NFC Modulation Explained : NRZ, Manchester and Modified Miller Coding
Modulation plays a crucial role in how NFC (Near Field Communication) transmits data wirelessly between devices. NFC relies on different modulation techniques to encode and transfer data efficiently and reliably. In this page, 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 variation of amplitude shift keying modulation namely 10% ASK, 100% ASK and load modulation types.
As we know NFC network consists of two devices known as initiator and target. The tag and reader can be one of these. These devices communicate using RF carrier frequency of 13.56 MHz using contactless electro-magnetic induction method. The devices shoule be very close for communication to happen (approx. less than 10cm). NFC is 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 the table-1 below.
| NFC technology Type | Polling or Listening |
NFC Modulation | NFC Coding |
|---|---|---|---|
| NFC-A | Polling | ASK 100% (Read explaination below) | Modified Miller |
| NFC-A | Listening | Load(ASK-Amplitude Shift Keying) | Manchester |
| NFC-B | Polling | ASK 10% (Read explaination below) | NRZ-L |
| NFC-B | Listening | Load (BPSK) | NRZ-L |
| NFC-F | Polling | ASK 10% | Manchester |
| NFC-F | Listening | Load modulation (ASK), read explaination below. | Manchester |
NFC NRZ-L Coding
The figure-1 depicts NRZ-L Coding type.
Fig-1:NRZ-L coding
As shown binary 1 is represented by high voltage(+V) and binary 0 is represented by low voltage (i.e. 0 volt). It is also referred as unipolar NRZ coding type.
NFC Manchester Coding
The 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 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 and Low to High transition is mapped as binary logic-0. Transition occurs exactly in the middle of bit period. AC type is known as bipolar and DC type is known as unipolar manchester codes.
NFC Modified Miller Coding
The 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 previous bit for mapping.
• While binary 0 is mapped according to previous bit condition i.e.
if previous bit was 1 then current zero (logic 0) is mapped as reverse of logic '1' mapping done.
if previous bit was 0 then current zero (logic 0) is mapped same as mapping done for logic '1'.
What is 10% ASK and 100% ASK modulation
The term 10% ASK refers to amplitude shift keying modulation having modulation index anywhere
between 8 and 14%.
Modulation index is expressed as follows:
Modulation index = (A-B)/(A+B)
Where,
A is unmodulated signal amplitude
B is Modulated signal amplitude
The term 100% ASK means ASK modulation in which modulation and carrier amplitude are almost equal.
In this modulation, modulation wave amplitude sometimes touches zero.
Refer 10% and 100% ASK modulation types.
What is Load Modulation ?
In NFC based communication between reader and transponder (passive), reader transfers data to transponder using ASK modulation while transponder transmits data back to reader using load modulation.
In load modulation, modulation resistance connected in parallel to the antenna is
switched ON and OFF at clock rate of the signal which need to be transmitted.
Often it is carried out using either bringing resistor or capacitor in and out of the antenna circuit.
Here modulated carrier frequency is 848 KHz.
Subcarrier is ASK modulated with manchester coded data at bit rate of 106Kbps.
This is specified in ISO/IEC 14443 standard.
This is known as load modulation.
Refer ASK active load modulation used in NFC.
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.