Wireless communication is a scientific gift that the world has started to take for granted now. Mobile phones, remote controls, WiFi networks, and sensors are some of the of the innumerable modern technologies that involve transfer of meaningful information, or data, wireless-ly, over fairly long distances.
Though extremely useful, wireless technology is far more difficult to implement and faces several challenges that a wired system does not have to. One of them is, of course, inconsistency in signal strength. In wireless technology, information is transferred over air, and air can be a very difficult medium to send signals. The signals do not travel in one direction, as in a wired system, and are far more dispersed.
They encounter several physical barriers, such as buildings and walls, and are bounced off, or deflected from, these, thereby affecting signal strength and quality. As a result, signal strength fluctuates constantly, something every mobile phone user in India is well aware of.
Another fairly common problem with wireless devices is their relatively large power consumption. Mobile phones can be charged relatively conveniently at home or office, or even in a car, bus or train, but this is not true for all sorts of devices or situations. A large number of sensors are being used for a variety of purposes these days, many of them in public places or even in remote areas. It is not possible to connect them to a charger once a day, and yet they have to keep working, without a break, for several weeks or even years. There are other challenges as well.
Despite these challenges, wireless networks and devices are extremely popular because of the convenience they offer. Scientists are constantly trying to improve the efficiency of wireless networks to ensure faster data transfers, increase in the number of users that can be served simultaneously, and reduced energy consumption. Improving efficiency often entails improving the algorithms that make conversations among various wireless devices possible.
Neelesh Mehta and his Next Generation Wireless Systems Lab at the Indian Institute of Science, Bengaluru, has been engaged in such an endeavour for over a decade, and has been able to make several significant contributions to improving the efficiency of wireless networks. Mehta was recently named one of the winners of this year’s Shanti Swarup Bhatnagar Prize.
Traditionally, the problem of weak signals in wireless networks has been dealt with by taking steps that increase signal strength or spectrum bandwidth. But bandwidth or power at the disposal of the network is not unlimited. Such measures, therefore, achieve only partial success.
Mehta and his team realised that variations in signal strength were not entirely a bad thing for the network and could be taken advantage of, in fact, in improving its functioning. Relying on what is known as “opportunistic selection”, Mehta was able to design distributed algorithms to exploit it and show that the overall efficiency of the network could be improved even if some users experienced very weak signals sometimes.
In opportunistic selection, the base station, or the source of information — such as the cellphone tower or the WiFi router — decides to communicate, or share data packets, only with those users who have strong signals while ignoring, for the time being, those with weaker signals, who can be “served” once their signals improve.
While this might seem like a discriminatory algorithm, Mehta insists that the overall efficiency of the network is vastly improved. By catering only to the requests of users with strong signals, the base station is able to communicate effectively with far more users at a time compared to a situation in which it has to communicate with weak-signal users as well. So instead of serving five users, three of whom could be with weak signals and thus act as a drag on the network, the base station can serve 10 other users with greater efficiency using opportunistic selection.
And, this preferential treatment happens over time-scales of a few milliseconds. It is not as if the users with weaker signals are being abandoned by the network, Mehta says. There are also fairness techniques by which those with weaker signals can be preferred over users with stronger signals over time, thus ensuring that all users are served — and served well. In fact, the general idea can be used to opportunistically prioritise users who are encountering long delays or backlogs in uploading their data.
Opportunistic selection is already being used by 4G wireless networks today. They are likely to become the backbone of 5G and subsequent generations of wireless technologies.