18_Reliability and Redundancy of Flexi Multiradio BTS

RELIABILITY COMMUNICATION MATERIAL FLEXI MULTIRADIO BTS BTS Systems and Configurations Reliability Version 3.0 4. February 2010

Document Code/Version Sharenet 445958/3.0

© 2010 Nokia Siemens Networks Proprietary and Confidential

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The information in this document is subject to change without notice and describes only the product defined in the introduction of this documentation. This documentation is intended for the use of Nokia Siemens Networks customers only for the purposes of the agreement under which the document is submitted, and no part of it may be used, reproduced, modified or transmitted in any form or means without the prior written permission of Nokia Siemens Networks. The documentation has been prepared to be used by professional and properly trained personnel, and the customer assumes full responsibility when using it. Nokia Siemens Networks welcomes customer comments as part of the process of continuous development and improvement of the documentation. The information or statements given in this documentation concerning the suitability, capacity, or performance of the mentioned hardware or software products are given "as is" and all liability arising in connection with such hardware or software products shall be defined conclusively and finally in a separate agreement between Nokia Siemens Networks and the customer. However, Nokia Siemens Networks has made all reasonable efforts to ensure that the instructions contained in the document are adequate and free of material errors and omissions. Nokia Siemens Networks will, if deemed necessary by Nokia Siemens Networks, explain issues which may not be covered by the document. Nokia Siemens Networks will correct errors in this documentation as soon as possible. IN NO EVENT WILL Nokia Siemens Networks BE LIABLE FOR ERRORS IN THIS DOCUMENTATION OR FOR ANY DAMAGES, INCLUDING BUT NOT LIMITED TO SPECIAL, DIRECT, INDIRECT, INCIDENTAL OR CONSEQUENTIAL OR ANY LOSSES, SUCH AS BUT NOT LIMITED TO LOSS OF PROFIT, REVENUE, BUSINESS INTERRUPTION, BUSINESS OPPORTUNITY OR DATA,THAT MAY ARISE FROM THE USE OF THIS DOCUMENT OR THE INFORMATION IN IT. This documentation and the product it describes are considered protected by copyrights and other intellectual property rights according to the applicable laws. The wave logo is a trademark of Nokia Siemens Networks Oy. Nokia is a registered trademark of Nokia Corporation. Siemens is a registered trademark of Siemens AG. Other product names mentioned in this document may be trademarks of their respective owners, and they are mentioned for identification purposes only. Copyright © Nokia Siemens Networks 2010. All rights reserved



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Nokia Siemens Networks

RELIABILITY COMMUNICATION MATERIAL FLEXI MULTIRADIO BTS BTS Systems and Configurations Reliability

Contents 1.

INTRODUCTION............................................................................................................................ 4

2.

RELIABILITY ENGINEERING........................................................................................................ 4

3.

4.

5.

2.1

Definition of reliability concepts ............................................................................................... 4

2.2

Reliability terminology and calculation principles .................................................................... 5

RELIABILITY METHODS TO OPTIMIZE DESIGN FUNCTIONALITY .......................................... 7 3.1

Redundancy ............................................................................................................................ 7

3.2

Recovery functionality ............................................................................................................. 7

3.3

Remote controllability .............................................................................................................. 7

FLEXI MULTIRADIO BTS RELIABILITY PRINCIPLES ................................................................. 8 4.1

Standard reliability functionalities ............................................................................................ 8

4.2

Reliability performance optimization during R&D work............................................................ 8

4.2.1

HW reliability .................................................................................................................... 8

4.2.2

Software reliability ............................................................................................................ 9

FLEXI MULTIRADIO BTS SYSTEM RELIABILITY...................................................................... 10 5.1

Flexi WCDMA BTS (HW Release 1) Reliability Performance................................................ 10

5.2

Flexi Multiradio BTS (HW Release 2) System Reliability Performance ................................. 11

5.2.1

MTBF values for one System Module and one 3-sector RF Module configurations ...... 11

5.2.2

MTBF values for one System Module and two 3-sector RF Module configurations....... 11

5.2.3

MTBF values for two System Module and one 3-sector RF Module configurations....... 12

5.2.4

MTBF values for two System Module and two 3-sector RF Module configurations ....... 12

5.2.5

MTBF values one System Module and 3x Remote Radio Head configurations............. 13

5.2.6

Redundancy and capacity definitions in Flexi Multiradio BTS ........................................ 14

5.2.7

System Availability and Downtime ................................................................................. 14

5.3

Prerequisites and assumptions for predicted data ................................................................ 15

5.4

Temperature trend for system MTBF and thermal protectioning ........................................... 16

5.5

RF-power effect to the system MTBF.................................................................................... 17



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1. INTRODUCTION Purpose of this document is to represent the Flexi Multiradio BTS product predicted reliability data and introduce the reliability and calculation methods. Document also introduces the Flexi BTS functionality reliability point of view.

2. RELIABILITY ENGINEERING 2.1 Definition of reliability concepts Reliability Engineering consists of the management and engineering tasks needed to specify, plan, manage, achieve and measure a product reliability. The term "reliability" as used in this document considers reliability in its broadest sense and comprises all aspects that ensure the product continues to meet its availability requirement. Availability performance is not only affected by product reliability but by maintainability performance and maintenance support strategies. The method of providing maintenance support varies from customer to customer depending on if a maintenance agreement is made and the type of agreement. In Figure 1., reliability (dependability) aspects and their relationships are presented in accordance with the definitions of international standard IEC 50(191).

Figure 1: Dependability concepts. Dependability The collective term used to describe the availability performance and its influencing factors: reliability performance, maintainability performance and maintenance support performance. This term is often replaced by “reliability” term. Availability performance The ability of an item to be in state to perform a required function at a given instant of time or at any instant of time within a given time interval, assuming that the external resources, if required, are provided. Reliability performance The ability of an item to perform a required function under given conditions for a given time interval. Maintainability performance The ability of an item under stated conditions of use, to be retained in, or restored to a state in which it can perform a required function, when maintenance is performed under given conditions and using stated procedures and resources. Document Code/Version Sharenet 445958/3.0


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Maintenance support performance The ability of a maintenance organization, under given conditions, to provide upon demand the resources required to maintain an item, under a given maintenance policy.

2.2 Reliability terminology and calculation principles Reliability of BTS system and modules is typically expressed with next terminologies and calculation formulas. Please note that terminologies and calculation principles can differ between organizations and reliability standards. To avoid conflicts between contract participants, the details of terms need to be understood and clarified more exactly to avoid misreading. Mean (operating) Time Between Failures (MTBF) The expectation of the operating time duration between two consecutive system failures of a repaired item. Only the failures that affect to system traffic capacity are included. System MTBF System MTBF indicates the expected failure rate for the defined configuration. MTBF value is stated for the full and partial sector capacity in nominal cases. MTBF for Repair comprises all failures requiring repair whether they affect the system functionality or not. It summaries the parts FITs (Failure In Time) and indicates the probability for unplanned site visit. Mean Repair Time (MRT) The expectation of active corrective maintenance time when repair actions are performed for an item. Consist of module replace time with actions to perform the replacement and power-up the module to initial state. • Flexi BTS MRT = 0.5 hours (30 minutes)

Mean Time To Recovery (MTTR) The expectation of the time interval during which an item is in a down state due to a failure. MTTR includes mean active repair time (MRT) and logistical and administrative delay times (MLD). Due to differing national conditions, logistic delay times may vary from country to country and therefore may not be subject to international recommendation. Therefore the availability performance values are calculated based on the active repair time only (MRT). MTTR is expressed in some standards as Mean Time To Restoration (same as Recovery), but also as Mean Time To Repair. To avoid conflicts, it is necessary to understand differences between operational and intrinsic availability. Availability (A) is the intrinsic availability (Ai) at time infinity for a mature product and assumes an ideal maintenance and operational conditions. Intrinsic availability includes only the active repair times (MRT) and excludes logistic delays caused by traveling, spare part availability and preventive maintenance actions, which are part of logistical delays (MLD). Intrinsic availability is sometimes spoken as inherent availability and is similar to steady-state availability. • Operational availability (Ao) can be calculated to represent total system downtime, but because the logistical delays depends on the operators maintenance actions, it is not relevant in the predictions. Intrinsic availability (Ai) calculation formula and parameters:

Ai = MTBF / (MTBF + MRT) •

MRT (Flexi BTS) o

= 0.5 hours (30 minutes)

To calculate operational availability (Ao) and total downtime, next formula and parameters shall be used.

Ao = MTBF / (MTBF + MTTR) Document Code/Version Sharenet 445958/3.0


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MTTR = MRT + MLD

MLD (Mean Logistical Delay) = 4,0 hours (average estimation).

Unavailability (U) presents the value to condition when a system is down and is used for mean down time calculation. U = 1 - Ai Mean Down Time (MDT) The expectation of the time interval during which an item is in a down state and cannot perform its function. MDT is calculated from the inherent availability including only the active repair times (MRT) and excludes logistic delays caused by traveling, spare part availability and preventive maintenance actions, which are part of logistical delays (MLD). Typically it is calculated in annual bases. MDT [min/year] = U * 8760h/year *60 minutes/h

Lifetime Design lifetime (design life, useful life) defines the expected minimum time for design functionality. Starts when the design is first time taken into use and continues without interruption to failure when the design is not anymore possible to repair with reasonable costs. Lifetime and MTBF are two separate terms and not confided to each other. Typically MTBF should be must higher than lifetime. Lifetime is defined for a large population installed components in service, not for a single component. Lifetime is expressed often as design life and service life and it is not same than a product life span or life-cycle. MTTF Median Time To Failure is the time from the moment of installation to the point when 50% of the component population have failed. This time is longer than the lifetime defined at a low failure probability. It is used for the components whose lifetime distribution is relatively narrow. MTTF is most relevant parameter for non-repairable parts and must be much higher than whole product lifetime. L10 life The L10 specifications states that less than 10% of a statistically significant numbers of cooling fans or similar kind of equipment will fail before the L10 life expectation in the specified conditions. Cooling fans contains typically both electrical and mechanical parts and because of the electrical parts, is it usual to define relevant MTBF for the fan also to estimate spare part need. L10 life need to be higher than the specified lifetime. Typically L10 is the value for rotor life in cooling fan design.



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3. RELIABILITY METHODS TO OPTIMIZE DESIGN FUNCTIONALITY To optimize a design robustness and possibility to maintain certain capacity level in the failure cases, it is possible to use several design techniques.

3.1 Redundancy Reliability predictions based on the module, unit and system level Reliability Block Diagrams (RBD) and take account unit and module level redundancy, which have significant effects on the complete system's functionality. Redundancy can be performed on next ways.

Duplication: If the spare unit is designated for only one active unit the software in the unit pair is kept synchronized so that taking the spare in use in fault situations (switchover) is very fast. The spare unit can be said to be in hot standby. This redundancy principle is called duplication, abbreviated "2N". Replacement: For less strict reliability requirements, the one or more spare units may also be designated to a group of functional units. One spare unit can replace any unit in the group. In this case the switchover is a bit slower to execute, because the spare unit synchronization (warming) is performed as a part of the switchover procedure. The spare unit is in cold standby. This redundancy principle is called replacement, abbreviated "N+1". Load sharing: A unit group may be allocated no spare unit at all, if the group acts as a resource pool. The number of units in the pool is selected so that there is some overcapacity. If a few units of the pool are disabled because of faults, the whole group can still perform its designated functions. This redundancy principle is called load sharing, abbreviated "SN+" or "N+1/L" Some functional units have no redundancy at all (do not need to be backed-up). This is because they are otherwise protected or offer non-critical services or a failure in them does not prevent the function or cause any drop in the capacity.

3.2 Recovery functionality To optimize BTS functionality it contains several recovery and remote control functionalities. Most common recovery actions are: • Restart: o Self-reset or forced reset to recover in the failure cases. • Diagnostics features: o Self-test and loop-tests to detect functionality errors. o Alarm generation in the error cases (internal and BTS level alarms). • Temperature control with alarm to shut down module or reduce electrical stress if overheated.

3.3 Remote controllability Every BTS and most of its modules are possible to control remotely to avoid site visits. Remote connection can be take by using specific element manager or from other core network element.



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4. FLEXI MULTIRADIO BTS RELIABILITY PRINCIPLES 4.1 Standard reliability functionalities Flexi WCDMA and LTE BTSs hava designed to achieve optimum reliability performance on system level. The design contains functionalities, which maintain operability in error and failure cases. During the R&D work several actions have been completed to achieve optimum reliability level.

4.2 Reliability performance optimization during R&D work Flexi BTS is highly integrated and high capacity BTS in small size. When combining functionality of several modules to one entity, it increases critical parts amount and thus number of single point failures in that package. Benefit of high integration level is that it makes possible to combine certain functionalities to one part and reduce signal routing between parts and sub-modules. This reduces component and connection amounts. This is one reason for better HW-reliability expectations, because the design contains less parts and contacts, risk for HW-level failures is smaller. HW-level reliability (MTBF for repair value) can be expected to be about three times better than in older BTS’s in minimum. This will be seen in reduced site visit needs. High integration level improves the system level reliability performance also, but there the improvement is not so significant as in the part level HW-reliability. Small size and high RF-power makes temperature behavior more critical and that is emphasized in R&D work as well actions to ensure good durability of mechanical design. Normal R&D work contain several actions to ensure adequate design margins and verify the reliability performance. Component and material selection. All the materials are selected to fulfill durability over 10 years lifetime. Derating. To ensure the components functionality over the design lifetime their accepted electrical stress is analyzed and limited to safe performance level when seen reasonable. Thermal design. The design internal temperature behavior is verified and design actions completed to keep the components thermal stress in safe level. This contains the active components junction temperature analysis as part of derating analysis and reliability testing to verify results. Failure modelling and risk analysis. The design failure modelling and risk analysis starts in the beginning of the design work to prevent or minimize the design risk. Reliability testing. Module and system level reliability testing is completed to verify the design functionality in the specified conditions and ensure adequate margins for the limits. Typical reliability testing contains different kind of accelerated or highly accelerated life tests (ALT/HALT), which are completed for functional design during the R&D-phases. Trialing and piloting. All new products are in normal cases taken to trial or pilot tests in the real field conditions to ensure their maturity before volume production.

4.2.1 HW reliability HW-reliability means the actions to prevent field failures caused by mechanics and electrical HW-design. •

Redundancy. To minimize single point failures and minimize severity of most critical failures, it is analysed possibilities to duplicate functionality in the design. Redundancy can be arranged by pooling the baseband capacity and arrange redundant RF-paths (diversity and parallel RF-parts).

•

Baseband pooling. All the signal processing is in the pool ensuring flexible baseband capacity usage (Rel.2 feature).



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•


Thermal protection. o Mechanical thermal design to optimize location of high thermal parts in the cooling optimized chassis. o Possibility to use cooling fans for forced air routing with adjustable rotation speed. o Internal temperature sensors for over-temperature protection to be used for temperature alarm and for module shut down or electrical stress reduction. o Over-voltage and short circuit protection. o Prevention to install modules for wrong slot or cables to wrong connection.

4.2.2 Software reliability •

BTS diagnostics. Modules and BTS SW contains internal diagnostics to trace functionality, perform recovery actions and make possible to detect failed part.

•

Recovery actions: o o o

Alarms. In the failure cases modules are able to send an alarm messages to start internal or external failure action. Self-tests (BIST) are run in the start-up’s and in some cases in via element manager to verify functionality and detect possible failure. Automatic or forced restarts are possible to run to get module or BTS to initial state.

•

Remote control. Possibility to control and run recovery actions for BTS from the other network element or locally by using the element manager or other special tool and connection.

•

Special features: o o o o

Memory handling to prevent exceeding of memory budget. Storing of old memory until new release is loaded and activated correctly with data check. Task monitoring and rejuvenation. Saving the internal reporting (log information) for later analysis purposes.



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5. FLEXI MULTIRADIO BTS SYSTEM RELIABILITY Estimated reliability of Flexi Multiradio BTS for the main configurations is presented in this chapter. Estimated values contains the values of MTBF dependency to the RF-power. That is explained more in chapter 5.5. o Tables in this chapter presents the predicted values for most common configurations. MTBF values in table are hours/years. Redundancy is taken account in system level calculations, more details in chapter 5.3.1. All the values are calculated for +25°C ambient temperature. Temperature effect to MTBF is presented in graph 1 in chapter 5.4. For transmission modules it is used one typical value, which is based on the FTEB values. 5.1 Flexi WCDMA BTS (HW Release 1) Reliability Performance System MTBF 3-sectors of 3

BTS Configuration

80 000 h

1+1+1@ 20/40W

System MTBF 2-sectors of 3 120 000 h

FSMB + 2-sector RF + 1-sector RF

System MTBF 3-sectors of 3

BTS Configuration 2+2+2@ 20W 2*FSMB + 2-sector RF + 1-sector RF


BTS Configuration 2+2+2 @ 40W 2*FSMB + 3x 2-sector RF

50 000 h


BTS Configuration 1+1+1 @ 20W FSMB + 3x 1-sector RF


50 000 h

70 000 h


System MTBF 2-sectors of 3 140 000



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5.2 Flexi Multiradio BTS (HW Release 2) System Reliability Performance 5.2.1 MTBF values for one System Module and one 3-sector RF Module configurations

System MTBF 3 sectors of 3


1+1+1 60W

96 000

192 000

1+1+1 40W

104 000

208 000

1+1+1 20W

117 000

233 000

1+1+1 60W

92 000

176 000

1+1+1 40W

99 000

190 000

1+1+1 20W

110 000

211 000

BTS Configurations FSMD + 3-sector RF TX RX

TX RX

RX

TX RX

RX

RX

RF-Common

FSME + 3-sector RF BB 250 250 BB BB-Common FSMD

BB BB 250 250 BB 250 BB-Common FSME

OR

5.2.2 MTBF values for one System Module and two 3-sector RF Module configurations



2+2+2 60W / 1+1+1 60 + 60W MIMO

114 000

200 000

2+2+2 40W / 1+1+1 40 + 40W MIMO

125 000

218 000

2+2+2 20W / 1+1+1 20 + 20W MIMO

133 000

231 000

2+2+2 60W / 1+1+1 60 + 60W MIMO

113 000

202 000

2+2+2 40W / 1+1+1 40 + 40W MIMO

125 000

223 000

2+2+2 20W / 1+1+1 20 + 20W MIMO

144 000

257 000

BTS Configurations

TX RX

RX

TX RX

RX

TX RX

TX

RX

RX

RF-Common

BB 250 250 BB BB-Common FSMD

RX

TX RX

RX

TX RX

RX

RF-Common

OR



FSMD + 2x 3-sector RF

FSME+ 2x 3-sector RF


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5.2.3 MTBF values for two System Module and one 3-sector RF Module configurations

TX

TX

RX

RX

RX

RX



1+1+1 60W

99 000

192 000

BTS Configurations for Flexi

TX RX

RX

2x FSMD + 3-sector RF

RF-Common

1+1+1 40W

104 000

204 000



1+1+1 20W

113 000

220 000

OR

OR

1+1+1 60W

100 000

200 000

BB BB 250 250 BB 250 -Common BB FSME

BB BB 250 250 BB 250 -Common BB FSME

1+1+1 40W

105 000

215 000

1+1+1 20W

115 000

240 000

2x FSME + 3-sector RF

Notice 1. Dual FSMx requires redundant SM feature. Notice 2. FSMx values are for one of two FSMx is functional.

5.2.4 MTBF values for two System Module and two 3-sector RF Module configurations



2+2+2 60W / 1+1+1 60 + 60W MIMO

122 000

215 000

BTS Configurations TX RX

RX

TX RX

RX

TX RX

RX

RF-Common

TX RX

RX

TX RX

RX

TX RX

RF-Common

RX

2x FSMD + 2x 3-sector RF

2+2+2 40W / 1+1+1 40 + 40W MIMO

131 000

232 000



2+2+2 20W / 1+1+1 20 + 20W MIMO

147 000

260 000

OR

OR

2+2+2 60W / 1+1+1 60 + 60W MIMO

125 000

220 000



2+2+2 40W / 1+1+1 40 + 40W MIMO

135 000

240 000

2+2+2 20W / 1+1+1 20 + 20W MIMO

150 000

270 000


2x FSME + 2x 3-sector RF

Notice 1. Dual FSMx requires redundant SM feature. Notice 2. FSMx values are for one of two FSMx is functional.


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5.2.5 MTBF values one System Module and 3x Remote Radio Head configurations



1+1+1 60W

58 000

118 000

1+1+1 40W

66 000

136 000

1+1+1 20W

80 000

170 000

BTS Configurations FSMD + 3x RRH 1TX TX RX

TX RX

RX

RF-Common

TX RX

RX

RF-Common

RX

RF-Common

FSME + 3x RRH 1TX BB 250 250 BB BB-Common

OR

FSMD

BB BB 250 250 BB 250 BB-Common

1+1+1 60W

56 000

114 000

FSME

1+1+1 40W

64 000

132 000

1+1+1 20W

75 000

164 000

1+1+1 40 + 40W

119 000

210 000

1+1+1 20 + 20W

133 000

235 000

1+1+1 40 + 40W

112 000

200 000

1+1+1 20 + 20W

127 000

222 000

FSMD + 3x RRH 2TX TX RX

RX

TX RX

RX

RF-Common

TX RX

RX

TX RX

TX

RX

RF-Common

RX

RX

TX RX

RX

RF-Common

FSME + 3x RRH 2TX BB 250 250 BB BB-Common OR FSMD


250 BB BB 250 BB 250 BB-Common FSME


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5.2.6 Redundancy and capacity definitions in Flexi Multiradio BTS Flexi Multiradio BTS contains redundancy in System Module, RF Modules and Remote Radio Heads. System Module has redundancy with two parallel signal processor cards in FSMD and tree cards in FSME. Baseband capacity (BB capacity) in system values is calculated in every case for > 50% capacity allowing one signal processor card failure in FSMD or FSME. RF Module internal redundancy is in the form of parallel identical TRXs. 3-sector RF Module is able to maintain 67% RF capacity if one sector fails. With two 3 sector RF Modules is possible to have full 1+1 redundancy for 3-sector BTS site. New 2TX RRH has internal redundancy in the form of two parallel identical TRXs. RRH is able to maintain 50% RF capacity if one TRX fails. Redundant two System Module configuration requires redundant System Module feature, please check WCDMA or LTE release roadmap. Next pictures clarifies redundancy principles in BTS reliability calculations.

BB- capacity

BB-capacity

2 baseband cards 100% = Both cards in use. >50% = One card in use.

3 baseband cards 100% = All 3 cards in use. >50% = Two cards in use.

BB 250 250 BB BB - Common FSMD


Figure 2: System Module capacity principles from reliability point of view

Single TRX

Dual TRX

Triple TRX

Y

Y

Y

Y

Y

Y

TX

TX

TX

TX

TX

TX

RX RX

RX RX RX RX

RX RX RX RX RX RX

Common

Common

Common

RF-capacity

RF-capacity

RF-capacity

100% = Div RX can fail

100% = “Div RX can fail

100% = Div RX’s can fail

≥ 50% = 1 of 2 TRX works

> 50% = 2 of 3 TRX works

≥ 50% = NA

Figure 3: RF Modules and RRHs RF capacity principles from reliability point of view 5.2.7 System Availability and Downtime The presented systems intrinsic availability Ai is expected to be 99.999% in minimum for the conditions mentioned in the chapter 5.3 and calculated with formulas defined in chapter 2.2. Five nines (99.999%) availability is indicating better than 5.5 minutes yearly downtime (MAIDT). Document Code/Version Sharenet 445958/3.0


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5.3 Prerequisites and assumptions for predicted data When the field failure rates (MTBF) are compared to predicted value, it is absolutely necessary to notice the difference of component failures used in prediction and the field failures. To make the field MTBF value comparable to predicted one, it is needed to use next restrictions for data: • • • •

•

• •

Module must have about 10 000 000 hours (1 100 years) field use hours before the field results is able to state accurate enough MTBF level. o This can be reached when > 1000 modules is one year on the field. Predicted value is valid during the product lifetime only (BTS 10 years). Failure rates (MTBF) are defined for electrical parts only and typically only for repairable items. Nonrepairable items reliability is assumed to fulfill 10 years lifetime if not otherwise stated and they are not included to predicted value typically. Predicted values are valid in the calculation conditions only, not in the product specified conditions directly. o BTS air (ambient) temperature, which is used based on the common climate conditions: • Humid Temperate, average +15°C. • Warmest month > 10 C (> 50 °F). • Coldest month > 0 C but < 18 C (> 32 °F but < 64.4 °F ). • Most populated, about 55 % of world's population. • Like Mid-European, Mediterranean, Mid Asia and North-America costal climates. • Value is also valid for typical air-conditioned indoor site. • Tropical/Sub-tropical climate, average +25°C. • Coldest month > 18 C (> 64.4 °F ). • Like South Asia, most India and equator climates. • Daily and annual temperature range about +10…+50°C. o Assumes an average RF-output power. • Long-term RF-power load about 50% of maximum. o Operating more than 30 minutes in temperatures over +40°C is not recommended to avoid errors or total failure in operation. Excludes the failures, which are not direct design weaknesses. o “No Fault Found” (NFF’s). o External and misuse failures. • Equipment handling and usage errors. • Failures caused by other network element or external system. Excludes systematic failures caused by 3rd party. o Systematic part failures caused by manufacturer process weakness. Cooling fans are included if not otherwise stated.

For selected units for which MTBF for repair values are not relevant (like non-repairable items mostly), design lifetime, MTTF (Mean Time To Failure) or L10 life or similar life values are given. It is expected that nonrepairable items can work in normal conditions over the product lifetime.



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5.4 Temperature trend for system MTBF and thermal protectioning Next graph is showing an average temperature trend for system MTBF. It is based on the module level thermal analysis. Thermal analysis is done for the worst case conditions and assumes the maximum RFpower in the maximum temperature. Fanless modules are usually more critical for temperature rise, but with derating, component selection and mechanical design structure solutions their thermal stress can be kept near the modules with fans. Modules with cooling fans have higher and more stable raliability performance over the design lifetime. In higher temperatures cooling fans enforce the air flow and keep the module internal temperature lower when compared to fanless module. BTS modules are equipped with thermal protectioning. It’s purpose is to detect an over-heating condition, create temperature alarm and reduce RF-power and capacity at first step. If temperature safe limit is exceeded, module is shut-down to avoid permanent design failure. Module and system should start to operate after possible thermal shut-down immediately when the safe conditions are restored. NSN uses the +25°C temperature as a reference temperature. That temperature corresponds in outdoor conditions the tropical climate, where daily temperature can rise over +40°C on day time and be less than 10°C during night time. That temperature is most common for indoor sites also as a constant temperature. It is assumed that more than 50% of BTS sites are locating in more favourable climates than tropical one (subtropical and moderate) and presented value should cover all normal site locations.

MTBF x 1.8

Module with fans x 1.3

Module without fans

x 1.0 x 0.7 x 0.5

< +15

+25

+ 40

+ 55

°C

Graph 1. Effect of BTS ambient temperature to the system MTBF in average.



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5.5 RF-power effect to the system MTBF Next graphs are showing the average RF-power effect of RF Modules and RRHs. Higher RF-power increases the RF Module and RRHs internal temperatures and thus reduces the MTBF on some part. Because site and BTS usage conditions are different often, it is impossible to prove an exact model for RF-power level and system reliability dependencies. In BTS systems, where the System Module is also included, the effect of RF-power is lower. That is presented in graph 3. RF-power effect to MTBF is lower from 40W to 60W mode, because thermal design has been optimized for high temperatures to avoid permanent component failures and more stable system functionality in hot climate areas.

MTBF x 1.5

x 1.20

RF Module

x 1.0

RRH

x 0.7

20 W

40 W

60 W

RF-power

Graph 2. The RF-power effect to the module MTBF in average, 60W module.



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MTBF x 1.3

x 1.10

RF Module x 1.00

RRH x 0.85

20 W

40 W

60 W

RF-power

Graph 3 The RF-power average affect to the system MTBF, 60W module. Note! 40W RF behaves similarly to 60W and graph is valid to 40W module assuming same change from 40W to 60W.



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18_Reliability and Redundancy of Flexi Multiradio BTS

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