Teaching Communication Systems with Simulink and the USRP

This paper discusses a system to demodulate and decode radio signals in real-time using the universal software radio peripheral (USRP) in combination with Simulink in an undergraduate teaching environment. Several previous papers have shown the value of using real world radio signals to inspire and motivate students in traditional communication systems courses and laboratories. Recent versions of Simulink now support a direct connection to the newer versions of the USRP, and Simulink can connect to the original USRP1 through an existing open source software package. This capability allows users to rapidly prototype radio systems that run in real-time on the host computer, which enables more realistic and interesting projects, laboratory experiments, and demonstrations. To illustrate this capability, a Simulink model is presented that implements a real-time FM radio that demodulates and plays the audio signal, and it also decodes the digital Radio Data System (RDS) signal and prints out the RadioText messages which usually convey the name of the artist and song that is currently playing. The use of this receiver as a demonstration in an undergraduate communication systems course is discussed, and its effectiveness is assessed. The source files for this receiver are available for free from the author. Background Communication systems classes usually contain highly mathematical and theoretical material which is sometimes difficult for students to learn. It can be very helpful to motivate the students by showing how the material is used in real systems. Simulations are an effective and easily deployed method, but they do not have as direct connection to the real world as real systems. However most real communication systems are very complex, and it is often difficult to utilize them in a class. Some laboratory experiments have been based on the USRP because they can be used to build communication systems that run in real-time without the need for building low level hardware and software 1,2 . The USRP is an open source hardware platform provided by Ettus Research 3 . It converts radio frequency (RF) signals to digital signals that can be processed by a computer, and also converts digital signals from a computer to RF signals. Its function is analogous to a sound card, except that it can capture and generate high frequency RF signals. Using the USRP allows the students to work with RF signals from existing communication systems and to generate their own RF signals. P ge 25243.2 Simulink is a programming environment that works in conjunction with MATLAB 4 . It allows the user to build a program by connecting blocks that represent basic functions into a block diagram. This programming environment is often used to run simulations, but it can also process data on a standard computer in real-time as long as the system does not require too much computation. Simulink now has the ability to take data directly from the USRP and deliver data to the USRP, which allows a real-time communication system to be constructed using a high level language like Simulink and commercially available hardware. Since it greatly simplifies the process of implementing a communication system, this platform is very useful for demonstrating communication systems in a lecture format and for experimenting with communication systems in a laboratory. The Radio Data System (RDS), which is also called Radio Broadcast Data System (RBDS) in the United States, is a digital signal that is sent along with the analog audio signals in FM radio broadcasts. This signal is usually used to transmit the title of the song, the name of the artist, the name of the program, and other information. The RDS signal, which is added to the analog audio signals before frequency modulation, uses a carrier frequency of 57 kHz, bi-phase (or Manchester) encoding, a raised cosine pulse shape, and a bit rate of 1187.5 bits/sec 5,6 . The RDS signal is useful in teaching communication systems because it can be demodulated and decoded with a reasonably simple receiver, and because it is a signal that is familiar to most students 7 . Hardware In order to demonstrate the abilities of Simulink and the USRP in an educational setting, an FM receiver was developed that not only demodulates and plays the audio signal from an FM radio station, but it also demodulates the digital RDS signal and prints out the RadioText messages that usually contain the title of the song, name of the artist, and other information. The hardware consists of an antenna, USRP1 with an appropriate daughterboard such as the TVRX or the WBX, and a standard PC or laptop as shown in Figure 1. The antenna can be a length of wire or an FM antenna. The author used the USRP1, which costs about $700 and is one of the inexpensive versions of the USRP 3 , but it should be possible to use the other versions of the USRP as well.