Overview
The NFC RF interface is designed to keep the interface simple, reduce power consumption, and maintain reliable communication. The RF interface must also accommodate communication between active and passive devices.
To achieve this, simple modulation techniques and encoding are used, making the modulation formats and the overall RF interface relatively straightforward.
NFC RF Signal Parameters
NFC uses the global 13.56 MHz allocation, which is an unlicensed RF ISM band.
Amplitude-shift keying (ASK) is used as the NFC modulation format. Most RF energy is concentrated within the permitted 14 kHz bandwidth, although sidebands can extend to ±1.8 MHz.
Encoding and Modulation
NFC uses two different encoding schemes on the RF signal to transmit data. In most cases, Manchester encoding is used with a 10% modulation depth. However, for active devices transmitting at 106 kbps, an enhanced Miller coding scheme with 100% modulation is used. For all other cases, Manchester encoding uses a 10% modulation depth.
| Summary of NFC RF Signal Encoding and Modulation | ||
|---|---|---|
| Data Rate (kbps) | Active Device | Passive Device |
| 106 | Modified Miller, 100%, ASK | Manchester, 10%, ASK |
| 212 | Manchester, 10%, ASK | Manchester, 10%, ASK |
| 424 | Manchester, 10%, ASK | Manchester, 10%, ASK |
Manchester Encoding
Manchester encoding is used for most NFC communications. It relies on two possible transitions within a bit period. A low-to-high transition represents a 0 bit, while a high-to-low transition represents a 1 bit.
To meet these conditions, a transition is sometimes required at the middle of the bit period. A transition at the start of the period is ignored.
Manchester encoding
Modified Miller Encoding
The modified Miller code is less intuitive, but it provides an efficient encoding form. It is characterized by pauses in the carrier at different positions within a bit period. Encoding of a bit depends on the information to be transmitted. The encoding for a high or "1" is always the same, while the encoding for a low or "0" depends on the previous bit content.
Modified Miller encoding used for NFC data transfer, applied to 106 kbps active device transmissions
Conclusion
The NFC RF signal and physical layer are intentionally kept simple to accommodate simple passive cards, allowing information to be read and decoded as needed. The low data rates also help keep the RF interface as simple as possible.
