Elements to consider – Frequency-hopping

By John Fillinger, Marketing Manager

The success of any mobile automatic meter reading (AMR) system hinges on the ability of the receiver to hear signals from a transmitter to capture the data, while not missing any reads.

Making this happen depends on three equally critical components integral to all AMR systems: the timing interval, the sensitivity of the receiver and power level of the transmitter. A system designed with a shortcoming in any one of these three components can negatively affect overall network performance. With many AMR systems available on the market today with frequency-hopping capabilities, selecting a network that is strong in the three areas of timing interval, receiver sensitivity and power level is crucial for optimal performance.

The Relationship between AMR and Line of Sight

To understand how an AMR system works is to understand line of sight, which is the path between the data signal being broadcast from the transmitter on the meter to the transceiver. AMR systems work best if there are no major obstructions that could interrupt the signal's path between the transmitter and transceiver - such as vehicles, trees, bushes or buildings. If the line of sight is free from obstacles, the transceiver will easily pick up the signal sent from the transmitter, even if broadcast at a lower power. However, a line of sight that is interrupted by an obstacle can cause the transceiver to miss hearing the signal when it is broadcast from the transmitter.

Timing Interval

Understanding the relationship between an AMR system and line of sight will help illustrate the importance of timing intervals, receiver sensitivity and power levels.

In examining timing intervals, most AMR systems broadcast data at intervals between four and 13 seconds. Systems broadcasting at shorter intervals, such as every four seconds, stand a much better chance at getting picked up by a transceiver in a passing vehicle than systems broadcasting signals at longer intervals. That's because shorter interval systems are broadcasting their data more frequently, thus increasing the opportunity to be picked up by the transceiver.

Problems can arise in transmitters that broadcast data every 12 or 13 seconds. For example, if the line of sight is impeded by an obstruction, the transceiver may fail to hear and record the data signal. Since the transmitter will not ping again for another 12 or 13 seconds, the transceiver in a moving vehicle may be past the home before the signal is sent again. The opportunity to hear and record a signal increases when the interval is shorter.

Receiver Sensitivity

Not all transceivers are made the same. Some are built to be more sensitive than others, and when it comes to an AMR system, sensitive is better.

A transceiver that is "sensitive" is more likely to hear the signals being broadcast from the transmitter, especially if an obstacle is impeding the line of sight. Of course a transceiver that is "not as sensitive" should easily hear unobstructed signals, and may even pick up signals with a line of sight that are partially impeded. However a more sensitive transmitter is more likely to hear signals with a line of sight that is obstructed, thus increasing the chances of picking up the data read on the first time.

Power Level and Frequency Hopping

The power in which an AMR system is broadcasting can have an effect in the ability for the transceiver to pick up the data signal. Most narrow-band AMR systems use one milliwatt of power to broadcast their signals over one channel. Narrow-band AMR systems work well and have decent performance capacity. However, a growing trend within the industry is designing frequency-hopping systems that are powered by either 10 milliwatts or 100 milliwatts of power.

A frequency-hopping system uses 25 channels between the 902 and 928 MHZ frequency band to broadcast its signals as opposed to a single channel (916.45 MHz) used in narrow band. The advantage a frequency-hopping system brings is the ability for the transmitter to "hop" to any one of 24 other channels to send the signal. For example, if one frequency band has static or some type of interference, the transmitter is programmed to hop to the next band to hear the signal. A frequency-hopping system improves the chances of receiving signals, which ultimately maximizes performance.

Along with frequency-hopping comes additional power - either 10 or 100 milliwatts. Using a higher power to broadcast extends the range in which transceivers can hear signals. For example, if the line of sight was unobstructed between the transmitter and transceiver, a signal sent with 100 milliwatts of power will travel farther than a signal sent with one milliwatt of power. Many transmitters on the market today that use 100 milliwatts of power are the ones that broadcast data every 12 or 13 seconds. Why is that? The reason is because there is a trade off between power and the timing interval. To ensure its transmitters conform to an industry-standard 10 year warranty, they save battery life by limiting broadcast intervals to 12 or 13 seconds.

But while some manufacturers tout 100 milliwatts of power as their system's main advantage, it's important to look at the relationship of power to real-world scenarios. Most of the time the path between transmitter and transceiver has some level of line of sight obstruction, and when that happens, the advantages that come with a higher level of power such as 100 milliwatts are minimized. Trees, vehicles, brush and buildings are just some of the obstacles that are found in most AMR system scenarios, and in these cases, 10 milliwatts of power performs just as effectively in broadcasting signals.

Frequency-hopping AMR systems do increase performance through the availability of broadcasting signals over 25 channels, as opposed to one channel in a narrow-band system. However, utilities should closely examine an AMR system's specifications of timing intervals, receiver sensitivity and power level to ensure those factors will meet their application's need.