Digital Modulation Techniques

Digital Modulation Techniques Course

Introduction

In electronics, modulation is the process of varying one or more properties of a high frequency periodic waveform, called the carrier signal, with respect to a modulating signal. This is done in a similar fashion as a musician may modulate a tone (a periodic waveform) from a musical instrument by varying its volume, timing and pitch. The three key parameters of a periodic waveform are its amplitude ("volume"), its phase ("timing") and its frequency ("pitch"), all of which can be modified in accordance with a low frequency signal to obtain the modulated signal.

Order custom essay Digital Modulation Techniques with free plagiarism report

450+ experts on 30 subjects Starting from 3 hours delivery

Get Essay Help

Typically a high-frequency sinusoid waveform is used as carrier signal, but a square wave pulse train may also occur. Aim The aim of digital modulation is to transfer a digital bit stream over an analog passband channel, for example over the public switched telephone network (where a bandpass filter limits the frequency range to between 300 and 3400 Hz), or over a limited radio frequency band. The aim of analog modulation is to transfer an analog baseband (or lowpass) signal, for example an audio signal or TV signal, over an analog passband channel, for example a limited radio frequency band or a cable TV network channel. Analog modulation methods In analog modulation, the modulation is applied continuously in response to the analog information signal.  A low-frequency message signal (top) may be carried by an AM or FM radio wave. Common analog modulation techniques are: Amplitude modulation (AM) (here the amplitude of the carrier signal is varied in accordance to the instantaneous amplitude of the modulating signal) o Double-sideband modulation (DSB)  Double-sideband modulation with carrier (DSB-WC) (used on the AM radio broadcasting band).

Double-sideband Suppressed-Carrier Transmission

Double-sideband suppressed-carrier transmission (DSB-SC) ? Double-sideband reduced carrier transmission (DSB-RC) o Single-sideband modulation (SSB, or SSB-AM). Angle modulation o Frequency modulation (FM) (here the frequency of the carrier signal is varied in accordance to the instantaneous amplitude of the modulating signal) o Phase modulation (PM) (here the phase shift of the carrier signal is varied in accordance to the instantaneous amplitude of the modulating signal) The accompanying figure shows the results of (amplitude-)modulating a signal onto a carrier (both of which are sine waves).

At any point along the y-axis, the amplitude of the modulated signal is equal to the sum of the carrier signal and the modulating signal amplitudes. Simple example of amplitude modulation.

Digital modulation methods

In digital modulation, an analog carrier signal is modulated by a digital bit stream. Digital modulation methods can be considered as digital-to-analog conversion, and the corresponding demodulation or detection as analog-to-digital conversion. The changes in the carrier signal are chosen from a finite number of M alternative symbols (the modulation alphabet).

A simple example: A telephone line is designed for transferring audible sounds, for example tones, and not digital bits (zeros and ones). Computers may however communicate over a telephone line by means of modems, which are representing the digital bits by tones, called symbols. If there are four alternative symbols (corresponding to a musical instrument that can generate four different tones, one at a time), the first symbol may represent the bit sequence 00, the second 01, the third 10 and the fourth 11. If the modem plays a melody consisting of 1000 tones per second, the symbol rate is 1000 symbols/second, or baud.

Since each tone (i. e. , symbol) represents a message consisting of two digital bits in this example, the bit rate is twice the symbol rate, i. e. 2000 bits per second. This is similar to the technique used by dialup modems as opposed to DSL modems. . According to one definition of digital signal, the modulated signal is a digital signal, and according to another definition, the modulation is a form of digital-to-analog conversion. Most textbooks would consider digital modulation schemes as a form of digital transmission, synonymous to data transmission; very few would consider it as analog transmission. Fundamental digital modulation methods The most fundamental digital modulation techniques are based on keying:

• In the case of PSK (phase-shift keying), a finite number of phases are used.
• In the case of FSK (frequency-shift keying), a finite number of frequencies are used.
• In the case of ASK (amplitude-shift keying), a finite number of amplitudes are used.
• In the case of QAM (quadrature amplitude modulation), a finite number of at least two phases, and at least two amplitudes are used.

FSK The two binary states, logic 0 (low) and 1 (high), are each represented by an analogue waveform. Logic 0 is represented by a wave at a specific frequency, and logic 1 is represented by a wave at a different frequency. Below shows the basic representation. With binary FSK, the centre or carrier frequency is shifted by the binary input data. Thus the input and output rates of change are equal and therefore the bit rate and baud rate equal. The frequency of the carrier is changed as a function of the modulating signal (data), which is being transmitted. Amplitude remains unchanged. Two fixed-amplitude carriers are used, one for a binary zero, the other for a binary one. Uses of FSK.

Today FSK Modems are used for short haul data communication over private lines or any dedicated wire pair. These are many used for communication between industrial applications like railroad signalling controls and mobile robotic equipment. The short haul modem offers the following specs; - Speeds of up to 9600 bps - Full-duplex or half duplex operation. - Distance up to 9. 5 miles In the past FSK was used in the Bell 103 and Bell 202. These were the first data modem but due to their low bit rate there not being used any more. The Bell 103 had a data rate of only 300 bauds. This modem was predominant until the early 1980s.

Phase Shift Keying

PSK Phase shift keying (PSK) is a method of transmitting and receiving digital signals in which the phase of a transmitted signal is varied to convey information. The simplest form of PSK has only two phases, 0 and 1. It is therefore a type of ASK with ¦(t) taking the values -1 or 1, and its bandwidth is the same as that of ASK. The digital signal is broken up time wise into individual bits (binary digits). The state of each bit is determined according to the state of the preceding bit. If the phase of the wave does not change, then the signal state stays the same (low or high).

If the phase of the wave changes by 180 degrees, that is, if the phase reverses, then the signal state changes (from low to high or from high to low) If the phase of the wave changes by 180 degrees, that is, if the phase reverses, then the signal state changes (from low to high or from high to low). Because there are two possible wave phases, this form of PSK is sometimes called bi-phase modulation. If two or more of the same logic level are received in secession the frequency will remain the same until the logic level changes. Variants of PSK. -Binary Phase Shift Keying (BPSK): – Use alternative sine wave phase to encode bits Simple to implement, inefficient use of bandwidth Binary Phase Shift Keying (BPSK) demonstrates better performance than ASK and FSK. PSK can be expanded to a M-array scheme, employing multiple phases and amplitudes as different states. Phase Shift Keying - PSK Phase shift keying (PSK) is a method of transmitting and receiving digital signals in which the phase of a transmitted signal is varied to convey information. The simplest form of PSK has only two phases, 0 and 1. It is therefore a type of ASK with  taking the values -1 or 1, and its bandwidth is the same as that of ASK.

The digital signal is broken up time wise into individual bits (binary digits). The state of each bit is determined according to the state of the preceding bit. If the phase of the wave does not change, then the signal state stays the same (low or high). If the phase of the wave changes by 180 degrees, that is, if the phase reverses, then the signal state changes (from low to high or from high to low) If the phase of the wave changes by 180 degrees, that is, if the phase reverses, then the signal state changes (from low to high or from high to low).

Because there are two possible wave phases, this form of PSK is sometimes called bi-phase modulation. If two or more of the same logic level are received in secession the frequency will remain the same until the logic level changes. Variants of PSK. a)Binary Phase Shift Keying (BPSK): – Use alternative sine wave phase to encode bits – Simple to implement, inefficient use of bandwidth. Binary Phase Shift Keying (BPSK) demonstrates better performance than ASK and FSK. PSK can be expanded to a M-array scheme, employing multiple phases and amplitudes as different states.

Filtering can be employed to avoid spectral spreading. b)Quadrature Phase Shift Keying (QPSK): – Multilevel modulation technique: 2 bits per symbol – More spectrally efficient, more complex receiver Quadrature Phase Shift Keying is effectively two independent BPSK systems (I and Q), and therefore exhibits the same performance but twice the bandwidth efficiency. Output waveform is sum of modulated; ± Cosine and ± Sine wave. Variants of QPSK - Conventional QPSK has transitions through zero (i. e.. 180o phase transition). Highly linear amplifier required. In Offset QPSK, the transitions on the I and Q channels are staggered. Phase transitions are therefore limited to 90o - ? /4-QPSK the set of constellation points are toggled each symbol, so transitions through zero cannot occur. This scheme produces the lowest envelope variations.

Uses of PSK Binary Phase Shift Keying (BPSK) - BPSK is mainly used in deep space telemetry and also cable modems Quadrature Phase Shift Keying (QPSK) and it Variants Satellites - CDMA, (Code-Division Multiple Access) refers to any of several protocols used in so-called second-generation (2G) and third-generation (3G) wireless communications. TETRA, Terrestrial Trunked Radio) is a set of standards developed by the European Telecommunications Standardization Institute (ETSI) that describes a common mobile radio communications infrastructure throughout Europe This infrastructure is targeted primarily at the mobile radio needs of public safety groups (such as police and fire departments), utility companies, and other enterprises that provide voice and data communications services. LMDS, (Local Multipoint Distribution System) is a system for broadband microwave wireless transmission direct from a local antenna to homes and businesses within a line-of-sight radius, a solution to the so-called last-mile technology problem of economically bringing high-bandwidth services to users. . 3. 3 List of common digital modulation techniques The most common digital modulation techniques are:

• Phase-shift keying (PSK): o Binary PSK (BPSK), using M=2 symbols Quadrature PSK (QPSK), using M=4 symbols o 8PSK, using M=8 symbols o 16PSK, using M=16 symbols o Differential PSK (DPSK) o Differential QPSK (DQPSK) o Offset QPSK (OQPSK) o ? /4–QPSK
• Frequency-shift keying (FSK): o Audio frequency-shift keying (AFSK) o Multi-frequency shift keying (M-ary FSK or MFSK) o Dual-tone multi-frequency (DTMF) o Continuous-phase frequency-shift keying (CPFSK)
• Amplitude-shift keying (ASK)
• On-off keying (OOK), the most common ASK form M-ary vestigial sideband modulation, for example 8VSB
• Quadrature amplitude modulation (QAM) - a combination of PSK and ASK: o Polar modulation like QAM a combination of PSK and ASK. [citation needed]
• Continuous phase modulation (CPM) methods: o Minimum-shift keying (MSK) o Gaussian minimum-shift keying (GMSK)
• Orthogonal frequency-division multiplexing (OFDM) modulation: o discrete multitone (DMT) - including adaptive modulation and bit-loading.
• Wavelet modulation Miscellaneous modulation techniques
• The use of on-off keying to transmit Morse code at radio frequencies is known as continuous wave (CW) operation.
• Adaptive modulation
• Space modulation A method whereby signals are modulated within airspace, such as that used in Instrument landing systems.

Refrences

1. J. R. Barry, E. A. Lee, D. G. Messerschmidt, Digital Communication, Kluwer Academic Publishers, 2004. www. wikipedia. org Communication systems (Simon Haykin), Wiley India

Don't let plagiarism ruin your grade

Run a free check or have your essay done for you

Check my essay Hire Writer