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Chapter 5: Analog Transmission - Coggle Diagram
Chapter 5: Analog Transmission
Digital to Analog
What would we use this for? To send data from one computer to another over a public access phone line.
A modem is needed
Amplitude shift keying (ASK)
Quadrature amplitude modulation (QAM)
Due to PSK limits (what is the limit?) we can combine PSK with ASK to get QAM
Usually more phase changes compared to amplitude, example are: 4-QAM, 8-QAM, and etc.
Same bandwidth as PSK
Questions
Compute the bit rate for a 1000-baud 16-QAM signal.
-Carrier amplitude is changed to represent binary. Carrier frequency remains unchanged. -Prone to Noise
In ASK Bit rate = Baud rate
Binary Amplitude Shift keying (BASK) or On-Off Keying (OOK)
In here B (bandwidth) = (1+d(roll-off factor)) x S (signal rate...Baud rate)
The middle of the bandwidth is at f_c, the carrier frequency
Min bandwidth needed = Baud Rate = S (signal rate)
Implementation of binary ASK:
Questions
We have an available bandwidth of 100 kHz which spans from 200 to 300 kHz. What are the carrier frequency and the bit rate if we modulated our data by using ASK with d = 1?
Multilevel ASK
There are more than two level for modulated signal. Bit Rate > Baud Rate
frequency shift keying (fsk
-Carrier frequency is varied to represent binary 1 or 0; peak amplitude & phase are constant -Better performance than ASK (less prone to noise, EMI) -Bit rate = Baud Rate
Binary fSK (BfSK)
Two carrier frequencies f1 & f2, f1 for data element 0, and f2 for data element 1
Like 2 ASK Spectra centered around f1 & f2
Bandwidth = B(1+d)xS + 2Delta f
Delta f = Absolute difference between 2 carrier frequencies
Questions
We have an available bandwidth of 100 kHz which spans from 200 to 300 kHz. What should be the carrier frequency and the bit rate if we modulated our data by using FSK with d = 1?
Multilevel fSK (MfSK)
Use more than 2 frequencies, like f1,f2,f3 to send a group of two bits
Bit Rate in the example given = 2 * Baud Rate
Phase shift keying (PSK)
Carrier phase is varied b/w binary 1 and 0. Peak amplitude and frequency are constant.
PSK is more reliable than ASK & fSK
We can have 2 >= phase shifts, like 2PSK = Binary PSK, 4PSK = QPSK
Phase 0 degree -> binary 0, Phase 180 degree -> binary 1
Bigger 4 PSK
Bandwidth = Same as ASK = B = (1 + d ) x S
Questions
Find the bandwidth for a signal transmitting at 12 Mbps for QPSK. The value of d = 0.
Important Vocabulary
Bit rate: Number of bits transmitted per seconds
Bit rate = Baud rate x # bits represented by a signal unit
In case of binary systems, each signal unit carries one bit, so baud rate = bit rate
Bit Rate >= Baud Rate
Baud Rate: Number of signal units per second used to represent the bits. (A signal unit may represent one or more bits)
It is more efficient to use ewer signal units to represent the bits (less bandwidth to send)
Carrier Signal: In analog transmission, the sending device produces a high-frequency signal that acts as a basis for the information signal.
Its frequency is called carrier frequency
Questions:
An analog signal carries 4 bits in each signal unit. If 1000 signal units are sent per second, find the baud rate and the bit rate.
The bit rate of a signal is 3000. If each signal unit carries 6 bits, what is the baud rate?
Analog to Analog Modulations
Conversion is the representation of analog information to analog signal.
Why is it needed? Modulation is needed if the medium is bandpass in nature or if only a bandpass channel is available to us.
Modulations to: 1. Efficient transmission | 2. Sharing resources | 3. Suit the medium
Amplitude Modulation
Carrier signal is modulated so that its amplitude varies wit the Changing amplitudes of the modulating signal
Only amplitude changes
Bandwidth = 2B
frequency modulation
Carrier Signal is modulated so that its frequency varies with the changing amplitudes of the modulating signal.
Only frequency changes
Bandwidth = 2(1+Bbeta)B
Beta is a factor depends on modulation technique with a common value of 4
Phase modulation
Phase of the carrier signal is modulated to follow the changing voltage level of the modulating signal.
As amplitude changes, phase changes
PM is similar to AM, in PM age instantaneous change in the carrier frequency is proportional to the derivative of the amplitude o the modulating signal
Bandwidth = 2(1+Bbeta)B
Value of Beta is lower in PM, varies from 1 - 3.
