WO2004077662A1

WO2004077662A1 - Power amplifying device for communication system

Info

Publication number: WO2004077662A1
Application number: PCT/CN2003/000144
Authority: WO
Inventors: James Peroulas
Original assignee: Huawei Technologies Co., Ltd
Priority date: 2003-02-25
Filing date: 2003-02-25
Publication date: 2004-09-10
Also published as: CN1695297A; AU2003213984A1; CN100474762C

Abstract

A power-amplifying device for a communication system is disclosed. It comprises a decomposer, two D/A converters, two power amplifiers and a signal combiner. It further includes another power amplifier before the decomposer and a power detection & gain factor calculation device that measures the average power of the source signal of the power-amplifying device and adjusts the gain factors of the three power amplifiers accordingly. With this power amplifying device, a complex network is not needed to compensate for amplifier imbalance and neither is a complex measurement device required to measure the absolute performance of every component. With this invention, lighter power cables can be used, fewer backup batteries are needed, and fewer heat sinks are needed. Also, the gain factors of the amplifiers can be adjusted by changing their DC supply voltage. This will reduce their power consumption.

Description

POWER AMPLIFYING DEVICE FOR COMMUNICATION SYSTEM

FIELD OF THE INVENTION

The invention relates to a signal transmission system, and more particularly to a power amplifying device for a communication system.

BACKGROUND OF THE INVENTION

The final step in a communication system involves the amplification of a low power signal to the power level that is required by the transmission medium. As an example, in a radio communication transmitter, the source information signal may have a power level of about 1 milliwatt whereas the radio channel may require a transmit power ten thousand times greater at about 10 watts. The process of amplifying such a low power signal to a high power level is called power amplification and is accomplished by using a power amplifier or PA.

The power amplifier has some very difficult requirements. It must accurately amplify the source signal, it must not distort the source signal, it must not transmit energy outside the frequency range of the source signal, it must act on the entire bandwidth of the source signal, and it must do all of the above in a very efficient manner. For a fixed architecture and process technology, the classical tradeoffs have been between bandwidth, efficiency, and linearity. Improving one parameter necessarily reduces the performance of the other parameters .

In the past, power amplifier design problems have been avoided (not solved) by modulating the information signal in a way that the result of the modulation was easy to amplify. As an example, a digital source signal that has been FM modulated can be amplified to high power levels using cheap and efficient class C amplifiers. Unfortunately, modulation schemes that are easy to amplify are out of favor nowadays because these modulation schemes are typically either spectrally inefficient (ex. FM) or inefficient in their use of transmitted power (ex. AM) .

Modern modulation schemes such as QAM do not allow any tricks to be used and the power amplification problem can no longer be avoided. Modern transmitters must accept wide bandwidth signals where the information is carried both in the amplitude and in the phase of the transmitted signal . Several architectures have been proposed to create an efficient, wide bandwidth, linear amplifier and one such architecture is called the LINC architecture, which stands for Linear amplification using Nonlinear Components. This architecture also goes by the names "Outphasing" and "Chireix" .

As shown in Fig.l, the fundamental principle of the LINC architecture is that the digital source signal s (n) is decomposed by a decomposer 10 into two digital component signals s_x (n) and s₂ (n) such that s (n) =s_λ (n) +s₂ (n) . The two digital component signals Si (n) and s₂ (n) each have the property that for all values of n, their magnitude is always constant. No matter what the digital source signal s (n) looks like, no matter what the power of the digital signal s (n) is, the magnitude (and hence the power) of the digital component signals s₁ (n) and s₂ (n) will always be exactly the same. In words, the digital source signal s (n) , which contains both amplitude and phase variations, has been decomposed into two signals that have only phase variations. The following equations describe the signal decomposition operation and assume that the digital source signal s (n) is constrained so that its magnitude is less than or equal to 1.

The two digital component signals s (n) and s₂ (n) are converted into two analog signals Sη_.(t) and s₂(t) by means of a first D/A converter 11 and a second D/A converter 12. Preferably, the two analog signals s_x(t) and s₂(t) are modulated to a desired carrier frequency f_c by means of a modulator 13.

The .modulated signals are respectively amplified by a first power amplifier 15 and a second power amplifier 16. The amplified signals x_x(t) and x₂(t) are combined by a signal combiner 17 to obtain an output signal x(t) to be transmitted through an antenna.

The reason why this architecture has received attention is that although it requires two power amplifiers, the power amplifiers in this architecture are much easier to construct than a single power amplifier because the signals going into these power amplifiers have characteristics that are easier to amplify. Specifically, the signals going into the power amplifiers are constant magnitude signals that can be amplified using simple, cheap amplifiers, such as, but not limited to, Class C amplifiers. This architecture has only limited real world use because it is very sensitive to gain and phase imbalances between the two amplifiers. Fig.2a shows an example, the X-axis ranges from -fs/2 to +fs/2, where fs is the sampling rate of the D/A converters and is also equal to the sampling rate of the digital source signal s (n) as well as the two component signals s_x (n) and s₂ (n) . The Y-axis, standing for x(ζ), is measured in dB and is useful for relative measurements, i.e., since the passband is at 0 dB, one can say that the noise energy begins to appear 30 dB below the passband. The graph in Fig.2b is the result of zooming in on the passband of the graph in Fig.2a. The X-axis in Fig.2b ranges from approximately -0.025fs/2 to approximately 0.3fs/2.. The two graphs in Fig.2a and Fig.2b are the result of plotting the same data, but on different scales. -

It can be seen from Fig.2a and Fig.2b that a 0.25 dB gain imbalance causes out of band noise to begin to appear about 30 dB below the passband. Furthermore, for a given gain imbalance, the noise introduced does not scale with the input power. For example, in the above scheme with a full power source signal, the out of band emissions are 30 dB below the passband. If the source power is reduced by 12 dB (for example) , the out of band emissions will not go down by 12 dB. They will stay at a fixed absolute power level. Therefore, the out of band emissions will be 18 dB below the passband.

Fig.3a shows the relationship of a source signal, an intermediate signal (a combined signal of the two analog signals Sι(t)+s₂(t)) and an output signal x(t) of the conventional power amplifying device for a communication system where the digital source signal is a full power source signal. Fig.3b shows the relationship of a source signal, an intermediate signal (a combined signal of the two analog signals Sχ(t)+s₂(t)) and an output signal x(t) of the conventional power amplifying device for a communication system where the digital source signal is a weak source signal. In Fig.3a and Fig.3b, the two rippled lines indicate the introduced output noise.

It can be seen from Fig.3a and Fig.3b that the introduced output noise is always added at an absolute power level regardless of the input power level . This is because the noise produced by the amplifier imbalance is relative to the power of the analog signals s_x(t) and s₂(t). The power of these signals is constant, and hence the noise caused by the amplifier imbalance is also going to be constant. It does not depend on the strength of the input signal . Thus, because the noise does not move up and down along with the power of the source signal, the requirements on the out of band noise are not just the dBc requirement for transmission. The requirement is actually the dBc requirement plus the expected dynamic range of the output signal. If the average power of the output signal is expected to vary by 20 dB and the out of band emission requirements are 30 dBc, then the noise introduced by the amplifier imbalances must be 50 dBc below the maximum output power .

The sensitivity to gain imbalance has usually been solved either with a complex network to compensate for the amplifier imbalance. Or with a complex measurement device to measure the performance of every component to make sure that only devices with a similar performance are used.

Another problem with this architecture is that the two amplifiers are always operating at full power. For instance, if 20 watts are needed at the antenna, then each amplifier will produce 10 watts and all the energy will be sent to the antenna. However, if only 1 watt is needed, each amplifier will still produce 10 watts of output. 1 watt will be sent out of the antenna and 19 watts will be wasted and dissipated as heat in the combiner.

The fact that the amplifiers must always work at full power reduces the efficiency of the amplifier and requires the use of heavy cables to supply the power needed, it requires the use of large backup batteries, and it requires the use of a heat sink in the combiner.

SUMMARY OF THE INVENTION

The object of the invention is to solve the above- mentioned problems in the prior art by providing a power amplifying device for a communication system, in which no complex network is needed to compensate for amplifier imbalance, no complex measurement device is required to measure the absolute performance of every power amplifier, the power consumption of the power amplifiers can be reduced, lighter power cables can be used, smaller/fewer backup batteries are needed, and a smaller heat sink can be used.

In order to achieve the above object, a power amplifying device is provided. The power amplifying device for a communication system according to the invention comprises a decomposer for decomposing a digital source signal into a first digital component signal and a second digital component signal; a first D/A converter for converting the first digital component signal into a first analog signal; a second D/A converter for converting the second digital component signal into a second analog signal; a first power amplifier for amplifying the first analog ,- signal to obtain a first amplified signal; a second power amplifier for amplifying the second analog signal to obtain a second amplified signal; a signal combiner for combining the first amplified signal and the second amplified signal to obtain an output signal to be transmitted through an antenna. The power amplifying device for a communication system further comprises a power detection & gain factor calculation device and a third amplifier for amplifying the digital signal to be decomposed by said decomposer; said power detection & gain factor calculation device detects the power value of said digital signal, adjusts the gain factor of the third amplifier and the gain factor of the first power amplifier and the second power amplifier based on the detected power value of said digital signal.

In brief, this invention involves adding a circuit that measures the average power of the digital source signal and calculates two gain factors, adding a gain device before the decomposer, and adding gain control on the two power amplifiers.

In operation, if the power detection & gain factor calculation device detects that the power of the digital source signal is low, it will increase the gain of the signal going into the decomposer so that at all times, the decomposer is receiving a full power signal. The first and the second amplifiers, on the other hand, will have their gain factor reduced by the same amount that the gain of the signal going into the decomposer was increased, so that the overall gain factor of the system is constant.

Thus, the amplifiers will always be producing a signal that will use the full power of both the amplifiers. The noise added by the amplifiers will always be relative to a full power signal, regardless of the power of the input signal. Since full power is not always needed at the output, the gain factor of the power amplifiers will be adjusted to lower both the signal power and the noise power to the desired transmit power level . This way, the requirements on the balance between the amplifiers are reduced. Continuing with the example presented earlier, even if the input power has a dynamic range of 20 dB, the requirements on amplifier balancing are still only 30 dB. The amplifier balancing will take care of the adjacent channel noise and the power detection and gain factor calculation device will take care of increasing and reducing both the signal power and the noise power. Thus, a complex network is not needed to compensate for amplifier imbalance. Neither is a complex measurement device required to measure the absolute performance of every power amplifier.

Furthermore, the gain factor of the amplifiers can be adjusted by changing their DC supply voltage. This will also reduce their power consumption. For example, if only 1 watt is needed at the antenna, then the DC supply of the power amplifiers will be reduced so that they only produce 1 watt of power in total . In the conventional architecture, if only 1 watt was needed, the power amplifiers would still produce 9 watts of power.

Thus, because the efficiency of the power amplifiers is increased, heavy cables are not needed to supply the required power. The size of the backup batteries is reduced. And it removes the need for a heat sink in the combiner. BRIEF DESCRIPTION OF THE DRAWINGS

Fig.l is a schematic drawing of a conventional power amplifying device for a communication system; Fig.2a and Fig.2b are graphs showing the out of band noise introduced due to gain imbalance of the two power amplifiers in the conventional power amplifying device for a communication system; Fig.3a shows the relationship of a source signal, the intermediate signal and the output signal of the conventional power amplifying device for a communication system where the digital source signal is a full power source signal; Fig.3b shows the relationship of a source signal, the intermediate signal and the output signal of the conventional power amplifying device for a communication system where the digital source signal is a weak power source signal; Fig. is a schematic drawing of a power amplifying device for a communication system according to the invention;

Fig.5 (a) -Fig.5 (d) show the relationship of a source signal, the intermediate signal, the out of band noise introduced due to gain imbalance and the output signal of the power amplifying device for; a communication system according to the invention where the digital source signal is a weak power source signal .

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Fig.4 shows schematically a power amplifying device for a communication system according to the invention. It is preferred that the digital source signal of the power amplifying device for a communication system is a full power input signal. For instance, if the peak power of the digital source signal is assumed to be 0 dB, and the peak to average ratio is assumed to be 10 dB, the largest average value of the input power will be -10 dB . With such an average input power, the gain of the amplifiers is chosen so that the output power in watts is equal to the desired level of output power. For instance, if the desired output power is 10 watts or 40 dBm, this amount of power will be produced when the input signal power is at -10 dB.

As shown in Fig.4, the power amplifying device for a communication system according to the invention comprises a third amplifier 22 for amplifying the digital source signal s (n) and outputting an amplified digital source signal to a decomposer 10 as its input signal.

The decomposer 10 decomposes the amplified digital source signal into a first digital component signal Sx (n) and a second digital component signal s₂ (n) . A first D/A converter 11 converts the first digital component signal s (n) into a first analog signal s_x (t) .

A second D/A converter 12 converts the second digital component signal s₂ (n) into a second analog signal s₂ (t) . A modulator 13 modulates respectively the first analog signal Sχ(t) and the second analog signal s₂(t) to become a first modulated signal m_x(t) and a second modulated signal m₂(t) each having a desired carrier frequency f_c.

A first power amplifier 15 amplifies the first modulated signal _ι(t) to obtain a first amplified signal x_x (t) .

A second power amplifier 16 amplifies the second modulated signal m₂(t) to obtain a second amplified signal x₂ (t) .

A signal combiner 17 combines the first amplified signal x_x(t) and the second amplified signal x₂(t) to obtain an output signal x(t) to be transmitted. It should be noted that the modulator 13 is optional . Even if the desired carrier frequency f_c is zero, the power amplifying device for a communication system will work. In other words, the modulation operation does not affect any of the properties of the power amplifying device, which are equally valid for a carrier frequency of 0Hz as they are valid for a carrier frequency of 2 GHz, for example.

The power amplifying device for a communication system according to the invention further comprises a power detection & gain factor calculation device 20. The power detection & gain factor calculation device 20 detects the average power of the digital source signal s (n) , adjusts the gain factor g_x of the third amplifier 22 and the gain factor g₂ of the first power amplifier 15 and the second power amplifier 16 based on the detected average power of the digital source signal s (n) .

Assume that the largest average power of the digital source signal s (n) will be -10 dB. In this case, the measurement of the average power of the digital source signal s (n) is used to adjust the gain factors g₁ and g₂ to make sure that the amplified digital source signal going into the decomposer 10 always has a power level of -10 dB. For instance, if the average power of the digital source signal is measured to be -32 dB, the gain factor g_x of the third amplifier 22 will be so set that the amplified digital source signal has a power level of -10 dB. Thus, the imbalanced first and second power amplifiers will add noise to a signal that they think is a full power signal.

The gain factor g₂ of the first power amplifier 15 and the second power amplifier 16 is used to scale the average power of the output signal x(t) so that it is at the desired setting. Continuing with the example, the gain factor g₂ will be set to a suitable value so that the overall gain will -be constant.

The power amplifying device for a communication system according to the invention further comprises two reconstruction filters (not shown) , one of which is connected between the first D/A converter 11 and the modulator 13, and the other of which is connected between the second D/A converter 12 and the modulator 13. Any time that a digital to analog conversion happens, what is produced is the desired output spectrum, and images of that spectrum that keep repeating every fs . The reconstruction filters are used to filter out the images that are not wanted. According to the invention, the digital source signal s (n) could be a multi-carrier signal or a single carrier signal . Further the modulation of the digital source signal s (n) could be anything (GSM, CDMA, OFDMA, etc)

There are several ways to estimate the power of the source signal . One example is to examine the source signal itself, and estimate the power of the source signal, i.e., one can calculate abs(s(n)²) for every input sample and then average these values over a certain averaging period. This averaging operation can be performed, for example, either with an alpha filter, or by directly summing N number of abs(s(n)²) values and then dividing by N.

Another method is to use some information about how the signal itself was generated. For instance, if it is known that s (n) is the result of adding three signals, each with a power level of X, therefore, one can deduce that the power of s (n) is 3X. This is a very real solution in a CDMA system, since the signal s (n) is actually composed of the sum of hundreds of sequences, and we know the power of each of those sequences. This method would require different signals be sent into the power estimation module. The source signal s (n) is not needed for this method,.

Other methods exist that can be used to calculate the average power of a signal and this invention is not limited to the use of one of the above examples. For the gain factor calculation, assume that M is the maximum RMS power of the source signal s (n) that is possible, and S is the measured RMS power of the source signal s (n) (Note that the RMS power of a signal v(n) is the square root of the average of abs(v(n)²)) . In this case, the gain factor g_x is equal to M/S.

Assume g_n is the nominal gain factor of the two power amplifiers when the source signal is at full power, i.e., if the source signal is at full power, g_x will be 1 and g₂ will be g_n to produce a signal on the output that is at the required power level. If the source signal is not at full power, g₂ will be equal to S/M*g_n.

Fig.5 (a) -Fig.5 (d) show the relationship of a source signal, the intermediate signal, the out of band noise introduced due to gain imbalance and the output signal of the power amplifying device for a communication system according to the invention where the digital source signal is a weak power source signal . As shown in Fig.5 (a) and Fig.5(b), in the case of weak power source signal s (n) , the intermediate analog signal, formed by combining the first analog signal s_x(t) and the second analog signal s₂(t), is scaled up due to the amplifying effect of the third amplifier 22. If the gain factor g_x of the third amplifier is set as required, the signal going into the decomposer 10 is always at full scale. The power amplifiers will always be reconstructing a full scale signal, and hence the output out of band spectral emissions will be relative to the input signal power. The output signal x(t) being sent to the antenna is determined by the common gain factor g₂ of the first and the second power amplifiers 15 and 16.

Provided that the gain factor g₂ of the first power amplifier 15 is set to g_n and the second power amplifier 16 is equal to g_n, the output signal x(t) would be that as shown in Fig.5(c) and the SNR after the first and the second power amplifiers is still very high, as if the input signal was at full power. However, according to the invention, the gain factor g₂ of the first power amplifier 15 and the second power amplifier 16 is set based on the relationship gx=M/S, g₂=g_n*S/M, and thus the output signal x(t) (including the passband and the noise) is scaled down, as shown in Fig.5(d) . According to the invention, the digital source signal does not have to be centered around zero frequency. It can be centered around an intermediate frequency (so-called IF frequency) .

Primarily, this invention can and will be used in the field of radio communications, but it is not limited to this use. This invention can be used with other transmission media such as media that carries an optical signal, or even a cable that carries an RF signal . Furthermore, the invention will work for all modulations schemes including, but not limited to CDMA, OFDMA, FM, QAM, DQPSK, and future modulation schemes .

Furthermore, the invention will work whether the source signal s (n) is composed of a single signal, or if it is a multi-carrier signal composed of several carriers .

Furthermore, several, different methods can be envisioned to calculate the power of the incoming signal including simple averaging over N samples, or IIR averaging, or some form of predictive filtering. This invention is not limited by the power calculation algorithms described above.

Claims

WHAT IS CLAIMED IS:

1. A power amplifying device for a communication system comprising: a decomposer (10) for decomposing a digital source signal (s (n) ) into a first digital component signal (s_x(n)) and a second digital component signal (s₂(n))_; a first D/A converter (11) for converting the first digital component signal (s_x(n)) into a first analog signal (s (t)); a second D/A converter (12) for converting the second digital component signal (s₂(n)) into a second analog signal (s₂(t)); a first power amplifier (15) for amplifying the first analog signal (s_x(t)) to obtain a first amplified signal (xχ(t) ) ; a second power amplifier (16) for amplifying the second analog signal (s₂(t)) to obtain a second amplified signal (x₂(t)); a signal combiner (17) for combining the first amplified signal (xχ(t)) and the second amplified signal (x₂(t)) to obtain an output signal x(t) to be transmitted through an antenna; wherein said power amplifying device for a communication system further comprises a power detection & gain factor calculation device (20) and a third amplifier (22) for amplifying said digital signal (s(n)) to be decomposed by said decomposer (10) ; said power detection & gain factor calculation device (20) detects the power value of said digital signal (s(n)), adjusts the gain factor (g_x) of the third amplifier (22) and the gain factor (g₂) of the first power amplifier (15) and the second power amplifier (16) based on the detected power value of said digital signal (s(n)) .

2. The power amplifying device for a communication system according to claim 1, wherein said power amplifying device further comprises a modulator (13) for modulating respectively the first analog signal (Sχ(t)) and the second analog signal (s₂(t)), to be sent respectively to the first power amplifier (15) and the second power amplifier (16) , to a desired carrier frequency (f_c) .

3. The power amplifying device for a communication system according to claim 1 or 2, wherein said power detection & gain factor calculation device (20) adjusts the gain factor (g_x) of the third amplifier (22) so that said decomposer (10) receives a full power signal.

4. The power amplifying device for a communication system according to claim 3, wherein said power detection & gain factor calculation device (20) adjusts the common gain factor (g₂) of the first power amplifier (15) and the second power amplifier (16) so that the overall gain factor of said power amplifying device is constant.

5. The power amplifying device for a communication system according to claim 1 or 2 , wherein said digital source signal (s(n)) is a multi-carrier signal .

6. The power amplifying device for a communication system according to claim 1 or 2 , wherein said digital source signal (s (n) ) is a single carrier signal .

7. The power amplifying device for a communication system according to claim 1 or 2 , wherein the modulation of said digital source signal (s(n)) is any one of GSM, CDMA, OFDMA etc.

8. The power amplifying device for a communication system according to claim 1 or 2 , wherein said power amplifying device further comprises two reconstruction filters, one of which is connected between the first D/A converter (11) and the modulator (13) , and the other of which is connected between the second D/A converter (12) and the modulator (13) .

9. The power amplifying device for a communication system according to claim 4 , wherein the gain factor (g_x) of the third amplifier (22) , and the common gain factor (g₂) of the first power amplifier (15) and the second power amplifier (16) are so adjusted that the gain factor (g_x) of the third amplifier (22) is equal to M/S and the common gain factor (g₂) of the first power amplifier (15) and the second power amplifier (16) is equal to S/M*g_n, where M is the maximum RMS power of the source signal (s(n)), S is the measured RMS power of the source signal (s(n)), and g_n is the nominal gain factor of the two power amplifiers when the source signal (s(n)) is at full power.