Radio range is an important factor in planning wireless networks
In practical terms, radio range is infinite, as long as you have a very powerful transmitter and a very sensitive receiver. Recently, you would have heard news reports on very sensitive radio antennae listening to an amazingly powerful radio “transmission” at a range of 130 million light years.
The event was the cataclysmic collision of two Neutron stars that has created and scattered gold and platinum across the universe and was powerful enough to cause ripples in the space-time continuum. The range, 130 million light years, is quite a distance, a lot more than the 200m to 800m radio range we commonly deal with when talking WirelessHART; however, astronomy antennae are extremely sensitive receivers and the “transmission” from the colliding stars was an explosion with the power of a billion suns.
To put the range into perspective we tend to think of radio communication as instantaneous, and it takes just under 3 micro seconds for a radio message to travel 800m. As for the neutron star collision, we are monitoring it now, but it took place 130 million years ago. We think that the famed Tyrannosaurus Rex roamed the Earth a long time ago, but that was “only” 65 million years into the past, when the radio signal from the neutron star collision was already halfway along its journey to us.
Here in our more mundane lives, away from exploding stars and killer lizards the size of a bus, we work with radios that have mundane ranges confined by mundane limitations. The maximum permitted transmission strength is generally regulated by local authorities, and the minimum useful signal strength that is often more dependent on the ambient radio noise than how sensitive our radio receivers are.
The Limits on Radio Range
The maximum permitted radio signal strength varies, depending in where you live, and what frequency you are broadcasting on. For the 2400MHz spectrum, used by WirelessHART, the limit can be as high as 4W of power in the USA but only 100mW in Europe. If you are working to a global standard, then you want to keep everything under 100mW. This is often expressed is decibels of milliwatts, dBm, rather than mW, so 36dBm for the USA and 20dBm for Europe. The lower bound is meant to be the sensitivity of your receiver, however, in practical terms, you need to have a reception that is clear of any local noise. For example, you may have a receiver that is sensitive down to -96dBm, but if you are in the midst of many other transmitters, then you will need higher signal strength just to be heard above the din of the other radio messages, the local “noise floor”. In some cases, the noise can also be your own radio signal that is reflecting off other nearby surfaces and scattering. The difference between your received signal strength and the noise floor is called the “link margin”.
A low link margin will result in a reduced transmission quality, where you will start to see corrupted messages that must be resent. Most communications protocols are designed to handle some message corruption and will continue to operate without any major issues. At some point, however, the quality will drop to the point that insufficient error free messages are received and due to continuous retries, the link will slow down and become unusable.
The radio range that you can achieve between two points can be estimated using the Friis equation that calculates the free space loss, or loss due only to distance between the transmitter and receiver. If we start with a regulated maximum of 20dBm transmission strength in an area with a noise floor of -85dBm, there is 105dB to play with; the difference between 20dBm and -85dBm. If we allow 10dB for the link margin, then we are left with a maximum loss of 95dB due to the distance between the transmitter and receiver. Using the Friis equation, for a 2400MHz radio signal, there is a loss of 95dB at 559m, so your maximum range is 559m. If you are in a quieter environment with a -90dBm noise floor, then you can extend that out to 994m, but in a noisier environment with a -80dBm noise floor your link margin will start to suffer beyond 314m.
Ultimately, it is the regulated upper limit and the noise floor lower limit that restrict your range. More powerful radios will break transmission regulations and more sensitive receivers will just be better at picking up noise. You can also squeeze the link margin, if you do not intend to send too many messages, and bandwidth is not a limiting factor for you.
Of course, if can take all the shackles off, then you can achieve very long distances. Our record for WirelessHART is 20,000m, by our partners in Canada. They were operating in an environment with almost no ambient noise and were permitted to boost the transmission strength with powered amplifiers on their antennae. When limits are relaxed, anything is possible.
Another limit we have not yet considered, is the attenuation of the signal due to obstructions. In addition to the free space loss, any obstructions will attenuate the signal strength and therefore limit the radio range. If you can’t get clear line of sight, and your range is suffering, then one solution is a repeater to redirect your signal pathway around the obstruction.
Some obstructions do not fully cancel the signal, but simply weaken it. An internal plaster wall would block line of sight, but radio waves can penetrate it, with a small loss to signal strength, about 3dB for each plater board. Again, a repeater helps again by picking up the weakened signal and retransmitting it at full strength.
Is Range Really Relevant?
Radio range is an important factor in planning wireless networks. For WirelessHART we quote maximum ranges 200m and 800m as rules of thumb for our devices that broadcast at around 10dBm and 16dBm respectively. In our wireless planning tool, we also allow for obstructions or noise that might limit range down to 150m, 75m or in the case of heavy industrial sites with dense pipe work, only 30m. Whilst these sound short, we have to allow for the reality of noise and obstructions in an industrial environment. The beauty of WirelessHART is that all transmitters also act as repeaters and relay messages automatically. This means that after a few short hops in dense, obstructed areas, messages will eventually reach repeaters with clear line of sight to the destination. In an industrial environment, this is a massive advantage over point to point and star pattern networks that rely solely on clear line of sight.
In the process industry, long distance radio range lacks relevancy. The physical environment that wireless is expected to operate within will block or attenuate direct pathways, leaving weakened radio signals to disappear into the ambient noise. A meshed network of short range hops works better, allowing for messages to be received and re-transmitted, always keeping the signal strength above the noise floor. It also allows for message pathways to bend around obstructions that would block a point to point signal. WirelessHART uses a meshed topology for these very reasons and it is WirelessHART that after a decade of real world use, continues to dominate as the preferred wireless instrument protocol standard.
Craig Abbott studied Computer Science and Information Technology at the University of Western Australia and has spent over 20 years (17 years with Emerson Automation Solutions) travelling across the Asia Pacific region, working on industrial SCADA and remote monitoring projects. For the last 5 years, Craig has been focused on utilising wireless within plants to gather previously inaccessible data points to support business improvement initiatives, based on increased efficiency, productivity, safety and reliability.
Image # 1 – Tall metal structures will attenuate signals
Image # 2 – Industrial Installations are harsh
Image # 3 – Wireless Pressure Transmitter on a remote installation
Image # 4 – Craig Abbott