A Roadmap for Spectrum Sharing
This is the first in a series of articles by Elliott Drucker, dealing with technology issues, that we will be presenting on this site. Elliott was previously a long-time contributing writer for the print edition of Wireless Week. A pioneer in the industry, he has been involved in wireless technology analysis and development since 1979. Elliott is president of the consulting firm Drucker Associates. He can be reached at firstname.lastname@example.org.
The use of 4G networks in the U.S. appears to be hitting its stride with a general consensus that we will be needing a steady supply of new spectrum to help manage growing demand. So far, commercial cellular networks, including 3G and 4G broadband, have exclusively utilized dedicated spectrum. That is, in each designated geographic service area each network operates on spectrum that is free from other, potentially interfering, users. However, the growing difficulty in identifying new spectrum for the exclusive use of wireless networks, and the length of time the reallocation process takes, has precipitated a growing interest in using shared spectrum for offloading some broadband network traffic.
We have been using a rudimentary form of such shared spectrum offloading for some time in the form of Wi-Fi hotspots, wherein user devices, on their own, select operation on public (or private) Wi-Fi networks where they are available. This is hardly a perfect solution. For one thing, the amount of spectrum available for Wi-Fi operation is quite limited, so in high traffic areas like airport terminals Wi-Fi performance can be pretty bad. More significantly, because it uses unlicensed spectrum that performance is largely out of the control of network operators.
A more sophisticated spectrum sharing concept is for wireless broadband networks to move “overload” traffic in a particular area onto spectrum that is vacant or underutilized in that location. One obvious example is unassigned television channels – so-called “white spaces.” A key impediment to this idea is that there are already localized users of such white spaces that could be severely impacted by (and could themselves interfere with) co-frequency operation of broadband cellular networks. In addition, there is a clamor to at least partially open white spaces to unlicensed uses, mainly for relief of Wi-Fi congestion, which would pose similar interference problems with co-frequency broadband network operation.
What is needed to enable reliable and efficient use of such shared spectrum is a means for coordinating and managing, in real time, multiple unrelated users in a given region so that they can operate without interfering with one another. As it happens, just such a means already exists in the form of…broadband wireless networks! Well, OK, maybe the full management system isn’t available yet, but the key parts of the infrastructure certainly are.
To illustrate how this would work, consider a theatrical wireless microphone system, one of the typical incumbent TV white space users.If this system was equipped with a simple 4G machine-to-machine modem it could communicate with a coordinating server associated with the wireless network. That communication could include measurements of signal strength from nearby cells, allowing the server to determine potential interferences between the wireless mic system and the broadband network, and to coordinate use of the shared spectrum accordingly. Such a coordination function is really just an extension of the radio resource management wireless networks do to manage their internal interference.
Requiring localized users of shared spectrum to communicate with and be coordinated by an overlying broadband network would allow efficient use of such spectrum with minimal additional regulation. Network operators could be required to provide such coordination services as part of the price of using the shared spectrum for traffic offloading. Hard to find a loser here.