Uh-oh! What happens to the future of wireless when we run out of spectrum space? That may seem impossible, but let me tell you something: We are already well along that path.
Practically speaking, all of the spectrum from DC to visible light has already been used, allocated, or assigned in some way. If you don’t believe this, take a look at NTIA’s famous spectrum chart.
If you look closely, you’ll see how the various segments of spectrum are assigned from 0 to 300 GHz. Some of them may not actually be in use, true, but they are spoken for and cannot be used. Since the electromagnetic spectrum is our playground, how do we implement our wireless application or product when we cannot find a piece of bandwidth to use? Is this our spectrum apocalypse?
As it turns out, we are already dealing with this problem. The Internet of Things (IoT) vendors are hell-bent on putting radios on practically everyone and everything. If the lack of spectrum doesn’t kill us, the massive cloud of interference and noise will. I am not anti-IoT, by the way, but I do worry about the unmitigated use of the spectrum with little thought or concern for the future.
Nor are the IoT designers the only culprits: The cellular operators are also spectrum hogs. As rich as they are, they can easily pay billions of dollars for prime spectrum. Yet it never seems to be enough. This quest for more space will continue as more smartphone users whine for faster video and other high-speed data applications.
Who decides the use of spectrum? The government, of course. They decide things like trading off better video baby monitors for improved electronic-warfare (EW) gear, or vice-versa.
In the U.S., spectrum is managed and assigned by the Federal Communications Commission (FCC) for commercial and personal communications, with the National Telecommunications and Information Administration (NTIA) playing that role for the government and military. Most of the spectrum is licensed—that is, designated for a specific use. There is some assigned unlicensed spectrum that anyone can use if they meet the regulatory guidelines.
However, this spectrum is heavily used and users often suffer interference problems. The best example is the unlicensed band from 2,400 to 2,483.5 MHz used by Wi-Fi, Bluetooth, ZigBee, cordless phones, drone camera links, and a half dozen or more radio technologies, not to mention your microwave oven.
So what do we do? As I worry about this problem, I am also thinking about the solutions, including those already in play. Here is a list of techniques that make our use of the spectrum more efficient.
Go higher in frequency. Over the past decade, that has been the plan for many wireless users. When you run out of VHF space, jump to UHF. When UHF space is depleted, move to microwave. Or jump from microwave to millimeter waves. There is obviously more bandwidth at the higher frequencies, but that is being rapidly consumed as the demand for high speed data and video grows. The limit or downside to this approach is the lack or cost of components to build at these higher frequencies. As far as I know, there are no terahertz ICs commercially available…yet.
Go optical. Keep going higher in frequency by using the optical spectrum. Infrared (IR) is probably the most useful, but visible light may also work. It’s certainly worth a shot for some applications.
Share the space. That is being done right now with the industrial-scientific-medical (ISM) unlicensed bands. Examples are the 902-928 MHz, 2.4 to 2.483 GHz, and 5.8 GHz bands. Billions of devices share these frequencies. Luckily, most of the radios are low power and short range, so they don’t bother one another. Co-existence techniques like compatible channel assignments can minimize interference. Look for more of this as we get squeezed even more.
Frequency Reuse. Different radios or applications can use the same frequency if we keep them apart. This means greater physical spacing or increased use of directional antennas to isolate each user. Cellular basestations use sectorized antennas to separate users who do use the same frequencies without interference. The new 5G New Radio cellular systems will use adaptive beamforming to make efficient use of the spectrum, as well as to boost radiated power and range.
Reassignment. While most spectrum chunks are spoken for, some are not actively used. If these can be identified, it is possible that the spectrum license can be bought or reallocated to a new owner or service.
Multiplex. Multiplexing, or multiple access methods, let multiple users share a spectrum band. FDMA, TDMA, and OFDMA are all examples. But don’t forget that direct sequence and frequency hopping spread spectrum with their unique coding, and UWB techniques let many users share a band. The same thing occurs with spatial multiplexing in the form of MIMO.
Cognitive radio. A cognitive radio (CR) is a software-defined radio (SDR) where most of the radio functions are implemented in software. SDRs are already a reality, but there is more to come. Cognitive radios make up systems that are aware of their environment and internal state and can make decisions about their operating conditions. CRs are adaptive radios that automatically adjust frequency, power, or other factors to optimize their operation.
One example is the CRs used in white space applications where unused TV channels can be accessed. The availability of a channel is determined by listening or by way of a database query, so that no interference will be caused. Maybe AI and machine learning techniques can be adopted to better manage the spectrum with CRs.
What did I forget? Anyway, you get the idea. As a wireless engineer, you are probably already familiar with most of these techniques. Keep them in mind, use them, and definitely invent some new solutions. Kick the can down the road to put off our day of reckoning as long as possible.