Complex signal formats and rapidly-changing radio architectures are demanding more sophisticated tools and techniques for signal analysis. If you're working on up-to-date communications technology, chances are good that you're either making these measurements today, or will be needing them in the very near future.
CCDF - with the peak-to-average ratios of some new signal formats exceeding 10 dB, it's no longer practical to design in enough amplifier headroom to accommodate every possible peak. Complementary Cumulative Distribution Function (CCDF) analysis provides a statistical display of signal peaks versus probability, so you can choose your headroom while knowing precisely the percentage of signal peaks that will thus be clipped.
Integrated phase noise - in earlier single-carrier systems such as GSM, the impact of phase noise could be derived from a few key points on a traditional phase noise plot of dBc-vs-Hz. In broadband multi-carrier systems such as OFDM, system SNR is less related to the shape of this plot, and more closely tied to the total power under the curve, as integrated between offsets of symbol rate/10 and channel BW. Phase noise measurement techniques now need to provide this integrated power as one of their standard results.
Modulation flatness - with modulation bandwidths now commonly exceeding 20 MHz, verifying signal flatness (frequency response) is both an imperative and a challenge. Traditional swept-frequency network analyzers can test individual components, but the net result of all stages working together can only be verified on the modulated signal itself, directly at the transmitter (or receiver baseband) output. In effect, frequency response has now become one of the basic measures of modulation quality, to be verified at the same time and with the same tools as used for power, EVM, spectral mask, etc.
MIMO analysis - multi-transmitter / multi-receiver technology is already being deployed in devices ranging from unlicensed consumer electronics to next-generation cellular comms and broadband wireless access. While basic RF measurements can verify power levels and spectral masks, verification of modulation quality, crosstalk, and basic signal structure usually require analyzers with built-in MIMO demodulators as sophisticated as the hardware under test. In many cases, you'll need a signal analyzer equipped with multiple receiver inputs that are fully coherent in frequency, phase and time.
Analog measurements at digital test points - a recent architectural trend has been the migration of the ADC and DAC chips from the baseband/MAC board to the IF/RF board, meaning that the baseband signals are often accessible only as digital bitstreams. The data may be provided in generic parallel bus format, or via one of the emerging new comms-oriented serial standards, such as DigRF, CPRI or OBSAI. In any event, your next set of receiver measurements may require you to find a way to "port" the already- digitized signal directly into your test instrument's CPU, so as to bypass the now-unnecessary analog stages.
Combined PHY/MAC analysis - in many new systems, the lowest protocol layers have increased "authority" to directly manage significant aspects of the physical signal, such as bandwidth, modulation and coding types, power level, etc. For the RF designer, signal verification now includes such issues as whether the MAC layer has chosen the correct configuration for the given link conditions. Thus, even routine measurements can be very challenging to set up and interpret unless the signal analyzer also provides some basic insights into the MAC layer messaging that accompanies the signal.
Reprints
