ZigBee Standard (IEEE 802.15.4)(Digital Demod)

ZigBee (IEEE Institute of Electrical and Electronics Engineers. A US-based membership organisation that includes engineers, scientists, and students in electronics and related fields. The IEEE developed the 802 series wired and wireless LAN standards. Visit the IEEE at http://www.ieee.org 802.15.4) is a standard for low data rate, low duty cycle applications. This standard was designed to meet the unique needs of sensors and control devices. Sensors and controls don’t need high bandwidth but they do need low latency and very low energy consumption for long battery lives and for large device arrays.

The standard specifies different modulation types for different frequency bands:

Band (MHz Megahertz: A unit of frequency equal to one million hertz or cycles per second.)	Modulation Type	Pulse Shaping Filter
868	BPSK Binary phase shift keying - A type of phase modulation using 2 distinct carrier phases to signal ones and zeros.	Raised Cosine
915	BPSK	Raised Cosine
2450	OQPSK Offset Quadrature Phase Shift Keying: A type of QPSK modulation that offsets the bit streams on the I and Q channels by a half bit. This reduces amplitude fluctuations and helps improve spectral efficiency.	Half Sine

Option AYA provides the following analysis tools required to make ZigBee measurements in the 868, 915 and 2450 MHz bands:

Standard Presets for all three bands
Half Sine filter for the 2450 MHz band

The Half Sine pulse shaping filter changes the vector diagram of OQPSK from a square to a circle. It eliminates any amplitude variations in the OQPSK signal making it a constant envelope signal. This makes the OQPSK resemble MSK Minimum Shift Keying.

The definition of EVM Error vector magnitude (EVM): A quality metric in digital communication systems. See the EVM metric in the Error Summary Table topic in each demodulator for more information on how EVM is calculated for that modulation format. in the ZigBee standard corresponds to the Offset EVM displayed by the VSA. The standard says to measure EVM for 1000 chips, but to achieve this the VSA should be set to a Result Length of 1001. This is necessary because the VSA doesn’t use the last half of the last OQPSK symbol, so it computes Offset EVM only on Result Length - 1 symbols.

State Definitions

The state definitions used for the ZigBee 2450 MHz standard are different from the default OQPSK state definitions so to get the correct bits to display in the Symbol Table, you will need to recall a custom state definitions file.

Here are the ZigBee 2450 standard state definitions:

!# State definitions for ZigBee 2450 MHz OQPSK

! "S_OQPSK_1","01","11"

! "S_OQPSK_2","00","10"

To load these state definitions into the 89600 VSA Digital Demodulator, follow these steps:

Create a new text file .
Copy and paste the text above into the file and save the file.
Preset the demodulator to Wireless Networking > ZigBee 2450 MHz.
Click File > Recall > Recall State Definitions.
Find the file you created in step 1 and click Open.

The symbol table should now show the correct bits when demodulating ZigBee 2450 MHz signals.