Method and a device for adjusting power amplifier properties

[0001] This is the U.S. National Stage of International Application Serial Number PCT/FI2004/000098 filed 25 Feb. 2004 and published in English on Sep. 10, 2004 based on Finnish patent application 20030272 filed Feb. 23, 2003 from which priority is claimed.

[0002] 1. Technical Field

[0003] The present invention relates generally to a method and a device for adjusting power amplifier properties. Particularly the invention concerns techniques exploiting PAR (Peak to Average Ratio) in the adjustment.

[0004] 2. Discussion of Related Art

[0005] Many traditional communication systems utilize modulation formats, for example GSMK (Gaussian Minimum Shift Keying) in the case of GSM (Global System for Mobile communications), wherein amplitude envelope stays constant resulting in low, in principle zero, PAR values making possible the use of affordable and non-linear amplifiers. As a generalization, if the PAR value is high, which is due to the large variance of the signal amplitude, amplifier circuit linearity must be improved to achieve low enough ACLR (Adjacent Channel Leakage power Ratio) values. Therefore, an amplifier must have enough amplification headroom, so called “back-off”, to avoid distortions in an output signal. The distortions in the signal include e.g. second- and third-order harmonics which leak into the adjacent channels. This leakage is identical to the intermodulation caused by two sinusoidal signals input to an amplifier. Moreover, high PAR values are undesirable as they place considerable requirements for the dynamic range of A/D and D/A converters without forgetting large power dissipation in the highly linear and large back-off amplifiers. The circuit linearity can be measured as a back-off number from the amplifier input compression point or some other specific point in the radio chain.

[0006] FIG. 1 discloses Pinput-Poutput characteristics of a power amplifier, wherein active power ranges of two signals, one of signal 102 with higher PAR value PARh and another of signal 104 with lower PAR value PAR1, both signals having a common average power Pavg, are illustrated with dotted lines. As indicated by the figure, the amplifier has to be designed according to maximum possible Pavg and PAR values, which is not optimal whenever the actual operational range is something less demanding. The reduction in efficiency resulting from too high back-off can be estimated by the following equation, which defines the efficiency η of the class-B amplifier as a function of the output power Poutput
η=η_max√{square root over (P_output/P_max)}
wherein Pmax is the maximum output power and η_max is the efficiency at Pmax. Considering an ideal class-B amplifier with a maximum efficiency of π/4 and applying Pmax/Pavg (PAR) value of 3.2 dB, being typical PAR for WCDMA (Wideband Code Division Multiple Access) PAs, and 5.5 dB, respectively common for HSDPA (High Speed Downlink Packet Access) PAs, efficiencies of 54.3% and 41.7% are achieved. Thus, the efficiency of the WCDMA PA is reduced by the factor of 0.768 if the HDSPA PA is utilized for both purposes.

[0007] In WCDMA and multicarrier, e.g. OFDM (Orthogonal Frequency Division Multiplexing), systems amplifier design and control is particularly relevant issue as the modulation techniques thereof are non-constant in order to achieve higher data rates than constant techniques like GMSK can provide. The fundamental idea behind the WCDMA is to allow several users to share the same spectrum by spreading the data across the available frequency band. This can be achieved if the user's data signal is multiplied with a pseudorandom “chip” sequence of a higher bit rate to fill the frequency band. In the receiving end the single user's data can be retrieved from the overall received signal by cross-correlating the received signal with a synchronized copy of the chip signal. The procedure is called dispreading. Naturally, chip sequences should be designed to be orthogonal so as to be maximally separable in the reception sense. On the other hand, OFDM is a multicarrier system wherein the available frequency band includes several narrowband carriers with overlapping sidebands. If each carrier's spectrum is ideally shaped like a sinc function having zero crossings every f0, and the carrier separation is f0 as well, the resulting overall spectrum is spectrally efficient (dense), and at the same time, the co-channel interference is retained to be low. In practice, OFDM transceivers convert a data signal to PSK (Phase Shift Keying) or QAM (Quadrature Amplitude Modulation) symbols after which the serial stream is converted to a parallel form, modulated with inverse FFT (Fast Fourier Transform), serialized and finally converted to analog for transmission. The receiver executes the aforesaid process in reverse order utilizing e.g. FFT.

[0008] The difference between the average and peak power of the signal, which can be presented via PAR, results from the fact that multiple carriers may add together either constructively to form a high level signal or destructively to form a very low level signal. Therefore, as the number of sub-carriers is increased, also the PAR inevitably increases. The same phenomenon occurs in the WCDMA system as the PAR typically increases with the number and types of codes transmitted. A WCDMA system supports multirate data transfer in two major ways, namely by using a variable spreading factor and by utilizing a multicode approach, wherein higher data rates are achieved via use of several parallel codes. In addition, forthcoming HSDPA based on a HS-DSCH (High Speed Downlink Shared Channel) is a packet data service in WCDMA downlink to transmit data up to 20 Mbps (MIMO, Multiple Input Multiple Output, systems with multiple transmitter and receiver antennas). It features a concept of AMC (Adaptive Modulation and Coding) enabling utilization of adaptive modulation and coding techniques in order to maximize the data throughput in changing channel conditions. The channel conditions can be estimated, for example, based on feedback from the receiver and use AMC instead of fast power control to compensate for the variations in the channel estimate. Users close to the base station may be assigned higher order modulation with higher code rates (e.g. 64 QAM and R=¾ turbo codes), but the lower modulation-order, e.g. QPSK, and code rate will be taken into use as the distance from the base station increases and/or the degradation in the channel conditions is detected.

[0009] It's obvious that with an adaptive system like AMC in WCDMA HSDPA, PAR values may be high as the system intermittently changes transmission parameters affecting also the amplitude envelope and extends its possible range. It's admitted that many solutions have been proposed for minimizing PAR, see, for example, U.S. Pat. Nos. 6,294,956, 6,292,054 and 5,894,498 for reference. However, despite the existing solutions, PAR fluctuations, which normally originate from the changes in the utilized transmission techniques with possibly varying modulation types and/or number of multicodes, cannot be completely removed. Also, in the case of multicode transmission, PAR of the amplified signal may vary based on gain factors i.e. power offsets between different codes. This is the case especially in HSDPA (High Speed Downlink Packet Access) service. These gain factors between codes are signalled by higher layers in the network. As presented above, in the worst case the PAR in HSDPA uplink signal can even be ˜2.3 dB higher compared to the standard WCDMA scenario. Recalling that the PAR value directly affects to the power consumption of a PA and considering, for example, mobile terminals' quite limited power supply in a form of compact and relatively low capacity batteries, it surely isn't a surprise that power amplifiers are one of the most power hungry building blocks in a terminal as their output power back-off is typically designed according to the worst case PAR and Pavg values possible in the system.

[0010] The object of the present invention is to alleviate aforesaid problems originated from PAR variations. The object is achieved with a method and a device enabling the adaptive tuning of PA properties according to the PAR value of that moment. For example, in order to increase talk time remarkably in a mobile terminal, the power amplifier back-off can be adjusted as a function of the PAR value of the transmitted signal. A device utilizing the invention, such as a transceiver in a base station or mobile terminal, is, e.g. in the case of WCDMA, aware of power offsets between codes and number of transmitted codes. Consequently, the device can be programmed and configured so that it knows the PAR value of the transmitted signal during the connection to a network. Additional detectors or signals are thus not necessarily required. Based on the knowledge of the current PAR, the device is able to tune the amplifier's back-off without computationally exhaustive calculations.

[0011] The invention offers several advantages over prior art. Reduction in power consumption is in the best case (HSDPA) ˜60 mA and also heat problems decrease. Correspondingly, amplifier efficiency is increased. Furthermore, the proposed approach for amplifier adjustment is relatively simple to implement and applicable to already existing solutions without need to twiddle with complex DSP SW based control algorithms.

[0012] A method according to the invention for tuning a power amplifier to be performed by an electronic device, comprises the steps of

• • [0013] obtaining a maximum peak-to-average value related to an input signal of the amplifier, and
  • [0014] adjusting at least one amplifier parameter according to said peak-to-average value.

[0015] In another aspect of the invention a circuit arrangement for tuning a power amplifier, comprises control means for obtaining a peak-to-average value related to the amplifier input signal and providing control information based on said peak-to-average value, and tuning means for receiving said control information and adjusting at least one amplifier parameter, said amplifier functionally connected to the tuning means, according to said control information.

[0016] In a further aspect of the invention a wireless communications device operable in a telecommunications network, comprises processing means and memory means for processing and storing instructions and data, control means for obtaining a peak-to-average value related to an input signal of an amplifier and providing control information based on said peak-to-average value, and tuning means for adjusting at least one amplifier parameter, said amplifier functionally connected to the tuning means, according to said control information.

[0017] The term “peak-to-average” (PAR) refers herein to a value which is the ratio of the peak envelope power to the average power. Sometimes the term “crest factor” is used exactly in the same manner but also a partly differing interpretation exists according to which the crest factor is the ratio of signal maximum value to its RMS (Root Mean Square Value). Then the resulting value is 3 dB greater than the corresponding PAR. However, as there undoubtedly are varying ways to represent substantially the same properties of the signal, the term PAR shall be interpreted broadly.

[0018] In an embodiment of the invention a WCDMA mobile terminal starts transferring data over the air interface. Prior to data transmission the terminal deduces the PAR of the signal to be transmitted by means of the transfer mode (WCDMA/HSDPA) information and tunes PA back-off to reduce the power consumption thereof.

[0025] FIG. 1 was already described in conjunction with the description of the prior art. Whenever PAR and/or average power of the signal vary, the amplifier causes unnecessary power consumption as the PA back-off is gratuitously high.

[0026] Referring to FIG. 2, the basic inventive concept is depicted as components of a circuit arrangement like a tunable transmitter for a communication device transferring signals of variable PAR. Basically similar means which have been used to improve the efficiency of a power amplifier operating at back-off can be used for improving the efficiency of an amplifier operating at signals with variable PAR ratios. However, the control concept needed in this case is totally different than the one prior art solutions exploit when improving the efficiency as a function of the power level.

[0027] The control signals 228, 230, 232 generated by the PAR control logic block 206 are used to adjust other transmitter components according to the peak-to-average ratio PAR 226 of the transmitted signal. PAR 226 may be received from the network via signalling information and possibly passed to the control logic 206 by some other element of the terminal like a radio DSP. Alternatively, PAR 206 value can be derived from implicit information like a transfer mode if, for example, the logic 206 stores a table to link different transfer modes to different PAR values. The same control block 206 can also be utilized for tuning the system according to the average power level if necessary. The ultimate goal is to control the power amplifier 203 in a way that optimizes its performance in most situations. To approach the optimum scenario in this particular example three parameters Vcc 216, Ibias 218 and Zload 220 are adjusted according to the transmitted signal by tuning the DC/DC-converter 210 (Direct Current to Direct Current) providing battery voltage Vbatt 224 or some other suitable supply voltage to the PA 203, bias control 208 which also takes a regulating voltage Vreg 222 as an input, and tunable output matching network (OMN) 204 included in the PA module 202. In a general case all the parameters can be controlled simultaneously but typically at least one or more parameters are used if, for example, some of the blocks are not available. Additionally, FIG. 2 includes a TX (transmitter) modulator 212 and an isolator, a switch and/or a filter 214, all typical elements of an RF (Radio Frequency) transmitter.

[0028] In the following, numeric values for a WCDMA power amplifier current consumption are calculated to find out how significant this effect is in practice.

[0029] Vcc=3.3 V

[0030] Poutput=24 dBm (max. Pavg)

[0031] Efficiency=40%

[0032]
Pdc=Poutput/η=628 mW and current taken from the battery Idc=Pdc/Vcc=190 mA.

[0033]
Pdc=Poutput/(0.768η)=818 mW and Idc=248 mA.

[0034] Calculations above show that when the amplifier designed for the requirements of HSDPA is used in the ‘normal’ WCDMA case, 58 mA current is wasted at maximum Poutput compared to the optimal solution. At low power levels the factor of reduction remains the same though the absolute difference in battery currents decreases. It is noted that in practice the reduction in efficiency is even higher because class-AB operation has to be used to meet the linearity requirements.

[0035] The most common way to control the operation of the power amplifier is to adjust the bias currents of amplifier stages. Therefore the bias signal Ibias 218, output of the bias controller block 208 is not necessarily only one signal but a group of signals used to adjust the operation of the amplifier 203. When maximum possible PAR decreases, it is often possible to reduce the bias currents as well and still meet the linearity requirements. From the realization point of view it may be advantageous to integrate the bias control 208 into the PAR control block 206.

[0036] Another possibility is to drop the collector (drain) voltage of the amplifier stages (typically output stage only) by using a DC/DC-converter 210. In case of switching the PAR-value from 5.5 dB to 3.2 dB and keeping Pavg constant, it is possible to reduce the value of Vcc 216 by the factor of 0.768. If the supply voltage Vcc 216 in the first case is 3.3 V, it can be dropped down to 2.6 V for lower PAR. Because DC/DC-converters can operate with an efficiency of over 90%, this is a very effective way of reducing the current taken from the battery.

[0037] The third alternative is to adjust load impedance Zload 220 of the power amplifier 203 in a way that it increases as the peak-to-average ratio decreases. For example, the load impedance could be increased by a factor of 1.7 when the PAR changes from 5.5 dB to 3.3 dB. In practice this can be realized by switching different matching network elements, typically capacitors, on and off by using either pin-diode or High Electron Mobility Transistor HEMT (basically a FET, Field-Effect Transistor, incorporating a high-mobility layer to enhance performance) switches. In future MEMS (Micro Electro Mechanical Systems) technology will provide more options for manufacturing high-quality components to realize low-loss tunable output matching circuits.

[0038] FIG. 3A discloses a high-level flow diagram of the described embodiment of the invention wherein PAR control logic 206 embedded in a WCDMA mobile terminal supervises the adjustment of PA module 202 properties like PA 203 back-off by utilizing PAR value determined for the connection type. Upon connection initiation the terminal is aware of the connection parameters including transfer mode (HSDPA/normal WCDMA) information and therefore also related maximum PAR value in order to tune the transmitter properties. Also average output power information Pavg can be exploited in the adjustment procedure. In phase 302 the control logic 206 receives notification of a new connection to be established. The terminal deduces the required PAR for the transmission 304 based on the connection type/transfer mode information. If the determined value is equal to a maximum possible value, which is checked during phase 306, then the data transmission can be initiated immediately 310 without actual adjustment procedures as in this case the PAR is set maximum by default. Otherwise, PA back-off is first tuned during phase 308. The data transmission is ceased 312 by the connection release.

[0039] FIG. 3B discloses a more detailed flow diagram of the actual PAR adjustment phase 308 internal events. Upon recognizing a back-off tuning request 320, PAR control logic 206 checks whether the OMN is available for the load impedance adjustment or not in phase 322. In the first case the OMN is tuned via PA module control means 232 according to the PAR and/or possibly Pavg 324, in the latter case the OMN tuning phase is not possible, e.g. not available, and thus skipped. Next, if amplifier output stage switching or bypassing is applicable 326, output stages are bypassed and/or cells switched off 328 if PAR (and/or Pavg) is under a predetermined limit. Finally, DC/DC converter output voltage, being in fact the amplifier supply voltage 216, and bias current 218 are set to predetermined levels by the corresponding tuning blocks 210, 208 which are controlled by the PAR control logic 206 via the control signal connections 230, 228. After above-presented adjustment measures the tuning phase is ended 332 and the overall data transmission process continues as proposed in FIG. 3A.

[0040] PAR control and tuning means 204, 206, 208, 210, may, for example, tune the amplifier 203 based on mappings between different peak-to-average and amplifier parameter values. The resulting parameter value can be either directly or inversely proportional to the peak-to-average value and a decrease in the obtained peak-to-average value is typically converted to a decrease but sometimes also, e.g. in the case of the amplifier output impedance 220, to an increase in the adjusted amplifier parameter value. Accordingly, a decrease in the peak-to-average value converts to a decrease in both the bias current 218 and the supply voltage 216. On a higher level, a decrease in the peak-to-average value is turned into a decrease in the amplifier back-off by the adjusting procedure to optimize the amplifier usage.

[0041] PAR control logic 206 can be implemented, for example, as a new standalone chip connected to other components providing the necessary input for the control via data buses, or as a dedicated software process embedded in an existing chip or to a plurality of existing chips, e.g. microcontrollers or DSPs (Digital Signal Processor) with optional external memories inside the terminal. Also programmable logic circuits may be applied. The checking of contemporary connection state, connection/transmission type/mode, available resources etc. shall be carried out with simple memory access operations by the control logic if the process is included in an existing chip already maintaining that information. Otherwise the information must be transferred from the other components first. The predetermined data including various threshold values for the decision making is stored in the control logic 226 or an accessible external memory chip. Additionally, the updated values may be received from the far-end of the connection as well. Message, e.g. PAR control requests and possible control messages more complex than just a control voltage, transfer between different software processes inside a single chip or between several at least partially independent components may be implemented as desired, however taking selected components' interfaces and other properties into account.

[0042] As proposed hereinbefore, basically three parameters Vcc, Ibias and Zload are adjusted based on the PAR value to alter the PA back-off. Additionally, amplifier output stages/cells may be bypassed/switched off. Concerning the implementation and realization aspects in practice, the following publications are referred to as providing advantageous advice on the related issues. U.S. Pat. No. 5,493,255 discloses a solution for bias current adjustment, namely a circuit for adjusting RF power amplifier bias-voltages as a function of power level, wherein the control by power level can be replaced by the PAR based control, or alternatively, a parallel PAR control may be presented in addition to the existing power level control. U.S. Pat. No. 6,301,467 concerns transmitter efficiency in relation to the PA output matching (Zload). PA supply voltage (Vcc) tuning is relatively well-known issue as such, and, for example, U.S. Pat. No. 4,442,407 discloses a one possible solution for the purpose as it suggests monitoring of input signal properties, e.g. signal amplitude, arriving to an RF amplifier and adjusting the amplifier supply voltage based on the gathered information with a control circuit.

[0043] Referring to FIG. 4, one option for the basic components of an electronic device, for example, a wireless communications device like a mobile terminal or a PDA (Personal Digital Assistant), capable of adjusting the PA properties as disclosed hereinbefore are depicted. A memory 402, divided into one or more physical memory chips and possibly combined with a processing unit 406, comprises instructions, e.g. in a form of a computer program/application, for executing the proposed method. Processing unit 406 is required e.g. for the control of various events performed by the terminal in accordance with the stored instructions. A display 404 and a user interface 412 are typically needed for providing necessary device control and data visualization means to the user. Transceiver 408 handles data transmission over air interface and includes also the components for executing the method of the invention disclosed in FIG. 2. Alternatively, PAR control logic of FIG. 2 may be at least partially included in the processing unit 406 to gain possible benefits, e.g. space and/or power consumption savings, achieved from concentrating the terminal controls in a single element. Audio parts 410 include e.g. transducers and amplifiers necessary for interfacing acoustic signals such as speech and music with said device.

[0044] The scope of the invention can be found in the following independent claims. However, utilized devices, method steps, data structures etc may vary significantly depending on the current scenario, still converging to the basic ideas of this invention. For example, it is clear that the invention may also be exploited in other devices than wireless communication devices if these devices include power amplifiers preferably controlled and controllable by input signal's PAR value. Furthermore, the presented embodiment suggested adjustment of PA properties like back-off in conjunction with a transmission start, e.g. a connection setup, but it may as well be performed intermittently during the data transfer if needed. Parameters, which are to be adjusted by the PAR, are not limited to the ones of the disclosed embodiment either. Finally, in some occasions the PAR control logic and PA tuning means may advantageously be at least partially combined. Therefore, the invention is not strictly limited to the embodiments described above.