FIELD OF THE INVENTION

This invention generally relates to an amplification apparatus for a wireless broadcast transmitter, and more particularly to a wireless broadcast transmitter having such an amplification apparatus, a broadcast network having such a transmitter, and a method of adding at least one service to an existing broadcast network.

BACKGROUND TO THE INVENTION

The governments of many nations have stated a desire to switch off existing analogue TV and FM radio transmissions as soon as digital radio service is available to 95-97% of the population. These governments are actively encouraging replacement of analogue TV and FM with digital devices, setting targets for analogue equivalent digital coverage and ensuring all new cars are fitted with digital radio equipment.

In parallel many existing analogue TV and FM transmission systems are now reaching the end of their 30 year service life. The network operators are faced with a dilemma: to replace analogue services with new equipment or to expand the digital network and actively transition users over to digital services.

Achieving above 90% geographical coverage, including provision of national and local services and involving sharing of spectrum between digital radio operators, is difficult. This is because digital radio is built using old analogue methodologies involving big sites, high power and large channel specific filters. A traditional architecture for a site transmitting multiple services or channels (1 1C, 1 1 D, 12B, 12D) is illustrated in Figure 1. Here the individual amplifier, filter and analogue combiner elements are used to create a composite output containing all the site transmission signals needed for the services. The filter and combiner for example may be physically very large and/or expensive, and may be expensive to move or replace, e.g., building extension works may be required.

In addition, high power gives greater site coverage, but also leads to significant cross system interference - the receiver blocking issue. This is illustrated in Figure 3 - here the addition of a new high power service creates high signal levels that prevent receivers near the site from receiving a much weaker distant signal. Furthermore, the use of high rejection channel filtering also places significant constraints on site design: such filter arrangements are often physically large, and require individual amplification of each carrier and expensive analogue combining elements. In addition this architecture requires dedicated planning and mechanical installation to expand site services.

Generally, to achieve dense and/or high coverage, broadcast network operators desire more modern solutions for the overall network and/or particular network elements such as transmitters. Desired advantages may include, e.g., scalable, flexible spectrum use, low interference impact, power efficiency, small site footprint and/or low site cost, etc..

SUMMARY

According to a first aspect of the present invention, there is provide an amplification apparatus for a wireless broadcast transmitter, comprising: a digital modulation stage to receive data streams and to output baseband signals each modulated according to a respective said data stream, each said baseband signal comprising a control component such that the baseband signals are at least partially correlated, each said data stream further comprising error correcting code for forward error correction; a mixing stage to shift each said baseband signal to a respective carrier frequency to generate a respective carrier signal; a signal combiner to combine the carrier signals to output a combined signal; a peak reduction stage to reduce peak-to-average power ratio of the combined signal; and an amplifier to amplify the peak reduced combined signal from the peak reduction stage.

In a preferred embodiment, such an apparatus is provided in a transmitter for over- the-air broadcasting from an antenna - for example, for broadcasting RF-converted OFDM signals. The transmitter may have a filter to limit the frequency spectrum to be broadcast by the antenna. Such a filter may alone (rather than in parallel with other filters) filter the peak reduced combined signal, thus effectively filtering all of the carriers. The filter may feed the antenna input, preferably through an impedance matching network.

In this regard, and taking into account that significant power peaks may be found in a multi-carrier signal formed from correlated signals, an embodiment of the apparatus may nevertheless allow cost- and/or power-efficient multi-carrier amplification in a broadcast transmitter. The peak reduction may allow efficient amplification, while the use of error correcting codes such as Viterbi, Reed Solomon or Turbo coding may nevertheless allow the transmitted datastreams to be successfully received at a receiver. This may be achieved even where the control components, e.g., pilot and/or sychronisation sequences, of the baseband signals are identical, i.e., give rise to a high degree of correlation.

Given the above, multiple amplifiers in parallel may not necessary in order to provide multi-carrier transmission. The specified amplifier may be the sole amplifier of any particular amplification stage following the carrier signal combination. This may be of considerable advantage for the realisation of broadcast networks, for example for allowing dense and/or high coverage, scalability, power efficiency, small site footprint and/or low site cost, etc..

There may further be provided the amplification apparatus, having at least one decorrelation stage configured to apply at least one decorrelation transform to signals prior to the combining, to reduce correlation of the signals, wherein the signals comprise at least one of: the baseband signals; and the carrier signals.

The decorrelation transform may at least reduce correlation resulting from the control components, prior to the signal combiner. This may again assist with regard to power peaks - noting that correlated components of the baseband signals may increase Peak-to-Average Power Ratio (PAPR) in the combined signal input to the amplifier, due to beating effects of the correlated components in the combiner. Thus, application of one or more decorrelation techniques, preferably directly to the datastreams, the baseband signals and/or the carrier signals prior to the combiner, may be advantageous. The decorrelation transform(s) may selected to reduce the beating effect and/or distortion of a power probability density function of the combined signal.

There may further be provided the amplification apparatus, when a said decorrelation transform applies different timing offsets to respective said signals. Such a transform may be applied to baseband signal(s) and/or carrier signal(s) to time-shift the signals (forwards and/or backwards). For example, the timing of OFDM symbols in the output signal(s) of the modulation stage and/or mixing stage may be adjusted. Preferably, each time-shifted signal at a particular stage of the apparatus prior to the combining shifts each parallel signal by a different amount, the time shifts for example being within a predefined range such as +-2us. The time offsets may be determined locally or by a network operator, core network or Operational Support System (OSS)). Such an amplification apparatus may be configured to select each of the different timing offsets to be less than a time duration limit, wherein at least one said baseband signal modulation according to a said respective data stream comprises OFDM modulation and the time duration limit is a guard interval of the OFDM modulation or a part thereof, e.g., 1/4 or 1/2, of the guard interval.

The amplification apparatus may be configured to select each of the different timing offsets to be less than a predefined time duration limit, wherein the time duration limit is a propagation time of a broadcast signal from the amplification apparatus (or a transmitter having that apparatus) to a nearest receiver for the broadcast signal. The over-the-air propagation time can be determined locally at the amplification apparatus or remotely, based on a physical distance to a nearest receiver site (e.g., 50km) and the known free space propagation speed of radio signals (about 3.00x10 ^s m/s).

The amplification apparatus may be configured to select each of the different timing offsets to be not greater than the inverse of a narrowest signal bandwidth of the carrier signals to be combined. In this regard, the apparatus may comprise circuitry to measure the bandwidth and input this as a parameter to the timing offset selection. Alternatively, the value of the narrowest bandwidth may be a preset parameter.

Preferably, the amplification apparatus (or remote entity such as the network operator, core network and/or OSS) selects the various timing offsets specifically to reduce the correlation, the beating effect and/or distortion of a power probability density function of the combined signal.

There may further be provided the amplification apparatus, wherein each of the signals to have a said decorrelation transform applied to (generally, the baseband or carrier signals the decorrelation transform is applied to) comprises an OFDM signal, wherein a said decorrelation stage is configured to apply different sets of phase offsets to respective said OFDM signals, and wherein the decorrelation stage is configured to apply to each sub-carrier of the respective OFDM signal a different said phase offset of the respective set. Preferably each set is consistently applied over time to the respective OFDM signal. As for the above timing offsets, the phase offsets may only be re-determined (e.g., adjusted) when a new service or site is added to the network.

Such an amplification apparatus may be configured to select the different phase offsets to reduce the correlation, the beating effect and/or distortion of a power probability density function of the combined signal. There may further be provided the amplification apparatus, wherein the signals to have a said decorrelation transform applied to comprise at least one of: a subset of the baseband signals output by the digital modulation stage; and a subset of the carrier signals generated by the mixing stage. In this regard, a subset may mean a predefined proportion, e.g., 1/4, 1/3 or a majority of the relevant signals. The potential use only of a subset recognises that it may not be necessary to decorrelate all of the parallel signals at any particular pre-combiner stage of the apparatus. A subset may be sufficient such that the applied decorrelation transform still sufficiently decorrelates the components of a combined signal.

There may further be provided the amplification apparatus, wherein a plurality of said modulated baseband signals are OFDM signals and each symbol of the OFDM signals comprises: an internal symbol comprising a constant number of samples; a cyclic prefix consisting of K-N samples of one end of the internal symbol, where K is a constant; and a suffix consisting of N samples of the other end of the internal symbol, wherein the apparatus is configured to determine values of N to be applied to all symbols of respective ones of said OFDM signals.

Such an amplification apparatus may have the decorrelation stage configured to determine the values of N to reduce the correlation, the beating effect and/or distortion of a power probability density function of the combined signal.

The amplification apparatus may be implemented wherein the values of N are all different.

The amplification apparatus may be implemented wherein the OFDM signals are a subset of modulated OFDM baseband signals output by the digital modulation stage. Similarly as above, the subset may be, e.g., a majority, or at least determined to be sufficient such that the decorrelation transform application still sufficiently decorrelates.

There may further be provided the amplification apparatus, wherein the modulation comprises orthogonal frequency division multiplexing (OFDM). The modulated baseband signal may then comprise modulated sub-carriers generated by the OFDM. There may further be provided the amplification apparatus, wherein each said control component comprises a pilot signal. Where the modulation is OFDM, each OFDM symbol may have its own pilot signal, and the pilot signal may be an OFDM sub- carrier. Each frame may have symbols comprising pilots at different frequencies, positions and/or phases. More generally, the pilot signal may comprise a continuous tone of certain phase angle.

There may still further be provided the amplification apparatus, wherein each said control component comprises synchronisation information. Such information may comprise a synchronisation data sequence. In an OFDM signal, the synchronisation information may comprise a whole OFDM symbol or a bit sequence forming part of a symbol.

There may further be provided a wireless broadcast transmitter comprising the amplification apparatus, the transmitter configured to broadcast the combined signal amplified by the amplifier of the amplification apparatus. Thus, the output of the amplifier may be fed (indirectly, e.g., via a further mixing stage for RF conversion and/or via a filter) to an antenna.

There may further be provided a broadcast network comprising at least one such transmitter, the network further comprising at least one receiver to demodulate a said broadcast combined signal to derive data, and configured to apply Forward Error Correction based on a said error correcting code to correct the derived data. In such an embodiment, the error correcting code may allow the corresponding datastream to be derived without any errors.

Such a broadcast network may be a digital television broadcasting network configured to operate (at one or more transmission sites having such a transmitter, and at least one corresponding receiver) in accordance with at least one of DVB, ATSC, ISDB, DTMB, DMB, DVB-T, ATSC 2.0 and ISDB-T.

Similarly, such a broadcast network may be a digital audio broadcasting network configured (at one or more transmission sites having such a transmitter, and at least one corresponding receiver) to operate in accordance with at least one of DAB, DAB+ and DMB. The broadcast network may further comprise a controller, wherein: the controller is configured to adjust / determine a plurality of time-to-send values of respective datastreams to be modulated by a said modulation stage of the amplification apparatus of a said transmitter, said adjustment / determination to reduce (preferably remove) alignment of the control components of the modulated datastreams; the controller is configured to send the adjusted time-to-send values to the transmitter; and the amplification apparatus of the transmitter is configured to adjust, in accordance with the sent adjusted time-to-send values, timing of signals having the modulated respective datastreams, wherein the signals are at least one of modulated baseband signals and carrier signals of the amplification apparatus.

The controller may be the network operator, the core network and/or an OSS, and the adjusted values may be sent together with corresponding data streams (raw data) to the transmitter(s) of the network. Preferably, the controller determines the various time-to-send values (alternatively referred to as time-to-air parameters) to achieve greater or complete decorrelation across the network. There may be significant flexibility in adjusting the time-to-send values, e.g., the time shift may be upto 10msec, 0.25sec or 0.5sec.

More generally, a time-to-send value (parameter) may be received at a transmission site in the form of a Global Network Setting. A network operator, preferably by means of an OSS, may ensure that the global settings received at one or more, e.g., all, sites are such that correlation is reduced/avoided. Different time-to-air values may be chosen depending on the relevant modulation parameters, for example taking into account broadcast standard (e.g., DAB), modulation type (e.g., OFDM), frame length and/or pilot structure.

Local adjustment of time-to-air values at the transmitter may additionally or alternatively be implemented to reduce/avoid correlation. Corresponding procedures for those set out for the network operator may be applied locally for adjusting time-to- air values. The values before adjustment are preferably global setting parameters received at the transmitter.

Such a broadcast network may be provided, wherein the modulation to be applied to the respective datastreams by the modulation stage of the transmitter is OFDM, and wherein the adjusted time-to-send values are all different and each differ by one or more OFDM symbol period from any other said adjusted time-to-send value. Such a period is the duration of a single symbol. In such a broadcast network, the controller may be configured to select as the respective datastreams only datastreams that are to be modulated according to one particular modulation method, e.g., OFDM. Thus, the datastreams may be selected from among datastreams to be modulated by different modulation methods.

The broadcast network may be provided, wherein the modulation to be applied to the respective datastreams by the modulation stage is OFDM and the timing adjustment varies timing of OFDM symbols of the datastreams.

The broadcast network may comprise more than one transmission site having a said transmitter, wherein the controller is configured to send adjusted time-to-send values to each said amplification apparatus of the transmitters, the adjustment to reduce alignment of the control components of the modulated datastreams at each said transmission site.

The broadcast network may be implemented such that the controller is configured to determine, e.g., re-adjust, all of the adjusted time-send-parameters when at least one of a site and a service is added to the network. Thus, one set of time-to-send values may be applied to at least one transmitter (preferably all transmitters across the whole network) until a new channel is added.

There may further be provided a method of adding at least one service or channel to an existing broadcast network, wherein an existing, first transmission site of the broadcast network broadcasts at least one first service to at least one receiver, the method comprising: adding a second transmission site comprising a transmitter of claim 17 to broadcast at least one additional service (channel) to a said receiver, the second site closer to the said receiver than the first site, wherein the signal combiner of the amplification apparatus of said transmitter combines carrier signals of the first and additional services and the peak reduction stage of said amplification apparatus applies a peak factor reduction function to the combined signal; and broadcasting at a first power level from the second transmission site frequency components of the peak reduced combined signal that correspond to the at least one first service and broadcasting at a second power level from the second transmission site frequency components of the peak reduced combined signal that correspond to the at least one additional service, wherein the first power level is lower than the second power level.

Thus, the first service/channel may be transmitted from both the existing site and the added site. In this regard, the method may allow addition of new service(s) or channels(s), without causing receiver blocking or reduced quality of receiving / demodulating of the first channel at an existing receiver. Broadcasting of the first service(s) at the added site may be achieved based on listening to the first channel as received from the existing site and generating at the added site a baseband or carrier signal accordingly. Alternatively, a baseband signal corresponding to the first channel may be generated based on raw data received at the new site, e.g., from an OSS.

The existing site may, or may not, use an amplification apparatus as described herein for multi-carrier transmission, to broadcast at least the first channel / service to receiver(s).

Preferably, the method involves the amplification apparatus being controlled to set the first power level at a minimum power level to prevent blocking of the at least one first service at the receiver. The first power level may be at least 16dB, 20dB or 30dB lower than the second power level, and further preferably no more than 35dB lower than the second power level.

The method may involve the peak reduction stage of the amplification apparatus of the second site implementing a peak reduction function that peak reduces frequency components corresponding to the at least one first service less than frequency components corresponding to the at least one second service. The peak reduction function (e.g., maximum signal amplitude(s) of peaks) may be selectable by an input to the peak reduction stage. The function may be selected to result in a distribution of distortion against frequency characteristic that creates lower distortion of components corresponding to the first channel.

The method may be performed such that the peak reduction function of the amplification apparatus of the second site causes less distortion of the at least one first service than of the at least one second service. Thus a clipping algorithm of the added site peak reduction stage may be biased to reduce or minimise distortion of lower power carriers or frequency components. It may similarly be preferable to choose a peak reduction algorithm that creates less noise in the carrier(s) / components at the lower level - taking into account that crest factor reduction (CRF) for example may generate noise.

The method may be performed such that the broadcasting of the at least one first service by the second site does not increase geographical coverage of at least one said first service. BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which:

Fig. 1 shows a known broadcast transmit architecture (wherein the analogue combiner provides the input to an antenna labelled 'x' and services are broadcast on respective channels 1 1C, 11 D, 12B and 12D);

Fig. 2 shows an example multi-carrier broadcast transmitter architecture. Preferably, the filter is a single large filter and thus may filter multiple combined carrier signals from the amplifier. A control input (not shown) may be provided to control (e.g. to select an algorithm for) mathematical combining by the digital combiner;

Fig. 3 illustrates cross-system interference due to receiver blocking;

Fig. 4 illustrates interference mitigation with a multi-carrier DAB (MC-DAB) system;

Fig. 5 illustrates a peak reduction operation with shaped distortion;

Fig. 5a shows Complementary Cumulative Density Function (CCDF) of original and peak reduced signals;

Fig. 6 shows peak reduction applied in an example multi-carrier transmitter. The peak reduction may be provided by a Crest Factor Reduction (CFR) unit or other limiter, and broadly speaking may be configured to reduce amplitude peaks (e.g. cap at a predefined maximum amplitude) of the signal from the combiner. Such peak reduction may result in shaped distortion across the spectrum of the combined signal;

Fig. 7 shows a composite multi-carrier DAB signal with beating effect;

Fig. 7a shows Complementary Cumulative Density Function (CCDF) of the single component, composite and reduced MC signals;

Fig. 8 is a constellation diagram showing an effect of correlated Peak-to-Average Power Ratio (PAPR) reduction on synchronisation carriers (e.g. OFDM sub-carriers). The effect on demodulation from carrier signals having components in common (e.g., pilot and/or synchronisation sequences) compared to the cleaner results of demodulation from uncorrelated carrier signals may thus be understood; Fig. 9 shows an example of decorrelation for peak reduction in a multi-carrier transmitter using an implementation of the Technique 2' disclosed herein. Similarly as above, the peak reduction may be provided by a CFR unit or other limiter;

Fig. 10 is a constellation diagram showing distortion improvement with preconditioning for multi-carrier peak reduction. This may be compared for example to Fig. 8;

Fig. 1 1 shows an example network architecture;

Fig. 12 illustrates an OFDM frame structure;

Fig. 13 illustrates a structure of a DVB modulation frame;

Fig. 14 illustrates multiple DAB services with control signal information alignment;

Fig. 15 illustrates an example of DAB OFDM symbol cyclic prefix generation;

Fig. 15a illustrates cyclic prefix rotation;

Fig. 16 shows a network comprising existing and new sites;

Fig. 17 shows signal levels at a receiver;

Fig. 18 shows multi-carrier amplification with low power service addition for interference mitigation;

Fig. 19 shows a post peak reduction transmitted signal;

Fig. 20 shows a post peak reduction transmitted signal;

Fig. 21 illustrates a network in which any embodiment described herein, e.g., the multi-carrier transmitter or amplification apparatus, may be implemented;

Fig. 22 shows a preferred embodiment of a transmitter for wireless broadcasting. A filter (not shown) may be placed between the amplifier and antenna; and

Fig. 23 illustrates an example peak reduction process.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

One approach to dealing with, e.g., interference, cost and/or coverage when providing / adding broadcast services is the deployment of lower cost, smaller sites that transmit all services using a single amplifier and filter arrangement - this may be provided using multi-carrier transmitter(s). Existing types of multi-carrier transmitter are commonly found in mobile telephone networks where small and medium size cellular transmission sites transmit multiple carriers using a single amplifier and filter for each antenna. In contrast to existing broadcast systems however, mobile telephone systems are well suited to multi- carrier transmission, and the techniques needed to perform such transmission are well understood for application to mobile telephony. Deployment of multi-carrier transmission within mobile telephony systems is largely due to the specially 'designed in' characteristics of the underlying modulation, system setup and RF band allocations.

By contrast, existing digital broadcast transmission systems - which generally do not support multi-carrier transmission for example due to the lack of colourisation to prevent mixing between multiple carrier signals passing through a single amplifier, and where it would not be practical to implement colourisation in transmission and further modify all existing broadcast receivers to remove unwanted signal components that may result from such colourisation - have evolved with design considerations which make multi-carrier transmission more difficult, for example:

1) Individual carriers within the broadcast network using pilot and synchronisation information in common. These may be directly correlated between carriers which can produce severe cross system interference in a multi-carrier amplification process.

2) Broadcast networks mix sites having large coverage and high transmit power with sites having medium or low powers on the same frequencies; this creates significant potential for inter-site interference in the form of receiver blocking. Large coverage sites may effectively have 'holes punched' in their coverage by small sites, so that in some areas a receiver will not receive the signal from the large coverage site.

As a result, directly applying to broadcast networks the multi-carrier amplification methods used in mobile telephone systems can result in impaired network performance or even severe degradation. A different approach is necessary.

In light of the above, we now describe an approach to amplification of multi-carrier broadcast signals. By using multi-carrier transmitter(s) such as the preferred embodiment of Fig. 22, a broadcast network may achieve high geographical coverage, scalability, flexible spectrum use, managed interference mitigation, small site footprint and/or low site cost. The transmitter(s) preferably (i.e., optionally) use Orthogonal Frequency Division Multiplexing (OFDM). The preferred transmitter embodiment of Fig. 22 has a modulation stage 200, decorrelation stage 300, mixing stage (comprising mixers 401 , 402), signal combiner 403, peak reduction stage 500 and amplifier 700. Any one or more of these stages may be implemented with Digital Signal Processing (DSP). Such stage(s) may be provided in a single chip such as a Digital Signal Processor.

A simpler, outline multi-carrier transmitter architecture for application to digital broadcast transmission is illustrated in Figure 2. A single output block comprising a signal combiner, single amplifier and single filter may be provided with a wide bandwidth to encompass multiple carrier frequencies. Advantageously, the block may further comprise a peak reduction stage (now shown). A control input may be provided to the combiner to allow selection of frequencies for combination and/or selection of a peak reduction algorithm; such an algorithm may considered to be performed by the combiner or a following peak reduction stage (not shown; e.g., Crest Factor Reduction (CFR) unit).

Regarding the peak reduction, it is noted that when a signal has both amplitude and phase variation in the time domain, its average transmitted power varies from its instantaneous and maximum transmitted power level. The variation between the signal's average value and its maximum value, which is described by a parameter known as the Peak to Average Power Ratio (PAPR), affects the overall efficiency and/or cost of the amplifier design.

In digital modulation systems the peak to average ratio of the underlying modulation may be as high as, e.g., 16-20dB, due to infrequent peak power events. As these high power events are statistically infrequent, peak large events can in an embodiment effectively be removed from the system and the resulting distortion may be recovered by means of digital modulation Forward Error Correction (FEC) coding.

The removal process is known as peak reduction or Crest Factor Reduction (CFR). Such a process is illustrated in Figure 5.

By applying peak reduction and allowing for error recovery, preferably by means of CFR at a transmitter and processing of received FEC coding at a receiver, a multi- carrier broadcast system may be achievable.

Considering the 'multi-carrier' nature of the transmission, a process of creating a composite multi-carrier signal for amplification is shown in Figure 6. The embodiment has an amplification apparatus comprising: a modulation stage having modulators 1 to N (201); a mixing stage comprising mixers 202; a signal combiner 203; a peak reduction stage 204; and an amplifier 206. Any one or more of the stages (e.g., modulation, mixing, combiner, peak reduction, optionally further functions such as time scheduling and/or application of a decorrelation technique / function) may be implemented with Digital Signal Processing (DSP). Such stage(s) may be provided in a single chip such as a Digital Signal Processor.

Here, the modulators modulate received data streams (such as that at point a) to output baseband signals (such as that at point b). Individual baseband carriers from the multiple modulation sources (201) are mixed to appropriate offset frequencies f1 - fN, to generate carrier signals (such as that at point c). The resulting carrier signals are then combined (summed) in signal combiner 203 before passing through a peak reduction stage 204, e.g., a Crest Factor Reduction unit. The resulting composite (combined) peak reduced signal may then be RF converted (see mixer 205) prior to amplification by the amplifier 206. The transmitter may further comprise a filter (not shown) provided between the amplifier 206 and antenna.

In the system illustrated in Figure 6 it is understood that the overall mixing of the modulation source baseband carriers may be to offset frequencies, an intermediate frequency and/or to a final output frequency, prior to the combining and peak reduction process. The combined and peak reduced composite signal therefore potentially being at a baseband, intermediate or the final output frequency.

With regard to power peaks present in the baseband signals, the peak reduction stage may allow improved power efficiency and/or cost of the amplifier. While such peak reduction may further introduce distortion. This may be overcome at least to some extent at a receiver by use of error correcting coding added to the baseband signals. For example, Forward Error Correction coding may be added by the modulation stage and/or may be received in the data stream(s) for modulation.

It is further noted however that correlated components of the baseband signals may increase PAPR in the combined signal input to the amplifier, due to beating effects of the correlated components in the combiner. Thus, application of one or more decorrelation techniques, preferably to the data streams and/or baseband signals output by the modulation stage, may be advantageous.

In this regard, if the carrier signals entering the combiner 203 from respective mixers 202 had spectrums similar to white noise, i.e., fully de-correlated, this may result in similarly uncorrelated / white signals output from the combiner and peak reduction (e.g., CFR) stages. However, input signals modulated for broadcast may have identical signal components, e.g., pilot signals and/or synchronisation sequences. While these components are generally offset in frequency, they may nevertheless beat, thus increasing amplitude variation of the combined signal; see Fig. 7).

With regard to mobile telephone systems, where signals are fully de-correlated due to network colorization as present in mobile telephony standards, the composite multi-carrier signal prior to peak reduction will generally have similar or identical signal characteristics to that of a signal carrier. Under these conditions each of the individual carriers within the composite multi-carrier signal will generally incur similar degradation to the equivalent peak reduction process with a single carrier system.

In broadcast systems however, the presence of common synchronisation, pilot and/or control signal information (collectively referred to as control components) in baseband signals, and thus carrier signals entering the combiner, may result in the composite signal having a significant increase in both the probability and overall power of peaks above the average value - thereby modifying the probability density function (PDF). This bias of the signal characteristics or PDF towards higher peak powers may occur due to the beating effect of the common correlated signal components at the different carrier offsets.

This effect is illustrated in Figure 7 for a multi-carrier broadcast signal based on the Digital Audio Broadcast (DAB, DAB+ or DMB) standard - here common synchronisation symbols result in beating during the synchronisation period. Similar effects may occur for other broadcast standards such as DVB, ISDB and DTMB, ATSC with pilot structures in common within their OFDM symbols. In Fig. 7, no synchronisation information is present in a first time period (time up to T1), i.e., the signals on the different carriers are uncorrelated. Thereafter, synchronisation signals are present and beat such that power levels above the peak level of the first time period (amplitude about 2.3) occur.

This bias of the composite signal toward high signal levels may be observed in the composite signals Complementary Cumulative Density Function (CCDF) as shown in Figure 7a. The CCDF shows the probability of an instantaneous signal level being above a specified power level. When the summed component signals are correlated, the resulting composite signal is more likely to be at high signal levels (curve 302) than the original component signals (curve 301).

When passed through a peak reduction process (e.g. CFR), an effect of this 'peak bias' may be to increase the distortion present on the underlying modulation to a level significantly higher than would be observed for a single carrier system. This degradation is clearly demonstrated by the resulting constellation diagrams for a DAB multi-carrier modulation signal, as shown in Figure 8.

To reduce or avoid signal degradation in a broadcast system, it may be advantageous to de-correlate the component broadcast signals making up the composite multi-carrier signal prior to peak reduction and amplification.

However, it may not be possible to simply colorize the individual carriers similarly as implemented in mobile systems, as this may prevent successful reception by standard broadcast receiver equipment. A preferred approach is to perform a decorrelation process using a technique that does not prohibit or degrade the overall reception by standard install receiver equipment.

(The term 'Colorise' generally refers to the concept of applying different scrambling and/or coding sequences to control and data at different transmission sites - where the selected sequences are preferably orthogonal and/or de-correlated for all observable sites. This may allow the receiver to apply an inverse operation that selects only the site it wants. Thus, 'colorisation' relates to the fact that the scrambled sites can be described as having different 'colours', and the receiver can apply a filter that selects only the colour it wants, thereby removing the observable effects of all other sites.

Further in this regard, individual carriers within a mobile network are generally intentionally 'colourised' to prevent cross interference within amplification and/or receiver equipment. Such colourisation may involve applying a carrier specific scrambling sequence that is orthogonal to other carriers to achieve coding gains, as in 3G and 4G systems. This scrambling in spread spectrum modulation techniques (3G) and sub-carrier scrambling in OFDM (4G) will reduce or fully de-correlate the different service carriers).

Where a new multi-carrier transmission site is to be added to an existing network having a large installed base of legacy receiver equipment, it may be a desirable that the decorrelation technique(s), e.g., decorrelation function(s) applied thereby, is transparent and/or has minimal impact on reception in all existing installed end user equipment, preferably without prior knowledge of the providence and/or implementation of that equipment.

In this respect, a preferred embodiment implements a decorrelation technique, e.g., selects / applies a decorrelation function (and preferably an associated implementation), that has characteristics that appear identical in effect to a time of arrival, phase of arrival and/or multi-path transmission effect on the components making up the broadcast multi-carrier signal. The mitigation of such 'delay and propagation' effects is generally a standard requirement for all receiver devices. In this regard, any decorrelation function that meets this criteria would generally provide an effective mechanism for multi-carrier transmitters to be deployed in a practical system. In embodiments, this may allow existing receiver performance to be maintained.

In a practical broadcast system, a core network (e.g. Operational Support System (OSS) thereof) may distribute one or more data streams comprising broadcast services to be sent from multiple RF transmission sites. The distribution network may take many forms, for example optical cable, satellite link, microwave link, or a combination thereof.

Each transmission site may be configured to receive a combination of data streams relating to specific broadcast services, and to modulate and transmit these services on planned frequency allocations (carriers). Depending on the transmission site location, different services may be transmitted at any given site, however some services may be common to two or more transmission sites, taking into account for example regional or national services.

It may be advantageous that services in common are delivered from different sites in a timely manner, such that users who can observe two different sites do not notice any delay in reception times between these sites. In this regard, it is noted that most broadcast standards using OFDM modulation (e.g., DAB, DVB, ISDB) support the use of Single Frequency Networks (SFN) where different sites send the same services on the same frequency allocations. For such an SFN system, it is generally preferably that the signals from each site arrive at the receiver within a fixed time of each other, this time preferably being the OFDM Guard Interval. The signals may then combine at the receiver with minimal or no interference with each other.

In practice, different transmission sites generally receive their data streams by different routes through the core distribution network that can have delays much greater than the maximum arrival delay of an SFN system. To resolve this issue, transmission sites may have a common time reference (e.g., GPS) and the distribution network may send control information on the time to transmit a given service at the transmission site. The transmission site may then buffer the received data stream and schedule the on air transmission at the specified time. An example network architecture in this regard is shown in Figure 1 1. Regarding a suitable method to mitigate the correlation effects, we first consider how the existing network configuration and/or addition of new services may result in such destructive correlation for a multi-carrier amplifier site using peak reduction.

Within the broadcast network, a transmission site may use a modulator element of a modulation stage, to convert service data (of an input data stream) into modulation symbols, preferably using a defined standard (DAB, DVB, ISDB) that is suitable for RF transmission to end user receiver equipment. These modulation symbols may comprise the original service data, added error detection and/or correction information, as well as preferably additional synchronisation and/or control information that allows the receiver to detect and decode the transmission to recover the original service data. (Where the term 'control information' may be used to refer to both synchronisation and control information components added during modulation and not dependant on the unique service data content being modulated.)

To ensure effective use of the transmission resources, the modulation symbols may be arranged into frames. Each frame may comprise a defined grouping of two or more modulation symbols that distributes added synchronisation and/or control information across multiple modulation symbols at pre-defined locations. These locations are generally defined by the modulation standard, therefore the frame structure and control information locations may be known a priori to the receiver equipment.

For example, in a DAB system for digital audio, the first two symbol locations of the frame may contain a null symbol followed by a synchronisation symbol with the remaining frame symbols then containing the service data and error control information.

This structure is shown in Figure 12. Here, the modulation process generates an OFDM symbol, a symbol structure made up of orthogonal sub-carriers, and the resulting DAB frames consists of a sequence of N+2 such OFDM symbols.

In contrast, digital video transmission standards using the same OFDM modulation scheme such as DVB and ISDB, may distribute the control and synchronisation information into a subset of the OFDM sub-carriers known as pilots. (See Fig. 13) In this structure, some pilots locations may be fixed and common to all symbols within the frame, while other pilots may be varied for different symbols within the frame.

In broadcast systems, the contents of the control information is generally defined by the modulation standard and may be identical in the modulation of all services encoded and transmitted throughout the network. (In contrast to mobile standards where the control information is also scrambled or colourised, to make it unique for each carrier and site in the network.)

Such common control information that may result in the beating or peak bias effect for a composite multi-carrier signal. This may be understood with regard to Figure 14 showing control signal information alignment for a DAB system transmitting four services at a site. Here, Services 1-4 are scheduled by the network to be transmitted at times ΤΊ - T ₄ . In these circumstances, the presence of the common synchronisation symbol in each DAB may result in beating and distortion during the period T ₅ -T ₆ similarly as shown in Figure 7.

Three techniques, which may reduce or prevent destructive correlation being applied to the multi-carrier peak reduction, are proposed. Any one or more of these techniques may be implemented in a transmitter or amplification apparatus embodiment described herein:

Technique 1 :

Where the broadcast network contains one or more multi-carrier transmission sites, network timing may be configured such that the required air transmission time (time- to-air parameter or time-to-send value) specified by the core network (e.g. from an OSS) for each individual service component of the composite signal transmitted at any multi-carrier transmission site is time adjusted to minimise or reduce (preferably remove) possible correlations of the component carriers at the multi-carrier site.

For example, the network operator may determine the services that are to be transmitted at each multi-carrier site and, where they include service components using a modulation type in common (e.g., OFDM), select or set the core network timing to minimise or prevent beating or correlation of these services at one or more, e.g., all, multi-carrier site(s). A single set of network timing parameters may be selected to increase or ensure decorrelation of services at all multi-carrier sites, even where these sites may transmit different services. This may be of particular advantage where most services are common to all sites.

As previously described, the control information present in each modulation symbol will generally vary across the transmission frame. Therefore, selecting network timing so that services with the same modulation type will not transmit the same symbols of each frame at the same time may reduce beating and peak correlation prior to the peak reduction process.

As a concrete example, for the DAB system of Figure 14 the network time to air parameters ΤΊ - T ₄ may be chosen or adjusted such that each time parameter is offset by at least one OFDM symbol time (T _S YMBOL) from any other time to air parameter present at the transmission site. This method is applicable for example where the pilot or synchronisation information applied in each symbol varies across the modulation frame. The network operator may be able to configure the overall network scheduling and/or RF transmission timing to ensure correlated alignment of individual signal components is not present when combining into a composite signal for peak reduction and amplification.

However, it is notable that ensuring that the same symbols of a frame for different services are not transmitted at the same time will generally reduce correlation and beating prior to peak reduction. Some modulation standards, such as DVB and ISDB, either partially or fully repeat the pilot allocations assigned to synchronisation data within their frame structures. In this instance a modulation analysis or simulation tool, or a set of tables generated by such a tool, may be employed in the planning process to select suitable offsets between carriers that will minimise correlation.

In an implementation, a network operator and/or control facility may send different time-to-send parameters for each service to the different nodes or sites (or particular transmitters) of the network.

When new services are added to the network, the process may be repeated to determine a suitable core network time alignment for the new added service carrier, preferably without adjustment of the existing service time assignments. Similarly, the various time-to-send values may be applied consistently over time, for example until new site(s) or service(s) are added to the network when the values may be redetermined / -adjusted.

Technique 2:

A decorrelation function may be applied to one or more of the component carriers, e.g., to at least some of the baseband signals and/or to at least some of the carrier signals, prior to the combining process. Such a method is illustrated in Figure 9, wherein a decorrelation stage comprises blocks 206 that apply an overall transform to the modulator outputs such that the composite signal at the output of the combiner block (203) that is applied to the input of the peak reduction block (204) has components that are suitably de-correlated so as to minimise distortion in peak reduction.

An effect of a decorrelation function involving at least one such transform in an embodiment is shown in the constellation diagram of Fig. 10.

Examples of suitable transforms, each of which may be applied to baseband signals or to carrier signals, and may be applied as alternatives or together to provide an overall decorrelation function, are described below:

Transform 1) Time adjustment:

A different time offset is applied to each component carrier (e.g., each carrier signal or each baseband signal) to be combined. The offsets may be selected so as to minimise the corresponding beating effect and/or PDF distortion. An example effect of a time offset on a symbol is shown in the middle diagram of Fig. 15a, where a symbol including a cyclic prefix is offset to start at an earlier time and ends correspondingly earlier.

To ensure correct network operation without interference to standard receiver sites in the network, it is preferable to have a requirement that the site transmit the service(s) very close to, e.g., within a predefined range of, the on-air time(s) (time-to-send values) specified by the core network. (In some embodiments, this may represent a difference to an implementation of Method 1 previously described). As such, it may not be possible or appropriate to select an arbitrary offset such as one whole symbol period as in an embodiment of previously described in Method 1. Instead, an embodiment may be restricted such that only a fixed or limited delay, e.g., less than one quarter of the Guard Interval for OFDM modulation, or a fraction (such as one tenth or less) of the propagation time between sites for other modulation types can be selected. Preferably a delay of not greater than the inverse of signal bandwidth of the narrowest carrier is preferred.

An implementation may provide different timing offsets to each of multiple baseband and/or carrier signals preceding the combiner stage. This may result in the respective carrier signals being effectively transmitted in the broadcast output earlier and/or later than otherwise. Preferably, such timing offsets are applied to all baseband and/or carrier signals of a site but are not required to be applied at every transmission site of a broadcast network. A maximum time offset (time duration limit) to be applied in an embodiment may be, e.g., 2msec or 1 msec or less.

The different time offsets may be applied consistently over time, for example until new site(s) or service(s) are added to the network when the time offsets may be redetermined.

Transform 2) Phase Adjustment:

For a component carrier (baseband signal or carrier signal) comprised of OFDM modulation symbols, an embodiment may vary the modulation symbol phase with frequency. This (or at least an effect thereof) may be removable by an equalisation process at a receiver. In this regard, application of different (preferably differing / non- constant) phase offsets among the component carriers may increase decorrelation. The effect of the phase offsets at the receiver may be substantially the same as that of time and/or phase variations that might have resulted due to the broadcast environment, e.g., due to multipath variation. Thus, advantageously existing receivers may be used without modification.

In an embodiment, the transform may apply different sets of phase offsets to respective OFDM signals (baseband signals or carrier signals), and each such set may comprise different phase offsets to be applied to respective sub-carriers of the OFDM signal. Thus, the constituent phase offsets of a set may all be different to each other. The sets may be applied consistently applied over time, e.g., only redetermined / adjusted when a new service or site is added to the network.

With either of the transforms 1) and 2) above, the selection of the time or phase adjustments applied to the individual carriers may be chosen so as to minimise the corresponding beating and/or PDF distortion above that of a single carrier, prior to the peak reduction process.

Technique 3:

Where the individual signal components use modulation based on the OFDM modulation technique with a cycle prefix (e.g., as for standard DAB as shown in the upper diagram of Fig. 15), an individual modulation process for a datastream may introduce a cyclic rotation of the OFDM symbol samples either prior to, or after the addition of, the cyclic prefix element. Preferably the same amount of rotation (value of N, i.e., number of samples) is applied for all symbols within any one frame on an OFDM baseband signal. However, different values of N may be applied at any given time to respective OFDM signals.

The various values of N for the OFDM signals of one or more transmission sites may be chosen so as to minimise the corresponding beating and/or PDF distortion, prior to the peak reduction process. The values of N for each amplification apparatus may be determined at the apparatus or communicated remotely from the core network (e.g., OSS), and may be applied consistently over time to the OFDM signals, e.g., only changed when a new service or site is added to the network.

Considering an OFDM signal comprising multiple sub-carriers each having symbols, a cyclic prefix may be achieved by transferring or copying part of a symbol (e.g., a last 500 samples of a total 2000 samples of symbol) from the back to the front of the symbol (see upper diagram of Fig. 15). Additionally, and as shown in the lower diagram of Fig. 15, a part (N samples) may be copied from the front to the back of a symbol. This may effectively provide a suffix (or post-fix) in addition to the prefix The total number of samples of the prefix and suffix is preferably constant, e.g. 504, so that the total length of the overall symbol is the same as if no suffix were applied (upper diagram of Fig. 15). Specifically, the prefix length may be K-N whereas the suffix length may be N, thus a standard prefix may be shortened by the same number of samples (N) that are copied to the back for the suffix. The rotation is described as cyclic because, e.g., if N>504, this is equivalent to N = N-504, i.e., the symbol rotates back on itself. This value of N applied to the prefix and suffix may determine an effective rotation of the symbol in space and/or time.

To understand the advantage of a rotation with cyclic prefix we refer to Fig. 15a, wherein: the upper diagram represent non-adjusted cyclic prefixed symbol; the middle diagram represents such a symbol time-shifted by the transform 1) or Technique 2; and the lower diagram shows the symbol (without the time shift of the transform 1)) having a cyclic rotation. As can be seen, both the overall symbol (comprising prefix and IFFT) and internal symbol (IFFT) of the middle diagram are shifted to start and end earlier relative to the non-adjusted symbol. In contrast, the overall symbol of the lower diagram starts and ends at the same times as the non- adjusted symbol would have done while the internal symbol starts earlier than the internal symbol than that of the non-adjusted symbol. Thus, Fig. 15a shows the original symbol (upper diagram) and the same symbol (middle diagram) delayed by a delay of Tn seconds that is equivalent to N samples. In comparison, the lower diagram shows the effect of a cyclic rotation by N samples; from the comparison it can be understood that the cyclic rotation keeps the original signal boundaries, but effectively shifts the internal core IFFT (Internal FFT) symbol alignment equivalently to that of a time shift of the transform 1) of Technique 2. This may allow use of the same or equivalent de-correlation function as for transform 1) of Technique 2.

The cycle rotation may be effectively the same as a time delay / adjustment of the transform 1) of Technique 2 without altering the symbol boundaries. Therefore, we may select the number of rotated samples in the same way as the delays of Technique 2.

Any non-linear effect resulting from such rotation may be removable by a receiver as this will appear as carrier phase rotation which will generally be recoverable by a standard equalisation process.

This Technique (3) may have the advantage of preserving the absolute carrier or service timing and/or the carrier or service timing relative to existing network installation components, and may be preferred where the multi-carrier system must operate within an existing environment.

A combination of one or more of the above Techniques 1 - 3 can be deployed. Advantageously, such technique(s) may decorrelate the carrier signals and thus may allow peak reduction to be used in a multi-carrier transmitter without significant destruction of information of the baseband signals. The decorrelation function may be applied at a transmitter or elsewhere in the network (e.g. external to the site comprising the transmitter), depending on whether the transmitter is performing the baseband modulation in addition to the combining and amplifying. For example, where the modulation is not occurring at the transmitter, Technique 3 may be implemented elsewhere. Where the decorrelation function is applied to the carrier signals, this may be performed by the transmitter regardless. For example, the decorrelation function(s) may be implemented at before the combiner, or between the combiner and amplifier, of the transmitter. The baseband modulator, whether or not provided at the transmitter, may control timing of sending the output RF signal according to time-to-send parameters received preferably with associated raw data from a network facility, e.g., a core network element. Any one or more chosen decorrelation technique(s) above may be applied at any point between the modulation symbol output and the peak reduction input by applying as appropriate a suitable linear translation of the function (e.g., time shifts of Technique 1 , and/or time and/or phase shifts of Technique 2) to the corresponding point within the multi-carrier transmitter. This may be understood from Figure 6, which has labels 'a', 'b', 'c', 'd' & 'e' on the signal path, where point 'b' is the modulator output, and point 'd' is the peak reduction input. A set of decorrelation functions or transforms may be applied to the modulator baseband outputs at points 'b'. Similarly, an equivalent set of transforms may be applied to the carriers at point 'c', preferably by translating the original decorrelation functions or transforms from point 'b' using the same offset as applied by blocks 202. Similarly, a single decorrelation function may be applied at point 'd', which function is preferably the sum of the functions that would have been applied at point 'c'. Any one or more of these options may be applied, and/or a combination thereof that is mathematically equivalent. Advantageously, the final sum of all function(s) or transform(s) applied may result in decorrelation of the service components in the composite signal at the input to the peak reduction.

In view of the above description, a system may involve one or more of the following:

1) a broadcast digital modulation scheme containing synchronisation or pilot components in common that may correlate to modify the probability density function;

2) digital combining of two or more carriers having modulation characteristics in 1) for the purpose of multi-carrier amplification;

3) application of peak reduction to the composite multi-carrier signal to reduce the composite signal PAPR prior to amplification; and/or

4) one or more of the Techniques 1 - 3 described above, preferably to ensure the composite signal presented to the peak reduction process of step 3) has a suitable PDF that peak reduction does not degrade receiver reception due to correlation of the common synchronisation or pilot signals.

'Preferred Digital Audio Broadcast Embodiments'

One particularly preferred embodiment is application to Digital Audio Broadcast using one or a combination of any one or more of DAB, DAB+, and DMB standards, and/or video broadcast using second generation DVB-T modulation (generally referred to as DVB-T2) in a Single Frequency Network (SFN) having multiple transmission sites. All these transmission standards use the OFDM modulation scheme.

In this embodiment the network may consist of a mixture of traditional transmission sites with analogue combining for example as illustrated in Figure 1 and new digitally combined multi-carrier sites (for example as illustrated in Figure 22) comprising modulation, signal combining, peak reduction and amplification with a decorrelation stage (eg 300) preferably according to a combination of any two or more of Techniques 1 - 3 as previously described.

The embodiment may further include an apparatus for the decorrelation stage of these sites to be configured from the core network (e.g., OSS), and/or to set the time to air and/or chosen decorrelation function, and/or a common time reference which is distributed to all sites within the network (for example GPS).

Within the network it may be desired to transmit some signal from all sites (e.g., 'National' services to be received across the whole network area) and/or other signals from only a subset of sites (e.g., 'Local' services to be received in a subset of the network area). The total number of unique signals (known as 'services') to transmit transmitted across all network sites being K.

By way of example, in the United Kingdom three national services may be transmitted on channels 1 1 A, 1 1 D and 12B, and 12 or greater additional local services may be transmitted on the remaining channels in the channel band 10A to 12D.

For the prescribed modulation types (DAB, DAB+, DMB, and/or DVB-T2) each modulation frame will contain N OFDM symbols, and it may be observed that the first two OFDM symbols within the modulation frame contain correlated synchronisation data for all signals that will lead to high signal distortion in the peak reduction stage of the digitally combined multi-carrier sites, as illustrated for example in Figure 8. To minimise the distortion introduced by the peak reduction process, any one or more of the following methods 1) - 3) may be applied to determine the decorrelation function(s):

1) Where the total number of services to be transmitted in the network, K, is less than the total number of symbols within the modulation frame, N, each service may be allocated a time to air that is offset by at least one OFDM symbol duration but preferably not more than N OFDM symbols duration, from any other service. One advantage of such an embodiment may be that the synchronisation symbols of no two services overlap. These offsets being preferably applied according to Technique 1.

2) Where the total number of services, K, exceeds the number of OFDM symbols in the frame, N, and/or it is desirable for other network operational reasons to space the services more closely than with greater than one OFDM symbol separation, then each overlapping service may be allocated a unique decorrelation function preferably according to Technique 2 and/or 3.

3) Additionally or alternatively, where the total number of services, K, exceeds the number of OFDM symbols in the frame, N, and/or it is desirable for other network operational reasons to space the services more closely than with greater than one OFDM symbol separation, then each overlapping service may be allocated a decorrelation function unique to the frequency channel on which it will be transmitted preferably according to Technique 2 and/or 3.

Optionally, in the absence of unique allocations the network may choose to apply a mixture of methods 2) and 3) such that the same allocation does not apply to any two services that are transmitted in the same geographical area.

These selections being preferably applied across all sites within the network from the core network.

(However, it is to be noted that all of the description, claims and drawings provided in the present application may be understood in conjunction with, or in isolation from, the above description of 'Preferred Digital Audio Broadcast Embodiments'). We now focus on interference mitigation with regard to addition of new services. Understanding of interference mitigation techniques described herein may be assisted by, e.g., Figs. 16 - 23. In this regard, embodiments may address a so-called "near-far" issue wherein, when a new site is added to a broadcast network, the signal broadcast from the new site results in a nearby receiver no longer being able to successfully receive a signal from a pre-existing, more distant site. This may be due to relatively low power of such a signal received from the distant site. In other words, the signal received from the distant site may be drowned out by the new signal. (This is particularly the case where the network is a single frequency network or wherein different sites can transmit on the same frequency(s)). Advantageously, embodiments described herein may allow new (preferably localised) services to be provided in an existing broadcast network, without requiring any modification of the existing receivers and/or the addition of extra coverage sites.

Figure 16 provides an example of this issue. Here, Site A is an existing site providing a service on channel 12B and having an original coverage range shown by Curve A which includes Receiver A. At a later time two new additional services 1 1 D and 12D are added at Site B, which is much closer to Receiver A. After addition the strong signals from Site B now prevent Receiver A from receiving original channel 12B from Site A, this occurs due to a process known as receiver blocking.

To understand receiver blocking, it is helpful to also understand that a radio receiver may have characteristics that are very similar to a human ear. In quiet or low noise environments, the human ear may easily detect very faint sounds or signals (a pin drop, for example). In a noisy environment or the presence of much larger sounds, that faint sound (our pin drop) may no longer be detectable. This occurs because a human ear, similarly to a radio receiver, may have limited dynamic range; at any instant it may only be able to detect signals that are within a narrow power range of the loudest present sound, this range being much less than the total range of all detectable signal levels.

Figure 17 shows the receiver blocking effect at Receiver A with the addition of new services 1 1 D and 12D. The two new channels (1 1 D, 12D) from nearby Site B have much greater power than channel 12B which is from the more remote Site A. In this example, the power difference is 60dB (-30dBm for 1 1 D vs -90dBm for 12B) making 12B one one-millionth of the 11 D signal power, so that reception of channel 12B is no longer possible. One approach to improving or restoring reception of 11 B at the receiver may be to add an additional new site, which is less remote from the intended receiver than the original Site A and is configured to transmit 11 B at lower, but sufficient, power to prevent blocking at the receiver (an in-fill site). To minimise disruption to service 11 B by new services at Site B using this method it may be considered to either a) add an additional network site closer to Site B, or b) to add a new modulator, amplifier and combiner hardware for service 1 1 B at Site B which will restore the original coverage. However, such an approach generally adds cost and complexity when planning and deploying new services, both in the extra planning and capital outlay required to install the in-fill site for the 1 1 B service, and the ongoing operational costs of powering and maintaining the in-fill site.

As an improved approach to the near-far interference problem, the new services site may be implemented using a multi-carrier amplification broadcast site as previously described. The multi-carrier site may transmit the in-fill for the original service in addition to the new services without the addition of any new amplifier, filter or combining elements. Costs may thereby be significantly reduced.

In practice, adding an in-fill service at the multi-carrier amplification broadcast site may incur extra cost. Another service generally means the multi-carrier site transmitting more RF power, potentially requiring a larger amplifier element and/or having higher operational electricity cost. For this reason it is desirable to minimise the extra power that is transmitted for the in-fill service.

To ensure robust network operation, broadcast standards generally set out a minimum dynamic range requirement for receiver blocking - the first adjacent channel blocking specification - which every receiver within the network must meet. This specification is generally 35dB below the highest signal present or greater.

In practice, a power level may be chosen that is higher than the minimum receiver blocking specification. This may allow for other degradations introduced in the wireless channel and reception process, but preferably is not so high as to significantly increase the amplifier size and/or cost of the multi-carrier site. For example, selecting the in-fill service power level to be between 16dB and 35dB below the maximum power of any other service would ensure no receiver blocking occurred, but may only increase the total amplifier output power requirement and/or size by, e.g., 2.5%. Therefore, negligible additional cost may be added to the multi- carrier site. (An in-fill service at 16dB below the new services would represent a maximum 2.5% power increase.) With reference to Fig. 18, the new multi-carrier site is preferably set to transmit 12B at a power level just sufficient to prevent the drowning out of 12B from its other services at the afore-mentioned receiver.

The new, multi-carrier site may be configured to not increase geographical coverage of 12B or to provide the minimum increase in coverage as is necessary to prevent receiver blocking, and this may determine the power level of 12B as transmitted from the multi-carrier amplification site.

The new site may be configured to transmit 12B at not greater that 16dB below the maximum power of any of the other services transmitted from the site. This may allow minimal or no increase in the site amplifier size and/or cost.

As previously described, the multi-carrier amplification site preferably includes a peak reduction function. This may enable efficient amplification. The peak reduction process generally introduces distortion, e.g., produced by clipping, into the carrier components. Such distortion may be removable by means of the modulation's forward error correction coding.

With reference to Fig 19, peak reduction process (e.g. CFR or clipping) may apply different levels to components of the combined signal corresponding to different services. For example, components of the new services may be clipped to a lower maximum amplitude than components of the in-fill service(s). The distortion introduced by the peak reduction process to the new services (1 1 D & 12D) may be equal to or greater than the distortion level for the in-fill service (12B). In this case, if equivalent distortion were instead applied to the in-fill carrier 11 B as to the new service(s), severe degradation to the service reception could occur.

As a consequence, for the in-fill to be more or fully effective in providing interference mitigation, the peak reduction process in the multi-carrier amplification site is preferably set or is controllable to allow the distortion level of the in-fill carrier to be proportional to its transmitted power (and preferably further to allow the distortion level of the new service(s) to be similarly proportional to the power of their respective broadcast carriers).

In a preferred embodiment, the peak reduction function may allow the distortion introduced into each carrier of the combined signal to be set independently. The distortion level applied to the lower power carriers may then be chosen to be proportionally equivalent to or lower than that applied to the high power carriers as shown in Fig 19. Alternatively or alternatively, a peak reduction function may be chosen that provides substantially (e.g. exactly) no distortion to the low power carriers as depicted in Fig 20.

A preferred implementation may therefore involve any one or more of the following:

1. provision of new services in the network using a multi-carrier amplification site (e.g., new multi-carrier transmitter) as previously described.

2. the multi-carrier site being able to transmit different services at different power levels to provide different geographical coverage levels.

3. the multi-carrier site including a peak reduction function that can be adjusted to set the level of distortion introduced by the peak reduction process independently for each service transmitted.

4. the multi-carrier site where new services are transmitted at a relatively high power as previously described.

5. the multi-carrier site where a remote service that may otherwise be blocked is transmitted at a lower power selected to meet one or more of the conditions described above.

6. the multi-carrier site where peak reduction function is configured to introduce lower or no distortion into the low power service(s).

Regarding the application of different distortion levels, it is noted that carrier signals corresponding to both of the in-fill and new channels / services may enter the combiner of the transmission apparatus to form the combined, i.e., composite, signal. Indeed, the modulation stage and/or the mixing stage may each be involved in processing respective input signal(s) to generate such carrier signal(s) corresponding to the in-fill and/or new channels. The peak reduction stage may apply a peak reduction function or algorithm to the whole of the combined signal. The function / algorithm is preferably selected to unevenly distribute (across frequency) the distortion resulting from the peak reduction, such that frequency components that are output by the peak reduction stage and correspond to the in-fill channel(s) have been distorted to a lesser extent than such components that correspond to the new channel(s). The peak reduced combined signal is then input to the amplifier and eventually broadcast.

Fig. 23 illustrates an example shaped peak reduction method. The algorithm may include any one or more of the illustrated steps of this example: 1) Create frequency domain model of desired distortion levels.

2) Compute time limited peak reduction function using an Inverse FFT and window function.

3) Detect a peak greater than desired level in our composite signal.

4) Subtracted a weighted version of our peak reduction function from composite signal to remove the peak.

5) REPEAT this process (iteratively) to remove all peaks from the composite signals.

Note however, this is only an example - the skilled person reading the present specification will immediately recognise that other example are possible, for example based on peak reduction algorithms applied in the field of mobile telephony.

With reference for example to Fig. 22, to allow the frequency components corresponding to different channels / services to be broadcast at different power levels, signals corresponding to the respective channels may be available at the output of the modulator stage (201), the decorrelation stage (block 300, comprising 301+302) or the mixing stage (402) prior to summation at the combiner 403. The different power levels may be achieved by applying one or more different gain block(s) (401) prior to mixing and combining into the composite signal.

No doubt many other effective alternatives will occur to the skilled person. It will be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the spirit and scope of the claims appended hereto.