central office to a residential location, over which communication services, including the exchange of voice and data, may be provisioned. DSL devices can be referred to as modems, or, more accurately, transceivers, which connect the telephone company central office to the user, or remote location, typically referred to as the customer premises (CP). DSL communication devices use different formats and different types of modulation schemes and achieve widely varying communication rates. However, even the slowest DSL communications devices achieve data rates far in excess of conventional point-to-point modems.
Some of the available modulation schemes include quadrature-amplitude modulation (QAM), carrierless amplitude/phase (CAP) and DMT. In a DMT modulation scheme, a number of carriers, commonly referred to as “tones” are encoded with the information to be transmitted and communicated over the communications channel. This information, in the form of data words, is encoded into signal space constellations and then transmitted. In a typical DMT transmitter, 256 carrier tones are used to encode the data and are added together resulting in a very high peak signal power due to the high numerical peak resulting from the addition of the 256 tones. Power consumption is further increased due to the use of square signal space constellations used in conventional DMT transmitters and the allowed +2.5 dB to −14.5 dB power variation allowed on each carrier tone. Square signal space constellations have an inherently high peak signal power due to the location of the highest power signal point. The peak signal power in conventional DMT transmitters is sufficiently high to cause saturation or clipping of the transmitter in normal operation. Conventional DMT allows a probability of clipping of 10−7. To combat this inherent deficiency, conventional DMT transmitters use expensive Reed-Solomon forward error correction encoders combined with bit wise interleavers. Coders such as these induce a significant amount of throughput delay and are unsuitable for multi-point communication environments.
The number of bits encoded on each DMT carrier is selected in whole bit increments to bring the margin associated with each tone to within 3 dB of the specified margin. Contributing to the peak power problem, the DMT modulation scheme allows the power on individual tones to be increased by up to 2.5 dB to satisfy margin requirements. While increasing the power on some tones, the system reduces the power on other tones to maintain the specified transmit power. This scheme of tone power variation is useful in instances where it may be desirable to turn off specified tones and allocate their power to other tones. Unfortunately, this tone power variation results in spectrum management difficulties. Conventional DMT systems simply turn off specified tone carriers and increase others by the allowed 2.5 dB, but this results in the undesirable situation in which some carriers will be 2.5 dB hotter than necessary in certain spectral bands, resulting in undesirable cross-talk, while other carrier tones are switched off completely. The tones at the high end of the frequency spectrum are frequently switched off.
Crosstalk between wire pairs in cable bundles or in dense equipment cabinets is the major contributor to degradation in DSL networks. Spectrum management schemes have been proposed to limit cross-talk by specifying a Power Spectrum Density (PSD) for all DSL transmitters. DMT allows PSD pass band ripple of +3.5 dB to accommodate the +2.5 dB power variation on individual tones. While currently allowed, a boost of 2.5 dB in power will effectively lower channel capacity by 2.5 dB on neighboring wire pairs, thereby nullifying any true gain that may have been anticipated. It is desirable to have a DMT system that can transmit a specified PSD with pass band ripple of less than 1 dB.
Thus, it would be desirable to have a DMT communication system capable of transmitting fractional bit rates, while maintaining desirable spectral power requirements.