The explosion in mobile and multimedia usage that started some 15 years ago continues unabated, placing increasing demand on the available bandwidth required to transmit data. Simultaneously, the largest volumes of mobile and streaming traffic are generated in indoor environments, from homes and offices through to hospitals, airports and smart factories.
This scenario places huge pressures on existing indoor wireless communication technologies in terms of the performance level required for modern applications. Thus, it acted as a catalyst, inspiring a team of innovative researchers from the University of Cambridge’s Centre for Photonics Systems and LRDC, and the University of Bath, to look at employing optical wireless communication (OWC) technologies in new ways, in order to address the challenge. OWC was selected because it offers access to vast, license-free bandwidth in the visible and infrared regions of the electromagnetic spectrum, and by harnessing light rather than radio waves, it represents a powerful solution to mitigating the congestion that exists in the radio spectrum.
Indoor OWC systems mainly use visible-light LEDs for simultaneous lighting and data transmission, but their modulation bandwidth places limitations on data transmission rates. This has led to near-infrared vertical cavity surface emitting laser (VCSEL) emerging as a compelling alternative, offering bandwidth in excess of 5 GHz and the ability to be arranged in dense 2D arrays, providing terabit-per-second (Tb/s) indoor transmission at a lower cost. However, although VCSEL-based OWC systems can provide high data rates and long-range performance, they typically operate as point-to-point links, with limited spatial coverage due to strict alignment requirements.
This work saw the researchers investigate a VCSEL-based multi-beam OWC link and exploit a VCSEL array, using each element to address a non-overlapping attocell (ultra-small, low-power cellular base station), to deliver a multi-element transmitter that provides wide coverage and high aggregate throughput, and supports multiple fixed or low-mobility users within discrete attocells. A 5 × 5 VCSEL array-based, multi-beam OWC transmitter, generating square attocells with near-uniform power distribution and little optical inter-cell interference, tackled key limitations of previous approaches undertaken by others. Data transmission rate was demonstrated using orthogonal frequency division multiplexing (OFDM) with three independently modulated channels, and a low-cost receiver comprising an off-the-shelf silicon avalanche photo-diode (Si-APD) and an aspheric lens.
Delivering outstanding results in subsequent tests, the system prototype achieved a data transmission rate of 10 gigabits (Gb/s) per channel, at a bit error rate (BER) of <2 × 10−3, when the receiver was oriented in the centre of the beam and rotationally aligned with the boresight of the transmitter. The link also maintained ≥4 Gb/s with up to 12 cm lateral displacement without adjusting the angular alignment of the transmitter or receiver, displaying strong tolerance against misalignment.
Professor Haas, Director of the LRDC, said “The outcomes show that LiFi can not only harness unregulated and free optical spectrum, but also benefits from extremely high spectrum reuse capabilities, as lightwaves can be easily spatially controlled. When operating in segments – using a fly-eye-type structure – this enables large coverage with minimal light overlap between neighbouring beams. In our system, the same spectrum was reused 225 times within the same room, giving rise to a spectrum reuse improvement of 225 compared to Wi-Fi.”
To find out more about the team’s explorations, experiments, results and findings, read their co-authored technical paper: Yi Liu, Wajahat Ali, Rui Chen, Nikolaos Bamiedakis, Ian H White, Harald Haas, Michael Crisp, and Richard V Penty, A Scalable VCSEL-Array Optical Wireless Transmitter With Experimental Multi-Beam Prototype, in the Journal of Lightwave Technology Vol. 43, Issue 23, which describes and illustrates their collaborative work in detail.
This work was supported in part by UK EPSRC under Grant EP/S016570/1, Grant EP/Y037243/1 and Grant EP/X040569/1.
Image right shows experimental setup of OWC link.