MIMO-OFDM Interference Channel Testbed
As the the density of wireless networks increase, interference from within the network increases and quickly becomes a bottleneck in communication. Current wireless standards are heavily pursuing cooperative interference management strategies to overcome this limitation, an effort in which the use of multiple antennas at both the transmitter and receiver is proving to be extremely valuable. This, along with the lack of proper multi-user channel models, necessitates a flexible testbed on which to study the performance of emerging communication strategies. As a result, we have built a software defined MIMO interference channel testbed to study the performance of theoretical concepts in realistic physical channels.
This multi-user prototype was built using hardware from National Instruments and coded fully in LabVIEW. The testbed consists of three interfering wireless transmit-receiver pairs, each of which communicate across a two-by-two MIMO link. The system operates in the 2.4 GHz ISM band and the generated 16 MHz signal is divided into a variable number of subcarriers using orthogonal frequency division multiplexing (OFDM).
Adopting a software defined approach to design and build this system allows us the freedom to adjust most physical and MAC layer parameters including everything from FFT size to training time, pilot structure, and synchronization. Moreover, this software defined approach allows us to transparently add or change any MIMO signal processing blocks such as precoding, channel estimation, or equalization. This three user setup was later augmented using extra synchronization modules and power amplifiers to accommodate outdoor communication with ranges up to 300 ft.
This multiuser testbed was originally built to study the real world performance of emerging MIMO interference channel techniques such as interference alignment. Theoretical results are often studied in a theoretical environment where system functions and propagation channels are idealized and abstracted. This has been known to cause varying levels of discrepancy between performance on paper vs. the real work. Therefore, this testbed was used to obtain highly accurate multi-user channel measurements to later evaluate the performance of interference alignment offline in realistic channels which models can not fully capture. Through this we have shown that interference does in fact achieve the performance predicted in theory in a variety of indoor and outdoor channels. However, we have been able to uncover fundamental characteristics about wireless channels that affect the performance of such multi-user techniques. We proposed new multi-user channel correlation measures and showed their tight relationship with achieved sum rate. Our results are summarized in the following two papers:
O. El Ayach, S. W. Peters, and R. W. Heath, Jr., “Feasibility of Interference Alignment of Measured MIMO-OFDM Channels”, IEEE Trans. on Veh. Tech.. vol. 59. No. 9. pp. 4309-4321, Nov. 2010. Older preprint available at ArXiv.
O. El Ayach, S. W. Peters, and R. W. Heath, Jr., “Real World Feasibility of Interference Alignment Using MIMO-OFDM Channel Measurements”, Proc. of the Military Communications Conference, World Trade Center, Boston, Oct. 18-21, 2009, pp. 1-6.