Link Budget

Link Budget

A link budget is a systematic accounting of all gains and losses in the RF signal path from RFID reader transmitter through the air interface to the tag and back. It is the fundamental engineering tool for predicting read range, diagnosing performance problems, and specifying system components.

Forward Link (Reader to Tag)

The forward link determines whether enough power reaches the passive tag to activate its IC. The budget starts with reader transmit power (typically 30-33 dBm), adds antenna gain (6-12 dBi), and subtracts all losses:

• Cable loss: Coaxial cable between reader and antenna (0.5-3 dB depending on length and type)
• Connector loss: Each RF connector adds 0.1-0.5 dB
• Free-space path loss: Increases with distance and frequency (FSPL = 20log10(4pi*d/lambda))
• Material loss: Packaging, pallets, or products between reader and tag (0-20 dB)
• Polarization mismatch: Up to 3 dB loss for circular polarization or variable loss for linear polarization depending on tag orientation

The tag activates when the received power exceeds its sensitivity threshold (typically -18 to -24 dBm for modern UHF ICs).

Reverse Link (Tag to Reader)

The reverse link determines whether the backscattered signal from the tag is strong enough for the reader to decode. The tag's reflected power is much weaker than the forward signal -- typically 60-80 dB below the transmitted power. The reader's receiver sensitivity (around -80 dBm for modern readers) sets the detection floor.

In most UHF RFID systems, the forward link is the limiting factor: the tag runs out of activation power before the reverse link becomes undecodable.

Practical Application

Link budget analysis guides critical design decisions. If the calculated forward link margin is less than 6 dB, the system lacks sufficient headroom for environmental variations. Engineers can improve the margin by selecting a more sensitive tag IC, increasing antenna gain, reducing cable length, or using lower-loss cabling. The EIRP constraint sets an upper bound on the combined transmit power and antenna gain.

Environmental factors like humidity, temperature, and nearby metallic structures cause link budget variations of 3-10 dB in practice. Robust system designs build in at least 10 dB of margin above the minimum required for reliable communication.