The 450 MHz spectrum is ideally placed at the low end of the usable radio spectrum for unique applications, especially wide area communications between machines. Back in the 1980s it was used for the groundbreaking cellular system known as NMT (Nordic Mobile Telephony). But, by the 1990s, voice users were migrating to the higher capacity GSM system in the 900 MHz or higher frequency bands. GSM was not applicable to lower frequencies because the technology (Time Division Multiplexing) limited the sizes of cell sites, so the benefits of the frequencies in low-density areas could not be realized. This left the valuable 450 MHz spectrum underutilized, ripe for new applications.
Public wireless systems, such as cellular and PCS are generally found between about 400 MHz at the low end and 2 GHz at the high end. Lower frequencies tend to propagate better, meaning that cells are considerably larger and in-building penetration is better. Higher frequencies propagate shorter distances meaning that they are more suitable for urban areas, where it is desirable to pack cell sites more closely together to increase capacity.
Most attention in the past was placed on consumer wireless systems, with people talking, sending text messages, surfing the web, watching videos, and using an endless number of applications on their smart phones. For these applications operators struggle to get the most capacity out of their systems, especially in the highly competitive urban areas.
Today, attention is increasingly turning to the important field of intelligent, networked machines, known as Machine-to-Machine (M2M) communications or IOT (the Internet of Things). The communication needs of machines are generally much more efficient than that of humans – they tend to be more taciturn, reporting infrequent small packets under most circumstances. A voice user talking for one second will send about 8,000 bits (1’s and 0’s) and a smartphone data user might send about 100 or even 1,000 times more … in one second. On the other hand some machines are satisfied with 8, 80 or 800 bit packets. To them, that says all they need to say. They can report an alarm on a piece of equipment, a meter reading, a temperature, a pressure, a GPS location, and many other things, just in a handful of bits. For routine communications, such as hourly, daily or monthly reports, many bits of information can be squeezed into one tiny packet.
Machines also tend to not to congregate quite so much in urban areas. Those that are in rural areas often are unable to take advantage of wired forms of communications or private radio systems. It is machines that are monitoring vehicles, transmission lines, oil wells and pipelines that have the greatest unmet needs for communications. There are always satellites, but they require a bulky and expensive (both to buy and to use) radio, with coverage that is not always reliable without a huge antenna. Cellular promises wide area communications with a small, cheap to buy and cheap to use device, with a small antenna and, if battery powered, modest power requirements.
What has proven to be a more usable technology to repurpose the 450 MHz frequency bands has been CDMA, in which users are separated by orthogonal mathematical codes rather than time or frequency. This technology has been proven in both the widely used CDMA2000 cellular system (the voice technology in use by major operators such as Verizon, Sprint, China Telecom and KDDI) and by UMTS, in use by most GSM operators as their 3G technology, while they transition to LTE. It is expected that, as LTE matures, more and cheaper LTE devices will be available for this band, and gradually M2M applications in the 450 MHz band will migrate from CDMA to LTE.
The 450 MHz Alliance has a mission to bring operators and manufacturers together, to produce a wider variety of equipment and systems that are cost-effective, flexible, interoperable, secure and scalable.
David Crowe, Telecom Consultant, David.Crowe@cnp-wireless.com