CA2904775C - Transmitters, receivers and methods of transmitting and receiving with scattered and continuous pilots in an ofdm system - Google Patents

Abstract

A receiver recovers data from Orthogonal Frequency Division Multiplexed (OFDM) symbols, the OFDM symbols including a plurality of sub-carrier signals. Some of the sub-carrier signals carrying data symbols and some of the sub-carrier signals carrying pilot symbols, the pilot symbols comprising scattered pilots symbols and continuous pilot symbols. The continuous pilot symbols are distributed across the sub-carrier signals in accordance with a continuous pilot symbol pattern and the scattered pilot symbols are distributed across the sub-carrier signals in accordance with a scattered pilot signal pattern. The receiver comprises a demodulator configured to detect a signal representing the OFDM symbols and to generate a sampled digital version of the OFDM symbols in the time domain. A Fourier transform processor is configured to receive the time domain digital version of the OFDM symbols and to form a frequency domain version of the OFDM symbols, from which the pilot symbol bearing sub-carriers and the data symbol bearing sub-carriers can be recovered. A detector is configured to recover the data symbols from the data bearing sub-carriers of the OFDM symbols and to recover the pilot symbols from the pilot bearing sub-carriers of the OFDM symbols in accordance with the scattered pilot symbol pattern and the continuous pilot symbol pattern. The scattered pilot symbol pattern is one of a plurality of scattered pilot symbol patterns and the continuous pilot pattern is independent of the scattered pilot symbol pattern. The detector comprises a memory configured to store a master continuous pilot pattern and a processor configured to detect the number of sub-carrier signals in the plurality of sub-carrier signals and to derive the continuous pilot pattern from a master pilot pattern based on the number of sub-carrier symbols.

Description

TRANSMITTERS, RECEIVERS AND METHODS OF TRANSMITTING AND
RECEIVING WITH SCATTERED AND CONTINUOUS PILOTS IN AN OFDM
SYSTEM
Field of the Disclosure The present disclosure relates to transmitters, receivers and methods of transmitting and receiving in an OFDM communications system.
Background of the Disclosure There are many examples of radio communications systems in which data is communicated using Orthogonal Frequency Division Multiplexing (OFDM). Systems which have been arranged to operate in accordance with Digital Video Broadcasting (DVB) standards for example, utilise OFDM.
OFDM can be generally described as providing K narrow band sub-carriers (where K is an integer) which are modulated in parallel, each sub-carrier communicating a modulated data symbol such as Quadrature Amplitude Modulated (QAM) symbol or Quadrature 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 a 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.
To facilitate detection and recovery of the data at the receiver, the OFDM
symbol can include pilot sub-carriers, which communicate data-symbols known to the receiver. The pilot sub-carriers provide a phase and timing reference, which can be used to estimate an impulse response of the channel through which the OFDM symbol has passed and perform tasks such as channel estimation and correction, frequency offset estimation etc. These estimations facilitate detection and recovery of the data symbols at the receiver. In some examples, the OFDM symbols include both Continuous Pilot (CP) carriers which remain at the same relative frequency position in the OFDM symbol and Scattered Pilots (SP). The SPs change their relative position in the OFDM
symbol between successive symbols, providing a facility for estimating the impulse response of the channel more accurately with reduced redundancy. However, the location of the pilots is required to be known at the receiver so the receiver can extract the pilot symbols from the correct locations across the OFDM sub-carriers.
The development of communications system which utilise OFDM symbols to communicate data can represent a significant and complex task. In particular, the optimisation of communications parameters particular in respect of frequency planning and network deployment can present a significant technical problem requiring considerable effort to identify the communications parameters which are suitable for a communications system which utilises OFDM. As will be appreciated much work has been performed to optimise the parameters of DVB standards and in particular DVB T2.
Summary of the Disclosure A receiver recovers data from Orthogonal Frequency Division Multiplexed (OFDM) symbols, the OFDM symbols including a plurality of sub-carrier signals. Some of the sub-carrier signals carrying data symbols and some of the sub-carrier signals carrying pilot symbols, the pilot symbols comprising scattered pilots symbols and continuous pilot symbols. The continuous pilot symbols are distributed across the sub-carrier signals in accordance with a continuous pilot symbol pattern and the scattered pilot symbols are distributed across the sub-carrier signals in accordance with a scattered pilot signal pattern. The receiver comprises a demodulator configured to detect a signal representing the OFDM symbols and to generate a sampled digital version of the OFDM symbols in the time domain. A Fourier transform processor is configured to receive the time domain digital version of the OFDM symbols and to form a frequency domain version of the OFDM symbols, from which the pilot symbol bearing sub-carriers and the data symbol bearing sub-carriers can be recovered. A detector is configured to recover the data symbols from the data bearing sub-carrier signals of the OFDM
symbols and to recover the pilot symbols from the pilot bearing sub-carrier signals of the OFDM
symbols in accordance with the scattered pilot symbol pattern and the continuous pilot symbol pattern. The scattered pilot symbol pattern is one of a plurality of scattered pilot symbol patterns and the continuous pilot pattern is independent of the scattered pilot symbol pattern. The detector comprises a memory configured to store a master continuous pilot pattern and a processor configured to detect the number of sub-carrier signals in the plurality of sub-carrier signals and to derive the continuous pilot pattern from a master pilot pattern based on the number of sub-carrier signals.
The provision of continuous pilot patterns that are independent of scattered pilot patterns means that fewer continuous pilot patterns have to be stored in memory when there is a plurality of scattered pilot patterns. Furthermore, the ability to derive continuous pilot patterns from a master pilot pattern dependent on the number of sub-carriers may allow fewer continuous plot patterns to be stored in memory when the number of sub-carriers varies from symbol to symbol.
In some examples the number of sub-carrier signals in the plurality of sub-carrier signals is one of a set of sub-carrier signal numbers and the master pilot symbol pattern is the pilot symbol pattern for the continuous pilot symbols for OFDM symbols which include the highest number of sub-carrier signals from the set of sub-carrier signal numbers.
The provision of a master pilot pattern which is for the highest order sub-carrier mode means that the pilot sub-carrier patterns for modes with fewer subcarriers can be derived without storing separate pilot patterns. This therefore may allow a single pilot pattern to be stored that covers all possible sub-carrier numbers, thus saving memory anywhere a continuous pilot pattern is required to be stored for each mode.
In some examples the set of sub-carrier numbers includes approximately 8k, 16k, and 32k sub-caniers, the master pilot pattern being provided for the 32k sub-carriers, and the continuous pilot pattern for the 8k and 16k sub-carriers being derived from the 32k sub-carrier continuous pilot pattern.
Various further aspects and features of the present technique are defined in the appended claims and include a transmitter for transmitting OFDM symbols, a method for transmitting OFDM
symbols and a method for receiving OFDM symbols.
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Brief Description of the Drawings Embodiments of the present invention will be now be described by way of example only with reference to the accompanying drawings where like parts are provided with corresponding reference numerals:
Figure 1 provides a schematic diagram of an example OFDM transmitter;
Figure 2 provides an example OFDM super frame;
Figure 3 provides a schematic diagram of an example OFDM receiver, Figure 4 provides a diagram of part of an example OFDM frame;
Figure 5 provides a graph illustrating the distribution of continuous pilot locations in a DVB-T2 system that do not coincide with scattered pilot positions.
Figure 6 provides a table of continuous pilot symbol sub-carrier locations for an 8k mode in accordance with an example of the present disclosure;
Figure 7 provides an illustration of continuous pilot symbol sub-carrier locations for an 8k mode in accordance with an example of the present disclosure;

2 Figure 8 provides a histogram of the spacing of continuous pilot symbol sub-carrier locations for an 8k mode in accordance with an example of the present disclosure;
Figure 9 provides a histogram of dither applied to the continuous pilot symbol sub-carrier locations in accordance with an example of the present disclosure;
Figure 10 provides a table of continuous pilot symbol sub-carrier locations for a 16k mode in accordance with an example of the present disclosure;
Figure 11 provides an illustration of continuous pilot symbol sub-carrier locations for a 16k mode in accordance with an example of the present disclosure;
Figure 12 provides a histogram of the spacing of continuous pilot symbol sub-carrier locations for a 16k mode in accordance with an example of the present disclosure;
Figure 13 provides a table of continuous pilot symbol sub-carrier locations for a 32k mode in accordance with an example of the present disclosure;
Figure 14 provides an illustration of continuous pilot symbol sub-carrier locations for a 32k mode in accordance with an example of the present disclosure;
Figure 15 provides a histogram of the spacing of continuous pilot symbol sub-carrier locations for a 32k mode in accordance with an example of the present disclosure;
Figure 16 provides a flow diagram of the operation of a transmitter in accordance with an example of the present disclosure; and Figure 17 provides a flow diagram of the operation of a receiver in accordance with an example of the present disclosure.
Description of Example Embodiments Figure 1 provides an example block diagram of an OFDM transmitter which may be used for example to transmit video images and audio signals in accordance with the proposed ATSC 3 standard or DVB-T, DVB-T2 or DVB-C2 standards. In Figure I a program source generates data to be transmitted by the OFDM transmitter. A video coder 2, and audio coder 4 and a data coder 6 generate video, audio and other data to be transmitted which are fed to a program multiplexer 10.
The output of the program multiplexer 10 forms a multiplexed stream with other information required to communicate the video, audio and other data. The multiplexer 10 provides a stream on a connecting channel 12. There may be many such multiplexed streams which are fed into different branches A, B etc. For simplicity, only branch A will be described.
As shown in Figure 1, an OFDM transmitter 20 receives the stream at a multiplexer adaptation and energy dispersal block 22. The multiplexer adaptation and energy dispersal block 22 randomises the data and feeds the appropriate data to a forward error correction encoder 24 which performs error correction encoding of the stream. A bit interleaver 26 is provided to interleave the encoded data bits which for the example in a DVB-T2 system is the LDCP/BCH
encoder output. The output from the bit interleaver 26 is fed to a bit into constellation mapper 28, which maps groups of bits onto a constellation point of a modulation scheme, which is to be used for conveying the encoded data bits. The outputs from the bit into constellation mapper 28 are constellation point labels that represent real and imaginary components. The constellation point labels represent data symbols formed from two or more bits depending on the modulation scheme used. These can be referred to as data cells. These data cells are passed through a time-interleaver 30 whose effect is to interleave data cells resulting from multiple LDPC code words.
The data cells are received by a frame builder 32, with data cells produced by branch B etc. in Figure 1, via other channels 31. The frame builder 32 then forms many data cells into sequences to be conveyed on OFDM symbols, where an OFDM symbol comprises a number of data cells, each data cell being mapped onto one of a plurality of sub-carriers. The number of sub-carriers will depend on

3 the mode of operation of the system, which may include one or more of 8k, 16k or 32k, each of which provides a different number of sub-carriers and therefore fact Fourier transform (F.t,T) sizes.
The sequence of data cells to be carried in each OFDM symbol is then passed to the symbol interleaver 33. The OFDM symbol is then generated by an OFDM symbol builder block 37 which introduces pilot and synchronising signals generated by and fed from a pilot and embedded signal former 36 according to pilot symbol pattern(s). An OFDM modulator 38 then forms the OFDM
symbol in the time domain which is fed to a guard insertion processor 40 for generating a guard interval between symbols, and then to a digital to analogue convertor 42 and finally to an RF
amplifier within an RF front end 44 for eventual broadcast by the COFDM
transmitter from an antenna 46.
Frame Format For the system of Figure 1, the number of sub-carriers per OFDM symbol can vary depending upon the number of pilot and other reserved carriers. An example illustration of a "super frame" is shown in Figure 2.
For example, in DVB-12, unlike in DVB-T, the number of sub-carriers for carrying data is not fixed. Broadcasters can select one of the operating modes from lk, 2k, 4k, 8k, 16k, 32k each providing a range of sub-carriers for data per OFDM symbol, the maximum available for each of these modes being 1024, 2048, 4096, 8192, 16384, 32768 respectively. In DVB-T2 a physical layer frame is composed of many OFDM symbols. Typically the frame starts with a preamble or P1 symbol as shown in Figure 2, which provides signalling information relating to the configuration of the DVB-T2 deployment, including an indication of the mode. The P1 symbol is followed by one or more P2 OFDM symbols 64, which are then followed by a number of payload carrying OFDM
symbols 66.
The end of the physical layer frame is marked by a frame closing symbols (FCS) 68. For each operating mode, the number of sub-carriers may be different for each type of symbol. Furthermore, the number of sub-carriers may vary for each according to whether bandwidth extension is selected, whether tone reservation is enabled and according to which pilot sub-carrier pattern has been selected.
Receiver Figure 3 provides an example illustration of an OFDM receiver which may be used to receive signals transmitted from the transmitter illustrated in Figure 1. As shown in Figure 3, an OFDM
signal is received by an antenna 100 and detected by a tuner 102 and converted into digital form by an analogue-to-digital converter 104. A guard interval removal processor 106 removes the guard interval from a received OFDM symbol, before the payload data and pilot data is recovered from the OFDM
symbol using a Fast Fourier Transform (FFT) processor 108 in combination with a channel estimator and corrector 110, an embedded-signalling decoding unit 111 and pilot symbol pattern(s). The demodulated data is recovered from a de-mapper 112 and fed to a symbol de-interleaver 114, which operates to effect a reverse mapping of the received data symbol to re-generate an output data stream with the data de-interleaved. Similarly, the bit de-interleaver 116 reverses the bit interleaving performed by the bit interleaver 26. The remaining parts of the OFDM receiver shown in Figure 3 are provided to effect error correction decoding 118 to correct errors and recover an estimate of the source data.
Embodiments of the present technique provide a communication system which utilises OFDM to transmit data and reuses much of the system design and configuration parameters which have been adopted for the DVB-T2 standard. However the communication system is adapted to transmit OFDM symbols within channels of 6 MHz rather than the 8 MHz which is used for the DVB
T2 standard and utilise 8k, 16k and 32k modes. Accordingly, the present disclosure presents an

4 adaptation of the parameters for an OFDM system for 6 MHz but rationalising where possible the parameters that were developed for the DVB T2 standard to simplify architecture and implementation of a communications system.
Pilot Symbols In addition to signalling data and a payload data, OFDM frames and the cells they include may also comprise pilot symbols which have been inserted at the transmitter.
These pilot symbols may for instance have been generated by the pilot and embedded signal former 36 and inserted by the symbol builder 37. Pilot symbols are transmitted with a known amplitude and phase and the sub-carriers upon which they are transmitted may be termed pilot sub-carriers.
Pilot symbols may be required for a range of different purposes at the receiver, for example, channel estimation, synchronisation, coarse frequency offset estimation and fine frequency offset estimation. Due to the a priori knowledge of the pilot symbols' amplitude and phase, the channel impulse response may be estimated based on the received pilot symbols, with the estimated channel then being used for =
purposes such as equalisation.
In order for the receiver to receive the pilot symbols and differentiate the pilots signals from other signalling symbols and data symbols, the pilot symbols may be distributed across the subcarriers and symbols of an OFDM frame according to a sub-carrier pilot symbol pattern.
Consequently, if the receiver has knowledge of pilot symbol pattern and is synchronised with the OFDM frame, it will be able to extract the received pilot symbols from the appropriate locations or sub-carriers in the OFDM
symbols and frame.
The distribution of pilots with respect to OFDM sub-carriers may fall into two categories:
continuous pilots and scattered pilots. Continuous pilots are formed from pilot symbols whose location relative to the sub-carriers does not change from symbol to symbol with the result that they are transmitted on a same sub-carrier each time. Scattered pilots broadly describe pilot symbols whose location changes from symbol to symbol, possibly according to some repeating pattern.
Figure 4 illustrates a series of OFDM symbols where the circles represent OFDM
cells and shaded circles represent pilot symbols. In Figure 4 the horizontal direction represents frequency or the sub-carrier number, and the vertical direction represents time or the symbol number. Continuous pilot symbols 120 are located on the same subcarrier (CP) each time whereas scattered pilots 122 are located on different sub-carriers from symbol to symbol. The repetition of the scattered pilots can be represented by variables Dx and Dy. Dx represents a separation between scattered pilots in the frequency domain from one OFDM symbol to another, so that the scattered pilot symbols on a first OFDM symbol is displaced by a number of sub-carriers equal to Dx in the frequency domain on a subcanier in the next OFDM symbol. Dy represents a parameter indicating a number of OFDM
symbols before the same subcanier is used again to carry a pilot symbol on the next occasion. For instance, in Figure 4 the location of the scattered pilots symbols may be represented by Dy = 8, and Dx = 10. Scattered pilots are an efficient way of providing pilot symbols because channel estimates for sub-carriers and symbols in between scattered pilot symbols can be estimated by interpolation in both time and frequency from the known pilot symbols or channel estimates.
Consequently, pilot symbols may not be required to be present on all sub carriers in order to obtain channel estimates for each sub-carrier and cell within an OFDM frame.
Pilot symbols occupy sub-carriers and cells which may otherwise be carrying data, therefore pilot symbols adversely affect the capacity of a system and it may be advantageous to minimise the number of pilot symbols. Consequently, a well-designed pilot pattern that enables channel estimates etc. to be obtained across the entire OFDM frame whilst using a small number of pilot symbols is desirable.

The scattered pilot pattern chosen for an OFDM signal may be dependent upon a number of factors, such as the rate of channel variation with respect to time and frequency. For instance, the density of the pilots must fulfil the sampling theorem in both time and frequency if accurate channel estimates are to be obtained i.e. the maximum channel impulses response length determines the pilot symbol repetition in the frequency direction, and the maximum Doppler frequency of the channels determines the pilot symbol repetition in the time domain. In some example OFDM systems the guard interval is determined by the length of the channel impulse response and therefore the pilot symbol repetition in the frequency direction may also be dependent upon the guard interval duration.
It may be beneficial if the location of continual pilot symbols and scattered pilot symbols do not overlap or coincide so that there is an approximately constant number of pilot symbols per frame and there are no significant "blind spots". In OFDM frames where there is large number of neighbouring cells which do not include a pilot symbol, this area may be referred to as a blind spot. It is generally desirable to avoid such situations because they may lead to reduced accuracy channel estimation and interpolation as well as a possible inability to detect and compensate for coloured noise such as analogue TV or other narrow band interference. Figure 5 provides a graph of continuous pilot locations which do not coincide with scattered pilot positions in a DVB-T2 system and illustrates the aforementioned problems, where blind spots 124 are shown as regions where there is a lack of pilot symbols. Also shown in Figure 5 are the edges of the frequency band 126 where measurements taken via pilot symbols on these regions may be subject to increased noise and attenuation and should therefore be avoided if possible.
A measure of the extent which continual pilot symbols and scattered pilots symbol coincide may be referred to as a utilisation ratio, and can be calculated using the formula below Number of CPnSP
Utilisation Ratio = _________________________ x 100%
Total Number of CP
where CPnSP represents the number of continual pilot symbols which do not coincide with scattered pilots sub-carriers during an OFDM frame. Consequently, due to the reasons given above, it may be beneficial to try and maxim' ise the utilisation ratio. There are also a number of other factors which may have to be taken into account when determining the scattered pilot and continual pilot patterns, for instance, it may not be useful to have pilot symbols close to the outer sub-carriers of an OFDM
signal because it is likely that these sub-carriers may be within the transition band of tuner filters and be subject to extra noise as mentioned above. It may also be beneficial to randomise the location of pilot symbols to some extent in order to ensure that interference is adequately modelled and reliable channel estimates obtained. Furthermore, due to the dependence of the scattered pilot patterns on factors such as the guard interval duration and Doppler spread, an OFDM system may have a plurality of scattered pilot patterns available to use, each specified by the repetition rates DX and Dy.
Due to the possible variation of scattered pilot patterns, in order to maximise the utilisation ratio, minimise blind spots and avoid pilots symbols being located close to the outer sub-carriers, different continuous pilot patterns may be required for one or more of the scattered pilots patterns. For instance, in DVB-T2 in some modes there are eight scattered pilot patterns and eight corresponding continuous pilot patterns. In some OFDM systems there may be more than one pattern per mode and different patterns across different modes so that in total there may be a sign i Remit number of pilot patterns.
The pilot signal embedder 36 which embeds the pilot symbols at the transmitter and the pilot signal extractor 111 which extracts the pilot symbols at the receiver require knowledge of the pilot patterns. Consequently, it is Rely that all the pilot patterns which may be used in a system will have to be stored in ROM at both the transmitter and the receiver, thus requiring a significant amount of memory if there are multiple modes and multiple pilot patterns per mode. This memory requirement is particularly relevant to the receiver in a broadcast system because there is likely to be a large number of receivers compared to transmitters and the cost of the receivers is likely to be lower than that of the transmitters. Consequently, reducing memory requirements will likely be beneficial, especially in the receiver side of a system.
In addition to memory requirements, utilising a large number of different scattered and continuous pilot patterns in a system also makes the system more complex because the transmitter has to select which pilot pattern is most appropriate for the current channel conditions and signal properties, and the receiver needs to identify the pilot pattern which is being used. The receiver may do this via the signalling information which specifies the pilot pattern(s) and mode of operation, or the receiver may detect the mode and pilot patterns via charaetelistics of the signal. However, both of these approaches become more complex and have larger overheads when more pilot patterns are available in a system. Therefore, it would be desirable to reduce the number of pilot patterns which are used in a system whilst maximising the utilisation ratio, avoiding blinds spots and minimising the number of pilots near to the outer sub-carriers.
In accordance with an example of the present technique, an OFDM system with a 6MHz bandwidth and 8k, 16k, and 32k modes has a single continuous pilot sub-carrier pattern for each mode, which is suitable for use with a plurality of different scattered pilot symbol patterns within each mode. In one example, there is a continuous pilot pattern which is suitable for use with one or more of the scattered pilot patterns given in Table 2 the below Scattered Dx DY
Pilot Pattern P4,2 4 2 P4,4 4 4 P8,2 8 2 P16,2 16 2 P32,2 32 2 Table 2: Scattered Pilot Patterns In an 8k mode (normal or extended) of an OFDM system that utilises the scattered pilots sequence given in Table 2 above, the distribution of the continuous pilots may be given by the table in Figure 6. The same locations as given in Figure 6 are also given by 41, 173, 357, 505, 645, 805, 941, 1098,1225, 1397, 1514, 1669, 1822, 1961, 2119,/245, 2423, 2587, 2709,2861, 3026, 3189,3318, 3510, 3683, 3861, 4045, 4163, 4297, 4457, 4598, 4769, 4942, 5113, 5289, 5413, 5585, 5755, 5873, 6045, 6207, 6379, 6525, 6675, 6862 in terms of sub-carrier locations in the extended bandwidth mode. For operation in normal 8k mode the pilot pattern may be derived by discarding the final sub-carrier location. The location of the continuous pilot symbols relative to the sub-carriers given in Figure 6 do not coincide with the location of the scattered pilots given in Table 2 above and therefore the continuous pilot pattern obtains a utilisation ratio of 100%. Figure 7 graphically illustrates the location of the continuous pilots of Figure 6 for the extended 8k mode and shows that there is a substantially uniform distribution of continuous pilots across the subcarriers of the extended 8k mode without any substantial blind spots. Figure 8 provides a histogram of the continuous pilot symbol spacing with respect to sub-carriers. The histogram once again shows that there is a substantially consistent distribution of continuous pilot symbols across the sub-carriers, thus reinforcing the absence of blind spots. Although the distribution of pilot symbols across the sub-carriers is substantially uniform, their location has been randomised to some extent by the introduction of dither.
Figure 9 illustrdtes the dither which has been applied to the placement of the continuous pilot symbols in Figure 6.
In a 16k mode (normal or extended) of an OFDM system that utilises the scattered pilots sequence given in Table 2 above, the distribution of the continuous pilots may be given by the table in Figure 10. The same locations as given in Figure 10 are also given by 82, 243, 346, 517, 714, 861, 1010, 1157, 1290, 1429, 1610, 1753, 1881, 2061, 2197, 2301, 2450, 2647, 2794,2899, 3027, 3159, 3338, 3497, 3645, 3793,3923, 4059,4239, 4409, 4490, 4647, 4847, 5013, 5175, 5277, 5419, 5577, 5723, 5895, 6051, 6222, 6378, 6497, 6637, 6818, 7021, 7201, 7366, 7525, 7721, 7895, 8090, 8199, 8325, 8449, 8593, 8743, 8915, 9055, 9197, 9367, 9539, 9723, 9885, 10058, 10226, 10391, 10578, 10703, 10825, 10959, 11169, 11326, 11510, 11629, 11747, 11941, 12089, 12243, 12414, 12598, 12758, 12881, 13050, 13195, 13349, 13517, 13725, 13821in terms of sub-carrier locations in the extended bandwidth mode. For operation in normal 16k mode the pilot pattern may be derived by discarding the final two sub-carrier locations. The location of the continuous pilot symbols relative to the sub-carriers given in Figure 10 do not coincide with the location. of the scattered pilots given in Table 2 above and therefore the continuous pilot pattern obtains a utilisation ratio of 100%. Figure 11 graphically illustrates the location of the continuous pilots of Figure 10 for the extended 16k mode and shows that there is a substantially -uniform distribution of continuous pilots across the subcarriers of the extended 16k mode without any substantial blind spots. Figure 12 provides a histogram of the continuous pilot symbol spacing with respect to sub-carriers. The histogram once again shows that there is a substantially consistent distribution of continuous pilot symbols across the sub-carriers, thus reinforcing the absence of blind spots. As for the 8k mode, although the distribution of pilot symbols across the sub-carriers is substantially uniform, their location has been randomised to some extent by the introduction of dither. The same dither as applied to the 8k continuous pilot symbol placement has also been applied to the 16k continuous pilot symbol placement and therefore Figure 9 illustrates the dither which has been applied the placement of the continuous pilot symbols in Figure 10.
In a 32k mode of an OFDM system that utilises the scattered pilots sequence given in Table 2 above, the distribution of the continuous pilots may for example be given by the table in Figure 13.
The same locations as given in Figure 13 are also given by 163, 290, 486, 605, 691, 858, 1033, 1187, 1427, 1582, 1721, 1881, 2019, 2217, 2314, 2425, 2579, 2709, 2857, 3009, 3219, 3399, 3506, 3621, 3762, 3997, 4122, 4257, 4393, 4539, 4601, 4786, 4899, 5095, 5293, 5378,5587, 5693, 5797, 5937, 6054, 6139, 6317, 6501, 6675, 6807, 6994, 7163, 7289, 7467, 7586, 7689, 7845, 8011, 8117, 8337, 8477, 8665, 8817, 8893, 8979, 9177, 9293, 9539, 9693, 9885, 10026, 10151, 10349, 10471, 10553, 10646, 10837, 10977, 11153, 11325, 11445, 11605, 11789, 11939, 12102, 12253, 12443, 12557, 12755, 12866, 12993, 13150, 13273, 13445, 13635, 13846, 14041, 14225, 14402, 14571, 14731, 14917, 15050, 15209, 15442, 15622, 15790, 15953, 16179, 16239, 16397, 16533, 16650, 16750, 16897, 17045, 17186, 17351, 17485, 17637, 17829, 17939, 18109, 18246, 18393, 18566, 18733, 18901, 19077, 19253, 19445, 19589, 19769, 19989, 20115, 20275, 20451, 20675, 20781, 20989, 21155, 21279, 21405, 21537, 21650, 21789, 21917, 22133, 22338, 22489, 22651, 22823, 23019, 23205, 23258, 23361, 23493, 23685, 23881, 24007, 24178, 24317, 24486, 24689, 24827, 25061, 25195, 25331, 25515, 25649, 25761, 25894, 26099, 26246, 26390, 26569, 26698, 26910, 27033, 27241, 27449, 27511, 27642, 27801 in terms of sub-carrier locations in the extended bandwidth mode. For operation in normal 32k mode the pilot pattern may be derived by discarding the final four sub-carrier locations. The location of the continuous pilot symbols relative to the sub-carriers given in Figure 14 do not coincide with the location of the scattered pilots given in Table 2 above and therefore the continuous pilot pattern obtains a utilisation ratio of 100%.
Figure 14 graphically illustrates the location of the continuous pilots of Figure 13for the extended 32k mode and shows that there is a substantially uniform distribution of continuous pilots across the subcarriers of the extended 8k mode without any substantial blind spots. Figure 15 provides a histogram of the continuous pilot symbol spacing with respect to sub-carriers. The histogram once again shows that there is a substantially consistent distribution of continuous pilot symbols across the sub-carriers, thus reinforcing the absence of blind spots. As for the 8k and 16k modes, although the distribution of pilot symbols across the sub-carriers is substantially uniform, their location has been randomised to some extent by the introduction of dither. The same dither as applied to the 8k and 16k continuous pilot symbol placement has also been applied to the 32k continuous pilot symbol placement and therefore Figure 9 illustrates the dither which has been applied the placement of the continuous pilot symbols in Figure 13.
As previously mentioned, the proposed continuous pilot patterns described above may also achieve substantially a 100% utilisation ratio, however, they also achieve a capacity loss which is approximately 0.65% in a system such as a proposed ATSC 3 system as previously described.
The continuous pilot patterns specified above may provide advantages over existing continuous pilot patterns because only a single continuous pilot pattern is required to operate with all five of the scattered pilots patterns specified in Table 2. Furthermore, these pilot patterns also reduce the number of blind spots in comparison to continuous pilot patterns such as those specified in DVB-T2. Since only one continuous pilot pattern is required to be stored at both the transmitter and the receiver compared to five if conventional continuous pilot patterns were used, memory requirements have been reduced by approximately 80%. However, memory for multiple continuous pilot patterns may still be required when there is more than one mode of operation e.g. 8k, 16k, 32k, and both normal and extended modes are available. Consequently, in a system such as a proposed ATSC 3 system where there are three modes, it is likely that three continuous pilot patterns are still required to be stored.
In accordance with another example of the present technique, the continuous pilots patterns illustrated in Figures 6, 10, and 13 are related such that the continuous pilot patterns of the 8k and the 16k modes are derivable from the 32k mode continuous pilot symbol pattern.
This therefore allows a transmitter and a receiver to store only a single master continuous pilot pattern for the highest order mode then derive the continuous pilot patterns for lower order modes when they are required.
For instance, at the transmitter the pilot and embedded signal former 36 may comprise a processor which is operable to detect or receive data which conveys the operating mode of the OFDM
system and then derive the appropriate continuous pilot pattern from a master pilot pattern based on the number of sub-carriers, where the master pilot pattern is stored in a memory at the pilot and embedded signal former 36. In the case of the continuous pilot patterns discussed above, the master continuous pilot pattern would be the 32k pilot pattern and the 16k continuous pilot pattern and the 8k continuous pilot pattern would be derived from the 32k pilot pattern by the processor according to the following equations below where the master pilot pattern is given by the following sub-carrier locations for the extended bandwidth mode 163, 290, 486, 605, 691, 858, 1033, 1187, 1427, 1582, 1721, 1881, 2019, 2217, 2314, 2425, 2579, 2709, 2857, 3009, 3219, 3399, 3506, 3621, 3762, 3997, 4122, 4257, 4393, 4539, 4601, 4786, 4899, 5095, 5293, 5378,5587, 5693, 5797, 5937, 6054, 6139, 6317, 6501, 6675, 6807, 6994, 7163, 7289, 7467, 7586, 7689, 7845, 8011, 8117, 8337, 8477, 8665, 8817, 8893, 8979, 9177, 9293, 9539, 9693, 9885, 10026, 10151, 10349, 10471, 10553, 10646, 10837, 10977, 11153, 11325, 11445, 11605, 11789, 11939, 12102, 12253, 12443, 12557, 12755, 12866, 12993, 13150, 13273, 13445, 13635, 13846, 14041, 14225, 14402, 14571, 14731, 14917, 15050, 15209, 15442, 15622, 15790, 15953, 16179, 16239, 16397, 16533, 16650, 16750, 16897, 17045, 17186, 17351, 17485, 17637, 17829, 17939, 18109, 18246, 18393, 18566, 18733, 18901, 19077, 19253, 19445, 19589, 19769, 19989, 20115, 20275, 20451, 20675, 20781, 20989, 21155, 21279, 21405, 21537, 21650, 21789, 21917, 22133, 22338, 22489, 22651, 22823, 23019, 23205, 23258, 23361, 23493, 23685, 23881, 24007, 24178, 24317, 24486, 24689, 24827, 25061, 25195, 25331, 25515, 25649, 25761, 25894, 26099, 26246, 26390, 26569, 26698, 26910, 27033, 27241, 27449, 27511, 27642, 27801.
In order to derive the 16k continuous pilot locations from the 32k pilot positions given in Figure 13 and above, every other 32k continuous pilot position is taken, the position divided by two and the result rounded up. In terms of a computer implementable equation, this is given by CP16K_pos = round(CP_32K__pos(1:2:last 32k cp_pos)/2).
In order to derive the 8k continuous pilot locations from the 32k pilot positions given in Figure 13 every four of the 32k continuous pilot positions is taken, the taken position divided by four and the result rounded up. In terms of a computer implementable equation, this is given by CP_8K_pos = round(CP32K_pos(1:4:last_32k cp_pos)/4).
Using the equations above it is possible that the 8k, 16k and 32k continuous pilot patterns may be derived from a single master set and therefore an OFDM system is effectively able to operate with a single continuous pilot pattern across all modes and all scattered pilot patterns. This may therefore simplify the operation of an OFDM system in terms of memory requirements but also the processing required because it is no longer necessary to switch between independent continuous pilot patterns which are unrelated.
Although in the preceding paragraphs the derivation of the continuous pilot patterns takes place at the transmitter, a similar process may also be performed at the receiver. For instance, the embedded signal decoding unit 111 may also comprises a processor which is substantially similar the processor described with reference to the pilot and embedded signal former 36.
The processor would be operable to detect or receive data which conveys the operating mode of the OFDM system i.e.
number of sub-carriers per OFDM symbol, and then derive the appropriate continuous pilot pattern from a master pilot pattern as previously described.
Due to the computational simplicity of the derivation processes described above, a decrease in ROM memory requirements i.e. the memory required to store the 8k and 16k continuous pilot patterns, may be achieved with only a small increase computational complexity.
In some examples in accorrinnce with the present technique, the derivation in the transmitter and the receiver may be performed by existing computational elements within the pilot related elements and therefore no additional components would be required in these cases.
In other examples in accordance with the present technique, the continuous pilot patterns for 8k, 16k and 32k modes may be used in an OFDM system, such as an ATSC 3.0 system for example, in order to exploit the intrinsic advantages of the continuous pilot symbol patterns. For instance, the advantages relating to the regular distribution of the pilot locations and the reduction in pilot locations near the outer sub-carriers can be achieved by one of the continuous pilot sub-carrier patterns with the following indices:
41, 173, 357, 505, 645, 805, 941, 1098, 1225, 1397, 1514, 1669, 1822, 1961, 2119, 2245, 2423, 2587, 2709, 2861, 3026, 3189, 3318, 3510, 3683, 3861, 4045, 4163, 4297, 4457, 4598, 4769, 4942, 5113, 5289, 5413, 5585, 5755, 5873, 6045, 6207, 6379, 6525, 6675,(6862) for the 8k mode;
82, 243, 346, 517, 714, 861, 1010, 1157, 1290, 1429, 1610, 1753, 1881, 2061, 2197, 2301, 2450, 2647, 2794, 2899, 3027, 3159, 3338, 3497, 3645, 3793, 3923, 4059, 4239, 4409, 4490, 4647, 4847, 5013, 5175, 5277, 5419, 5577, 5723, 5895, 6051, 6222, 6378, 6497, 6637, 6818, 7021, 7201, 7366, 7525, 7721, 7895, 8090, 8199, 8325, 8449, 8593, 8743, 8915, 9055, 9197, 9367, 9539, 9723, 9885, 10058, 10226, 10391, 10578, 10703, 10825, 10959, 11169, 11326, 11510, 11629, 11747, 11941, 12089, 12243, 12414, 12598, 12758, 12881, 13050, 13195, 13349, 13517, (13725, 13821) for the 16k mode; and 163, 290, 486, 605, 691, 858, 1033, 1187, 1427, 1582, 1721, 1881, 2019, 2217, 2314, 2425, 2579, 2709, 2857, 3009, 3219, 3399, 3506, 3621, 3762, 3997, 4122, 4257, 4393, 4539, 4601, 4786, 4899, 5095, 5293, 5378,5587, 5693, 5797, 5937, 6054, 6139, 6317, 6501, 6675, 6807, 6994, 7163, 7289, 7467, 7586, 7689, 7845, 8011, 8117, 8337, 8477, 8665, 8817, 8893, 8979, 9177, 9293, 9539, 9693, 9885, 10026, 10151, 10349, 10471, 10553, 10646, 10837, 10977, 11153, 11325, 11445, 11605, 11789, 11939, 12102, 12253, 12443, 12557, 12755, 12866, 12993, 13150, 13273, 13445, 13635, 13846, 14041, 14225, 14402, 14571, 14731, 14917, 15050, 15209, 15442, 15622, 15790, 15953, 16179, 16239, 16397, 16533, 16650, 16750, 16897, 17045, 17186, 17351, 17485, 17637, 17829, 17939, 18109, 18246, 18393, 18566, 18733, 18901, 19077, 19253, 19445, 19589, 19769, 19989, 20115,20275, 20451, 20675, 20781, 20989, 21155, 21279, 21405, 21537, 21650, 21789, 21917, 22133, 22338, 22489, 22651, 22823, 23019, 23205, 23258, 23361, 23493, 23685, 23881, 24007, 24178, 24317, 24486, 24689, 24827,25061, 25195, 25331, 25515, 25649, 25761, 25894, 26099, 26246, 26390, 26569, 26698, 26910, 27033, 27241, (27449, 27511, 27642, 27801) for the 32k mode, where the values in brackets relate to the extended bandwidth modes.
Summary of Operation An example flow diagram illustrating the operation of a transmitter according to the present technique is shown in Figure 16, an operation of a receiver to detect and recover data from a received OFDM symbol is provided in Figure 17. The process steps illustrated in Figure 15 are summarised as follows:
Si: As a first step to transmitting data using OFDM symbols a data formatter receives the data for transmission and forms the data into sets of data symbols for each of the OFDM symbols for transmission. Thus the data symbols are formed into the sets each have a number of data symbols corresponding to an amount of data which can be carried by an OFDM symbol.
S2: An OFDM symbol builder then receives each of the sets of data symbols from the data formatter and combines the data symbols with pilot symbols according to predetermined scattered and continuous pilot patterns. In accordance with the present technique, the pilot patterns are given by Table 2 for scatted pilots and Figures 6, 10, and 13 for continuous pilots, where the sub-carrier locations in Figure 6 and Figure 10may be derived from the locations given in Figure 13. The predetermined pattern sets out the subcarriers _of the_ OFDM symbol which are to carry the pilot symbols. The remaining subcarriers of the OFDM symbol carry the data symbols.
The OFDM
symbols therefore each include a plurality of subcarrier symbols, some of the subcarrier symbols carrying data symbols and some of the subcarrier symbols carrying pilot symbols.
S4: A modulator maps the data symbols and the pilot symbols onto modulation symbols in accordance with the value of the data symbols and the pilot symbols. With the modulation symbols each of the subcarriers is then modulated to form the OFDM symbols in the frequency domain.
S6: An inverse Fourier transformer then converts the OFDM symbols in the frequency domain into the time domain within a bandwidth of the communication system which is 6 MHz or approximately 6 MHz.
S8: A guard interval inserter adds a guard interval to each of the time domain OFDM symbols by copying a part of the OFDM symbols which is a useful part containing data symbols or pilot symbols and appending the copied part sequentially in the time domain to the OFDM symbols. The part which is copied has a length which corresponds to a guard interval which is a predetermined guard interval duration.
S10: A radio frequency transmission unit then modulates a radio frequency carrier with the time domain OFDM symbols and transmits the OFDM symbols via an antenna of the transmitter.
The operation of a receiver to detect and recover data from the OFDM symbols transmitted by the method of transmission is presented in Figure 17 which are summarised as follows:
S12: A demodulator receives a signal from an antenna and a radio frequency down converter and detects a signal representing the OFDM symbols. The demodulator generates a sampled digital version of the OFDM symbols in the time domain. A bandwidth of the OFDM symbols in the frequency domain in accordance with the present technique is substantially 6 MHz, that is approximately 6MHz.
S14: A guard interval correlator correlates the set of samples corresponding to the guard interval of the OFDM symbols to detect a timing of a useful part of the OFDM
symbols. A section of the received signal samples corresponding to the guard interval are copied and stored and then correlated with respect to the -same received signal samples in order to detect a correlation peak identifying where the repeated guard intervals are present in the useful part of the OFDM
symbols.
S16: A Fourier transform processor then transforms a section of the time domain samples of the received signal for a useful part of the OFDM symbols identified by the timing detected by the guard interval correlator into the frequency domain using a Fourier transform. From the OFDM symbols in the frequency domain the pilot symbols can be recovered from the pilot symbol bearing subcarriers and data symbols can be recovered from data bearing subcarriers. In accordance with the present technique, the pilot sub-carrier locations are given by Table 2 for scatted pilots and Figures 6, 10, and 13 for continuous pilots, where the sub-carrier locations in Figure 6 and Figure 10 may be derived from the locations given in Figure 13.
S18: A channel estimation and correction unit estimates an impulse response of a channel through which the OFDM symbols have passed from the recovered pilot symbols and corrects the received data symbols bearing subcarriers using the estimated channel impulse response.
Typically this is in accordance with the equalisation technique where the received signal in the frequency domain is divided by a frequency domain representation of the channel impulse response.
S20: A de-mapper recovers the data symbols from the data bearing subcarriers of the OFDM
symbols by performing a reverse mapping to that which was performed at the transmitter.
As will be appreciated the transmitter and receiver shown in Figures 1 and 3 respectively are provided as illustrations only and are not intended to be limiting. For example, it will be appreciated that the present technique can be applied to a different transmitter and receiver architecture.
As mentioned above, embodiments of the present invention find application with an ATSC
standard such as ATSC 3Ø For example embodiments of the present invention may be used in a transmitter or receiver operating in accordance with hand-held mobile terminals. Services that may be provided may include voice, messaging, internct browsing, radio, still and/or moving video images, television services, interactive services, video or near-video on demand and option. The services might operate in combination with one another.

Claims (18)

Claims

1. A receiver for recovering data from Orthogonal Frequency Division Multiplexed (OFDM) symbols, the OFDM symbols including a plurality of sub-carrier signals, some of the subcarrier signals carrying data symbols and some of the sub-carrier signals carrying pilot symbols, the pilot symbols comprising scattered pilots symbols and continuous pilot symbols, the continuous pilot symbols being distributed across the sub-carrier signals in accordance with a continuous pilot symbol pattern and the scattered pilot symbols being distributed across the sub-carrier signals in accordance with a scattered pilot symbol pattern, the receiver comprising a demodulator configured to detect a signal representing the OFDM symbols, and to generate a sampled digital version of the OFDM symbols in the time domain, a Fourier transform processor configured to receive the time domain digital version of the OFDM symbols and to form a frequency domain version of the OFDM symbols, from which the pilot symbol bearing sub-carriers and the data symbol bearing sub-carriers can be recovered, and a detector configured to recover the data symbols from the data bearing sub-carrier signals of the OFDM symbols and to recover the pilot symbols from the pilot bearing sub-carrier signals of the OFDM symbols in accordance with the scattered pilot symbol pattern and the continuous pilot symbol pattern, wherein:
the scattered pilot symbol pattern is one of 4. plurality of scattered pilot symbol patterns, and the detector comprises a memory configured to store a master pilot pattern and a processor configured to detect the number of sub-carrier signals in the plurality of sub-carrier signals and to derive the continuous pilot symbol pattern from the master pilot pattern based on the number of sub-carrier signals.

2. The receiver as claimed in claim 1, wherein the number of sub-carrier signals in the plurality of sub-carrier signals is one of a set of sub-carrier signal numbers and the master pilot pattern is the pilot symbol pattern for the continuous pilot symbols for OFDM symbols which include the highest number of sub-carrier signals from the set of sub-carrier signal numbers.

3. The receiver as claimed in claim 2, wherein the set of sub-carrier signal numbers includes approximately 8k, 16k, and 32k sub-carrier signals, the master pilot pattern being provided for the 32k sub-carrier signals, and the continuous pilot symbol pattern for the 8k and 16k sub-carrier signals being derived from the continuous pilot symbol pattern for the 32k sub-carrier.

4. The receiver as claimed in claim 3, wherein the continuous pilot symbol pattern for 8k sub-carriers in terms of sub-carrier signal locations is given by 41, 173, 357, 505, 645, 805, 941, 1098, 1225, 1397, 1514, 1669, 1822, 1961. 2119. 2245, 2423, 2587, 2709, 2861, 3026, 3189, 3318, 3510, 3683, 3861, 4045, 4163, 4297. 4457, 4598. 4769, 4942, 5113, 5289, 5413, 5585, 5755, 5873, 6045, 6207, 6379. 6525, 6675, 6862.

5. The receiver as claimed in claim 3, wherein the continuous pilot symbol pattern for 16k sub-carrier signals in terms of sub-carrier signal locations is given by 82, 243, 346, 517, 714, 861, 1010, 1157, 1290, 1429, 1610, 1753, 1881, 2061, 2197, 2301, 2450, 2647, 2794, 2899, 3027. 3159, 3338, 3497, 3645, 3793, 3923, 4059, 4239, 4409, 4490, 4647, 4847, 5013, 5175, 5277, 5419, 5577, 5723, 5895, 6051, 6222, 6378, 6497, 6637, 6818, 7021, 7201, 7366, 7525. 7721, 7895, 8090, 8199, 8325, 8449, 8593, 8743, 8915, 9055, 9197, 9367, 9539, 9723, 9885, 10058, 10226. 10391, 10578, 10703, 10825, 10959, 11169, 11326, 11510, 11629, 11747, 11941, 12089, 12243, 12414, 12598, 12758, 12881, 13050, 13195, 13349, 13517, 13725, 13821.

6. The receiver as claimed in claim 3, wherein the continuous pilot symbol pattern for 32k sub-carrier signals in terms of sub-carrier signal locations is given by 163, 290.
486, 605, 691, 858, 1033, 1187, 1427, 1582, 1721, 1881, 2019, 2217, 2314, 2425, 2579, 2709, 2857, 3009, 3219, 3399, 3506, 3621, 3762, 3997, 4122, 4257, 4393, 4539, 4601, 4786, 4899, 5095, 5293, 5378, 5587, 5693, 5797, 5937, 6054. 6139, 6317, 6501, 6675, 6807, 6994, 7163, 7289, 7467, 7586, 7689, 7845, 8011, 8117, 8337, 8477, 8665, 8817, 8893, 8979, 9177, 9293, 9539. 9693, 9885, 10026, 10151, 10349, 10471, 10553, 10646, 10837, 10977, 11153, 11325, 11445, 11605, 11789, 11939, 12102, 12253, 12443, 12557, 12755, 12866, 12993, 13150, 13273, 13445, 13635, 13846, 14041, 14225, 14402, 14571. 14731, 14917. 15050. 15209, 15442, 15622, 15790, 15953, 16179, 16239, 16397, 16533, 16650, 16750, 16897, 17045, 17186, 17351, 17485, 17637, 17829, 17939, 18109, 18246, 18393, 18566, 18733, 18901, 19077, 19253, 19445, 19589, 19769, 19989, 20115, 20275, 20451, 20675, 20781, 20989, 21155, 21279, 21405, 21537, 21650, 21789, 21917, 22133. 22338, 22489, 22651, 22823, 23019, 23205, 23258, 23361, 23493, 23685, 23881, 24007, 24178, 24317.
24486, 24689, 24827, 25061, 25195, 25331, 25515, 25649, 25761, 25894, 26099, 26246, 26390, 26569, 26698, 26910, 27033, 27241, 27449, 27511, 27642, 27801.

7. The receiver as claimed in claim 6, wherein the number of sub-carrier signals is approximately 16k and the processor is configured to derive the 16k continuous pilot symbol pattern according to the equation C_13_16K_pos = round(C13_32K_pos(1:2:last_32k_cp_pos)/2).

8. The receiver as claim in claim 6, wherein the number of sub-carrier signals is approximately 8k and the processor is configured to derive the 8k continuous pilot symbol pattern according to the equation = round(CP_8K_pos=1:4:last_32kp_pos)/4).

9. The receiver as claimed in claim 1, wherein the plurality of scattered pilot symbol patterns include scattered pilot symbol patterns; Dx = 4, Dy = 4; Dx = 8, Dy = Dx = 16, Dy = 2; and Dx = 32, Dy = 2.

10. The receiver as claimed in claim 1, wherein locations of the scattered pilot symbols across the plurality of scattered pilot symbol patterns and locations of the continuous pilot symbols with respect to the sub-carrier signals substantially do not coincide.

11. A method for receiving and recovering data from Orthogonal Frequency Division Multiplexed (OFDM) symbols, the OFDM symbols including a plurality of sub-carrier signals, some of the sub-carrier signals carrying data symbols and some of the sub-carrier signals carrying pilot symbols, the pilot symbols comprising scattered pilots symbols and continuous pilot symbols, the continuous pilot symbols being distributed across the sub-carrier signals in accordance with a continuous pilot symbol pattern and the scattered pilot symbols being distributed across the sub-carrier signals in accordance with a scattered pilot symbol pattern, the method comprising detecting a signal representing the OFDM symbols., generating a sampled digital version of the OFDM symbols in the time domain;
receiving the time domain digital version of the OFDM symbols and forming a frequency domain version of the OFDM symbols, from which the pilot symbol bearing sub-carrier signals and the data symbol bearing sub-carrier signals can be recovered;
recovering the data symbols from the data bearing sub-carrier signals of the OFDM
symbols; and recovering the pilot symbols from the pilot bearing sub-carrier signals of the OFDM
symbols in accordance with the scattered pilot symbol 'pattern and the continuous pilot symbol pattern, wherein the scattered pilot symbol pattern is one of a plurality of scattered pilot symbol patterns, and the method comprises detecting the number of sub-carrier signals in the plurality of sub-carrier signals; and deriving the continuous pilot symbol pattern from a stored master pilot pattern based on the number of sub-carrier signals.

12. A transmitter for transmitting Orthogonal Frequency Division Multiplexed (OFDM) symbols, the OFDM symbols including a plurality of sub-carrier signals, some of the sub-carrier signals carrying data symbols and some of the sub-carrier signals carrying pilot symbols, the pilot symbols comprising scattered pilots symbols and continuous pilot symbols, the continuous pilot symbols being distributed across the sub-carrier signals in accordance with a continuous pilot symbol pattern and the scattered pilot symbols being distributed across the sub-carrier signals in accordance with a scattered pilot symbol pattern, the transmitter comprising a pilot signal former configured to generate pilot symbols.
a symbol builder configured to receive a frequency domain data symbol stream and embed the generated pilot symbols from the pilot signal former into the sub-carrier signals of the data symbol stream pilot symbols in accordance with the scattered pilot symbol pattern and the continuous pilot symbol pattern, and an OFDM modulator configured to generate a time domain version of the signal embedded with pilot symbols, wherein the scattered pilot symbol pattern is one of a plurality of scattered pilot symbol patterns, and the pilot signal former comprises a memory configured to store a master pilot pattern and a processor configured to detect the number of sub-carrier signals in the plurality of sub-carrier signals and to derive the continuous pilot symbol pattern from the master pilot pattern based on the number of sub-carrier signals.

13. A method for transmitting Orthogonal Frequency Division Multiplexed (OFDM) symbols, the OFDM symbols including a plurality of sub-carrier signals, some of the sub-carrier signals carrying data symbols and some of the sub-carrier signals carrying pilot symbols, the pilot symbols comprising scattered pilots symbols and continuous pilot symbols, the continuous pilot symbols being distributed across the sub-carrier signals in accordance with a continuous pilot symbol pattern and the scattered pilot symbols being distributed across the sub-carrier signals in accordance with a scattered pilot symbol pattern, the method comprising generating pilot symbols;
receiving a frequency domain data symbol stream and embedding the generated pilot symbols into the sub-carrier signals of the data symbol stream in accordance with the scattered pilot symbol pattern and the continuous pilot symbol pattern; and generating a time domain version of the symbol stream embedded with the pilot symbols, wherein the scattered pilot symbol pattern is one of a plurality of scattered pilot symbol patterns, and the method comprises detecting the number of sub-carrier signals in the plurality of sub-carrier signals; and deriving the continuous pilot symbol pattern from a stored master pilot pattern based on the number of sub-carrier signals.

14. A physical memory haying stored thereon computer executable instructions, which when executed by a computer causes the computer to perform the method according to claim I I .

15. The receiver as claimed in claim 1, wherein the continuous pilot symbol pattern is independent of the scattered pilot symbol pattern.

16. The transmitter as claimed in claim 12, wherein the continuous pilot symbol pattern is independent of the scattered pilot symbol pattern.

17. The method as claimed in claim 11, wherein the continuous pilot symbol pattern is independent of the scattered pilot symbol pattern.

18. The method as claimed in claim 13, wherein the continuous pilot symbol pattern is independent of the scattered pilot symbol pattern.