The present invention is concerned with predistorting a modulated signal prior to its transmission over a wired or wireless channel in such a way that the distortion incurred by the signal from the channel is reduced. An example of such a channel is a satellite communication channel. Note that in satellite communications a performance gain of 0.1 dB (e.g. obtained by predistortion) is valued to be quite large because of the large cost of producing, launching and maintaining a satellite. Optimal satellite resources exploitation requires transmitting a signal that uses the satellite amplifier at or close to its saturation point. This is typically the case in single-carrier per transponder scenarios where the transponder operates in ALC (automated level control) mode in order to keep the satellite amplifier at or close to its saturation point. When amplified close to saturation, the transmitted signal typically incurs distortion from the non-linear behaviour of the satellite amplifier, thus reducing the reliability of the communication.
The transmitter output in a digital communication system, particularly a satellite communication system, can be seen as a transmit signal which is a pulse train modulated by a sequence of complex symbols. This modulation is typically performed by applying the symbols to a pulse shaping filter (PSF). Each symbol is selected (referred to as mapping) from an allowed set of complex values, represented by an in-phase and quadrature component (I and Q, respectively). The set of possible symbols is called a constellation. Several mapping strategies can be envisaged in satellite communications, including quadrature amplitude modulation (QAM), phase shift keying (PSK) and amplitude and phase shift keying (APSK). These mapping strategies employ different types of constellations. For example, in the APSK mapping scheme the constellation points are located on two or more concentric rings. The combination of a constellation and a forward error correcting code (FEC) is referred to as a modulation and coding or a modcod. As different prior art documents often use other notations to denote the same physical entity, the notation is explicitly recalled in this document. The complex (I,Q) values provided to the PSF are referred to as transmit symbols. These symbols may or may not be predistorted by a symbol predistorter. The PSF output is a complex signal and can be applied to a signal predistorter or not. The output of the PSF (and possibly the signal predistorter) is denoted as the transmit signal.
The above digital communication system is a single-carrier communication system. The extension to a multi-carrier communication system where all carriers are transmitted from one device is trivial. Typically, more than one pulse shaping filter is used. After the combination of all carriers, a signal predistorter can possibly be applied.
An important feature in satellite communications is adaptive code and modulation (ACM), see ETSI EN 302 307 v1.2.1: Second generation framing structure, channel coding and modulation systems for Broadcasting, Interactive Services, News Gathering and other broadband satellite applications. In the remainder of this document this reference is referred to as the DVB-S2 standard. In ACM the modcod can dynamically change on a per-frame basis, based on the channel quality, i.e., the link budget. That is, given a change in the link margin (e.g. due to fading), another modcod is chosen such that the link margin is again similar as before. For example, in one frame 8-PSK with FEC coding rate 2/3 can be used and in the next frame 16APSK with FEC coding rate 2/3. Another important mode in satellite communications is constant code and modulation (CCM). In CCM the chosen modcod is fixed and cannot change. Thus, sufficient link margin is taken to accommodate for fading (e.g. due to rain) without losing frames. CCM is typically applied for TV signal broadcasting.
In FIG. 1 a satellite communication link with its main components is shown as an example of a transmission link. In this example structure the complex transmit signal (which is typically obtained after a PSF or after a predistortion block following the PSF) is I/Q modulated onto a carrier waveform. Before transmission, the carrier waveform is amplified by the ground station high power amplifier (HPA). The signal is received by the satellite's transponder, the operation of which is illustrated in the simplified schematic drawing of FIG. 2. The transponder's incoming signal is passed to a bandpass input multiplexer filter (IMUX), typically amplified by a travelling wave tube amplifier (TWTA) and filtered again by a bandpass output multiplexer filter (OMUX). A transponder contains other components as well, such as up- and down-converters. The transponder output signal travels to a plurality of receivers. One such receiver amplifies the signal through a low-noise amplifier (LNA), I/Q demodulates the amplified signal to yield the complex receive signal. The receive signal is typically provided to a receive filter (typically a PSF) that outputs the received symbols. When referring to a transmission link in the following, the structure shown in FIG. 1 is referred to.
In the absence of channel distortion and noise, the receive signal is equal to the transmit signal. On a transmission link of practical use, however, channel non-linearities change the phase and amplitude of the transmit signal as it passes through the transmission link, and thus generate distortion.
The non-linearities of the transmission link can be modelled by an AM/AM and AM/PM curve, where AM and PM refer to the magnitude and phase of a complex signal, respectively. The AM/AM curve returns the magnitude of the receive signal versus the magnitude of the transmit signal and the AM/PM curve returns the phase rotation of the transmit signal incurred during amplification in the transmission link versus the magnitude of the transmit signal. The absolute phase of the receive signal thus equals the phase of the transmit signal plus the phase rotation applied by the channel. These AM/AM and AM/PM curves are often normalised, such that the saturation point (i.e. the maximum) of the AM/AM curve is (1,1). The ordinate and abscissa of such normalized curves are then the inverses of the output backoff (OBOlin) and input backoff (IBOlin) of the on-board TWTA, respectively. The subscript lin refers to the fact that here these values are shown in linear scale. An example of AM/AM and AM/PM curves is given in FIG. 3.
The distortion caused by the non-linearities is best illustrated by plotting the location of the received symbols, which is referred to as a scatter plot at the receiver side (in the following, simply denoted as a “scatter plot”). The distortion mainly has two consequences:    (1) in a scatter plot, each constellation point becomes a cluster, caused by inter-symbol interference (ISI) due to the memory in the channel, and    (2) constellation warping occurs, which causes the mass points of the clusters to be no longer on the original constellation grid.
Such a scatter plot for the channel given in FIG. 3 and for 16-APSK rate 2/3 from the DVB-S2 standard is illustrated in FIG. 4. Note that no noise is added and only channel distortion is taken into account.
Techniques to mitigate the distortion effects, caused by the satellite transponder, by manipulating the signal in the transmitter are generally referred to as predistortion. It is important to distinguish satellite TWTA predistortion from ground station HPA predistortion. The main difference is that the wireless link towards the TWTA should comply with a spectral mask which limits the occupied bandwidth of the signal. When referring to predistortion in this text, satellite TWTA predistortion is meant. Predistortion can yield significant gains and is thus highly valuable in satellite communications, especially because one predistorter in the hub, can improve the performance of millions of terminals receiving the signal from one satellite transponder. The first publications on predistortion date from the 1970s (see amongst others “Modeling and Performance Evaluation of Nonlinear Satellite Links-A Volterra Series Approach”, Benedetto, Biglieri, and Daffara, IEEE Tr. on Aerospace and Electronic Systems, Vol. AES-15, No. 4, pp. 494-507, July 1979 and “Adaptive Cancellation of Nonlinear Intersymbol Interference for Voiceband Data Transmission”, Biglieri, Gersho, Gitlin, Leong Lim, IEEE J. Sel. Areas In Comm., Vol. SAC-2, No. 5, pp. 765-777, September 1984). Early and recent publications focussed especially on a Volterra series representation of the non-linear channel. In general, prior art predistortion techniques introduce a unit in the transmitter that generates “anti-distortion” for the distortion caused by the channel. The combination of the distortion from the channel and the “anti-distortion” generated at the transmitter ideally should minimize the overall distortion at the receiver. The most relevant techniques can be classified in two categories: signal predistortion (also known as fractional predistortion or sample-level predistortion) and symbol predistortion (a.k.a. data predistortion). Symbol predistortion aims at subtracting from the transmitted symbols the interference expected at the receive side. This can for example be done by (statically or dynamically) computing a new constellation from which the transmitted symbols are selected, while maintaining the original constellation for demapping at the receiver. The new constellation can for example be a non-linear transformation of the original constellation (in the case of static symbol predistortion). Signal predistortion aims at performing the inverse operation of the transmission link on the signal provided by the PSF. Ideally, the inverse operation of the transmission link and the transmission link itself are applied consecutively on the transmit signal, as illustrated in FIG. 5. In the ideal case the corresponding overall AM/AM and AM/PM curves of the cascade of the predistortion unit and the transmission link are those of a hard-limiter channel, as shown in FIG. 6.
However, performing the inverse operation of the channel on the signal provided by the PSF is a non-linear operation and causes spectral regrowth, i.e., the occupied frequency bandwidth of the transmit signal becomes larger. Until very recently, signal predistortion was thought not to be applicable for satellite communications, because the spectral regrowth does not comply with the spectral mask on the transmit signal. For example, it is explicitly mentioned that fractional predistortion cannot be used in satellite communications in U.S. Pat. No. 6,963,624B1 and in the papers “Constellation Design for Transmission over Nonlinear Satellite Channels” (Montorsi et al., IEEE Global Communications Conference (GLOBECOM), pp. 3401-3406, December 2012) and “Joint precoding and predistortion techniques for satellite telecommunication systems” (M. Álvarez-Diaz et al., Int'l Symposium on Wireless Communication Systems, September 2005, pp. 688-692).
High performance symbol predistortion is complex in logic and/or memory, especially for higher order constellations. In most of the literature it is argued that constellations larger than 32-APSK cannot be predistorted using symbol predistortion.
Besides the above mentioned problems for signal and symbol predistortion, it was hard to estimate the unknown TWTA non-linear amplifier characteristics (see for example “Adaptive Fractional Predistortion Techniques for Satellite Systems based on Neural Networks and Tables”, M. Berdondini et al., Vehicular Technology Conference, April 2007, pp. 1400-1404). For example, for signal predistortion the channel characteristics must be known in order to be able to apply the inverse of these channel characteristics on the transmit signal fed from the PSF.
For the above-mentioned reasons predistortion was not much applied in satellite communications, despite it being a relatively long studied problem. Only recently, some prior art techniques, disclosed for example in WO02/73920 and in international patent application PCT/EP2014/051947, have applied symbol predistortion in a memory-efficient way. Application WO2013/012912 proposes a solution to estimate the unknown TWTA characteristics.
In WO2013/012912A1 the TWTA characteristics are estimated during a calibration stage in which a transmitter sends a signal to a receiver. At the receiver the difference between the transmit signal (which is known through decoding and remodulation of the received signal) and corresponding receive signal is used to estimate the channel characteristics. The estimated channel characteristics are fed back to the transmitter as an input to the inverse channel model. This prior art solution is illustrated in FIG. 7.
However, an important problem remains to be solved. The estimated TWTA characteristics do not depend on the modcod, so the predistortion parameters used at the transmitter do not depend on the modcod either. As a consequence, non-linear predistortion is cumbersome to use in combination with ACM using the technique disclosed in WO2013/012912A1. That is, when the modcod changes dynamically while the predistortion unit is static, the predistortion unit will not be well parameterized anymore and even the output power of the modulator will change. Due to this changing output power, the ALC on the satellite might introduce level variations across the baseband frames with significant performance degradation as a consequence.
Consequently, there is a need for a predistortion unit that is compatible with ACM.