Beam forming in a smart antenna array makes use of a number of individual antennas and associated signal processors to create a desired transmission radiation patter.
The major benefits to using a smart, active antenna system come from a reduction in overall system power, reduction in communication interference, increase in system capacity and increase in power efficiency.
Also, please read our companion article for additional information on Smart Antenna Fundamentals .
The simplest implementation is a an linear array of at least 4 to 8 independent antennas. This allows for a general coverage of an angular area. A more effective array is built in a matrix configuration. Two common approaches make use of either a Butler Matrix or a Blass Matrix. While the technical details are outside of the scope of this article, we do provide a very brief description of each: The Butler uses a combination of 64 antennas in an 8 by 8 pattern, with 45 degree placement offsets to implement phase shifts in the transmission signal. The Blass matrix can also be realized with 64 antennas. These are placed orthogonally and along the z-axis.
For transmission, the signal processing algorithms are used to calculate beam forming vectors by simultaneously activating one or more of the antennas in the array. As the name implies, beam forming creates a transmission signal with a spatial signature to maximize signal reception at the receiver. If there is two way communication from the receiver back to the transmitter, this can also enable location and tracking of the receiver. This is done by constructively adding the phases of the transmission signals in the desired direction of the while simultaneously nulling the patterns where there is interference.
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More complex antenna arrays are adaptive, which allows for a much greater possible number of beam patterns, which can continuously modified. One significant advantage is that by using sophisticated Direction Of Arrival (DOA) estimation algorithms and a high speed processor, the transmission beam can effectively be steered in any direction. Using this beam forming method, it’s also possible to nullify any interfering signals. Adaptive beam forming also provides a transmission pattern that can track a moving object to maintain a constant signal strength to the receiver.
Building on this concept, an advanced version of switched beam arrays uses “Diversity Combining Techniques” to improve performance in noisy or failing communications channels. At its simplest, using feedback from the receiver it is possible to monitor the signal-to-noise ratio of the transmitted signal for each antenna element in the array. At periodic intervals, the element with the best SNR is selected. Diversity combining takes this one step further by instantaneously evaluating each element and assigning a weighted value to its SNR. A signal processor at the transmitter then extrapolates an optimum beam pattern and creates this using the a time delay transmission over the selected elements. While this is computationally intensive, it allows for the overall signal to be properly co-phased from the aggregate of the elements using variable power output on each element.
The radiation pattern achieved with adaptive beam forming will typically have an excellent high gain center with very low side lobes. These are achieving a 10dB to 15 dB advantage over a standard array configuration, with up to 30 dB in the next 5 years.
Beam forming smart antennas allow for greater performance than is achievable with a simpler beam switching or switch diversity implementation (both of which are in turn better than a standard, fixed antenna system). There is a greater cost associated with the signal processing requirements as well as the number of antennas required. However, implementing a smart antenna array using adaptive beam forming is a very cost effective method to provide a greater quality of service (QOS) to customers while potentially increasing the overall system capacity and data rates.
For more information on this topic, please read these articles:
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