Smart antennaFrom Wikipedia, the free encyclopedia

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 calculatebeamforming 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.

Contents

 [hide]

1   Direction of arrival (DOA) estimation

2   Beamforming

3   Types of smart antennas

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

Marketplace

5   Extension of smart antennas

6   References

7   See also

[edit]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.

[edit]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].

[edit]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.

[edit]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 -- 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]

[edit]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.

DEFINITION -

A smart antenna is a digital wireless communications antenna system that takes advantage of diversity effect at the source (transmitter), the destination (receiver), or both. Diversity effect involves the transmission and/or reception of multiple radio frequency (RF) waves to increase data speed and reduce the error rate.In conventional wireless communications, a single antenna is used at the source, and another single antenna is used at the destination. This is calledSISO (single input, single output). Such systems are vulnerable to problems caused by multipath effects. When an electromagnetic field (EM field) is met with obstructions such as hills, canyons, buildings, and utility wires, the

wavefronts are scattered, and thus they take many paths to reach the destination. The late arrival of scattered portions of the signal causes problems such as fading, cut-out (cliff effect), and intermittent reception (picket fencing). In a digital communications system like the Internet, it can cause a reduction in data speed and an increase in the number of errors. The use of smart antennas can reduce or eliminate the trouble caused by multipath wave propagation.Smart antennas fall into three major categories: SIMO (single input, multiple output), MISO (multiple input, single output), and MIMO (multiple input, multiple output). In SIMO technology, one antenna is used at the source, and two or more antennas are used at the destination. In MISO technology, two or more antennas are used at the source, and one antenna is used at the destination. In MIMO technology, multiple antennas are employed at both the source and the destination. MIMO has attracted the most attention recently because it can not only eliminate the adverse effects of multipath propagation, but in some cases 

Smart antennasMar 1, 2008 12:00 PM, ALDO CUGNINI

         

                

The analog cutoff is now less than one year away, and with that change will come new reception issues for terrestrial broadcast viewers. Although the FCC has sought to replicate analog service in its digital channel allocation plan, most broadcasters will have a digital channel assignment different from their analog one. Inherently, this means that the RF field conditions at the viewer's location will be considerably different from those of the analog service. Smart antennas offer a convenient way to minimize the impact on the viewer.

Click on image to enlarge.

Because terrestrial television receivers must potentially receive signals from various locations, a fixed antenna cannot provide optimum reception across the available channels. In addition, community antennas using the same transmitting site may cause receivers to experience different multipath reception conditions across different channels. While indoor antenna re-aiming may not affect an analog viewer — or the viewer simply tolerated a lower SNR on some channels — re-aiming an antenna for optimum digital service could be quite burdensome.

Electronically steerable smart antennas that automatically optimize the preferred signal direction for each particular broadcast emission were developed years ago for military applications and are increasingly being used in cellular telephone base stations. This optimization can take into account various signal quality factors, such as signal strength, multipath energy and BER.

DIGITAL SIGNAL RECEPTION VARIES WIDELY

Terrestrial television reception is subject to many transmission path impairments, including multipath interference, where delayed echos of the transmitted signal can arrive at the antenna because of reflections off large objects in the receiving space. Moderate to severe multipath can lead to an increase in BER, which could compromise video and audio or, in the worst case, result in no reception at all.

While this situation can often be remedied by physically re-aiming the receiving antenna, this adjustment may not be ideal for all received stations because of their different transmission powers, frequencies and locations.

Figure 1. Simple smart antenna system, with selectable element phase and overall gainClick on image to enlarge.

These difficulties are compounded because of the cliff effect,wherein the BER increases catastrophically below a certain C/N ratio or D/U interference ratio. As such, antenna adjustment can be problematic under many reception conditions. The situation is equally inconvenient with outdoor antennas (requiring a rotator) or indoor ones (requiring frequent trips to the television).

It is now practical to use this same technology for consumer digital television reception. By providing an automatic mechanism to adjust the antenna, the direction and gain (amplification) of the antenna can be electronically changed, with no need for user intervention or physical adjustment of the antenna. This type of antenna functions by changing the relative gain and phase (delay) of the internal elements. While offering a high degree of optimization for both signal capture and interference rejection, this kind of adaptive antenna is somewhat complex and hence expensive to implement.

PRACTICAL SMART ANTENNAS

An alternate type of smart antenna is the so-called switched beam antenna system. In this system, multiple fixed elements within the antenna are selectively used so that a primary receiving direction is favored. At the same time, strong sources of multipath can be negated. An optimization algorithm can perform a trade-off between the two factors. The user simply plugs the antenna in to a suitably equipped DTV receiver or converter box, and the receiver automatically adjusts the antenna for optimal reception of each DTV station.

One example of such a system is shown schematically in Figure 1. The optimization algorithm is typically executed by the CPU in the receiving device and is done once during initial setup. In addition to selecting different azimuth directions, units can operate with different levels of RF amplification. This is useful in areas of high signal strength to avoid overload of the receiver front end, which could otherwise result in high intermodulation distortion.

Selecting an antenna direction and gain setting for optimum signal reception involves assessing the signal quality over the operating extent of the antenna. Various parameters of the received signal can be evaluated and weighed, including signal strength, mean squared error of the channel equalizer, spectral flatness and unwanted interference.

Depending on the system architecture, this optimization process can be tightly integrated with the demodulator or implemented separately. The combination of direction and gain can also be used in a more sophisticated algorithm that anticipates third-order intermodulation interference from strong UHF taboo channels, or from the n ± 1, two-channel pairs where tuner RF selectivity may be minimal.

A STANDARDIZED SMART ANTENNA INTERFACE

While a smart antenna can be an option to the consumer, it will only function if the appropriate interface is available at the receiver. Such an interface has been developed and standardized by the Consumer Electronics Association, and is known as CEA-909A, “Antenna Control Interface.”

This standard describes how a compliant receiver can operate with any compliant antenna, regardless of manufacturer. The standard also specifies the data format used, the connection standards and other requirements.

Figure 2. A smart antenna can be a small, attractive and affordable solution.

The antenna configuration is neither specified nor implied, leaving specific design considerations to the manufacturer. As such, an elaborate system can even be designed using a full-blown antenna farm. The more practical design allows for the realization of an affordable, attractive antenna with a small form factor, as seen in Figure 2.

The CEA R4-WG4 working group has also defined a control protocol that works over the antenna coax, resulting in two options for the CEA-909A standard: one that uses a separate connector for the control signal, and one that shares the RF signal connector/coax.

SMART ANTENNA DESCRIPTION

There are two basic types of smart antennas. As shown in Fig. 6.1, the first type is the phased array or multibeam antenna, which consists of either a number of fixed beams with one beam turned on towards the desired signal or a single beam (formed by phase adjustment only) that is steered toward the desired signal. The other type is the adaptive antenna array as shown in Fig. 6.2, which is an array of multiple antenna elements, with the received signals weighted and combined to maximize the desired signal to interference plus noise power ratio. This essentially puts a main beam in the direction of the desired signal and nulls in the direction of the interference.

A smart antenna is therefore a phased or adaptive array that adjusts to the environment. That is, for the adaptive array, the beam pattern changes as the desired user and the interference move; and for the phased array the beam is steered or different beams are selected as the desired user moves.

Nearly every company the WTEC panel visited is doing significant work in smart antennas. Indeed, some companies placed strong emphasis on this research. In particular, researchers at NEC and NTT stated that they felt that smart antenna technology was the most important technology for fourth generation cellular systems. Researchers at Filtronics and other companies agreed that smart antenna technology was one of the key technologies for fourth generation systems. The reasons appear below.

Fig. 6.1. Phased array.

Fig. 6.2. Adaptive array.

IntroductionA Smart Antenna is an antenna system which dynamically reacts to its environment to provide better signals and frequency usage for wireless communications. There are a variety of smart antennas which utilize different methods to provide improvements in various wireless applications. This report aims to explain the main types of smart antennas and there advantages and disadvantages.

Smart AntennasContrary to the name smart antennas consist of more than an antenna. A smart antenna is a system involving multiple antenna elements and a signal processor (Usually digital) to adjust the radiation and or elements of the antenna (IEC 2004). Smart antennas usually use arrays of antennas linked to a control unit or digital signal processor (Lehne et al. 1999, p. 4). Current efforts into smart antenna development are for wireless digital networks, for use with mobile telephones and computer networks. The smart antennas developed for mobile telephone networks are focused on replacing standard base station antennas to communicate with standard antenna mobile phones, however in the future mobile phone handsets may also contain smart antennas.

 

 

Fig 1 Smart Antenna System 

The theory behind smart antennas has been around for some time. The technique of two parties ‘targeting’ each other with there transmissions being used in world war II to prevent jamming and eavesdropping form third parties. However smart antennas have recently become a viable option for

large digital networks as computer processors now have sufficient computational power to process wireless signals in real time. (004p3)

Conveniently the emergence of powerful enough processors has also coincided with the need by communications carriers to be able to use their frequency space more efficiently. As the frequency space is limited and expensive to purchase carriers needed a way to support more users within the same frequency space.

Smart antennas work on the premise that interferes (devices which are also transmitting on the same frequency) are rarely in the same physical location as another device. Smart antennas exploit this by targeting antenna gain (sensitivity) in the direction individual devices (004p3). This means that signals from interferers are not received or much weaker than the signal we are transmitting and receiving with the desired device. Targeting devices also reduces the interference radiated to other devices, this means that other devices can use the same frequency. Having more than one set of devices using a frequency means that the capacity of the frequency space is effectively increased. A smart antenna is similar in function to a switch on a traditional wired computer network, sending signals only to the intended recipients.

 

Fig 2 Device with interfering device 

There are several advantages and disadvantages to the use of smart antennas, which have been expanded upon below.

Advantages

Increased number of users

Due to the targeted nature of smart antennas frequencies can be reused allowing an increased number of users. More users on the same frequency space means that the network provider has lower operating costs in terms of purchasing frequency space.(Lehne et al. 1999, p. 5),(SYMENA 2004, p. 5)

Increased Range

As the smart antenna focuses gain on the communicating device, the range of operation increases. This allows the area serviced by a smart antenna to increase. This can provide a cost saving to network providers as they will not require as many antennas/base stations to provide coverage.(Lehne et al. 1999, p. 5)

Geographic Information

As smart antennas use ‘targeted’ signals the direction in which the antenna is transmitting and the gain required to communicate with a device can be used to determine the location of a device relatively accurately. This allows network providers to offer new services to devices. Some services include, guiding emergency services to your location, location based games and locality information. (Lehne et al. 1999, p. 5)

Security

Smart antennas naturally provide increased security, as the signals are not radiated in all directions as in a traditional omni-directional antenna. This means that if someone wished to intercept transmissions they would need to be at the same location or between the two communicating devices.(Gadh et al. 2003)

Reduced Interference

Interference which is usually caused by transmissions which radiate in all directions are less likely to occur due to the directionality introduced by the smart antenna. This aids both the ability to reuse frequencies and achieve greater range.

Increased bandwidth

The bandwidth available increases form the reuse of frequencies and also in adaptive arrays as they can utilize the many paths which a signal may follow to reach a device.

Easily integrated

Smart antennas are not a new protocol or standard so the antennas can be easily implemented with existing non smart antennas and devices.

Disadvantages

Complex

A disadvantage of smart antennas is that they are far more complicated than traditional antennas. This means that faults or problems may be harder to diagnose and more likely to occur.(Lehne et al. 1999, p. 5)

More Expensive

As smart antennas are extremely complex, utilizing the latest in processing technology they are far more expensive than traditional antennas. However this cost must be weighed against the cost of frequency space.

Larger Size

Due to the antenna arrays which are utilized by smart antenna systems, they are much larger in size than traditional systems. This can be a problem in a social context as antennas can be seen as ugly or unsightly.(Lehne et al. 1999, p. 6)

Location

The location of smart antennas needs to be considered for optimal operation. Due to the directional beam that ‘swings’ from a smart antenna locations which are optimal for a traditional antenna are not for a smart antenna. For example in a road context, smart antennas are better situated away from the road, unlike normal antennas which are best situated along the road.(Lehne et al. 1999, p. 6)

Types of Smart AntennasThere are three main types of smart antenna. They can be seen as evolutional types with each type being more complex and or smart then its predecessor.

Normal

A normal antenna is not a smart antenna and simply attempts to radiate as much signal as possible uniformly around the antenna element.

 

 

Fig 3 Normal antenna radiation pattern

Switched Lobe

A switched lobe antenna has an array of directional antenna elements all covering different areas. The element in the direction of the device that communication is to take place is then used to transmit and receive. If a device moves out of the beam of an element the antenna must switch to transmit and receive on an element that does reach the device. Switched lobe antennas integrate well with existing normal antennas. They are also less complex than the other smart antennas meaning they may be more cheaply implemented. This may be a good antenna to use in an older network where the infrastructure will not remain in use for long enough to justify the outlay on more advanced antennas.

 

Fig 4 Switched lobe antenna radiation pattern

 

A switched lobe antenna provides some improvement over a normal antenna

Dynamically Phased Array

Dynamically phased array antennas form a beam to a device digitally. The array forms a beam by activating certain omnidirectional elements in the array which have a multiplying effect to form a beam. This beam can then be ‘steered’ or pointed in the direction of a device by phasing the transmission of the signal in the elements and adjusting the gain on each antenna element. A dynamically phased array steers the created beam at the desired device. As the beams are formed digitally the same array of elements can target beams at multiple devices on multiple frequencies. Assuming two beams do not ‘illuminate’ the two devices they can use the same transmission frequency.

 

Fig 5 Dynamically phased array antenna radiation pattern

 

Digital beam forming requires a great deal of processing power, and so have only recently become a feasible technique.(Howard 2003, p. 7)

Fig 6 Digitally formed beam

Adaptive Array

An adaptive array is similar to a dynamically phased array however it performs more it is more ‘intelligent’ taking into account a greater number of factors. An adaptive array adapts to its environment by taking into account other interfering devices and multiple signal paths. Interfering devices can be ‘blocked’ by reducing the signal received form the antenna elements in that direction and increasing it in others. Multiple signal paths can be utilized by forming beams in the directions of signal paths meaning a combined signal can be formed from multiple beams. This provides a much better signal to noise ration giving clearer communication to a device.

 

Fig 7 Adaptive array antenna radiation pattern

 

Another technique which is being developed using smart antennas on both transmission devices is to use the multiple signal paths to transmit different data and create multiple data streams on the same frequency. This allows a higher bandwidth for communication between the two devices. This technology is called multiple input, multiple output (MIMO) and is being integrated into wireless LAN systems. (Lingblom 2003, p. 5)(Temme 2004, p. 33)(Cox 2004, p. 12)

 

 

Fig 8 MIMO radiation pattern

 

ConclusionSmart antennas vastly improve the efficiency of wireless transmission and are likely to become the standard in use for connections between wireless devices. As the technology becomes cheaper it is likely that all devices will utilise smart antennas. This transition could be compared to the use of hubs in wired computer networks and there replacement with switching technology as the costs of implementation reduced.