KOMUNIKASI DATA Materi Pertemuan 10

Encoding and Modulation Techniques Have already noted in Ch 3 that both analog and digital information can be encoded as either analog or digital signals: Digital data, digital signals: simplest form of digital encoding of digital data Digital data, analog signal: A modem converts digital data to an analog signal so that it can be transmitted over an analog Analog data, digital signals: Analog data, such as voice and video, are often digitized to be able to use digital transmission facilities Analog data, analog signals: Analog data are modulated by a carrier frequency to produce an analog signal in a different frequency band, which can be utilized on an analog transmission system Stallings DCC8e Fig 5.1 emphasizes the process involved in this. For digital signaling, a data source g(t), which may be either digital or analog, is encoded into a digital signal x(t). The basis for analog signaling is a continuous constant-frequency fc signal known as the carrier signal. Data may be transmitted using a carrier signal by modulation, which is the process of encoding source data onto the carrier signal. All modulation techniques involve operation on one or more of the three fundamental frequency domain parameters: amplitude, frequency, and phase. The input signal m(t) may be analog or digital and is called the modulating signal, and the result of modulating the carrier signal is called the modulated signal s(t). 2/45

Digital Signaling Versus Analog Signaling Digital or analog data is encoded into a digital signal Encoding may be chosen to conserve bandwidth or to minimize error Analog Signaling Digital or analog data modulates analog carrier signal The frequency of the carrier fc is chosen to be compatible with the transmission medium used Modulation: the amplitude, frequency or phase of the carrier signal is varied in accordance with the modulating data signal by using different carrier frequencies, multiple data signals (users) can share the same transmission medium 3/45

Digital Signaling Digital data, digital signal Simplest encoding scheme: assign one voltage level to binary one and another voltage level to binary zero More complex encoding schemes: are used to improve performance (reduce transmission bandwidth and minimize errors). Examples are NRZ-L, NRZI, Manchester, etc. Analog data, Digital signal Analog data, such as voice and video Often digitized to be able to use digital transmission facility Example: Pulse Code Modulation (PCM), which involves sampling the analog data periodically and quantizing the samples 4/45

Analog Signaling Digital data, Analog Signal A modem converts digital data to an analog signal so that it can be transmitted over an analog line The digital data modulates the amplitude, frequency, or phase of a carrier analog signal Examples: Amplitude Shift Keying (ASK), Frequency Shift Keying (FSK), Phase Shift Keying (PSK) Analog data, Analog Signal Analog data, such as voice and video modulate the amplitude, frequency, or phase of a carrier signal to produce an analog signal in a different frequency band Examples: Amplitude Modulation (AM), Frequency Modulation (FM), Phase Modulation (PM) 5/45

INTRODUCTION

DIGITAL TO ANALOG TRANSMISSION

Types of Digital to Analog Conversion

Modulation in wireless communication Translate digital data to analog signal (baseband) Shifts center frequency of baseband signal up to the radio carrier example carrier frequency 802.11b/g: 2.4 GHz, 802.11a (5 GHz), GSM: 1.9 GHz Why? Antenna size: on the order of signal’s wavelength More bandwidth available at higher carrier frequency Medium characteristics (path loss, shadowing, reflection, scattering, diffraction) depend on signal’s wavelength

Modulation at sender modulation analog baseband signal digital data radio carrier analog baseband signal 101101001

Amplitude Shift Keying (ASK) Pros: simple Cons: susceptible to noise Example: optical system, infra-red 1 1 t

Describe Amplitude Shift Keying Amplitude-shift keying (ASK) is a form of modulation that represents digital data as variations in the amplitude of a carrier wave.

Amplitude Shift Keying (ASK) The simplest and most common form of ASK operates as a switch, using the presence of a carrier wave to indicate a binary one and its absence to indicate a binary zero. This type of modulation is called on-off keying,

Frequency Shift Keying (FSK) Pros: less susceptible to noise Cons: requires larger bandwidth 1 1 t

Describe Frequency Shift Keying (FSK) Frequency-shift keying (FSK) is a frequency modulation scheme in which digital information is transmitted through discrete frequency changes of a carrier wave. The simplest FSK is binary FSK (BFSK).

Describe Frequency Shift Keying (FSK) Two binary digits represented by two different frequencies near the carrier frequency where f1 and f2 are offset from carrier frequency fc by equal but opposite amounts

Phase Shift Keying (PSK) Pros: Less susceptible to noise Bandwidth efficient Cons: Receiver must synchronize in frequency and phase w/ transmitter t 1 1

Describe Phase Shift Keying (PSK) Phase-shift keying (PSK) is a digital modulation scheme that conveys data by changing, or modulating, the phase of a reference signal (the carrier wave).

Describe Phase Shift Keying (PSK) Uses two phases to represent binary digits

Variant of phase shift keying Q I 1 BPSK (Binary Phase Shift Keying): bit value 0: sine wave bit value 1: inverted sine wave very simple PSK low spectral efficiency robust, used in satellite systems 11 10 00 01 Q I A t QPSK (Quadrature Phase Shift Keying): 2 bits coded as one symbol needs less bandwidth compared to BPSK symbol determines shift of sine wave Often also transmission of relative, not absolute phase shift: DQPSK - Differential QPSK

Quadrature Amplitude Modulation (QAM) combines amplitude and phase shift keying It is possible to code n bits using one symbol 2n discrete levels bit error rate increases with n 0000 0001 0011 1000 Q I 0010 φ a Example: 16-QAM (4 bits = 1 symbol) Symbols 0011 and 0001 have the same phase φ, but different amplitude a. 0000 and 1000 have same amplitude but different phase Used in Modem

What is transmitted in air? radio wave (baseband modulated w/ carrier radio) high-frequency, short wavelength wave length * frequency = speed of light  3x108m/s e.g., 802.11b, wavelength 0.1m

Frequency range (bandwidth) need a wide spectrum e.g., 802.11b bandwidth 20 MHz w/o noise: Nyquist’s result w/ noise (e.g., thermal noise, background radiation) Shannon’s channel capacity theorem: the maximum number of bits that can be transmitted per second by a physical channel is: W: frequency range S/N: signal noise ratio

Frequencies for communication twisted pair coax cable optical transmission 1 Mm 300 Hz 10 km 30 kHz 100 m 3 MHz 1 m 300 MHz 10 mm 30 GHz 100 m 3 THz 1 m 300 THz VLF LF MF HF VHF UHF SHF EHF infrared visible light UV VLF = Very Low Frequency UHF = Ultra High Frequency LF = Low Frequency SHF = Super High Frequency MF = Medium Frequency EHF = Extra High Frequency HF = High Frequency UV = Ultraviolet Light VHF = Very High Frequency

Frequency regulation ITU-R holds auctions for new frequencies, manages frequency bands worldwide (WRC, World Radio Conferences)