Over time, the telephone replaced the telegraph, and now cellular and voice over Internet protocol (VoIP) technology are replacing the landline. However, as more communication is done wirelessly and over the Internet, we are becoming more interested in increased bandwidth. This is because bandwidth places a limit on how quickly we can send information through a channel such as an optical fiber or a section of the radio spectrum.
If we look back at work done on the telegraph a century ago, we can gain some insight into why bandwidth is important and why we need mmWave today. At that time, there was a scientist working at AT&T’s Department of Development and Research named Harry Nyquist. Out of his work Certain Topics in Telegraph Transmission Theory came the notion of the Nyquist Rate – the theoretical minimum system bandwidth needed to detect Rs symbols per second is Rs/2 hertz.
A way to look at this is to think about how signals behave in time when they are band limited. A signal such as a series of pulses that is band limited in the frequency domain loses components and gets distorted and smeared out in time. To stop these smeared-out pulses from overlapping and becoming indistinguishable from one another at a detector, there needs to be enough bandwidth. It turns out, the amount of bandwidth we need gives us the Nyquist Rate. The Nyquist Rate suggests that for things to make sense at the other end of a transmission, we can send pulses as fast as twice the channel bandwidth, but no faster.
All things being equal, such as signal-to-noise ratio, encoding, modulation schemes, etc., a system with 10 times the bandwidth can help users do one of two things – send more at a faster rate, or send the same things in a fraction of the time. For example, with more bandwidth, users can send the same amount of stuff in a tenth of the time. This is the part of 5G that gets covered by the press because users can say, “Oh look, a two-hour movie downloaded in four seconds.” Or, users can send 10 times the stuff in the same time. So, if it used to take five minutes to send one movie to one person, we can now send that movie to 10 people in five minutes. This is particularly important because time is money.
In addition to the increased data transfer rates, there is another major benefit to increased bandwidth and moving to mmWave, that may perhaps be the real reason why mmWave is not just convenient but inevitable. The cost per bit for the operator is fundamentally lower. Some estimates suggest it is actually more than 60x lower at mmWave than it is at LTE.
Overall, once mature, mmWave will be a far cheaper way to communicate. And just think, all the reasons why we are focused on expanding bandwidth in the 2020s date back to the 1920s and the maturity of the telegraph!