Out-of-band emissions of digital transmissions using Kahn EER technique.pdf
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1、IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 50, NO. 8, AUGUST 20021979 Out-of-Band Emissions of Digital Transmissions Using Kahn EER Technique Dietmar Rudolph AbstractThe Kahn envelope elimination and restoration (EER) technique allows for linear RF power amplification by combining no
2、nlinear, but efficient, RF and AF power amplifiers (PAs). In order to use the EER technique for digital signals, a coordinate transform from the original Cartesian in-phase and quadrature mode into a polar mode has to take place, yielding an envelope (or amplitude) and a PM RF signal. This coordinat
3、e transform is extremely nonlinear and thereby broadens the spectra of the original signals. In the final PA stage, both signals are recombined. However, since this recombination process is imperfect, out-of-band (OOB) emissions come up, also known as adjacent channel power or spectral regrowth. In
4、this paper, the impact of the broadening of the amplitude and phase signals on OOB emissions is investigated with respect to imperfect restoration due to signal delays and limited bandwidth of the amplitude path. It is shown that the amount of OOB emissions can significantly be reduced if the modula
5、tion scheme shows a “hole” at the origin in its vector diagram. Index TermsACPR, amplifier, band-limited communications, delay effects, EER, Kahn technique, PM, spectral regrowth, transmitter. I. INTRODUCTION D IGITAL cellular transmission systems, as well as digital broadcasting systems, widelyuse
6、modulation formats that do not have a constant envelope, e.g.,DQPSK, OQPSK, and additionally with zero crossings in their vector diagram, e.g., QPSK, eight phase shift keying (8PSK), amplitude phase shift keying (APSK), orthogonal frequency division multiplex (OFDM), CDMA. If the amplitude of the di
7、gitally modulated signal is not constant, the transmitter power amplifier (PA) has to operate in linear mode. Linear operation mode can either be established by a linear amplifier, which suffers from low efficiency 2, or by a transmitter that linearizes a high-effi- ciency PA, e.g., by using the env
8、elope elimination and restora- tion (EER) technique proposed by Kahn in 1952 111. In an EER transmitter, basically, the RF signal is split into a PM and an AM signal. The PM signal is directly amplified by a PA that runs in a saturated or even switching mode. In order to re- store the amplitude, the
9、 supply voltage of the PA is modulated by the AM signal. Thereby, although the PA itself is operated in a nonlinear high-efficiency mode, the total transmitter shows linear behavior while maintaining the high efficiency. The ben- efits of the EER technique with respect to the transmitters ef- ficien
10、cy are without any question. The EER technique, how- ever,introducesacoordinatetransformofthedigitalsignalfrom Manuscript received August 15, 2001. The author is with T-Systems Nova GmbH, Berkom, D-10589 Berlin, Germany (e-mail: dietmar.rudolpht-systems.de). Publisher Item Identifier 10.1109/TMTT.20
11、02.801349. Cartesian to polar form, and the PA stage of an EER transmitter afterwards produces the inverse coordinate transform back. In this way, linear distortions within the “polar” branches of the transmitteraretransformedbackintononlineardistortionsinthe RF signal, even if the transmitter itsel
12、f had no additional hard- warenonlinearities.Thecoordinatetransformationprocessisan additional extreme nonlinearity within the transmission chain. Therefore, the EER technique also has its drawbacks, which are the out-of-band (OOB) emissions. For the case of an analog two-tone signal, Raab 6 has ana
13、lyzed the intermodulation distortion (IMD) spectra. In this paper, however, the investigation is limited to the effects produced by the additional EER nonlinearities. As a signal, a band-limited complex Gaussian noise is used. This signal is very similar to an OFDM modulation. With respect to its ve
14、ctor diagram, it has a maximum of zero crossings, and might be in so far together with its high Crest factor the worst case of all digital modulations for EER transmitters. Based on numerical simulations, this paper first examines the broadening of the internal phase and amplitude spectra with respe
15、ct to the modulation scheme. The influence of unequal time delay in the amplitude and phase branch on OOB emissions and the required bandwidth of both branches are then discussed. It turns out that a “hole” in the vector diagram of the modulation scheme significantly reduces the OOB emissions. As a
16、result, the amount of unwanted emissions depends on the type of digital modulation used. A modulation scheme that already has such a “hole,” like digital offset modulations, is better suited for EER techniques than others. Also, the bigger the “hole,” the better. II. CARTESIAN-TO-POLARCONVERSION The
17、 structure of a transmitter using EER is shown in Fig. 1. It is subdivided into two building blocks, i.e., the digital mod- ulator and AM transmitter. In the digital modulator, the signal is split into a phase signal (RF-P) and an amplitude (A) signal, which serve as the input signals of the AM tran
18、smitter. The dig- ital signal (“Digital Signal”) at the input of the modulator shall already describe the transmit symbols, and all coding shall have been done in stages prior to the digital modulator, not shown in Fig. 1. In the first step, the digital modulator forms in-phase and quadrature (I & Q
19、) symbols, consisting of I & Q baseband symbols, and shapes them properly. The spectra of these digital signals are band limited to half the channel bandwidth and have shoulder distances of approximately 70 dB. When modulated, they fit exactly into the channel bandwidth. These Cartesian I & Q signal
20、s are then converted into polar RF-P & A signals. This process introduces the nonlinearity that is typical for EER 0018-9480/02$17.00 2002 IEEE 1980IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 50, NO. 8, AUGUST 2002 Fig. 1.Structure of an EER transmitter. techniques. The AM transmitter
21、 has a conventional structure. The RF path is a chain of RF driver and RF power output stage, both operating in classes C, D, E, or F, respectively, with high efficiency. The PA receives its supply voltage via an amplitude modulator, which operates in class S, switched mode. A partial drive modulati
22、on to overcome amplitude-to-phase (A/Phi) con- versions in the PA, suggested by Raab 8, is also possible, but is not shown in Fig. 1. In this paper, the modulation process in the PA is idealized and, thus, does not suffer from A/Phi con- versions. In the EER application, the I & Q symbols are only i
23、nternal Cartesian signals, which have to be converted to polar signals. According to Fig. 1, the conversion to polar signals is done all digitally in the following way. Generating asignal explicitly is avoided because the phase angle is not limited, and an overflow could happen. Alternatively, two s
24、ignals andare used, which are confined to1. Thecomponent is multiplied byand the component is multiplied by, andis the RF carrier frequency. When using “Cartesian” I & Q signals, the complex dig- ital-modulated signalbecomes (1) The EER technique has to furnish the same digitally modu- lated signala
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