Device and method for communications signalling through a fragmented spectrum

Abstract

The invention relates to a device for transmission of data on a frequency spectrum divided into a plurality N.sub.f of spectrum fragments (f.sub.1, f.sub.2) each of which covers a frequency band, the frequency bands being discontiguous. The device comprises a packet generator configured to generate a data packet comprising a payload and at least one occurrence of a constant envelope signalling sequence. Said sequence, for example a modified Zadoff-Chu sequence, comprises N complex symbols and consists of a plurality of complex symbol sets each associated with one of the spectrum fragments. Each set comprises N/Nf complex symbols and each complex symbol of a set comprises a scaling term to the frequency band covered by the spectrum fragment associated with this set and a spectral transposition term in the frequency band covered by the spectrum fragment associated with this set.

Claims

1. A device for transmission of data on a frequency spectrum divided into a plurality N.sub.f of spectrum fragments each covering a frequency band, the frequency bands being discontiguous, comprising a packet generator configured to generate a data packet comprising a payload and at least one occurrence of a constant envelope signalling sequence, said sequence comprises N complex symbols and consists of a plurality of complex symbol sets each associated with one of the spectrum fragments, each set comprising N/Nf complex symbols and each complex symbol of a set comprises a spectral transposition term that distribute the complex symbol in the frequency band covered by the spectrum fragment associated with this set and a scaling term that distribute uniformly the complex symbols of the set in the frequency band covered by the spectrum fragment associated with this set, wherein each symbol S[n] of the set associated with the i-th spectrum fragment corresponds to Sca(Δf.sub.i)*TF(f.sub.li) wherein Sca(Δf.sub.i) is the scaling term and TF(f.sub.li) is the spectral transposition term, wherein $\frac{\mathrm{iN}}{N_f}\le n<\frac{\left(i+1\right)N}{N_f}$ and wherein the i-th spectrum fragment covers a frequency band that is delimited by a low frequency f.sub.li, a high frequency f.sub.hii and has a width of Δf.sub.i=f.sub.li−f.sub.hi.

2. The device according to claim 1, further comprising a modulator configured to implement a Turbo-FSK (frequency shift keying) modulation.

3. The device according to claim 1, wherein $\mathrm{Sca}\left(\Delta \mathrm{fi}\right)=\exp \left(\frac{j2{\pi \left(n-\frac{\mathrm{iN}}{\mathrm{Nf}}\right)}^2\Delta f_i}{{\mathrm{Nf}}_{\mathrm{ech}}}\right)$ and TF(f.sub.li)=exp(j2πν.sub.lin), in which f.sub.ech is a sampling frequency of the transmitted data and ν.sub.li is a reduced frequency defined by f.sub.li/f.sub.ech.

4. The device according claim 1, wherein the data packet generated by the packet generator comprises a preamble and the packet generator is configured to insert the at least one occurrence of said sequence as a synchronisation sequence in the preamble.

5. The device according to claim 4, wherein the preamble carries a plurality of occurrences of said sequence, in succession.

6. The device according to claim 1, wherein the packet generator is configured to insert the at least one occurrence of said sequence in the payload as a pilot sequence.

7. A method for reception of data from a frequency spectrum divided into a plurality N.sub.f of spectrum fragments each covering a frequency band, the frequency bands being discontiguous, comprising a synchronisation step and a channel estimation step, and wherein at least one of said steps uses a known constant envelope signalling sequence, said sequence comprises N complex symbols and consists of a plurality of complex symbol sets each associated with one of the spectrum fragments, each set comprising N/Nf complex symbols and each complex symbol of a set comprises a spectral transposition term that distribute the complex symbol in the frequency band covered by the spectrum fragment associated with this set and a scaling term that distribute uniformly the complex symbols of the set in the frequency band covered by the spectrum fragment associated with this set, wherein each symbol S[n] of the set associated with the i-th spectrum fragment corresponds to Sca(Δf.sub.i)*TF(f.sub.li) wherein Sca(Δf.sub.i) is the scaling term and TF(f.sub.li) is the spectral transposition term, wherein $\frac{iN}{N_f}\le n<\frac{\left(i+1\right)N}{N_f}$ and wherein the i-th spectrum fragment covers a frequency band that is delimited by a low frequency f.sub.li, a high frequency f.sub.hii and has a width of Δf.sub.i=f.sub.li−f.sub.hi.

8. A device for reception of data from a frequency spectrum divided into a plurality N.sub.f of spectrum fragments each covering a frequency band, the frequency bands being discontiguous, said device comprising a synchronisation unit and a channel estimation unit and wherein at least one of said units is configured to exploit a constant envelope signalling sequence known to said device, said sequence comprises N complex symbols and consists of a plurality of complex symbol sets each associated with one of the spectrum fragments, each set comprising N/Nf complex symbols and each complex symbol of a set comprises a spectral transposition term that distribute the complex symbol in the frequency band covered by the spectrum fragment associated with this set and a scaling term that distribute uniformly the complex symbols of the set in the frequency band covered by the spectrum fragment associated with this set, wherein each symbol S[n] of the set associated with the i-th spectrum fragment corresponds to Sca(Δf.sub.i)*TF(f.sub.li) wherein Sca(Δf.sub.i) is the scaling term and TF(f.sub.li) is the spectral transposition term, wherein $\frac{iN}{N_f}\le n<\frac{\left(i+1\right)N}{N_f}$ and wherein the i-th spectrum fragment covers a frequency band that is delimited by a low frequency f.sub.li, a high frequency f.sub.hii and has a width of Δf.sub.i=f.sub.li−f.sub.hi.

9. A method for transmission of data on a frequency spectrum divided into a plurality N.sub.f of spectrum fragments each covering a frequency band, the frequency bands being discontiguous, wherein said method comprises generation of a data packet comprising a payload and at least one occurrence of a constant envelope signalling sequence, said sequence comprises N complex symbols and consists of a plurality of complex symbol sets each associated with one of the spectrum fragments, each set comprising N/Nf complex symbols and each complex of a set comprises a spectral transposition term that distribute the complex symbol in the frequency band covered by the spectrum fragment associated with this set and a scaling term that distribute uniformly the complex symbols of the set in the frequency band covered by the spectrum fragment associated with this set, wherein each symbol S[n] of the set associated with the i-th spectrum fragment corresponds to Sca(Δf.sub.i)*TF(f.sub.li) wherein Sca(Δf.sub.i) is the scaling term and TF(f.sub.li) is the spectral transposition term, wherein $\frac{iN}{N_f}\le n<\frac{\left(i+1\right)N}{N_f}$ and wherein the i-th spectrum fragment covers a frequency band that is delimited by a low frequency f.sub.li, a high frequency f.sub.hii and has a width of Δf.sub.i=f.sub.li−f.sub.hi.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other aspects, purposes, advantages and characteristics of the invention will be better understood after reading the detailed description given below of preferred embodiments of the invention, given as non-limitative examples, with reference to the appended drawings on which:

(2) FIG. 1 is a diagram of a fragmented spectrum;

(3) FIG. 2 represents a data frame carrying signalling sequences;

(4) FIG. 3 is a diagram of the frequential spectrum of a signalling sequence according to the invention adapted to a fragmented spectrum comprising two discontiguous bands;

(5) FIG. 4 represents the auto-correlation function of a signalling sequence according to the invention adapted to a fragmented spectrum comprising two discontiguous bands and the auto-correlation function of a signalling sequence disclosed in the above mentioned paper by F. Dehmas et al, for comparison purposes;

(6) FIG. 5 is a diagram of the frequential spectrum of a signalling sequence according to the invention adapted to a fragmented spectrum comprising four discontiguous bands;

(7) FIG. 6 represents the auto-correlation function of a signalling sequence according to the invention adapted to a fragmented spectrum comprising four discontiguous bands and the auto-correlation function of a signalling sequence disclosed in the above mentioned paper by F. Dehmas et al, for comparison purposes;

(8) FIG. 7 is a diagram of a unit of a data transmission device according to the invention configured to use a Turbo-FSK modulation;

(9) FIG. 8 is a diagram of a unit of a data reception device configured to use a Turbo-FSK demodulation;

(10) FIG. 9 compares the packet error rate as a function of the signal to noise ratio of the Turbo-FSK modulation depending on whether the spectrum is or is not fragmented;

(11) FIG. 10 compares the spectral shape of the Turbo-FSK modulation with that of a signalling sequence according to the invention adapted to a fragmented spectrum comprising two discontiguous bands;

DETAILED DESCRIPTION

(12) The invention relates to a device for transmission of data on a fragmented spectrum, namely a frequency spectrum divided into a plurality N.sub.f of spectrum fragments each of which covers a frequency band, the frequency bands being discontiguous. FIG. 1 represents the case of a fragmented spectrum with two spectrum fragments f.sub.1, f.sub.2 covering the frequency bands delimited in frequency by f.sub.l1, f.sub.h1 and f.sub.l2, f.sub.h2 respectively, where f.sub.l2>f.sub.h1.

(13) The device according to the invention comprises a packet generator configured to generate a data packet comprising a payload and at least one occurrence of a signalling sequence. As illustrated on FIG. 2, a packet thus generated is composed of a preamble Pe and pilots Pi uniformly distributed in the data D (i.e. the payload). The preamble Pe is used for synchronisation (both temporal and frequential) and the pilots are used for channel estimation. The preamble Pe and/or each of the pilots Pi comprise(s) at least one occurrence of the signalling sequence. In one embodiment, the preamble Pe comprises several occurrences of the signalling sequence, for example successive occurrences of the signalling sequence.

(14) The packet generator of the device according to the invention is configured more particularly to generate a packet for which the at least one signalling sequence S is a constant envelope sequence. This constant envelope sequence S comprises N complex symbols, each denoted S[n] when n is an integer between 0 and N−1. The N complex symbols are distributed into a plurality N.sub.f of sets of complex symbols, each of these sets being associated with one of the spectrum fragments and comprising N/N.sub.f complex symbols.

(15) For each set of complex symbols, the constant envelope sequence S comprises more particularly a scaling term to the frequency band covered by the spectrum fragment associated with this set and a spectral transposition term in the frequency band covered by the spectrum fragment associated with this set.

(16) Consider a fragmented spectrum composed of N.sub.f spectrum fragments, in which each fragment n°i, in which 0≤i≤N.sub.f−1, covers a frequency band delimited by a low frequency f.sub.li and by a high frequency f.sub.hi and has a width Δf.sub.i=f.sub.li−f.sub.hi. The packet generator associates a set of N/N.sub.f symbols with the spectrum fragment n°i of the fragmented spectrum, in which each symbol comprises a scaling term to the frequency band Δf.sub.i covered by the corresponding spectrum fragment and a spectral transposition term in the frequency band [f.sub.li; f.sub.hi] covered by the corresponding spectrum fragment.

(17) In one possible embodiment, for

(18) $\frac{\mathrm{iN}}{N_f}\le n<\frac{\left(i+1\right)N}{N_f},$
a set of complex symbols is defined associated with fragment n°i and for which S[n]=Sca(Δf.sub.i)*TF(f.sub.li) in which Sca(Δf.sub.i) is a scaling term to the frequency band Δf.sub.i that makes it possible to distribute symbols uniformly on this band and TF(f.sub.li) is a spectral transposition term in the frequency band [f.sub.li; f.sub.hi] which makes it possible for symbols to be distributed in this band.

(19) Starting from a CAZAC sequence, for example a Zadoff-Chu sequence or a Bjorck sequence, the Sca(Δf.sub.i) term makes it possible, by a phase interpolation, to obtain a modified CAZAC sequence that occupies the right bandwidth and that has a constant amplitude. The term TF(f.sub.li) translates this modified CAZAC sequence into the right band.

(20) To guarantee a constant envelope, we could for example choose

(21) $\mathrm{Sca}\left(\Delta \mathrm{fi}\right)=\exp \left(\frac{j2{\pi \left(n-\frac{\mathrm{iN}}{\mathrm{Nf}}\right)}^2\Delta f_i}{{\mathrm{Nf}}_{\mathrm{ech}}}\right)$
and TF(f.sub.li)=exp(j2πν.sub.lin), in which f.sub.ech is the sampling frequency of the data to be transmitted and ν.sub.li is the reduced frequency f.sub.li/f.sub.ech. In this example, the signalling sequence S is a modified Zadoff-Chu sequence that is expressed as follows:

(22) $S\left[n\right]=\{\begin{array}{cc}\exp \left(\frac{j2\pi n^2\Delta f_0}{{\mathrm{Nf}}_{\mathrm{ech}}}\right)\times \exp \left(j2\pi v_{l0}n\right)& \mathrm{for}n<\frac{N}{N_f}\\ \mathrm{.Math.}& \mathrm{.Math.}\\ \exp \left(\frac{j2{\pi \left(n-\frac{\mathrm{iN}}{N_f}\right)}^2\Delta f_i}{{\mathrm{Nf}}_{\mathrm{ech}}}\right)\times \exp \left(j2\pi v_{\mathrm{li}}n\right)& \mathrm{for}\le n<\frac{\left(i+1\right)N}{N_f}\\ \mathrm{.Math.}& \mathrm{.Math.}\\ \exp \left(\frac{j2{\pi \left(n-\left(N_f-1\right)\times \frac{N}{N_f}\right)}^2\Delta f_{N_f-1}}{{\mathrm{Nf}}_{\mathrm{ech}}}\right)\times \exp \left(j2\pi v_{{\mathrm{lN}}_f-1}n\right)& \mathrm{for}n\ge \left(N_f-1\right)\times \frac{N}{N_f}\end{array}$

(23) Taking the example in FIG. 1 with a fragmented spectrum composed of two discontiguous bands, in which

(24) $v_{l1}=\frac{f_{l1}}{f_{\mathrm{ech}}},v_{l2}=\frac{f_{l2}}{f_{\mathrm{ech}}}$
are the reduced frequencies, the signalling sequence is expressed as:

(25) $S\left[n\right]=\{\begin{array}{cc}\exp \left(\frac{j2\pi n^2\Delta f_1}{{\mathrm{Nf}}_{\mathrm{ech}}}\right)\times \exp \left(j2\pi v_{l1}n\right)& \mathrm{for}n<\frac{N}{2}\\ \exp \left(\frac{j2{\pi \left(n-\frac{N}{2}\right)}^2\Delta f_2}{{\mathrm{Nf}}_{\mathrm{ech}}}\right)\times \exp \left(j2\pi v_{l2}n\right)& \mathrm{for}n\ge \frac{N}{2}\end{array}$

(26) Such a sequence has a constant envelope (|S[n]|=1) and therefore the PAPR is 0 dB. This sequence can thus be used with constant envelope wave shapes while keeping the energy efficiency of the transmitter power amplifier.

(27) FIG. 3 represents the frequential spectrum (the result of a Fast Fourier Transform, FFT, in dB as a function of the frequency, F, in kHz) of this signalling sequence in which f.sub.ech=15.36 MHz, ν.sub.l1=300 kHz, ν.sub.l2=6000 kHz, N=1024 and a same band width for the two bands Δf.sub.1=Δf.sub.2=480 kHz. It is found that this sequence comfortably covers the two required bands.

(28) FIG. 4 also shows the auto-correlation function A of this signalling sequence according to the invention (in continuous lines) and that of the signalling sequence adapted to a single band as disclosed in the above-mentioned paper by F. Dehmas et al. (in dashed lines). It can be seen that the auto-correlation properties are not degraded in comparison with the single band according to prior art; the width of the principal peak is kept and the secondary peaks have the same level for an identical total band width.

(29) FIGS. 5 and 6 illustrate that these properties are kept in the case of a signalling sequence according to the invention adapted to a fragmented spectrum composed of four spectrum fragments. Thus, FIG. 5 represents the frequency spectrum (resulting from a Fast Fourier Transform, FFT, in dB as a function of the frequency, F, in kHz) of such a signalling sequence in which Δf.sub.1=Δf.sub.3=120 kHz, Δf.sub.2=240 kHz, Δf.sub.4=480 kHz, f.sub.ech=15.36 MHz, ν.sub.l1=300 kHz, ν.sub.l2=3000 kHz, ν.sub.l3=5250 kHz, ν.sub.l4=7500 kHz and N=1024. This sequence comfortably covers the required bands. FIG. 6 also represents the auto-correlation function A of this signalling sequence (in continuous lines) and that of the signalling sequence adapted to a single band as disclosed in the above-mentioned paper by F. Dehmas et al. (in dashed lines), for comparison purposes. It is found that the auto-correlation properties are similar to the case of the non-fragmented spectrum for the same total band width.

(30) In one preferred embodiment, and as represented on FIG. 7, the transmission device according to the invention comprises a modulator capable of implementing the Turbo-FSK modulation. This modulator is composed of λ stages, each encoding a differently interlaced version of Q information bits. At each stage, the Q information bits are interlaced by an interlacer π.sub.0, π.sub.1, . . . , π.sub.λ−1 then grouped into blocks of q bits, each block being sequentially encoded using a parity accumulator acc.sub.0, acc.sub.1, . . . , acc.sub.λ−1. The q+1 resulting bits are associated with a code word in the FSK coplanar alphabet using an encoder cod.sub.0, cod.sub.1, . . . , cod.sub.λ−1, in other words associated with a phase modulation with order N.sub.L (N.sub.L−PSK) and a frequency modulation with order N.sub.⊥ (N.sub.⊥−FSK), in which q+1=log.sub.2(N.sub.LN.sub.⊥)). Due to the accumulator, each FSK coplanar symbol is connected to its predecessor. A parallel to serial converter CPS terminates the chain to transmit the symbols in the channel.

(31) FIG. 8 represents a data reception device configured to use a Turbo-FSK demodulation. This device comprises a serial to parallel converter CPS to reconstruct the λ transmitted stages. A detector det.sub.0, det.sub.1, . . . , det.sub.λ−1 makes the estimation of the probabilities of each possible code word. These probabilities are supplied to a decoder dec.sub.0, dec.sub.1, . . . , dec.sub.λ−1 that uses them as observations, outputs from other decoders being used as a priori information. A modified BCJR algorithm, as described by L. Bahl, J. Cocke, F. Jelinek, and J. Raviv, “Optimal decoding of linear codes for minimizing symbol error rate (corresp.),” IEEE Trans. on Information Theory, vol. 20, No. 2, pp. 284-287, March 1974, is used to obtain a posteriori probabilities of the information bits.

(32) This reception device also comprises a synchronisation unit and a channel estimation unit. At least one of these units is configured to exploit the previously described signalling sequence, that is known to the reception device. The synchronisation unit can thus be configured to correlate a received signal with the known signalling sequence. And the channel estimation unit can thus be configured to make a deconvolution of the signal received by the known signalling sequence.

(33) The following considers the example of a fragmented spectrum comprising two fragments, one covering the 300 kHz to 765 kHz band and the other covering the 6000 kHz to 6465 kHz band (these two bands are given in base band, the RF band being a translation of these bands towards the RF band used). A Turbo-FSK modulation is used, characterised by N.sub.⊥=64, with 32 possible frequencies in the first band and 32 possible frequencies in the second band, N.sub.L=16 and λ=4. Simulations were made on an “Extended Pedestrian A” (EPA) propagation channel used by the 3GPP. It corresponds to a residential environment with ranges of the order of a kilometre. The packets are compose of 1008 useful bits.

(34) FIG. 9 compares the packet error rate PER as a function of the signal to noise ratio SNR of the Turbo-FSK modulation depending on whether the spectrum is fragmented (curve Sf) is or is not fragmented (curve 50). This FIG. 9 illustrates the gain in frequential diversity obtained by using a fragmented spectrum. In particular, there is a gain of 7 dB for a packet error rate of 1%.

(35) FIG. 10 compares the spectral shape (FFT, in dB) of the Turbo-FSK modulation (curve TFSK) with that of a signalling sequence according to the invention adapted to the fragmented spectrum composed of two discontiguous bands (curve S). It can be seen that these spectral shapes are similar both inside the bands used (where the amplitudes are very close) and outside the bands. Outside the band, the maximum difference is 6 dB for a hole of about 38 dB. The levels of sequence rejection, admittedly not quite as good as the modulation, are nevertheless comparable.

(36) The invention thus discloses the construction of a preamble using the signalling sequence described above, this sequence possibly being repeated so as to improve synchronisation performances while keeping good auto-correlation and PAPR properties. Similarly, this sequence can be used as a pilot by repeating it at different locations inf the packet. The invention can then be used to obtain a physical layer with a constant envelope (PAPR=0 dB), on data as on the signalling part (preamble and pilots). Since the auto-correlation properties are similar to the case of the non-fragmented spectrum (see FIGS. 4 and 6), the synchronisation performances are similar in the fragmented spectrum and non-fragmented spectrum cases. The same applies for the channel estimation due to a practically flat spectral amplitude in the useful bands.

(37) The invention is not limited to the data transmission device, but it also relates to a data reception device from a fragmented spectrum, said device comprising a synchronisation unit and a channel estimation unit, at least one of said units being configured to use the previously described signalling sequence. The invention also relates to a data transmission method comprising the generation of packets carrying this signalling sequence and a method of reception of data comprising a synchronisation step and a channel estimation step, at least one of said steps using this signalling sequence.