When investigating the nuances of Advanced Metering Infrastructure (AMI), there is much to consider and it’s easy to have information overload. However, the one aspect you shouldn’t overlook is the outsized role that radio frequency (RF) plays in communications.
A resilient and reliable grid is heavily dependent upon having a robust and secure communication network. Most AMI systems today operate on either the 400 MHz or 900 MHz spectrum bands. Both will work, but several differences need to be acknowledged. Even within individual spectrum bands.
Private spectrum and the 900 MHz band
Let’s set the record straight—not all 900 MHz spectrum is the same. For example, many AMI providers rely on public spectrum (902-928 MHz) for their meter communications. This is the same spectrum band shared by garage door openers, baby monitors, smart thermostats, and even the Ring Doorbell. The challenge with public spectrum is three-fold: the inevitability of RF interference, the inherent security risks of sharing bandwidth, and a one watt limit in transmission strength.
However, within the 900 MHz band, there are also primary-use, licensed frequencies used for AMI. By having exclusive private rights, the risk of RF noise and interference is significantly less. There is no radio competition with third-party smart home devices. Additionally, private frequencies have the backing and protection of the FCC—which can levy fines up to $10,000 a day for offenders. These private frequencies can also transmit at two watts which allows the messages to reach much further. These same advantages apply to licensed spectrum AMI systems that operate on the 400 MHz band.
900 MHz vs. 400 MHz
To reiterate—both 900 MHz and 400 MHz bands will work for AMI applications. The key is to use a vendor with primary frequency rights and doesn’t rely on the public spectrum. When comparing 900 MHz and 400 MHz bands, there are five key differences. For this exercise, we are focusing solely on private spectrum.
Both bands are licensed, which helps tremendously with interference. Generally, the 400 MHz band does well penetrating through solid objects. However, it is challenged with noise from vehicles, fixed-to-mobile voice and data communication, and more.
The antennas used in a 400 MHz system must be twice as long to achieve the same efficiency as their 900 MHz counterparts. This additional length is rarely achievable inside a meter; therefore, the efficiency is typically half of what can be achieved in a 900 MHz endpoint.
- Primary-use vs. Secondary-use
Not all licensed spectrum is the same. For example, the 450 to 470 MHz range can also be used for police, EMS, and tow trucking communications which can interfere with the AMI system. It’s essential to make this distinction. With primary use, no other entities are allowed to use RF frequencies without permission.
- Link Budget
A 900 MHz system has a better link budget, which considers the power gains and losses that a communication signal experiences during the journey from transmitter to receiver. This complex equation takes into account antennas, wattage, noise floor, and more—tabulating it into a final number.
Imagine a swimming meet that doesn’t have designated lanes roped off. Athletes would be bumping into each other, and confusion would reign. This analogy applies to channelization within an RF system. Systems within the 400 MHz band are not able to easily channelize their messages.
It’s important to realize that not every message has the same level of importance. A regular meter read does not carry the same weight as an outage alert. This is why channelization is vitally important. In a licensed 900 MHz system, a utility can dedicate applications to separate RF channels to ensure that all applications are adequately supported and do not impede each other. For example, one frequency can be reserved for normal metering operations, another for alarms, and another for SCADA applications.
Radio frequency plays an increasingly significant role in utility communications, and it’s essential to realize that not all spectrum is the same. While acknowledging that AMI has been successfully deployed across many different radio frequencies, the advantages of licensed spectrum systems are hard to overlook. And, AMI networks that operate within the licensed 900 MHz band have clear communication advantages over 400 MHz systems.