Lightning Protection for Fiber-to-the-Antenna

Choosing the best surge protection for 4G and 5G installations.

4G: Fiber to the Antenna Configuration

The telecommunications world is quickly moving from third-generation (3G) to fourth-generation (4G) configurations, with announcements for 5G already starting. The 4G has acted as a stimulus for a paradigm shift of the cell tower structure, which has included the implementation of fiber-to-the-antenna (FTTA) architecture. FTTA architecture has enabled lower power requirements, distributed antenna sites, and  a reduced base station footprint than conventional tower sites.

In 3G operating systems, the power, cooling, electronics, and voltage converter equipment are all located within the base station, as shown in Figure 1. Examples of 3G systems include those using a global system for mobile communications (GSM), Wi-Fi, code division multiple access (CDMA), universal mobile telecommunications system (UMTS), and high-speed downlink packet access (HSPDA).

The communication signal is fed from the base station via coaxial cables to the antenna mounted on a mast 30 to 100 meters above the base station. This tower cell configuration is evolving, as the data content and speed requirements of the 4G communications systems continue to increase.

FTTA has served as an enabling architecture for the deployment of 4G mobile communications systems.¹ The acceptance of FTTA as a viable architecture is largely due to the separation of power and signal components from the base station and their relocation to the top of the tower mast in a remote radio head (Figure 2).

This separation has allowed cell tower operators to comply with the performance requirements of 4G systems, created a smaller and greener footprint for the reconfigured base station, and reduced the power/cost/weight for the overall tower structure.

The Remote Radio Head Paradigm

The remote radio head (RRH) is the most important change in the architecture of the cell site. The RRH concept shifts the entire high-frequency and power electronic segments from the base station to a location adjacent to the antenna (Figure 3).

The specifics of these changes include maintaining the control and base band signaling components in the base station, the last unit in the base station signal generation being a small form-factor pluggable (SFP) transceiver, which converts the electrical signal to an optical signal. The optical signal is then transmitted via fiber optic cable from the base station to one or more remote radio heads located adjacent to antenna feeds. The optical signal is received at the input to the radio head via another SFP transceiver, then converted back into an electrical signal where it is amplified and converted into a carrier frequency and fed to the antenna via a short coaxial cable jumper.

The shifting of high-frequency and power electronic segments to an RRH has improved the efficiency of the cell tower and reduced the footprint of the cell site:

• Elimination of the coaxial cable feed from the base station to the antenna.
• Eliminating the electrical losses associated with the coaxial cable reduces the tower’s power consumption. The reduced weight of the fiber-optic cable compared to the coaxial cable reduces the structural load on the tower.
• Reduction in electrical power requirements due to the relocation of power amplifiers from the base station to the RRH.
• This change allows the use of -48 VDC, as opposed to 110 VAC, in power transmission to the RRH. This eliminates the 110 VAC cable transmission loss, reduces power cable transmission weight, and improves efficiency of the power amplifiers.
• Recent product improvements in this area have included the development of a hybrid cable solution consisting of optical fiber and -48 VDC power wires housed in a single cable that includes armoring, shielding and grounding options.
• This configuration allows a one-cable feed from the base station to the RRH, with each cable containing multiple power conductors and fibers.
• The opportunity to drive multiple antenna systems from one base station, a concept also known as distributed antenna systems (DAS).
• This concept reduces the physical footprint and power requirements of the base station when compared to traditional base station designs. This configuration (Figure 4) is also used in low-power deployment of base stations and DAS on rooftops, poles, etc.

RRH Environmental Factor

While deployment of the FTTA concept has improved many operational facets of the cell tower, it has also changed the environmental and electromagnetic design requirements of the components located within the RRH.

Specifically, high-frequency and power electronic components located in the RRH will now be subject to the harsher environmental and electromagnetic conditions found at the top of the antenna mast, as opposed to the more benign environmental and electromagnetic conditions found within the base station. The components will require a wider temperature range with higher vibration and shock ratings. They will also need to withstand the hostile electromagnetic environment associated with the unprotected antenna mast.

Table I compares quantifiable environmental specifications between base station components and top-of- the-mast components. Components located at the top of the mast have significantly higher requirements for solar radiation, air velocity, vibration and exposure to moisture.

A variety of different packaging designs are being used to lessen the impact of the environmental factors at the top of the mast. One such design features an aerodynamically dome-shaped housing sealed per IP68 requirements to keep out moisture and dust. This structure can contain power and fiber components in a single housing. This approach minimizes the number of devices mounted at the top of the tower and reduces installation time. It is being used in FTTA applications around the world.

Table 1 – Comparison of environmental specifications for base stations and remote head components²

RRH Electromagnetic Environment

The change in electromagnetic environment due to the relocation of the components from the base station to the top of the mast can be visualized by referring to the concept of lightning protection zones (LPZ), as presented in IEC 61312-1. The lightning protection zones define where direct or indirect strikes are possible and give a measure of relative field strengths (Figure 5).

A brief summary of the LPZ definitions follows:

• LPZ OA – The zone where a direct hit is possible and where objects must be capable of carrying the full lightning current.
• LPZ OB – The zone where a direct hit is not possible, but the unattenuated electromagnetic field is present.
• LPZ 1 – A zone where a direct hit is not possible, and the currents in all conductive components are lower than in LPZ OA and LPZ OB. In this zone, the electromagnetic field is attenuated according to the screening measures applied.
• LPZ 2 – A zone where a direct hit is not possible, and the currents in all conductive components are lower than in LPZ 1. In this zone, the electromagnetic field is attenuated through multiple screening measures.

In this case, the exposure of power and signal line components changes from the LPZ 2 zone at the base of the tower to the LPZ 1 zone at the top of the mast. The differences in LPZ classification discussed above are directly proportional to the amount of electromagnetic energy present in the two different locations during a lightning event.

Surge protective devices (SPDs) are used to shunt lightning discharge currents to ground during a lightning strike. Figure 6 relates the current magnitude present during a lightning strike to the class/type of waveform the SPDs are tested to during electrical evaluation. The high energy content 10/350 µs waveform is used to simulate peak lightning test currents. The lower energy content 8/20 µs waveform is used to simulate leading edge rate of rise maximums.

Table 2 – Comparison of environmental specifications for base station and remote radio head components⁵

Table 2 shows a comparison of typical SPD test parameters for SPDs used to protect electronic components in either base station or top-of-the-mast positions. As Table II explains, SPDs used at top-of-the mast locations have more stringent performance requirements in terms of maximum discharge surge current and peak lightning test current.

SPD manufacturers have used three different designs to meet the electrical requirements of SPDs used in FTTA applications:

Spark gaps in combination with metal oxide varistors (MOV) (Figure 7)

This combination of components is associated with the highest lightning test current peak values, typically 100 kA. It has a high voltage protection level, approximately 1,000 V. Open-air spark gaps have a wide variation of triggering voltage, due to variations in humidity, temperature and altitude. To mitigate the triggering voltage variations, many manufacturers now use a triggered spark gap design, which provides a tighter band of triggering voltage values.

Gas discharge tubes in combination with MOVs (Figure 8)

This combination of components is associated with lightning test current peak values in the range of 50 kA and voltage protection values of approximately 800 V. Unlike the open-air spark gap, the gas discharge tube is environmentally sealed. It also has a tightly controlled trigger voltage level, faster response time and a lower discharge surge current rating than the spark gap.

Parallel MOVs (Figure 9)

This combination of components is associated with lightning current peak values in the range of 25 to 40 kA and the lowest voltage protection values, typically in the range of 400 V. This combination has the fastest surge current reaction time and the lowest lightning current peak values of the three SPD styles.

Cell tower designers have a range of performance and cost options available when selecting SPDs to protect their systems from direct lightning strikes and induced power surges. An individual system generalized performance criteria given in the above discussion should be refined and adapted to the specific requirements of an individual system.

Selection should also consider installation and replacement complexities. All three surge protection technologies mentioned above will degrade over time and will need replacement. The use of pluggable and hot-swappable protectors will lower installation costs and maintain power to the RRH at all times.

Conclusion

While the 4G and 5G telecommunications systems have made improvements in the overall reliability of the cellular network, the shift to FTTA architecture has come with risks. High-frequency and power electronics, previously located in the base station, are now at the top of the mast, where they are subject to harsher environmental and electromagnetic conditions. Lightning strikes present a particular threat to this sensitive equipment. While different towers have different requirements, understanding the LPZ concept and the different SPD technologies available is a starting point. With the right SPD properly applied, it is possible to prevent the expensive damage that lightning and other power surges can cause.
 
References

[1]    OSP Magazine, LTE – “Change is in the air.” February 2011, pp. 22-27.
[2]    European Telecommunication Standard ETS 300 019-1-3 and ETS 300 019-1-4, Environmental Conditions and Environmental Test for Telecommunication Equipment, Stationary Use, class 3.2 and 4.1E.
[3]    Hermi d.o.o.: About Surge Protection, 2011.
[4]    International Electrotechnical Commission, IEC 62305-1, Part 1 Protection of Structures Against Lightning, General Principles.
[5]    International Electrotechnical Commission, IEC 62305-4, Part 4 Electrical and Electronic Systems within Structures.

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Filed Under: Aerospace + defense, M2M (machine to machine), Breakers + surge protection

 

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