Modeling Wireless Communications Systems in LabVIEW
PrintDid you know that you can use the National Instruments Modulation Toolkit for LabVIEW to quickly create graphical models of complex wireless communications systems? Because the NI Modulation Toolkit for LabVIEW provides various models to abstract the complex physical channel, you can easily observe the response of a communications system to various impairments.
Wireless digital communications systems use discrete changes in the phase, frequency, or amplitude of an RF carrier signal to transmit digital information. Each of the discrete changes are referred to as “symbols,” and each symbol represents a unique state of the carrier signal. One common modulation scheme, quadrature amplitude modulation (QAM), can use anywhere from four to 256 symbols. Unfortunately, the presence of noise in a communications channel can prevent the receiver from accurately detecting the state of the RF carrier. In addition, higher-order modulation schemes such as 256-QAM are susceptible to channel noise because changes to the modulated carrier are much more subtle.
To explore this and other similar relationships, you can use the Modulation Toolkit for LabVIEW to interactively model a wireless communications system. The model simulates one common impairment to the physical channel by applying additive white Gaussian noise (AWGN) to the modulated carrier signal. See Figure 1 for a sample communications system block diagram. One of the easiest ways to visualize the effect noise has on a communications system is with a LabVIEW constellation plot. Using this plot, you can observe the effect of AWGN on higher-order modulation schemes. As seen in Figure 2, lower-order modulation schemes (such as 4-QAM) are more immune to channel noise than higher-order modulation schemes (such as 256-QAM).
Figure 1. This LabVIEW block diagram shows AWGN applied to a digital communications system.
As Figure 2 shows, individual symbols of a communications channel become difficult to distinguish as the order of the modulation scheme increases. In this channel model example, you can see that 256-QAM fails when the signal-to-noise ratio (SNR) approaches 30 or less. As the SNR increases, the constellation plot begins to spin – suggesting that the receiver is unable to detect each individual symbol (also known as losing carrier lock).
Figure 2. You can see the effects of noise for various orders of modulation on constellation plots.
With the Modulation Toolkit for LabVIEW, you can model a variety of signal impairments including AWGN, phase noise, frequency offset, fading profiles, and more. Because LabVIEW easily connects with hardware, you also can translate software models to a physical system that uses real-world signals with little additional effort.
This article first appeared in the Q2 2007 issue of Instrumentation Newsletter.
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