SIST ES 203 997 V1.1.1:2024

ETSI STANDARD
Environmental Engineering (EE);
Requirements and use cases of liquid cooling and high energy
efficiency solutions for 5G BBU in C-RAN mode

2 ETSI ES 203 997 V1.1.1 (2024-05)

Reference
DES/EE-0112
Keywords
5G, RAN
ETSI
650 Route des Lucioles
F-06921 Sophia Antipolis Cedex - FRANCE

Tel.: +33 4 92 94 42 00  Fax: +33 4 93 65 47 16

Siret N° 348 623 562 00017 - APE 7112B
Association à but non lucratif enregistrée à la
Sous-Préfecture de Grasse (06) N° w061004871

Important notice
The present document can be downloaded from:
https://www.etsi.org/standards-search
The present document may be made available in electronic versions and/or in print. The content of any electronic and/or
print versions of the present document shall not be modified without the prior written authorization of ETSI. In case of any
existing or perceived difference in contents between such versions and/or in print, the prevailing version of an ETSI
deliverable is the one made publicly available in PDF format at www.etsi.org/deliver.
Users of the present document should be aware that the document may be subject to revision or change of status.
Information on the current status of this and other ETSI documents is available at
https://portal.etsi.org/TB/ETSIDeliverableStatus.aspx
If you find errors in the present document, please send your comment to one of the following services:
https://portal.etsi.org/People/CommiteeSupportStaff.aspx
If you find a security vulnerability in the present document, please report it through our
Coordinated Vulnerability Disclosure Program:
https://www.etsi.org/standards/coordinated-vulnerability-disclosure
Notice of disclaimer & limitation of liability
The information provided in the present deliverable is directed solely to professionals who have the appropriate degree of
experience to understand and interpret its content in accordance with generally accepted engineering or
other professional standard and applicable regulations.
No recommendation as to products and services or vendors is made or should be implied.
No representation or warranty is made that this deliverable is technically accurate or sufficient or conforms to any law
and/or governmental rule and/or regulation and further, no representation or warranty is made of merchantability or fitness
for any particular purpose or against infringement of intellectual property rights.
In no event shall ETSI be held liable for loss of profits or any other incidental or consequential damages.

Any software contained in this deliverable is provided "AS IS" with no warranties, express or implied, including but not
limited to, the warranties of merchantability, fitness for a particular purpose and non-infringement of intellectual property
rights and ETSI shall not be held liable in any event for any damages whatsoever (including, without limitation, damages
for loss of profits, business interruption, loss of information, or any other pecuniary loss) arising out of or related to the use
of or inability to use the software.
Copyright Notification
No part may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying and
microfilm except as authorized by written permission of ETSI.
The content of the PDF version shall not be modified without the written authorization of ETSI.
The copyright and the foregoing restriction extend to reproduction in all media.

© ETSI 2024.
All rights reserved.
ETSI
3 ETSI ES 203 997 V1.1.1 (2024-05)
Contents
Intellectual Property Rights . 4
Foreword . 4
Modal verbs terminology . 4
Executive summary . 4
Introduction . 5
1 Scope . 6
2 References . 6
2.1 Normative references . 6
2.2 Informative references . 6
3 Definition of terms, symbols and abbreviations . 6
3.1 Terms . 6
3.2 Symbols . 7
3.3 Abbreviations . 7
4 Description of the cooling solutions . 7
5 Immersion and spray liquid cooling technology . 8
5.0 General . 8
5.1 Immersion liquid cooling . 9
5.2 Spray liquid cooling . 11
5.3 Other liquid cooling technologies . 13
6 Key indicators of immersion and spray liquid cooling system . 13
6.1 Requirements of the Liquid coolant . 13
6.1.0 General . 13
6.1.1 Requirements of the equipment operation . 13
6.1.2 Requirements of the physical properties . 13
6.1.3 Requirements of chemical properties . 13
6.1.4 Requirements of safety . 13
6.1.5 Requirements of environmental protection . 13
6.1.6 Requirements of liquid disposal . 14
6.1.7 Requirements of control and monitoring . 14
6.2 Requirements of other key indicators . 14
7 Management procedure and energy efficiency measurement method . 14
7.1 Management procedure . 14
7.2 Partial Energy efficiency measurement for BBU liquid cooling system . 15
7.3 Total Energy efficiency measurement including liquid cooling system . 15
Annex A (informative): Comparison between liquid cooling and air cooling . 18
Annex B (informative): On-line monitor function of the liquid cooling system . 19
Annex C (informative): Bibliography . 20
History . 21

ETSI
4 ETSI ES 203 997 V1.1.1 (2024-05)
Intellectual Property Rights
Essential patents
IPRs essential or potentially essential to normative deliverables may have been declared to ETSI. The declarations
pertaining to these essential IPRs, if any, are publicly available for ETSI members and non-members, and can be
found in ETSI SR 000 314: "Intellectual Property Rights (IPRs); Essential, or potentially Essential, IPRs notified to
ETSI in respect of ETSI standards", which is available from the ETSI Secretariat. Latest updates are available on the
ETSI Web server (https://ipr.etsi.org/).
Pursuant to the ETSI Directives including the ETSI IPR Policy, no investigation regarding the essentiality of IPRs,
including IPR searches, has been carried out by ETSI. No guarantee can be given as to the existence of other IPRs not
referenced in ETSI SR 000 314 (or the updates on the ETSI Web server) which are, or may be, or may become,
essential to the present document.
Trademarks
The present document may include trademarks and/or tradenames which are asserted and/or registered by their owners.
ETSI claims no ownership of these except for any which are indicated as being the property of ETSI, and conveys no
right to use or reproduce any trademark and/or tradename. Mention of those trademarks in the present document does
not constitute an endorsement by ETSI of products, services or organizations associated with those trademarks.
DECT™, PLUGTESTS™, UMTS™ and the ETSI logo are trademarks of ETSI registered for the benefit of its

Members. 3GPP™ and LTE™ are trademarks of ETSI registered for the benefit of its Members and of the 3GPP
Organizational Partners. oneM2M™ logo is a trademark of ETSI registered for the benefit of its Members and of the ®
oneM2M Partners. GSM and the GSM logo are trademarks registered and owned by the GSM Association.
Foreword
This ETSI Standard (ES) has been produced by ETSI Technical Committee Environmental Engineering (EE).
Modal verbs terminology
In the present document "shall", "shall not", "should", "should not", "may", "need not", "will", "will not", "can" and
"cannot" are to be interpreted as described in clause 3.2 of the ETSI Drafting Rules (Verbal forms for the expression of
provisions).
"must" and "must not" are NOT allowed in ETSI deliverables except when used in direct citation.
Executive summary
Liquid cooling systems are mainly used for processing capability of the high thermal power density, which exceeds the
physical limits of air cooling methods, to support more and more application scenarios where manufacturers are
creating competitive advantages. Liquid cooling can provide heat transfer capabilities several orders of magnitude
higher than that of air cooling, and applications dealing with high heat density in the core and edge computing as well
as access network will increasingly require the support of liquid cooling technology.
The present document identifies the requirements for liquid cooling and high energy efficiency solutions for 5G BBU in
Centralized-RAN mode, including requirements of immersion and spray liquid cooling technology, key indicators of
immersion and spray liquid, safety requirements of immersion and spray liquid cooling system, management procedure
and energy efficiency measurement method, and use cases of liquid cooling solutions.
ETSI
5 ETSI ES 203 997 V1.1.1 (2024-05)
Introduction
The power consumption of 5G BBU increases significantly compared with that of 4G BBU. On the one side, in
Centralized-RAN mode, BBU is centrally installed in the cabinet, and the number of BBU in one cabinet can reach as
many as 10. Besides, in the air cooling system of the BBU, the airflow goes in from the left side and out from the right
side (or in from the right side and out from the left side). All of the factors mentioned above make it difficult to
dissipate the heat generated from BBU, resulting in a significant increase in air conditioning cooling capacity and power
consumption required for heat dissipation of BBU equipment compared with the 4G one. On the other side, the internal
stability of the equipment becomes poor and the failure rate increases because the internal temperature of the BBU is
too high. From the perspective of equipment safety as well as energy saving and carbon reduction, exploring more
efficient and energy-saving technical methods is crucial. In order to solve the heat dissipation problem of 5G BBU in
Centralized-RAN mode, it is necessary to introduce liquid cooling technology to provide a better heat dissipation effect
for equipment with high power density and complex airflow conditions.
Liquids have a much larger thermal capacity than that of gases, which makes them ideal as heat dissipation media in
high-density devices, and therefore liquid cooling has been already heavily used in the server cooling of data centres. In
the liquid cooling system, there is no compressor, instead, it can directly use the heat dissipation of outdoor air as a
natural cold source. The CoolEff of the liquid cooling server has been proved to be reduced to 1.1-1.2 practically.
Though the entire power of BBU is less than that of the server, the volume power density is higher compared with that
of the server, which makes it suitable to utilize liquid cooling. This recommendation focuses on the solution of liquid
cooling method being used in the 5G BBU.
The present document was developed jointly by ETSI TC EE and ITU-T Study Group 5. It is published respectively by
ITU and ETSI as Recommendation ITU-T L.1326 [i.1] and ETSI ES 203 997 (the present document), which are
technically-equivalent.
ETSI
6 ETSI ES 203 997 V1.1.1 (2024-05)
1 Scope
The present document provides requirements for liquid cooling and high energy efficiency solutions for 5G BBU in
Centralized RAN mode (C-RAN), including: requirements of immersion and spray liquid cooling technology, key
indicators of immersion and spray liquid, safety requirements of immersion and spray liquid cooling system,
management procedure and energy efficiency measurement method, and use cases of cooling solutions.
2 References
2.1 Normative references
References are either specific (identified by date of publication and/or edition number or version number) or
non-specific. For specific references, only the cited version applies. For non-specific references, the latest version of the
referenced document (including any amendments) applies.
Referenced documents which are not found to be publicly available in the expected location might be found at
https://docbox.etsi.org/Reference/.
NOTE: While any hyperlinks included in this clause were valid at the time of publication, ETSI cannot guarantee
their long term validity.
The following referenced documents are necessary for the application of the present document.
Not applicable.
2.2 Informative references
References are either specific (identified by date of publication and/or edition number or version number) or
non-specific. For specific references, only the cited version applies. For non-specific references, the latest version of the
referenced document (including any amendments) applies.
NOTE: While any hyperlinks included in this clause were valid at the time of publication, ETSI cannot guarantee
their long term validity.
The following referenced documents are not necessary for the application of the present document but they assist the
user with regard to a particular subject area.
[i.1] Recommendation ITU-T L.1326 (08/2023): "Requirements and use cases of liquid cooling
solutions and high energy efficiency solutions for 5G BBU in Centralized-RAN mode".
[i.2] ETSI TS 103 586: "Environmental Engineering (EE); Liquid cooling solutions for Information and
Communication Technology (ICT) infrastructure equipment".
3 Definition of terms, symbols and abbreviations
3.1 Terms
For the purposes of the present document, the following terms apply:
Cloud RAN (C-RAN): Radio Access Network (RAN) where functions are partially or completely centralized, with two
additional key features: pooling of baseband/hardware resources, and virtualization through general-purpose processors
Distributed RAN (D-RAN): network development where Radio Access Network (RAN) processing is fully performed
at the site, as in 4G
ETSI
7 ETSI ES 203 997 V1.1.1 (2024-05)
3.2 Symbols
Void.
3.3 Abbreviations
For the purposes of the present document, the following abbreviations apply:
4G fourth Generation
5G fifth Generation
AHU Air Handling Unit
BBU BaseBand Unit
CDU Coolant Distribution Unit
CoolEff Cooling Effectiveness
CPU Central Processing Unit
CRAC Computer Room Air Conditioner
GSM Global System for Mobile communications
GTMU GSM Transmission & Timing & Management Unit
GWP Global Warming Potential
ICT Information and Communications Technology
IT Information Technology
ODP Ozone Depletion Potential
PCB Printed Circuit Board
RAN Radio Access Network
UPEU Universal Power and Environment interface Unit
UPS Uninterruptible Power Supply
4 Description of the cooling solutions
In the past few years, the air cooling system makes it possible to accommodate higher heat density cooling requirements
by bringing the cold source closer to the heat source or by hot-aisle/cold-aisle containment. However, as rack power
density increases to above 20 kW (Figure 1), the benefits of these methods gradually diminish. A variety of liquid
cooling technologies have emerged to meet the cooling requirements of high heat density cabinets.

Figure 1: Rack power density and cooling solutions
5G BBU is deployed in the cabinet in Centralized-RAN mode, which not only has a large total power (10 BBUs reach
5-6 kW), but also because the BBU air flow sometimes is insufficient, which makes it difficult to cool the cabinet.
ETSI
8 ETSI ES 203 997 V1.1.1 (2024-05)
The traditional cooling mode in data centre is not suitable for the cooling demand of 5G BBU in Centralized-RAN
mode. The BBU equipment with liquid spray can cool the BBU chip/board with high density and the main board
without relying on any air flow channel.
Using immersion and spray liquid cooling technology can not only solve the problem of low energy efficiency for 5G
base station (BBU centralized deployment-the C-RAN mode), but also solve the problem of high density of BBU chip
and difficulty of heat dissipation. However, as a new technology completely different from the traditional air cooling
technology, it needs a complete system design and safety protection mechanism; otherwise unexpected safety risks may
appear and damage the whole system.
5 Immersion and spray liquid cooling technology
5.0 General
Nowadays, there are three main types of liquid cooling technology for ICT equipment, i.e. liquid cooling of cold plates,
immersion, and spray. The configuration of different parts in 5G BBU is extremely dense and the overall dimension of
the 5G BBU device is 2 u high, containing 4 layers of BBU board with each board about 2 cm thick, and the gap
between one board and the other is not more than 5 mm. The typical configuration of the BBU is shown in Figure 2.
The corresponding slots of the BBU are listed in Table 1. And therefore neither pasting the heat exchange plate on the
heat-generating chip of the board nor adding liquid flow copper tubes on the PCB board is feasible. Based on the factors
mentioned above, the 5G BBU can only utilize immersion or spray liquid cooling methods other than cold plate liquid
cooling.
Universal Power and The GSM Transmission &
Fan Baseband board
Environment Interface Timing & Management
Unit (UPEU) Unit for BBU (GTMU)

Figure 2: Typical Configuration of the BBU
Table 1: Slots of the BBU
Slot 0 Slot 4
Power
Slot 1 Slot 5
FAN
Slot 2 Slot 6
Power
Slot 3 (Baseband board) Slot 7

ETSI
9 ETSI ES 203 997 V1.1.1 (2024-05)
5.1 Immersion liquid cooling
In an immersion liquid cooling solution, all BBU components are immersed in a flowing thermally conductive and
electrically insulating liquid. By this method, the flowing liquid takes away the heat generated by all BBU components,
which maximizes the heat conduction characteristics of the coolant and is the most energy-efficient liquid cooling
method. In a single-phase immersion liquid cooling system, the entire BBU device is installed vertically with the front
side up in the thermally conductive and electrically insulating coolant. The coolant is in direct contact with all BBU
components and absorbs heat from them, after which the coolant is carried by a pump to a heat exchanger in the CDU
(coolant distribution unit). Inside the heat exchanger, the heat is transferred between the refrigerant and the coolant
resulting in temperature decreases of the coolant, after which the low-temperature coolant can participate in the next
circulation of heat absorption of BBU components and heat release in the heat exchanger in the CDU. As for the heat
absorbed by the refrigerant, it can finally be taken to the outdoor heat dissipation equipment through the heat exchanger
in the CDU. The detailed coolant circulation and heat transfer in single-phase immersion liquid cooling systems are
demonstrated in Figure 3. CDU is usually installed near the BBU device cabinet or outside the data centre room.
Single-phase immersion
liquid cooling system
Open data centre server
Final heat
rack, filled with circulating
Coolant distribution unit (CDU)
dissipation option
Electronosafe coolant
Evaporative cooling
tower
Coolant Coolant-water
pump heat exchanger
Dry condenser
Chilled water
circulation
The coolant is in direct contact with all BBU components and absorbs heat from them, after which the
coolant is carried by a pump to a heat exchanger in the CDU where the coolant’s temperature decreases.
The coolant finally flows out of the heat exchanger back to the rack at a user-specified temperature.

Figure 3: Coolant circulation and heat transfer in single-phase
immersion liquid cooling systems
An
...

SLOVENSKI STANDARD
01-september-2024
Okoljski inženiring (EE) - Zahteve in primeri uporabe rešitev za tekočinsko hlajenje
in visoko energetsko učinkovitost za 5G BBU v načinu C-RAN
Environmental Engineering (EE) - Requirements and use cases of liquid cooling and
high energy efficiency solutions for 5G BBU in C-RAN mode
Ta slovenski standard je istoveten z: ETSI ES 203 997 V1.1.1 (2024-05)
ICS:
19.040 Preskušanje v zvezi z Environmental testing
okoljem
35.020 Informacijska tehnika in Information technology (IT) in
tehnologija na splošno general
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

ETSI STANDARD
Environmental Engineering (EE);
Requirements and use cases of liquid cooling and high energy
efficiency solutions for 5G BBU in C-RAN mode

2 ETSI ES 203 997 V1.1.1 (2024-05)

Reference
DES/EE-0112
Keywords
5G, RAN
ETSI
650 Route des Lucioles
F-06921 Sophia Antipolis Cedex - FRANCE

Tel.: +33 4 92 94 42 00  Fax: +33 4 93 65 47 16

Siret N° 348 623 562 00017 - APE 7112B
Association à but non lucratif enregistrée à la
Sous-Préfecture de Grasse (06) N° w061004871

Important notice
The present document can be downloaded from:
https://www.etsi.org/standards-search
The present document may be made available in electronic versions and/or in print. The content of any electronic and/or
print versions of the present document shall not be modified without the prior written authorization of ETSI. In case of any
existing or perceived difference in contents between such versions and/or in print, the prevailing version of an ETSI
deliverable is the one made publicly available in PDF format at www.etsi.org/deliver.
Users of the present document should be aware that the document may be subject to revision or change of status.
Information on the current status of this and other ETSI documents is available at
https://portal.etsi.org/TB/ETSIDeliverableStatus.aspx
If you find errors in the present document, please send your comment to one of the following services:
https://portal.etsi.org/People/CommiteeSupportStaff.aspx
If you find a security vulnerability in the present document, please report it through our
Coordinated Vulnerability Disclosure Program:
https://www.etsi.org/standards/coordinated-vulnerability-disclosure
Notice of disclaimer & limitation of liability
The information provided in the present deliverable is directed solely to professionals who have the appropriate degree of
experience to understand and interpret its content in accordance with generally accepted engineering or
other professional standard and applicable regulations.
No recommendation as to products and services or vendors is made or should be implied.
No representation or warranty is made that this deliverable is technically accurate or sufficient or conforms to any law
and/or governmental rule and/or regulation and further, no representation or warranty is made of merchantability or fitness
for any particular purpose or against infringement of intellectual property rights.
In no event shall ETSI be held liable for loss of profits or any other incidental or consequential damages.

Any software contained in this deliverable is provided "AS IS" with no warranties, express or implied, including but not
limited to, the warranties of merchantability, fitness for a particular purpose and non-infringement of intellectual property
rights and ETSI shall not be held liable in any event for any damages whatsoever (including, without limitation, damages
for loss of profits, business interruption, loss of information, or any other pecuniary loss) arising out of or related to the use
of or inability to use the software.
Copyright Notification
No part may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying and
microfilm except as authorized by written permission of ETSI.
The content of the PDF version shall not be modified without the written authorization of ETSI.
The copyright and the foregoing restriction extend to reproduction in all media.

© ETSI 2024.
All rights reserved.
ETSI
3 ETSI ES 203 997 V1.1.1 (2024-05)
Contents
Intellectual Property Rights . 4
Foreword . 4
Modal verbs terminology . 4
Executive summary . 4
Introduction . 5
1 Scope . 6
2 References . 6
2.1 Normative references . 6
2.2 Informative references . 6
3 Definition of terms, symbols and abbreviations . 6
3.1 Terms . 6
3.2 Symbols . 7
3.3 Abbreviations . 7
4 Description of the cooling solutions . 7
5 Immersion and spray liquid cooling technology . 8
5.0 General . 8
5.1 Immersion liquid cooling . 9
5.2 Spray liquid cooling . 11
5.3 Other liquid cooling technologies . 13
6 Key indicators of immersion and spray liquid cooling system . 13
6.1 Requirements of the Liquid coolant . 13
6.1.0 General . 13
6.1.1 Requirements of the equipment operation . 13
6.1.2 Requirements of the physical properties . 13
6.1.3 Requirements of chemical properties . 13
6.1.4 Requirements of safety . 13
6.1.5 Requirements of environmental protection . 13
6.1.6 Requirements of liquid disposal . 14
6.1.7 Requirements of control and monitoring . 14
6.2 Requirements of other key indicators . 14
7 Management procedure and energy efficiency measurement method . 14
7.1 Management procedure . 14
7.2 Partial Energy efficiency measurement for BBU liquid cooling system . 15
7.3 Total Energy efficiency measurement including liquid cooling system . 15
Annex A (informative): Comparison between liquid cooling and air cooling . 18
Annex B (informative): On-line monitor function of the liquid cooling system . 19
Annex C (informative): Bibliography . 20
History . 21

ETSI
4 ETSI ES 203 997 V1.1.1 (2024-05)
Intellectual Property Rights
Essential patents
IPRs essential or potentially essential to normative deliverables may have been declared to ETSI. The declarations
pertaining to these essential IPRs, if any, are publicly available for ETSI members and non-members, and can be
found in ETSI SR 000 314: "Intellectual Property Rights (IPRs); Essential, or potentially Essential, IPRs notified to
ETSI in respect of ETSI standards", which is available from the ETSI Secretariat. Latest updates are available on the
ETSI Web server (https://ipr.etsi.org/).
Pursuant to the ETSI Directives including the ETSI IPR Policy, no investigation regarding the essentiality of IPRs,
including IPR searches, has been carried out by ETSI. No guarantee can be given as to the existence of other IPRs not
referenced in ETSI SR 000 314 (or the updates on the ETSI Web server) which are, or may be, or may become,
essential to the present document.
Trademarks
The present document may include trademarks and/or tradenames which are asserted and/or registered by their owners.
ETSI claims no ownership of these except for any which are indicated as being the property of ETSI, and conveys no
right to use or reproduce any trademark and/or tradename. Mention of those trademarks in the present document does
not constitute an endorsement by ETSI of products, services or organizations associated with those trademarks.
DECT™, PLUGTESTS™, UMTS™ and the ETSI logo are trademarks of ETSI registered for the benefit of its

Members. 3GPP™ and LTE™ are trademarks of ETSI registered for the benefit of its Members and of the 3GPP
Organizational Partners. oneM2M™ logo is a trademark of ETSI registered for the benefit of its Members and of the ®
oneM2M Partners. GSM and the GSM logo are trademarks registered and owned by the GSM Association.
Foreword
This ETSI Standard (ES) has been produced by ETSI Technical Committee Environmental Engineering (EE).
Modal verbs terminology
In the present document "shall", "shall not", "should", "should not", "may", "need not", "will", "will not", "can" and
"cannot" are to be interpreted as described in clause 3.2 of the ETSI Drafting Rules (Verbal forms for the expression of
provisions).
"must" and "must not" are NOT allowed in ETSI deliverables except when used in direct citation.
Executive summary
Liquid cooling systems are mainly used for processing capability of the high thermal power density, which exceeds the
physical limits of air cooling methods, to support more and more application scenarios where manufacturers are
creating competitive advantages. Liquid cooling can provide heat transfer capabilities several orders of magnitude
higher than that of air cooling, and applications dealing with high heat density in the core and edge computing as well
as access network will increasingly require the support of liquid cooling technology.
The present document identifies the requirements for liquid cooling and high energy efficiency solutions for 5G BBU in
Centralized-RAN mode, including requirements of immersion and spray liquid cooling technology, key indicators of
immersion and spray liquid, safety requirements of immersion and spray liquid cooling system, management procedure
and energy efficiency measurement method, and use cases of liquid cooling solutions.
ETSI
5 ETSI ES 203 997 V1.1.1 (2024-05)
Introduction
The power consumption of 5G BBU increases significantly compared with that of 4G BBU. On the one side, in
Centralized-RAN mode, BBU is centrally installed in the cabinet, and the number of BBU in one cabinet can reach as
many as 10. Besides, in the air cooling system of the BBU, the airflow goes in from the left side and out from the right
side (or in from the right side and out from the left side). All of the factors mentioned above make it difficult to
dissipate the heat generated from BBU, resulting in a significant increase in air conditioning cooling capacity and power
consumption required for heat dissipation of BBU equipment compared with the 4G one. On the other side, the internal
stability of the equipment becomes poor and the failure rate increases because the internal temperature of the BBU is
too high. From the perspective of equipment safety as well as energy saving and carbon reduction, exploring more
efficient and energy-saving technical methods is crucial. In order to solve the heat dissipation problem of 5G BBU in
Centralized-RAN mode, it is necessary to introduce liquid cooling technology to provide a better heat dissipation effect
for equipment with high power density and complex airflow conditions.
Liquids have a much larger thermal capacity than that of gases, which makes them ideal as heat dissipation media in
high-density devices, and therefore liquid cooling has been already heavily used in the server cooling of data centres. In
the liquid cooling system, there is no compressor, instead, it can directly use the heat dissipation of outdoor air as a
natural cold source. The CoolEff of the liquid cooling server has been proved to be reduced to 1.1-1.2 practically.
Though the entire power of BBU is less than that of the server, the volume power density is higher compared with that
of the server, which makes it suitable to utilize liquid cooling. This recommendation focuses on the solution of liquid
cooling method being used in the 5G BBU.
The present document was developed jointly by ETSI TC EE and ITU-T Study Group 5. It is published respectively by
ITU and ETSI as Recommendation ITU-T L.1326 [i.1] and ETSI ES 203 997 (the present document), which are
technically-equivalent.
ETSI
6 ETSI ES 203 997 V1.1.1 (2024-05)
1 Scope
The present document provides requirements for liquid cooling and high energy efficiency solutions for 5G BBU in
Centralized RAN mode (C-RAN), including: requirements of immersion and spray liquid cooling technology, key
indicators of immersion and spray liquid, safety requirements of immersion and spray liquid cooling system,
management procedure and energy efficiency measurement method, and use cases of cooling solutions.
2 References
2.1 Normative references
References are either specific (identified by date of publication and/or edition number or version number) or
non-specific. For specific references, only the cited version applies. For non-specific references, the latest version of the
referenced document (including any amendments) applies.
Referenced documents which are not found to be publicly available in the expected location might be found at
https://docbox.etsi.org/Reference/.
NOTE: While any hyperlinks included in this clause were valid at the time of publication, ETSI cannot guarantee
their long term validity.
The following referenced documents are necessary for the application of the present document.
Not applicable.
2.2 Informative references
References are either specific (identified by date of publication and/or edition number or version number) or
non-specific. For specific references, only the cited version applies. For non-specific references, the latest version of the
referenced document (including any amendments) applies.
NOTE: While any hyperlinks included in this clause were valid at the time of publication, ETSI cannot guarantee
their long term validity.
The following referenced documents are not necessary for the application of the present document but they assist the
user with regard to a particular subject area.
[i.1] Recommendation ITU-T L.1326 (08/2023): "Requirements and use cases of liquid cooling
solutions and high energy efficiency solutions for 5G BBU in Centralized-RAN mode".
[i.2] ETSI TS 103 586: "Environmental Engineering (EE); Liquid cooling solutions for Information and
Communication Technology (ICT) infrastructure equipment".
3 Definition of terms, symbols and abbreviations
3.1 Terms
For the purposes of the present document, the following terms apply:
Cloud RAN (C-RAN): Radio Access Network (RAN) where functions are partially or completely centralized, with two
additional key features: pooling of baseband/hardware resources, and virtualization through general-purpose processors
Distributed RAN (D-RAN): network development where Radio Access Network (RAN) processing is fully performed
at the site, as in 4G
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3.2 Symbols
Void.
3.3 Abbreviations
For the purposes of the present document, the following abbreviations apply:
4G fourth Generation
5G fifth Generation
AHU Air Handling Unit
BBU BaseBand Unit
CDU Coolant Distribution Unit
CoolEff Cooling Effectiveness
CPU Central Processing Unit
CRAC Computer Room Air Conditioner
GSM Global System for Mobile communications
GTMU GSM Transmission & Timing & Management Unit
GWP Global Warming Potential
ICT Information and Communications Technology
IT Information Technology
ODP Ozone Depletion Potential
PCB Printed Circuit Board
RAN Radio Access Network
UPEU Universal Power and Environment interface Unit
UPS Uninterruptible Power Supply
4 Description of the cooling solutions
In the past few years, the air cooling system makes it possible to accommodate higher heat density cooling requirements
by bringing the cold source closer to the heat source or by hot-aisle/cold-aisle containment. However, as rack power
density increases to above 20 kW (Figure 1), the benefits of these methods gradually diminish. A variety of liquid
cooling technologies have emerged to meet the cooling requirements of high heat density cabinets.

Figure 1: Rack power density and cooling solutions
5G BBU is deployed in the cabinet in Centralized-RAN mode, which not only has a large total power (10 BBUs reach
5-6 kW), but also because the BBU air flow sometimes is insufficient, which makes it difficult to cool the cabinet.
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The traditional cooling mode in data centre is not suitable for the cooling demand of 5G BBU in Centralized-RAN
mode. The BBU equipment with liquid spray can cool the BBU chip/board with high density and the main board
without relying on any air flow channel.
Using immersion and spray liquid cooling technology can not only solve the problem of low energy efficiency for 5G
base station (BBU centralized deployment-the C-RAN mode), but also solve the problem of high density of BBU chip
and difficulty of heat dissipation. However, as a new technology completely different from the traditional air cooling
technology, it needs a complete system design and safety protection mechanism; otherwise unexpected safety risks may
appear and damage the whole system.
5 Immersion and spray liquid cooling technology
5.0 General
Nowadays, there are three main types of liquid cooling technology for ICT equipment, i.e. liquid cooling of cold plates,
immersion, and spray. The configuration of different parts in 5G BBU is extremely dense and the overall dimension of
the 5G BBU device is 2 u high, containing 4 layers of BBU board with each board about 2 cm thick, and the gap
between one board and the other is not more than 5 mm. The typical configuration of the BBU is shown in Figure 2.
The corresponding slots of the BBU are listed in Table 1. And therefore neither pasting the heat exchange plate on the
heat-generating chip of the board nor adding liquid flow copper tubes on the PCB board is feasible. Based on the factors
mentioned above, the 5G BBU can only utilize immersion or spray liquid cooling methods other than cold plate liquid
cooling.
Universal Power and The GSM Transmission &
Fan Baseband board
Environment Interface Timing & Management
Unit (UPEU) Unit for BBU (GTMU)

Figure 2: Typical Configuration of the BBU
Table 1: Slots of the BBU
Slot 0 Slot 4
Power
Slot 1 Slot 5
FAN
Slot 2 Slot 6
Power
Slot 3 (Baseband board) Slot 7

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5.1 Immersion liquid cooling
In an immersion liquid cooling solution, all BBU components are immersed in a flowing thermally conductive and
electrically insulating liquid. By this method, the flowing liquid takes away the heat generated by all BBU components,
which maximizes the heat conduction characteristics of the coolant and is the most energy-efficient liquid cooling
method. In a single-phase immersion liquid cooling system, the entire BBU device is installed vertically with the front
side up in the thermally conductive and electrically insulating coolant. The coolant is in direct contact with all BBU
components and absorbs heat from them, after which the coolant is carried by a pump to a heat exchanger in the CDU
(coolant distribution unit). Inside the heat exchanger, the heat is transferred between the refrigerant and the coolant
resulting in temperature decreases of the coolant, after which the low-temperature coolant can participate in the next
circulation of heat absorption of BBU components and heat release in the heat exchanger in the CDU. As for the heat
absorbed by the refrigerant, it can finally be taken to the outdoor heat dissipation equipment through the heat exchanger
in the CDU. The detailed coolant circulation and heat transfer in single-phase immersion liquid cooling systems are
demonstrated in Figure 3. CDU is usually installed near the BBU device cabinet or outside the data centre room.
Single-phase immersion
liquid cooling system
Open data centre server
Final heat
rack, filled with circulating
Coolant distribution unit (CDU)
dissipation option
Electronosafe coolant
Evaporative cooling
tower
Coolant Coolant-water
pump heat exchanger
Dry condenser
Chilled water
circulation
The coolant is in direct co
...