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Optoelectronics & Communications

Indoor freespace optical links for multi-gigabit wireless communications

A novel wavelength-routed optical data communications system can provide wireless communications in high bandwidth "hotspots."
7 February 2006, SPIE Newsroom. DOI: 10.1117/2.1200601.0060

There is little doubt that radio communications will provide sufficient wireless bandwidth for many applications in the near future. ‘WiFi’ standards such as IEEE 802.11a/b/g and the upcoming IEEE 802.11n provide enough capacity to meet near-term growth requirements. But within the radio research community, it's becoming apparent that more bandwidth will ultimately be required, and research into LANs using higher frequencies (such as 60GHz) is already in progress. Using these higher frequencies can be problematic as the shorter wavelengths only support line-of-sight signal propagation and require a short, relatively clear path between terminals. Future networks may therefore employ a two-tiered approach, providing low-rate WiFi service for area coverage supplemented by ‘hotspots’ running Gigabit-level services on short-range high-frequency links.

RF propagation at these higher frequencies begins to take on the characteristics of optical channels and, given that a line-of-sight link is already required, there are many advantages\break\newpage\noindent in using optical frequencies directly. Here we describe a novel passive distribution scheme that offers optical data transmission to users at multi-gigabit rates.

The downlink

Figure 1 shows a schematic of the system, showing the base station (BS) and mobile terminals (MTs) with separate uplink and downlink systems. The system uses wavelength spatial division to route information to a desired destination. The downlink employs an optical element that maps a different wavelength to an angle or position within the required coverage area. This is achieved using a diffraction grating to separate wavelengths in one dimension. An array of parabolic mirrors is then used to map the one dimensional pattern into a two-dimensional coverage area. When operating within a building, a range of wavelengths is distributed and a coupler taps off power to illuminate each coverage zone. Light then passes into the passive hub and each wavelength is mapped to a small zone within the room, allowing each user to be addressed by a different wavelength. This concept has the advantage that it is transparent to data rate, allowing tens of Gbits/s to be distributed to users within the room. The technology also shows commercial promise since the downlink hub is passive and has the potential for low-cost volume manufacture.1

 
Figure 1. Click to expand
(a) Optical downlink. Light from a fiber is routed via a passive ‘hub’ to a particular point in the coverage area, depending on the wavelength of the radiation. (b) The uplink uses a combination of diffractive steering at the terminal and adaptive diffractive optics at the base station to route light back into the optical fiber.
 

Figure 2 shows the results of a preliminary experiment where an array of plane mirrors is used to map 16 wavelengths to a 4×4 array of points. The figure also shows an open eye diagram showing data transmission at 1Gb/s for a receiver placed at one of the center points.2–4

 
Figure 2. Click to expand
(a) Shown are the desired positions (blue) and measured positions (red) of each of the 16 wavelengths routed by the passive-hub demonstration. (b) The received eye diagram for a receiver placed at a spot position, with a 1Gb/s non-return-to-zero transmitted waveform.
 
The uplink

The link from BS to MT is relatively straightforward, but routing light from a user into the fiber infrastructure is extremely challenging. Two solutions have been investigated: the first is to use an RF uplink in combination with the optical downlink. Work performed at Oxford5 shows that the natural asymmetry in most network traffic (where the ‘downstream’ data flow from the network to the user predominates) allows a slower uplink to be combined with a faster downlink with little loss in network performance.

A more direct approach is shown in Figure 1(b). Here, an active beamsteering device is used at the base station in order to divert light back into the fiber distribution system. Here, the BS is no longer passive, but can remain transparent to the data that is passing through it. Preliminary simulations indicate that a combination of fixed and programmable optical elements (implemented using a spatial light modulator) can achieve this.

Conclusions

The demand for wireless bandwidth will continue to increase, and will likely be met by a heterogeneous mix of standards and technologies. The work described here shows the potential for free-space optical networking, using infrastructure that is transparent to data rate and can scale to future needs. Future work will focus on achieving a high data-rate uplink, and simplifying the design of the required elements.


Authors
Haiyan Shi and Dominic O'Brien
Department of Engineering Science, University of Oxford
Oxford, United Kingdom