An Najah National UniversityTelecommunication Engineering Departmentcomparison of binary shift keying techniquesPrepeared by : Suhad MalayshiRaghad foqhaOla mashaqisubmitted to Dr. Allam Mousa
What is digital modulation?The techniques used to modulate digital information so that it can be transmittedvia microwave, satellite or down a cable pair are different to that of Analoguetransmission .
Fundamental digital modulation methods:Binary Amplitude shift keying (on-off keying)advantage: simplicity•disadvantage: ASK is verysusceptible to noise interference–noise usually (only)affects theamplitude, therefore ASK is themodulation technique mostaffected by noise•application: ASK is used totransmit digital data over opticalfiber
Frequency Shift KeyingFrequency Shift Keying (FSK) is a frequency modulation scheme (conveysinformation over a carrier wave by varying its instantaneous frequency)in which digital information is transmitted through discrete frequencychanges of a carrier wave .
FSK is the most common form of digital modulation in the high frequency radiospectrum, and has important applications in telephone cct. A modem converts the binary data from a computer to FSK for transmissionover 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 ispresent at a given timeadvantage:FSK is less susceptible to errors than ASK – receiver looks for specific frequencychanges over a number of intervals, so voltage (noise) spikes can be ignoreddisadvantage:FSK spectrum is 2 x ASK spectrumapplication:over voice lines, in high-freq. radio transmission, etc.
BFSK ( Binary Frequency Shift Keying )General form of BFSK is :0),2cos(1),2cos()(21binarytfAbinarytfAtsHere , in BFSK, the “1” is called the mark frequency and the “0” is called the spacefrequency.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 orthogonalsignaling scheme because each of the M tone detection filters at the receiver responds onlyto 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 givenprobability of error decreases as M increases.Eb/N0 It’s useful when comparing the bit error rate (BER) performance of different digitalmodulation 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 rateB is the channel bandwidth As M approaches infinity the required Eb/N0 ratio decreases asymptotically tothe 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 changesin the frequency of an audio tone, yielding an encoded signal suitable fortransmission 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) tosend and receive data at rates up to about 1200 bits per second.Some early microcomputers used a specific form of AFSK modulation, theKansas City Standard(Byte standard), to store data on audio cossets.AFSK is not always used for high-speed data communications, since it is far lessefficient both power and bandwidth than most other modulation modes.
• Phase-shift keying (PSK) is a method of digital communication in whichthe phase of a transmitted signal is varied to transmit information. There areseveral methods that can be used to do PSK.Phase Shift Keying•The simplest PSK technique is called binary phase-shift keying (BPSK). It uses twoopposite signal phases (0 and 180 degrees). The digital signal is broken up timewiseinto individual bits (binary digits).
The “PSK” in 8PSK refers to the use of Phased Shift Keying.Phased Shift Keying : is a form of phase modulation which isaccomplished by the use of a discrete number of states. 8PSKrefers to PSK with 8 sates.With half that number of states, you will have QPSK. Withtwice the number of states as 8PSK, you will have 16PSK
Differential phase shift keying is a no coherent formof phase shift keying ; which avoids the need for acoherent reference signal at the receiver.No coherent receivers are easy and cheap to build, andhence are widely used in wireless communications. InDPSK system, the input binary sequence is firstdifferentially encoded and then modulated using aBPSK modulator.
DPSK → COHERENT→ NON-COHERENT Coherent systems need carrier phase information atthe receiver and they use matched filters (orcorrelation receiver) to detect and decide what datawas sent while non coherent systems do not need carrierphase information and use methods like square law(push detection or energy detection) to recover thedata.
coherent vs noncoherent In terms of performance coherent systems are muchbetter than non coherent systems. DPSK: non-coherentQPSK & BPSK & MPSK : coherent In coherent phase shift keying different phasemodulation 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 transmittedsignal changes very rapidly costing amplitude change . This signal may be distorted when is passed throughthe filter or nonlinear amplifier. To solve the amplitude change problem, we proposethe offset QPSK. Offset QPSK delay the data in quadrature componentby T/2 seconds (half of symbol). In the offset QPSK, the phase of the signal can changeby 90 or 0 degree only ,,, while in the QPSK the phaseof the signal can change by 180; 90 or 0 degree.
A wireless local area network (WLAN) links two ormore devices using some wireless distributionmethod ,and usually providing a connection throughan access point to the wider Internet. This gives usersthe mobility to move around within a local coveragearea and still be connected to the network. Mostmodern WLANs are based on IEEE 802.11 standards,marketed under the Wi-Fi brand name. WLANs wereonce called LAWNs (for local area wireless network)by the Department of Defense.
Wireless LANs have become popular in the home dueto ease of installation, and in commercial complexesoffering wireless access to their customers; often forfree
How to choose ??There are many other types of modulation you coulduse and which one would work best would dependon your project
DifferencesFSK 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 pskbut 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 dashedline circle around the signal.
FSK vs PSK noise immunity?you need to define a quantitative metric that has a level playingfield between the two, such as measuring the systems bit-errorrate versus the signal to noise ratio ”