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Wikipedia defines smart antennas as follows

Smart antennas (also known as adaptive array antennas, multiple antennas and, recently, MIMO) are antenna arrays with smart signal processing algorithms used to identify spatial signal signature such as the direction of arrival (DOA) of the signal, and use it to calculate beamforming vectors, to track and locate the antenna beam on the mobile/target.

And phased arrays as follows

In antenna theory, a phased array is an array of antennas in which the relative phases of the respective signals feeding the antennas are set in such a way that the effective radiation pattern of the array is reinforced in a desired direction and suppressed in undesired directions.

From these definitions it seems that two terms define similar things. And yet the article on smart antennas do not reference (in the text) phased arrays and vice versa. To contribute to my confusion, there is a book Adaptive Antennas and Phased Arrays for Radar and Communications by Alan J. Fenn, which has two major parts: one on adaptive antennas and one on phased arrays. These parts address different problems: smart antennas for nulling out interference sources and phased arrays to scan an antenna field of view. And again the core structure looks the same to me.

So what exactly is the difference between smart antennas and phased arrays?

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Simply put: both are different ways of harnessing the properties of using multiple antennas.

In a Phase array the antennas are usually placed in a repetitive pattern like a matrix or circles or hexagons. Often the distance between the antennas is always the same. Phased arrays are used to make these many antennas behave as one antenna with certain properties like a more narrow beam pattern (where the antenna is sensitive and/or emits more power).

I find smart antennas a misnomer as the antennas are not smart at all, the "smart" bit is in the signals which are used with the antennas. This can be used for improving reception by beam-forming or the reverse: enhanced sensitivity in a certain direction.

This "smart antenna" signal processing is also used with Phased Arrays.

You could say that some smart Antenna systems are a simple form of Phased Array. Where a simple Phased array might use 9 antennas, a smart antenna system might use 3 antennas.

You could call any Phased array with signal processing a Smart Antenna system but I would not call any smart Antenna system a phase array. for example some laptops have 2 or 3 Wifi antennas for beamforming and Mimo. You would not call that a phase array but I think it would be OK to call that a smart antenna system.

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Smart antennas and phased arrays do their magic by taking the RF signal and linearly combining copies of it with different phases and amplitudes. Depending on the combination, it points its beam to a different direction.

A phase array antenna usually does that choosing the phases and / or amplitudes (usually it is only the phases) from a fixed, finite set of values. This is implemented with a matrix, called a feeding matrix or feeding network, of passive elements like phase shifters arranged in a suitable manner. A Butler matrix is an example of such a matrix. The matrix has N inputs and N outputs, and the choice of the input determines the direction of the beam.

Smart antennas, on the other hand, is more versatile. It chooses the phase and / or amplitude values according to a given algorithm and implements them with electronics, and hence is able to get a much larger number of combinations.

This translates to the smart antenna being able to point its irradiation beam to many more directions (potentially, infinite) than a phased array. On the other hand, it costs more.

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There is much convergence between the two types of antenna, and the main difference is one of history.

Phased arrays have been around for a long time, as an electrical method of making the equivalent of a rotating or steerable beam, both for transmission and reception. The military have developed these for a long time, much of the development being secret, hidden inside radomes. Typically a phased array will have 100s if not 1000s of elements. Cost was no object.

Smart antennae have been around only a decade or so, as an evolution of mobile communications. One antenna is subject to fading, this is much improved by having two, and two antennae were standard for a long time. DSP was used to use the two receive chains intelligently. Initially, a receiver would switch to the strongest signal, but as more DSP became available cheaply, using both receivers to estimate the data independently, then soft-combining them, gave a better error rate. This was the first 'smart' antenna.

More antennae got added to the system, as it was realised that crude beamforming could increase the data rate. Under ideal conditions, a 4 antenna MIMO system may give as much as three times the channel capacity of a single antenna by 'spatial multiplexing', employing different scattering paths to the other end of the link. As the number of active antennae increases, the hardware certainly begins to look like the classic phased array hardware.

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Not knowing anything about "Smart Antenna" before reading the Wiki article, But working the RF Communications Field for the better part of 2 decades, I would suggest that Electronically Steerable Phased Arrays are synonymous with the term "smart antenna." The difference is likely similar to saying you got a 3dB diversity gain by using 2 antennas, versus using a 2 omni element array. They are for all intents, the same thing.

The fundamentals of array processing are that you use amplitude and phase to either change your antenna response, both rx and TX (i.e: Beam-steer, null steer,sidelobe rejection), or to glean info about the incoming signal (i.e: direction, velocity.)

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