Smart Antennas Systems

Smart Antenna Systems Tutorial

DEFINITION: A smart antenna system combinesmultiple antenna elements with asignal-processing capability toautomatically optimize its radiationand/or reception pattern in responseto the signal environment.

Tutorial Overview:

What does an antenna do in a telecommunications system? It is the portthrough which radio frequency (RF) energy is coupled from the transmitterto the outside world and, in reverse, to the receiver from the outside world.

Antennas have been the most neglected of all the components in personalcommunications systems. Yet, the manner in which energy is distributed intoand collected from surrounding space has a profound influence upon theefficient use of spectrum, the cost of establishing new networks, and theservice quality provided by those networks.

The following tutorial is an introduction to the essential concepts of smartantenna systems and the important advantages of smart antenna systemdesign over conventional omnidirectional approaches. The discussion alsodifferentiates between the various and often dissimilar technologies broadlycharacterized today as "smart antennas." These range from simple diversityantennas to fully adaptive antenna array systems.

TOPICS

1. A Useful Analogy for Adaptive Smart Antennas 2. Antennas and Antenna Systems 3. What Is a Smart Antenna System? 4. The Goals of a Smart Antenna System 5. Signal Propagation: Multipath and Co-Channel Interference 6. The Architecture of Smart Antenna Systems 7. Who Can Use Smart Antenna Technology 8. Progress: Self-Test 9. Glossary of Terms and Acronyms

10. Products, Services, and Hot Link

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Smart Antenna Systems Tutorial: Index http://www.webproforum.com/arraycomm/full.html

1. A Useful Analogy for Adaptive SmartAntennas

For an intuitive grasp of how an adaptive antenna system works, close youreyes and converse with someone as they move about the room. You willnotice that you can determine their location without seeing them because:

you hear the speaker's signals through your two ears -- your "acousticsensors." the voice arrives at each ear at a different time. your brain, a specialized "signal processor," does a large number ofcalculations to correlate information and compute the location of thespeaker.

Your brain also adds the strength of the signals from each ear together, soyou perceive sound in one chosen direction as being twice as loud aseverything else.

Adaptive antenna systems do the same thing, using antennas instead of ears.As a result, 8 or 10 or 12 "ears" can be employed to help fine-tune and turnup signal information. Also, because antennas both listen and talk, anadaptive antenna system can send signals back in the same direction fromwhich they came. This means that the antenna system cannot only hear 8 or10 or 12 times louder but talk back more loudly and directly as well.

Going a step further, if additional speakers joined in, your internal "signalprocessor" could also tune out unwanted noise (interference) and alternatelyfocus on one conversation at a time. Thus, advanced adaptive array systemshave a similar ability to differentiate between desired and undesired signals.

2. Antennas and Antenna Systems

Antennas

Radio antennas couple electromagnetic energy from one medium (space) toanother (e.g., wire, coaxial cable, or waveguide). Physical designs can varygreatly:

Omnidirectional Antennas

Since the early days of wireless communications, there has been the "simple"dipole antenna, which radiates and receives equally well in all directions. Tofind its users, this single-element design "broadcasts" omnidirectionally in apattern resembling ripples radiating outward in a pool of water. While

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adequate for simple RF environments where no specific knowledge of theusers' whereabouts is available, this unfocused approach scatters signals,reaching desired users with only a small percentage of the overall energy sentout into the environment.

Figure 1: Omnidirectional Antenna and Coverage Patterns

Given this limitation, omnidirectional strategies attempt to overcomeenvironmental challenges by simply boosting the power level of the signalsbroadcast. In a setting of numerous users (and interferers), this makes a badsituation worse in that the signals that miss the intended user becomeinterference for those in the same or adjoining cells.

In uplink applications (user to base station), omnidirectional antennas offerno preferential gain for the signals of served users. In other words, users haveto "shout" over competing signal energy. Also, this single-element approachcannot selectively reject signals interfering with those of served users and hasno spatial multipath mitigation or equalization capabilities.

Omnidirectional strategies directly and adversely impact spectral efficiency,limiting frequency reuse. These limitations force system designers andnetwork planners to devise increasingly sophisticated and costly remedies. Inrecent years, the limitations of broadcast antenna technology on the quality,capacity, and coverage of wireless systems have prompted an evolution in thefundamental design and role of the antenna in a wireless system.

Directional Antennas

A single antenna can also be constructed to have certain fixed preferentialtransmission and reception directions. As an alternative to the "brute force"method of adding new transmitter sites, many conventional antenna towerstoday split, or "sectorize" cells. A 360° area is often split into three 120°subdivisions, each of which is covered by a slightly less broadcast method oftransmission.

All else being equal, sector antennas provide increased gain over a restrictedrange of azimuths as compared to an omnidirectional antenna. This iscommonly referred to as "antenna element gain" and should not be confusedwith the processing gains associated with smart antenna systems.

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While sectorized antennas multiply the use of channels, they do notovercome the major disadvantages of standard omnidirectional antennabroadcast, such as co-channel interference, which will be described morefully in Section 5.

Figure 2: Directional Antenna and Coverage Pattern

Antenna Systems

How can an antenna be made more intelligent? First, its physical design canbe modified by adding more elements. Second, the antenna can become anantenna system that can be designed to shift signals before transmission ateach of the successive elements so that the antenna has a composite effect.This basic hardware and software concept is known as the phased arrayantenna.

The following summarizes antenna developments in order of increasingbenefits and intelligence.

Sectorized Systems

Sectorized antenna systems take a traditional cellular area and subdivide itinto "sectors" that are covered using directional antennas looking out fromthe same base station location. Operationally, each sector is treated as adifferent cell whose range is greater than in the omnidirectional case. Sectorantennas increase the possible reuse of a frequency channel in such cellularsystems by reducing potential interference across the original cell, and theyare widely used for this purpose. As many as six sectors per cell have beenused in practical service. When combining more than one of these directionalantennas, the base station can cover all directions.

Figure 3: Sectorized Antenna and Coverage Patterns

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Diversity Systems

In the next step toward smart antennas, the diversity system incorporatestwo antenna elements at the base station whose slight physical separation("space diversity") has been used historically to improve reception bycounteracting the negative effects of multipath (see Section 4).

Diversity offers an improvement in the effective strength of the receivedsignal by using one of two methods:

Switched Diversity: Assuming that at least one antenna will be in afavorable location at a given moment, this system continually switchesbetween antennas (connects each of the receiving channels to the bestserving antenna) so as always to use the element with the largestoutput. While reducing the negative effects of signal fading, they do notincrease gain since only one antenna is used at a time. Diversity Combining: This approach corrects the phase error in twomultipath signals and effectively combines the power of both signals toproduce gain. Other diversity systems, such as maximal ratiocombining systems, combine the outputs of all the antennas tomaximize the ratio of combined received signal energy to noise.

Because macrocell-type base stations historically put out far more power onthe downlink (base station to user) than mobile terminals can generate onthe reverse path, most diversity antenna systems have evolved only toperform in uplink (from the user to the base station).

Figure 4: Switched Diversity Coverage, with Fading andSwitched Diversity

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Figure 5: Combined Diversity Effective Coverage Pattern,with Single Element and Combined Diversity

Diversity antennas merely switch operation from one working element toanother. Although this approach mitigates severe multipath fading, its use ofone element at a time offers no uplink gain improvement over any othersingle-element approach. In high-interference environments, the simplestrategy of locking onto the strongest signal or extracting maximum signalpower from the antennas is clearly inappropriate and can result incrystal-clear reception of an interferer rather than the desired signal.

The need to more efficiently transmit to numerous users withoutcompounding the interference problem led to the next step of the evolutionantenna systems that "intelligently" integrate the simultaneous operation ofdiversity antenna elements.

"Smart"

The concept of using multiple antennas and innovative signal processing tomore intelligently serve cells has existed for many years. In fact, varyingdegrees of relatively costly smart antenna systems have already been appliedin defense systems. Until recent years, cost barriers have prevented their usein commercial systems. The advent of powerful low-cost digital signalprocessors, general-purpose processors (and ASICs), as well as innovativesoftware-based signal-processing techniques (algorithms) have made"intelligent antennas" practical for cellular communications systems.

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Today, when spectrally efficient solutions are increasingly a businessimperative, these systems are providing greater coverage area for each cellsite, higher rejection of interference, and substantial capacity improvements.

3. What Is a Smart Antenna System?

In truth, antennas are not smart -- antenna systems are smart. Generallyco-located with a base station, a smart antenna system combines an antennaarray with a digital signal-processing capability to transmit and receive in anadaptive, spatially sensitive manner. In other words, such a system canautomatically change the directionality of its radiation patterns in responseto its signal environment. This can dramatically increase the performancecharacteristics (such as capacity) of a wireless system.

How Many Types of Smart Antenna Systems AreThere?

Terms commonly heard today that embrace various aspects of a smartantenna system technology include intelligent antennas, phased array,SDMA, spatial processing, digital beamforming, adaptive antenna systems,and others. Smart antenna systems are customarily categorized, however, aseither "switched beam" or "adaptive array" systems.

The following are distinctions between the two major categories of smartantennas regarding the choices in transmit strategy:

Switched Beam: A finite number of fixed, pre-defined patterns orcombining strategies (sectors) Adaptive Array: An infinite number of patterns (scenario-based) thatare adjusted in real time

What are Switched Beam Antennas?

Switched beam antenna systems form multiple fixed beams with heightenedsensitivity in particular directions. These antenna systems detect signalstrength, choose from one of several predetermined, fixed beams, and switchfrom one beam to another as the mobile moves throughout the sector.

Instead of shaping the directional antenna pattern with the metallicproperties and physical design of a single element (like a sectorized antenna),switched beam systems combine the outputs of multiple antennas in such away to form finely sectorized (directional) beams with more spatial selectivitythan can be achieved with conventional, single-element approaches.

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Figure 6: Switched Beam System Coverage Patterns(Sectors)

What Are Adaptive Array Antennas?

Adaptive antenna technology represents the most advanced smart antennaapproach to date. Using a variety of new signal-processing algorithms, theadaptive system takes advantage of its ability to effectively locate and trackvarious types of signals to dynamically minimize interference and maximizeintended signal reception.

Both systems attempt to increase gain according to the location of the user;however, only the adaptive system provides optimal gain whilesimultaneously identifying, tracking, and minimizing interfering signals.

Figure 7: Adaptive Array Coverage: A RepresentativeDepiction of a Main Lobe Extending Toward a User with aNull Directed Toward a Co-Channel Interferer

What Do They Look Like?

Omnidirectional antennas are obviously distinguished from their intelligentcounterparts by the number of antennas (or antenna elements) employed.Switched beam and adaptive array systems, however, share many hardwarecharacteristics and are distinguished primarily by their adaptive intelligence.

To process information that is directionally sensitive requires an array ofantenna elements (typically 4 to 12), the inputs from which are combined toadaptively control signal transmission. Antenna elements can be arranged inlinear, circular, or planar configurations and are most often installed at the

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base station, although they may also be used in mobile phones or laptopcomputers.

What Makes Them So Smart?

A simple antenna works for a simple RF environment. Smart antennasolutions are required as the number of users, interference, and propagationcomplexity grow. Their "smarts" reside in their digital signal-processingfacilities.

Like most modern advances in electronics today, the digital format formanipulating the RF data offers numerous advantages in terms of accuracyand flexibility of operation. Speech starts and ends as analog information.Along the way, however, smart antenna systems capture, convert, andmodulate analog signals for transmission as digital signals and reconvertthem to analog information on the other end.

In adaptive antenna systems, this fundamental signal-processing capability isaugmented by advanced techniques (algorithms) that are applied to controloperation in the presence of complicated combinations of operatingconditions.

The benefit of maintaining a more focused and efficient use of the system'spower and spectrum allocation can be significant.

4. The Goals of a Smart Antenna System

The dual purpose of a smart antenna system is to augment the signal qualityof the radio-based system through more focused transmission of radiosignals while enhancing capacity through increased frequency reuse. Morespecifically, the features of and benefits derived from a smart antenna systeminclude those listed in Table 1.

Table 1: Features and Benefits of Smart Antenna Systems

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Feature: Benefit:

Signal Gain: Inputs from multipleantennas are combined to optimizeavailable power required toestablish given level of coverage.

Better range/coverage:Focusing the energy sent out intothe cell increases base stationrange and coverage; lower powerrequirements also enable agreater battery life andsmaller/lighter handset size.

Interference Rejection:Antenna pattern can be generatedtoward co-channel interferencesources, improving thesignal-to-interference ratio of thereceived signals.

Increased capacity: Precisecontrol of signal nulls quality andmitigation of interferencecombine to frequency reusereduce distance (or cluster size),improving capacity. Certainadaptive technologies (such asspace division multiple access)support the reuse of frequencieswithin the same cell.

Spatial Diversity: Compositeinformation from the array is usedto minimize fading and otherundesirable effects of multipathpropagation.

Multipath rejection can reducethe effective delay spread of thechannel, allowing higher bit ratesto be supported without the useof an equalizer.

Power Efficiency: Combines theinputs to multiple elements tooptimize available processing gainin the downlink (toward the user).

Reduced expense: Loweramplifier costs, powerconsumption, and higherreliability will result.

5. Signal Propagation: Multipath AndCo-Channel Interference

A Useful Analogy for Signal Propagation

Envision a perfectly still pool of water into which a stone is dropped. Thewaves that radiate outward from that point are uniform and diminish instrength evenly. This pure omnidirectional "broadcasting" equates to onecaller's signal -- originating at the terminal and going uplink. It is interpretedas one signal everywhere it travels.

Picture now a base station at some distance from the wave origin. If the

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pattern remains undisturbed, it is not a challenge for a base station tointerpret the waves. But as the signal's waves begin to bounce off the edges ofthe pool, they come back (perhaps in a combination of directions) to intersectwith the original wave pattern. As they combine, they weaken each other'sstrength. These are "multipath" interference problems.

Now, picture a few more stones being dropped in different areas of the pool,equivalent to other calls starting. How could a base station at any particularpoint in the pool distinguish which stone's signals were being picked up andfrom which direction? This multiple-source problem is called "co-channel"interference.

These are two-dimensional analogies; to fully comprehend the distinctionbetween callers and/or signal in the earth's atmosphere, a base station mustpossess the intelligence to place the information it analyzes in a true spatialcontext.

Multipath

Multipath is a condition where the transmitted radio signal is reflected byphysical features/structures, creating multiple signal paths between the basestation and the user terminal.

Figure 8: The Effect of Multipath on a Mobile User

Problems Associated with Multipath

One problem resulting from having unwanted reflected signals is that thephases of the waves arriving at the receiving station often do not match. Thephase of a radio wave is simply an arc of a radio wave, measured in degrees,at a specific point in time. The following is an illustration of two out-of-phasesignals as seen by the receiver.

Figure 9: Two Out-of-Phase Multipath Signals

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Conditions caused by multipath that are of primary concern are:

Fading: When the waves of multipath signals are out of phase,reduction in signal strength can occur. One such type of reduction iscalled a fade; the phenomenon is known as "Rayleigh fading" or "fastfading."

A fade is a constantly changing, three-dimensional phenomenon. Fadezones tend to be small, multiple areas of space within a multipathenvironment that cause periodic attenuation of a received signal forusers passing through them. In other words, the received signalstrength will fluctuate downward, causing a momentary, but periodic,degradation in quality.

Figure 10: A Representation of the Rayleigh Fade Effect ona User Signal

Phase Cancellation: When waves of two multipath signals arerotated to exactly 180° out of phase, the signals will cancel each otherout. While this sounds severe, it is rarely sustained on any given call(and most air interface standards are quite resilient to phasecancellation). In other words, a call can be maintained for a certainperiod of time while there is no signal, although with very poor quality.The effect is of more concern when the control channel signal iscanceled out, resulting in a "black hole" -- a service area in which callset-ups will occasionally fail.

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Figure 11: Illustration of Phase Cancellation

Delay Spread: The effect of multipath on signal quality for a digitalair interface (e.g., TDMA) can be slightly different. Here, the mainconcern is that multiple reflections of the same signal may arrive at thereceiver at different times. This can result in intersymbol interference(or bits "crashing" into one another) which the receiver cannot sort out.When this occurs, the bit error rate rises and eventually causesnoticeable degradation in signal quality.

Figure 12: Multipath: The Cause of Delay Spread

While switched diversity and combining systems do improve the effectivestrength of the signal received, their use in the conventional macrocellpropagation environment has been typically reverse-path limited due to apower imbalance between base station and mobile unit. This is because"macrocell-type" base stations have historically put out far more power thanmobile terminals were able to generate on the reverse path.

Co-Channel Interference: One of the primary forms of man-madesignal degradation associated with digital radio, co-channelinterference occurs when the same carrier frequency reaches the samereceiver from two separate transmitters.

Figure 13: Illustration of Co-Channel Interference in aTypical Cellular Grid

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As we have seen, both broadcast antennas as well as more focused antennasystems scatter signals across relatively wide areas. The signals that miss anintended user can become interference for users on the same frequency in thesame or adjoining cells.

While sectorized antennas multiply the use of channels, they do notovercome the major disadvantage of standard antenna broadcast --co-channel interference. Management of co-channel interference is thenumber-one limiting factor in maximizing the capacity of a wireless system.To combat the effects of co-channel interference, smart antenna systems notonly focus directionally on intended users, but in many cases direct "nulls" orintentional non-interference toward known, undesired users (see Section 6).

Sources

Neil J. Boucher, The Cellular Radio Handbook, Second Edition,Mendicino, California: Quantum Publishing, Inc., 1992.

George Calhoun, Wireless Access and the Local TelephoneNetwork, Norwood, Massachusetts: Artech House, Inc., 1992.

6. The Architecture of Smart AntennaSystems

How Do Smart Antenna Systems Work?

Traditional switched beam and adaptive array systems enable a base stationto effectively customize the beams they generate for each remote user bymeans of internal feedback control. Generally speaking, each approach formsa main lobe toward individual users and attempts to reject interference ornoise from outside of the main lobe.

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Listening to the Cell (Uplink Processing)

It is assumed here that a smart antenna is only employed at the base stationand not at the handset or subscriber unit. Such remote radio terminalstransmit using omnidirectional antennas, leaving it to the base station toselectively separate the desired signals from interference.

Typically, the received signal from the spatially distributed antenna elementsis multiplied by a "weight," a complex adjustment of an amplitude and aphase. These signals are combined to yield the array output. An adaptivealgorithm controls the weights according to predefined objectives. For aswitched beam system, this may be primarily maximum gain; for an adaptivearray system, other factors may receive equal consideration. These dynamiccalculations enable the system to change its radiation pattern for optimizedsignal reception.

Speaking to the Users (Downlink Processing)

The task of transmitting in a spatially selective manner is the major basis fordifferentiating between switched beam and adaptive array systems. Asdescribed below, switched beam systems communicate with users bychanging between preset directional patterns, largely on the basis of signalstrength. In comparison, adaptive arrays attempt to understand the RFenvironment more comprehensively and transmit more selectively.

The type of downlink processing used depends on whether thecommunication system uses time division duplex (TDD), which transmitsand receives on the same frequency (e.g., PHS and DECT) or frequencydivision duplex (FDD), which uses separate frequencies for transmit andreceiving (e.g., GSM). In most FDD systems, the uplink and downlink fadingand other propagation characteristics may be considered independent,whereas in TDD systems the uplink and downlink channels can beconsidered reciprocal. Hence, in TDD systems uplink channel informationmay be used to achieve "spatially selective" transmission. In FDD systems,the uplink channel information cannot be used directly and other types ofdownlink processing have to be considered.

Switched Beam Systems

In terms of radiation patterns, switched beam is an extension of the currentmicrocellular, or cellular sectorization, method of splitting a typical cell. Theswitched beam approach further subdivides macro-sectors into severalmicro-sectors as a means of improving range and capacity. Each micro-sectorcontains a predetermined fixed beam pattern with the greatest sensitivitylocated in the center of the beam and less sensitivity elsewhere. The design ofsuch systems involves high-gain, narrow azimuthal beamwidth antennaelements.

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The switched beam system selects one of several predetermined fixed-beampatterns (based on weighted combinations of antenna outputs) with thegreatest output power in the remote user's channel. These choices are drivenby RF or baseband digital signal-processing (DSP) hardware and software.The system switches its beam in different directions throughout space bychanging the phase differences of the signals used to feed the antennaelements or received from them. When the mobile user enters a particularmacro-sector, the switched beam system selects the micro-sector containingthe strongest signal. Throughout the call, the system monitors signal strengthand switches to other fixed micro-sectors as required.

Figure 14: Beamforming Lobes and Nulls that SwitchedBeam (Red) and Adaptive Array (Blue) Systems MightChoose for Identical User Signals (Green Line) andCo-Channel Interferers (Yellow Lines)

Smart antenna systems communicate directionally by forming specificantenna beam patterns. When a smart antenna directs its main lobe withenhanced gain in the direction of the user, it naturally forms side lobes andnulls or areas of medium and minimal gain respectively in directions awayfrom the main lobe. Different switched beam and adaptive smart antennasystems control the lobes and the nulls with varying degrees of accuracy andflexibility.

Adaptive Antenna Approach

The adaptive antenna systems approach communication between a user andbase station in a different way, in effect adding a dimension of space. Byadjusting to an RF environment as it changes (or the spatial origin ofsignals), adaptive antenna technology can dynamically alter the signalpatterns to near infinity to optimize the performance of the wireless system.

Adaptive arrays utilize sophisticated signal-processing algorithms tocontinuously distinguish between desired signals, multipath, and interferingsignals as well as calculate their directions of arrival. This approachcontinuously updates its transmit strategy based on changes in both thedesired and interfering signal locations. The ability to smoothly track userswith main lobes and interferers with nulls ensures that the link budget isconstantly maximized because there are neither micro-sectors nor

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pre-defined patterns.

Figure 15 illustrates the relative coverage area for conventional sectorized,switched beam, and adaptive antenna systems. Both types of smart antennasystems provide significant gains over conventional sectored systems. Thelow level of interference on the left represents a new wireless system withlower penetration levels. The significant level of interference on the rightrepresents either a wireless system with more users or one using moreaggressive frequency re-use patterns. In this scenario, the interferencerejection capability of the adaptive system provides significantly morecoverage than either the conventional or switched beam system.

Figure 15: Coverage Patterns for Switched Beam andAdaptive Array Antennas

Relative Benefits/Tradeoffs of Switched Beam and AdaptiveArray Systems:

Integration: Switched beam systems are traditionally designed toretrofit widely deployed cellular systems. It has been commonlyimplemented as an "add-on" or "appliqué" technology that intelligentlyaddresses the needs of mature networks. In comparison, adaptive arraysystems have been deployed with a more fully integrated approach thatoffers less hardware redundancy than switched beam systems butrequires new build-out. Range/Coverage: Switched beam systems can increase base stationrange from 20% to 200% over conventional sectored cells depending onenvironmental circumstances and the hardware/software used. Theadded coverage can save an operator substantial infrastructure costsand means lower prices for consumers. Also, the dynamic switchingfrom beam to beam conserves capacity because the system does notsend all signals in all directions. In comparison, adaptive array systemscan cover a broader, more uniform area with the same power levels as aswitched beam system. Interference Suppression: Switched beam antennas suppressinterference arriving from directions away from the active beam'scenter. Because beam patterns are fixed, however, actual interference

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rejection is often the gain of the selected communication beam patternin the interferer's direction. Also, they are normally used only forreception because of the system's ambiguous perception of the locationof the received signal (the consequences of transmitting in the wrongbeam being obvious). Also, because their beams are predetermined,sensitivity can occasionally vary as the user moves through the sector.

Switched beam solutions work best in minimal to moderate co-channelinterference and have difficulty in distinguishing between a desired signaland an interferer. If the interfering signal is at approximately the center ofthe selected beam and the user is away from the center of the selected, theinterfering signal can be enhanced far more than the desired signal. In thesecases, the quality is degraded for the user.

Adaptive array technology currently offers more comprehensive interferencerejection. Also, because it transmits an infinite, rather than finite, number ofcombinations, its narrower focus creates less interference to neighboringusers than a switched-beam approach.

SDMA: The Smartest Antenna Approach: Among the mostsophisticated utilizations of smart antenna technology is spatialdivision multiple access (SDMA), which employs advanced processingtechniques to, in effect, locate and track fixed or mobile terminals,adaptively steering transmission signals toward users and away frominterferers. This adaptive array technology achieves superior levels ofinterference suppression, making possible more efficient reuse offrequencies than the standard fixed hexagonal reuse patterns. Inessence, the scheme can adapt the frequency allocations to where themost users are located.

Figure 16: Fully Adaptive Spatial Processing, SupportingTwo Users on the Same Conventional ChannelSimultaneously in the Same Cell

Utilizing highly sophisticated algorithms and rapid processing hardware,spatial processing takes the reuse advantages that result from interferencesuppression to a new level. In essence, spatial processing dynamically createsa different sector for each user and conducts a frequency/channel allocation

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in an ongoing manner in real time.

Adaptive spatial processing integrates a higher level of measurement andanalysis of the scattering aspects of the RF environment. Whereas traditionalbeam-forming and beam-steering techniques assume one correct direction oftransmission toward a user, spatial processing maximizes the use of multipleantennas to combine signals in space in a method that transcends a "oneuser-one beam" methodology.

7. Who Can Use Smart AntennaTechnology?

Smart antenna technology can significantly improve wireless systemperformance and economics for a range of potential users. It enablesoperators of PCS, cellular, and wireless local loop (WLL) networks to realizesignificant increases in signal quality, capacity, and coverage.

Operators often require different combinations of these advantages atdifferent times. As a result, those systems offering the most flexibility interms of configuration and upgradeability are often the most cost-effectivelong-term solutions.

Applicable Standards

Smart antenna systems are applicable, with some modifications, to all majorwireless protocols and standards, including those in Table 2:

Table 2: Applicable Standards

Access Methods analog Frequency Division Multiple Access(FDMA) (e.g., AMPS, TACS, NMT)

digital Time Division Multiple Access (TDMA)(e.g., GSM, IS-136)Code Division Multiple Access (CDMA)(e.g., IS-95)

Duplex Methods Frequency Division Duplex (FDD)Time Division Duplex (TDD)

Transparency to the Network

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The flexibility of adaptive smart antenna technology allows for the creation ofnew value-added products and services that give operators a significantcompetitive advantage. Adaptive smart antennas are not restricted to anyparticular modulation format or air-interface protocol. They are compatiblewith all current air-interface modulation schemes.

Added Advantages of Spatial Processing

A wide range of wireless communication systems may benefit from spatialprocessing, including high-mobility cellular systems, low-mobilityshort-range systems, wireless local loop applications, satellitecommunications, and wireless LAN. By employing an array of antennas, it ispossible to multiplex channels in the spatial dimension just as in thefrequency and time dimensions. To increase system capacity, spatiallyselective transmission as well as spatially selective reception must beachieved.

Improved algorithms and low-cost processors make sophisticated spatialprocessing practical alternatives for an increasing number of wireless systemmanufacturers and operators. Many agree that the unique benefits of spatialprocessing will ultimately affect all aspects of wireless system design.

8. Progress Self-Test

Multiple Choice

1. Switched beam antenna systems "cover" a cell area:

a. by dividing the 360° cell into three 120° micro-sectors

b. by switching to the antenna with the best gain at the givenmoment

c. with a finite number of predefined patterns or combiningstrategies

2. Adaptive array systems "cover" a cell area with:

a. a finite number of predefined patterns or combining strategies

b. an infinite number of patterns that are adjusted in real time

c. dividing the 360º cell into three 120º micro-sectors

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3. Which type of smart antenna system is typically employed asan "appliqué" or retrofit to an existing cellular system?

a. switched beam

b. adaptive array

c. switched diversity

4. Which type of smart antenna system is typically implementedas an integrated approach with less hardware redundancy?

a. adaptive array

b. switched beam

c. combined diversity

5. The "smarts" of a smart antenna system are chiefly derivedfrom:

a. the multiple elements of the antenna array

b. frequency re-use

c. the digital signal-processing capability

6. The propagation condition whereby a signal arrives at areceiver both directly and by bouncing off physical structuresis called:

a. multipath

b. Rayleigh fading

c. co-channel interference

7. A "black hole" in a reception area occurs when:

a. signals from the same source arrive at the antenna out ofsequence

b. the antenna system generates a null toward the user

c. two multipath signals arrive 180º out of phase, canceling eachother out

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8. The major disadvantage of omnidirectional antennabroadcast that smart antennas try to overcome in a cellularnetwork is:

a. fading

b. co-channel interference

c. nulls

9. Switched beam antenna systems break a coverage area intomicro-sectors in order to improve:

a. range

b. capacity

c. both

10. Interference suppression has a major impact on:

a. call quality

b. capacity

c. both

11. The type of smart antenna system technology that currentlyoffers the most advanced capabilities for interferencesuppression and frequency reuse is:

a. spatial division multiple access (SDMA)

b. switched beam

c. adaptive array

12. Between uplink and downlink, which direction of wirelesscommunication extends from the base station to the user?

a. uplink

b. downlink

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SELF-TEST CORRECT ANSWERS

Multiple Choice correct answers are in BOLDFACE

1. Switched beam antenna systems "cover" a cell area:

a. by dividing the 360° cell into three 120° micro-sectorsb. by switching to the antenna with the best gain at the given momentc. with a finite number of predefined patterns or combiningstrategies

2. Adaptive array systems "cover" a cell area with:

a. a finite number of predefined patterns or combining strategiesb. an infinite number of patterns that are adjusted in realtimec. dividing the 360º cell into three 120º micro-sectors

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3. Which type of smart antenna system is typically employed asan "appliqué" or retrofit to an existing cellular system?

a. switched beamb. adaptive arrayc. switched diversity

4. Which type of smart antenna system is typically implementedas an integrated approach with less hardware redundancy?

a. adaptive arrayb. switched beamc. combined diversity

5. The "smarts" of a smart antenna system are chiefly derivedfrom:

a. the multiple elements of the antenna arrayb. frequency re-usec. the digital signal-processing capability

6. The propagation condition whereby a signal arrives at areceiver both directly and by bouncing off physical structuresis called:

a. multipathb. Rayleigh fadingc. co-channel interference

7. A "black hole" in a reception area occurs when:

a. signals from the same source arrive at the antenna out of sequenceb. the antenna system generates a null toward the userc. two multipath signals arrive 180º out of phase, cancelingeach other out

8. The major disadvantage of omnidirectional antennabroadcast that smart antennas try to overcome in a cellularnetwork is:

a. fadingb. co-channel interferencec. nulls

9. Switched beam antenna systems break a coverage area intomicro-sectors in order to improve:

a. range

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b. capacityc. both

10. Interference suppression has a major impact on:

a. call qualityb. capacityc. both

11. The type of smart antenna system technology that currentlyoffers the most advanced capabilities for interferencesuppression and frequency reuse is:

a. spatial division multiple access (SDMA)b. switched beamc. adaptive array

12. Between uplink and downlink, which direction of wirelesscommunication extends from the base station to the user?

a. uplinkb. downlink

9. Glossary of Terms And Acronyms

Algorithm An automatic signal-processingstrategy that varies the way in whichmultiple antenna elements areemployed as a function of operationalscenarios.

Amplitude The magnitude of the high and lowpoints of a waveform or signal. Alsocalled wave "height" or signalstrength.

Azimuth Horizontal direction expressed indegrees as the angular distancebetween the direction of a fixed point(as the observer's heading) and thedirection of the object.

Backhaul To take communications channeltraffic beyond its destination andback, most often because it is cheaper

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to go the long way instead of goingdirectly. Tapping into a pre-existingsystem can take advantage ofestablished efficiencies. Backhaulingis often a temporary re-routingalternative to more direct connectionused from moment to momentaccording to changing systemconditions.

Bandwidth The range of a channel's limits. Thebroader the bandwidth, the faster andmore varied the data that can be sent.A wideband circuit, for example, cancarry a TV channel. (The bandwidthused for one video channel is roughlyequal to 1,200 voice telephonechannels.)

Base Station (Cell Station) A radio transceiver(transmitter/receiver) that usesprocessing hardware/software and anantenna array to control and relaysignals between the central office andthe remote handset or subscriber unit(fixed or mobile). It connects wirelessusers to a phone network, public orprivate.

bps (bits per second) The rate of datatransmission pertaining to thenumber of pulses in one second.

Co-Channel Interference One of the primary forms ofman-made signal degradationassociated with radio, co-channelinterference occurs when the samecarrier frequency reaches the samereceiver from two separatetransmitters as a result of "spillingover" from an adjoining cell.

Code Division (CDMA) A spread spectrum accesstechnology that assigns a code to allMultiple Access speech bits, sendsscrambled transmission of theencoded speech over the air, andreassembles the speech to its originalformat.

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Cross Talk A condition of wirelesscommunication that occurs when oneuser hears someone they are nottalking to, commonly resulting froman inaccurately aimed or accidentallybounced signal transmission.

Delay Spread An effect of multipath for a digital airinterface in which multiple reflectionsof the same signal arrive at thereceiver at different times, creating anoticeable degradation in signalquality.

Directive Transmission Directionally focused signaltransmission from a base station to aremote caller made possible bycertain smart antenna systems withdigital signal processing capabilities.These base stations use informationobtained during reception to transmitsignals selectively toward certainusers and away from others.

Diversity Diversity antenna systemsincorporate two slightly spacedantenna elements with theassumption that one or the other'slocation will be better at any givenmoment for reception of a user'ssignal. Switched diversity systemsalternately operate with one or theother antenna to maintaincommunication; diversity combiningsystems combine signals to increasegain.

Downlink Refers to the connection from thebase station to the remote user site(alternately from the central office tothe base station).

Fading The reduction in signal intensity ofone or several of the components of aradio signal, typically caused by thereflective or refractive effects ofmultipath.

Fast Fading See Rayleigh Fading.

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Frequency The rate at which an electrical currentalternates, expressed as the numberof cycles per unit of time (from crestto crest in a sine wave pattern).Frequency is typically measured inhertz (Hz), or cycles per second.

Frequency DivisionDuplex (FDD)

The simultaneous exchange of uplinkand downlink information ondifferent frequencies. (See timedivision duplex.)

Modulation The process of impressinginformation on a carrier wave bychanging some of the wave'scharacteristics (such as amplitude,frequency, or phase) to reflect thechanges in the information it delivers.

Multipath Copies of the desired signal that havearrived at the antenna after bouncingfrom objects between the signalsource and the antenna. These signalscan either cancel or reinforce eachother.

Phase The relationship between a signalwave and its horizontal axis. Thephase between two identical signals isthe ratio of the timing difference tothe period of time it is measured indegrees of a 360 period.

Phased Array A type of antenna design thatincorporates two or more elementsthat integrate signal informationreceived from the spatially separateelement locations and transmit in acoordinated manner (eithersimultaneously or alternately).

Radio Frequency (RF)Interference

Basically, undesired signals from thestandpoint of any particular listener.Those signals that "miss" theirdesired user become interferenceenergy to users in the same oradjacent cells.

Rayleigh Fading (also called Fast Fading) A multipathcondition affecting users in motion

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whereby the user receives a signalboth directly and reflected offstructures or landscape features. Thereflected signals will have traveleddifferent distances, and the combinedsignals at the mobile can add andsubtract out of phase, causing thesignal to momentarily fade.

Re-use/FrequencyRe-use

The utilization of frequency(channels) more than once in awireless network. Equated primarilywith the basic cellular grid design,where each cell uses each channelonce within its boundaries and isinsulated from other cells using thatfrequency to allow for anticipatedinterference. Due to the shortcomingsof conventional transmissiontechniques, frequency re-use inadjacent cells has been largelyimplausible until the recentdevelopment of spatial processingtechnology, which can enablesame-cell frequency re-use.

Selective Reception A characteristic of spatial processingthat monitors incoming signals anddistinguishes between desirableinformation and interference. Byfiltering out interfering signals andappropriately combining thereception from all the antennas in thearray, this approach providessignificant improvement in signalquality.

Spatial Diversity An antenna configuration of two ormore elements that are physicallyspaced (spatially diverse) to combatsignal fading and improve signalquality. The desired spacing dependson the degree of multipath anglespread.

Spatial Division MultipleAccess

A complement (not an alternative) toCDMA and TDMA, this technologyincreases the number of users thatcan access an existing wireless phone

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system by exploiting the spatialcharacteristics of the channel itselfthrough highly developedimplementation of an intelligentantenna system's capabilities forreceiving and transmitting.

Subscriber Unit The fixed, typically wall-mountedequipment used by the subscriber in awireless local loop system to send andreceive messages. A standardtelephone is attached to it by wire tocomplete the connection to the user.

Time Division Duplex(TDD)

The method of multiplexingtransmit/receive (uplink/downlink)parts of a wireless communicationslink together. The exchange of uplinkand downlink information takes placeon the same frequency, but isdistinguished by time-slotcharacteristics. (See frequencydivision duplex.)

Time Division MultipleAccess

(TDMA) A method used in wirelesstechnology to separate multipleconversations over set frequenciesand bandwidth. It is used to allocate adiscrete amount of frequencybandwidth to each user. To permitmany simultaneous conversations,each caller is assigned a specific timeslot for transmission.

Uplink Refers to the return signal from theuser to the base station (alternatelyfrom the base station to the centraloffice).

Wireless Local Loop (WLL) In conventional wiredsystems, the local loop refers to theconnection that runs from thesubscriber's telephone set or PBX ortelephone system to the telephonecompany's central office. As the nameimplies, a WLL connects potentialusers to the central office bysubstituting a wireless, basestation-handset component for thelocal loop connection. WLL service is

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the most advantageous alternative forparts of the world that can leapfrogexpensive and time-consuming wireinstallations in establishing moderntelecommunications systems.

10. Products, Services, and Hot Link

ArrayComm Products and Technologies

IntelliCell Processing ArrayComm's unique, patented approach to wireless access enables operators

of PCS, cellular, and wireless local loop networks to realize unparalleledincreases in signal quality, capacity, and coverage by automatically adapting

to the complexities of the environment and continuously enhancing theusers' signals in real time. Specifically, the technology extends base station

range, increases system capacity, improves signal quality, and providesgreater network flexibility. IntelliCell processing forms the basis of the

company's product line.

IntelliWave WLL System ArrayComm's wireless local loop product is a complete system solution that

offers high-capacity, high-quality PSTN access at a fraction of the cost oftoday's wireless and wireline technologies. The compact IntelliWave WLLsystem consists of: base station(s) equipped with the IntelliCell processor;

low-cost subscriber units that interface to a standard PSTN or PBX through aflexible E1-based network interface unit; and a comprehensive element

manager.

IntelliCell-enhanced Base Station Modems ArrayComm's IntelliCell processor is serving as the basis for a new

generation of base stations serving most of the personal handyphone system(PHS) PCS network in Japan. The PHS standard has been adopted or

implemented in more than 14 countries and is now undergoing extensivefield tests in many other countries across the globe.

Software Services and Licensing The company's innovative wireless system solutions can be incorporated intotelecommunications systems that span the full range of wireless standards,

including TDMA, CDMA, PCS-1900, AMPS, DECT/GSM, and PHS.

Spatial Channels

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ArrayComm has pioneered Spatial Channels, which allows many users tosimultaneously occupy the same conventional traffic channel in the same or

adjoining cell. The Spatial Channels feature dramatically enhances thecapacity of today's air interface standards to meet the burgeoning demand for

wireless local loop service worldwide.

Contacting ArrayComm

Please visit ArrayComm's Web site at http://www.arraycomm.com forinformation on leading-edge adaptive telecommunications products and

services.

For additional information, contact our global headquarters at:

ArrayComm, Inc.3141 Zanker Road

San Jose, CA 95134phone (408) 428-9080

fax (408) 428-9083e-mail: [email protected]

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