Distributed Base Stations

Application Note:
Distributed Base Stations
architecture changes are being implemented to correct some of
these long-standing drawbacks.
The most popular type of Wireless Base Station deployment
(cell site) consists of a Base Transceiver Station (BTS) located
in close proximity to the antenna tower. This BTS connects
to both the Mobile Switching Center (MSC), which directs
hand-off between towers for mobile users, and the Radio
Frequency (RF) transmitters/receivers antenna located on
the tower structure. The “hut” at the base of the tower or
in the basement of a tall building is configured with the RF
transceivers and RF amplifiers, along with the baseband
processing unit, test and alarm unit, ac power, battery back-up
systems, and a backhaul transport unit (MSC connection), all of
which are typically installed in a single rack enclosure. The RF
amplifiers drive through the cables to the antenna located at the
top of the elevated tower. This typical setup requires climate
controls for the entire building structure, a large building site
footprint, and a hefty back-up system (large, bulky batteries); it
also is subject to high signal and power losses in the cable due
to the length of the cable between the RF amplifiers and the
transmitter/receiver antennas mounted at the top of the tower.
Tower Mounted Amplifiers (TMAs) are sometimes required
to boost this RF signal when the distance between the towermounted antenna and the BTS location is too great. Some
Five basic Base Station architectures are in use today:
1. Legacy architecture, with all of the equipment located
inside the BTS hut, with a coax connection to the top
of the tower and a fiber/copper connection to the MSC
(illustrated in Figure 1).
2. Split architecture design, with the BaseBand Unit
(BBU) located indoors and a Remote Radio Unit (RRU)
located on the tower (illustrated in Figure 2).
3. “Hoteling” approach that uses a single BTS hut but
connects to multiple towers (illustrated in Figure 3).
4. All-outdoor, zero-footprint BTS, with all components
located on the tower (essentially multiple boxes on
the tower that travel via a combination of coax to the
antennas and fiber/copper to the MSC without a BTS
hut in between, as illustrated in Figure 4).
5. Capacity Transfer System (wireless BTS repeater
concept) (illustrated in Figure 6).
Transmier
Receiver #1
Receiver #2
= Lielfuse protecon opportunity
BTS Hut
RF
Amplifiers
AC
Power
RF Combiners
Power
Supply
Test and
Alarm Units
Baeries
To Mobile Switching
Center ( MSC)
{
Voice
Voice
Voice
Data
Control
Scanning
Base
Staon
Controllers
Data
Transceivers
Receiver
Mul-coupler
Radio tower and BTS equipment used in a typical cell site locaon.
Legacy BTS drawbacks:
Figure 1. Legacy BTS (cell site). Radio •tower
equipment
a typical
BTS hutand
mustBTS
be physically
close used
to the in
tower
to avoidcell
the site location.
need for Tower Mounted Amplifiers (TMAs)
• Large footprint requirement
• BTS hut must be physically close to the tower to avoid the need for Tower Mounted Amplifiers (TMAs)
• Structurally reinforced roo‰ops needed to support BTS hut
• Large footprint requirement
• Lack of suitable size locaon in highly populated areas
Parametertosecurity
requirements
• Structurally reinforced rooftops•needed
support
BTS hut
• Nuisance appearance in local neighborhoods
Legacy BTS drawbacks:
• Lack of suitable size location in highly populated areas
• Parameter security requirements
• Nuisance appearance in local neighborhoods www.littelfuse.com
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The Distributed Base Station architecture illustrated in Figure 2
places the RF transceivers on the tower. This arrangement
requires an optical fiber to connect the digital baseband signals
inside the BST hut with the tower mounted RRU. This allows
making a much shorter coax connection between the RRU
and the transmitters and receivers on the top of the tower.
This arrangement consumes much less RF power due to
Distributed Base Stations
the reduced losses that result from using the shorter coaxial
cable and the optical fiber. It also allows greater flexibility in
selecting the location of the BTS hut with respect to the tower.
The BTS hut and the tower currently may be up to 20 km (12
miles) apart; in the near future, this may be as much as 40 km
(25 miles).
Transmier
Receiver #1
Receiver #2
= Lielfuse protecon opportunity
Coax
Remote Radio Units (RRUs)
Higher exposure for RRUs
BTS Hut
AC
Power
To Mobile
Switching
Center ( MSC)
{
Power
Supply
Baeries
Voice
Data
Test and
Alarm Units
Base
Staon
Controllers
Fiber/Coax
Voice
Voice
Data
Control
Scanning
Transceivers
Radio tower and Distributed BTS equipment
Figure 2. Distributed BTS Architecture
Distributed BTS architecture advantages:
Distributed
BTScan
architecture
advantages:
• Hut
be physically
remote from antenna site; no TMAs required, more flexibility on
placement
•hutHut
can be physically remote from antenna site; no TMAs required, more flexibility on
• Smaller
footprint requirements (lower power requirements): no special reinforced
hut placement
rooftops,
parameter
security(lower
measures,
reduced
nuisance appearance
• Smallerreduced
footprint
requirements
power
requirements):
no special reinforced rooŠops,
There arereduced
no RF amplifiers
contained
within
the
BTS
hut
or
TMAs
because
the RRU
parameter security measures, reduced nuisance
appearance
performs this function in this architecture. However, because this function is now located on
the tower, it has increased exposure to lightning induced surges.
There are no RF amplifiers contained within the BTS hut or TMAs because the RRU performs this funcon
in this architecture. However, because this funcon is now located on the tower, it has
increased exposure to lightning induced surges.
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Outdoor LED Lighting
Distributed Base Stations
and 4G Base Stations in densely populated downtown districts.
Placing all of the hardware on the tower (see Figures 4 and 5)
makes a zero-footprint design possible.
This Distributed Base Station concept can be further expanded
by using a central remote “hotel” for multiple tower sites (see
Figure 3). This approach dramatically reduces the required
footprint, which allows for an easier expansion of the new 3G
Transmier
Receiver #1
Receiver #2
= Lielfuse protecon opportunity
Coax
Remote Radio Units (RRUs)
Transmier
Receiver #1
Receiver #2
Transmier
Receiver #1
Receiver #2
Coax
Remote Radio Units (RRUs)
Fiber/Coax
Coax
Remote Radio Units (RRUs)
Transmier
Receiver #1
Receiver #2
Coax
Fiber/Coax
Higher exposure for RRUs
Remote Radio Units (RRUs)
BTS Hut
AC
Power
To Mobile
Switching
Center ( MSC)
{
Power
Supply
Baeries
Voice
Data
Test and
Alarm Units
Base
Staon
Controllers
Fiber/Coax
Voice
Voice
Data
Control
Scanning
Transceivers
Radio tower and Distributed Base Staon equipment
Figure 3. “Hoteling” Distributed BTS Architecture
“Hoteling” Distributed Base Staon Architecture advantages:
“Hoteling” Distributed Base Station Architecture advantages:
• Single hut can be physically remote from mulple antenna sites
• required
Single hut
can be
physically
remote
multiple antenna sites
• No TMAs
because
RRUs
substute
for thisfrom
feature
• More flexibility
on hutrequired
placement
due to smaller
footprint for this feature
• No TMAs
because
RRUs substitute
• Lower power
requirements
• More
flexibility on hut placement due to smaller footprint
• No special reinforced rooŒops requirements
• parameter
Lower power
requirements
• Reduced
security
measures
• nuisance
No special
reinforced rooftops requirements
• Reduced
appearance
• Reduced parameter security measures
• Reduced nuisance appearance
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Outdoor LED Lighting
Distributed Base Stations
Transmier
Receiver #1
Receiver #2
Coax
Remote Radio Units (RRUs)
Higher exposure for RRUs and CTBP units
Control, transport, Baseband, & power (CTBP)
To Mobile
Switching
Center ( MSC)
{
Fiber and Power
= Lielfuse protec‹on opportunity
Radio tower and Distributed Base Sta‹on equipment
Figure 4. Zero-footprint BTS Architecture
Zero-footprint Architecture advantages:
• No TMAs required, most flexibility
Zero-footprint Architecture advantages:
• No footprint requirements except for tower (this equipment may be installed
• onNo
TMAs required, most flexibility
the top floor of a parking garage without need of a tower)
•
No
footprint requirements except for tower (this equipment may be installed on
• Lowest power requirements
the
top floor of a parking garage without need of a tower)
• No special reinforced rooftops
•
Lowest
power requirements
• No physical security measures (depending on specific location of equipment)
• No special reinforced roo‰ops
• Minimized nuisance appearance
• No physical security measures (depending on specific loca‹on of equipment)
•
Minimized
nuisance
appearance
Another variation on the Distributed BTS concept is the
Figure 5 shows a zero-footprint BTS
installed on
the top floor
of
capacity transfer system, in which a single BTS with a digital
connection to the BSC (Base Station Controller) is connected
to additional tower sites via microwave frequency carriers to
extend its footprint coverage (see Figure 6).
the parking garage at the Littelfuse, Inc. headquarters building
in Chicago, Illinois, USA.
The RRUs are powered by either a shielded or unshielded dc
power cable. Because they are now located on the tower,
their exposure to nearby lightning strikes is greatly increased.
Therefore, appropriate overvoltage protection must be
considered for these new architectures. ITU K.56 provides
some basic recommendations for the BTS hut; however, it
was issued before the concept of Distributed BTSs started.
New efforts are underway in ITU Study Group 5 to define the
lightning protection needs of this new architecture.
The power supplies and the tower mounted equipment
require both over-voltage and over-current protection. Figure 7
illustrates the recommendation for protecting the power supply
interface as a block diagram. Given that this dc supply is most
likely a 48-volt supply, the stand-off voltage for the protection
Figure 5: Zero-footprint BTS installed on the top floor of a
parking garage.
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Outdoor LED Lighting
Distributed Base Stations
Figure 6
BTS repeater concept (Capacity Transfer System)
FR
CTR1
.2 to .5 W
V
BTS-R
FR
2 3 4
1
TRX
FR
CTR2
CTR3
X3
X2
X1
X2
X1
X1
Power
Supply
Power
Supply
Power
Supply
Digital unit
Power
Supply
LTE
LTE
1
LTE
2
3
CTR1
BTS
LTE
4
CTR2
CTR3
X
X
CTR - Capacity Transfer Repeater
BTS system with
a single
connection
to theconcept
central BTS-R
(digital unit)
and then
RF connections
Figure
6. BTS
repeater
(Capacity
Transfer
System)
between the BTS-R and CTR1, CTR2, and CTR3 (repeaters).
BTS system with a single connection to the central BTS-R (digital unit) and then
RF connections between the BTS-R and CTR1, CTR2, and CTR3 (repeaters).
Table 1: Lightning Protection Levels (LPLs).
is easily defined. The worst-case surge resistibility may be
defined as a 40 kA 8/20 event for an unshielded system and 20
kA for a shielded cable (Table 1).
Lightning Protection Level
DC power cable
Current
(kA)
Protection
module
RRU
I
II
III-IV
Unshielded cable
40
30
20
Shielded cable
20
15
10
8/20 µs peak current
To meet the worst-case situation for the unshielded cable, each
individual SPD shown in Figure 8 would have to consist of
three (3) AK15-058C devices, but to meet the minimum case
for a shielded cable (10kA), a single AK10-058C could be used
for each SPD position. Table 2 shows the various surge rated
AK devices available with a 58-volt stand-off parameter.
Figure 7: Recommendation for protecting the power supply
interface.
Figure 8
This protection module has three possible solutions as
Protection Module Implementations
illustrated in Figure 8.
-48V
-48V
-48V
SPD
SPD
RTN
RTN
SPD
a
SPD
SPD
b
RTN
SPD
SPD
c
Figure 8: Protection module implementations.
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Distributed Base Stations
Table 2: AKxx-058 Series Electrical Characteristics.
Part Number
VSO
IR
V
µA
VBR
Min
IT
Max
VCL @ IPP (8/20µs)
AK6-058C
Max Cap @ 0 V bias 10 kHz
%/°C
(nF)
15000
AK15-058C
AK10-058C
Max Temp Coefficient
IPP
VCL
58
20
64
70
10
12
10000
110
6000
3000
AK3-058C
For over-current protection of these over-voltage devices, the
LVSP20/30/40 power fuses would be appropriate for the 20
kA/30kA/40 kA categories of the LPL classes from Table 1
so that excessive lightning induced events nor excessive
power fault events do not cause a safety-related issue with the
AK devices (this fuse is placed in series with the AK device,
NOT in series on the power supply line). However, the design
engineer must be aware of the I2t rating for each fuse because
the “lightning rating” is so high. For example, the LSVP20 has
a nominal I2t of 4,940A 2S. See Table 3 for a list of available
Littelfuse options.
8/20 Rating
I 2 t melting
(A 2 s)
I 2 t clearing
(A 2 s)
LVSP 5
5,000
359
981
LVSP10
10,000
1,300
3,210
LVSP15
15,000
3,267
8,235
LVSP20
20,000
4,940
11,710
LVSP30
30,000
11,950
35,325
LVSP40
40,000
20,550
61,700
8
6
Figure 9
Protection
module
DC power cable
Rectifier
Figure 9: Protection module.
The dc voltage feeder cable between the RRU and the
transmitter/receiver located at the top of the tower should not
require an additional protection module if the RRU dc voltage
feeder has been protected sufficiently and there is sufficient
distance between the RRU and the top of the tower.
Table 3: LVSP fuse
Part Number
6.5
0.1
One can quickly see from Equation 1 that the ZT l factor must be
a significant value to result in a peak surge voltage of concern
(such as non-Distributed BTS architectures where the distance
between the tower top and the radio unit is significant). If this
feeder uses the same conductor as the RF feed between these
two points, then a low capacitance solution would have to be
used to prevent any negative impacts on the high frequency
content. If this feeder carries the dc power feed only, then the
protection choice may include the AK series.
Equation 1 is useful in determining the peak voltage on this dc
voltage feeder cable.
The rectifier located within the hut that is supplying this dc
power should also be protected and comply with ITU K.56. The
protection module illustrated in Figure 9 would use the same
options as shown in Figure 7 and Figure 8 (a single SPD, two
SPDs, or three SPDs). Refer to the Littelfuse Radio Base Station
Protection Summary article for full details.
V T = I LPLa T aF Z T l
Eq. 1
where:
ILPL is the peak lightning current associated with the application.
The lightning protection level rating as given in Table 4 based
on the 10/350 waveshape.
l is the length of the feeder cable.
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Outdoor LED Lighting
Distributed Base Stations
The value of a T is determined by the tower and feeder
geometry. Typical values are:
Tubular tower (mast): a T = 0.30
Three legs tower:
a T = 0.20
Four legs tower:
a T = 0.15
Equation 2 provides an approximate value of a F , where n is the
number of cables in the feeder tray.
1
aF = ______
n
+ 3.5
Eq. 2
Table 4: Lightning flash parameters from [IEC 62305-1] are based
on a 10/350 mS waveshape
Lightning Protection Level (LPL)
Parameter
Units
I
II
III
IV
Max peak
current
kA
200
150
100
100
Table 5: Typical values of DC resistance of the external conductor
of coaxial feeder cables (ZT).
External
diameter (mm)
7.8
10.2
13.7
27.5
39.0
50.3
59.9
DC resistance
(W /km)
6.6
5.3
3.4
1.04
0.62
0.47
0.31
The various data communication and long haul ports located
inside the Base Station hut or on the tower such as Ethernet
ports, T1/E1 ports, or xDSL ports should also be protected
accordingly. Refer to the Littelfuse Ethernet Protection
Design Guide for more details on the Ethernet port protection
recommendations and the “Reference Designs” section of the
Littelfuse SIDACtor Product Catalog and Design Guide for other
port protection recommendations.
Figure 10 provides an overview of how the BTS connects
to the MSC.
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Outdoor LED Lighting
Distributed Base Stations
PSTN
BTS
= Lielfuse protecon opportunity
MSC
BTS
BTS
BTS
MSC
AC Power
Power
Supply
Customer
Database
Controller
Baeries
Cell
Site
Controller
SWITCH
BTS
Control
To BTSs
Home
Visitor
SS7
Controller
Control
{
}
To Telephone
Network (PSTN)
Voice/Data
Voice/Data
Radio tower and Distributed BTS equipment
Figure 10. This MSC (Mobile Switching Center) connects mobile users to mobile users or
mobile
users
to wireline
users.
This MSC
(Mobile
Switching
Center) connects mobile users to mobile users or mobile
users to wireline users.
Littelfuse, Inc.
8755 West Higgins Road, Suite 500
Chicago, IL 60631 USA
Phone: (773) 628-1000
www.littelfuse.com
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