IL143877A

IL143877A - Method for transmitting data blocks without prefix in the guard interval wherein said data blocks are demodulated by means of fft with a length greater or equal to the symbol period

Info

Publication number: IL143877A
Application number: IL143877A
Authority: IL
Original assignee: Ericsson Telefon Ab L M
Priority date: 1998-12-21
Filing date: 2001-06-20
Publication date: 2006-04-10
Also published as: ATA212898A; WO2000038387A1; AU1958200A; EP1142248A1; AT408396B; CN1336063A; IL143877A0

Description

FFT Method For Transmitting Data Blocks Without Prefix In The Guard Said Data Blocks Are Demodulated By Means Of FFT With A Length Greater Or Equal The Symbol Period Telefonaktiebolaget LM Ericsson AT 009900311 AMENDED SHEET Method for transmitting data blocks without prefix in the guard said data blocks are demodulated by means of FFT with a length greater than or equal to the symbol period The invention relates to a method of transmitting data by means of a multiple carrier DMT in a transmitter in which the data are combined in a transmitter into a transmitter signal in the form of transmitter blocks with the same number M of information are modulated and transmitted by an Inverse Fast Fourier Transform of the transformation length M and are demodulated in a receiver by the Fast Fourier Transform on the transmitter one guard interval for equalization on the side of the receiver is inserted each between the transmitter blocks and transmitted together with said guard interval having a length P that is greater than or equal to the memory length of the transmission and wherein demodulation is carried out the receiver by means of the Fast Fourier Transform with a receiver transformation length L that is greater than or equal to the sum of the transformation length M and the length P of the guard Many of the known transmission methods use the available frequency range of a transmission channel by properly modulating the data to be In the frequency spectrum is divided into several slots through which information is Such methods have become known under the designations multiple carrier Orthogonal Frequency Division Multiplex and Discrete Multitone Technique A broad frequency band is thereby subdivided into a host of very narrow subchannels over which data are For this the data are combined in a transmitter into information blocks of the same length and are modulated by an Inverse Fast Fourier Transform that effects a filtration of the subchannels with versions of a prototype The transmitter serially transfers the thereby generated transmitter block onto the transmission The memory of the dispersive transmission channel generally causes consecutive blocks on the receiver side to In order to avoid overlapping on the receiver a guard interval must be inserted between the discrete blocks on the transmitter Demodulation of the data occurs in the receiver by means of a Fast Fourier Transform the input samples being transformed in blocks into spectral When using the FFT in the equalization can be considerably simplified by also transmitting in the guard interval a cyclical prefix consisting of a number of recurrent data from each said data being transmitted within the guard interval before the block with respect to The transformation length L of the FFT thereby equals the length M of the data blocks In order to obtain efficient the guard interval or the cyclical prefix respectively must be greater than or equal to the length of the The advantage of the relatively easy equalization entails however the disadvantage of the data being transmitted in the prefix signal without any gain of information and requiring part of the transmitting power available for In the OFDM method for radio transmission of data which is indicated in US Patent 5 357 the transmission of the information is carried out by means of N orthogonal carrier frequencies that are modulated by an Inverse Fast Fourier In order to avoid interferences between the discrete carrier the time windows of these filters are not chosen to be rectangular like in the conventional OFDM they are rather selected according to the Nyquist N data values at a time are thereby combined into information blocks according to the number of carrier Figure 4 of this document shows the Nyquist interval used for a guard interval in which a prefix with useless information is also transmitted being left free between the transmitter EP 0 682 426 A discloses an OFDM transmission system with Fast Fourier Transform and quadrature amplitude modulation a pilot signal being transmitted together with a portion of the block of information in the guard interval of each block of information It is therefore the object of the invention to indicate a method of the type mentioned herein above that makes equalization of the transmitted transmitter signal on the receiver side possible without transmitting useless information at the same thus increasing the transmitting power available for data This is achieved according to the invention in that the signal values of the transmitter signal contained in the guard interval have a signal amplitude of The advantage of the method according to the invention is that in the guard interval no signal or power needs to be which entails that the mean transmitting power is thereby but that the equalization of the signal transmitted can be carried out with relatively little Assuming a predetermined power it is therefore possible to increase the transmitting power for the blocks of information within a transmission According to an exemplary embodiment of the demodulation may advantageously be conducted in segmenting the receiver signal in the receiver into blocks of the length and in lengthening every single block by appending zeros to the receiver transformation length In another embodiment of the invention there may be provided that the receiver transformation length L of the Fast Fourier Transform equals the double transformation length According to still another embodiment of the invention there may be provided that the guard interval is transmitted each before or after a transmitter The invention will be described more fully herein after with the help of the embodiment illustrated in the Figure 1 shows a transmitter signal when using a cyclical prefix according to prior Figure 2 shows the decomposition into blocks of the length M of a receiver signal generated by the transmitter signal according to Figure Figure 3 shows a transmitter signal according to an embodiment of the method according to the Figure 4 shows the decomposition into blocks of the length of a receiver signal generated by the transmitter signal according to Figure 3 and Figure 5 shows the demodulation of the receiver signal according to Figure 4 by an FFT of the length 2 on page 3 of the original When transmitting data by means of a multiple carrier DMT the data to be transmitted are combined in a transmitter into the blocks with the same number M of information symbols represented herein block A I block block The thus combined data are modulated and transmitted by an M points Inverse Fast Fourier Transform The transmitter block reads al and is emitted in series at the transmitter output The memory of the transmission channel generally causes consecutive blocks to interfere on the receiver To avoid the state of the art introduces a guard interval with a cyclical prefix between the discrete at the beginning of each the last P data of said block are transmitted once that is each block is cyclically carried If demodulation of the data in the receiver is carried out by means of a Fast Fourier Transform equalization in the receiver may be considerably simplified using a cyclical the transmitter signal has the following T j aM a2 l The the elements M P to M 1 of the vector is a schematic illustration of the transmitter signal when a cyclical prefix is The receiver signal yn is the convolution of transmitter signal and p the channel and has P 1 The receiver divides the input sequence into blocks of the length M P and rejects the first P values of each see The mtn block has a range of indices of n P A Fast Fourier Transform of the length is now used on each of these For the block we have y P n P n The term the FFT of the channel the coefficients to being It would now be desirable to have equation that is to have it resolved into the product of the FFT of hk and of another It cannot directly be seen that equation may indeed be resolved into because the summation index k of the first sum also appears in the second sum of equation If it can be demonstrated that the value of the second sum is still independent of equation may be The expression represents the Ith value of the FFT of the sequence n 1 k M Considering that the value range for k is limited to 1 it may be seen from equation that the limits of the summation always remain in the Because the transmitter block consists of summation is made over exactly one complete period In equation it is true that is independent of This is to be made clear with the help of a simple M 3 P 2 M 0 a2 a0 The identity of importance for the above mentioned equation is the FFT of the block which in turn is the FFT of the data block is nothing else than the piece of data AmM If this result is inserted in equation the following result is obtained As it has already been the remaining sum represents the FFT of the length M of the channel Yi equation is nothing else than the piece of data of the multiplied by that is the spectrum of the channel hk interpreted at the frequency In this M equalization is particularly each receiver value only needs to be multiplied by the reciprocal value of The transformation length L of the FFT is identical with the length of the data blocks M whereas the length P of the guard or of the cyclical prefix is greater than or equal to the memory length of the transmission To save the cyclical prefix of the transmitter there is provided according to the invention to transmit the guard interval free from signal or without demodulation being conducted by means of the Fourier Transform with a length L which is greater than or equal to the sum of the length of the information block M and of the length P of the guard The guard interval may hereby be transmitted each either before or after one block of the data to be transmitted are combined into blocks of the length in the same way as in the known method of Modulation is also carried out by means of an points amM Instead of cyclically repeating in the known way the last P values of each block empty guard intervals of the length P are this time zeros are transmitted in these time In this the transmitter signal reads 0 0 0 Op is the zero vector of the length 3 shows the thus formed transmitter If the guard interval is P symbols long and if information symbols are blocked at a time in the the incoming data are combined in the receiver into blocks of the length as it is shown in The block has a range of indices n m An FFT with a block length L of at least is used on each of these blocks of the length The transformed signal is now combined in the vector YL Like in the method of transmission equalization of the dispersive transmission channel occurs in the frequency After the L elements of the vector YL are divided by samples of the spectrum of the The vector XL resulting therefrom is the FFT of the data block x i presently transmitted XL Because the modulation in the transmitter is conducted with an the FFT of the actual transmitter block x equals the transmitted data The FFT XM must therefore be computed from The calculation of the vector from XL is clearly but the choice of L determines the If the memory length of the channel is smaller than or equal to M it makes sense to choose the transformation length L of the Fourier Transfonm to equal the double length of the information block 2 M as it is illustrated in Fig Since the FFT of the transformation length 2M only needs to be interpreted at the a very efficient implementation is The block to be which is only M P is lengthened to 2M by appending P For the block the following is obtained P n k P n Depending upon the value of the summation begins over n for k 0 at n 0 until n at k that is to P Except for all these values are equal to zero as a result of the zeros in the guard the summation may always be started at n irrespective of In dependence on the upper summation limit can accept the values M 1 to M P the corresponding signal elements are to T0 however again fall into a guard interval and accordingly again equal For this for the upper summation M may always be Insertion of these summation limits into equation yields the following being Hk 0 for k P and 0 for n h is the impulse response of the The vector is the I FFT of the length M of the data block to be accordingly The expression is examined more Interpretation of the above mentioned expression for 2r yields l l H 1 With this equation becomes The 2M FFT of interpreted at 2r accordingly is the symbol of the multiplied by the spectrum of the channel H at the frequency 2r For the same 2M method may be used as when a cyclical prefix is being Since in equation the indices only are of the FFT of the length 2M in equation may easily be brought back to an FFT of the length The block on which the FFT of the length 2M is used has a length of it is extended to 2M by means of 2 As may be seen from equation the indices of a FFT can be calculated by way of an FFT of the length The only additional step to be taken is to sum up the two Considering that the second block only contains P of zero different P additional additions are insufficientOCRQuality

Claims

1. 1 International Patentapplication PCT/AT99/00311 Applicant: Telefoniesktiebolaget LM Ericsson et al. Amended sheet Method of transmitting data by a multiple carrier method, e.g. DMT (Discrete Multitone) in a transmission channel in which the data are combined in a transmitter into a transmitter signal in the form of transmitter blocks with the same number M of information symbols, are modulated and transmitted by an Inverse Fast Fourier Transform (IFFT) of the transformation length M and are demodulated in a receiver by the Fast Fourier Transform (FFT), wherein, on the transmitter side, one guard interval for equalization on the side of the receiver is inserted each between the transmitter blocks and transmitted together with them, said guard interval having a length P that is greater than or equal to the memory length of the transmission channel, and wherein demodulation is carried out in the receiver by means of the Fast Fourier Transform (FFT) with a receiver transformation length L that is greater than or equal to the sum of the transformation length M and the length P of the guard interval, wherein the signal values of the transmitter signal contained in the guard interval have a signal amplitude of zero. Method according to claim 1, wherein the receiver signal is segmented in the receiver into blocks of the block length M+P and that each of these blocks is lengthened by appending zeros to the receiver transformation length L. Method according to claim 1 or 2, wherein the receiver transformation length L of the Fast Fourier Transform (FFT) equals the double transformation length

2.M. Method according to claim 1, 2 or 3, wherein the guard interval is transmitted each time before or after a transmitter block. FOR THE APPLICANT Dr. Yitzhak Hess & Partners