Kwonhue Choi

Sampling and Reconstruction

This chapter shows how the sampling changes the signal spectrum. It reviews reconstruction of a signal from its sampled version using low pass filtering and implements frequency up-conversion using sampling and a band pass filter. The chapter provides step-by-step code exercises and instructions to implement execution sequences. Starting Simulink and opening a new mdl/slx file and adding the blocks is a first step in customizing the analog filter design. These blocks can be easily identified by searching for them in the Simulink library using block names. The chapter provides a simple block diagram of a sampling and reconstruction system to restore the original signal from its sampled version by using an LPF. It is designed to help teach and understand communication systems using a classroom-tested, active learning approach.

Hilbert Transform, Analytic Signal, and SSB Modulation

This chapter reviews generation of analytic signals and single-side band (SSB) modulation signals using the Hilbert transform. The Hilbert transform is a linear operator. The system that performs the Hilbert transform (called a Hilbert transformer) is a linear system with a frequency transfer function. The analytic signal is a conceptual complex signal, but it is widely used in signal analysis. The chapter examines the spectral characteristics of the analytic signal. The impulse response of the Hilbert transformer can be calculated by taking the inverse Fourier transform of frequency transfer function. The sampled vector of the Hilbert transform is generated from the sampled vector of an audio signal. The chapter describes the implementation of an SSB modulation and demodulation system using a band pass filter (BPF). It is designed to help teach and understand communication systems using a classroom-tested, active learning approach.

Near-Ultrasonic Wireless Orthogonal Frequency Division MULTIPLEXING MODEM DESIGN

This chapter discusses transmission of an image file over a near-ultrasonic (NUS) wireless channel. The image file is orthogonal frequency division multiplexing (OFDM) modulated and transmitted from the speaker of a phone over an NUS wireless channel. The microphone in a PC samples the received signal and demodulates it to restore the image data. The chapter investigates transmit and receive algorithms for NUS OFDM systems. It also analyzes the waveforms and spectra of NUS systems at major processing stages. To demodulate each OFDM symbol, one needs to separately perform fast Fourier transform (FFT) on the properly partitioned portions of rcbb. The vector rcbb is the sampled version of the frequency-offset compensated complex baseband signal. Proper partitioning of rcbb requires locating the starting sample of each OFDM symbol. The chapter is designed to help teach and understand communication systems using a classroom-tested, active learning approach.

Correlation and Spectral Density

This chapter discusses communication concepts and algorithms, which are explained using simulation projects, accompanied by MATLAB and Simulink. It provides step-by-step code exercises and instructions to implement execution sequences. The chapter first calculates the correlation function of two time functions using numerical integration. It then estimates the shape and parameters of unknown periodic signals in severe noise using correlation. With the ideal model (unrealistic), the background noise has infinite power and thus infinite energy. The chapter also investigates the relationship between the correlation function and the spectral density. The energy spectral density (ESD) is calculated using numerical integration. A method is developed to systematically determine the shape and estimate the parameters of an unknown periodic signal distorted by an additive noise. The chapter is designed to help teach and understand communication systems using a classroom-tested, active learning approach.

Orthogonal Frequency Division Multiplexing Over Multipath Fading Channels

This chapter creates the impulse response of a multipath channel and analyzes the relationship between the power-multipath magnitude profile and channel response. It then focuses on generation of orthogonal frequency division multiplexing (OFDM) signals with cyclic prefix (CP) added and demodulation of these signals in multipath fading environments. The chapter also analyzes the bit error rate (BER) performance of OFDM systems over multipath fading channels. It then provides step-by-step code exercises and instructions to implement execution sequences. The chapter subsequently focuses on OFDM systems. In order to minimize the impacts of ICI and ISI, a guard interval with a minimum length equal to the channel excess delay can be left in front of each OFDM symbol. In addition, the last portion of each OFDM symbol of the same length as the guard interval is copied and inserted in the guard interval.

Voltage-Controlled Oscillator and Frequency Modulation

Like some components in a communications system transmitter, the amplifiers and analog-to-digital converters in the receiver also have their linear operation regions. When the instantaneous input signal exceeds the dynamic operation region of these components, for example, due to slow automatic gain control responses, the input signal will be clipped. This chapter analyzes the impact of signal clipping (due to components/processes in the transmitter or in the receiver) in amplitude modulation (AM) systems. For practical applications, however, maintaining a strong received signal to meet a high received signal-to-noise ratio (SNR) might not always be possible. For example, component nonlinearity might cause signal clipping if the input signal is too strong. The chapter then focuses on voltage-controlled oscillator (VCO) operation by using a Simulink design. It also discusses a frequency modulation transmitter and demodulator. The chapter is designed to help teach and understand communication systems using a classroom-tested, active learning approach.

Quadrature Multiplexing and Frequency Division Multiplexing

In this chapter, the authors design a modulation and demodulation system for quadrature multiplexing (QM) amplitude modulation (AM). They also design a frequency division multiplexing (FDM) system and analyze the effects of phase and frequency errors in QM systems. The chapter considers three audio signals x(t), y(t), and z(t) all having the same bandwidth of 4 kHz. The chapter provides step-by-step code exercises and instructions to implement execution sequences. It reviews stereo sound effects using intentional frequency error in QM methods. If the design is correct, the sound volumes of the two audio signals, a voice signal and a music signal, will not change but one of the audio signals will sound like moving from left to right and the other one moving right to left. The chapter is designed to help teach and understand communication systems using a classroom-tested, active learning approach.