6 th Karlsruhe Workshop on Software Radios 83 IEEE 802.11p Transmission Using GNURadio P. Fuxjäger * , A. Costantini *† , D. Valerio * , P. Castiglione * , G. Zacheo * , T. Zemen * , F. Ricciato *† * Forschungszentrum Telekommunikation Wien, Donau-City-Strasse 1, A-1220 Vienna, Austria † University of Salento, 73100 Lecce, Italy E-mail: {fuxjaeger, costantini, valerio, castiglione, zacheo, zemen, ricciato}@ftw.at Abstract—In this work we present an implementation of a fully functional IEEE 802.11p transmitter in software-defined radio. We describe the rapid-prototyping methodology that was used to implement the frame-encoder within the open-source GNU Software Radio (GNURadio) platform [1]. The encoder generates OFDM frames in digital complex base-band represen- tation and uses the USRP2 [2] as digital-to-analog front-end for up-conversion and final transmission. Since the actual encoding process involves a large number of complex steps we split the development approach into three sequential stages. First, a reference-encoder in a high-level language (MATLAB) is derived from the IEEE standard documents. Second, the individual blocks of the MATLAB encoding chain are progressively ported to GNURadio, cross-checking with the reference after each step. Finally, standard compliance is verified by conducting compar- ative over-the-air measurements with an early prototype of a commercial 11p transceiver. Initial measurement results indicate that the fidelity of the resulting GNURadio implementation is on par with non-software-defined radio industry solutions and capable of generating truly standard-compliant OFDM frames. The encoder presented here has been released under GPLv3 and is also capable of encoding frames according to the 11a and 11g amendments, thus making it a valuable building block for upcoming software-defined radio projects. I. I NTRODUCTION AND RELATED WORK The IEEE 802.11p standard (which will be finalized in late 2010 [3]) aims at providing reliable wireless communication for vehicular environments. It will serve as an underlying protocol for future car-to-car and car-to-infrastructure appli- cations worldwide. At the physical layer it has essentially the same structure as 802.11a and 802.11g: the modulation format, based on orthogonal frequency-division multiplexing (OFDM), the forward-error-correction (FEC), the structure of the preamble-sequences and the pilot-symbol schemes are identical. Furthermore, 802.11p uses the same medium ac- cess scheme common to all IEEE 802.11 standards, known as carrier sensing multiple access with collision avoidance (CSMA/CA) [4]. In the current draft version of the standard, the frame encoding procedure for IEEE 802.11p differs from 11a and 11g only in two key aspects: the operating frequency-band is shifted to around 5.9GHz and the duration of OFDM symbols is doubled from 4μs to 8μs. The rationale behind these modifications is the following: first, using a dedicated part of the spectrum reduces interference with legacy systems, The Telecommunications Research Center Vienna (FTW) is supported by the Austrian Government and the City of Vienna within the competence center program COMET. The work of Paul Fuxäger, Andrea Costantini, Danilo Valerio, Giammarco Zacheo, Thomas Zemen and Fabio Ricciato has been supported by the FTW projects I-0 and N-0. The work of Paolo Castiglione has been supported by the Austria Science Fund (FWF) through grant NFN SISE (S106). second, doubling the symbol-time also means doubling the cyclic-prefix-duration, i.e. decreasing the OFDM inter-symbol- interference (ISI) in outdoor channels. Given that the original IEEE 802.11a/g standards have been designed for low mobility and indoor usage, the question arises whether these two (minor) changes are sufficient to make 802.11p suitable for vehicular communication. The research community has already started analyzing 802.11p link-layer performance by using simulation tools but we believe that only real-world experiments can reliably evaluate the robustness of the standard in high-mobility scenarios. Due to the current lack of commercial 802.11p chipsets, using a software-radio prototype is an attractive basis for conducting these empirical measurements. We present the implementation of an IEEE 802.11p frame- encoder on the open-source GNURadio platform and outline the methodology that was used during the development pro- cess. The encoder generates OFDM frames in digital complex base-band representation and uses the USRP Version 2 [2] as digital-to-analog front-end to up-convert and transmit them in the 5.9GHz band that has been allocated for dedicated short range communication (DSRC) for vehicular applications. GNURadio-based encoders for other communication stan- dards are already publicly available (e.g. [5]). To the best of our knowledge, this is the first implementation that is capable of generating and transmitting frames for the OFDM-based IEEE 802.11p standard. Additionally, since the aforemen- tioned differences to the original standards are so marginal, the encoder we built is also capable of generating frames for 11a and 11g just by changing two parameters (interpolation factor and carrier frequency) in the front-end. II. DEVELOPMENT METHODOLOGY The first step in the development procedure was to create a reference-encoder for OFDM frames in MATLAB, using the detailed instructions of the encoding scheme given in the IEEE 802.11-2007 standard document [4]. The reason for taking this intermediate step using MATLAB is that it speeds up the development process as it provides a valuable debugging tool for the final GNURadio-based encoding chain. We followed the encoding recipe outlined in [4, paragraph 17.3.2.1] and subsequently validated the resulting MATLAB code by comparing its output with the reference-frame (a table of 881 complex numbers) that is included in [4, Annex G]. A preliminary test-measurement was done by saving the MATLAB output to disk, using this file as input-source for the USRP2, and decoding the transmitted signal with a conven- tional WiFi receiver. During these trial-runs we discovered that