Introductory Communications Systems

The purpose of this introductory laboratory exercise is to ensure that students have a working installation of LabVIEW Communications on their computers and know how to connect to the USRP software defined radio.

This laboratory exercise has two objectives. The first is to gain a firsthand experience in programming the USRP to act as a transmitter and a receiver. The second is to investigate classical analog amplitude modulation and the envelope detector.

In this laboratory exercise students will investigate sending multiple messages on a single carrier by frequency-division multiplexing.

This laboratory exercise illustrates the image problem in superheterodyne receivers. Image rejection is carried out using complex filtering. This lab introduces a processing technique that is straightforward in a software defined radio, but is virtually unavailable in a conventional hardware- based radio.

This laboratory exercise introduces suppressed-carrier modulation. A simple scheme for phase and frequency synchronization is introduced in implementing the demodulator.

This laboratory exercise introduces frequency modulation. This lab exercise is a nice illustration of the utility of the software defined radio approach, since the algorithms for creating and demodulating FM in software are much simpler than those used in the traditional hardware approach.

In this lab project, design of the transmitter and design of the receiver each present challenges and opportunities for investigation. The lab project is consequently divided into several parts. The transmitter part investigates creation of the ASK signal and the effect of transmitted pulse shape on the bandwidth of the transmitted signal. The receiver part investigates demodulation, matched filtering, and signal detection. There is a third part investigating alignment of the receiver and transmitter bit streams.

In this lab, students will learn about frequency-shift keying. In frequency-shift keying (FSK), a 1 is represented by a tone at a specific frequency, known traditionally as the “mark” frequency, while a 0 is represented by a tone at a different frequency, known as the “space” frequency. FSK owes part of its popularity to the fact that a tone is always being transmitted, even when the source generates a long string of zeros. This makes it easy for the receiver to distinguish between a transmitter that is idling and a transmitter that has stopped transmitting. FSK also has the property that the transmitted signal has a constant amplitude. This allows a very efficient nonlinear power amplifier to be used for transmission, a very important consideration when the transmitter is battery-powered.

In phase-shift keying (PSK), information is encoded on the phase of the transmitted carrier, rather than on its amplitude (ASK) or its frequency (FSK). In binary phase-shift keying (BPSK) there are two phase values, 0 and 180, which means that an unmodified carrier is transmitted to represent one binary data value, while an inverted carrier is transmitted to represent the other binary data value.

In this lab project, students will examine how the eye diagram changes when intersymbol interference (ISI) is present in the communication channel, and students will learn how to make quantitative measurements of the amount of ISI from the eye diagram.

In this lab project, students will use the BPSK transmitter and receiver that they created for Lab 9 as a “test bed” system. The Channel.gvi that students used in the eye diagram lab project will create the intersymbol interference. The equalizer that we will investigate is provided in the Modulation Toolkit.

In this lab project we introduce quadrature phase-shift keying (QPSK), a variation on binary phase-shift keying that encodes two bits of data into each symbol. Using QPSK, we can transmit data at twice the rate, without increasing the channel bandwidth. There will be some loss in performance, however, as somewhat more transmitted power will be needed to maintain a given bit error rate in the presence of noise.