UNIT 2 DIGITAL COMMUNICATION DIGITAL COMMUNICATION-Introduction...

Course Material (Lecture Notes)

CS6304-Analog & Digital Communication UNIT-2

UNIT 2

DIGITAL COMMUNICATION

DIGITAL COMMUNICATION-Introduction

The techniques used to modulate digital information so that it can be transmitted via microwave, satellite or down a cable pair is different to that of analogue transmission. The data transmitted via satellite or microwave is transmitted as an analogue signal. The techniques used to transmit analogue signals are used to transmit digital signals. The problem is to convert the digital signals to a form that can be treated as an analogue signal that is then in the appropriate form to either be transmitted down a twisted cable pair or applied to the RF stage where is modulated to a frequency that can be transmitted via microwave or satellite.

The equipment that is used to convert digital signals into analogue format is a modem. The word modem is made up of the words ―modulator‖ and ―demodulator‖.

A modem accepts a serial data stream and converts it into an analogue format that matches the transmission medium.

There are many different modulation techniques that can be utilized in a modem. These techniques are:

x Amplitude shift key modulation (ASK) x Frequency shift key modulation (FSK) x Binary-phase shift key modulation (BPSK) x Quadrature-phase shift key modulation (QPSK) x Quadrature amplitude modulation (QAM)

Amplitude Shift Key Modulation

In this method the amplitude of the carrier assumes one of the two amplitudes dependent on the logic states of the input bit stream. A typical output waveform of an ASK modulator is shown in the figure below. The frequency components are the USB and LSB with a residual carrier frequency. The low amplitude carrier is allowed to be transmitted to ensure that at the receiver the logic 1 and logic 0 conditions can be recognized uniquely.

The ON-OFF signaling variety

𝑏 𝑡 = 𝐸𝑏, 𝑓𝑜𝑟 𝑏𝑖𝑛𝑎𝑟𝑦 𝑠𝑦𝑚𝑏𝑜𝑙 10, 𝑓𝑜𝑟 𝑏𝑖𝑛𝑎𝑟𝑦 0

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𝑠 𝑡 = 2𝐸𝑏𝑇𝑏 cos(2𝜋𝑓𝑡) , 𝑓𝑜𝑟 𝑏𝑖𝑛𝑎𝑟𝑦 𝑠𝑦𝑚𝑏𝑜𝑙 1

0, 𝑓𝑜𝑟 𝑏𝑖𝑛𝑎𝑟𝑦 0

The average transmitted signal energy is (the two binary symbols must by equiprobable)

Eav = Eb /2

FREQUENCY SHIFT KEYING





If the bit intervals and the phases of the signals can be determined (usually by the use of a phase-lock loop), then the signal can be decoded by two separate matched filters:

The first filter is matched to the signal s1.t/, and the second to s2.t/. Under the assumption that the signals are mutually orthogonal, the output of one of the matched filters will be E and the other zero (where E is the energy of the signal). Decoding of the bandpass signal can therefore be achieved by subtracting the outputs of the two filters, and comparing the result to a threshold.



Sri Vidya College of Engineering & Technology, Virudhunagar Course Material (Lecture Notes)


If the signal s1.t/ is present then the resulting output will be CE, and if s2.t/ is present it will be E. Since the noise variance at each filter output is E =2, the noise in the difference signal will be doubled, namely 2 D E . Since the overall output variation is 2E, the probability of error is

The overall performance of a matched filter receiver in this case is therefore the same as for ASK.

The frequency spectrum of an FSK signal is difficult to obtain — this is a general characteristic of FM signals. However, some rules of thumb can be developed. Consider the case where the binary message consists of an alternating sequence of zeros and ones. If the two frequencies are each multiples of 1= T (eg. f1 D m= T and f2 D n= T ) and are synchronised in phase, then the FSK wave is a periodic function:

FSK signal

This can be viewed as the linear superposition of two OOK signals, one delayed by T seconds with respect to the other. Since the spectrum of an OOK signal is




Phase Shift Key Modulation

With this method the phase of the carrier changes between different phases determined by the logic states of the input bit stream.

There are several different types of phase shift key (PSK) modulators.

x Two-phase (2 PSK)

x Four-phase (4 PSK)

x Eight-phase (8 PSK)

x Sixteen-phase (16 PSK)

x Sixteen-quadrature amplitude (16 QAM)

The 16 QAM is a composite modulator consisting of amplitude modulation and phase modulation. The 2 PSK, 4 PSK, 8 PSK and 16 PSK modulators are generally referred to as binary phase shift key (BPSK) modulators and the QAM modulators are referred to as quadrature phase shift key (QPSK) modulators.

Two-Phase Shift Key Modulation

In this modulator the carrier assumes one of two phases. A logic 1 produces no phase change and a logic 0 produces a 180° phase change. The output waveform for this modulator is shown below.




Four-Phase Shift Key Modulation

With 4 PSK, 2 bits are processed to produce a single phase change. In this case each symbol consists of 2 bits, which are referred to as a dibit. The actual phases that are produced by a 4 PSK modulator are shown in the table below.




Eight-Phase Shift Key Modulation

With this modulator 3 bits are processed to produce a single phase change. This means that each symbol consists of 3 bits.

8 PSK Modulator

Figure 10 above shows a typical circuit for the 8 PSK modulator. With this modulator bit A controls the output polarity of the first digital-to-analogue converter (DAC1). Bit B is used to control the output polarity of the second DAC 2 and bit C is used to control the output amplitude of both DACs.

Digital to Analogue Conversion Condition for 8 PSK modulator

The conditions shown in the table above produce the positions shown in table below for all the different permutations.




Input permutations and Positions

The constellation diagram can be drawn according to the above table and is shown below.

Sixteen-Phase Shift Key Modulation

With this modulator 4 bits are processed to produce a single phase change. This means that each symbol consists of 4 bits. The constellation for this modulator scheme is shown below.




16 PSK Modulation Constellation

Quadrature Phase Shift Keying (QPSK)

An important goal of digital communication is the efficient utilization of channel bandwidth




Quadrature Amplitude Modulation (QAM)

Overview

This tutorial is part of the National Instruments Measurement Fundamentals series. Each tutorial in this series teaches you a specific topic of common measurement applications by explaining the theory and giving practical examples. This tutorial covers an introduction to RF, wireless, and high-frequency signals and systems. For the complete list of tutorials, return to the NI Measurement Fundamentals main page, or for more RF tutorials, refer to the NI RF Fundamentals main subpage. For more information about National Instruments RF products, visit www.ni.com/rf. A variety of communication protocols implement quadrature amplitude modulation (QAM). Current protocols such as 802.11b wireless Ethernet (Wi-Fi) and digital video broadcast (DVB), for example, both utilize 64-QAM modulation. In addition, emerging wireless technologies such as Worldwide Interoperability for Microwave Access (WiMAX), 802.11n, and HSDPA/HSUPA (a new cellular data standard) will implement QAM as well. Thus, understanding QAM is important because of its widespread use in current and emerging technologies. QAM involves sending digital information by periodically adjusting the phase and amplitude of a sinusoidal electromagnetic wave. Each combination of phase and amplitude is called a symbol and represents a digital bitstream. This tutorial first covers the hardware implementation required to constantly adjust the phase and amplitude of a carrier wave. The tutorial also discusses the binary value associated with each symbol.

Hardware Implementation

Quadrature amplitude modulation (QAM) requires changing the phase and amplitude of a carrier sine wave. One of the easiest ways to implement QAM with hardware is to generate and mix two sine waves that are 90 degrees out of phase with one another. Adjusting only the amplitude of either signal can affect the phase and amplitude of the resulting mixed signal.

These two carrier waves represent the in-phase (I) and quadrature-phase (Q) components of our signal. Individually each of these signals can be represented as: I = A cos(φ) and Q = A sin(φ).

Note that the I and Q components are represented as cosine and sine because the two signals are 90 degrees out of phase with one another. Using the two identities above and the following trigonometric identity cos(α + β) = cos(α)cos(β) – sin(α)sin(β),




rewrite a carrier wave A cos(2πfct + φ) as A cos(2πfct + φ) = I cos(2πfct) – Q sin(2πfct).

As the equation above illustrates, the resulting identity is a periodic signal whose phase can be adjusted by changing the amplitude of I and Q. Thus, it is possible to perform digital modulation on a carrier signal by adjusting the amplitude of the two mixed signals.

Figure 1 shows a block diagram of the hardware required to generate the intermediate frequency (IF) signal. The ―Quadrature Modulator‖ block shows how the I and Q signals are mixed with the local oscillator (LO) signal before being mixed together. The two LOs are exactly 90 degrees out of phase with one another.

Hardware Used to Generate the IF Signal

The next section discusses exactly how the I and Q components are used to represent actual digital data. To do this, the following section details the relationship between the QAM symbol map and the actual real-world signal.

QAM Symbol Map

As stated previously, QAM involves sending digital information by periodically adjusting the phase and amplitude of a sinusoidal electromagnetic wave. Four-QAM uses four combinations of phase and amplitude. Moreover, each combination is assigned a 2-bit digital pattern. For example, suppose you want to generate the bitstream (1,0,0,1,1,1). Because each symbol has a unique 2-bit digital pattern, these bits are grouped in two’s so that they can be mapped to the corresponding symbols. In our example, the original bitstream (1,0,0,1,1,1) is grouped into the three symbols (10,01,11).

In the following figure, 4-QAM consists of four unique combinations of phase and amplitude. These combinations—called symbols—are shown as the white dots on the constellation plot in Figure 2. The red lines represent the phase and amplitude transitions from one symbol to another. Labeled on the constellation plot is the digital bit pattern that each symbol represents. Thus, a digital bit pattern can be sent over a carrier signal by generating unique combinations of phase and amplitude.




QAM Symbol Map

How do these digital bit patterns correspond to I/Q data? Figure 3 shows both the I/Q data (top) and the 2-bit digital pattern (in a constellation plot, bottom) relating to 4-QAM. The black dashed line in the top graphic corresponds to the black square marker in the bottom graphic, so you can follow the phase and amplitude changes as the I/Q parameters also change.

QAM Baseband I/Q Data (bottom) 4-QAM Constellation Plot




The constellation plot in Figure 2 shows the symbol map of 4-QAM with each possible phase (Θ) and amplitude (A) of a carrier signal in polar coordinate form. Notice how in the bottom graphic of Figure 3 the square marker passes over each symbol at least once.

While it is possible to send up to two bits per symbol when using 4-QAM modulation, it is also possible to send data at even higher rates by increasing the number of symbols in our symbol map. By convention, the number of symbols in a symbol map is called the symbol map ―M‖ and is considered the ―M-ary‖ of the modulation scheme. In other words, 4-QAM has an M-ary of four and 256-QAM has an M-ary of 256. Moreover, the number of bits that can be represented by a symbol has a logarithmic relationship to the M-ary. For example, we know that two bits can be represented by each symbol in 4-QAM. While this makes sense intuitively, it is defined by the equation bits per symbol = log2 (M).

Using this equation, each symbol in 256-QAM can be used to represent an 8-bit digital pattern (log2 (256) = 8). Because the M-ary of a QAM modulation scheme affects the number of bits per symbol, the M-ary has a significant affect on the actual data transmission rate.

Bandwidth Efficiency refers to the information rate that can be transmitted over a given bandwidth in a specific communication system

QAM bits per symbol

The advantage of using QAM is that it is a higher order form of modulation and as a result it is able to carry more bits of information per symbol. By selecting a higher order format of QAM, the data rate of a link can be increased.

The table below gives a summary of the bit rates of different forms of QAM and PSK.

MODULATION BITS PER SYMBOL SYMBOL RATE

BPSK 1 1 x bit rate QPSK 2 1/2 bit rate 8PSK 3 1/3 bit rate

16QAM 4 1/4 bit rate 32QAM 5 1/5 bit rate 64QAM 6 1/6 bit rate





Comparison of various Digital Communication System (ASK – FSK – PSK – QAM).



UNIT 2 DIGITAL COMMUNICATION DIGITAL COMMUNICATION-Introduction...

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