Hydra – Physical Layer
Block Diagram of Physical Layer
Physical Layer Design
The PHY design implements a 2×2 Multiple-Input Multiple-Output (MIMO) Orthogonal Frequency Division Multiplexing (OFDM) system that is based on the IEEE 802.11a standard. The ability to support multiple antennas allows Hydra to not only exploit the capacity advantages of MIMO, but it also gives Hydra the flexibility to operate in various multiple antenna configurations (MIMO, SISO, SIMO, and MISO). Recently OFDM has become a popular modulation scheme used in many communication standards (IEEE 802.11a/g, HIPERLAN/2, ADSL, IEEE 802.15.3a (UWB), etc.). Because of the multiple subchannels inherent to OFDM, this modulation technique gives Hydra an additional degrees of freedom in the allocation of subchannels for data and control. This design of the PHY provides a great deal of flexibility to researchers who may wish to implement a variety of systems and algorithms.
The PHY design also features multiple MIMO algorithms (i.e. spatial multiplexing and transmit diversity). Through the use of these MIMO algorithms and various coding and modulation schemes, the PHY design can support variable data rates up to 108 Mbps. This added flexibity gives us the opportunity to use Hydra in the research of rate adaptive protocols. Finally, in order to facilitate cross-layer design, the PHY employs a flexible interface to the MAC for the communication of various physical layer information (e.g. channel estimates, signal-to-noise ratio (SNR), etc.). The PHY is implemented in National Instruments software and hardware, which is discussed in the next sections.
The physical layer is implemented using National Instruments LabVIEW. NI LabVIEW provides the programmer with a simple graphical programming paradigm in which modular elements are wired together to create a virtual instrument. This development tool allows us to visualize the flow of information and control in our design and to easily build a reconfigurable PHY. NI LabVIEW also allows us to easily interface with and configure NI hardware. A variety of communication toolsets from NI (such as the Modulation Toolkit) can also be leveraged in LabVIEW. Together these features allow for a tremendously flexible PHY implementation that allows researchers to implement a wide array of physical layer algorithms.
Physical Layer Hardware
The NI hardware used to implement the PHY lives on an NI PXI-Chassis (shown in the above picture). The chassis houses:
- an embedded PC (which runs NI LabVIEW),
- a Digital-to-Analog (D/A) converter (NI PXI-5421 Arbitrary Waveform Generator),
- and an Analog-to-Digital (A/D) converter (NI PXI-5122 High-Speed Digitizer).
The hardware allows for a great deal of flexibility in adjusting sampling rates and bandwidth. These high quality A/D and D/A boards are normally designed for signal aquisition and analysis; they provide us with high resolution sampling which increases the fidelity of our PHY system. The NI hardware also provides us with a range of timing and sychronization options. For example we can configure the nodes in Hydra to allow out of band synchronization which would not be possible in systems engineered for the real world.