Multipath Effect and Equalizer

Objective

In this experiment, we will investigate the impact of multipath effects on signals and how to use equalizers to eliminate these effects. The main principle used in the equalizer here is the Constant Modulus Algorithm (CMA) in adaptive algorithms.

System architecture file: Qpsk_stage4.grc

Steps and Instructions

1. Continuing from the previous unit, use Linear Equalizer after Polyphase Clock Sync to process the received signal. Here, Linear Equalizer uses a CMA type filter.
2. Observe the characteristics of the time domain, frequency domain and constellation diagram of the signal after being processed by Linear Equalizer.

System Simulation

GNU Radio QPSK Tutorial

GNU Radio QPSK Tutorial

Additional Notes

Causes of Multipath Effect

• In most communication environments, the signal travels from the transmitter to the receiver not only through one path. Any object that can reflect the signal, such as buildings, signs, trees and people, can create new paths.
• These reflected paths arrive at the receiver at different times depending on their path length and add up at the receiver, causing constructive or destructive interference, leading to signal distortion.

Equalizer: Adaptive Algorithm and Constant Modulus Algorithm (CMA)

• Equalizers are used to compensate for the distortion and distortion of the channel to the signal, restoring the original signal.
• CMA is a blind equalization algorithm suitable for signals with constant amplitude. The constellation points of QPSK are all located on the unit circle, so it is a good application object.
• The system uses both clock synchronization and equalizer modules, which converge individually, but each stage affects the next stage. The combined work of the two can effectively restore the signal quality and reduce interference caused by multipath effects.
• The signal spectrum after the equalizer becomes more flat.

Exercise 5

Simulate different multipath environments, observe the convergence process of the CMA equalizer under different conditions, and analyze its impact on signal quality.