According to 2014 Pew research on U.S. cell usage, 72 percent of Americans experience dropped calls, and 32 percent report loss of service several times a week. It’s not surprising that traditional macro cell towers just can’t keep up with the traffic. These numbers account for the rapid growth of small cells and distributed antenna systems (DAS), which are helping to fill local coverage and service gaps while providing faster service for more users.

Small cells are wireless base stations on a smaller scale, covering a much reduced area and supporting tens to hundreds of users, rather than the more traditional macro-cells with potentially thousands of users. 

DAS are like a base station with many antennas, each serving a small area, adding up to a medium-scale coverage area. This also enables frequency re-use and increases capacity and coverage. DAS are well suited to indoor locations, while small cells are being installed in many densely populated urban environments.

Powering Remote Nodes–Power Versus Reach Trade-Offs

Users expect wireless service to be available even during power outages, so backup power is a necessity. To provide a small uninterruptible power supply (UPS) at each small cell location can become a cost prohibitive expense, so a centralized provision of backup power is advantageous. Yet powering devices at large distances from a centralized UPS can present challenges. Copper wires incur power losses in direct proportion to the distance of the run, courtesy of Ohm’s Law. In North America, the provision of power circuits is governed by the National Electrical Code (NEC), which ensures safety of circuits. If the voltage and power levels are low (<100 watts (W)), the NEC Class 2 provision allows for power-limited circuits to be installed without the need for protective conduit, which dramatically reduces the installation complexity and cost. At higher voltages, a similar provision exists allowing up to ±200-volt circuits limited to <100 W to be similarly installed.

High Voltage Approaches

The high voltage approach employs a power-limiting DC/DC converter to generate ±190 volts, limited to <100 W per circuit, and feed small gauge wire circuits feeding remote loads. The reach of a single circuit is dependent upon the wire gauge selected and the power required by the load, as shown in Figure 1. 

At the remote end of the circuit, a “down converter” is used to return the ±190 volts to the voltage required by the load equipment, typically 48 or 12 volts. The outputs of the down converters may be paralleled to enable load power in excess of the 100 W limit per circuit or to increase the reach at lower power levels.

Figure 1: Reach calculations for different wire gauges; power delivered to load per converter circuit.

Low Voltage Approaches

Power delivery to remote equipment at low voltage DC, typically 48 volts, can be achieved using NEC safety practices designated “Class 1 or 2.” 

Class 1 circuits are not limited in power allowed in each circuit, but circuits must be installed in protective conduit to comply with fire and safety codes. Class 1 circuits can carry higher currents and reach longer distances but are more expensive, both in material and installation. 

Class 2 circuits require that power be limited to 100 W per circuit for safety and fire purposes but can be installed without protective conduit, significantly reducing material and installation costs. Each circuit requires a protective device that can limit total power to 100 W per circuit; this is typically an active current-limiting device.

Reach calculations are based on Ohms Law, which governs the power loss in a resistive circuit. Just as with the high voltage approach, we can calculate the reach of a power-limited circuit for different load power requirements. The red curve on Figure 2 shows this calculated reach for different loads. 

The calculation must be performed using a starting voltage (at the source) of 42 volts, since it drops to this value as the battery discharges if utility power fails. The use of a boost converter to keep the starting voltage at a constant 57 V allows much longer reaches, as shown by the blue curve of Figure 2.

Figure 2: Reach calculations; power limited low voltage DC with and without boost converter.

Unlike the high voltage approach, a Class 2 circuit is already low voltage compatible with load equipment, so a down converter is not required at the remote end of the circuit. 

If higher power or longer reach is required, multiple circuits cannot be directly paralleled at the load, since this negates the safety provisions of a Class 2 circuit. To parallel circuits, a combiner must be installed at the remote end to isolate the individual circuits and maintain Class 2 safety integrity. Similarly, to maintain Class 2 circuit safety, the current limiter should not allow user replaceable current-limiting elements (such as fuses) to prevent erroneous replacement.

Figures 1 and 2 show that the high voltage approach can provide significantly longer reach for a given load and can use a smaller gauge wire than the low voltage approach. Using the high voltage approach, a 50 W load can be powered on a 22 American wire gauge (AWG) wire pair out to 6.2 miles, whereas a Class 2 low voltage circuit using a larger 12 AWG wire pair would only reach 4,500 feet. 

For longer reaches, one might choose the high voltage system, but for shorter reaches the low voltage system without the need for down converter may be a more cost effective choice.