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comm.ChannelFilter

Filter signal using multipath gains at specified path delays

Description

Use the comm.ChannelFilter System object™ to filter a signal using multipath gains at specified path delays.

To filter a signal using multipath gains:

  1. Create the comm.ChannelFilter object and set its properties.

  2. Call the object with arguments, as if it were a function.

To learn more about how System objects work, see What Are System Objects?.

Creation

Description

example

chanFilt = comm.ChannelFilter creates a multipath channel filter System object to filter an input signal with path gains at the specified path delays

chanFilt = comm.ChannelFilter(Name,Value) sets properties using one or more name-value pairs. For example, 'SampleRate',1e6 sets the sampling rate to 1 MHz. Enclose each property name in quotes.

Properties

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Unless otherwise indicated, properties are nontunable, which means you cannot change their values after calling the object. Objects lock when you call them, and the release function unlocks them.

If a property is tunable, you can change its value at any time.

For more information on changing property values, see System Design in MATLAB Using System Objects.

Sample rate of the input signal, specified as a real, positive scalar.

Data Types: double

Delays of the discrete paths in seconds, specified as a real scalar or vector.

Data Types: double

Channel filter delay source, specified as either 'Auto' or 'Custom'.

  • Set FilterDelaySource to 'Auto' to specify the channel filter delay as the minimum possible value.

  • Set FilterDelaySource to 'Custom' to specify the channel filter delay as a custom value. The custom value cannot be smaller than the minimum possible value.

Data Types: char

Channel filter delay in samples, specified as a real, non-negative, integer scalar.

Dependencies

To enable this property, set the FilterDelaySource property to 'Custom'. The specified value must be no smaller than the automatically determined channel filter delay when you set FilterDelaySource to 'Auto'.

Data Types: double

Normalize outputs by number of receive antennas, specified as either 'true' (1) or 'false' (0).

Data Types: logical

Usage

Description

y = chanFilt(x,g) filters input signal x, through a multipath channel with path gains g, at the path delay locations specified by the PathDelays property.

Input Arguments

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Input signal, specified as a matrix. The argument x must be a Ns-by-Nt matrix, where Ns is the number of samples and Nt is the number of transmit antennas.

Data Types: double
Complex Number Support: Yes

Path gain, specified as an array. The input G must be a Ns-by-Np-by-Nt-by-Nr or 1-by-Np-by-Nt-by-Nr array, where Nr is the number of receive antennas and Np is the number of paths, i.e., the length of the PathDelays property.

Data Types: double
Complex Number Support: Yes

Output Arguments

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Channel output, returned as a Ns-by-Nr matrix.

Data Types: double
Complex Number Support: Yes

Object Functions

To use an object function, specify the System object as the first input argument. For example, to release system resources of a System object named obj, use this syntax:

release(obj)

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infoReturn characteristic information about channel filter
stepRun System object algorithm
releaseRelease resources and allow changes to System object property values and input characteristics
resetReset internal states of System object

Examples

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In a distributed MIMO system, send the same signal from two geographically separate transmitters (Tx) and combine the received signals at one single receiver (Rx) to explore spatial diversity. The two Txs are not co-located and they experience different multipath (path delays) to the Rx. Specify the path delays respectively.

delay1 = [0 1.5 2.3 5.2 6.6];
delay2 = [0 3.7 6.2];

Configure one channel filter object per Tx.

chanFilt1 = comm.ChannelFilter('PathDelays', delay1);
chanFilt2 = comm.ChannelFilter('PathDelays', delay2);

The two channel filters get different filter delays.

fd1 = chanFilt1.info.ChannelFilterDelay
fd1 = 6
fd2 = chanFilt2.info.ChannelFilterDelay
fd2 = 4

In order to combine the two channel outputs at Rx, the two channel filters must have the same filter delay. Customize the filter delay for each object.

set(chanFilt1, ...
    "FilterDelaySource", 'Custom', ...
    "FilterDelay", max(fd1, fd2));
set(chanFilt2, ...
    "FilterDelaySource", 'Custom', ...
    "FilterDelay", max(fd1, fd2));

Set up system parameters. It is assumed one antenna at each Tx and Rx.

[Nt, Nr] = deal(1);
Ns  = 30720;
Np1 = length(chanFilt1.PathDelays);
Np2 = length(chanFilt2.PathDelays);
M   = 64;

Generate random 64-QAM signal and complex path gains for the two channels.

x  = qammod(randi([0, M-1], Ns, Nt), M);
g1 = complex(rand(Ns, Np1, Nt, Nr), rand(Ns, Np1, Nt, Nr));
g2 = complex(rand(Ns, Np2, Nt, Nr), rand(Ns, Np2, Nt, Nr));

Perform channel filtering and Rx combining. The combined 2x1 MIMO channel has a filter delay of max(fd1, fd2).

y = chanFilt1(x, g1) + chanFilt2(x, g2);

Construct a channel filter object with the LTE Extended Vehicular A model (EVA) delay profile.

chanFilt = comm.ChannelFilter( ...
    'SampleRate', 30.72e6, ...
    'PathDelays', [0 30 150 310 370 710 1090 1730 2510]*1e-9);

Set up system parameters. There are two transmit and receive antennas.

[Nt, Nr] = deal(2);
Ns = 30720;
Np = length(chanFilt.PathDelays);
M  = 256;

Generate random 256-QAM signal and complex path gains.

x = qammod(randi([0, M-1], Ns, Nt), M);
g = complex(rand(Ns, Np, Nt, Nr), rand(Ns, Np, Nt, Nr));

Filter the signal with path gains for the EVA delay profile.

y = chanFilt(x, g);

Algorithms

The channel filter implements a fractional delay (FD) finite impulse response (FIR) bandpass filter with a length of 16 coefficients for each candidate fractional delay at 0, 0.02, 0.04, …, 0.98.

Each discrete path is rounded to its nearest candidate fractional delay, so the delay error limit is 1% of the sample time. To achieve a group delay bandwidth exceeding 80% and a magnitude bandwidth exceeding 90%, the algorithm selects the optimal FIR coefficient values for each fractional delay, while satisfying the following criteria:

  • Group delay ripple ≤ 10%

  • Magnitude ripple ≤ 2 dB

  • Magnitude bandedge attenuation = 3 dB

The plots show bandwidths that satisfy the design criteria for group delay ripple, magnitude ripple, and magnitude bandedge attenuation.

For additional information, see the article A Matlab-based Object-Oriented Approach to Multipath Fading Channel Simulation at MATLAB® Central.

Extended Capabilities

C/C++ Code Generation
Generate C and C++ code using MATLAB® Coder™.

Introduced in R2020b