Channel simulators increase system quality while decreasing costs
StorySeptember 23, 2009
Steve Williams
RT Logic
Channel simulators create realistic signals using precision, physics-based Radio Frequency (RF) propagation models. They enable true, real-world hardware-in-the-loop tests for R&D, QA, and production verification, reducing R&D and test schedule and cost for RF Communication Systems (COMMS) and components used with satellites, Unmanned Aerial Vehicles (UAVs), weapons, targets, test ranges, ground systems, and aircraft.
Radio Frequency (RF) Communication Systems (COMMS) for satellites, Unmanned Aerial Vehicles (UAVs), weapons, targets, test ranges, ground systems, and aircraft – especially those that protect lives or relate to expensive, unique, or secure assets – must perform flawlessly in increasingly crowded, interference-laden, low-power and high-noise RF signal environments.
As these COMMS systems and components are designed and tested, performance assurance is initially achieved through well-known circuit, software/firmware, and system simulation along with rigorous link budget analysis. As development proceeds, increased assurance comes from the use of Test and Measurement (T&M) instrumentation for stimulus and analysis.
Full and final verification is normally achieved through numerous system insertions, and test and debug cycles with all other interacting system hardware and software components are included – over a full span of nominal and worst-case signal scenarios. Often, these scenarios include those expensive or dangerous to create, time consuming to replicate, or theoretical and cannot – or should not – be produced in the real world. However, in marked contrast to these expensive and tedious tests conducted without them, tests that employ channel simulators sharply reduce risks associated with under-design and over-design – and save time, reduce cost, and increase test coverage and quality. Channel simulators enable this same level of rigorous testing and are essential to comprehensive COMMS system and component test, yet are much more economical.
Our discussion describes channel simulator use in testing wireless COMMS systems. We explore how channel simulators provide physics-obedient, phase-continuous carrier and signal Doppler shift, range delay, and range attenuation to test signals, resulting in signal environments that precisely duplicate what will be encountered in actual system deployment.
Wireless COMMS testing and channel simulators
For stimulus and analysis, robust wireless COMMS R&D and test activities utilize spectrum analyzers, signal generators, oscilloscopes, data generators, Bit Error Rate (BER) testers, and other instruments specific to the mission and to the hardware, firmware, and/or software under test.
Especially in wireless COMMS systems where receivers and transmitters will be in motion and at distance with respect to one another, test instrumentation must generate test signals that include time-varying Doppler shift, delay, and attenuation, along with noise and interference impairments.
When testing receivers, demodulators, and decoding algorithms, for example, it must be verified that these remain locked, maintaining acceptable BER performance, even as the received signal frequency shifts, and as the received data rate changes due to the time-varying relative motion of the transmitter and receiver. As well, Digital Signal Processing (DSP) algorithms must be proven able to extract the desired signal from noise and interference as its amplitude varies over time.
Channel simulators create the signals, nominal and worst-case, that faithfully model nature and enable such testing. They perform their functions in a fully phase-continuous manner with smooth interpolation and high output resolution. This ensures that no data errors are introduced by the instrument as a result of waveform discontinuities, inappropriate phase shifts, flat spots, or other anomalies.
Channel simulators perform inherently difficult tasks but buffer users from this complexity. Simple, yet flexible and powerful scenario creation and visualization are core attributes, as are the technical capabilities explained in the next sections.
Channel simulator – Doppler shift
RF signals between moving transmitters and receivers are affected by a number of RF propagation effects. The first of these is Doppler shift.
Equation 1 describes Doppler shift based on an actually transmitted frequency and the relative velocity between the transmitter and the receiver:
Fs = Fa * V/c (1)
where Fs = Doppler shift in Hz; Fa = actually transmitted frequency in Hz; V = relative velocity between transmitter and receiver in km/s; and c = speed of light (~299,792.458 km/s).
Equation 1 is plotted in Figure 1 for a typical LEO satellite in an 800 km circular orbit, with a 1.5 GHz RF COMMS link. Acquisition Of Signal (AOS) and Loss Of Signal (LOS) points are shown.
Figure 1: Example Doppler shift curve
(Click graphic to zoom)
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Observed with a spectrum analyzer, a channel simulator-generated signal for this scenario smoothly moves from right to left (higher to lower frequency) as time progresses, exactly like a signal from an on-orbit LEO. This is referred to as “carrier Doppler shift.”
As well, the channel simulator applies appropriate Doppler shift across its bandwidth. This means that the higher-frequency edge of a 30 MHz-wide QPSK signal, for example, will receive a higher Doppler shift than the components of the QPSK signal that are lower in frequency. This is a subtle yet important aspect of carrier Doppler shift.
In addition to carrier Doppler shift, “signal Doppler shift” is based on the data modulation rate. The data rate varies similarly to Figure 1, but with a data rate vertical scale.
Well-designed channel simulators implement fully phase-continuous carrier and signal Doppler shift. If either is missing, because a channel simulator is not utilized, or an incompletely implemented one is, the instrument might introduce bit errors.
Channel simulator – Range delay
All COMMS systems incur some form of propagation delay between transmitter and receiver, whether they are wired systems, optical systems, or wireless radio systems.
In each, the propagation velocity is related to the dielectric constant of the medium through which the signal passes. Propagation velocity is expressed as a percentage of the speed of light. In vacuums (dielectric constant = 1) and in air (dielectric constant = 1.00054), propagation velocity can be considered to be 100 percent of the speed of light for most practical purposes.
Therefore, in wireless COMMS systems, the propagation delay between a transmitter and receiver can be calculated by dividing the straight line distance between the transmitter and the receiver by the speed of light, as shown in Equation 2:
D = R / c (2)
where D = delay in seconds (s); R = range in km; and c = speed of light (299,792.458 km/s).
For the LEO satellite discussed earlier, the range delay profile of Figure 2 applies.
Figure 2: Example delay curve for a LEO satellite
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When observed with an oscilloscope triggered with a suitable time synchronization signal, a reference point in a channel simulator-generated output signal smoothly moves from right to left (larger to smaller delay), then from left to right as time progresses, exactly like a signal from an on-orbit LEO.
Channel simulators implement smooth and phase-continuous range delay that precisely matches what would be encountered on a real-world RF link such as with the described LEO satellite. Tests without channel simulators capable of such fidelity, high delay resolution, and range might unnaturally exercise the systems or components being tested.
Channel simulator – Range attenuation
COMMS system performance also depends on the power level of the received signal. Often, transmitters are low-power systems at great distances from receiver systems. Verification with dynamic signal power levels and validating signal reception under worst-case conditions are key COMMS system tests.
The power level of a received signal is affected by free-space path loss, which can be calculated from Equation 3:
L = 32.4 + 20 log F + 20 log R (3)
where L = free-space path loss in dB; F = frequency in MHz; and R = range in km.
For the LEO satellite discussed, the path loss profile in Figure 3 can be expected.
Figure 3: Example path loss curve for a LEO satellite
(Click graphic to zoom)
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When observed with a spectrum analyzer, the amplitude of a channel simulator-generated satellite signal moves smoothly, moves lower (higher to lower power), then back up as time progresses, exactly representing the scenario requested.
Channel simulators increase system assurance
With its dynamic, physics-compliant application of Doppler shift, delay, and attenuation – in combination and singly – the channel simulator is a vital tool for generating realistic test signals that exercise tested hardware, firmware, and software in realistic as well as worst-case fashion. As a result, channel simulator-enhanced testing significantly increases system quality while decreasing costs.
Steve Williams is a business area manager at RT Logic, responsible for R&D and business development activities for RT Logic’s channel simulator, range test, and signal geolocation systems. His 28-year digital and RF instrumentation career has also included R&D, management, and business positions at Hewlett-Packard, Agilent Technologies, and precisionWave Corporation, which he cofounded. Steve has a BSEE from the University of Illinois and is an Extra Class amateur radio licensee (KØSRW). He can be reached at swilliams@www.rtlogic.com.
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