The IEEE 802.15.4 Physical Layer (PHY) specifies the spread spectrum techniques, modulation schemes, and frame structure standards for low-power wireless personal area networks (LR-WPANs) operating across the 868/915 MHz and 2.4 GHz bands, enabling critical functions including Energy Detection (ED), Link Quality Indication (LQI), and Clear Channel Assessment (CCA).
1. Introduction to the Main Functions of the PHY Layer
Radio Transmission and Reception Activation and Deactivation
Energy Detection (ED) within the Current Channel
Link Quality Indication (LQI) for Received Packets
Free Channel Assessment (CCA) for Carrier Sense Multiple Access/Collision Avoidance (CSMA-CA)
Channel Frequency Selection
Data Transmission and Reception
2. Introduction to Physical Parameters
868/915 MHz DSSS (Direct Sequence Spread Spectrum) uses BPSK (Binary Phase Shift Keying) modulation
868/915 MHz DSSS (Direct Sequence Spread Spectrum) uses O-QPSK modulation
868/915 MHz PSSS (Parallel Sequence Spread Spectrum) uses BPSK and ASK (Amplitude Shift Keying) modulation
2450 MHz DSSS (Direct Sequence Spread Spectrum) uses O-QPSK modulation
| PHY (MHz) | Frequency band (MHz) | Chip rate (kchip/s) | Modulation | Bit rate (kb/s) | Symbol rate (ksymbol/s) | Symbols |
|---|---|---|---|---|---|---|
| 868/915 | 868–868.6 | 300 | BPSK | 20 | 20 | Binary |
| 868/915 | 902–928 | 600 | BPSK | 40 | 40 | Binary |
| 868/915 (optional) | 868–868.6 | 400 | ASK | 250 | 12.5 | 20-bit PSSS |
| 868/915 (optional) | 902–928 | 1600 | ASK | 250 | 50 | 5-bit PSSS |
| 868/915 (optional) | 868–868.6 | 400 | O-QPSK | 100 | 25 | 16-ary Orthogonal |
| 868/915 (optional) | 902–928 | 1000 | O-QPSK | 250 | 62.5 | 16-ary Orthogonal |
| 2450 | 2400–2483.5 | 2000 | O-QPSK | 250 | 62.5 | 16-ary Orthogonal |
3. Introduction to PHY Frame Format
| Field Category | Subfield | Length (Octets) | Description |
|---|---|---|---|
| SHR | Preamble | - | Synchronization header (including preamble) |
| SFD (Start Frame Delimiter) | - | Start Frame Delimiter | |
| PHR | Frame length (7 bits) | 1 (total) | Physical layer header, total 8 bits |
| Reserved (1 bit) | Reserved bit | ||
| PHY payload | PSDU | variable | Physical layer data payload (variable length) |
SHR: Synchronization Header (including Preamble and SFD), allows a receiving device to synchronize and lock onto each bit of the stream simultaneously.
PHR: PHY Frame Header (including Frame Length and Reserved), containing frame length information.
PHY Payload: Variable-length payload carrying data frames from the MAC sublayer.
The SHR field, Preamble, is used by the radio transceiver to extract chip and synchronization identifiers from the received data. The length of the Preamble field is determined by physical parameters, as shown in the following figure:
| PHY | Length (Octets) | Length (Symbols) | Duration (µS) |
|---|---|---|---|
| 868–868.6 MHz BPSK | 4 octets | 32 symbols | 1600 |
| 902–928 MHz BPSK | 4 octets | 32 symbols | 800 |
| 868–868.6 MHz ASK | 5 octets | 2 symbols | 160 |
| 902–928 MHz ASK | 3.75 octets | 6 symbols | 120 |
| 868–868.6 MHz O-QPSK | 4 octets | 8 symbols | 320 |
| 902–928 MHz O-QPSK | 4 octets | 8 symbols | 128 |
| 2400–2483.5 MHz O-QPSK | 4 octets | 8 symbols | 128 |
SFD indicates the end of SHR and the beginning of a data packet. Different physical parameters have different lengths, as shown in the figure below:
| PHY | Length (Octets) | Length (Symbols) |
|---|---|---|
| 868–868.6 MHz BPSK | 1 octet | 8 symbols |
| 902–928 MHz BPSK | 1 octet | 8 symbols |
| 868–868.6 MHz ASK | 2.5 octets | 1 symbol |
| 902–928 MHz ASK | 0.625 octets | 1 symbol |
| 868–868.6 MHz O-QPSK | 1 octet | 2 symbols |
| 902–928 MHz O-QPSK | 1 octet | 2 symbols |
| 2400–2483.5 MHz O-QPSK | 1 octet | 2 symbols |
FAQ
Q1: What are the primary responsibilities of the IEEE 802.15.4 PHY layer?
A: The PHY layer is responsible for the activation and deactivation of the radio transceiver, Energy Detection (ED) within the current channel, Link Quality Indication (LQI) for received packets, providing Clear Channel Assessment (CCA) for the CSMA-CA algorithm, channel frequency selection, and the physical transmission and reception of data bits.
Q2: How do the modulation schemes differ between the 868/915 MHz and 2.4 GHz bands?
A: The 868/915 MHz bands support DSSS (using BPSK or O-QPSK modulation) as well as PSSS (using BPSK and ASK modulation). In contrast, the 2450 MHz (2.4 GHz) band exclusively utilizes DSSS with O-QPSK modulation to ensure global compatibility and higher data rates.
Q3: What is the specific function of the Synchronization Header (SHR) in a PHY frame?
A: The SHR, which consists of a Preamble and a Start Frame Delimiter (SFD), allows the receiving device to synchronize with the incoming bit stream and extract chip identifiers. The Preamble facilitates bit-level synchronization, while the SFD signals the end of the header and the beginning of the actual data packet.
