WO2004091159A1

WO2004091159A1 - Method and apparatus for channel equalization in multi-carrier communication devices using discrete cosine modulated filter bank or wavelet packet modulation

Info

Publication number: WO2004091159A1
Application number: PCT/EP2004/050486
Authority: WO
Inventors: Stephan Horvath; Antony Jamin
Original assignee: Acn Advanced Communications Networks Sa
Priority date: 2003-04-08
Filing date: 2004-04-07
Publication date: 2004-10-21
Also published as: EP1649651A1

Abstract

A method for channel equalization in a data communication system comprising at least two cosine modulated filter bank-based multi-carrier communication devices or at least two wavelet packet modulation-based multi-carrier communication devices being connected over a single transmission channel (103), the channel equalization being a recursive time-domain equalization comprising a finite impulse response equalization part (301) and an infinite impulse response equalization part (304, 304’), as well as an apparatus for channel equalization for use in one or more cosine modulated filter bank-based multi-carrier communication devices or in one or more wavelet packet modulation-based multi-carrier communication devices, the apparatus being a recursive time-domain channel equalizer (207) comprising a finite impulse response part (301) and an infinite impulse response part (304, 304’).

Description

Method and apparatus for channel equalization in multi-carrier communication devices using discrete cosine modulated filter bank or wavelet packet modulation

Technical field of the invention

The present invention relates to an equalization method to be used in cosine modulated filter bank-based multi-carrier communication devices and/or in wavelet packet modulation-based multi-carrier communication devices in order to reduce the adverse effects of the transmission channel and to an apparatus using this method.

Background art and assessment thereof:

Most prior art multi-carrier communication devices use inverse orthogonal frequency division multiplexing (OFDM) modulation. A cycle prefix, chosen to be preferably longer than the channel impulse response, is commonly added to each transmitted symbol in order to avoid inter-symbol interferences. If the channel impulse response is longer than the cycle prefix, an additional time-domain equalizer is used at the receiver's side for shortening the channel impulse response. By exploiting the properties of the circular convolution realized, a simple frequency domain equalizer with complex coefficients can be employed in the receiver after the OFDM modulation in order to retrieve the sent data.

Fig. 1 shows for example the block-diagram of a communication system using such prior art multi-carrier communication devices. The data 10 to be sent over the channel 103 is first fed to the serial to parallel converter 101 before being converted from the frequency domain into the time-domain by inverse discrete Fourier transform (DFT) in the OFDM modulator 102. The modulated data 12 out of the sender 11 is then sent over the transmission channel 103 with the channel noise 104.

The transmission channel's output 13 at the input of receiver 14 is fed into an optional time-domain finite impulse response (FIR) equalizer 105 for shortening the channel impulse response such that the shortened channel impulse response becomes shorter than or equal to the cycle prefix. The output of the FIR equalizer 105 is then fed to the OFDM demodulator

106 which will convert the data using DFT from the time-domain back into the frequency-domain. The demodulator's output is connected to the frequency domain equalizer (FEQ) 107, the output of which is then converted in the parallel to serial converter 108. The output 15 of the converter 108 is thus an estimate of the sent data 10.

Such prior art multi-carrier communication devices have a major drawback in that OFDM modulation has a poor stop-band attenuation. This results in significant inter-channel interferences when adjacent channels are used simultaneously in a real communication system. In order to avoid or to reduce these inter-channel interferences, the different channels must be chosen sufficiently far apart from each other, thus resulting in a poor spectral efficiency of the communication system, that is in an inefficient usage of the communication system's available bandwidth.

In order to increase the communication system's spectral efficiency, other multi-carrier communication devices can be used, such as for example cosine modulated filter bank-based multi-carrier communication devices or wavelet packet modulation-based multi-carrier communication devices, which provide higher stop-band attenuation, leading to lower inter-channel interference. However, the frequency domain combiner needed by the receiver of such communication devices in order to estimate the sent data is considerably more complex than the frequency domain equalizer 107 needed for example in the case of OFDM modulation, making such multi-carrier communication devices unattractive for a practical implementation in a data communication system. An example of a frequency combiner for use in a multi-carrier transmission system using wavelet modulation is for instance described in Patent US 5,636,246. Summary of the invention

An aim of the present invention is thus to propose a method and an apparatus for efficient channel equalization in cosine modulated filter bank-based multi-carrier communication devices and/or in wavelet packet modulation-based multi-carrier communication devices.

Another aim of the present invention is to propose a method and an apparatus for channel equalization in cosine modulated filter bank- based multi-carrier communication devices and/or in wavelet packet modulation-based multi-carrier communication devices that can be implemented in a data communication system at lower costs than prior art methods or apparatus.

These aims are achieved by an apparatus and a method having the characteristics of the respective independent claim, variant embodiments being given by the dependent claims.

These aims are achieved in particular by a method for channel equalization in a data communication system comprising at least two cosine modulated filter bank-based multi-carrier communication devices or at least two wavelet packet modulation-based multi-carrier communication devices being connected over a single transmission channel, the channel equalization being a recursive time-domain equalization comprising a finite impulse response equalization part and an infinite impulse response equalization part, as well as by an apparatus for channel equalization for use in one or more cosine modulated filter bank-based multi-carrier communication devices or in one or more wavelet packet modulation-based multi-carrier communication devices, the apparatus being a recursive time- domain channel equalizer comprising a finite impulse response part and an infinite impulse response part.

The present invention assumes a slowly varying transmission channel and noise. According to a preferred embodiment of the invention, the transmission channel is preferably symmetrical. The present invention is thus particularly adapted for use over wired networks, such as for example electric power distribution networks.

The present invention uses a time-domain recursive equalizer to compensate the distortions due to the transmission channel. According to a preferred embodiment of the invention, a recursive equalizer is placed before the cosine modulated filter bank or wavelet packet modulation at the receiver. The coefficients of the recursive equalizer are determined in such a way that the channel transfer function is compensated by the time- domain equalizer's transfer function. The coefficients are first calculated using a training sequence known to the receiver during an initialization procedure and are updated periodically using transmitted pilot symbols.

According to an embodiment of the invention, the time-domain recursive equalizer has a rational transfer function having poles and zeros to better compensate the transmission channel transfer function. Such equalizers are usually avoided in prior art systems because they dramatically amplify the channel noise and decision feedback equalizers are preferably used instead. According to the present invention, the channel noise amplification is avoided by performing, for data transmissions in both directions, a channel pre-equalization at the sender using the infinite impulse response (MR) part of the time-domain recursive equalizer, the finite impulse response (FIR) part of the equalizer being used at the receiver to reduce the remaining channel distortions.

The time-domain recursive equalizer according to the present invention is less complex than the prior art frequency combiners proposed for use in cosine modulated filter bank-based multi-carrier communication devices or in wavelet packet modulation-based multi-carrier communication devices.

The equalization method according to the present invention allows a near perfect reconstruction of the sent data in the cosine modulated filter bank-based multi-carrier communication devices or in the wavelet packet modulation-based multi-carrier communication devices. Near perfect reconstruction means that the carriers are orthogonal before demodulation. Thanks to the equalization method of the present invention, cosine modulated filter bank-based multi-carrier communication devices and/or wavelet packet modulation-based multi-carrier communication devices having intrinsically a better spectral efficiency than other multi-carrier communication devices are made attractive for an implementation in data transmission systems.

Description of the Drawings

A better understanding of the present invention can be obtained when the following detailed description of embodiments of the invention is considered in conjunction with the following drawings, in which

Already discussed Fig. 1 shows a block diagram of a communication system using prior-art OFDM-based multi-carrier communication devices. Fig. 2 is a block diagram during initialization of a data communication system using cosine modulated filter bank-based multi- carrier communication devices or wavelet packet modulation-based multi- carrier communication devices, in accordance with a preferred embodiment of the present invention.

Fig. 3a is a detailed block diagram of a time-domain recursive equalizer according to an embodiment of the present invention.

Fig. 3b is a detailed block diagram of a time-domain recursive equalizer according to another embodiment of the present invention.

Fig. 4 is a block diagram during operation of a data communication system using cosine modulated filter bank-based multi- carrier communication devices or wavelet packet modulation-based multi- carrier communication devices, in accordance with a preferred embodiment of the present invention. Description of the invention

Fig. 2 shows a block diagram of a communication system with multi-carrier communication devices in accordance with a preferred embodiment of the present invention during an initialization or a training phase. A sender 21 of a first communication device sends a known sequence of training symbols 20 to a receiver 24 of a second communication device. The training symbols 20 are first converted by a serial to parallel converter 201 and then converted from the frequency domain into the time domain by an inverse transform in a modulator 202. The inverse modulation employed in the modulator 202 is preferably inverse cosine modulated filter bank modulation or inverse wavelet packet modulation. The output of the sender 21 is a modulated training sequence 22 which is sent over the transmission channel 103 with the channel noise 104.

The channel output 23 received by the receiver 24 is first fed into a time-domain recursive equalizer 207 having a rational transfer function with poles and zeros. In order to avoid amplifying the channel noise 104 in the recursive equalizer 207, the known modulated training sequence 22, which is for example stored in a memory area 211, is simultaneously fed by the receiver 24 into the recursive path of the equalizer 207, as explained below in more details.

The output 27 of the time-domain equalizer 207 is then fed into the demodulator 209. The modulation performed in the demodulator 209 preferably is either a cosine modulated filter bank modulation or a wavelet packet modulation in accordance with the inverse transform used in the modulator 202. The parallel output of the demodulator 209 is fed into a parallel to serial converter 210, the output of which is a good estimate 25 of the data 20 sent by the sender 21.

As represented in Fig. 3a, the time-domain recursive equalizer 207 comprises a finite impulse response (FIR) part 301 and an infinite impulse response (MR) part 304. The channel's output 23, made of the modulated training sequence 22 transmitted over the channel 103 and the channel noise 104, is fed into the FIR part 301. In order to avoid infinitely amplifying the channel noise 104, the MR part 304 is fed with a noiseless reference modulated training sequence 22 instead of being recursively fed with the noisy output 27 of the equalizer. During this initialization or training phase, the coefficients a₀ to a_n and bi to b_m of the equalizer 207 are preferably determined by a calculator 208 using an error criterion like for instance minimum mean square error (MMSE) in order to minimize the difference between the output 27 of the equalizer 207 and the reference modulated training sequence 22, so as to make the output signal 25 represented on Fig. 2 a good estimate of the sent data 20.

Fig. 3b is the block diagram of the time-domain recursive equalizer 207 according to another embodiment of the invention. According to this embodiment, the MR part 304' is realized using a lattice form digital filter. During the initialization or training phase, the coefficients a₀ to a_n and ki to k_m are determined as previously explained for the embodiment illustrated in Fig. 3a. The advantage of the lattice form digital filter is that it is easily stabilized as its transfer function is always stable as long as the absolute value of all coefficients ki to k_m is inferior to 1.

According to a preferred embodiment of the invention, illustrated by its block diagram during operation in Fig. 4, a communication system is built by connecting at least two inventive multi-carrier communication devices over a transmission channel 103. Each communication device preferably comprises a sender 21, a receiver 22 and a hybrid circuit 404. The sender 21 and the receiver 22 are connected to the hybrid circuit 404 over which the communication device is connected to the transmission channel 103.

Before sending information data to each other, the communication devices perform an initialization phase as described above. The initialization or training phase is preferably performed at least once with each communicating sender-receiver pair. Each receiver 21 thus determines all necessary coefficients for an optimal channel equalization. During operation, that is while information data is transmitted over the channel 103, only the FIR part 301 of the channel equalization is performed in the receivers 24. The IIR part 304 or 304" is preferably performed in the senders 21 which thus perform a pre-equalization of the channel's transfer function on the signal before transmitting it. This avoids the channel noise 104 to be amplified by the IIR part 304 or 304', as only the FIR part 301 of the equalization is performed on the received signal.

As the transmission channel 103 is assumed to be symmetrical, the coefficients bi to b_m or ki to k_m determined for the IIR part 304 or 304' by each receiver 24 can be used to tune the IIR part 304 or 304' within the same communication device. After the training phase, the coefficients calculated within the receiver 24 are thus preferably transferred to the IIR part 304 of the sender 21 within the same communication device. The one skilled in the art will however recognize that it is possible within the frame of the invention to compute the coefficients bi to b_m or ki to k_m in the receiver of one communication device and send them over the channel 103 to the sender of another communication device for tuning its IIR part. In some variant embodiments of the invention, the transmission of the coefficients over the channel can even be required, for example if the communication devices only comprise either a receiver or a sender, or if the transmission channel is not symmetrical.

The transmission channel 103 is preferably assumed to be slowly varying over time. The training phase thus doesn't need to be frequently performed in order to keep the equalizer in optimal operating conditions. Only a very small proportion of the communication system's transmission capacities are thus required for training purposes.

The inventive equalization and apparatus as described above according to a preferred embodiment are thus particularly adapted to be used over a wired and symmetrical transmission network, such as for example an electrical power distribution network comprising the area between the low-voltage transformer station and the house connection unit and/or the electricity distribution within the house.

Claims

1. Method for channel equalization in a data communication system comprising at least two multi-carrier communication devices being connected over a single transmission channel (103), said at least two multi- carrier communication devices being cosine modulated filter bank-based multi-carrier communication devices or wavelet packet modulation-based multi-carrier communication devices, said channel equalization being a recursive time-domain equalization comprising a finite impulse response equalization part (301) and an infinite impulse response equalization part (304, 304").

2. Method according to claim 1, said transmission channel (103) being an electrical power distribution network.

3. Method according to one of the claims 1 or 2, said infinite impulse response equalization part (304, 304') being performed before transmitting said data over said transmission channel (103) and said finite impulse response equalization part (301) being performed after transmitting said data over said transmission channel (103).

4. Method according to one of the claims 1 to 3, said infinite impulse response equalization part (304') comprising a lattice form digital filter.

5. Method according to one of the claims 1 to 4, comprising the step of transmitting a training sequence (20) from one of said at least two multi-carrier communication devices to another one of said at least two multi-carrier communication devices.

6. Method according to claim 5, the parameters (ao to a_n, bi to b_m, ki to km) of said channel equalization being computed during said step of transmitting a training sequence (20).

7. Method according to claim 6, said parameters comprising the parameters (a₀ to a_n) of said finite impulse response equalization part (301).

8. Method according to one of the claims 6 or 7, said parameters comprising the parameters (bi to b_m, ki to k_m) of said infinite impulse response equalization part (304, 304').

9. Method according to one of the claims 6 to 8, said parameters (a₀ to a_n, bi to b_m, ki to k_m) being determined using an error criterion between said training sequence (20) and the output (25) of said another one of said at least two multi-carrier communication devices.

10. Method according to claim 9, said error criterion being minimum mean square error.

11. Method according to one of the claims 5 to 10, said infinite impulse response equalization part (304, 304") being fed with a reference training sequence (22) during said step of transmitting a training sequence (20).

12. Apparatus for channel equalization for use in one or more cosine modulated filter bank-based multi-carrier communication devices or in one or more wavelet packet modulation-based multi-carrier communication devices, said apparatus being a recursive time-domain channel equalizer (207) comprising a finite impulse response part (301) and an infinite impulse response part (304, 304').

13. Apparatus according to claim 12, for use in one or more multi- carrier communication devices adapted to be connected over an electrical power distribution network.

14. Apparatus according to one of the claims 12 or 13, said finite impulse response part (301) being implemented in a receiver (24) of said one or more multi-carrier communication devices, said infinite impulse response part (304, 304') being implemented in a sender (22) of said one or more multi-carrier communication devices.

15. Apparatus according to one of the claims 12 to 14, said infinite impulse response equalization part (304") comprising a lattice form digital filter.

16. Apparatus according to one of the claims 12 to 15, the parameters (ao to a_n, bi to b_m, ki to k_m) of said apparatus being computed using a modulated training sequence (22) transmitted from one of said at least two multi-carrier communication devices to another one of said at least two multi-carrier communication devices.

17. Apparatus according to claim 16, said parameters comprising the parameters (ao to a_n) of said finite impulse response equalization part (301).

18. Apparatus according to one of the claims 16 or 17, said parameters comprising the parameters (bi to b_m, ki to k_m) of said infinite impulse response equalization part (304, 304').

19. Apparatus according to one of the claims 16 to 18, said parameters (a₀ to a_n, bi to b_m, ki to k_m) being computed using an error criterion between said modulated training sequence (22) and the output (27) of said apparatus.

20. Apparatus according to claim 19, said error criterion being minimum mean square error.

21. Apparatus according to one of the claims 16 to 20, said parameters (a₀ to a_n, bi to b_m, ki to k_m) being computed using a reference modulated training sequence (22).

22. Apparatus according to claim 21, said infinite impulse response part (304, 304') being connected so as to be fed with said reference modulated training sequence (22) while said parameters (a₀ to a_n, bi to b_m, ki to km) are being computed.

23. Data communication system comprising at least two multi- carrier communication devices being connected over a power line communication channel (103), said at least two multi-carrier communication devices being cosine modulated filter bank-based multi- carrier communication devices or wavelet packet modulation-based multi- carrier communication devices, said at least two multi-carrier communication devices comprising an apparatus according to one of the claims 12 to 21.

24. System according to claim 22, at least one of said at least two multi-carrier communication devices comprising a sender (21).

25. System according to one of the claims 22 or 23, at least one of said at least two multi-carrier communication devices comprising a receiver (24).