Method and device in a communication system

The present invention concerns a method and a device for the synchronization of a transmitter (1) and at least one receiver (3) in multi-carrier modulated communication systems in which FFT technology is used for the modulation and demodulation of data transmitted between the transmitter (1) and the receiver (3). According to the invention the transmitter (1) transmits synchronization symbols (15, 16) as training symbols (15, 16) at the beginning of a transmission, until a result is obtained that may indicate where a synchronization symbol starts. The result is used for the adjustment of the symbol rate in the receiver (3). Data symbols may then be transmitted after the synchronization symbols (15, 16).

TECHNICAL FIELD
 The present invention relates to a method and a device for recovery of
 sampling rate and symbol rate in multi-carrier modulated communication
 systems, preferably using copper wires as a transmission medium.
 STATE OF THE ART
 Multi-carrier modulation is a known method for transmitting broadband
 information over copper wire or radio connections. The information may be,
 for example, video, Internet or telephony. Very briefly explained, for
 example, the bits of a digitally encoded video signal that are to be
 transmitted, are encoded as complex numbers in a transmitter, before an
 Inverse Fast Fourier Transform (IFFT) is carried out.
 The IFFT gives, in the modulation, a sum of orthogonal carriers or tones,
 the amplitudes and phase displacement of which are determined by the
 values and phases of the complex numbers. These carriers are then
 transmitted in time slots at constant time intervals and are called
 symbols. In a receiver a Fast Fourier Transform (FFT) is carried out
 instead. In this way, the original bits are retrieved. Attenuation and
 phase displacement may be easily compensated for, by multiplication by a
 complex number for each carrier.
 Two similar methods in the above mentioned technology are Orthogonal
 Frequency Division Multiplex (OFDM), used in radio applications, and
 Discrete Multitone (DMT), which is used in copper wires.
 In both cases the receiver must be able to adjust the correct sampling rate
 and to determine the beginning and the end of the transmitted symbols.
 In WO 95/03656 OFDM is used. To adjust the symbol rate, a transmitter
 transmits synchronization frames at known intervals, that is,
 synchronization symbols having a pseudo random sequence of known
 frequencies and phase displacements, and also known time intervals in
 special time slots. The receiver carries out a number of FFT calculations
 over the position in time in which the synchronization frame is presumed
 to be found. For each FFT calculation a cross correlation calculation is
 made in the frequency plane, using the known frequency function of the
 synchronization frame. The correlation maximum is detected, which
 determines the time slot containing the synchronization frame.
 SUMMARY OF THE INVENTION
 The problem associated with transmitting synchronization symbols at known
 intervals is that it takes up time in which data could have been
 transmitted. Also, a complex procedure of cross correlation calculations
 is required to detect and analyze the synchronization symbols.
 The object of the present invention is to solve the above problem by
 transmitting training symbols before the start of a data transmission.
 Each training symbol comprises at least a period of a pilot tone and is
 transmitted using 180.degree. phase jumps between the symbols. The use of
 this simple training symbol makes it easy to detect the beginning and the
 end of the symbol. An FFT calculated over the length of a symbol gives the
 value zero at a maximally erroneous position, that is, with the phase jump
 in the middle of the calculation, and a maximum/minimum at the ideal
 position, that is, half way between two phase jumps. The simplest method
 is probably to look for the position in which the result of the FFT
 calculation is zero and then move a distance of half a symbol.
 An advantage of the present invention is that the symbol rate may be
 restored in a fast and simple way even before the beginning of a data
 transmission. During the transmission it may then be sufficient to use a
 method known in the art for retrieving the sampling rate, because if
 something locks the sampling rate, the symbol rate is automatically kept
 constant. Another advantage is that the inventive method is simple and
 inexpensive to implement.
 The invention will be described in more detail in the following, by means
 of preferred embodiments and with reference to the enclosed drawings.

PREFERRED EMBODIMENTS
 Multi-Carrier Modulation
 FIG. 1 shows, schematically, how the main parts of a prior art system for
 multi-carrier modulation may look. In a transmitter 1 modulation of data
 bits, for example, from a digitally encoded video signal, is performed.
 The bits to be transmitted are encoded in the transmitter 1 as N complex
 numbers before a hermit symmetry operation is carried out in a calculation
 block 4. 2N complex numbers are obtained having a symmetric real part and
 an asymmetric imaginary part.
 An Inverse Fast Fourier Transform (IFFT) is then performed in an IFFT
 calculation unit 5, as a modulation. As the imaginary part becomes zero,
 it may be eliminated, and a real signal remains, which passes a parallel
 to serial converter 6 and a digital to analogue converter 7.
 This gives a sum of orthogonal carriers or tones, the amplitudes and phases
 of which are determined by the values and phases of the original complex
 numbers. These carriers are then transmitted on a channel 2 at constant
 time intervals/time slots and are called symbols.
 In a receiver 3 the data, in the opposite way, passes an analogue to
 digital converter 8, a serial to parallel converter 9 and an FFT
 calculation unit 10, in which an FFT is carried out, as a demodulation.
 This gives 2N complex numbers. For symmetry reasons, for example, the
 upper half of the 2N complex numbers may be discarded, leaving a number N
 of complex numbers.
 Finally, an equalizer 11 is used, which compensates for attenuation and
 phase displacement by multiplying the different numbers with complex
 numbers so that finally the same data bits are obtained that were
 transmitted to begin with.
 For each new symbol a discontinuity occurs in the carriers. To minimize the
 effects of this a so called cyclic prefix (not shown in the figure) may be
 used. This means copying the last part of the symbol and transmitting it
 just before the start of the symbol. In this way there is time for the
 effect of the discontinuity to fade out before the actual symbol starts.
 Symbol Rate
 In order to synchronize the transmitter 1 and the receiver 3, according to
 the invention first the sampling rate is adjusted so that the transmitter
 1 and the receiver 3 sample at approximately the same times and so that
 the first sample taken is approximately zero. This will be described in
 more detail later.
 Then training symbols are transmitted so that the receiver 3 will know
 where data symbols transmitted later will begin and end. It is appropriate
 to transmit the training symbols only at the beginning of the
 transmission. During transmission it may then be sufficient to use some
 method known in the art for restoration of the sampling rate, since if the
 sampling rate is locked the symbol rate is automatically maintained.
 When both the sampling rate and the symbol rate have been restored, the
 transmitter 1 and the receiver 3 are synchronized and the data
 transmission may begin.
 FIG. 2a shows a training symbol 15 which, according to the invention, is
 used to restore the symbol rate. The training symbol 15 comprises a number
 of periods of a pilot tone or carrier, in this case, for the sake of
 illustration, six periods. This training symbol 15 is transmitted with a
 180.degree. phase jump for each new symbol so that every other symbol 16
 is inverted.
 To detect the symbol position a series of time shifted FFT calculations
 13a, 13b, 13c are carried out during a time interval of the same length as
 a training symbol according to FIG. 2a or in a similar way. The result of
 the FFT calculations 13a, 13b, 13c will then vary approximately as shown
 in FIG. 2b. The maximum and the minimum, respectively, of the result in
 FIG. 2b is achieved when the FFT calculation 14 in FIG. 2a is carried out
 exactly on a symbol or an inverted symbol, respectively, that is, in the
 desired position.
 The maximum or minimum may, however, be difficult to detect. It is
 considerably less difficult to detect when the FFT calculation 13c is
 totally wrong, as the result then becomes zero. The most appropriate
 solution may therefore be to time shift the FFT calculations 13a, 13b, 13c
 until a value relatively close to zero is calculated and then indicate the
 start of a symbol half a symbol away from this.
 Note that if a cyclic prefix is used, this must be accounted for. Depending
 on the direction in which it is desired to move to find the start of the
 symbol, either the distance moved should be half a symbol, as usual, or
 half a symbol plus the length of the cyclic prefix.
 An example of the implementation of the embodiment for looking for the
 point where the FFT calculation becomes zero, is shown, schematically, in
 FIG. 3. The data sampled in the receiver is successively shifted into a
 shift register 21 or a similar set of memory units. From there, at
 different time intervals as shown below, parallel data corresponding to
 the length of a symbol is read to a calculation unit 22 in which an FFT
 calculation of the parallel data, for example 1024 points, is carried out.
 The result of the FFT calculation is then placed in a register 23, from
 which data corresponding to the frequency of the phase jumping pilot tone
 may be retrieved in one of the memory positions 23a.
 This data is forwarded to a calculation block 24, in which the imaginary
 component of the frequency of the pilot tone is preferably obtained for
 future adjustment to zero. During every other symbol the sign of the
 imaginary component is changed, or only every other symbol is calculated.
 This is done because every other symbol is inverted.
 The imaginary component adjusted in this way is compared to a threshold
 value. If the value of the adjusted imaginary component is smaller than or
 equal to the threshold value, the calculation block 24 emits a check value
 k equal to zero as the phase jump is then located approximately in the
 middle of the data on which the FFT calculation was carried out
 If the adjusted imaginary part is greater than the threshold value, the
 calculation block 24 emits a check value, which may suitably be equal to
 the number of samples in a period of the pilot tone, for example, four.
 If, on the other hand the adjusted imaginary part is smaller than the
 negative threshold value, the calculation block 24 in a corresponding way
 emits a check value k, which is, in this case minus four.
 The check value k is forwarded to a counter 25 which controls when the
 calculation unit 22 is to perform a new FFT calculation. If the number of
 samples is 1024, then the counter 25 counts down from 1023+k to zero,
 where k is the check value. This causes the successive shifting of the
 start position of the FFT calculation until the phase jump is located
 approximately in the middle of the samples that were subjected to the FFT
 calculation. This may be compared to the successive FFT calculations 13a,
 13b and 13c in FIG. 2a where each symbol, for clarity, only comprises
 6*4=24 samples.
 The symbol start needs then only be moved half a symbol to find the optimal
 position for the reading of the data to be transmitted.
 Sampling Rate
 Before the symbol rate is adjusted the sampling rate should be adjusted.
 The simplest way to do this is to use one of the carriers as a pilot tone,
 that is, transmitting a constant tone all the time, while the receiver
 locks to this tone.
 FIGS. 4a-4c show an example in which the transmitter transmits a pilot tone
 with four samples 21a, 21b, 21c, 21d for each period and where the
 receiver, in the same way, reads the pilot tone with four samples 22a,
 22b, 22c, 22d for each period.
 In order to synchronize the transmitter and the receiver, for every four
 samples one, for example sample 22a, is taken out as a first sample in the
 receiver. The receiver then tries to adjust the sampling of the first
 sample 22a so that it takes place the first time the pilot tone passes
 zero. If the first sample 22a is positive, the sampling is 20 shifted, so
 that the first sample 22a is taken a little earlier next time, see FIG.
 4a. If on the other hand, the sample 22a is negative, the sampling is
 shifted so that the sample 22a is taken a little later next time, see FIG.
 4b.
 The final, desired, result is shown in FIG. 4c, in which the samples 21a,
 21b, 21c, 21d and 22a, 22b, 22c, 22d of the transmitter and the receiver,
 respectively, are transmitted and received at approximately the same time.
 Instead of the pilot tone a phase jumping pilot tone is then transmitted as
 training symbols as described above. In order for the adjustment of the
 receiver not to change directions for every other symbol, the receiver
 instead locks to the phase jumping pilot tone. The simplest way to achieve
 this is probably by adjusting the sign of the first sample 22a with the
 sign of a second sample 22b before or after the first sample 22a, for
 example, at a distance of a quarter of a period. The second sample 22b
 will change signs for every other symbol, which will give an indication as
 to whether the pilot tone being sampled is inverted or non-inverted.
 FIG. 5 shows, schematically, how the above may be achieved in practice. In
 the receiver there is a voltage controlled oscillator (VCO) 31 controlling
 an analogue to digital converter 35 to take a first sample 22a that is
 delayed in a delay circuit 32, and a second sample 22b. The sign of the
 second sample 22b is identified and "multiplied" with the first sample 22a
 in a sign correction unit 33. Of course, no real multiplication is needed;
 only the sign is changed, if necessary.
 The sign correction unit 33 emits a control signal Q which passes a digital
 to analogue converter 34 and then controls the sampling through the
 oscillator 31 in the way described above.
 Of course it is possible to instead delay the second sample 22. Then it is
 necessary to "multiply", in a corresponding way, the first sample 22a by
 the reverse sign of the second sample 22b.
 During Data Transmission
 The above method functions as a "training" before the transmission of data.
 Of course it would be possible to interrupt the data transmission from
 time to time at known intervals to transmit training symbols again, but it
 would probably be better to use a known method for the retrieval of
 sampling rate, for example in the frequency domain, at least if the data
 is to be transmitted over copper wires. If the sampling rate is locked the
 symbol rate will be maintained automatically. The time may then be used to
 transmit data symbols instead of training symbols.
 These frequency domain techniques are usually slower than the method
 described above, but when the data transmission begins, approximately the
 correct sampling rate and symbol rate have already been adjusted, so that
 no major adjustments will be needed.