Hydra – A Wireless Multihop Testbed (Phase 2)
Wireless ad hoc networks provide rapidly deployable, self-configuring networks which operate without infrastructure. These networks will have far reaching applications in various areas, such as military applications (e.g. battlefield scenarios) and commercial applications (e.g. ubiquitous connectivity, extending cellular network range). Multiple antennas can be used in these networks to not only increase capacity, but also to improve reliability and spatial reuse.
In order to investigate multiple-input-multiple-output (MIMO) ad hoc networking, we have developed Hydra, a flexible wireless networking testbed. Hydra’s development is motivated by the gap between theoretical research and practical systems. Most research is supported by the analysis and simulation in which it is often necessary to make assumptions that are difficult to maintain in real systems (such as perfect synchronization and channel estimation). This prototype will allow us to study the effects of breaking these assumptions to see how current theories hold up in the real world.
Flexible Node Design
To maximize the utility of Hydra for researchers, our primary design goal is to make the prototype as flexible and easy-to-modify as possible. To this end the physical layer (PHY) and medium access control layer (MAC) have been implemented in reconfigurable software that runs on a general purpose Linux PC, as seen in the figure above. The RF front end, while primarily a hardware device, is controlled by an Altera FPGA, and different daughterboards can be used with it to operate in different frequency ranges. A block diagram of the entire system is shown at right. The RF front end is implemented using the Universal Software Radio Peripheral (USRP) from Ettus Research. Each board features four high-speed A/D converters and D/A converters and interfaces with a general purpose Linux PC through a USB 2.0 interface. Each USRP board can have up to two RF daughterboards, each built for a specific frequency range, and multiple USRP boards can be used synchronously by a single PC for MIMO communication.
The PHY is designed around OFDM and MIMO using PHY algorithms based on the IEEE 802.11n standard. The PHY is implemented using the GNU Radio software defined radio framework, which consists of signal processing blocks written in C++ connected together using Python as a glue language. Our system currently supports variable data rates of up to 5.4 Mbps (limited by the bandwidth of USB 2.0) in a SISO configuration, and our next step is to implement MIMO algorithms such as spatial multiplexing and transmit diversity. The PHY communicates to the MAC over a local IP connection.
The MAC layer design for Hydra is the DCF mode of 802.11. This basic CSMA scheme employs handshaking to control medium access. The flexible interface with the PHY enables the use of cross-layer algorithms. The MAC is implemented using the Click modular router software development tool created by the MIT Parallel and Distributed Operating Systems group. This software enables a flexible and modular MAC and routing layer design that easily interfaces to the IP network stack. This allows our MAC to interoperate with applications, enabling end-to-end tests.
K. Mandke, Soon-Hyeok Choi, Gibeom Kim, R. Grant, R. Daniels, Wonsoo Kim, R. W. Heath, Jr., and S. Nettles, “Early Results on Hydra: A Flexible MAC/PHY Multihop Testbed,” Proc. of IEEE Vehicular Tech. Conf. , Dublin, Ireland, April 23 – 25, 2007.
National Science Foundation under grant CNS-626797, Office of Naval Research (ONR) under grant number N00014-05-1-0169, and the DARPA IT-MANET program, Grant W911NF-07-1-0028. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the aforementioned sponsors.