FR2737366A1 - Packet data transmission method for wideband network - using distinct and simultaneous subcarriers for data field to divide data into symbol groups for OFDM and perform modulation of subcarriers - Google Patents

Abstract

The method involves dividing the information for a field into successive groups of N symbols which are allocated to N sub-carriers by OFDM. The N sub-carriers are respectively modulated to obtain a modulating signal. A signal is generated at a second carrier frequency, varies linearly with time according to a ramp and is modulated by the modulating signal. On reception, the information field is mixed with a ramp signal similar to the second carrier frequency to obtain a signal corresponding to the modulating signal and to extract the information field.

Description

 The present invention relates to the transmission, in particular in broadband networks, of packet data, these packets comprising a header at a first carrier frequency followed by an information field at a second carrier frequency, the start of transmission of the field being linked to the start of transmission of the header of the corresponding packet.

 It is known to transmit data in packets each comprising a header followed by an information field, the information provided by the header allowing, among other things, the recognition and taking into account of the packet by its recipients. .

 It is known, having a given frequency band for transmitting packets in a network, to define different transmission channels in the band and to allocate them to the transmitters of the network, either in a predetermined manner or according to the needs. resignation; it turns out that such a method does not allow optimal use of the frequency band and is impractical especially when a receiver is likely to receive packets on any of the channels and must therefore monitor the transmission of packets on all channels.

 It is also known to assign, in the transmission frequency band of a network, a channel to the headers; the receivers only have to monitor this channel and the information provided by the headers makes it possible to receive the fields, the latter being transmitted in the rest of the useful band on a carrier frequency which can be constant or vary in a predetermined manner , for example by hopping, for the duration of the packet. It is within the framework of a transmission of the field on a carrier frequency which varies that the invention is situated. The object of the invention is to improve the transmission conditions, in particular with regard to spectrum spreading, bit rate, ease of operation.

 In this document we are talking about symbols. It is recalled that these are groupings of binary information called bits; these groupings can be expressed in different forms of modulation (amplitude, frequency, phase), each value of the grouping being represented by a state of the constellation in the complex plane. As an example, a grouping of m = 3 bits can be expressed through modulation at N = 2m = 23 = 8 phase states, each phase state being located on the unit circle in multiples of 7tu4. And, more particularly, in the case of a two-state modulation, the symbol corresponds to a bit.

By using, for the information field, a signal with carrier frequency which varies according to a linear ramp, this aim is obtained by a modulation of said signal not by a subcarrier but by several subcarriers, these subcarriers being , themselves, modulated by the data to be transmitted according to an orthogonal frequency division multiplexing technique, generally called OFDM technique after the acronym which, in the Anglo-Saxon literature, means Orthogonal Frequency
Multiplex Division. It should be noted that, in what follows, the OFDM technique covers both simple OFDM and coded OFDM also called technique
COFDM after the acronym which, in Anglo-Saxon literature, means
Coded Orthogonal Frequency Division Multiplex; it is recalled on this subject that, considering a binary train, in OFDM technique the symbols are transmitted by group of N symbols, with N integer greater than one, respectively on N subcarriers and for a time equal to the time to receive the N symbols. It is recalled that the COFDM technique is only a variant of the OFDM technique in the sense that in addition there is associated, in the COFDM technique, a coding function making it possible to obtain, from the N symbols of input, N outputs each composed of a weighting of the N input symbols.

According to the invention, a method of transmitting packet data is proposed, these packets comprising a header at a first carrier frequency followed by an information field at a second carrier frequency, the start of transmission of the field being linked to the start of transmission of the header of the corresponding packet, characterized in that it consists, for the transmission of the field, of using N, with N integer greater than 1, distinct and simultaneous subcarriers, with split the data to be transmitted in the field into successive groups of N symbols, to assign the N symbols respectively to the N subcarriers by multiplexing
OFDM and to modulate these N subcarriers respectively by these N symbols in order to obtain a modulating signal made of the N subcarriers thus modulated, to generate a signal at the second carrier frequency varying according to a linear ramp in time, to modulate the signal at the second carrier frequency by the modulating signal, and, on reception, mixing the signal corresponding to the information field with a ramped signal similar to the signal at the second carrier frequency in order to obtain a signal corresponding to the signal modulating and extracting data from the information field.

 According to the invention, a transmitter is proposed for implementing the method, characterized in that it includes means for developing groups of symbols representative of the data to be transmitted in a field, first means of modulation in OFDM technique. to develop N, with N integer greater than 1, subcarriers modulated by the groups in order to generate a modulating signal, a ramp generator for generating a carrier signal whose frequency varies according to a linear ramp in time and second means of modulation to receive the signals generated by the first modulation means and the ramp generator and perform modulation.

 According to the invention there is also proposed a receiver for implementing the method, characterized in that it comprises a mixer with a first input for receiving a signal transmitted by the method, a second input and an output, a generator for ramp to deliver, on the second input of the mixer, a signal whose frequency varies linearly over time and, at the output of the mixer, a frequency-time operator followed by a sub-carrier demodulation circuit.

The present invention will be better understood and other characteristics will appear from the following description and the corresponding figures which represent: - Figures 1 to 3, time diagrams relating to packets
transmitted according to the invention, - Figure 4, the diagram of a transmitter according to the invention, - Figure 5, the diagram of a receiver according to the invention.

 In the diagrams, the precise synchronization devices, which are part of current technology, have not been shown in order to make the drawings clearer and to simplify the description.

Figure 1 is a schematic diagram showing how, in the context of the invention, two successive data packets are transmitted.
Each packet has a header E1, E2 followed by an information field
C1, C2.

 The header is transmitted on a fixed carrier frequency Fe and occupies a transmission channel whose limits are two frequencies FO, F1.

The information field is transmitted simultaneously over four subcarriers which modulate a carrier Fc and this carrier has a ramp variation, that is to say that the value of its frequency is a linear function of time t; in FIG. 1, four ramps Fc + fo, Fc + 2fo,
Fc + 3fo, Fc + 4fo constitute a symbolic representation of each of the information fields C1 and C2.

 The information field thus occupies a frequency channel of constant width, which moves continuously in time between the value F1 and a value which, for the longest packets, is at most equal to F2, the band F0 -F2 being the frequency band allocated to the network in which the packets considered are exchanged.

 It should be noted in the various diagrams of this document the proportions between the various time intervals and between the various frequency intervals are given only by way of examples since they depend on the applications; moreover, the proportions are not respected between the different diagrams.

The transmission of data in the information field in the network is done in groups; the modulation used on each subcarrier can be of different types; two-state phase modulations known as DPSK, according to their acronym in the Anglo-Saxon literature which stands for Differential Phase-Shift Keying, are the simplest and most economical for the transmission of this type of packets; modulations with more than two phase states, in particular with 8 so-called phase states
D8PSK, allow higher data rates to be achieved, however at the cost of lower noise immunity. In the case, for example, of four subcarriers for the field, the groups are of four symbols which are reduced to four bits in the case of a modulation with two phase states.

These groups are transmitted in simple OFDM or coded OFDM technique, that is to say COFDM, depending on how the network is to be operated.

 Thus the overall throughput is shared between the subcarriers and can easily be increased or decreased by increasing or decreasing the number of subcarriers.

 As an example, a set of 16 subcarriers has been used on an instantaneous total frequency band of 8 MHz, each subcarrier making it possible to transmit approximately 250 kbaud / s, that is, in the case of QPSK modulation, a bit rate 500 kb / s per subcarrier and an overall speed of 8 Mb / s. Each symbol time therefore lasts 2ps. With a ramp which lasts approximately 500ps for a total band of approximately 250MHz, the frequency variation between the start and the end of a symbol time is then 1 MHz.

Figure 2 is intended to show how, due to its constitution, the information field is not very sensitive to fading, more commonly called fading. Indeed if a frequency band Ff1-
Ff2 is blocked by selective fading, a duration symbol ts2-tsl, emitted by modulation of a frequency ramp, is affected by this fading only at the time of the passage in the band Ff1-Ff2 that is to say say only during the time tp2-tpl and this time is inversely proportional to the slope of the ramp used. It thus appears that the problem of selective fading is reduced to a problem of temporal fading whose effects are then reduced by modulation in OFDM technique which makes it possible to lengthen the duration of the symbol time, at equivalent bit rate, compared to a conventional transmission. on single subcarrier.

 It should also be noted that, due to the distribution of the transmission energy not over a single subcarrier but over several and due to the linear ramp modulation of the carrier, the emission spectrum of the fields information is spread out and undergoes a translation corresponding to the modulation of the frequency ramp. This is illustrated by FIG. 3 which represents, at a given instant, the amplitude IA of the main lines corresponding to the subcarriers, as a function of the transmission frequencies; when the carrier frequency varies between its two extreme values, the spectrum, whose envelope is represented by an interrupted curved line, undergoes a sliding represented by a horizontal arrow in figure 3. The case represented in this figure is that of eight under carriers.

 FIG. 4 is the diagram of a transmitter for implementing the packet transmission method which has just been described with the aid of FIGS. 1 to 3. The example described relates to a transmission on eight subcarriers in technique COFDM or simple OFDM; on the diagram, a circuit 50, drawn in dashed lines, represents the coding function which makes it possible to pass from the OFDM technique to the COFDM technique.

 The information to be transmitted is applied to the input of a shift register 1, the eight outputs of which are connected to the input of a buffer register 2. This information consists of symbols delivered at the rate of 8fo which are grouped in successive groups of eight symbols each, thanks to the shift register-buffer register set; the duration of a group is therefore 8T = 8 / 8fo = 1 / fo.

 The content of register 2 is applied to the first terminals of an electronic switch 3 which, if it were feasible in mechanical version, would be a switch with eight wafers and two positions.

 In what follows it will first be considered that the switch 3 is directly connected to the eight inputs of a digital generator, 5, of eight modulated subcarriers, of respective frequencies fo, 2fo, 3fo, ..., 8fo ; these relationships between the frequency values are given by way of nonlimiting example. In the case of the example described, where an oversampling by a factor of 8 of the fastest subcarrier is carried out, the generator 5 comprises a trigonometric table sampled at the frequency 64.fo by angle differences of 27d64 for the frequency fo, of 2x.2 / 64 for the frequency 2fo, of 2x.3 / 64 for the frequency 3fo, ..., of 2toc.8 / 64 for the frequency 8fo. In the example described, each subcarrier is assigned a multiplicative coefficient +1 or -1, for a two-state phase modulation, depending on whether the bit corresponding to the subcarrier considered has the value 1 or 0. L 'sampling is done in sine and cosine so as to generate two quadrature components for each subcarrier. The eight components I and Q are transmitted, respectively to two accumulators 6a, 6b in which the eight components of the same time index are summed to provide, at the output of accumulator1 a binary number; in the example described this binary number is made of eight bits. The accumulators 6a, 6b are respectively connected to two digital-analog converters 7a, 7b, working at the rate of 64.fo and provided with low-pass output filters.

 An analog I and Q modulator 8 receives on two first inputs the signals of the converters 7a, 7b and on two second inputs the signals I and Q of a digital sawtooth generator with analog outputs. The modulator 8 thus receives on its first inputs two modulation signals of the form sin b, cos b and on its second inputs two signals carrying the form sin a, cos a; it performs the operation cos a.cos b - sin a.sin b and therefore outputs a signal of the form cos (a + b), that is to say a carrier modulated by eight subcarriers. The output signal of the modulator 8 is then amplified, in a linear amplification chain not shown, before being transmitted.

 The switch 3, according to FIG. 4, makes it possible to insert groups of test data between the groups of information data coming from register 2. These groups of test data have a configuration which is known to the receivers to which the packets of data are intended; they allow, in a conventional manner, the adjustment of the receivers in order to take account, in particular, of the impulse response generated by the various multipaths of the signal between a receiver and a transmitter.

In Figure 4 a rectangle has been drawn in broken lines; this rectangle represents a transformation matrix 50 which is inserted between the switch 3 and the subcarrier generator 5 when it is desired that the transmission is carried out not in simple OFDM technique as has been considered with the transmitter as described so far, but in technique
COFDM; the matrix 50 transforms the binary data of the input data group into a set of output signals with interdependent polynomials, with a view to reducing, in a conventional manner, transmission errors.

 In the above it has been considered that the symbols of the groups represent bits 0 or 1 but, of course, they can also represent values of modulation constellations such as phase, frequency or phase modulations, with N states ; and the symbols then represent complex values in amplitude and in phase, each symbol being able to be written in the form AkeiXk where Ak represents an amplitude, e the Euler number, j the imaginary unit, fk an angle and k denotes the symbol with 0 <k <Nl if N designates the number of modulation states.

 In the transmitter according to FIG. 4, each subcarrier is constructed temporally, sample after sample, taking into account the binary information to be transmitted; it is a direct generation of subcarriers. A classic alternative consists in carrying out the transmitter so as to work out the theoretical spectrum with the modulated subcarriers and to carry out an inverse discrete Fourier transform, like the Inverse Fast Fourier Transform or FFT-1 of the Anglo-Saxon literature so generating time signals each of which is valid for the duration of a group of N symbols.

 FIG. 5 is the diagram of a receiver for receiving packets transmitted by the transmitter according to FIG. 4.

 In what follows, for all that is conventional in the transmission of packets consisting of a header and an information field, in the technique of echo discrimination and in that of correction of transmission errors , the explanations will be given without encumbering details of implementation within the reach of those skilled in the art in order to better identify what, in the receiver, is specific to the invention.

 The signal received by the receiver according to FIG. 5 is continuously analyzed by a synchronization circuit 10. The circuit 10 includes a clock which is synchronized with the headers of the packets; for this, when a first correlation peak exceeds the detection threshold, the analysis of the following peaks, during a given time interval, preferably a symbol time, makes it possible to refine the synchronization of the clock and thus to give a precise time reference for the reception of the information field. When this synchronization is acquired the received signal is mixed, in a mixer 11, with the output signal of a ramp generator 12 which provides, on a single output, an analog signal with the same slope and the same frequencies as the output signals from the slope generator 9 of the transmitter according to FIG. 4; the ramp generator 12 is triggered in coherence with the synchronization signal obtained in the synchronization circuit 10. The mixer 11 includes an output filter to eliminate the terms "sum frequency".

A time-frequency operator 13 receives the signals from the mixer II; in the example described, this is an operator of the transformed type of
Fourier, like the Fast Fourier Transform or FFT of Anglo Saxon literature. The operator 13, also consistent with the synchronization tops developed by the synchronization circuit 10, provides, in the example described, a set of 64 analysis signals making it possible to extract the information carried by the eight subcarriers of the information field signal. These 64 analysis signals are supplied either to a circuit 15 for calculating and controlling impulse response correction and frequency deviations when the data transmitted are test data, or to a circuit 14 for demodulating the subcarriers when the transmitted data is information data. Circuit 15 provides impulse response correction signals to circuit 14 and frequency offset correction signals to ramp generator 12
The following three paragraphs concern remarks as to the faults likely to affect the transmission of data in a network implementing the transmission method which has just been described with, by way of example, a transmitter according to FIG. 4 and a receiver according to FIG. 5.

 Multipaths between a transmitter and a receiver, insofar as the differences between their respective durations are short compared to the duration of a symbol, are not troublesome by the very fact of the use of the OFDM technique. It is however possible to make an estimate of these multiple paths in order either to choose, between the signals corresponding to the different paths, the one which has the most energy, or to combine all or part of the signals relating to the multiple paths in order to increase the energy of the demodulated signal and thus improve the robustness of the process. This involves an estimation of the impulse response of the channel, the components being used according to their amplitude and their phase to find the original signal; strictly this estimate must be repeated throughout the ramp since the value of the carrier frequency changes continuously but in practice it is sufficient to make this estimate on a few regularly spaced portions of the frequency ramp. To make this estimate it is possible, for example, to use test groups made of 1 on all the sub-carriers and to determine, by a frequency analysis of the output signal of the mixer 11, the phase and the amplitude of each of the components of the spectrum, for each subcarrier.

 It is also possible to estimate and correct the frequency difference between the transmitter and the receiver; this frequency difference is due to the drifts existing between the local oscillators of the transmitter and the receiver, to the differences brought by the Doppler effect, to the residual frequencies generated by an imperfect synchronization on the header; each of the subcarriers is affected by the same frequency drift. To determine this drift in order to be able to take it into account, it is possible, for example, to emit at the start of the ramp or regularly in the ramp, a group of known symbols such as a series of 1 in order to obtain a set of sub -pure carriers which facilitates the detection of frequency drift using a spectrum analyzer.

 The invention is not limited to the description which has just been made and extends more generally to all the variants within the reach of those skilled in the art, in particular as regards the circuits for the implementation of the method, the frequencies used, the number of subcarriers of the information field; it is also understood that the header can also be transmitted using not just one but several subcarriers and that the values of the frequencies of these subcarriers may not be regularly distributed.

Claims (5)

 1. Method for transmitting data in packets, these packets comprising a header at a first carrier frequency followed by an information field at a second carrier frequency, the start of transmission of the field being linked to the start of transmission the header of the corresponding packet, characterized in that it consists, for the emission of the field, of using N, with N integer greater than 1, distinct and simultaneous subcarriers, splitting the data to be transmitted in the field into successive groups of N symbols, assigning the N symbols respectively to the N subcarriers by OFDM multiplexing and modulating these N sub- carriers respectively by these N symbols in order to obtain a modulating signal made of the N subcarriers thus modulated, to generate a signal at the second carrier frequency varying according to a linear ramp in time, to modulate the signal at the second carrier frequency by the signal modulating, and, on reception, mixing the signal corresponding to the information field with a ramped signal similar to the signal at the second carrier frequency in order to obtain a signal corresponding to the modulating signal and to extract the data from the field of information.  2. Method according to claim 1, characterized in that it consists in effecting the modulation by direct generation of the subcarriers by constructing, from a table of trigonometric lines whose values are multiplied by the data of the group of symbols , representative samples of the N subcarriers modulated by the group data, summing these samples with the same time index and converting the results of the summation of the samples from digital to analog in order to obtain the modulating signal.  3. Transmitter for the implementation of the method according to claim 1, characterized in that it comprises means for generating groups of symbols (1, 2) representative of the data to be transmitted in a field, first modulation means in OFDM technique (5, 6a, 6b, 7a, 7b) to develop N, with N integer greater than 1, subcarriers modulated by the groups in order to generate a modulating signal, a ramp generator (9) for generating a carrier signal whose the frequency varies according to a linear ramp in time and second modulation means (8) to receive the signals generated by the first modulation means and the ramp generator and to carry out a modulation.  4. Transmitter according to claim 3, for the implementation of the method according to claims 1 and 2, characterized in that the first modulation means comprise in series a digital generator of modulated subcarriers (5), summation means (6a, 6b) and means for converting digital to analog (7a, 7b).  5. Receiver for implementing the method according to claim 1 or 2, characterized in that it comprises a mixer (11) with a first input for receiving a signal transmitted by the method, a second input and an output, a ramp generator (12) for delivering, on the second input of the mixer, a signal whose frequency varies linearly over time and, at the output of the mixer, a frequency-time operator (13) followed by a demodulation circuit of subcarriers (14).