RSSI (received signal strength indicator) is the reader’s measurement, in dBm, of how strongly a tag’s reply arrives: it helps you find a tag by hotter–colder hunting, but it cannot measure distance in metres. The GS1 LLRP 2.0 standard defines its PeakRSSI field as the peak received power of the EPC backscatter.
Short answer: RSSI is a relative strength reading taken by the reader. It is useful for comparison and close-range hunting, and unreliable as a ruler.
- Good for: hunting one tag on a handheld (in CX Flow Pro, lower the RF power and follow the live RSSI), spotting stray and marginal reads, and breaking ties between neighbouring read points.
- Poor for: distance in metres, an x–y position on the floor, and fixed “good value” thresholds copied from another site.
| Use | What RSSI tells you | Reliability | What to pair it with |
|---|---|---|---|
| Locate (one tag, handheld) | Hotter or colder than the last few reads | Good at short range with power lowered; reflections can mislead | Lower RF power; a second approach direction |
| Diagnose (stray or weak reads) | Which antenna sees a tag, and how strongly | Reliable as a comparison within one antenna and channel | Antenna ID, channel, timestamps, read count |
| Zone (shelf or bay) | A tie-breaker when two read points see one tag | Only as good as the antenna layout | Hysteresis and dwell; RF phase for direction |
How do RSSI, RF phase and angle of arrival compare?
| Signal | Unit | What it measures | What it tells you | Main limits |
|---|---|---|---|---|
| RSSI | dBm (whole numbers in standard LLRP) | Received power of the tag’s reply at the reader | Relative strength | Skewed by orientation, materials and multipath; no conversion to metres |
| RF phase | Degrees or radians | Phase of the tag’s reply | Direction of movement; when a tag crosses the zone centre | Wraps every half wavelength of radial movement, about 17.3 cm; vendor extension, not standard LLRP |
| Angle of arrival | Degrees | Phase differences across an antenna array | Bearing to the transmitter | Needs a dedicated array or anchor |
In free space, the phase of a tag’s reply turns through 360 degrees for every half wavelength of radial movement, towards or away from the antenna (Nikitin et al., IEEE RFID 2010). Half a wavelength, c/f/2, is about 17.3 cm on India’s four 2 W channels, so a single phase reading repeats every 17 cm or so of range. In the same paper, tracking phase over time was the most dependable use: even in a real warehouse portal, direction of travel and the centre crossing were easy to identify. In the portal, phase-based ranging was strongly affected by multipath and bearing could not be tested with that antenna layout; both worked well in an anechoic chamber, without reflections. LLRP 2.0 has no standard tag-phase field, so phase comes from vendor extensions and SDKs.
What does RSSI actually measure, and why is it a negative number?
The reader measures it, not the tag. A passive tag has no transmitter: it backscatters part of the reader’s signal, and the reader measures how strongly that reply arrives. LLRP 2.0 §14.2.3.7 makes PeakRSSI a signed integer in dBm, with possible values from −128 to +127.
dBm means decibels relative to one milliwatt (ITU-R V.574-5). The scale is logarithmic: 10 dB higher is ten times the power, and about 3 dB higher is roughly double. Tag replies are far weaker than a milliwatt; the footnote to Table IV of G.S.R. 853(E) says tags respond “at a very low power level” of −20 dBm e.r.p. That is what a tag radiates, not a reading to expect, but it is why reader values are negative.
When a reader rolls several sightings into one report, LLRP 2.0 §14.2.3.1 requires PeakRSSI to be the highest among them, so a number can be a peak over a window. The report can also carry antenna ID, channel index, first- and last-seen timestamps and a seen count. An accumulated report can span channels, with ChannelIndex giving at most the first channel seen, so to compare per channel, report every read rather than accumulated reports.
Identium is a BIS and WPC approved RFID manufacturer in New Delhi. Its readers expose RSSI in several places: the CX1500N’s native UHF SDK (with frequency, phase, read count and antenna), ReaderSense in active mode, the IHZ4 4-port reader’s developer APIs, and the ID-EA500’s per-read events, which also carry phase and antenna.
Can RSSI measure the distance to a tag?
No, not reliably. Nikitin et al. found that the surroundings and the tagged item affect RSSI so strongly that no simple path-loss model converts it to distance in general, and that calibrating it in absolute terms is hard (IEEE RFID 2010). The tag adds variation too: the same paper notes that its backscatter changes with the power reaching it, because the chip’s input impedance depends on that power.
Orientation moves the number as well. With a linear antenna, turning a tag 90 degrees can make the read collapse; circular polarisation reads at almost any angle, at a cost of roughly 3 dB against a matched linear antenna, a rule of thumb rather than a site figure (antenna gain and polarisation guide). Metal shelving and liquids change it again. There is no metres-per-dB rule: two tags at different distances can report the same value.
What makes RSSI fluctuate on a stationary tag?
- Channel changes. India’s rules permit 2 W e.r.p. only on four channels of at most 200 kHz, centred at 865.7, 866.3, 866.9 and 867.5 MHz, with at most 4 s of continuous transmission on a channel and at least 100 ms between consecutive transmissions on the same channel (G.S.R. 853(E)). A reader working continuously moves between channels, and reflections add up differently on each.
- Moving reflectors. Close-range reflections depend on exactly where nearby objects sit (Nikitin et al.), so people, forklifts and doors moving about change the reading.
- Reporting. A peak over a window differs from a single read, and whole-dBm steps make a value near a boundary flip by 1 dB.
- Power changes. Lowering reader power changes what reaches the tag, and so its backscatter.
Compare like with like: one antenna, one channel, over a window of reads. Set floors from your own data.
What happens inside a handheld find-tag (Geiger) mode?
- Address one tag. Gen2 lets an interrogator use a Select command to choose tags by values in tag memory (GS1 EPC Gen2, Release 3.0.1), so a handheld can ask for one EPC.
- Keep it answering. Each Gen2 session keeps an inventoried flag, and Table 6-21 of the standard sets how long it persists. In S0 the flag lasts only while the tag stays powered, so the tag answers again after any break in the reader’s carrier. In S1 it lapses on its own after 500 ms to 5 s at nominal temperature, even while powered. In S2 and S3 it holds indefinitely while energised and for more than 2 s after power is lost, so an inventoried tag stays quiet unless the reader flips the target. A find-tag mode can therefore keep the tag replying by alternating Target A and B, re-selecting the tag, or letting the carrier drop between rounds.
- Turn reads into a cue. Software converts the stream into a bar, number or tone. In Zebra’s documentation, one vendor example, locating runs on one antenna and reports relative distance from 0 to 100, where 100 is closest; the beep frequency varies in proportion, which Zebra calls “Geiger counter” (Zebra RFID API). It is a relative scale, not metres.
On Identium’s CX1500N handheld, CX Flow Pro offers adjustable RF power and a proximity power sweep: drop the power, follow the live RSSI, and keep lowering it as you close in. Cutting power shrinks the read zone, which isolates one file or tool instead of the whole rack. It also exposes Gen2 Session (S0–S3) and Target (A/B) control. Rotate the handheld as you go, since orientation moves the reading independently of distance, and if the value peaks beside a metal upright, approach from a second direction.
Does RSSI give shelf-level or bay-level location?
Only as a tie-breaker. Passive UHF reports the read zone a tag was last seen in, not a coordinate: a doorway portal gives room level, an antenna per bay gives bay level, and the last metre is normally closed with a handheld as the signal strengthens (RFID file tracking). Where two neighbouring antennas both see a tag, an RSSI comparison can arbitrate, with hysteresis and dwell so that a movement is logged only once it is real.
The ID-EA500 edge array reader lists every tag in view with RSSI, antenna and last-seen, places tags in zones with hysteresis and hold-times, and reports direction of travel from RF phase. It does not produce x, y coordinates, and no coverage figure is published; Identium offers a site trial instead.
How do you use RSSI to diagnose stray reads and misses?
Work from the raw per-read stream. ReaderSense in active mode pushes every read with EPC, antenna, RSSI, frequency and timestamp. A tag seen at similar strength every cycle is usually sitting still in the field; one seen for a single cycle is more likely passing by.
Fix strays physically first: reduce power within the legal ceiling, re-aim antennas, move staged stock. Add an RSSI floor per antenna and channel only afterwards, knowing it also discards genuine distant reads (troubleshooting guide). If a second reader shares the space, see two UHF readers in one room; if one port reads weaker for every tag, check the antenna cable run and connectors (cable loss and read range).
What should you use when you need a position, not a hint?
Bluetooth angle of arrival is a separate system from passive UHF. Bluetooth Core 5.1 introduced direction finding (Bluetooth SIG); in AoA a single-antenna transmitter is received by an antenna array that calculates the signal’s direction (Bluetooth SIG). The receiver samples a Constant Tone Extension into IQ samples, and an antenna matrix is required (Nordic nRF Connect SDK). It does not locate passive UHF tags.
- Passive UHF zoning: read points per room, bay or door, with the ID-EA500 adding zone presence, dwell and phase-based direction.
- BLE gateways in RSSI mode: one ID-BGW-02 per area you need to tell apart, because RSSI gives a zone rather than a coordinate.
- BLE AoA anchors: dedicated ID-AOA-GW25 anchors with AoA-capable beacons for finer positioning; figures are on the BLE beacons page. AoA-capable beacons also work in RSSI mode, so pilot with RSSI and upgrade zones to AoA without re-tagging.
The RTLS comparison of active RFID, BLE and passive UHF works through the choice. If you are unsure whether you need a zone, a direction or a position, talk to our team about a site trial with the ID-EA500 and ReaderSense on your own goods.