ask_fsk_msk

An Introduction

PASSBAND DATA TRANSMISSION

Intended Learning Outcome

To explain technically passband data transmission

Discuss and differentiate Amplitude Shift Keying (ASK)

Frequency Shift Keying (FSK) and Minimum Shift Keying in

terms of their:

a. definition

b. generation/transition model

c. detection

d. Advantages and disadvantages

Evaluation of the bandwidth requirement and signal space

diagram of each transmission bandwidth requirement

Definition Passband Data Transmission - A communication that is obtained by

modulating a baseband analog or digital signal into a carrier

Baseband waveform has a spectral magnitude that is nonzero forfrequencies in the vicinity of the origin and negligible elsewhere

Bandpass waveform has a spectral magnitude that is nonzero forfrequencies in some band concentrated about a frequency f=+fc wherefc>>0Hz and fc=carrier frequency

Modulation the process of imparting the source information onto a bandpass signal with a carrier frequency fc by the introduction of amplitude or phase perturbations or both.

modulated signal bandpass signal

modulating signal baseband source signal

Definition

Digital Modulation the transmittal of digitally modulated

analog signals (carriers) between two or more points in a

communication system

Digital Radio Modulation the process of imparting the

source information onto a bandpass signal with a carrier frequency

fc by the introduction of amplitude or phase perturbations or

both.

Different Forms of Digital ModulationAMPLITUDE SHIFT KEYING (ASK)

a digitally modulated signal where in the information signal is digital andthe amplitude of the carrier is varied proportional to the informationsignal

FREQUENCY SHIFT KEYING (FSK)

a digitally modulated signal where in the information signal is digital andthe frequency of the carrier is varied proportional to the informationsignal

Minimum Frequency Shift Keying (MSK)

a special type of continuous phase-frequency shift keying (CPFSK) with h=0.5. A modulation index of 0.5 corresponds to the minimum frequency spacing that allows two FSK signals to be coherently orthogonal, and the name minimum shift keying implies the minimum frequency separation (i.e. bandwidth) that allows orthogonal detection

Parameters for Digital Communication

M-ary Encoding derived form bi-nary where M=2 and it

represents a digit that corresponds to the number of

conditions, level, or combinations possible for a given

number of binary variables

N= log2 M M=number of condition, level, combination possible

N= number of bits needed

Baud - a rate of change; however it refers to the rate of change of a

signal on the transmission medium after encoding and

modulation have occurred

Note: In digital modulation, the baud and the ideal minimum Nyquist

bandwidth have the same value and are equal to the bit rate

divided by the number of bits encoded

Nyquist Formulation for Channel Capacity

-binary digital signal can be propagated through an ideal noiseless

transmission medium at a rate equal to twice the bandwidth of the

medium

- The actual bandwidth depends on several factors including the type

of encoding the type of encoding and modulation used, the type of

filter, system noise and desired error performance

fb = 2B log2M where: fb = channel capacity (bps)

B = Min. Nyquist bandwidth

M=number of discrete signal/voltage

Simplifying:

B= (fb/log2M) = (fb/N)

AMPLITUDE SHIFT KEYING

AMPLITUDE SHIFT KEYINGWaveform:

Baud rate: Baud = fb/N (baud) where: N=1

Baud = fb/1

Bandwidth: B= fb/N (hertz) where: N=1

B= fb/1

Sample Problem

1. Determine the baud and minimum bandwidth necessary to

pass 10kbps signal using amplitude shift keying

2. What can you conclude about the performance of ASK

with respect to the transport of digital information?

FREQUENCY SHIFT KEYING

FREQUENCY SHIFT KEYING

FREQUENCY SHIFT KEYING

Sample Problem

1. Determine (a) peak frequency deviation, (b) minimum

bandwidth, and (c) baud for a binary FSK signal with a

mark frequency of 49kHz, a space frequency of 51khz and

an input bit rate of 2 kbps

1. Using the Bessel table, determine the min. bandwidth for

the same FSK in example 1.

Board work

1. Determine the maximum bit rate for an FSK signal with a

mark frequency of 48khz, space frequency of 52kHz and an

available bandwith of 10 kHz

2. Determine the bandwidth and baud rate for an FSK signal

with a mark frequency of 32 Khz, and a space frequency of

24 khz, and a bit rate of 4kbps.

3. Compare FSK and ASK in terms of their performance

FSK Transmitter

Similar to conventional FM and often s voltage-controlled

oscillator (VCO)

The peak frequency deviation of the carrier is equal to the

difference between the carrier rest frequency and either

the mark or space frequency ( half difference between the

mark and space frequency).

The VCO FSK modulator can be operated in the sweep

mode where the peak frequency deviation is the product

of the binary input voltage and Deviation sensitivity of

the VCO

FSK Transmitter

FSK ReceiverA. Non-coherent FSK Receiver

there is no frequency involved in the demodulation process

Input signal is simultaneously applied to both BPF of the mark and space frequency

These respective filter passes only the mark and space frequency on its respective envelop detector

The envelop detector indicate the total power in each passband

The comparator responds to the largest of the two power

B. Coherent Receiver

The incoming FSK signal is multiplied by a recovered carrier signal that has the exact same frequency and space as the transmitter reference.

C. Phase-Locked Loop

As the input of the PLL shifts between the mark and space frequencies, the dc error voltage at the output of the phase comparator follows the frequency shift

FSK ReceiverA. Non-coherent FSK Receiver B. Coherent Receiver

C. Phase-Locked Loop

Continues Phase Frequency-Shift

Keying Mark and space frequency are

synchronized with the input bit

rate and are separated from the

center frequency by an exact

multiple of one-half bit rate

There is a precise time

relationship

There is a smooth transition of

analog signal between the space

and mark frequency

Has better bit-error performance

but is expensive to implement

MINIMUM SHIFT KEYING (MSK)

Improves the noise performance of receiver in comparison

with coherent detection of BFSK signal

MSK Signals should satisfy two conditions

1. The modulating pulse must be symmetrical about t + Tb/2

and 0 otherwise

2. The modulated carrier has to have a constant envelope

Signal using MSK modulation

the modulating data signal changes the frequency of the signal and there are no phase discontinuities

the frequency difference between the logical one and logical zero states is always equal to half the data rate.

modulation index, and it is always equal to 0.5.

M SK General Equation

S(t) = (2Eb/Tb) * Cos [2f1t + (0)] for symbol 1

= (2Eb/Tb) * Cos [2f2t + (0)] for symbol 0

S(t) is defined in the region 0 t Tb

Eb - transmitted signal energy per bit,

Tb- is the bit duration.

The phase (0)- denoting the value of phase at time 0 , sums up the past history of the modulation process up to time 0

the frequencies

f1 and f2- sent in response to binary symbols 1 and 0 appearing

at the modulation input respectively.

M SK General Equation

S(t) = (2Eb/Tb) * Cos [2fct + (t)]

Where :

(t) - phase of the signal and it is a continuous function of time.

- increases or decreases linearly with time during each bit duration of Tb

(t) = (0) ( h / Tb) * t , 0 t Tb.

The plus sign indicates to sending bit 1 and the minus sign corresponds to sending a 0 bit.

fc = ( f1 + f2 ) / 2

h = Tb ( f1 - f2 )

Where: fc - nominal carrier frequency

h - which is referred to as the deviation ratio

Generating MSK 1st Method : Conventional Method

Filtering the modulating signal using Gaussian filter and apply the signal to

the frequency modulator with modulation index equal to 0.5

2nd Method: Quadrature Modulation

Generating MSK using Gaussian filter and VCO that uses one signal with

phase 90deg difference that maintains modulation index at 0.5

Advantages of MSK

Improved spectral efficiency when compared to other phase

shift keyed modes

Can be amplified by a non-linear amplifier and remain

undistorted

Immune to amplitude variation and more resilient to noise

Assignment

1.Explain the relationship between bits per second and baud

rate for an FSK and MSK.

2. Give at least three practical application for each of FSK, PSK

and MSK

3. Provide the transmitter and receiver block diagram for

CPFSK and give an explanation for each block.

4. Discuss the bandwidth requirement of MSK. Present at least

two problems about the bandwidth consideration of MSK

and provide the proper solution