GB2488168A - Additional signalling for Digital Video Broadcasting - Google Patents

Abstract

Methods and apparatus are described for transmitting data comprising one or more frames (100 Figure 1), such as Digital Video Broadcast data. These frames comprise a preamble section (110 Figure 1) and a data section (120 Figure 1), the preamble section having a synchronization function and carrying at least signalling information. Transmitting and receiving examples are described that operate on a superimposed sequence 770 (also 960 Figure 9) comprising a second modulated sequence 765 (also 955 Figure 9) that is superimposed upon a first modulated sequence 735 (also 935 Figure 9). The second modulated sequence is based on a second modulation scheme capable of non-coherent detection that provides continuous variation of the second modulated sequence across the length of the first modulated sequence. Examples using hierarchical differential phase shift keying and repeated sequences (945 Figure 9) of chirps and tones are further described. Described examples have an advantage of increasing the capacity of a preamble section to accommodate future signalling modes, whilst maintaining the synchronization function of the preamble section as well as backward compatibility. The term superimposing may involve multiplication of the two sequences, ANDâ ing them or performing bit-addition.

Description

Additional S ignailingfor Digital Video Broadcasting

Field of thc Invention

The present invention relates to the transmission and reception of synchronisation and signalling data, and more specifically, but not exclusively, to a method and apparatus relating to transmission and reception of synchronisation and signalling information in digital video broadcast systems.

Background of the Inventioij,

A wireless broadcast system, such as a digital video broadcasting (DVB) system, may transmit data in the form of a sequence of frames, wherein each frame comprises a prcamble section and a data section. A digital video broadcasting system may, for example, operate according to a DVB-T2 (Terrestrial 2 Generation) standard, or for example, to the following families of standards: ATSC (Advanced Televisions Systems Committee), TSDB (Integrated Services Digital Broadcasting), or DMB (DigitaE multimedia Broadcasting) The preamble section typically comprises control data and the data section typically comprises content data. Further information concerning the DVB-T2 standard may be found in the published European Telecommunications Standards Institute (ETSI) standard EN 302 755.

Wireless broadcast systems, and in particular digital video broadcast systems, are constantly evolving. This has to be balanced with the aim of maximising compatibility between current and new systems at the levels of services, network infrastructures, and hardware platforms. New developments in signalling technology and hardware mean that the number of signalling modes available increases over time. For example, further standards may be introduced that require additional data to be carried by the preamble section. It is desired that new systems co-exist with, and be integrated into, existing systems in the same spectrum. It is further desired to maximise reuse of available infrastructures and platforms. However, a preamble of a transmitted signal may need to have a fixed format to enable a synchronisation function, rapid initial detection, and the production of standard transmission and reception equipment.

There is thus a need in the art to accommodate future changes in signalling technology, while maintaining backward compatibility with existing hardware.

It is an object of the invention to mitigate these problems with the prior art systems.

Summary of the Invention

In accordance with a first aspect of the present invention, there is provided a method of transmitting data comprising one or more frames, said frames comprising a preamble section and a data section, the preamble section having a synchronisation function and carrying at least signalling information, the method comprising: modulating a carrier waveform with a first data sequence of the preamble section to generate a first modulated sequence, said modulating using a first non-coherent modulation scheme; modulating a second data sequence of the preamble section to generate a second modulated sequence, said modulating using a second modulation scheme capable of non-coherent detection that provides continuous variation of the second modulated sequence across the length of the first modulated sequence; superimposing the second modulated sequence upon the first modulated sequence to produce a superimposed sequence; and transmitting the superimposed sequence.

An advantage of this method is that the capacity of a preamble section is increased to accommodate future signalling modes, whilst maintaining the synchronisation function of the preamble section and backward compatibility with existing equipment adapted to receive the first data sequence.

In an embodiment of the invention, the carrier waveform comprises a plurality of sub-carriers transmitting the superimposed sequence.

In an embodiment of the invention, the first data sequence carries a first field (SI) and a second field (52), the first field being modulated on a first set of sub-carriers and the second field being modulated on a second set of sub-carriers.

In an embodiment of the invention, the second modulation scheme comprises hierarchical differential phase shift keying (fl-DPSK), wherein the second modulation scheme may comprise providing a rotation of up to ±/3 radians between consecutive symbols of the second data sequence. For example, differential binary phase shift keying (D-BPSK) may be used wherein /3 is less than 2t/2 radians.

An advantage of hierarchical (binary) differential phase shift keying is that existing components that are arranged to modulate the first data sequence may be reused to modulate the second data sequence. Additionally, hierarchical (binary) differential phase shift keying enables non-coherent detection and maintains the synchronisation function of the preamble.

In an embodiment of the invention, a first rotation of /Jj radians is used between consecutive symbols of the first field (SI) and a second rotation of /32 radians is used between consecutive symbols of the second field (52).

An advantage of using two values of for the two data fields is that modulation can be optimised based on the particular transmission properties of each field, thus lowering signal degradation. Additionally, each value may be optimised according to a different set of optimisation criteria.

In an embodiment of the invention, the second data sequence carries a third field (S3) in a chip sequence, the chip sequence being repeated and/or interleaved across the second data sequence. In this case, the chip sequences carrying the third field (S3) may be derived from a set of sequences that are used to generate the first data sequence carrying the first and second fields (Sl, S2).

An advantage of deriving the sequences from existing sequences is that existing components may be reused.

In an embodiment of the invention, the second modujation scheme comprises the generation of a plurality of short sequences carrying a third field (S3) that are repeated across the length of the first modulated sequence to form the second modulated sequence. The sequence may be referred to as "short" as it is of a length Nchip that is less than the length of the first modulated sequence In an embodiment of the invention, the phase of each short sequence varies across the length of the short sequence, the difference in phase between S two consecutive samples of the short sequence being less than the difference in phase between two consecutive samples of the first modulated sequence. This has an advantage of avoiding degradation to the first modulated sequence.

In an embodiment of the invention, the short sequences are repeated and/or are interleaved across the second modulated sequence. This has an advantage of, in the particular embodiment, ensuring continuous variation across the length of the second modulated sequence.

In an embodiment of the invention the plurality of short sequences comprise one of: tones, wherein part of the third field (S3) is carried in a phase value of the short sequence; or chirps, wherein part of the third field (S3) is carried in a frequency sweep of the short sequence.

In an embodiment of the invention, each short sequence is tuned so that the phase difference between the end of one short sequence and the beginning of an adjacent short sequence is less that a predetermined threshold.

In an embodiment of the invention, a set of tones superimposed onto the part of the first modulated sequence carrying the first field (Si) is different from a set of tones superimposed onto the part of the first modulated sequence

carrying the second field (52).

In an embodiment of the invention, a set of chirps superimposed onto the part of the first modulated sequence carrying the first field (Si) is different from a set of chirps superimposed onto the part of the first modulated sequence

carrying the second field (S2).

Superimposing differentiated tones or chirps has an advantage of allowing differentiated optimisation of parameters with respect to portions of the first modulated sequence carrying the first field and portions of the first

modulated sequence carrying the second field.

In accordance with a second aspect of the present invention, there is provided a method of receiving data comprising one or more frames, said frames comprising a preamble section and a data section, the preamble section having a synchronisation function and carrying at least signalling information, the method comprising: receiving a signal comprising a second modulated sequence superimposed on a first modulated sequence; demodulating the signal to extract a first data sequence of the preamble section from the first modulated sequence, said demodulating using a first non-coherent demodulation scheme; removing the superimposition to provide the second modulated sequence; and demodulating the second modulated sequence using a second demodulation scheme to extract a second data sequence of the preamble section, wherein the second demodulation scheme is configured to provide non-coherent detection of the second data sequence.

Features of the embodiments of the first aspect may also be provided with the second aspect, with appropriate allowances for a change from modulating the signal to demodulating the signal. For example, embodiments of the second aspect may assume features of the embodiments of the first aspect are present to enable appropriate demodulation. Any advantages such features confer also apply to the second aspect of the invention.

In an embodiment of the invention, the step of demodulating using a second demodulation scheme comprises: determining a phase difference between consecutive samples of the second demodulated sequence to produce a plurality of phase estimates; averaging the phase estimates to determine an average phase estimate for each sequence; and using the average phase estimate to extract phase or frequency sweep values representing encoded data.

In an embodiment of the invention, removing the superimposition to provide the second modulated sequence comprises: reconstructing the first modulated sequence from the extracted first data sequence; and subtracting the reconstructed first modulated sequence from the signal to provide the second modulated sequence.

In accordance with a third aspect of the present invention, there is provided a transmitter for transmitting data comprising one or more frames, said frames comprising a preamble section and a data section, the preamble section having a synchronisation function and carrying at least signalling information, the transmitter being arranged to: modulate a carrier waveform with a first data sequence of the preamble section to generate a first modulated sequence, said modulating using a first non-coherent modulation scheme; modulate a second data sequence of the preamble section to generate a second modulated sequence, said modulating using a second modulation scheme capable of non-coherent detection that provides continuous variation of the second modulated sequence across the length of the first modulated sequence; superimpose the second modulated sequence upon the first modulated sequence to produce a superimposed sequence; and transmit the superimposed sequence.

Features of the embodiments of the first aspect, relating to a method of transmitting, may also be used to appropriately arrange the transmitter of the third aspect. Any advantages such features confer also apply mutatis mutandis to the third aspect of the invention.

In accordance with a fourth aspect of the present invention, there is provided a receiver for receiving data comprising one or more frames, said frames comprising a preamble section and a data section, the preamble section having a synchronisation function and carrying at least signalling information, the receiver being arranged to: receive a signal comprising a second modulated sequence superimposed on a first modulated sequence; demodulate the signal to extract a first data sequence of the preamble section from the first modulated sequence, said demodulating using a first non-coherent demodulation scheme; remove the superimposition to provide the second modulated sequence; and demodulate the second modulated sequence using a second demodulation scheme to extract a second data sequence of the preamble section, wherein the second demodulation scheme is configured to provide non-coherent detection of the second data sequence.

Features of the embodiments of the second aspect, relating to a method of receiving, may also be used to appropriately arrange the receiver of the fourth aspect. Any advantages such features confer also apply mutatis mutandis to the fourth aspect of the invention.

In accordance with a fifth aspect of the present invention, there is provided a signal arranged to carry one or more frames of data, said frames comprising a preamble section and a data section, the preamble section having a synchronisation function and carrying at least signalling information; the signal being arranged to carry a first data sequence of the preamble section according to a first non-coherent modulation scheme and a second data sequence of the preamble section according to a second modulation scheme, the second modulation scheme being capable of non-coherent detection and providing a continuous variation of the second modulated sequence across the length of the first modulated sequence; the signal comprising a second modulated sequence produced by the second modulation scheme based on the second data sequence superimposed upon a first modulated sequence produced by the first modulation scheme based on the first data sequence.

Features of the embodiments of the first aspect may also used to appropriately arrange the signal of the fifth aspect. Any advantages such features confer also apply mutatis mutandis to the fifth aspect of the invention.

In accordance with a sixth aspect of the present invention, there is provided a non-transitory computer readable program product, comprising a computer usable medium having a computer readable program code embodied therein, the computer readable program code adapted to be executed to implement a method of transmitting data comprising one or more frames, said frames comprising a preamble section and a data section, the preamble section having a synchronisation function and carrying at least signalling information, the method comprising: modulating a carrier waveform with a first data sequence of the preamble section to generate a first modulated sequence, said modulating using a first non-coherent modulation scheme; modulating a second data sequence of the preamble section to generate a second modulated sequence, said modulating using a second modulation scheme capable of non-coherent detection that provides continuous variation of the second modulated sequence across the length of the first modulated sequence; superimposing the second modulated sequence upon the first modulated sequence to produce a superimposed sequence; and transmitting the superimposed sequence.

In accordance with a seventh aspect of the present invention, there is provided a non-transitory computer readable program product, comprising a computer usable medium having a computer readable program code embodied therein, the computer readable program code adapted to be executed to implement a method of receiving data comprising one or more frames, said frames comprising a preamble section and a data section, the preamble section having a synchronisation function and carrying at least signalling information, the method comprising: receiving a signal comprising a second modulated sequence superimposed on a first modulated sequence; demodulating the signal to extract a first data sequence of the preamble section from the first modulated sequence, said demodulating using a first non-coherent demodulation scheme; removing the superimposition to provide the second modulated sequence; and demodulating the second modulated sequence using a second demodulation scheme to extract a second data sequence of the preamble section, wherein the second demodulation scheme is configured to provide non-coherent detection of the second data sequence.

Both the sixth and seventh aspects of the invention may be adapted as described with relation to the first and second aspects of the invention.

Further features and advantages of the invention will become apparent from the following description of preferred embodiments of the invention, given by way of example only, which is made with reference to the accompanying drawings.

Brief Descriptjon o [the Drawings Figure 1 is a schematic diagram of a frame in an exemplary digital video broadcast system; Figures 2a to 2d are four tables showing the possible contents of two data fields from an exemplary first preamble signalling section of the frame of Figure 1; Figure 3 is a schematic diagram showing an exemplary receiver system for receiving a first preamble signalling section; Figure 4 is a schematic diagram showing a receiver system according to an aspect of the invention; Figure 5 is a schematic diagram of first and second modulation schemes according to a first embodiment of the invention; Figure 6 is a schematic diagram showing a receiver system according to a first embodiment of the invention; Figure 7 is a schematic diagram showing a method of transmitting a signal according to a first embodiment of the invention; Figure 8 is a schematic diagram showing a receiver system according to a second embodiment of the invention; S Figure 9 is a schematic diagram showing a method of transmitting a signal according to a second embodiment of the invention; Figure 10 is a schematic diagram showing a method of receiving a signal according to an aspect of the invention; Figure Il I shows the results of two simulations of a modulation scheme according to a first embodiment of thc invention; Figure 12 shows the results of two simulations of an alternate modulation scheme according to a variation of the first embodiment of the invention; Figure 13 illustrates the properties of an exemplary symbol transmitted according to the second embodiment of the prcsent invention; Figure 14 shows the results of two simulations of a modufation scheme according to the second embodiment of the invention; Figure 15 shows a number of possible modulation patterns; and Figures 1 6a to 1 6c illustrates example use cases of data fields generated according to the present invention.

Detailed Description of th e Invention

By way of example, embodiments of the invention will now be described in the context of a Digital Video Broadcasting Next Generation Handhcld (DVB-NGH) standard, which is based on the 2 generation terrestrial DVB-T2 system. However, it will be understood that this is by way of example only and that other embodiments may involve other communication systems where control data is transmitted and received; embodiments are not limited to the transmission of wireless broadcast signals such as digital video signals. Thc invention will be described in relation to transmission and reception of a signal; it will be understood that these two operations are complementary and relate to a common inventive concept.

An exemplary frame is illustrated in Figure 1. Each frame 100 typically comprises a preamble section 110 and a data section 120, the preamble section and the data section being time-multiplexed. The data section 120 may carry data that is arranged in the form of a number of data streams that may be referred to as physical layer pipes (PLP). A physical layer pipe may carry, for example, a service such as a video channel provided to a user. Reception of data from the frames, and reception of the data streams, may be assisted by signalling, which may typically be carried in the preamble of the frame. The signalling may be referred to as physical layer signalling, or Layer I (LI) signalling. The signalling may indicate a modulation or coding scheme to be used for decoding data, and it may for example indicate sections of a data field to be decoded, or the location of a data stream within the data section 120.

Although reference to frames is made in the present description, the present invention may also apply to alternate signal structures wherein control data is transmitted and/or received.

The preamble section 110 may comprise multiple signalling sections. A first preamble signalling section 130, referred to as Preamble 1 (P1) in the DYB-T2 standard, may be used to synehronise the reception of a transmitting data stream, identify the preamble section 110 itself and provide information for initial fast recognition of a broadcast signal. Information 150 regarding transmission and reception parameters may also be provided. A second preamble section 140, referred to as Preamble 2 (P2) in the DVB-T2 standard, may provide more detailed pre-signalling 160 and post-signalling 170 parameters for the physical layer.

The first preamble signalling section 130 has four main ftinetions. First, it is used during an initial signal sean for fast recognition of a broadcast signal, for which just the detection of the first preamble signalling section 130 is enough.

The first preamble signalling section 130 has a particular time domain structure that enables synchronisation following detection. This means that the first preamble signalling section 130 is necessarily limited in length. and complexity.

The second purpose of the first preamble signalling section 130 is to identify the preamble itself as a preamble relating to a particular format or standard. The third task is to signal basic transmission parameters that are needed to decode the rest of the preamble, which can help during an initialization process. The fourth purpose of the first preamble signalling section 130 is to enable a receiver to detect and correct frequency and timing synchronization.

The integration of the DVB-Next Generation Handheld (NGI4) standard that is designed to deliver mobile television (TV) services with the DVB-Seeond Generation Terrestrial (T2) standard that is designed to deliver standard definition (SD) and high definition (I-ID) fixed TV services, imposes constraints on the signalling structure of a wireless broadcast signal. For example, it may mandate the use of the same preamble symbol from DVB-T2. This means that the signalling capacity of the preamble symbol (equal to 128 modes) will have to be distributed amongst both DVB-T2 and integrated DVB-NGI-I systems.

After discounting capacity that cannot be used for technical reasons or that is reserved for future use, there is limited signalling capacity left for the DVB-NGH system; typically this limited capacity is not sufficient to signal all the modes required by DVB-NGH. This raises the problem of signalling capacity shortage, and stresses the need to find an effective way to add an extra signalling capacity to accommodate and integrate future extensions and developments.

The present invention is directed towards a modified preamble for a communications signal. The term preamble is used to describe a portion of the signal that has a control function, for example, that allows signal synchronisation and/or that carries signalling information. A preamble is typically distinguished from a portion of the signal that carries content data, for example a data section comprising broadcast video data.

In an exemplary implementation, the preamble comprises a fixed-length pilot symbol located in the beginning of a frame of a signal within each radio-frequency channel. A fixed-length enables the preamble to be used to synchronise reception of the signal. The performance of the preamble is independent of any data carried by the signal. To provide robust detection of the pilot symbol a minimum Signal-to-Noise-Ratio (SNR) of -3dB is required and the Frame Error Rate (FER) is 0.01 for an SNR greater than -4dB The preamble may further providc protection against interference, such as inter-symbol-interference (ISI) and echoes, tolerate large frequency shifts of up to +1-500 kI-Iz, provide coarse time andlor frequency synchronisation, and have a low Peak-to-Average Power Ratio (PAPR). Any modification of the preamble should, as far as possible, maintain at least one of these properties; i.e. should maintain the synchronisation function of the preamble and minimise any degradation in preamble detection. For example, the degradation introduced by any modification may need to be less than a certain tolerable threshold. In the DVB-T2 standard a P1 pilot symbol may comprise a 1K orthogonal frequency-division multiplexing (OFDM) symbol with two modified guard intervals that have a length of half the primary symbol. The two modified guard intervals comprise frequency shifted versions of the main symbol which are correlated in parallel during detection of the preamble. This produces a particular time domain format that enables the preamble to provide a synchronisation function.

The frequency power distribution of the main symbol must follow a specific profile to enable detection, i.e. the number of active subcarriers is fixed. This enables the correction of the integer number of frequency offsets to provide coarse frequency synchronisation. As well as a synchronisation function the pilot symbol of the preamble may carry signalling data. The content of this signalling data is discussed below.

Figures 2a to 2d illustrate the possible contents of two data fields that may be carried within an exemplary first preamble signalling section 130, as for example illustrated in Figure 1. A first data field SI is shown. in Figure 2a. In the DVB-T2 standard, the first data field SI comprises a 3-bit data sequence that identifies whether the preamble section 110 conforms to the T2 standard. The first data field Si also identifies whether a second preamble signalling section 140 is transmitted in Single input Single Output (SISO) format or Multiple Input Single Output (MISO) format. Out of the eight possible bit sequences, only three are used, the rest are reserved for future use, Hence, the first data field SI can represent one of three different signalling modes in the T2 standard, Figures 2b to 2d show the possible contents of a second data field 52 according to the DVB-T2 standard. In the present example, the second data field S2 is 4 bits in length. Figures 2c and 2d show the possible contents of these bits when the preamble conforms to the T2 standard: the first three bits signalling a Fast Fourier Transform (FFT) size and a guard interval (UI) for the broadcast signal following the preamble section as shown in Figure 2c, arid the last bit indicating whether the preamble sections of a current transmission are of the I 0 same type as the current preamble, i.e. not mixed, or are of different types, i.e. mixed, as shown in Figure 2d. Figure 2b shows how particular non-T2 bit patterns for the second data field 52 are reserved for Future Extension Frames (FEF) or other future use. Hence, if the signal conforms to the T2-standard, the first data field S2 can represent one of eight different signalling modes. As the mixed bit cannot be freely used, the total number of modes that can be signalled is 64, of which 24 (3 times 8) are defined by the T2 standard.

Although embodiments of the invention will be described below with relation to two data fields that are carried within a first preamble signalling section, it will be understood that a single data field, or three or more data fields, of any desired length could be used. These data fields would need to be of a size that eou].d be accommodated by the fixed-length of the preamble. Additionally, it will be understood that the first preamble section could carry a data sequence of a length other than 7 bits depending on the transmission standard that is used, for example standards other than DVB-T2 may use a symbol for synchronisation of a different size, which may or may not be followed by further preamble signalling sections. Furthermore, the preamble section need not comprise multiple signalling sections; the invention can apply to a preamble section with one or multiple signalling sections.

An exemplary receiver system for receiving a first preamble signalling section 130 is shown in Figure 3. This receiver system is adapted to provide a discrete signal that is modelled as: y(i) = h(i)d12(i) + n(i) wherein y is a received broadcast signal, h is a sub-carrier function modelling the channel effects, d12 is a data sequence generated from data fields 51 and 52, n is a modelled noise term, and i = 1 to N, N being the length of the signal. The DVB-NGI-1/T2 standards use OFDM, or OFDM-based variations, and as such the length of the signal is equal to the number of subcarriers used to transmit the signal. When a 1K FFT size is used with 384 active subcarriers, N = 384. Sub-carriers are then further modulated using the data sequence d12. Even though OFDM-based systems are preferred, the invention as described below may be used with non-OFDM-based systems.

The exemplary receiver system comprises a Fast Fourier Transform (FFT) component 310, a Carrier Distribution Sequence (CD S) Component 320, a Descrambler 330, a Differential-Binary Phase Shift Keying (D-BPSK) Demodulator 340 and a Correlator 350. A wireless broadcast signal PIA(l.. .1024) spread across 1024 sub-carriers comprising the first preamble signalling section 130 is received and input into FF1 component 310, which is arranged to apply an FFT algorithm to the received signal. In the example, an FFT size of 1K (1024) is used; however, other FFT sizes may alternatively be used depending on the implementation. Following the application of the FFT a frequency-domain signal is input into CDS Component 320. CDS Component 320 uses a carrier distribution table to identify active sub-carriers that carry the first preamble signalling section 130, i.e. it uses a Carrier Distribution Sequence that specifies which subearriers are used to carry data in order to isolate those subearriers. In certain embodiments all sub-carriers may be active sub-carriers and so the CDS Component 320 may be omitted. In an example that uses a 1K FFT size there may be 853 useable sub-carriers of which 384 are used in the DVB-T2 standard, with the remaining sub-carriers being set to zero. The FFT size and any guard interval (GI) that is used are signalled in the second data field (52). Typically, the used carriers occupy a subset of available signal bandwidth.

The output of CDS Component 320 comprises a modulated data sequence, in this case of length 384: i.e. y(l.. .384). In certain embodiments the wireless broadcast signal may be optionally scrambled, in which case the output of CDS Component 320 is passed to Descrambier 330. Descrambler 330 applies a descrambling function to provide an unscrambled modulated data sequence. In one embodiment, scrambling may be applied by bit-by-bit multiplying by a 384-bit scrambling sequence an.d dcserambling applied by an appropriate multiplicative descrambler. As well as the descrambier, additional decryption components may also be optionally provided at this stage. Following descrambling the modulated data sequence is passed to D-BPSK Demodulator 340 which demodulates the modulated data sequence to provide a demodulated data sequence s, in this example of length 384. The modulated data sequence is demodulated according to the following model: (i) 1° if j(L(y(i)) -L(y(i -i)) «= rr/2 12 otherwise i.e. a 0 is detected if the magnitude of the change in phase between successive symbols is less than ff12 radians, wherein s12 (i) represents the i-th detected symbol. In other embodiments, alternative demodulators and demodulation schemes may be used; however, it is important that such demodulators and demodulation schemes are capable of non-coherent demodulation; the existing synchronisation function of the preamble would not operate successfully if a coherent (de)modulation scheme was used as a synchronised receiver clock would be required. The demodulated data sequence is then input into Correlator 350 wherein the bit values of the two data fields, Si and S2 are extracted.

An embodiment of the present invention utilises a superimposed data sequence to increase the capacity of a preamble of a transmitted signal, in this case the first preamble signalling section 130. A third data field S3 of bit length n3 is used to produce a further modulated data sequence that is superimposed on a modulated data sequence that carries data fields S 1 and S2.

According to an embodiment of the present invention, a second modulated data sequence d3 is superimposed on a first modulated data sequence d12 according to the model: y(i) = h(i)d12(i)d3(i) + n(i) The first modulated sequence d12 (i) may be produced from a first data sequence by a first modulation scheme; for exampLe, the modulation scheme used to modulate the data sequence that is extracted by the receiver of Figure 3.

In a similar manner, the second modulated sequence c13 (1) may be produced from a second data sequence by a second modulation scheme. The term modulation is used to refer to a process by which a first sequence, waveform or signal is transformed into a second sequence, waveform or signal by modification of one or more parameters in accordance with a set rule or function. The first and second data sequences respectively carry the two data fields SI and S2, and the third data field Si They may comprise the bit sequences of the fields themselves or suitably prepared bit patterns produced from the bit sequences, as is described in more detail below. Following modulation, superimposition and transmission, a first data sequence may be extracted from a received signal using a first demodulation scheme, for example using a D-BPSK scheme as described with relation to Figure 3. This may be achieved without altering legacy receiver systems. The second modulated sequence may then also be extracted from the received signal using a second demodulation scheme after the superimposition has been removed.

The properties of the second modulation scheme and the data sequence d3 (1) are chosen such that data fields S 1 and S2 can be accurately extracted by the receiver system of Figure 3 using the first (de)modulation scheme. To fulfil a synchronisation function, the first (dc)modulation scheme needs to be non-coherent; i.e. needs to be able to demodulate the first modulated signal without a reference clock signal that is phase synchronised with a carrier waveform. To enable legacy detection of the first and second data fields, the second modulation scheme is selected so as to maintain non-coherent detection of the first modulated sequence. This in turn requires that the second modulation scheme be such to enable demodulation of the second modulated sequence without a synehronised reference clock signal. Successful legacy extraction of data fields S 1 and 52 may be achieved by minimising the changes between adjacent symbols in the data sequence d12(i) produced by the superimposition of data sequence d3 (i), Data sequences with constant amplitude, continuous variation and/or slow variation of phase when compared to data sequence d12 (Q, for example complex exponential functions, chirp sequences, and Zadoff-Chu functions, may all produce data sequences for d3 (i) that result in minimum degradation to the first modulated sequence such that Si and S2 may be successfully detected. The term continuous variation is used to describe the manner in which the second modulated sequence varies across the length of the first modulated scqucncc; i.e. the second modulated sequence must continue to vary across the length of the first modulated sequence without abrupt changes or discontinuities to avoid degradation.

Figure 10 shows an exemplary method 1000 of receiving a transmitted signal according to an aspect of the present invention. At step 1010 the detection of a P1 symbol that comprises the first preamble signalling section 130 is attempted. If the detection is unsuccessful, i.e. the P1 symbol is not found, then at step 1020 the detection at step 1010 is repeated. The detection may be repeated a set number of times or for a set time period until a time-out event occurs. Jf detection is successful then the method proceeds from step 1020 to step 1030, wherein coarse time and frequency synchronisation is performed.

This synchronisation is deemed to he coarse as further refinement of timing and frequency parameters is performed following detection of data fields SI and 52 and other possible preamble signalling sections. The coarse time and frequency synchronisation then allows the decoding of data fields Si and S2 at step 1040, as for example described with relation to Figure 3. At step 1050 an optional check is made as to whether the legacy standards are being used or whether an additional data field S3 is being transmitted. This may be detennined by the receiving equipment, i.e. particular equipment may be set up to always extract a third data field where present, and/or may be set in the bit patterns of SI reserved for future use. If, for example, legacy equipment is being used, data fi&d 53 is not present andlor the decoding of the third data field 53 is not required then the first and second data fields SI and S2 are extracted. They may be used to set signalling parameters to allow reception and decoding of a digital video broadcast carried in a data section that follows the preamble section.

If the third data field S3 is being transmitted as well as data fields Si and 52, then a number of detection steps 1060 are performed. At step 1070 the extracted data fields SI and S2 are re-encoded to reconstruct the first modulated sequence. At step 1080 an equalisation of the received signal is performed. This may comprise the removal of the reconstructed first modulated sequence from a superimposed sequence comprising the first and second modulated sequences.

At step 1090 the detection of the third data field 53 is performed, which may comprise the demodulation of the second modulated sequence that results from the equalisation step 1080. Following detection of the third data field 53, all three data fields S 1, S2, and 53 may be output for use in setting signalling parameters to allow a data stream to be received and decoded.

To receive a signal as generated according to an aspect of the present invention a modified receiver system may be provided. Figure 4 illustrates a suitable adaptation of the receiver system of Figure 3. FFT component 410, CDS Component 420, Descrambler 430, Differential-Binary Phase Shift Keying (D-BPSK) Demodulator 440 and Correlator 450 are conserved from the receiver system of Figure 3. According to an aspect of the invention an additional data field detector (referred to herein as an S3 detector) 460 is provided which receives an output sequence from the CDS Component 420 and the extracted data fields SI and 52. The S3 detector 460 then extracts the third data field. S3 from the CDS Component output sequence using the extracted data fields SI and 52, for example as set out in step 1090.

The operation of the 53 detector 460 is described in more detail below in relation to two particular embodiments of the present invention.

A first embodiment of the present invention uses hierarchical D-BPSK to provide the second modulation scheme. Figure 7 illustrates an exemplary method of transmitting a wireless broadcast signal according to the first embodiment. It will be understood that following a description of the transmission method, a suitable transmitter comprising means to perform the processing of steps 710 to 770 may be provided.

Steps 710, 720 and 730 represent a method of producing a first modulated sequence that is compatible with the receiver system of Figure 3. At step 710 the first and second data fields SI and 52 arc encoded to produce a first and second modulation pattern. These modulation patterns may be required to provide OFDMbased transmission. In the present example, patterns to encode the first data field SI are based on 8 orthogonal sets of 8 complementary sequences of length 8 (total length of each SI pattern is 64), while patterns to encode the second data field S2 arc based of 1.6 orthogonal sets of 16 complementary sequences of length 16 (total length of each S2 pattern is 256).

These patterns have two main properties. Firstly, the sum of the auto-correlations of all the sequences of the set is equal to a Kroneckcr delta, multiplied by a KN factor, K being the number of the sequences of each set and N being the length of each sequence. In the case of Si K=N=8; in the case of S2, KN= 16. Secondly, each set of sequences arc mutually uncorrelated (also called I!matesr?) Typically, modulation patterns for each bit sequence of Si and S2 are provided in a look-up table in hexadecimal format, e.g. bit sequence 110 of data field S 1 maps onto modulation pattern 2E7B]D4821 741247. An exemplary look-up table is shown in Figure 15. The bit Sequences CSS51 (CSS510 CSS5163) and CSS52 (C5S520... CSS52255) for given values of SI and S2 respectively are obtained by taking the corresponding hexadecimal sequence from left to right and from most significant bit (MSB) to least significant bit (LSI3), i.e. CSS510 is the MSB of the first hexadecimal digit and CSS51,63 is the LSB of the last digit of the Si sequence. A first data sequence 715 carrying the first and second data fields is then produced by concatenating two modulation patterns for the first data field SI with a single modulation pattern for the second data field S2 in the form {CSS51, CSS52, CSS5i}. In the present example, this first data sequence has a length of 384. In this regard, the first data sequence is

said to carry the first and second data fields.

At step 720 D-BPSK modulation is applied to the first data sequence to produce a first modulated sequence 725. Differential Binary Phase Shift Keying is a known modulation technique wherein the bit patterns of a data sequence are used to change the phase of a carrier waveform. Changes in the phase of the carrier waveform are then used to demodulate a received signal. Constellation diagram 510 of Figure 5 illustrates the symbol encoding. At step 730 the first data sequence is optionally scrambled to produce a scrambled first modulated sequence 735.

Steps 740 to 760 represent a method of producing a second modulated sequence 765 according to the first embodiment. At step 740 a second data sequence 755 is produced that encodes the 123 bits of the third data field S3. In the present example, if 123 3 bits or 123= 4 bits the modulation patterns for the first data field SI (in the ease of n3 3) or the second data field S2 (in the case of 123 4) may be re-used to produce a chip sequence of length NhI less than the length Ncms of the first modulated sequence, i.e. a short sequence. As the length of the chip sequence is less than the length of the first modulated sequence then the chip sequence may be repeated Nr times over the frill length of the first modulated sequence. The chip sequence may also be interleaved as shown in Figure 7 to produce the second data sequence 755. For example: 53(1 + k x = sW; I = 1... NCh; k = 1. Nrep Nfms/N E.gforn3 = 3; Nhjp = 64;N15 = 384;k = 6 In a case where 123= 4, and =256, one full chip sequence and one half chip sequence may be concatenated to generate the second data sequence. The repetition and/or the interleaving of the chip sequence across the length of the first modulated sequence ensures that all sequences have maximum diversity and equally affect the detection of the first and second data fields SI and S2. In other embodiments, sequences other than those provided by the modulation patterns of data fields SI and S2 may be used, although it is preferred that such sequences maintain good cross-correlation properties, i.e. enable cross-correlation operations on the data. Using the modulation patterns fer data fields SI or 52 does provide an implementation advantage in that existing sequence generation and/or detection modules can be used to provide the same function for the third data field Si This avoids the need for new hardware and further reduces cost.

At step 750 the second data sequence 755 is modulated using a hierarchical (/9) D-I3PSK modulation scheme. The constellation diagram 520 for an exemplary hierarchical /9-D-BPSK modulation scheme is shown in Figure 5.

The second data sequence is encoded according to the following model: d3(i) = { i = 2...N,N = 384; d3(0) 1 i.e. a rotation of /1? radians is provided between consecutive symbols. This produces a second modulated sequence 765. Although this example is described in relation to a modified D-BPSK system, in practice any modulation scheme that enables non-coherent detection can be used, including other PSK schemes (QPSK, 8-PSK etc) wherein an additional rotation of /1? radians from the normal symbol values is provided.

At step 770 the second modulated sequence 765 is superimposed on the first modulated sequence 735 to generate a superimposed sequence d123. In the present example, the first data sequence d12 (i) and the second data sequence d3(i) are multiplied. This is possible as both sequences are complex signals with a magnitude of one, wherein information is carried in the phase of the signal. In other embodiments, depending on the form of the sequences, other superimposition operations could alternatively be used, such as bit-addition, AND operations etc.. In the current example, d123 is of length 384 and may be transmitted using known transmission methods.

An exemplary receiver system according to the first embodiment of the present invention is shown in Figure 6. Figure 6 illustrates in more detail components of the S3 detector 460 utilised in the first embodiment. Features not explicitly described are assumed to be conserved from Figures 3 and 4. In this case signal P1 A(l... 1024) may comprise a received version of transmitted sequence d123.

The S3 detector 460 of Figure 6 comprises an Si, 52 Encoder 660, a hierarchical f3-D-BPSK Demodulator 670, an S3 Deinterleaver 680 and an 53 Correlator 690. The Si $2 Encoder 660 receives the extracted data fields Si and S2 from th.e SI, S2 Correlator 650 and reconstructs the first modulated sequence d12. This reconstructed sequence is then applied to superimposed sequence y(I * . .384) to remove the superimposition. In the present example, this is achieved by subtracting the first modulated sequence dt2 from the superimposed i 0 sequence y(l.. .384). This may be achieved by multiplying the superimposed sequence y(l. . .384) by a negative version of the reconstructed first modulated sequence d12. In other embodiments, alternatives to the Si,S2 Encoder 660 and subtractor may be used that remove the superimposition of the first and second modulated sequences.

Following the removal of the superposition the resultant sequence is input into the hierarchical /3D-BPSK Demodulator 670. The resultant sequence represents the second modulated sequence, plus signal noise due to transmission through a communication channel. The hierarchical fl-D-BPSK Demodulator 670 demodulates the second modulated sequence assuming the symbol encoding described above with respect to Figure 7, i.e. assuming a rotation of /3 radians between the symbols of 0 and 1, wherein each symbol is offset from standard BPSK symbols by /3/2 radians. In one embodiment 6' = 2ir/3 radians. In other embodiments that use other modulation schemes to modulate the second data sequence, the Demodulator 670 may be altered accordingly. The result is an estimated. secon.d data sequence:?t (1... 384). This estimated second data sequence is input into 83 Deinterleaver 680, which removes any interleaving of the second data sequence to leave a series of repeated chip sequences: 5k (1... NChL) where k = I.. Nrep. These chip sequences are correlated by S3 Correlator 690 to extract the bits of the third data field 53.

Figure 11 shows simulation results for the reception of signal preambles constructed according to the first embodiment. The graphs show Bit Error Rate (ERR) versus Signal to Noise Ratio (in decibels -dB) for data fields SI and S2 in the existing T2 standard and an adapted NGH standard that incorporates the extra third data field 83 provided by the first embodiment, wherein /3 1.5 radians. References to "old" values in the graphs refer to the data fields Si and 82 when using the T2 standard, whereas references to "new" values refer to the data fields Si and S2 when using the embodiments of the present invention. A first graph 1110 shows an Additive Gaussian White Noise (AWGN) noise model and a second graph 1120 shows a TU6-60kmph noise model, wherein TU6 is a Typical Urban Mode 6 channel model with an assumed terminal speed of 60km/h. When fl 3, the capacity of the first preamble signalling section 130 is increased by 43% (10 bits are carried as opposed to 7 bits with the first and second fields alone) at the expense of a 1.5 to 2 dB degradation of data fields Si and 52. This degradation is slight and enables full detection of the first and second data fields SI and 82 with existing equipment, i.e. full backward compatibility. When n3 4 the degradation increases by 0.5dB; however, this still allows successful detection of the existing data fields as well as the

additional. data field S3.

By optimising the value of /3 in the hierarchical D-BPSK it is possible to improve performance. In a variation of the first embodiment two values of /3 are used: a first rotation of radians for data sequence values relating to the first data field Sl and a second rotation of f3 radians for data sequence values relating to the second data field 82. Following from the example described above, this corresponds to the use of a first rotation value for subcarriers relating to the first data field Si and the use of a second rotation value for subearriers

relating to the second data field S2. For example:

exp(j ç \ 2J jS3tj-i=2...N,N=384; d3(0)1 (cxp (-i-f) if 53(t) 1 wherein: i [2... 64) U [321... 384) -, f3 = Psi 1[65.,,32O)4fliflsz The values of the parameters fl and /2 may be tuned to align the detection performance of all three data fields. For example, Figure 12 shows simulation results for the reception of signal preambles constructed according to the present variation, wherein P13 = 4, /3 = 1.S radians and /2 = 2 radians.

References to "old" values in the graphs refer to the data fields S l and 52 when using the T2 standard, whereas references to "new" values refer to the data fields 51 and 52 when using the embodiments of the present invention. Again, a first graph 1210 shows an AWGN noise model and a second graph 1220 shows a TU6-60kmph noise model. This results in a 0.5 to 0.7 dB gain as compared to a case where only a single value of /3 is used. Compared to an existing preamble symbol, the modified preamble according to the present variation of the first embodiment provides an increase in capacity of 57% (11 bits as opposed to 7 bits) with a 1.5dB degradation in data fields Si and 52. With such an example, the performance gap between all three data fields SI, 52 and S3 is less than 0.25dB.

It is also possible to modify the optimisation criteria used to produce tuncd values of /3 and/or /?. For example, if one or more of the three fields required different levels of robustness, different tuned values could be accordingly calculated; e.g. a degradation in the detection of data field S2 may be more acceptable than a degradation in the detection of data field SI.

A second embodiment of the present invention will now be described.

The second embodiment uses a second modulation scheme wherein a plurality of repeated sequences encodes the third data field S3. The repeated sequences will be referred to as short continuous phase modulated (CPM) sequences. The term short is used to denote that these sequences have a length Nhp less than the length Nfms of the first modulated sequence; typically, is 16 or 32 whereas in a 1k FFT, DVB-T2 implementation = 384. The term continuous phase modulated is used to denote a slow yet continuous variance in phase across the length of the second modulated sequence, which is equal in length to the first modulated sequence. A variance in phase is slow if, in the time domain, the time taken for a change in phase of the second modulated sequence is greater than the time taken for a change in phase of the first modulated sequence; with an optimum difference between timings being one that allows detection of data fields SI and 52 from the first modulated sequence. A variance is continuous if changes in the phase occur without abrupt or discontinuous changes across the whole length of the second modulated sequence, such that when it is superimposed on the first modulated sequence, such changes occur smoothly and continuously across the whole length of the first modulated sequence. This results in short CPM sequences wherein the phase of each sequence varies across the length of the sequence and the difference in phase between two consecutive samples of the short CPM sequence is less than the difference in phase between two consecutive samples of the first modulated sequence. These properties of a second modulated sequence according to the second embodiment prevent degradation to the first modulated sequence such that said sequence can be accurately demodulated to provide data fields S I and S2.

Figure 9 illustrates an exemplary method of transmitting a wireless broadcast signal according to the second embodiment. It will be understood that following a description of the transmission method, a suitable transmitter comprising means to perform the processing of steps 910 to 960 may be provided. Steps 910 to 930 correspond substantially to steps 710 to 730 of Figure 7, as such a first data sequence 915 is generated from data fields SI and S2, modulated at step 920 using D-BPSK to generate a first modulated sequence 925, and optionally scrambled at step 930 to produce a scrambled first modulated sequence 935, which is equivalent to sequence 735.

The second embodiment differs from the first embodiment in the manner of the second modulation scheme, i.e. the manner in which the second modulated sequence is produced. In Figure 9, k short sequences, 53k' are generated based on a function which provides sequences with constant amplitude, slow variation of phase andlor a smooth transition between adjacent short sequences. Preferably the function meets all three requirements. Examples of exemplary functions are tones, chirps, complex exponential functions or Zadoff-Chu functions. Tones comprise data sequences wherein information is carried in the phase of the sequence and chirps comprise data sequences wherein information is carried in the frequency of the sequence. For example, a tone sequence may be generated according to a first model: 53k (1) = exp (I -I = 0... (NC -1), 9k E [9, ..., 24 -J \ chip! and a chirp sequence may be generated according to a second model: / o/2\ = exp U2M) I = 0... (N -1), 0k f0, ,.,,02n4_1} \ 1"chtp/ wherein / denotes the /-th component of sequence S3k, and Ok5 a phase modifier for the k4h sequence. Figure 13 shows the real and imaginary components of an exemplary chirp sequence. The chirp sequence is repeated three times across 100 samples and from the real and imaginary plots (1310 and 1320) it can be seen how the frequency of the chirp waveform increases towards the end of each sequence (i.e is "swept").

In one variation of the second embodiment a set of tones or chirps superimposed onto the part of the first modulated sequence carrying the first data field (SI) is different from a set of tones or chirps superimposed onto the part of the first modulated sequence carrying the second data field (S2). As with the use of two /3 values in the variation of the first embodiment, the use of different tones or chirps for sequence portions corresponding to the different first and second data fields SI and S2 allows optimisation based on the particular characteristics of the SI and S2 portions of the first modulated sequence. It also enables a more selective degradation of the detection of the first and second data fields, for example, degradation to the detection of the first data field SI may be preferred over degradation to the detection of the second

data field S2 (and vice versa).

A smooth transition between adjacent short sequences is achieved by appropriately tuning the sequences so that all sequences start and finish with similar phases, i.e. the phase difference between the end of a sequence and the start of an adjacent sequence is less than a predetermined threshold; the threshold being selected so that data fields S 1 and S2 can still be successfully detected. As a slow modification of phase between samples is required to prevent degradation to the existing data fields, the use of sequences in which information is carried in the phase or frequency is somewhat counterintuitive.

However, it has been found that such sequences can be successfully used.

S Each short sequence S3k is repeated times across the length of the first modulated sequence, which in the present case corresponds to the full spectrum of active subcarriers of the preamble. This is illustrated in the second modulated sequence 955, wherein Nrep = x Nc) and K = and n3 and n4 are two bit size parameters. The repetition has two main functions.

First, it enables the noise term introduced by the communication channel to components of each sequence to be filtered, effectively reducing the noise level by a factor equal to I +]Vrep. Second, the repetition ensures that alE components experience similar fading effects within the communication channel, so that no channel estimation is required and the third data field 53 can be detected non-coherently. Assuming the communication channel is reasonably flat across the subcarriers carrying the short sequence, the good cross-correlation properties of the short sequences is preserved, i.e. the ability to produce accurate cross-correlation results is maintained. The short sequences 53k may also further be interleaved across the second modulated sequence 955.

In one variation of the second embodiment different repetition patterns may be used. For example, a standard repetition pattern may simply repeat the components of the chip sequences in order, for example: [do... dN.] } [do... dNI I [d0... dN.11 [do... dN.1]. A modified pattern may then reverse the components in alternate repetitions, for example: [do... dNJJ [dN. ... do] [do... dN..[] [dNj d3]. The modified pattern may be used to average out any constant shift which would remain in the equivalent channel of adjacent components d and d1+1.

Once a second modulated sequence 955 has been generated, it is superimposed on the first modulated sequence 735 at step 960, in a similar manner to step 770 of Figure 7, to produce a superimposed sequence d123, which may he transmitted using known transmission methods.

Figure 8 shows an exemplary receiver system according to the second embodiment of the present invention. As for Figure 6, Figure 8 illustrates in more detail the components of the 53 detector 460. Features not explicitly described are assumed to be conserved from Figures 3 and 4. In this case signal P1 A(l... 1024) may comprise a received version of transmitted sequence din, for example that produced by step 960 above.

The S3 detector 460 of Figure 8 Comprises an SI, S2 Encoder 860, a Phase Detector 870, an Averaging Component 880 and an Estimator 890. The Si,S2 Encoder 860 has substantially the same function as Si,S2 Encoder 660, i.e. to reconstruct the first modulated sequence from data fields Si and S2 so as to remove the superimposition of the first and second modulated sequences.

Again, a subtraetor may be used to subtract a reconstructed sequence from a received superimposed sequence y(I.. .384). Hence, the input to the Phase Detector 870 comprises a received second modulated sequence, plus any noise introduced by the communication channel: z(t) = h(1)S3k(l) + W(l) The operation of the Phase Detector 870, Averaging Component 880 and Estimator 890 will now be described in relation to two examples: a first using tones and a second using chirps. The tones and chirps may be constructed according to the models presented above.

In the Phase Detector 870 the phase difference between successive samples of the second modulated sequence is measured in order to estimate the phase modifier term ° as repeated across each set of short sequences.

Assuming that noise terms are filtered out, the received second modulated signal will be modelled as: x(1) = h(l)53ft(l) wherein for tone sequences the phase difference p. (1) of an 1-th component being of repetition r is estimated according to the following calculation: x(1)x(1 + 1) = h(1)th(1 + 1) exp ( O1) exp + = h(O*h(1+1)exp(J o1.o(1+1)) 2NchLp = h(lfh(L +1) exp (i 2N ((1+1) -1)) cMp / r = h(L)*h(1+1)exp(j k \ 2Nc or = \ where 1 0... Nh1p -2; and for chirp sequences the phase difference 4r(/) of an 1-th component is estimated according to the following calculation: I O/12 / 9,(1+1)2 Pr(D =x(1)*x(1I1) =h(L)*h(1+ 1)cxp( -j Jexp(j \ 2NC,1Pj \. ZNctp = h(1)*h(1 +1) exp (- + 9(1 + 1)2) / or = h(1)*h(1 + 1) expfj-((1 + 1)2_12) \ / r = h(1)*h(L + 1) exp j"(2L + 1) \ 2Ncip / or = Ih(1)IIh(l+1)Iexp(j k (21+1)+e \ where 1 0... A'hl -2.

The phase difference estimates c°r (/) are then used to estimate the phase modifier term O. This is achieved using the following calculation for tone sequences: NchipZ N&t2 Nchcpl 0 ______ arg 9r(1) +E1 (1Vchtp 2) + = 2N (ivChIP -2) chfp and the following calculation for chirp sequences: Nchip2 Nchip-2 1c' v1 (8 arg L cor(1)= L (=0 1=0 -o ( (C -2)(NCh1 -1) -2N -+(NChIP-l) chipk.

NchLpi + X E12 1=1 chip The output of Phase Detector 870 thus comprises a number of phase modifier estimates O. These estimates are then averaged over the repetitions (r) of each sequence S3k by Averaging Component 880. This may be achieved for tone sequences using the following calculation: Nrep Nchjp2 2NChp Np(Njp -2) arg r(1) and for chirp sequences using the following calculation: Nrep Nch1p2

_______________ ST ST

--arg Pr(l) rept, chip -) r=1 1=0 The averaging over each set of repeated sequences performed by the Averaging Component 880 filters, i.e. removes, the noise n(i) introduced by the communication channel. The output of the Averaging Component 880 comprises a number of phase modifier estimates for each set of short sequences, 8kP... n3). These are input into the Estimator 890 which extracts the corresponding bit values of the third data field, i.e. S3(l... n3). In one embodiment Estimator 890 comprises a maximum likelihood (ML) estimator wherein a transmitted S3k field is estimated by computing the distance to all possible transmitted values: argminf(Bk -This process is repeated for all values of It from 1 to NrE,, obtaining n4 bits on each repetition until all 113 bits are extracted.

The choice of function to produce the repeated short sequences depends on each implementation. Comparing the averaging expressions for tone and chirp sequences it is apparent that chirp sequences offer better noise filtering as the denominator term Niep ( NChLV -1)2 from the chirp calculation is larger than the denominator term Nrep ( NChI -2) from the tone calculation. However, a chirp function may introduce a high frequency component at the end of each chirp sequence that may introduce a larger degradation to the detection of data

fields SI and 52 than tone sequences.

Figure 14 shows simulation results for the reception of signal preambles constructed according to the second embodiment. The graphs show Bit Error Rate (BER) for each data field SI, 52, S3 and Frame Error Rate (FER) for a P1 symbol versus Signal to Noise Ratio (in decibels -dB) for the existing T2 standard (solid line plots) and an adapted NGH standard (dashed line plots) that incorporates the extra third data field S3 provided by the second embodiment, the short sequences being based on a chirp function and n3 = 4. Frame Error Rate is used for the P1 symbol as in the present example there is one P1 symbol per frame and a check is made to see if the complete P1 symbol has been received correctly. A first graph 1410 shows an AWGN noise model and a second graph 1420 shows a TTJ6-6Okmph noise model. In the illustrated simulations, the degradation in detection of the first and second data fields SI and S2 is around 1.5dB when it3 = 3 and less than 2dB when /? 4.

Figures 1 óa, I 6b and I 6c show an exemplary use of the new third data field 53 in the DVB-NGU standard. These uses are provided as examples only and should not be seen as limiting. The Figures show possible bit patterns for data fields SI, S2 and S3, wherein 53 is of length (it3) 4 bits, 3 bits and 2 bits respectively. Figure 1 6a illustrates a case where P13 4 bits. Building on the bit patterns for data fields SI and S2 shown in Figures 2a to 2d, an S I bit pattern of 1 Ox indicates that the DVR-NGH standard is being used. The least significant bit of data field SI then indicates SISO/MISO as for the T2 standard. Bit I of field I of data field S2 indicates the NGH profile (NGH Prof) being used (for example T2 Mobile, MGH, etc.) and the last two bits of field I indicate FFT parameters. The "mixed" bit of data field 82, i.e. field 2, is signalled as shown in Figure 2d. The first bit of the new third data field then indicates the waveform used, for example, OFDM, Single Carrier (SC)-OFDM etc.. The last three bits of the third data field 83 then fully signal the guard interval (GI) such that S Cyclic Prefix (CP) correction is not required. In Figure 16b, where it3 = 3 bits, a bit pattern of 011 may be used for the first data field 81 and the SISO/MISO parameter may be signalled by the first bit of the third data field 53. The second bit may then indicate the waveform, as described above. Finally, the third bit of the third data field 83 may provide a hint for the guard interval which is fully I 0 resolved with subsequently transmitted information; for example, the hint may prime the reception apparatus for a particular subset of intervals. Figure 1 6c shows a n3 = 2 ease, wherein the SISO/MISO parameters are signalled by the third bit of the first data field SI and the third data field 53 provides a waveform indication and guard interval hint.

The above embodiments are to be understood as illustrative examples of the invention. Further embodiments of the invention are envisaged. For example, the invention may be adapted to be used with different forms of data signals that use different transmission and reception standards. The invention may be applied to alternative data included in a transmitted signal, wherein limitations in the signal format mean that further signal capacity is required, for example, relating to alternate forms of control data. The examples described above demonstrate that superimposing a new sequence on an existing sequence generates extra capacity while providing a tolerable degradation to the detection performance of the existing sequence. Certain described examples provide an increase in capacity of over fifty-percent, while only introducing a degradation of around 1.5dB.

Advantages of the invention will be apparent to those in the art from the description. In particular, the present invention increases capacity while maintaining full backward compatibility. Both embodiments can be simply implemented as part of next generation NGT-T or T2-mobile receivers without complex and expensive components. The detection of the first and second data fields Si and S2 uses existing system components and at least the first embodiment allows the re-use of those components to detect the third data field 53. The described solution also adds capacity without affecting the timing and/or frequency synchronisation properties of a preamble symbol. The Peak-to-Average Power Ratio (PAPR) of the preamble section is substantially maintained, for example in some embodiment a degradation of only around 0.5dB is introduced.

The method of transmitting andlor receiving data according to the present invention may be implemented using dedicated circuits or appropriately programmed components. In addition, embodiments of the invention can also be implemented through computer readable code/instructions inlon a medium, e.g., a computer readable medium, to control at least one processing element. The medium can correspond to any medium/media permitting the storage and/or transmission of the computer readable code. The computer readable code can be recorded/transferred on a medium in a variety of ways, with examples of the medium including recording media, such as magnetic storage media (e.g., ROM, floppy disks, hard disks, etc.) and optical recording media (e.g., CD-ROMs, or DVDs), and transmission media such as Internet transmission media.

Furthermore, the media may also be a distributed network, so that the computer readable code is stored/transferred and executed in a distributed fashion.

Embodiments of the invention are described in terms of functional block components and various processing steps. Such functional blocks may be realized by any number of hardware and/or software components configured to perform the specified functions. For example, an embodiment of the invention may employ various integrated circuit components, e.g., memory elements, processing elements, logic elements, look-up tables, and the like, which may carry out a variety of ftmctions under the control of one or more microprocessors or other control devices. Similarly, where the elements of an embodiment of the invention arc implemented using software programming or software elements the invention may be implemented with any programming or scripting language such as C, C++, Java, assembler, or the like, with the various algorithms being implemented with any combination of data structures, objects, processes, routincs or other programming elcmcnts. Furthermore, an embodiment of the invention could employ any number of conventional techniques for electronics configuration, signal processing and/or control, data processing and the like.

For the sake of brevity, conventional electronics, control systems, software development and other functional aspects of the systems (and components of the individual operating components of the systems) may not be described in detail. Furthermore, the connecting lines, or connectors shown in l U the various figures presented arc intended to represent exemplary functional relationships and/or physical or logical couplings between the various elements.

It should be noted that many alternative or additional functional relationships, physical connections or logical connections may be present in a practical device.

It is to be understood that any feature described in relation to any one I S embodiment may be used alone, or in combination with other features described, and may also be used in combi.nation with one or more features of any other of the embodiments, or any combination of any other of the embodiments.

Furthermore, equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined in the accompanying claims.

Claims (45)

1. Claims 1. A method of transmitting data comprising one or more frames, said frames comprising a preamble section and a data section, the preamble section having a synchronisation function and carrying at least signalling information, the method comprising: modulating a carrier waveform with a first data sequence of the preamble section to generate a first modulated sequence, said modulating using a first non-coherent modulation scheme; modulating a second data sequence of the preamble section to generate a second modulated sequence, said modulating using a second modulation scheme capable of non-coherent detection that provides continuous variation of the second modulated sequence across the length of the first modulated sequence; superimposing the second modulated sequence upon the first modulated sequence to produce a superimposed sequence; and transmitting the superimposed sequence.
2. 2. The method of claim 1, wherein the carrier waveform comprises a plurality of sub-carriers transmitting the superimposed sequence.
3. 3. The method of claim 2, wherein the first data sequence carries a first field (Si) and a second field (S 2), the first field being modulated on a first set of sub-carriers and the second field being modulated on a second set of sub-carriers.
4. 4. The method of any one of the preceding claims, wherein the second modulation scheme comprises hierarchical differential phase shift keying (/3-DPSK).
5. 5. The method of claim 4, wherein the second modulation scheme comprises providing a rotation of up to ±/3 radians between consecutive symbols of the second data sequence.
6. 6. The method of claim 5, wherein the differential phase shift keying comprises differential binary phase shift keying (D-BPSK) and wherein /3 is less than ic/2 radians.
7. 7. The method of claim 4, when dependent on claim 3, wherein a first rotation of /3j radians is used between consccutivc symbols of the first field (Si) and a second rotation of /32 radians is used between consecutive symbols ofthe second field (S2).
8. 8. The method of any one of the preceding claims, wherein the second data sequence carries a third field (53) in a chip sequence the chip sequence being repeated and/or interleaved across the second data sequence.
9. 9. The method of claim 8, when dependent on claim 3, wherein the chip sequences carrying the third field (S3) are derived from a set of sequences that are used to generate the first data sequence carrying the first and secondfields (SI, S2).
10. 10. The method of any one of claims I to 3, wherein the second modulation scheme comprises the generation of a plurality of short sequences carrying a third field (S3) that are repeated across the length of the first modulated sequence to form the second modulated sequence.
11. Ii. The method of claim 10, wherein each short sequence is of a length Nchip that is less than the length of the first modulated sequence n
12. 12. The method of claim 10 or claim 11, wherei.n a phase of each short sequence varies across the length of the short sequence, the diffcrcnee in phase between two consecutive samples of the short sequence being less than the difference in phase between two consecutive samples of the first modulated sequence.
13. 13. The method of any one of claims 10 to 12, where the short sequences are repeated and/or are interleaved across the second modulated sequence.
14. 14. The method of any one of claims 10 to 13, wherein the plurality of short sequences comprise one of tones, wherein part of the third field (S3) is carried in a phase value of the short sequence; or chirps, wherein part of the third field (53) is carried in a frequency sweep of the short sequence.
15. 15. The method of any one of claims 10 to 14, wherein each short sequence is tuned so that a phase difference between the end of one short sequence and the beginning of an adjacent short sequence is less that a predetermined threshold.
16. 16. The method of claim 14, when dependent on claim 3, wherein a set of tones superimposed onto a part of the first modulated sequence carrying the first field (51) is different from a set of tones superimposed onto a part of the first modulated sequence carrying the second field (52).
17. 17. The method of claim 14, dependent on claim 3, wherein a set of chirps superimposed onto a part of the first modulated sequence carrying the first field (SI) is different from a set of chirps superimposed onto a part of the first modulated sequence carrying the second field (S2).
18. 18. A method of receiving data comprising one or more frames, said frames comprising a preamble section and a data section, thc preamble section having a synchronisation function and carrying at least signalling information, the method comprising: receiving a signal comprising a second modulated sequence superimposed on a first modulated sequence; demodulating the signal to extract a first data sequence of the preamble section from the first modulated sequence, said demodulating using a first non-coherent demodulation scheme; removing the superimposition to provide the second modulated sequence; and demodulating the second modulated sequence using a second demodulation scheme to extract a second data sequence of the preamble section, wherein the second demodulation scheme is configured to provide non-coherent detection of the second data sequence.
19. 19. The method of claim 18, wherein the signal comprises a plurality of sub-carriers transmitting a superimposed sequenee
20. 20. The method of claim 19, wherein the first data sequence carries a first field (S 1) and a second field (S2), the first field being demodulated from a first set of sub-carriers and the second field being demodulated from a second set of sub-carriers.
21. 21. The method of any one claims 18 to 20, wherein the second demodulation scheme comprises hierarchical differential phase shift keying (/1-DPSK).
22. 22. The method of claim 21, wherein the second demodulation scheme assumes a rotation of up to ±f3 radians between consecutive symbols of the second data sequence.
23. 23. The method of claim 22, wherein the differential phase shift keying comprises differential binary phase shift keying (D-BPSK) and wherein /1 is less than t/2 radians.
24. 24. The method of claim 21, when dependent on claim 20, wherein a first rotation of /3; radians is assumed between consecutive symbols of the first field (S 1) and a second rotation of /32 radians is assumed between consecutivesymbols of the second field (52).
25. 25. The method of any one of claims 18 to 24, wherein the second data sequence carries a third field (S3) in a chip sequence the chip sequence being repeated andlor interleaved across the second data sequence.
26. 26. The method of claim 25, when dependent on. claim 20, wherein the chip sequences carrying the third field (53) are derived from a set of sequences that are used to generate the first data sequence carrying the first andsecond fields (Si, 52).
27. 27. The method of any one of claims 18 to 20, wherein the second modulated sequence comprises a plurality of short sequences carrying a third field (S3) that are repeated across the length of the first modulated sequence.
28. 28. The method of claim 27, wherein each short sequence is of a length that is less than the length of the first modulated sequence Nfms.
29. 29. The method of claim 27 or claim 28, wherein a phase of each short sequence varies across the length of the short sequence, the difference in phase between two consecutive samples of the short sequence being less than the difference in phase between two consecutive samples of the first modulated sequence.
30. 30. The method of any one of claims 27 to 29, where the short sequences are repeated and/or are interleaved across the second modulated sequence.
31. 31. The method of any one of claims 27 to 30, wherein the plurality of short sequences comprise one of: tones, wherein part of the third field (53) is carried in a phase value of the short sequence; or chirps, wherein part of the third field (53) is carried in a frequency sweep of the short sequence.
32. 32. The method of any one of claims 27 to 31, wherein each short sequence is tuned so that a phase difference between the end of one short sequence and the beginning of an adjacent short sequence is less that a predetermined threshold.
33. 33. The method of claim 30, when dependent on claim 20, wherein the second modulated sequence comprises a set of tones superimposed onto a part of the first modulated sequence carrying the first field (S 1) that are different from a set of tones superimposed onto a part of the first modulated sequencecarrying the second field (52).
34. 34. The method of claim 30, dependent on claim 20, wherein the second modulated sequence comprises a set of chirps superimposed onto a part of the first modulated sequence carrying the first field (SI) that are different from a set of chirps superimposed onto a part of the first modulated sequencecarrying the second field (52).
35. 35. The method of claim 30, wherein the step of demodulating using a second demodulation scheme comprises: determining a phase difference between consecutive samples of the second modulated sequence to produce a plurality of phase estimates; averaging the phase estimates to determine an average phase estimate for each sequence; and using the average phase estimate to extract phase or frequency sweep values representing encoded data.
36. 36. The method of any one of claims 18 to 35, wherein removing the superimposition to provide the second modulated sequence comprises: reconstructing the first modulated sequence from the extracted first data sequence; and subtracting the reconstructed first modulated sequence from the signal to provide the second modulated sequence.
37. 37. A transmitter for transmitting data comprising one or more frames, said frames comprising a preamble section and a data section, the preamble section having a synchronisation function and carrying at least signalling information, the transmitter being arranged to: modulate a carrier waveform with a first data sequence of the preamble section to generate a first modulated sequence, said modulating using a first non-coherent modulation scheme; modulate a second data sequence of the preamble section to generate a second modulated sequence, said modulating using a second modulation scheme capable of non-coherent detection that provides continuous variation of the second modulated sequence across the length of the first modulated sequence; superimpose the second modulated sequence upon the first modulated sequence to produce a superimposed sequence; and transmit the superimposed sequence.
38. 38. The transmitter of claim 37, wherein the carrier waveform comprises a plurality of sub-carriers transmitting the superimposed sequence.
39. 39. The transmitter of claim 38, wherein the first data sequence carries a first field (SI) and a second field (S2), the first field being modulated on a first set of sub-carriers and the second field being modulated on a second set of sub-carriers.
40. 40. The transmitter of any one of claims 36 to 39, wherein the second modulation scheme comprises hierarchical differential phase shift keying (/3-DPSK).
41. 41. The transmitter of claim 40, wherein the second modulation scheme comprises providing a rotation of up to ±73 radians between consecutive symbols of the second data sequence.
42. 42. The transmitter of claim 41, wherein the differential phase shift keying comprises differential binary phase shift keying (D-BPSK) and wherein /3 is less than m/2 radians.
43. 43. The transmitter of claim 40, when dependent on claim 39, wherein a first rotation of /3, radians is used between consecutive symbols of the first field (SI) and a second rotation of /32 radians is used between consecutivesymbols of the second field (S2).
44. 44. The transmitter of any one of claims 37 to 43, wherein the second data sequence carries a third field (S3) in a chip sequence, the chip sequence being repeated andlor interleaved across the second data sequence.
45. 45. The transmitter of claim 44, when dependent on claim 39, wherein the chip sequences carrying the third field (53) are derived from a set of sequences that are used to generate the first data sequence carrying the first andsecond fields (Si, S2).D46. The transmitter of any one of claims 37 to 39, wherein the second modulation scheme comprises the generation of a plurality of short sequences carrying a third field (53) that are repeated across the length of the first modulated sequence to form the second modulated sequence.47. The transmitter of claim 46, wherein each short sequence is of a length N0h that is less than the length of the first modulated. sequence Nims.48. The transmitter of claim 46 or claim 47, wherein a phase of each short sequence varies across the length of the short sequence, the difference in phase between two consecutive samples of the short sequence being less than the difference in phase between two consecutive samples of the first modulated sequence.49. The transmitter of any one of claims 46 to 48, where the short sequences are repeated and/or are interleaved across the second modulated sequence.50, The transmitter of any one of claims 46 to 49, wherein the plurality of short sequences comprise one of: tones, wherein part of the third field (S3) is carried in a phase value of the short sequence; or chirps, wherein part of the third field. (SB) is carried in a frequency sweep of the short sequence.51. The transmitter of any one of claims 46 to 50, wherein each short sequence is tuned so that a phase difference between the end of one short sequence and the beginning of an adjacent short sequence is less that a predetermined threshold.52. The transmitter of claim 50, when dependent on claim 39, wherein a set of tones superimposed onto a part of the first modulated sequence carrying the first field (SI) is different from a set of tones superimposed onto a part of the first modulated sequence carrying the second field (52).53. The transmitter of claim 50, dependent on claim 39, wherein a set of chirps superimposed onto a part of the first modulated sequence carrying the first field (SI) is different from a set of chirps superimposed onto a part of the first modulated sequence carrying the second field (52).54. A receiver for receiving data comprising one or more frames, said frames comprising a preamble section and a data seetion the preamble section having a synchronisation function and carrying at least signalling information, the receiver being arranged to: receive a signal comprising a second modulated sequence superimposed on a first modulated sequence; demodulate the signal to extract a first data sequence of the preamble section from the first modulated sequence, said demodulating using a first non-coherent demodulation scheme; remove the superimposition to provide the second modulated sequence; and demodulate the second modulated sequence using a second demodulation scheme to extract a second data sequence of the preamble section, wherein the second demodulation scheme is configured to provide non-coherent detection of the second data sequence.55. The receiver of claim 54 wherein the signal comprises a plurality of sub-carriers transmitting a superimposed sequence.56. The receiver of claim 55, wherein the first data sequence carries a first field (SI) and a second field (S2), the first field being demodulated from a first set of sub-carriers and the second field being demodulated from a second set of sub-carriers.57. The receiver of any one claims 54 to 56, wherein the second demodulation scheme comprises hierarchical differential phase shift kcying (/3-DPSK).58. The receiver of claim 57, wherein the second demodulation scheme assumes a rotation of up to ±/3 radians between consecutive symbols of the second data sequence.59. The receiver of claim 58, wherein the differential phase shift keying comprises differential binary phase shift keying (D-BPSK) and wherein /Jis less than n/2 radians.60. The receiver of claim 57, when dependent on claim 56, wherein a first rotation of /3 radians is assumcd between consecutive symbols of the first field (S 1) and a second rotation of /12 radians is assumed between consecutivesymbols of the second field (S2).61. The receiver of any one of claims 54 to 60, wherein the second data sequence carries a third field (53) in a chip sequence, the chip sequence being repeated and/or interleaved across the second data sequence.62. The receiver of claim 6t, when dependcnt on claim 56, wherein the chip sequences carrying the third field (S3) are derived from a set of sequences that are used to generate the first data sequence carrying the first andsecond fields (SI, S2).63. The receiver of any one of claims 54 to 56, wherein the second modulated sequence comprises a plurality of short sequences carrying a third field (83) that are repeated across the length of the first modulated sequence.64. The receiver of claim 63, wherein each short sequence is of a length that is less than the length of the first modulated sequence Nrms.65. The receiver of claim 63 or claim 64, wherein a phase of each short sequence varies across the length of the short sequence, the difference in phase between two consecutive samples of the short sequence being less than the difference in phase between two consecutive samples of the first modulated sequence.66. The receiver of any one of claims 63 to 65, where the short sequences are repeated and/or are interleaved across the second modulated sequence.67. The rcceiver of any one of claims 63 to 66, wherein the plurality of short sequences comprise one of tones, wherein part of the third field (S3) is carried in a phase value of the short sequence; or chirps, wherein part of the third field (83) is carried in a frequency sweep of the short sequence.68. The receiver of any one of claims 63 to 67, wherein each short sequence is tuned so that a phase difference between the end of one short sequence and the beginning of an adjacent short sequence is less that a predetermined threshold.69. The receiver of claim 66, when dependent on claim 56, wherein the second modulated sequence comprises a set of tones superimposed onto a part of the first modulated sequence carrying the first field (SI) that are different from a set of tones superimposed onto a part of the first modulated sequencecarrying the second field (S2).70. The receiver of claim 66, dependent on claim 56, wherein the second modulated sequence comprises a set of chirps superimposed onto a part of thc first modulated sequence carrying the first field (Si) that are different from a set of chirps superimposed onto a part of the first modulated sequencecarrying the second field (S2).71. The receiver of claim 66, wherein the receiver is further arranged to: determine a phase difference between consecutive samples of the second modulated sequence to produce a plurality of phase estimates; average the phase estimates to determine an average phase estimate for each sequence; and use the average phase estimate to extract phase or frequency sweep values representing encoded data.72. The receiver of any one of claims 54 to 71, wherein the receiver is further arranged to: reconstruct the first modulated sequence from the extracted first data sequence; and subtract the reconstructed first modulated sequence from the signal to provide the second modulated sequence, to remove the superposition.73. A signal arranged to carry one or more frames of data, said frames comprising a preamble section and a data section, the preamble section having a signalling function and carrying at least signalling information; the signal being arranged to carry a first data sequence of the preamble section according to a first non-coherent modulation scheme and a second data sequence of the preamble section according to a second modulation scheme, the second modulation scheme being capable of non-coherent detection and providing a continuous variation of the second modulated sequence across the length of the first modulated sequence; the signal comprising a second modulated sequence produced by the second modulation scheme based on the second data sequence superimposed upon a first modulated sequence produced by the first modulation scheme based on the first data sequence.74. The signal of claim 73, wherein the signal comprises a plurality of sub-carriers.75. The signal of claim 73, wherein the first data sequence carries a first field (SI) and a second field (52), the first field being modulated on a first set of sub-carriers and the second field being modulated on a second set of sub-carriers.76. The signal of any one of claims 73 to 75, wherein the second modulation scheme comprises hierarchical differential phase shift keying (/3-DPSK).77. The signal of claim 76, wherein the second modulation scheme comprises providing a rotation of up to ±71 radians between consecutive symbols of the second data sequence.78. The signal of claim 77, wherein the differential phase shift keying comprises differential binary phase shift keying (D-BPSK) and wherein flis less than rr/2 radians.79. The signal of claim 78, when dependent on claim 75, wherein a first rotation of flj radians is used between consecutive symbols of the first field (Si) and a second rotation of /32 radians is used between consecutive symbols ofthe second field (S2).80. The signal of any one of claims 73 to 79, wherein the second data sequence carries a third field (S3) in a chip sequence, the chip sequence being repeated and/or interleaved across the second data sequence.81. The signal of claim 80, when dependent on claim 75. wherein the chip sequences carrying the third field (S3) are derived from a set of sequences that are used to generate the first data sequence carrying the first and secondfields (51, S2).82. The signal of any one of claims 73 to 75, wherein the second modulation scheme comprises the generation of a plurality of short sequences carrying a third field (S3) that arc repeated across the length of the first modulated sequence to form the second modulated sequence.83. The signal of claim 82, wherein each short sequence is of a length that is less than the length of the first modulated sequence Nims.84. The signal of claim 82 or claim 83, wherein a phase of each short sequence varies across the length of the short sequence, the difference in phase between two consecutive samples of the short sequence being less than thc difference in phase between two consecutive samples of the first modulated sequence.85. The signal of any one of claims 82 to 84, where the short sequences are repeated and/or arc interleaved across the second modulated sequence.S86. The signal of any one of claims 82 to 85, wherein the plurality of short sequences comprise one of: tones, wherein part of the third field (53) is carried in a phase value of the short sequence; or chirps, wherein part of the third field (53) is carried in a frequency sweep of the short sequence.87. The signal of any one of claims 82 to 86, wherein each short sequence is tuned so that a phase difference between the end of one short sequence and the beginning of an adjacent short sequence is less that a predetermined threshold.88. The signal of claim 86, when dependent on claim 75, wherein a set of tones superimposed onto a part of the first modulated sequence carrying the first field (Si) is different from a set of tones superimposed onto a part of the first modulated sequence carrying the second field (52).89. The signal of claim 86, dependent on claim 75, wherein a set of chirps superimposed onto a part of the first modulated sequence carrying the first field (Si) is different from a set of chirps superimposed onto a part of the first modulated sequence carrying the second field (S2).90. A non-transitory computer readable program product, comprising a computer usable medium having a computer readable program code embodied therein, the computer readable program code adapted to be executed to implement a method of transmitting data comprising one or more frames, said frames comprising a preamble section and a data section, the preamble section having a synchronisation function and carrying at least signalling information, the method comprising: modulating a carrier waveform with a first data sequence of the preamble S section to generate a first modulated sequence, said modulating using a first non-coherent modulation scheme; modulating a second data sequence of the preamble section to generate a second modulated sequence, said modulating using a second modulation scheme capable of non-coherent detection that provides continuous variation of the second modulated sequence across the length of the first modulated sequence; superimposing the second modulated sequence upon the first modulated sequence to produce a superimposed sequence; and transmitting the superimposed sequence.91. A non-transitory computer readable program product, comprising a computer usable medium having a computer readable program code embodied therein, the computer readable program code adapted to be executed to implement a method of receiving data comprising one or more frames, said frames comprising a preamble section and a data section, the preamble section having a synchronisation function and carrying at least signalling information, the method comprising: receiving a signal comprising a second modulated sequence superimposed on a first modulated sequence; demodulating the signal to extract a first data sequence of the preamble section from the first modulated sequence, said demodulating using a first non-coherent demodulation scheme; removing the superimposition to provide the second modulated sequence; and demodulating the second modulated sequence using a second demodulation scheme to extract a second data sequence of the preamble section, wherein the second demoduation scheme is configured to provide non-coherent detection of the second data sequence.4822-5633-3064, v. 4-5633-3064, v. 3-5633-3064, v. 3-5633-3064, v. 2