Yihenew

gr-SatelliteModem

2018-08-12 12:54:50 -0400T00:00:00-00:00

gr-SatelliteModem - GNU Radio-based satelltie modem

gr-SatelliteModem is a GNU Radio module that offers single-carrier satellite communications capablility. The module contains GNU Radio blocks which can be used to transmit and/or receive data over the air (eg. over satellite frequencies). Currently, the module supports differential BPSK and QPSK modulation, rate-1/2, 3/4 and 7/8 convolutional coding rates. A range of data rates can be achieved through use of different combination of modulations, coding rates and sampling rates.

Installation

Before installing SatelliteModem, make sure that you have already installed the latest version Gnuradio. The module has been tested against Gnuradio v3.7.9. Any radio front-end can be used as long as it is supported by Gnuradio and has both TX and RX antenna ports. UHD-based example can be found in examples/ folder. The module was created using Gnuradio templating engine called gr_modtool which automates creation of out-of-tree modules. The installation process follows standard CMake project installation.

$ cd gr\_SatelliteModem
$ mkdir build
$ cd build 
$ cmake ../ 
$ make
$ make test
$ sudo make install

Transceiver Design

Modulation

Currently, supported modulation formats are Differencial BPSK (DBPSK) and Differential QPSK (DQPSK). The modulator converts data bytes (each byte contains 8 data-bits) into symbols. The number of bits per symbol is either 1 (for DBPSK) or 2 (for DQPSK). Then each of the symbols is mapped to one of the constellation points (complex symbols). Finally, the complex symbols are fed to a pulse-shaping filter. The filter is Root-Raised Cosine (RRC) filter, and exact replica of this filter is used at the receiver such that all the signal energy is coherently combined without causing Inter-Symbol interference (ISI). Figure below shows both the modulator and demodulator connected over a fading channel simulator. In practice, communication over a radio channel suffers from signal distortion caused by the propagation medium and hardware imperfections such as clock drift. For successful reception, the receiver has to compensate for such distortions. Oscillators used by the transmitter and the receiver are far from being ideal. A mismatch between these oscillators introduces carrier frequency offset and clock skew. Moreover, the receiver may receive multiple reflections of the transmitted signal each arriving at the receiver with slightly different delays and phases. Hence, the receiver has to adaptively compensate (equalize) for channel fading caused by such multi-path propagation. Once this is done, a matched-filter (RRC filter) is used to down-sample the signal and recover transmitted symbols. DPSK demodulator de-maps the complex symbols back to bits. Output of the demodulator is a stream of floating-point numbers, or soft-bits, (instead of hard-decision 1’s and 0’s). The magnitude of a soft-bit corresponds to the reliability of the demodulated bit.

Synchronization

Without a time-synchronization mechanism, the receiver is not able to identify the starting point of a data packet. Moreover, Viterbi decoder may not be able to decode the data. For example, assume that the coding rate is 3/4. This implies that, the channel encoder produces 4 bits for every 3 data bits where the 4 output bits are serial contatenated. The decoder, on the other hand, gropus received soft bits into a block of 4 consecutive soft bits. However, there are four possible starting points when grouping the soft bits. To solve these problems, a pseudo-random sequence of bits (preamble) which is known by both the transmitter and the receiver is appended to each transmitted packet. The receiver correlates received soft bits against the preamble sequence and marks the frame boundaries by looking at places where correlation peak is detected. SatelliteModem module comes with the following two blocks as shown in the figure below.

SyncPreamble This block appends preamble bytes at the beginning of each encoded packet.
PreambleDetector This block correlates soft bits against the preamble sequence. After detection of frame boundaries, PreambleDetector block removes the preamble bits and passes soft bits which belong to an encoded packet.

Forward Error-Correcting Code (FEC)

Error-correcting codes such as Viterbi (convolutional) encoding add extra redundancy to transmited data such that the receiver is able to correct errornous bits. The coding rate determines the maximum number of errornous bits that can be corrected. SatelliteModem’s encoder supports two channel encoders: Viterbi and Low-Density Parity Check (LDPC) codes. Viterbi encoder can be configured to one of three coding rates: 1/2, 3/4 and 7/8. Currently, LDPC supports only one coding rate which is 0.42.

Packet Encoder

Packet encoder serves two main functions. First, it packs stream of data bytes into blocks of fixed size. Secondly, each block, frame/packet is prepended with a 6-byte header as shown in the figure below.

A 16-bit CRC is added in order to detect errornous packets. SatelliteModem has two alternative packet encoder/decoder blocks. One of them is Packetizer/Depacketizer. These blocks are written in Python. The Packetizer block reads bytes from a UDP port and encodes them into packets. The size of UDP packets should not exceed MTU. In case, the size of a UDP packet is less than MTU, padding bytes are appended at the end. However, the CRC is computed only for the payload part excluding padding bytes. The Depacketizer block, on the other hand, validates the CRC header, and writes the payload into another UDP port.

Alternatively, C++ based implementations, Framer/Deframer, can be used. These blocks do not access the UDP port directly. Instead, messaging blocks are used to connect Framer/Deframer with UDP Source/UDP Sink blocks. This isolation might help in reducing latency.

Note that both Packetizer and Framer have two modes of operation: blocking and non-blocking. In blocking mode, the transmitter is suspended until data is available at the UDP port. In non-blocking mode, a dummy packet is transmitted when there is no data transmit. This eliminates discontineous transmissions. During over-the-air operation, it is recommended to use non-blocking mode such that the receiver stays synchronized to the transmitter’s clock.

Examples

There are two example applications located in examples/ folder. The first one is transceiverLoopbackSimulator which simulates both the transmitter and the receiver communicating over a fading channel. The second is transceiverLoopbackUHD that implements a complete transceiver when connected to UHD-compatible frontend. This application requires a radio front-end that is supported by a UHD driver. Make sure to set both TX and RX freqeuencies appropriately. The transceiver can be tested in loopback mode by using identical TX and RX frequencies.

gr-SatelliteModem was originally published by Yihenew beyene at Yihenew on August 12, 2018.

Creative art using Matlab

2017-12-04 14:17:50 -0400T00:00:00-00:00

While working with complex I/Q data captured from radio signals, some of the graphic plots ended up being pieces of art work by accident. So, it might be worth sharing it with you. Matlab is a powerful software that is used in various fields such as engineering, mathematics, economics, automation systems. It has a rich set of libraries for signal processing and data visualization. Though Matlab’s graphics engine is not meant for high quality artwork, it is fun to play with it. The script is available on github.

Here are sample outputs.

If you are serious artist, however, I recommend you to check other softwares such as Processing and Cinder which are platforms for professional creative artwork design.

Creative art using Matlab was originally published by Yihenew beyene at Yihenew on December 04, 2017.

Youconduct (Virtual orchestra)

2017-12-04 14:18:26 -0400T00:00:00-00:00

YouConduct is a small python project that allows you to conduct an orchestra of virtual instruments. YouConduct is based on two libraries: fluidSynth and OpenCV. FluidSynth is a software that synthesizes music in real-time from a soundfont file. OpenCV (Open Source Computer Vision Library) is known for its powerful real-time graphics processing capabilities. The code is available on github.

In order to use youConduct, you need to install the following libraries first:

fluidSynth
opencv2
python binding for opencv

For debian distributions, you can install the above as follows:

sudo apt-get install libopencv-dev python-opencv fluidsynth

colorSound.py

colorSound lets you conduct a virtual orchestra using colors. It processes images captured from a webcamera and tracks three colors (Green, Blue, and Red). colorSound uses the OpenCV library for image processing and tracking of colors. When you run the script, it shows a video stream captured from your camera. Currently, colorSound detects and tracks three colors. Only one instance of each color with largest size is tracked, and each color plays different instrument. The soundfont that comes with youConduct (soundFont1.sf2) has three instrument presets.

Green - Yamaha Grand Piano
Blue - Violin
Red - Ahh Choir

Just grab anything that has at least one of the three colors and conduct the orchestra as professionals do. The software can play three instruments simultaneously if it sees all the three colors. Colors can be detected from your shirt too!

You can play other/more instruments by modifying the script to use different soundfont file. If you use different soundfont, make sure that you select the appropriate program and bank for each instrument.

Youconduct (Virtual orchestra) was originally published by Yihenew beyene at Yihenew on December 04, 2017.

LAMP stack webserver

2017-12-04 16:11:53 -0400T00:00:00-00:00

After experimenting with the Amazon’s EC2 compute instance, I decided to setup a webserver on a CentOS machine and deploy a Wordpress website. While doing this, I documented the steps I followed from start to finish. Though there are plenty (and better) of online resources and tutorials, I will share with you what I did in case you find it useful.

This is a quick guide for setting up LAMP stack and installing Wordpress on a CentOS machine. If you are new to web technologies and do not know what LAMP stack is, check this post for a brief introduction. In order to follow this guide, you need to have access to a CentOS machine either locally or remotely (for example, through ssh connection).

CentOS has a long (10 years) release support. Each version of CentOS receives feature enhancements and new hardware support until 7 years, and security updates until 10 years after general availability. However, support for point releases (eg. CentOS 6.3) will no longer be available once a new point release (eg. CentOS 6.4) is available for the same version. At the time of this writing 7.3 is the latest version. However, 6.8 is the final point release for version 6. Hence, it is better use CentOS 6.8 since the final point release for version 7 is yet unknown. CentOS is available in two images: minimal and full DVD (contains all packages available for the respective version). The following instructions assume that you have installed the minimal version of CentOS.

STEP 1: INITIAL SERVER SETUP

After a fresh install of CentOS, it is recommended to install updates.

$ sudo yum update

You may want to change the root password. To do that run the following command.

$ sudo passwd root

The following instructions are optional. You can continue from STEP 2.

If you want to create a new user account, eg. john, then run this command.

$ sudo adduser john

Then you can set password for john (eg 123456) as follows

$ sudo passwd john 123456

Even though you login with john’s credentials, you still need the root password in order to run some commands (eg. installing packages and making changes to system configurations). You can avoid this by adding john into the sodoers group. The sudoers group name in CentOS is wheel.

$ sudo gpasswd -a john wheel

Now every time you run commands with sudo, you will not be prompted for the root password. Make sure that you use a strong password for any user (especially the ones that are in sudoers group).

Step 2: LAMP SERVER SETUP

In this step we will install and configure Apache HTTP server, PHP and MariaDB database server. MariaDB is an enhanced, drop-in replacement for MySQL database server. From now on, all commands need to be executed as root (or with sudo privilege). If you don’t want to type sudo for ever command, then switch to the root user.

$ sudo su root

Now the command prompt sign changes from $ to #.

First, install Apache server

# yum install httpd

Start apache server

# systemctl start httpd.service

Enable apache to start on boot

# systemctl enable httpd.service

Configuration file for Apache is located at /etc/httpd/conf/httpd.conf. Open this file and search for "DocumentRoot". By default, you will see the following

DocumentRoot "/var/www/html/"

This means, Apache will serve websites that are hosted in /var/www/html directory. By default, if Apache doesn’t find an index file (eg. index.html), it will show directory listing of all files in /var/www/html directory. This will expose all the files in this director to the public. In order to disable directory listing, open /etc/httpd/conf/httpd.conf file and within <Directory "/var/www/html"> directive, remove "Indexes" from the list of Options. Eg. If you find something like the following

Options Indexes, FollowSymLinks

then change it to

Options FollowSymLinks

i.e remove Indexes. You need to restart apache in order to apply configuration changes

# systemctl restart httpd.service

The next step is to install MariaDB database server.

# yum install mariadb-server mariadb

Start the database server

# systemctl start mariadb

By default, MySQL has EMPTY root password and comes with a test database. Run the following script and follow the instructions to override dangerous DEFAULT and lock down access to the database.

# mysql_secure_installation

Enable MariaDB to start on boot

# systemctl enable mariadb.service

Finally, install PHP

# yum install php php-mysql

Restart Apache

# systemctl restart httpd.service

STEP 3: INSTALL WORDPRESS

First, create database for wordpress. To do that, login to mysql server as a root

# mysql -u root -p

Then create a database. For example, the following command creates a database called wp

> CREATE DATABASE wp;

Then create a user account which Wordpress will use to access the database. For example, the following command creates an account with username wpUser and password wpPass123

> CREATE USER wpUser@localhost IDENTIFIED BY 'wpPass123';

Grant the user access to the database we created earlier.

> GRANT ALL PRIVILEGES ON wp.* TO wpUser@localhost IDENTIFIED BY 'wpPass123';

Flush privileges and close the connection.

> FLUSH PRIVILEGES;
> exit;

Install a module that allows wordpress to create tumbnails for uploaded images

# yum install php-gd
# systemctl restart httpd.service

Install wget which is used to download files from the command line. This is a handy tool when you configure your server remotely.

# yum install wget

Then download and extract latest Wordpress source files

# wget http://wordpress.org/latest.tar.gz
# tar xzvf latest.tar.gz

Extracted files are located at wordpress folder in the current working directory. Copy files from wordpress/* to a directory where you want to host your website. By default, Apache serves files that are hosted in/var/www/html directory. In order to keep this directory clean, create a sub directory /var/www/html/wp and copy Wordpress files to this directory. Use rsyncto copy the files. This is because rsync keeps file permissions.

# mkdir /var/www/html/wp
# rsync -avP wordpress/ /var/www/html/wp

Create folder in wp to store uploaded files.

# mkdir /var/www/html/wp/wp-content/uploads

Grant ownership of all files in wp to Apache’s user and group.

# chown -R apache:apache /var/www/html/wp/*

Create wordpress configuration file (Copy from the sample).

# cd /var/www/html/wp
# cp wp-config-sample.php wp-config.php

Open wp-config.php and change DB_NAME, DB_USER and DB_PASSWORD accordingly.

Now change Apache’s document root directory to the one we created. To do that, open /etc/httpd/conf/httpd.conf and change Apache’s document root as follows

DocumentRoot "/var/www/html/wp"

Finally, open a browser and type the server’s IP address. If you have local server, then type http://localhost to the URL. Then finish the installation of WordPress throught the web browser.

LAMP stack webserver was originally published by Yihenew beyene at Yihenew on December 04, 2017.

Web Application 101

2017-12-03 14:11:53 -0400T00:00:00-00:00

In order to deploy a dynamic web site on a server machine, one needs an operating system, an HTTP server, a database, and a server-side engine that interacts with the database and processes requests that come from clients (web browsers). There are three commonly used alternative protocol stacks (combination of web servers, frameworks and platforms). These stacks are based on the three most popular HTTP servers:

LAMP Stack

LAMP is an acronym for Linux operating system, Apache HTTP Server, MySQL relational database and PHP programming language. Since Apache, MySQL and PHP can run on other platforms such as Windows, there are also some variants LAMP stack. WAMP stack, which is based on Windows operating system, is one of them. PHP powers back-end (server-side) functionalities such as handling HTTP requests, and iteracting with MySQL database. The front-end is mainly implemented using HTML and javascript.

MEAN Stack

MEAN stands for MongoDB database, Express javascript-based web framework, AngularJS front-end framework, and Node.js HTTP server. Node.js is a JavaScript runtime built on Chrome’s V8 JavaScript engine. Node.js uses an event-driven, non-blocking I/O model that makes it lightweight and efficient. Morever, since both the front-end and the back-end are powered by javascript, it simplifies web development process.

Ruby on Rails

Rails is a complete web application development framework written in Ruby language. It is designed to make programming of web applications easier by making assumptions about what every developer needs to get started. It abstracts routine works and allows you to write less code while accomplishing more than many other languages and frameworks.

Web Application 101 was originally published by Yihenew beyene at Yihenew on December 03, 2017.

SoftRadar

2016-12-04 14:17:50 -0400T00:00:00-00:00

SoftRadar is a software defined radio that can transmit and receive (synchronoulsy) radar waveforms. Currently, it targets Universal Software Radio Peripheral (USRP) based hardwares only that support simultaneous Tx and Rx. The main streaming program is written in C++ and is responsible for configuring the hardware, transmitting and receiving radar signals. Some of the hardware settings (frequency, gain, bandwidth,…) are configured via a configuration file. Other real-time controls are handled through a UDP socket for possible remote control. You can download the source code from here.

SoftRadar can be run from the commnand-line or through a web-interface that can be accessed remotely. Remote access is possible only when the host computer is connected to the network and is visible from the other side where one want to access the radar. Picture below shows snapshot of the graphical interface.

The graphical interface is also used to upload new waveform file at any time even when the radar is streaming. The device manager records events that occur while streaming. The web interface has a tool that generates a python template code which can be downloaded and modified. The tool is designed as a basis for generating SMPRF radar waveforms. Picture below show snapshot of the interface.

SoftRadar was originally published by Yihenew beyene at Yihenew on December 04, 2016.