capacity enhancement techniques for wimax networks

8/3/2019 Capacity Enhancement Techniques for WIMAX Networks

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Capacity Enhancement Techniques for WIMAX Networks

By:Imad Memon & Ayaz Ahmad


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Introduction

Initial design

Monitoring & Optimization

Network expansion


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Introduction

Spectrum Allocation

Channel Size selection

Capacity dimensioning

Initial Design

Marketing Inputs

Product specs

Design Requirements

Simulations

Monitoring &

Optimization

Actual Capacity

Factors affecting Capacity

Capacity Improvement techniques

Drive Test

Network

Expansion

Future growth & Way forward

Capacity enhancement techniques

Guard band Selection & WiMAX Forum Recommendation

Spectrum deployment scenarios

Challenges and design considerations


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Spectrum Allocation

Channel size selection



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The frequency spectrum is allocated by theFrequency Allocation Board ( FAB).

An operator is bound to operate within thespectrum bandwidth allocated.

FAB in conjunction with Pakistan

Telecommunication Authority (PTA) ensuresthis.


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Spectrum is scarce and costly resource andneeds to be efficiently utilized.

The spectrum is divided into a number ofchannels.

Adequate Guard band is kept to avoid

interference.


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Based on the spectrum bandwidth, appropriatechannel sizes are selected.

The Wi-MAX forum defines the following channel

sizes (MHz): 3.5

5

7

10

The transmission capacity increases with thechannel size, but increasing channel size has itsown pros and cons.


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DuplexingMode

TDDvsFDD

TDD Configurations

Modulation Schemes

Capacity Calculations


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The duplexing mode is chosen on the basisof spectrum allocated.

There are two implementations of WiMaxstandard, TDD and FDD.


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Due to the symmetric nature of FDD transmission channels, andthe FDD legacy as duplex method of choice for TDM voiceapplications, FDD transmission channels are always of equal size(50% for UL and 50% for DL). In applications such as Internetaccess, which can be very asymmetric in nature, a largepercentage of the available UL bandwidth remains unused and is,therefore, wasted.

A guard band about two times the size of the UL or DL channel isrequired to separate the UL and DL channels. This amounts toan additional 50% loss in spectrum.

Once the channel bandwidth is granted by the regulator, the

UL/DL allocation cannot be changed. This leads to unusedspectrum for asymmetric operations, i.e., for last-mileapplications, where typically the UL traffic is a fraction of the DLtraffic.


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Guard Band Guard Time (FDD)

Hardware cost

Dynamic bandwidth allocation Latency (FDD)

AAS/MIMO


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TDD ratio configurations

75:25

60:40

55:45


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WiMax supports different modulation and

coding schemes for downlink and Uplinkdirection.


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QPSK 1/2 QPSK 3/4

16 QAM 1/2

16 QAM 3/4 64 QAM 2/3

64 QAM 3/4

64 QAM 5/6


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QPSK 1/2 QPSK 3/4

16 QAM 1/2

16 QAM 3/4


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QPSK QPSK Rep 2

QPSK Rep 4

QPSK Rep 6


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Each MCS provides different capacity.

Total capacity is calculated on the basis ofdifferent MCS percentages supported in thenetwork.


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Theoretical calculations of Downlink data Rate

Determine the DL/UL ratio and channel size.

Multiply the DL active sub-carriers by DL symbols by frame rate to find thetotal number of active DL sub-carriers per second.

Multiply the number of DL active sub-carriers per second by the modulationbits per sub-carrier.

Multiply by the (FEC) coding rate to find the DL data bit rate


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Calculations of bit rate using above steps for a 5 MHz channel, 60/40 DL/ULratio, at 64QAM 5/6 is as follows:

420 active sub-carriers x (29-6) symbols x 200 frames per second =1.932 million active DL sub-carriers

1.932 million active DL sub-carriers x 6 bits per sub-carrier = 11.592

Mbps 11.592 Mbps x 5/6 = 9.66 Mbps.

Calculate data rate for all MCS.

Calculate the percentage of MCS in the network.

Calculate the capacity based on the weighted average of data rate and %ageof MCS.


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The actual capacity figures will vary due to the various margins incorporated( re-transmission, drops, etc)


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Introductio

n

Spectrum Allocation



Initial

Design

Marketing Inputs

Product Specs

Design Requirements

Simulations

Monitoring

&

Optimizatio

n

Actual Capacity



Drive Test

Network

Expansion







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Marketing Inputs

Subscriber and Traffic Requirements Capacity Requirements Coverage Requirements

Product Specs

Base station and Customer premises equipment

Design requirements

Simulations


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Example:

50,000 Customers base are required assuming thatthe customer base is about 20% of the number ofhouseholds in the different clutter types.

The Network should accommodate the requiredsubscribers count (50,000) at 70% utilization.

Therefore, the system has to be designed to support

a total of 72,000 subscribers at 100% utilization.


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Marketing provides information about variouspackages and their percent distribution in theNetwork from which Minimum User Rate is calculated.

With the help of simulations, the distribution of MCS

is calculated which gives us the Average Capacity perSite

Based on the above information, the number ofCapacity sites needed to support the Subscribers withMinimum User Rate is calculated.


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The potential maximum data rate is 768 Kbps

Packages Distribution*

Average

data rate (kbps)

Pkg 1 256 K 20% 51.2

Pkg 2 512 K 40% 204.8

Pkg 3 1024 K 30% 307.2

Pkg 4 2048 K 10% 204.8

* Random numbers


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Contention ratio is the ratio of the potentialmaximum demand to the realistic capacityrequirement.

The higher the contention ratio, the greater the

number of users that may be trying to use the actualbandwidth instantaneously resulting in lower offeredbandwidth, especially at peak hours.


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Minimum user rate is calculated

The minimum data rate per user is 38 Kbps

Packages Distribution*

Average

data rate (kbps)

ContentionData rate

(kbps)

Pkg 1 256 K 20% 51.2 20 2.56

Pkg 2 512 K 40% 204.8 20 10.24

Pkg 3 1024 K 30% 307.2 20 15.36

Pkg 4 2048 K 10% 204.8 20 10.24

* Random numbers


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No. of Sites = Network Capacity requirementsPer Site Capacity

No. of Users / Site = Per Site CapacityMin User rate


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Simulated MCS Distribution for an AOI

Modulation % DistributionAvg Data Rate

(Mbps)Wt Avg (Mbps)

QPSK 1/2 R6 0.0 0.35 0.000

QPSK 1/2 R4 0.0 0.25 0.000

QPSK 1/2 R2 0.0 0.5 0.000

QPSK 1/2 8.0 1 0.080QPSK 3/4 2.0 1.8 0.036

16 QAM 1/2 20.0 2.46 0.492

16 QAM 3/4 5.0 3.5 0.175

64 QAM 1/2 0.0 4 0.000

64 QAM 2/3 25.0 4.6 1.150

64 QAM 3/4 25.0 5.2 1.300

64 QAM 5/6 15.0 5.8 0.870

Avg Throughput per sector 4.10

Avg Throughput per site 12.31

* Random numbers


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Marketing provides area to be covered.

The chalked out Area of Interest (AOI) is then used toclassify the clutter distribution (e.g., Dense Urban,Suburban, Rural, Open, etc).

Based on the product information and link budget,coverage radius of site is calculated thoughsimulations.


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No. of Sites = Area to be covered

Coverage Radius / Site


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Class Name Count Percent Area Area Sq KM Category

open 158087 15.55 3,952,175.00 3.952175 SUsea 0 0 0 0 SU

inland_water 1643 0.16 41,075.00 0.041075 SU

residential_low 0 0 0 0 MU

mean_urban 33319 3.28 832,975.00 0.832975 MU

dense_urban 0 0 0 0 DU

buildings 131 0.01 3,275.00 0.003275 DU

village 0 0 0 0 SUindustrial/Commercial 10912 1.07 272,800.00 0.2728 MU

open_in_urban 1339 0.13 33,475.00 0.033475 SU

forest 0 0 0 0 SU

Parks 520 0.05 13,000.00 0.013 SU

dense_urban_high 0 0 0 0 DU

block_buildings 5256 0.52 131,400.00 0.1314 DU

dense_block_buildings 0 0 0 0 DU

residential_high 23812 2.34 595,300.00 0.5953 MU

dense-urban_low 128564 12.65 3,214,100.00 3.2141 MU

semi_open 2212 0.22 55,300.00 0.0553 SU

open_wet_area 0 0 0 0 SU

sparse_forest 30256 2.98 756,400.00 0.7564 SU

agriculture_fields 3411 0.34 85,275.00 0.085275 SU

Example:


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Based on the simulations the coverage radius of the site is calculatedwhich is dependent of the product specifications.

Clutter Clutter Area (Sq-KM) Site coverage Area (Sq-KM) Sites RADIUS (KM)

Product specific Product specific Product specific

DU 0.50 0.49 1.02 0.40

MU 7.49 1.10 6.83 0.60

SU 2.00 7.80 0.26 1.20

Total 9.99 8.11~ 9

Example:*

* Random numbers


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After the initial designing of coverage and capacity sites iscompleted, certain margins are kept to avoid unforeseencircumstances.

Example: If the number of coverage sites to cover a given AOI based

on Simulations comes out to be 100, the actual number ofsites budgeted would be 105.


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BS Antenna

Parameters Values

Antenna Type Directional antenna

Frequency range (MHz) 3400~3800Polarization Dual-polarized

Gain (dBi) 17.5

Half-power beam width 60

Vertical beam width 5.4

Antenna Structure4 Ports / 2 columns

Tx Power(Max) 48 dBm


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CPE

Features CPE

SS Type (CPE, USB) C750i

Antenna gain (dB) 7

Noise figure 5

TX power (dBm) 27

Number of TX antennas 1

TX diversity gain 0

RX diversity gain (dB) 4.7


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Parameter ValueFrequency 3.5 GHz

Channel Size 5MHz (512 FFT)

Duplex Scheme TDD

DL / UL Ratio 60:40 (29:18 DL / UL Symbols)

Target Cell-Edge Data Rates 1.4Mbps DL / 144kbps UL

DL Cell-Edge MCS QPSK

UL Cell-Edge MCS QPSK

MAP MCS QPSK Rep 2


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Simulations are performed to predict coverage radius of siteagainst each clutter class

Simulations also tell the MCS distribution within AOI which givesus the Average capacity per site

GIS maps (Digital Elevation + Clutter) are bought for specificgeographic region where coverage is to be provided.

The Design requirements + Product specs are made part of Linkbudget (which also lists down all the gains and losses in thesystem)

The Project configuration for Simulation tool is based on the Linkbudget, GIS maps, and tuned Prediction models for specificclutter type.


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Example:


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Introductio

n

Spectrum Allocation



Initial

Design

Marketing Inputs

Product specs

Design Requirements

Simulations

Monitoring

&

Optimizati

on

Actual Capacity



Drive Test

Network

Expansion

Future growth






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Actual Capacity

Factors Affecting Capacity

MCS Re-Configuration for Control Channels

CPE Placement

Drive test


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Date MBytes *MB at max util *Mbps DL Util Control util *Total sites *Total cells *Mbps per site *Mbps per cell

10/10/2011 4000000 23638042 2188.71 34.4817 23.6517 215 645 10.18 3.39

10/11/2011 4200000 23641785 2189.05 35.0748 23.7051 215 645 10.18 3.39

10/12/2011 4100000 23307663 2158.12 34.9386 23.6805 215 645 10.04 3.35

*Calculated fields highlighted

The current DL throughput and RF Utilization is measured from stats.

Based on the current stats the throughput is up scaled to the max utilization.

This gives the max achievable capacity in the network.

This information is critical for future planning as it gives a realistic figure ofcapacity.


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Another approach of calculating the actual capacity is toidentify the current percentage of MCS in the network fromstats.

Based on each MCSs capacity figure, average weightedcapacity is calculated.


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Simulated MCS Distribution for an AOI*

Modulation % Distribution Avg Data Rate(Mbps) Wt Avg (Mbps)

QPSK 1/2 R6 0.0 0.35 0.000

QPSK 1/2 R4 0.0 0.25 0.000

QPSK 1/2 R2 0.0 0.5 0.000

QPSK 1/2 8.0 1 0.080

QPSK 3/4 2.0 1.8 0.03616 QAM 1/2 20.0 2.46 0.492

16 QAM 3/4 5.0 3.5 0.175

64 QAM 1/2 0.0 4 0.000

64 QAM 2/3 25.0 4.6 1.150

64 QAM 3/4 25.0 5.2 1.300

64 QAM 5/6 15.0 5.8 0.870

Avg Throughput per sector 4.10

Avg Throughput per site 12.31

* Random numbers


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Indoor Coverage

Out of Coverage Area Sales

Number of Users/sector

RF Conditions of users


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Total capacity is calculated on the basis of differentMCS percentages supported in the network.

If the objective is to provide indoor coverage, theresidential areas should be targeted while calculating

the percentage mix of MCS during the planningphase.

For indoor coverage the losses will be different whichneeds to be catered.

MCS distribution will greatly differ for Indoor andOutdoor.


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As shown in the previous slide, MCS is selected onthe basis of SNR.

Users out of coverage area will have degraded SNRand hence lowest MCS.

If the Percentage of Low MCS is more, the overallsector capacity will reduce.


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The networks are best effort and hence theresources are shared.

As the number of users increase, the sector

capacity is shared amongst them.

This degrades the capacity / user.


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Network capacity is based on certain design assumptionsduring planning (e.g. CPE location and height)

The CPE placement is not always according to the designassumptions which affects the RF conditions and hence thecapacity.


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CPE Placement / RF Optimization

MCS Re-Configuration for Control Channels


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Sector Level Interference

Coverage Footprint

Capacity Distribution

CPE/Subscriber Level CPE Orientation & Placement

Serving Cell

CPE Type (wifi, Outdoor)


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The MCS used for Control Channels, such as QPSK rep2 can be re-

configured as QPSK which will increase capacity for TrafficChannels.

Aggressive MCS for DL control channels (Map + Broadcast) shouldonly be configured selectively on highly utilized sectors provided theusers are in good RF conditions (share of 64 QAM is higher). As

implementing this change on network level can cause camping issuefor the users with poor RF conditions.


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As the drive test tool is moved (driven) from point to point, the tool locates,identifies and synchronizes with APs; tracks the DL and UL characteristics;collects the GPS coordinates for each measurement point; and builds thedrive test map.

There are two types of drive test:

Scanned Mode

Dedicated Mode


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A Drive test is the process of network access and measurement over anoutside route typically in a key network coverage area.

The Drive Test tool operates as a subscriber station and predicts the usersquality of experience.

The following are the software/hardware requirements:

Drive test software ( Agilent, NEMO, XCAL, Genex Probe, etc)

Post processing software ( XCAP, MapInfo)

Operator device/ hardware ( USB, CPE, Vendor specific hardware)

GPS device ( Garmin, etc)


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Drive tests are carried out for various reasons. Some common ones are:

Network Planning

Performance goals

Simulation

Network implementation

Special Drive test


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Drive tests can give much information about a network and the main thingsthat are measured are listed below:

RSSI

CINR

Throughput DL & UL

Latency

Jitter

MCS


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No network entry

Preamble RSSI/CINR scanning only

Interference/ overshoot detection

Competitor benchmarking

Coverage footprint

Model tuning


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Legend:


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Legend:


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Complete network entry (6-7 step procedure) Tap into segments of problem during any of the steps.

DL/UL Throughput

RSSI/CINR recording

MCS assignment

Interference detection

Coverage footprint


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Sequence

Number Time Latitude Longitude Preamble ID Base Station ID Instantaneous CINR Current RSSI DL Throughput UL Throughput DL Burst Data FEC Scheme UL Burst Data FEC Scheme

713 04:15.5 33.63759 73.08712 41 00-00-0f-10-5d-4a 26 -56 10696 202 QPSK (CTC) 1/2 16-QAM (CTC) 3/4

711 04:12.5 33.63758 73.08713 41 00-00-0f-10-5d-4a 26 -56 10539 224 64-QAM (CTC) 3/4 16-QAM (CTC) 3/4








732 04:10.5 33.63758 73.08714 41 00-00-0f-10-5d-4a 29 -55 9397 173 64-QAM (CTC) 2/3 16-QAM (CTC) 3/4706 03:58.4 33.63754 73.08722 41 00-00-0f-10-5d-4a 27 -56 9386 174 64-QAM (CTC) 2/3 16-QAM (CTC) 3/4









799 51:48.0 33.63625 73.08868 41 00-00-0f-10-5d-4a 31 -50 8691 146 64-QAM (CTC) 5/6 16-QAM (CTC) 3/4761 49:50.0 33.6369 73.08797 41 00-00-0f-10-5d-4a 29 -53 8638 156 64-QAM (CTC) 3/4 16-QAM (CTC) 3/4






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Introductio

n

Spectrum Allocation



Initial

Design

Marketing Inputs

Product specs

Design Requirements

Simulations

Monitoring

&

Optimizatio

n

Actual Capacity



Drive Test

Network

Expansion







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With the growth in users, capacity sites are requiredin addition to the already deployed sites.

With additional sites, inter-site distance will decrease,frequency re-use will increase, increasing theinterference and hence reducing the capacity of thesite.


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New methods/ techniques are used to effectively utilizethe existing spectrum.

New spectrum/ clean channels are required to improve thecapacity further.


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TDD Ratio Optimization

7/10 MHz Channels

Multicarrier & 4th Sector

Percentage of Improvement


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Changing the TDD ratio can provide morecapacity in either direction as required.

Increasing the percentage for downlink

direction will add more slots to the downlinkand hence more capacity.


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The TDD Ratio is optimized based on the analysisof current UL/DL traffic distribution of thenetwork.

Usually downlink traffic in a network is 4-5 times

the uplink traffic.

In such cases the TDD ratio, 75:25, will be mostsuitable.

Care must be taken during the analysis as theuplink capacity is reduced and can lead tocongestion on the uplink.


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Pros: Number of Slots increases

Medium scale optimization required to optimize the coverage

No additional hardware requirement-minimum cost and implementationoverhead.

Cons: Coverage degradation planning challenge

BW (MHz) TotalSubcarriers DataSubcarriersSub channels

DL UL7 or 10 1024 840 30 35

5 512 420 15 17

Bandwidth Change Coverage Change

5 -> 10 0.73

5 -> 7 0.86

7 -> 10 0.85


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For multi-carrier & 4th Sector a new frequency carrier is used.

In Multicarrier, this new carrier is introduced over the congested sector,provided the existing hardware supports this feature.

Addition of 4th sector requires additional hardware.


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Pros: Congestion Relief in only the high Utilization cells giving at max 50%

more capacity to the sector.

The sector with multicarrier will have High MCS distribution due tominimum interference.

No reduction in network coverage instead it improves for the CPEscamped on the 4th sector.

Cons: Traffic distribution/balancing between the existing and new carrier

present an optimization challenge.

Additional RF heads/modems required for 4th Sector additional costand implementation overhead.


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Techniques Capacity Improvement Cost Implementation OptimizationCoverage

Improvement

5 MHz TDD 75/25 20% in DL Low Low Low Negative

7 MHz TDD 60/40 43% in DL Slots Low Low Medium Negative

5 MHz 4thChannel Overlay 50% on overlaid Sectors Medium Medium Medium Medium

1*3*3 to 1*3*4 7% in DL and UL Low Low Low Low

1*3*3 to 1*4*4 33 % in DL and UL High High High High


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Interference in the network can degrade thecapacity.

Guard band is required to avoid the interference

Guard band is the spectrum left unused at thepoints where interference is expected.

Vendors recommend leaving guard band equal to

half the channel size selected, on each side ofthe spectrum.


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The RF filter at the BTS plays a vital role in theselection of guard band.

Antennas with narrow band filters reject

adjacent channel interference to a greaterextent as opposed to wide band filters.

The selection of RF filters is vendor specific.


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There are no stringent requirements on theamount of guard band for the operator.

However WiMax forum recommends to have aminimum guard band which varies product toproduct.

It has given the figures for Adjacent channelrejection and co-channel rejection.

Based on the operators selection of product theguard band can be adjusted.


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Option - 1: Non-Contiguous Band (2 blocks of 21 MHz) Option - 2: Contiguous Band (1 block of 42 MHz)

ComparisonRecommended Guard

Band

Reduced Guard Band Recommended Guard Band Reduced Guard Band

Possible channel BW Only 5MHz and 7MHz All: 5MHz, 7MHz and 10MHz All: 5 MHz, 7 MHz and 10 MHzAll: 5 MHz, 7 MHz and 10

MHz

Possible configurationscenarios

1) 6 x 5 MHz channel 1) 8 x 5 MHz channel 1) 7 x 5 MHz channel 1) 8 x 5 MHz channel

2) 4 x 7 MHz channel 2) 4 x 7 MHz channel 2) 5 x 7 MHz channel 2) 5 x 7 MHz channel

3) 4 x 10 MHz channel 3) 3 x 10 MHz channel 3) 4 x 10 MHz channel

Effective band

5 MHz channel: 30 MHz 5 MHz channel: 40 MHz 5 MHz channel: 35 MHz 5 MHz channel: 40 MHz

7 MHz channel: 28 MHz 7 MHz channel: 28 MHz 7 MHz channel: 35 MHz 7 MHz channel: 35 MHz

10 MHz channel: 40 MHz 10 MHz channel: 30 MHz 10 MHz channel: 40 MHz

Guard Band required5 MHz channel: 10 MHz

(5+5)5 MHz channel: 2 MHz (1+1)

5 MHz channel: 5 MHz(2.5+2.5)

5 MHz channel: 2 MHz(1+1)

(Ideally, half thechannel BW for eachside of each block)

7 MHz channel: 14 MHz(7+7)

7 MHz channel: 14 MHz (7+7)7 MHz channel: 7 MHz

(3.5+3.5)7 MHz channel: 7 MHz

(3.5+3.5)

10 MHz channel: 2 MHz (1+1) 10 MHz channel: 10 MHz (5+5)10 MHz channel: 2 MHz

(1+1)

Un-utilized Band5 MHz channel: 12 MHz

(7+7)5 MHz channel: 2 MHz (1+1)



(Available BW at eachside of block)

7 MHz channel: 14 MHz(7+7) 7 MHz channel: 14 MHz (7+7)



10 MHz channel: 2 MHz (1+1) 10 MHz channel: 12 MHz (6+6)10 MHz channel: 2 MHz

(1+1)


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Case Channel SectorsNo. of

Channels

DL CapacityImprovement

(%)

AdjacentBand

SeparateBand

CAPEX Comments

1 5 3 6 19%No CAPEX required.

2 5 4 6 46%Cost for additional sector

hardware (Antenna, ModemCard)

Can be used to offerCapacity Relief in specific

cases.

3 7 3 4 55%

No CAPEX required. 50%gain in capacity at the cost

for slightly reducedcoverage.

4 7 4 4 97% Cost for adjacent band, Costfor additional sector hardware

(Antenna, Modem Card)

Can be used to offer

Capacity Relief in specificcases. Coverage loss is an

issue

55

(Overlay)3 6 100%

New BS hardware needed (inaddition to Antennas &Cabinets, Transmission

equipment, etc). Additionalspace on OMO towers needs to

be negotiated.


cases. Not an efficientutilization of resources.

65 (multi-carrier)

3 6 119%Software upgrade costinvolved per site basis


cases

7 10 3 3 138% Cost for adjacent band, Swapof old BS hardware not

supporting 10 MHz Channel Network is redesigned.


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Some of older Base stations dont support 10 MHz channel band width and Multi-carrierfeature. They need to be swapped with newer Base station versions.

Multi-carrier feature can in some cases result in 3 dB loss in coverage (e.g., 4Tx4Rbecomes 2Tx2R).

With newer Software release, it may be possible to overcome 3 dB loss in UL withEnhanced Multi-carrier feature. So essentially, 4Tx4R will become 2Tx4R.

Either Multi-carrier or Sector Overlay can be planned to offer Capacity relief. However,Multi-carrier is much cheaper option than physical Sector overlay.

There will be around 25% of coverage shrinkage when the operator moves to 10 MHz

channel bandwidth from 5 MHz and 16% with 7 MHz from 5 MHz. 10 MHz channel bandwidth is suitable for offering high data rate packages. With 5 MHz

Multi-carrier, the operator can support more users but may not be able to offer newservices requiring bandwidth.

For physical sector overlay, (2m) vertical antenna separation is recommended. It may notbe possible to achieve at some sharing sites.

Moving network from 3 to 4 sectors requires major network redesign. Again, there may

not be adequate space available for 4th sector on some sites.

In case of non-contiguous band, 7 MHz channel size is least efficient in terms ofeffective bandwidth usage.

5 & 10 MHz channel bandwidth with reduced Guard-band offers most efficient use ofspectrum.


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capacity enhancement techniques for wimax networks

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