The easiest way to understand the issues involved is to think back at how we used wireless devices in the past. Anybody that ever fiddled with an older 802.11n WiFi router using 2.4 GHz remembers directing different devices in the home to channels 1,6 or 11. While the 2.4 GHz band has 11 separate available channels, most wireless router manufacturers limited the use to those three channels in order to avoid cross-channel interference. They knew that if a home only used these three channels they’d likely not see such interference and would get the maximum performance on each channel. However, the decision to use only those three channels limited the amount of bandwidth that can be utilized. In peak usage situations only 3 of the 11 channels of 2.4 GHz are carrying bandwidth – avoiding interference meant not using much of the available frequency.
It’s easy to think of the channels within a wireless frequency as separate channels, because that’s how they are defined at the FCC. Cable companies are able to create distinct channels of frequency within the controlled confines of a coaxial cable in way to limit interference between channels. But when transmitted in the wild through the air all sorts of interference arises. Anybody old enough to remember watching TV in the 50s can remember times when you could see ghosts of a nearby channel when you were watching one of the low channel numbers.
Our cellular networks have been designed in a similar fashion to the WiFi channels. Within each of the frequencies used for cellular service are channels predefined by the FCC, with buffers between each channel. However, even with the buffers there is cross-channel interference between neighboring channels, and so the cellular carriers have selectively chosen to spread the actual use of frequency in ways similar to how we used channels 1,6 and 11 for WiFi.
Flexible numerology is new goal for 5G that was published with the 3GPP Release 15 standard. Flexible numerology is part of a system for allocating frequency in a new way that is intended to get the most and best use of the spectrum.
5G will use the same underlying method for modulating signals as 4G LTE – orthogonal frequency division multiplexing (OFDM). The OFDM scheme is the current way to try to get the best use of frequency and with OFDM a data stream is split across several separate narrowband channels to reduce interference, much in the same way that we used the three channels of WiFi.
Flexible numerology is going to give the cell site the option to create much smaller narrowband channels within the channels described in the OFDM standard. That’s the magic sauce that will enable 5G to communicate with huge number of devices without creating massive interference.
Consider a situation of two users at a 5G site. One is an IoT sensor that wants to trickle small amounts of data to the network and the other is a gamer that needs bursts of huge amounts of bandwidth. In the LTE network both devices would be given a narrowband channel – the IoT device for perhaps a tiny amount of time and the gamer for longer bursts. That’s an inefficient use of frequency since the IoT device is transmitting only a tiny amount of data. For even the short time that the cell site communicates with that device, in an LTE network the device commands as much bandwidth as any other user.
Flexible numerology will allow assigning a tiny slice of frequency to the IoT device. For example, the cell site might elect to assign 1/64th of a channel to the IoT device, meaning the remaining 63/64ths of the frequency can be assigned to some other purpose to be used at the same time that the IoT device is demanding bandwidth. In a 5G network the IoT device might grab a tiny slice of frequency for a short period of time and barely create a ripple in the overall use of frequency at the cell site.
The cellular network might treat the gamer the same as today but has numerous new options with flexible numerology to improve the gaming performance. It might separate sent and received data and size each path according to needs. It might create a connection for a longer time period than normal to efficiently transmit the needed packages. Essentially, flexible numerology lets the cell site treat every customer differently depending upon their specific needs.
This implementation of flexible numerology for 5G is complicated and will require new algorithms that ultimately get built into the chips for 5G devices. It’s always interesting to watch how new standards are implemented in the industry. I’ve seen numerous papers on the web over the last few months from labs and universities looking at the challenges of flexible numerology. These investigations will eventually get translated into lab trials of devices, and, if those trials are successful make it into the production for both cell sites and cellular devices. This is why a new standard like 5G can’t be implemented immediately. Standards define the problem, and then scientists, engineers and manufacturers take a shot at making the new ideas work (or sometimes find out that they don’t work). It’s likely to be years until the flexible numerology is made to work good enough to be in everyday use in cell sites – but when it does the utilization of frequency will be significantly improved, which is a key goal for 5G.