Materi 10 - Repository UNIKOM
Materi Pertemuan 10
Encoding and Modulation Techniques Encoding and Modulation Techniques
Digital Signaling Versus Analog Signaling
Digital Signaling Versus Analog Signaling
Digital 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
Digital Signaling
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
Analog Signaling
Analog Signaling
Digital data, Analog Signal
- be transmitted over an analog line
A modem converts digital data to an analog signal so that it can
- 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)
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 car
- 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 digital data modulation radio carrier analog baseband signal 101101001
- Pros: simple
- Cons: susceptible to noise
- Example: optical system, infra-red
1
1 t
Amplitude Shift Keying (ASK)
Describe Amplitude Shift Keying
Amplitude-shift keying (ASK) is a form of that represents as variations in the The simplest and most common form of ASK operates as a switch, using the presence of a carrier wave to indicate a one and its absence to indicate a binary zero.
This type of modulation is called on-off keying,
Amplitude Shift Keying
(ASK) 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
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
BPSK (Binary Phase Shift
I Keying): 1 bit value 0: sine wave bit value 1: inverted sine wave
Q
10
11 very simple PSK low spectral efficiency
I robust, used in satellite systems
QPSK (Quadrature Phase Shift
00
01 Keying): A 2 bits coded as one symbol needs less bandwidth compared to BPSK t symbol determines shift of sine wave
Often also transmission of relative,
01
11
10
00 not absolute phase shift: DQPSK -
Quadrature Amplitude Modulation
(QAM)
combines amplitude and phase shift keying
It is possible to code n bits using one symbol n
2 discrete levels
bit error rate increases with n
Q
- Example: 16-QAM (4 bits = 1
0010 0001 symbol)
0011 0000 φ • Symbols 0011 and 0001 have the a same phase φ, but different
I amplitude a. 0000 and 1000 have same amplitude but different phase
1000
- Used in Modem
air?
- radio wave (baseband modulated w/ carrier radio)
- high-frequency, short wavele
- wave length * frequency = speed of light
8 3x10 m/s
- e.g., 802.11b, wavelength 0.1m
Frequency range (bandwidth)
- need a wide spectrum
- e.g., 802.11b bandwidth 20>w/o noise: Nyquist’s result
- w/ noise (e.g., thermal noise, background radiat
- Shannon’s channel capacity theorem: the maximum number of bits that can be transmitted per second by a physical channel is:
)
1 ( log2 N S W
W: frequency range S/N: signal noise ratio
Frequencies for
communication twisted coax cable optical transmission pair1 Mm 10 km 100 m 1 m 10 mm 100 1 m 300 30 kHz
30 GHz 300 m Hz
3 THz THz
VHF UHF SHF EHF infrared visible UV light
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)