An Najah National University

Telecommunication Engineering Department

comparison of binary shift keying techniques

Prepeared by : Suhad Malayshi

Raghad foqha Ola mashaqi

submitted to Dr. Allam Mousa

What is digital modulation?

The techniques used to modulate digital information so that it can be transmitted via microwave, satellite or down a cable pair are different to that of Analogue

transmission .

Fundamental digital modulation methods:

Binary Amplitude shift keying (on-off keying)advantage: simplicity

•disadvantage: ASK is very susceptible to noise interference–

noise usually (only)affects the amplitude, therefore ASK is the

modulation technique most affected by noise

•application: ASK is used to transmit digital data over optical

fiber

Frequency Shift KeyingFrequency Shift Keying (FSK) is a frequency modulation scheme (conveys information over a carrier wave by varying its instantaneous frequency) in which digital information is transmitted through discrete frequency changes of a carrier wave .

FSK is the most common form of digital modulation in the high frequency radio spectrum, and has important applications in telephone cct.

A modem converts the binary data from a computer to FSK for transmission over telephone lines, cables, or wireless media.

Shifts are usually in the range of 50 to 1000Hz.

demodulation: demodulator must be able to determine which of two possible frequencies is present at a given time

advantage:

FSK is less susceptible to errors than ASK – receiver looks for specific frequency changes over a number of intervals, so voltage (noise) spikes can be ignored

disadvantage:FSK spectrum is 2 x ASK spectrum

application:over voice lines, in high-freq. radio transmission, etc.

BFSK ( Binary Frequency Shift Keying )

General form of BFSK is :

0),2cos(

1),2cos()(

2

1

binarytfA

binarytfAts

Here , in BFSK, the “1” is called the mark frequency and the “0” is called the space frequency.

The optical field of FSK form is :

The total B.W of FSK is given by :2Δf + 2B, where B is the bit rate.

Multiple FSK ( MFSK) :

MFSK is a variation of FSK that uses more than 2 frequencies.

MFSK is a form of M-ary orthogonal modulation. MFSK is classed as an M-ary orthogonal signaling scheme because each of the M tone detection filters at the receiver responds only to its tone and not at all to the others; this independence provides the orthogonality.

M, the size of alphabet, is usually a power of 2 , so that, each symbol represent log2M bits.

The required Eb/N0 ratio (energy per bit to noise power spectral density ratio ) for a given probability of error decreases as M increases.

Eb/N0

It’s useful when comparing the bit error rate (BER) performance of different digital modulation schemes without taking bandwidth into account.

Eb/N0 is closely related to the carrier- to- noise ratio (CNR or C/N), i.e. the signal-to-noise-ratio (SNR) of the received signal, after the receiver filter but before detection:

wherefb is the channel data rate

B is the channel bandwidth

As M approaches infinity the required Eb/N0 ratio decreases asymptotically to the Shannon limit of -1.6 dB.

Audio FSK (AFSK) and it’s applications :

AFSK is a modulation technique by which digital data is represented by changes in the frequency of an audio tone, yielding an encoded signal suitable for transmission via radio or telephone.

Normally, the transmitted audio alternates between two tones: one, the "mark", represents a binary one; the other, the "space", represents a binary zero.

Most early telephone-line modems used audio frequency-shift keying (AFSK) to send and receive data at rates up to about 1200 bits per second.

Some early microcomputers used a specific form of AFSK modulation, the Kansas City Standard(Byte standard), to store data on audio cossets.

AFSK is not always used for high-speed data communications, since it is far less efficient both power and bandwidth than most other modulation modes.

• Phase-shift keying (PSK) is a method of digital communication in which the phase of a transmitted signal is varied to transmit information. There are several methods that can be used to do PSK.

Phase Shift Keying

•The simplest PSK technique is called binary phase-shift keying (BPSK). It uses two opposite signal phases (0 and 180 degrees). The digital signal is broken up timewise into individual bits (binary digits).

BPSK QPSK or( 4-PSK

`

The “PSK” in 8PSK refers to the use of Phased Shift Keying.

Phased Shift Keying : is a form of phase modulation which is accomplished by the use of a discrete number of states. 8PSK refers to PSK with 8 sates. With half that number of states, you will have QPSK. With twice the number .of states as 8PSK, you .will have 16PSK

Differential phase shift keying is a no coherent form of phase shift keying ; which avoids the need for a coherent reference signal at the receiver. No coherent receivers are easy and cheap to build, and hence are widely used in wireless communications. In DPSK system, the input binary sequence is first differentially encoded and then modulated using a BPSK modulator.

DPSK → COHERENT → NON-COHERENT

Coherent systems need carrier phase information at the receiver and they use matched filters (or correlation receiver) to detect and decide what data was sent

while non coherent systems do not need carrier phase information and use methods like square law (push detection or energy detection) to recover the data.

coherent vs noncoherent In terms of performance coherent systems are much

better than non coherent systems. DPSK: non-coherent QPSK & BPSK & MPSK : coherent

In coherent phase shift keying different phase modulation schemes will be covered i.e. binary PSK, quadrature phase shift keying and M-ary PSK

OFF-SET PSK

At 180 phase-shift, the amplitude of the transmitted signal changes very rapidly costing amplitude change .

This signal may be distorted when is passed through the filter or nonlinear amplifier.

To solve the amplitude change problem, we propose the offset QPSK.

Offset QPSK delay the data in quadrature component by T/2 seconds (half of symbol).

In the offset QPSK, the phase of the signal can change by 90 or 0 degree only ,,, while in the QPSK the phase of the signal can change by 180; 90 or 0 degree.

Applications on PSK

The wireless LAN standard

A wireless local area network (WLAN) links two or more devices using some wireless distribution method ,and usually providing a connection through an access point to the wider Internet. This gives users the mobility to move around within a local coverage area and still be connected to the network. Most modern WLANs are based on IEEE 802.11 standards, marketed under the Wi-Fi brand name. WLANs were once called LAWNs (for local area wireless network) by the Department of Defense.

Wireless LANs have become popular in the home due to ease of installation, and in commercial complexes offering wireless access to their customers; often for free

How to choose ??

There are many other types of modulation you could use and which one would work best would depend

on your project

Differences FSK vs PSK less susceptible to interference . easier to generate with modern electronics BFSK requires twice the bandwidth of BPSK power fsk is better compared to psk but it is expensive

ASK Simplicity low implementation costs conserving power.

How to Imagine noise ?

ф1 and ф 2 are vectorsEach diagram represents the additive noise with a dashed line circle around the signal.

FSK vs PSK noise immunity?

you need to define a quantitative metric that has a level playing field between the two, such as measuring the systems' bit-error rate versus the signal to noise ratio ”

Error performance

Eb/N0 (the energy per bit to noise power spectral density ratio

THANK YOU

Questions