GB2510651A - Spread spectrum communication system with separate spreading codes for header and payload portions - Google Patents

Abstract

The invention discloses a spread spectrum communication system for framed data. It may be used to improve satellite broadcasting, e.g. DVB-S2 broadcasts. Payload data 222 of a frame is passed through a modulator to map the data onto modulation symbols. The output from this modulator is spread using a selected first spreading code S1 selected from a plurality of first spreading codes which have mutually different spreading factors. This allows payload data to be transmitted with a variable spreading factor. A header of the frame contains 231 physical layer signaling data (PLS) and an indication of the selected first spreading code. At least a portion of this header is spread using a second spreading code S2. The spreading factor of the second spreading code is greater than or equal to the spreading factor of each of the first spreading codes. This ensures that the header can always be received, even when the SNR is poor. The header may additionally include a start of frame (SOF) indication 203, spread using a third spreading code S3. The frame is modulated onto an RF carrier for transmission. There are transmitter and receiver embodiments. In an embodiment the receiver differentially encodes a received baseband signal (see Fig. 14) and the differentially encoded signal is correlated with a stored expected version of the differentially encoded signal in order to detect the start of the frame.

Description

DATA PROCESSING APPARATUS AND METHOD

Ficid of Disclosure

The present disclosure relates to transmitters for transmitting data using a radio signal and receivers for detecting the radio signal and recovering the data from the S radio signal.

Background of thc Disclosure

Wireless communication systems communicate data for various applications using radio signals. For example, terrestrial broadcast systems transmit data representing sound, images or data to receivers using a terrestrial frequency band.

Cellular mobile conin-iunications networks transmit and receive data for various applications to and from mobile devices which roam within a coverage area of each of a plurality of cells which provide a wireless access interface for communicating data to and/or from the mobile terminals within a relatively short range of a base station serving the cell.

It is known to transmit and receive data from a satellite for example located in a stationary position above the Earth. Radio signals transmitted from the satellite may therefore he received by receivers on the ground. Data to he broadcast may be transmitted on an uplink from a ground station to the satellite which then re-transmits the data as radio signals to receivers disposed on the ground in a downlink. Example satellite broadcast transmissions for transmitting data as well as video and audio signals are those which arc configured in accordance with the DVB-S standards such as DVB-S2. The DVB-S standards provide an efficient arrangement for communicating audio, video and data for various applications and is particularly useful For applications in which the receivers may be disposed on the ground in remote locations where it is difficult to provide infrastructure equipment.

As will be appreciated, in some applications the receivers may be disposed on the ground in remote locations and improving a likelihood of receiving data from the radio signals transmitted, for example, from a satellite presents a technical problem.

Summary of Disclosure

According to an aspect of the present technique there is provided a transmitter for transmitting data using a radio signal. The transmitter comprises a data formatter configured to form the data into frames for transmission as payload data of the radio signal, a modulator configured to map the payload data of the transmission frame onto modulation symbols using a predetermined modulation scheme, and a spectrum spreader configured to combine the modulation symbols of the transmission frame with a spreading code to form a spreading code modulated signal. The spectrum of the radio signal to be transmitted is spread according to a spreading factor determined by 1 0 the spreading code. A frame former adds a header to the spreading code modulated signal to form the frames for transmission, the header providing in one part header data which has been modulated using the predetermined modulation scheme. The one part of the header may therefore provide physical layer data, whereas in another part of the header a start of frame sequence may be provided. A radio frequency transmitter is 1 5 configured to modulate the radio frequency carrier signal with the spreading code modulated signal, and a controller is configured to the content of the header data in accordance with the payload data of each frame. The spectrum spreader is configured to spread the spectrum of the payload data using a first spreading code, the first spreading code being selected to spread the spectrum of the payload data by a variable factor determined in accordance with a predetermined set of possible first spreading codes. The one part of the header data is combined with a second spreading code to spread the spectrum of the header data using the second sprcading code, the second spreading code spreading the spectrum of the header data by an amount which is greater than or equal to any of the first spreading codes for spreading the spectrum of the payload data by the variable factor. The controller is configured to generate the at least part of the header data which is spread spectrum encoded with the second spreading code, the generated header data providing an indication of one or more of the first spreading codes used to spread the spectrum of the payload data by the variable factor.

Embodiments of the present technique can provide a transmitter and a receiver which are configured to communicate data using radio signals. In one example the transmitter forms part of a satellite and the receivers are located on the ground. for example in remote locations. The radio signal is generated by the transmitter so that the receiver can detect the header data at low signal to noise ratios, as a result of the second spreading code providing a greater spreading factor which is greater than or equal to any of the first spreading codes. Therefore even if the signal-to-noise ratio falls to a low-level, the payload data may still be detected at the receiver, because the receiver can dc-spread the header data using the second spreading code.

In some embodiments a part of the header may include a start of frame indication which is combined with a third spreading code, which spreads the spectrum of the start of frame by an amount which is greater than the first spreading code and greater than or equal to the second spreading code.

In some example embodiments the transmitter may also include an error correction encoder and a time interlcavcr. [he time interleaver is configured to interleave the error correction encoded data to a depth which is determined in accordance with a spreading factor of the first spreading codes, a baud rate of the communications channel and a likelihood of a fading duration of the deep fades. As such, by error correction encoding the payload data and interleaving the payload data in accordance with the determined depth which is matched to the likelihood of the duration of thc fades, even if the signal to noise ratio temporally falls to a low-level, thc receiver may still be able to recover the payload data, if it can recover the header data.

Various other aspects of the present technique are defined in the appended claims and include a receiver, a receiving method and a transmitting method.

Description of Preferred Embodiments

Embodiments of the present disclosure will now be described by way of example only with reference to the accompanying drawings, wherein like parts are provided with corresponding reference numerals, and in which: Figure 1 providcs a schematic diagram of an example DVB-S2 system; Figure 2 provides a schematic block diagram of an example DVB-82 transmitter; Figure 3 provides a schematic block diagram of transmitter in accordance with

an embodiment of the present disclosure;

Figure 4 provides a plot of modulation constrained Shannon limits; Figure 5 provides a plot of fade duration vs fade probability for a number of fade depths for the example network of Figure 1 over the Q/V bands; Figure 6 provides an example of spreading the payload data of a TFECFRAME in accordance with an embodiment of the present disclosure; Figure 7 provides an example structure of a frame in accordance with DVB-S2; Figure 8 provides a plot of SOF field correlation efficiency against SNR; Figures 9a and 9b provide example structures of a frame in accordance with an

embodiment of the present disclosure.

Figures ba and lOb provide plots of FEC capacity and maximum spreading factors; Figure 11 provides a plot of FER for varying payload spreading factors and a 1/3 code rate; Figure 12 provides a plot of FER for varying payload spreading factors and a 1/4 code rate; Figure 13 provides an example of a reeciver in accordance with an example of

the present disclosure;

Figure 14 provides a schematic diagram of a differential encoder; and Figurcs 15 to 17 provide schematic diagrams of differential matched filter correlators in accordance wilh examples of the present disclosure.

Descilption of Example Embodiments Figure 1 providcs an example block diagram illustrating an arrangement in which a broadcast signal or multicarrier signal is transmitted from a satellite 1 to receivers on the ground 3 which transmit and rcccive signals via a satellite dish 3. An 1: earth station 4 is arranged to transmit an uplink signal 6 to a receiving satellite dish 8 on the satellite I for transmission onto the receivers 2 via a downlink I 0. l'he downlink comprises a forward link 10.1 and a reverse link 10.2 in which the receivers 2 transmit data on a reverse link to the satellite 1. Thus the satellite 1 includes a transmitter 12 and a receiver 14. The receiver is configured to receive both the signal transmitted from the earth station 4 via the uplink 6 and also signals received from tile reverse link 10.2 from the receivers on the ground 3.

DVB-S2 Subsystem Architecture An example block diagram of a DVB-S2 transmitter in accordance with a known arrangement is shown in Figure 2. In Figure 2 either a single input stream 20 or multiple input streams 22 generate payload data which is fed respectively to an input interface 24, 26 for each of the single input streams and the multiple input streams.

The payload data is then formed into a stream which is synchronised by a synchronising unit 28, 30 and then null-packet deletion is perfonned by a unit 32, 34.

A cyclic redundant check of 8 bits is then performed by a CRC-8 encoder 36, 38 which is again performed respectively for both the single input stream 20 and the multiple input streams 22. The payload data from both the single input stream and the multiple input streams may then be buffered and fed to a merger slicer 40 which is adapted to form the input streams 20 22 into a single format stream for transmission.

The output from the merger slicer 40 is fed to a switch 42 which is arranged to switch into the strewn baseband signalling from a baseband signalling generation unit 44.

Thus at the input to a stream adaptation unit 46 the payload data is provided from an input channel 48 fed from the switch 42. Within the stream adaptation unit 46 the stream of payload data is first fed to a padder unit 50, which adds additional dais symbols to match a data rate of the frame and then to a baseband sbrambler 52, which scrambles the payload data. The output of the stream adaptation unit is fed to a forward error correction (FEC) encoding unit 54 which comprises a BCH encoder 56, an LDPC encoder 58 and a bit interleaver 60. The output of the FEC encoding unit 54 is then fed to a modulator 62 which includes a bit to modulation symbol mapper 64 which maps the FEC encoded payload data onto modulation symbols in accordance with one of four modulation schemes which are QPSK. APSK, 16 APSK and 32 APSK. The output of the modulator 62 is received at a frame former 66 which combines the modulation symbols of the payload data with physical layer signalling information and pilots from a signalling and pilot insertion unit 68, and dummy frames from a dummy frame insertion unit 70. The combined data is then fed to a physical layer scrambler 72. Finally the modulation symbols are output for the frame of data from a channel 74 and are fed to a radio frequency modulator 76 for up conversion and transmission from the satellite to the receivers 2. In Figure 2 boxes 80 with dotted lines are sub-systems which are not relevant for single transport stream broadcasting applications.

Transmission Architecture for Low Signal to Noise Ratios Embodiments of the present technique have been devised to provide in one application an arrangement for transmitting signals in remote locations or using small aperture antenna dishes 3. In some examples the present technique can be adapted to form a DVB-SX system which is arranged to evolve from and may supersede the DVB-S2 system architecture.

Embodiments of the present technique have been developed in order to provide an improvement over the DYB-S2 transmission architecture shown in Figure 2 so that, for example, payload data and signalling data can be detected and recovered by ground stations 2 during periods of low signal-to-noise (SNR) ratio. The improvement over DVB-S2 may assist the receivers 2 to maintain a minimal connection with the satellite 1 during periods of low SNR so that they do not have to re-establish communications with the satellite due to a lost connection. For example, it has been discovered that in certain applications an amount of fading produced on a satellite broadcasting channel, for example, by rain and other disturbances can cause a sigpalling-to-noise ratio at the receivers to drop to a very low value. In applications where the receivers are provided with an application layer which requires the receiver to remain attached or logged into a transmission service for the downlink the receivers need to remain synchronised to the transmission service or an application layer programme should be configured to maintain an active link with the transmitter. However, if the signalling-to-noise ratio falls to a low value then the receiver may not remain synchronised or connected to the transmitter and accordingly, each of the receivers may attempt to reconnect with the transmitter by transmitting information on a reverse channel. However, if each of the receivers attempts to access the reverse channel simultaneously then this can lead to congestion and a reduction in connectivity for all users. Accordingly, by spreading the spectrum of the header data which provides the signalling information by a one or more spreading codes which are greater than or equal to any of the first spreading codes, the header data identifying the spreading lactor used to spread spectrum encode the payload data with one of the first spreading codes can, at the receiver, still at least be detected at low signal-to-noise ratios and therefore the receiver remain synchrornsed and recover from the total or substantial loss of payload data temporarily as a result of a deep fade in the received signal. In some examples, a first part of the header will be spread by a third spreading factor (S3), a second part of the header will be spread by a second spreading factor (S2) and the payload data spread by a first spreading factor (S3). where S3 is greater than or cqual to S2, and S2 is greater than or equal to S 1. Consequently. the spreading factor length with which each portion of the frame is spread reflects the importance of the dab conveyed by each portion of the frame. For example, the first part of the header has to he detected if the rest of the frame is to be detected and decoded; therefore the first part of the header is spread by the highest spreading factor.

Figure 3 conforms substantially to the transmitter architecture shown in Figure 2 principally because the present technique provides an adaptation or evolution of the DVB-S2 architecture. Accordingly, only differences with respect to the block diagram shown in Figure 2 will be described with reference to Figure 3. In Figure 3 the symbol stream of the payload data is fed from an input 48 to a stream adaptation unit 46 which adds padding bits using a padder 101 and then feeds the signal to a haseband scrambler 102 before a FEC unit 104. The FEC unit 104 comprises a BCH encoder 106, an LDPC encoder 108 and a hit interlcaver 110. The BCH encoder 106, the LDPC encoder 108 and the bit intcrleavcr 110 corresponds substantially to the operation performed for the DVB-S2 architecture shown in Figure 2 for the corresponding units 56, 58 and 60. however, in accordance with the present tcclmiquc the coding rate of the enor conection encoder for LDPC encoding 108 is arranged to adapt the encoding rate to rates 1⁄4, 1/3, 2/5, and 1⁄2 in accordance with a state of the channel at each of the receivers 2. Accordingly, the FEC encoder 104 is connected to a controller 112 which is configured to select the code rates for the FEC encoding. The FEC encoded payload data is then fed to a modulator 114 which maps the bits of the payload data onto the constellations of modulation symbols in accordance with a predetermined modulation scheme. The modulation scheme proposed in accordance with the present technique is QPSK which has been selected for reasons explained in the following section.

Following the modulator 114 is a lime intcrlcaver 118 which interleaves the modulation symbols in time for both the complex and imaginary components. The interleaved data is then fed by a channel 120 to the input of a spectrum spreader 122.

The spectrum spreader 122 is adapted to spread the spectrum of the payload data using direct sequence spreading codes by different factors (Si) which are for example 1, 2, 3, 4, 5, 6, 7, 8, 9. and 10. The spreading factors are selected under the control of the controller 112 and are selected in accordance with a current state of the communications channel with the ground receivers 2. For instance the selection of the first spreading codes (Sl) may he dependent upon the current SNR of the link or the level of fading. A frame forming unit 124 is then configured to receive the spread modulation symbols from a channel 126 and then feed ti-ic symbols to a physical layer header and pilot spreading unit 128 after the physical layer signalling and pilot insertion unit 130 has added physical layer signalling and pilots into the spread spectrum encoded symbol stream, where the spreading unit 128 may spread the header with one or more spreading codes As will be explained shortly however, in one embodiment no pilots are inserted. After the physical layer header and pilots have been inserted by the physical layer header inserter 128 the payload data which is then formed into frames is fed to radio frequency modulator 132 which up converts the baseband modulation stream to the radio frequency signal for transmission from the satellite to the ground receivers 2. in Figure 3 the boxes 180 with dotted lines are sub-systems which are not relevant for a single transport stream broadcasting applications.

As noted in Figure 3 and explained above the physical layer header and pilot spreading unit 128 adds a header to the fonned frame. In accordance with the preseut technique the data and at least part of the header is spread using a second spreading code which is greater than or equal to any of the first spreading codes used to spread the payload data.

In a more a generalised form of the system architecture described above, embodiments of the present technique can provide an arrangement in which a signal transmitted, for example, from a satellite is formed by a transmitter with the effect that the payload data is encoded and modulated and then the spectrum of the payload data is spread using a first spreading code (SI). A formatter in the transmitter under the control of a controller is arranged to insert a header into the transmitted signal to form with the payload data frames of data so that in each frame a header is provided which includes physical layer signalling (PLS) information relating to parameters with which tile payload data has been modulated and encoded and a start of frame (SOP) sequence. The transmitter is configured to spread the spectrum of the payload data using a first spreading code which is selected from a plurality of first spreading codes (Si) each of which is arranged to provide a different spreading factor therefore providing an arrangement in which the payload data is spread by a variable factor.

Each of the frames is formed from the payload data and the header. The transmitter is further configured to spread the spectrum of at least part of the header data using a second spreading code (S2) and the SOF sequence is spread using a third spreading code (53) where these spreading codes may be generated in accordance with any known method, for example, an rn-sequence generated from a generator polynomial.

Part of the header data spread with a second spreading code (52) includes physical layer signalling data which provides the parameters with which the payload data is modulated and encoded and the spreading factor in accordance with one of the first spreading codes which is chosen. The signalling data transmitted in the header data is therefore spread using the second spreading code. According to the present tecimique the second spreading code produces a spreading factor which is greater than or equal to any of the spreading faclors produced by the first spreading codes. A receiver is configured to detect the header data and use the header data to recover the payload data in accordance with the parameters with which the payload data has been modulated and the spreading code in accordance with one of the first spreading codes used to spread the spectrum of the payload data.

More detail of the selection of various parts of the transmitter shown in Figure 3 will be explained in the following paragraphs.

Selection of Modulation Scheme (OPSK) In "BBC TN R&D 3420 vl.0, J Stott, CM and BICM Limits for rectangular constellations, Aug 2012" it is explained that the capacity we seek, as defined in the requirements for a DVB-S2 and DVD-S2x, is the mutual information (I) between the transmitted data (X) and the received data (Y) and can he represented in the following expression f p(x1,y) I(X;Y) = j It is assumed that all constellation points (ii.) occur with equal probability and therefore the probability distributions can then be re-written as; p(x,y) = p(yIxP(x1) = V'P(YIXk) n and (y_x1)Z C 20-2 p(yIx1) = ___ V2mc Where the later equation is the "normql probability density function since S Gaussian noise is assumed. In Figure 4 the modulation constrained capacity for BPSK, QPSK and I 6QAM modes are shown.

As previously mcntioned, the proposed system is to operate at low SNR levels, for instance -9.5dB. Based on the curves of Figure 4 and the preceding equations it can be seen the maximum theoretical capacity at low SNRs is similar for BPSK to 8PSK.

However for the previously mentioned code rates it can be shown that QPSK presents the optimum constellation type and therefore it is preferred that present disclosure is implemented with QPSK.

leavin The proposed system, as illustrated in Figure 3, without the Time-interleaving block is designed to cope with fades resulting in CINref levels between -3dB to -10dB, and as will be explained in subsequent paragraphs this is primarily achieved via the use of a new physical layer framing structure and the application of direct spreading technology with low rate LDPC codes to the header and data payload portions of the transmitted frames. In "Submission to DVB: Markets for Low SNR Satellite Links" measurements performed by DLR of Q/V beacons flown on Italsat were presented and show the relationship between fade power, fade duration and the probability of occurrence. It ear be seen in the Figure 5 that for a 99.9% link availability the system must be able to cope with fades of over 20dB.

In some examples the payload data is error correction encoded and interleaved by an interleaver with the effect that with an increasing probability a depth of interleaving is chosen to ensure that the payload data is interleaved over a period which is greater than a most likely length of fade. Accordingly receivers cannot only recover the header data providing an indication of demodulation and spreading code used to spread the spectrum of the payload data in accordance with a first spreading codes but also to recover the data in the presence of a fade which would otherwise result in loss of the data and synchronisation of the receiver to the transmitter.

Therefore by dimensioning the time-interleaver accordingly it is possible to extend the system dynamic performance range. This extension may for instance be of use in order to maintain session links in a predetermined duration of excessive and intermittent fades or to reduce the adverse effects of interference. It will he beneficial to use a convolutional time-interleaving block so as to reduce the size of the memory required.

The interleaving depth required can be calculated as depth = (1 -R1 X X D. Where depth is the interleaving depth (cells), R is the Effective Code Rate, B is the Channel Baud rate, S is the Spreading factor and D is the Duration of fade (sec).

As an example, if the condition during transmission is such that the resulting SNR dips to -15dB for a duration of 10 Seconds in a 1⁄4 rate system employing a spreading factor of S and a channel baud rate of 30.9MBaud, then the time-interleaving depth required can be calculated as; &th = (4/3) x 30.9e6/8 x 10 = 51.5 MCells. The table below shows the interleaver requirement in a 30.9MBaud channel for different fading durations.

Interleaving Depth (Cells) _______________________ ______________________________ Rain Fade Duration (5cc) Rate 1/3 Rate 1/4 l.45M 1.29M 0.25 2.89M 2.SSM 5.79M 5.l5M 1 11.59M 10.3DM 2 38.97M 25.75M 5 57.94M 51.5DM 10 86.90M 77.25M 15 115.87M 103.00M 20 173.81M 154.SOM 30 347.62 309.00 60 Table 1; Calculated interleaving depths for 30.9MBaud channel vs fade duration The use of convolutional interleaving will halve the memory requirement, and in addition it is known that for the QPSK constellation only 4 bits of information needs to be stored (per I/Q cell). These two factors will aid in greatly reducing the hardware requirements of the time interleaver implementation. Furthermore, the table I above only shows the possible requirements without making any recommendations as to what the maximum targeted duration should be. Consequently, the transmitter and the receiver are configured in accordance with a likely fade duration that the communications system is designed to cope with respect to certain constraints such as an available memory size to implement the interleaver, Physical Layer Frame Structure and Spreading In embodiments of the present technique and in accordance with Figure 3, the data contained in the FECFRAME is modulated using QPSK modulation having a constant signal envelope, after which it is then interleaved in time across several FECFRAMES before being segmented into TFECFRAMES, which have the same length as a FECERAME, before being spread to construct the SFECFRAME.

Figure 6 shows the TFECFRAME data is spread, for example, using an M-Sequence code with a repetition rate much longer than the length of the frame (32400), however other codes may also be used such as Gold codes, Hadamard codes etc. Assuming a spreading factor of Slit follows that the length of the SFECFRAME will be a spread factor multiple of the parent TFECFRAME.

It is envisaged that the M-sequcncc used is generated using a degree 21 generator polynomial, however, other generating polynomials may also be used. An example sequence may be constructed using the primitive (over (GF (2)) polynomial; 1 + x3 + x5 + x6 + x12 + x18 + x19 + x2° + x21 Where the above polynomial results in a repetition length of 221 and is capable of spreading 64800 modulated cells with a spreading factor of 16.

As shown in Figure 6 several FEC frames are fonned and segmented into a TFEC frame 200 before being spread, by a factor of 8 in this example, to construct the SFEC frame 208. This process corresponds to the spreader 122 of Figure 3. As shown in Figure 6 a TFEC frame comprises 32,400 symbols comprising IIQ cells of QPSK modulated symbols. Each of the TFEC frames is then combined with a spreading code. A combiner 202 receives a spreading code in accordance with one of the specified codes on a first input 204. The I/Q samples of each of the modulation symbols are then formed into an SFEC frame 208 which comprises a plurality of sub-frames each of which is provided for one of the ground receivers 2.

After the spreading and the modulation described above, the physical layer frame is then constructed with the aim of maintaining its frame synchronisation capability at the new low SNR levels from -3dB to -l 1dB so that receivers 2 can maintain a connection with a satellite I during low SNR periods and the previously described connection re-establishment problems avoided. This is primarily achieved via a second and in some for examples of the present technique a third layer of spreading that concentrates on the header and its signalling data.

Figure 7 provides an example illustration of current DVB-S2 physical layer frame. As shown in Figure 7 the physical layer frame comprises a physical layer header 220 of 90 symbols and a plurality o slots comprising 90 symbols 222, where there may be any number of slots and the number of slots lila FECFRAME is given by dividing the length of the FECFRAME by 90. Pilots 224 are inserted every 16 slots where the pilot symbols are made up of 36 pilot symbols 224. In accordance with one example of the present technique, all or part of the header is spread with a spreading a factor which is the highest of the available spreading factors (for example 1 0)which are arranged to spread the data carrying portions of the frame. however, it is necessary to determine an appropriate spreading factor for the header in order to achieve the required performance at the low SNR levels specified above.

Frame synchronisation in DVB-S2 is usually performed by colTelation and its performance is susceptible to channel noise and so for low SNR levels. A receiver according to the present technique is configured, spreading in accordance of the header in accordance with the present technique will improve the detection ratio and header data decoding at low SNRs.

Figure 8 shows the performance of the correlation of the start of frmne (S OF) field of a frame ii) the presence of noise where the SOF data is spread by varying amounts. As can be seen in Figure 8 the correlation results of the current SOF field (spread = 1) has a detection ratio close to zero at levels less than -6.5dB SNR. In contrast, the use of a spreading factor of 16 ensures a 100% detection ratio even to levels of -12.8dB SNR. Therefore in examples in accordance with the present technique it is envisaged that the SOF sequence of the header can spread by a factor of 16 whereas the header data in the PLS code field is spread by the highest spreading factor (81) used to generate the SFECFRAME as described above.

Figure 9 provides an example illustration of a frame structure adapted in accordance with the present technique. As shown in Figure 9a a new frame structure is provided which does not include pilot symbols. As explained above, in some examples the pilots are not required because the signal to noise ratio allows the QPSK and spread spectrum modulated signals to be recovered and so the payload data can be recovered without the use of a pilot. This may be achieved by arranging for the spreading codes itself or other known parts of the signal to be used to determine a channel impulse response of the transmitted signal. As for the example explained with reference to Figures 9a and 91,, 1 6 time slots are provided 222 which transmit data to each of the ground receivers 2. However, the header of the physical layer frame includes a section of data 230 (e.g. SOF code) which is spread spectrum modulated with a spreading code producing a spreading factor of 16 (S3) and a PLS portion 231, which conveys signalling data referring to the payload data, spread by a second spreading factor (52) where 83 is greater than or equal to S2. The remainder of the symbols of the frame arc spread by a different spreading code which is indicated as a factor Si, where Si is a spreading factor less than the spreading factor 16 used to spread the data of section 230 and in some examples of the present technique, Si is less than or equal to 82. Consequently, 64 x S2 symbols are transmitted to produce the header data (PLS) code and 16 x 90 x Si symbols are transmitted in the data bearing slots 222, where the transmitted symbols have a duration equal to the bits of the spreading sequences as is the case for the spread SOF sequence. In the example fI-ame structure given by Figure 8 it is envisaged that the spreading factor Si will take a value from the following spreading factors, 1, 2, 3, 4, 5, 6, 7, 8, 9 and 10. However, the spreading code 52 used for the PLS header data 231 may be variable between and including the spreading factors SI and S3.

The example frame shown in Figure 9b corresponds to that shown in Figure 9a except for the example shown in Figure 9b it is necessary to transmit the pilot symbols in section 232. In this second example the pilots may be used for frequency offset correction and channel estimation etc. such that existing rcceivcr algorithms may be used with the existing frame structure. Also in the case of the frame shown in Figure 9b. the frequency of the phase tracking algorithm can he reduced by a factor of Si due the spreading of symbols in the data slots 222.

As described in the preceding paragraphs, since it is possible to have different spreading and coding options or adaptive spreading and coding it is necessary to include the required signalling in order for the receiver to identifS' the current mode of operation. A receiver configured to use a low signal to noise ratio in accordance with the present disclosure may be different to other receivers because it will be structured to look for the physical layer heading based on the newly proposed physical layer frame as illustrated in Figure 8.

t'he PLS of the DVB-S2 system is done with 7 bits and placed in the PLS code.

The PLS code is constructed using a (32, 6) block code which is furthcr processed by a phases repetition of the bits based on the value of the 7th bit, effectively converting it to a (64, 7) code, In an example of the present teclmique it is proposed that this structure remains unchanged and hence will mean that the new system which uses only QPSK modulation will be able to utilise the bits according to the example shown in table 2.ilowever, it will be appreciated that not all of the modes in table 2 may be used in an examples system in accordance with present technique. The block coding of the PLS portion 231 of the header also provides extra robustness to the PLS header data, therefore even though the PLS data may he spread by a lower spreading factor than the SOF portion of the header, it still has an increased robustness so that the PLS data (Si, code rate etc.) can he detected and the payload data decoded even in low SNR scenarios.

Spread Spread Spread Spread Mode Mode Mode Mode Code Code Code Code QPSK1⁄4 QPSK1/3 QPSK2/5 QPSK1/2 ID 9D 171) 25D Spread = 1 Spread I Spread = 1 Spread = 2 QPSK 1⁄4 QPSK 1/3 QPSK2/5 QPSK 1/2 2D IOD 1SD 26D Spread 2 Spread = 2 Spread 2 Spread 3 QPSK1⁄4 QPSK 1/3 QPSK2/5 QPSK 1/2 3D liD 19D 27D Spread = 3 Spread = 3 Spread 3 Spread = 4 QPSK 1⁄4 QPSK 1/3 QPSK2/5 QPSK 1/2 41) 121) 20D 281) Spread 4 Spread 4 Spread = 4 Spread = 5 QPSK 1⁄4 QPSK 1/3 QPSK 2/5 QPSK 1/2 51) 13D 211) 29D Spread = 5 Spread = 5 Spread = 5 Spread = 6 QPSK 1⁄4 QPSK 1/3 -QPSK 2/5 QPSK 1/2 61) 141) 22D 30D Spread6 Spread=6 Spreadó Spread8 QPSK 1⁄4 -QPSK 1/3 QPSK 2/5 QPSK 1/2 71) 1SD 23D 31D Spread-8 Spread8 Spread8 Spread 10 QPSK 1/3 QPSK 2/5 QPSK1⁄4 DUIVIIvIY SD Spread 161) Spread = 24D 01) Spread = 10 PLFRAME 10 Table 2: Example signalling table of the middle 5 bits of the physical layer header.

As with the current DVB-S2 standard, it is proposed that the current technique will retain both the MSB and LSB usage. The MSB is used to signal the FEC length (0 = normal 64800 bits, 1 = short 16200 bits) while the LSB is used to signal whether or not pilots are used (e.g. 0 = no pilots, 1 = pilots) Although various spreading factors and coding rates that can be covered by the physical later header have been given in Table 2, it is useful to identifiy the limits of the spreading factor that can be utilized in the system design. This maximum spreading factor is defined as the factor required so that the maximum transmission rate is not exceeded, this rate being defined as the number of bits per symbol. Figures lOa and lOb illustrates plots of these maximum rates over different coding rates and spreading factors. It follows that if the spreading factor exceeds this maximum value then either the channel bandwidth needs to be increased to maintain the data capacity or the channel bandwidth remains the same consequently resulting in a data capacity reduction.

In Figure 11 and 12 the frame error rate (FER) of FEC Frames is shown for a range of spreading factors where the modulation scheme is QPSK. Figure 11 illustrates the FER when 1/3 coding FEC coding is used and Figure 12 illustrates the FER when A coding is used. Based on the results in Figures 11 and 12 six preferred modes of operation may be established, however, there are other combinations of coding rates, spreading factors may also be used.

ModesI,2&3 Constellation = QPSK Code Rate = 1/3 Data Spreading Factor (SI) = 6, 8 & 10 PL Header Spreading Factor = 16 FEC = Long or Short AWGN SNR target = [-9.5dB. -10.84dB & -11.74dB] Pilots = Optional (effeetiveness+ reduced by Si if pilots are included) Mode 4,5 &6 Constellation = QPSK CodeRatel/4 Data Spreading Factor (Si) = 4, 5 & 6 PL header Spreading Factor = 16 FEC Long or Short AWGN SNR target = -9.24dB, -10.24dB & -l 1dB Pilots = Optional (effectiveness+ reduced by SI if pilots are included) The data capacity of the system designed and operating in accordance with the present technique may be calculated by first identifying the drop (or rise) in capacity due to the use of a spreading factors Si. This drop is calculated by dividing the maximum spread factor allowable (Figure 10) by the factor 51. This ratio is then applied to the theoretical capacity limit from which the maximum spread factor was derived (Figure 4) and is in this instance the modulation constrained Shannon capacity for the QPSK constellation.

Table 3 provides performance results for the six modes specified above.

F 1/3 Rate 1/4 -Data. Data Capaciw Capacity C/Nref Cap C/Nrcf Cap Mode Spread (bits/sec/ (dB) (3o.9M Mode Spread (bits/sec/H (ffl) (309M _____________________________________ Bd) z) Baud) (Mbits) (Mbits) 6 0.0918 -9.5 5,67 4 4 0.1029 -9.24 6.36 2 8 0.0787 -10.84 4.86 5 5 0.0823 -10.24 5.09 3 10 0.0688 -11.74 4.25 6 --6 0.0686 -11.00 4.24 Table 3: Mode performance results It is possible to include the use of an adaptive spreading system in the proposed technique based on interactions with a return channel thereby increasing the data rate possible at the higher SNR ranges. AU the spreading factors of this adaptive system will be in line with the corresponding 52 QPSK with repetition equivalent modes for SNR levels of -3dB and below. Typically, such an adaptive system will make use of a return channel through which the optimum coding and spreading factors can he set, Receiver An example of a receiver architecture for one of the example ground receivers 2 is shown in Figure 13. In Figure 13 the transmitted downlink signal on the forward channel is received by an antenna dish 3 and fed to an RE to baseband converter 300.

The radio frequency to baseband converter 300 down converts the received radio frequency signal to basehand symbols which are fed to a physical layer signalling (PLS) header despreader 302 where frame synchronisation is performed via the correlation of the received signal. In applications where no frequency offsets exist in the channel it is sufficient to synchronisc to the start of the frame using known techniques such as coherent correlation using a matched filter. This approach involves stoting the expected SOP sequence andlor spreading code in a buffer or memory and cross-correlating the stored sequence with the received sequence in order to identi' the location of the start of each frame i.e. the SOP location. Figure 8 illustrates the performance that may he obtained when this approach is ulilised with a variety of spreading factors. However, in applications where a frequency ofThet may be present in the received signal, the aforementioned approach to frame synchronisation may not be possible because the phase rotation of the received signal means that matching of the received signal with the expected signal may not be performed reliably. A method to overcome this problem and allow matching to be perfonned is to differentially encode the received spread SOF signal and symbols such that the phase rotation of the received signal is removed. Differential encoding entails multiplying the presently received symbol of the spread SOP sequence with the previously received symbol of the spread SOF sequence, where a symbol of the spread sequence refers to the a symbol or sample of duration equal to a bit of spreading code. However, although this process allows matching to take place, the carrier-to-noise ratio (C/N) may deteriorate because of the additional noise terms which are introduced in to the signal.

Consequently the detection performance degrades from that shown in Figure 8, This can be seen in the equations representing differential encoding given below, where the phase rotation has been cancelled in the first term but the three terms in the square brackets represent the additional noise components.

Sym1 = A1eJ(°1*A) + AieI(1 Sym2 = A2eJ(°2 + Diff = Sym1Sytn2 = A1A2eJ(12) + [A2A1e cGz) + AiA2e 1a2) + AiA2eJ12)] A first solution to the degraded performance is to increase the length of the underlying SOF sequence so that there are more symbols/samples in the spread sequence. However, this may adversely affect the capacity of the system. Furthermore, the increase in SOF length needed to achieve a (C/N) comparable to those achieved using coherent correlation is significant, for example, in order to achieve a C/N of - 10.8dB a SOF length of 7200 is required.

A second solution to the performance degradation is to utilise the SOF, PLS header and data fields to perform differential matching and correlation of spreading codes that they are spread with so that a sequence significantly longer than the SOF may be utilised. For example, the entire length of the frame and therefore all spread symbols may be used. To achieve this, the received signal is first required to be differentially encoded. This may be performed using a differential decoder as illustrated in Figure 14, where the differential encoder comprises a delay element 1401 which delays by the duration of one spreading code symbol (spread symbol sample) the data/signal input on the input 1402, a conjugator 1403 which conjugates data from the input 1402, a multiplier 1404 which multiplies the delayed and conjugated input data to form a differential encoded signal at an output 1405. The encoded signal has one fewer symbols or samples than the input data, for example if the length of the spread SOP sequence in spread symbol samples is K, the length of the differentially encoded sequence will he K-i, Similarly, for the PLS and payload data their respective lengths will be (NxS2) -i and T -1, where T is the number of symbols in the spread payload data 90 x No. of slots x Sl. The output of thc differential encoder may then be sent to a differential matched filter.

Figure 15 provides an illustration of a differential matched uilter correlator 1500 for the differentially encoded SOF sequence. The correlator 1500 is configured to detect thc start of a frame by correlating the differentially encoded received signal with a stored expected version of the differentially encoded signal, which has been generated with knowledge of the SOP sequence and spreading code, and where the values W are samples of the stored expected signal. Alternatively, the correlation may be done with the differentially encoded spreading code where W are the samples of the differentially encoded spreading code S3. The differentially encoded signal is input into the K-i delay elements 1401 and thc multipliers 1404. These multipliers multiply samples from the stored expected differentially encoded sequence with the differentially encoded received signal and the products are summed by a summer 1501. [he output of the summer is then compared to a threshold by a threshold detector 1502 and if a certain threshold is exceeded it can be decided that the SOF sequence and therefore the start of a frame has been detected. A signal conveying this detection may then be sent to the controller or other appropriate element in the receiver.

Figure 16 provides an illustration of a differential matched filter correlator for the differential encoded SOP and PLS header where the SOF correlator 1 500 is concatenated with the PLS header correlator 1600. i'he structure of the PLS eorrelator 1600 is similar to that of the SOF correlator 1500 and comprises (NxS2)-1 delay elements 1401 and multipliers 1404. However, because the data of the PLS header is unknown, correlation is performed between the differentially encoded received signal and a stored differentially encoded version of the known spreading sequence S2, which may also be referred to as an expected stored signal. Selected symbols are not used th the correlation in order to remove the ambiguity arising from the unknown data. For example, correlation is not performed on every one in 52 samples in the received and stored expected signals because the change in the underlying unknown data in the received signal renders the differential encoding useless at these underlying symbol transition points as it is not only the spreading code which is changing. The skipping of these symbols may improve the performance of the correlation and may also reduce the number of delay elements and multipliers required. In Figure 16, the value W represent the samples from the stored expected differentially encoded spreading sequence S2.

Figure 17 illustrates an overall differential matched filter correlator where the correlation of the differentially encoded SOF, PLS header mid payload data is performed. The structure of the payload data differential matched filter correlator 1700 is again similar to that of the SOF correlator 1500, however, because both the underlying data and the spreading factor of the spreading code S 1 that the data is spread with are unknown, a number of adaptations are required. Firstly, if the spreading factor Si is greater than 1 then knowledge of the spreading factor of Si is not required, consequently, it is assumed that spreading factor of Sl is greater than 1 for the correlation procedure. This simplification is possible because in order to overcome the changes in the underlying data which render the associated differentially encoded samples useless with regards to correlating the spreading code (as for the PLS eorrclator 1600), selected symbol/samples must not be included in the correlation process. The maximum number of symbols/samples to be skipped and not included would be every one in two when a spreading factor of 2 is used. Flowever, skipping every other symbol will also ensure that when the spreading factor is greater than 2 the symbols associated with underlying data symbol transitions will still not be included in the correlation process. Although not including every other symbol will remove more than the minimum samples or symbols required for spreading factors greater than 2 and therefore may reduce correlation performance, the greater length of the payload data compared to the SOF and PLS compensates for this removal in terms of correlation performance. Given a total data field length of T' samples all even spreading factors (Si = 2,4,6,8...) will have an effective number of correlation samples of T/2 and for odd spreading factors (SI = 3,5,7,9...) the effective number of' correlation samples is equal to (T/2)x(1-1/S1).

The differentially encoded received signal is correlated with a stored differential encoded version of the of the longest Si spreading code, where the later signal may also be referred to as the stored expected signal, and every other symbol from each signal is not included in the correlation. Correlation of the payload data in this manner is possible because although the spreading factor of Si is unknown, all the spreading codes which Si may be are derivable from the longest SI spreading code e.g. the symbols of the SI with a spreading factor of 2 are the first two symbol of SI with a spreading factor of 10. When using the combined diffcrcntial matched filter correlator illustrated in Figure 17 the stored expected sequence represented by W is derived from the three spreading codes SI to 53 and the total number of effective correlation samples i.e. number of differentially encoded symbols with duration of the spreading code symbols, is therefore given by Men = (K-XS1 x (i -))+ and MF0 = (K -1) + (N X Si X (1-E)) + (i x (i -The differential matched filter correlator frame synchronisation method described with reference to Figures 14 to 17 allows frame synchronisation to be performed when frequency offsets are present and without reducing the data capacity of the system -unlike the hcadcr extension approach previously described. Using the differential matched filter correlator frame, synchronisation can he performed down to SNR levels below -17dB in the presence of a frequency offset and without any additional signalling required by the transmitter. The differential matched filter correlator may lead to increased latency due to the delay elements and therefore it may be beneficial in terms of performance to use it only when frequency offsets are present in the received signal and use conventional coherent correlation when frequency offsets are not present. Furthermore, although the stored expected signal may be derived from all the spreading codes i.e. Si to S3 as described above, it may also be beneficial to utilise only one or two of the SOF, PLS and payload data differential matched eorrelators dependent on the system SNR so that the expected signal may be derived from one or more of the spreading codes, For instance, if the SNR is relatively high it may be sufficient to use only the SOF correlator or the PLS correlator in order to achieve frame synchronisation and therefore the stored expected signal may only be derived from the spreading codes S3 or S2. Consequently, with this approach a reduction in the number of computations required and the acquisition delay may be achieved compared to using the entire correlator shown in Figure 17 whilst not sacri [icing perfbrrnance.

Referring baelc to Figure 13, the SOP may he spread by a factor that is greater than or equal to the spreading factor used for the PLS code. For example the spreading code for SOF may be 16, while the spreading code for the PLS may he 16 or 10. Once the SOP is identified the header dcsprcader 302 also despreads the PLS portion of the header 231 sprcad with the second spreading factor 52 in to order obtain the information on the spreading and encoding of the payload data. In some embodiments the spreading factor S2 used for the PLS may be known at the receiver, as is the ease for the spreading factor S3 used for SOF. 1-lowever, as previously mentioned, in some examples in accordance with the present technique the spreading factor for the PLS may he unknown at the receiver. In this case the receiver would then correlate for possible known spreading factors, e.g. 1, 2, 3... 10 until valid signalling data is obtained. The number of known spreading factors is finite. This spreading code detection may be achieved by parallel processing circuitry with a branch for each known spreading factor. Such an approach permits greater flexibility, with a small increase in in processing. The number of spreading factors determines the memory requirements. Such correlation techniques may however introduce noise which may not be commensurate with any the low SNR requirements.

Once the SOP and PLS data has been detected and dcspread, the partially dcsprcad stream may then be fed to a dummy frame removal module if required. As explained above the header of the frame has been spread spectrum encoded with one or more spreading factors whereas the payload data has been spread with a different, lower or, in some examples in accordance with the present technique, a spreading factor equal to one of the spreading factors of thc header such as 52. The spread spectrum despreader for tile header data 302 can therefore despread the header data at a lower SNR then the payload data despreader 306 which is arranged to despm-ead the data in accordance with a spreading code which has been used to spread the spectrum of the payload data. 1-lowever. the spreading code (Si) used to spread spectrum of the payload data is variable and this information in carried by the header data and in particular the PLS code 231. Accordingly, the despread PLS data is fed to a controller 308 which configures the payload data despreader 306 to despread the payload data according to thc spreading code specified in the header. Furthermore, because the header data is spread by a one or more spreading factors higher than that used for the payload data and the block coding of the I'LS data, the reception and decoding of the header data will he more reliable. Therefore even during a low signal to noise ratio period thc receiver will stiJl be able to receive correlation (SOF) and signalling information (PLS) correctly such that it can maintain connection with the satellite even if little or no payload data can be received or transmitted. The despread payload modulation data symbols are then fed to a time de-interlcaver 310 which deintcrleaves the interleaved data frames and feeds the output to the modulation symbol demapper 314 via an input feed 312 in order to recover the payload data which has been FEC encoded. The encoded payload data is therefore fed to an FEC decoder unit for error correction decoding, where the FEC decoder unit comprises a bit dc-interleaver 320 and an error correction LDPC dccodcr 322 and a BCI-1 decoder 324 which combine to form inner and outcr decoding of the encoded data as explained above in accordance with different encoding rates. As previously mentioned, a number ol encoding rates may be used and information on the coding rate at which the payload data has been encoded is conveyed in the header. Consequently, the controller is also configured to control the LDPC decoded such that it decodes the encoder payload data using the appropriate code rate.

The output of the FEC decoding unit may be then passed through a number of intermediate blocks at point 322 which perform corresponding functions as inverse functions to the blocks contained in the mode adaptation block of Figure 3. Finally, the payload data will be dc-multiplexed and the appropriate streams of data passed on to subsequent blocks for processing appropriate to the data information conveyed in the payload data.

Various further aspects in features of the present disclosure are defined in the independent claims. Various modifications may he made to the embodiments described above without departing from the scope of the present disclosure. In particular, it will be appreciated that the application to satellite transmissions is not limiting and the techniques applied above can be applied to other forms of communications system.

References 1. ETSI TS 102 441: "Digital Video Broadcasting (DVB); DVB-S2 Adaptive Coding and Modulation for Broadband Hybrid Satellite Dialup Applications".

2. ETSI TB. 102 376: "Digital Video Broadcasting (DVB) User guidelines for the second generation system for broadcasting, Interactive Services, News Gathering and other broadband satellite applications (DVB-S2)".

3. TM-S2-0l22r1: "Call for technologies (Cif) for the evolutionary subsystem for the 52 system".

4. BBC TN R&D 3420 vl.0, J Stott, CM and BICM Limits for rectangular constellations, Aug 2012.

5. Submission to DVB: "Markets for Low SNR Satellite Links".

6. ETSI EN 302 755: "Digital Video Broadcasting (DVB); Frame structure channel coding and modulation for a second generation digital terrestrial television broadcasting system (DVB-T2)"

Claims (37)

1. CLAIMS1. A transmitter for transmitting data using a radio frequency carrier signal, the transmitter comprising a data formatter configured to form the data into frames for transmission as payload data of the radio signal, a modulator configured to map the payload data of the transmission frame onto modulation symbols using a predetermined modulation scheme, and a spectrum spreader configured to combine the modulation symbols of the transmission frame with a spreading code to form a spreading code modulated signal, the spectrum of the radio signal to he transmitted being spread according to a spreading factor determined by the spreading code, a frame former configured to add a header to the spreading code modulated signal to form the frames for transmission, the header providing signalling data which has been modulated using the predetermined modulation scheme, 1 5 a radio frequency mmsniitter configured to modulate the radio frequency carrier signal with the spreading code modulated signal, and a controller for adapting the content of the header data in accordance with the payload data of each frame, wherein the spectrum spreader is configured to spread the spectrum of the payload data using a first spreading code, the first spreading code being selected to spread the spectrum of the payload data by a variable factor determined in accordance with a predetermined set of possible Iirst spreading codes and the header data is combined with a second spreading code to spread the spectrum of at least part of the header data using the second spreading code, the second spreading code spreading the spectrum of the at least part of the header data by an amount which is greater than or equal to any of the first spreading codes for spreading the spectrum of the payload data by the variable factor, and the controller is configured to generate the at least part of the header data which is spread spectrum encoded with the second spreading code, the generated header data providing an indication of one or more of the first spreading codes used to spread the spectrum of the payload data by the variable factor.
2. 2. A transmitter as claimed in Claim 1, wherein the controller is configured to select the first spreading code from the set of first spreading codes in accordance with a state of a communication channel between the transmitter and one or more receivers of the transmitted signal.
3. 3. A transmitter as claimed in Claim 1 or 2, wherein the transmitter includes a reverse channel receiver for receiving a signal transmitted by one or more of the receivers of the transmitted radio signal providing the indication of the state of the channel from the transmitter to the one or more of the receivers of the payload data from the transmitted signal, the first spreading code being selected for each of the one or more receivers which are to receive the payload data.
4. 4. A transmitter as claimed in any of Claims 1, 2 or 3, wherein the header includes a start of frame sequence, the start of frame sequence being spread by a third spreading code, the third spreading code providing a spreading factor which is greater than that of the first spreading code and greater than or equal to that of the sccond spreading code.
5. 5. A transmitter as claimed in any of Claims 1, 2, 3 or 4, wherein the first spreading code is an M-sequence derived using a degrcc 21 generator polynomial according to; 1 + x3 + x5 + r6 + x12 + x18 + x19 + x20 + x21
6. 6. A transmitter as claimed in any of Claims I to 4, wherein the spreading factor of the third spreading code is a factor of sixteen.
7. 7. A transmitter as claimed in any of Claims 1 to 6, wherein the predetermined modulation scheme is quadrature phase shifi kcying, QPSK.
8. 8. A transmitter as claimed in any of Claims 1 to 7, comprising an error correction encoder configured to receive the payload data and to encode the payload data with an error correction code, the modulator being configured to mapping the error correction encoded payload data on to the modulated symbols in accordance with the predetermined modulation scheme, and a time interleaver which is configured to interleave the modulation symbols in accordance with an interleaving depth, wherein the interleaving depth is determined in accordance with a coding rate of the error correction encoder, a baud rate of the communications channel between the transniltted and the receiver, the spreading factor of the first spreading code and a probability of possible fade durations of the radio signal when received.
9. 9. A transmitter as claimed in Claim 8, wherein the time interleaver is a convolutional interleaver.
10. 10. A transmitter as claimed in any of Claims 8 or 9, wherein the first spreading codes and coding rates of the error correction encoder are determined in accordance with the following modes: Mode Sprd Mode Sprd Mode Sprd Mode Sprd Cod Cod Cod Cod QPSKY4 1D QPSK 9D QPSK 17D QFSK 25D Spread = 1/3 2/5 112 1 Spread = Spread = Spread = 1 1 2 QPSK% 2D QPSK 1OD QPSK 1BD QPSK 26D Spread= 1/3 2/5 1/2 2 Spread = Spread = Spread = 2 2 3 QPSKY4 3D QPSK liD QPSK 19D QPSK 27D Spread= 1/3 2/5 1/2 3 Spread = Spread = Spread = 3 3 4 QPSK% 4D QPSK 12D QPSK 20D QPSK 28D Spread= 1/3 215 1/2 4 Spread = Spread = Spread = 4 4 5 QPSK1h SD QPSK 13D QPSK 21D QPSK 29D Spread 113 2/5 1/2 Spread = Spread = Spread = 5 6 QPSK A SD QPSK 14D QPSK 22D QPSK 30D Spread= 1/3 2/5 1/2 6 Spread = Spread = Spread = 6 6 8 QPSK'I4 7D QPSK 15D QPSK 23D QPSK 31D Spread = 1/3 2/5 1/2 & Spread = Spread = Spread = 8 8 10 QPSK% 8D QPSK 16D QPSK 24D DUMMY OD Spread = 1/3 2/5 PLFRAM Spread= Spread= E 10
11. 11. A transmitter as claimed in any of Claims 1 to 10, wherein the transmitter is a satellite transmitter.
12. 12. A transmitter as claimed in any of Claims 1 to 11, wherein the radio signal transmitted by the transmitter is transmitted in accordance with a DVB-Sx standard.
13. 13. A method of transmitting data from a transmitter using a radio frequency carrier signal, the method comprising forming the data into frames for transmission as data payload of the radio signal, mapping the payload data and the header data of the transmission frame onto modulation symbols using a predetennined modulation scheme, and combining the modulation symbols of the transmission frame with a spreading code to form a spreading code modulated signal, the spectrum of the radio signal to be transmitted being spread in accordance with a factor determined by the spreading code, adding a header to the spreading code modulated signal to form the frame for transmission, the header providing signalling data which has been modulated using the predetermined modulation scheme to form the header of the radio signal, modulating the radio signal with the spreading code modulated signal. and adapting the content of the header data in accordance with the spreading code of the payload data, wherein the combining the modulation symbols of the transmission frame with the spreading code includes selecting a first spreading code to spread the spectrum of the payload data by a variable factor determined from a predetennined set of possible first spreading codes, combining the modulation symbols of the payload data with the first spreading code, combining the header data with a second spreading code to spread the spectrum of at least part of the header data using the second spreading code, the second spreading code spreading the spectrum of the at least part of the header data by an amount which is greater than or equal to any of the first spreading codes for spreading the spectrum of the payload data by the variable factor, and generating the at least part of the header data which is combined with the second spreading code, the generated header data providing an indication of one or more of the first spreading codes which is used to spread the spectrum of the payload data by the variable factor.
14. 14. A method as claimed in Claim 13, wherein the selecting the first spreading code includes selecting the first spreading code from the set of first spreading codes in accordance with a state of a communication channel between the transmitter and one or more receivers of the transmitted signal.
15. 15, Method as claimed in Claim 13 or 14, comprising receiving at the transmitter from a reverse channel a signal transmitted by one or more of the receivers ol the transmitted radio signal providing the indication of the state of the channel from the transmitter to the one or more of the receivers of the payload data from the transmitted radio signal, wherein the selecting the first spreading code includes selecting the first spreading code for each of the one or more receivers which are to receive the payload data.
16. 16. A method as claimed in any of Claims 13, 14 or 15, wherein the header includes a start of frame sequence, the start of frame sequence being spread by a third sprcading code, the third spreading code providing a spreading factor which greater than the first spreading code and greater than or equal to the second spreading code.
17. 17. A receiver for receiving and recovering data transmitted as a payload of a radio signal, the receiver comprising a radio frequency receiver configured to detect the radio frequency signal and to generate a base band version of the received signal, a spectrum de-spreader configured to detect modulation symbols of the transmission frame which have been spread with a spreading code by correlating the received base band signal with the spreading code to fomi the modulated symbol, a dc-modulator configured to map the modulation symbols of the transmission frame into received payload data symbols according to a predetermined modulation scheme, and a controller configured to control the spectrum dc-spreader to detect the modulation symbols in accordance with the spreading code, wherein the received radio frequency signal comprises a plurality of frames of the payload data, the payload data having been spread spectrum encoded with a first spreading code, which is one of a predetermined set of first spreading codes providing a variable spreading factor and each of the frames of the received radio signal includes a header, the header carrying signalling data identifying the first spreading code which has been used to spread spectrum encode the payload data and the header has been combined with a second spreading code to spread the spectrum of at least part of the header data using the second spreading code, the second spreading code spreading the spectrum of the at least part of the header data by an amount which is greater than or equal to any of the first spreading codes for spreading the spectrum of the payload data by the variable factor, and the controller is contigured in combination with the spectrum dc-spreader and the dc-modulator to detect the at least part of the header data which is spread by the spectmm spread with the second spreading code, the header data providing an indication of one or more of the first spreading codes which has been used to spread the spectrum of the payload data, and to control the spectrum dc-spreader to detect and recover the payload data by correlating the data with the identified first spreading code.
18. 18. A receiver as claimed in Claim 17, comprising a feedback transmitter configured to communicate data via a reverse channel to the transmitter from which the radio signal was received, wherein the conftoller is configured to generate from the received radio signal channel state information providing an indication of a current state of the channel via which the radio signal was received from the transmitter, and to control the feedback transmitter to transmit the channel state information to the transmitter, the transmitter selecting the first spreading code from the first set of spreading codes in accordance with the received channel state information,
19. 19. A receiver as claimed in any of Claims 17 or 18, wherein the header includes a start of frame sequence, the start of frame sequence being spread by a third spreading code, the third spreading code providing a spreading factor which is greater than the first spreading code and the second spreading code, and the controller in combination with the dc-spreader is configured to detect the start of frame sequence by correlating the header with the third spreading code.
20. 20. A r?ceivcr as claimed in any of Claims 17 or 18, wherein the hcadcr includes a start of frame sequence, the start of frame sequence being spread by a third spreading code, the third spreading code providing a spreading factor which is weater than the Iirst spreading code and the second spreading code; and the receiver comprises a differential encoder configured to differentially encode the received baseband signal, and a correlator configured to detect the start of the frame by correlating the differentially encoded received signal with a stored expected version of the differentially encoded signal.
21. 21. A receiver as claimed in Claim 20, wherein the expected version of the differentially encoded signal is derived from the spreading codes and selected samples of the expected vcrsion of the differentially encoded signal are not included in the correlation of the differentially encoded signals, the selection of the samplcs being dependent on the spreading code from which the samples are derived.
22. 22. A receiver as claimed in Claim 21, wherein every second sample of the samples derived from the first spreading code is not included in the correlation.
23. 23. A receiver as claimed in Claims 21 or 22, wherein every one in S2 samples of the samples derived from the second spreading code is not included in the correlation, where S2 represents the spreading factor of the second spreading code.
24. 24. A receiver as claimed in any of Claims 17 to 23, wherein the first spreading code is an M-sequence derived using a degree 21 generator polynomial according to; 1 + x3 + x5 + x6 + x12 + x18 + x19 + x20 + x21
25. 25. A receiver as claimed in any of Claims 17 to 24, wherein the spreading factor of the third spreading code is factor of sixteen.
26. 26. A receiver as claimed in any of Claims 17 to 25, wherein the predetermined modulation scheme is quadrature phase shift keying, QPSK.
27. 27, A receiver as claimed in any of Claims 17 to 26, wherein the received payload data of the received signal has been error correction encoded and time interleaved, the demodulator being configured to map the modulation symbols of the transmission frame into received error correction encoded payload data symbols according to the predetermined modulation scheme, and the receiver comprises a time de-interleaver configured to deinterleave the error correction encoded payload data, and an error correction decoder configured to receive dc-interleaved error correction encoded payload data and to estimate the payload data by error correction decoding in accordance with the error colTeetion code, wherein the interleaving depth is determined in accordance with a coding rate of the error correction code, a baud rate of the communications channel between the transmitted and the receiver, the spreading factor of the first spreading code and a likelihood of possible lade durations of the received radio signal.
28. 28. A receiver as claimed in Claim 27, wherein the time de-interleaver is a convolutional de-interleaver.
29. 29. A receiver as claimed in any of Claims 27or 28, wherein the first spreading codes and coding ratcs of the error correction code are determined in accordance with the following modes: Mode Sprd Mode Sprd Mode Sprd Mode Sprd Cod Cod Cod Cod QPSK% 1D QPSK 9D QPSK 17D QPSK 25D Spread= 1/3 2/5 1/2 Spread = Spread = Spread = 1 1 2 QPSK% 2D QPSK 1OD QPSK 18D QPSK 26D Spread= 1/3 2/5 1/2 2 Spread = Spread = Spread = 2 2 3 QPSK1⁄4 3D QPSK liD QPSK 19D QPSK 27D Spread 1/3 2/5 1/2 3 Spread = Spread = Spread = 3 3 4 QPSKY4 4D QPSK 12D QPSK 20D QPSK 28D Spread 1/3 2/5 1/2 4 Spread = Spread = Spread = 4 4 5 QPSK3⁄4 SD QPSK 13D QPSK 21D QPSK 29D Spread = 1/3 2/5 1/2 Spread = Spread = Spread = 5 6 QPSK 3⁄4 6D QPSK 14D QPSK 22D QPSK 30D Spread = 1/3 2/5 1/2 6 Spread = Spread = Spread = 6 6 8 QPSK3⁄4 7D QPSK 15D QPSK 23D QPSK 31D Spread= 1/3 2/5 1/2 8 Spread = Spread = Spread = 8 8 10 QPSK% SD QPSK 16D QPSK 24D DUMMY OD Spread = 1/3 2/5 PLFRAM Spread = Spread = E 10
30. 30. A receiver as claimed in any of Claims 17 to 29, wherein the radio signal carrying the payload data is transmittcd from a satellite transmitter.
31. 31. A receiver as claimed in any of Claims 17 to 30, wherein the radio signal received by the receiver has been transmitted in accordance with a DVB-Sx standard.
32. 32. A method of receiving and recovering data transmitted as a payload of a radio signal at a receiver, the method comprising detecting the radio frequency signal and to generate a base band version of the received radio signal, detecting modulation symbols of the transmission frame which have been spread with a spreading code by correlating the received base band signal with the spreading code to form the modulated symbol, mapping the modulation symbols of the transmission frame into received payload data symbols according to a predetermined modulation scheme, and controlling the detecting the modulation symbols in accordance with the spreading code, wherein the received radio frequency signal comprises a plurality of frames of the payload data, the payload data having been spread spectrum encoded with a first spreading code, which is one of a predetermined set of first spreading codes providing a variable spreading factor and each of the frames of the received radio signal includes a header, the header carrying signalling data identiing the first spreading code which has been used to spread spectrum encode the payload data and the header has been combined with a second spreading code to spread the spectrum of at least part of the header data using the second spreading code, the second spreading code spreading the spectrum of the at least part of the header data by an amount which is greater than or equal to any of the first spreading codes for spreading the spectrum of the payload data by the variable factor, and detecting the at least part of the header data which is spread by the spectrum spread with the second spreading code by correlating the header with the second spreading code, to identil the one or more of the first spreading codes which has been used to spread the spectrum of the payload data, and detecting and recovering the payload data by correlating the data with the identified first spreading code.
33. 33. A method as claimed in Claim 32, comprising communicating data from the receiver via a reverse channel to the transmitter from which the radio signal was received, generating from the received radio signal channel state information providing an indication of a current state of the channel via which the radio signal was received from the transmitter, and transmitting the channel state information to the transmitter, the transmitter sclccting the first spreading code from the first set of spreading codes in accordance with the received channel state in[brmation.
34. 34. A method as claimed in any of Claims 32or 33, wherein the header includes a start of frame sequence, the start of frame sequence being spread by a third spreading code, the third spreading code providing a spreading factor which greater than the first spreading code and the second sprcading code, and the method includes detecting the start of frame sequence by correlating the header with the third spreading code.
35. 35. A method as claimed in any of Claims 32 or 33, wherein the header includes a start of frame sequence, the start of frame sequence being spread by a third spreading code, the third spreading code providing a spreading factor which is greater than the first spreading code and the second spreading code; and the method includes differentially encoding the received baseband signal, and detecting the start of the frame by correlating the differentially encoded received signal with a stored expected version of the differentially encoded signal.
36. 36. A transmitter or a receiver substantially as hereinbefore described with reference to the drawings.
37. 37. A method of transmitting or receiving substantially as hereinbefore described with reference to the drawings.