As the previous discussion describes, channel impairments are a significant challenge that engineers must address in design validation. As a result, the validation process typically involves a combination of simulation and/or drive testing to ensure that the receiver will perform adequately in the final environment.

In the simulation phase, a channel emulator is used to apply some of the same impairments previously described. While this method presents the receiver with a model of what the final environment might look like, it cannot account for many environmental conditions. Thus, channel emulation is usually supplemented with a series of drive tests as well.

In the drive-testing phase, a prototype is tested in an environment that is known to be particularly troubling for receivers. This approach to receiver design enables engineers to ensure that a receiver will function correctly in the harshest physical environments. However, drive tests have two notable drawbacks. First, this method is costly and time consuming, particularly when testing involves multiple physical locations. Second, field testing can produce inconsistent results because it is difficult to re-create variables like traffic, weather and antenna orientation. As a result, it is difficult for RF engineers to make repeatable measurements between each revision of a receiver design.

Because of these challenges, one increasingly popular validation approach is to use PCI extensions for instrumentation (PXI) RF record and playback systems to record the real signal. With this approach, an RF vector signal analyzer is set up in the deployment environment where it acquires the spectrum of interest. The signal is saved to a redundant array of inexpensive disks (RAID) volume as a baseband waveform. Once saved, the recorded signal can be generated in a lab environment with a vector signal generator as a test stimulus.

RF record and playback systems enable designers to perform validation in a manner that is both accurate and repeatable. Because recorded signals contain impairments such as interference and multipath propagation, these waveforms provide a more realistic representation of the deployment environment.

PXI RF record and playback systems are only possible through the combination of several innovations in instrument technology. These innovations, including improvements in databus speed and disk speed, enable a new paradigm for waveform storage. One of the most notable recent innovations is the adoption of the PCI Express data bus, which enables data transfers at up to 1 Gbyte per second to each instrument.

Traditional RF instruments use embedded RAM as a mechanism for waveform storage, limiting the maximum waveform size to several hundred megabytes. However, the evolution of faster bus speeds enables PXI instruments to use high-speed RAID to store much larger waveforms. Using external RAID arrays, PXI instruments can generate or acquire waveforms that are up to several terabytes in length at the full rate of the instrument.

In NI LabVIEW, RF stream-to-disk applications achieve best results with a concurrent loop structure called producer-consumer loop. In this case, the producer loop acquires baseband data from a vector signal analyzer and passes it to a queue structure. The queue structure passes each record to a second loop, the consumer, which writes data to disk. The diagram in Figure 7 illustrates how this system can be configured in software.

The parallel programming approach shown in Figure 7 enables RF stream-to-disk applications at the full bandwidth of the instrument. As an example, the new NI PXIe-5672 can support continuous generation of up to 20 MHz of RF bandwidth. When combined with a 2 TB RAID volume, the instrument can continuously generate waveforms for five hours or more.

Wireless channels provide inherent challenges to communications systems. As a result, receiver validation and verification requires design and test engineers to model or otherwise account for various conditions. Historically, various characteristics of the wireless environment were modeled mathematically with impairments such as AWGN, adjacent-channel interference, and Rician fading. Today, RF stream-to-disk systems provide an alternative approach to creating the “perfect simulation.” Using recorded real-world signals, RF engineers can perform validtion of wireless receivers with greater accuracy and repeatability than ever before.

David A. Hall is the RF product manager at National Instruments. He holds a bachelors of science with honors in computer engineering from Penn State University.