Error detection/correction and fault detection/recovery – Pulse or data error handling – Digital data error correction
Reexamination Certificate
2000-06-02
2003-07-22
Decady, Albert (Department: 2133)
Error detection/correction and fault detection/recovery
Pulse or data error handling
Digital data error correction
C714S776000
Reexamination Certificate
active
06598200
ABSTRACT:
TECHNICAL FIELD
This invention relates to a method and apparatus for transmitting data frames in frequency domain data transmission systems, and particularly for implementing forward error correction in orthogonal frequency division multiplexed (OFDM) data transmission systems.
BACKGROUND OF THE INVENTION
It is known to transport telecommunications signals over a medium in a plurality of separate frequency channels at the same time. This can be achieved using a frequency domain modulation scheme such as OFDM or coded orthogonal frequency division multiplexing (COFDM) and is desirable in transmission media which carry other signals in predetermined frequency bands or suffer from noise in particular frequency bands. Examples of such media are the use of powerlines (PL) for transporting telecommunications data to and from subscribers, cable television (CATV) systems and fixed wireless access (FWA) systems.
Use of OFDM for signal transmission in PL media is known from U.S. Ser. No. 09/419,209, which is incorporated herein by reference. U.S. Ser. No. 09/419,209 also describes the use of OFDM for signal transmission in more than one frequency band, as follows.
One of the problems with using power lines as a communications medium is that they are subject to noise and interference, for example due to cables picking up radio signals such as broadcast AM radio signals and amateur radio band transmissions, or electrical noise from electrical equipment coupled to the power lines. Noise propagates along the power lines and can corrupt communications signals.
CATV and FWA suffer similar problems, though not necessarily from the same sources. For example, in CATV noise can result from ingress at the consumer connection and inter-modulation products from the TV carriers.
FIG. 1
shows typical background noise on an underground power line across the frequency band 0-10 MHz. It is known to be advantageous to transmit communications signals within the frequency bands 2.2-3.5 MHz (PLT
1
) and 4.2-5.8 Mz (PLT
2
) because these bands fall between the medium wave and short wave bands used for broadcast radio transmissions and avoid the radio amateur band at 3.5-3.8 MHz. There is a reduced level of background noise in these bands and the radiation of power line communications signals in this frequency band causes minimum interference with radio receiver equipment at subscriber premises. Other frequency bands in the range of, for example, 2-30 MHz can be used although it is preferred to use the lower frequencies because attenuation over the distribution cables is lower.
Upstream and downstream transmissions preferably share a common frequency band with the upstream and downstream transmissions occupying different times.
The use of OFDM provides flexibility to fit into non-uniform and non-contiguous frequency allocations, while maintaining reasonable spectral efficiency. This results from the intrinsic nature of OFDM which is composed of a large number of simultaneously transmitted sub-carriers which are staggered in frequency each individually occupying a low bandwidth, as illustrated in FIG.
2
. The scheme's flexibility comes about from the ability of designate which sub-carriers are to be activated and which are not. Regarding spectral efficiency, the signal composition results in an intrinsic spectrum fall-off outside of the active bandwidth commensurate with the bandwidth of each sub-carrier rather than with the total spectrum width. Thus relatively low excess bandwidths can be achieved.
Therefore, the spectral attributes of OFDM represent a major advantage in favor of its selection for use in power line telecommunication systems.
A further difficulty of transmission over certain media in more than one frequency band is frequency selective fading, particularly if the frequency bands are non-contiguous. An OFDM transmission system is again suited to such systems because of the possibility for adaptively selecting which portions of the frequency spectrum are to be used, enabling the avoidance not only of mutual interference but also of areas of poor transmission capability in the frequency spectrum which may arise from time to time due to frequency selective fading.
A problem then arises as to how to encode framed data for transmission over a variable bandwidth in an OFDM system requiring use of a constant OFDM symbol length because, as the available bandwidth varies, so does the total capacity of each symbol. A further problem then arises as to how to apply effective error detection or correction to frames of variable capacity.
SUMMARY OF THE INVENTION
An object of the invention is to provide an efficient method and apparatus for handling data for transmission over a variable bandwidth transmission channel or a transmission channel in which transmissions are carried in two or more non-contiguous channel portions.
A further object of the invention is to provide effective error correction for a transmission over a variable bandwidth transmission channel or a transmission channel in which transmissions are carried in two or more non-contiguous channel portions.
The invention provides a method and an apparatus for transmitting a data frame between a transmitter and a receiver in a frequency domain data transmission system using a transmission channel from which one or more channel portions are selected, by adaptive or preemptive selection or by allocation, for data transmission such that the channel contains one or more unused channel portions, comprising the steps of;
inserting data bits into a payload portion of the data frame;
padding with a predetermined bit sequence a portion of the payload;
applying error correction to the data bits and the padding bits and inserting error detection bits into an error detection portion of the frame to form a data frame of a predetermined length;
mapping the data frame into the transmission channel such that the payload and the error detection bits are mapped to and transmitted in the one or more selected channel portions and the padding bits are mapped to the one or more unused channel portions and are not transmitted;
receiving the data bits and the error detection bits from the one or more selected channel portions;
reinserting the padding bit sequence to restore the predetermined-length frame, subject to any transmission errors;
applying error correction using the error detection bits to correct the transmission errors in the data bits, where possible; and
extracting the data bits.
In further aspects the invention provides a transmission method and a reception method for carrying out the method above, and a transmitter and a receiver for implementing these methods, as defined in the appended claims.
In a further aspect, the invention provides a reduced-length data frame, comprising the payload and the error detection bits as transmitted and received by the method and apparatus of the invention.
Advantageously, the data padding may comprise zero padding, in the form of a sequence of zero bits.
The invention may operate advantageously with any block coding error correction scheme, such as Reed-Solomon coding.
In a transmission system embodying the invention, after the step of applying error correction and before transmission, a padded data frame may undergo conventional techniques such as interleaving, inner coding and energy dispersal. Such techniques may rearrange the bits within a frame but are predictable, so that they can be taken into account in the step of mapping the payload and the error-detection bits to the adaptively-selected channel portion or portions.
The transmission channel into which the invention maps a data frame is a contiguous frequency band. Within that band, one or more channel portions are adaptively selected to carry the payload and error detection portion of the data frame; padding bits within the payload map into any unused channel portions(s). In various aspect the invention may therefore find a variety of applications. For example, if two or more non-contiguous channel portions, or sub-channels, are available for data transmi
Carotti David
Greenwood John Christopher
Barnes & Thornburg
De'cady Albert
Nortel Networks Limited
Torres Joseph D.
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