EE456 Digital Communications Ha H. Nguyen September 2014 EE456 - - PowerPoint PPT Presentation

EE456 – Digital Communications

Ha H. Nguyen September 2014

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INTRODUCTION

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Communication Systems (Analog or Digital)

Source (User) Transmitter Channel Receiver Sink (User) Message signal Transmitted signal Received signal Detected signal Distortion and noise

Source: Originates a message (e.g., human voice, TV picture, email message) and converts it to an electrical waveform, referred to as a message signal. Transmitter (Modulator): Modifies the message signal for efficient transmission. Channel: A physical medium of choice that can convey the electrical signals at the transmitter output over a distance. Receiver (Demodulator): Processes the signal received from the channel by reversing the signal modifications made at the transmitter and removing the distortion made by the channel. Sink: Converts the electrical signal at the output of the receiver to its original form – the message.

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Analog, Digital, Continuous-Time, Discrete-Time?

x(t) (a) Analog continuous-time signal Ts t Ts x(t) t t t (b) Digital continuous-time signal (c) Analog discrete-time signal (d) Digital discrete-time signal

In signal classification, the adjectives “analog” and “digital” refer to the amplitude property of the signals, while “continuous-time” and “discrete-time” refer to the time property. “continuous-time” and “discrete-time” are also commonly used to refer to the type of signal processing in hardware.

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What is Digital Communication?

The message has to be in a digital format, i.e., it can be represented as a string

• f bits 0 and 1. A message can be analog (such as human voice) or digital (such

as a computer-generated text document). An analog message can always be represented in a digital format through the processes of sampling and quantization. Digital Modulation: Over a finite duration, the transmitted signal belongs to a finite set of continuous-time waveforms. Note that each waveform in the set itself can be an analog continuous-time signal!

0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 −1 1 t Analog message 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 −1 1 t AM signal 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 −1 1 t Digital message 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 −1 1 t BASK signal EE456 – Digital Communications 5/10

Why Digital Communications?

0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 −1 1 t Transmitted AM 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 −2 2 t Received AM 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 −1 1 t Transmitted BASK 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 −2 2 t Received BASK

Since the transmitted signal in digital communications belong to a finite set of waveforms → The distorted signal can be recovered to its ideal shape, hence removing all the noise.

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Analog-to-Digital Conversion = Sampling + Quantization

Sampling does not introduce information loss if it satisfies the Nyquist sampling theorem. Quantization always introduces information loss, but the loss can be made arbitrarily small by increasing the number of quantization levels (i.e., using more bits).

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Types of Modulation and Hardware Realization (Implementation)

Continuous-time implementation is mostly adopted in this course (EE456) to explain the theory of digital communications (e.g., why do we use a certain modulation and demodulation methods?) Discrete-time implementation is most common in practice, but only briefly discussed in this course. Discrete-time implementation has to do with how efficiently one can build modulation/demodulation methods in hardware. This is the focus of EE465 in Term 2.

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Block Diagram of a Digital Communication System

Source (User) Transmitter Channel Receiver Sink (User) Synchronization Transmitter Receiver

Source Encoder Channel Encoder Modulator De- modulator Channel Decoder Source Decoder

(a) (b)

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Digital vs. Analog Communications

Advantages: Digital signals are much easier to be regenerated. Digital circuits are less subject to distortion and interference. Digital circuits are more reliable and can be produced at a lower cost than analog circuits. It is more flexible to implement digital hardware than analog hardware. Digital signals are beneficial from digital signal processing (DSP) techniques. Disadvantages: Heavy signal processing. Synchronization is crucial. Larger transmission bandwidth. Non-graceful degradation.

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