Zigbee Physical Layer (PHY) Explained

There are two Physical layer formats, one each for the 868/915MHz and 2450MHz bands, in the Zigbee standard.

• 868-868.6 MHz zigbee band delivers about 20 Ksymbol/s with BPSK modulation employed.
• 902-928 MHz band delivers about 40 Ksymbol/s with BPSK modulation.
• 2400-2483.5 MHz delivers about 62.5 Ksymbol/s with O-QPSK modulation.

868/915 PHY Layer:

The PHY layer, as depicted in the figure, is largely self-explanatory. The first block is a differential encoder. $E_n = E_{n-1} \oplus R_n$. Where, $R_n$ is the raw data bit being encoded, $E_n$ is the corresponding differentially encoded bit, and $E_{n-1}$ is the previous differentially encoded bit.

• Bit to chips will convert each bit to 15 chip values, as mentioned in IEEE 802.15.4-2003 standard in section 6.6.2.3.
• These chip values are BPSK modulated using the BPSK modulator block.
• The modulated signal is passed through the Raised cosine filters in the chain.
• This filtered signal is modulated using an RF carrier and transmitted over the air.

2450 PHY Layer:

• Each octet is divided into two nibbles of 4bits each. These nibbles are converted to decimal value 0-15.
• This is fed as input to symbol to chip and based on Symbol-to-chip mapping table these decimal values are converted into chips.
• Later chips are passed through the OQPSK modulator block and half sine wave pulse shaping filter before being converted to an RF modulated waveform.

The PPDU packet format which is transmitted is mentioned below.

Zigbee PPDU Frame

The Zigbee PPDU Frame consists of SHR (Preamble 4bytes, SFD 1 byte) + PHR(Frame length 1 bit, Reserved 1 bit) + PHY Payload(PSDU variable length)

The Preamble is composed of 32 zeros and is used for chip and symbol synchronization at the receiver part of the transceiver.

SFD is an 8-bit field used to segregate between the preamble and the actual physical layer data.

Frame length is 7 bits long and indicates the total number of octets contained in the PSDU (i.e. PHY payload).

The PSDU field carries the PHY packet and is variable in length. All the packet types of length 5 octets or greater than 7 octets, the PSDU contains the MAC sub-layer frame (i.e. MPDU).

Zigbee References

Apart from this zigbee tutorial’s contents if reader wants to gain more knowledge they can read IEEE standard mentioned below.

1. IEEE 802.15.4-2003 standard

Pl. note that all the figures and tables mentioned on this page have been taken from IEEE standard to demonstrate zigbee concept.