review of smart antennas
DESCRIPTION
Review of Smart AntennasTRANSCRIPT
2
Introduction
On overview of the smart antenna technology will be presented, and the different waysin which it is foreseen to influence mobile communication systems.
Simple definitions for sharing the radio channel.
A broad definition of smart antennas shall be introduced.
The basic principle and level of intelligence.
Improvements and benefits/ drawbacks.
Implementation Transmitter/ Receiver.
Conclusions.
3
The cellular radio concept
In cellular systems the radio communication isbetween the user and a base station, which providesa radio coverage within a certain area called cell.Capacity in such a system can be defined as the totalbit rate per unit bandwidth per unit area, bit/s/Hz/
Because the available frequency band islimited, the capacity is given by the celldensity. Techniques for increasing the capacityin cellular systems include using smaller cells,so called microcells, and frequency hopping, atechniques that disperse interferences andaverages the fading rate.
Fig. 1: The uplink/down link trafficchannels in a mobile communicationsystem
2m
4
Evolution of wireless communications
Traditionally, users communicating via the same basestation have been separated by either one of the multipleaccess schemes:
Frequency Division Multiple Access (FDTD), in whichdifferent channels are assigned to different frequencybands (users).
Time Division Multiple Access (TDMA), where eachchannel occupies a cyclically repeating time slot.
Code Division Multiple Access (CDMA), in whicheach channel is assigned a unique signature sequencecode.
A system could also combine all three of these multipleaccess strategies.
Fig. 2: Three multiple access schemesused in mobile communication.
5
What is a smart antenna?
Its an array of antennas which is able to changeits antenna pattern dynamically to adjust tonoise, interference, and multipath. They canadjust their pattern to track a portable user.Smart antennas are used to enhance receivedsignals and may also be used to form beams fortransmission.
The difference between the fixed and the smartantenna concept is shown in fig. 3. Smartantennas will lead to a much more efficient useof the power and spectrum, increasing the usefulreceived power as well as reducing theinterference. Fig. 3: Illustration of the difference
between a traditional base stationradiation pattern and a smart antennabase station.
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Basic principles
The term “antenna” has an extendedmeaning to it than the usual understanding. Itconsists of a number of radiating elements, acombining/dividing network and a controlunit. The control unit can be called the smartantennas intelligence,normally realized usingdigital signal processing (DSP). Theprocessor controls feeder parameters of theantenna, based on several inputs based, inorder to optimize the communication link.
Different optimization criteria can be used.
Fig. 4: Principle of a smart antenna
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Levels of intelligence
Based on the previous definition, thelevel of intelligence can be defined:
Switched lobe (SL): This is alsocalled switched beam. It contains onlya basic switching function betweenseparate directive antennas orpredefined beams of an array. Thesetting that gives the best performance,usually in terms of power, is chosen.
Dynamically phased array (PA): Byincluding a direction of arrival (DoA)algorithm for the signal received fromthe user.
Adaptive array (AA): In this case, aDoA algorithm for determining thedirection towards interference sourcesis added. The radiation pattern can thenbe adjusted to null out the interference. Fig. 5: Different smart antenna concepts
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Improvements and benefits
Capacity Increase: the principle reason for the growing interest in smart antennas is thecapacity increase. In densely populated areas mobile systems are normally interference-limited,meaning that the interference from other users is the main source of noise in the system. Thismeans that the signal to interference ration, SIR, is much larger than the signal to thermal noiseratio, SNR. Smart antennas will on average, by simultaneously increasing the useful receivedsignal level and lowering the interference level, increase the SIR.
Range increase: in rural and sparsely populated areas radio coverage rather than capacity willgive the premises for base station deployment. Because the smart antenna will be more directivethan traditional antennas, a range increase potential is available. This means that base stations canbe placed further apart. Potentially leading to a more cost efficient development.
Security: it is more difficult to tap a connection when smart antennas are used. To successfullytap a connection, the intruder must be positioned in the same direction as the user as seen from thebase station.
Reduced multipath propagation: by using a narrow antenna beam at the base station, themultipath propagation can be somewhat reduced. The actual reduction depends on the scenario, andit is not always significant.
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Drawbacks and cost factors
Transceiver complexity: it is obvious that a smart antenna’s transceiver is much morecomplex than a traditional base station transceiver. The antenna will need separate transceiverchains for each array antenna elements and accurate real-time calibration of each of them. Inaddition, the antenna beam forming is a computationally intensive process. This means thatsmart antenna base station must include very powerful numeric processors and controlsystems. There will be a growing need for real-time optimizing, and signal tracking.
Resources management: smart antennas are mainly a radio technology, but they will also putnew demands on network functions such as resources and management.
Physical size: for the smart antenna to obtain a reasonable gain, and array antenna element isnecessary. Typical arrays consisting of 6 to 10 horizontally separated elements have beensuggested for outdoor mobile environment. The necessary element spacing is 0.4-0.5 wavelengths. This means that an eight-element antenna would be approximately 1.2 m wide at 900MHz. And 60 cm at 2 GHz. With the growing public demand for less visible base stations, thiscould be a problem.
Fig. 6: Picture of an 8-element array antenna at 1.8GHz. (Antenna property of Telia Research AB,Sweden).
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Implementation
The technology is based on array antennaswhere the radiation pattern is altered byadjusting the amplitude and relative phaseon the different array elements.
Electronically steerable antenna patternsare most often generated using arrayantennas. Theses are antennas consistingof a number of antenna elements on whichthe signal is divided/combined in bothphase and amplitude.
Arrays can be one-, two-, and three-dimensional depending on the dimensionof the space one wants to access.
The total radiation pattern is given by theelement type, the relative positions andthe excitation (amplitude and phase).
Fig. 7: Different array geometries forsmart antennas: a) uniform linear array; b)circular array; c) 2 dimensional gridarray; d) 3 dimensional grid array.
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are the spherical coordinates of the observation point
Linear equidistance array:The simplest geometry is the one-dimensional linearequidistance array. All the array elements are placed along aline with equal distance between them.
The fare field expression of the electrical filed from a singlearray element is given by
Fig. 8: Geometry of theequidistant linear array.
(1)
where: In is the complex element current,
( , )e θ ϕ is the element antenna pattern, usually called element factor.
The total field from an array of N elements is the superposition of the elements contributions;
' cos cosnnr ndψ θ=where:
is the difference in distance for element n compared to element 0 towards the observation point inthe direction θ due to spatial separation in the array.
(2)
(3)
, , andr θ ϕ
`1
cos
0
( , , ) ( , )n n
jkrNjkr
nn
n
eE r I e e
rψθ ϕ θ ϕ
−−
=
=∑
( , , ) ( , )jkr
nne
E r I er
θ ϕ θ ϕ−
=
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d: is the element spacing
: is the distance between the coordinate systems origin and element n
In the case of an array of equal elements, the fare field expression becomes
The sum expression in equation (4) is the array factor which is independent of both the elementtype and the distance from the antenna. It is only dependent on the array geometry and elementexcitation. Thus, the total field is given by the product of the element factor and the array factor.
Example:
Consider that it is desired to steer the main lobe of the
antenna in a certain direction thus we want the fare field
signals from all the elements of the array to be added in phase
in the wanted direction. If element number 0 is given the
reference phase angle 0, then the phase angle of element
number 1 must be shifted (in this case negatively)with a
value of , where is the wave constant.
'nr
(4)
Fig. 9: Using the phase angle of each
element to steer the lobe
ϕ
cosknd θ 2k πλ=
1cos
0
( , , ) ( , )jkr N
jkndn n
n
eE r e I e
rθθ ϕ θ ϕ
− −
=
= ⋅ ∑
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In order to obtain maximum gain in the direction of , the amplitudes of all the elements mustbe equal. This example is called uniform excitation
The resulting array factor is shown in the figure 10.
A drawback of the linear array is the
front-back ambiguity. A mirror of the antenna
pattern will appear around the antenna axis,
i.e., in the direction
In practice, the linear array is only useful in
a sector of at most
ϕ
Fig. 10: The resulting array factor with uniformexcitation
0 1 1(| | | | ... | | ... | |)n NI I I I −= = = = =
, (cos cos( ))ϕ ϕ ϕ− = −
120
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Using a non-uniform excitation with an amplitude roll off towards the end of the arrayresults in both a wider main lobe and a lower side lobe level (SLL).
Fig. 11: Examples of non-uniform excitation and resulting far-field
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Receiver
Figure 12 shows schematically the elements ofthe reception part of a smart antenna:
The antenna array consists of N elements. TheN signals are combined into one signal, which isfed to the rest of the receiver (channel decoding,etc…)
The smart antenna reception part consists offour units. In addition to the antenna its self itcontains a radio unit, a lobe forming unit, and asignal processing unit (SPU).
The radio unit consists of down-conversionchains and complex A/D converters. There mustbe N down conversion chains, one for each arrayelement.
The SPU will, based on the received signal,calculate the complex weights w1-wN with whichthe received signal from each of the arrayelements is multiplied. These weights will decidethe antenna pattern in the uplink direction. Fig. 12: Reception part of a smart antenna
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The method for calculating the weights willdiffer depending on the type of optimizationcriteria used : maximization of the receivedsignal from the desired user (e.g., switchedlobe SL or phased array PA) or maximizationof the SIR by suppressing the signal frominterference sources (adaptive array AA).
The actual weighting of the receivedsignal from each of the array elements isdone in the lobe forming unit.
the three mentioned methods (SL, PA, andAA) differ in complexity. Generally, SL isless complex than PA, which in turn is lesscomplex than AA. Conditions that influencethis are, for example, the required accuracyand performance in terms of power or gain. Itis also expected that the radio access methodmakes a difference. Fig. 13: Resulting antenna patterns for
SL, PA, and AA for measured signaland interference channels. The figurealso shows a so-called fixed lobe (FL)conventional antenna pattern forcomparison
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Transmitter
The transmission part of the smart antenna willbe schematically very similar the the receptionpart.
the signal is split into N branches, which areweighted by the complex weights z1-zN in thelobe forming unit.
the weights decide the radiation pattern in thedownlink direction, are calculated by the SPU.
the radio unit consists of D/A conversers andthe up converter chain.
the most frequently beamforming techniqueused is the geometrical approach of estimatingthe Direction-of-Arrival (DoA). The assumptionis directional reciprocity, i.e., the direction fromwhich the signal arrived on the uplink is thedirection in which the signal should betransmitted to reach the user on downlink. Thisdirection is used on the downlink by choosingthe weights z1-zN so that the radiation pattern isa lobe directed toward the desired user. Fig. 14: Transmission part of a smart antenna
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Conclusions
An overview of smart antenna technologies with respect to mobile systems has beenpresented.
It is obvious that the mart antennas at the base station will be an important technology toprovide the necessary capacity and coverage. It also helps realize new services, e.g.,based on user location.
Beamforming are critical to enable multiple users to transmit simultaneously in afrequency band. Research showed that the beamforming algorithms were able tominimize the interference that a terminal experiences from other simultaneouslytransmitting users.
The techniques for beamforming with array antennas are well known, and must beemployed in both duplex directions for the improvements to be substantial. However,with the rapid channel variations it is not a trivial task to provide optimum beamformingespecially for the downlink direction.
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References
[1] Joseph C. Liberti, JR. Theodore S. Rappaport, Smart Antennas for WirelessCommunications, Prentice Hall PTR, 1999.
[2] Garret T. Okamoto, Smart Antenna Systems and Wireless LANs, Kluwer academicpublishers, 1999.
[3] John G. Proakis, Digital Communications, McGraw-Hill Inc., 1989.
[4] Per H. Lehne and Magne Petter, Telenor Research and development “ An Overview ofSmart Antenna Technology for Mobile Communications Systems,” IEEECommunication Surveys, vol. 2, no. 4, fourth quarter 1999.