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What is Antenna Diversity - The Basics

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Environmental factors cause the vast majority of problems people experience when using wireless radio technology for communication. Designers also often experience difficulties reproducing environmentally related issues because the environment is constantly changing.

 

The presence of people, vehicles, humidity, trees, foliage and movement can all affect the way a radio signal works.

 

WiConnect incorporates a number of techniques to minimise many of these environmental factors, which in turn can increase the range, robustness and reliability of a communications data link. Antenna Diversity is one of the techniques employed by WiConnect and may be implemented with very little incremental cost to the design of a connected device.


Background

Environmental factors impact the range and robustness of all radio links and this is due to a few key factors:

  • Multipath / Fading
  • Antenna Polarization
  • Movement of transmitters / receivers relative to one another
The Antenna Diversity technique makes use of two antennas and algorithms in the WiConnect software to evaluate the signal quality on both antennas before selecting a particular antenna to use when sending or receiving a data packet. Antenna diversity increases the likelihood that an antenna is physically positioned to 'see' a strong signal.
In the diagram below, an RF signal is represented with a simple sine wave, a radio transmitter is shown at the far left, and a receiver is shown near the middle with two antennas: a 'left' antenna and a 'right' antenna. At a given distance from the transmitter the 'left' antenna of the receiver is located in one of the peaks while the 'right' antenna is located in a trough.
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If the transmitter moves, and the sine wave peak moves away from the 'left' antenna, that antenna would see a reduction in any available signal strength. In a single antenna design, the reduced signal level may result in dropped packets or a dropped connection. However if the design incorporated antenna diversity and the second antenna (the 'right' antenna) was positioned at 1/4 wavelength from the 'left' antenna, then the 'right' antenna would be in a new peak. The antenna diversity algorithm in WiConnect would then automatically switch the radio to use the 'right' antenna as the new primary antenna.
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In addition to positioning the two diversity antennas at a 1/4 wavelength apart, it is also useful to consider the radiation profile of the antennas. In the case of a simple whip antenna such as the ACA-4HSRPP-245, a typical profile is similar to that of a donut around the antenna itself.
The donut profile of the whip antenna means that the antenna transmits and receives very well to the sides but does not radiate as well up and down. In a single antenna design positioned with a vertical antenna, as in the diagram to the left, this would create an ideal situation for a particular floor in a building, but would not be ideal for communicating with devices on a higher or lower level floor. image

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If that same device was rotated 90 degrees, the performance above and below would improve but the performance to the side would degrade.
If the device had antenna diversity, using two antennas positioned at 90 degrees to each other, the performance in both directions would be improved.
In an indoor environment the situation is more complex due to reflections from the walls/ceiling/floor and other objects (a phenomena known as multipath) but the basic principles described here still apply and aid understanding.
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In summary WiConnect's Antenna Diversity feature is designed to choose the best antenna to counter the effects of multipath and antenna polarization, improving both link and connection quality. This benefit can increase coverage, range and eliminate dead spots.

 

WiConnect automatically manages antenna diversity without needing help from a host microcontroller as the antenna diversity algorithms are built-in. The built-in algorithms also offer additional system power savings, since the host microcontroller can remain asleep during the operation and a given range can often be achieved at a lower transmit power setting than a single antenna design.