Design and Implementation of a Novel MSK-based Frequency-Domain SWIPT Multiplexing Scheme Rahul Gupta, Konstantinos Ntougias, and Ioannis Krikidis Department of Electrical and Computer Engineering, University of Cyprus Email: {gupta.rahul, ntougias.konstantinos, krikidis.ioannis}@ucy.ac.cy Abstract—In this paper, we propose a novel frequency-domain simultaneous wireless information and power transfer (FD- SWIPT) waveform which allows for rectifiers based on local oscillators (LO) and eliminates the need for receive (demulti- plexing) filters. This is achieved via appropriate multiplexing a multitone energy signal with a minimum shift keying (MSK) data signal. We analytically derive the average error probability for a point-to-point link in the presence of i.i.d. Rayleigh fading, additive white Gaussian noise, and multitone interference. The performance of the proposed waveform is evaluated via numerical simulations, assuming either a LOs-based receiver or a power splitting (PS)-based one for comparison purposes. The simulation results show that the proposed scheme substantially increases both the data rate and the harvested energy in comparison with conventional PS-SWIPT at the cost of negligible performance loss in data detection. Finally, the proposed concept is experimentally validated in a realistic indoor propagation environment. Index Terms—Frequency-domain simultaneous wireless infor- mation and power transfer (FD-SWIPT), minimum shift keying (MSK) modulation, multitone energy signal, waveform design, experimental validation. I. I NTRODUCTION Simultaneous wireless information and power transfer (SWIPT) integrates wireless energy supply in the downlink to prolong the lifetime of power-constrained end devices, such as wireless sensors and Internet-of-Things (IoT) nodes, via energy harvesting (EH) [1]. In practice, though, the receiver commonly orthogonalizes the information decoding (ID) and EH processes in the power [2] or time [3] domain, due to implementation issues. This sharing of resources among the information and energy streams results in performance loss. Frequency-domain (FD) SWIPT represents an alternative that addresses this issue via frequency multiplexing of the in- formation and energy streams at the transmitter and separation of them at the receiver with the assistance of complementary filters. In [4], the transmitter superimposes a direct current (DC) energy signal onto an orthogonal frequency division multiplexing (OFDM) data signal. A corresponding frequency splitting receiver architecture that makes use of a 3-port circulator and a notch filter to separate the data and energy signals is proposed in [5]. In [6], a multitone energy signal is embedded on a continuous-phase modulation (CPM) data signal. In this work, the authors use a combination of peak and notch filters to separate the information and energy signals over an additive white Gaussian noise (AWGN) channel. They also utilize a linear rectifier based on local oscillators (LO) operating at the tones’ frequencies instead of diodes. The main drawback of FD-SWIPT schemes is the require- ment for using receive filters, since practical filters are non- ideal and near-ideal filters are expensive. In this paper, we design a novel FD-SWIPT waveform, wherein a multitone energy signal is frequency multiplexed with a minimum-shift keying (MSK) data signal. The use of the former is dictated by spectral regulations, which might prevent the transmission of a single high-power tone, while the utilization of the latter is motivated by its spectral characteristics [7]. Specifically, by placing the tones of the energy signal at the spectral nulls and/or low-power sub-bands that characterize the power spectral density (PSD) of MSK signals, we minimize the interference incurred to the data signal by the energy signal, thus ensuring similar information transfer performance with the MSK-only case and simplifying the separation of these signals at the receiver. In fact, with appropriate multitone signal design, we can avoid the use of notch/peak filters. An example of the proposed MSK-SWIPT waveform is illustrated in Fig. 1. In this case, an energy signal with N = 4 tones f i (i =1,...,N ) placed in the frequency interval [0.45/T b ,..., 0.75/T b ], where T b is the bit duration, is superimposed on an MSK signal with first frequency null at ft b =0.75. The tone spacing is Δ=0.1/T b and the i-th tone corresponds to a signal-to-interference-ratio (SIR) SIR i . The unique contributions of this paper that set it apart from similar studies, such as the work in [6], are listed below: • we derive the average error probability of the MSK- SWIPT waveform for a point-to-point wireless link in the presence of i.i.d. Rayleigh fading, AWGN, and multitone interference; • we evaluate via numerical simulations the performance of MSK-SWIPT with either an LOs-based receiver without notch/peak filters or a power splitting (PS)-based one (for comparison purposes), using various EH models in the latter scenario to capture the non-linear behavior of the EH circuit; and • we validate the proposed concept via measurements per- formed in a realistic indoor propagation environment. Numerical simulations highlight the substantial performance gains of MSK-FD-SWIPT, in terms of the achieved data rate and harvested power, over MSK-PS-SWIPT. Furthermore, the simulation results indicate that the performance loss in MSK detection attributed to the application of the proposed SWIPT scheme is negligible.