“LiFi” – a proposal for massive-MIMO visible-light communication networks
National Instruments (NI) and Professor Harald Haas, lead researcher at the University of Edinburgh have described development of a test bed that aims to to dramatically improve indoor wireless communications capacity. The project is a massive multiple input, multiple output (MIMO) technique, referred to as spatial modulation, that potentially provides a highly energy-efficient capacity increase in another step on the path toward fifth-generation (5G) wireless communications.
Haas and Professor Cheng-Xiang Wang, head of the Advanced Wireless Technologies Lab at Heriot-Watt University, recently used NI PXI Express hardware and NI LabVIEW system design software to create the first working prototype showing spatial modulation techniques over a wireless RF channel. In 2011, Haas demonstrated a concept, nicknamed LiFi, using visible light communication over a single-channel, point-to-point link. He now plans to combine these technologies to create even higher density optical wireless networks—called optical attocell networks—that will harness massive MIMO gains in both the optical and RF domains for energy-efficient indoor wireless communications.
“We’ve known for a long time that decreasing cell size can significantly increase cellular capacity and user data rates, but it’s not been clear how we could facilitate that given current spectrum, energy and interference limitations,” said Haas. “RF wireless and optical wireless networks that work together using spatial modulation and massive MIMO approaches could allow us to effectively mitigate interference and significantly increase energy efficiency, coverage and capacity using existing infrastructure.”
The Edinburgh team is extending its research capabilities with the LabVIEW reconfigurable I/O (RIO) architecture for rapid prototyping (Photograph). Using the NI FlexRIO Software Defined Radio Bundle with reconfigurable FPGAs and interchangeable I/O adapter modules, the team is building prototypes that operate beyond the rates of a commercial RF wireless system. The team recently achieved 3.5 Gbit/sec from a single colour LED, allowing them to create an ultra-realistic test bed.
“We are excited to collaborate with Professor Haas, who has been pioneering visible light communications and spatial modulation for nearly a decade,” said Erik Luther, Wireless Communications Group Manager at NI. “Through the LabVIEW RIO architecture, Haas can rapidly prototype a first-of-its-kind wireless test bed.”
Prof. Haas says that the energy-efficiency factor is hard to quantify, since he proposes using LEDs that will be powered for room illumination in any case. Today’s lighting LEDs are mostly blue-emitters that energise broad-spectrum phosphors – his results of 3.5 Gbit/sec are achieved with a selective blue filter to capture the blue component from the LED emitter, and using OFDM at up to 64 QAM – using OFDM provides the expected (and needed) multipath immunity, he says. Today’s sensing and detection devices already offer more-than-adequate performance for such a system Haas has found. If the lighting is implemented with RGB emitters, then potentially, and with optical filtering, his results could be multiplied on a per-colour basis. Asked about the back-channel, Haas comments, “We’re not proposing abandoning RF entirely; [with future mobile devices] as now, the uplink bandwidth requirement will be much less than the downlink.”
National Instruments; www.ni.com
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