Patent Publication Number: US-10326557-B2

Title: Transmitter and method of transmitting and receiver and method of receiving

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation Application of U.S. application Ser. No. 15/348,542, filed Nov. 10, 2016, which is a continuation application Ser. No. 14/969,418, filed Dec. 15, 2015 (now U.S. Pat. No. 9,520,966), which claims priority to United Kingdom Application 1422307.7 filed on Dec. 15, 2014 and to United Kingdom Application 1504207.0 filed on. Mar. 12, 2015. The entire contents of the above-identified applications are incorporated herein by reference. 
    
    
     FIELD OF THE DISCLOSURE 
     The present disclosure relates to transmitters and methods of transmitting data and receivers and methods of receiving data, which has been interleaved in accordance with a convolutional interleaving scheme. 
     BACKGROUND OF THE DISCLOSURE 
     There are many examples of radio communication systems in which data is communicated by encoding the data to improve the likelihood of recovering the data at a receiver. As those acquainted with error correction encoding will appreciate, error correction encoding and decoding to recover payload data being communicated can perform better when the encoded data is interleaved so as to disperse the effect of any bursts of errors in the data symbols, so that the errors are separated as far as possible in the received symbol stream. 
     For example some systems use Orthogonal Frequency Division Multiplexing (OFDM) to transmit the data. For example Digital Video Broadcasting (DVB) standards, use OFDM, which is also being proposed for the Advanced Television System Communications (ATSC) standard of 3 rd  generation (ATSC3.0). OFDM can generally be described as providing K narrowband sub-carriers (where K is an integer) which are modulated in parallel, each sub-carrier communicating a modulated data symbol, such as a Quadrature Amplitude Modulated (QAM) symbol or a Quaternary Phase-shift Keying (QPSK) symbol. The modulation of the sub-carriers is formed in the frequency domain and transformed into the time domain for transmission. Since the data symbols are communicated in parallel on the sub-carriers, the same modulated symbols may be communicated on each sub-carrier for an extended period, which can be longer than the coherence time of the radio channel. The sub-carriers are modulated in parallel contemporaneously, so that in combination the modulated carriers form an OFDM symbol. The OFDM symbol therefore comprises a plurality of sub-carriers, each of which has been modulated contemporaneously with different modulation symbols. During transmission, a guard interval filled by a cyclic prefix of the OFDM symbol precedes each OFDM symbol. When present, the guard interval is dimensioned to absorb any echoes of the transmitted signal that may arise from multipath propagation or other transmitters transmitting the same signal from a different geographic location. However one characteristic of OFDM is that errors in the received data stream can occur in bursts and so some form of interleaving is appropriate as mentioned above. The interleaving can be performed at the symbol level in which data symbols which are to be mapped onto the sub-carriers of the OFDM symbols are interleaved before being modulated onto the sub-carriers. The interleaving can also be performed at the bit level in which the bits of data before mapping onto the symbols for modulating the sub-carriers are interleaved/deinterleaved. Usually, communication systems include interleaving both at bit and symbol levels. 
     The simplest form of interleaving is block interleaving, in which data is written into memory row-wise and read-out column-wise or vice versa. The depth of such an interleaver can roughly be defined as the number of codewords that can be interleaved together. The effectiveness of an interleaver depends on its depth. Therefore, with large code words and a requirement for a large depth, the interleaver can demand a significant amount of memory, which can increase the cost of implementing receivers for interleaved data, which require a corresponding block deinterleaver. For this reason convolutional interleaving has been proposed, which as will be explained requires typically about half of the memory size of a block interleaver for the same interleaving depth. However for systems, such as television broadcast systems, in which data is transmitted in frames, convolutional interleavering can present some technical challenges. 
     SUMMARY OF DISCLOSURE 
     According to an aspect of the present disclosure there is provided a transmitter for transmitting data using Orthogonal Frequency Division, OFDM, symbols. The transmitter comprises a forward error correction encoder configured to encode the data to form forward error correction encoded frames of encoded data cells. A service frame builder is configured to form a service frame for transmission comprising a plurality of the forward error correction encoded frames. A convolutional interleaves comprising a plurality of delay portions and configured to convolutionally interleave the encoded data cells of the plurality of the forward error correction encoded frames of the service frames, a modulation symbol mapper configured to map the interleaved and encoded data cells of the service frames onto modulation cells, and a modulator configured to modulate one or more carriers with the modulation cells for transmission. A controller is configured to form signalling data to be transmitted with the service frames to include an indication of an identified first of the encoded data cells of a first of the plurality of forward error correction encoded frames of a service frame which can be decoded from encoded data cells received from the service frame or the service frame and one or more other service frames following after the service frame. 
     Embodiments of the present technique can be arranged to detect the first cell of a first of the error correction encoded frames of a new service frame which does not have any data cells in one or more previous service frames as a result of the convolutional interleaving then a receiver, which has acquired the new service frame but none of the one or more previous service frames can decode this first forward error correction encoded frame of the new service frame and ignore the other forward error correction encoded frames earlier in the service frame. Therefore for example a receiver may power on or change channels during a previous service frame and be directed to only decode a forward error correction encoded frame that it can receive completely. 
     According to some embodiments, the controller is configured to calculate a displacement value (A) representing a minimum displacement from the first cell of the service frame of a first cell of a first of the forward error correction frames, for which none of the data cells of the forward error correction frame will be transmitted in one or more previous service frames; and to form the signalling data to be transmitted to the receiver to include the identified first cell of the first of the forward error correction frame of the service frame which can be decoded from cells received from the new service frame. According to a further aspect of the present technique there is provided a receiver for recovering data symbols from a received signal. The receiver comprises a demodulator configured to detect the received signal comprising an interleaved symbol stream comprising a sequence of service frames (ATSC frames), each service frame including one or more forward error correction encoded frames of data cells, which have been interleaved by a convolutional interleaver. The receiver includes a convolutional deinterleaver circuitry configured to perform a convolutional deinterleaving of the received data cells, a forward error correction decoder configured to decode the encoded frames of data cells, and controller circuitry. The controller circuitry is configured to detect a new service frame, to detect from which of a plurality of delay portions of the convolutional deinterleaver a first service frame data symbol of the new service frame is read, to detect the number of the service frame data cells of the new service frame after which a first forward error correction frame is read, and to detect, based upon the detected new service frame, the delay portion from which the first new service frame data symbol of the new service frame is read, and the number of the service frame data cells of the new service frame after which a first forward error correction frame is read, whether any data cells of the first forward error correction frame were transmitted in a previous service frame. If any of the data cells of the first forward error correction frame were transmitted in a previous service frame not received by the receiver, not to decode the first forward error correction frame, or if no data cells of the new forward error correction frame were transmitted in the previous service frame, or if any data cells of the new forward error correction frame were transmitted in a received previous service frame, to decode the new forward error correction frame. 
     Embodiments of the present technique can provide an arrangement in which the receiver is arranged to detect a condition in which after initially acquiring the start of a new service frame and an indication of a point in the service frame at which a new error correction encoded frame begins, a receiver cannot recover all of the symbols which have been encoded in the error correction encoded frame and therefore discards the error correction encoded frame and awaits the next error correction encoded frame before decoding the next error correction encoded frame to recover the service data. This is because, as a result of the convolutional interleaving, one or more data cells of the error correction encoded frame have been transmitted in a previous service frame, which may not have been received. The present technique therefore recognises a problem in which when a receiver is initially powered up or changes channel to the received symbol stream and detects a start of a new service frame, and, as a result of a latency in the convolutional interleaver, if a new error correction encoded frame starts too close to the start of the service frame then not all of the symbols of the error correction encoded frame can be recovered from the new service frame because some of these have been transmitted in the previous service frame as a result of the convolutional interleaving. The condition for detecting whether the new error correction encoded frame can be recovered from the new service frame is dependent upon a position in which the convolutional encoder starts in respect of a new row of the interleaver with respect to the new service frame. 
     In some examples, the receiver determines a row of the convolutional deinterleaver for the new service frame in accordance with a pre-determined arrangement in which a previous service frame finishes at a commutator of the convolutional interleaver on a pre-determined row whereas other examples provide an arrangement for signalling the row of the convolutional interleaver in which the first symbol or cell of the new frame be input. Furthermore, in some examples, the first symbol or cell of the new service frame in which the new error correction encoded flame starts is signalled to the receiver whereas in other examples the start of the error correction encoded frame within the new service frame is detected by the receiver in accordance with the pre-determined arrangement. 
     As will be explained below, calculation of the displacement of the first encoded data cell of a forward error correction encoded frame of a service frame from the first encoded data cell of the service frame, which ensures that the error correction encoded data frame can be decoded differs for different forms of the convolutional interleaver and deinterleaver. 
     Various further aspects and features of the disclosure are defined in the appended claims, which include a method of receiving, a data processing apparatus and a television receiver. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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: 
         FIG. 1  is a schematic diagram illustrating an arrangement of a broadcast transmission network: 
         FIG. 2  is a schematic block diagram illustrating an example transmission chain for transmitting broadcast data via the transmission network of  FIG. 1 ; 
         FIG. 3  is a schematic block diagram showing parts of the transmitter shown in  FIG. 2 , which perform error correction encoding and interleaving; 
         FIG. 4  is a schematic illustration of OFDM symbols in the time domain which include a guard interval; 
         FIG. 5  is a schematic block of, a typical receiver for receiving data broadcast by the broadcast transmission network of  FIG. 1  using OFDM; 
         FIG. 6  is a schematic illustration of a transmission frame for transmitting broadcast data including payload data and signalling data; 
         FIG. 7  is a schematic illustration of a part of the transmission frame shown in  FIG. 5 , showing an example arrangement in which physical layer pipes are arranged within subintervals of a service frame, such as an ATSC frame; 
         FIG. 8  is a schematic block diagram showing parts of the receiver shown in  FIG. 4 , which perform deinterleaving and error correction decoding; 
         FIG. 9  is a schematic block diagram of a convolutional interleaver and a convolutional deinterleaver, which form pan of the transmitter and the receiver of  FIGS. 2 and 4 ; 
         FIG. 10  is a part schematic block diagram, part schematic illustration of a function of the convolutional interleaver of  FIG. 9 ; 
         FIG. 11  is a schematic representation illustrating an effect of convolution interleaving in the transmitted symbol stream; 
         FIG. 12  is a schematic representation illustrating an effect of convolution interleaving in the transmitted symbol stream and an arrangement for performing convolutional deinterleaving between a new service frame and a first new error correction encoded frame in the new service frame; 
         FIG. 13  is a schematic representation illustrating an effect of convolution interleaving in the transmitted symbol stream and an arrangement for performing convolutional deinterleaving between a new service frame and a first new error correction encoded frame in the new service frame, in which a technical problem is illustrated; 
         FIG. 14  is a schematic representation illustrating the technical problem illustrated in  FIG. 13  of performing convolutional deinterleaving between a new service frame and the first new error correction encoded frame in the new service frame; 
         FIG. 15  is a schematic representation illustrating an effect of convolution interleaving in the transmitted symbol stream and an arrangement for performing convolutional deinterleaving between a new service frame and a first new error correction encoded frame in the new service frame, in which a technical solution is illustrated according to the present technique; 
         FIG. 16  is a schematic block diagram showing parts of the receiver shown in  FIG. 4 , which perform deinterleaving and error correction decoding and including a controller which is configured to control the error correction decoding in accordance with the present technique; 
         FIG. 17  is a flow diagram providing an example illustration of a method of controlling error correction decoding for data which has been convolutionally interleaved by the convolutional interleaver shown in  FIG. 9  and in which a new service frame is being received in accordance with the present technique; 
         FIG. 18  is a flow diagram providing illustration of a method of transmitting data, including generating signalling data which indicates the first forward error correction encoded frame in a service frame, in which none of the data cells have been transmitted in a previous service frame when interleaved by the convolutional interleaver shown in  FIG. 9 ; 
         FIG. 19  is a schematic block diagram of a further alternative arrangement for a convolutional interleaver and a convolutional deinterleaver, which could form the transmitter and the receiver of  FIGS. 2 and 4 ; 
         FIG. 20  is a flow diagram providing an example illustration of a method of controlling error correction decoding for data which has been convolutionally interleaved for the example of the convolutional deinterleaver shown in  FIG. 19  in accordance with the present technique; 
         FIG. 21  is a flow diagram providing illustration of a method of transmitting data, including generating signalling data which indicates the first forward error correction encoded frame in a service frame, in which none of the data cells have been transmitted in a previous service frame for the example convolutional interleaver shown in  FIG. 19 . 
     
    
    
     DESCRIPTION OF EXAMPLE EMBODIMENTS 
     Embodiments of the present disclosure can be arranged to form a transmission network for transmitting signals representing data including video data and audio data so that the transmission network can, for example, form a broadcast network for transmitting television signals to television receiving devices. In some examples the devices for receiving the audio video of the television signals may be mobile devices in which the television signals are received while on the move. In other examples the audio/video data may be received by conventional television receivers which may be stationary and may be connected to a fixed antenna or antennas. 
     Television receivers may or may not include an integrated display for television images and may be recorder devices including multiple tuners and demodulators. The antenna(s) may be inbuilt to television receiver devices. The connected or inbuilt antenna(s) may be used to facilitate reception of different signals as well as television signals. Embodiments of the present disclosure are therefore configured to facilitate the reception of audio/video data representing television programs to different types of devices in different environments. 
     An example illustration of a television broadcast system is shown in  FIG. 1 . In  FIG. 1  broadcast television base stations  1  are shown to be connected to a broadcast transmitter  2 . The broadcast transmitter  2  transmits signals from base stations  1  within a coverage area provided by the broadcast network. The television broadcast network shown in  FIG. 1  operates as a so called single frequency network in which each of the television broadcast base stations  1  transmit the radio signals conveying audio/video data contemporaneously so that these can be received by television receivers  4  as well as mobile devices  6  within a coverage area provided by the broadcast network. For the example shown in  FIG. 1  the signals transmitted by the broadcast base stations  1  are transmitted using Orthogonal Frequency Division Multiplexing (OFDM) which can provide an arrangement for transmitting the same signals from each of the broadcast stations  2  which can be combined by a television receiver even if these signals are transmitted from different base stations  1 . Provided a spacing of the broadcast base stations  1  is such that the propagation time between the signals transmitted by different broadcast base stations  1  is less than or does not substantially exceed a guard interval that precedes the transmission of each of the OFDM symbols then a receiver device  4 ,  6  can receive the OFDM symbols and recover data from the OFDM symbols in a way which combines the signals transmitted from the different broadcast base stations  1 . Examples of standards for broadcast networks that employ OFDM in this way include DVB-T, DVB-T2, ATSC 3.0 and ISDB-T. 
     An example block diagram of a transmitter forming part of the television broadcast base stations  1  for transmitting data from audio/video sources is shown in  FIG. 2 . In  FIG. 2  audio/video sources  20  generate the audio/video data representing television programmes, each of which can be thought of as source data for transmission via different physical layer pipes (PLP). The audio/video data is encoded using forward error correction encoding by an encoding/interleaver block  22  which generates forward error correction encoded data frames. A stream of forward error correction (FEC) encoded data frames therefore forms a physical layer pipe. Encoded data symbols, also referred, to in the following description as data cells, are formed into FEC frames are therefore transmitted as a PLP within each of the transmission frames, which are referred to as service frames, or according to the ATSC standard. ATSC flames. 
     The encoded data cells of the FEC frames are fed to a modulation unit  24  which maps the encoded data onto modulation symbols which are used to modulate OFDM symbols. The modulation symbols are generated in accordance with a modulation scheme such as mQAM, which may be uniform or non-uniform and where m is typically equal to 2 n , where a is an integer. Accordingly, in the following description in order to avoid confusion with OFDM symbols, the modulation symbols will be referred to as cells or QAM cells, although it will be appreciated that embodiments of the present technique are not limited to QAM. 
     Depicted on a separate lower arm, signalling data providing physical layer signalling for indicating for example the format of coding and modulation of the audio/video data is generated by a physical layer signalling unit  30  and after being encoded by an encoding unit  32  the physical layer signalling data is then modulated by a modulation unit  24  as with the audio/video data. 
     A frame builder  26  is arranged to form the data to be transmitted with the physical layer data into a frame for transmission, which combines a plurality of physical layer pipes (PLPs) each of which carries the data from one of the audio/video television programme sources  20 . The service frame includes a time divided section having a preamble in which the physical layer signalling is transmitted and one or more data transmission sections, which transmit the audio/video data generated by the audio/video sources  20 . A symbol or frequency interleaver  34  may interleave the data which is formed into symbols for transmission before being modulated by an OFDM symbol builder  36  and an OFDM modulator  38 . The OFDM symbol builder  36  receives pilot signals which are generated by a pilot and embedded data generator  40  and fed to the OFDM symbol builder  36  for transmission. An output of the OFDM modulator  38  is passed to a guard insertion unit  42  which inserts a guard interval and the resulting signal is fed to a digital to analogue convertor  44  and then to an RF front end  46  before being transmitted by an antenna  48 . 
       FIG. 3  shows a block diagram of example data processing elements which may form part of the encoding/interleaving, block  22  shown in  FIG. 2 . In  FIG. 2 , the audio/video source block  20  provides the source data for transmission to an FEC encoder  201 , possible via intermediate elements. As explained above, a forward error correction (FEC)  202  encoder performs error correction encoding of the data. The output of the forward error correction (FEC) encoder  202  is a vector of bits that may then be processed by other blocks, for example a bit interleaver  202  may be arranged to preform interleaving of the encoded data bits for transmission. The output of the bit interleaver  202  is fed to a QAM mapper  203 , which feints the encoded data bits into groups of bits in accordance with a modulation scheme, which has been selected by the controller  208 . The groups of bits may, for example define a cell such as a QAM cell, because the content of the group of data bits defines the QAM modulation symbol, which is used to modulate a carrier signal, such as for example a sub-carrier of an OFDM symbol. The QAM cells are fed from the output of the QAM mapper  203  to a time interleaver  206 . The output from the QAM mapper  203  may comprise the groups of one or more bits for selecting constellation points or constellation points represented by real and imaginary components. As mentioned above, the constellation points represent cells representing the one or more bits depending on the modulation scheme used, which may be, for example, BPSK, QPSK, QAM, 16QAM etc., wherein the constellation points are either uniformly or non-uniformly spaced. 
     The stream of QAM cells is then input to a time interleaver  203  which interleaves the symbols from various FEC frames across time. The time inlerleaver  203  interleaves input the QAM cells from across one or more FEC code words or frames, thereby creating a more uniform distribution of errors in cases where errors may occur in bursts in time or frequency. 
     The interleaved symbols then enter the modulator  24  as shown in  FIG. 2  and are converted into modulation cells and then fed to a frame builder  205 , which combines these QAM cells from other sources with other symbols which are used to convey information about the format of the transmission, into a set of cells which will be referred to as a physical layer frame or service frame. A frame may then be processed by further blocks which modulate the frame onto for example a set of OFDM symbols, which are then used to modulate an RF signal for transmission over a radio channel or a cable channel. 
     Although  FIG. 3  depicts four elements, the transmitter  2  may also comprise a number of other elements common to the transmission of a digital signal, such as a multiplexer, a compression coder and signal amplifiers etc. 
     As with a conventional arrangement, OFDM is arranged to generate symbols in the frequency domain in which QAM cells to be transmitted are mapped onto sub carriers which are then converted into the time domain using an inverse Fourier Transform. Thus the data to be transmitted is formed in the frequency domain and transmitted in the time domain. As shown in  FIG. 4  each time domain symbol is generated with a useful part of duration Tu and a guard interval of duration Tg. The guard interval is generated by copying a part of the useful part of the symbol in the time domain. By correlating the useful part of the burst with the guard interval, a receiver can be arranged to detect the useful part of the OFDM symbol Tu, from which data can then be recovered from an OFDM symbol by triggering a Fast Fourier Transform to convert the time domain symbol samples into the frequency domain. Such a receiver is shown in  FIG. 5 . 
     In  FIG. 5  a receiver antenna  50  is arranged to detect an RF signal which is passed via a tuner  52  and converted into a digital signal using an analogue to digital converter  54  before the guard interval is removed by a guard interval removal unit  56 . After detecting the optimum position for performing a fast Fourier Transform (FFT) to convert the time domain samples into the frequency domain, an FFT unit  58  transforms the time domain samples to form the frequency domain samples which are fed to a channel estimation and correction unit  60 . The channel estimation and correction unit  60  then estimates the transmission channel for example by using pilot sub-carriers which have been embedded into the OFDM symbols. After excluding the pilot sub-carriers, all the data-bearing sub-carriers are fed to a frequency deinterleaver  62  which deinterleaves the QAM cells received from the sub-carriers of the OFDM symbols, to reverse the operation performed by the frequency interleaver  34  in the transmitter. A time deinterleaver  63  performs an operation to deinterleave the QAM cells in the received service frame. A de-mapper unit  64  then extracts the data bits from the QAM cells provided from the time and frequency de-interleaved sub-carriers of the OFDM symbol. The data bits are fed to a bit deinterleaver  66 , which performs the de-interleaving so that the error correction decoder  68  can correct errors in accordance with a conventional operation. 
     Framing Structure 
       FIG. 6  shows a schematic of the framing structure according to an example embodiment of the present technique.  FIG. 6  illustrates different physical layer frames, some targeted for mobile reception whilst others are targeted for fixed roof-top antenna reception. The system can be expanded in future to incorporate new types of frames, for the current system, these potential new types of frames are simply known as future extension frames (FEFs). 
     One requirement for fixed reception frames is an improved spectral efficiency which may be assured by such features as adopting a higher order modulation, for example 256QAM, and higher code rates, for example greater than half rate, because of relatively benign channel conditions, and a high number of sub-carriers per OFDM symbol (FFT size) such as 32K. This reduces the capacity loss due to the guard interval fraction. However, a higher number of sub-carriers can make such OFDM symbols unsuitable for mobile reception because of lower tolerance to high Doppler frequency of the received signal. On the other hand, the main requirement for mobile reception frames could be robustness in order to ensure a high rate of service availability. This can be improved by adopting such features as a low order modulation for example QPSK or BPSK, low code rates, a low number of sub-carriers per OFDM symbol (FFT size) and a high density scattered pilot pattern etc. A low number of sub-carriers for OFDM symbols can be advantageous for mobile reception because a lower number of sub-carriers can provide a wider sub-carrier spacing and so more resilience to high Doppler frequency. Furthermore a high density pilot pattern eases channel estimation in the presence of Doppler. 
     The framing structure shown in  FIG. 6  is therefore characterised by frames which may each include payload data modulated and encoded using different parameters. This may include for example using different OFDM symbol types having different number of sub-carriers per symbol, which may be modulated using different modulation schemes, because different frames may be provided for different types of receiver. However each frame may include at least one OFDM symbol carrying signalling data, which may have been modulated differently to the one or more OFDM symbols carrying the payload data. Furthermore the signalling OFDM symbol may be a different type to the OFDM symbol(s) carrying the payload data. The signalling data is required to be recovered so that the payload data may be de-modulated and decoded. 
       FIG. 7  provides an alternative view of an example of a physical layer frame or service frame structure, which may be used to deliver the services via physical layer pipes (PLP) provided by the transmitted of  FIG. 1 . As shown in  FIG. 7  an ATSC frame or service frame  100 , which may therefore correspond in one example to the stationary TV frame  100  shown in  FIG. 6 , comprises a plurality of sub-slices  160 , each of which comprises in time and frequency a section of OFDM sub-carriers  162 ,  164 ,  166 ,  168 ,  170  on which each of five physical layer pipes are carried ( 1 ,  2 ,  3 ,  4 ,  5 ). Also shown in  FIG. 7  and of particular relevance in the following explanation, in some examples physical layer signalling (layer  1  signalling) can provide an indication of the sub-carrier and a time at which a first of the symbols or cells of a PLP is transmitted (PLP_START)  174 . The physical layer signalling may also provide an indication of the first symbol or cell of the first forward error correction (FEC) word of the service that starts in the current frame or sub-slice interval (PLP_TI_FECFRAME_START)  176  and an indication of a number of blocks to the end of the PLP (PLP_TI_NUM_BLOCKS_TO_END)  178 . 
       FIG. 8  provides a block diagram of example data processing elements that may comprise the receivers  4 ,  6  where the receivers  4 ,  6  are operable to receive the signal transmitted by the transmitter  2 , representing a sequence of FEC frames shown in  FIGS. 1 and 2 . The transmitted signal comprising the symbols of one or many FEC frames from one or several audio/video (AV) sources is separated, after potentially performing channel estimation and correction, into symbol streams formed in accordance with the AV sources to which the streams belong. Each stream is then de-interleaved. The frequency deinterleaver  62  is configured to deinterleave the QAM modulation symbols (cells) received from the sub-carriers of the OFDM symbols. The time deinterleaver  63  performs a time deinterleaving of the QAM cells of the received service frame for the PLP and reverses the time interleaving performed by the time interleaver  206 . The output of the time deinterleaver  63  then may pass through a QAM demapper  64 , which converts the QAM cells (symbols) to a stream of bits. The received stream of bits are then deinterleaved by a bit deinterleaver  66  before being passed to a FEC decoder  68  which attempts to correct any errors in the data. 
     Interleaving is often used in digital communication systems to improve the performance of forward error correcting codes. In many communication channels, errors often occur in bursts rather than in isolation or independently. If the number of errors within a code word exceeds the error-correcting code&#39;s capability, the error correction process may fail to recover the original code word. Interleaving ameliorates this problem by shuffling source symbols across several code words, thereby creating a more uniform distribution of errors. These code words are generally transmitted sequentially in time; hence the name “time-interleaving” because this stream of symbols is then interleaved in time. However, interleaving may also be done in the frequency domain so that in OFDM systems symbols may be distributed across different frequencies. 
     One method of performing interleaving is called block interleaving. This is used to implement the time-interleaving in many digital communication systems, for example DVB-T2. In a block interleaver blocks of, symbols are interleaved and these blocks may be referred to as interleaving-frames. Another method of performing interleaving is convolutional interleaving. Convolutional interleaving is also known and used in digital communication systems, for example ISDB-T. The advantage of a convolutional interleaver is that it generally uses half the amount of memory that is required for block interleaving, whilst giving similar performance. There are, however, disadvantages of convolutional interleaving when compared to block interleaving. For example, the resulting interleaved data is continuous, rather than being naturally divided into discrete interleaving frames. This means that it may not be possible to change the interleaver parameters whilst the interleaver or deinterleaver is running. Secondly, the size of a convolutional interleaver needs to be related to other parameters of the communication system in which it is operating so the deinterleaver at the receiver can be synchronised with the interleaver at the transmitter. 
       FIG. 9  illustrates the internal operation of a convolutional interleaver, such as the time interleaver  206  depicted in  FIG. 3 , which would typically be found in the transmitter side of a digital communication system as depicted in  FIG. 1 .  FIG. 9  also illustrates the internal operation of a convolutional deinterleaver, such as the time deinterleaver  63  depicted in  FIGS. 5 and 8 , which would typically be found in the receiver side of a digital communication system as depicted in  FIG. 1 . However as, explained below, there is an alternative arrangement which could be used for the convolutional interleaver  206  and the convolutional deinterleaver  63 , which is illustrated in  FIG. 19 , in which the rows of delay elements are inverted with respect to the arrangement shown in  FIG. 9 . 
     The interleaver  206  and the deinterleaver  63  each comprise an input commutator  401 , an output commutator  402  and a plurality of delay portions  403 , where the delay portions may also be referred to as interleaver/deinterleaver rows due to their layout in the interleaver and deinterleaver visualisations. The input commutator  401  is operable to input symbols into a delay portion and the output commutator  402  is operable to read out symbols from the delay portions. In some examples the input and output commutators  401 ,  402  may be synchronised such that they input and read out symbols from the same delay portion, however, they may also be unsynchronised such that they input and read out symbols from different delay portions. The commutators  401 ,  402  may for example become unsynchronised in the interleaver when output commutator  402  does not read out a symbol from a delay portion but the input commutator  401  continues to input a symbol to said delay portion. As will be appreciated, the term commutator may be replaced with the term ‘selector’ or ‘commuter’ without any intended change of meaning or function. 
     The plurality of delay portions  404  are arranged to delay symbols by different amounts from when a symbol is input into a selected delay portion by the input commutator  401  and read out from the selected delay portion by the output commutator  402 . At least one of the delay portions is made up of one or more memories or memory elements  404  where the delay introduced by a delay portion is partially dependent upon the number of memory elements the delay portion comprises. The number of symbols a delay portion is operable to store or delay at any one time is dependent on the number of memory elements the delay element comprises. In some examples the arrangement of the memory elements  404  may be said to form columns which are analogous to the interleaver/deinterleaver rows mentioned above. Each memory element may be associated with a memory location in an interleaver/deinterleaver memory which is operable to store a symbol input into a delay portion. The delay portions  403  are arranged in a sequence such as that illustrated in  FIG. 9 , where the delay portions  403  in the interleaver  206  are arranged in an order corresponding to decreasing delay i.e. decreasing number memory elements, and the delay portions  403  in the deinterleaver  302  are arranged corresponding to increasing delay i.e. increasing number of memory elements. However, the sequence of delay portions  403  may be arranged in any sequence depending upon the interleaving pattern or sequence required. The input commutator  401  is configured to input symbols from an input sequence in accordance with the sequence of the delay portions  404  as shown by the dashed line  405 . Likewise, the output commutator  402  is configured to read symbols from the delay portions in accordance with the sequence of the delay portions  403  as shown by the dashed lines  406  to form a deinterleaved symbol stream. 
     The input to the convolutional interleaver  206  may be a stream of bits, symbols, blocks of bits or symbols etc. The term symbol used throughout this description includes all these cases, or the term cell may be used because the data symbol may have been carried by a sub-carrier of an OFDM symbol, such as in the case of this example. Each symbol from a symbol stream is input by an input commutator  401 , the commutators advance one delay portion downwards after the input of a symbol, wrapping round back to the top after the bottom delay portion. Each time a symbol is read out of or input to a delay portion  403  symbols remaining in the delay portions  403  are shifted towards the output commutator  402  by one or more memory elements such that at least the memory element nearest to the input commutator becomes vacant i.e. no longer contains useful data. During normal operation the next symbol from the interleaved or deinterleaved stream of symbols is input into the vacant memory element by the input commutator  401 . In the case of the delay portion, which does not comprise any memory elements  404 , the symbol input by the input commutator  401  is immediately output or read out by the commutator  402  before the commutators advance one delay portion downwards. Consequently, the delay between the input of a symbol from the stream of symbols into a delay portion and the output or reading-out of the same symbol from the delay portion is dependent upon the number of memory elements  404  in the delay portion and the frequency that symbols are input or read out from the delay portion  403 . Although in the description above the commutators  401 ,  402  move in a downward direction, they may also move in an upwards direction after the input and reading-out of symbols from each delay portion  403 . In some circumstances a symbol may be output, without the use of the output commutator, from a delay portion in response to the input of a symbol to the delay portion by the input commutator  401  and is not read directly into the interleaved/deinterleaved symbol stream. 
     As can be seen in  FIG. 9 , as a result of the different delays introduced by the delay portions  403  of the interleaver  206 , the stream of symbols input into the interleaver  206  or deinterleaver  64  will be read-out in a different interleaved or deinterleaved order, respectively, which is dependent on the number of delay portions, the delay introduced by each delay portion, and the sequence of the delay portions. As can also be seen in  FIG. 9 , the deinterleaver  64  has a structure that corresponds to that of the interleaver  206  such that the interleaved symbols inputted into the deinterleaver are read-out in the deinterleaved order of the stream of symbols that were input into the interleaver at the transmitter. 
     As will be appreciated from the structure of the interleaver, each row provides a difference delay. As such, one symbol entering the last row will be transmitted without delay, while the next input symbol (to the 1 st  row) will be output with a long delay. Thus it may happen that an FEC Frame start is transmitted in a particular service (ATSC) frame, but following symbols, belonging to the same FEC Frame, are transmitted in previous service frames. The receiver may then see the start of a FEC Frame in one frame, but is still not be able to recover the complete FEC Frame, because one or more symbols of the FEC frame have already been transmitted in previous service frames. A problem will therefore occur, when a receiver switches initially to this service, although it does not occur, when the receiver is in “steady-state”. 
     Although in  FIG. 9  the interleaver  206  and the deinterleaver  302  are illustrated as comprising rows and columns of memory elements, convolutional interleaving may also be represented via other visualisation. For example, they may be represented via the use of matrices where the size of a matrix and its elements determine the delay and therefore the interleaving/deinterleaving of the input/output symbol stream. Furthermore, the elements of the interleaver and deinterleaver described above and throughout the application may be implemented ort a multipurpose or dedicated processor chip, where the functionality of the interleaver and deinterleaver is equivalent to that described but the implementation differs from the visualisations that are illustrated and described. For instance, the delaying of symbols and the input and output of symbols may be performed via memory address manipulation as opposed to the physical transfer of data representing symbols between memory elements and locations. 
     Further example arrangements of convolutional interleavering/de-interleaving are disclosed in our co-pending patent applications PCT/GB2014/050971 and GB 1305943.1 the contents of which are incorporated herein by reference. 
       FIG. 10  provides a representation of the convolutional interleaver shown in  FIG. 9 , which is presented to provide an explanation of the formulae used to explain the operation of the interleaver. As shown in  FIG. 10 , the interleaver provides an arrangement for determining the output symbol Y given an input symbol X n  as presented by the following equations:
 
 y   n =int( x   n )= x   m  with  m=n −[( N   r −1)−( n  mod  N   r )]* N   r  (output  y   n  is given by  x   m )
 
 x   n =int −1 ( y   m ) with  m=n +[( N   r −1)−( n  mod  N   r )]* N   r  (output  x   n  is interleaved to  y   m ) group of subsequent  N   r  cells  y   n  described by above rule next group: all indices shifted by  N   r  
 
     As shown in  FIG. 10  the convolutional interleaver  206  as it appears in  FIG. 9  is shown with reference to a number of rows N r  of delay elements  1001  and N c =N r −1 columns of delay elements  1002  in order to explain the terminology used in the following explanation. As shown the function of the convolutional interleaver is to perform a time interleaving (TI), of the input symbols X n  to form the output symbol stream Y n . As shown in  FIG. 10  in a first row of cell indices n  1004  with respect to n mod N r  indices  1006  the input signals X n  as they appear at the output of the interleaver Y n  are shown within a group of N r  cells  1008  which represents a cycle of interleaving performed by the interleaver  206 . 
     As will be appreciated with an input to the time interleaver being a sequence of cells g p , the time interleaved sequence will consists of N r  delay lines, with the k-th line having N r −k−1 memory units, k=0, 1, N r −1. Thus, the number of rows in  FIG. 10  is N r −1. The input and output commutators, cyclically switch downwards after one symbol is written in or read out, respectively. At each cycle, they will be located in the same position k. The total number of memory units is N r ·(N r −1)/2. If the input commutator is located at position k, an input symbol g q  will be written to this delay line. The delay elements from this line will shift their memory content to the right-most memory element, respectively, and the content from the right-most memory element will be output via the output commutator. Next, the input symbol g q  can be written to the left-most delay element of this line. Both commutators will then move cyclically to the next line (k+1) modulo N r . In case, the commutators have been located at the last line, k=N r −1, the input symbol is directly read out via the output commutator. 
     In some examples, the initial content of the N r ·(N r −1)/2 memory elements shall be a CAZAC sequence, to allow for constant transmit power on the one hand, and to minimize peak-to-average-power-ratio (PAPR) effects after IFFT. In other examples the memory elements may be filled with “dummy insertion” symbols, as disclosed in section 8.3.8 of the DVB-T2 specification, the contents of which are incorporated herein by reference. However, the interleaved version of this sequence may not be optimum in terms of PAPR. In another example the memory elements may be initially filled with 0 (or whatever value, which will never appear). Then, each 0 after interleaving may be replaced by the sequence from dummy cell insertion (or CAZAC or whatever), rather than the initial input to the interleaver. 
     As shown in  FIG. 11  the effect of the convolutional interleaver, for example, with the number of rows being N r =4, is to map groups of input symbols into the output symbols stream in groups of 4 where in each group a displacement of each input symbol with respect to the output is equal to N r +1. Therefore, as shown in  FIG. 11  each of the groups of symbols  1108  have 4 symbols corresponding to the number of rows N r  and each input symbol is displaced with respect to the corresponding position in the group for the next group as represented by the arrow  1102  by a number N r . Furthermore, each input symbol is displaced with respect to its neighbouring symbol within the group by N r +1. 
     A technical problem addressed by the present technique, which has been referred to above, will now be explained with reference to  FIGS. 12 to 17 . In  FIG. 12  the presentation is given of a stream of received symbols, which is received at corresponding cell positions represented by the numbers within each of the boxes  1201  in the sequence of symbols or cells  1204 . As for the example given in  FIG. 11  the output signal stream is divided into groups of symbols  1208  each of which contains N r =4 symbols. According to this representation the received symbol stream  1204  represents the sequence of symbols or cells which have been interleaved and are presented to the input of the deinterleaver  64  which could represent bits or symbols depending on whether the interleaving is performed at the symbol level or the bit level. As shown in  FIG. 12  along side the received symbol stream  1204  is a sequence of numbers representing the cell indices numbering the sequential order of the received cells  1210 . A dashed line  1212  indicates the start of a new service frame  1214 . As for the example shown above the service frame  1214  could be an ATSC or a DVB frame representing a sequence of transmitted data cells providing a new time frame for a service data or a physical layer pipe (PLP). Thus, the new service frame  1214  is represented by an arrow which starts at position  1212 . Furthermore, for the example shown in  FIG. 12  the PLP start row  1218 , which is the start of the new service frame  1214  corresponds to, a position of the row  1  of the convolutional deinterleaver or interleaver. In other words, with regard to the numbering of the rows of the convolutional deinterleaver  64  shown in  FIG. 10 , the parameter PLP_TI_START_ROW shown in  FIG. 12  identifies the row number of the convolutional deinterleaver from which the first cell or symbol of the new service frame is output. 
     As explained above within each service frame  1214  is transmitted a sequence of error correction encoded frames  1216 , which will be referred to as forward error correction frames (FEC frames). Each of the FEC frames belongs to a physical layer pipe which is to be decoded by an error correction decoder  68  as shown in  FIG. 8 . Therefore, as shown in  FIG. 12  a new service frame  1214  includes as part of that service frame many FEC frames  1216 , which are to be decoded to recover part of the data of a physical layer pipe. 
     For the example shown in  FIG. 12  the start row of the convolutional deinterleaver of the PLP is shown within box  1218  (PLP_TI_START_ROW) to be the first row that is row  1 . Also for the example shown in  FIG. 12 , the first cell of the new service frame  1214  in which the first FEC frame  1216  starts is cell  14  and shown in box  1220  (PLP_TI_FECFRAME_START). Accordingly, in the received symbol stream  1204 , that is before deinterleaving, the first cell number of the FEC frame is  27  and the subsequent cells in sequence are  28 ,  29  and  30  of the FEC frame  1216  as shown within brackets  1222 . For this example, in which the start row of the new service frame for the first cell output by the convolutional deinterleaver is row  1  and the FEC frame starts at cell  14  of the new service frame, which is cell  27  of the received (interleaved) symbol stream, then within the interleaved symbol stream  1204  all of the subsequent cells carrying data cells of the FEC frame will occur within the new service frame, which is after the new service frame indicator  1212 . However, this is not always the case, as will be explained with reference to  FIG. 13 . 
       FIG. 13  corresponds to the examples shown in  FIG. 12  except that there are different parameters for the interleaving and the start of the FEC frame within the new service frame  1314 . As shown in  FIG. 13  the new service frame  1314  starts at cell  23  in the interleaved received symbol stream  1304  after the start of new service frame indicator  1212 . As shown in the boxes  1318 ,  1320  the PLP start row for this example is row  3  (PLP_TI_START_ROW=3) and the FEC frame start cell  1320  is cell  2  of the new service frame (PLP_TI_FEC_FRAME_START=2) so that as shown within the brackets  1322  the new FEC frame is provided from cells  17 ,  18 ,  19 ,  20  and onwards because start of the FEC frame occurred in cell  2  of the new service frame which comprises cell  17  and onwards in the interleaved received symbol stream  1304 . However, there is a problem with the parameters according to this arrangement because cells  18  and  19  occur, in the received interleaved symbol stream before the start of the new service frame indicator  1312  in the previous service frame  1324 ,  1326 . Therefore, if a receiver has been switched on in the previous service frame then the symbols  18  and  19  can not been received because the receiver could not have started receiving data until the new service frame  1312  has been detected. Accordingly, the error correction encoder would be unlikely to recover the FEC frame. Therefore it would be better not to decode this FEC frame in order to save on energy such as battery power or to signal to an application layer that this FEC frame has not been received (treated as an erasure), which may trigger a recovery process in which a previous frame is repeated. 
     According to the present technique it is possible to calculate a safe distance value A with respect to which the first cell or symbol of the new FEC frame should be separated from the start of the new service frame  1314 , as a function of the start row of the convolutional deinterleaver (PLP_TI_START_ROW) and the number of rows in the deinterleaver (N r ) as shown in  FIG. 14 . As can be seen in  FIG. 14  because the start row of the convolutional deinterleaver is row  3  (PLP_TI_START_ROW=3), there are other symbols separated from the first cell recovered from the deinterleaver in previously transmitted cells such as shown in  FIG. 14  by a distance  1402 ,  1404 . Thus, as shown by boxes  1406 ,  1408  groups of cells or symbols in the received interleaved symbol stream  1304  contain symbols which makeup the first new FEC frame which was received within the new service frame  1314 . As such, in order to detect whether the previous frame includes symbols of the first new FEC frame of the new service frame  1314 , it is possible to determine as a function of the start row of the convolutional deinterleaver (PLP_TI_START_ROW) and the first cell within the new service frame of the first new FEC frame (PLP_TI_FECFRAME_START) a distance between the first cell of the new service frame and the cell within the service frame in which the first cell of the FEC frame is received, whether or not there are any data cells of the first new FEC frame in the interleaved received symbol stream, which have been transmitted in the previous service frame. Accordingly, a decoder can be de-activated so as to avoid trying to decode a first new FEC frame, thereby saving energy or signalling to a higher application layer that the FTC frame or the first new service frame cannot be recovered (erased frame). 
     As shown in  FIG. 15  according to the present technique a distance between the first cell of the FEC frame and the start of the new service frame is compared to a calculated safe distance value A, which is a function of the row of the interleaves in which the first cell of the new service frame is output. If the first cell of the FEC frame is greater than the distance A from the start of the new service frame, then the receiver can decode the FEC frame because none of the FEC frame cells were transmitted in the previous service frame. However, if there were cells transmitted in the previous service frame then the receiver, cannot decode the FEC frame. This may apply to the first one of the FEC frames or one or more subsequent frames in the new service frame, because these may comprise cells which were transmitted in the previous service frame. 
     Transmitter Embodiment 
     According to some embodiments of the present technique, therefore, layer  1  signalling may be used to convey, amongst other parameters, the cell index of an FEC frame start symbol after the first cell index of the new service frame, which is in other words the position after the first cell index of the new service frame of a first symbol belonging to a FEC Frame, preferably one of the first FEC frames in this service frame. According to one example embodiment a transmitter is configured to determine the safe distance value A and to adjust the cell index of the first FEC frame after the first cell index of the new service frame to indicate to a receiver the first FEC frame which can be recovered entirely from the new service frame. To this end, the controller  210  in the encoding processor  22  shown in  FIG. 3 , for example, is configured
         to calculate the safe distance value A;   to identify the first cell of a first of the FEC frames, after the cell index of the first cell of the new service frame, which is more than a displacement A from the first cell of the new service frame;   to form the layer  1  signalling to be transmitted to the receiver to include the identified first cell of the first of the FEC frames of the service frame which can be decoded from cells received from the new service frame according to the two above steps.       

     According to this arrangement the transmitter is configured to adjust the signalling start frame of the first FEC frame, so that receivers, when detecting the first FEC frame can be sure to receive it completely after deinterleaving, thus to decode and to recover data from the FEC frame. 
     Receiver Embodiments 
     As an alternative arrangement, the transmitter may not transmit an indication of the first cell index of the first FEC frame after the first cell index of the new service frame, although the transmitter may in any case transmit, as part of the layer  1  signalling, the start row of the convolutional deinterleaver. However, the transmitter may not identify, if this first cell index is larger than the “safety distance value”. For this example, embodiments of the present technique can provide an arrangement in which the receiver can perform a detection of whether the indicated first cell is within or outside the safe distance value. 
     According to the present technique, a receiver may therefore start with the first cell of a frame and inserts it to the convolutional deinterleaver in the start row, which has been identified from the layer  1  signalling. The receiver will then count until the cell index of the first cell or symbol of the FEC Frame. After this point the receiver can determine when symbols entered into the convolutional deinterleaver will leave the convolutional deinterleaver. This will be the beginning of a new FEC Frame, to be forwarded to the following parts of the receiver which are usually, a QAM demapper to demap the symbols to bits or soft bits (like LLRs—log likelihood ratios), followed by a bit deinterleaver, followed by a FEC decoder etc. However as mentioned above, the interleaved FEC Frame start-symbol may occur “too early” in the service frame, and as a result some of the cells of the FEC frame may have already been transmitted in the previous service frame. A receiver switching, to this service (initial acquisition) will try to reconstruct this FEC frame after convolutional deinterleaving, but will not be able to receive it completely (missed the symbols from the previous frame). Its error correction decoder will run unnecessary processing steps or may even end up in an invalid state. As mentioned above for the transmitter embodiment, this problem does not occur because the signalled first cell index for the first FEC frame already includes an offset so that only the first FEC frame of the new service frame can be decoded. 
     With regard to the receiver embodiment, in which the offset for the safe distance value A has not been taken into account at the transmitter side, so that the signalled first FEC cell of the new start frame may be within the safe distance value A. Accordingly, embodiments of the present technique can provide a receiver, which detects a first cell of a first of the FEC frames which is outside the safe distance value A. This is done by a controller  310  shown in  FIG. 16 , which can calculate the safe distance value A. As explained above the value A represents the amount of cells in the interleaved sequence before the occurrence of a certain symbol, which needs to be collected, in order to guarantee that after de-interleaving, all following symbols to this particular symbols, are included as well. Thus, the cell index of a potential first symbol of a FEC frame after convolutional interleaving (cell index inside the service frame) must be larger or equal than A to guarantee that this FEC frame can be received completely. 
     As shown in  FIG. 16  parts of the receiver which are arranged to receive and decode the FEC frames in a service frame corresponding to the diagram shown in  FIG. 8  includes the controller  310  which in accordance with the present technique serves to disable the FEC decoder  304  in accordance with received parameters in respect of the start of the FEC frame within the new service frame and the row of the interleaver in which the first cell of the FEC frame is output. 
     The operation of the controller  310 , which forms part of a receiver in accordance with the present technique is presented in  FIG. 17  which is summarised as follows:
     S 1 . As a first step S 1  the receiver is powered up or reset in some way or retunes or zapps to a new channel and therefore needs to recover a different PLP,   S 2 . In step S 2  the receiver first detects the start of the new service frame index.   S 4 . The receiver then detects the start of the row of the convolutional interleaver in which the first cell or symbol of the service frame will be output (PLP_TI_START_ROW). The receiver also detects the cell index of the service frame in which the first or cell or symbol of the first new FEC frame starts in the service frame (PLP_TI_FECFRAME_START). In some examples, the positon of the cell index of the service frame in which the new FEC frame starts is explicitly signalled to the received by the transmitter whereas in other examples the first cell of the FEC frame in the new service frame is detected in accordance with the pre-determined conditions, such as a specific number of cells or symbols from the start of the service frame. Signalling the PLP start row for the convolutional interleaver could be determined by ensuring that at the end of each service frame the interleaver finishes at a determined row of the interleaver. As explained below, in one example, the receiver decodes the L1-pre and post, configure the convolutional deinterleaver memory according to PLP_NUM_TI_ROWS. The controller then sets the commutators at row PLP_TI_START_ROW, before inserting the first data symbol of the current ATSC Frame.   S 6 . The receiver then calculates the distance A representing the safe distance from the start of the new service frame to the first symbol or cell in the service frame in which the first new FEC frame begins.   S 8 . The receiver then detects whether the FEC frame start cell or symbol in the new service frame, from the start of the new service frame is greater than or equal to the distance A. If is it not, then the FEC decoder  68  is reset by the controller  310  in that is does not attempt to decode the first new FEC frame of the service frame. If the FEC frame start cell is greater than the distance A with respect to the first new cell service frame then the decoder is allowed to proceed to decode the first new FEC frame. The controller  810  then writes the data cells into the convolutional deinterleaver. Note that in one example the convolutional deinterleaver resembles the convolutional interleaver structure from  FIG. 9 , except that the memory units constitute a lower-right triangle, compared with the upper-left triangle of the convolutional deinterleaver.   

     Effectively therefore the receiver is configured to forward the first complete FECFRAME to the next blocks at time t+N r *[(N r −1)−Rs], where t is the time index, when PLP_TI_FECFRAME_START will enter the Rs-th row of the TIMI. The delay until this symbol can be read out is thus N r *[(N r −1)−Rs] for the  FIG. 9  convolutional deinterleaver, although this expression changes to R s *N r  for the convolutional encoder of  FIG. 19 . 
     In contrast the controller  210 , which forms part of the encoder  22  in the transmitter may perform a process according to  FIG. 18  in the transmitter which is summarised as follows:
     S 20 . As a first step the transmitter detects the cell index in which the first cell of a FEC frame new service frame is to be transmitted.   S 22 . The start of the row or delay portion of the convolutional interleaver is detected, in which the first cell or symbol of the service frame was output (PLP_TI_START_ROW).   S 24  The cell index of the service frame in which the first cell or symbol of the first new FEC frame starts is detected in the service frame (PLP_TI_FECFRAME_START). In some examples, the position of the cell index of the service frame in which the new FEC frame starts is detected from the output of the convolutional interleaver or detecting a delay through the convolutional interleaver for cells being interleaved from the first new FEC frame.   S 26 . The receiver then calculates the safe distance value or displacement value A representing the safe distance from the start of the new service frame to the first symbol or cell in the service frame in which the first new FEC frame begins.   S 28 . The receiver then determines at decision point S 28 , whether the FEC frame start cell or symbol in the new service frame, from the start of the new service frame is greater than or equal to the distance A. If is it not, then the processing proceeds to step S 30 , but if it is then processing proceeds to process step S 30 .   S 30 . If a displacement between the first cell of the new FEC frame after the first cell of the new service frame, and the first cell of the service frame exceeds the safe distance/displacement value A, then at step S 30 , the layer  1  signalling is arranged to include an indication of the first cell of the FEC frame for detection by the receiver.   S 32 . If the displacement between the first cell of the new FEC frame after the first cell of the new service frame, and the first cell of the service frame is less than the safe distance/displacement value A, then the next FEC frame is detected and the cell index of the first cell of the next FEC frame in the new service frame is identified. Processing then proceeds back to step S 26  and the process steps S 26 , S 28  and S 32  repeated until the condition of the decision point S 28  satisfied to reach S 30 .   S 34 . If step S 30  has been executed then processing terminates in step S 34 .   

     Other Example Embodiments 
     As explained above, the present technique can be applied to any convolutional interleaver/de-interleaver to ensure that a synchronising receiver is able to detect and decode a FEC frame after tuning to a new channel. As will be appreciated therefore the order of the rows of the convolutional/de-interleaver can be inverted in the sense that the convolutional intereleaver presented above and illustrated in  FIG. 9  is arranged with the row with the maximum delay in the first row and the row with no delay in the last row. However it will be appreciated that the first row could provide no delay, whereas the final row could provide the maximum delay. A corresponding arrangement could be provided for the convolutional de-intereleaver. Such an alternative arrangement for a convolutional interleaver/de-interleaver is shown in  FIG. 19 . As shown in  FIG. 19 , effectively the structure of the interleaver and de-interleaver are reversed, with respect to the arrangement shown in  FIG. 9 . As such in the interleaver in  FIG. 19 , the first row has no delay, whereas the final row has the maximum number of delay elements  1504 . As for the example shown in  FIG. 9 , the successively increasing number of delay elements  1504  are arranged to connect an input terminal connecting a commutator  1501  which successively rotates as represented by the arrow  1506 . Output connectors  1503  connect the input terminals via the delay elements  1504  to output terminals, which are connected by an output commutator  1502 . 
     Correspondingly the de-interleaver  206  shown in  FIG. 19  is provided with an input commutator  1501 , which is arranged to connect input terminals via rows with successively decreasing numbers of delay elements  1504  to output terminals provided by connectors  1503 . At the output, an output commutator  1502  connects the output terminals successively as represented by arrow  1506 . 
     In accordance with the present technique, for an example embodiment in which a first row of the interleaver  63  has no delay elements  1504  and the final row has the maximum number of delay elements, then the calculation of the a controller in the transmitter is changed in order to provide a displacement value (A) representing a minimum displacement from the first cell of the service frame of a first cell of a first of the forward error correction frames, for which none of the data cells of the forward error correction frame will be transmitted in one or more previous service frames. According to the present technique, the calculation of the displacement value A for the convolutional interleaver/de-interleaver shown in  FIG. 19  changes to be: A=R s *(N r +1) whereas with the corresponding expression for the convolutional interleaver/de-interleaver shown in  FIG. 9 , N r  is the number of delay portions, and R s , as for the  FIG. 9  convolutional interleaver/de-interleaver, is calculated in accordance with R s =(PLP_TI_FECFRAME_START+PLP_TI_START_ROW) (mod N r ) where PLP_TI_FECFRAME_START is the number of service frame data cells of the new service frame after which the first data symbol of the first forward error correction frame is read and PLP_TI_START_ROW is the delay portion from which the first service frame data symbol of the new service frame is read. 
     The operation of a receiver in accordance with the present technique for an example of a convolutional interleaver/de-interleaver shown in  FIG. 19  is as shown in  FIG. 20 . Correspondingly the operation of a transmitter embodying the present technique for the example of the convolutional interleaver/de-interleaver of  FIG. 19  is shown in  FIG. 21 . As will be appreciated from the above explanation, the only difference between the operations presented in  FIGS. 20 and 21  for a respective receiver and a transmitter according to the present technique, which uses a convolutional interleaver/de-interleaver of  FIG. 19  from the operations of the respective receiver and transmitter shown in  FIGS. 17 and 18  for the convolutional interleaver/de-interleaver of  FIG. 9  concerns the calculation of the displacement value A, which changes to the expression presented above. Accordingly, the only difference between the flow diagram presented in  FIG. 20  to that of  FIG. 17  is in process step S 6 , which changes to the abovementioned expression, with A=R s *(N r +1). Similarly the only difference between the operation of the transmitter between the flow diagrams presented in  FIGS. 18 and 21  is process step S 26 , which has also changed to the same expression for the displacement value A. 
     Signalling Parameters for Time Interleaving for Single PLP for the Example Embodiment According to FIG.  19   
     The following paragraphs provide some example illustrations for parameters which may be used in example applications of the present technique. 
     L1—Detail Time Interleaver (TI) Parameters 
     The following signalling parameters may be used to signal the use and parameters for convolutional time interleaver for a given PLP:
         L1D_plp_TI_extended_interleaving: This flag shall indicate whether extended interleaving is to be used for this PLP. A value of 1 shall indicate that extended interleaving is used. A value of 0 shall indicate that extended interleaving is not used. A value of 1 shall only be selected when the modulation of the PLP is QPSK. A value of 1 shall not be selected when LDM is used.
 
Convolutional Time Interleaver Mode Parameters
       

     The following parameters shall indicate the configuration of the convolutional time interleaver in CTI mode.
         L1D_CTI_depth: This field shall indicate the number of rows used in the convolutional time interleaver, including the row without any delay. L1D_CTI_depth shall be signaled according to the Table below:       

     
       
         
           
               
             
               
                 TABLE 
               
             
            
               
                   
               
               
                 Signaling format for L1D_CTI_depth 
               
            
           
           
               
               
            
               
                 Value 
                 Number of Rows 
               
               
                   
               
            
           
           
               
               
            
               
                 000 
                 1 (no time interleaving in CTI mode) 
               
               
                 001 
                 Reserved for future use 
               
               
                 010 
                 Reserved for future use 
               
               
                 011 
                  512 
               
               
                 100 
                  724 
               
               
                 101 
                  887 or 1254 (extended interleaving) 
               
               
                 110 
                 1024, or 1448 (extended interleaving) 
               
               
                 111 
                 Reserved for future use 
               
               
                   
               
            
           
         
       
     
     L1D_CTI_start_row: This field shall indicate the position of the interleaver selector at the start of the subframe.
         L1D_CTI_fecframe_start: This field shall indicate the start position of the first complete FEC frame in the subframe. The start position is the index of the first cell in the FEC frame. In order to signal only FEC frames which will begin in the current subframe, the following condition shall be fulfilled L1D_CTI_fecframe_start=Rs·(Nrows+1), where Rs is the row index for the L1D_CTI_fecframe_start-th cell inside the subframe and is defined as Rs:=(L1D_CTI_fecframe_start+L1D_CTI_start_row) modulo Nrows. If the start position of a FEC frame does not fulfil the above condition, that is if cells belonging to the same FEC frame appear also in previously transmitted subframes (due to the delaying nature of the convolutional time interleaver) the next FTC frame start position shall be checked and used if it meets the conditions, and so on until a FEC frame which meet the conditions, is found. When LDM is used, this field shall be signaled for both core layer and enhanced layer PLPs since the start positions of the first complete FEC frames of core-layer and enhanced-layer PLPs that have been layered division multiplexed together are in general different.   L1D_CI_fecframe_start: This field shall indicate the start position of the first complete FEC frame, leaving the CTI in the current or subsequent subframes. The position counts the number of cells from current subframe start and is computed as follows:
           Detect the first FEC frame entering the CTI after the subframe start. Denote the cell position as k0 (e.g., k0=3 indicates a FEC frame enters the CTI after 3 cells, strafing counting from 0 from current subframe start).   Then L1D_CI_fecframe_start=k0+Rs*Nrows, where   Rs:=(k0+L1D_CI_start_row) modulo Nrows is the row index for this FEC frame.   The counter L1D_CI_fecframe_start may exceed subframe boundaries.   
               

     As an alternative embodiment, for a single PLP (S-PLP), several signalling parameters may be changed, compared with the case of multiple PLPs (M-PLP) case, due to two differences: a) different interleaver structure, b) no need to signal parameters, which are needed for M-PLP. These differences in signalling parameters will be explained in the following paragraphs in more detail: 
     a) Signalling Parameters for Convolutional Interleaver 
     The main parameter to configure the TI for S-PLP is the number of rows N rows , which has been called N r  previously. This will be signaled in the configurable L1-post part as follows: PLP_NUM_TI_ROWS—This 2-bit field shall indicate the number of rows used in the time de-interleaver including the row without any delay, PLP_NUM_TI_ROWS shall be signalled according to the table below:
 
Signalling Format for the PLP_NUM_TI_ROWS Field
 
                                             Value   number of rows N rows                                                      00   1024           01   887           10   724           11   512                        
For initial acquisition, the receiver needs to know the following two parameters, which are signaled in the dynamic L1-post part.
 
PLP_TI_START_ROW—This 11-bit field shall indicate the position of the (de-)interleaver commutator at the start of the ATSC Frame.
 
PLP_TI_FECFRAME_START—This 15-bit field shall indicate the start position of the first complete FECFRAME in the ATSC Frame. The start position is the cell index inside the ATSC Frame. In order to signal only FECFRAMEs, which will appear completely in the current or upcoming ATSC Frames, the following condition needs to be fulfilled: PLP_TI_FECFRAME_START≥R s ·(N rows +1) (for the  FIG. 19  convolutional interleaver/deinterleaver, or for the  FIG. 9  convolutional interleaver/de-interleaver then≥[(N r −1)−Rs]*(N r −1)), where
 
R s :=(PLP_TI_FECFRAME_START+PLP_TI_START_ROW) modulo N rows  is the row index for the PLP_TI_FECFRAME_START-th cell inside the ATSC frame.
 
If the start position of a FECFRAME does not fulfil the above condition, i.e., if cells belonging to the same FECFRAME appear also in previously transmitted ATSC Frames due to the delaying nature of the convolutional interleaver, the next FECFRAME start position shall be checked and so on.
 
The time de-interleaver (TDI) at the receiver will thus perform the following steps at initial acquisition:
         Decode L1-pre and post, configure the TDI memory according to PLP_NUM_TI_ROWS   Set the commutators at row PLP_TI_START_ROW, before inserting the first data symbol of the current ATSC Frame   Write the data cells to the TDI. Note that the TDI resembles the TI structure from  FIG. 19 , except that the memory units constitute an upper-left triangle, compared with the lower-left triangle of the TI.   Forward the first complete FECFRAME to the next blocks at time t+N rows ·R s , where t is the time index, when PLP_TI_FECFRAME_START will enter the R s -th row of the TDI. The delay until this symbol can be read out is thus N rows ·R s .       

     b) Limited Parameters for S-PLP Case 
     The following parameters from dynamic L1-post shall not be signalled for S-PLP to reduce the overhead, since they do not apply to the S-PLP case: 
     
         
         
           
             SUB_SLICES_PER_FRAME, SUB_SLICES_INTERVAL, TYPE_2_START—apply only to Type 2 PLPs of M-PLP case 
             PLP_START—Each ATSC frame will immediately start with data PLP. 
             PLP_NUM_BLOCKS—For a sheer convolutional interleaver, there is no need to have an integer number of FECFRAMEs per ATSC Frame. Instead, the complete frame capacity shall be used to transmit an arbitrary number of data cells of the S-PLP. This avoids the need to insert dummy cells at the end of each ATSC Frame and thus significantly reduces the overhead. 
           
         
       
    
     It will be appreciated that the above description for clarity has described embodiments with reference to different functional units, circuitry and/or processors. However, it will be apparent that any suitable distribution of functionality between different functional units, circuitry and/or processors may be used without detracting from the embodiments. 
     Described embodiments may be implemented in any suitable form including hardware, software, firmware or any combination of these. Described embodiments may optionally be implemented at least partly as computer software running on one or more data processors and/or digital signal processors. The elements and components of any embodiment may be physically, functionally and logically implemented in any suitable way. Indeed the functionality may be implemented in a single unit, in a plurality or units or as part of other functional units. As such, the disclosed embodiments may be implemented in a single unit or may be physically and functionally distributed between different units, circuitry and/or processors. 
     Although the present disclosure has been described in connection with some embodiments, it is not intended to be limited to the specific form set forth herein. Additionally, although a feature may appear to be described in connection with particular embodiments, one skilled in the art would recognize that various features of the described embodiments may be combined in any manner suitable to implement the technique. 
     The following numbered paragraphs provide further example aspects and features of the present disclosure: 
     Paragraph 1. A transmitter for transmitting data, the transmitter comprising 
     a forward error correction encoder configured to encode the data to form forward error correction encoded frames of encoded data cells, 
     a service frame builder configured to form a service frame for transmission comprising a plurality of the forward error correction encoded frames, 
     a convolutional interleaver comprising a plurality of delay portions and configured to convolutionally interleave the encoded data cells of the plurality of the forward error correction encoded frames of the service frames, 
     a modulation symbol mapper configured to map the interleaved and encoded data cells of the service frames onto modulation cells, and 
     a modulator configured to modulate one or more carriers with the modulation cells for transmission, and 
     a controller configured 
     to form signalling data to be transmitted with the service frames to include an indication of an identified first of the encoded data cells of a first of the plurality of forward error correction encoded frames of a service frame which can be decoded from encoded data cells received from the service frame or the service frame and one or more other service frames following after the service frame. 
     Paragraph 2. A transmitter according to paragraph 1, wherein the controller is configured 
     to calculate a displacement value (A) representing a minimum displacement from the first of the encoded data cells of the service frame to a first encoded data cell of a first of the forward error correction frames, for which none of the encoded data cells of the forward error correction frame will be transmitted in one or more previous service frames, and 
     to form the indication of the identified first encoded data cell from the first cell of the service frame providing a first of the forward error correction encoded frames in the service frame which has a encoded data cell index equal to or after the calculated displacement value. 
     Paragraph 3. A transmitter according to paragraph 2, wherein the controller is configured 
     to detect the service frame, 
     to detect from which of the plurality of delay portions of the convolutional interleaver circuitry a first encoded data cell of the service frame is read, 
     to calculate the displacement value (A) based upon the detected service frame and the delay portion from which the first encoded data cell of the service frame is read, 
     to identify the first encoded data cell of a first of the forward error correction frames, after the encoded data cell index of the first encoded data cell of the service frame, which is more than the displacement value (A) from the first encoded data cell of the service frame; and 
     to form the signalling data with the indication of the first encoded data cell of the first of the forward error correction encoded frames of the service frame which can be decoded from encoded data cells received from the service frame. 
     Paragraph 4. A transmitter according to paragraph 2 or 3, wherein each of the plurality of delay portions of the convolutional interleaver is arranged to delay the encoded data cells from an input to an output by a different amount, the delay portions being arranged in a sequence, the convolutional interleaver comprising 
     an input commutator configured to input the encoded data cells to the delay portions so that successive data cells are input in accordance with the sequence of the delay portions, and 
     an output commutator configured to read the encoded data cells from the delay portions by successively selecting the data cells from the outputs of the delay portions in accordance with the sequence of the delay portions to form the interleaved and forward error correction encoded data cells. 
     Paragraph 5. A transmitter according to paragraph 4, wherein the displacement value A is based on a multiplication of a factor R s  by the number of delay portions N r , where
 
 R   s =(PLP_TI_FECFRAME_START+PLP_TI_START_ROW) (mod  N   r )
 
and where PLP_TI_FECFRAME_START is the number of service frame data cells of the new service frame after which the first data symbol of the first forward error correction frame is read and PLP_TI_START_ROW is the delay portion from which the first service frame data symbol of the new service frame is read.
 
     Paragraph 6. A transmitter according to paragraph 5, wherein a first of the plurality of delay portions of the sequence of the convolutional interleaver is arranged to delay the encoded data cells from the input to the output by the greatest amount compared to the other delay portions in the sequence, and a last of the delay portions in the sequence is arranged to delay the encoded data cells by the least amount, and the displacement value A is calculated in accordance with A=((N r −1)−R s )*(N r −1). 
     Paragraph 7. A transmitter according to paragraph 5, wherein, a first of the plurality of delay portions of the sequence of the convolutional interleaver is arranged to delay the encoded data cells from the input to the output by the least amount compared to the other delay portions in the sequence, and a last of the delay portions in the sequence is arranged to delay the encoded data cells by the greatest amount, and the displacement value A is calculated in accordance with A R s *(N r +1). 
     Paragraph 8. A transmitter according to paragraph 5, 6 or 7, wherein controller is configured to form the layer  1  signalling data to include PLP_TI_FECFRAME_START and PLP_TI_START_ROW. 
     Paragraph 9. A transmitter according to any of paragraphs 1 to 8, wherein each of the delay portions includes one or more memory elements and the controller is configured to set the initial content of the memory elements of the interleaver before inputting the encoded data cells with predetermined values. 
     Paragraph 10. A transmitter according to paragraph 9, wherein the predetermined values include symbols derived from a constant amplitude zero auto-correlation, CAZAC sequence, 
     Paragraph 11. A transmitter according to any of paragraphs 1 to 10, wherein the number of memory elements is N r ·(N r −1)/2, where N r  is the number of rows of the convolutional interleaver. 
     Paragraph 12. A method of transmitting data, the method comprising encoding the data to form forward error correction encoded frames of encoded data cells, 
     forming a service frame for transmission comprising a plurality of the forward error correction encoded frames, 
     convolutionally interleaving the encoded data cells of the plurality of the forward error correction encoded frames of the service frames using a convolutional interleaver comprising a plurality of delay portions, 
     mapping the interleaved and encoded data cells of the service frames onto modulation cells, 
     modulating one or more carriers with the modulation cells for transmission, and 
     forming signalling data to be transmitted with the service frames to a receiver to include an indication of an identified first of the encoded data cells of a first of the plurality of forward error correction encoded frames of a service frame which can be decoded from encoded data cells received from the service frame or the service frame and one or more other service frames following after the service frame. 
     Paragraph 13. A method according to paragraph 12, wherein the forming the signalling data comprises 
     calculating a displacement value (A) representing a minimum displacement from a first of the encoded data cells of the service frame to a first encoded data cell of a first of the forward error correction frames, for which none of the encoded data cells of the forward error correction frame will be transmitted in one or more previous service frames and forming the indication of the identified first encoded data cell from the first cell of the service frame providing a first of the error correction encoded frames in the service frame which has a encoded data cell index equal to or after the calculated displacement value. 
     Paragraph 14. A method according to paragraph 13, wherein the calculating the displacement value (A) comprises 
     detecting the service frame, 
     detecting from which of the plurality of delay portions of the convolutional interleaver circuitry a first encoded data cell of the service frame is read, 
     calculating the displacement value (A) based upon the detected service frame and the delay portion from which the first encoded data cell of the service frame is read, 
     identifying the first encoded data cell of a first of the forward error correction frames, after the encoded data cell index of the first encoded data cell of the service frame, which is more than the displacement value (A) from the first encoded data cell of the service frame, and 
     forming the signalling data with the indication of the first encoded data cell of the first of the forward error correction encoded frames of the service frame which can be decoded from encoded data cells received from the service frame. 
     Paragraph 15. A method according to paragraph 12, 13 or 14, wherein each of the plurality of delay portions of the convolutional interleaver is arranged to delay the encoded data cells from an input to an output by a different amount, the delay portions being arranged in a sequence, the convolutional interleaving comprising 
     inputting the encoded data cells to the delay portions so that successive data cells are input in accordance with the sequence of the delay portions, and 
     reading the encoded data cells from the delay portions by successively selecting the data cells from the outputs of the delay portions in accordance with the sequence of the delay portions to form the interleaved and forward error correction encoded data cells. 
     Paragraph 16. A method according to paragraph 15, wherein the displacement value A is based on a multiplication of a factor R s  by the number of delay portions N r , where
 
 R   s =(PLP_TI_FECFRAME_START+PLP_TI_START_ROW) (mod  N   r )
 
and where PLP_TI_FECFRAME_START is the number of service frame data cells of the new service frame after which the first data symbol of the first forward error correction frame is read and PLP_TI_START_ROW is the delay portion from which the first service frame data symbol of the new service frame is read.
 
     Paragraph 17. A method according to paragraph 16, wherein a first of the plurality of delay portions of the sequence is arranged to delay the encoded data cells from the input to the output by the greatest amount compared to the other delay portions in the sequence, and a last of the delay portions in the sequence is arranged to delay the encoded data cells by the least amount, and the displacement value A is calculated in accordance with A=((N r −1)−R s )*( N   r −1). 
     Paragraph 18. A method according to paragraph 16, wherein a first of the plurality of delay portions of the sequence is arranged to delay the encoded data cells from the input to the output by the least amount compared to the other delay portions in the sequence, and a last of the delay portions in the sequence is arranged to delay the encoded data cells by the greatest amount, and the displacement value A is calculated in accordance with A=R s *(N r +1). 
     Paragraph 19. A method according to paragraph 16, 17 or 18, wherein the forming the layer  1  signalling data includes forming the layer  1  signalling data to include PLP_TI_FECFRAME_START and PLP_TI_START_ROW. 
     Paragraph 20. A method according to any of paragraphs 12 to 19, wherein each of the delay portions includes one or more memory elements and the method comprises setting the initial content of the memory elements of the interleaver, before inputting the encoded data cells, with predetermined values. 
     Paragraph 21. A method according to paragraph 20, wherein the predetermined values include symbols derived from a constant amplitude zero auto-correlation, CAZAC, sequence. 
     Paragraph 22. A method according to any of paragraphs 12 to 21, wherein the number of memory elements is N r ·(N r −1)/2, where N r  is the number of rows of the convolutional interleaver. 
     Paragraph 23. A receiver for recovering data symbols from a received signal, the receiver comprising 
     a demodulator configured to detect the received signal comprising a convolutionally interleaved symbol stream comprising a sequence of service frames, each service frame including one or more forward error correction encoded frames of encoded data cells, 
     convolutional deinterleaver circuitry comprising a plurality of delay portions and configured to convolutionally deinterleave the received encoded data cells, 
     a forward error correction decoder configured to decode the encoded frames of encoded data cells, and 
     controller circuitry configured 
     to detect a service frame, 
     to detect from which of the plurality of delay portions of the convolutional deinterleaver circuitry a first encoded data cell of a service frame is read, 
     to detect the number of the service frame encoded data cells of the service frame after which, a first forward error correction frame is read, and 
     to detect, based upon the detected service frame, the delay portion from which a first encoded data cell of the service frame is read, and the number of the service frame encoded data cells of the service frame after which a first encoded data cell of one of more of the forward error correction frames is read, whether any encoded data cells of the one or more forward error correction frames were transmitted in a previous service frame, and 
     if any of the encoded data cells of the one or more of the forward error correction frames were transmitted in a previous service frame not received by the receiver, not to decode the forward error correction frame, or 
     if no encoded data cells of one or more of the forward error correction frame were transmitted in the previous service frame, or if any encoded data cells of the new forward error correction frame were transmitted in a received previous service frame, 
     to decode the forward error correction frames to recover the stream if data symbols. 
     Paragraph 24. A receiver according to paragraph 23, wherein the receiver is configured to switch to detect the received signal during the previous service frame. 
     Paragraph 25. A receiver according to paragraph 23 or 24, wherein the number of encoded data cells of the service frame after which a first encoded data cell of the first forward error correction frames of the service frame can be read in order for a first of the forward error correction frames to be decoded is greater than or equal to a displacement value A determined in accordance with the start row of the convolutional deinterleaver and the number of rows in the deinterleaver. 
     Paragraph 26. A receiver according to paragraphs 23, 24 or 25, wherein each of the plurality of delay portions of the convolutional deinterleaver is arranged to delay symbols from the symbol stream from an input to an output by a different amount, the delay portions being arranged in a sequence, the convolutional deinterleaver comprising 
     an input commutator configured to input the encoded data cells to the delay portions so that successive encoded data cells are input in accordance with the sequence of the delay portions, and an output commutator configured to read the symbols from the delay portions by successively selecting the encoded data cells from the outputs of the delay portions in accordance with the sequence of the delay portions to form a deinterleaved stream of encoded data cells from the received interleaved stream of encoded data cells. 
     Paragraph 27. A receiver as claimed in paragraph 26, wherein the displacement value A is based on a multiplication of a factor R s  by the number of delay portions N r , where
 
 R   s =(PLP_TI_FECFRAME_START+PLP_TI_START_ROW) (mod  N   r )
 
and where PLP_TI_FECFRAME_START is the number of service frame data cells of the new service frame after which the first data symbol of the first forward error correction frame is read and PLP_TI_START_ROW is the delay portion from which the first service frame data symbol of the new service frame is read.
 
     Paragraph 28. A receiver according to paragraph 27, wherein a first of the plurality of delay portions of the sequence of the convolutional deinterleaver is arranged to delay the encoded data cells from the input to the output by the least amount compared to the other delay portions in the sequence, and a last of the delay portions in the sequence is arranged to delay the encoded data cells by the greatest amount, and the value A is calculated in accordance with A=((N r −1)−R s )*(N r −1). 
     Paragraph 29. A receiver according to paragraph 27, a first of the plurality of delay portions of the sequence of the convolutional deinterleaver is arranged to delay the encoded data cells from the input to the output by the greatest amount compared to the other delay portions in the sequence, and a last of the delay portions in the sequence is arranged to delay the encoded data cells by the least amount, and the displacement value A is calculated in accordance with A=R s *(N r +1). 
     Paragraph 30. A receiver according to paragraph 28 or 29, wherein PLP_TI_FECFRAME_START and PLP_TI_START_ROW are received by the receiver as signalled data. 
     Paragraph 31. A receiver according to paragraph 28 or 29, wherein PLP_TI_FECFRAME_START and PLP_TI_START_ROW are received by the receiver as predetermined values. 
     Paragraph 32. A receiver according to any of paragraphs 23 to 31, wherein each of the delay portions includes one or more memory elements and the controller is configured to set the initial content of the memory elements of the deinterleaver before inputting the symbols of the received signal with predetermined values. 
     Paragraph 33. A receiver according to paragraph 32, wherein the predetermined values include symbols derived from a constant amplitude zero auto-correlation, CAZAC, sequence. 
     Paragraph 34. A receiver according paragraph 32 or 33, wherein the number of memory elements is N r ·(N r −1)/2, where N r  is the number of rows of the convolutional deinterleaver. 
     Paragraph 35. A method for recovering data symbols from a received signal, the method comprising 
     detecting the received signal, the received signal comprising an interleaved stream of encoded data cells comprising a sequence of service frames, each service frame including one or more forward error correction encoded frames of encoded data cells, 
     convolutional deinterleaving the stream of encoded data cells using a plurality of delay portions, 
     decoding the encoded frames of encoded data cells, wherein the decoding the encoded frames comprises 
     detecting a service frame, 
     detecting from which of the delay portions a first service frame data symbol of the new service frame is read for the convolutional deinterleaving, 
     detecting the number of the encoded data cells of the service frame after which a first forward error correction frame is read, 
     detecting, based upon the detected service frame, the delay portion from which a first encoded data cell of the service frame is read, and the number of the encoded data cells of the service frame after which a first encoded data cell of one or more of the forward error correction frames is read, whether any encoded data cells of one or more of the forward error correction frames were transmitted in a previous service frame, and 
     if any of the encoded data cells of one or more of the forward error correction frames were transmitted in a previous service frame, not decoding the forward error correction frame, or 
     if no encoded data cells of the one or more of the forward error correction frames were transmitted in a previous service frame, decoding the forward error correction frame to recover the stream if data symbols. 
     Paragraph 36. A method according to paragraph 35, comprising 
     switching to detect the received signal during the previous service frame. 
     Paragraph 37. A method according to paragraph 35 or 36, wherein the number of encoded data cells of the service frame after which a first encoded data cell of the first forward error correction frames of the service frame can be read in order for a first of the forward error correction frames to be decoded is greater than or equal to a displacement value A determined in accordance with the start row of the convolutional deinterleaver and the number of rows in the deinterleaver. 
     Paragraph 38. A method according to paragraph 37, wherein the convolutional deinterleaving comprises 
     inputting the encoded data cells to the delay portions so that successive encoded data cells are input in accordance with the sequence of the delay portions, each of the delay portions being arranged to delay encoded data cells from an input to an output by a different amount, the delay portions being arranged in a sequence, and 
     reading the symbols from the delay portions by successively selecting the encoded data cells from the outputs of the delay portions in accordance with the sequence of the delay portions to form a deinterleaved stream of encoded data cells. 
     Paragraph 39. A method according to paragraph 38, wherein the displacement value A is based on a multiplication of a factor R s  by a number of delay portions N r , where
 
 R   s =(PLP_TI_FECFRAME_START+PLP_TI_START_ROW) (mod  N   r )
 
and where PLP_TI_FECFRAME_START is the number of service frame data cells of the new service frame after which the first data symbol of the first forward error correction frame is read and PLP_TI_START_ROW is the delay portion from which the first service frame data symbol of the new service frame is read.
 
     Paragraph 40. A method according to paragraph 39, wherein a first of the plurality of delay portions of the sequence of the convolutional deinterleaver is arranged to delay the encoded data cells from the input to the output by the least amount compared to the other delay portions in the sequence, and a last of the delay portions in the sequence is arranged to delay the encoded data cells by the greatest amount, and the value A is calculated in accordance with A=((N r −1))−R s )*(N r −1). 
     Paragraph 41. A method according to paragraph 35, a first of the plurality of delay portions of the sequence of the convolutional deinterleaver is arranged to delay the encoded data cells from the input to the output by the greatest amount compared to the other delay portions in the sequence, and a last of the delay portions in the sequence is arranged to delay the encoded data cells by the least amount, and the displacement value A is calculated in accordance with=R s *(N r +1). 
     Paragraph 42. A method according to paragraph 39, 40 or 41, the method comprising 
     receiving PLP_TI_FECFRAME_START and PLP_TI_START_ROW by the receiver as signalled data. 
     Paragraph 43. A method according to paragraph 39, 40 or 41, the method comprising receiving PLP_TI_FECFRAME_START and PLP_TI_START_ROW by the receiver as predetermined values. 
     Paragraph 44. A method according to any of paragraphs 35 to 43, wherein each of the delay portions includes one or more memory elements and the method comprises setting the initial content of the memory elements of the deinterleaver before inputting the symbols of the received signal with predetermined values. 
     Paragraph 45. A method according to paragraph 44, wherein the predetermined values include symbols derived from a constant amplitude zero auto-correlation, CAZAC, sequence, 
     Paragraph 46. A method according to any of paragraphs 35 to 45, wherein the number of memory elements is N r ·(N r −1)/2, where N r  is the number of rows of the convolutional deinterleaver. 
     Paragraph 47. A non-transitory computer-readable medium storing a computer program having computer executable instructions, which when loaded on to a computer causes the computer to perform the method according to any of paragraphs 12 to 22 and 35 to 47. 
     Various further aspects and features of the present disclosure are defined in the appended claims. Various combinations of features may be made of the features and method steps defined in the dependent claims other than the specific combinations set out in the attached claim dependency. Thus the claim dependencies should not be taken as limiting.