blog

Key IoT Hardware Considerations: Wireless Device Range

  • 08.09.17
  • ::
  • By David Wilde

Cortet | Blog | Wireless Range

 

With the number of wireless applications growing faster than ever, one of the most often asked questions of the wireless device manufacturers is, “How far away from a wireless access point will your devices work?”

If you get an answer that sounds lower than expected, you are probably getting an answer from an engineer. If you get an answer that sounds too good to be true, you are probably getting the answer from the marketing department (I’m only half-joking here).

It can be difficult to get an honest answer because the operating range of your wireless devices is dependent on a wide range of parameters, and the parameters in any two settings are rarely the same.

Ultimately, the distance over which your wireless devices will work comes down to how far apart the transmitter and receiver are situated such that the received signal is above the sensitivity of the receiver itself.

HOW ANTENNAS AFFECT “REALIZED LINK BUDGET”

You may often hear something called “link budget” used to compare wireless devices.

Link budget is simply the transmit-power minus the receive-sensitivity. So, for a transmitter capable of transmitting 20dBm to a receiver with a sensitivity of -105dBm, the link budget is 125dB.

Still, the transmit-power and the receive-sensitivity are often conducted measurements that neglect the effect of the antenna. Antennas can have either gain or loss based on their construction and implementation. A typical quarter wave dipole antenna can have +2.5dB of gain.

If the same antenna is used on the transmitter and receiver, this increases the link budget by 5dB over a link budget based on conducted measurements alone. Similarly, if a PCB trace antenna is used, it might have an antenna gain of -2.5dB resulting in a 5dB decrease in the link budget.

Taking these two systems and simply changing the antenna results in a 10dB difference in “realized link budget.”

DETERMINING SIGNAL LOSS

Antennas are, however, only one aspect of the system that significantly affects link budget. Signal loss through space typically has the greatest impact on device communication distance.

Most of the time, this involves signals passing through free space. So, a frequently referenced calculation is some form of the Friis transmission formula, where P is the power transmitted in watts, R is the distance between the transmitter and the receiver in meters, and the antennas are omnidirectional and lossless:

 

Cortet | Received Power Formula

 

Manipulating the formula to obtain the ratio of transmitted-power to receive-power will obtain the Free Space Path Loss, where f is the frequency of operation in hertz, c is the speed of light in meters-per-second, and d is the separation distance in meters:

 

Cortet | Formula  for Free Space Path Loss

 

When the environment changes from free space to either urban, suburban, or an indoor environment, the exponent in the previous equation can increase over the free space value of 2.

This results in an exponentially increasing path loss, which in turn means an exponentially decreasing signal level at the receiver. Simply put, this means an exponentially decreasing range of the associated wireless devices.

HOW ENVIRONMENTS AFFECT RANGE

To demonstrate how dramatic this can be, the table below demonstrates how rapidly the range decreases as the exponent increases. The following parameters were held constant in the Path Loss Equation and only the exponent was increased (the antennas are assumed ideal omnidirectional):

 

f = 2440MHz
transmit power = 20dBm
receive sensitivity = -105dBm

The table below shows that if the exponent simply increases from 2 to 2.1 due to the environment, the reception range of your wireless device is effectively cut in half.

 

Cortet | Distance Vs. Path Loss Exponent

 

Most office deployments typically have multiple walls separating work spaces. Some have cinder blocks, cement, or other thick and dense building materials, which makes it difficult to know what the exponent for the path loss really is. This makes it difficult to estimate the expected range or even know if the wireless device is working properly.

Studies have been done to determine the correct value for the path-loss exponent for different environments. The table below was presented by Ayushi Gagneja in a study done on propagation models, which demonstrates that it is not unrealistic for the exponent to increase from 2 up to 4 in buildings with obstructions.

This validates that the range of wireless devices is a highly variable number that is very much based on its environment.

 

Cortet | Propogation Models

Propagation Models presented by Ayushi Gagneja

 

Rather than quoting range, Cortet prefers to state the Total Radiated Power (TRP) of its small wireless devices. As the name implies, this is a radiated-power measurement rather than a conducted measurement.

The total radiated power includes any effects of the antenna and its implementation, and is therefore a much more realistic measurement for determining if the wireless device is working optimally.

UP NEXT...

In our next blog we will explain the TRP measurement we use at Cortet in greater detail. In the meantime, feel free to post any questions in the comments section below.


up next

Is IoT Smart Lighting Worth It? Growing and Diversifying Your Revenue Through Smart Lighting Control

  • Erik Davidson
  • ::
  • 07.06.17

subscribe