Experiences With a Platform for Frequency-Agile Techniques

One of the challenges impeding the development of frequency-agile interfaces is the lack of a robust platform that allows fine grain frequency control. As part of an NSF program to develop an experimental platform for software defined radio, we have developed two platforms that al- low frequency control. The first, MultiMAC, which was reported in DYSPAN 2005, provides limited controls for spectrum sharing. In this paper we describe the second platform, which is an FPGA-based software defined radio usinganOFDM waveform. The currentplatformcantrans- mit OFDM packets using a 256-FFT with sufficient robust- ness to be received by commodity 802.11g radios. Now that "backward compatibility" has been implemented, the platform is being evolved to provide a number of profiles more suitable for frequency-agile experimentation. In par- ticular, we provide interfaces to control individual subcar- riers using specific modulations(BPSK, QPSK, QAM) and to aggregate "bundles" of subcarriers into distinct radio in- terfaces that can be controlledat the application layer. This paper focuses on the architectural and design choices made on the transmit end of the radio to support this function- ality; suprisingly, the system is difficult to build using ex- isting SDR development frameworks such as GNU Radio. The end result is a radio system that lets applications "dial down" the bandwidth they use or to sense and react to sub- carrier specific fades and interference. We believe this plat- form will enable expanded experimentation in frequency agile research.