4G LTE remains the backbone of cellular IoT in the UK. Despite 5G rollout, LTE connects the majority of deployed IoT devices - CCTV, smart meters, industrial controllers, fleet trackers and remote monitoring equipment. Getting the 4G signal test right at installation is the single most important factor in long-term deployment reliability.
How 4G LTE works - what installers need to know
LTE operates using OFDMA (Orthogonal Frequency Division Multiple Access). The base station - called an eNodeB - transmits reference signals called CRS (Cell-Specific Reference Signals) that devices use to measure signal quality. These measurements produce RSRP, RSRQ and SINR. The device reports CQI back to the network, which uses it to schedule transmissions at the highest modulation order the channel supports.
LTE uses FDD (Frequency Division Duplex) in most UK deployments - separate uplink and downlink frequencies. Signal metrics from the modem reflect downlink quality. Both uplink and downlink benefit from a good external antenna because the antenna is bidirectional.
4G LTE frequency bands in the UK
| Band | Frequency | UK Operators | Characteristics | IoT suitability |
|---|---|---|---|---|
| B20 | 800 MHz | EE, Vodafone, Three | Best building penetration, long range, lower bandwidth | Excellent |
| B28 | 700 MHz | EE | Extended rural coverage, deep indoor penetration | Excellent |
| B3 | 1800 MHz | EE, Vodafone, O2, Three | Most widely deployed urban band, good balance of range and speed | Good |
| B1 | 2100 MHz | EE, Vodafone, O2 | Urban capacity band, moderate building penetration | Moderate |
| B7 | 2600 MHz | EE, Vodafone, Three | Dense urban capacity, poorest building penetration | Limited |
| B40 | 2300 MHz | Three (TD-LTE) | Time Division duplex, dense areas only | Limited |
For most IoT installations, B20 (800 MHz) and B3 (1800 MHz) are the primary bands. B20 provides the best combination of coverage, building penetration and multi-operator availability. When a router connects on B7 or B1, consider whether a directional antenna aimed at a lower-band sector would improve signal stability.
4G signal metrics explained
RSRP - Reference Signal Received Power
RSRP is the primary coverage indicator. It measures the power of the LTE CRS reference signals from the serving cell, in dBm per Resource Element. Always negative - closer to zero is stronger.
| RSRP (dBm) | Quality | Installer action |
|---|---|---|
| -80 or better | Excellent | Omni antenna sufficient. Proceed. |
| -80 to -90 | Good | Reliable for all IoT applications. Proceed. |
| -90 to -100 | Acceptable | External directional antenna recommended. Check SINR. |
| -100 to -110 | Poor | Directional antenna essential. Consider operator change. |
| Below -110 | No service | High-gain directional and operator change mandatory. |
RSRQ - Reference Signal Received Quality
RSRQ captures the ratio of the wanted signal to total received power including interference. Poor RSRQ with acceptable RSRP is the interference signature - the device hears its serving cell clearly but the channel is polluted. Target: -10 dB or better. Below -15 dB is poor. Solutions include band locking, a directional antenna to improve signal-to-interference ratio, or an operator change.
SINR - Signal to Interference plus Noise Ratio
SINR is the most direct predictor of achievable throughput. High SINR enables 256-QAM; low SINR forces QPSK. The throughput difference between +20 dB SINR and +3 dB SINR is more than 12x at the same RSRP. For CCTV and video requiring minimum bandwidth, SINR is the critical metric. Target for video: +13 dB or better. Reliable data: +6 dB or better. Below 0 dB: severely degraded regardless of RSRP.
CQI - Channel Quality Indicator
Scale 0-15. Reported by the modem to the network. CQI 15 = 256-QAM rate 5/6 (maximum throughput). CQI below 5 = QPSK only, very poor efficiency. Target: CQI 10 or above for reliable IoT performance.
Why handheld signal testers fail for IoT commissioning
Standard handheld 4G signal testers use their own internal modem with a stub antenna. The reading reflects what that device receives in the orientation the engineer holds it. A Teltonika RUT200 with an external LPDA mounted at rooftop level can show 15-20 dB more signal. The handheld result is not wrong - it just has nothing to do with the installation you are commissioning.
Automated 4G operator sweep
With a roaming or multi-network SIM, CellTester locks to each UK operator PLMN in sequence - EE (23430), Vodafone (23415), Three (23420), O2 (23410) - waits for full registration, captures all signal metrics and runs a speed test on the router WAN interface. The sweep takes 2-4 minutes and produces a ranked operator comparison. CellTester recommends the best primary and failover operators.
4G MIMO antenna commissioning
CellTester reads RSRP from each antenna port individually. For 2x2 MIMO installations the port delta is rated: 3 dB or under is excellent (full MIMO gain). 3-6 dB is good (check connector seating on the weaker port). 6-10 dB is marginal (investigate cable, connectors and antenna position). Above 10 dB is a fault - open circuit or connector failure causing severely degraded MIMO.