Digital takes over multipath simulation

The coming generation of wireless communications will employ digital signal processing at baseband frequencies in its transmitters and receivers. This shift to digital baseband processing has prompted manufacturers of test equipment to create all-digital simulators for one of the most complex wireless tests: multipath fading.

Traditional analog methods are giving way to simpler, digital multipath testing.

Traditionally, simulation for multipath testing required an analog signal source working at RF or baseband frequencies. This approach has two weak points: Conversion loss and noise calibration. Conversion loss is the error added to the test signal during A/D or D/A conversion. Conversions are necessary with an analog signal source because the fading profiles and noise signals are digitally generated. Because the errors come from the test equipment, not the channel, they make the results harder to interpret.

The addition of noise changes the signal's total power level as well as the carrier-to-noise ratio, so the noise sources in fading tests must be calibrated to the incoming signal power level. Calibration requires that the tester perform a statistical determination of the carrier power after fading, a complex and time-consuming process.

A new generation of multipath fading simulators, such as the Wave-3G SIM from Micronetics Wireless (Hudson, NH; http://www.micronetics.com) and the N5115A Baseband Studio from Agilent Technologies (Palo Alto, CA; http://www.agilent.com), work entirely within the digital domain, reducing conversion loss and simplifying noise calibration. These simulators create the fading profiles and noise signals digitally, then drive a vector signal generator to create the needed analog signal. Using a digital source reduces conversion loss because it eliminates one conversion step. It simplifies noise calibration because the digital fading profile already contains the faded carrier power statistics so there is no need for a separate determination. The Micronetics and Agilent instruments also aid in test setup—each has software that configures the signal generator for standards-based testing for W-CDMA, GSM, and other popular wireless communications schemes.

In addition, these new instruments provide test signals directly to the digital baseband signal processor, enabling the developer to perform tests early in the design cycle, when design modifications are easiest to make. The processor and its algorithms can be tested long before the RF sections are available to prototype. Testing can even be performed on device simulation models before any hardware is available.

The match of digital signal generation to the digital baseband processing will allow the new generation of testers to maintain currency with new communications standards. As requirements change, developers can simply reprogram the signal generators with the new test profiles.