Zigbee Physical Layer (PHY)

There are two Physical layer formats one each for 868/915MHz and 2450MHz bands in Zigbee standard. 868-868.6 MHz zigbee band delivers about 20Ksymbol/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:

PHY layer mentioned in the figure is self explanatory. The first block is differential encoder.
En = EX-OR of Rn and En-1.
Rn is the raw data bit being encoded,
En is the corresponding differentially encoded bit,
En-1 is the previous differentially encoded bit.

zigbee tutorial fig1

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

2450 PHY layer:

• Each octet is divided into two nibbles of 4bits each. This 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 OQPSK modulator block and half sine wave pulse shaping filter before being converted to RF modulated waveform.

zigbee tutorial fig2

PPDU packet format which is transmitted is mentioned below. Zigbee PPDU Frame consists of SHR (Preamble 4bytes, SFD 1 byte) + PHR(Frame length 1 bit, Reserved 1 bit) + PHY Payload(PSDU variable length)

zigbee PHY packet structure

Preamble is composed of 32 zeros and is used for chip and symbol synchronization at receiver part of transceiver. SFD is 8 bit field used to segregate between preamble and actual physical layer data. Frame length is 7 bits long and indicates total number of octets contained in the PSDU (i.e. PHY payload) PSDU field carries PHY packet and is variable in length. All the packet types of length 5 octets or greater than 7 octets, PSDU contains MAC sub-layer frame (i.e. MPDU). Refer PHY Packet format>>.

Zigbee References

Apart from this zigbee tutorial's contents if reader wants to gain more knowledge they can refer 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.

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