The power amplifier is the final active element in the radio transmission chain, and so a well-instrumented amplifier can reveal a lot about the state of the antenna and its associated feed cables.
The power amplifier is also the most power-hungry component in the basestation and accounts for most of the heat in the enclosure. Because of this, most basestations are designed around their amplifiers, and the amplifier dominates the health and performance of the basestation.
Forward and Reverse Power
When a radio signal traveling in a cable meets a discontinuity, some of the radio power gets reflected back, just like the partial reflection when light traveling through the air meets a piece of glass. (This is not just an analogy. It is exactly the same phenomena.) In an ideal radio system, all of the cables, connectors, and antennas are “impedance-matched”, with no electrical discontinuities, and there are no such reflections.
Nearly all basestation power amplifiers include instruments to measure the power that is reflected back into the antenna feed ports, so-called “reverse power”. The ratio of forward to reverse power is an important health indicator for the basestation. Excess reverse power indicates a problem in the antenna installation or in the cables and connectors between the antenna and the basestation.
The forward power measurement is also important for know exactly how much power the basestation is transmitting, for coverage cross-checks and to verify license compliance.
The temperature of the amplifier drives the temperature of the basestation. If the amplifier is hot, soon enough, everything else will also be hot. Excessive heat may indicate a problem in the amplifier itself, or a problem in the way the basetation is installed. Temperature history of the amplifier can also be used to predict its remaining service life.
The amplifier is usually instrumented to measure output RF power and input supply power. From these, we can calculate efficiency. For any given temperature, the expected efficiency of a particular amplifier can be known or modeled. It the amplifier does not match the expectation, this probably indicates a problem inside the amplifier.
Power Supply Measurements
In some ways, the power supply is like the central nervous system of the basestation. It measures the power consumed by different subsystems and can also turn them on and off. The power supply is also the most failure-prone subsystem in most basestation designs, and so monitoring its health is critical.
Temperature stresses in the power supply are probably the highest for any subsystem in the basestation. The temperature history of the power supply can be used to predict its service life. The power supply may also shut down other components in the basestation, especially the amplifier, to prevent permanent damage to itself due to excessive temperature.
The power supply measures its own input and output power flows, and from these can calculate efficiency. Like the amplifier, the power supply should have a predictable efficiency at a given temperature. If the power supply does not meet the expected efficiency, that is a sign of an internal problem, and possible coming failure.
Be measuring outgoing power flows, the power supply can identify circuit faults in basestation subsystems. Hardware or firmware circuit breakers in the power supply can limit damage by shutting down subsystems that are drawing excessive power. For example, our SatSite product has separate power measurements and controls for the amplifier, receiver LNAs, digital radio, and CPU sections.
The basestation’s amplifier and power supply provide several kinds of measurements that can be used to detect and diagnose hardware faults, and to predict the remaining service life of the unit. This information is critical in the short-term operation of the network and valuable in long-term network management and cost-planning. This article presents the measurements made in our own SatSite products, but they are typical of all basestations.