IEEE TRANSACTIONS ON INSTRUMENTATIONAND MEASUREMENT, VOL. 67, NO. 1, JANUARY 2018 241 RF Circuit Implementation of a Real-Time Frequency Spread Emulator Murat Karabacak , Graduate Student Member, IEEE , Ahmed Hossam Mohammed, Mehmet Kemal Özdemir, and Hüseyin Arslan, Fellow, IEEE Abstract— Despite their reliability, on-site measurements are time- consuming and costly actions for the evaluation of new devices. Channel emulators are widely utilized measurement instruments to generate desired environmental channel effects in laboratory environments. Within these instruments, baseband emulators are expensive, and reverberation chambers provide limited control of the channel. However, radio- frequency (RF) circuit implementation of channel emulators provides an affordable and easy tool to test performances of new systems and methods under different channel effects. In this paper, a new RF domain Doppler emulator, which is compact and easy to control, is presented for measuring signal characteristics under frequency dispersive channel conditions. The circuit has been implemented using variable attenuators, switches, and power splitters to emulate the Doppler spread of air-ground channels, and the performance is evaluated through measurements. It is observed that the emulator indeed generates the desired Doppler model close enough to replicate environmental channels for mobile applications in laboratory environments. Index Terms—Channel emulator, Doppler spread, fast fading emulator, high-speed communication, radio-frequency (RF) circuit. I. I NTRODUCTION O N-SITE measurements provide reliable and realistic results for the evaluation of prototype devices. However, it is time- consuming and costly to execute, especially for air-ground commu- nication due to the necessity of an airplane. In order to mitigate these challenges, measurement instruments that generate desired environmental channel effects are widely utilized as channel emu- lators to facilitate an affordable evaluation and verification tool in the laboratory environments. Channel effects, i.e., time dis- persion, frequency dispersion, and additive noise, can be emu- lated using the measurement instruments presented in the literature. Boutillon et al. [1] and [2] introduce hardware designs that add white Gaussian noise to baseband signals. Komninakis [3] proposes an infinite impulse response filter that introduces a time-varying channel to baseband signals. Yang et al. [4] present a Doppler emulation method with partially overlapping windows to overcome issues of finite response filters. Huang [5] implements the Rayleigh fading channel using low computational resources on a field-programmable gate array board. Mar et al. [6], Stephenne and Champagne [7], and Olmos et al. [8] demonstrate doubly dispersive emulators Manuscript received June 19, 2017; revised August 28, 2017; accepted August 29, 2017. Date of publication October 11, 2017; date of current version December 7, 2017. This work was supported by Savronik under Project SV.SOZ.E/0043.14.008’. The Associate Editor coordinating the review process was Dr. Huang-Chen Lee. (Corresponding author: Murat Karabacak.) M. Karabacak and H. Arslan are with the Department of Electrical Engi- neering, University of South Florida, Tampa, FL 33620 USA, and also with the School of Engineering and Natural Sciences, Istanbul Medipol University, 34810 Istanbul, Turkey (e-mail: murat@mail.usf.edu; arslan@usf.edu). A. H. Mohammed is with Electronics and Computer Engineering, Istanbul Sehir University, Uskudar, 34662 Istanbul, Turkey (e-mail: ahmedmohammed@std.sehir.edu.tr). M. K. Özdemir is with Electronics and Computer Engineering, Istanbul Sehir University, Uskudar, 34662 Istanbul, Turkey, and also with the School of Engineering and Natural Sciences, Istanbul Medipol University, 34810 Istanbul, Turkey (e-mail: mkozdemir@medipol.edu.tr). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TIM.2017.2757118 that affect the signal in time and frequency. The given emulator approaches introduce channel effects on digital baseband sig- nals to utilize flexibility. However, baseband emulators require a radio-frequency (RF) signal input to be downconverted, digitized, processed, and upconverted again. Thus, the baseband emulators introduce high processing delays on top of their complex structure and high cost. On the other side, RF domain channel emulators intro- duce channel effects to the signal without digitization or baseband conversion. Li et al. [9] introduce an approach using RF mixers to generate the time variation. Sorrentino et al. [10] present a method that controls the coherence time of the channel with stirrers in a reverberation chamber. Similarly, Guzelgoz et al. [11] present a method to manipulate the Doppler spread of a chamber. Because of their straightforward and simple design, reverberation chambers require larger forms that may exceed a room, and they have a limited control over the emulated channel. Complementing the RF circuit design for the time dispersive effect of a channel presented in [12], in this paper, generating frequency dispersion is focused. Thus, a doubly dispersive channel emulator can be implemented by combining both studies. Although Li et al. [9] already present a Doppler spread solution as an RF circuit, it requires a complicated process to generate desired effects using the mixers. Similarly, Goubran et al. [13] present a design on single chip to generate channel coefficients but applies them on a very narrow band signal. In this paper, a simple novel Doppler emulator design using variable attenuators, switches, and power splitters is proposed to provide low-cost and real-time solution with a small form factor. The channel variation has been implemented in the RF domain by manipulating the attenuation and switching between RF paths with different phases. Therefore, the input RF signal does not need to pass through a costly conversion process into and out of the digital domain. The prototype circuit is designed to introduce the Doppler spread of air-ground channels. Experimental measurements have been performed to validate the circuit and the results match with the desired environmental channel model. II. DOPPLER SPREAD IN WIRELESS CHANNELS A wireless channel can be modeled with its time-varying impulse response as h (t ) = M-1  i =0 α i (t )δ(t - ǫ i ) (1) where α i and ǫ i represent the i th tap time-varying channel coefficient and tap delay, respectively. The channel coefficient α(t ) is assumed to be wide sense stationary and the fading process for each tap is modeled as a complex Gaussian random process. The power spectral density (PSD) of this process depends on mobility, environmental scatters of the transmitted signal, and radiation pattern of trans- mitter/receiver antenna. For air-to-ground communication, Doppler spectrum has been analyzed through theory and measurements in [14] and [15], and the PSD of α i can be given as S( f ) = G B √ π exp  -(2 f 2 ) B 2  (2) 0018-9456 © 2017 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.