CA2330522A1 - Method of isolating a block frequency on the basis of a signal formatted in blocks - Google Patents
Method of isolating a block frequency on the basis of a signal formatted in blocks Download PDFInfo
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- CA2330522A1 CA2330522A1 CA002330522A CA2330522A CA2330522A1 CA 2330522 A1 CA2330522 A1 CA 2330522A1 CA 002330522 A CA002330522 A CA 002330522A CA 2330522 A CA2330522 A CA 2330522A CA 2330522 A1 CA2330522 A1 CA 2330522A1
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- signal
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- blocks
- frequency
- isolating
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Abstract
The invention relates to a method of isolating a block frequency on the basis of a received signal formatted in blocks. Preferably, the signal is formed by symbols having a coded modulation transmitted according to an orthogonal frequency division multiplexing (OFDM). The invention comprises means for delaying (12) the symbol blocks and for correlating (13) a symbol block with a delayed symbol block that corresponds thereto. One thus obtains a difference signal e(t) which is used, for example, for synchronization purposes.
Description
PHF 93.503 DIV
METHOD OF ISOLATING A BLOCK FREQUENCY ON THE BASIS OF A SIGNAL
FORMATTED IN BLOCKS.
Description The invention relates to a method of isolating; a frequency called block frequency on the basis of a received signal formatted in blocks. The invention also relates to a receiver intended to use such a method, and a transmission system comprising such a receiver.
The article entitled "Applications of the digital correlator" by John Eldon, published in the journal "Microprocessor and microsystems", vol. 12, no. 4, 1 May 1988, pages 214-224, describes the use of correlation means for detecting particular synchronization words and periodicities in a received signal.
The invention notably comprises applying such teaching for isolating a, frequency to signals formatted in blocks, said blocks being formed by symbols of a constellation and having redundancy information, that is to say, a same block contains a same information signal that is present a first and a second time.
They may be, for example, signals obtained from orthogonal N-frequency division multiplexing which consists of splitting up a transmit information signal by distributing same over a large number of low-rate elementary channels. One thus transforms a highly selective wideband channel into a large number of non-selective elementary channels. As the arrangement forms a wideband channel, it is not very likely that fading during transmission will simultaneously affect the whole channel. This technique specifically makes it possible to reduce intersymbol interference.
One frequency thus corresponds to each elementary channel, all the frequencies together being symmetrically distributed around a central carrier frequency. As it is hard to accept the use of selective filters at the receiver end, one prefers tolerating an overlap of the spectra but then requirements are imposed with respect to orthogonality between the frequencies to eliminate intersymbol interference at the sampling instants. The whole spectrum of an OFDM signal thus tends towards a rectangular spectrum.
The OFDM signal is subdivided into frames :formed by blocks of which certain blocks are service blocks and other blocks are data blocks. 'to avoid intersymbol interference, PHF. 93.50 DIV
METHOD OF ISOLATING A BLOCK FREQUENCY ON THE BASIS OF A SIGNAL
FORMATTED IN BLOCKS.
Description The invention relates to a method of isolating; a frequency called block frequency on the basis of a received signal formatted in blocks. The invention also relates to a receiver intended to use such a method, and a transmission system comprising such a receiver.
The article entitled "Applications of the digital correlator" by John Eldon, published in the journal "Microprocessor and microsystems", vol. 12, no. 4, 1 May 1988, pages 214-224, describes the use of correlation means for detecting particular synchronization words and periodicities in a received signal.
The invention notably comprises applying such teaching for isolating a, frequency to signals formatted in blocks, said blocks being formed by symbols of a constellation and having redundancy information, that is to say, a same block contains a same information signal that is present a first and a second time.
They may be, for example, signals obtained from orthogonal N-frequency division multiplexing which consists of splitting up a transmit information signal by distributing same over a large number of low-rate elementary channels. One thus transforms a highly selective wideband channel into a large number of non-selective elementary channels. As the arrangement forms a wideband channel, it is not very likely that fading during transmission will simultaneously affect the whole channel. This technique specifically makes it possible to reduce intersymbol interference.
One frequency thus corresponds to each elementary channel, all the frequencies together being symmetrically distributed around a central carrier frequency. As it is hard to accept the use of selective filters at the receiver end, one prefers tolerating an overlap of the spectra but then requirements are imposed with respect to orthogonality between the frequencies to eliminate intersymbol interference at the sampling instants. The whole spectrum of an OFDM signal thus tends towards a rectangular spectrum.
The OFDM signal is subdivided into frames :formed by blocks of which certain blocks are service blocks and other blocks are data blocks. 'to avoid intersymbol interference, PHF. 93.50 DIV
each block contains redundant information signals. Any block is formed at all the OFDM
frequencies the device uses, the frequencies being modulated by transmit symbols coming from a coded modulation, for example, a digital PSK or QA,M modulation. At the transmitter end, the symbols are coded with a certain timing, which is to be found back at the receiver end, so that they can be decoded correctly It is an object of the invention to isolate a signal that notably permits to find back the timing used at the transmitter end.
This object is achieved with a method as claimed in claim 1 of the present patent application.
Thus, the invention advantageously makes use of the redundancy of the data contained in each block to isolate a block-frequency signal.
Preferably, said correlation step comprises a delay step of delaying the received signal by a delay that separates said identical information signals, and a subtraction step of generating said difference signal by subtraction of the received and the delayed signal.
These various aspects of the invention and also others will be apparent and elucidated on the basis of the embodiments to be described hereinafter.
The invention will be better understood with reference to the following Figures given by way of non-limiting examples which represent in:
Fig. 1: a time diagram indicating the generation of a difference signal at the block rate; and Fig. 2: a diagram of an isolation device of a difference signal e(t).
The following description has been developed in the particular case of an OFDM signal, but also applies to other signals formatted in. blocks for which each block contains redundant information signals.
The OFDM technique consists of frequency multiplexing various orthogonal carriers modulated by the symbols. An OFDM symbol may be written as:
s(t)-Re ~ezc~fot ~xk ~C~k ~t~~ (1) kL=. ' l0 for j.T'S < t < (j+1)T'S
PHF 93.503 DIV
frequencies the device uses, the frequencies being modulated by transmit symbols coming from a coded modulation, for example, a digital PSK or QA,M modulation. At the transmitter end, the symbols are coded with a certain timing, which is to be found back at the receiver end, so that they can be decoded correctly It is an object of the invention to isolate a signal that notably permits to find back the timing used at the transmitter end.
This object is achieved with a method as claimed in claim 1 of the present patent application.
Thus, the invention advantageously makes use of the redundancy of the data contained in each block to isolate a block-frequency signal.
Preferably, said correlation step comprises a delay step of delaying the received signal by a delay that separates said identical information signals, and a subtraction step of generating said difference signal by subtraction of the received and the delayed signal.
These various aspects of the invention and also others will be apparent and elucidated on the basis of the embodiments to be described hereinafter.
The invention will be better understood with reference to the following Figures given by way of non-limiting examples which represent in:
Fig. 1: a time diagram indicating the generation of a difference signal at the block rate; and Fig. 2: a diagram of an isolation device of a difference signal e(t).
The following description has been developed in the particular case of an OFDM signal, but also applies to other signals formatted in. blocks for which each block contains redundant information signals.
The OFDM technique consists of frequency multiplexing various orthogonal carriers modulated by the symbols. An OFDM symbol may be written as:
s(t)-Re ~ezc~fot ~xk ~C~k ~t~~ (1) kL=. ' l0 for j.T'S < t < (j+1)T'S
PHF 93.503 DIV
with ~k(t) = e2'~~s for j .T'S <_ k S (j+1 ) T'S
where:
T's: total duration of an OFDM symbol, T'S = TS+D
Re: real part of a complex number k: index of the orthogonal carriers TS: useful period of an OFDM symbol D: guard interval N: maximum number of carriers fo: random frequency j: index of the OFDM symbol.
Thus between the instants j.T'S and (j+1)T'S, an OFDM signal is formed by a block of complex symbols xk, where each symbol xk modulates an orthogonal earner 0_<k <_N-1.
To avoid the problem of spectrum overlap and to facilitate the filtering at the receiver end, the sum corresponding to the equation (1) is made of N" earners where hT" is the number of useful carriers (N" < N).
To realise the OFDM modulation, that is to s;ay, form the signal s(t) of equation (1), a modulator is used which performs a calculation of an inverse Fast Fourier Transform (FFT-1). 'Therefore, a number of the form 2", where x is an integer is selected for N. Other check blocks intended for the transmission are also inserted.
The selected parameters are, for example, the; following:
T'S = 160 ~.s; TS = 128 p,s and O = 32 ~s, N = 1024 carriers, N" = 900 carriers.
The main role of the guard interval D is to absorb the echoes coming from the multipath channel and having delays which axe lower than n. During the guard interval (which is preferably equal to a quarter of the useful period) a signal is transmitted which is identical with part of the useful period.
The selection of N" = 900 comes from the fact that with the band axound each carrier being 1/TS = 7.81 kHz, 900 carriers are necessaxy'to have an effective bandwidth of the transmitted signal of about 7 MHz (the exact bandwidth being 7.031 MHz) The blocks at the output of a channel coder are transmitted in frames. Thus a frame regroups a plurality of time-division multiplexed OFDM blocks. An OFDM
block may PHF 93.503 DIV
where:
T's: total duration of an OFDM symbol, T'S = TS+D
Re: real part of a complex number k: index of the orthogonal carriers TS: useful period of an OFDM symbol D: guard interval N: maximum number of carriers fo: random frequency j: index of the OFDM symbol.
Thus between the instants j.T'S and (j+1)T'S, an OFDM signal is formed by a block of complex symbols xk, where each symbol xk modulates an orthogonal earner 0_<k <_N-1.
To avoid the problem of spectrum overlap and to facilitate the filtering at the receiver end, the sum corresponding to the equation (1) is made of N" earners where hT" is the number of useful carriers (N" < N).
To realise the OFDM modulation, that is to s;ay, form the signal s(t) of equation (1), a modulator is used which performs a calculation of an inverse Fast Fourier Transform (FFT-1). 'Therefore, a number of the form 2", where x is an integer is selected for N. Other check blocks intended for the transmission are also inserted.
The selected parameters are, for example, the; following:
T'S = 160 ~.s; TS = 128 p,s and O = 32 ~s, N = 1024 carriers, N" = 900 carriers.
The main role of the guard interval D is to absorb the echoes coming from the multipath channel and having delays which axe lower than n. During the guard interval (which is preferably equal to a quarter of the useful period) a signal is transmitted which is identical with part of the useful period.
The selection of N" = 900 comes from the fact that with the band axound each carrier being 1/TS = 7.81 kHz, 900 carriers are necessaxy'to have an effective bandwidth of the transmitted signal of about 7 MHz (the exact bandwidth being 7.031 MHz) The blocks at the output of a channel coder are transmitted in frames. Thus a frame regroups a plurality of time-division multiplexed OFDM blocks. An OFDM
block may PHF 93.503 DIV
contain data or may be a block particularly used for the synchronization (frame, clock, carrier synchronization) or used as a reference block for the differential modulation.
An example of the frame structure is given by:
Zer AF Wo EXT EXT Data Data Dat o C b 1 2 1 2 a < >
Ts = 160 ps < >
Tf = 20 ms The frame contains 125 OFDM blocks and has a duration T f of 20 ms: , - The first block is the zero block during which nothing is transmitted (xk = 0, k = 0, N-1). It is used for synchronizing the beginning of the frame.
- The second block is the AFC (Automatic Frequency Control) block used for synchronizing the frequency of the local oscillator of the receiver with that of the transmitter.
The third block is the wobulation block defined by:
z xk =~e~~'K,~k =rc ~ +~t14 The wobulation block is used as a reference block for the differential coding and also for estimating the channel impulse response for a precise synchronization of the beginning of the frame.
- The fourth and fifth blocks are additional blocks which may be used for transmitting service data.
- Finally, there are the 120 OFDM data blocks. , A frame contains 100 codewords generated by the channel coder.
The invention advantageously uses the existence of a guard interval in each block of an OFDM frame. Fig. 1 represents two consecutive blocks B l and B2.
The following explication applies to all the blocks. The blocks Et1/B2 are formed by a guard i i!
PHF 93.503 DIV
interval having duration D followed by a useful interval having duration TS
which contains useful data. In a transmitted block the data which appear at the end of the useful interval TS
are copied at the beginning of the block before being transmitted through the channel. It will thus be noted that in each block an identical information sil;nal appears at the beginning and 5 at the end of the block. In Fig. 1 the received signal is the signal r(t).
This signal r(t) is delayed by delay means by a period of time equal to the information signal at the beginning of the delayed signal rd(t) being in phase with the information signal at the end of the undelayed signal r(t). For the same block B 1 these two information signals are identical. By subtracting these two signals, a signal e(t) is obtained, which is formed by an interval having duration A where e(t) is zero preceded by an interval having duration TS where it is non-zero.
In Fig. 1 the signal e(t) is represented in the form of a square-wave signal, but in reality this signal presents a more complex and more sinusoidal shape, having a fundamental frequency equal to the block frequency. It is thus possible to isolate thus block frequency by filtering.
Fig. 2 represents a diagram of a device 11 according to the invention which permits isolating the block frequency. The device 11 comprises associated filter means 14.
The signal r(t) is delayed in delay means 12 for the signal r,d(t) to be produced. The signals r(t) and rd(t) are subtracted from each other in subtracter means 13 and produce the difference signal e(t). This difference signal e(t) has previously been filtered in a bandpass filter 14 brought in line with the block frequency used at a transmitter end.
An example of the frame structure is given by:
Zer AF Wo EXT EXT Data Data Dat o C b 1 2 1 2 a < >
Ts = 160 ps < >
Tf = 20 ms The frame contains 125 OFDM blocks and has a duration T f of 20 ms: , - The first block is the zero block during which nothing is transmitted (xk = 0, k = 0, N-1). It is used for synchronizing the beginning of the frame.
- The second block is the AFC (Automatic Frequency Control) block used for synchronizing the frequency of the local oscillator of the receiver with that of the transmitter.
The third block is the wobulation block defined by:
z xk =~e~~'K,~k =rc ~ +~t14 The wobulation block is used as a reference block for the differential coding and also for estimating the channel impulse response for a precise synchronization of the beginning of the frame.
- The fourth and fifth blocks are additional blocks which may be used for transmitting service data.
- Finally, there are the 120 OFDM data blocks. , A frame contains 100 codewords generated by the channel coder.
The invention advantageously uses the existence of a guard interval in each block of an OFDM frame. Fig. 1 represents two consecutive blocks B l and B2.
The following explication applies to all the blocks. The blocks Et1/B2 are formed by a guard i i!
PHF 93.503 DIV
interval having duration D followed by a useful interval having duration TS
which contains useful data. In a transmitted block the data which appear at the end of the useful interval TS
are copied at the beginning of the block before being transmitted through the channel. It will thus be noted that in each block an identical information sil;nal appears at the beginning and 5 at the end of the block. In Fig. 1 the received signal is the signal r(t).
This signal r(t) is delayed by delay means by a period of time equal to the information signal at the beginning of the delayed signal rd(t) being in phase with the information signal at the end of the undelayed signal r(t). For the same block B 1 these two information signals are identical. By subtracting these two signals, a signal e(t) is obtained, which is formed by an interval having duration A where e(t) is zero preceded by an interval having duration TS where it is non-zero.
In Fig. 1 the signal e(t) is represented in the form of a square-wave signal, but in reality this signal presents a more complex and more sinusoidal shape, having a fundamental frequency equal to the block frequency. It is thus possible to isolate thus block frequency by filtering.
Fig. 2 represents a diagram of a device 11 according to the invention which permits isolating the block frequency. The device 11 comprises associated filter means 14.
The signal r(t) is delayed in delay means 12 for the signal r,d(t) to be produced. The signals r(t) and rd(t) are subtracted from each other in subtracter means 13 and produce the difference signal e(t). This difference signal e(t) has previously been filtered in a bandpass filter 14 brought in line with the block frequency used at a transmitter end.
Claims (4)
1. A method of isolating a frequency called block frequency on the basis of a received signal formatted in blocks, said blocks coming from symbols of a constellation and comprising each information signals, called identical information signals, formed by part of the useful information transmitted in the block and a copy of this part put in shifted position in said block before transmission, said method comprising a correlation step between said identical information signals to generate a signal called difference signal, whose frequency is representative of the block frequency.
2. A method as claimed in claim 1, characterized in that said correlation step comprises a delay step of delaying the received signal by a delay separating said identical information signals, and a subtraction step of generating said difference signal by subtraction of the received and the delayed signal.
3. A receiver comprising means for implementing a method as claimed in one of the claims 1 or 2.
4. A transmission system comprising a signal transmitter and receiver, said receiver comprising means for implementing a method as claimed in one of the claims 1 or 2.
Applications Claiming Priority (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| FR9300539 | 1993-01-20 | ||
| FR9300539 | 1993-01-20 | ||
| CA002113766A CA2113766A1 (en) | 1993-01-20 | 1994-01-19 | Transmission system comprising timing recovery |
Related Parent Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002113766A Division CA2113766A1 (en) | 1993-01-20 | 1994-01-19 | Transmission system comprising timing recovery |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2330522A1 true CA2330522A1 (en) | 1994-07-21 |
Family
ID=25676938
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002330522A Abandoned CA2330522A1 (en) | 1993-01-20 | 1994-01-19 | Method of isolating a block frequency on the basis of a signal formatted in blocks |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA2330522A1 (en) |
-
1994
- 1994-01-19 CA CA002330522A patent/CA2330522A1/en not_active Abandoned
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|---|---|---|---|
| EEER | Examination request | ||
| FZDE | Dead |