Fading Testing: Understanding and Measuring Signal Degradation

This article explores the world of fading testing and measurement, covering the equipment used to simulate and analyze fading conditions. We’ll delve into what fading is, the different types, and the tools used by leading test solution providers to tackle this challenge.

What is Fading?

Introduction:

Fading refers to the fluctuations in the strength of a received signal over time, caused by various impairments along the transmission path. Several factors contribute to this phenomenon, including:

• The number of paths the signal takes between the transmitter and receiver.
• Path loss (signal weakening over distance).
• The operating frequency of the signal.
• The relative time and positions of the transmitter and receiver.

There are different types of fading, broadly categorized as large-scale fading and small-scale fading. Small-scale fading is further divided into:

• Slow fading: Gradual changes in signal strength.
• Fast fading: Rapid fluctuations in signal strength.
• Flat fading: Affects all frequencies in the signal bandwidth equally.
• Frequency selective fading: Affects different frequencies in the signal bandwidth differently.

For a deeper dive into the basics and types of fading, refer to resources on “Fading basics and types” in wireless communication.

Fading Test Setup

The core setup for fading measurement, regardless of the specific type of fading, remains largely the same, although the settings will vary.

• Signal Generator: This device creates an ideal modulated RF signal, tailored to the device under test (DUT). For example, if the DUT is WiMAX compliant, the signal generator will produce a WiMAX signal with the correct frequency, power level, and baseband physical layer parameters.
• Channel Simulator: This is where the magic happens. The channel simulator allows you to configure the fading parameters, following standard-specific profiles. This includes settings like:
  • The number of paths the signal takes
  • The attenuation (signal weakening) in each path
  • The Doppler frequency (frequency shift due to movement)
• SISO/MIMO Configuration: Depending on the test requirements, you’ll configure the setup to use either a Single-Input Single-Output (SISO) or a Multiple-Input Multiple-Output (MIMO) channel model.
• Pass Criteria: The ultimate goal is to ensure the receiver can process the faded RF signals with an acceptable Bit Error Rate (BER) or Packet Error Rate (PER). See resources on “BER vs PER” for more information.

Fading Test and Measurement Equipment

Here are some examples of fading test equipment from prominent vendors:

Other notable companies like Keysight Technologies, Octoscope, NI, and R&S also develop channel simulators or emulators used for fading testing.

Besides channel emulators, fading tests also require standard-compliant signal generators, tailored to the DUT’s specific test requirements. Companies like NI, Keysight, R&S, Anritsu, and Litepoint are key players in developing Vector Signal Generators (VSG) and Vector Signal Analyzers (VSA) used for generating and analyzing signals.