17-03-2014, 02:54 PM
Transmit Constellation Error
Purpose:
To verify that the relative constellation RMS error of the device under test (DUT) is
within the conformance limits.
Resource Requirements:
• Vector Signal Analyzer (VSA), Rohde & Schwarz FSQ26 or equivalent
• ERP-PHY Station with external antenna jack
• Associated ERP-PHY AP or station with external antenna jack (optional)
• RF Power Splitter, Picosecond 5333 or equivalent
• Controlling Software (Matlab)
Last updated: April 2008
Discussion:
Reference [1] describes the Extended Rate Physical Medium Dependent (ERP-PMD) sublayer
for the 2.4 GHz band, and subclause 19.4.7 specifies the PMD transmit specifications. ERP
devices are required to follow the orthogonal frequency division multiplexing (OFDM) transmit
specifications of Clause 17 with the exceptions listed in 19.4.7.
At any moment in time the transmitted signal magnitude and phase can be measured, and an
ideal or “reference signal” can be calculated. According to [1], modulation accuracy is defined as
the difference between the actual transmitted waveform and the ideal waveform. In practice, the
modulation accuracy is measured using error vector magnitude (EVM). EVM is based on the
concept that any impaired signal can be represented as the sum of an ideal signal and an error
signal. Since an error signal cannot be measured directly, test instrumentation determines the
error signal by reconstructing the ideal signal based on received data and then subtracting it from
the actual signal. This concept is depicted in Figure 7.
Procedure:
1. For DUTs that support continuous transmission, configure the DUT to transmit 100-byte
PSDU’s continuously on channel 6 at a bit rate of 11 Mbps at maximum output power.
Otherwise, configure the DUT to associate with the AP on channel 6 at a bit rate of 11
Mbps at maximum output power; configure the AP to repeatedly transmit 100-byte ICMP
echo request packets to the DUT.
2. Configure the VSA to capture an entire packet.
3. Demodulate the packet and compute the EVM using the methodology described in [1].
4. Repeat steps 1-4 for each channel and supported data rate.
Observable Results:
a. For ERP-DSSS, the worst-case EVM should not exceed 0.35 for the normalized sampled
chip data
b. For ERP-OFDM, the relative constellation RMS error should not exceed the values listed
in Table 3.