Why signal metrics matter for IoT installations
Consumer devices show signal strength as bars. Bars are not a measurement - they are a visual representation derived from RSRP, and different devices use different conversion algorithms. Two phones showing full bars can have very different actual signal levels. For IoT and M2M installations, bars are irrelevant. You need RSRP, RSRQ and SINR from the actual installed hardware.
RSRP - Reference Signal Received Power
RSRP measures the average power of the LTE Cell-Specific Reference Signals received from the serving cell, measured per Resource Element in dBm. It is always negative - closer to zero means stronger signal. RSRP is measured independently of channel bandwidth, making it directly comparable across different bands and channel widths.
| RSRP (dBm) | Signal level | What it means for IoT |
|---|---|---|
| -80 or better | Excellent | Strong signal, omni antenna typically sufficient |
| -80 to -90 | Good | Reliable for all IoT applications |
| -90 to -100 | Acceptable | External directional antenna recommended |
| -100 to -110 | Poor | Directional antenna essential, consider operator change |
| Below -110 | Very poor | High-gain antenna and operator change required |
RSRQ - Reference Signal Received Quality
RSRQ = N x RSRP / RSSI, where N is the number of Resource Blocks. It captures the ratio of the wanted reference signal to the total received power including interference from other cells and users. A device can have strong RSRP (good signal from its serving cell) but poor RSRQ (because of high interference from neighbouring cells). This is particularly common in dense urban areas and industrial sites near large cell tower clusters.
Target: -10 dB or better. Acceptable: -10 to -15 dB. Poor: below -15 dB. If RSRQ is poor while RSRP is good, the solution is typically a directional antenna pointed directly at the serving cell to reduce off-axis interference, or a band lock to a less congested frequency.
SINR - Signal to Interference plus Noise Ratio
SINR is the ratio of the wanted signal power to the sum of interference and thermal noise. It directly determines which modulation scheme the network assigns to the device.
| SINR (dB) | Modulation | Approximate downlink efficiency | IoT impact |
|---|---|---|---|
| Above +20 | 256-QAM | Maximum throughput | Excellent for all applications |
| +13 to +20 | 64-QAM | High throughput | Good for video and high-data IoT |
| +6 to +13 | 16-QAM | Moderate throughput | Reliable for telemetry and M2M |
| 0 to +6 | QPSK | Low throughput | Marginal - antenna optimisation needed |
| Below 0 | QPSK (degraded) | Very low | Unreliable - connection may drop |
Full bars but slow internet - the SINR explanation
This is one of the most common IoT installation problems. A device shows strong signal (high RSRP, full bars) but delivers poor throughput. The cause is almost always low SINR. The device is close to the tower and receives a strong signal, but the cell is congested or there is significant co-channel interference from neighbouring cells. The network has no choice but to assign low-order modulation, and throughput collapses regardless of RSRP.
Solution: try a different operator (some cells are less congested), use band locking to move to a less loaded frequency, or test at a different time of day if traffic load is the cause.
RSSI, CQI and other metrics
RSSI (Received Signal Strength Indicator) is the total received power including noise and interference. It is less useful than RSRP for commissioning decisions because it does not isolate the wanted signal. High RSSI with low RSRP indicates a noisy RF environment.
CQI (Channel Quality Indicator, scale 0-15) is reported by the modem to the base station to indicate the highest modulation order it can currently support. CQI 15 = 256-QAM maximum rate. CQI below 5 = QPSK only, very poor efficiency. Target CQI 10 or above for reliable IoT performance.
PCI (Physical Cell Identity) and EARFCN (Evolved Absolute Radio Frequency Channel Number) identify which cell and frequency the device is connected to. Useful for cross-referencing with CellMapper to understand the physical cell tower the router is using.