WO2000038387A1 - Method for transmitting data blocks without prefix in the guard interval, said data blocks are demodulated by means of fft with a length greater or equal the symbol period - Google Patents

Abstract

The invention relates to a method for transmitting data by a multi-carrier method, e.g. DMT (discrete multitone). The data is combined in a transmitter into blocks having the same number of information symbols (M). Said data is modulated and transmitted by an inverse fast Fourier transformation (IFFT) and demodulated in a receiver by a fast Fourier transformation (FFT). An equalising guard interval in the receiver is co-transmitted and fitted into the transmitter between the respective blocks. The guard interval is greater than or equal to the memory length of the transmission channel and is transmitted without any signals or prefix and demodulation is carried out in the receiver by means of fast Fourier transformation (FFT) with a length (L) which is greater than or equal to the sum of the information block length (M) and the length (P) of the guard interval.

Description

METHOD FOR TRANSMITTING DATA BLOCKS WITHOUT A PREFIX IN THE GUARD INTERVAL AND THE MEANS FFT WITH A LENGTH LARGER OR EQUAL

SYMBOL DURATION WILL BE DEMODULATED

The invention relates to a method for the transmission of data by a multi-carrier method, e.g. DMT (Discrete Multitone) in a transmission channel in which the data in a transmitter are combined into blocks with the same number M of information symbols, modulated and transmitted by an Inverse Fast Fourier Transform (IFFT), and in a receiver by Fast- Fourier transformation (FFT) are demodulated, a guard interval for the receiver-side equalization being inserted and also transmitted between the blocks on the transmitter side, which guard interval has a length (P) which is greater than or equal to the memory length of the transmission channel.

Many of the known transmission methods use the available frequency range of a transmission channel by suitable modulation of the data to be transmitted. In the case of a frequency division multiplex transmission, a division into several frequency positions is carried out, via which the information is transmitted. Methods of this type have become known under the names multicarrier method, orthogonal frequency division multiplex (OFDM) and discrete multitone method (DMT).

A given broad frequency band is divided into a large number of narrow subchannels over which the data is transmitted. For this purpose, the data are combined in a transmitter into information blocks of equal length and modulated by an Inverse Fast Fourier Transform (IFFT), which filters the subchannels with frequency-shifted versions of a prototype filter. The resulting transmission block is output serially by the transmitter on the transmission line. As a result of the memory of the dispersive transmission channel, interference generally occurs between successive blocks on the receiving side. In order to avoid an overlap on the receiver side, a guard interval must be inserted between the individual blocks on the transmitter side. The data is demodulated in the receiver by a Fast Fourier Transform (FFT), the input samples being transformed block by block into spectral values. Equalization can be significantly simplified when using the FFT in the receiver if a cyclic prefix is also transmitted in the guard interval, which consists of a number of repeated data of each block, which are transmitted before the block within the guard interval. The transformation length L of the FFT is equal to the length M of the data blocks sent. In order to obtain an effective equalization, the guard interval or the cyclic prefix must be greater than or equal to the memory length of the channel. However, the advantage of the relatively simple equalization entails the disadvantage of the data transmitted in the prefix signal without gaining information, which takes up a portion of the available transmission power. The object of the invention is therefore to provide a method of the type mentioned at the outset which enables receiver-side equalization of the transmitted transmission signal without transmission of unusable information and thus an increase in the transmission power available for data transmission.

According to the invention, this is achieved in that the guard interval is transmitted without a signal or without a prefix, and in that the demodulation in the receiver is carried out by means of a Fourier transform (FFT) with a length L which is greater than or equal to the sum of the information block length M and the length P of the guard interval.

The advantage of the method according to the invention is that no signal or no power has to be sent in the guard interval, as a result of which the average transmission power is reduced, but at the same time the equalization of the transmitted signal can be carried out with relatively little effort. Therefore, assuming a predetermined power density within a transmission channel, the transmission power for the information blocks can be increased. Alternatively, according to a further feature of the invention, it can be provided that a useful signal, e.g. Pilot tones is transmitted, which is advantageous for clock recovery.

According to one embodiment of the invention, the demodulation can advantageously be carried out by extending the information block to be transformed in the receiver, which has the length M + P, by appending zeros to the transformation length L.

In a further embodiment of the invention it can be provided that the transformation length L of the Fast Fourier Transform (FFT) is twice the information block length 2-M. In this case, a very efficient implementation is possible.

According to a further embodiment of the invention, it can be provided that the guard interval is sent before or after an information block.

The invention is explained in detail below on the basis of the exemplary embodiment illustrated in the drawings. It shows

1 shows a transmission signal when using a cyclic prefix according to the prior art;

2 decomposing a received signal caused by the transmission signal according to FIG. 1 into blocks of length M;

3 shows a prefix-free transmission signal according to an embodiment of the method according to the invention;

4 breakdown of a received signal caused by the transmission signal according to FIG. 3 into blocks of length M + P and

5 demodulation of the received signal according to FIG. 4 by an FFT of length 2M. When data is transmitted using a multicarrier method, for example DMT (Discrete Multitone), the data to be transmitted are combined in a transmitter into blocks shown below with the same number M of information symbols.

O. Block A ₀ = [A ₀ A, ... A _{M. j} ] ^T

1. Block A _M = [A _M A _{M + 1} ... A _2M.1 ] ^T

m. Block A _mM = [A _πιM A _{mM + 1} ... A _{mM + M.}

The data summarized in this way are modulated and transmitted by an M-point inverse fast Fourier transform (IFFT). The send block is

a ₀ = [a ₀ a, ... a ^] ⁷ = IFFT _M {A ₀ } ^a M ⁼ f ^a M ^A M + 1 - ^A 2M-1 ^ ^{= IFFT} M ^ ^A M ^

T ^a mM ⁼ t ^a mM ^a mM + l - ^A mM + Ml] ⁼ ^^ M ^ M ^

and is output serially at the transmitter output. As a result of the memory of the transmission channel, interference generally occurs between successive blocks on the receiving side. In order to avoid this, according to the state of the art, a guard interval with a cyclic prefix is inserted between the individual blocks, the last P data of this block being transmitted again at the beginning of each block, so each block is continued cyclically. If the data is demodulated in the receiver using a Fast Fourier Transform (FFT), the equalization in the receiver can be significantly simplified if a cyclic prefix is used. The transmission signal then has the following form:

s T = [a _{M. p} a _M _ _{p + 1} ... a _M _ _j a _Q a, ... a _{M- 1} ] f ^a 2M-P ^a 2M-P + 1 ^{• • • a} 2M- 1 ^a M ^a M + 1 ^{• • • a} 2M- 1 J ( ^l ) 1

4

= [ ^a 0 < _M -P> ^a 0 ^a _h ( ^< M- ^{> a} MJ ^[ '

The station aJ <$ I _P > means the elements - P to M- 1 of the vector a _0. In Fm.1 the transmission signal is graphically represented when a cyclic pref is used

The received signal y _n is the convolution of the transmitted signal and channel p

V _n = {f> k * h _k } {n) = ∑h _k s _n -. _k (3) k-0

h _k is the channel and has P + 1 coefficients. The receiver splits the input sequence into blocks of length M + P and discards the first P values of each block, see FIG. 2

Υp = [VP VP + \ VM + P-I]

Υt _Λ + 2P ^~ [Vt + 2P 2M + 2P + 1 2/2 _M + 2P-1]

y _ra ( _{M +} P) + P = [y (M + P) + PJ / _m (M + P) + P + ly (m + l) (A +) -l]

The -th block has an index range of n = m (M + P) + P, m (M + P) + P + 1,, (m + l) (+ P) - 1 There is now a fast on each of these blocks Fourier transformation (FFT) of length M applied

K, = FFT {y _{rπ (} rt p _{) +} } (Z) (4)

M-i

= ∑ym {H ₊ P) ₊ p ₊ ne- ° ^nl (5) r- = 0

M-i P

= ∑ ∑ ι _fc s _m ( _{M +} p _{) + P + n} _ ^ e ^_J Ϊ "'ri = n - k (6) π = 0 k-0 P -Jt + M-1

⁼ Σ ^l * Σ 5 _{m (M + P) + P + n} , _e - ^J 7T < ^{n '+} *>' π = n '(7) = Q n' - k

P -.c-tM-1

= / ι _λ e- ^ ^w T s _{m (rt + P) + P + n} e- ^J H ⁿ '(8) fc = 0 n ---- /.

ist die M-Punkte FFT des Kanals h_k wobei die Koeffizienten h_P+ bis /._M_ι λull sind Wünschenswert wäre nun wenn Gl (8) faktonsierbar ist d h sich in das Produkt der FFT von h_k und eines weiteren Multiphkanten zerlegen laßt 1The term H, =

is the M-points FFT of the channel h _k with the coefficients h _{P +} to /. _M _ι λull are desirable if Gl (8) can be factored, ie can be broken down into the product of the FFT of h _k and another multiphase edge 1

5 That Eq. (8) can actually be divided multiplicatively, is not directly readable, because in the second sum of Eq. (8) also comes the summation index k of the first

Total before. If it can be shown that the value of the second sum is still independent of k, Eq. (8) can be factored. If you look at the expression

-fc + M-l

sζ_lN] besteht, wird jeweils über genau eine vollständige Periode

summiert.Sι (k) = ∑ s _{m (M + P) + P + n} e-> W ^nl , (9) n = - k so this represents the Z-th value of the FFT from the sequence s _{m (M + P) + P + n} , n = -k, -k + 1, ..., - k + M - 1 ₎ . Taking into account that the range of values for k is limited to 0, 1, ..., P from Eq. (1) it can be seen that the summation limits always remain in the -th block. Because the mth transmission block from [

sζ _lN ], is for exactly one complete period

summed up.

In Eq. (9) holds that S _t (k) is independent of k, S _t (k) = Si. This fact should now be made clear using a simple example. Example:

M = 3

P = 2 m = 0

5, (0) = 5 ₂ + s ₃ e- ^J - ^u + s ₄ e- ^J 3 ² '-: αo + α ₁ e ^J J "+ O ₂ e ^_J τ ² '

5, (1) = _{Sl e} ^J - ¹ '+ ₂ + S ₃ e ^J 3 ^{1 (}

= α ₂ e ^ ^u + a ₀ + a _l e ^~ ^ ^u = α ₀ + a _λ e ^~ ^ ^u + α ₂ e ^~ ^ ² '= 5, (0)

5, (2) = s ₀ e ^ - ²¹ + 5! e ^ ^u + s

= _Ql e ^ ^{2 /} + α ₂ e ^ ^u + ₀ = ₀ + α, e ^ "+ α ₂ e ^ x ² '= 5, (0)

The decisive factor for the above transformations is the identity e ^{~ J nl} = e ^J N ^(W_n) '.

Eq. (9) is therefore the FFT of the block a _mM , which in turn is the IFFT of the data block A _mM . (9) is nothing else than the date A _m ^ _{+ l} . If you put this result in Eq. (8) one, one gets

As already mentioned, the remaining sum represents the FFT of the length of the channel, p Yι = H _t A _{mM + l} with H / = J ^ ι _it e - 'π ^w (11)

/ c = 0

Eq. (4) is nothing more than the Z-th date of the m-th block, A _{mM + l} , multiplied by Hι, that is the spectrum of the channel h _k evaluated at the frequency l ^ -. In this case, equalization is particularly simple; each received value Y _t only has to be multiplied by the reciprocal of H. The transformation length L of the FFT is identical to the length of the data blocks M while the length P of the guard interval or the cyclic prefix is greater than or equal to the memory length of the transmission channel.

In order to save the cyclic prefix of the transmission signal, it is provided according to the invention that the guard interval is transmitted signal-free or without a prefix, the demodulation using a Fourier transform (FFT) having a length L which is greater than or equal to the sum of the information block length M and the length P of the guard interval. The guard interval can be sent before or after an information block.

First, as in the known transmission method, the data Afc, k = 0, 1, 2,... Are combined in blocks A _m M of length M. The modulation is also carried out using an M-point IFFT, a _m jyi = IFFTj ^ AmM} - Instead of repeating the last P values of each block sent cyclically in a known manner, empty guard intervals of length P are now inserted, ie in these time periods zeros are transmitted. In this case, the transmission signal is

s - [ao ai ... a _M _ι 0 0 ... 0] [a _M a _{M + 1} ... a ₂ Ml 0 0 ... 0] [... (12) = [a ₀ ^T 0 _P ^T a _M ^T ... 0 _P ^T ] (13)

0p is the zero vector of length P. FIG. 3 shows the transmission signal formed in this way. If the guard interval P symbols is long and M information symbols are blocked in the transmitter, the incoming data y _{n are first combined} in the receiver to form blocks of length M + P, as shown in FIG.

-ι]

-ι]

ym (M + P) ^T = [ym (M + P) ym (M + P) + l - y (m + l) (M + P) -L

The block m has an index area n = nv (M + P), m- (M + P) + l, ..., (m + l) - (M + P) -l. An FFT with a block length L of at least M + P is applied to each of these blocks of length M + P. The transformed signal is now combined in the vector YL = FFTL (y _m (M + P)}. The equalization of the dispersive transmission channel takes place in the frequency domain as in the known transmission method. After demodulation, the L elements of the vector YL are divided by samples of the spectrum of the channel. The resulting vector XL is the L points FFT of the currently sent data block x = [a _{m] yj} ^a mM + l - a _m M + Ml] ^T

X _L = FFT _L {x}.

Because the modulation takes place in the transmitter with an M-point IFFT,

x = IFFT _M {A _mM },

the M-points FFT of the current transmission block x is equal to the transmitted data A _m ] yj. The M points FFT X ^ = FFTM (X} = A ^ must be calculated from XL.

The calculation of the vector XM from XL is clearly possible, but the choice of L determines the complexity.

If the memory length of the channel is less than or equal to M (P <M), it makes sense to choose the transformation length L of the Fourier transform (FFT) equal to twice the information block length 2-M (L = 2M), as shown in Fig. 5 is shown. Because the FFT of the transformation length 2M only has to be evaluated on the even-numbered indices, a very efficient implementation is possible. The block to be transformed, which is only M + P long, is extended to 2M by appending M- P zeros. For block m you get

Y _l = FFT _2M {y _{m (M + P)} } (Z) (14)

M + P-l

= ∑ ym (M + P) + n e- ^{j nl} (15) π = 0 M + Pl P = ∑ ∑ h _k s _{m {M + P) + n} _ _k e ^{~ j} ik ^nl n '= n - k (16) n --- 0 A ---- 0

=. (18)

Depending on the value of k, the summation over n begins for k - 0 at n = 0 up to n = —P at k - P, ie s _m ( _{M + P} ) _ _P to s _{m (M + P)} . All these values except for s _m ^ _{M + P} are always identical zero due to the zeros in the guard interval. The summation can therefore always start at n = 0 regardless of k. J

8th

Depending on k, the upper summation limit can assume the values M - 1 to / vf + • P - 1, the associated signal elements are s _{m (} fl _{+ P) +} _ι to s _{m (} ^ _{+ P) +} M ₊ _ ₁ . Srn (M ₊ P) ₊ M to ⁵ -τ- (tf ₊ p) ₊ M + p-ι fall again into a guard interval and are therefore again identical zero. The upper summation limit can therefore always be f | - 1 can be written.

Insert these summation limits in Eq. (18) delivers

P M-i

Y _l = h _k e ^~ ^ ^kl ∑ s _{m {+ P) + n} e ^~ ι% ^nl (19) k-0 n = Ö

= FFT _2M {h} (Z) FFT _2M {a _mM } (Z), (20)

where h _k - 0 for k> P and s _m ( _{M +) + n} = 0 for n> M, h is the impulse response of the channel, h ^τ = [h ₀ hι. , , h _P ]. The vector S ^ _M is the IFFT of length AI of the data block A _mM to be transmitted, so it applies

y, = FFT _2rt {h} (Z) FFT _2M {IFFT _M {A _mM }} (Z). (21)

The expression FFT _2M {IFFT _M {A _mM }} (Z) is examined in more detail below.

FFT _2t1 {lFFT _M {A _mΛ ,}} (Z) (22)

_k = 0 π --- 0

Evaluating the above expression for even l = 2r gives

, M- M M M

FFT _2M {lFFT _M {A _mM }} (2r) = - £ ∑ A _{mM + ne} ^ ^k ^ ^~ ^ (24)

J

J

With this result Eq. (20) to

Y _2r = FFT _2M {h} (2r) A _{mM + r} . (29)

The 2 M FFT of y _m ( _{+ P} ) evaluated at position 2r is therefore the rth symbol of the mth block, - _m M _{+ r} , multiplied by the spectrum of channel h at frequency ^ 2r. The same method of equalization can be used as when using a cyclic prefix.

Because in Eq. (29) only the even-numbered indices are of interest, the FFT of length 2H in Eq. (14) can easily be reduced to an FFT of length M. The block to which the FFT of length 2M is applied has a length of + P, it is expanded to 2M with zeros.

2M-1

FFT _aM {y _{m (+ P)} } (2r) - ∑ ϊ / _m (M + P) + ne ^{j nτ} (30) n = 0

M-i 2 M-1

= Σ 2 m (M + P) + ne ^j i ^nr + ∑ ym ( ₊ P) ₊ n T \ ^nr (31)

= ^{M n) r •} (32)

-------- (2 / m (N \ + P) + n + ym (M + P) + M + n) e ^{3 nr} (33) n = 0

= FFT _M {y _{m (N + P)} < ₀ M ^{| ,,} --i ¹ -> + y _{πι (} W _{+ P} ) <-2 ² _M W ^W --1 ¹ >} () (34)

As from Eq. (34) can be seen, the even-numbered indices of a 2 FFT can be calculated by an FFT of length -M. The only additional effort is to add the two blocks. If it is taken into account that the second block contains only P elements other than zero, P additional additions are necessary.

Claims

PATENT CLAIMS 1. Method for the transmission of data by a multi-carrier method, e.g. DMT (Discrete Multitone) in a transmission channel, in which the data in a transmitter are combined into blocks with the same number of information symbols (M), modulated and transmitted by an Inverse Fast Fourier Transform (IFFT), and in a receiver Fast Fourier Transformation (FFT) are demodulated, a guard interval for the receiver-side equalization being inserted and also transmitted between the blocks on the transmitter side, which guard interval has a length (P) which is greater than or equal to the memory length of the transmission channel , characterized in that the guard interval is transmitted signal-free or without a prefix, and that the demodulation in the receiver is carried out by means of Fast Fourier Transformation (FFT) with a length (L) which is greater than or equal to the sum of the information block length (M ) and the length (P) of the guard interval. 2. The method according to claim 1, characterized in that the length to be transformed in the receiver, the length (M + P) having information block is extended by appending zeros to the transformation length (L). 3. The method according to claim 1 or 2, characterized in that the transformation length (L) of the Fast Fourier Transformation (FFT) is equal to twice the information block length 2-M. 4. The method according to claim 1, 2 or 3, characterized in that the guard interval is sent before or after an information block. 5. The method according to any one of claims 1 to 4, characterized in that a useful signal, e.g. Pilot tones, is transmitted.