Superfast indoor wireless networks

It is a growing source of frustration: poor indoor Wi-Fi coverage caused by the fact that a multitude of devices tries to connect to the same Wi-Fi router, which often is located inside a meter cabinet or utility room that attenuates the radio signal. Recently, two PhD students from the ECO-group of the department of Electrical Engineering at Eindhoven University of Technology defended their theses in which they propose new solutions to enable high data rates indoor.

The basic idea both of the PhD candidates started to work on is to transport the data through optical fibers from the central unit in the meter cabinet to antenna access points within the various rooms in the building (a residential home, office building, hospital, shop, …). Such a fibre backbone network thus establishes wireless links with the users in those rooms, either by using optical wireless beams or radio wireless beams. This way, the wireless signal doesn’t have to penetrate walls, and maximum signal quality and bandwidth can be guaranteed. An additional advantage is that the emitted wireless power can be minimized, thus elongating battery lifetimes of the user equipment.

Do it yourself in-home technology with plastic optical fibres
As part of the NWO-funded FlexCom project, the first PhD student, Italian Federico Forni, together with the company Genexis explored the use of plastic optical fibres to transport the data to the different antenna access points in rooms. ‘This set-up is aimed to be a relatively cheap, do it yourself solution for in-home networks,’ Forni explains. ‘Plastic optical fibres are low-cost and very easy to use. The fibres have a large core diameter and you don’t need any specific connectors. You just cut the fibre and plug it into the device, or put the bare fibres together in simple fibre-to-fibre connections. Furthermore, we use visible light to transmit the data, so if you see a red dot coming out of your fibre, you easily know it works.’ Another advantage of Forni’s set-up is that the central unit in the meter cabinet does all the processing, so that is the only component that has to be smart, and thus somewhat more expensive. The access units in the different rooms only need to contain a LED, a photodiode, an amplifier and an antenna, and can be relatively cheap.

Combining broadband services
The main problem Forni had to overcome was that plastic optical fibres in principle are only suitable to transport narrow bandwidth signals. ‘But to keep things simple, we want to use these fibres to transmit Wi-Fi, 4G or 5G wireless signals, and baseband signals – the ones you use for your fixed phone line or smart TV – all together in a single fibre, and exactly in the way they are. We optimized the optical source, the fibre and the receivers and developed signal processing methods to enable equal but high-quality transmission of all of these types of signals.’

In experiments, Forni showed that this concept works rather well. ‘We have successfully demonstrated the simultaneous transmission of both multi-gigabit baseband and radio signals over a 50 meter plastic fibre and 12 meter wireless link. I think plastic optical fibres are very promising to increase the available capacity of networks inside buildings and houses in the future in a cost-effective way.’ 

Ultrahigh data rate communication with infrared optical beams
His colleague Ketemaw Mekonnen focused on a different configuration to achieve high data rates in indoor wireless networks. In the ERC-funded Advanced Grant BROWSE-project, the Ethiopian PhD student developed a revolutionary set-up that combines infrared wireless optical data transmission with flexible 60 GHz radio communication techniques to realize networks with very high upstream and downstream capacities.

‘I investigated three different configurations to realize high quality, fast bidirectional communication: all-optical-wireless, hybrid optical/radio-wireless, and all-optical-wireless backed-up with 60 GHz radio techniques. We have demonstrated that with these configurations it is possible to achieve both upstream and downstream transmission rates up to 50 gigabits per second, per user,’ he says with pride.

Finding the user
One of the challenges of the proposed system is that the position of the user has to be known in order to setup the communication link. ‘The optical beams we use for the wireless communication are narrow, so we need to localize the users with adequate resolution, which is difficult to achieve using the current radio-based localization techniques alone.’ To overcome this problem, the student designed a radio-optical method in which the already mature radio localization techniques are employed to determine the estimated location of the access point with respect to the user terminal, and then the user sends out a wide-spectrum beam from an integrated transceiver unit towards the ceiling-mounted access point. The 2D gratings module at the access point in the ceiling then filters the appropriate wavelength according to the angle-of-arrival of the beam. This is facilitated by the narrow spatial filtering functionality of the gratings module which allows us to determine the wavelength required precisely. ‘Every user is assigned a different wavelength depending on his or her position, since the beam steering is wavelength-based. Every time the user moves to another location, the new wavelength is determined and the communication link is re-established.’

Another possible issue with optical wireless communication is that as soon as something blocks the line-of-sight between the user and the transceiver, the connection is lost. To guarantee continuous network connection, Mekonnen implemented a flexible 60 GHz radio system to provide the user with a lower speed back-up alternative in case of an optical link failure.

The indoor fibre backbone network then becomes crucial in orchestrating the overall functionality of the network, including the network management and control, the localization, and the optical and radio-wireless communication links in a dynamic and cost-efficient way, he says. Mekonnen developed a centralized network architecture, where costly components and all network control functions are kept at a central site somewhere in the building, where costs are shared. ‘This way the access points and user terminals can be made simpler and more cost-efficient, and the whole capacity can be dynamically routed to the rooms where and when it is needed.’

Increasing the bandwidth during nomadic behaviour
The system he developed is not meant to fully replace Wi-Fi, the PhD student emphasizes. This technology is meant for nomadic behaviour, for example when office workers are using laptops and want to move but do not need to communicate when moving. If everyone is using Wi-Fi, the communication channel becomes congested. With this system, you can have the speed of a wired optical system in a wireless fashion. Other mobile users using low-to-moderate speeds are not hindered by you, and they can share the Wi-Fi bandwidth.’

Federico Forni defended his thesis entitled ‘Indoor Optical Network Technologies for Multiple Services Provisioning’ on Tuesday June 26th at Eindhoven University of Technology. Promotor prof.ir. A.M.J. Koonen, co-promotors dr.ir. E. Tangdiongga and N.C. Tran PhD (Genexis BV).

Ketemaw Mekonnen defended his thesis entitled ‘Dynamic Ultrahigh-Capacity Indoor Wireless Communication Using Optical-Wireless and Millimeter-Wave Radio Techniques’ on Thursday June 28th at Eindhoven University of Technology. Promotor prof.ir. A.M.J. Koonen, co-promotor dr.ir. E. Tangdiongga.