Offset QPSK (Digital Demod)

Offset QPSK demodulates like the other demodulation formats. With Offset QPSK, two additional IS-95 filters are available to measure signals that conform to US CDMA Code Division Multiple Access: One of several digital wireless transmission methods in which signals are encoded using a specific pseudo-random sequence, or code, to define a communication channel. A receiver, knowing the code, can use it to decode the received signal in the presence of other signals in the channel. This is one of several "spread spectrum" techniques, which allows multiple users to share the same radio frequency spectrum by assigning each active user an unique code. CDMA offers improved spectral efficiency over analog transmission in that it allows for greater frequency reuse. Other characteristics of CDMA systems reduce dropped calls, increase battery life and offer more secure transmission. See also IS-95. digital cellular standard IS-95. To learn about these filters, see IS-95 base phase EQ and IS-95 baseband measured and reference filters.

To automatically configure the VSA for mobile measurements, display the Digital Demodulation Properties dialog box, then click Preset to Standard > Cellular and select CDMA Mobile (for mobile measurements).

When using Offset QPSK, note the following:

An even number must be specified for Points / Symbol.
Offset QPSK displays error data in the symbol table and uses the same computations as other demodulation formats with the following exceptions:
- Amplitude droop is not computed for Offset QPSK.
- The VSA uses two points-per-symbol (instead of one point-per-symbol) to compute the following:
- Offset EVM is computed in addition to the regular 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..
The symbol rate is the frequency of the I (or Q) clock, not the 2x frequency sometimes associated with the I to Q clock frequency.
Demodulated traces show a symbol at every I and Q detection (decision) point. Therefore, all demodulated traces (except the symbol table) show twice as many symbols as the Result Length, at twice the symbol rate.
When a Half-Sine reference filter is selected, the constellation changes from a square to a circle and the ideal state circles are moved to the I and Q axes.
When Low SNR Enhancement is selected, Offset QPSK does not support the Rotation parameter so the parameter will be grayed out.
When Low SNR Enhancement is selected and the Adaptive Equilizer Filter is used, measurement speed can become very slow when the filter length is long.
When Low SNR Enhancement is enabled, the maximum Result Length value is about 40000 symbols, otherwise it is 2048. In both cases this maximum may be limited by "Search Time" or by hardware capabilities.