Communications Blockset

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 in the Rayleigh case, as well as Doppler shifts that arise from relative motion between the transmitter and receiver. This section discusses

• How to categorize the possible paths along which a signal can travel from the transmitter to the receiver in the situation that you want to model
• How to choose and configure a fading channel block based on the categorization
• An example that uses fading channels

Categorizing Signal Paths

The following figure depicts the two types of paths between a moving transmitter and a stationary receiver. The solid line is a direct-line-of-sight path, which might or might not exist in your situation. Each dotted line is a reflected path that the signal travels when it is reflected from one of the shaded shapes. The shaded shapes represent obstacles such as buildings or trees.

The situation in the figure is just an example. In general, you should analyze your system by considering these questions:

• Are there any reflected paths along which a signal can travel from transmitter to receiver? If so, how many?
• Is there a direct path from transmitter to receiver?
• What is the relative motion between the transmitter and receiver?

The first two questions will help you choose which fading channel blocks to use in your simulation, while the third question will help you choose appropriate parameters for the blocks.

Choosing and Configuring a Fading Channel Block

Once you categorize the types of signal paths in the situation you want to model, use the table below to determine the appropriate block (or blocks) for your simulation.

Table 1-1: Choosing a Fading Channel Block Based on Signal Paths

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

If a signal can use more than one reflected path, then 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, then they must have the same length; if exactly one of these parameters is a scalar, then the block expands it into a vector whose size matches that of the other vector parameter.)

The relative motion between the transmitter and receiver influences the values of the blocks' parameters. For more details, see their reference pages, as well as the works listed in Selected Bibliography for Channels if necessary.

Example: Using Fading Channels

The reference page for the Multipath Rayleigh Fading Channel block includes an example that illustrates the channel's effect on a constant signal.

Another example is the following model, which uses both the Multipath Rayleigh Fading Channel and the Rician Fading Channel blocks in parallel. This combination of blocks simulates a mobile communication link in which the transmitted signal can travel to the receiver along a direct path as well as along three indirect paths. (The number of indirect paths is three because the Multipath Rayleigh Fading Channel block's Gain vector parameter is a vector of length three. Although the Delay vector parameter is a scalar, its value is applied to each of the three paths.)

To open the completed model, click here in the MATLAB Help browser. To build the model, gather and configure these blocks:

• DSP Constant, in the DSP Blockset DSP Sources library
• Set Constant value to j*ones(10000,1).
• Set Output to Frame-based.
• Set Frame period to .01.
• Rician Fading Channel, with default parameter values
• Multipath Rayleigh Fading Channel
• Set Delay vector to [0 2e-6 3e-6].
• Set Gain vector to [0 -3 1].
• Sum, in the Simulink Math library
• To Workspace, in the Simulink directory
• Set Save format to Array.

Connect the blocks as shown above. Also, from the model window's Simulation menu, choose Simulation parameters; then, in the Simulation Parameters dialog box, set Stop time to 0.6.

Note    To reduce execution time by logging less data to the workspace, set the Decimation parameter in the To Workspace block to 100. Then the variable simout will contain fewer entries, but its graph will look similar.

Run the model. After the simulation stops, plot the faded signal's power (versus sample number) by executing this command at the MATLAB prompt.

• simout = simout.'; plot(20*log10(abs(simout(:))))
  

The resulting plot is shown in the figure below.

AWGN Channel

Binary Symmetric Channel