1 IEEE 802.11 OFDM Software Defined Radio Beacon Frame Transmission Joseph Wamicha, Member, IEEE, Simon Winberg, Member, IEEE Faculty of Engineering and the Built Environment, University of Cape Town. Abstract—A transmit chain test bed is used in the project presented in this paper to test an 802.11g OFDM (Orthogonal Frequency Division Multiplexing) Software Defined Radio (SDR) base station beacon frame transmitter. Several prior Gnuradio test bed setups show how a Gnuradio SDR Transmitter is able to communicate with another Gnuradio SDR. This paper additionally shows how a Gnuradio SDR Transmitter is able to visibly com- municate with off-the-shelf 802.11 WLAN (Wireless Local Area Network) off-the-shelf chips. Results of the implementation show that a SDR base station is successfully able to communicate with traditional WLAN off-the-shelf communication chips found inside devices such as laptops, netbooks and mobile phones, which were used as the OFDM beacon frame receptors. Index Terms—Software defined radio, orthogonal frequency division multiplexing, wireless local area network, 802.11, beacon frame, medium access control, gnuradio, universal software radio peripheral 2, flex 2400 radio frequency board, symbols, frames, wifi, off the shelf. I. I NTRODUCTION R ADIO communication systems in production today are often implemented using application specific integrated circuits (ASICs) which support a single modulation method. Due to the rapidly evolving nature of wireless communications and the aggressive competition from market rivals, network operators are forced to upgrade their ASIC radio communications equipment, without getting the opportunity to fully realize their return on investment on the ASIC hardware installation. Despite this disadvantage, ASIC radio communications hardware devices are relatively cheap to mass produce [1] To address high radio communications hardware turnover rate problem, in his classic paper Joe Mitola proposes the partitioning of channel coding into the following functional components: antenna, radio frequency, analog to digital conversion (reception), digital to analog conversion (transmission) and digital signal processing [2]. Signal processing functions such as modulators, demodulators, mixers, filters and amplifiers, which vary between different communication protocols, could now be implemented using software. This would now allow a network operator’s base station to communicate using different communication protocols by reconfiguring the abstract software application layer, thereby overcoming the more prohibitively expensive option of having to replace the underlying communications hardware. This makes it possible for network operators to affordably upgrade mobile networks in underserved African rural areas to the latest wireless protocols, using the intial SDR hardware investment. In order to accurately model real life network conditions and to prove that software defined radios could replace the ASIC communications chip in future, the OFDM software defined radio implementation presented in this paper had to be compatible with off-the-shelf ASIC chips. So as to ensure this compatibility goal was met, an open standard had to be implemented on the Gnuradio-USRP2 (Universal Software Defined Radio Peripheral 2) software defined radio platform from Ettus Research. The potential open standards which could have been implemented were the IEEE 802.11 WLAN standard [3], the IEEE 802.16 WiMAX (Worldwide Interoperability for Microwave Access) standard [11] and the ETSI DVB-T2 (Digital Video Broadcasting - Second Generation Terrestrial) standard [12]. In the end the IEEE 802.11 WLAN standard was selected for the test implementation since off-the-shelf 802.11 devices are more readily available. The paper begins by detailing the hardware used for the test bed setup, followed by an explanation of the OFDM WLAN protocol and implementation used and ending with the presentation of the test bed results. The paper will often refer to the gr wlan code which was used to generate the 802.11g WLAN OFDM beacon frames. The gr wlan code wraps and refactors the Gnuradio OFDM python flow graph and C++ signal processing blocks from FTW and originally written by Andrea Constantini [17]. The python flow graph code in turn makes use of underlying C++ Gnuradio signal processing blocks by using SWIG (http://www.swig.org). The gr wlan code referred to in this paper can be downloaded from the following link: http://rrsg.ee.uct.ac.za/members/jwamicha/gr-wlan.tar.gz II. SOFTWARE DEFINED RADIO HARDWARE SETUP The SDR OFDM implementation presented in this paper was done using the USRP2 FPGA board working in conjunction with an RF (Radio Frequency) daughter board [9]. Both boards are manufactured by Ettus Research, a subsidiary of National Instruments. The USRP2 consists of an FPGA and mixed signal circuit section consisting of an ADC (Analog to Digital Converter) and DAC (Digital to Analog Converter) [8]. Since the 802.11g WLAN channels fall between a frequency range of 2.412 GHz and 2.484 GHz, the RFX2400 RF daughter board was used for the OFDM symbol transmission since it is able to transmit at between 2.25GHz and 2.9GHz. IEEE Africon 2011 - The Falls Resort and Conference Centre, Livingstone, Zambia, 13 - 15 September 2011 978-1-61284-993-5/11/$26.00 ©2011 IEEE