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802.11ax: AN Uplink MU-MIMO SDR Testbed

The proliferation of smartphones and growth of user-generated content has led to substantial increase in uplink traffic in use by mobile devices. Multi-input multi-output (MIMO) technology has been successfully adopted in past 802.11 standards, leading to high throughput and robust performance. Downlink MU-MIMO (DL MU-MIMO) was introduced in 802.11ac, where an access point (AP) with multiple transmit antennas is able to transmit multiple data streams to multiple spatially distributed stations (STAs) at the same time. To address the challenge of burgeoning uplink traffic is one of the primary objectives of the new IEEE 802.11ax. Uplink multi-user MIMO (UL MU-MIMO) can improve uplink capacity by enabling multiple spatially separated clients to access the channel at the same time and is especially useful in scenarios where the STAs have limited number of antennas (e.g. typical smartphones). The focus of this project is to build a software-defined radio (SDR) testbed for prototyping UL MU-MIMO and studying related design trade-offs. The project uses the Wireless Institute’s Software Defined Radio (SDR) Lab, namely the National Instruments (NI) USRP RIOs, the LabVIEW Communications System Design Suite, and NI’s 802.11 Application Framework.

The project is anticipated to have several phases. As the first step, we have already implemented two algorithms for UL MU-MIMO: Carrier Frequency Offset (CFO) correction at STAs, and multiuser detection (MUD) of users at the AP. The prototyping setup includes two NI USRP RIOs associated with their host computers, where the AP is implemented in one USRP and two stations are implemented in the second USRP. In comparison with the SISO system, we are able to get doubled system throughput. In the next step, we plan to add transmit power control functionality to the current framework to study the UL performance for the situations in which distances of stations to the AP are not similar. In addition, we will study the effect of asynchronous UL MU transmissions on the system performance. 

The 802.11ax project was started in 2015 and is sponsored by InterDigital and National Instruments via the BWAC I/UCRC, an NSF program. J. Nicholas Laneman is the PI for this project.