FORM-2
THE PATENTS ACT, 1970
(39 of 1970)
&
THE PATENTS RULES, 2006
PROVISIONAL SPECIFICATION
(See section 10 and rule 13)
METHOD FOR SENDING AND RETRIEVING SIDE INFORMATION IN A MULTI-CARRIER SIGNAL


(a) TATA CONSULTANCY SERVICES LTD.,
an Indian Company of Nirmal Building, 9th floor, Nariman Point, Mumbai 400 021, Maharashtra, India;
and
(b) COMMISSARIAT A L'ENERGIE ATOMIQUE,
a French Company
of 25 rue, Leblanc, Batiment « Le Ponant D»
75015 Paris, France
THE FOLLOWING SPECIFICATION DESCRIBES THE INVENTION


FIELD OF THE INVENTION
The present invention relates to the field of multi-carrier telecommunication systems.
Particularly, the present invention relates to the field of Orthogonal Frequency Division Multiplexing (OFDM) telecommunication systems.
BACKGROUND OF THE INVENTION
Orthogonal Frequency Division Multiplexing (OFDM) is a widespread technique used in telecommunication systems ranging from Wi-Fi (IEEE 802.11a/g) to WiMAX (IEEE 802.16), ADSL, DAB (Digital Audio Broadcasting) and DVB-T (Terrestrial Digital Video Broadcasting). As is well known, OFDM is a multi-carrier modulation method based on a large number of closely spaced orthogonal sub-carriers (also called an OFDM multiplex) which has the advantage of good spectral efficiency and high protection again frequency selective fading.
The data to be transmitted are represented by symbols which are divided into several parallel streams, one for each sub-carrier, each stream modulating a corresponding sub-carrier of the multiplex according to a conventional modulation scheme such as quadrature amplitude modulation (QAM) or phase shift keying (PSK). More precisely, information symbols representing the data are mapped into modulation symbols, i.e. symbols belonging to a modulation alphabet, for instance an M-ary modulation alphabet. Blocks of such modulation symbols are transformed from the frequency domain into the time domain by IFFT (Inverse Fast Fourier Transform) and then submitted to cyclic prefix extension for combating ISI. The OFDM symbols


thus obtained is upconverted from baseband to the channel frequency by modulation with an RF carrier.
At the receiving side, the OFDM signal is downconverted into baseband and the OFDM symbols are submitted to FFT, after being stripped of their prefix extensions. The blocks of modulation symbols thus obtained are then equalized in the frequency domain, after the channel has been estimated, and then de-mapped to retrieve the information symbols.
The OFDM receiver needs to know the configuration parameters of the physical layer of the link, hereinafter referred to as PHY configuration parameters, in order to demodulate and decode the data properly. These parameters are Mer aha, the type and order of the modulation constellation (for instance, BPSK, QPSK, 16-QAM, 64-QAM), the parameters of the interleaver, the type of the error control coding and the like.
Conventional OFDM systems use the same channel for transmitting the PHY configuration parameters and the data. To illustrate this, FIGURE 1 shows the structure of an OFDM signal, 100. It comprises a sequence of transmission frames, F, each frame being constituted by a given number L of consecutive OFDM symbols. A header constituted by a predetermined number H of OFDM symbols, is present at the start of each frame and carries the PHY configuration parameters. The header itself is coded and modulated according to a coding and modulating scheme known to the receiver.
One of the drawbacks of the existing OFDM systems is that, the header needs to be processed prior to the demodulation and decoding of the subsequent OFDM symbols. More precisely, the samples corresponding to


the subsequent symbols have to be buffered while the header is processed and the PHY configuration parameters extracted. The size of this buffer, LB, is proportional to the time delay needed for processing the header. To give an example, if the operation de-mapping and de-interleaving requires a time equivalent to 2 OFDM symbols and the Viterbi decoding has a latency of 2 symbols, the size of the buffer should be:
LB = 4 (N1+NQ)NSC (1)
where Ni and NQ denote the number of bits for quantifying the in-phase and quadrature samples respectively, Nsc is the number of sub-carriers of the OFDM multiplex. For instance, for Ni = NQ = 12, the size of the buffer is comprised between 768 bytes and 24576 bytes when NSc varies from 64 to 2048.
FIGURE 2 schematically illustrates an OFDM receiver according to the state of the art. The receiver 200 comprises an RF to baseband down-converter 210, followed by an A/D converter 220. The digital I and Q samples of the OFDM symbols are time aligned by synchronisation module 225, before being subjected to an FFT in 230. For the sake of simplicity, the sampling of the baseband signal, the removal of the cyclic prefix and the serial-to-parallel conversion have been omitted. The complex symbols in the frequency domain, output by the FFT module 230, are then equalised in 240. This equalisation is performed by a scalar multiplication with complex coefficients calculated by the channel estimation module, 245. The channel estimation is generally performed by using a training sequence, for instance, pilot tones in the header and/or embedded in the OFDM symbols.


The symbols thus equalised are buffered in buffer 250 before being demapped by de-mapper 260, into information symbols, de-interleaved in de-interleaver 270 (if the symbols were interleaved at the transmitter side) and finally decoded by the channel decoder 280 into information data.
The PHY configuration parameters are extracted from the OFDM symbols of the frame header by the extraction module, 285, at the output of the channel decoder. These parameters are obtained in the same way as data are obtained from the subsequent OFDM symbols. However, by contrast with the latter, the receiver has an a priori knowledge of the PHY configuration namely, the modulation and encoding schemes used for transmitting the header and therefore the side information can be easily retrieved. In general, the side information is encoded and modulated according to the most robust encoding and modulation schemes (for instance, the lowest coding rate and QAM, respectively), which means that the coding of the side information is not very efficient.
While the header is processed, the incoming symbols output by the equaliser 250 are buffered. Once the PHY configuration parameters have been extracted, the de-mapper 260 and decoder 280 can be correctly configured for de-mapping and decoding the subsequent (noisy modulation) symbols stored in the buffer, belonging to the subsequent OFDM symbols. The de-mapper 260, de-interleaver, 270, and decoder, 280, are configured by the extraction module 285 via the control lines 295, 296 and 297, respectively.
FIGURE 3 illustrates a conventional method for incorporating pilot tones into an OFDM symbol. The modulation symbols X(1),...,X(K) obtained by mapping of the information symbols are interspersed, in 340, with


predetermined complex symbols P(1),..,P(J), at predetermined frequencies also called pilot tones. In general, the pilot symbols belong to a robust modulation alphabet, for instance, QPSK. The pilot symbols and modulation symbols form together a block of (J+K) = 2N symbols, Z(l),...,Z(2N), which is subjected to IFFT, in 350. The samples z(l),..,z(2N) represent the OFDM symbol in the time domain and modulate the RF carrier frequency as already set out above.
The positions of the pilot tones in the OFDM symbols are the same throughout a frame. FIGURE 4 shows an example of such a frame where the horizontal axis represents time and the vertical axis represents the tones. The OFDM symbols are represented as columns of N modulation symbols in the frequency domain, namely prior to IFFT. The pilot tones are shown in black while the modulation symbols carrying the information data are left in white. The modulation symbols represented in grey belong to the header. The presence of the header and the pilot tones reduces the payload and therefore the useful data rate of the OFDM telecommunication system.
Therefore there is felt a need for a method for sending side information, for instance, PHY configuration parameters, in an OFDM telecommunication system and more generally in a multi-carrier telecommunication system that overcomes the drawbacks of the conventional systems including latency at the receiver side, slow rate of information data exchange in an OFDM frame and relatively low efficiency.


OBJECTS OF THE INVENTION
An object of the present invention is to provide a method for sending side information, for instance, PHY configuration parameters, in an OFDM telecommunication system and more generally in a multi-carrier telecommunication system, which reduces or even removes latency at the receiver side.
Another object of the present invention is to provide a method that enables an increase of the information data rate in an OFDM frame.
Still another object of the invention is to provide a method that transmits side information in an OFDM telecommunication system with a higher efficiency than in the prior art.
SUMMARY OF THE INVENTION
According to the present invention there is provided a method for sending and retrieving side information in a multi-carrier signal.
Typically, in accordance with the present invention, said multi-carrier signal is constituted of frames, each frame being constituted of a sequence of multi-carrier symbols, each multi-carrier symbol being obtained by a frequency to time conversion of a block of modulation symbols and comprising a plurality of pilot tones, characterised in that, at least for one multi-carrier symbol of a frame, at least some of the pilot tones are modulated by using said side information, prior to said frequency to time conversion.


Typically, in accordance with the present invention, for at least said one multi-carrier symbol, said at least some of the pilot tones are modulated by inserting into said block a plurality of symbols (PI,...,PQ) derived from said side information, according to a predetermined pattern (J])-
Typically, in accordance with the present invention, said one multi-carrier symbol is the first multi-carrier symbol of the frame.
Typically, in accordance with the present invention, said side information comprises a set of configuration parameters for configuring the physical layer of a channel over which the multi-carrier signal is transmitted.
Typically, in accordance with the present invention, said modulation symbols are obtained by mapping information symbols into constellation elements of a modulation scheme, a first parameter of said set being associated with said modulation scheme.
Typically, in accordance with the present invention, said information symbols are obtained by a source and/or channel encoding binary information data according to an encoding scheme, a second parameter of said set being associated with said encoding scheme.
Typically, in accordance with the present invention, the binary information data thus encoded is interleaved according to an interleaving scheme, prior to be modulated into said modulation symbols, a third parameter of said set being associated with said interleaving scheme.
Typically, in accordance with the present invention, said side information (co) is represented by an orthogonal sequence (a) belonging to a set of

orthogonal or quasi-orthogonal sequences (Σ), and that a predetermined symbol (P) is multiplied with each element of said orthogonal sequence, the symbols thus obtained (PI,...,PQ) being inserted into said block according to a predetermined pattern (II) so as to generate said modulated pilot tones.
Typically, in accordance with the present invention, said multi-carrier signal is an OFDM signal.
Typically, in accordance with the present invention, the multi-carrier symbols of said signal are subjected to time to frequency conversion and the symbols in the frequency domain thus obtained are equalised, the equalised symbols thus obtained are sampled according to said predetermined pattern (II)' so as to output a sequence of sampled symbols (r1,...,rQ), said sequence of sampled symbols being cross-correlated with each orthogonal sequence (o1,..., oM) of said set (2) of orthogonal sequences to produce a plurality of correlation values (1,..., M), the side information being retrieved (w(m0)) as the one corresponding to the orthogonal sequence (o- ) leading to the
highest correlation value.
Typically, in accordance with the present invention, the side information thus retrieved comprises a first parameter associated with a modulation scheme and that said equalised symbols are demodulated into information symbols according to said scheme.
Typically, in accordance with the present invention, the side information thus retrieved comprises a second parameter associated with a channel and/or source encoding scheme and that said information symbols are decoded according to this scheme.


Typically, in accordance with the present invention, the side information thus retrieved comprises a third parameter associated with an interleaving scheme, and that said information symbols are de-interleaved according to this scheme, prior to be decoded.
Typically, in accordance with the present invention, there is provided a transmitter for transmitting a multi-carrier signal over a communication channel, said transmitter comprising frequency to time conversion means (450) for transforming a block of modulation symbols into a multi-carrier symbol, characterised in that it comprises generating means (460) for generating modulated symbols from said side information and inserting means (440) for inserting said modulated symbols into said block according to a predetermined pattern, prior to performing said frequency to time conversion.
Typically, in accordance with the present invention, said transmitter comprises a source/channel encoding means for encoding binary information data into information symbols according to an encoding scheme, and mapping means to map said information symbols into modulation symbols according to a modulation scheme, said side information comprising parameters associated with said encoding scheme and said modulation scheme.
Typically, in accordance with the present invention, there is provided a receiver for receiving a multi-carrier signal comprising multi-carrier symbols, said receiver comprising:
- time to frequency conversion means (530) for transforming a


multi-carrier symbol into a sequence of symbols in the frequency domain;
- equalising means (540) for equalising said symbols in the frequency domain;
- side information extracting means (590) comprising:

- sampling means (610) for sampling the equalised symbols according to a predetermined pattern (n) to obtain a sequence of sampled symbols (r1,..,re);
- correlating means (6201,.., 620M) for cross-correlating said sequence with each of plurality of orthogonal sequences (a1,...,oM) to output a plurality of correlation
values;
- comparing means (630) for selecting the sequence (omo )
corresponding to the highest value; and
- a look-up table storing side information;
- said side information extracting means extracting a side
information (o(mQ)) stored in said look-up table at an address
associated with said selected sequence.
Typically, in accordance with the present invention, said receiver further comprises de-mapping means (560) to de-map said equalised symbols into information symbols, a channel/source decoding means (580) for decoding said information symbols, said side information comprising a first parameter (a) associated with a modulation scheme and a second parameter associated with a channel/source encoding scheme (p), said side information extracting


means (590) sending said first parameter to the de-mapping means and said second parameter to the channel/source decoding means.
BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS
The invention will now be described in relation to the accompanying drawings, in which,
FIGURE 1 illustrates a schematic representation of the structure of an OFDM signal known from the prior art;
FIGURE 2 illustrates a schematic representation of the structure of an OFDM receiver known from the prior art;
FIGURE 3 illustrates schematically how pilot tones are incorporated in an OFDM symbol;
FIGURE 4 illustrates a schematic representation of the structure of a frame of OFDM symbols in a conventional OFDM telecommunication system;
FIGURE 5 illustrates schematically how PHY configuration parameters are transmitted in an OFDM signal, according to an embodiment of the invention;
FIGURE 6 illustrates a schematic representation of the structure of a frame of OFDM symbols in an OFDM telecommunication system according to an embodiment of the invention;
FIGURE 7 illustrates schematically an OFDM receiver according to a first embodiment of the invention; and


FIGURE 8 illustrates a schematic representation of the detailed structure of the side information extraction module represented in FIGURE 7.
DESCRIPTION OF THE INVENTION
The drawings and the description thereto are merely illustrative and only exemplify the invention and in no way limit the scope thereof.
The basic idea underlying the invention is to transmit side information, in particular the PHY configuration parameters, by modulating pilot tones in some OFDM symbols of a frame. Therefore, the side information need not be contained in the OFDM frame header and the header can be removed or significantly downsized.
FIGURE 5 illustrates a method for transmitting side information according to an embodiment of the invention. The side information co is used to generate in 560 a plurality of symbols pi,..,pQ which respectively modulate Q predetermined pilot tones. In general, these Q tones form a subset of the pilot tones which are normally used at the receiving side for channel estimation. The symbols pi,.-,pQ are interspersed in 540 with the modulation symbols X(1),...,X(K) obtained by mapping the information symbols. The symbols pi,..,pQ and X(l),...,X(K) thus interspersed form a block of N symbols in the frequency domain which is then converted in the time domain, for instance, by an IFFT, as represented in 550.
The generating means 560 can be embodied in various ways. For example, it may use a multiplexer 520 to select an orthogonal sequence a, from a set of


orthogonal sequences £ = (a1.,am) depending upon the item of side information co to be transmitted. Preferably S is a set of orthogonal binary sequences, for instance, Walsh-Hadamard sequences or quasi-orthogonal sequences. The selected sequence c = (SI,...,SQ) modulates in 530 a predetermined pilot symbol P to provide a sequence of modulated pilot symbols pi,..,pQ = SIP,...,SQP. Preferably, Q is a power of 2, namely Q = 2V.
The various combinations of PHY configuration parameters, for instance, modulation, interleaving and decoding schemes may be mapped into the set £ of orthogonal sequences. More specifically, there is a one to one relationship between an item of side information co and an orthogonal sequence oCΣ. For example, a triplet co = (a, β, x), where a corresponds to a modulation alphabet, p corresponds to a channel coding scheme and X to an interleaving scheme is associated with one given orthogonal sequence.
Alternatively, the generating means 560 may comprise a look-up table addressed by items of side information co and outputting modulated pilot symbols PI,..,PQ stored therein.
FIGURE 6 shows the structure of a frame of OFDM symbols obtained with the method of transmission according to an embodiment of the invention. Here again, OFDM symbols are represented in the frequency domain, namely as columns, and the tones are represented as rows. Contrary to the frame illustrated in FIGURE 4, the frame F does not exhibit a header but comprises instead modulated pilot tones represented by pi,..,pQ which carry the side information. These modulated pilot tones may coexist, in the same frame and even in the same OFDM symbol with unmodulated pilot tones (represented in black) and which are used for the purpose of channel


estimation. The modulated pilot tones form a pattern II in the frame (here a pattern of eight symbols).
In the example illustrated in FIGURE 6, the first OFDM symbol of the frame exhibits pilot tones which are modulated to carry the side information. The second and subsequent OFDM symbols of the frame have their pilot tones unmodulated except the tenth OFDM symbol which also carries side information.
In contrast with the prior art, OFDM telecommunication systems, where the side information transmitted in the header was modulated with the lowest modulation order ^nd encoded with the lowest coding rate, it should be appreciated that the modulated pilot tones may be modulated here with the same modulation order as the remaining data. Hence, the side information is coded more efficiently and the payload is increased.
Furthermore, if the first OFDM symbol carries said side information, there is no need to store the first incoming samples in a buffer and the data rate can be increased. Indeed, if Dh is the data rate of the conventional OFDM signal and Dp is the data rate of the OFDM signal where the side information is carried by modulated pilot tones, it can be seen that Dp = (1 + LB/L) Dh where LB is the length of the buffer 250 and L is the length of an OFDM frame. Furthermore, the removal of the buffer leads to a simplified hardware and a lower power Consumption.
Finally, as each OFDM symbol may carry a pattern of modulated tones, the PHY configuration parameter may be made variable from one OFDM


parameter to the next, thereby providing transmission conditions of high flexibility.
FIGURE 7 illustrates an OFDM receiver according to an embodiment of the invention. The OFDM receiver 700 comprises a baseband downconverter 710, followed by an A/D converter 720. The digital I and Q samples of the OFDM symbols are time aligned by synchronisation module 725, before being subjected to an FFT in 730. The symbols thus obtained in the frequency domain are equalised in 740. As in the prior art, the equalisation is performed by scalar multiplication with complex coefficients calculated by the channel estimation module 745 on the basis of the unmodulated pilot tones.
The modulated pilot tones are selected from the block of the equalised symbols by extraction module 790, according to the pattern II, known by the receiver.
The extraction module is adapted to select the modulated pilot tones from the received frame and to extract therefrom the side information, in the present instance, the triplet co = (a, p, x) representing the PHY configuration parameters used by the OFDM transmitter.
The extraction module 790 forwards the value a over control line 795 to the de-mapper 760, the value P over control line 797 to the decoder, 780, and value % over control line 796 to the (optional) interleaver 770.
If the PHY configuration parameters are already present in the first OFDM symbol, there is no need for buffering the incoming modulation symbols. In


this instance, the de-mapper, de-interleaver, decoder can indeed be configured just at the start of the OFDM frame whereby the latency of the receiver is significantly reduced.
FIGURE 8 shows an example of a detailed structure of the extraction module 790.The block of equalized symbols output by equaliser 740, relative to an OFDM symbol, is sampled according to above-defmed patternII, thereby retaining the modulated pilot tones symbols, denoted r1,...,rQ. The sequence of these modulated pilot symbols is cross-correlated with all the possible orthogonal sequences o1,.. .0M in correlators 820i,...,820M. More precisely, the mth correlator 820m performs the calculation:

(2)
where the sequence am is defined by om = (s1m...SQm), sq-m is the complex
conjugate of sqm and P the conjugate of the pilot symbol P.
The correlation results Tm, m = 1,...,M are then compared in comparator 830
which outputs the index m0 corresponding to the highest correlation value,
namely:

The triplet co(m0) corresponding to m0, that is to sequence omQ, is retrieved from the look-up table 240.

Although, the present application has been more particularly described in the case where the PHY configuration parameters are the coding, the interleaving and the modulation schemes, it will be readily understood by the man skilled in the art that the invention may concern the transmission of any configuration parameter of the physical layer or any set of such parameters, and more generally, any side information.
TECHNICAL ADVANCEMENT
The technical advancements offered by the present invention include the realization of:
• a method for sending side information, for instance, PHY configuration parameters in an OFDM telecommunication system and more generally in a multi-carrier telecommunication system, which reduces or even removes latency at the receiver side;
• a method that enables an increase of the information data rate in an OFDM frame; and
• a method that transmits side information in an OFDM telecommunication system with a higher efficiency than in the prior art.
While considerable emphasis has been placed herein on the particular features of this invention, it will be appreciated that various modifications can be made, and that many changes can be made in the preferred embodiments without departing from the principles of the invention. These and other modifications in the nature of the invention or the preferred


embodiments will be apparent to those skilled in the art from the disclosure herein, whereby it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the invention and not as a limitation.
Dated this 23rd day of April, 2009

MOHAN DEWAN OfR. K. DEWAN & CO. APPLICANT'S PATENT ATTORNEY