On the reasonableness of nonlinear models for high power amplifiers and their applications in communication system simulations

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High power amplifier (HPA) models with inherent nonlinearities play an important role in analysis and evaluation of communication system performance in both theoretical and practical aspects. However, there are not so much discussions on the suitability to the use of such models in simulating HPA nonlinearity in communication systems. Kỹ thuật điều khiển & Điện tử ON THE REASONABLENESS OF NONLINEAR MODELS FOR HIGH POWER AMPLIFIERS AND THEIR APPLICATIONS IN COMMUNICATION SYSTEM SIMULATIONS Nguyen Thanh1,*, Nguyen Tat Nam2, Nguyen Quoc Binh1,3 Abstract: High power amplifier (HPA) models with inherent nonlinearities play an important role in analysis and evaluation of communication system performance in both theoretical and practical aspects However, there are not so much discussions on the suitability to the use of such models in simulating HPA nonlinearity in communication systems In this work, we investigate the reasonableness of well-known nonlinear models and propose two models that are both analytic and better than Cann’s new model in terms of approximating to the real-world data Examples with specific testing signals verify the relevance of the arguments and point out suitable alternatives for use Keywords: High power amplifier; Nonlinear modeling; Nonlinear distortion simulation INTRODUCTION Generally, for many communication systems such as satellite or mobile communications, power and/or bandwidth efficiencies are among the leading interests On the other hand, for high power efficiency, amplifiers behave nonlinearities unignored Nonlinear characteristics show an important influence for small-signal stages of a receiver since intermodulation products can strongly interfere with the desired signals However, with less dealing to the power efficiency problem than performance considerations, the small-signal amplifiers then should be well linearized Therefore, studies on the nonlinear characteristics commonly focus on the high power amplifiers (HPAs) Generally, there is a tradeoff between HPA’s maximum power efficiency that requires pushing its operating point well into saturation, and minimizing nonlinear distortion, namely, demanding that the HPA operates well below saturation for diminishing spectrum regrowth, nonlinear interference (ISI) and interchannel interference (ICI) [18]-[20] This problem has been discussed widely manifesting as the tradeoffs between output power back-off (OBO), linearization, adjacent channel power ratio (ACPR), However, these works mostly based on the envelope models while rarely considered the instantaneous models Different techniques are employed to operate the HPA at its highest possible power efficiency but satisfying the linear specifications, at the same time If the designed HPA does not fulfill the ACPR specification for a desired operating frequency, linearization techniques are usually applied to improve its linearity These procedures require extensive simulation work and reliable large-signal model is indispensable Similarly, other complex efficiency enhancing HPA design techniques also need large-signal model.As a simple method, the HPA nonlinear characteristics are usually measured at separated points based on one or two unmodulated carrier(s) Then, for system analysis or simulation purposes, interpolation/extrapolation should be carried out to retrieve the desired characteristics For these reasons, the approximated close-form model is a very convenient tool for the replacement However, for a long time, the suitablity of using such nonlinear models in simulating communication systems with HPA nonlinearity is not much investigated This could at least create a significant gap between theoretical research results and realities or more severely, might produce invalid research results 86 N Thanh, N T Nam, N Q Binh, “On the reasonableness of … system simulations.” Nghiên cứu khoa học công nghệ Looking back in the past, in 1980, Cann [5] proposed an instantaneous nonlinearity model for HPA with the convenient feature of variable knee sharpness, mostly suitable for both theoretical analysis and simulation However, until 1996, Litva [3] shown that this model give incorrect results for intermodulation products (IMPs) in the two-tone test Four years later, Loyka [9] diagnosed the reason: non-analyticity Other publications showed that no problem exists with typical real-world signals Recently, Cann [6] improved the original instantaneous model, totally eliminating the problem with minimal complexity augmentation However, to investigate its applicability as an envelope model for simulating nonlinearities in communication systems, there need careful analyses since the usage of the instantaneous models is quite different to that of the envelope models Moreover, with a rather structural form of formulation, the Cann’s new model is inherently less accurate in approximating to the real-world data Thus, there is a gap to fill in by more suitalble models that are analytic and better approximate to the reality Therefore, this work investigates the reasonableness of the current widespread-used nonlinear models, and proposes two models that are both analytic and better than Cann’s new model in terms of approximating to the real-world data Examples with specific testing signals verify the relevance of the arguments and point out suitable alternatives for use The rest of this paper is organized as follows The Cann’s original instantaneous model and improved version are introduced and analyzed in Section 2, emphasizing on the defects of non-analyticity and asymmetry Focusing on the same targets, Section carries out analyses for two proposed models and other extensively used envelope models Examples with numerical results are shown and discussed in Section revealing suitable models for use Section concludes the achievements CANN'S MODEL FOR INSTANTANEOUS SIGNALS Cann's original model To represent a signal passing through an HPA, in 1980, Cann [5] proposed the instantaneous nonlinear model with variable knee sharpness y Aos ·sgn( x) s 1/ s  gx 1/ s (1)   g | x | s  1       Aos   where, y is the output voltage, x is the input voltage, g is the small-signal (linear) gain, Aos is the output saturation level and s is the sharpness (smoothness) parameter This is   A   1   os     g | x |   one of the oldest nonlinear models for representing HPA [1] However, until 1996, Litva [3] found that this model gave incorrect results for the third and higher order IMPs in the two-tone test Four years later, Loyka [9] discovered that the reason was the use of modulus (|.|) function in (1), some of whose derivatives at zero not exist, are undefined, or are infinite In other words, the function is not analytic, despite the deceptively smooth appearance of the plotted curves Incidentally, in 1991, Rapp [8] introduced a complex envelope model for solid-state power amplifiers (SSPAs) that resembles to the Cann’s instantaneous model except for the modulus operator in the denominator and the exponent of 2s instead of s However, at the best of our knowledge, there are not so much discussions on the defect of this commonly used model Detail analysis on this topic will be given in the next section Tạp chí Nghiên cứu KH&CN quân sự, Số 55, 06 - 2018 87 Kỹ thuật điều khiển & Điện tử Cann's new model Based on the magnitude Bode plot of a simple lead network transfer function (1  jω) / (a  jω) which is analytic and symmetric regarding to variable ω , Cann [6] suggested the new nonlinear instantaneous model in the scaled normalized form as Aos  e s ( gx / Aos 1) (2) ln - Aos s  e s ( gx / Aos -1) with variables y , x , and parameters g , Aos , and s have the same meanings as what are y in the original model (1) It is not difficult to show that the derivatives of new model’s (2) exist and well behave, even with fractional s The reasonableness of the third and fifth order IMPs for the twotone test simulation using this model is illustrated in figure Here, the sharpnesses s vary in a quite large range revealing the model’s effectiveness It is observed that these lines have the expected slopes as what happening in a realworld experiment: dB/dB for third order (in figure 1.a)) and dB/dB for fifth order (in figure 1.b)) Moreover, the IMPs’ slopes not change for all sharpnesses This confirms the suitability of the new Cann’s model (2), yielding simulation results conforming to what happening in reality Therefore, model (2) totally eliminates the shortcomings of the previous one This is the analyticity and symmetry of the original lead network transfer function to resolve the problem -50 s=3 s=5 s=9 -100 -80 IMP5 Output [dB] IMP3 Output [dB] -40 -120 -160 -200 s=3 s=5 s=9 -150 -200 -250 -300 -240 -280 -80 -70 -60 -50 -40 -30 Input [dB] -20 -10 -350 -60 -50 -40 -30 Input [dB] -20 -10 (a) (b) Figure IMPs by the new Cann’s model (2): a) Third order; b) Fifth order ENVELOPE MODELS Envelope representation of bandpass signals Practically, to comply with spectral regulations, a communication system with nonlinear HPA often has a bandpass zonal filter that restricts the output to the first spectral zone, suppressing all harmonics and even-order IMPs Such a system is referred as narrowband or bandpass, meaning that the bandwidth is considerably less than the center frequency This attribute allows huge saving of computation since the required sampling rate is then determined not by the highest frequency of the signal but by its bandwidth (of course plus a suitable redundance for significant IMPs) The resulting model is the lowpass equivalent representation of the bandpass system and is regarded as envelope model 88 N Thanh, N T Nam, N Q Binh, “On the reasonableness of … system simulations.” Nghiên cứu khoa học công nghệ A narrowband radio-frequency (RF) signal can be represented as v(t )  A(t ) cos[t   (t )]  Re[ A(t )e j[t  ( t )] ] , (3) where, A(t ) is the amplitude modulation (AM) component, and  (t ) is the phase modulation (PM) component, both varying slowly regarding to the carrier frequency  When being observed in a reference plane rotating at the carrier frequency, the resulting signal is complex envelope x(t )  A(t )e j ( t )  A(t ) cos  (t )  jA(t ) sin  (t ) (4) It is noteworthy that the carrier  disappears but all modulating information (carried in both amplitude and phase) still exists in (4) Envelope model characteristics The envelope model is characterized by its complex transfer function F ( A)  y / x  Fa ( A)e jFp ( A ) , including the AM-AM transfer function Fa ( A) , output amplitude as a function of input amplitude, and the AM-PM transfer function Fp ( A) , phase shift as a function of input amplitude, all for a single frequency signal At rather low frequencies and small bandwidth, SSPA previously was considered as having little or no AM-PM and a constant transfer function over the passband [11] However, at higher frequencies and larger bandwidth, this assumption is no longer valid [13]-[17] Actually, measurements of these transfer functions are usually made at only a discrete set of points; therefore, to simulate the nonlinearity at a specific operating point, generally, the input-output relation is usually interpolated from measured data This can be carried out with great accuracy using series expansion or splines,… but a closed-form model can provide a convenient approximation and is often accurate enough Saleh model 60 1.2 Phase change [deg] Normalized output magnitude 50 0.8 0.6 0.4 Saleh (5) Mod Saleh (11) Mod Ghorbani (13) Rapp (7) 0.2 0 0.2 0.4 0.6 0.8 1.2 Normalized input magnitude 1.4 40 Saleh (6) Mod Saleh (12) Mod Ghorbani (14) Mod Rapp (15) 30 20 10 -10 1.6 -20 0.2 0.4 0.6 0.8 1.2 Normalized input magnitude 1.4 1.6 (a) (b) Figure Characteristics of typical nonlinear models: a) Amplitude; b) Phase In 1981, Saleh, a researcher working at Bell Labs in Crawford Hill, introduced a closeform model for traveling wave tube amplifiers (TWTAs) [7], which then has been widely used since it includes both AM-PM and AM-AM with typical turndown after saturation These AM-AM, AM-PM are formulated as: Fa ( A)  a A ,   a A2 Tạp chí Nghiên cứu KH&CN quân sự, Số 55, 06 - 2018 (5) 89 Kỹ thuật điều khiển & Điện tử Fp ( A)   p A2 ,   p A2 (6) where, A is the input amplitude, Fa ( A) is the output voltage, Fp ( A) is the phase shift,  a is the small-signal (linear) gain, together with  a ,  p ,  p forming the shape of amplitude and phase conversion curves,  a  ( a / Aos ) , Aos is the output saturation level This model is illustrated in figure with normalized linear gain and input saturation level,  a  1, Aos  [V] This figure also illustrates other typical AM-AM and AM-PM characteristics which are then discussed below Saleh reminded that the amplitude A might be negative, thus, (5) must be an odd function Noting that the Saleh model does not support adjusting the knee sharpness of AM-AM characteristic Otherwise, the curvature of (5) is too smooth regarding to the typical SSPAs’ AM-AM characteristics, which also not fall down after saturation Rapp model In 1991, in a work studying the effects of nonlinear HPA in digital broadcasting system, Rapp proposed an envelope model with variable knee sharpness for SSPAs as [8] Fa ( A)  gA 1/2 s   gA  s  1       Aos   , (7) where, A is the input magnitude, Fa ( A) is the output mangitude, g is the small-signal (linear) gain, and s is the curve’s sharpness 1.2 Output voltage [V] 0.8 0.6 Ideal limiter Rapp, s = Rapp, s = 1.4 Rapp, s = Rapp, s =  0.4 0.2 0 0.25 0.5 0.75 1.25 Input voltage [V] 1.5 1.75 Figure Amplitude characteristics of the Rapp model with different sharpnesses It is noteworthy that this model assumed zero AM-PM conversion and by changing the sharpness parameter s , the AM-AM characteristic could have any curvature Further, (7) is only odd (Saleh’s condition) for integer s Several examples of (7) with different knee sharpnesses s are illustrated in figure with normalized linear gain and output saturation level, g  1, Aos  [V] In addition to this, the normalized characteristic curve of the ideal limiter is included for reference 90 N Thanh, N T Nam, N Q Binh, “On the reasonableness of … system simulations.” Nghiên cứu khoa học công nghệ purpose This is an upper bound for any real-world amplifiers (with approximated exception of ideal predistorter-amplifier combination [17], [18]) Incidentally, the Rapp’s model resembles to the instantaneous model (1) excepting the absence of modulus operator in the denominator Thus, it seems to avoid the problem of (1) for the suitability of IMPs resulted by simulation, but this is not the case The Rapp’s model has been widely used for roughly a quarter of century without any notation for its reasonableness and also its suspicious results until the publication of Cann [6] Thorough investigation leads to the conclusion that the problem of (1) only manifests with signals that have their magnitude distribution concentrating around zero, such as the signal used in the two-tone test For real-world signals like M-FSK, M-PSK, M-QAM, MAPSK, OFDM,… the Rapp’s model behaves almost perfectly well Therefore, resembling to the case of instantaneous models, all envelope AM-AM models should ideally be odd and analytic over the expected amplitude range An envelope model, which is asymmetric and is not analytic at zero, should be used with caution and only for signal waveforms that are sufficiently complex to have a wide amplitude distribution However, non-analytic model is not a serious defect, because typical realworld signals with high spectral efficiency have large amplitude distribution It is well known that signal should be noise-like for maximizing the channel capacity Cann’s new model 35 30 Output [V] 25 Data Cann (2) Rapp (7) Polynomial (8) Polynomial (9) Polysine (10) 20 29.5 29 28.5 15 1.15 1.2 1.25 1.3 22 10 21 0 20 0.2 0.4 0.65 0.7 0.75 0.6 0.8 Input [V] 0.8 1.2 1.4 1.6 Figure Rapp, Cann, polynomial and polysine models’ amplitude characteristics fitted to measured data Although originally developed as an instantaneous model, (2) can be used equally as an envelope model This should find broad applications, like Rapp model, it has adjustable knee sharpness and does not turn down after saturation But, unlike the Rapp model, it is analytic everywhere and therefore valid for any signal waveform Moreover, if the phase convesion is significant, an AM-PM function, such as Saleh’s (6), can be included Resembling to the Rapp model (7), envelope model (2) could support any curvature, especially in the region above s  2.5 , suitable for AM-AM characteristics of most SSPAs [17] The approximations of model (7) and model (2) to the real-world data are verified by curve fitting of these functions to the measured data from the L band Quasonix 10W amplifier [12] Results are, for Rapp model (7): g  29.4 , Aos  30 [V], s  4.15 , for the new Cann model (2): g  29.4 , Aos  30 [V], s  8.9 , [6] For this particular Tạp chí Nghiên cứu KH&CN quân sự, Số 55, 06 - 2018 91 Kỹ thuật điều khiển & Điện tử HPA, Rapp model is little better fitted than Cann model Figure illustrates these fittings with the inclusion of other approximated curves discussed next Polynomial models Considering the measured data in figure 4, it is not difficult to recognized that there is a simple yet efficient method approaching the close-form characteristic function by approximation using polynomials In this case, the complex envelope nonlinearity F ( A)  y / x can be represented by a complex polynomial power series of a finite order N such that N N y   ak | x |k 1 x   ak  kP [ x] , k 1 (8) k 1 where,  kP [ x] | x |k 1 x are the basis functions of the polynomial model, and ak are the model’s complex coefficients Table Coefficients of polynomial models (8) and (9) a2 a3 a4 a5 Model a1 (10) 30.02 -8.665 33.68 -40.19 12.39 (11) 28.60 8.310 -15.06 a6 a7 a8 a9 0 6.257 0 -0.872 Obviously, model (8) is not analytic at A | x | by the existence of modulus operators However, if even order coefficients a2k vanish, then, for real-valued signals x(t ) , (8) turns into the odd order polynomial model of the form N N y   a2 k 1 | x |2( k 1) x   a2 k 1 x k 1 k 1 (9) k 1 Model (9) is clearly analytic at A | x | and is used as a counter example to model (8) in the applications section below The measured data of the L band Quasonix 10W amplifier is then used to fit the polynomial models (8) and (9) with the same number of coefficients N  Figure depicts the approximated characteristics with parameters shown in table It is not difficult to show that at large enough order, polynomial models are better fitted to the real-world data than Rapp model (7) and Cann model (2) Further, with the same N, higher order polynomial in (9) is smoother than lower order one in (8) resulting better fitting performance for the sooner Polysine model It can be seen that the sine/cosine functions are distinctly better than polynomial ones in terms of both analyticity and smoothness Thus, while remaining to be analytic, the sooners are better fitted to the real-world data than the laters Based on this argument, we propose the nonlinear model of the form N y   ak sin(bk x) , (10) k 1 where, ak and bk are correspondingly the amplitude annd phase coefficients The introduction of bk lets the function better addapting to the fitting data, thus improving the approximation performance 92 N Thanh, N T Nam, N Q Binh, “On the reasonableness of … system simulations.” Nghiên cứu khoa học công nghệ Using the Matlab curve fitting tool, (10) is fixed to the AM-AM characteristic of the L band Quasonix 10W amplifier data [12] in figure resulting in the parameters listed in table Table Coefficients of polysine model (10) Order k ak 30.73 bk 1.045 0.00955 -0.6586 -0.1061 0.1859 5.312 12.91 18.61 8.107 The fitting performances of these five models are quantified using Square Error Sum (SES) measure and are compared in table Odd-order polynomial model (9) and polysine model (10) are both analytic and much better fitted to the real data than Cann model (2) This is illustrated in figure with sub-figures focusing on segments with significant differences where the data is rather harder to fit The better fitting performance is the closer to the data these curves approach With almost one order of magnitude better in SES than the rest, the polysine model’s curve always coincide to all data points The fitting performance of these models will reflect in the nonlinearity simulation results that are then discussed bellow Table Fitting performance (SES  e2 ) of five models Model Cann (2) Rapp (7) SES 1.786 0.963 Polynomi al (8) 0.533 Polynomi al (9) 0.346 Polysine (10) 0.032 Other models Beside the AM-AM characteristic, updated envelope models for SSPAs at higher frequencies and larger bandwidth all consider the AM-PM conversion and generally better fit to the measured data than previous models However, it is not difficult to see that models discussed below are not analytic or symmetric at A  for most of the parameter sets and thus problem of (7) still exists The characteristics of these models are graphically illustrated in figure for comparison purpose Modified Saleh model The modified Saleh model [13] was proposed for popular LDMOS (Laterally diffused metal oxide semiconductor) power amplifiers (PAs), that are very common for the base station (BS) amplifiers of 2G, 3G and 4G mobile networks (in the L, S, C bands) The AM-AM and AM-PM conversion functions are a A Fa ( A)  Fp ( A)  , (11)  p , (12)   a A3 p  A4 where,  a  1.0536 ,  a  0.086 ,  p  0.161 ,  p  0.124 is a typical parameter set Modified Ghorbani model Tạp chí Nghiên cứu KH&CN quân sự, Số 55, 06 - 2018 93 Kỹ thuật điều khiển & Điện tử The modified Ghorbani model [14] that is suited for GaAs pHEMT FETs (Gallium arsenide pseudomorphic High-electron-mobility transistor Field-effect transistor) PAs that are operating at frequencies upto 26 GHz (K band) and are dominant in terms of production technologies and market shares compared to other power semiconductor techlogogies This model proposed the following charactertistics x1 A x2  x3 A x2 1 , Fa ( A)   x4 A x2 (13) y1 A y2  y3 A y2 1 , Fp ( A)   y A y2 (14) where, the model parameters are given by x1  7.851 , x2  1.5388 , x3  0.4511 , x4  6.3531 , y1  4.6388 , y2  2.0949 , y3  0.0325 , y4  10.8217 Modified Rapp model The modified Rapp model [16] was introduced for GaAs pHEMT/CMOS (Complementary metal-oxide-semiconductor) PA model at 60 GHz band, the new band for communication industry, with AM-AM function of (7) and AM-PM described as  Aq Fp ( A)  , (15)   A q2  1           where, parameter set are g  16 , Aos  1.9 , s  1.1 ,   345 ,   0.17 , q1  q2  APPLICATIONS This section describes the applications of envelope models investigated above for representing nonlinear HPA in communication systems and analyses typical experiments with test signals having discrete and continuous spectra to reveal their applicability and reasonableness Representation of envelope model Consider the finding of IMPs in a two-tone test with a signal consisting of two equalamplitude unmodulated sinusoid waveforms at frequencies f1 and f  f1 These testing signal could be equivalently regarded as a double-sideband suppressed carrier AM of the form xinst (t )  1 A0 [sin(2 f1t )  sin(2 f 2t )]  A0 cos(2 f mt ) sin(2 f c t ) , (16) where, f m  ( f  f1 ) is the modulating frequency, f c  ( f  f1 ) is the (center) carrier frequency Waveform (15) with f1  [Hz], f  10 [Hz] is illustrated in figure It is observed that the carrier f c manifests inside the envelope and is the average of f1 and f , while the envelope is the modulating signal at frequency f m 94 N Thanh, N T Nam, N Q Binh, “On the reasonableness of … system simulations.” Nghiên cứu khoa học công nghệ -1 -2 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Figure Two-tone signal waveform with f1 = [Hz], f2 = 10 [Hz] With the 90o phase shifting, xinst (t ) in (16) could be recast as xinst (t ) =A0 sin(2 f mt ) sin(2 f c t ) (17) Therefore, its envelope form is xenv (t )  A0 sin(2 f mt ) A(t )e j (t ) A(t ) (18) V (t )  (t ) V (t )e j ( t )  (t )  (t ) Figure Polar envelope model block diagram Because the envelope model requires non-negative input, thus, the sinusoid waveform of (18) is decomposited to the polar form as xenv (t )  A(t )e j (t )  A0 | sin(2 f mt ) | e j (t ) , (19) A(t )  A0 | sin(2 f mt ) | , (20) where, 0, sin(2 f mt )r (21) e (t )    , sin(2 f mt )  In other words, the amplitude component A(t ) is the full-wave-rectified sinusoid, and the phase component  (t ) is the 180o square wave When passing through the envelope model, the amplitude component is input to the model, while the phase component is bypassed as depicted in figure [1] The distorted amplitude output is then combined with the phase part, resulting the output waveform for analysis If AM-PM conversion is included, then the distorted phase is added up to the input phase  (t ) before combining Tạp chí Nghiên cứu KH&CN quân sự, Số 55, 06 - 2018 95 Kỹ thuật điều khiển & Điện tử Two-tone test Third-order IMPs Fifth-order IMPs -30 Cann (2) Rapp (7) Polynomial (8) Polynomial (9) Polysine (10) Output [dB] -90 -120 -50 -100 Output [dB] -60 -150 -180 -210 Cann (2) Rapp (7) Polynomial (8) Polynomial (9) Polysine (10) -150 -200 -250 -240 -300 -270 -80 -70 -60 -50 -40 Input [dB] -30 -20 -10 -80 -70 -60 -50 -40 Input [dB] -30 -20 -10 (a) (b) Figure Third (a) and fifth (b) order IMPs for five models depicted in figure Simulation procedure is as depicted in figure with the following parameters: simulation time [s], sampling rate 1000 [Hz], input signal waveform as in figure 5, five models depicted in figure are considered Output signals will be used for IMPs analysis 2.5 x 10 3000 2500 2000 1.5 1500 1000 0.5 0 500 0.25 0.5 0.75 1.25 1.5 1.75 2.25 0 0.2 0.4 0.6 0.8 1.2 1.4 1.6 1.8 (a) (b) Figure Histogram of the testing signals: a) Two-tone; b) 1+7 APSK The third and fifth order IMPs are correspondingly shown in figure 7.a) and 7.b) As observed, new Cann model (2), odd order polynomial model (9) and polysine model (10) result in the required slope of [dB/dB] and [dB/dB] correspondingly for the third and fifth order IMPs With almost the same structure as (9), however, the full order polynomial model (8) fails in simulating the odd IMPs, revealing the problem as found by Litva in [3] for the Cann’s instantaneous model (1) So does the Rapp model Further, there are constant gaps between IMPs created by models (2), (9) and (10) Obviously, smaller error in fitting approximation should result in better performance of simulation Thus, Cann model (2) produces less confident results than what created by odd-order polynomial model (9) and especially by polysine model (10) Reconsidering the processing in figure 6, it is recognized that the separator indirectly yields the modulus operation, causing the former problem Thus, to receive reasonable results for the two-tone test, the envelope model should be analytic at A  , as the same as found by Loyka [9] for the instantaneous model 96 N Thanh, N T Nam, N Q Binh, “On the reasonableness of … system simulations.” Nghiên cứu khoa học công nghệ For the apparentness of the defect of Rapp model (7) and polynomial (8) under the effect of the signal amplitude distribution to the IMPs, consider the histogram of the twotone signal amplitude as illustrated in figure 8.a) It is inferred that the very high concentration of signal amplitude around A  results in the failure of the non-analytic model Continuous spectrum test 10 Normalized power spectral density 0.5 -0.5 -1 Polynomial (9) Cann (2) -1 -0.5 0.5 -10 Cann (2) Rapp (7) Polynomial (8) Polynomial (9) Polysine (10) -43 -44 -45 -20 0.21 0.24 0.27 0.3 0.4 0.3 -30 -24 -40 -25 -50 -26 -60 -70 -0.5 -27 0.1 0.12 0.14 0.16 0.18 -0.4 -0.3 -0.2 -0.1 0.1 0.2 Normalized frequency 0.5 (a) (b) Figure Continuous spectrum test results: a) Receive constellations; b) Receive spectra Consider an updated real-world signal as the input for such models investigated above Amplitude-phase shift keying (APSK) is usually used for communication systems with considerations in spectral and power efficiencies 1+7 APSK is recently introduced as an efficient modulation scheme for satellite communications [21] The signal constellation includes one signal point at the origin ( A  ) and seven others evenly distributed in a circle Under the above argument flow, the test with this input signal could result in the fail of models (7) and (8), deceptively But the fact is more complicated With the inclusion of transmit shaping filter and receive matched filter, the simulated signal waveform is in the form of continuous spectrum with its magnitude distribution depicted in figure 8.b) It is seen that there is so less concentration at A  , totally different to the magnitude distribution counterpart of the two-tone waveform in figure 8.a) This somehow relieves the defect of non-analytic models investigated in the previous section Applying this test signal into system with five HPA models used in the previous section, the output signals are then analysed showing the spectrum regrowth Figure 9.a) illustrates the receive constellations for Cann (2) model and odd order polynomial model (9), manifesting the relatively strong effects of HPAs Figure 9.b) depicts the receive spectra corresponding to all five models Roughly, at high levels of spectra in the main lobe, these is almost no difference in results from all models, both analytic and non-analytic ones However, as the same as what can be observed in figure for the IMP3s and IMP5s in the two-tone test, there are divergences for the third- and fifth order spectrum regrowths in this case The gap is up about 0.5 dB between the Cann model’s curve and the polysine model’s one at the first sidelobe and is up about dB at the second sidelobe The closer coincidence of the oddorder polynomial model’s curve and the polysine model’s one reveals defect of Cann model (2) Tạp chí Nghiên cứu KH&CN quân sự, Số 55, 06 - 2018 97 Kỹ thuật điều khiển & Điện tử CONCLUSION In this paper, typical instantaneous and envelope models are investigated in detail for their suitability and applicabilities Cann’s new model eliminates the old one’s defect and can be used as an envelope model although first introduced as an instantaneous model However, odd order polynomial model and polysine model could be used as alternatives with the simplicity and much better accuracies Further, all models analyzed can be somehow safely used for real-world signal in simulations However, care should be taken into account for the case where small level IMPs and spectral regrowths are in consideration REFERENCES [1] Jeruchim, M., Balaban, P., and Shanmugan, K., Simulation of Communication Systems, Plenum Press, 2000 [2] Corazza, G E., Digital Satellite Communications, Chapter 7, Springer, 2007 [3] Litva, J and Lo, T K-Y, Digital Beamforming in Wireless Communications, Norwood MA: Artech House, 1996 [4] Alamouti, S.M., “A simple transmit diversity technique for wireless communications,” IEEE J on Sel Areas in Commun., Vol 16, No 8, pp 1451-1458, 1998 [5] Cann, A., “Nonlinearity model with variable knee sharpness,” IEEE Trans on Aerospace and Electronic Systems, Vol 16, No 6, pp 874-877, Nov 1980 [6] Cann, A., “Improved nonlinearity model with variable knee sharpness,” IEEE Trans on Aerospace and Electronic Systems, Vol 48, No 4, pp 3637 - 3646, Oct 2012 [7] A A M Saleh, “Frequency-independent and frequency-dependent nonlinear models of TWT amplifiers,” IEEE Trans on Commun., Vol 29, No 11, pp 1715-1720, 1981 [8] Rapp, C., “Effects of HPA-nonlinearity on a 4-DPSK/OFDM-signal for a digital sound broadcasting system,” in Proceedings of the Second European Conference on Satellite Communications, Liege, Belgium, Oct 22-24, 1991, pp 179-184 [9] Loyka, S., “On the use of Cann' model for nonlinear behavioral-level simulation,” IEEE Trans on Vehicular Tech., Vol 49, No 5, pp 1982-1985, Sep 2000 [10] Loyka, S and Mosig J., “New behavioral-level simulation technique for RF/microwave applications Part I: Basic concepts,” Int J of RF and Microwave Computer-Aided Engineering., Vol 10, No 4, pp 221-237, Jul 2000 [11] Van Nee, R and Prasad, R., OFDM for Wireless Multimedia Communications, Norwood MA: Artech House, 2000 [12] Shaw, C and Rice, M., “Turbo-coded APSK for aeronautical telemetry,” in Proceedings of IEEE Int Conf on Waveform Diversity and Design, Orlando FL, USA, Feb 2009, pp 317-321 [13] M O'Droma, S Meza, and Y Lei, “New modified Saleh models for memoryless nonlinear power amplifier behavioural modelling,” IEEE Commun Lett., Vol 13, No 6, pp 399-401, Jun 2009 [14] A Aghasi, A Ghorbani and H Amindavar, “Polynomial based predistortion for solid state power amplifier nonlinearity compensation,” in Proc 2006 IEEE North-East Workshop on Circuits and Systems, QC, Canada, Jun 18-21, 2006, pp 181-184 [15] Dragoslav D Siljak, Nonlinear Systems: Parameter Analysis and Design, John Wiley & Sons, 1969 [16] C.-S Choi, Y Shoji, H Harada, R Funada, S Kato, K Maruhashi, I Toyoda, and K Takahashi, “RF impairment models for 60GHz-band SYS/PHY simulation,” Tech Rep IEEE 802.15-06-0477-01-003c, Nov 2006 98 N Thanh, N T Nam, N Q Binh, “On the reasonableness of … system simulations.” Nghiên cứu khoa học công nghệ [17] Fadhel M Ghannouchi, Oualid Hammi, Mohamed Helaoui, Behavioral Modeling and Predistortion of Wideband Wireless Transmitters, John Wiley & Sons, 2015 [18] N Thanh, N T Nam, and N Q Binh, “Predistortion methods for nonlinear high power amplifiers in MIMO-STBC systems,” Journal of Science and Technology, Le Quy Don Technical University, No 188, pp 74-88, Feb., 2018 [19] N Thanh, N T Nam, and N Q Binh, “Automatic phase compensation in MIMOSTBC systems with nonlinear distortion incurred by high power amplifiers,” in Proc Advanced Technol for Commun - ATC 2017, Quy Nhon, Vietnam, Oct 18-20, 2017, pp 86-91 [20] N Thanh, N T Nam, and N Q Binh, “Performance of a phase estimation method under different nonlinearities incurred by high power amplifiers in MIMO-STBC systems,” in Proc Conference on Information and Computer Science - NICS 2017, Ha Noi, Vietnam, Nov 24-25, 2017, pp 42-47 [21].M Eroz and L-N Lee, “Method and apparatus for improved high order modulation,” US Patent No 8,674,758, Mar 2014 TĨM TẮT VỀ TÍNH HỢP LÝ CỦA CÁC MƠ HÌNH PHI TUYẾN CHO CÁC BỘ KHUẾCH ĐẠI CÔNG SUẤT LỚN VÀ ỨNG DỤNG TRONG MÔ PHỎNG CÁC HỆ THỐNG THƠNG TIN Các mơ hình khuếch đại cơng suất lớn (KĐCS) với đặc tính phi tuyến cố hữu đóng vai trò quan trọng phân tích đánh giá chất lượng hệ thống thơng tin khía cạnh lý thuyết thực tế Tuy nhiên, khơng có nhiều cơng trình thảo luận tính phù hợp sử dụng mơ hình mô đặc trưng phi tuyến KĐCS hệ thống thông tin Trong báo này, tác giả khảo sát tính hợp lý mơ hình phi tuyến tiêu biểu vốn sử dụng rộng rãi đồng thời đề xuất hai mơ hình phi tuyến vừa bảo đảm tính chất giải tích vừa tốt mơ hình Cann phương diện xấp xỉ theo liệu thực Các ví dụ với tín hiệu kiểm tra khác giúp kiểm chứng lập luận mơ hình sử dụng phù hợp Từ khóa: Khuếch đại cơng suất; MIMO-STBC; Mơ hình phi tuyến Received date, 26th March, 2018 Revised manuscript, 6th June, 2018 Published, 8th June, 2018 Author affiliations: Le Quy Don Technical University; Department for Standard, Metrology and Quality; Hung Yen University of Technology and Education * Corresponding author: ngthanh1210@yahoo.com Tạp chí Nghiên cứu KH&CN quân sự, Số 55, 06 - 2018 99 ... having discrete and continuous spectra to reveal their applicability and reasonableness Representation of envelope model Consider the finding of IMPs in a two-tone test with a signal consisting...   0.17 , q1  q2  APPLICATIONS This section describes the applications of envelope models investigated above for representing nonlinear HPA in communication systems and analyses typical experiments... semiconductor) power amplifiers (PAs), that are very common for the base station (BS) amplifiers of 2G, 3G and 4G mobile networks (in the L, S, C bands) The AM-AM and AM-PM conversion functions
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