Rapid MIMO-OFDM Prototyping

A Multiple Input - Multiple Output (MIMO) wireless system uses multiple transmit as well as multiple receive antennas. The addition of multiple antennas provides a substantial increase in the capacity of the wireless system without requiring an increase in the systemâs bandwidth. MIMO is often combined with Orthogonal Frequency Division Multiplexing (OFDM) as it greatly simplifies the channel equalization of the wireless system. As the research and the standardization efforts on MIMO-OFDM technology continues to progress, it becomes increasingly important to test the real world performance of MIMO-OFDM systems. To this effect we have constructed a MIMO-OFDM prototype with support from National Instruments, the National Science Foundation, and Intel. We are using our prototype for two purposes; the first is to implement a full physical layer MIMO-OFDM link as in IEEE 802.11N, while the second is to use perform MIMO propagation channel measurements.

Prototype Specifications

We are using National Instrumentsâ Radio Frequency (RF) hardware and LabVIEW for our prototype. The National Instrumentsâ platform gives us a unique advantage over other prototyping platforms: the flexibility to change many of our systemâs parameters easily through our software interface. Our current system implements a two-transmit two-receive antenna MIMO communication system with a bandwidth of 16MHz and an adjustable carrier frequency (currently set at 2.4GHz).

System Implementation

Using this platform, we have created a full MIMO-OFDM physical layer link over which we transmit an image. The link is configurable so that we are able to implement a number of different MIMO schemes (i.e. spatial multiplexing, transmit diversity, Alamouti coding, linear dispersion codes) to test and compare their real world performance. The difference among these various schemes is that some provide greater data rates while others provide greater signal robustness.

Channel Measurements

In a wireless system it is crucial to understand the effects of the wireless channel. Since the wireless signal propagates through open space, there are many ways the signal can be distorted and arrive unrecognizable at the receiver. As a result, we are using the prototype to understand the nature of wireless MIMO propagation channels. We can use this information in order to create realistic channel models for system simulations as well as to research various characteristics of the channel that we can exploit to increase capacity and signal strength.

Future Work

We are in the process of constructing a MIMO-OFDM ad hoc communication network. This network will consist of multiple nodes, each equipped with two transmit and two receive antennas. We will augment our current physical layer software with an advanced medium access control protocol and will experiment with both MIMO-OFDM ad hoc networks as well as multi-user cellular communication systems.

Download LabVIEW VIs

Documentation