Radio, spectrum and dBm

A signal reading is a power measurement

A dBm value is a power measurement against one milliwatt, which is why most readings are negative. Zero dBm equals 1 mW; −30 dBm equals one microwatt; −105 dBm is far below both. The phone reports the received power of a reference signal, so the number describes what arrived at the antenna, not what the network promised.

The decibel is a ratio, not a step on a linear scale. A change of 3 dB halves or doubles the power. A change of 10 dB multiplies or divides it by ten. A change of 20 dB multiplies or divides it by a hundred. That is why a reading of −105 dBm and a reading of −95 dBm differ by a factor of ten, not by ten points on a dial.

RSRP is the measurement usually quoted for LTE and 5G. It is the average received power of the reference signal, in dBm. RSRQ and SINR describe quality rather than strength: RSRQ compares the signal with the noise and interference around it, and SINR compares the wanted signal with the interference. A strong RSRP with poor RSRQ or SINR is a real and common state.

Decibels add when powers multiply. A signal that loses 20 dB through a wall and 10 dB through a window arrives 30 dB weaker than it left. This is why a single wall can turn a usable reading into a weak one, and why moving a device one meter can change the number more than a change of band.

Low band covers; high band carries

At a lower frequency, free-space path loss is lower for the same antennas, and the signal penetrates walls and bends around obstacles better, so it reaches farther. At a higher frequency, free-space path loss is higher for the same antennas, so a cell covers a smaller area. That is the trade: low band gives coverage and indoor reach, high band gives capacity in a smaller area.

High band carries more capacity because more contiguous spectrum is available there. A wide channel moves more data at the same signal quality. Low band has narrow channels and less room, so it serves more area with less capacity. Neither band is better; they solve different problems in the same network.

A band is a defined frequency range, paired for FDD and unpaired for TDD, not a speed label. Band 28 sits around 700 MHz and is used for coverage; n78 sits around 3.5 GHz and is used for capacity in cities. The actual radius of a cell depends on transmit power, antenna height, terrain, buildings and the device, so no table can give a distance that holds everywhere.

Read three numbers, not the bars

The bars on a phone are a device interface. The vendor decides where each bar starts and ends, and the thresholds differ between models and software versions. A phone with five bars and a phone with three bars can report the same RSRP. Use the bars to notice a change; use the measurement to explain it.

Read RSRP, RSRQ and SINR together. RSRP says how much signal arrived. RSRQ says how clean it is relative to the load around it. SINR says how much the wanted signal stands above interference. A device can show good RSRP and poor SINR in a crowded cell, and the result is a slow or unstable session.

The application decides what counts as usable. A sensor that sends a few hundred bytes a day can work at −105 dBm on a narrow, low-rate bearer. A video call needs sustained throughput and a better signal. NB-IoT trades rate for link budget: it reaches deeper into buildings and basements, but it will not carry video.

Measure in the same place and the same way twice. Hold the device, the antenna and the orientation constant, then change one variable: band, height or position. A reading taken beside a window and a reading taken in a basement are not comparable, and a vendor who compares them is not measuring the network.

Ask for the measurement, not the map

A coverage map is a prediction built from a model. A reading from the device is evidence. Ask for the reading, the band, the technology, the time and the location. Without those five items, a signal claim cannot be checked or repeated.

Ask which band the device used, not only which network it attached to. The same network can serve a device on 700 MHz at the edge of a cell and on 3.5 GHz near the center. The band explains why two devices in the same place report different readings.

A single reading is a snapshot. Ask for a short series: before the fault, during it and after a change. A series shows whether the problem is location, time, load or a failed component. One number can start a diagnosis; a series can finish it.