NFC Physical Layer: modulation & RF signal

The NFC RF interface has been kept simple but used a number of key features & modulation to provide the required performance.

NFC, Near Field Communication Includes:
What is NFC? NFC technology NFC RF signal Data exchange format NFC Tags & tag types NFC security

The NFC RF interface has been developed to keep the interface simple, reduce power consumption and retain reliable communication.

The NFC RF interface also needs to accommodate communication to and from both active and passive devices.

To achieve this the modulation format and overall RF interface has been made relatively straightforward using simple modulation techniques and encoding.

NFC RF signal parameters

NFC uses the global 13.56 MHz allocation as this is an unlicensed radio frequency ISM band.

Using ASK - amplitude shift keying, as the format for the NFC modulation, most of the RF energy is concentrated in the allowed 14 kHz bandwidth, although the sidebands may extend out as far as ± 1.8 MHz.

NFC RF signal coding & modulation

NFC employs two different coding systems on the RF signal to transfer data. In most cases a level of 10% modulation is used, with a Manchester coding format. However for an active device transmitting data at 106 kbps, a modified Miller coding scheme is used with 100% modulation. In all other cases Manchester coding is used with a modulation ratio of 10%.

Summary of NFC RF Signal Coding & 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

NFC and Manchester coding

Manchester coding is used for the majority of cases for the NFC communications. The Manchester coding utilises the two different transitions that may occur at the midpoint of a period. A low-to-high transition expresses a 0 bit, whereas a high-to-low transition stands for a 1 bit.

To achieve these conditions it is sometimes necessary to have a transition at the middle of a bit period. Transitions at the beginning of period are disregarded.

Manchester coding used for NFC data transfer — Manchester coding

NFC and Modified Miller coding

The modified Miller code is a little less intuitive, but provides an efficient form of coding. It is characterised by the pauses occurring in the carrier at di?erent positions of a period. Depending on the information to be transmitted, bits are coded as shown below. A high or "1" is always encoded inth e same way, but a low or "0" is encoded differently dependent upon what preceded it.

Modified Miller coding used for the NFC RF signal — Modified Miller coding used for NFC data transfer
used for 106 kbps active device transfers

The NFC RF signal and physical layer is purposefully kept simple and straightforward to ensbale simple passive cards to be able to read and decode the information as required. As low data rates are used, this also enables the RF inteface to be kept as simple as possible.