Computationally efficient protocols for VDSL system
Techniques which reduce the computational burden of the IFFT/FFT section of a DSL data transmission/reception apparatus, taking into account the nature of conventional IFFT/FFT algorithms. These algorithms operate using “butterflies” representing complex calculations performed on input values. The butterflies are here referred to as “simple” if all the input values are zero or if only one input value is non-zero. The invention proposes that the tones which have non-zero amplitudes are selected to maximize (or at least increase) the number of simple butterflies. This is done either in designing the band plans of the communication, or in designing the band plan of a mode of operation of the apparatus having a reduced data transmission rate. The IFFT/FFT section receives information indicating that the IFFT/FFT algorithm can be simplified based on knowledge of which of the input values will be zero, and changes its mode of operation accordingly.
This application relates to the following co-pending and commonly assigned patent applications, all filed concurrently herewith: Ser. No. ______, entitled “Allocating Data Between Tones in a VDSL System” (attorney docket number 2005 LW 2383), Ser. No. ______, entitled “VDSL Protocol with Low Power Mode” (attorney docket number 2005 LW 2385), and Ser. No. ______, entitled “Trellis Modulation Protocols for a VDSL System” (attorney docket number 2005 LW 2386), which applications are hereby incorporated herein by reference.
This application claims priority to Singapore Patent Application 200401383-5, which was filed Mar. 5, 2004, and is incorporated herein by reference.
TECHNICAL FIELDThe present invention relates to methods for transmitting data, in particular over telephone lines (typically, copper telephone lines) or similar lines. It further relates to systems arranged to perform the methods.
BACKGROUNDThe use of fast Internet connections has grown rapidly over the last few years, and consequently the demand for broadband (high-speed) connections is increasing.
One technology that is very well known in the market is Asymmetric Digital Subscriber Line (ADSL) technology. This employs the frequency spectrum indicated schematically in
In a typical ADSL modem, the main sections are (i) a Digital Interface (which may use asynchronous transfer mode (ATM)); (ii) a Framer (also referred to here as a framing unit); (iii) an DMT Modulator; (iv) the AFE (Analog Front End); and (v) a Line Driver.
The framer multiplexes serial data into frames, generates FEC (forward error correction), and interleaves data. FEC and data interleaving corrects for burst errors. This allows DMT-based ADSL technology to be suitable for support of MPEG-2 and other digital video compression techniques. For the transmit signal, an Encoder encodes frames to produce the constellation data for the DMT Modulator. It assigns the maximum number of bits per tone (based on measured SNR of each tone) and generates a QAM constellation where each point represents a digital value. Each constellation point is one of N complex numbers, x+iy, where x and y are the phase and amplitude components. The summation of bits in all carriers, multiplied by the frame rate (4 kHz), represents the data rate. For the receive signal, the decoder converts QAM symbols back into the data bitstream.
In the DMT Modulator, a frequency domain processor implements FFT/IFFT and associated processing. In the transmit path, the Inverse Fast Fourier Transform (IFFT) module accepts input as a vector of N QAM constellation points and duplicates each carrier with its conjugate counterpart so the 2N output samples are real. The 2N time domain samples may have for example the last 2N/16 samples appended as a cyclic extension (which may include a cyclic suffix, a windowing function and/or a cyclic prefix extension) for every symbol, and are then delivered to a DAC (digital-to-analog converter). The set of time domain samples represents a summation of all the modulated sub-channels, for the duration of one data frame. In the receive path, the first 2N/16 samples (cyclic prefix) from the ADC are removed from every symbol. A FFT module transforms the carriers back to phase and amplitude information (N complex QAM symbols). Correction for attenuation of the signal amplitude and phase shifts (i.e., overall distortion) is implemented. If the QAM constellation is thought of as points in a grid where rows and columns represent phase and amplitude information respectively, then the grid effectively rotates reference to the constellation points to correct for these distortions.
Based on the SNR, which has been established for the tones, they are classified based on the SNR such that a “path” is selected for each tone through the encoding device, and each of the tones is transmitted along to the framing unit through the corresponding selected transmission path. This is illustrated in
DMT technology also includes a feature known as “tone ordering”. This means that the encoder, in forming VDSL symbols (there may be multiple VDSL frames within one VDSL symbol), determines the order in which subcarriers are assigned bits. The term tone ordering is wide enough to include both (i) determining the order in which the subcarriers are assigned data transmitted along a given latency path; and (ii) the order in which the subcarriers are assigned data transmitted along the different latency paths.
Furthermore, the number of bits that are transmitted by each of the tones may be modified if the estimated SNRs of the tones are revised: increasing the number of bits stored per frame in some tones and correspondingly reducing the number of bits stored per frame in other tones. There could be other reasons to dynamically change the bit allocation for spectral reasons too. This process is known as “bit swapping.”
For further details of the ADSL2 standard, the reader is referred to the document ITU-T Recommendation G.992.3 published by the International Telecommunication Union, the disclosure of which is incorporated herein by reference in its entirety.
While ADSL provides Internet connections that are many times faster than a 56K modem, they still are not fast enough to support the integration of home services such as digital television and Video-on-Demand. However, another DSL technology known as very high bit-rate DSL (VDSL) is seen by many as the next step in providing a complete home-communications/entertainment package.
In contrast to ADSL, a conventional VDSL standard (here referred to as VDSL1) uses a number of bands, e.g., as shown in
Embodiments of the present invention aim to provide new and useful protocols for transmitting data through lines such as telephone lines. Typically these protocols have transmission rates of over 24 Mbps, and often much higher.
In general terms, embodiments of the invention propose techniques which reduce the computational complexity of the IFFT/FFT section that a data transmission/reception apparatus has to perform, taking into account the nature of conventional IFFT/FFT algorithms. These algorithms operate using “butterflies” representing complex calculations performed on input values, and composed of “radix calculations.” The radix calculations may be regarded as relatively “simple” if only one input value is non-zero and even simpler (in fact, trivial) if all the input values are zero. In certain circumstances (as described below) it is possible to use pre-knowledge of which tones that have zero amplitudes to simplify the calculations required, and thus save power for example. From one point of view, the invention proposes that the tones having non-zero amplitude are selected to maximize (or at least increase) the number of simple radix calculations, to increase the savings possible.
As in conventional systems, the total number of tones available for the bi-directional communication is generally a power of two. If the tones having non-zero amplitude are a set of consecutive tones, then the number of simple radix calculations is high (at least in the first stage) if there are about 2n consecutive tones in the set, where n is an integer.
Similarly, if the tones having non-zero amplitudes are two groups of consecutive tones, then the number of radix calculations having more than one non-zero input is low (at least in the first stage) if the groups of tones are spaced apart by about 2m tones, where m is an integer.
A first aspect of the invention proposes in general terms that the bands for the transmit and/or receive directions are selected so that the IFFT/FFT algorithms include a relatively large number of simple radix calculations.
The IFFT/FFT module uses the information that certain of the tones have zero amplitude to simplify the computation it performs.
A second aspect of the invention proposes in general terms that the data transmission/reception includes a first (“high data transmission rate”) mode and at least one second (“reduced data transmission rate”) mode in which the rate of data transmission is lower than in the high data transmission rate mode. In the reduced data rate transmission mode, data is only encoded in a subset of the total number of tones employed in the high transmission rate mode, and this subset is selected such that the IFFT/FFT operations include a large number of simple radix calculations.
The IFFT/FFT module (which, when performing the high data transmission rate mode, employs an algorithm designed to be capable of processing data specifying the full range of tones available for transmission in the corresponding direction) is switched, in the reduced data transmission rate mode(s), to a mode of operation that uses the information that certain of the tones are not used to simplify the computation it performs.
The reduced-power mode(s) may be useful when the volume of data that is to be transmitted is reduced, since the economies in the IFFT/FFT module lead to reduced power consumption and reduced memory requirements. It is motivated by the observation that while DSL communication cannot be interrupted completely, the volume of data that it is required to transmit varies considerably with time, and the possibility of a reducing power consumption at certain times is advantageous, for example, to reduce unwanted heat generation or unwanted spectral disturbances to other systems.
In two of the related applications referenced above, it is proposed that tones may be grouped (e.g., subsets of the tones may be defined; each of these subsets is composed only of tones that are used for transmission in the same direction). This grouping may have one of two functions. Firstly, the data that is to be transmitted using a given group of tones can be Trellis encoded together. Secondly, the group of tones can be used for one of more of (i) bit allocation; (ii) bit swapping; (iii) tone ordering; and/or (iv) gain allocation. The purpose of performing the five operations on groups of tones (rather than, for example, on all tones associated with data transmission in the same direction) is to reduce the computational and memory requirements of coding and decoding. This concept may be combined with the second aspect of the present invention. For example, in the high data rate transmission mode, the tones can be grouped such that those tones, which will be used in the reduced data transmission rate mode(s), are grouped together. Thus, the transition between the two modes means ceasing to transmit complete groups of tones, which minimizes the interference with the various uses of the groups described in the previous paragraph.
A third aspect of the invention relates to the IFFT/FFT unit employed in implementing the first and second aspects of the invention. As described above, in order to obtain power savings, the FFT/IFFT unit is capable of receiving control inputs that alter its mode of operation, selecting modes of operation that use the information that certain of the tones have a zero amplitude, and thus skipping certain of the stages required in a full IFFT/FFT algorithm.
BRIEF DESCRIPTION OF THE DRAWINGSPreferred features of the invention will now be described, for the sake of illustration only, with reference to the following figures in which:
Embodiments of the invention will now be described. For the sake of simplicity, many features of the protocol operated by these embodiments are not described, since they are according to the ADSL standard (which is described for example in the document ITU-T Recommendation G.992.3 published by the International Telecommunication Union and is incorporated herein by reference).
These embodiments make use of a novel IFFT/FFT module 10 shown in
Considering first the case that the module is used to perform an IFFT transform, the module includes a buffer 11, which receives amplitude data and which, following the IFFT transform operation, transmits frequency domain data. The IFFT transform is performed by a processing unit 12 which repeatedly reads data from the buffer 11, processes it according to a butterfly operation, and returns it. This is performed under the control of a processor 13 which controls the processing unit 12 to simplify (truncate) the algorithm it performs (alternatively in the implementation the processing unit 13 may be embedded as a part of processing unit 12). Specifically, when the processing unit 12 is performing a stage of the IFFT operation in which the inputs to number of the radix calculations are known to be zero, the processor 13 controls the processing unit 12 to simplify its algorithm by omitting at least some of the unnecessary calculations implied by the zero values.
The structure of a data transmission section of a VDSL transceiver incorporating this module is shown in
Turning now to the case of a data receiving section of the transceiver, the FFT can be performed using the module of
The structure of a data transmission section of a VDSL transceiver incorporating this module is shown in
Even with existing band plans, the IFFT/FFT module of
FFT transforms and IFFT transforms may be regarded as a plurality of stages, each involving a butterfly (“radix calculation”). As an illustration of the power saving available using the IFFT/FFT module of
The explanation above assumes that the processor of the IFFT/FFT module of
Other systems, which are embodiments of this invention, also include the feature of selecting the tones having non-zero amplitudes for the downstream and upstream directions so as to reduce the power of the FFT/IFFT module and have efficient computations. With the band plans appropriately defined, only a small number of computations will be required (at least in the first stages) and there will be a considerable power saving. The idea here is to propose a scheme such that the FFT/IFFT module only processes the non-trivial data, hence saving a large amount of power. The IFFT/FFT module is controllable to omit those calculations that are not required in view of the selected band plan. The power saving can be more than 50% depending on the number of tones that are used to carry the relevant data. Consequently, it lowers the heat dissipation requirements. This is particularly important for remote DSL equipment, where heat is a challenging problem.
If the downstream band is configured to a number of tones, which is a power of two, efficient implementation could be drawn and significant savings could be achieved. Due to the architecture of the FFT/IFFT, the samples taken for each butterfly computation in stage 0 involves at most one non-trivial sample. This significantly reduces the computation since the samples with value zero do not contribute to the computation and can be bypassed.
The following explanation is based on 4K tones and a radix-2 butterfly. There are eight stages of FFT/IFFT processing and a pre-processing or post-processing stage. In conventional VDSL band plans, tones 513 to 575 are used for all downstream transmission profile (TS 101 270-2 V2.0.8). By contrast in one embodiment of the present invention the band plan is adjusted as shown in
A second example of a suitable proposed band plan according to this embodiment is shown in
Note that while it is preferred in this embodiment for each of the bands to have exactly 2n tones, and be spaced apart by exactly 2m tones (n is not necessarily equal to m), as in FIGS. 8(a) and 9(a), exact conformity with this rule is not required, and lower (but still appreciable) savings compared to the band plan of
We now turn to embodiments of the invention that are protocols permitting a low power mode of operation, which permits a saving of power consumption in the IFFT in the case of the encoder, or the FFT in the case of the decoder.
In the second embodiment of the invention, the protocol switches from a high data transmission rate mode referred to here as L1, to a low data rate transmission mode referred to here as L2. This is illustrated in
In an embodiment of the invention, this is achieved by providing the possibility of a low power mode in which data is only transmitted on certain of the frequencies. The other tones are all zeros. The transmitter section of the embodiment again has the overall structure of
In the embodiment, the non-zero tones are selected such that in the first stage (“stage 0”) the samples taken for each butterfly computation involve at most one non-trivial sample, and all the trivial samples (involving coefficients which are all zero, or only 1 non-zero coefficient) are bypassed. The IFFT module (or FFT module in the case of the decoder) is controlled to modify its operation based on the saving that is possible due to the trivial samples.
The following explanation, with reference to
After the pre-processing, tones 512 to 575 and 3521 to 3583 are non-zero.
The radix-16 butterfly of
From
The second stage of the IFFT processing takes data in strides of 16, for example {Y1, Y17, Y33, Y49, Y65, Y81, Y97, Y113, Y129, Y145, Y161, Y177, Y193, Y209, Y225, Y241}). The first 4 samples and the last 4 samples are non-zero. Therefore, for each radix-4 butterfly in the first segment, two samples are non-zero and two are zeros. The number of complex additions is halved. There would be no saving in the second stage since the inputs to the four radix-4 butterflies are all non-zero.
There is no potential saving in the third stage. Table 1 shows the breakdown of the computation saving in each stage as a factor of the number of non-trivial tones. For the number of tones from 48 to 63, the pre-processing stage, stage 0 and stage 1 have a computation reduction of approximately 97%, 80% and 10% respectively (1 CMAC≈5 CADD). Overall saving is about 40%. For a number of tones less than 16, even stage 1 can be reduced by up to 40%.
There is no saving in the IFFT processing if the number of tones exceeds 127. Hence, it is preferable to limit the number of tones to this number, and specifically to the range from 513 to 639.
Here the term “BF4” means a radix-4 butterfly, BF2 mean a radix-2 butterfly, etc. CADD stands for a complex adder operation, and CMAC for a complex multiplier-accumulator.
We now analyze the power saving for the FFT processing. Assume that in low-power mode, the 64 tones from 960 to 1023 are non-trivial, as shown by shading in
We now briefly describe the process of switching between the modes in the embodiments of
As mentioned above, in two of the related applications referenced above, it is proposed that tones may be grouped (e.g. subsets of the tones may be defined; each of these subsets is composed only of tones that are used for transmission in the same direction). This grouping may have one of two functions. Firstly, the data which is to be transmitted using a given group of tones can be Trellis encoded together. Secondly, the group of tones can be used for one or more of (i) bit allocation; (ii) bit swapping; (iii) tone ordering; and/or (iv) gain allocation. The purpose of performing the five operations on groups of tones (rather than, for example, on all tones associated with data transmission in the same direction) is to reduce the computational and memory requirements of coding and decoding.
This concept may be combined with the present invention. For example, suppose that a given group is defined to be the frequencies 513 to 576 (i.e., the shaded tones of
Although only a few embodiments of the invention have been disclosed in this application, many variations are possible within the scope of the invention as will be clear to a skilled reader.
Claims
1. A method of two-directional communication of data over a line, the communication employing a bandwidth partitioned into a number of bands including at least one band associated with each of the two directions, the method including encoding the data carried in each direction by modulation of a plurality of tones defined within the at least one band associated with that direction, the bands being selected within the bandwidth such that all the bands in a given direction have a number of tones which is substantially equal to a power of 2, and the bands are spaced apart by a number of tones which is substantially equal to a power of 2.
2. The method according to claim 1 wherein the encoding of the data to be transmitted in at least one of the directions is performed using an IFFT section which includes a memory for receiving amplitude data specifying the amplitudes of the plurality of tones associated with said one direction, and an IFFT processing unit operating iteratively an algorithm including radix calculations on the data stored in the memory, the method including in at least one of the iterations determining that one or more input values of at least one radix calculation are zero, and accordingly simplifying the radix calculation.
3. The method according to claim 2 wherein the simplifying of the radix computation comprises omitting a calculation step of the radix calculation.
4. The method according to claim 1 and further comprising decoding of the data using an FFT section which includes a memory for receiving time domain data describing a received signal, and a processing unit including an FFT processing unit operating iteratively an algorithm composed of radix calculations on the data stored in the memory, the method including in at least one of the iterations determining that one or more input values of at least one radix calculation are zero, and accordingly simplifying the radix calculation.
5. The method according to claim 4 wherein the simplifying of the radix computation comprises omitting a calculation step of the radix calculation.
6. A method of two-directional communication of data over a line, the communication employing a bandwidth partitioned into a number of bands including at least one band associated with each of the two directions, the method including encoding the data carried in each direction by modulation of a plurality of tones defined within the at least one band associated with that direction, the method including operating selectively in one of at least two modes including:
- a first mode in which data is transmitted on all the tones associated with communication in the corresponding direction; and
- at least one second mode in which data is transmitted on a subset of the tones associated with the corresponding direction, the subset being selected as a set of 2n consecutive tones.
7. The method according to claim 6 wherein the encoding of the data transmitted in at least one direction is performed using an IFFT section which includes a memory for receiving amplitude data specifying the amplitudes of the plurality of tones associated with the one direction, and an IFFT processing unit iteratively performing an algorithm composed of radix calculations on the data stored in the memory, the method including in at least one of the iterations determining that one or more of inputs to at least one radix calculation are zero, and accordingly simplifying the radix calculation.
8. The method according to claim 7 wherein simplifying the radix computation comprises omitting a calculation step of the radix calculation.
9. The method according to claim 6 and further comprising decoding the data transmitted in one direction using an FFT section which includes a memory for receiving time domain data describing a received signal, and a processing unit including an FFT processing unit iteratively operating an algorithm composed of radix calculations on the data stored in the memory, the method including in at least one of the iterations determining that one or more inputs to at least one radix calculation are zero, and accordingly simplifying the radix calculation.
10. The method according to claim 9 wherein simplifying the radix computation comprises omitting a calculation step of the radix calculation.
11. A method of performing an IFFT transform as part of a method of encoding data for transmission along a line within a bi-directional communication protocol, the method comprising:
- receiving amplitude data specifying the respective amplitudes of a plurality of tones; and
- iteratively operating an algorithm composed of radix calculations on the data stored in the memory to generate time-domain data;
- the method including, in at least one of the iterations, determining that one or more inputs to at least one radix calculation are zero, and accordingly simplifying the radix calculation.
12. A method of performing an FFT transform as part of a method of decoding data transmitted along a line within a bi-directional communication protocol, the method comprising:
- receiving time domain data describing the received signal; and
- iteratively operating an algorithm composed of radix calculations on the data stored in the memory to generate amplitude data specifying the respective amplitudes of a plurality of tones;
- the method including, in at least one of the iterations, determining that one or more inputs to at least one radix calculation are zero, and accordingly simplifying the radix calculation.
13. A communication apparatus for use to implement a process of two-directional communication of data over a line, the communication employing a bandwidth partitioned into a number of bands including at least one band associated with each of the two directions, the apparatus including:
- a signal generation section for generating a signal by encoding the data carried in one direction by modulation of a plurality of tones defined within the at least one band associated with that one direction, the signal generation section including: an input section for receiving data and transforming it into frames; an encoder for using the frames to generate complex amplitude data characterizing the amplitude of the tones associated with the one direction; an IFFT processing unit for receiving the amplitude data, and iteratively operating an algorithm composed of radix calculations on the data stored in the memory to generate time-domain data; and a processor arranged, in at least one of the iterations, to determine that one or more inputs to at least one radix calculation are zero, and accordingly control the IFFT processing unit to simplify the radix calculation; and
- a signal transmission section including a line driver for transmitting a signal based on the time-domain data.
14. The communication apparatus according to claim 13 wherein the apparatus is operative to function in a selected one of:
- a first mode in which data is transmitted on all the tones associated with communication in a corresponding direction; and
- at least one second mode in which data is transmitted on a subset of the tones associated with the corresponding direction, the subset being selected as a set of 2n consecutive tones, the processor being arranged when the apparatus is in a said second mode to simplify the radix calculation in at least the first stage to omit radix calculations which do not take input values representing any of said set of tones.
15. The communication apparatus according to claim 14 wherein the processor is arranged, when the apparatus is in a said second mode, to simplify the radix calculation in at least the first stage to omit radix calculations which do not take as input values amplitude data representing amplitudes of any of said set of tones.
16. The communication apparatus according to claim 13 wherein said apparatus is operative to receive a signal in which there are a plurality of bands associated with said one direction, each of the bands having a number of tones which is substantially equal to a power of 2, and the bands being spaced apart by a number of tones which is substantially equal to a power of 2.
17. A communication apparatus for implementing a process of two-directional communication of data over a line, the communication employing a bandwidth partitioned into a number of bands including at least one band associated with each of the two directions, the apparatus including:
- a signal reception section for receiving a signal transmitted in one direction and encoding data by modulation of a plurality of tones defined within the at least one band associated with that one direction, the signal reception section including: an input section for receiving the signal and generating time domain data; an FFT processing unit for receiving the time domain data, and iteratively operating an algorithm composed of radix calculations on the data stored in the memory to generate amplitude data for the respective tones; a processor arranged, in at least one of the iterations, to determine that one or more input values to at least one radix calculation are zero, and accordingly control the FFT processing unit to simplify the radix calculation; and a decoder section for extracting data from the amplitude data.
International Classification: H04K 1/10 (20060101);