Magnitude and Relative Constellation Error Measurements
- IEEE 802.11a configurable signal source, adjustable data rate
- Adjustable sample rate
- Constellation display
- Integrated RF section
This design tests IEEE 802.11a transmit modulation accuracy and transmitter constellation error by measuring the EVM. The schematic for this design is shown in Figure 2-16.
Figure 2-16. WLAN_80211a_TxEVM.dsn Schematic
Measurements in this design are based on IEEE Standard 802.11a-1999 section 22.214.171.124. The transmit modulation accuracy test must be performed by instrumentation capable of converting the transmitted signal into a stream of complex samples at 20 Msamples per second or more, with sufficient accuracy in terms of I/Q arm amplitude and phase balance, dc offsets, phase noise, and so on. A possible embodiment of such a setup is converting the signal to a low IF frequency with a microwave synthesizer, sampling the signal with a digital oscilloscope and decomposing it digitally into quadrature components.
The sampled signal must be processed in a manner similar to an actual receiver, according to the following, or equivalent steps:
- Start of frame must be detected.
- Transition from short sequences to channel estimation sequences must be detected, and fine timing (with one sample resolution) must be established.
- Coarse and fine frequency offsets must be estimated.
- The packet must be de-rotated according to estimated frequency offset.
- The complex channel response coefficients must be estimated for each subcarrier.
- For each data OFDM symbol: transform the symbol into subcarrier received values, estimate the phase from the pilot subcarriers, de-rotate the subcarrier values according to estimated phase, and divide each subcarrier value with a complex estimated channel response coefficient.
- For each data-carrying subcarrier, find the closest constellation point and calculate the Euclidean distance from it.
- Calculate the RMS average of all errors in a packet:
- LP is the length of the packet
- Nf is the number of frames for the measurement
- (I0(i, j, k), Q0(i, j, k)) denotes the ideal symbol point of the ith frame, jth OFDM symbol of the frame, kth subcarrier of the OFDM symbol in the complex plane
- (I(i, j, k), Q(i, j, k)) denotes the observed point of the ith frame, jth OFDM symbol of the frame, kth subcarrier of the OFDM symbol in the complex plane (see Figure 2-17)
- P0 is the average power of the constellation.
The vector error on a phase plane is shown in Figure 2-17.
The test must be performed over at least 20 frames (Nf) and the RMS average must be taken. The packets under test must be at least 16 OFDM symbols long. Random data must be used for the symbols.
Figure 2-17. Constellation Error
The EVM and relative constellation RMS error, averaged over subcarriers, OFDM frames, and packets, cannot exceed a data-rate dependent value according to Table 2-3.
Table 2-3. Allowed EVM and Relative Constellation Error
Relative Constellation Error (dB) |
Simulation results displayed in WLAN_80211a_TxEVM.dds are shown in Figure 2-18 for EVM and relative constellation error of 54 Mbps. The EVM is less than 0.6%; the constellation error is approximately -45dB which is much smaller than the specification requirements given in Table 2-3.
Figure 2-18. EVM and Relative Constellation Error of 54 Mbps
- Hardware platform: Pentium III 450 MHz, 512 MB memory
- Software platform: Windows NT 4.0 Workstation, ADS 2001
- Simulation time: approximately 30 minutes
 IEEE Standard 802.11a-1999, "Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications: High-speed Physical Layer in the 5 GHz Band," 1999.