Transmitter signal processor

Abstract

A transmitter signal processor (TSP) for combining, amplifying and transmitting a plurality of independent signals while maintaining a substantially constant envelope, the processor comprising a feedback network, thereby calculating an error signal between a sum of the signals and an output of the constant envelope processing within the TSP, and for each input signal, the error signal correlating with the input signal, the feedback signal is generated by generating an error signal to simulate error effects in the receiver, and by multiplying each error signal by its corresponding input signal and summing the results of each such input signal, and then subtracting the feedback signal from the sum input. The constant envelope processing may be all-digital, such as normalization processing, and / or may include a portion of an analog transmission chain, such as an amplifier, a frequency converter, or a filter.

Description

Technical Field

[0001] The present invention relates to electronic amplifiers, and more particularly to amplifiers for amplifying signals for transmission, where multiple signals need to be transmitted together in an efficient manner. Background Art

[0002] Amplifiers are typically used in applications where the power efficiency of the amplifier is crucial. One such application is in satellite systems, where there is usually only a very limited power budget available to power all the on-board electronic systems. Not only does the power source (which typically includes solar panels) cost a large amount of money, but it also has an adverse effect on satellite performance in many ways due to the increase in size and weight, such as the size of the launch rocket, maneuverability, vulnerability, etc. In addition, for satellites in space, dissipating the waste heat caused by amplifier inefficiency is a serious challenge, again resulting in additional costs and weight.

[0003] Global Navigation Satellite Systems (GNSS), such as the United States Global Positioning System (GPS), the European Galileo system, or the Chinese Beidou system, each include multiple satellites (usually 15 - 30) in Earth orbit. The trend in modern GNSS is to transmit more signals from a single satellite. These signals can support multiple different GNSS "services", and for the purposes of this application, each service is associated with one or more signals.

[0004] Each individual signal can be a constant envelope signal, but when the signals are combined together, especially in cases where the combined signal is subsequently filtered, it is difficult to maintain a constant envelope. Compared to the ideal solution of generating a combined signal with a constant envelope that can then be fed into a constant envelope amplifier, the result may be the generation of unwanted intermodulation signals that waste energy.

[0005] One way to process multiple signals in a satellite signal transmitter is to use a separate transmit amplifier for each signal, where each signal is a constant envelope signal, to obtain maximum efficiency, and then combine these amplified signals before sending them to the transmit antenna. Thus, this provides constant envelope high-power amplification. However, the step of combining high-power signals for transmission through a single antenna is an engineering challenge, and overall, this solution has disadvantages in terms of size, cost, and weight. Note that a constant envelope signal is a signal whose amplitude is not modulated. It can be synonymously referred to as a constant amplitude signal.

[0006] Solutions to these problems have been sought that employ multiplexing of non-constant amplitude signals and modify the signals before they pass through the amplifier, with the aim of maximizing the output power portion carrying the desired signal, which correspondingly means minimizing the output power portion going into unwanted error signals (called intermodulation signals).

[0007] An alternative approach is to use a single high-power constant envelope amplifier. A known technique operating on this basis is called Phase Optimized Constant Envelope Transmission (POCET) and is described in U.S. Patent No. US8774315. It operates by using a pre-computed table of composite signal phase values and selecting a value from the table through an optimization process that minimizes or reduces the envelope variation of the phase-modulated carrier subject to in-signal constraints of multiple input signals.

[0008] International Patent Application WO01 / 058026 discloses a method in which signals from individual sources in a transmission system are combined and amplified in a power amplifier, while these signals are combined and subtracted from a portion of the amplified combined signal to provide an error signal that is a measure of the distortion produced by the amplification process. Then, in a feedforward system, the error signal is adjusted in phase and amplitude before being combined with the power-amplified signal to compensate for the distortion. SUMMARY OF THE INVENTION

[0009] It is an object of the present invention to provide an alternative method for effectively generating a transmission signal.

[0010] According to a first aspect of the present invention, there is provided a transmitter having a transmitter signal processor (TSP) that processes a plurality of signals for transmission to produce an output signal having a constant envelope, the transmitter signal processor having a sum input that includes the sum of the plurality of signals for transmission, and additional inputs and outputs for each signal for transmission, the TSP being arranged to calculate an error signal between the sum input and the output of the TSP, and for each input signal, to simulate the error effect in a receiver by correlating the error signal with the input signal, and to generate a correction signal using the sum of the effects of each input signal, the correction signal being fed back to the input of the TSP in a feedback loop.

[0011] Thus, in cases where a constant envelope is desired, embodiments of the present invention employ a feedback method to improve the distortion caused by the amplification of multiple signals.

[0012] Some embodiments of the present invention may include a transmitter in which the feedback loop generates a combined feedback signal that is subtracted from the sum input to produce a "constant envelope input signal", and for each input signal, the feedback loop includes:

[0013] a) a correlator arranged to generate a correlation between (i) the input signal and (ii) the error signal, the error signal comprising a measurement of the difference between the sum input and the output within the constant envelope processing or within the TSP; and

[0014] b) A multiplier arranged to multiply the correlation result of step (a) with the input signal to produce a multiplier output;

[0015] The signal processor is further arranged to: for each input signal, sum the outputs of the multiplier at step (b) to produce a combined feedback signal and feed the combined feedback signal back to the sum input of the signal processor.

[0016] Wherein the constant envelope processing includes a normalization function or either an analog signal processing chain or its digital model.

[0017] Embodiments may provide a transmitter capable of processing multiple (e.g., 2, 3, 4, 5 or more inputs) digital input signals for transmission to generate an output signal for transmission having a substantially constant envelope.

[0018] Those of ordinary skill in the art will realize that the correlator at (a) is performing a process similar to that which will be performed on the signal transmitted by the transmitter in the receiver. This is clear because the form of the sum input signal in (a) is similar to the form of the reference signal used in the correlator in a well-designed receiver and the transmitted signal is (partially) received at the receiver. Thus, the output of the correlator gives a representation of the distortion effects caused by the constant envelope processing as would be seen at the receiver. At (b), the correlator output is multiplied by the input signal to form (along with contributions from similar signals generated for each other input signal) a combined feedback signal which is subtracted from the sum input signal going to the TSP. Thus, in effect, the distortion error caused by the constant envelope processing is continuously subtracted from the input, which at least partially cancels the said distortion.

[0019] In some embodiments, the output signal may be a digital signal which is then converted to an analog signal in a digital-to-analog converter (DAC), amplified, and if necessary, upconverted before being transmitted.

[0020] Other embodiments may include the outputs of the DAC and amplifier and / or upconverter as part of the feedback loop of the transmitter signal processor (TSP), which enables the TSP to take into account the distortion added by these components and thus results in an improved, more constant envelope signal transmitted by the transmitter. Alternatively, digital models of one or more components (e.g., DAC and upconverter and / or amplifier) may be used in the feedback loop of the TSP instead of the corresponding analog components. Thus, in some embodiments of the present invention, the constant envelope processing includes an analog signal processing chain having at least one of an amplifier or upconverter, or includes its digital model.

[0021] In those embodiments that incorporate analog components (such as upconverters and / or amplifiers), the feedback loop further includes a digital-to-analog converter and a downconverter, where the upconverter is present in the constant envelope processing. This enables the analog signal (which may be at a frequency different from that of the signals provided to the TSP input) to have the same frequency as those input signals and to be more conveniently processed in the feedback loop.

[0022] In some embodiments, the processing within the TSP occurs entirely within the digital domain, and the constant envelope processing may include a normalizer that is arranged to calculate the output o(t) = βw(t) / |w(t)|, where β is the desired constant amplitude and w(t) is the sum input or a scaled version thereof.

[0023] Advantageously, in some embodiments, the correlator includes a multiplier that is arranged to multiply the complex conjugate of signal (i) with signal (ii) and accumulate the result in an accumulator.

[0024] Advantageously, a high-pass filter may be included in the correlator between the multiplier and the accumulator to remove low-frequency components from the output of the multiplier. This prevents any such low-frequency components (including DC or near-DC signals) from accumulating and causing errors.

[0025] Advantageously, the transmitter may further include a scaling member for scaling the input signals before the input signals are summed at the sum input. The scaling member may include one or more multipliers. The multipliers may preferably be implemented digitally. The multipliers may be arranged to scale each input signal independently. The scaling provided by the scaling member is adjustable in the operation of the TSP. The adjustment of the scaling may be determined by measuring the DC component removed by the high-pass filter.

[0026] According to a second aspect of the present invention, there is provided a method of combining a plurality of digital input signals for transmission by a transmitter, the method for generating an output signal having a constant envelope, and the transmitter having a transmitter signal processor (TSP) that is configured to: sum the digital inputs to produce a sum input, and calculate an error signal between the sum input of the TSP and the output, and for each digital input signal, simulate the error effect as would occur in a receiver by correlating the error signal with the digital input signal, and generate a correction signal using the sum of the simulated effects of each digital input signal, and feedback the correction signal in a feedback loop, and subtract the feedback signal from the sum input of the TSP.

[0027] Advantageously, in some embodiments, the method may further include a method in which the sum signal is fed to a constant envelope processor in the TSP, the constant envelope processor including at least a normalization function or an analog signal processing chain or a digital model thereof, and an output is produced;

[0028] The feedback signal is generated through the following steps:

[0029] i) Measure the instantaneous error signal by obtaining the difference between the sum input and the output;

[0030] ii) For each input signal, simulate the effect of the instantaneous error signal by correlating the instantaneous error signal with the input signal and multiplying the correlated output with the input signal;

[0031] iii) Sum the multiplication results generated for each input signal in step (ii), and this summation result includes the feedback signal;

[0032] where the constant envelope processing includes either a normalization function or any one of the analog signal processing chains.

[0033] Advantageously, the constant envelope processor can be arranged to normalize the constant envelope input signal using a normalizer.

[0034] In some embodiments, the constant envelope processing includes a digital - to - analog converter and at least one of an up - converter and an RF amplifier, and the output for the purpose of (i) in this second aspect is derived from a down - converted and digitized version of the output from the RF amplifier and / or the up - converter.

[0035] In some embodiments, the constant envelope processing includes a digital model of at least a part of the analog processing chain of the transmitter in which the method is implemented, and the analog processing chain includes at least a power amplifier. Advantageously, the digital model can have inputs from the analog processing chain and can be adapted to change the parameters of the digital model based on these inputs. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Embodiments of the present invention will now be described in more detail, by way of example only and with reference to the following drawings, in which:

[0037] Figure 1 shows a high - level representation of a transmitter arranged to transmit multiple independent signals through a common up - conversion and amplification process;

[0038] Figure 2 shows a high - level representation of a prior - art process for achieving a constant envelope output signal;

[0039] Figure 3 shows a high - level representation of an improved prior - art process for achieving a constant envelope output signal;

[0040] Figure 4 shows a high - level architecture of an embodiment of the present invention;

[0041] Figure 5 Shows a more detailed architecture of an embodiment of the present invention that can operate entirely in the digital domain;

[0042] Figure 6 Shows partial details of an alternative embodiment of the present invention, where some components can operate in the analog domain; and

[0043] Figure 7 Shows partial details of an alternative embodiment of the present invention, where digital models of analog components are used. Detailed Description

[0044] Figure 1 Shows a simple transmitter architecture 100, where three independent digital input signals s1(t) - s3(t) are scaled in multipliers α1–α3 and then summed in summer 102 to produce a combined digital signal w(t). Then, it is converted to analog form in digital-to-analog converter (DAC) 104, then up-converted by mixing with a local oscillator signal (not shown) in mixer 106, and amplified in amplifier 108. Then it is fed to an antenna (not shown) for transmission. Generally, in a given system, there can of course be a different number of input signals s i (t). Typical applications can include GNSS satellites, and the signal s i (t) can include different navigation signals, such as open navigation signals, as well as commercial, public regulatory service, and life safety navigation signals and pilot signals, which can be at different frequencies, bandwidths, or phases with respect to each other.

[0045] The signal w(t) is the signal to be transmitted. However, due to the independence of the signals s i (t), the envelope of w(t) may vary, for example when each signal itself varies in amplitude or phase. The variation in the envelope of w(t) results in inefficiency, especially within RF amplifiers, such as the generation of intermodulation signals mentioned earlier, which can be costly in terms of additional cooling requirements or additional power input required to achieve a given performance when the amplifier is in a challenging environment such as a satellite.

[0046] Figure 2 Shows a simple prior art 200 for generating a constant envelope signal. It is similar to Figure 1architecture, but with a new block added in the signal path between the summer 102 and the DAC 104. The new block includes a scaling member χ (which can be ignored for the purposes of this paragraph), and a "constant envelope" (CE) block which takes the signal w(t) and produces an output o(t) = βw(t) / |w(t)| with a constant (or more constant than its input) envelope. Thus, it is used to normalize the input signal to an amplitude β, and all known techniques for producing a constant envelope distort the output signal to some extent, but this simple method performs poorly compared to other techniques including embodiments of the present invention.

[0047] Measurement of the error produced by this method can be done by subtracting the output signal of the CE block from the input signal in the subtractor 202 to produce an instantaneous error signal e0(t). When implementing Figure 2 the technique shown, the power of the error e0(t) will typically be minimized in the design by adjusting the scaling factor χ such that the mean square value of w(t) is equal to the mean square value of o(t), i.e., β 2 .

[0048] χ 2 {E[|α1| 2 |s1(t)| 2 +E[|α2| 2 |s2(t)| 2 +E[|α3| 2 |s3(t)| 2} = β 2

[0049] Alternatively and equivalently, the three scaling factors α1, α2, and α3 can be adjusted by a common scaling factor χ. Assuming the scaling factors are fixed, this is all set only once. It should be noted that Figure 2 any known implementation of a system of the type shown does not actually use the error signal e0(t) in any real-time correction process - which is shown here merely to illustrate where the error will be measured.

[0050] This simple prior art method has two limitations:

[0051] ● There is no control over the power density spectrum of the noise added by the action of the CE block; and

[0052] ● There is no control over the actual signal power seen by the receivers that match the respective signals.

[0053] Figure 3An improved prior art 300 is shown. This is the technique used in the prior art document US8774315 which, as described above, uses pre-computed tables to generate output signals based on the respective input signals presented to the CE component by the dotted line connections 302. These tables take the possible combinations of the input signals, for example each combination having a discrete number of different phases (for phase modulated signals), and pre-compute the ideal output signal for each possible input combination.

[0054] Figure 4 A system 400 according to an embodiment of the present invention is shown in a top-level manner. The basic principle of this embodiment and all embodiments of the present invention is to use negative feedback of the error signal e0(t) to correct the error introduced by constant envelope processing. The error signal e0(t) is equivalent to the error signal as Figure 2 shown, but in this embodiment, the error signal is used to generate a feedback signal as explained below.

[0055] The signals s1(t) - s3(t) are the input signals to be combined and are transmitted from a power amplifier. They are scaled by the corresponding scaling factors α1 - α3 and summed in a summer 402 to provide a summation output. Further combined scaling can be performed on the summation output at 404 or alternatively, this scaling can be done by incorporating an appropriate common factor into the individual scaling factors α1 - α3. Its output is the signal w1(t) which provides a first input to a summing node 406 to compute the error signal e0(t). The instantaneous error caused by the conversion to constant envelope (CE) is thus computed.

[0056] The signal w1(t) is also provided to another summing node 408 in the transmission path where a feedback signal is subtracted from the signal to produce the signal w2(t). This signal is then fed to CE processing to produce an output o(t) which, in this embodiment, is given by o(t) = w2(t)|w2(t)|). The signal o(t) is fed to the summing node 406 as its second input where it is subtracted from the signal w1(t) to produce e0(t). In this embodiment, the signal o(t) is a constant envelope (digital) signal which is then, as required, converted to an analog signal, up-converted, and amplified in a power amplifier for transmission.

[0057] By obtaining the error signal and for each individual input signal s i (t), applying it to a correlator 410 to produce a feedback signal which the correlator 410 correlates with s i (t) to simulate the effect of the error on the correlation process that will occur in a receiver which of course has (in GNSS and other direct sequence spread spectrum applications) its own locally stored signal s iA reference copy of (t). Note that Figure 4 only the correlation and feedback processes associated with the signal s1(t) are shown, and in fact, each signal s i (t) will have its own correlator to provide its own contribution to the feedback signal, as explained later. The correlator output is then multiplied by the signal s i (t) in the multiplier 412 to become the contribution of s i (t) to the feedback correction signal. The corrections themselves are distorted by the CE processing as they are subtracted from the signal w1(t), but the continuous negative feedback still reduces the cumulative error effect.

[0058] Figure 5 The generation of the feedback signal is shown in more detail. The illustrated embodiment 500 is the same as Figure 4 but shows additional elements that further clarify the operation of the system. Like reference numerals indicate like functional blocks. As previously explained, the instantaneous transmitter error e0(t) is calculated and correlated with each reference signal. This is only shown in the figure for the signal s1(t); the processing of other signals uses replicated blocks. For example, the arrow 502 indicates the feeding of the correlator for other signals s i (t), and the arrow 504 indicates the summing of the results of the processing of these signals to create the total feedback signal. The correlator 410 includes a conjugator 506 for conjugating the reference signal before multiplying it by the error signal in the multiplier 508. It is then accumulated in the accumulator 510 to produce the error signal e1(t).

[0059] An ideal system would add a correction to the output signal o(t) for the next sample that, when demodulated by the associated receiver correlator, would be equal to the negative of the cumulative error of that signal. This would make the cumulative error of that signal zero after the next sampling. This is the purpose of the multiplier 412, which multiplies the cumulative error from the accumulator 510 by the reference signal s1(t), then sums the result with the results of a similar process for the other input signals s i (t), and then subtracts the summed result from the desired signal w1(t) in the adder 406, giving the corrected desired signal w2(t).

[0060] The distorting operation of the CE block means that the corrections are also distorted, so the actual correction level achieved on the next sample is reduced. However, a significant amount of correction does survive the CE block, and better analog performance than the prior art has been achieved.

[0061] A detail to note is that unless the scaling factor χ at 404 (or the equivalent adjustment to α1 - α3) can be optimally adjusted simultaneously, for the signal component s iIn fact, the output amplitudes that can be achieved may not be equal (χα i s i ). Therefore, the error associated with this signal component may grow without limit. This problem is solved by including a high-pass filter at the point indicated by the asterisk 0 in the correlator 410. By measuring the DC component removed by the high-pass filter, the source signal scaling factor (α i ) can be adjusted by a slow adaptive algorithm (e.g., the LMS steepest gradient algorithm).

[0062] The above embodiments operate throughout in the digital domain because all processing, as well as CE processing, also operates in this domain and provides its output to a DAC, which then feeds any required upconversion and amplification for transmission. Other embodiments may use CE processing that operates at least partially in the analog domain, where the feedback path comes from elements within the analog path, typically after an amplification stage. This allows the processing that occurs in the feedback path to directly account for the defects within the amplifier. These defects may include any soft clipping that occurs in the amplifier, and / or any AM-to-AM and AM-to-PM distortion that may occur, where AM is amplitude modulation and PM is phase modulation.

[0063] Figure 6 Part 600 of an embodiment of the present invention is shown in top-level form, which has CE processing in analog form and includes a part of the transmission chain of the transmitter. Note that the details of the feedback processing are the same as those of the above embodiments and will not be described in further detail with respect to this figure.

[0064] Therefore, this figure only shows Figure 5 the analog equivalent of the (digital) CE block in, as well as the error summing block 406. The connections from w1(t) and o(t) to the summing block 406 and the output from the summing block 406 show the path to the feedback circuit, which includes, for example, the feedback circuit as explained with respect to Figure 4 .

[0065] Here, the signal w1(t) is presented to the DAC, and before being passed to the RF amplifier 606 to be amplified to the desired transmission output power, the resulting analog signal is upconverted to the transmission frequency in the upconversion mixer 604 to produce an analog power signal 608 for subsequent transmission via the antenna. This is the step where most of the distortion correction is performed by the method of the present invention. The coupler 610 extracts a small amount of this transmission energy from the output of the RF amplifier, brings it back to the baseband signal in the downconversion mixer 612, and converts it back to a digital signal in the analog-to-digital converter 614. Then, it is fed into the feedback loop to generate the error signal e0(t), as in the previous embodiments.

[0066] Unlike the all-digital method of the previous embodiment, this embodiment can introduce a small delay in the signal path. In GNSS applications, where the signal s i (t) includes a chip sequence, then assuming that any additional delay is small compared to one chip duration (which is approximately 0.1 μs at higher GNSS chip rates), this delay can be ignored without any significant problems. However, compensating for such a delay is not difficult, especially if it is approximately constant. To this end, the delay is measured during the design and test phases of system production. Then, at the conjugate box 506 in Figure 5 , this amount of delay is inserted into the digital correction circuit (although this is shown in an all-digital scenario, in this embodiment, the generation of the feedback signal will be the same as described previously).

[0067] Figure 6 Another potential problem with the embodiment of Figure 7 is that the RF measurement process of separating the signal from the output of the amplifier (in coupler 610), downconverting it, and converting it into digital form may introduce noise. Figure 6 shows an alternative embodiment aimed at reducing the impact of any such noise that may be present. This embodiment is similar to the embodiment of Figure 6 in that the output of the analog transmission chain is used to affect the feedback process. The analog transmission chain, as well as the coupler, downconverter, and digitizer, are similar to those shown in Figure 6 and thus have the same reference numerals. However, in this embodiment, instead of directly using the output of digitizer 614, it is fed into a digital amplifier model 616. This digital amplifier model is a model of the RF amplifier. Since the characteristics of the RF amplifier do not change rapidly, it has adjustable parameters that are slowly adapted over a "relatively long" period of time based on the input of the downconverted and digitized signal from the actual power amplifier. This period of time can be measured, for example, in tens or hundreds of milliseconds or seconds. Their adaptation is controlled by error feedback, where the error is the difference between the (downconverted and digitized) amplifier output p(t) and the model output o(t).

[0068] The adjustable parameters can take the form of an input envelope to output envelope mapping function and an input envelope to output phase error mapping function. Alternatively, they can be the parameters of a Volterra function model of the power amplifier. Such a model is mentioned in ""The Evolution of PA Linearization"" by Allen Katz, John Wood, and Daniel Chokola, IEEE Microwave Magazine, Feb 2016, p. 32".

[0069] This embodiment provides the advantage that any noise or error in the coupled output o(t) is not fed directly into the feedback process described above, but is smoothed by long-term adaptation within the model. The model itself is digital and operates with low output noise.

[0070] Those of ordinary skill in the art will understand that the new techniques described herein are completely different from existing methods because, although they use pre-computed tables, the new methods use real-time feedback of errors.

[0071] Embodiments of the present invention will typically be implemented in software, such as in one or more digital signal processors or microprocessors, the one or more processors being programmable hardware controlled by computer code, or may operate in firmware / hardware, such as in one or more application specific integrated circuits (ASICs) or field programmable gate arrays (FPGAs). As will be understood by those of ordinary skill, such devices will typically include suitable memory and storage means, or be connected to suitable memory and storage means. Accordingly, the present invention extends to software programs arranged to be stored in a computer memory, the software program comprising instructions to cause a processor to implement the various elements described herein. Some embodiments may be implemented in a combination of hardware or software, and the transmitter signal processor may include a combination of analog and digital circuitry arranged to process signals in both the analog and digital domains. In particular, the analog circuitry forming part of the transmitter signal processor may include one or more amplifiers (including power amplifiers), frequency shifters, filters, etc., which form part of the signal processing chain of the signal to be transmitted by the antenna. Embodiments of the present invention have utility in many fields of signal transmission, typically in space or airborne applications, such as in GNSS satellites, and may also be more widely used in terrestrial radio receivers (where increased power efficiency gives longer battery life and reduced waste heat).

Claims

1. A transmitter having a transmitter signal processor (TSP) that processes a plurality of signals for transmission to produce an output signal having a constant envelope, the transmitter signal processor having a sum input that includes the sum of the plurality of signals for transmission, and additional inputs and outputs for each signal for transmission, the TSP being arranged to calculate an error signal between the sum input and the output of the TSP, and for each input signal, to simulate the error effect in a receiver by correlating the error signal with the input signal, and to generate a correction signal using the sum of the effects of each input signal, the correction signal being fed back in a feedback loop to the input of the TSP.

2. The transmitter according to claim 1, wherein the feedback loop generates a combined feedback signal, the combined feedback signal being subtracted from the sum input to produce a "constant envelope input signal", and for each input signal, the feedback loop comprises: a) a correlator arranged to generate a correlation between (i) the input signal and (ii) an error signal, the error signal comprising a measure of the difference between the sum input and the output of the constant envelope processing within the TSP; and b) a multiplier arranged to multiply the correlation result of step (a) by the input signal to produce a multiplier output; The signal processor is further arranged to: for each input signal, sum the outputs of the multiplier at step (b) to produce a combined feedback signal, and feed the combined feedback signal back to the sum input of the signal processor; wherein the constant envelope processor comprises either a normalization function or an analog signal processing chain or a digital model thereof.

3. The transmitter according to claim 2, wherein the constant envelope processor comprises an analog signal processing chain having at least one of an amplifier or an upconverter, or comprises a digital model thereof.

4. The transmitter according to claim 3, wherein the feedback loop further comprises a digital-to-analog converter and a downconverter, wherein there is an upconverter in the constant envelope processing.

5. The transmitter according to claim 1 or claim 2, wherein the processing within the signal processor is performed entirely within the digital domain, and the constant envelope processing comprises a normalizer arranged to calculate an output o(t) = βw(t) / |w(t)|, where β is the desired constant amplitude and w(t) is the sum input or a scaled version thereof.

6. The transmitter according to any of the preceding claims, wherein the correlator comprises a multiplier arranged to multiply the complex conjugate of signal (i) by signal (ii) and to accumulate the result in an accumulator.

7. The transmitter according to claim 6, wherein a high-pass filter is included in the correlator between the multiplier and the accumulator to remove low-frequency components from the output of the multiplier.

8. The transmitter according to any of the preceding claims, wherein the transmitter further comprises a scaling member for scaling the input signals before they are summed at the sum input.

9. A method of combining multiple digital input signals for transmission by a transmitter, the method for generating a constant envelope output signal, and the transmitter having a transmitter signal processor (TSP) configured to: sum the digital inputs to produce a sum input, and calculate an error signal between the sum input of the TSP and the output, and for each digital input signal, by correlating the error signal with the digital input signal, simulate the error effect as would occur in a receiver, and use the sum of the simulated effects of each digital input signal to generate a correction signal, and feedback the correction signal in a feedback loop, and subtract the feedback signal from the sum input of the TSP.

10. The method according to claim 9, wherein the sum signal is fed to a constant envelope processor in the TSP, the constant envelope processor comprising at least a normalization function or an analog signal processing chain or a digital model thereof, and producing an output; wherein the feedback signal is generated by the following steps: i) measuring an instantaneous error signal by obtaining the difference between the sum input and the output; ii) for each input signal, simulating the effect of the instantaneous error signal by correlating the instantaneous error signal with the input signal and multiplying the correlated output with the input signal; iii) summing the multiplication results produced for each input signal in step (ii), the result of the summing including the feedback signal.

11. The method according to claim 10, wherein the constant envelope processor includes a normalizer for normalizing the constant envelope input signal.

12. The method according to claim 10 or claim 11, wherein the constant envelope processing further includes a digital-to-analog converter and at least one of an upconverter and an RF amplifier, and wherein the output for the purpose of (i) of claim 9 is derived from a downconverted and digitized version of the output from the RF amplifier and / or the upconverter.

13. The method according to claim 10 or claim 11, wherein the constant envelope processing further includes a digital model of at least a part of an analog processing chain of the transmitter in which the method is implemented, the analog processing chain including at least a power amplifier.

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