Multi-carrier transmission systems

Multiplex communications – Communication techniques for information carried in plural... – Combining or distributing information via frequency channels

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Details

370294, H04J 100

Patent

active

061608203

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION



Field of the Invention

The invention relates to multi-carrier transmission systems and, in particular, a method for the allocation of tones in such systems, and a heterogeneous telecommunications network including a multi-carrier transmission system.


DISCUSSION OF THE BACKGROUND

The demand for provision of multi-media and other bandwidth services over telecommunications networks has created a need to transmit high bit rate traffic over twisted copper pairs. This requirement has led to the development of a number of different transmission schemes, such as, ADSL and VDSL. One of the more likely modulation systems for all these transmission schemes is DMT, which bears some resemblance to orthogonal frequency division multiplex, and is a spread spectrum transmission technique.
In discrete multi-tone transmission, the available bandwidth is divided into a plurality of sub-channels each with a small bandwidth, 4 kHz perhaps. Traffic is allocated to the different sub-channels in dependence on noise power and transmission loss in each sub-channel. Each channel carries multi-level pulses capable of representing up to 11 data bits. Poor quality channels carry fewer bits, or may be completely shut down.
Because inter pair interference in copper pair cables is higher where data is transmitted in both directions, i.e. symmetric duplex, the use of asymmetric schemes has been proposed in which high data rates are transmitted in one direction only. Such schemes meet many of the demands for high bandwidth services, such as, video-on-demand but, in the long term, symmetric duplex systems will be required.
VDSL (Very high bit-rate Digital Subscriber Lines) technology resembles ADSL to a large degree, although ADSL must cater for much larger dynamic ranges and is considerably more complex as a result. VDSL is lower in cost and lower in power, and premises VDSL units need to implement a physical layer media access control for multiplexing upstream data.
Four line codes have been proposed for VDSL: Amplitude Modulation (QAM), for passive Network termination (NT) configurations, CAP would use QPSK upstream and a type of Time Division Multiple Access (TDMA) for multiplexing (although CAP does not preclude a Frequency Division Multiplexing (FDM) approach to upstream multiplexing); Transforms to create and demodulate individual carriers, for passive NT configurations; DMT would use FDM for upstream multiplexing (although DMT does not preclude a TDMA multiplexing strategy); Transforms to create and demodulate individual carriers, DWMT also uses FDM for upstream multiplexing, but also allows TDMA; and filters the base band and restores it at the receiver, for passive NT configurations; SLC would most likely use TDMA for upstream multiplexing, although FDM is possible.
Early versions of VDSL will use frequency division multiplexing to separate downstream from upstream channels and both of them from POTS and ISDN. Echo cancellation may be required for later generation systems featuring symmetric data rates. A rather substantial distance, in frequency, will be maintained between the lowest data channel and POTS to enable very simple and cost effective POTS splitters. Normal practice would locate the downstream channel above the upstream channel. However, the DAVIC specification reverses this order to enable premises distribution of VDSL signals over coaxial cable systems.
Modern multi-carrier techniques using orthogonal carriers with high order QAM constellations for the transmission of a plurality of bits per carrier and symbol, use some method for the determination of an inverse channel model to be used in an equalization process. In the frequency domain, this is normally based on an estimate of the channel attenuation and phase for each carrier.
In a heterogeneous network, a number of users with different channel characteristics will coexist on the same cable and there is a need, with such networks, to be able to utilize the capacity of the network in a manner whereby coverage is optimized.


SUMMAR

REFERENCES:
patent: 5838667 (1998-11-01), Bingham et al.
patent: 6014412 (2000-01-01), Wise et al.
William Y. Zou, et al., IEEE Transactions on Broadcasting, vol. 41, No. 1, pp. 1-8, "COFDM: An Overview", Mar. 1995.

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