Hardware Testbed for Sidelink Transmission of 5G Waveforms without Synchronization David Garcia-Roger, Josue Flores de Valgas, Jose F. Monserrat, N. Cardona Universitat Polit` ecnica de Val` encia, iTEAM Research Institute Valencia, Spain Email: {dagarro,joflode1,jomondel,ncardona}@iteam.upv.es Nicolo Incardona Politecnico di Milano Milano, Italy Email: nicolo.incardona@mail.polimi.it Abstract—This paper details a hardware testbed conceived for studying the impact of the lack of synchronism between trans- mitters on several 5G waveform candidates, which is of special relevance for car-to-car communications. The experimental re- sults show that a proper fitting of waveforms in a software-defined platform would permit increasing the range of communication between cars without peer-to-peer synchronization, paving the way for the real development of collision avoidance messages. I. I NTRODUCTION IEEE 802.11p, also known as wireless access in the ve- hicular environment (WAVE), is an amendment to the IEEE 802.11 standard that extends its applicability to vehicular environments, including short-range communications for data exchange between vehicles (V2V) and between vehicles and the roadside infrastructure. IEEE 802.11p is based on orthog- onal frequency-division multiplexing (OFDM) [1], being its multiple access counterpart, OFDMA, widely used to divide the spectrum in multiple and orthogonal parallel sub-bands, as it happens in long term evolution (LTE) technology [2]. How- ever, as it turns out, the orthogonality property of OFDMA is only verified under ideal conditions, that is to say, under perfect frequency synchronization and precise time alignment during the cyclic prefix (CP). This issue imposes critical requirements, in such a way that, in order not to introduce inter-carrier interference (ICI) or inter-symbol interference (ISI), the terminals should be prevented from transmitting if a central entity does not verify that both time and frequency alignment are being respected. However, in the current design of LTE-Vehicle (LTE-V) the synchronization requirements are becoming a crucial issue. For example, while vehicles may be synchronized with elements of the roadside infrastructure (i.e. base stations), it is not expected that the synchronism between vehicles in a V2V sidelink communication scenario would be supported by such elements; thus specific synchronization signals are now under discussion for the support of this direct V2V communication. Moreover, the concept of timing advance (TA) may not be reused because it only assists in the determination of the distance to a common element (the base station) and is useless when determining the distance between two vehicles with varying relative distances. Consequently, it is of the utmost benefit to assess alternatives to OFDM, which should be able to tolerate a lower degree of synchronization without introducing additional ICI or ISI. The filter bank multi-carrier (FBMC) [3] and the universal filtered OFDM (UF-OFDM), also known as universal filtered multi-carrier (UFMC) [4], modulation schemes are two of the alternative fifth-generation (5G) waveform candidates cur- rently under consideration for designing a multicarrier physical (PHY) layer. Like OFDM, FBMC and UF-OFDM schemes segment the spectrum into multiple orthogonal sub-bands. However, unlike OFDM, FBMC performs a per-subcarrier fil- tering and UF-OFDM applies a filter to blocks of subcarriers. As a result, with FBMC and UF-OFDM the sidelobes are attenuated and thus the ICI and ISI issues are less critical than with OFDM for transmissions not perfectly synchronized. 5G waveform candidates have been subject of research efforts for several years, but so far mainly from the theoretical viewpoint, with only a few experimental testbeds [5]–[7], which do not address the synchronization requirements of the studied waveforms. This paper is focused on using a hardware testbed to evaluate a set of 5G waveform candidates, explaining the usefulness of the proposed hardware testbed in checking the feasibility of each candidate for sidelink communications, and collecting the results of evaluating the impact of the lack of synchronization and MIMO performance. The remaining sections of the paper are structured as follows. Section II describes the waveforms implemented, briefly introducing OFDM, FBMC, and UF-OFDM. Section III explains the V2V scenario studied in the testbed. Section IV gives an overview of the experimental hardware testbed. Section V provides additional details about its software com- ponents and Section VI presents the measurement results. Finally, Section VII draws the main conclusions of this work. II. WAVEFORMS UNDER STUDY A. OFDM A transmitter uses a baseband constellation mapper to map a bit stream to a QAM symbol called X. N parallel QAM symbols X[0],X[1],...,X[N - 1] represent the symbols transmitted at each subcarrier. By modulating these symbols with the inverse fast Fourier transform (IFFT) the transmitter generates the discrete baseband OFDM symbol x[n]= F -1 {X[i]} = 1 √ N N-1  i=0 X[i]e j 2π N ni , 0 ≤ n ≤ N - 1, (1)