Patent Description:
In satellite communications, there is an increasing demand to support higher data throughput. In communication payloads, DC-to-RF power conversion efficiency is an important consideration and most of the DC power is consumed by the RF Power Amplifier (PA). The maximization of PA efficiency while maintaining low signal distortion is desirable.

Signal transmission systems are known generally that include a base station to emit a signal and a relay station remote from the base station to relay the original signal further than would be possible with the base station alone. For example, this arrangement is often used in terrestrial mobile telecommunication systems. The arrangement is also used in satellite communications, where the relay station typically comprises a satellite in orbit around the Earth.

There is increasing requirement to send large volumes of data as quickly as possible and bandwidth is necessarily restricted to pre-allocated channels. Further, there is a desire to increase the efficiency of signal transmission systems, especially on board satellites where the power supply is often restricted.

In order to increase the signal strength emitted by a power amplifier, one option is to simply increase the power at which they are operated. When power amplifiers are operated in the upper part of their range, their behaviour is non-linear.

<CIT> discloses power amplifier capable of operating over broader frequency ranges.

<CIT> relates to pre-distortion of signals that are to be transmitted to satellite transmission systems.

"<NPL>, discloses the results of dynamic load modulation on a high power amplifier.

<CIT> provides improved methods for computing and using pre-distortion coefficient values in wireless systems.

According to an aspect of the present invention, there is provided a transmitter according to claim <NUM>.

According to another aspect of the present invention, there is provided a method of transmitting a signal with a transmitter according to claim <NUM>.

Advantageously, the use of load modulation allows efficient flow of power through the system. Further advantages include that the simple design (i.e. depends on the output voltage swing and required capacitance range), adds a very little complexity to the system in terms of weight, volume, and cost compared.

There follows a detailed description of the accompanying drawings, in which:.

By way of further background explanation of the principles behind the invention, <FIG> includes three schematic graphs showing how the DPD can offset the non-linearity of the PA. The DPD has a linearization effect on the PA output. In small fractional bandwidth systems, it is not feasible to filter out out-of-band spectral regrowth due to the required high-Q filter. PA DPD-based linearization is achieved by digitally processing in-phase (I) and quadrature (Q) baseband data so that frequency components are generated within a bandwidth equal to that of the spectral regrowth (normally <NUM> times the modulated signal bandwidth) to compensate for the distortion due to PA nonlinearities. Thus, a wideband transmitter should be used. This digital "pre-processing" allows the PA to be operated up to saturation point and mitigates the in-band and out-of-band distortions due to nonlinear behaviour. Hence, output power back-off can be significantly reduced.

As shown in <FIG>, power back-off techniques can also be used to help achieve linearity in the output of the PA and to cope with signals having a high peak to average power ratio. To maintain linear amplification for high PAPR signals using linear PAs, two power back-offs can be utilised; the first to avoid the nonlinear part of the gain curve and the second which is to deal with the PAPR. Even if DPD is used to alleviate the 1st back-off, either supply or load modulation could only mitigate the 2nd back-off to achieve acceptable overall power added efficiency (PAE).

<FIG> shows a block diagram for a typical DPD+PA hardware test setup. This setup is for implementing a method of modelling the response of a transmission system that utilises DPD using a model coefficient extraction procedure. I and Q data of a test signal can be generated on a PC using Matlab then downloaded on an arbitrary waveform generator (AWG). These data modulate an RF carrier in a Vector Signal Generator (VSG) where signal upconversion is achieved. The modulated RF carrier feeds the PA and a driver amplifier (DA) may be used for high power PAs (HPA). A Vector Signal Analyzer (VSA) downconverts then demodulates the RF modulated carrier. This allows extraction of the DPD model coefficients (in the PC) by comparing the demodulated I and Q data of the original (PA is removed) and distorted signal (i.e. signal as amplified by the PA). The DPD+PA performance can be verified by downloading the predistorter I and Q data on the AWG and measuring the PA output.

Future high throughput satellites, where a large fractional bandwidth is expected, could benefit from adopting band-limited-DPD. These benefits, compared to using a conventional DPD, could be: less hardware complexity and less processing power as a result of processing a bandwidth comparable to the original modulated signal bandwidth compared to <NUM> times bandwidth in conventional DPD.

<FIG> shows a signal transmission system <NUM>. The system <NUM> comprises means <NUM> for producing a signal that is desired to be transmitted by a satellite, e.g. a film or television program, and a means <NUM> for transmitting an amplified output signal, such as an antenna <NUM>. A first channel <NUM> or electrical path of the system leads from the signal production means <NUM> to the antenna <NUM>, via a first digital predistortion device <NUM> and an amplifier <NUM> to the antenna <NUM>. The first digital predistortion device <NUM> produces a non-linearity in the signal that cancels out the non-linearity produced by the amplifier <NUM>. A second channel <NUM> or electrical path is able to isolate an envelope of the signal. The second channel includes a second digital predistortion device <NUM> which is connected to an envelope amplifier for amplifying the envelope signal. The second digital pre-distortion device <NUM> applies a non-linearity to the envelope signal that is cancelled out by the non-linearity of the envelope amplifier.

<FIG> shows a more detailed version of the signal transmission system shown in <FIG>. The varactor-based matching network is modulating the output matching network based on the input modulated signal. The DPD+PA architecture shown in <FIG> advantageously increases the overall average power added efficiency of the system while minimizing the distortion in the driver amplifier (DA) stage. Load modulation is applied at the PA output using a varactors-based matching network where varactors can be placed in parallel to cope with high PA output power. The matching network can also be connected to an antenna for transmitting the output amplified signal. The bandwidth through path DPD1, upconverter, DA, and PA is limited to the original modulated signal bandwidth. Envelope tracking is applied to the driver amplifier using an envelope amplifier (EA) with additional DPD block (DPD2) to compensate for nonlinearities at EA output. Switching between the DA and PA is possible for low input power which further improves the average power added efficiency.

<FIG> shows a spectrum produced by a DPD modelling process, in particular, a NARMA-based (non-linear auto-regressive moving average) DPD model for <NUM> <NUM>-QAM ultra wideband signal modulated on a <NUM> carrier using MGA-545P8 PA model on Agilent ADS (RTM) software. Three iterations are done to attain further adjacent channel power ratio (ACPR) improvement. <FIG> shows the original signal spectrum (<NUM>), PA distorted output (<NUM>), and DPD+PA output for the three iteration (<NUM>, <NUM> and <NUM>). The vertical and horizontal axes are set to <NUM> to -<NUM> dBm and <NUM> to <NUM>, respectively. Approximately <NUM> dB improvement in ACPR is achieved while -23dB NMSE is maintained.

<FIG> show spectra produced in a test setup as follows: A demonstration low noise amplifier (ZFL-500LN+ Mini-Circuits (RTM)) was used to demonstrate the effect of band limited DPD on the DPD+PA performance. The following equipment was used: Agilent (RTM) N5182B MXG RF Vector Signal Generator (VSG), Agilent (RTM) N9030A PXA Vector Signal Analyser (VSA), and a TTi EL302Tv triple power supply. The VSG and VSA are connected through a network switch for control and data exchange via a PC. Synchronization is established by connecting a <NUM> reference, trigger, and event ports. <FIG> show the measured spectra of original signal, distorted PA output, and NARMA based DPD+PA for three different modulated test signals:.

The spectra shown in <FIG> are each normalised to the level of the original signal to facilitate comparison of the signals. In <FIG>, line <NUM> represents the original signal, line <NUM> represents the amplified signal without DPD, and line <NUM> represents the signal as amplified and pre-distorted. In <FIG>, line <NUM> represents the original signal, line <NUM> represents the amplified signal without DPD, and line <NUM> represents the signal as amplified and pre-distorted. In <FIG>, line <NUM> represents the original signal, line <NUM> represents the amplified signal without DPD, and line <NUM> represents the signal as amplified and pre-distorted.

The adjacent channel power ratio (ACPR) and normalised mean square error (NMSE) for DPD+PA were measured for each modulated signal and are summarized in Table <NUM>. It is to be noted that a good NMSE could be achieved in all cases while a good ACPR is achieved only for the <NUM> BW signal. This is justified as follows; due to the limited analysis bandwidth at the PA output, i.e. <NUM>, no sufficient information about the spectral regrowth arrives to the DPD. Thus, the ACPR gets worse as the signal bandwidth becomes larger. However, DPD still copes with the in-band distortion.

The data thus indicate a most optimal performance for the <NUM> LTE DL signal.

Spectral re-growth can be filtered out for large fractional bandwidth signals (e.g. in L-Band) and for this reason, the ACPR constraint is significantly relaxed. To allow reliable reception of the transmitted signal over a satellite communication link a link budget-determined ratio of the signal energy over the spectral noise density, i.e. Es/No, has to be maintained at the receiver side assuming perfect signal transmission. EVM at the transmitter side decreases this ratio and has to be kept at minimum by employing DPD.

As a result of the heritage in space technology, nonlinear (switch) PAs, although power efficient, are not commonly used whereas linear PAs (power inefficient) are used. Thus high spectral density modulation techniques are avoided. An advantage of the present invention is that using DPD plus load and supply modulation on space (and ground) segments guarantees efficient usage of power. Moreover, a greater amount of data can be pushed into the link assuming the same power budget for a transmitter.

The figure of merit for the proposed DPD+PA should be achieving a lower EVM and high throughput with fixing the power consumption.

It is possible to use a training sequence to update the DPD model: in X-band payloads, the transmitter is on for a short period of time to transfer data when the satellite is in the visibility zone of the station. However, this does not necessarily happen for each orbit. Consequently, one of the orbits can be freed to transmit a training sequence to the data reception station. This received data could be compared, offline, to the ideal training sequence and an update for the DPD model coefficient could be extracted. This updated coefficient could be transmitted to the satellite through the TT&C transponder and used to configure the DPD model onboard. In other words, an offline adaptation could be made to cope with any unexpected very slow time variation of the PA.

DPD techniques for terrestrial communications as proposed in the present invention advantageously maximize the overall PAE while high PAPR signals can be used. A further advantage of embodiments of the present invention is that it allows less expensive (in terms of volume, mass, and cost) space and ground segment transmitters.

<FIG> shows a signal transmission system in accordance with a third aspect of the invention comprising a base station <NUM> and a relay station <NUM>. An end user <NUM> receives signals from the system, using for example a telephone or a television, or any other suitable receiving device. The base station <NUM> comprises a base power amplifier (PA) <NUM> which amplifies a signal "x" to be transmitted. The signal x is transmitted by an antenna (not shown) to the relay station <NUM>, which in the present embodiment is on board a satellite. The relay station <NUM> comprises a relay PA <NUM>, for amplifying the signal as it is relayed.

The base station <NUM> includes a digital pre-distortion means <NUM> which applies pre-distortion to the signal x to compensate for the non-linearity of the power output by the PA <NUM>. The pre-distortion means <NUM> also applies pre-distortion to signal x to compensate for the non-linearity of the power output by the relay PA <NUM>.

The digital pre-distortion means <NUM> comprises first and second digital pre-distortion devices <NUM> and <NUM>. The first pre-distortion device <NUM> applies pre-distortion to the signal x, to compensate for the non-linearity of the relay PA <NUM> at the relay station <NUM>. Where the relay station is a satellite, the relay PA <NUM> is typically part of a satellite transponder system. The second pre-distortion device <NUM> pre-distorts the signal x to compensate for the non-linearity of the base PA <NUM>.

Each pre-distortion device <NUM>, <NUM> has a response characteristic that is the inverse function of the response characteristics or gain curve of the PA.

<FIG> illustrates schematically an embodiment, which does not correspond to the claimed invention, whereby load modulation is used to further increase the efficiency of the system. The figure shows a part of the base station <NUM> shown in <FIG>, with some further detail added. In view of the high power requirements of the system, the base PA <NUM> is driven by a driver amplifier (DA) <NUM>. To address the fact that both the PA <NUM> and the DA <NUM> are subject to non-linearity issues near the top of their operating ranges, the digital pre-distortion device <NUM> comprises first and second digital pre-distorters <NUM>, <NUM>, the first pre-distorter <NUM> applying pre-distortion to linearize the DA <NUM> and the second pre-distorter <NUM> applying pre-distortion to linearize the PA <NUM>. In addition, the system makes use of load modulation to increase the efficiency of the system. The load on the DA <NUM> is matched to the input impedance of the PA by a matching network <NUM>. Similarly the load on the PA <NUM> is matched to the impedance of an output antenna <NUM> by a further matching network <NUM>. Further use of the digital pre-distorters <NUM>, <NUM> is made to compensate for distortion arising due to the use of load modulation.

In this regard, the first and second pre-distorters <NUM>, <NUM> are connected to the respective matching networks <NUM>, <NUM>, wherein signals reflecting the levels of load modulation are passed from the matching networks to the digital pre-distorters <NUM>, <NUM> forming a control feedback loop. A switch <NUM> can be used to bypass the PA <NUM> when signals of a low level are required.

The matching networks are varactor-based, where a control signal is used to vary the output impedance instantaneously, and isolators at the output of each amplifier. DPD is applied to compensate for the distortion due to load modulation in the signal path (i.e. DA+PA). This is achieved by characterizing the DA and PA separately (i.e. two consecutive DPD blocks).

An embodiment, which corresponds to the claimed invention is shown in <FIG> of the accompanying drawings. In this embodiment, corresponding features to those of <FIG> have been indicated with corresponding reference numerals raised by <NUM>.

In this embodiment, both envelope tracking and load modulation are applied to both driver amplifier <NUM> and power amplifier <NUM>. Although linearising the driver amplifier <NUM> would result in improved spectral performances (important from thermal management and reliability points of view, especially for high power applications, e.g. <NUM> Watts upwards, as well as to feed a linear signal to the power amplifier <NUM>), linearising the power amplifier contributes the most towards the overall efficiency. While load modulation could maximise the power transfer to the load by changing the varactor based network impedance, envelope modulating the power amplifier <NUM> supply at the same time can further improve the power efficiency. This is because it will provide instantaneous DC power to the power amplifier <NUM> according to the envelope of the baseband signal (shown as "modulator" in <FIG>).

Claim 1:
A transmitter for transmitting a signal comprising:
a power amplifier (<NUM>); and
a driver amplifier (<NUM>), an output of the driver amplifier being connected to an input of the power amplifier via a first load modulation device comprising a first varactor-based matching network (<NUM>) operable to match the impedance of the driver amplifier output with the impedance of the power amplifier input, further comprising a second load modulation device comprising a second varactor-based matching network (<NUM>) connected to the output of the power amplifier and operable for matching the impedance of the power amplifier output with a connectable input impedance of a further device; and
means (<NUM>) for applying envelope tracking to the power amplifier and the driving amplifier;
a digital pre-distortion device (<NUM>) operable to apply pre-distortion to the signal to compensate for non-linearity of the driver amplifier and the power amplifier and the varactor-based matching networks, wherein the digital pre-distortion device comprises:
a first digital pre-distorter (<NUM>) connected to the second varactor-based matching network and arranged to provide a control signal thereto, and operable to apply pre-distortion to compensate for non-linearity of the power amplifier (<NUM>) and the second matching network (<NUM>), wherein the second varactor-based matching network is arranged to pass a signal reflecting the load modulation at the output of the power amplifier back to the first digital predistorter for the formation of a control feedback loop; and
a second pre-distorter (<NUM>) being consecutive to the first digital predistorter (<NUM>), connected to the first varactor-based matching network and arranged to provide a control signal thereto, and
operable to apply pre-distortion to compensate for non-linearity of the driver amplifier (<NUM>) and the first varactor-based matching network (<NUM>), wherein the first varactor-based matching network is arranged to pass a signal reflecting the load modulation at the output of the driver amplifier back to the second digital pre-distorter for the formation of the control feedback loop.