Originally posted Apr 18, 2013
Big clues and little ones
In the previous post the signal impairment was confined to a narrow range of frequencies and therefore a frequency-specific error measurement (error vector spectrum) was the key to spotting the error and deducing its source. Since the impairment was constant over time, the time-specific error measurement (error vector time) wasn’t much help.
Now let’s look at a different OFDM example, a prototype fixed WiMAX (IEEE 802.16) signal. Here’s a measurement of error vector spectrum, the display type that was so useful in the previous post:
Error spectrum of an impaired WiMAX OFDM signal. The error is generally constant across the subcarrier frequencies and is much higher for one modulation type.
The small squares are individual symbol errors and knowing that the analyzer (Agilent 89600 VSA) represents different modulation types with different colors provides the first big clue. The large errors are almost all green, while the blue squares and the orange ones behind them indicate that the other modulation types have much smaller errors.
Here’s where adding your own knowledge of the fixed WiMAX scheme comes in. Multiple modulation types can be used in a single burst, and they are transmitted sequentially. Time (or symbol number) and modulation type are the key. A glance at two other common displays, constellation and error vector time, reveal the error clearly:
Composite constellation (left) and error vs. time (right) of a fixed WiMAX signal.
The clearest anomaly is in the error vector time trace on the right. The sudden increase in error at the end of the frame corresponds to the switch from 16QAM in blue to 64QAM in green. The average error indicated by the heavy white line also reflects the increased error.
The constellation diagram on the left is also very useful, even though the difference in error is not as apparent. This constellation is a stacked display of all symbols and modulation types, using the same modulation color coding as the other displays. The small white circles are ideal symbol locations, showing that the BPSK, QPSK and 16QAM symbols are being transmitted correctly. By contrast the outer (largest amplitude) symbols of the 64QAM modulation all show an amplitude that is smaller than it should be. Is this a result of compression (a typical RF error) or overall scaling (perhaps a baseband error) of the 64QAM modulation?
A less obvious clue from the constellation display settles the issue. If we examine the BPSK pilots which are used to track received amplitude and scale the demodulation we see that their amplitude is correct. It’s a little hard to see from this small display but the same is true of the BPSK, QPSK and 16QAM data. As for 64QAM, close inspection shows that the inner symbols also have reduced amplitude. This strongly suggests that the error is from overall scaling of 64QAM and not compression or limiting, which would mostly affect the outer constellation states. The defect is probably in the digital baseband of the transmitter and will reduce performance or margin even if it does not cause bit errors by itself.
We’ve discussed larger clues here, and some smaller ones. Another small clue is the low EVM of the pilot subcarriers in the first display above, and the fact that it does not increase along with the 64QAM data subcarrier error. All these clues can be helpful and are generally clearer in one display type than another. As with the example in the previous post it’s a reason to use multiple traces and a large display if available. Combined with knowledge of systems or signals, pattern recognition is a powerful thing for experienced RF engineers!
You can experiment with this signal yourself, at no charge, using the 89600 VSA software and included recordings. Just download the VSA software and accept the trial or demo license. Then recall the demo recording, which includes a setup state and an explanation of the signal. Dozens of other recorded signals and setups are also included.
Download the software at www.keysight.com/find/vsa.
You’ll find a complete installation guide here.
For this example just select File – Recall – Demo – WiMAX Fixed – WiMAX_5MHz_Impaired.htm