Smart antenna

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. The antenna could optionally be any sensor.

Smart antenna techniques are used notably in acoustic signal processing, track and scan RADAR, radio astronomy and radio telescopes, and mostly in cellular systems like W-CDMA and UMTS.

Smart antennas have two main functions: DOA estimation and Beamforming.

Direction of arrival (DOA) estimation

The smart antenna system estimates the direction of arrival of the signal, using techniques such as MUSIC (Multiple Signal Classification), estimation of signal parameters via rotational invariance techniques (ESPRIT) algorithms, Matrix Pencil method or one of their derivatives. They involve finding a spatial spectrum of the antenna/sensor array, and calculating the DOA from the peaks of this spectrum. These calculations are computationally intensive.

Matrix Pencil is very efficient in case of real time systems, and under the correlated sources.

ABSTRACT

The technology of smart or adaptive antennas for mobile communications has received enormous interest worldwide in recent years. The purpose of this article is to give an overview of the technology and of the different ways in which it is foreseen to influence mobile communications systems. A broad definition of smart antennas is used in this article, namely base station antennas with a pattern that is not fixed, but adapts to the current radio conditions. The article starts with an explanation of the basic principles of smart antennas. Different levels of intelligence are introduced, ranging from simple switching between predefined beams to optimum beam-forming. Also, an evolutionary path for the development of smart antennas is suggested. A discussion of the consequences for mobile systems is given. The principle reason for introducing smart antennas is the possibility for a large increase in capacity: an increase of three times for TDMA systems and five times for CDMA systems has been reported. Other advantages include increased range and the potential to introduce new services. Major drawbacks and cost factors include increased transceiver complexity and more complex radio resource management. The basic principles for implementation of smart antennas at the base stations are explained. First, the general principles for beam-forming using array antennas are explored, then the special cases of different smart antenna implementations are discussed. Also, the fundamental difference between performing beam-forming on up link (base station receiving) and down link (base station transmitting) is dealt with. Special attention is given to the critical factors and technological challenges, including achieving equal performance on up link and down link as well as real-time calibration of the receiver and transmitter chains. The article concludes with a short description of some of the trials and testbeds that have been implemented. In the autumn of 1998 Ericsson, in cooperation with Mannesmann Mobil funk in Germany, became the first to allow commercial traffic through a base station employing smart antennas.

Beamforming

Beamforming is the method used to create the radiation pattern of the antenna array by adding constructively the phases of the signals in the direction of the targets/mobiles desired, and nulling the pattern of the targets/mobiles that are undesired/interfering targets. This can be done with a simple FIR tapped delay line filter. The weights of the FIR filter may also be changed adaptively, and used to provide optimal beamforming, in the sense that it reduces the MMSE between the desired and actual beampattern formed. Typical algorithms are the steepest descent, and LMS algorithms [4].

Types of smart antennas

Two of the main types of smart antennas include switched beam smart antennas and adaptive array smart antennas. Switched beam systems have several available fixed beam patterns. A decision is made as to which beam to access, at any given point in time, based upon the requirements of the system. Adaptive arrays allow the antenna to steer the beam to any direction of interest while simultaneously nulling interfering signals [3]. Beamdirection can be estimated using the so-called direction-of-arrival (DOA) estimation methods [6].

In 2008, the United States NTIA began a major effort to assist consumers in the purchase of digital television converter boxes.[1] Through this effort, many people have been exposed to the concept of smart antennas for the first time. In the context of consumer electronics, a "smart antenna" is one that conforms to the EIA/CEA-909 Standard Interface.

Limited Choice of EIA/CEA-909A Smart Antennas in the Marketplace

Prior to the final transition to ATSC Digital television in the United States on June 11, 2009, two smart antenna models were brought to market:

• RCA ANT2000 -- no longer available from retailers
• DTA-5000 -- manufactured by Funai Electric, marketed under the "DX Antenna" brand name, sometimes associated with the Sylvania brand name; no longer available from retailers

And two models are causing consumer confusion:

• Although the Apex SM550 is capable of connecting to a CEA-909 port for the purpose of drawing electrical power, it is not a true smart antenna.[2]
• The unfortunately-named Channel Master 3000A SMARTenna is a conventional antenna, not a smart antenna.[3]

Extension of smart antennas

Smart antenna systems are also a defining characteristic of MIMO systems, such as the IEEE 802.11n standard. Conventionally, a smart antenna is a unit of a wireless communication system and performs spatial signal processing with multiple antennas. Multiple antennas can be used at either the transmitter or receiver. Recently, the technology has been extended to use the multiple antennas at both the transmitter and receiver; such a system is called a multiple-input multiple-output (MIMO) system. As extended Smart Antenna technology, MIMO supports spatial information processing, in the sense that conventional research on Smart Antennas has focused on how to provide a beamforming advantage by the use of spatial signal processing in wireless channels. Spatial information processing includes spatial information coding such as Spatial multiplexing and Diversity Coding, as well as beamforming.

References

1. http://www.stanford.edu/group/sarg/ : Smart Antenna Research Group.
2. http://www.mprg.org : Virginia Tech, Mobile and Portable & Radio research group
3. Frank B. Gross, "Smart Antennas for Wireless Communications with Matlab", McGraw-Hill, 2005
4. http://broadcastengineering.com/hdtv/smart_antennas/
5. Handbook on Advancements in Smart Antenna Technologies for Wireless Networks, IGI 2008
6. http://users.ece.utexas.edu/~bevans/courses/ee381k/lectures/13_Array_Processing/lecture13/lecture13.pdf : Array Processing Tutorial

See also

• Antenna (radio)
• Array processing
• Phased array
• CEA-909
• Mimax
• diversity combining
• Active antenna