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. INTRODUCTION 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. D
EVELOPMENT 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
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