Page 2 of 9 APPLICATION NOTE Power Design Tool for Remote Radio Heads POWER DIMENSIONING FOR REMOTE RADIOS Power dimensioning for remote radio heads (RRH) is an important task for system design engineers. For network roll-outs, upgrades of existing networks or re-use of existing cable infrastructure the following questions arise: What is the maximum allowable RRH power consumption for a given cable configuration? What is the minimum required cable cross section to supply a specific RRH at a given tower height? What is the maximum allowed cable length for a prespecified cable cross section and RRH? E.g., a 500 W remote radio with 6 mm2 power cable. A power unit at the base station supplies the mast-mounted RRHs with a constant DC voltage UBTS. The remote radio has a constant power consumption PRRH at a given minimum operating voltage URRH. The power supply cable causes electrical losses due to the electrical resistance of copper leading to a cable drop voltage U = UBTS URRH. The electrical resistance of the copper cable depends on the cable length L, the copper cross section A, and the electrical conductivity (T) which is influenced by the temperature T RL L (T ) A , whereas (20°C) = 5.46 107 S/m. The resulting drop voltage is U U BTS U RRH 2 I RL 2 I L 2 PRRH A U RRH L A since the power consumption PRRH is the product of the electrical current I and the remote radio voltage URRH. These formulas are used for power dimensioning in the HUBER+SUHNER power design tool. HUBER+SUHNER disclaims any liability resulting from incorrect calculation including any damages resulting from the use of of the power design tool.
Page 3 of 9 APPLICATION NOTE Power Design Tool for Remote Radio Heads DROP VOLTAGE A power supply system needs to be dimensioned for the worst-case scenario to guarantee reliable operation under all conditions. The nominal supply voltage UBTS is 48V, however, the minimum supply voltage is determined by the battery backup system which takes over the voltage supply in case of electrical power outage. The ETSI standard EN3001321 defines the minimum supply voltage of a battery backup as 40.5V. Many mobile operators have their own standard for minimum battery supply voltage (potential) which is usually slightly higher than 40.5V, for example 42V. In case that the operator has no own minimum voltage standards, a minimum supply voltage of UBTS, min = 40.5V should be used for power dimensioning. The minimum operating voltage of remote radios is typically in the range of URRH,min = 36V to 40V, which means that the maximum allowed drop voltage is typically in the range of U = UBTS,min - URRH,min = 2 to 5V. Please note that the drop voltage U is an important system parameter which needs to be defined before any power dimensioning can be done. The drop voltage has a strong influence on, for example, the required copper cross section and therefore needs to be carefully determined. IMPORTANT: A wrong definition of the drop voltage can lead to wrong system dimensioning and therefore potential system failure. As a rule of thumb for “standard remote radio heads”, a cable cross section of 6mm2 is required to feed power over a distance up to 60m, while the cross section needs to be increased to 10mm2 for distances up to 100m. PRRH, max UBTS, min URHH, min 250 W 380 W “Rule of thumb” 40.5V 38V 42V 38V Umin 2.5V 4.0V Lmax for 6mm2 60m 61m 64m Lmax for 10mm2 100m 101m 106m ______________ 1 ETSI EN 300 132-2, Environmental Engineering (EE); Power supply interface at the input to telecommunications and datacom (ICT) equipment; Part 2: Operated by 48 V direct current (DC). The drop voltage is a system parameter which needs to be defined for power dimensioning. REMOTE RADIO SPECIFICATIONS The following table provides an overview of the maximum power consumption and minimum operating voltage of RRHs from different system vendors. The maximum power consumption values corresponds to 100% of the maximum RF (radio frequency) output power, however, typical power consumption values, which are based on a realistic and typical traffic distributions, are in the range of 60 to 80% of the maximum PRRH, max. HUBER+SUHNER disclaims any liability resulting from incorrect calculation including any damages resulting from the use of of the power design tool.