Multiplex communications – Generalized orthogonal or special mathematical techniques
Reexamination Certificate
1998-12-22
2003-04-01
Ton, Dang (Department: 2666)
Multiplex communications
Generalized orthogonal or special mathematical techniques
C370S468000
Reexamination Certificate
active
06542460
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a multi-directional communication system, such as a duplex system. Such a system may, for example, be used for data communication using modem links over a subscriber line, network, or other communication link. The invention is particularly suitable for implementation using conventional wire links, but the invention is equally applicable to other communication mediums, for example, wire-less links and optical fibre links.
BACKGROUND OF THE INVENTION
A number of data transmission techniques have been developed with the aim of providing high speed communication over existing communication mediums, for example, a subscriber line. Examples include Integrated Services Digital Network (ISDN) and Asymmetric Digital Subscribers Line (ADSL). ISDN provides full duplex communication with the same capacity in either direction of communication. ADSL provides asymmetric capacities (for example, for applications such as Internet access in which more information is expected to flow in one direction than the other).
Reference is made to U.S. Pat. No. 5,668,802 which describes a high speed digital subscriber line employing discrete multiple tone (DMT) signals. DMT is an example of orthogonal frequency division multiplexing, in which the signal on the line consists of encoded multiple harmonic carriers, each carrier representing a “communication channel”. The carriers are modulated with a signal representing a time-varying data frame in a discrete frequency domain. The capacity of the line in either direction is fixed; a first group of the channels (e.g. the odd numbered channels) are used exclusively for communication in one direction, and a second group of the channels (e.g. the even numbered channels) are used exclusively for communication in the other direction.
In devising the present invention, it has been appreciated that the above techniques, which employ fixed data rates in either direction, can only use the communication system efficiently when the demand in each direction closely matches the respective data rate capacity fixed for that direction. If the demand in either direction does not match that provided by the communication link, the efficiency of communication may be significantly reduced, since redundant capacity in one direction is not usable to increase capacity in the other direction. Not only does this result in increased communication time (and the consequent increased expense of, for example, a subscriber line), it can also require the need for expensive large buffers to accommodate temporary buffering of data bottlenecking while awaiting transmission.
SUMMARY OF THE INVENTION
In contrast to the prior art technique discussed above, in which channels are permanently reserved for information communication in specific directions, one aspect of the present invention is to enable control of the relative capacities of first and second channels for a multidirectional communication system by controlling the relative number of sub-channels (from a communication band) allocated to each channel.
The term “channel” is used herein in a broad sense to denote a stream of information being carried by one or more information carrying sub-channels representing the channel.
Preferably, the allocation is controllable dynamically, such that the relative capacities of the channels can be varied, for example, to suit temporal demand for communication capacity in each channel.
With this aspect of the invention, the efficiency of multidirection communication can be improved significantly by enabling redundant capacity in one channel to be used to increase the capacity of another channel when demand for capacity in that other channel is high. For example, in times of heavy capacity demand in the first channel, the capacity of the first channel relative to the second channel can be increased by allocating more sub-channels to the first channel than to the second. Likewise, in times of heavy demand in the second channel, the capacity of the second channel can be increased relative to the first by allocating (or re-allocating) a greater number of sub-channels to the second channel.
Preferably, the capacity is dependent on the number of sub-channels allocated to the channel, the capacity increasing with a greater number of available sub-channels.
The determination of capacity demand for each channel can be made in a number of ways, for example, by monitoring the whether the “live” traffic in each channel matches the current channel capacity, or by monitoring the size of input buffer files for data awaiting transmission to each channel.
Preferably, the communication system employs multi-carrier modulation. Preferably, each carrier corresponds to a sub-channel of the communication band.
More preferably, the system employs orthogonal frequency division modulation (OFDM) in which the carriers (also referred to as sub-carriers) are separated in frequency by the baud rate of the modulation (or by a multiple thereof) such that the sub-carriers are theoretically independent. Most preferably, the system employs coded orthogonal frequency division multiplexing (COFDM) in which one or more redundant coding techniques are used to enable at least limited error correction.
In a closely related aspect, the invention provides apparatus for use in a multi-directional communication system for enabling control of the relative capacities of first and second communication channels by controlling the relative number of sub-channels allocated for each channel.
In a further closely related aspect, the invention provides a method of operating a multi-directional communication system, the method comprising controlling the relative capacities of first and second channels in the system by controlling the relative number of sub-channels allocated to each channel.
In a yet further aspect, the invention provides a multidirection communication system including means for controlling the relative capacities of first and second channels thereof in response to the demand for capacity for at least one of the channels. Preferably, the control means is operable to vary the relative capacities in response to changes in the demand.
Preferably, the relative capacities are controlled in response to the demand for capacity for each channel.
Preferably, the system comprises means for determining the demand for capacity for each channel.
The capacity of a channel can be controlled by varying the bandwidth of the channel, for example, by varying the number of sub-channels allocated to the channel.
In a closely related aspect, the invention provides apparatus for use in a multi-directional communication system, the apparatus comprising means for controlling the relative capacities of first and second channels of the system in response to the demand for capacity for at least one of the channels.
In yet further closely related aspect, the invention provides a method of operating a multi-directional communication system, the method comprising controlling the relative capacities of first and second channels of the system in response to the demand for capacity for at least one of the channels.
In a further aspect, the invention relates to an OFDM system, and to synchronisation techniques for improving operation.
Referring again to U.S. Pat. No. 5,668,802, special time synchronisation schemes are described in order to permit the transmitters at the opposite ends of the subscriber line to transmit a data frame according to a common “line” timing. A first scheme involves adding a post-cursor to a data frame to repeat data from the beginning of the frame; a second scheme involves a master station commanding the slave to introduce “slip” between the reception and transmission frames (at the slave end).
In developing this aspect of the present invention, it was appreciated that interference between OFDM transmissions in each direction can be reduced by synchronising each of the OFDM transmitters/receivers using a common source.
In accordance with this aspect, the invention provides an OFDM communication
LSI Logic Corporation
Maiorana PC Christopher P.
Ton Dang
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