Originally posted Feb 10, 2014
Balancing RF performance, processing power and throughput
OFDM is a demanding modulation scheme for RF equipment. The numerous subcarriers are so closely spaced that phase noise is a potentially serious problem. In addition, the combination of these independently modulated—but time-aligned—subcarriers results in an RF signal with a high peak-to-average power ratio (PAPR) that challenges amplifier linearity and power efficiency.
One solution is to simply design or use synthesizers with very low phase noise and power amplifiers with extremely high linearity. But those approaches are expensive, and wireless engineers and system architects are cleverer than that. They recognize not only the growing power and power efficiency of advanced DSP but also its ability to change the balance of . . . um . . .power between RF cost/performance and processing algorithms.
A good example is the use of pilots in OFDM transmissions and pilot-tracking routines in OFDM receivers. Other interesting examples include PAPR-reduction techniques, relevant to OFDM and a good topic for a future post.
In many OFDM systems, pilots are used in the form of dedicated subcarriers that are transmitted continuously during the data portion of frames or subframes. These pilots displace subcarriers that would normally carry payload data and instead transmit a prearranged sequence, typically with low-order modulation such as BPSK or QPSK. Pilots are thus conceptually similar to equalizer training sequences: they compensate for non-ideal RF performance, but at the cost of carrying capacity and additional processing power.
The plot below shows OFDM error versus subcarrier, with the pilot symbols in white.
This error vector spectrum trace plots error versus frequency or subcarrier. Four of the 52 subcarriers, shown in white, are continuous pilots that carry reference information for demodulation rather than payload data. Note the comparatively high error associated with one subcarrier, caused by spurious interference.
The essential thing to understand about pilots and OFDM demodulation is that the signal is demodulated relative to the pilots, which are presumed to generally share the impairments of the rest of the modulated signal. This part of the demodulation process is called pilot tracking and it allows the demodulation process to track signal errors and correct for them on a symbol-by-symbol basis.
Because the receiver knows the intended I/Q or amplitude/phase values of the pilot symbols, it can adjust all other symbols by the same amount required to fix the error it sees in the pilots. In this way, errors of signal amplitude, phase and symbol timing can be removed or “tracked out” as shown in the measurements below.
OFDM demodulation controls in the 89600 VSA software allow pilot tracking types to be individually selected as a way to isolate errors when troubleshooting. This signal has both amplitude droop and phase drift, shared by the pilots (white) and the data symbols (red). Correcting the pilot symbol locations corrects the data symbols as well.
Switching off the amplitude and phase elements of pilot tracking reveals significant errors in this 16QAM signal. Note that the BPSK pilots show the same error as the data symbols, and that both the amplitude and phase portions of the error appear greater at higher signal amplitudes. This suggests that straightforward correction of the data I/Q values, scaled to the pilot values, will correct the data symbols properly.
These selectively configured demodulation options in the 89600 VSA software are obviously useful in determining the source of errors. The pilot error quantities themselves can also be displayed in the form of common pilot error (CPE) measurement traces, in which the demodulation software isolates errors common to all pilots. CPE results quantify the performance of core circuits and algorithms in real-world operation.
For the wireless engineer, these pre- and post-tracking results are also useful in making system optimization choices. Signal impairments that can be readily corrected through pilot tracking—such as close-in phase noise—may not justify the cost or power required to reduce them below a particular magnitude. I discussed an example of pilot tracking and phase noise in a previous post Phase Noise and OFDM: Adding the Right Amount in the Right Place.
There are other interesting aspects of pilot tracking, including pilot schemes that are more complicated than the one shown here. I’ll try to cover them in future posts.
If you’re interested in more demodulation and troubleshooting techniques, you may want a copy of our latest application note Successful Analysis of Modulated Signals in Three Steps.
By the way, I should mention that the spurious interference in the first figure above is not much of a concern for demodulation, and that robustness is an important benefit of OFDM. However, if the interference affected one of the pilot subcarriers it would be much more significant!