NFC Modulation Explained: NRZ, Manchester, and Modified Miller Coding

Modulation plays a crucial role in how Near Field Communication (NFC) transmits data wirelessly between devices. NFC relies on different modulation techniques to encode and transfer data efficiently and reliably.

In this article, we will delve into key modulation schemes used in NFC: Non-Return-to-Zero (NRZ), Manchester coding, and Modified Miller coding. Understanding these techniques is essential to grasp how NFC ensures seamless and secure communication in various applications.

We will also touch upon variations of amplitude shift keying (ASK) modulation, namely 10% ASK, 100% ASK, and load modulation types. As we know, an NFC network consists of two devices known as the initiator and the target. The tag and reader can be one of these. These devices communicate using an RF carrier frequency of 13.56 MHz using a contactless electro-magnetic induction method. The devices should be very close for communication to happen (approximately less than 10cm). NFC is a low data rate and short-range wireless technology.

Based on different data rates as well as modulation & coding, there are three standards viz. NFC-A, NFC-B, and NFC-F. The NFC modulation and coding are mapped to these NFC standards as mentioned in Table 1 below.

Table 1: NFC Technology Types, Modulation, and Coding

NFC NRZ-L Coding

Figure 1 depicts NRZ-L Coding type.

Fig-1: NRZ-L coding

As shown, binary 1 is represented by a high voltage (+V) and binary 0 is represented by a low voltage (i.e., 0 volts). It is also referred to as a unipolar NRZ coding type.

NFC Manchester Coding

Figure 2 depicts Manchester Coding type.

Fig-2: Manchester coding

As shown, there are two types of Manchester codes i.e., AC and DC type. Here, both logic 1 and logic 0 are represented by a transition from high to low and low to high within the same bit period duration.

• High to Low transition is mapped as binary logic-1
• Low to High transition is mapped as binary logic-0

Transition occurs exactly in the middle of the bit period. AC type is known as bipolar and DC type is known as unipolar Manchester codes.

NFC Modified Miller Coding

Figure 3 depicts Modified Miller Coding type.

Fig-3: Modified Miller coding

• As shown, binary logic 1 is always represented by high to low and high within the bit period. It does not depend on the previous bit for mapping.
• While binary 0 is mapped according to the previous bit condition i.e., if the previous bit was 1 then the current zero (logic 0) is mapped as the reverse of the logic ‘1’ mapping done. If the previous bit was 0 then the current zero (logic 0) is mapped the same as the mapping done for logic ‘1’.

What is 10% ASK and 100% ASK modulation?

The term 10% ASK refers to amplitude shift keying modulation having a modulation index anywhere between 8 and 14%. The Modulation index is expressed as follows:

$Modulation \; index = \frac{A-B}{A+B}$

Where:

• A is the unmodulated signal amplitude
• B is the Modulated signal amplitude

The term 100% ASK means ASK modulation in which modulation and carrier amplitude are almost equal. In this modulation, the modulation wave amplitude sometimes touches zero.

What is Load Modulation?

In NFC-based communication between the reader and transponder (passive), the reader transfers data to the transponder using ASK modulation while the transponder transmits data back to the reader using load modulation. In load modulation, modulation resistance connected in parallel to the antenna is switched ON and OFF at the clock rate of the signal which needs to be transmitted.

Often it is carried out using either bringing a resistor or capacitor in and out of the antenna circuit. Here, the modulated carrier frequency is 848 KHz. The Subcarrier is ASK modulated with Manchester coded data at a bit rate of 106Kbps. This is specified in the ISO/IEC 14443 standard. This is known as load modulation.

Conclusion

NFC modulation techniques like NRZ, Manchester, and Modified Miller coding each serve specific purposes to enhance data transmission efficiency and reliability. While NRZ simplifies data encoding, Manchester coding and Modified Miller coding are essential for minimizing errors and improving synchronization.