www.greenpacket.com

APPLICATION NOTE

Optimizing Cell Edge Coverage Of Suburban/

Rural Deployment With WiMAX Outdoor CPE

Abstract

Wireless broadband operators constantly face challenges of balancing the economics to deploy a robust and reliable

wireless broadband service in suburban-rural areas or at cell-edges. Till to date, a large portion of these areas still

remains hugely underserved or receiving unreliable coverage. Most available studies have shown that WiMAX is indeed

an ideal technology candidate not only for a suburban-rural deployment, but as a wireless broadband technology on an

overall, with most of the studies focusing on infrastructure performance.

This paper in retrospect functions to further solidify the technological advantage of WiMAX, where we explore and proof

the concept of suburban-rural deployment through the optimal pairing of a proper CPE strategy. We will show that the

best solution for suburban-rural area and cell-edge performance can be extended four times through deployment via

outdoor CPEs.

This paper explores deep into the technical and implementation advantages of an outdoor CPE solution, including the

concept of antenna gain, transmission power, integrated POE design and environment-proof enclosure; and the effective

advantage of Line of Sight performance and decreased penetration loss amongst other features of an outdoor CPE. We

will also explore the misconception behind the high incremental cost, difficulty of engineering installation and service

challenges in operations with the implementation of an outdoor CPE which has been the lead culprit towards the low

adoption of this CPE strategy. Greenpacket will share three reference cases where deployment through the OX series

outdoor CPE has shown great success in Australia, Africa and Haiti, from a performance and business implementation

perspective.

APPLICATION NOTE

01

IntroductionOne major challenge for wireless broadband operators is to successfully deploy in a suburban-rural morphology. This

challenge is inherent with disregard of the type of wireless technologies implemented, be it through WiMAX, Satellite, 3G,

CDMA and similarly in the future, with LTE. In our previous white paper “Improving Indoor Coverage: How WiMAX

Modems can Play a Major Role in the Subsystem” (WPWIC10), we have described technologies to improve user

experience when they are farther away from the base station, or in the cell-edge area. We can see that in general,

cell-edge represents the area where poor consistency in connectivity is prevalent, with typical modulation being QPSK

where cell-edge users generally experience low and in many cases, inconsistent throughput speeds.

Figure 1 : UL and DL Modulation Relative to Cell Radius

The figure below represents a typical wireless cell that has a cell-center with radius ‘r’ and cell edge with a radius of ‘2r’.

Tabulating the coverage area, the cell-edge would give us an area of coverage equivalent to 4πr², while the cell-center is

only πr². Showing the distinct differences in coverage area achieved between the cell-edge and cell-center. This direct

difference implies that the efficiency of spectrum utilization by the base station is not able to achieve its intended

maximum coverage and capacity capability unless higher modulation scheme can be extended further into the cell-edge

coverage area; without which, a decent robust connectivity is next to impossible by just relying on the base station itself.

APPLICATION NOTE

Downlink

16-QAM

16-QAM

64-QAM

64-QAM QPSK

Uplink

Coverage cut off point

B C D

Point ALoss of uplink connection (1TX without MIMO and Beamforming)

Point BLoss of downlink connection (without MIMO and Beamforming)

Point CLoss of downlink connection with MIMO A

Point DLoss of downlink connection with Beamforming

Legend

• uplink connection

• downlink connection

A

QPSK

F

F

F1cell center

cell edge

F2 F3

F r

2r

02

WiMAX As The Best Solution With Outdoor CPE Challenges For Rural Broadband Deployment

Broadband penetration in rural areas across the world has lagged far behind than that in urban areas, both in developed

countries as well as in developing countries. This is mainly due to multiple reasons which make providing coverage in

these areas economically non-viable. The main reasons for this low penetration lie within the following TOP 3

roadblocks:

• As population density in rural areas is relatively low, setting up and maintaining copper or fiber lines to provide

broadband service across the “last mile” incurs high costs for Internet Service Providers (ISPs)

• Capacity on the backhaul or the “middle mile” from the ISP to the Internet backbone is often very limited and

expensive as it is provided over facilities designed for voice telephony or cable television

• The difference in per capita income between rural areas and urban regions means that broadband services need to

be offered at a lower-price point in the rural areas in order to gain customer adoption.

This combination of high CAPEX/OPEX and low Average Revenue per User (ARPU) has traditionally made the rural

broadband market less lucrative for service providers, but the advent of new high speed mobile broadband technologies

such as WiMAX has rapidly changed the situation.

APPLICATION NOTE

03

WiMAX As The Ideal Solution For Rural Broadband Deployments

As operators around the world evaluate multiple technologies for rural broadband deployments, WiMAX is emerging as

the de-facto standard. The advantages offered by WiMAX have time and again proven its commercial viability, among

others:

• Infrastructure Effectiveness: WiMAX offers a highly cost effective solution to address the rural broadband conundrum

as physical infrastructure need not be rolled out to individual customer premises on the last mile. This significantly

reduces the overall cost and the time-to-market in deploying the network.

• Flexible Network Deployment: WiMAX allows operators to surgically choose deployment options and focus on

specific areas where there is strong demand.

• High Data Throughput: 802.16e has reached a stage of technology and standards maturity with field proven results

that shows data throughput of over 40Mbps (@10MHz channel bandwidth).

• Spectrum Concerns: WiMAX spectrum has typically been less expensive worldwide as compared to 3G licenses.

Additionally, with mobile WiMAX being on licensed band, operators can deploy rural broadband networks with no

spectrum-related concerns.

The largest challenge facing operators in deploying a suburban/rural service is to create a balanced business case that

allows them to maximize the access infrastructure capability from a coverage and capacity perspective and at the same

time to deliver a robust and reliable service with a positive revenue return. Greenpacket believes that this can be realized

effectively if ‘Line Of Sight’ (LOS) can be achieved through the pairing of Outdoor CPE to the network, where we will

investigate the technology, performance and reveal a detailed deep dive reference case study based on a suburban/rural

environment profile.

APPLICATION NOTE

04

Advantages Of Outdoor CPEThe right CPE strategy for cell-edge and suburban/rural area is to deploy Outdoor CPE, and from our experience,

outdoor CPE have the following advantages.

i. Improved Uplink Technologies

ii. High Transmission Power And High Gain Directional Antenna

iii. Zero Penetration Loss

iv. Further Improvements Due To Line Of Sight

v. When Compared To External Antenna Option, Minimal Cable Loss

vi. Environmentally Robust

vii. Easy Installation

viii. Plug And Play

(i) Improved Uplink Technologies

We have stated a comprehensive analysis of the importance of uplink technologies in our previous white paper “A

Practical Approach to Improve WiMAX Indoor Coverage.” Here, we briefly describe the most important technology Dual

transmitter (2TX) which is implemented in our OX series Outdoor CPE.

Dual Transmitter (2Tx) (CDD)

The simplest Dual Transmitter technology employs Cyclic Delay Diversity (CDD) algorithm and signals are transmitted via

a single data stream through two antennas and two power amplifiers – hence, both antennas are switched on and used

for transmission. The combined power from two PAs provides approximately an additional perceived 3dB gain with

potentially lower power consumption (if PA’s are tuned to a lower power). This arrangement improves uplink performance

and lengthens coverage through better signal strength; however, it increases the cost of the modem as more

components are used.

Dual Transmitter (2Tx) STC Matrix (Uplink MIMO A)

The second generation Dual Transmitter technology further employs Space Time Coding signaling algorithm (Uplink

MIMO A) instead of just (CDD) algorithm as previously mentioned. However, in order to support this new algorithm, base

stations need to be upgraded to support WiMAX R1.5. The hardware implementation is the same as Dual Transmitter

(CDD) where transmission is conducted through two antennas and two power amplifiers. The combined power from two

PAs provides approximately an additional perceived 4dB gain, which is better than the Dual Transmitter (CDD).

Figure 2: Dual Transmitter (2Tx) component arrangement for CDD or STC

APPLICATION NOTE

Baseband RF

PA

PA

Antenna 2

Antenna 1

05APPLICATION NOTE

(ii) High Transmission Power And High Gain Directional Antenna

Comparatively, Outdoor CPEs are designed with higher maximum transmit EIRP. Greenpacket OX series Outdoor CPE

has a 14dBi antenna, Dual transmitters (2Tx) with a combined Tx power rated at 27dBm, thus a maximum transmit EIRP

equivalent to 41dBm, making it the most powerful mobile WiMAX CPE in the market today. Directional antenna is

conventionally implemented to optimize the antenna performance specifically aligned to face towards the base station.

Different from an indoor or USB based device, operator will normally assign service engineers to the premise where

installation is based on the optimum facing direction and upon locating it, the outdoor antenna is locked into place.

Figure 3 shows a sample antenna pattern for the 2.5GHz model. The diagram shows a concentrated RF energy radiated

in a single direction (+- 10 degrees within the pink line). This specific radiation pattern plays the highest role of effectively

increasing the coverage area.

Figure 3: Antenna patterns for OX250 main antenna

Figure 4 : Improvements of Outdoor CPE over indoor CPE. We can see that OX series can have 16dB improvements from

indoor USB dongles

USB donglesAverage 23dBm2dBi Antenna

Indoor CPE

5dB improvement 11dB improvement

Average 25dBm5dBi Antenna

Outdooe CPE (OX series)Average 27dBm (2Ti)14dBi Antenna

06

(iii) Zero Penetration Loss

Throughout our deployment experience, depending on the depth and material of the building, Glass windows and

concrete walls induces a 6dB and 12dB penetration loss respectively where the implementation through indoor CPEs

will normally endure. An Outdoor CPE, being placed normally on the roof of a specific building has the advantage of

being subjected to zero penetration loss, even without line-of-sight (LOS).

Figure 5: Frequency Vs Depth of penetration (Pd) for representative profiles of clay soils of pune at different soil moisture conditions (weight %)

(iv) Further Improvements With Line Of Sight (LOS)

The key advantage for Outdoor CPE is that it can easily achieve Line of Sight (LOS) with proper mounting. Having

achieved Line of Sight (LOS), there is no longer a multi-path problem and we can ensure optimal RF path between the

CPE and the base station. Figure 6 shows how Line of Sight can be achieved by moving the CPE above the roof with a

pole.

Figure 6: Improvement of LOS with o utdoor CPE

In addition, studies show that for a full featured base station. To achieve the same transmission data rate of 11Mbps,

LOS cell radius can be as far as 30km, while Non Line of Sight (NLOS) cell radius is only limited to 4km. This accounts

for five times the radius or about 25 times more coverage area for the same base station. LOS is a very critical

factor that affects the performance of the network, where an Outdoor CPE is proven to be the ideal solutions

to achieve LOS.

APPLICATION NOTE

LOSCPE (OutDoor)

NLOSCPE (InDoor)

07

Table 1: LOS and NLOS performance comparison using Full featured Base Station

Figure 7 : Coverage Distance Differerence for LOS and NLOS deployment

(v) Outdoor CPE VS Indoor CPE + External Antenna: Minimal Cable Loss

Another common solution to achieve LOS performance is to use outdoor external antenna to connect to an indoor CPE.

However, using external antenna introduces cable losses. For example, we can see in the below diagram, a typical cable

used for external antenna is RG316, which introduces about 1.5dB loss per meter length. Therefore, for an outdoor

extension of 10m, this contributes to 15dB loss, which has similar magnitude as the typical outdoor external Antenna

gain. This limitation is not absolute, where external antenna solutions with super high gain (greater than 15dBi) and super

low loss cable (less than 0.5dB/m) can be found, the question is however extended to economic sense of forming a

realistic solution rather than the best solution.

APPLICATION NOTE

LOS 30 to 50km

NLOS 4 to 9km

Indoor self-install 1 to 2km

Full Featured

Cell radius (Km)

Maximum throghput per sector (Mbps)

LOS

NLOS ( Ereg-Flat)

Indoor self-install CPE

From To

30 50

4 9

1 2

Download

uplink

11.3 8

11.3 8

Maximum throghput per CPE at CellEdge (Mbps)

Download

uplink

11.3 8

0.7 0.175

Maximum number of subscribers More

08APPLICATION NOTE

(vi) Outdoor CPE VS Indoor CPE + External Antenna: Minimal Cable Loss

Another common solution to achieve LOS performance is to use outdoor external antenna to connect to an indoor CPE.

However, using external antenna introduces cable losses. For example, we can see in the below diagram, a typical cable

used for external antenna is RG316, which introduces about 1.5dB loss per meter length. Therefore, for an outdoor

extension of 10m, this contributes to 15dB loss, which has similar magnitude as the typical outdoor external Antenna

gain. This limitation is not absolute, where external antenna solutions with super high gain (greater than 15dBi) and super

low loss cable (less than 0.5dB/m) can be found, the question is however extended to economic sense of forming a

realistic solution rather than the best solution.

Table 2 : Typical cable loss for Coaxial RG316 cable which is used for external antenna

Table 3 : Comparison between a 2.5GHz Indoor CPE with and without a 14dBi external antenna, and a 2.5GHz Outdoor CPE

mounted on a pole 6m above the ground

Matrix Of Attenuation and Power CWCoefficients:

Coaxial Cable :RG_316_/U

a=0.7727 , b=0.0972 , Fmax =3 , Pwt 1GHz = 135

2.12.32.42.62.7

2.93.0

Frequency(GHz) (dB/ft)

1.321.401.431.501.53

1.601.63

(dB/m)sea level

25˚ C amblenttemperature

Nom attenuation

0.4020.4270.4360.4570.466

0.4880.497

sea level25˚ C amblenttemperature

Nom attenuation

9389878482

8278

sea level40˚ C amblenttemperature

(watt)Max. CW power

Indoor CPE

Antenna Gain 5dBi Not used Not used

Cable Loss (RG316) 0 -9dB 0

Outdoor CPEIndoor CPE with external Ant

External antenna None 14dBi None

Penetration Loss -6dB None NoneTotal Ant gain -1dB 5dB 14dB

Line of Sight NO YES YESPerformance Poor OK Best

09APPLICATION NOTE

Uncover The Myth about Outdoor CPE

There exist general misconceptions about outdoor CPE implementation in terms of cost, having extreme restriction on

being heavy, performing unreliably under changing weather conditions, and extremely difficult to install and service.

New advancement on design and architecture has changed all this, Greenpacket’s OX series outdoor CPE is an example

where cost is effectively affordable, weighing in at less than 2kg, flexibly mountable on a wall or pole or roof edge and

capable of enduring temperatures between - 40°C ~ 60°C while in operations.

Most importantly, OX series enclosure is certified to be IP67 compliant which comes with a protection against the effects

of immersion in water up to depth of 1 meter. POE (Power over Ethernet) cable is also an important feature which

ensures that only a single cable (RJ45) is stretched from the outdoor unit (ODU) directly indoors to the end user, where

both the power and data is carried.

(i) Environmentally Robust

In addition, OX series outdoor CPE is equipped with a robust mounting kit which has been certified to withstand severe

windward and crosswinds. Figure 8 show the test result for OX series outdoor CPE withstanding hurricane rated winds

(windward/crosswind under 60m/s for 10 minutes, Beauford Wind Force Scale 17)

Figure 8 : Greenpacket OX series under Hurricane-force Winward Test

10APPLICATION NOTE

(ii) Easy Installation

OX series outdoor CPE mounting kit provides complete flexibility for installation. The pole-mounting and wall-mounting

brackets can be selected according to field needs. The multi-angle mounting kit can also be adjusted where the antenna

can be positioned to face toward the base station, receiving Line-of-Sight (LOS) signals. Figure 8 illustrates the quick

installation using the standard mounting kit, withal done within five minutes.

Figure 9 : Examples of Pole Mount (vertical or horizontal) installation

(iii) Plug And Play, Auto Provisioning, And On-Site Service Free

Greenpacket ensures that operators are free from having to manage the provisioning of devices. The device

comes pre-provisioned to allow end subscribers to plug and be automatically connected to the operator’s

base stations for an immediate play.

Nut

Bracket W/L Tab

Bracket with Tab

Flat washer

Split washer

Screw (60mm)

Screw (35mm)

Nut

Flat washer

Split washer

Lockwasher

X2 Needed

11APPLICATION NOTE

The Rural Leapfrog – ENDING THE WAIT FOR RURAL CONNECTIVITY

Verifying It On The Field

Understanding the concept and technological advantage of mobile WiMAX coupled with the service offering via an

Outdoor Modem validates it as a viable solution option for rural connectivity. The bigger question remains on the

performance capability of the WiMAX outdoor modem when deployed in a commercial field that is subjected to

subscriber loading and the environmental uncertainties knowing historically the inherent limitation of implementation via

a wireless technology.

Vivid Wireless, is the first WISP to launch a mobile WiMAX network in Australia has had a successful service uptake since

its launch in April 2010 covering the urban and suburban areas in cities of Perth, Sydney and Melbourne via Indoor IADs

and USB modem. Looking to extend its service coverage to the fringing suburban and semi-rural areas within the city,

Vivid Wireless conducted a citywide evaluation to benchmark the performance of the OX outdoor modem. Vivid Wireless

wanted the performance test to be conducted on its commercially live network to ensure that the end results obtained

is strictly similar to the end user experience. Choosing to compare it against the current commercially offered Green

Packet high gain DX Indoor IAD, Vivid Wireless has the objective:-

1. To ensuring that the OX meets the technical specification as stated in its product datasheet.

2. To record the additional gain in coverage, and

3. To record the throughput speeds experienced

Table 4 : Technical Specification between Indoor IAD and Outdoor Modem

DX230 was mounted inside the test drive vehicle on the passenger seat while the OX230 is mounted on the roof of the

car. Both RSSI and CINR measurement were recorded in both line-of-sight (LOS) and non-line-of-sight (NLOS) positions.

The test drive was performed at multiple locations that varied between 200 meters and 9.5 kilometers to the base

station.

DX 230-Indoor IAD OX 230-Outdoor Modem

Frequency 2.3GHz 2.3GHz

28dBm (Combined 2Tx) 27dBm (Combined 2Tx)

Intergrated Omni with 5dBi gain Dual polarized with 14dBi gain

Average 5dB higher than WiMAX Forum RCT speci�cation

Average 5dB higher than WiMAXForum RCT Speci�cation

Maximum RF Transmit Power

Antenna Type

Received sensitivity

MIMO Matrix A & B Matrix A & B

12APPLICATION NOTE

Figure 10 : Coverage planning comparison between Indoor IAD and Outdoor Modem

On completion, Vivid Wireless observed significant performance increase from the OX outdoor modem with the OX

having an average 12dBm more coverage, reaching a radius of over 9.5 kilometer with LOS. Proving that OX meets the

specification as stipulated even on the field. This significant coverage gained will allow Vivid access to an additional

72,000 new subscribers on its existing infrastructure, allowing it to maximize and monetize its infrastructure investment

further.

Table 5 : Summary test result comparison between Indoor IAD and Outdoor Modem

Numerous operators globally have had commercially successful deployments through similar strategies. Greenpacket

approaches most of its deployment together with the operator in a collaborative manner, seeing multiple successful

deployments in suburban and rural scenarios.

DX 230-Indoor IAD

-43250 26.6 -32 26.4 11 -0.2

-60.5840 24.4 -52.1 19.5 8.4 -4.9

-641100

Yes

LOS

No

No 10.9 -50.3 20.3 13.7 9.4

-841900 8.1 -74 7.6 10 -0.5

-54.62870 9.7 -37.1 14.7 17.5 5

-516400

No

Yes

Yes 11.1 -41 8.3 10 -2.8

-80.69500Yes 6.2 -64.5 4.7 16.1 -1.5

OX 230-Outdoor Modem

Indoor / Outdoor(Delta)

RSSI(dBm)

Distance toCell(m)

CINR(dBm)

RSSI(dBm)

CINR(dBm)

RSSI(dBm)

CINR(dBm)

Purple shade represents existing network coverage area

Green shade indicates increased coveage area when using OX230 modem

13APPLICATION NOTE

Reaching Within Remote Africa

Direct-on-PC, a the leading operator in Nigeria has been deploying mobile WiMAX under the service name of Unwired

by extending its wireless broadband voice and data services covering sub-urban and rural developing communities in

selective areas of Lagos, Abuja and Port-Harcourt as a complimentary broadband access network to their existing VSAT

and fixed wireless networks. The flexibility of mobile WiMAX has allowed the operator to grow its service coverage area

flexibly as service demand grows.

The strategy of service implementation via outdoor based CPE creates the best service advantage as it allows

maximization of coverage making the service robust and reliable, perfect to the harsh conditions of the West African

nation.

With mobile WiMAX having the capacity efficiency that is 10x higher with an upfront subscriber acquisition cost that is

1/5 of current satellite network, the business case of providing affordable voice and broadband service is achievable. This

has created confidence that the 4G network plans is set to lay the foundation for growth, both for the company and to

the development of the underserved market.

Bringing Broadband To The Remote Islands

The islands of the Caribbean are a well known holiday destination with a developing community and economy. Providing

broadband services in islands with harsh environmental terrain has always been a challenge to service providers.

Digicel International has been the champion serving over 11 million customers across 32 markets mainly on the islands

of the Caribbean and the Pacific. Having launched mobile WiMAX in Haiti, Digicel has had a great success with its

pay-as-you surf mobile WiMAX services in suburban areas of Haiti currently providing robust fast broadband services

through Greenpacket indoor and USB based devices.

Looking to extend the services offering in rural and remote areas, with the service quality having similar robustness is

something that Digicel insist to maintain, in order to achieve this while not having to incur huge incremental CAPEX

investment by deploying additional infrastructure to these subscribers located mainly in fringing cell edge of the network.

The outdoor modem was evaluated and results has proved to be the perfect solution extending the coverage within a

healthy modulation scheme to achieve broadband speeds and consistent service.

Even with a higher subscriber acquisition cost via the outdoor modem implementation, the huge CAPEX/OPEX savings

from infrastructure spending allows Digicel to scale the deployment needs with the number of subscription growth.

APPLICATION NOTE

Explore A Blue Ocean Of Opportunity In Rural Connectivity Today

As a leading developer of Next Generation Mobile Broadband solutions, Greenpacket has both the experience and best in class 4G WiMAX

devices specifically designed for operators looking to maximize their investment in delivering a robust, reliable and fast 4G broadband for

RURAL CONNECTIVITY.

Free Consultation

If you would like a free consultation on selecting WiMAX modems for your fixed broadband network, please contact us

at marketing.gp@greenpacket.com (kindly quote the reference code, DAP0411 when you contact us).

14

15APPLICATION NOTE

References 1. IRIX123 User Manual, Greenpacket

2. WiMAX’s technology for LOS and NLOS environments, WiMAX Forum Release

3. OX Series User Manual, Greenpacket

4. OX250 Antenna report, Greenpacket

5. Datasheet for Coaxial cable RG316, Huber + Suhner

6. K.S.Rao, Girish Chandra and P.V.Narasimha Rao, Study on penetration depth and its dependence on frequency, soil

moisture, texture and temperature in the context of microwave remote sensing

7. http://www.itu.int/newsarchive/press/WTDC98/Feature1.html

8. http://it.tmcnet.com/topics/it/articles/36595-internet-usage-gaps-developing-countries-challenges-solutions.htm

About GreenpacketGreenpacket is the international arm of the Green Packet Berhad group of companies which is listed on the Main Board

of the Malaysian Bourse. Founded in San Francisco’s Silicon Valley in 2000 and now headquartered in Kuala Lumpur,

Malaysia, Greenpacket has a presence in 9 countries and is continuously expanding to be near its customers and in

readiness for new markets.

We are a leading developer of Next Generation Mobile Broadband and Networking Solutions for Telecommunications

Operators across the globe. Our mission is to provide seamless and unified platforms for the delivery of user-centric

multimedia communications services regardless of the nature and availability of backbone infrastructures.

At Greenpacket, we pride ourselves on being constantly at the forefront of technology. Our leading carrier-grade

solutions and award-winning consumer devices help Telecommunications Operators open new avenues, meet new

demands, and enrich the lifestyles of their subscribers, while forging new relationships. We see a future of limitless

freedom in wireless communications and continuously commit to meeting the needs of our customers with leading edge

solutions.

With product development centers in USA, Shanghai, and Taiwan, we are on the cutting edge of new developments in

4G (particularly WiMAX and LTE), as well as in software advancement. Our leadership position in the Telco industry is

further enhanced by our strategic alliances with leading industry players.

Additionally, our award-winning WiMAX modems have successfully completed interoperability tests with major WiMAX

players and are being used by the world’s largest WiMAX Operators. We are also the leading carrier solutions provider in

APAC catering to both 4G and 3G networks and aim to be No. 1 globally by the end of 2010.

For more information, visit: www.greenpacket.com.

Copyright © 2001-2011 Green Packet Berhad. All rights reserved. No part of this publication may be reproduced, transmitted, transcribed, stored in a retrieval system, or translated into any language, in any form by any means, without the written permission of Green Packet Berhad. Green Packet Berhad reserves the right to modify or discontinue any product or piece of literature at anytime without prior notice.

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