Getting Started with RFID

A UHF RFID system has three parts: a reader, one or more antennas, and tags. The reader generates a 920–925 MHz radio signal and sends it through the antenna. When a passive tag enters the antenna's field, it harvests energy from the radio wave to power its tiny microchip (typically needing just ~10 microwatts). The chip then modulates the incoming signal and backscatters it — essentially reflecting a modified version back. This reflected signal carries the tag's unique Electronic Product Code (EPC).

The entire read cycle — from transmitting the query to receiving the tag's response — takes about 1–3 milliseconds. This is what enables a single reader to inventory 200+ tags per second using the EPC Gen2 anti-collision protocol. The round-trip signal loss is significant (-40 to -80 dB), which is why reader TX power (typically 30 dBm / 1 watt) and tag chip sensitivity (down to -22 dBm) are such critical specifications.

RFID spans multiple frequency bands, but UHF (860–960 MHz) dominates commercial applications because it offers the best balance of read range, speed, and tag cost. LF (125 kHz) reads within 10cm at ~1 tag/sec — good for animal tracking but too slow for logistics. HF/NFC (13.56 MHz) reaches ~1m at ~50 tags/sec — great for payments and access cards. UHF reaches 1–12+ meters at 200+ tags/sec — ideal for supply chain, retail, and asset tracking.

Within the Vietnam 920–925 MHz band, readers use Frequency Hopping Spread Spectrum (FHSS) across multiple channels. The formula is: frequency = 920.0 + (channel_index × 0.5) MHz. A typical configuration uses 6 channels [0, 2, 4, 6, 8, 10] spanning 920.0 to 925.0 MHz for maximum channel separation.

Channel Index → Frequency (MHz)   Formula: f = 920.0 + (idx × 0.5)

Ch 0  → 920.0    Ch 4  → 922.0    Ch 8  → 924.0
Ch 1  → 920.5    Ch 5  → 922.5    Ch 9  → 924.5
Ch 2  → 921.0    Ch 6  → 923.0    Ch 10 → 925.0
Ch 3  → 921.5    Ch 7  → 923.5

Typical: use [0, 2, 4, 6, 8, 10] for max channel separation

Every UHF RFID tag has two essential components: an antenna pattern (etched or printed aluminum on a PET substrate) and a microchip (IC). The antenna captures the reader's signal and the chip processes commands and returns data. Chip sensitivity is the minimum power the chip needs to activate — a chip rated at -22.1 dBm can wake up with just ~6.3 microwatts. Lower (more negative) = better sensitivity = longer read range.

Common chip families include: NXP UCODE 9 (-22.1 dBm, 128-bit EPC, no user memory — dominant in retail), Impinj M700 series (-22.1 dBm, 128-bit EPC — strong in logistics), and Quanray QStar-7U (-21.0 dBm, 128-bit EPC, 512-bit user memory — ideal when you need to store data directly on the tag).

Tag form factors: Dry Inlays (raw tag on PET, ¢3–8, for converting into labels), Wet Inlays (with adhesive, ¢5–12, ready to apply), Sticker Labels (printable, ¢8–25, with branding), Hard Tags ($1–15, ruggedized for harsh environments), and Woven/Fabric labels (¢15–40, sewn into garments). Nextwaves manufactures dry inlays from 35×17mm to 95×8mm and sticker labels in matching sizes.

EPCglobal Gen2 (ISO 18000-6C) governs how UHF readers communicate with tags. The key innovation is the slotted-ALOHA anti-collision algorithm that lets one reader inventory hundreds of tags simultaneously without them interfering with each other.

Here's how an inventory round works: The reader sends a Query with parameter Q (creating 2^Q time slots). Each tag picks a random slot and waits. When a tag's slot arrives, it responds with a 16-bit random number. If only one tag responds, the reader ACKs and receives the full EPC. If multiple tags collide, the reader skips that slot. After all slots, Q is adjusted — up if too many collisions, down if too many empty slots — and the round repeats.

Practical Q settings: Q=2 (4 slots) for 1–5 tags, Q=4 (16 slots) for 5–20 tags, Q=5 (32 slots) for 20–100 tags, Q=6 (64 slots) for 100–500 tags, Q=7 (128 slots) for 500+ tags. Higher Q means fewer collisions but slower rounds.

Session persistence controls how long a tag remembers it was already read. Session S0 resets instantly (for continuous monitoring). S1 persists 0.5–5 seconds (standard inventory). S2/S3 persist ≥2 seconds (dock doors and conveyors where you want each tag counted once per pass). Rule of thumb: use S0 for shelf monitoring, S2/S3 for portals.

Tag Count → Q Value → Slots → Use Case

  1-5       Q=2       4       fast, low overhead
  5-20      Q=4       16      good balance
  20-100    Q=5       32      warehouse shelves
  100-500   Q=6       64      pallet scanning
  500+      Q=7       128     dock doors, bulk

Higher Q = fewer collisions but slower rounds

Every Gen2 tag has 4 memory banks. Reserved (Bank 00): Kill password + Access password, 64 bits total. EPC (Bank 01): CRC-16 + Protocol Control word + your EPC identifier, typically 96–128 bits. TID (Bank 10): Factory-burned unique chip ID that can never be changed — invaluable for anti-counterfeiting. User (Bank 11): Optional custom data storage (0 to 512+ bits depending on chip), useful for batch numbers, inspection dates, or sensor data.

When a reader inventories tags, each notification contains: antenna ID (which port), RSSI raw value (0–255, convert to dBm via: dBm = -100 + round(raw × 70 / 255)), the EPC data (12+ bytes), and the frequency channel index. This data is what your application processes to map physical tag reads to business events like 'item shipped' or 'pallet received'.

[ANT] [RSSI] [EPC ×12 bytes ..................] [CH]
 01    B4     30 34 25 7B F7 19 4E 40 00 00 1A 85  06

Antenna:  1 (port 1)
RSSI:     180 → dBm = -100 + round((180×70)/255) = -51 dBm
EPC:      3034257BF7194E4000001A85 (SGTIN-96)
Channel:  6 → 920.0 + (6×0.5) = 923.0 MHz
GTIN-14:  80614141123458  Serial: 6789

Here's a practical checklist for setting up your first RFID system, with specific guidance at each step.

Input:  GTIN-14=08600000232451  Serial=1001  Prefix=7 digits
Output: 30 14 1A 80 0E 98 78 00 00 00 03 E9  (12 bytes)