The communication systems

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Abstract

Introduction

Nowadays communication plays a main role in our life. The main purpose of the communication is to deliver the message signal from the source to the user destination. Communication can be divided in two different types, digital and anloge that consisting of three component, transmitter, channel and receiver. Transmitter modifies transmitted signal to the suitable form to pass over the channel .This modification is obtained by a process which is called modulation. The original signal after passing over the channel is achieved in the receiver by a process which is called demodulation. Modulation process can be organized in to pulse modulation and continues-wave (CW) modulation. In the phase modulation, the carrier includes a periodic sequence of rectangular pulses while in the CW modulation, the carrier is a sinusoidal. CW modulation can itself be of a digital modulation or analog modulation type .In the analog modulation, a baseband signal is transmitter over the analog passband channel such as TV signals or audio signals . this modulation is included to frequency modulation FM, when the frequency of the carrier is changed , amplitude modulation AM and phase modulation , in which the amplitude and angle of the carrier is varied respectively .Whereas, in the digital modulation, a stream bit of digital is transmitted over the analog passband channel . More common digital modulation are FSK, PSK, QAM and ASK. This essay is going to analyse and compare M-ary PSK and M-ary QAM which are altered modulation level of the PSK and QAM to transmit respectively. In M-ary scheme instead of one bit, more than one bit is transmitted in each symbol.

Literature review

Bit rate and bound rate

M-ary signal scheme means, sending "any one of M possible signals S1 (t), S2 (t),...Sm(t) during each signaling interval of duration T." (Haykin, 2001:345).

In the M-ary part we can refer in two mean terms in data communication which are rate bit and bound rate. Rate bit is the number of bit transferred in one second. Bound rate, is in the number of unit signal transmitted in on second which can be included in two or more bits. Bite rate play the significant role in data communication because we needs to know how long a part of information takes to process. A bound which is included two bits called Dibit and a bound included 3 bits called Tribit, etc. as figure 2.1 shows the Dibit system has double bit rate of a bound rate and in Triibit system, bit rate is three time larger in a bound (symbol). (Forouzan, 2004)

Passnaband transmission model

Digital modulation included two different types: basebad data transmission and passband data transmission. In the baseband data transmission, there is no carrier and data transmit directly without modulation Baseband data transmission uses in short distance while passbad transmission data uses in long distance. In Passband data transmission, data can be modulated by three different modulation kinds contained frequency shift keying (FSK), amplitude shift keying (ASK) and phase shift keying (PSK) ( chitode, 2008) . In the PSK, the phase of carrier in the digital data is varied to represent binary 0 or 1 but frequency and amplitude is constant. Moreover the digital modulation are called FSK and ASK, when the frequency and amplitude of the carrier is varied respectively .In addition, chitode (2008) claimed that there are two different types of scheme to detection passband signal which is included Coherent detection and non-coherent detection. Non-coherent compared to coherent detection has high error probability .In the coherent detection, to improve detection or demodulator performance, phase is locked to the carrier wave while in non-coherent detection, there is no need to lock phase to the carrier.

As shown in the figure 2.2, mi is the message source which belongs to an alphabet of M symbols. mi produce one symbol at the rate of T seconds . Therefore we will have m1,m2,m3.........mM.. In the transmitter part, the M-ary out put signal gets to the signal transmission encoder to produce a vector si. Si is made up to N element which is equal to M or less than M. Afterward vector si is presented to the modulator to create a si(t) which is a distinct signal duration T second for every symbol of mi. si(t) is transmitted over the bandpass communication channel which links transmitter to the receiver. The Communication channel presumes to have two features. The linear channel which has enough bandwidth to accommodate signal si(t) and the channel noise which means a channel with white Gaussion noise of zero mean and power spectral density of N0/2 .in the receiver part, received signal x(t) is demodulated by a detector to obtain a approximate vector signal. The approximation of vector signal X is presented to decoder to get the approximation of symbol.( Haykin,2001)

M-ary PSK modulation and M-ary QAM modulation

Phase shift keying modulation PSK

Phase shift keying modulation is a digital to analog modulation. In the phase shift keying modulation, the phase of the carrier is changed to get a suitable signal to transmit over the channel. In this scheme, when phase of carrier is changed, both frequency and amplitude of carrier is constant. Moreover, depending on the number of bits in each symbol M, different M-ary phase shift keying can be generated which is included Binary Phase -Shift keying (BPSK), 4-psk or Qutruture Phase-shift keying (QPSK) and High- ordered or M-ary PSK which.

Binary phase shift keying modulation BPSK

In BPSK, there is one bit n in each symbol M,( M=2). In this scheme, phase carrier is changed to represent binary 0 or 1. In the BPSK, phase of carrier is changed in to two phases, phase .Figure 3.1.1 shows conceptual and constellation diagram (signal-space diagram) of BPSK.

Quadrature phase-shift keying (QPSK)

4-PSK consisting to 2 bits in each symbol, M=4. In this system phase of carrier is changed in to four phases which take one of four spaced values similarly .Each phase represent the Gray-coded set of dibits. Gray-code is the form of binary code but in different method. In this rules, to transfer from one value to another value just one bit is changed. For this reason Grad coding use in error checking performance because when more than one bit is changed means that the value (data) is uncorrected.

In M-ary PSK the binary sequence in the input is divided in two other sequence including odd-number and even-number which represent two component in-phase component and quadrature component, and respectively.

M-ary PSK (High-order) the number of bits n in each symbol M included n=3.n=4.etc. the spectral efficiency can be increase by increasing number of bits in each symbol. On the other hand by increasing M, this scheme spend more energy per to get the same bit rate error in the output of demodulation .For this reason, in high order modulation, when M is equal to 16 or larger than 16, hybrid amplitude and phase shift keying is one of the best performance to achieve less energy to get rate bite error. (Maral and Bousquet, 2009) Moreover , as Haykin (2001) claimed, In a M-ary PSK scheme , quadrature component and in-phse component are interrelated which means that the envelope constrained to stay constant which also cause the message points to have a a circular constellation . On the other hand Haykin(2001) also mentioned that we can get to two independent components , in-phase and quadrature component , if the constraint is removed. Therefore, a new modulation scheme is achieved which is called M-ary quadrature amplitude modulation ( M-ary QAM). QAM is hybrid carrier of amplitude and phase modulation. QAM will be explained in the next section.

Quadrature- amplitude modulation (QAM)

Quadrature- amplitude modulation is the one method of digital -to-analog and linear modulation. It is usually used in ADSL. QAM is combined in to ASK and PSK. There is several numbers of possible variations in QAM system such as 4-QAM, 8-QAM, 16-QAM and etc.This scheme enables to transfer two difference signals in the same channel bandwidth and in the same time. These signals have similar frequency of carrier. For this reason, this scheme uses two copy of frequency of carrier waves but differing in phase by 90 degrees. Consequently with two separate modulators or demodulators, one signal multiplies to the carrier wave called in-phase component and the other signal multiplies to the carrier wave which is named quadrature component. These components are independent. As a result this, system will have two spread channels with independent signal in each channel.

In this figure, are two different messages used two separates product modulator to generated two double sideband-suppressed DSB-SC modulation, v1 (t) and v2 (t) respectively. DSB-SC modulation is the form of the linear modulation and in this form the upper and lower sidebands are transmitted. In Linear modulation, wasteful of power is less than amplitude modulation. Involves a channel bandwidth 2W, where W is the bandwidth of the messages. and are the in- phase component and the quadrature component in S(t) respectively.

In the receiver part are recovered by two separates coherent detection which is the kind of demodulator. In this figure to generate , in put signal multiplies in the carrier wave and then pass from low pass filter to remove high frequency terms of the out put signal of demodulator . To Generate , input signal S(t) are multiplies in the same carrier wave of frequency but differing in phase 90 degree and it also pass from low pass filter to cut-off high frequency term of . (venkata Rao ,2009)

M-ary quadrature- amplitude modulation (M-ary QAM)

M-ary quadrature- amplitude is a type of modulation system which "have been designed to efficiently use available frequency spectrum" (Sasaoka, 2002:89) which is done by altering modulation levels in QAM. This system is used in the coherent system commonly and it is an example of linear modulation. M-ary QAM can be combined considerable number of phases and considerable number of amplitudes .As Forouzan (2004: 125 ) mentioned, "shift phase used by QAM system always larger than the number of amplitude shifts". The diagram of M-ary QAM is divided in to Q-channel (qutureture component) and I-channel (in-phase component). Figure 3.2.1 Shows conceptual and constellation diagram (signal-space diagram) of 8- QAM.

The separation different message signal points in the constellation diagram are called Euclident distance (figure 3.2.1.b). Larger separation distance cause to increase the number of signal points therefore the constellation is denser. On the other word, "the distance between states is smaller thus susceptibility to noise and transmission path distortion is greater". Haykin (2001: 369) believes that "M-ary QAM is two- dimensional generation of M-ary pulse amplitude modulation (PAM)". For this reason M-ary QAM formulation occupies two orthogonal pass band basis functions, (Haykin, 2001:369)

Comparative between M-ary QAM and M-ary PSK

In the QAM the amplitude and phase of the signal are varied while the signal in the PSK has the constant amplitude. By this I mean in PSK, information are encoded in phase only while in QAM information are encoded in both phase and amplitude. Therefore QAM are susceptible to noise in amplitude as well as phase. Moreover in the M-ary PSK steady amplitude in the whole time cause all message points lie in the circumference of the circle. Whereas in the M-ary QAM, changing the amplitude cause the message point signals lie in the inside of circle and also on the signal space diagram which are illustrated in figure 4.1. For this reason noise immunity of the QAM system can be increased (Chitode, 2008).

Moreover, according to parts 3.1 and 3.2, this point cab be caught that M-ary QAM and M-ary PSK have different signal constellation. For M-ary QAM, the constellation of signal is rectangular whereas, for PSK is circle. The minimum channel bandwidth which is needed to transmit M_ary signals PSK and QAM are the same, which is given by, . Therefore efficiency of bandwidth can be defined by where is the bit rate. By increasing M we can boost the efficiency of bandwidth but to be sure that there is no degradation in error performance, the E0/N should be increase. By increasing this value we can decrease bite error rate BER = (Haykin.2001) .Therefore, by comparing figure 4.2 , M-ary QAM has superior error performance compared to the M-ary PSK.

Smith (1999) believes this difference in error performance can be as a result of difference signal constellation in M-ary QAM and M-ary PSK. In the same value of M, distance between message points in the QAM is smaller than distance between message points in the M-ary PSK (figure 4.1) . On the other hand Haykin (2001:420) points out that "the superior of M-ary QAM can be realized only if the channel is free of nonlinearities".

In addition, for high level systems, QAM compared to PSK provide better spectrum efficiency. Consequently QAM scheme is used in mobile communication commonly. because it can decrease congestion of frequency spectrum. However in the satellite using the QAM is not common, because the RF signal suffer from function of the amplitude of M-ary QAM which is non-linarite. (Richharia, 1999)

Conclusion,

According to the analyses and comparison of two types of digital modulation, PSK and QAM, show that Binary modulation provides a good error performance, while nedded bandwidth in this scheme is lack of the efficiency bandwidth. On the other hand by M-ary modulation, the necessary efficiency bandwidth can be provided. In QAM, information are encoded in both phase and amplitude while in PSK information are encoded just in phase .Furthermore in-phase and quadrature component are independent while in the PSK, in-phase and quadrature component are interrelated . Moreover error performance of the M-ary QAM is greater than M-ary PSK which can be as a result of value of E0/N0 and distance between the messages points. The distance between the messages points in the M-ary QAM constellation is smaller than distance between message points in the M-ary PSK constellation.

References,

  • Haykin ,S.,(2001). Communication systems. 4th edition. New York : Johan Wiley & sons.
  • Forouzan,B.A.,(2003). Data communication and networking .3rd edition. New York: McGraw-Hill
  • Sasaoka ,H.,(2000). Mobile Communication. England: IOS press
  • Chitode,J.S.,(2008). Digital Communication. India: technical publications
  • Maral,G.and Bousquet,M.,(2009). Satellite Communications Systems: Systems, Techniques and Technology. 5th Edition. United Kingdom: Johan Wiley & sons Ltd
  • Smith,D.R., (1993). Digital transmission systems. Second Edition. USA: Springer
  • Richaria,M.,(1999). Satellite communication systems: design principles. Second Edition. United stated of American : McGraw-Hill Professional
  • ETT080 Signals and Communication,( 2009).

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