Enhancing Communications Testing with a Channel Simulator

By Mark Lombardi, RT Logic

RF communication systems are subject to many variables due to the media in which they transmit. When the systems are moving, the number of variables increases still more. Communication systems that may work fine when the transmitter and receiver are stationary can encounter problems when exposed to the RF signal impairments that result from motion.

Engineers implementing RF communication systems typically account for variables such as carrier frequency movement due to Doppler shift and time delays due to the distance between the transmitter and receiver.   However, testing has always been difficult since suitable platforms (such as ships, planes, or other vehicles) are difficult (and expensive) to obtain, and measurements are awkward to take and reproduce. Channel simulators, a new category of test instruments, can model the electrical disturbances that would appear in a link when the transmitter and receiver are moving. And they can do this while the system is sitting stationary on a test bench.   

Whatever the platform, the physical dynamics will affect the communication system. Doppler shift, delay, path loss, and noise can degrade or completely disrupt a link. Therefore all phases of communication system development from early simulation through final testing can benefit from including nominal and worst RF channel effects during design, development, and test. A Channel Simulator can be placed in the IF or RF path, utilizing an antenna or cable connection. Testing then can proceed much as it normally would, with a link being established between the transmitter and receiver.   Typically at this point static margin testing might be done utilizing various modulation types and data rates. The carrier frequency may also be shifted to simulate Doppler effects. However, this approach cannot account for the fact that the Doppler shift in moving systems affects the entire modulated signal. Not only does the carrier frequency shift, so too does every frequency component of the modulated signal, as well as the data rate itself. Before Channel Simulators became available, the only way to test such effects was to actually put the communication system components onto moving platforms. 

For example, RT Logic’s T400CS Channel Simulator can apply phase-continuous distortions in a physics-realistic manner. This provides on-the-bench emulation of the exact propagation effects a system in motion experiences. Such effects include smooth, phase-continuous carrier and signal Doppler shift, range delay, range attenuation, fading, and noise.

When an RF communication system is subjected to motion, the physics affecting the signals are real and unavoidable. Understanding these effects and knowing how to design and test a system that will ultimately be put into motion is essential. Tools like the Channel Simulator provide designers with a unique, cost effective approach.  They can supplement or guide costly tests with actual moving platforms and allow testing to proceed even before the actual platform is built or when no platform is readily available.

Mark Lombardi is T400CS Project Manager at RT Logic. You can reach him at mlombardi@rtlogic.com.