GB2514083A - Data processing apparatus and method - 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.

Description

DATA PROCESSING APPARATUS AND METHOD

Field 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 radio signal.

Backuround of the Disclosure

Wir&ess 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 communications 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 andior from the mobile terminals within a relatively short range of a base station serving the eeH.

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 be received by receivers on the ground. Data to be 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 downhnk. Example satellite broadcast transmissions for transmitting data as well as video and audio signals are those which are 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 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 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 spreading 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 embodimcnts a part of the headcr may include a start of frame indication which is combined with a third spreading code, which sprcads thc 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 interleaver. The 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 intcrlcaving the payload data in accordance with the determined depth which is matched to the likelihood of the duration of the fades, even if the signal to noise ratio temporally falls to a low-level, thc receivcr may still be able to recover thc 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 thc prcscnt disclosurc will now be dcscribcd 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 provides a schematic diagram of an example DVB-S2 system; Figure 2 provides a schematic block diagram of an example DVB-S2 transmitter; Figure 3 provides a schematic block diagram of receiver 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 1 Oa and I Ob 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; and Figure 13 provides an example of a receiver in accordance with an example of

the present disclosure.

Description of Example Embodiments

Figure 1 provides 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 receive signals via a satellite dish 3. An earth station 4 is arranged to transmit an uplink signal 6 to a receiving sateHite dish 8 on the satellite 1 for transmission onto the receivers 2 via a downlink 10. The 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 I 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 the 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.

S The payload data is then formed into a stream which is synchronised by a synchronising unit 28, 30 and then null-packet deletion is performed 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 pcrformed respectively for both thc 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 stream 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 data symbols to match a data rate of the frame and then to a baseband scrambler 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 interleavcr 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 DVB-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 I 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 signalling-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 arc greater than or equal to any of the first spreading codes, the header data identifying the spreading factor 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 synchronised and recover from the total or substantial loss of payload data temporarily as a result of a deep fade in the rcccivcd 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 equal to S2, and S2 is greater than or equal to SI. Consequently, the spreading factor length with which each portion of the frame is spread reflects the importance of the data conveyed by each portion of the frame. For example, the first part of the header has to be detected if the rest of the frame is to be detected and decoded; therefore the first part of the header is spread by the highcst sprcading 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 wifl 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 baseband scrambler 102 before a FEC unit 104. The FEC unit 104 comprises a BCH encoder 106, an LDPC encoder 108 and a bit interleaver 110. The BCH encoder 106, the LDPC encoder 108 and the bit interleaver 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 technique the coding rate of the error correction 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 time interleaver 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 arc 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 (SI) may be 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 the 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 formed frame. In accordance with the present 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 the payload data has been modulated and encoded and a start of frame (SOF) 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 anangement 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 (S3) 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 (S2) 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 technique the second spreading code produces a spreading factor which is greater than or equal to any of the spreading factors 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 transrnitter 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 (T) between the transmitted data (X) and the received data (Y) and can be represented in the following expression f p(x1,y) I(X;Y)1 It is assumed that all constellation points (n) occur with equal probability and therefore the probability distributions can then be re-written as; p(xjy) = p(yIx3P(x) = ii C P(YIXk) Ti and (y-x2 e 2a2 p(yIx1) = ___ Vzira Where the later equation is the "normal probability density frmnction since Gaussian noise is assumed. In Figure 4 the modulation constrained capacity for BPSK, QPSK and I6QAM modes are shown.

As previously mentioned, 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 SPSK.

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.

Interleavinti The proposed system, as illustrated in Figure 3, without the Time-interleaving block is designed to cope with fades resulting in C/Nref 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 can 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 fir st 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 S to maintain session links in a predetermined duration of excessive and intermittent fades or to reduce the adverse effects of interference. It will be 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 -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 (see).

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 1⁄2 rate system employing a spreading factor of 8 and a channel baud rate of 30.9MBaud, then the time-interleaving depth required can be calculated as; 1dqlth = (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 (See) Rate 1/3 Rate 1/4 1.45M l.29M 0.25 2.89M 2.58M 0.5 5.79M 5.15M 1 l1.59M 10.30M 2 38.97M 25.75M 5 57.94M 51.50M 10 86.90M 77.25M 15 11 5.S7M I 03.OOM 20 173.81M 154.5GM 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 arc 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 FECFRAME, 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 Si it follows that the length of the SFECFRAME will be a spread factor multiple of the parent TFECFRAME.

It is envisaged that the M-sequence 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 + x'2 + 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 formed 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 I/Q 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 -11dB so that receivers 2 can maintain a conncction with a satellite 1 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 of slots comprising 90 symbols 222, where there may be any number of slots and the number of slots in a 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 10) which arc 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 correlation 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 ill 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 frame (SOF) field of a frame in 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 (Si) 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 cxamples the pilots arc not rcquircd because thc signal to noisc 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 9b, 16 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 (53) and a PLS portion 231, which conveys signalling data refening to the payload data, spread by a second spreading factor (S2) where S3 is greater than or equal to S2. The remainder of the symbols of the frame are spread by a different spreading code which is indicated as a factor Si, where Si is a spreading factor less than the spreading factor i6 used to spread the data of section 230 and in some examples of the present technique, Si is less than or equal to S2. Consequently, 64 x 52 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. in the example frame 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 S2 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 receiver 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 be 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 S include the required signalling in order for the receiver to identift 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.

The 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 further 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 technique 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. However, it will be appreciated that not all of the modes in table 2 may be uscd 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 be 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 be detected and the payload data decoded even in low SNR scenarios.

Spread Spread Spread Spread Mode Mode Mode Mode Code Code Code Code QPSK 1⁄4 QPSK 1/3 QPSK2/5 QPSK 1/2 ID 9D I7D 25D Spread = 1 Spread = 1 Spread = 1 Spread = 2 QPSK 1⁄4 QPSK 1/3 QPSK2/5 QPSK 1/2 2D IOD ISD 26D Spread = 2 Spread = 2 Spread = 2 Spread = 3 QPSK1⁄4 QPSKI/3 QPSK2/5 QPSKI/2 3D lID 19D 27D Spread = 3 Spread = 3 Spread = 3 Spread = 4 QPSK1⁄4 QPSKI/3 QPSK2/5 QPSKI/2 4D 12D 20D 28D Spread = 4 Spread = 4 Spread = 4 Spread = 5 QPSK1⁄4 QPSKI/3 QPSK2/5 QPSKI/2 511) 131) 2111) 29D Spread = 5 Spread = 5 Spread = 5 Spread = 6 QPSK 1⁄4 QPSK 1/3 QPSK2/5 QPSK 1/2 611 1411 22D 30D Spread = 6 Spread = 6 Spread = 6 Spread = 8 QPSK 1⁄4 QPSK 1/3 QPSK2/5 QPSK 1/2 711 1511 2311 3111 Spread = S Spread = 8 Spread = 8 Spread = JO QPSK 1/3 QPSK 2/5 QPSK 1⁄4 DUMMY SD Spread = 1611 Spread = 2411 011 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 identifr 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 1 Ob 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 1⁄4 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.

Modes l,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 SNRtarget= [-9.5dB, -10.84dB & -11.74dB] Pilots = Optional (cffectivencss+ reduced by SI ifpilots are included) Mode 4, 5 & 6 Constellation = QPSK Code Rate = 1/4 Data Spreading Factor (51) = 4, 5 & 6 PL Header Spreading Factor = 16 FEC = Long or Short AWGN SNR target = -9.24dB, -10.24dB & -I 1dB Pilots = Optional (cffectivencss+ reduced by SI ifpilots arc included) The data capacity of the system designed and operating in accordance with the present technique may be calculated by first identi'ing the drop (or rise) in capacity due to the usc of a spreading factors 51. This drop is calculated by dividing the maximum spread factor allowable (Figure 10) by the factor Si. 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.

________ ________ Rate 1/3 _________ _________ _______ ________ Rate 1/4 _________ _________ Dana Dala Capacity C'N f Cap Capacity C'N f Cap Mode Spread (hits/see! (30.9M Mode Spread (bits/sec/H (30.9M H.) Baud) z) Baud) __________ ___________ ______________ ____________ (Mbits) __________ ___________ ______________ ____________ (Mhits) 1 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 usc 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. All the spreading factors of this adaptive system will be in line with the corresponding S2 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 be 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 thc received radio frequency signal to baseband symbols which are fed to a physical layer signalling (PLS) header despreader 302 where the SOF portion of the header 230 is despread and correlated using the sequence corresponding to 53 such that the start of the frame can be detected and the receiver synehronised to the frame structure. The SOF may be spread by a factor that is greater than or equal to the spreading factor used for the PLS code. For exarnple the spreading code for SOF rnay be 16, while the spreading code for the PLS may be 16 or 10. Once the SOF is identified the header despreader 302 also despreads the PLS portion of the header 231 spread with the second spreading factor S2 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 case for the spreading factor S3 used for SOF.

However, as previously mentioned, in some exarnples in accordance with the present technique the spreading factor for the PLS may be unknown at the receiver. In this case the receiver would then correlate for possible known spreading factors, e.g. 1,2,3 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 cornmensurate with any the low SNR requirements.

Once the SOF and PLS data has been detected and despread,the partially despread 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 the header such as S2. The spread spectrum despreader for the header data 302 can therefore despread the header data at a lower SNR then the payload data dcspreader 306 which is arranged to desprcad the data in accordance with a spreading code which has been used to spread the spectrum of the payload data. However, 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 thc PLS codc 231. Accordingly, thc dcsprcad PLS data is fcd to a controller 308 which configures the payload data dcsprcadcr 306 to dcsprcad the payload data according to the 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 PLS data, the reception and decoding of the header data will be more reliable. Therefore even during a low signal to noise ratio period the receiver will still 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 arc then fed to a time de-intcrleavcr 310 which deinterleaves 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 de-interleaver 320 and an error correction LDPC decoder 322 and a BCH decoder 324 which combine to form ilmer and outer decoding of the encoded data as explained above in accordance with different encoding rates. As previously mentioned, a number of 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 proccssing appropriatc to thc data information conveyed in thc payload data.

Various further aspects in features of the present disclosure are defined in the indcpcndcnt claims. Various modifications may bc madc 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 systcm.

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

2. ETSI TR 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-52)".

3. TM-52-0122r1: "Call for technologies (Cii') for the evolutionary subsystem for the S2 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 (26)

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 be 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, a radio frequency transmitter 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 first 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 thc rcccivcrs of thc transmitted radio signal providing thc indication of thc statc 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 second 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-scquence derived using a degree 21 generator polynomial according to; 1 + x3 + r5 + r6 + r12 + x18 + r'9 + r2° + r2'
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 I to 6, wherein the predetermined modulation scheme is quadrature phase shift keying, QPSK.
8. 8. A transmitter as claimed in any of Claims Ito 7, comprising an en-or coneetion 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 transmitted 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 c, 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 QPSK% ID QPSK 9D QPSK 17D QPSK 25D Spread = 1/3 2/5 1/2 1 Spread = Spread = Spread = 1 1 2 QPSK% 2D QPSK 10D QPSK 18D QPSK 26D Spread= 1/3 2/5 1/2 2 Spread = Spread = Spread = 2 2 3 QPSK1h 3D QPSK liD QPSK 19D QPSK 27D Spread 1/3 2/5 1/2 3 Spread = Spread = Spread = 3 3 4 QPSK 1⁄4 4D QPSK 12D QPSK 20D QPSK 28D Spread 1/3 2/5 1/2 4 Spread = Spread = Spread = 4 4 5 QPSK1h 5D QPSK 13D QPSK 21D QPSK 29D Spread= 1/3 2/5 1/2 Spread = Spread = Spread = 5 6 QPSK1h 6D QPSK 14D QPSK 22D QPSK 30D Spread 1/3 2/5 1/2 6 Spread = Spread = Spread = 6 6 8 QPSK1h 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 OPSK 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 I 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 thc payload data and the header data of the transmission frame onto modulation symbols using a predetermined 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 predetermined 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 S 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 thc 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. A 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 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 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 spreading 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 dc-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 form the modulated symbol, a dc-modulator configured to map thc 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 idcntiring 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 sprcading 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 controfler is configured 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 spectrum 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 controller 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. Areceiver as claimedinanyofClaims 17 or 18, whereintheheader includes a start of frame sequence, the start of fine 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 spreading code, and the controller in combination with the de-spreader to detect the start of frame sequence by correlating the header with the third spreading code.
20. 20. A reccivcr as claimed in any of Claims 17, 18, or 19, wherein thc first spreading code is an M-sequence derived using a degree 21 generator polynomial according to; 1 + x3 + x5 + x6 + x12 + x18 + x19 + x2° + x21
21. 21. A receiver as claimed in any of Claims 17 to 20, wherein the spreading factor of the third spreading code is factor of sixteen.
22. 22. A receiver as claimed in any of Claims 17 to 21, wherein the predetermined modulation scheme is quadrature phase shift keyin QPSK.
23. 23. A receiver as claimed in any of Claims 17 to 22, 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 dc-interleave 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 correction 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 codc and a likelihood of possible fadc durations of the received radio signal.
24. 24. A receiver as claimed in Claim 23, wherein the time de-interleaver is a convolutional de-interleaver.
25. 25. A receiver as claimed in any of Claims 23 or 24, wherein the first spreading codes and coding rates 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 YD 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 QPSK% 3D QPSK lID QPSK 19D QPSK 27D Spread= 1/3 2/5 1/2 3 Spread = Spread = Spread = 3 3 4 QPSK 1⁄4 4D QPSK 12D QPSK 20D QPSK 28D Spread 1/3 2/5 1/2 4 Spread = Spread = Spread = 4 4 5 QPSK1⁄4 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 QPSK1h 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
26. 26. A receiver as claimed in any of Claims 17 to 25, wherein the radio signal carrying the payload data is transmitted from a satellite transmitter.S27. A receiver as claimed in any of Claims 17 to 26, wherein the radio signal received by the receiver has been transmitted in accordance with a DVB-Sx standard.28. 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 hcadcr, thc header carrying signalling data idcntilring 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 identify 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.29. A method as claimed in Claim 28, 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 selecting the first spreading code from the first set of spreading codes in accordance with the received channel state information.30. A method as claimed in any of Claims 28 or 29, 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 spreading code, and the method includes detecting the start of frame sequence by correlating the header with the third spreading code.31. A transmitter or a receiver substantially as hereinbcfore described with reference to the drawings.32. A method of transmitting or receiving substantially as hereinbefore described with reference to the drawings.