Smart Antenna Design With Switched Beam Arrays And Diversity Techniques
An array of multiple antennas, or elements, can be controlled with a signal processor to implement beam forming in a smart antenna configuration.
This architecture is typically used for base stations and fixed transmitters, but as both the technology cost and size is reduced, it is becoming more prevalent in mobile devices as well. (Also, please read our companion article for additional information on Smart Antenna Fundamentals ).
The purpose for a smart antenna design is to ensure a consistent and reliable communications channel from the transmitter to the receiver.
There are many obstacles to contend with in the communications channel, especially for mobile users, including physical (buildings, trees, the metal of a automobile or train) which can alter the signal (e.g., multipath) or diminish its intensity (e.g., fading) and electromagnetic interference. With mobile receivers the change in distance, as well as the rate of change (e.g., the speed ) complicates reception of the transmission.
This most basic implementation for a switched beam array is one with a small number of available fixed beam patterns. In this implementation, there are 3 to 8 fixed position antenna elements of varying lengths and orientation. A beam pattern is created by selecting one or more of the individual elements. Depending on a number of factors – including physical path obstructions, electromagnetic interference, whether there is relative movement between the transmitter and receiver, etc.- the signal processor will make a determination as to which beam pattern to select. The rate at which the beam selection is made, and how often to change beams, is based upon the inherent system requirements.
Switched beam methods provide a coarse adjustment to the spectral quality, but a definite improvement over a simple fixed antenna. This is also relatively simple to implement and with very low incremental cost to the transmitter design.
The first, and most well know, beam antenna array was invented in 1926 by Dr. Shintaro Uda. The “Yagi-Uda Array”, commonly referred to as a “Yagi Antenna” is comprised of 3 or 4 fixed elements. Due to its simple configuration it functions in a rather narrow bandwith, making it most suitable for what is now the non-commercial radio frequency spectrum. This has been modified and improved upon over the years and the newer Yagi antennas have a larger number of elements of different sizes and spacing providing significant gain within the radio and television broadcast frequencies. However, it still has the drawback of being highly directional.
Diversity in the antenna selection is another method of selecting the antenna which provides the best path from the transmitter to the receiver. The basis behind this is that a signal can experience a large drop from one antenna due to obstructions or multipath fading but have minimal fading (or even an increase in received power) at a nearby antenna. By selecting the appropriate antenna, it’s possible to increase the range and reliability of a system. As this also doesn’t require a large amount of processing power, it’s also relatively inexpensive to implement. The major restriction is in how much geographical, or distance, diversity there is in the transmitter antenna array, and the number of unique antennas to select from.
Beam forming is also referred to as spatial signal processing. By taking into account everything within the space between the transmitter to the receiver, and adjusting the transmit signal accordingly, smart antennas can increase overall quality of service (QOS) within a constrained geography and frequency spectrum. In conjunction with QOS, a wireless provider can also increase system capacity to provide both higher quality voice as well as higher data rates by optimizing the signal to interference ratio (SIR) of the environment.
Using a switched beam transmission signal will typically generate a 1 dB to 7 dB advantage over a standard singular fixed antenna.
For more information on this topic, please read these articles:
Multiple-Input Multiple-Output Antennas
Beam Forming and Adaptive Array Antennas
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