WO2014177839A1

WO2014177839A1 - Papr reduction in ofdm by using tone reservation

Info

Publication number: WO2014177839A1
Application number: PCT/GB2014/051217
Authority: WO
Inventors: Gholam Hosein ASJADI
Original assignee: Sony Corporation; Sony Europe Limited
Priority date: 2013-05-02
Filing date: 2014-04-17
Publication date: 2014-11-06
Also published as: GB201307931D0; TW201515413A; GB2513630A

Abstract

A transmitter for transmitting signals representing data using Orthogonal Frequency Division Multiplexing (OFDM) symbols with a predetermined number of OFDM subcarriers. The transmitter comprises a data source configured to provide payload data, a reserved sub-carrier data source configured to provide reserved sub-carrier data, and an OFDM modulator configured to modulate the subcarriers of the OFDM symbol with the payload data and the reserved sub-carrier data to form a frequency domain OFDM symbol, the payload data being modulated onto data sub-carriers and the reserved sub-carrier data being modulated onto reserved sub-carriers. The transmitter also comprises an inverse Fourier transformer configured to transform the frequency domain OFDM symbol into a time domain OFDM symbol and a peak-to-average-power-ratio (PAPR) reduction circuit. The PAPR reduction circuit is configured to reduce a peak-to-average-power ratio of the time domain OFDM symbol to below a threshold by to detecting a peak in the power of the time domain OFDM symbol exceeding the threshold and combining a kernel signal with the time-domain OFDM symbol to reduce the amplitude of the peak to form a PAPR processed time domain OFDM symbol. The kernel signal is formed from data on the reserved sub-carriers of the OFDM symbol and the reserved sub-carriers are distributed across the OFDM sub-carriers in accordance with a predetermined pattern, the predetermined pattern being dependent upon the number of sub- carriers each OFDM symbol comprises.

Description

PAPR REDUCTION IN OFDM BY USING TONE RESERVATION

FIELD OF DISCLOSURE

The present technique relates to transmitters for transmitting signals and receivers for receiving signals in Orthogonal Frequency Division Multiplexing (OFDM) Systems.

DESCRIPTION OF THE RELATED ART

Radio communications systems have evolved from early analogue communications networks to complex digital communications systems which are now integral to the everyday life. In the field of terrestrial television broadcasting, basic analogue terrestrial broadcast television systems of the 20^th century have been replaced with complex digital systems which have increased capacity, spectral efficiency and robustness. The development of the underlying wireless techniques has been central to the development of these complex systems, for example the transition from single carrier analogue system to multicarrier digital systems. Orthogonal Frequency Division Multiplexing has been selected as the wireless technique for the current generation of terrestrial television broadcast standards such as DVB-T2 and very likely in the emerging standards such as ATSC 3.0 and is also utilised in other communications networks such as 3GPP LTE mobile communications networks. OFDM has a number of advantages over single carriers systems used in previous ATSC generations (ATSC1.0 and ATSC2.0), for instance, OFDM offers increased robustness to intersymbol interference (ISI). However, the multicarrier nature of OFDM systems results in signals which have a higher peak- to-average-power-ratio (PAPR). If the transmission network is not carefully designed for, this characteristic may cause distortion when a signal is amplified in preparation for transmission.

Distortion of the signal may cause decoding errors at a receiver, thus degrading the performance of the system. Consequently, reducing the PAPR of an OFDM signal prior to amplification and transmission may be necessary if amplification distortion is to be reduced. A number of approaches exist to reduce the PAPR of OFDM signal but these approaches often adversely affect the capacity and complexity of an OFDM system.

SUMMARY

According to one example of the present technique, a transmitter transmits signals representing data using Orthogonal Frequency Division Multiplexing (OFDM) symbols with a predetermined number of OFDM subcarriers. The transmitter comprises a data source configured to provide payload data, a reserved sub-carrier data source configured to provide reserved sub-carrier data, and an OFDM modulator configured to modulate the subcarriers of the OFDM symbol with the payload data and the reserved sub-carrier data to form a frequency domain OFDM symbol, the payload data being modulated onto data sub-carriers. The reserved carriers do not carry any data information and are only filled with a peak-reduction signal. The transmitter also comprises an inverse Fourier transformer configured to transform the frequency domain OFDM symbol into a time domain OFDM symbol and a peak-to-average-power-ratio (PAPR) reduction circuit. The PAPR reduction circuit is configured to reduce the peak-to-average-power ratio of the time domain OFDM symbol to below a predetermined threshold by detecting one or a number of peak/s in the power of the OFDM symbol exceeding the predetermined threshold and combining a kernel signal with the OFDM symbol to reduce the amplitude of the peak to form a PAPR-processed OFDM symbol. The kernel signal is formed from data on the reserved sub-carriers of the OFDM symbol and the reserved sub-carriers are distributed across the OFDM sub-carriers in accordance with a predetermined pattern, the predetermined pattern being dependent upon the predetermined number of sub- carriers of each OFDM symbol.

According to another example embodiment the transmitter comprises a digital to analogue converter configured to convert the PAPR processed time domain OFDM symbol into an analogue time domain OFDM signal. The sub-carrier indices of the predetermined pattern are determined in accordance with on a ratio between an amplitude of a second largest peak in the kernel signal to a largest peak in the kernel signal formed from data on the reserved sub-carriers, the determination of the reserved sub-carrier pattern indices having been performed at a sampling rate higher than the sampling rate of the OFDM signal.

Performing the determination of the reserved sub-carrier pattern indices at a higher sampling rate ensures that the formed kernel signal is more accurate and more closely resembles the signal which will have an effect on the PAPR and the resulting analogue signal which will be transmitted by the transmitter. This therefore allows an improved set of reserved subcarriers to be obtained compared to performing the determination at the standard sampling rate of the OFDM system. The improved set of subcarriers results in a kernel signal which has fewer secondary peaks and therefore is more effectively at reducing the PAPR of the signal which is to be transmitted by the transmitter and ensuring less contamination of the data sub-carriers at the same time.

Various further aspects and embodiments of the disclosure are provided in the appended claims, including but not limited to a receiving for receiving signals in an OFDM system.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present disclosure will now be described by way of example only with reference to the accompanying drawing in which like parts are provided with corresponding reference numerals and in which:

Figure 1 provides a schematic block diagram of an example OFDM communications system;

Figure 2 provides a schematic block diagram of an example OFDM transmitter;

Figure 3 provides a schematic block diagram of an example peak cancellation processor;

Figure 4A provides an illustrative graphical plot of power with respect to time illustrating a first example of a kernel signal used in the peak cancellation processor of Figure 3; Figure 4B provides an illustrative graphical plot of power with respect to time illustrating a second example of a kernel signal used in the peak cancellation processor of Figure 3; and Figure 4C provides an illustrative graphical plot of power with respect to time illustrating a third example of a kernel signal used in the peak cancellation processor of Figure 3;

Figure 5 provides a schematic block diagram of an OFDM receiver;

Figure 6 provides a table of subcarrier indices of continual pilot patterns which are in accordance with an example embodiment;

Figure 7 provides a table of subcarrier indices of reserved subcarrier patterns which are in accordance with an example embodiment;

Figures 8a and 8b provide illustrative plots of an example DVB-T2 peak cancellation reference kernel for an 8k mode;

Figures 9a and 9b provide illustrative plots of a peak cancellation reference kernel which is in accordance with an example embodiment for an 8k mode;

Figures 10a and 10b provide illustrative plots of an example DVB-T2 peak cancellation reference kernel for a 16k mode; Figures 11a and lib provide illustrative plots of a peak cancellation reference kernel which is in accordance with an example embodiment for a 16k mode;

Figures 12a and 12b provide illustrative plots of an example DVB-T2 peak cancellation reference kernel for a 32k mode;

Figure 13a and 13b provide illustrative plots of a peak cancellation reference kernel which is in accordance with an example embodiment for a 32k mode;

Figure 14 provides an illustrative plot of peak-to-average-power-ratio (PAPR) simulation results for a system in an 8k mode in accordance with an example embodiment;

Figure 15 provides an illustrative plot of PAPR simulation results for a system in a 16k mode in accordance with an example embodiment; and

Figure 16 provides an illustrative plot of PAPR simulation results for a system in a 32k mode in accordance with an example embodiment;

DETAILED DESCRIPTION OF THE EMBODIMENTS

Figure 1 illustrates a communications system 100 arranged for broadcasting and receiving communication signals representing data, where the signals may for example be terrestrial television signals. The communications system comprises a core network 101, signal processing apparatus 102, a transmit antenna 103 and a plurality of such user devices 104. The user devices may be television sets in a television system but in some examples they may also be mobile handsets, personal video recorders or other devices operable to receive a communications signal. Each of the user devices also includes a means for receiving a signal such as an antenna 105, a cable connection or an antenna feed. The core network 101 includes a signal source such as for example a television studio camera that captures video and audio data and converts the data into a signal which is transmitted to the data processing apparatus 102. The data processing apparatus 102 processes the signal received from the core network 101 to transform the signal into a form suitable for transmission from the antenna 103.

Although the system illustrated by Figure 1 transmits data using radio frequency signals, being a wireless transmission system provided by the core network 101 and processed by the data processing apparatus 102, in other examples the signals carrying the data may be transmitted to the user devices 104 over a cable connection or a satellite link. The system 100 may use any one or a variety of transmission techniques to communicate the data to the user devices, for example the system may use a single or multicarrier techniques.

Orthogonal Frequency Division Multiplexing (OFDM) is an example of a multicarrier transmission technique that may be used in a system such as that illustrated in Figure 1. OFDM systems operate by dividing an available bandwidth into a plurality of orthogonal narrowband subcarriers each of which is a predetermined bandwidth, where the total number of subcarriers is N. Data which is to be transmitted is divided into a plurality of parallel streams and modulated onto the plurality of subcarriers in parallel to form OFDM symbols which extend across a plurality of the available subcarriers. OFDM provides a number of advantages over other transmission techniques in terms of capacity and improved robustness to multipath fading and intersymbol interference (ISI). For example, due to the plurality of subcarriers, data symbols on each subcarrier can be transmitted for an extended period of time compared to a single carrier system, therefore allowing multipath interference to be averaged out over the subcarrier, thus reducing ISI. Due to the advantages of OFDM, a number of broadcast standards designed for the transmission for standard and high- definition television signals utilise OFDM as a transmission technique. For instance, the set of DVB (DVB-T and DVB-T2) standards utilise OFDM over a bandwidth of up to 8MHz and it has been proposed that the Advanced Television Systems Committee (ATSC 3.0) standard utilise OFDM over a bandwidth of 6MHz. Furthermore, in order to take advantage of the standardisation efforts and proposals associated with DVB-T2, the ATSC 3.0 standard is to be based on the DVB-T2 but with the characteristics of the DVB-T2 standard adapted for implementation with the ATSC 3.0 parameters such as the reduced bandwidth. It will be appreciated that reference to ATSC 3.0 throughout this document refers to a proposed ATSC3.0 standard or future generations thereof at the time of filing.

Figure 2 provides a diagram of a simplified structure of an example data processing apparatus 102. Data from the core network 101 is provided to the data source 201 which is then input into a multiplexer or modulator 204 which modulates the subcarriers with data from the data source. Subcarriers of an OFDM signal in a system as described above may be separated into data subcarriers which convey payload data and control data, pilot subcarriers which convey pilot data, and reserved subcarriers which do not convey any useful data but are instead modulated with redundant data which enables properties of the transmit signal to be adjusted. The multiplexer positions data from the data source 201, pilot data source 202 and the reserved subcarrier data source 203 onto their respective subcarrier locations. The multiplexed signal is then passed to an inverse fast Fourier transform 205 which transforms the time domain digital signal into a frequency domain digital signal. The size of the IFFT is dependent upon the number of subcarriers, for example, in DVB-T2 and possibly ATSC 3.0, the FFT and IFFT sizes include approximately 8k, 16k and 32k. The frequency domain signal is then passed through a parallel to serial converter 206, a peak cancellation processor 207, a digital to analogue converter 208 and a high power amplifier (HPA) 209 which outputs a high power signal suitable for transmission from the antenna 103. The data processing apparatus 102 may also comprise a number of other data processing stages which are not illustrated in Figure 2, for instance there may also be a guard interval insertion processor, error correction coding, an interleaver, and other data processing stages common to OFDM transmitters, details of which may for example be found in the DVB-T2 standard.

When forming an OFDM signal suitable for transmission, a number of factors may be required to be taken into account. One such factor is a peak-to-average-power-ratio (PAPR) of the signal which is to be amplified by the HPA 209. OFDM signals commonly have a higher PAPR than single carrier signals and therefore this characteristic of OFDM signals needs to be taken into account when designing OFDM systems. HPA have a limited power envelope in which they can operate in a linear fashion i.e. without significantly distorting or clipping the amplified signal, and a cost of an HPA 209 is often dependent upon the size of its operating envelope. Consequently, if a signal with a high PAPR such as an OFDM signal is amplified by an HPA, it is likely that distortion will be introduced or the cost of a suitable HPA 209 which does not introduce distortion will be high. The issue of PAPR is also particularly relevant to the development of the ATSC 3.0 standard. Unlike DVB- T2, previous generations of the ATSC standard were single carrier systems and therefore the signals transmitted by these systems had a low PAPR and the HPA within the transmitters likely to have a small operating envelope in order to reduce the cost of the transmitters. However, as previously explained, the next generation ATSC 3.0 system is very likely to be a multicarrier OFDM system and therefore if the same transmission infrastructure is to be used for the transmission of ATSC 3.0 signals, the PAPR of the signals may be required to be reduced if significant distortion of the amplified signal is to be avoided.

In order to address the problem of high PAPR, OFDM transmitters may include a PAPR reduction processing, one form of which is a peak cancellation processor 207 as illustrated in Figure 2. A process of performing peak cancellation serves to reduce the PAPR of an OFDM signal which is to be amplified by an HPA by introducing or combining a peak cancellation signal with the OFDM signal that reduces the PAPR of the OFDM signal. In DVB-T2 a technique termed tone reservation is used as part of PAPR reduction, where selected subcarriers of the OFDM signal are reserved to carry the introduced peak cancellation signal which helps to reduce the PAPR of the OFDM signal. These subcarriers are termed reserved subcarriers where the distribution of the reserved subcarriers is in accordance with a reserved subcarrier pattern. PAPR is explained in more detail in the DVB-T2 standard document ETSI TS 102 831 Vl.2.1 (2012-08) at page 143 section 9.3.8. The following paragraphs provide a summary of this process.

Figure 3 provides an illustration of a known example peak cancellation process 207 which is utilised in DVB-T2 transmitters and detailed in the DVB-T2 standard, and is therefore envisaged also to be used in the ATSC 3.0 standard. The PAPR of a time domain signal x from the parallel to serial converter 206 is first calculated and if it exceeds a PAPR threshold the signal is iteratively analysed and peaks in the power of the signal above a certain threshold detected at a peak detection unit 301. The peak detection unit 301 detects the amplitude, location and phase of peaks in the OFDM signal which are above a pre-determined threshold V_djp and therefore contribute to the OFDM signal exceeding a PAPR threshold. If a peak is detected, compensation in the form of a signal c which is formed from data on the reserved subcarriers is introduced to reduce the power of the peak. A first step in reducing the PAPR is to limit the amplitude of the signals on the reserved carriers so that the signal produced by the reserved carrier data is appropriate to the desired PAPR and the power of the reserved subcarriers do not exceed a maximum level.

A signal which is introduced into the OFDM signal to reduce the PAPR is derived from a predetermined signal termed a reference kernel, which can be formed from data on the reserved subcarriers and where an ideal reference kernel is a signal which has a single impulse-like peak and zero amplitude elsewhere. Presuming an ideal reference kernel, the reference kernel is first shifted to coincide with detected peak by a circular shifter 304, and then scaled and phase rotated by a scaler and phase rotator 305 to match the detected peak. The shifted, scaled and rotated reference kernel is then combined with the OFDM signal by adding or subtracting the kernel to or from the OFDM signal by an adder 306 in order to reduce the amplitude of the detected peak to below V_dip. Once the reference kernel has been added/subtracted the PAPR of the peak cancelled OFDM signal is calculated by a PAPR calculator 307 and if the PAPR of the signal still exceeds the threshold the controller 308 configures the peak detector 301 to detect any remaining peaks which exceed V_dip and the compensation process is performed again. These steps are then repeated in an iterative fashion until no further peaks exceeding V_C|_ip remain or a maximum number of iterations is reached. The extent of the PAPR reduction may be controlled by the level at which V_C|_ip is set but also the maximum number of iterations of the process. In a peak cancellation process as described above, the reserved subcarriers are initially modulated with a null data stream. It is the OFDM signal including the reserved subcarrier data as well as the payload, control and pilot data which is analysed by the peak detector. Data provided by the reserved data source is not directly altered by the peak cancellation process but it is instead effectively altered by the introduction of the reference kernel into the OFDM signal because the reference kernel is a signal which is formed only from redundant data on the reserved subcarriers. This method of reducing the PAPR effectively alters the data content of the reserved subcarriers in the frequency domain by introducing a time domain signal that can be formed solely from the redundant data on the reserved subcarriers. This approach has an equivalent effect to adjusting the reserved subcarrier data output from the reserved subcarrier data source but does not require feedback to the reserved carrier data source, thus reducing any possible latency implications.

In order to obtain an ideal reference kernel it would be necessary to utilise an infinite number of reserved subcarriers because an impulse in the time domain has an infinite frequency spectrum. However, utilising a large number of reserved subcarriers would adversely impact the capacity of a communications system because there will be fewer data subcarriers to convey payload data. In order to reduce capacity loses arising from reserved carriers the number of reserved subcarriers is often limited, for instance in DVB-T2 the number of reserved subcarriers represents a 1% drop in capacity compared to where there are no reserved carriers i.e. when tone reservation is not implemented. Limiting the number of reserved subcarriers means that it is no longer possible to obtain an ideal reference kernel and therefore a reference kernel will have a number of other secondary peaks. A consequence of this is that when the reference kernel is introduced into the OFDM signal it is not only the detected peak which is affected, other parts of the signal are also affected by the secondary peaks of the reference kernel. In some examples the introduction Df the reference kernel may reduce the detected peak to below V_dip but may increase others peaks to above V_C|_ip such that they will be detected at the next iteration of the peak cancellation process. This unintended growth of other peaks in the OFDM signal is termed side peak growth.

The peak cancellation process described above when utilising a non-ideal reference kernel is illustrated in Figures 4a to 4c. An input OFDM signal 401 before PAPR reduction is shown in Figure 4a, a shifted reference kernel 403 is shown in Figure 4b and the compensated or peak cancelled OFDM signal 405 is shown in Figure 4c. In Figure 4a it can be seen that the peak 402 in the input single 401 exceeds V_C|_ip and therefore it would be beneficial to reduce the height of this peak in order to reduce the PAPR of the OFDM signal. To reduce the peak 402, the reference kernel 403 shown in Figure 4b is introduced into the input signal 401 where reference kernel has been shifted, scaled and rotated so that the peak 404 coincides with the peak 402. In Figure 4c one can see that the peak 402 in the peak cancellation signal 405 has been reduced to below V_C|_ip but, due to a non-ideal reference kernel, other peaks 406 in the signal have also been affected by the reference kernel and side peak growth has occurred. Although in Figure 4c no further peaks exceed V_C]_ip, this may not always be the case and therefore introducing a reference kernel can also cause problems as well as solutions to the PAPR problem. The cause of side peak growth is the presence of secondary peaks in the reference kernel, thus increasing the ratio between the height of the primary peak and second peaks in the reference kernel improves a likelihood of reducing a peak in the OFDM signal without causing other signal components to grow. Alternatively, if the power of the reference kernel has been normalised with respect to the primary peak, it is a matter of examining height of the secondary peak and attempting to reduce the height of the secondary peak.

Referring back to Figure 3, the OFDM signal amplified by the HPA is analogue and therefore it is the PAPR in the analogue domain which determines whether the signal may be distorted by the HPA. However, the peak cancellation as described above is performed in the digital domain.

Therefore it is necessary to ensure that the PAPR calculation and processing performed in the digital domain will produce a signal in the analogue domain which has a PAPR below the require PAPR threshold. In order to reduce any discrepancies between the PAPR measurement in the digital domain and the actual PAPR in the analogue domain, the digital PAPR calculation is performed on an up-sampled OFDM signal. It is a known common practice to use a typical up-sampling ratio of four to model the D/A process before measuring the PAPR of the OFDM signal.

Figure 5 illustrates an OFDM receiver which may be present in the user devices 104 and is operable to receive an OFDM signal and output data corresponding to the payload data conveyed by the OFDM signal. A transmitted OFDM signal is received by the antenna 105 and down converted by a baseband converter 501 and then sampled by an analogue to digital converter 502 to form a digital baseband signal. The digital baseband signal is transformed into the frequency domain by a fast Fourier transform 503, pilot data extracted from the frequency domain signal by a pilot data extractor 504, and channel equalisation performed on the frequency domain signal by a channel equaliser 505. The equalised signal is then processed by a serial to parallel converter 506 before the payload data and control data is demultiplexed and/or demodulated by a demultiplexer 507 and extracted by a data extractor 508. There may also be other processing steps in an OFDM receiver which are not illustrated in Figure 5, for example the receiver may also comprise a guard interval remover, a low noise amplifier, deinterleaver, and an error correction decoder. As described above, when tone reservation/PAP reduction is in operation at the transmitter, a number of subcarriers are devoted to carrying reserved subcarrier data i.e. reference kernel data as opposed to payload or control data. Consequently, it is necessary for the receiver to have knowledge of the location of these subcarriers so that it does not interpret reserved subcarrier data as payload data. In order to inform the receiver of this, there may be a data field in the control data which indicates via an indicator to the receiver whether tone reservation is in operation or not. The reserved subcarrier pattern may be known at the receiver and therefore a single bit may be sufficient to indicate the operation of tone reservation to the receiver. In order to control the demultiplexer not to demodulate the data which is on the reserved subcarriers the receiver may also comprise a controller 509 which is configured to receive control data comprising the indicator from the demutliplexer and control the demultiplexer not to demodulate data from the reserved subcarriers. As previously mentioned, knowledge of the reserved subcarrier pattern is required at the receiver and therefore the reserved subcarrier pattern may also be stored in a memory 510 at the controller.

An ideal reference kernel will have an infinite primary to second peak ratio and therefore when utilising an ideal reference kernel there will be no side peak growth in the peak cancelled signal. However, as previously described this is not possible when a limited number of reserved subcarriers are available from which to form the reference kernel. Consequently, maximising the primary to secondary peak ratio in the reference kernel is an important design consideration when considering PAPR reduction. If the number of reserved subcarriers is limited, as is the case in most OFDM systems, the means by which the primary to secondary peak ratio of the reference kernel can be optimised is by optimising the location of the reserved subcarriers from the IM subcarriers available positions, The location selection of reserved subcarriers may be achieved by simulating an OFDM system and selecting the reserved subcarriers that produce a reference kernel with a low primary to secondary peak ratio. However, this process is complicated by a number of factors such as the placement of continual and scattered pilots whose position may have been optimised to ensure adequate channel estimation and frequency offset estimation. In DVB-T2, in order to provide flexibility in the placement of the reserved subcarriers and obtain a reference kernel with a high primary to second peak ratio, the reserved subcarrier pattern may change from symbol-to-symbol to accommodate the staggered scattered pilots. The time shift nature of the reserved subcarriers means that they can be positioned around the scattered pilot subcarriers which are also subjected to time shift (staggered), and the set of reserved subcarriers can be optimised based on the current positions of the pilot subcarriers. Furthermore, the conditions under which an optimised reference kernel and reserved carriers are derived may also impact of the optimality of the reference kernel and therefore peak cancellation.

In accordance with an example of the present technique selection of a reference kernel and the reserved subcarriers from which the reference kernel is formed are performed via simulations at a higher sampling rate than the standard sampling rate of the system in which they are intended to be utilised. The up-sampling or over-sampling allows the selection of the reserved subcarriers and calculation of the kernel to be performed at a sampling rate which produces a digital signal that more closely resembles the analogue signal which is amplified by the HPA. This technique may produce a set of reserved carriers which are different from those that may be produced if the standard sampling of the system is used to determine an optimum reference kernel and associated subcarriers. Referring back to Figure 3, the processing blocks operate at the same sampling rate as before but the reserved carriers that form the reference kernel have been derived from simulations operating at a higher sampling rate. In one example, the sampling rate may be four-times higher than the standard sampling rate, such that the sampling rate corresponds to the up-sampling performed in the peak cancellation process 307 when the PAPR of the OFDM signal is calculated. Although described with reference to proposed ATSC 3.0 and DVB-T2, the technique above may be used in a wide variety of systems. In one example in accordance with the present technique the simulation and optimisation process may be performed by implementing an OFDM system as described with reference to Figure 2 and randomly selecting an appropriate number of reserved subcarriers which do not coincide with pilot subcarriers from the plurality of subcarriers. A reference kernel with a maximum possible primary to secondary peak power ratio is then formed using the randomly selected subcarriers and the primary to secondary peak power ratio recorded. This random selection process is then repeated a number of times and the set of reserved subcarriers which on average produces a reference kernel with the highest primary to secondary peak power ratio is then chosen as the optimised set of reserved subcarriers/reserved subcarrier pattern.

For each mode (8k, 16k, 32k) the capacity loss is approximately 1% and a reserved subcarrier pattern has been optimised around a number of continuous and scattered pilot patterns where the reserved subcarriers do not coincide with the pilot subcarriers. The subcarrier indices of the continual pilots are given in Figure 6 and the scattered pilot patterns are given by P(Dx,Dy) = (4,4),(8,2),(16,2),(32,2) . 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 subcarrier in the next OFDM symbol. Dy represents a parameter indicating a number of OFDM symbols before the same subcarrier is used again to carry a pilot symbol on the next occasion.

For the 8k mode the 72 reserved subcarrier locations are given in Figure 7 and by the following index numbers identifying the sub-carriers of the OFDM symbol where each of the sub- carriers of the OFDM symbol have been numbered sequentially:

131, 263, 267, 611, 639, 696, 782, 811, 829, 860, 861, 930, 982, 1029, 1380, 1476, 1571, 1589, 1594, 1609, 1793, 1836, 2131, 2215, 2538, 2540, 2717, 2966, 2975, 2995, 3058, 3171, 3399, 3483, 3666, 3698, 3786, 4004, 4015, 4057, 4181, 4241, 4292, 4326, 4356, 4516, 4772, 4929, 5027, 5069, 5214, 5234, 5306, 5307, 5310, 5400, 5494, 5500, 5602, 5685, 5717, 5858, 5911, 6133, 6139, 6213, 6217, 6241, 6266, 6341, 6686, 6708.

The reference kernel produced by the above reserved carrier locations in an envisaged ATSC 3.0 system is compared against the reference kernel produced by the known reserved subcarrier pattern in a DVB-T2 system in Figures 8 and 9. Figures 8a and 8b show the DVB-T2 reference kernel over a longer and a shorter time period respectively, and Figures 9a and 9b show the proposed ATSC 3.0 reference kernel, which is formed from the reserved subcarrier pattern that is in accordance with the present technique, over a longer and shorter time period respectively. As one can see the two reference kernels have similar primary to secondary peak power ratios.

For the 16k mode the 144 reserved subcarrier locations are given in Figure 7 and by the following index numbers identifying the sub-carriers of the OFDM symbol where each of the sub- carriers of the OFDM symbol have been numbered sequentially:

140, 152, 180, 271, 277, 282, 285, 316, 406, 494, 794, 895, 1021, 1060, 1085, 1203, 1318, 1324, 1414, 1422, 1597, 1674, 1890, 1907, 1931, 2004, 2125, 2348, 2356, 2834, 2854, 2870, 2913, 2917, 3017, 3183, 3225, 3311, 3491, 3549, 3566, 3715, 3716, 3949, 3987, 4054, 4127, 4147, 4422, 4534, 4585, 4597, 4687, 4759,4879, 4882, 5015, 5097, 5114, 5412, 5527, 5571, 5639, 5661, 5666, 5737, 5795, 5997, 6014, 6029, 6084, 6108,6122, 6170, 6251, 6500, 6609, 6620, 6721, 6774, 6842, 7162, 7247, 7503, 7770, 7775, 7890, 7932, 7985, 8043, 8089, 8092, 8104, 8156, 8233, 8241, 8329, 8362, 8371, 8518, 8603, 9022, 9090, 9190, 9400, 9453, 9544, 9598, 9769, 10022, 10043, 10150, 10347, 10568, 10849, 10895, 11313, 11394, 11401, 11574, 11583, 11726, 11822, 12120, 12244, 12276, 12438, 12451, 12540, 12574, 12654, 12671, 12806, 12898, 12899, 12950, 13021, 13107, 13153, 13293, 13303, 13430, 13433, 13494.

The reference kernel produced by the above reserved carrier locations in an envisaged ATSC 3.0 system is compared against the reference kernel produced by the known reserved subcarrier pattern in a DVB-T2 system in Figures 10 and 11. Figures 10a and 10b show the DVB-T2 reference kernel over a longer and a shorter time period respectively, and Figures 11a and lib show the proposed ATSC 3.0 reference kernel, which is in formed from the reserved subcarrier pattern that is in accordance with the present technique, over a longer and shorter time period respectively. As one can see the two reference kernels have similar primary to secondary peak power ratios.

For the 32k mode the 288 reserved subcarrier locations are given in Figure 7 and by the following index numbers identifying the sub-carriers of the OFDM symbol where each of the sub- carriers of the OFDM symbol have been numbered sequentially:

275, 303, 526, 537, 584, 648, 722, 779, 794, 859, 867, 1182, 1238, 1251, 1263, 1336, 1389, 1399, 1420, 1471, 1498, 1541, 1587, 1628, 1719, 1822, 2055, 2068, 2181, 2242, 2248, 2346, 2350, 2359, 2502, 2525, 2834, 2889, 2937, 3018, 3091, 3124, 3160, 3480, 3489, 3512, 3707, 3718, 3734, 3861, 4079, 4255, 4371, 4484, 4499, 4550, 4588, 4850, 4854, 4935, 4946, 5084, 5110, 5127, 5169, 5178, 5345, 5480, 5623, 5649, 5805, 6236, 6265, 6346, 6397, 6447, 6482, 6500, 6644, 6735, 6796, 6934, 6956, 7113, 7146, 7186, 7194, 7215, 7226, 7266, 7507, 7736, 7757, 7942, 8056, 8471, 8516, 8567, 8583, 8613, 8868, 8883, 9000, 9005, 9134, 9163, 9242, 9310, 9348, 9358, 9378, 9586, 9682, 9697, 9732, 9825, 9883, 10037, 10792, 10830, 10872, 10879, 10892, 10894, 11007, 11030, 11103, 11129, 11186, 11461, 11466, 11585, 11912, 11937, 11970, 12035, 12185, 12392, 12406, 12424, 12529, 12666, 12842, 12982, 13004, 13115, 13182, 13226, 13356, 13407, 13554, 13681, 13743, 14114, 14119, 14247, 14369, 14458, 14468, 14629, 14655, 14694, 14724, 14796, 14815, 15084, 15143, 15249, 15420, 15530, 15655, 15666, 15703, 15844, 15882, 15970, 16019, 16065, 16215, 16420, 16695, 16705, 16707, 16764, 16818, 16855, 16913, 16925, 16958, 16965, 17048, 17233, 17441, 17624, 17729, 18157, 18181, 18193, 18433, 18451, 18506, 18665, 18858, 18988, 19085, 19098, 19286, 19374, 19659, 19707, 19717, 19878, 19890, 19979, 19983, 20008, 20241, 20295, 20527, 20801, 20943, 21015, 21067, 21163, 21175, 21191, 21201, 21293, 21337, 21608, 21656, 21674, 21755, 21758, 22145, 22380, 22788, 22847, 22856, 22878, 22889, 22924, 23069, 23099, 23103, 23151, 23897, 23997, 24033, 24039, 24063, 24127, 24163, 24217, 24302, 24386, 24434, 24854, 24910, 24946, 25017, 25199, 25256, 25455, 25513, 25555, 25565, 25772, 25778, 25782, 25889, 25943, 26034, 26161, 26167, 26289, 26409, 26492, 26498, 26620, 26683, 26709, 26758, 26821, 26842, 26920, 26952, 26968.

The reference kernel produced by the above reserved carrier locations in an ATSC 3.0 system is compared against the reference kernel produced by the known reserved subcarrier pattern in a DVB-T2 system in Figures 12 and 13. Figures 12a and 12b show the DVB-T2 reference kernel over a longer and a shorter time period respectively, and Figures 13a and 13b show the ATSC 3.0 reference kernel, which is in formed from the reserved subcarrier pattern that is in accordance with the present technique, over a longer and shorter time period respectively. As one can see the two reference kernels have similar primary to secondary peak power ratios. In comparison to DVB-T2, the combination of the pilot symbol patterns given above and in Figure 6, and the reserved subcarrier patterns given above for proposed ATSC 3.0 present an equal set of subcarriers which do not coincide with either of scattered and continual pilots and achieve equivalent performance. Figures 14 to 16 provide plots comparing the complementary cumulative distribution function (CCDF) performance of the proposed reserved subcarrier patterns given above in an envisaged ATSC 3.0 system against the existing DVB-T2 reserved subcarrier patterns where the parameters of the system have been matched as far as is possible. The ATSC 3.0 system is operating with the proposed reserved carrier pattern in bandwidth extension mode, has a modulation scheme of 16QAM, a guard interval fraction of 96/512 and a scattered pilot pattern of P(4,4). The DVB-T2 system is operating with its conventional reserved carrier patterns and is in bandwidth extension mode, has a modulation scheme of 16QAM, a guard interval fraction of ¾, and pilot pattern PP1. The plots illustrate the probability that the PAPR of a signal will exceed a certain level, for instance, in Figure 14 an OFDM system in 8k mode is illustrated and it can be seen that there is approximately 1/1000 probability that the PAPR of the peak cancelled signal will exceed 11.5dB. In each plot the performance of a DVB-T2 system with and without peak cancellation is given along with the proposed 6MHz ATSC 3.0 system with and without peak cancellation with the proposed reserved subcarrier patterns. In all of Figures 14 to 16 it can be seen that the peak cancellation, whether in a DVB-T2 or ATSC 3.0 system, significantly reduces the probability the PAPR of the OFDM signal exceeding a threshold by as much as a factor 100. In all of Figures 14 to 16 it can also be seen that, the performance of the system without peak cancellation are equivalent and the capacity loss for each system is equal. As will be appreciated from the above explanation, as a result of the presence of the scattered pilot symbols which are staggered throughout the OFDM symbol and change from OFDM symbol to OFDM symbol, the reserved tone sub-carriers may also be shifted from OFDM symbol to OFDM symbol. The OFDM symbol-by-symbol shift is required to avoid coincidence with the staggered scattered pilot positions. For the proposed ATSC3.0, the tone reservation algorithm in one example is arranged to calculate the shift which may be different from symbol to symbol in accordance with the scattered pilot pattern selected. In the proposed system, the shift is difference from DVB-T2 because the pilot patterns are different.

Various example aspects and features of the present technique are identified in the following numbered clauses:

1. A transmitter for transmitting signals representing data using Orthogonal Frequency

Division Multiplexing (OFDM) symbols, the transmitter comprising:

a data source configured to provide payload data,

a reserved sub-carrier data source configured to provide reserved sub-carrier data, an OFDM modulator configured to modulate a predetermined number of subcarriers of each OFDM symbol with the payload data and the reserved sub-carrier data to form frequency domain OFDM symbols, the payload data being modulated onto data sub-carriers and the reserved sub- carrier data being modulated onto reserved sub-carriers,

an inverse Fourier transformer configured to transform the frequency domain OFDM symbols into a time domain OFDM symbol,

a peak-to-average-power-ratio (PAPR) reduction circuit configured to reduce a peak-to- average-power ratio of the time domain OFDM symbols to below a predetermined threshold by detecting a peak in the power of the time domain OFDM symbol exceeding the predetermined threshold and combining a kernel signal with the time-domain OFDM symbol to reduce the amplitude of the peak to form a PAPR processed time domain OFDM symbol, wherein the kernel signal is formed from data on the reserved sub-carriers of the OFDM symbol and the reserved sub-carriers are distributed across the OFDM sub-carriers in accordance with a predetermined pattern, the predetermined pattern being dependent upon the predetermined number of sub-carriers of the OFDM symbols.

2. A transmitter according to clause 1, wherein the transmitter comprises a digital to analogue converter configured to convert the PAP processed time domain OFDM symbol into an analogue time domain OFDM signal, and the sub-carrier indices of the predetermined pattern are determined in dependence on a ratio between an amplitude of a second largest peak to a largest peak in the kernel signal formed from data on the reserved sub-carriers, the determination of the reserved sub-carrier pattern indices having been performed at a sampling rate higher than the sampling rate of the OFDM signal.

3. A transmitter according to clause 2, wherein a sampling rate at which the sub-carrier indices of the reserved sub-carrier pattern is determined is a factor of four higher than a sampling rate of the OFDM signal.

4. A transmitter according to any of clauses 2 or 3, wherein the transmitter comprises a pilot data source and the modulator is configured to modulate pilot data from the pilot data source onto pilot-subcarriers of the OFDM signal, and the reserved sub-carrier indices do not coincide with the pilot sub-carrier indices.

5. A transmitter according to any of clauses 1 to 5, wherein the number of OFDM sub- carriers is approximately 8k and the sub-carrier indices of the sub-carriers of the predetermined reserved sub-carrier pattern are 131, 263, 267, 611, 639, 696, 782, 811, 829, 860, 861, 930, 982, 1029, 1380, 1476, 1571, 1589, 1594, 1609, 1793, 1836, 2131, 2215, 2538, 2540, 2717, 2966, 2975, 2995, 3058, 3171, 3399, 3483, 3666, 3698, 3786, 4004, 4015, 4057, 4181, 4241, 4292, 4326, 4356, 4516, 4772, 4929, 5027, 5069, 5214, 5234, 5306, 5307, 5310, 5400, 5494, 5500, 5602, 5685, 5717, 5858, 5911, 6133, 6139, 6213, 6217, 6241, 6266, 6341, 6686, 6708.

6. A transmitter according to any of clauses 1 to 5, wherein the number of OFDM sub- carriers is approximately 16k and the sub-carrier indices of the sub-carriers of the predetermined reserved sub-carrier pattern are 140, 152, 180, 271, 277, 282, 285, 316, 406, 494, 794, 895, 1021, 1060, 1085, 1203, 1318, 1324, 1414, 1422, 1597, 1674, 1890, 1907, 1931, 2004, 2125, 2348, 2356, 2834, 2854, 2870, 2913, 2917, 3017, 3183, 3225, 3311, 3491, 3549, 3566, 3715, 3716, 3949, 3987, 4054, 4127, 4147, 4422, 4534, 4585, 4597, 4687, 4759,4879, 4882, 5015, 5097, 5114, 5412, 5527, 5571, 5639, 5661, 5666, 5737, 5795, 5997, 6014, 6029, 6084, 6108,6122, 6170, 6251, 6500, 6609, 6620, 6721, 6774, 6842, 7162, 7247, 7503, 7770, 7775, 7890, 7932, 7985, 8043, 8089, 8092, 8104, 8156, 8233, 8241, 8329, 8362, 8371, 8518, 8603, 9022, 9090, 9190, 9400, 9453, 9544, 9598, 9769, 10022, 10043, 10150, 10347, 10568, 10849, 10895, 11313, 11394, 11401, 11574, 11583, 11726, 11822, 12120, 12244, 12276, 12438, 12451, 12540, 12574, 12654, 12671, 12806, 12898, 12899, 12950, 13021, 13107, 13153, 13293, 13303, 13430, 13433, 13494.

7. A transmitter according to any of clauses 1 to 5, wherein the number of OFDM sub- carriers is approximately 32k and the sub-carrier indices of the sub-carriers of the predetermined reserved sub-carrier pattern are 275, 303, 526, 537, 584, 648, 722, 779, 794, 859, 867, 1182, 1238, 1251, 1263, 1336, 1389, 1399, 1420, 1471, 1498, 1541, 1587, 1628, 1719, 1822, 2055, 2068, 2181, 2242, 2248, 2346, 2350, 2359, 2502, 2525, 2834, 2889, 2937, 3018, 3091, 3124, 3160, 3480, 3489, 3512, 3707, 3718, 3734, 3861, 4079, 4255, 4371, 4484, 4499, 4550, 4588, 4850, 4854, 4935, 4946, 5084, 5110, 5127, 5169, 5178, 5345, 5480, 5623, 5649, 5805, 6236, 6265, 6346, 6397, 6447, 6482, 6500, 6644, 6735, 6796, 6934, 6956, 7113, 7146, 7186, 7194, 7215, 7226, 7266, 7507, 7736, 7757, 7942, 8056, 8471, 8516, 8567, 8583, 8613, 8868, 8883, 9000, 9005, 9134, 9163, 9242, 9310, 9348, 9358, 9378, 9586, 9682, 9697, 9732, 9825, 9883, 10037, 10792, 10830, 10872, 10879, 10892, 10894, 11007, 11030, 11103, 11129, 11186, 11461, 11466, 11585, 11912, 11937, 11970, 12035, 12185, 12392, 12406, 12424, 12529, 12666, 12842, 12982, 13004, 13115, 13182, 13226, 13356, 13407, 13554, 13681, 13743, 14114, 14119, 14247, 14369, 14458, 14468, 14629, 14655, 14694, 14724, 14796, 14815, 15084, 15143, 15249, 15420, 15530, 15655, 15666, 15703, 15844, 15882, 15970, 16019, 16065, 16215, 16420, 16695, 16705, 16707, 16764, 16818, 16855, 16913, 16925, 16958, 16965, 17048, 17233, 17441, 17624, 17729, 18157, 18181, 18193, 18433, 18451, 18506, 18665, 18858, 18988, 19085, 19098, 19286, 19374, 19659, 19707, 19717, 19878, 19890, 19979, 19983, 20008, 20241, 20295, 20527, 20801, 20943, 21015, 21067, 21163, 21175, 21191, 21201, 21293, 21337, 21608, 21656, 21674, 21755, 21758, 22145, 22380, 22788, 22847, 22856, 22878, 22889, 22924, 23069, 23099, 23103, 23151, 23897, 23997, 24033, 24039, 24063, 24127, 24163, 24217, 24302, 24386, 24434, 24854, 24910, 24946, 25017, 25199, 25256, 25455, 25513, 25555, 25565, 25772, 25778, 25782, 25889, 25943, 26034, 26161, 26167, 26289, 26409, 26492, 26498, 26620, 26683, 26709, 26758, 26821, 26842, 26920, 26952, 26968.

8. A method of transmitting signals representing data using Orthogonal Frequency Division Multiplexing (OFDM) symbols, the method comprising:

receiving payload data for transmission,

providing reserved sub-carrier data,

modulating a predetermined number of subcarriers of each OFDM symbol with the payload data and the reserved sub-carrier data to form frequency domain OFDM symbols, the payload data being modulated onto data sub-carriers and the reserved sub-carrier data being modulated onto reserved sub-carriers,

performing an inverse Fourier transform to transform the frequency domain OFDM symbols into a time domain OFDM symbol,

reducing a peak-to-average-power-ratio (PAPR) of the time domain OFDM symbols to below a predetermined threshold by detecting a peak in the power of the time domain OFDM symbol exceeding the predetermined threshold and combining a kernel signal with the time-domain OFDM symbol to reduce the amplitude of the peak to form a PAPR processed time domain OFDM symbol, wherein

the kernel signal is formed from data on the reserved sub-carriers of the OFDM symbol and the reserved sub-carriers are distributed across the OFDM sub-carriers in accordance with a predetermined pattern, the predetermined pattern being dependent upon the predetermined number of sub-carriers of the OFDM symbols.

9. A receiver for detecting and recovering data from signals representing the data which has been modulated on to Orthogonal Frequency Division Multiplexing (OFDM) symbols with a predetermined number of OFDM subcarriers, the data including payload data and control data modulated onto data subcarriers and reserved subcarrier data modulated onto reserved subcarriers, the receiver comprising:

a Fourier transformer configured to transform a received digital time domain OFDM symbol into a frequency domain OFDM symbol,

an OFDM demodulator configured to demodulate the data from the OFDM subcarriers of the received frequency domain OFDM symbol, and

a controller configured to control the demodulator in accordance with control data demodulated from the received frequency domain OFDM symbols, wherein the control data includes an indicator indicating the presence of reserved subcarrier data on the reserved subcarriers, and controller is configured, in response to the indicator indicating that reserved subcarrier data is present on the reserved subcarriers,

to control the demodulator not to demodulate the data on the subcarriers of the received OFDM symbol that are reserved subcarriers, the reserved subcarriers being distributed across the subcarriers of the received OFDM symbol in accordance with a predetermined pattern, the predetermined pattern being stored in a memory for access by the controller and dependent on the predetermined number of subcarriers of the received OFDM symbol.

10. A receiver according to clause 9, wherein the number of OFDM sub-carriers is approximately 8k and the sub-carrier indices of the sub-carriers of the predetermined reserved sub-carrier pattern are 131, 263, 267, 611, 639, 696, 782, 811, 829, 860, 861, 930, 982, 1029, 1380, 1476, 1571, 1589, 1594, 1609, 1793, 1836, 2131, 2215, 2538, 2540, 2717, 2966, 2975, 2995, 3058, 3171, 3399, 3483, 3666, 3698, 3786, 4004, 4015, 4057, 4181, 4241, 4292, 4326, 4356, 4516, 4772, 4929, 5027, 5069, 5214, 5234, 5306, 5307, 5310, 5400, 5494, 5500, 5602, 5685, 5717, 5858, 5911, 6133, 6139, 6213, 6217, 6241, 6266, 6341, 6686, 6708.

11. A receiver according to clauses 9 or 10, wherein the number of OFDM sub-carriers is approximately 16k and the sub-carrier indices of the sub-carriers of the predetermined reserved sub-carrier pattern are 140, 152, 180, 271, 277, 282, 285, 316, 406, 494, 794, 895, 1021, 1060, 1085, 1203, 1318, 1324, 1414, 1422, 1597, 1674, 1890, 1907, 1931, 2004, 2125, 2348, 2356, 2834, 2854, 2870, 2913, 2917, 3017, 3183, 3225, 3311, 3491, 3549, 3566, 3715, 3716, 3949, 3987, 4054, 4127, 4147, 4422, 4534, 4585, 4597, 4687, 4759,4879, 4882, 5015, 5097, 5114, 5412, 5527, 5571, 5639, 5661, 5666, 5737, 5795, 5997, 6014, 6029, 6084, 6108,6122, 6170, 6251, 6500, 6609, 6620, 6721, 6774, 6842, 7162, 7247, 7503, 7770, 7775, 7890, 7932, 7985, 8043, 8089, 8092, 8104, 8156, 8233, 8241, 8329, 8362, 8371, 8518, 8603, 9022, 9090, 9190, 9400, 9453, 9544, 9598, 9769, 10022, 10043, 10150, 10347, 10568, 10849, 10895, 11313, 11394, 11401, 11574, 11583, 11726, 11822, 12120, 12244, 12276, 12438, 12451, 12540, 12574, 12654, 12671, 12806, 12898, 12899, 12950, 13021, 13107, 13153, 13293, 13303, 13430, 13433, 13494.

12. A receiver according to any of clauses 9 to 11, wherein the number of OFDM sub- carriers is approximately 32k and the sub-carrier indices of the sub-carriers of the predetermined reserved sub-carrier pattern are 275, 303, 526, 537, 584, 648, 722, 779, 794, 859, 867, 1182, 1238, 1251, 1263, 1336, 1389, 1399, 1420, 1471, 1498, 1541, 1587, 1628, 1719, 1822, 2055, 2068, 2181, 2242, 2248, 2346, 2350, 2359, 2502, 2525, 2834, 2889, 2937, 3018, 3091, 3124, 3160, 3480, 3489, 3512, 3707, 3718, 3734, 3861, 4079, 4255, 4371, 4484, 4499, 4550, 4588, 4850, 4854, 4935, 4946, 5084, 5110, 5127, 5169, 5178, 5345, 5480, 5623, 5649, 5805, 6236, 6265, 6346, 6397, 6447, 6482, 6500, 6644, 6735, 6796, 6934, 6956, 7113, 7146, 7186, 7194, 7215, 7226, 7266, 7507, 7736, 7757, 7942, 8056, 8471, 8516, 8567, 8583, 8613, 8868, 8883, 9000, 9005, 9134, 9163, 9242, 9310, 9348, 9358, 9378, 9586, 9682, 9697, 9732, 9825, 9883, 10037, 10792, 10830, 10872, 10879, 10892, 10894, 11007, 11030, 11103, 11129, 11186, 11461, 11466, 11585, 11912, 11937, 11970, 12035, 12185, 12392, 12406, 12424, 12529, 12666, 12842, 12982, 13004, 13115, 13182, 13226, 13356, 13407, 13554, 13681, 13743, 14114, 14119, 14247, 14369, 14458, 14468, 14629, 14655, 14694, 14724, 14796, 14815, 15084, 15143, 15249, 15420, 15530, 15655, 15666, 15703, 15844, 15882, 15970, 16019, 16065, 16215, 16420, 16695, 16705, 16707, 16764, 16818, 16855, 16913, 16925, 16958, 16965, 17048, 17233, 17441, 17624, 17729, 18157, 18181, 18193, 18433, 18451, 18506, 18665, 18858, 18988, 19085, 19098, 19286, 19374, 19659, 19707, 19717, 19878, 19890, 19979, 19983, 20008, 20241, 20295, 20527, 20801, 20943, 21015, 21067, 21163, 21175, 21191, 21201, 21293, 21337, 21608, 21656, 21674, 21755, 21758, 22145, 22380, 22788, 22847, 22856, 22878, 22889, 22924, 23069, 23099, 23103, 23151, 23897, 23997, 24033, 24039, 24063, 24127, 24163, 24217, 24302, 24386, 24434, 24854, 24910, 24946, 25017, 25199, 25256, 25455, 25513, 25555, 25565, 25772, 25778, 25782, 25889, 25943, 26034, 26161, 26167, 26289, 26409, 26492, 26498, 26620, 26683, 26709, 26758, 26821, 26842, 25920, 26952, 26968.

13. A method of detecting and recovering data from signals representing the data which has been modulated on to Orthogonal Frequency Division Multiplexing (OFDM) symbols with a predetermined number of OFDM subcarriers, the data including payload data and control data modulated onto data subcarriers and reserved subcarrier data modulated onto reserved subcarriers, the method comprising:

performing a Fourier transform on signals representing a received digital time domain OFDM symbol to form a frequency domain OFDM symbol,

demodulating the data from the OFDM subcarriers of the received frequency domain OFDM symbol, and

controlling the demodulating in accordance with control data demodulated from the received frequency domain OFDM symbol, wherein

the control data includes an indicator indicating the presence of reserved subcarrier data on the reserved subcarriers, and the controlling the demodulating includes

in response to the indicator indicating that reserved subcarrier data is present on the reserved subcarriers, controlling the demodulating so as not to demodulate the data on the subcarriers of the received OFDM symbol that are reserved subcarriers, the reserved subcarriers being distributed across the subcarriers of the received OFDM symbol in accordance with a predetermined pattern, the predetermined pattern being stored in a memory and dependent on the predetermined number of subcarriers of the received OFDM symbol.

Various modifications to the example embodiments described above can be made. For example whilst the embodiments have been described with reference to the DVB-T2 and ATSC 3.0 standards it will be appreciated that embodiments of the present technique can find application with any OFDM system or communications standard.

Claims

1. A transmitter for transmitting signals representing data using Orthogonal Frequency Division Multiplexing (OFDM) symbols, the transmitter comprising:

a data source configured to provide payload data,

a peak-to-average-power-ratio (PAP ) reduction circuit configured to reduce a peak-to- average-power ratio of the time domain OFDM symbols to below a predetermined threshold by detecting a peak in the power of the time domain OFDM symbol exceeding the predetermined threshold and combining a kernel signal with the time-domain OFDM symbol to reduce the amplitude of the peak to form a PAPR processed time domain OFDM symbol, wherein

2. A transmitter as claimed in Claim 1, wherein the transmitter comprises a digital to analogue converter configured to convert the PAPR processed time domain OFDM symbol into an analogue time domain OFDM signal, and the sub-carrier indices of the predetermined pattern are determined in dependence on a ratio between an amplitude of a second largest peak to a largest peak in the kernel signal formed from data on the reserved sub-carriers, the determination of the reserved sub- carrier pattern indices having been performed at a sampling rate higher than the sampling rate of the OFDM signal.

3. A transmitter as claimed in Claim 2, wherein a sampling rate at which the sub-carrier indices of the reserved sub-carrier pattern is determined is a factor of four higher than a sampling rate of the OFDM signal.

4. A transmitter as claimed in Claim 2, wherein the transmitter comprises a pilot data source and the modulator is configured to modulate pilot data from the pilot data source onto pilot- subcarriers of the OFDM signal, and the reserved sub-carrier indices do not coincide with the pilot sub-carrier indices.

5. A transmitter as claimed in Claim 1, wherein the number of OFDM sub-carriers is approximately 8k and the sub-carrier indices of the sub-carriers of the predetermined reserved sub- carrier pattern are 131, 263, 267, 611, 639, 696, 782, 811, 829, 860, 861, 930, 982, 1029, 1380, 1476, 1571, 1589, 1594, 1609, 1793, 1836, 2131, 2215, 2538, 2540, 2717, 2966, 2975, 2995, 3058, 3171, 3399, 3483, 3666, 3698, 3786, 4004, 4015, 4057, 4181, 4241, 4292, 4326, 4356, 4516, 4772, 4929, 5027, 5069, 5214, 5234, 5306, 5307, 5310, 5400, 5494, 5500, 5602, 5685, 5717, 5858, 5911, 6133, 6139, 6213, 6217, 6241, 6266, 6341, 6686, 6708.

6. A transmitter as claimed in Claim 1, wherein the number of OFDM sub-carriers is approximately 16k and the sub-carrier indices of the sub-carriers of the predetermined reserved sub-carrier pattern are 140, 152, 180, 271, 277, 282, 285, 316, 406, 494, 794, 895, 1021, 1060, 1085, 1203, 1318, 1324, 1414, 1422, 1597, 1674, 1890, 1907, 1931, 2004, 2125, 2348, 2356, 2834, 2854, 2870, 2913, 2917, 3017, 3183, 3225, 3311, 3491, 3549, 3566, 3715, 3716, 3949, 3987, 4054, 4127, 4147, 4422, 4534, 4585, 4597, 4687, 4759,4879, 4882, 5015, 5097, 5114, 5412, 5527, 5571, 5639, 5661, 5666, 5737, 5795, 5997, 6014, 6029, 6084, 6108,6122, 6170, 6251, 6500, 6609, 6620, 6721, 6774, 6842, 7162, 7247, 7503, 7770, 7775, 7890, 7932, 7985, 8043, 8089, 8092, 8104, 8156, 8233, 8241, 8329, 8362, 8371, 8518, 8603, 9022, 9090, 9190, 9400, 9453, 9544, 9598, 9769, 10022, 10043, 10150, 10347, 10568, 10849, 10895, 11313, 11394, 11401, 11574, 11583, 11726, 11822, 12120, 12244, 12276, 12438, 12451, 12540, 12574, 12654, 12671, 12806, 12898, 12899, 12950, 13021, 13107, 13153, 13293, 13303, 13430, 13433, 13494.

7. A transmitter as claimed in Claim 1, wherein the number of OFDM sub-carriers is approximately 32k and the sub-carrier indices of the sub-carriers of the predetermined reserved sub-carrier pattern are 275, 303, 526, 537, 584, 648, 722, 779, 794, 859, 867, 1182, 1238, 1251,

1263, 1336, 1389, 1399, 1420, 1471, 1498, 1541, 1587, 1628, 1719, 1822, 2055, 2068, 2181, 2242, 2248, 2346, 2350, 2359, 2502, 2525, 2834, 2889, 2937, 3018, 3091, 3124, 3160, 3480, 3489, 3512, 3707, 3718, 3734, 3861, 4079, 4255, 4371, 4484, 4499, 4550, 4588, 4850, 4854, 4935, 4946, 5084, 5110, 5127, 5169, 5178, 5345, 5480, 5623, 5649, 5805, 6236, 6265, 6346, 6397, 6447, 6482, 6500, 6644, 6735, 6796, 6934, 6956, 7113, 7146, 7186, 7194, 7215, 7226, 7266, 7507, 7736, 7757, 7942, 8056, 8471, 8516, 8567, 8583, 8613, 8868, 8883, 9000, 9005, 9134, 9163, 9242, 9310, 9348, 9358, 9378, 9586, 9682, 9697, 9732, 9825, 9883, 10037, 10792, 10830, 10872, 10879, 10892, 10894, 11007, 11030, 11103, 11129, 11186, 11461, 11466, 11585, 11912, 11937, 11970, 12035, 12185, 12392, 12406, 12424, 12529, 12666, 12842, 12982, 13004, 13115, 13182, 13226, 13356, 13407, 13554, 13681, 13743, 14114, 14119, 14247, 14369, 14458, 14468, 14629, 14655, 14694, 14724, 14796, 14815, 15084, 15143, 15249, 15420, 15530, 15655, 15666, 15703, 15844, 15882, 15970, 16019, 16065, 16215, 16420, 16695, 16705, 16707, 16764, 16818, 16855, 16913, 16925, 16958, 16965, 17048, 17233, 17441, 17624, 17729, 18157, 18181, 18193, 18433, 18451, 18506, 18665, 18858, 18988, 19085, 19098, 19286, 19374, 19659, 19707, 19717, 19878, 19890, 19979, 19983, 20008, 20241, 20295, 20527, 20801, 20943, 21015, 21067, 21163, 21175, 21191, 21201, 21293, 21337, 21608, 21656, 21674, 21755, 21758, 22145, 22380, 22788, 22847, 22856, 22878, 22889, 22924, 23069, 23099, 23103, 23151, 23897, 23997, 24033, 24039, 24063, 24127, 24163, 24217, 24302, 24386, 24434, 24854, 24910, 24946, 25017, 25199, 25256, 25455, 25513, 25555, 25565, 25772, 25778, 25782, 25889, 25943, 26034, 26161, 26167, 26289, 26409, 26492, 26498, 26620, 26683, 26709, 26758, 26821, 26842, 26920, 26952, 26968.

receiving payload data for transmission,

providing reserved sub-carrier data. modulating a predetermined number of subcarriers of each OFDM symbol with the payload data and the reserved sub-carrier data to form frequency domain OFDM symbols, the payload data being modulated onto data sub-carriers and the reserved sub-carrier data being modulated onto reserved sub-carriers,

reducing a peak-to-average-power-ratio (PAP ) of the time domain OFDM symbols to below a predetermined threshold by detecting a peak in the power of the time domain OFDM symbol exceeding the predetermined threshold and combining a kernel signal with the time-domain OFDM symbol to reduce the amplitude of the peak to form a PAPR processed time domain OFDM symbol, wherein

a controller configured to control the demodulator in accordance with control data demodulated from the received frequency domain OFDM symbols, wherein

the control data includes an indicator indicating the presence of reserved subcarrier data on the reserved subcarriers, and controller is configured, in response to the indicator indicating that reserved subcarrier data is present on the reserved subcarriers,

10. A receiver as claimed in Claim 9, wherein the number of OFDM sub-carriers is approximately 8k and the sub-carrier indices of the sub-carriers of the predetermined reserved sub-carrier pattern are 131, 263, 267, 611, 639, 696, 782, 811, 829, 860, 861, 930, 982, 1029, 1380, 1476, 1571, 1589, 1594, 1609, 1793, 1836, 2131, 2215, 2538, 2540, 2717, 2966, 2975, 2995, 3058, 3171, 3399, 3483, 3666, 3698, 3786, 4004, 4015, 4057, 4181, 4241, 4292, 4326, 4356, 4516, 4772, 4929, 5027, 5069, 5214, 5234, 5306, 5307, 5310, 5400, 5494, 5500, 5602, 5685, 5717, 5858, 5911, 6133, 6139, 6213, 6217, 6241, 6266, 6341, 6686, 6708.

11. A receiver as claimed in Claim 9, wherein the number of OFDM sub-carriers is approximately 16k and the sub-carrier indices of the sub-carriers of the predetermined reserved sub-carrier pattern are 140, 152, 180, 271, 277, 282, 285, 316, 406, 494, 794, 895, 1021, 1060, 1085, 1203, 1318, 1324, 1414, 1422, 1597, 1674, 1890, 1907, 1931, 2004, 2125, 2348, 2356, 2834, 2854, 2870, 2913, 2917, 3017, 3183, 3225, 3311, 3491, 3549, 3566, 3715, 3716, 3949, 3987, 4054, 4127, 4147, 4422, 4534, 4585, 4597, 4687, 4759,4879, 4882, 5015, 5097, 5114, 5412, 5527, 5571, 5639, 5661, 5666, 5737, 5795, 5997, 6014, 6029, 6084, 6108,6122, 6170, 6251, 6500, 6609, 6620, 6721, 6774, 6842, 7162, 7247, 7503, 7770, 7775, 7890, 7932, 7985, 8043, 8089, 8092, 8104, 8156, 8233, 8241, 8329, 8362, 8371, 8518, 8603, 9022, 9090, 9190, 9400, 9453, 9544, 9598, 9769, 10022, 10043, 10150, 10347, 10568, 10849, 10895, 11313, 11394, 11401, 11574, 11583, 11726, 11822, 12120, 12244, 12276, 12438, 12451, 12540, 12574, 12654, 12671, 12806, 12898, 12899, 12950, 13021, 13107, 13153, 13293, 13303, 13430, 13433, 13494.

12. A receiver as claimed in Claim 9, wherein the number of OFDM sub-carriers is approximately 32k and the sub-carrier indices of the sub-carriers of the predetermined reserved sub-carrier pattern are 275, 303, 526, 537, 584, 648, 722, 779, 794, 859, 867, 1182, 1238, 1251, 1263, 1336, 1389, 1399, 1420, 1471, 1498, 1541, 1587, 1628, 1719, 1822, 2055, 2068, 2181, 2242, 2248, 2346, 2350, 2359, 2502, 2525, 2834, 2889, 2937, 3018, 3091, 3124, 3160, 3480, 3489, 3512, 3707, 3718, 3734, 3861, 4079, 4255, 4371, 4484, 4499, 4550, 4588, 4850, 4854, 4935, 4946, 5084, 5110, 5127, 5169, 5178, 5345, 5480, 5623, 5649, 5805, 6236, 6265, 6346, 6397, 6447, 6482, 6500, 6644, 6735, 6796, 6934, 6956, 7113, 7146, 7186, 7194, 7215, 7226, 7266, 7507, 7736, 7757, 7942, 8056, 8471, 8516, 8567, 8583, 8613, 8868, 8883, 9000, 9005, 9134, 9163, 9242, 9310, 9348, 9358, 9378, 9586, 9682, 9697, 9732, 9825, 9883, 10037, 10792, 10830, 10872, 10879, 10892, 10894, 11007, 11030, 11103, 11129, 11186, 11461, 11466, 11585, 11912, 11937, 11970, 12035, 12185, 12392, 12406, 12424, 12529, 12666, 12842, 12982, 13004, 13115, 13182, 13226, 13356, 13407, 13554, 13681, 13743, 14114, 14119, 14247, 14369, 14458, 14468, 14629, 14655, 14694, 14724, 14796, 14815, 15084, 15143, 15249, 15420, 15530, 15655, 15666, 15703, 15844, 15882, 15970, 16019, 16065, 16215, 16420, 16695, 16705, 16707, 16764, 16818, 16855, 16913, 16925, 16958, 16965, 17048, 17233, 17441, 17624, 17729, 18157, 18181, 18193, 18433, 18451, 18506, 18665, 18858, 18988, 19085, 19098, 19286, 19374, 19659, 19707, 19717, 19878, 19890, 19979, 19983, 20008, 20241, 20295, 20527, 20801, 20943, 21015, 21067, 21163, 21175, 21191, 21201, 21293, 21337, 21608, 21656, 21674, 21755, 21758, 22145, 22380, 22788, 22847, 22856, 22878, 22889, 22924, 23069, 23099, 23103, 23151, 23897, 23997, 24033, 24039, 24063, 24127, 24163, 24217, 24302, 24386, 24434, 24854, 24910, 24946, 25017, 25199, 25256, 25455, 25513, 25555, 25565, 25772, 25778, 25782, 25889, 25943, 26034, 26161, 26167, 26289, 26409, 26492, 26498, 26620, 26683, 26709, 26758, 26821, 26842, 26920, 26952, 26968.

performing a Fourier transform on signals representing a received digital time domain OFDM symbol to form a frequency domain OFDM symbol, demodulating the data from the OFDM subcarriers of the received frequency domain OFDM symbol, and

14. A transmitter substantially as hereinbefore described with reference to the drawings.

15. A receiver substantially as hereinbefore described with reference to the drawings.