Coexisting Isolation

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2008 Nokia Siemens Networks. All rights reserved.

Nokia Siemens Networks

Mobile WiMAX coexistence

When WiMAX is deployed in spectrumadjacent to other technologies

Leonid Bogod

30.07.2008

Technical White paper


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Mobile WiMAX coexistence1. Executive summary

Mobile WiMAX operates at frequencies higher than 2GHz as a Time Division Duplex(TDD) system. In most cases, Mobile WiMAX deployment occurs with FrequencyDivision Duplex (FDD) systems in the adjacent bands, for example, with Long TermEvolution (LTE) or Wideband Code Division Multiple Access (WCDMA) systems in the2.5GHz band and with Fixed WiMAX in the 3.5GHz band. Simultaneous operation oftransmitters and receivers from unsynchronized systems may cause interference anddegrade receiver sensitivity. It is very important to know in advance the mutual effect ofinterference between systems and how to minimize it.

For the purpose of increasing frequency utilization, local regulators want to minimize theguard band between TDD and FDD systems. The regulator can specify a block-edgemask and an external guard band. However, an operator may introduce an internalguard band based on product parameters and on allowed receiver desensitization. Onone hand, the internal guard band helps the operator meet the block-edge mask for thetransmitter and reduce the requirements for a receiver filter. On the other hand, theinternal guard band wastes spectrum. To find the optimum internal guard band value isone of the goals of this study (3.5GHz case). Another aim of the study is to provideinformation on the required coupling loss between TDD and FDD base stations.

To minimize capital expenditures (CAPEX) and operational expenditures (OPEX),operators quite often want to share the same sites, a deployment called co-siting. In

this case, antenna placement needs to be done very carefully to achieve maximumdecoupling. In addition, the decoupling value cannot be arbitrarily large for theseparated sides. To maximize revenue, the operators would like serve the same areaswith high population, which means the distance between different network sites cannotbe more than a half of cell radius on average, i.e., around 0.5km, and antennaplacement should support maximum antenna pointing loss.

Coexistence of two TDD networks is achievable without any additional coupling loss ifboth uplink and downlink are synchronized, i.e., transmitted and received time framesare the same as well as the starting points. If some of these conditions are not valid,then the two TDD base stations impact each other. In the case of two unsynchronizedTDD systems, the requirements for coexistence are quite similar to those for TDD/FDDsystems.

This paper analyzes Mobile WiMAX coexistence allocation in the 3.5GHz and 2.5GHzbands in the presence of FDD systems in the adjacent channels. Coexistenceconditions have been calculated assuming that the network product parameters are incompliance with the most relevant coexistence standards and regulatorrecommendations.


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1.1 List of abbreviations

3GPP 3rd Generation Partnership Project

ACLR Adjacent Channel Leakage power Ratio

ACS Adjacent Channel Selectivity

BS Base Station

BW Bandwidth

CAPEX Capital Expenditure

CEPT Conference of European Postal & Telecommunications

DL Downlink

ECC Electronic Communication Committee

ESD Equivalent Sensitivity Degradation

EGB External Guard Band

EIRP Emission Isotropic Radiated Power

ETSI European Telecommunications Standards Institute

FDD Frequency Division Duplex

FSL Free Space Loss

GB Guard Band

IMP Intermodulation Product

IGB Internal Guard Band

IRC Interference Rejection Combining

ITU International Telecommunication Union

LOS Line-of-Sight

LTE Long Term Evolution

MRC Maximum Ratio Combining

NF Receiver Noise Figure

OPEX Operational Expenditure

Rx Receiver

SEM Spectrum Emission Mask

SINR Signal to Interference-plus-Noise Ratio

TDD Time Division Duplex

Tx Transmitter

UL Uplink

WCDMA Wideband Code Division Multiple Access

WiMAX Worldwide Interoperability for Microwave Access


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2. Introduction

The purpose of this paper is to specify minimum requirements to support coexistingdeployment of Mobile WiMAX Networks with other unsynchronized systems. Theserecommendations are based on the latest available standards and regulations.

The WiMAX Forum, ITU, ECC and regulators have actively studied the same topic.However, those studies are based on theoretical base station (BS) parameters and theydo not correspond necessarily to real situations. Additionally, the studies include anabstract mathematical analysis that makes them difficult to use in practice.

Most cellular networks today use the FDD method, while Mobile WiMAX is a TDDsystem. Synchronization is typically not assumed for FDD systems; here we assumethe same for co-existence with Mobile WiMAX

No timing synchronization.

To increase revenue from a spectrum auction, local frequency regulators are sellingfrequency blocks without any guard bands. However, operators need to introduce aninternal guard band to full fill out-off-band emission requirements and to protect ownreceiver from an intolerable desensitization value

How much an internal guard band is needed?

All operators try to cover areas with the highest population density to increase revenue.This means unsynchronized (TDD and FDD) networks will likely be built in the same

geographical area

What is the minimum geographical separation needed amongthe sites?

Alternatively, to save capital expenditure (CAPEX) and operating expenditure (OPEX),operators always try to re-use existing sites. For example, Mobile WiMAX BSs could beplaced at the same site with FDD BSs

How to place antennas to achieve secure decoupling loss?


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3. Technical background

The BS transmitter (aggressor) from one network may impact the BS receiver from anothernetwork when these networks cover the same geographical area.

In this case several deployment scenarios are possible:

a) Two unsynchronized BSs are placed at separate sites (separated case)

b) Co-sited deployment refers to transmitting facilities that operate at the same site using acommon tower or mast

.

Depending on the deployment case, interference will decrease by propagation loss or by antennacoupling loss for separated and co-sited case, respectively.

The mutual interference is caused by non-idealities of the transmitter (aggressor) and the victimreceiver. These effects can be analyzed depending on the frequency separation of the systems:

1. Aggressor transmitter and victim receiver are in adjacent channels. The mutual effects canbe calculated by Adjacent Channel power Leakage Ratio (ACLR) of the transmitter andAdjacent Channel Selectivity (ACS) of the receiver.

2. Aggressor transmitter and victim receiver channels are separated by at least two times thechannel bandwidths. In this case, the spurious emissions of the transmitter and the blockingcharacteristics of the receiver must be considered when allocating bandwidth.

Within this White paper, we consider only the first case, which corresponds to a worst-case

situation.

Additionally, it is important to carefully calculate the effect of the intermodulation distortion product,which is caused by receiver non-linearity. This calculation should be based on the receiverparameters and frequency plan.

3.1 Interference sources

Radio signals are difficult to restrain both in space and in frequency, especially for widebandtransmission. As shown in Figure 1, there are three main sources of co-channel interference(blocking effect is not shown):

Out-of-band interference (interference 1), resulting from the modulation process and non-linearity in the transmitter and represented by ACLR. Interference 1 is calculated as asubtraction (in dB) of output power and ACLR value.

Interference caused by non-ideality of a receiver filter (Interference 2). This measurementshows how much unwanted power leaks to the receiver against the ideal receiver filter(shaded blue area), or how well the receiver filter can reject the dominant signal from anadjacent channel, i.e., Adjacent Channel Selectivity (ACS). Interference 2 is calculated as asubtraction (in dB) of output power and ACS value.


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Figure 1: Interference sources (blocking effect is not shown).

3.2 Interference mitigation techniques

Several interference mitigation methods are used for deployments where WiMAX needsto coexist with other systems without inter-operator synchronization.

3.2.1 External filter

An external filter can improve the transmitter emission mask and the receiver filterselectivity. Unfortunately, the external filter may be bulky and expensive. Additionally,the external filter has approximately 1.5dB insertion loss that will have an effect ontransmission power and receiver sensitivity.

3.2.2 Frequency planning

A frequency plan can mitigate interference avoiding the use of adjacent frequencychannels by an unsynchronized transmission system.

The simulation results presented in Sections 4.4 and 5.3 show that it is not possible toachieve a technology-neutral deployment for two networks operated in the samegeographical area without a frequency guard band. Guard bands are needed tofacilitate external filtering by providing a transition band for filter roll-off.

Adjacentband

ACLR

ACS

In-band

Tx SEM

Interfererence1Interfererence2

Rx filterVictim Rx Tx Pout (aggressor)


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3.2.3 Antenna pointing loss

Deployment of two BSs with three-sector antennas is presented in Figure 2. 3dBpointing loss can be achieved easily with the 65 half-power, beam-width antennapattern. Additional isolation may be available from the vertical down tilt of the antennas,however, this is not considered further in the following worst-case analysis.

Figure 2: Base station deployment and pointing loss.

3.2.4 Antenna decoupling

To save CAPEX and OPEX, network operators try to reuse existing sites, for example,by co-siting WiMAX BS antennas with other antennas. These antennas can be placedon the same mast (vertical separation) or on the same level (horizontal separation).

The 3rd Generation Partnership Project (3GPP) standards usually assume a 30dBcoupling loss between unsynchronized TDD base stations for a co-siting deployment(Reference 4). However, according to measurements described in Reference 1 andinformation provided by the antenna manufacturer Kathrein, a decoupling value of 56dB(vertical separation) and 50dB (horizontal separation) can be achieved with reasonable1-2m separation and careful antenna installation.

3.2.5 Baseband methods

Receiver diversity channels in a combined implementation in the base station can giveseveral dB gain in the link budget. The combining solutions can be Maximum RatioCombining (MRC) or Interference Rejection Combining (IRC). MRC is the optimalsolution when the interference is mainly white Gaussian noise, while IRC providesadditional Signal to Interference-plus-Noise Ratio (SINR) gain in the case of dominantinterferers.

gain

17dBi gain17dBi

gain17dBi

gain17dB

gain


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4. Analysis approach

4.1 Equivalent sensitivity degradation

An operator will see an impact of the interference as a reduction of the cell coverage andthroughput capacity. From a technical point of view, the interference increases the noise floor thatcauses BS receiver degradation.

Equivalent Sensitivity Degradation (ESD) is the subtraction between the receiver sensitivity withoutand with interference. ESD is described in Reference 1 and can be calculated as Equation 1:

[ ]

+=

1010

int

10110floorNC

LogdBESD (Equation 1)

Where:

Cint: Interferer contribution into receiver [dBm]

Nfloor: Noise floor of receiver [dBm]

Cint is the sum of the interference that falls within the receiver filter. Nfloorat room temperature is

calculated as Equation 2:

[ ] NFBWLogdBN flloor ++= )(10174 10 (Equation 2)

Where:

BW: Operating bandwidth in MHz

NF: Receiver noise figure in dB

These calculations show that ESD does not depend on the network service type and it can beapplied to any network.

It shall be noted that no intra-system interference (e.g. due to emissions from co-channel mobiles)was assumed in the definition of the ESD. Hence, the ESD, as defined here, corresponds to thethermal noise limited case i.e. worst case conditions as far as the relative impact due to adjacentchannel interference is concerned.

4.2 Interference source contribution


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As shown in Section 2.1, there are several interference sources. The interferer contribution can becalculated as the linear sum (in mW) of the various interference sources as descried in 3.1

dBBW

dffmWeInterferncdffmWceInterferen

MHzdBmtotalInterfer

f

f

f

f_

1

)(_2)(_1

)/(_

2

1

3

2

+

=

Equation 3

Where:

BW1= f2-f1 assigned channel; (f3-f2) = bandwidth determinable by an actual RX filter across theadjacent channel

_mW =dB conversion to mW; _dB = mW conversion to dB

Equation 3 can be rewritten in a more common manner (Equation 4):

dBmWACSmWACSmWACLRmWACLR

mWPoutdBmtotalInterfer _))_2

1

_1

1

_2

1

_1

1(*_()(_ +++=

(Equation 4)

Where:

Pout: Aggressor output power in mW;

ACLR1_mW and ACLR2_mW: Adjacent Channel Leakage Ratio for 1st

and 2nd

adjacent channels,respectively, in linear formACS1_mW and ACS2_mW: Adjacent Channel receiver Selectivity for 1st and 2nd adjacentchannels, respectively, in linear form

An interference level on the receiver is a subtraction from the transmitter interference level andlosses that depend on the deployment scenario (Section 2.2). To calculate the interference level atthe receiver, several parameters need to be taken into account (Equation 5):

( ) +++= OTHFLAPLGFSLtotalInterferdBRxInterfer ant_)(_ (Equation 5)

Where:

Gant: total antenna gain of aggressor and victim combinedFSL: Free Space LossAPL: Antenna pointing lossFL: Fading lossOTH: Others


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4.3 Propagation model

In this paper, the propagation loss is calculated by the most commonly used Free SpacePropagation Loss model (Reference 2) that gives minimum attenuation for Line-of-Sightpropagation (Equation 6):

)(20)(2044.32 1010 DLogFLogFSL ++= (Equation 6)

Where:

F is the operating frequency (in GHz)

D is the distance (in meters)

The operator can modify the results to add fading loss, rain effect, reflection and other factorsaccording to real allocation conditions.


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5. Mobile WiMAX deployment at 3.5GHz

For Mobile WiMAX operation, the most commonly available frequency bands in many Europeancountries are 3400-3600 MHz and 3600-3800 MHz. Previously, licenses for those bands wereissued for Fixed WiMAX. The Fixed WiMAX is a FDD system while the Mobile WiMAX is a TDDsystem. It means that to Mobile WiMAX (TDD) BS needs to coexist with Fixed WiMAX (FDD) BS inthis band. In other words, the Mobile WiMAX deployment should support a technology-neutralallocation.

5.1 Frequency licenses

In the 3.5GHz band, frequency licenses are often given for Broadband Wireless Access withoutspecifically mentioning the access method. This licensing has been the case in Germany and Italyamong other countries.

Figure 3 shows the frequency license band allocations that were issued in Italy. In certaingeographical areas, each block (two times 21MHz) A, B or C was given to only one operator. Forexample, block A consists of 3437-3458MHz and 3537-3558MHz. The operator can use thesebands for a FDD system with 100MHz downlink (DL) and uplink (UL) separation, or for TDDsystems as two separate carriers.

Figure 3: Frequency band allocation (3.5GHz) for Broadband Wireless Access in Italy.

To harmonize FDD and TDD deployment in the same geographical area, local regulators in mostEuropean countries require that a base stations block-edge-spectrum-emission masks are in

compliance with ECC RECOMMENDATION (04)05 (Reference 3).

5.2 Block-edge-spectrum-emission mask

The ECC Recommendation (04)05 specifies the maximum in-band Emission Isotropic RadiatedPower (EIRP) and out-of-band or block-edge mask. The recommendation includes someassumptions about the internal and external guard bands, but exact values for these bands dependon operator requirements, the deployment scenario and network equipment specifications.


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According to ECC Recommendation (04)05, the transmitter Block-Edge-Spectrum-Emission Mask(SEM) should be above the red line (block-edge mask) as shown in Figure 4. Those requirements

are not possible to be achieved without an external filter and a guard band towards the aggressor.It is clear that in the case of a more linear transmitter and a stronger filter, a Tx guard band couldbe smaller.

On the other hand, a block-edge mask determines how much interference falls into the victimreceiver. With a decrease in the internal guard band, the victim receiver will move to the aggressorband and total interference will increase. To achieve minimum receiver degradation, the Rx guardband should be as big as 35% of the aggressor assigned block.

Figure 4 shows the following case, which is also discussed as case 1 in Section 4.3:

Assigned block is 21MHz

Internal and external guard band is 3.5MHz Occupied band is 2x7MHz

Figure 4: The base stations block-edge-spectral-density mask defined in ECCRecommendation (04)05. (assigned block is 21MHz)


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5.3 Channel allocation

Assuming the operator can use the 21MHz band, many different channel allocation scenarios canbe considered. The following cases are the most relevant:

1. Unsynchronized aggressor system on both sides of the assigned band:a. Occupied band: 14MHz, Internal Guard Band (IGB): 3.5MHz, External Guard Band

(EGB): 3.5MHz

b. Occupied band: 15MHz, IGB: 3.0MHz, EGB: 3.0MHz

2. Unsynchronized aggressor system on one side of the assigned band:a. Occupied band: 15MHz, IGB: 6.0MHz, EGB: 3.0MHz

b. Occupied band: 17MHz, IGB: 4.0MHz, EGB: 3.0MHz

c. Occupied band: 19MHz, IGB: 2.0MHz, EGB: 2.0MHz

21MHz 21MHz

7MHz 7MHz 7MHz

Operator A (victim) Operator B (Aggressor)IGB EGB

7MHz

21MHz21MHz

5MHz 5MHz5MHz 5MHz 5MHz5MHz

Operator A (victim) Operator B (Aggressor)IGB EGB

21MHz

EGB3.0MHz

IGB 6.0MHz

5MHz 5MHz 5MHz

Operator A (victim)Operator B (Aggressor)

5MHz

21MHz

5MHz

Operator A (victim)

5MHz 5MHz 5MHz5MHz7MHz

Operator B (Aggressor)

5MHz

5MHz

IGB4MHz

EGB3.0MHz

21MHz 21MHz


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The cases 2b and 2c have unequal channels inside the occupied band. In those cases, the channelwith the minimum bandwidth should be allocated on the border with the most sensitive aliensystems in order to decrease out-of-band emission.

5.4 Simulation results

The simulation results represent a deterministic approach. i.e., without any probability distributionof the base stations.

As shown in Section 3:

Total interference is the integral of all interferers which fall into the receiver filter band Total interference depends on an internal guard band and receiver bandwidth and

selectivity

Receiver selectivity is the aggregation of internal receiver parameters and external filterresponse

The simulation calculations used Equations 2-5, with the block-edge-spectral-density mask definedin Reference 3 and receiver typical values defined in Table 1.

According to explanatory notes from Australias regulator (Reference 7), a co-located deployment(BSs) consists of a co-sited deployment and it refers to transmitting facilities that provide a serviceto the same geographical area but may use multiple towers and masts on one or more sites.

In this paper, a co-located deployment is divided into two cases:

a) Two unsynchronized BSs in the same geographical area (separated sites)b) Co-sited deployment refers to transmitting facilities that operate at the same site

using a common tower or mast

Common parameters used in the calculation of receiver sensitivity degradation are shown in Table1.

Table 1: Parameters for calculating receiver sensitivity degradation.

Parameter Value

Spectrum Emission Mask According to ECC(04)05

Output power 10 W

NF 6 dB

ACS Depends on IGB, min. 40 dB

External filter frequencyresponse

To fulfill Spectrum Emission Mask

IGB2MHz

5MHz 7MHz7MHz

EGB2MHz

Operator A (victim) Operator B (Aggressor)

7MHz 5MHz7MHz

21MHz21MHz


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External filter insertion loss 1.5 dB

Antenna gain 17 dB

Antenna pointing loss 3 dB

5.4.1 Separated sites

In the case where two BSs are located on separated sites, the interference will beincreased by the antenna gain and attenuated by the antenna pointing loss andpropagation loss.

Figure 5 shows that BS receiver degradation depends on coupling loss between twounsynchronized BSs for the cases described in Section 4.3. For demonstrationpurposes, the coupling loss is converted to the separation distance between BSs by

using Equation 6 and a total 6dB antenna pointing loss. One can see from Equation 6that the Free Space propagation Loss (FSL) for certain operating frequencies dependsonly on the separation distance.

Receiver Sensitivity degradation

0.0

3.0

6.0

9.0

12.0

15.0

18.0

10 0 20 0 3 00 4 00 500 6 00 70 0 8 00 9 00 1 00 0 1 10 0 12 00

BS separation distance, m

Sd

egrad

ation,

dB

BW=14MHz,IGB=3.5MHz,EGB=3.5MHz

BW=15MHz,IGB=3MHz,EGB=3MHz


BW=17MHz,IGB=4 EGB=3MHz


Figure 5: WiMAX BS receiver degradation in the presence of anunsynchronized BS in the same geographical area (excluding external filterloss of approximately 1.5dB).

Based on knowledge of the distance between a site and an aggressor BS, as well asband plans and an acceptable receiver degradation value, the operator can determinethe internal guard-band value needed (Figure 5).

For example, if:


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The aggressor is 200m from the BS site being studied And the aggressor is neighboring on one side of a block (case 2 in Section 4.3)

The acceptable receiver degradation is 3dB

Then, in accordance with Figure 5, the operator can use a 17MHz operating band andleave 4MHz as the internal guard band.

5.4.2 Co-siting deployment

Co-siting deployment, the case where two or more BSs share the same site, ispreferred by operators to save CAPEX and OPEX.

In the co-siting case, the total interference at the receiver will be decreased only bycoupling loss.

The simulation results for the cases described in Section 4.3 are shown in Figure 6.

Receiver Sensiti vit y degradation

0.0

3.0

6.0

9.0

12.0

15.0

18.0

21.0

24.0

27.0

30.0

33.0

36.0

39.0

30 35 40 45 50 55 60 65 70 75 80 85 90 95 100

BS isolation, dB

Sd

egradation,

dB

BW=14MHz,IGB=3.5,EGB=3.5MHz





Figure 6: WiMAX BS receiver desensitization for co-siting deployment with anunsynchronized BS (excluding external filter loss of approximately 1.5dB).

When calculating limiting requirements (e.g., spurious emissions, ACLR), the 3rdGeneration Partnership Project (3GPP) standards usually assume a 30dB coupling lossbetween unsynchronized TDD base stations for a co-siting deployment (Reference 4).


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However, according to measurements shown in Reference 1 and information from theantenna manufacturer Kathrein, a decoupling value of 56dB (vertical separation) and

50dB (horizontal separation) can be achieved with reasonable 1-2m distance. Acomparison of these values with Figure 6 shows the operator should use a minimuminternal guard band of 4MHz to achieve less than 3dB receiver desensitization.

A co-siting deployment for uncoordinated BSs with less than 3MHz internal guard bandis possible if:

1. The operator accepts more than 4dB desensitization or2. An interference mitigation technique is used (see Section 2.2) or3. Antennas are placed on different floors

The situation will be different if the assigned block has another size, e.g., 14 MHz in

Hungary and 15 MHz in France. The smaller block size will require a stronger block-edge mask that will cause less interference from the aggressor transmitter and lessESD of the victim receiver. For example, achieving 3dB ESD with 3.5GHz IGB can bedone for a 14MHz block with 52dB antenna coupling loss and for a 21MHz block with58dB antenna coupling loss. Of course, the useful frequency band for the 14MHz blockis almost two times less.


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6. Mobile WiMAX deployment in 2.5GHz

The 2.5GHz band is expected to be the most popular frequency band for Mobile WiMAXdeployment in European countries. Mobile WiMAX may be deployed within the frequency gapbetween the FDD downlink and uplink.

6.1 Frequency channeling arrangement

In the CEPT (Conference of European Postal & Telecommunications) band plan proposed by theECC (04)05 decision, the channeling arrangement shown in Figure 7 is allowed for Mobile WiMAXdeployment in the 2.5GHz band. TDD systems such as Mobile WiMAX or LTE-TDD could beallocated within the gap between the FDD downlink and uplink.

Figure 7: CEPT band plan proposed by ECC Decision (05)05.

In many European countries, only 40MHz will be available for the TDD (WiMAX or LTE) allocation

and a total of 10MHz (i.e. 5 MHz at each FDD/TDD transition frequency) will be used as guardbands to mitigate the interference effect of aggressor transmitters on a victim receiver (Figure 7).

However, in some countries such as the UK and Norway, the local regulator is proposing a flexibleFDD/TDD transition frequency. This proposal is in line with a new regulation for the 2.6 GHzfrequency band, published by the European Commission in July, 2008 (Reference 9).

When the TDD system (WiMAX) resides within the gap between the FDD downlink and uplink,there are mutual effects on both systems: WiMAX (TDD) is affected by transmissions from theunsynchronized FDD BS (i.e., WCDMA or LTE) and WiMAX (TDD) could cause interferenceproblems for the FDD receiver.

Most likely the TDD band will be shared among several operators. To avoid interference, thenetworks operated in the geographical area should be synchronized. Otherwise, mutualinterference between them will be similar to TDD-FDD interference.

2570 MHz 2690 MHz2620 MHzFDD- UL FDD- DLWiMAX TDD

FDD BS TDD BS TDD BS FDD BS

2500 MHz


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6.2 Regulations

As of mid-2008, two documents regulate coexisting deployments in the 2.5GHz band in EUcountries: ETSI EN 302 544-1 (Reference 5) and CEPT Report 019 (Reference 6) (based on theECC SE42 Recommendation).

In the EU, a network product must comply with ETSI EN 301 544-1 (Reference 5) in order to beplaced on the market. However, a preliminary analysis shows that even if WiMAX BSs are incompliance with the specification in Reference 5, some coexistence issues remain as the spuriousemission level of -45dBm/MHz applies only for a frequency gap of more than two time the channelbandwidths between the unsynchronized systems.

The local regulator could therefore require a more restrictive Block edge mask (BEM) limit fornetwork deployment, such as the EIRP limit specified in ECC SE42 Recommendation (as e.g. is

the case in Sweden). Figure 8 shows an example of a TDD EIRP BEM for a 20MHz TDD licenseblock that is adjacent to a FDD uplink (UL) spectrum block.

Figure 8: TDD BEM for a 20MHz TDD license adjacent to FDD uplink (UL) spectrum block(Reference 6).

5MHz guard band is applied on both TDD/FDD transition frequencies: the 2570-2575MHz guardband protects the FDD uplink from TDD emissions and the 2615-2620MHz guard band facilitatesthe stringent TX filtering requirements for the FDD BS. Both guard bands are taken from the TDDregion. At frequencies below those guard bands, a -45dBm/MHz EIRP value must be fulfilled byany aggressor transmitter.

All following calculations are made according to the CEPT 019 Report (ECC SE42Recommendation).

6.3 Simulation results

Figure 9 helps operators understand and calculate the interference that falls into the receiver bandsin the case where WiMAX (TDD) and LTE (FDD) co-exist on adjacent frequencies.

2500 2520 2540 2560 2580 2600 2620 2640 2660 2680

40

20

0

20

40

60

4 dBm/MHz

-45 dBm/MHz

FDD-UL FDD-DL


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Figure 9: Interference levels (according to SE42) and filters related to co-existing FDD/TDD(10MHz BW), Antenna Gain w ith cable loss is 17dB)

Table 2 shows common parameters of WiMAX (typical values) and LTE (according to 3GPP TS36.104v8) that are used for determining selectivity degradation on the victim receiver.

Table 2: Parameters fo r the coexistence calculation.

LTEWiMAX WiMAX LTE Comments

Channel bandwidth(for both, WiMAX

and LTE)

5MHz 10MHz 5MHz 10MHz

Output Powerspectral density,

dBm/MHz

36 33 36 33 Pout=20W

Rx bandwidth, MHz 4.8 9.6 4.5 9.0

External Filter (Rx)attenuation from

the channel edge,

dB

50 at5MHz

60 at10MHz

50 at5MHz

60 at10MHz

To rejectblocking

ACS1, dB 42 42 45.7 45.7

ACS2, dB 67 67 54.7 54.7

Noise floor, dBm -102 -99 -102.5 -99.5 NF=5dB

Antenna gain 17 17 17 17

Pointing loss, dB 3 3 3 3

36dBm/MHz

-13dBm/MHz

-62dBm/MHz

Powerdensityatante

nna

FDD(LTE)-UL WiMAX TDD FDD LTE -DL

TDD Rxchannel filter

FDD Rx channelfilter

TDD External fi lterFDD Rx External fi lter


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Interference of a WiMAX transmitter to a LTE receiver and interference of a LTE transmitter to aWiMAX receiver have been calculated by using Equation 4.

The calculations show if WiMAX and LTE are occupied on adjacent channels with 5MHz guardband, 3dB receiver sensitivity degradation can be achieved only with significant external filtrationon the receiver side. For WiMAX, the same filter can be also used for Spectral Emission Mask(SEM) shaping, but for LTE-FDD system it requires an additional RX filter.

Filter requirements can be relaxed if:

aggressor emission to in guard band is less Guard band is more than 5MHz, e.g. if the systems are not adjacent Actual implemented ACS value of the BS is larger More than 3dB receiver sensitivity degradation is allowed or allowance for intra-system

interference is made

Two coexisting deployment scenarios are considered in the following calculations (the scenariosare described in Section 4.4):

a) Separated sites andb) Co-sited sites

6.3.1 Separated sites

In the case of two BSs that are located at separated sites, the interference will be increased byantenna gain and attenuated by antenna pointing loss and propagation loss.

Figures 10 a) and b) show the effect of BS Tx LTE on the BS WiMAX receiver and BS Tx WiMAXon the BS LTE receiver, respectively. The effect depends on coupling loss. For demonstrationpurposes, the coupling loss is converted to the separation distance between the BSs by usingEquation 6 and the total 6dB antenna pointing loss is taken into account.


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a) b)

Figure 10: Receiver Equivalent Sensitivity Degradation (ESD) depends on relativeBS separation. a) LTE Tx to WIMAX Rx. b) WiMAX Tx to LTE Rx.

According to the simulation results (Figure 10), the interference effects of LTE Tx toWiMAX Rx and WiMAX Tx on LTE Rx quite similar for the 5MHz and 10MHz bandwidth.

The separation distance between LTE and WiMAX base station should be between 140

and 160 m, to achieve 3dB Rx sensitivity desensitization when taken into account non-ideality of the victim receiver. The separation distance can be decreasedapproximately on 50m if ACS1 and ACS2 are 50dB and 70dB, respectively.

Of course results will vary depending on:

- Tx output power- Antenna gain and pointing loss- Actual ACS and external filter values

6.3.2 Co-siting caseIn the co-siting case, total interference at the receiver will decrease only with antenna couplingloss.

The simulation results are shown in Figure 11.

20W LTE Tx in adjacent channel

0.0

3.0

6.0

9.0

12.0

15.0

18.0

21.0

0 100 200 300 400 500

BS to BS Separation, m

WiMAXReceiverESD,dB

5MHz BW

10MHz BW

20W WiMAX Tx in adjacent channel

0.0

3.0

6.0

9.0

12.0

15.0

18.0

21.0

0 100 200 300 400 500

BS to BS Separation, m

LTEReceiverESD,dB

5MHz BW

10MHz BW


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a) b)

Figure 11: Receiver Equivalent Sensitivity Degradation (ESD) depends onantenna decoupl ing at the BSs a) LTE Tx to WIMAX Rx b) WiMAX Tx to LTE Rx.

One can see that in order to achieve 3dB receiver sensitivity degradation will require51-52dB antenna decoupling.

According to experimental results such decoupling values can be achieved with carefulantenna installation.

7. Conclusions

This paper has analyzed the coexistence of Mobile WiMAX with FDD systems in the most popularallocation bands (3.5GHz and 2.5GHz). Unsynchronized WIMAX was assumed as an alien systemin the 3.5GHz case and LTE in the 2.5GHz case. These two cases were handled in a slightlydifferent way.

For the 3.5GHz case, an assigned block of 21MHz is assumed and a transmitter block-edge-emission mask in compliance with Reference 3. The required internal guard band (IGB) andmaximum operated band have been calculated based on several factors:

1. An alien system on one or both sides of the assigned block2. Allowable WiMAX victim receiver desensitization3. Co-sited or separate sites4. Coupling loss between base stations

Table 5 shows the summary of the simulations that allowed 3dB receiver desensitization.

20W LTE Tx in adjacent channel

0.0

3.0

6.0

9.0

12.0

15.0

18.0

30 35 40 45 50 55 60 65 70 75 80

BS to BS decoupling, dB

WiMAXReceiverESD,dB

5MHz BW

10MHz BW

20W WiMAX Tx in adjacent channel

0.0

3.0

6.0

9.0

12.0

15.0

18.0

30 35 40 45 50 55 60 65 70 75 80

BS to BS decoupling, dB

LTEReceiverESD,dB

5MHz BW

10MHz BW


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Table 5: Calculated required BS separation and antenna decoupling for different deployments(3.5GHz band).

OperatingBandwidth

Internal GB(+External GB)

Required BSto BS

separation*), m

Required antennadecoupling, dB

(co-siting) *)

Aggressor on

1 2x7MHz 3.5 (+3.5) MHz 140 57 both sides

2 3x5MHz 3.0 (+3.0) MHz 230 63 both sides

3 3x5MHz 6.0 (+3.0) MHz


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8. References:

1. GSM900, GSM1800 and UMTS FDD cositing analysis, COST 273TD(03)121, D.Bouguet,May, 2003

2. IEEE L802.16-07/070r1, R. Arefi, 15.11.2007 (propagation)3. ECC RECOMMENDATION (04)054. 3GPP TS 25.105v7.7.0 (2007-10) p.325. ETSI EN 302 544-1 v1.1.0, 2008-56. CEPT Report 019 (Draft), December 20077. http://www.acma.gov.au/WEB/STANDARD/1001/pc=PC_918568. ETSI TS 36.104v8, 2007-129. http://www.bbwexchange.com/pubs/2008/07/09/page1423-2862561.asp

Coexisting Isolation

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