Testing Wireless Receivers with Recorded RF Spectrum
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In 1899, Italian inventor Guglielmo Marconi conducted one of the first communications experiments by transmitting an electromagnetic wave 31 miles across the English Channel from France to England.
At the time, his experiment was groundbreaking research that proved the possibility of wireless communications. Ironically, this original experiment would not be successful today. Over the last century, Marconi’s success has contributed to a world where wireless systems are so widespread that interference from other devices would make repetition of his original experiment impossible. In today’s world, using wireless channels introduces many challenges such as fluctuations in signal strength, frequency offsets caused by mobility (called the Doppler effect), and multipath propagation in urban environments. This article explains the theory behind multipath propagation, its effect on wireless receiver design, and test strategies for receiver validation and verification.
Figure 1. Transmitted waves can reflect off of physical structures, thereby affecting the total distance traveled.
Understanding Multipath Interference
To give you an idea of how multipath propagation affects a receiver, imagine yourself listening to a voice booming from the front of a large room. In this scenario, the speaker’s voice travels along multiple paths as it bounces off of walls, chairs, and other objects. Because some paths take longer to travel than others, your ear (the receiver) hears sound that has been slightly distorted.
In a wireless communications channel, the echo characteristic described above is also true of electromagnetic waves. Moreover, because a transmitter and receiver might be miles or even thousands of miles apart, multipath fading can be quite substantial. See Figure 1 for a diagram that illustrates multipath propagation.
Because each path taken by the transmitted signal arrives at a slightly different time, these variations produce intersymbol interference (ISI) in modulated communications signals. In addition, multipath fading can cause periodic fluctuations in signal strength. Because of wireless challenges such as multipath fading, wireless receivers must be tested through a combination of channel emulation and drive testing in the final environment.
Figure 2. This image illustrates a high-level block diagram of a wireless receiver.
Designing for a Wireless Environment
As you might expect, wireless channel impairments have a significant effect on the design of wireless receivers. As an overview, wireless receivers use components such as low-noise amplifiers (LNAs), multiple types of filters, and sophisticated baseband processing algorithms to ensure that any transmitted message signal can be recovered (see Figure 2).
As Figure 2 shows, you can use the LNA to compensate for fluctuations in signal strength. In most receivers, the LNA is part of an automatic gain control (AGC) circuit, which measures the signal strength and applies the appropriate gain. AGC is crucial in multipath environments because fading can result in significant signal strength variation.
Validating Receivers with Real-World Data
Because of the wireless challenges described above, engineers typically use a combination of software models and drive testing to validate a wireless receiver’s performance. As an example, you can use the NI Modulation Toolkit for LabVIEW to emulate Rician or Rayleigh fading profiles. While mathematical models can emulate a channel to a degree, they cannot account for all environmental conditions. Moreover, drive testing is expensive, and it produces results that are often not repeatable. As a result, there is a growing trend to use real-world recorded signals for wireless receiver validation.
With the NI PXI-5661 vector signal analyzer, you can record up to 20 MHz of RF bandwidth (as IQ data) for more than five hours. Once recorded, you can generate waveforms in a laboratory environment with the new NI PXIe-5672 vector signal generator. By using recorded RF bandwidth, natural impairments are introduced. This produces the following benefits:
- Extended test waveforms for long-duration error detection
- Repeatable real-world data sets through multiple design cycles of the receiver
- Ability to use environmental conditions that are most similar to the deployment environment
Using RF record and playback systems, you can prototype next-generation receivers with greater efficiency, accuracy, and repeatability than previously possible.
Figure 3. This PXI system shows an external RAID hard drive configuration. Using external RAID arrays, PXI instruments can generate or acquire waveforms up to several terabytes in length at the full rate of the instrument.
RF Stream-to-Disk System Technology
RF record and playback systems are only possible through several innovations in instrument technology. Because traditional RF instruments use embedded RAM as a mechanism for waveform storage, maximum waveform size is typically limited to several hundred megabytes. However, with the evolution of faster bus speeds, PXI instruments can use high-speed redundant array of inexpensive disks (RAID) technology to store much larger waveforms. Using external RAID arrays, PXI instruments can generate or acquire waveforms up to several terabytes in length at the full rate of the instrument. For reference, a PXI system with an external RAID array (NI HDD-8264) is shown in Figure 3.
As an example, the NI PXIe-5672 can support continuous generation of up to 20 MHz of RF bandwidth. This requires an IQ sample rate of 25 MS/s, and, at 4 B per sample, it translates to a total data throughput of 100 MB/s. Using a 2 TB RAID volume, you can sustain this data rate for more than five hours.
Greater Accuracy at a Lower Cost
The ability to stream RF signals to and from hard disk for extended periods of time provides a revolutionary approach to instrument waveform storage. More importantly, with this technology, you can perform receiver validation and verification with greater accuracy, better repeatability, and at a lower cost than before.
David Hall is a product manager for RF and communications. He holds a bachelor’s degree in computer engineering from Pennsylvania State University.
This article first appeared in the Q4 2007 issue of Instrumentation Newsletter.
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