Physical Layer

Hydra uses the GNU Radio framework for PHY implementation. This open-source software framework handles all hardware interfacing, multithreading, and portability issues, thus freeing the implementor to focus on the signal processing aspects of the software radio design. To compose a protocol in GNU Radio users first build signal processing blocks in C++ and then connect these blocks together using the Python language to form a flow graph. In addition to the relative ease of programming in C++, this approach makes it easy to transition blocks that are part of a PHY simulator into GNU Radio. In fact, the blocks that make up Hydra were initially developed and verified as part of a LabVIEW simulation. GNU Radio is also an attractive development platform because of its growing community of users, who range from amateur radio enthusiasts to university researchers.

To investigate advanced physical layer concepts (such as feedback in MIMO systems, OFDMA, and coding), Hydra's PHY supports OFDM and channel coding, and support for multiple antennas is being added. The PHY also has a very flexible interface to the MAC, implemented over UDP through a local IP connection. This low latency connection provides an efficient interface for cross-layer communication, and facilitates experiments in cross-layer design.

A single-antenna OFDM physical layer based on the IEEE 802.11a PHY was used to perform rate-adaptive experiments. The physical layer supports data rates of up to 5.4 Mbps. This rate is limited by the bandwidth of USB 2.0, but higher bandwidth interfaces to the USRP are being investigated for future revisions of the front-end.