Patent Application
20060040624
The present invention relates to a method and a device for performing peak power limitation. More specifically, the invention relates to performing peak power limitation in a transmitter stage that includes a power amplifier pool with two or more individual power amplifiers.
Wireless cellular communications is continuing to grow unabated. As wireless applications become increasingly widespread, the pressure on network operators to increase the capacity of their networks becomes more intense.
There are a number of ways of enhancing capacity in a wireless cellular network, including frequency hopping, power control, micro cells, and the introduction of adaptive and sector antennas. Adaptive and sector antennas have thus become the subject of increasing interest in recent years.
Unlike conventional cellular antennas, which broadcast energy over an entire cell, adaptive and sector antennas confine the broadcast energy to a narrow beam or a sector. The advantages of directing the broadcast energy into a narrow beam or a sector are increased signal gain, greater range of the signal path, reduced multipath reflection, improved spectral efficiency and increased network capacity.
Several different adaptive antenna architectures can be used for directing energy in the form of a narrow beam toward a mobile device. According to a so-called multibeam or switched-beam approach for example, a particular beam is selected from a set of fixed beams to reach a particular mobile device. According to an alternative approach, a steerable beam is directed toward the mobile device. The main advantage of this latter approach is that beam forming in the downlink direction is not limited to a fixed set of beams or beam shapes. In either case, the direction of the downlink beam is usually derived by estimating the direction of arrival of an uplink beam from the mobile device.
Adaptive and sector antenna solutions require that special care is taken of the power amplifier arrangement in the downlink direction. If for example a separate power amplifier is provided for each beam or sector, the individual power amplifiers have to be designed for the worst case, i.e. the case that all mobile devices are located in a single beam direction or sector. Such a worst case design of the individual power amplifiers makes the amplifiers rather expensive. On the other hand, if the individual power amplifiers are not designed for the worst case the capacity gain resulting from adaptive or sector antennas can not be fully exploited.
In order to avoid the tradeoff between high capacity gain and cost efficient power amplifier design, power amplifier pooling is desirable. By means of power amplifier pooling the accumulated power of a plurality of beams or sectors is distributed among a plurality of power amplifiers according to a specific distribution scheme. If for example the distribution scheme specifies that the accumulated power of all beams or sectors is to be equally distributed among the individual power amplifiers, each power amplifier can be designed for only 1/n of the accumulated power of all beams or sectors, n being the number of beams or sectors.
There is a need for a method and a device for efficiently performing power amplifier pooling in particular in a narrow beam or sectorized environment.
As regards a method, this need is satisfied according to the invention by combining power amplifier pooling with peak power limitation. More specifically, the method comprises superimposing two or more digital beam or sector signals to generate combined signals that allow to distribute the accumulated power of beams or sectors according to a specific distribution scheme among the power amplifiers, individually subjecting the combined signals to peak power limitation, and individually amplifying the peak power limitated signals in the power amplifiers.
Thus, the plurality of superimposed digital beam or sector signals included in an individual combined signal may jointly be subjected to peak power limitation. During peak power limitation of an individual combined signal information regarding other combined signals can be taken into account.
Peak power limitation may be performed in the digital domain, e.g. prior to a digital-to-analog conversion. Various mechanisms for peak power limitation can be implemented. As examples, clipping mechanisms or cancellation mechanisms that are based on an estimate of the signal at an input of an individual power amplifier can be mentioned.
The invention may be practiced in a single carrier or in a multiple carrier scenario. In a multicarrier scenario the individual digital beam or sector signals are preferably superimposed carrier-wise, i.e. for each carrier separately. Peak power limitation in the multiple carrier scenario may be performed separately for each individual carrier signal on the basis of parameters that have been generated taking several or all carrier signals into consideration.
According to a preferred variant of the invention, the power distribution is performed in combination with digital predistortion. As regards the signal path of a particular combined signal, the power amplifier output signal can be exploited as feedback signal for the purpose of digital predistortion. However, in principle digital predistortion may be performed in the absence of a feedback signal also. The distribution of the accumulated power of the beams or sectors already in the digital domain enables to apply the inherent advantages of digital predistortion, i.e. compensation of non-linear power amplifier effects, to the individual amplifiers of a power amplifier pool.
The distribution scheme according to which the accumulated power is distributed among the power amplifiers can be predefined or can be selected according to e.g. the current network traffic. Various different distribution schemes can be implemented. According to a first variant of the invention, the distribution scheme specifies that the accumulated power of all beams or sectors is equally distributed among all or a subset of the available power amplifiers. According to a second variant, the distribution scheme specifies a non-equal distribution of the accumulated power among the individual power amplifiers. Such a non-equal distribution is particularly advantageous for e.g. beam forming purposes and allows to use differently designed power amplifiers for the power amplifier pool.
The individual combined signals, which have been generated by superimposing two or more digital beam or sector signals, may be identical or not. Thus, the combined signals may deviate from each other with respect to the superimposed digital beam or sector signals comprised therein. Preferably, however, the same set of digital beam or sector signals is used to generate all combined signals. The generation of the combined signals may include calculating the amplitude and phase weighted sum of all digital beam or sector signals.
Generation of the combined signals can be performed prior to at least one of digital predistortion and peak power limitation. An efficient solution will be to integrate the generation of the combined signals with the weighting and combining that is performed for example in wideband code division multiple access (WCDMA) schemes.
Different technical realizations for distributing the accumulated power among the power amplifiers can be implemented. For example the combined signals may be generated using a dedicated digital coupler matrix. Alternatively or additionally, a digital network for beamforming or sectorshaping can be used. In some cases it might be necessary to extract individual analog beam or sector signals corresponding to the digital beam or sector signals from the output signals of the power amplifiers. To that end, a signal transformer in the form of e.g. an inverse (analog) coupler matrix may be used.
In the case that analog beam or sector signals are to be derived from the power amplifier output signals, and in many other cases, it might be necessary to adjust (e.g. compensate) the relative delays present in the analog domain between e.g. individual upconverted RF signals or the individual power amplifier output signals. Delay control is preferably performed such that in a first step the relative delay between the signals in the analog domain is determined and in a second step the delay is adjusted in the digital domain in accordance with the determined relative delay.
Although the method according to the invention can be performed for various purposes, a preferred variant of the invention relates to performing the method in context with the generation of antenna input signals. Thus, the power amplifier output signals or signals obtained from the power amplifier output signals (like the analog beam signals mentioned above) can be fed to antenna elements that may include one or more individual antenna arrays or sector antennas.
The invention can be implemented as a hardware solution or as a computer program product comprising program code portions for performing the steps of the invention when the computer program product is run on a computing device. The computer program product may be stored on a computer-readable recording medium like a data carrier in fixed association with or removable from the computing device.
As regards the hardware solution, the invention is directed to a transmitter stage for a wireless communications system. The transmitter stage comprises a distributing component for distributing the accumulated power of beams or sectors according to a specific distribution scheme among a plurality of power amplifiers. Additionally, a power amplifier pool and peak power limiting components are provided. The distributing component includes a plurality of signal inputs for digital beam or sector signals and a plurality of signal outputs for combined signals that have been generated by superimposing two or more of the digital beam or sector signals. Individual transmitter units of a transmitter block may be coupled to the outputs of the peak power limiting components and may be configured to perform at least digital-to-analog conversion and RF upconversion. Individual power amplifiers of the power amplifier pool are coupled to the peak power limiting components preferably via the individual transmitter units and amplify the peak power limited signals. Digital predistortion components may be arranged between the signal outputs of the distributing component and the components of the transmitter units that perform digital-to-analog conversion. The digital predistortion components preferably tap the output signal of the associated power amplifiers to obtain a feedback signal.
The transmitter units may be configured as transceiver components, i.e. as components having both a transmitter and a receiver function. The digital predistortion components may be integrated in the transmitter units or may be configured as components separate from the transmitter units.
The transmitter stage may additionally include at least one of a signal transformer for transforming the power amplifier output signals into individual analog beam or sector signals and a delay controller for adjusting relative delays between the individual upconverted RF signals or power amplifier output signals. The distributing component, which may be configured as a digital network for beamforming or sectorshaping or as a digital coupler matrix, is preferably arranged in a signal path after or within a processing unit for generating the digital beam or sector signals, like a baseband spreader unit for spreading the beam or sector signals. Such a spreader unit will for example be required if a WCDMA scheme is to be implemented.
The transmitter stage is advantageously included in an antenna system, e.g. a sectorized antenna system or an adaptive antenna system of the type that has a multibeam or switched-beam architecture or of the type that generates a steerable beam.
In the following the invention will be described with reference to exemplary embodiments illustrated in the figures, in which:
In the following description, for purposes of explanation and not limitation, specific details are set forth, such as particular embodiments, circuits, signal formats, etc. in order to provide a thorough understanding of the present invention. It will be apparent to one skilled in the art that the present invention may be practiced in other embodiments that depart from these specified details. In particular, while the different embodiments are described herein below incorporated with a WCDMA adaptive antenna system, the present invention is not limited to such an implementation, but for example can be utilized in any transmission environment that allows amplifier pooling and in particular in combination with a sectorized antenna system. Moreover, those skilled in the art will appreciate that the functions explained herein below may be implemented using individual hardware circuitry, using software functioning in conjunction with a programmed microprocessor or general purpose computer, using an application specific integrated circuit (ASIC), and/or using one or more digital signal processors (DSPs).
In
The inputs 181 of the digital coupler matrix 18 are coupled to the downlink baseband processing unit 16 and the corresponding outputs 182 to individual peak power limiting components 221 . . . 224 of the peak power limiting block 22. Various exemplary mechanisms for performing peak power limitation will be explained in more detail below.
The peak power limiting block 22 is coupled to the transmitter block 24 which includes a plurality of transceiver units 241 . . . 244. More specifically, each transceiver unit 241 . . . 244 is coupled via an individual peak power limiting component 221 . . . 224 to one of the signal outputs 182 of the digital coupler matrix 18.
The power amplifier pool 26 including a plurality of power amplifiers 261 . . . 264 is arranged in a signal path behind the transmitter block 24. As becomes apparent from
Outputs of the power amplifiers 261 . . . 264 are coupled to the inputs 201 of the inverse coupler matrix 20. The corresponding outputs 202 of the inverse coupler matrix 20 are coupled to the plurality of antenna elements 12.
The configuration of a single one of the plurality of transceiver unit/power amplifier combinations shown in
From
An input of the power amplifier 261 is coupled to an output of the analog transmitter part 36 and an output of the power amplifier 261 is coupled via a directional coupler 42 to the input of the analog receiver part 38. The signal path between the directional coupler 42 and the digital predistortion component 32 including the receiver path 44 thus constitutes a feedback path for an output signal of the power amplifier 261. The function of this feedback path will be described in more detail below. It should be noted that the digital predistortion component 32 also perform its task without receiving a feedback from the output of the power amplifier 261. In such a case the feedback path can thus be omitted.
Now the function of the first embodiment of an adaptive antenna system will be described with reference to
The combination of multiple traffic channel signals into a single WCDMA signal or of independent WCDMA signals into a combined WCDMA signal is performed in the downlink baseband processing unit 16. From user or control data input into the downlink baseband processing unit 16 a plurality of complex baseband beam signals including an in-phase component and a quadrature component are generated. In the exemplary embodiment depicted in
The four digital beam signals output by the downlink baseband processing unit 16 are input to the digital coupler matrix 18 via the individual coupler matrix inputs 181. In the exemplary embodiment depicted in
The digital coupler matrix 18 generates a combined signal by calculating the phase weighted sum of the four digital beam signals. In other words, the individual digital beam signals are superimposed in a non-correlated manner. The combined signal thus obtained is output via each of the four signal outputs 182 of the digital coupler matrix 18. Thus, the total power of all beams is equally distributed among four individual branches shown in
The combined signal which has been generated in the digital coupler matrix 18 for the purpose of equally distributing the accumulated power of the beams among the power amplifiers 261 . . . 264 is input into the peak power limitation component 221 for performing joint multiple carrier peak limitation. Various limitation mechanisms can be implemented by the peak power limitation component 221. For example, the peak power limitation component 221 may perform baseband clipping as described in U.S. Pat. No. 6,266,320 B1, herewith incorporated by reference as far as an exemplary baseband clipping scheme Is concerned. During baseband clipping, the combined signal is digitally limited to thereby limit the peak-to-average power ratio. This, in turn, is accomplished by measuring the instantaneous amplitude for the in-phase and quadrature components that make up to the combined signal, deriving a maximum amplitude based on the instantaneous amplitude measurements, and then deriving one or more scaling factors based on the maximum amplitude. The one or more scaling factors are then applied to the in-phase and quadrature components.
An alternative approach for performing peak power limitation that may be implemented by the peak power limitation component 221 is described in WO 02/11283, herewith incorporated by reference as far as peak power limitation is concerned. The alternative approach includes estimating the amplitude of the analog RF signal that will be input to the power amplifier 261 and, based on the estimation, adjusting within the peak power limitation component 221 the amplitude of the combined signal such that the amplitude of the analog RF signal input to the power amplifier 261 will remain below a predefined threshold.
Of course, other mechanisms like the clipping approach disclosed in EP 1 217 779 A1, herewith incorporated by reference as far as clipping is concerned, could be used as well by the peak power limitation component 221 for peak power limitation purposes.
Due to the fact that the peak power limitation components 221 . . . 224 are arranged in a signal path behind the digital coupler matrix 18, each individual peak power limitation component 221 . . . 224 jointly limits a plurality of superimposed digital beam signals. Thus, the high peak-to-average power ratio of WCDMA multiuser signals can efficiently be reduced. From the point of view of reducing the peak-to-average power ratio, the superimposed digital beam signals input into each peak power limiting component 221 . . . 224 behave like a single WCDMA signal. In the present embodiment reduction of the peak-to-average power ratio is thus not effected by the number of digital beam signals included in a particular combined signal.
This would be different if the superposition of the beam signals was performed in the analog domain, i.e. after the transmitter block 24. In such a case the individual digital beam signals would have to be jointly peak power limited in the same way as described in U.S. Pat. No. 6,266,320 B1 for multiple carriers. However, since the subsequent superposition of the individual beam signals in the analog domain causes the amplitude peaks to regrow, it has been found that joint peak power limitation is limited to approximately four beam signals. Such a limitation does not exist in the embodiment depicted in
The combined signal that has been peak power limited in the peak power limitation component 221 is fed as digital input signal to the transceiver unit 241. As can be seen from
In the present embodiment the output signal of the power amplifier 261 is tapped by the directional coupler 42 and a signal is fed back via a feedback path to the digital predistortion component 32. The feedback signal is down converted in the analog receiver part 38, converted to the digital domain by the analog-to-digital converter 40 and input to the digital predistortion component 32. By comparing the feedback signal with the combined signal received from the digital transmitter part 30, the predistortion parameters of the digital predistortion component 32 are adapted for an optimal linearisation. As has been mentioned before, digital predestortion could also be performed if no feedback signal was available.
The predistorted combined signal output by the digital predistortion component 32 is converted to analog by the digital-to-analog converter 34 and then upconverted to RF by the analog transmitter part 36. The upconverted RF signal output by the analog transmitter part 36 is input to the power amplifier 261.
As can be seen from
Due to imperfections of the analog components of the transmitter stage 14, relative delays between the individual analog signals input to the inverse coupler matrix 20 can occur. Therefore, a digital delay controller 48 is provided which allows to digitally adjust the absolute delay of each of the four branches in such a way that the inverse coupler matrix 20 receives the analog output signals of the power amplifiers 261 . . . 264 substantially simultaneously. The digital delay controller 48 may be configured as described in EP 1 217 779 A1, herewith incorporated by reference as far as the construction and operation of the digital delay controller 48 is concerned. It should be noted that for clarity reasons the individual control lines associated with the digital delay controller 48 are not shown in
In
As can be seen from
For beamforming purposes, the combined signals distribute the accumulated power of the beams according to a specific beamforming scheme among the power amplifiers 261 . . . 264. This means that contrary to the embodiment depicted in
An advantage of the digital distributing components, i.e. the digital coupler matrix 18 of
In the embodiments depicted in
According to a steered beam approach downlink control of the adaptive antenna systems 10 depicted in
While the present invention has been described with respect to particular embodiments, those skilled in the art will recognize that the present invention is not limited to the specific embodiments described and illustrated herein. Therefore, while the present invention has been described in relation to its preferred embodiments, it is to be understood that this disclosure is only illustrative. Accordingly, it is intended that the invention be limited only by the scope of the claims appended hereto.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP02/14680 | 12/20/2002 | WO | 5/25/2005 |
