About IEEE 802.11b Filtering (802.11b/g DSSS/CCK/PBCC)

This topic discusses some of the unique measurement and reference filtering requirements and related parameters used by the VSA for 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 802.11b modulation measurements. This information will describe how the different filtering types affect 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. (and other related measurements) and how it relates to satisfying the standard test specifications.

There is ambiguity in determining what the appropriate VSA filtering type should be. This is because the IEEE 802.11b standard has inconsistencies in its definition. First, the standard does not specify what type of filtering the transmitter must use. Instead the standard imposes a spectral mask requirement. This does imply however that the transmitter is using some type of filtering in order to satisfy the standard. Second, the standard specifies "not" to use a reference filter when computing EVM. Thus the problem, the EVM is being computed with incorrect knowledge of the correct transmitter filtering.

The 89600 VSA computes EVM as specified by the IEEE 802.11b standard and does not use a reference filter. To emulate this the VSA uses a "Rectangular" filter. This causes the EVM for a "perfect" 802.11b signal to have a nonzero EVM level. This may be an unexpected result, but it is computed as specified by the standard. Also, because the exact transmitter filtering is unknown, the VSA does not support the display of multiple points per chip. This is because the EVM for these in-between points would always be high and unrepresentative of the actual signal. Fortunately, the in-between points are not required to accurately demodulate the signal.

The VSA provides limited support for reference filter selections: Rectangular, Gaussian, or Raised Cosine reference filter type. The VSA default is the Rectangular filter, so that it's compatible with the IEEE 802.11b standard. The Gaussian filter specifies the "BT Bandwidth Time Product" value. The Raised Cosine filter is used in 802.11b measurements for channel 14.

Regardless of which filter is used, the 802.11b demod optionally supports the use of an equalizer. The equalizer will essentially remove any effects of the transmit filter, as well as affects of the frequency response of the transmit channel. Using the equalizer is sometimes the best way to characterize the quality of a signal, but it is turned off by default because the IEEE 802.11b standard does not specify one.

Using an Keysight ESG and Signal Studio Software

The Keysight Signal Studio software for the Keysight ESG defaults to using a Gaussian transmit filter with a BT value of 0.5. This makes the Signal Studio signal satisfy the 802.11b spectral mask requirement. When generating 802.11b signals with Signal Studio, changing the BT value to 0.7 will result in lower EVM when computing EVM according to the standard.