Patent Application
20050118965
1. Field of the Invention
The present invention is related to a wireless transmitter.
2. Description of the Prior Art
Linear operation is generally required for a radio frequency power amplifier used for transmitting signal power to an antenna when a signal modulation method that accompanies amplitude modulation is used, especially when a M-ary modulation method such as a QAM(Quadrature Amplitude Modulation) method is used. For this reason, class A or class AB operation has been conventionally used for a radio frequency power amplifier.
However, on the other hand, communication technologies that use multiple carriers such as an OFDM (Orthogonal Frequency Division Multiplex) technology have become commonly used in accordance with the development of broadband communications. Consequently, in such a case, high operational efficiency cannot be expected any more for a conventional radio frequency power amplifier that is operated at class A or class AB. More precisely, a large signal power may be generated instantly and at random in the OFDM scheme that allows sub-carrier spectrum overlaps. It therefore leads to a large peak to average power ratio (PAPR) between the average power and the peak power of modulated signal for the amplifier operation. For this reason, a large DC power should always be maintained in the amplifier so that it may linearly amplify a peak power considerably larger than the average power. As a result, the DC power given by multiplication of the current and a voltage difference between a peak voltage that ensures the peak power and a average voltage that ensures the average power will almost be wasted as heat, except for a time period during which the peak power is outputted. This means that the power supply efficiency of the amplifier will be severely lowered.
For a portable radio system that uses, for example a battery as a power supply, the above situation is practically unacceptable since its available time of continuous operation becomes short. The Envelope Elimination and Restoration (EER) method that is known as Kahn Technique has proposed to solve such a problem as is explained above (see, for example U.S. Pat. No. 6,256,482 (
As an example of OFDM signals, an OFDM signal specified by IEEE standard 802.11a for wireless LAN applications is included. In order to satisfy the Error Vector Magnitude (EVM) specification of 5.6% specified by the standard, the radio frequency power amplifier 605 must operate at 7 dB lower than its maximum saturation power level (at the back-off of 7 dB), in the example. It means that the radio frequency power amplifier 605 utilizes only 20% of its output power capability.
If the amplifier operates at class A, the maximum drain efficiency (the ratio of the output radio frequency signal power and the input DC power) of the amplifier is 50%, and its power consumption is constant regardless of its output radio frequency signal power. As a result, when an OFDM modulated signal in this example is supplied to the amplifier, its power efficiency will be 10% at the maximum. As explained above, an ordinary class A radio frequency power amplifier cannot attain high power efficiency.
In order to solve the problem, in the EER method, the phase modulation component with a constant envelope amplitude is applied to the signal input terminal RFin of the radio frequency power amplifier 605, and the amplitude component is applied to the power supply input terminal VDD of the radio frequency power amplifier 605. Amplitude information and phase information are then multiplied at an output terminal RFout of the radio frequency power amplifier 605, so that the original OFDM vector signal will be recovered.
By using such a configuration explained above, the radio frequency power amplifier 605 can operate at the back-off of 0 dB, thereby making it possible to realize the maximum drain efficiency of nearly 50%, when it operates at, for example class A. An amplifier which is operated at a non-linear class of operation such as class B, C, E, or F may also be used. Since the drain efficiency of such an amplifier is larger than the drain efficiency of the class A operating amplifier, higher drain efficiency can be expected for the amplifier operating at one of these classes.
Generally, in the EER method, when deploying the radio frequency power amplifier 605 operating at one of the especially high efficiency classes mentioned above, it is necessary to apply a correction to the amplitude component and the phase component of the input signal so that the radio frequency power amplifier 605 may produce a correct output signal. In a distortion-compensation type amplifier that realizes aforementioned conditions, the output signal of a radio frequency power amplifier 704 is fed back to the input side by down-converting a frequency of an output signal into an intermediate frequency (IF) by using a frequency converter 705 as shown in
However, since a compensation value optimization requires a complicated circuit and additional power consumption is also required for the feed back loop that has no relation to signal power amplification, it is difficult to realize a signal transmitter with low power consumption by using the composition of
However, when correcting the tables according to the ambient temperature of the radio frequency power amplifier 804, the ambient temperature is easily affected by the operation environment of the amplifier. This means that the measured temperature does not always represent the real temperature within a channel of the radio frequency power amplifier 804.
In the EER method in particular, when the signal is separated into the amplitude component and the phase component, since bandwidth limitations of the D-A converter (DAC) and the quadrature modulator is further added thereto, there is a tendency that an alternative adjacent channel power ratio especially is generally deteriorated as compared with a spectrum when performing class A linear operation. Imperfect distortion compensation cannot be allowed in the EER method since it further increases the alternative adjacent channel power ratio.
Accordingly, although it is necessary to reflect the radio frequency amplifier output signal to the correction, complicated processing and circuits used for the purpose as shown in the second prior art hinder an improvement in power efficiency of the amplifier, and even offset an improvement effect by the EER method, especially when the EER method is used for improvement in the signal transmitter power efficiency.
The object of the invention is to provide a signal transmitter that can compensate for thermal characteristics of a radio frequency power amplifier efficiently and correctly.
In order to attain above object, a signal transmitter of a first invention comprises radio frequency (RF) power coupling means of taking out an RF power proportional to an output power of the radio frequency power amplifier to which a modulated signal is supplied at an output of the radio frequency power amplifier, and means of detecting an output voltage of the RF power coupling means. Moreover, it comprises storage means of recording a correction datatable for applying a change to the modulated wave signal supplied into the radio frequency amplifier corresponding to an output result of the above means of detecting the output voltage, and means of applying a change to the modulated wave signal supplied into the radio frequency power amplifier with reference to corresponding correction data from the storage means according to the output result of the means of detecting the output voltage.
According to this configuration, since the output power of the radio frequency power amplifier is detected, it is possible to correctly detect the channel temperature of the radio frequency power amplifier. Accordingly, a correction is carried out correctly. Additionally, a simple configuration does not allow an increase in power consumption. Hence, a correct compensation of the temperature characteristics of the amplifier can be carried out while maintaining high efficiency of the signal transmitter.
Hereafter, description will be made in detail. Whereas the ambient temperature of the radio frequency power amplifier is estimated in the prior art, according to the signal transmitter of the first invention, since the output power of the radio frequency power amplifier is detected, it is possible to correctly detect the channel temperature of the radio frequency power amplifier. Consequently, a correction is carried out correctly.
Also in the other prior art, the output signal of the high output power amplifier is subjected to a frequency conversion and then a quadrature detection, and the compensation value table is updated by resultant I and Q data (IQ data). As opposed to this, according to the signal transmitter of the first invention, the output voltage is detected, so that a configuration is simple and an increase in power consumption will be suppressed. Hence, a correct compensation of the temperature characteristics of the amplifier can be carried out while maintaining high efficiency of the signal transmitter.
Moreover, in order to achieve the above object, a signal transmitter of a second invention comprises modulated wave signal generating means of generating at least a modulated wave signal, phase-amplitude separation means of separating the modulated wave signal supplied from the modulated wave signal generating means into a phase component and an amplitude component as expressed on a polar coordinate. It also comprises a constant voltage source, and DC-DC conversion means into which the amplitude component and an output voltage of the constant voltage source are supplied to perform a voltage conversion of the voltage of the constant voltage source to a voltage proportional to the amplitude component. It further comprises a radio frequency power amplifier which supplies the phase component into a signal input terminal thereof, supplies the output of the DC-DC conversion means into a power supply terminal thereof, and produces a resultant modulated wave which is obtained by multiplying the amplitude component by the phase component.
In addition to the above configuration, it comprises RF power coupling means of taking out an RF power proportional to an output power of the radio frequency power amplifier at an output of the radio frequency power amplifier, and means of detecting an output voltage of the RF power coupling means. It still further comprises storage means of recording a correction datatable for applying a change to the amplitude component and the phase component supplied into the radio frequency amplifier corresponding to an output result of the means of detecting the output voltage, and means of applying a change to an output of the phase-amplitude separation means with reference to corresponding correction data from the storage means according to an output result of the means of detecting the output voltage.
According to this configuration, since the output power of the radio frequency power amplifier is detected, it is possible to correctly detect the channel temperature of the radio frequency power amplifier, and since a configuration is simple, an increase in power consumption will be suppressed, thereby making it possible to correctly compensate the temperature characteristics while maintaining highly efficiency of the signal transmitter using the EER method.
Hereafter, description will be made in detail. Whereas the ambient temperature of the radio frequency power amplifier is estimated in the prior art, according to the signal transmitter of the second invention, since the output power of the radio frequency power amplifier is detected, it is possible to correctly detect the channel temperature of the radio frequency power amplifier. Consequently, a correction is carried out correctly.
In the other prior art, the output signal of the high output power amplifier is subjected to a frequency conversion and then a quadrature detection, and the compensation value table is updated by using the I and Q data (IQ data) obtained by quadrature detection. As opposed to this, according to the signal transmitter of the second invention, since a configuration for detecting the output power amplitude is simple, an increase in power consumption will be suppressed. Hence, a correct compensation of the temperature characteristics of the amplifier can be carried out while maintaining high efficiency of the signal transmitter. Additionally, a temperature correction can be performed without spectral degradation in the EER method having few spectrum mask margin.
In the signal transmitter of the above second invention, it is preferable to have frequency conversion means between the radio frequency power amplifier and an output of the phase component of the phase-amplitude separation means.
Generally, since the operating bandwidth of the phase-amplitude separation means (including DAC) is hundreds of MHz at most, when the carrier frequency exceeds a GHz order, it cannot manage this frequency order. However, according to this configuration, by using a quadrature modulator or the like which is, for example frequency converting means, it becomes easily possible to upconvert the carrier frequency into the GHz band or the like.
Hereafter, referring to the drawings, description will be made of embodiments according to the present invention. In this embodiment, QPSK (Quadrature Phase Shift Keying) is considered as a modulated wave. The QPSK method is a four-ary digital modulation scheme that maps an input digital signal onto X-Y coordinates by a vector synthesis of an In-Phase signal component (I) that has the same vector direction as an X-axis and a Quadrature signal component (Q) that is shifted by 90 degrees from the In-Phase signal component and has the same vector direction as a Y-axis.
The above QPSK signal generating means 101 generates a QPSK signal, and corresponds to modulated wave signal generating means of generating a modulated wave signal.
The selector 107 selects from the correction table 108 correction table data corresponding to an output power level of the radio frequency power amplifier 104. The configuration is as follows, for example. That is, an analog voltage supplied from the output detecting section 106 is digitizes by an A/D converter integrated in the selector 107. A digitized voltage is then compared with respective voltage data at the time of obtaining a plurality of correction tables which are included inside the selector 107. An address of the correction table 108 corresponding to a voltage data closest to the digitized voltage is then specified.
Alternatively, the analog voltage from the output detecting section 106 is compared with a plurality of voltage levels at the time of obtaining the correction table by a plurality of comparators. An address of the correction table 108 is then specified according to output patterns of a plurality of comparator outputs.
The correction table 108 supplies one of correction data sequences A through D represented by correction A through D by addressing from the selector 107. The correction data sequences A through D are determined as follows, for example.
Symbol A is a correction data sequence obtained as a result of evaluation in an ambient (room) temperature, symbol B is a correction data sequence obtained as a result of evaluation at the highest temperature specified in an absolute rating, symbol C is a correction data sequence obtained as a result of evaluation at the lowest temperature specified in the absolute rating, and symbol D is a correction data sequence obtained as a result of evaluation at a temperature between the absolute rating temperature and the ambient temperature.
Each of the correction data sequences A through D records a supply voltage which should be supplied to the radio frequency power amplifier 104 and a phase rotation which should be supplied to the phase component for a signal amplitude which is desired to be produced from the radio frequency power amplifier 104. Ideally, in the radio frequency power amplifier 104, an output signal amplitude may perfectly be proportional to the supply voltage, and even when the supply voltage is changed, a phase difference between an output signal and an input signal may be constant. However, since it will not actually happen, it is required to correct the supply voltage and the phase of the input signal for the signal amplitude which is desired to be produced.
The IQ signal correction means 102 receives an output from the QPSK signal generating means 101, namely as an amplitude and a phase of a signal which is desired to be produced from the radio frequency power amplifier 104, it obtains an output of a correction result of an IQ signal corrected by an optimum correction table sequence of the correction table 108 which is selected by the selector 107.
The quadrature modulator 103 performs a frequency conversion of a phase component supplied from the IQ signal correction means 102 and obtains an output of an IQ modulated wave.
The output coupling section 105 couples an RF power proportional to the output power of the radio frequency power amplifier 104 (for example, 10 dB coupling) to supply to the output detecting section 106.
In the output detecting section 106, a circuit which is composed of, for example a diode and a smoothing capacitor detects an envelope of the RF power proportional to the output power of the radio frequency power amplifier 104.
The radio frequency power amplifiers 104 may be operated at, for example class F, and supplies a modulated wave signal supplied from the quadrature modulator 103 into a signal input terminal and performs a desired power amplification to produce it.
Hereinafter, description will be made of the operation using
Description will be made of a process sequence using
In the output detecting section 106, the RF power taken out by the output coupling section 105 is rectified by a diode and is then smoothed by a smoothing capacitor to be supplied as an average voltage. The average voltage taken out by the output detecting section 106 is compared with output average voltages measured at various environmental temperatures of the radio frequency power amplifier 104 in advance inside the selector 107. A correction table at an environmental temperature corresponding to an output average voltage closest to the average voltage taken out by the output detecting section 106, for example the correction B is then selected from the correction table 108. These processes are performed within an allowable range of a preamble signal which exists, for example in an early stage of a burst, and the correction data selected in the early stage are used during subsequent burst intervals.
According to a configuration described above, in the first embodiment, since the table data is selected by using the output of the radio frequency power amplifier 104, a channel temperature in operation of the radio frequency power amplifier 104 can be reflected correctly. Thereby making it possible to perform the correction of the radio frequency power amplifier 104 correctly. Moreover, since quadrature detection is not used for the output detecting section 106, a correction circuit and an algorithm can be simplified, thereby making it possible to achieve reduction in power consumption.
Hereafter, referring to the drawings, description will be made of a second embodiment according to the present invention. In this embodiment, an OFDM modulated wave signal is considered as the modulated wave signal. As a system using OFDM, it includes a wireless LAN system of IEEE802.11a specification, for example. In a wireless LAN system, respective 52 subcarriers which perpendicularly intersect to each other are modulated using, for example 64 QAM technology, and respective modulated subcarriers are transformed by using a inverse Fourier transform scheme and are then multiplexed to generate an OFDM modulated signal. The 52 subcarriers are separated to each other by 312.5 kHz, respectively, and occupy a bandwidth of 52×312.5=16.25 MHz.
The above OFDM signal generating means 301 generates an OFDM signal and corresponds to modulated wave signal generating means of generating a modulated wave signal.
The phase-amplitude separation means 302 separates the OFDM modulated wave signal (OFDM vector signal, namely IQ signal) generated by the OFDM signal generating means 301 into a phase component and an amplitude component.
The selector 310 selects from the correction table 311 correction table data corresponding to an output power level of the radio frequency power amplifier 305. The configuration is as follows, for example. That is, an analog voltage supplied from the output detecting section 309 is digitizes by an A/D converter integrated in the selector 310. A digitized voltage is then compared with respective voltage data at the time of obtaining a plurality of correction tables which are included inside the selector 310. An address of the correction table 311 corresponding to a voltage data closest to the digitized voltage is then specified.
Alternatively, the analog voltage of the output detecting section 309 is compared with a plurality of voltage levels at the time of obtaining the correction table by a plurality of comparators, respectively. An address of the correction table 311 is then specified according to output patterns of a plurality of comparator outputs.
The correction table 311 supplies one of correction data sequences A through D represented by correction A through D by addressing from the selector 310. The correction data sequences A through D are determined as follows, for example.
Symbol A is a correction data sequence obtained as a result of evaluation in an ambient temperature, symbol B is a correction data sequence obtained as a result of evaluation at the highest temperature specified in an absolute rating, symbol C is a correction data sequence obtained as a result of evaluation at the lowest temperature specified in the absolute rating, and symbol D is a correction data sequence obtained as a result of evaluation at a temperature between the absolute rating temperature and the ambient temperature.
Each of the correction data sequences A through D records a supply voltage which should be supplied to the radio frequency power amplifier 305 and a phase rotation which should be supplied to the phase component for a signal amplitude which is desired to be produced from the radio frequency power amplifier 305. Ideally, in the radio frequency power amplifier 305, an output signal amplitude may perfectly be proportional to the supply voltage, and even when the supply voltage is changed, a phase difference between an output signal and an input signal may be constant. However, since it will not actually happen, it is required to correct the supply voltage and the phase of the input signal for the signal amplitude which is desired to be produced.
The phase-amplitude correction means 303 receives an output of the OFDM signal generating means 301, namely an amplitude and a phase of a signal which is desired to be produced from the radio frequency power amplifier 305 and obtains an output of a correction result of an OFDM signal corrected by an optimum correction table sequence of the correction table 311 which is selected by the selector 310.
The quadrature modulator 304 performs a frequency conversion of a phase component supplied from the phase-amplitude correction means 303 and obtains an output of an OFDM modulated wave.
The DC-DC converter 308 adjusts an offset voltage and an amplitude so that the amplitude component may swing between the supply voltage maximum rating of the radio frequency power amplifier 305 and 0 V with respect to the amplitude component supplied from the phase-amplitude correction means 303. It then converts a DC voltage supplied from the supply voltage 307 into an adjusted amplitude component.
The output coupling section 306 couples an RF power proportional to the output power of the radio frequency power amplifier 305 (for example, 10 dB coupling) to supply to the output detecting section 309.
In the output detecting section 309, a circuit which is composed of, for example a diode and a smoothing capacitor detects an RF power proportional to the output power of the radio frequency power amplifier 305.
The radio frequency power amplifiers 305 may be, operated at, for example class F, and a modulated wave signal supplied from the quadrature modulator 304 is supplied into a signal input terminal and subsequently amplified in power. Moreover, the amplitude component to which an voltage conversion ha been performed by the DC-DC converter 308 is supplied from a power supply terminal. As a result, the radio frequency power amplifier 305 produces an original OFDM signal modulated wave, the phase and the amplitude of which are both modulated, namely the amplitude and the phase of which are multiplied.
Hereinafter, description will be made of the operation using
A format of a preamble signal of 802.11a used as an example is shown in
Description will be made of a process sequence using
In the output detecting section 309, the RF power taken out by the output coupling section 306 is rectified by a diode and is then smoothed by a smoothing capacitor, and produced as an average voltage of preamble time. The average voltage taken out by the output detecting section 309 is compared with output average voltages measured at various environmental temperatures of the radio frequency power amplifier 305 in advance inside the selector 310. A correction table at an environmental temperature corresponding to an output average voltage closest to the average voltage taken out by the output detecting section 306, for example the correction B is then selected from the correction table 311. These processes are completed within about {fraction (7/10)} of 8 μsec and a correction result will be reflected to an OFDM signal during a subsequent burst interval.
According to a configuration described above, in the second embodiment, since the table data is selected by using the output of the radio frequency power amplifier 305, a channel temperature in operation of the radio frequency power amplifier 305 can be reflected correctly. Thereby making it possible to perform the correction of the radio frequency power amplifier 305 correctly. Moreover, since quadrature detection is not used for the output detecting section 309, a correction circuit and an algorithm can be simplified, thereby making it possible to achieve reduction in power consumption.
Moreover, according to a configuration of the present invention, the EER method can perform an accurate temperature compensation without degradation to an alternative adjacent channel power ratio.
Moreover, by using the EER method, the radio frequency power amplifier can be operated near a saturation point. Therefore, the radio frequency power amplifier can be operated at a mostly theoretical maximum drain efficiency, thereby making it possible to achieve high efficiency.
Industrial Availability
A signal transmitter of the present invention is useful in radio communications systems which require reduction in power consumption, such as a cellular phone and a wireless LAN, as an highly efficient signal transmitter.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2003-402615 | Dec 2003 | JP | national |
