OTFS Modem SDR Implementation and Experimental Study of Receiver Impairment Effects Tharaj Thaj and Emanuele Viterbo ECSE Department, Monash University, Clayton, VIC 3800, Australia Email: {tharaj.thaj, emanuele.viterbo}@monash.edu Abstract—This paper presents a software defined radio (SDR) Design and Implementation of an orthogonal time frequency space (OTFS) modem. OTFS is a novel modulation scheme based on multiplexing information symbols over localized pulses in the delay–Doppler signal representation. Traditional OFDM modula- tion operates in the frequency-time domains. In contrast, OTFS modulation operates in the delay spread-Doppler plane domains, which are related to frequency and time by the symplectic Fourier transform (similar to a two-dimensional discrete Fourier trans- form). OTFS is shown to perform very well under the 5G usage scenarios such as high speed vehicle to vehicle communication with wide Doppler spreads, where the traditional OFDM system performance degrades. Like any other communications system, the OTFS modem is not free from receiver impairments such as DC offset and carrier frequency offset, which affects the channel estimation and hence the decoding process. We study the effects of these receiver impairments on the receiver performance from real time experiments conducted on the implemented OTFS modem in a real indoor wireless channel. We also compare the performance of OTFS modulation and OFDM modulation using the same hardware setup and environment for the real frequency selective and partially emulated doubly selective channel. Index Terms—Delay–Doppler channel, OTFS, modem, Soft- ware Defined Radio I. I NTRODUCTION Wireless multipath fading channel can be modelled as time varying impulse response or as a time varying frequency response. This is the appropriate representation for wireless OFDM-based systems like LTE. In LTE the frequency re- sponse is estimated every OFDM symbol in order to equalize the channel. Higher mobility results in faster variation of the multipath components. Since constructive and destructive addition of these multipath components causes signal fading, faster variation of these components leads to more rapid fluctuations in the channel. The frequency response rate of variation is also proportional to the signal carrier frequency. Thus, the faster the reflectors, transmitters, and/or receivers move, the higher the frequency band, the more rapidly changes in the channel frequency response occur. As the channel coherence time in the time-frequency do- main is the inverse of its Doppler, the impulse response for this channel varies rapidly over a fraction of a millisecond. Hence, in an LTE/OFDM system there is not sufficient time to estimate the channel, let alone provide feedback of the channel state to the transmitter. As compared to the time varying impulse response, or time varying frequency response, the delay Doppler representation of the channel varies much slower over a longer observation time. Orthogonal Time Frequency and Space (OTFS) is a new 2D modulation technique that transforms information symbols in the delay–Doppler coordinate system to the familiar time- frequency domain [1], [2]. By spreading all the information carrying symbols (e.g., QAM) over both time and frequency to achieve maximum diversity. As a result, the time-frequency selective channel is converted into an invariant, separable and orthogonal interaction, where all received QAM symbols experience the same localized impairment and all the delay- Doppler diversity branches are coherently combined. Software defined radio (SDR) is a radio communication system where all or most of the physical layer functions have been implemented in software. Traditional hardware based radio devices limits cross functionality and needs to be physically modified each time a different waveform standard is proposed, which leads to high production costs and low flexibility. On the other hand, a SDR handles a lot of the signal processing functions in a general purpose processor, which allows for transmitting and receiving a wide variety of waveforms and protocols. For our implementation, we use the National Instruments Universal Software Radio Peripheral (USRP) device. Like any typical radio, SDR is also affected by DC offset and carrier frequency offset (CFO), that can degrade the receiver performance. OTFS is expected to be robust towards CFO. This is due to the fact that it will be sensed in channel estimation phase as an additional Doppler shift and will be very simply corrected. On the other hand, a DC offset can severely corrupt the channel estimation. In this paper we study the effects of CFO and DC offset on channel estimation and hence on receiver performance using real time experiments conducted on the implemented OTFS modem inside a real indoor wireless channel. Further we will discuss how we can correct in the delay-Doppler domain, using the pilot symbols, CFO and DC Offset in the case when both remains constant for the duration of one OTFS frame. The paper is organized as follows. In Section II, we discuss the implementation aspects of the OTFS modem. In Section III, we discuss the pilot information extraction, channel esti- mation and effects of receiver impairments on estimating the channel and hence the receiver performance . The experimental setup and the results are provided in Section IV. Section V contains our concluding remarks. 978-1-7281-2373-8/19/$31.00 ©2019 IEEE Authorized licensed use limited to: Monash University. Downloaded on August 19,2020 at 14:33:00 UTC from IEEE Xplore. Restrictions apply.