Learn more about Communications Blockset

Channels

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Section Overview AWGN Channel Fading Channels Binary Symmetric Channel Selected Bibliography for Channels

Section Overview

Communication channels introduce noise, fading, interference, and other distortions into the signals that they transmit. Simulating a communication system involves modeling a channel based on mathematical descriptions of the channel. Different transmission media have different properties and are modeled differently. In a simulation, the channel model usually fits directly between the transmitter and receiver, as shown below.

This blockset provides several channel models for binary, real, and complex signals. Open the Channels library by double-clicking its icon in the main Communications Blockset library.

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AWGN Channel

An AWGN channel adds white Gaussian noise to the signal that passes through it. Gaussian noise is discussed on the reference page for the Gaussian Noise Generator block. The AWGN Channel block can process either sample-based or frame-based data, and it lets you specify the variance of the noise in one of four ways:

• Directly as a mask parameter

• Directly as an input signal

• Indirectly via a signal-to-noise ratio parameter

• Indirectly via an E_s/N_o parameter

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Fading Channels

The Channels library includes Rayleigh and Rician fading blocks that can simulate real-world phenomena in mobile communications. These phenomena include multipath scattering effects, as well as Doppler shifts that arise from relative motion between the transmitter and receiver. This section discusses

• How to use other blocks in the model to compensate for fading

• How to choose and configure a fading channel block

• How to plot Rayleigh channel characteristics using the channel visualization tool

For more information about fading channels in general, see Overview of Fading Channels in the Communications Toolbox documentation.

Note   To model a channel that involves both fading and additive white Gaussian noise, use a fading channel block connected in series with the AWGN Channel block, where the fading channel block comes first.

Compensating for Fading

A communication system involving a fading channel usually requires component(s) that compensate for the fading. Here are some typical approaches:

• Differential modulation or a one-tap equalizer can help compensate for a frequency-flat fading channel.

• An equalizer with multiple taps can help compensate for a frequency-selective fading channel.

See Equalizers to learn how to implement equalizers in this blockset. See the reference page for the M-DPSK Modulator Baseband block to learn how to implement differential modulation.

For an example that can help you visualize why compensating for a fading channel is necessary, see the plots in the commeqeml demo.

Choosing and Configuring a Fading Channel Block

The table below indicates the situations in which each fading channel block is appropriate.

Signal Path	Channel Block
Direct line-of-sight path from transmitter to receiver	Multipath Rician Fading Channel
One or more major reflected paths from transmitter to receiver	Multipath Rayleigh Fading Channel

In the case of multiple major reflected paths, a single instance of the Multipath Rayleigh Fading Channel block can model all of them simultaneously. The number of paths that the block uses is the length of either the Delay vector or the Gain vector parameter, whichever length is larger. (If both of these parameters are vectors, they must have the same length; if exactly one of these parameters is a scalar, the block expands it into a vector whose size matches that of the other vector parameter.)

Choosing appropriate block parameters for your situation is important. For more details about the parameters of fading channel blocks, see

• The reference pages for the Multipath Rayleigh Fading Channel block and the Multipath Rician Fading Channel block

• The Choosing Realistic Channel Property Values section under "Configuring Channel Objects" in the Communications Toolbox documentation

• The works listed in Selected Bibliography for Channels

Visualizing a Multipath Rayleigh Fading Channel

A multipath Rayleigh fading channel's characteristics can be plotted using the channel visualization tool. There are two ways to do this for a model that contains a Multipath Rayleigh Fading Channel block.

One method is to double-click the block during a simulation. The second method is to select the Open channel visualization at start of simulation check box in the block dialog box. In subsequent simulations, the channel visualization tool appears. The tool is also used if it was left open from a previous simulation.

For Communications Blockset, this channel visualization feature is available for the Multipath Rayleigh Fading Channel block and the Multipath Rician Fading Channel block.

For details, see Using the Channel Visualization Tool in the Communications Toolbox User's Guide.

Examples Using Fading Channels

The following demonstration models provide examples of the use of fading channels:

• Rayleigh Fading Channel, which illustrates the channel's effect on a QPSK modulated signal

• Adaptive Equalization Using Embedded MATLAB™

• IEEE 802.11a WLAN Physical Layer

• cdma2000 Physical Layer

• Defense Communications: US MIL-STD-188-110B

• WCDMA End-to-End Physical Layer

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Binary Symmetric Channel

Binary error channels process binary signals by adding noise modulo 2. This library contains the Binary Symmetric Channel block, which either preserves or perturbs each vector element independently. It requires a probability that applies independently to each noise element. An example using the Binary Symmetric Channel block is in the section Example: A Rate 2/3 Feedforward Encoder.

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Selected Bibliography for Channels

[1] Fechtel, Stefan A., "A Novel Approach to Modeling and Efficient Simulation of Frequency-Selective Fading Radio Channels," IEEE Journal on Selected Areas in Communications, Vol. 11, April 1993, pp. 422–431.

[2] Jakes, William C., ed., Microwave Mobile Communications, New York, IEEE Press, 1974.

[3] Lee, William C. Y., Mobile Communications Design Fundamentals, 2nd ed., New York, John Wiley & Sons, 1993.

[4] Proakis, John G., Digital Communications, 3rd ed., New York, McGraw-Hill, 1995.

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Communications Filters

MIMO

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