Note: Descriptions are shown in the official language in which they were submitted.
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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,
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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:
N-1
s(t)-Re ~ezc~fot ~xk ~C~k ~t~~ (1)
kL=. ' l0
for j.T'S < t < (j+1)T'S
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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
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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
120
< >
Ts = 160 ps
< >
Tf = 20 ms
TABLE 1
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!
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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.
