comm.ChannelFilter

Filter signal using multipath gains at specified path delays

Since R2020b

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

Syntax

chanFilt = comm.ChannelFilter

chanFilt = comm.ChannelFilter(Name,Value)

Description

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

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.

SampleRate — Sample rate

1 Hz (default) | real positive scalar

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

Data Types: double

PathDelays — Discrete path delays

0 (default) | real scalar | real vector

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

Data Types: double

FilterDelaySource — Channel filter delay source

'Auto' (default) | 'Custom'

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

FilterDelay — Channel filter delay

7 (default) | real non-negative integer scalar

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

NormalizeChannelOutputs — Normalize outputs by number of receive antennas

1 or true (default) | 0 or false

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

Data Types: logical

Usage

Syntax

y = chanFilt(x,g)

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

x — Input signal

matrix

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

Data Types: double | single
Complex Number Support: Yes

g — Path gain

array

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

Data Types: double | single
Complex Number Support: Yes

Output Arguments

y — Channel output

matrix

Channel output, returned as a N_s-by-N_r matrix. If either x or g are input as data type single, the output data type is single.

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)

Specific to comm.ChannelFilter

info	Return characteristic information about channel filter

Common to All System Objects

step	Run System object algorithm
release	Release resources and allow changes to System object property values and input characteristics
reset	Reset internal states of System object

Examples

Explore Spatial Diversity of Channel in Distributed MIMO System

In a distributed MIMO system, explore spatial diversity by transmitting the same signal from two geographically separated transmitters and combining the received signals at one receiver. Use ray tracing to analyze the propagation paths and gains from each transmitter to receiver.

Perform Ray Tracing

Import buildings data for Chicago into siteviewer from an OpenStreetMap® (osm) file. For more information about the osm file, see [1]. Place two transmitter sites and one receiver site in the city.

sv = siteviewer("Buildings","Chicago.osm"); 
rx = rxsite("Name","Receiver", ...
    "Latitude",41.878543,"Longitude",-87.630599, ...
    "AntennaHeight",1.5);
show(rx)
tx1 = txsite("Name","Transmitter #1", ...
    "Latitude",41.878996,"Longitude",-87.629361); 
show(tx1)
tx2 = txsite("Name","Transmitter #2", ...
    "Latitude",41.880142,"Longitude",-87.630850);
show(tx2)

Perform ray tracing from the transmitter sites to the receiver site. Then, plot the propagation paths. By default, the raytrace function uses the shooting and bouncing rays (SBR) method and calculate paths with up to two reflections.

rays = raytrace([tx1 tx2],rx);
plot([rays{:}])

Ray tracing finds several ray paths to the receiver from each transmitter. From the map we can visually see the first transmitter is closer to the receiver than the second transmitter. We can also see the first transmitter has more reflected paths to the receiver. Display the propagation delays associated with each transmitter.

pd1 = [rays{1}.PropagationDelay]

pd1 = 1×10
10-6 ×

    0.3830    0.3839    0.5476    0.6482    0.5485    0.5486    0.6572    0.6945    0.7140    0.8874

pd2 = [rays{2}.PropagationDelay]

pd2 = 1×5
10-6 ×

    0.5967    0.5973    0.6059    0.6066    0.6255

Construct one channel filter for each transmitter site. Specify a sample rate of 30 MHz and use the minimum delay among the seven rays as the reference of time 0.

chanFilt1 = comm.ChannelFilter( ...
    "SampleRate",30e6, ...
    "PathDelays",pd1-min([pd1, pd2]))

chanFilt1 = 
  comm.ChannelFilter with properties:

                 SampleRate: 30000000
                 PathDelays: [0 8.7598e-10 1.6459e-07 2.6516e-07 1.6553e-07 1.6565e-07 2.7417e-07 3.1151e-07 3.3105e-07 5.0442e-07]
          FilterDelaySource: 'Auto'
    NormalizeChannelOutputs: true

chanFilt2 = comm.ChannelFilter( ...
    "SampleRate",30e6, ...
    "PathDelays",pd2-min([pd1, pd2]))

chanFilt2 = 
  comm.ChannelFilter with properties:

                 SampleRate: 30000000
                 PathDelays: [2.1372e-07 2.1434e-07 2.2294e-07 2.2357e-07 2.4247e-07]
          FilterDelaySource: 'Auto'
    NormalizeChannelOutputs: true

The individual channel filters for the two transmitters yield different filter delay values. Use the info object function of comm.ChannelFilter to show the filter delay of the two channel filters.

fd1 = chanFilt1.info.ChannelFilterDelay

fd1 = 7

fd2 = chanFilt2.info.ChannelFilterDelay

fd2 = 1

The two channel filters must have the same filter delay to combine the channel outputs at the receiver site. Customize the filter delay for each channel filter to use the larger value of the individually computed delay values.

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

Apply Receive Signal Combining

Set up system parameters, assigning only one isotropic antenna at each site.

Nt = 1;    % Number of transmit elements
Ns = 1000; % Samples per frame
M  = 64;   % Modulation order 

Retrieve path gains from the computed rays. Assume the sites are static and no Doppler shift is introduced.

pg1  = 10.^(-[rays{1}.PathLoss]/20) .* ...
    exp(1i*[rays{1}.PhaseShift]);
pg2  = 10.^(-[rays{2}.PathLoss]/20) .* ...
    exp(1i*[rays{2}.PhaseShift]);

Generate a frame of random 64-QAM signals. Perform channel filtering for each transmitter site and receive signal combining. The combined 2x1 distributed MIMO channel has a filter delay of max(fd1,fd2).

x  = qammod(randi([0, M-1],Ns,Nt),M);
y = chanFilt1(x,pg1) + chanFilt2(x,pg2);

Appendix

[1] The osm file is downloaded from https://www.openstreetmap.org, which provides access to crowd-sourced map data all over the world. The data is licensed under the Open Data Commons Open Database License (ODbL), https://opendatacommons.org/licenses/odbl/.

Perform Channel Filtering for an LTE 2x2 EVA Profile

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);

Reciprocal Downlink and Uplink Transmissions in MIMO Channel

Using one MIMO channel System object™ and two identically configured channel filter System objects, switch a link-level simulation between 3-by-2 downlink and reciprocal 2-by-3 uplink signal transmissions.

Define system parameters.

modOrder = 256;        % Modulation order
Nant1 = 3;             % Number of 'transmit' antennas
Nant2 = 2;             % Number of 'receive' antennas   
Rs = 1e6;              % Sample rate 
pd = [0 1.5 2.3]*1e-6; % Path delays
frmLen = 1e3;          % Frame length

Create a MIMO channel System object™, configuring it for path gain generation by disabling channel filtering.

chan = comm.MIMOChannel( ...
    'SampleRate',Rs, ...
    'PathDelays',pd, ...
    'AveragePathGains',[1.5 1.2 0.2], ...
    'MaximumDopplerShift',300, ...
    'SpatialCorrelationSpecification','none', ...
    'NumTransmitAntennas',Nant1, ...
    'NumReceiveAntennas',Nant2, ...
    'ChannelFiltering',false, ...
    'NumSamples',frmLen);

Create identical channel filter System objects for both transmission directions: one channel filter for the Nant1-by-Nant2 downlink channel (3 transmit antennas to 2 receive antennas) and a reciprocal channel filter for the Nant2-by-Nant1 uplink channel (2 transmit antennas to 3 receive antennas).

chanFiltDownlink = comm.ChannelFilter( ...
    'SampleRate',Rs, ...
    'PathDelays',pd);
chanFiltUplink = clone(chanFiltDownlink);

Downlink Transmission

Generate random path gains for one frame of the downlink 3-by-2 channel. Pass randomly generated 256-QAM signals through the 3-by-2 downlink channel.

pgDownlink = chan();
x = qammod(randi([0 modOrder-1],frmLen,Nant1),modOrder);
yDL = chanFiltDownlink(x,pgDownlink);

Uplink Transmission

Switch the link direction. Run the channel object to generate another frame of path gains, permuting its 3rd (Tx) and 4th (Rx) dimensions for the reciprocal uplink 2-by-3 channel. Pass randomly generated 256-QAM signals through the 2-by-3 reciprocal uplink channel.

pgUplink = permute(chan(),[1 2 4 3]);
x = qammod(randi([0 modOrder-1],frmLen,Nant2),modOrder);
yUL = chanFiltUplink(x,pgUplink);

Downlink and Uplink Array Dimensions

Show the sizes of the downlink and uplink path gain arrays returned by the MIMI channel object as an ${\mathit{N}}_{\mathrm{S}}$-by-${\mathit{N}}_{\mathrm{P}}$-by-${\mathit{N}}_{\mathrm{T}}$-by-${\mathit{N}}_{\mathrm{R}}$ array.

• ${\mathit{N}}_{\mathrm{S}}$ is the number of samples.

• ${\mathit{N}}_{\mathrm{P}}$ is the number of path delays.

• ${\mathit{N}}_{\mathrm{T}}$ is the number of transmit antennas. Nant1 for downlink and Nant2 for uplink.

• ${\mathit{N}}_{\mathrm{R}}$ is the number of receive antennas. Nant2 for downlink and Nant1 for uplink.

size(pgDownlink)

ans = 1×4

        1000           3           3           2

size(pgUplink)

ans = 1×4

        1000           3           2           3

Show the size of the channel output matrices returned by the MIMI channel object as an ${\mathit{N}}_{\mathrm{S}}$-by-${\mathit{N}}_{\mathrm{R}}$ matrix. ${\mathit{N}}_{\mathrm{S}}$ is the number of samples. ${\mathit{N}}_{\mathrm{R}}$ is the number of receive antennas.

size(yDL)

ans = 1×2

        1000           2

size(yUL)

ans = 1×2

        1000           3

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™.

Version History

Introduced in R2020b