Advanced Antenna WiMAX 071129

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Advanced Antenna Systemsfor WiMAX


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A number of different antennasolutions can be applied to improvethe radio performance specificallycoverage and capacity. The IEEE802.16e-2005 standard and WiMAXForum Certification Wave 2 stronglysupport advanced antenna systems,including receive diversity, MultipleInput Multiple Output (MIMO),transmit diversity and InterferenceRejection Combining (IRC).Those antenna features can beimplemented with a compact cross-

polarized antenna with a width ofless than 15 centimeters.

The capacity can be later enhancedwith soft capacity features, includingadvanced packet scheduling anddynamic multicarrier operation.Dynamic multicarrier refers to thedeployment of a second 10 MHzcarrier per sector while sharing acommon power amplifier. The softcapacity features can be upgradedremotely with software. The phototo the right shows a typical simple,3-sector site configuration, including

the system module and RadioFrequency (RF) head.

Adaptive beamforming with a linearantenna array is a promising futuretechnology for enhancing the radioperformance with four and eightantennas per sector. Further workis required in the WiMAX Forum toenable full support of adaptivebeamforming. Before this work iscompleted, full adaptive beamforming

benefits cannot be enjoyed due tothe lack of terminal support. Also thesignificantly larger and more complexantenna structures related to adaptivebeamforming may easily lead to highcost increases in site constructionand operation. This cost increase isoff-setting potential benefits thetechnology promises to deliver.

Contents

02 1. Introduction

03 2. Advanced Antenna SystemTechnologies

05 3. Advanced Antenna Systemsin WiMAX

08 4. Pros and Cons of AdvancedAntennas

09 5. Other WiMAX CapacityEnhancements

10 6. Conclusions and PreferredSolutions for WiMAX Operators

11 7. Abbreviations and Terminology

11 8. References

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Typically, coverage is the numberone challenge in the first deploymentphase, while capacity will be thechallenge later when the amount oftraffic in the networks increases.

The operators capital expenditure(capex) and operating expenditure(opex) tend to be related to thenumber of base station sites.Enhancing the coverage and capacityperformance of each base stationsite would be attractive in terms of

reducing costs.

ExecutiveSummary

1. Introduction

This paper presents the advancedantenna systems for enhancing theWiMAX radio performance. Thetheoretical gains, the practicallimitations as well as the businessaspects are considered. Otherperformance enhancement solutions

are also presented. An operatorspreferred coverage and capacityevolution is introduced.


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2. Advanced AntennaSystem Technologies


Undesiredinterference

Adaptive signal processing

Desiredsignal

W2

W1

x

x

+

Figure 1: Beamforming with Interference Rejection Combining (IRC)

The wireless research community hasactively studied advanced antennasfor a number of years. A summary canbe found in [1]. The main antennasolutions are summarized below:

Receiver diversity, possibly

combined with optimum combiningcalled Interference RejectionCombining (IRC) [2]

Transmit diversity with or withoutfeedback

Multiple Input Multiple Output(MIMO) multistream transmissionwith or without feedback

Adaptive beamforming with lineararray antenna

The term beamforming can also beapplied to the IRC solution, to thetransmit diversity with feedback andto MIMO with feedback. The reason

is that the signal processing isessentially generating a beam byoptimizing the received signal-to-noiseratio (see Figure 1).

The receive and transmit diversityand 2x2 MIMO can utilize a normalcross-polarized antenna. The adaptivebeamforming uses linear arrayantennas with columns spaced athalf a wave length from each other.The practical antenna configurationat 2.5 GHz is illustrated in Figure 2for a three-sector case. The antennaheight is typically 1.3 meter. In the

case of a 3.5 GHz deployment, theantenna sizes are smaller.

Receiver Diversity and OptimalCombiningBase station receiver diversity iswidely used in all 2nd and 3rdgeneration cellular mobile systemsincluding GSM, UMTS and CDMAusing cross-polarized antennas.The combining solution can beMaximum Ratio Combining (MRC)or Interference Rejection Combining(IRC). MRC is the optimal solutionwhen the interference is spatially

white Gaussian, while IRC is optimalin case of dominant interferers.

Cross-polarized antenna

< 15 cm

Linear array antenna with 8 columns

50 cm

Figure 2: Typical 3-sector antenna configuration at 2.5 GHz

Receiver diversity is also applied insome of the terminals where the sizeand the power consumption allow animplementation with dual antennas,

for example in integrated laptopsolutions and PCMCIA cards.

Transmit diversity open andclosed loopBase station transmit diversity canbe utilized to enhance the downlinkcoverage and capacity. The transmitdiversity should preferably besupported by a standard to get thefull benefit. The transmit diversitycan be based on an open loop orclosed loop approach. The closedloop approach uses feedback fromthe terminal to optimize the downlink

transmission to that terminal. Theclosed loop approach is also calledadaptive beamforming.

The transmit diversity can utilize thesame cross-polarized antenna that isused for receive diversity. Therefore,the transmit diversity is an easy

solution from the site solution pointof view.

Multiple Input Multiple Output(MIMO) open and closed loop2x2 MIMO uses two transmitantennas at the base station and tworeceive antennas at the terminals.Closed loop MIMO transmission canalso utilize feedback from the terminalto create effective downlink beams.


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Wimax Forum system profile definesthe following MIMO modes:

Space time coding = Matrix A =Diversity transmission

Spatial multiplexing = Matrix B =Dual stream transmission

Spatial multiplexing transmits twoparallel data streams to double thedata rate. Spatial multiplexing is alsoknown as MIMO Matrix B in WiMAXspecifications. If the channel qualityis not good enough to maintainthe dual stream approach, thetransmission can be switched toMatrix Adiversity mode. The transmitdiversity mode is based on spacetime coding. The aim is to make thesignal more robust against fadingand interference, and to increase theeffective data rate by using highermodulation and coding schemes.

The utilization of MIMO modes isillustrated in Figure 4. Similar MIMOtechnology is utilized also in 3GPPHSPARelease 7 and Long TermEvolution (LTE) Release 8.

Downlink MIMO can utilize the samecross-polarized antenna that is usedfor receive diversity. Therefore, MIMOis an easy solution from the sitesolution point of view.

Throughput

SNR

Spatial multiplexing

mode (Matrix B)

Diversity mode(Matrix A)

Figure 4: Utilization of MIMO modes

Space timeencoder

+

Base station

+

Space timeencoder

Terminal

Figure 3: Principle of MIMO transmission

RF calibration

Figure 5: Adaptive beamforming

MIMO in the uplink faces the problemthat two power amplifiers would beneeded in the terminal, increasingthe cost, size and power consumptionof the devices. Therefore, similarMIMO is not defined for uplink as fordownlink. The uplink MIMO solutionis called Collaborative MIMO instead.Collaborative MIMO transmissionconfigures the transmissions fromtwo terminals so that their signalstogether utilize the MIMO scheme toincrease the cell peak rate and the cell

throughput, but it does not increasethe single user peak data rate.Collaborative MIMO works with asingle power amplifier in the terminal.

Adaptive beamformingAdaptive beamforming has beena promising option for boosting theradio coverage and capacity in theresearch community for all radiosystems. Adaptive beamforming isdefined here as the transmissionfrom a linear array antenna. Weightsassigned to the signal passingthrough each antenna element makeit possible to point the resultantsignal into a narrow beam towardsthe terminal. The downlink beam

direction is determined based on theuplink received signal using direction-of-arrival estimates. Beamformingrequires continuous calibration of theantenna system and RF chains ofthe base station because the signalprocessing unit must know the phaserotation caused by the receive andtransmit paths. Adaptive beamformingis illustrated in Figure 5.


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The alternative solution is the so-called Uplink sounding zone, wherethe WiMAX terminal sends a knownsignal covering the whole bandwidth.That signal can be applied for thedownlink beamforming.

The ideal capacity gain from adaptivebeamforming with N antennas is Ntimes capacity. Four antennas ideallyshould give 300% capacity gain oversingle antenna. The capacity gainswith the realistic assumptions tendto be 100-200%; see for example[3]. That capacity gain still representsa significant boost in the networkcapacity. For more details ofperformance gains of differentsolutions please refer to Table 4.


In a Time Division Duplex (TDD)based system, the uplink anddownlink use the same spectrum.Mobile WiMAX uses the TDD principle.Ideally, the received uplink signalcould be used to achieve optimaldownlink beamforming in TDD.In practice, however, the signal inOrthogonal Frequency DivisionMultiple Access (OFDMA) basedsystems, like in WiMAX, uses just apart of the total bandwidth and theuplink signal cannot be directly usedto control downlink beamforming.

MAP (Media Access Protocol)transmit diversity can be appliedwith Wave 1 devices using delaydiversity. MAP is the broadcastmessage from the base station togive the capacity allocations forthe terminals in uplink and indownlink. It can be a determiningfactor for the practical cell rangeas all devices have to receive theMAP message in order to stayconnected.

IO-BF is a group of inter-operableoptional features related to

Beamforming (BF) operation.Inter-operable Beamforming(IO-BF) in general is supportedin Wave 2. WiMAX equipmentcan, however, be certified withoutverifying the operation of thefeature. The support includes:- Dedicated pilot symbols, which

allow user-specific beamforming.- Uplink sounding. The base station

can request the terminal to senda sounding signal over the wholebandwidth. The base station canutilize the received signal for thedownlink beamforming in a TDD-

based system.

3.1 WiMAX Forum SupportThis section considers the usageof advanced antenna systems inWiMAX. IEEE 802.16 has createdthe specifications [4] while WiMAXForum has defined the systemprofiles [5] containing a subset offeatures to be used in the inter-operability testing and in the realimplementations. Generally, thenetwork deployments can utilize onlythose features that are defined inWiMAX Forum profiles. A few of thekey features for the use of advanced

antennas in WiMAX are listed below:

Receive diversity does not requiredirect support from standards.Some of the WiMAX Forumperformance requirements doinclude two receive antennas [6].

Downlink 2x2 open loop MIMOand uplink collaborative MIMO aredefined in WiMAX Forum Wave 2as mandatory features of theterminals. Closed loop MIMO isnot part of Wave 2. IEEE802.16-2005 includes MIMO up to fourstreams, 4x4 MIMO.

3. Advanced AntennaSystems in WiMAX


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Private MAP and Diversity MAPScan allow a user-specific MAPwhich helps to increase thecoverage of the MAP messages byapplying beamforming technology.Private MAP or Diversity MAPScan are not supported in WiMAXForum profiles.

SDMA (Space Division MultipleAccess) allows reusing of thesame resources several times inthe sector. SDMA is required totake the full capacity benefit frombeamforming. SDMA is notsupported in WiMAX Forum profiles.

The support of the key features inWiMAX Forum profiles is summarizedin Table 1. Wave 1 has beencompleted during 2007 while Wave2 certification is expected to startaround mid-2008.

In short, Wave 1 provides a minimumset of features without any supportfor advanced antennas. Receivediversity as well as MAP transmitdiversity can be used with Wave 1terminals since those features donot require terminal support. Wave 2brings support for open loop MIMOin downlink and collaborative MIMOin uplink. Wave 2 contains a set offeatures which enable beamforming,but a number of essentialbeamforming features are notincluded.

The mixture of Wave 1, Wave 2 andterminals based on future WiMAXForum releases in the network willcreate some challenges for thenetwork operation when the newadvanced antenna features areintroduced. Different zones need tobe reserved in the frame for thedifferent terminal capabilities.

WiMAX Forum is currently planning tosplit Wave 2 into two phases to bringsome of the key Wave 2 features tothe market as early as possible.

3.2 MAP CoverageCoverage extension is the first targetwhen deploying a new radio network.Large adaptive arrays combined withadaptive beamforming can providehigh coverage gains for user data.The coverage gains may be limitedby the coverage of the commonchannels. MAP (Media Access

Protocol) messages must be receivedby all terminals and the beamforminggain cannot be applied for MAPunless user-specific MAP is availablein the system (see Section 3.1). TheMAP coverage issue is illustrated inFigure 6.

Table 2 illustrates the link budgetsfor downlink MAP and uplink userplane data with 8 W base stationpower and Table 3 with 20 W basestation power. Uplink data requiresa signal-to-noise ratio of 3.5 dB with

fast layer 1 retransmissions, whiledownlink MAP requires a signal-to-noise ratio of 6.0 dB since noretransmissions can be applied forthe common channels. MAP repetitionof two is assumed. MAP uses QPSKwith 1/2 rate coding. The uplink directionuses full sub-channelization gain toboost the coverage. With an 8-Wbase station (2x 4W per MIMO branch)the downlink MAP coverage is 4 dBweaker than the uplink coverage.With a 20-W base station (2x 10Wper MIMO branch) the downlink MAPcoverage is balanced with uplink data.

If we utilize beamforming, the uplinkdata coverage is boosted while thedownlink MAP coverage is notimproved.

Table 1: Support of key features in WiMAX Forum profiles

Wave 1 Wave 2

Receive diversity (1) (1)

Downlink MIMO open loop - Yes

Downlink MIMO closed loop - -

Uplink collaborative MIMO - Yes

Downlink MIMO with AMC - -

MAP transmit diversity (1) (1)

Beamforming - Yes

Private MAP and diversity MAP scan - -

Space Division Multiple Access (SDMA) - -

(1) No need for standard support

MAP coverage

capacity gain

not useablecoverageextension

Figure 6: Beamforming improves user datacoverage, but not MAP coverage


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Table 2: Link budgets for downlink MAP and uplink user data with 2x4 W base station power

Table 3: Link budgets for downlink MAP and uplink user data with 2x10 W base station power

Downlink MAP Uplink data Formula

Transmitter

a Transmit power [W] 8.0 0.20(2x4 per MIMO branch)

b Cable, combiner, filter, body loss [dB] 1 0

c Antenna gain [dBi] 18 0

e Transmit EIRP [dBm] 56 23 = 10*log10(a)-b+c+d

Receiver

f Carrier bandwidth [MHz] 10 10

g MAP repetition 2

h MAP boosting (1=no boosting) 1

i Occuped bandwidth [MHz] 9.2 9.2 =9.2/h

j Thermal noise floor [dBm] -104 -104 = -174+10*log10(i)+60

k Receiver noise figure [dB] 7 3

l Required SNR including rx diversity 6.00 3.50 From simulations

m Receiver sensitivity [dBm] -94.4 -97.9 =j+k+l-10*log10(g)

n Receiver antenna gain [dBi] 0 18

o Cable, combiner, filter, body loss [dB] 0 0

p Max. number of subchannels 30 35

q Number of subchannels used 10 1

r Subchannelization gain [dB] 15.44 =10*log10(p/q)

s Link budget [dB] 150.4 154.3 =e-m+n-o+r

Downlink MAP Uplink data Formula

Transmitter

a Transmit power [W] 20.0 0.20(2x10 per MIMO branch)

b Cable, combiner, filter, body loss [dB] 1 0

c Antenna gain [dBi] 18 0

e Transmit EIRP [dBm] 60 23 = 10*log10(a)-b+c+d

Receiver

f Carrier bandwidth [MHz] 10 10

g MAP repetition 2h MAP boosting (1=no boosting) 1

i Occuped bandwidth [MHz] 9.2 9.2 =9.2/h

j Thermal noise floor [dBm] -104 -104 = -174+10*log10(i)+60

k Receiver noise figure [dB] 7 3

l Required SNR including rx diversity 6.00 3.50 From simulations

m Receiver sensitivity [dBm] -94.4 -97.9 =j+k+l-10*log10(g)

n Receiver antenna gain [dBi] 0 18

o Cable, combiner, filter, body loss [dB] 0 0

p Max. number of subchannels 30 35

q Number of subchannels used 10 1

r Subchannelization gain [dB] 15.44 =10*log10(p/q)

s Link budget [dB] 154.4 154.3 =e-m+n-o+r



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Antenna arrays are complex andheavy constructions, whichincreases the cost of the site. Moreexpensive antennas, more robustmast construction to carry largerweights, greater expenditure oninstallation material and cabling, etc.all result in roughly doubling the sitepreparation costs compared to sites

where a standard cross-polarizedantenna is used.

Antenna arrays are more complexto install and require more laboriousRF optimization. While the wholeinstallation of a Nokia Siemens Flexisite typically takes up to 4 hoursfor a single engineer, installing asite with antenna array structureis easily one full day for twoengineers.

Antenna arrays not only weigh a lotmore than standard cross-polarizedantennas, but are also large insize, thus increasing the space

they occupy per site. This typicallyincreases site rental fees.


The optimal usage of the adaptiveantenna solutions is discussed in thissection. The receive and transmitdiversity and MIMO options can berealized with compact antennas. Theinteroperability is guaranteed withWiMAX Forum Wave 2 support.Receive diversity can be expandedto four branches for boosting the

uplink data coverage. The adaptivebeamforming with large linear arrayantennas offers further capacitygains beyond 2x2 MIMO. The fullbenefit of the beamforming requiresfurther work in WiMAX Forum profilesto avoid proprietary implementations.The large antenna size and weight,combined with the wind load, mayprevent the usage of large linear arraysin some sites. Adaptive antennashave future potential and scale wellbeyond two and four branches, butlack today the full support in WiMAXForum and have challenges with site

solutions. In general, the followingimpacts of beamforming for the sitepreparation need to be considered:

The MAP coverage can be furtherimproved with MAP repetition andMAP boosting (concentration). MAPrepetition and boosting provideessentially trade-offs betweencoverage and capacity: if we repeatMAP symbols, then there is lessroom for data symbols. The principleof MAP repetition is illustrated inFigure 7. MAP is assumed to betwo symbols in this example withoutrepetition. If there is a large numberof low bit rate connections, like VoIP,the MAP overhead is clearly higher.Therefore, MAP boosting and MAPrepetition solutions with VoIPapplications will greatly limit themaximum capacity.

Some regulators and localcommunities restrict the size ofantennas because of environmentalconcerns.

In summary, while adaptive downlinkbeamforming offers interesting toolsto improve mobile WiMAX radionetwork performance, further work

in technology and standardizationis required before any benefit canmaterialize. Also the cost at whichthese potential future benefits can beenjoyed should be carefully analyzedin each case first. According to NokiaSiemens Networks calculations,downlink adaptive beamforming doesnot result in a lower site count, butsuggests a significant increase in siteinvestments: up to twice as expensivesite construction compared to standardcross-polarized antenna sites.

Additionally, the more complex sitestructure will lead to a clear increase

(15-20%) in network opex, which inmany cases further reduces theattractiveness of this solution.

4. Pros and Consof Advanced Antennas

MAP repetition-2

No MAPrepetition

MAP repetition-2andMAP boosting

= preamble= MAP

= downlink data= uplink data

Figure 7: Principle of MAP repetition and MAP boosting


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Transmit on those subcarriers that are not fadedFrequency

Carrier bandwidth

Resource block

Figure 8: Frequency domain scheduling

Table 4: Characteristics of different advanced antenna solutions

5.1 Frequency Selective SchedulingThe idea in the frequency selectivescheduling is to send the data onthose subcarriers that are not fadedand to avoid transmission on thefaded subcarriers (see Figure 8).The frequency selective schedulingcan be achieved in WiMAX by usingAdjacent MultiCarrier (AMC) allocation.

Frequency selective scheduling isshown to enhance system capacityby up to 30% compared to PUSC(Partial Use of SubCarriers). AMC isdefined in Wave 1. The combinationof MIMO and AMC is not a mandatoryrequirement for the terminals.

5.2 Multicarrier Capacity ExtensionThe capacity can be expanded withmultiple frequencies. The secondfrequency can be introduced withwideband power amplifiers where thepower can be shared between multiplefrequencies. The capacity can be

upgraded simply by remote softwareupgrade. Multilayer radio resourcecontrol can balance the loadingbetween the carriers to provideoptimized utilization of radio resources.

The multicarrier capacity upgradabilitybecomes important with beamforming.If the power amplifiers support just asingle frequency, the capacity upgraderequires the installation of a second

set of power amplifiers. If the poweramplifiers are integrated with thebeamforming antennas up in themast, the upgrading implies costlyantenna modification.

5. Other WiMAX Capacity

Enhancements


Receive/transmit diversity MIMO Adaptive beamforming

Antenna Cross-polarized Cross-polarized Linear array

Number of receive branches 2-4 2-4 4-8

Number of transmit branches 2 2-4 4-8

WiMAX Forum support Wave 1 Wave 2 Partly in Wave 2

Peak data rate gain1 - 2-4 times higher -peak data rate

Coverage gain1,3 - - Yes, in uplink2

No, in downlink

Capacity gain1 - Considerable Considerable(Some gain over MIMO)

Site capex excluding BTS Modest Modest High

Site opex excluding BTS Modest Modest High

1 The reference case is receive/transmit diversity2 The coverage gain is obtained by having more receiver branches3 The coverage gain assumes the same total combined transmit power


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The WiMAX deployment should takeadvantage of the advanced antennasolutions, including downlink 2x2MIMO, uplink Interference RejectionCombining (IRC) reception, uplinkcollaborative MIMO and MAPtransmit diversity. All these solutionscan be implemented with singlecross-polarized antennas per sector,and they have support in WiMAXForum Wave 2.

The capacity can be later enhancedwith soft capacity features, includingfrequency selective scheduling anddynamic multicarrier operation. Thecapacity can be enhanced by usinganother 10 MHz carrier per sector.It is possible to utilize the existingpower amplifier to share the powerbetween two carriers in an optimizedway (see Figure 9). The soft capacityfeatures can be upgraded remotelywith software.

The preferred performance featuresfor the WiMAX operators areillustrated in Figure 10. Whendesigning the initial coverage, the 2x2MIMO and MAP transmit diversitycan be applied to boost the downlinkcoverage together with high power RFmodule. The uplink coverage can beenhanced with subchannelizationgain and with low noise figure in thebase station. In the case of fixedwireless terminals, the network

coverage can be improved by usinghigh gain directional antennas in theterminals. The fixed antennas canbe located by the windows or on therooftop for maximized coverage.

The system capacity can be laterboosted with advanced scheduling,Collaborative MIMO and dynamicmulticarrieroperation. All theseevolution steps can be implementedwith remote software downloads.


6. Conclusions andPreferred Solutions

for WiMAX Operators

Frequency 1

Frequency 2

Figure 9: Dynamic power sharing in multicarrier operation

High power RF module

2x2 MIMO

MAP transmit diversity

Frequency-selectivescheduling

Dynamic multicarrieroperation

Excellent receiversensitivity andsubchannelization

Interference rejectioncombining (IRC)

Collaborative MIMO

Frequency-selectivescheduling

Dynamic multicarrieroperation

Downlink UplinkCompact sectorantenna at 2.5 GHz

0.12 m

1.30 m

Figure 10: Evolution path for enhancing WiMAX coverage and capacity


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7. Abbreviationsand Terminology

[1] Hottinen, A., Kuusela, M., Hugl, K., Zhang, J. and Raghothaman, B. Industrial Embrace of Smart Antennas and

MIMO, IEEE Wireless Communications, August 2006.

[2] J. Winters, Optimum combining in digital mobile radio with cochannel interference, IEEE Journal on SelectedAreas in Communications, vol. 2, no. 4, pp. 528539, 1984.

[3] Pedersen, K., Mogensen, P. and Ramiro-Moreno, J. Application and Performance of Downlink BeamformingTechniques in UMTS, IEEE Communications Magazine, October 2003.

[4] http://www.ieee802.org/16/tge

[5] http://www.wimaxforum.org/home/

[6] WiMAX Forum Mobile Radio Conformance Tests Amendment: Wave 2 Tests, 2007-07.

8. References

AAS Adaptive Antenna Systems. General term for a number of antenna solutions in WiMAX,including MIMO and beamforming.

AMC Adjacent Multi-Carrier, referring to the localized usage of the subbands which enablesfrequency domain scheduling.

BF Beamforming.Collaborative MIMO Also known as Virtual MIMO and V-MIMO.DL_MAP Downlink Media Access Protocol. The broadcast message in downlink giving the capacity

allocations for downlink and for uplink.MIMO Multiple-Input Multiple-Output, using a minimum of two base station antennas and two

terminal antennas to enhance the data rate and the transmission robustness.PUSC Partial Usage of Subcarriers, referring to the distributed usage of the subbands which provides

frequency diversity, but does not allow frequency domain scheduling.

RF Radio Frequency.SDMA Space Division Multiple Access, referring to the usage of the same time and frequencyresources multiple times within one sector.

Uplink beamforming Also known as IRC (Interference Rejection Combining).WiMAX Worldwide Interoperability for Microwave Access.


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Advanced Antenna WiMAX 071129