Using Antenna Diversity To Create Highly Robust Radio Links

in Antenna
Introduction

Radio, to many people, is a black box mystery. The challenges in design may be many and the learning curve can be high, and it is probably better left to other people. This misconception is heard many times over; but, in reality, given the high levels of integration from RFIC vendors, the task is surprisingly simple.

Initially many designers concern themselves with the complications surrounding the issue of regional regulation since the regulations tend to differ throughout the world. Understanding and addressing the different regional regulations however is mostly a function of research since in each region there is usually a government agency responsible for publishing documents to explain the rules associated with intentional transmitters.

Environmental Considerations

The primary environmental factors that can influence the consistent robustness of a radio link are phenomena called multipath/fading and antenna polarization/diversity. These phenomena can be either constructive or deconstructive to the quality of the radio link depending on many given
circumstances. It is the infinite number of conceivable circumstances that cause difficulties when trying to understand the specific environmental conditions acting upon a radio link at any one time and the resultant link quality that is achieved.

Antenna Polarization / Diversity

The phenomenon known as antenna polarization is due to the directional properties of any given antenna. While the effects of antenna polarization may be interpreted as a reduction in the quality of some radio links, some radio designers often make use of this property to tune an antenna to
their needs by restricting transmission or reception to signals on a limited number of vectors. This is done as antennas do not radiate equally in all directions and making use of this property can mask a system from other sources of RF noise.

Multipath/Fading

Fading is the phenomena often observed when small movements by either a transmitter or a receiver can lead to large differences in link quality. This happens as an antenna moves in and out of the peaks of a signal.

Multipath expands on this concept. As radio waves are transmitted, they may not be received by the receiver through one path, rather the signal may come from multiple paths through reflections off other objects such as walls and trees multipath. The signals received from each of these sources are likely to arrive at slightly different time intervals meaning slight phase shifts may
occur. When these signals combine they may result in a form of cancellation fading.

Antenna Diversity

Antenna diversity is a technique that is often used to recover signal integrity. Antennas in a product that implements antenna diversity usually have their antenna mounted at 90 degrees to one another such that the effects of polarization/directionality do not reduce the quality of the potential radio link.

In addition to mounting antennas at 90 degrees to one another, antennas in a product that implements antenna diversity have their antennas mounted at a distance of at least wavelength apart, this ensures that at least one antenna is in a peak of the waveform.

While antenna diversity is useful at recovering signal integrity and retaining link budget from the effects of the environment, many designers opt not to use it as the trade offs can be considered quite high in their applications. In most cases the trade off comes from the increased MCU overhead as the MCU has to remain awake for longer periods of time to evaluate the antenna signals. The increased MCU activity usually leads to a greater specification and a more costly MCU, the MCU also has extended on-times that result in shorter battery lives. In other cases the extra space used to implement a two antenna solution or the additional code expertise required restricts the engineers to a single antenna design.

Coding an antenna diversity system adds a substantial coding burden onto a design. Many antenna diversity systems are optimized to operate in a synchronized manner. The MCU on the receiver maintains a timer that allows the receiver to know when to start receiving data. Under these circumstances, the MCU can immediately start evaluating the signal on both antennas. To
evaluate the signal, the MCU would switch between each antenna and evaluate the RSSI levels. of a packet since preambles can be misinterpreted as noise (or vice versa). Unfortunately, strong
noise on a given antenna can result in the start of packets being missed.

Longer preambles are often used to provide the MCU and its antenna diversity algorithm time to detect and evaluate the signal on each antenna ensuring that a true preamble is found, but shorter preambles are preferred as they reduce MCU on-time and in turn reduce MCU current consumption in both the transmit and receive sides of a radio link. Engineers often try to find a
compromise by tuning their antenna diversity algorithm to reduce preamble lengths, but do so at the risk of causing other radio related issues since preamble sequences are usually optimized to provide fast bit clock recovery.
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Using Antenna Diversity To Create Highly Robust Radio Links

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This article was published on 2011/01/25