Originally posted Apr 12, 2013
Choose the best view or use several at once
In my last post I used a geology imaging example from mars to explain different measurement views and how these views can meet different measurement needs.
Well, I admit I also used the images because I’m fascinated by how a scene from Mars might look on Earth.
Now it’s time for an example or two from our home field of test and measurement. Let’s consider two measurements of OFDM. Over the past decade OFDM has become the single most important transport scheme in wireless, supporting a variety of modulation types through a large number of independent subcarriers. The scheme provides practical benefits for RF transmission, though its multicarrier nature poses some test challenges.
Consider the two measurements of modulation error shown below.
Two error displays of the same OFDM signal frame. Error is plotted vs. time or symbol number (left) and vs. frequency or subcarrier number (right). One display clearly shows the source of the major error: narrowband interference.
These are orthogonal modulation error displays of the same signal. Specifically they show the magnitude of the error vector (EVM) for every symbol and subcarrier in an IEEE 802.11a frame. The difference between them is that the X-axis on the left is time or symbol number and the X-axis on the right is frequency or subcarrier number. Thus the left display is error vector time and the right display is error vector spectrum. In both displays the white symbol dots (small squares) are pilot symbols from the 4 pilot subcarriers and the heavy white lines are averages by symbol (left) or by subcarrier (right).
The biggest difference between the displays is their ability to reveal the source of a modulation error. The total modulation error of the signal is relatively low, about 1%, and a constellation display would look good. However the display on the left shows that a few symbols have higher error that is relatively constant over the frame or burst.
The display on the right is much more useful, clearly showing that the larger errors all belong to a single subcarrier. This subcarrier is impaired much more than the others, up to about 5% peak. Such a narrowband interfering signal is likely a spur but could also be a harmonic of a narrow modulated signal from another band.
Thus an error that is obscure in one display is clear in another, making it much easier to spot a problem and determine a cause. Tools such as Agilent’s 89600 VSA software support many different signal and error traces at once, along with large monitors, allowing simultaneous displays of multiple signals and multiple measurements. This makes the best use of your visual processing, pattern recognition and system knowledge to discover and isolate problems quickly.
In this example the error spectrum trace is the key, but in others the error time trace can reveal a problem more clearly. I’ll describe just such a situation in my next post.
By the way, the display on the right also illustrates one of the practical benefits of OFDM: It’s resistance to narrowband interference or narrow spectral nulls from multipath. With forward error correction to compensate for the loss of data from one or several subcarriers OFDM handles the kind of impairment that would cripple an equivalent signal using traditional single carrier modulation. Its only one of many benefits of OFDM, which will be discussed in future posts.
For more on OFDM impairments see Bob Cutler’s classic article “Effects of physical layer impairments on OFDM systems” at http://rfdesign.com/images/archive/0502Cutler36.pdf. Note the typo in the article, however, where the rightmost column label of the table on page 40 (3rd page) should be “single carrier modulation.”