Characteristics Of Wireless Communications Computer Science Essay

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Wireless communications have some special characteristics which includes wireless communications relies on a scarce resource -such as , radio spectrum - the property rights for which were traditionally vested with the state. The next one is use of spectrum for wireless communications required the development of key corresponding technologies; especially those that allowed higher frequencies to be utilised more efficiently. Most of the applications require use of digital modulation techniques in wireless communication for efficient utilization of spectrum . Digital modulation techniques enable to achieve higher data rate and good efficiency. The techniques are Amplitude Shift keying (ASK), Frequency Shift keying (FSK) and Phase Shift Keying (PSK). This report includes the discussion of Phase Shift Keying (PSK) based modulation technique i.e ; BPSK. In BPSK the message signal ( which is of low frequency ) is modulated with the carrier signal of high frequency .the resulting modulated signal contains the information of message signal in terms of phase changes. Phase changes describes the discontinuity in the digital message signal. Further the working of BPSK is demonstrated through the simulation of BPSK technique using Matlab. The simulation results show that how the digital data is transmitted.

Modulation is the process of converting low frequency signal (base band) to high frequency signal for long transmission through wired or wireless medium. This is also called Carrier communication [1].

Types of modulation

There are two main types of modulations i.e. analogue and digital.

Analogue modulation

In analogue modulation the amplitude, frequency and phase of the carrier signal is varied in accordance with the amplitude, frequency and phase of analogue message signal. The resulting analogue modulation techniques are Amplitude Modulation (AM), Frequency Modulation (FM) and Phase Modulation (PM). FM and PM are also called angle modulation [1].

Digital modulation

In Digital modulation the amplitude, frequency and phase of the high frequency carrier signal is varied in accordance with the amplitude, frequency and phase of a digital message signal. The resulting digital modulation techniques are Amplitude Shift Keying (ASK), Frequency Shift Keying (FSK) and Phase Shift Keying (PSK) [1].

Transmitting Digital Data with Analog Signals:

There are three basic techniques:

Amplitude shift keying

Frequency shift keying

Phase shift keying

Amplitude Shift Keying:

One amplitude encodes a 0 while another amplitude encodes a 1 (a form of amplitude modulation).

Frequency Shift Keying:

One frequency encodes a 0 while another frequency encodes a 1 (a form of frequency modulation).

Phase Shift Keying:

One phase change encodes a 0 while another phase change encodes a 1 (a form of phase modulation).

Transmitting Digital Data with Digital Signals: Digital Encoding Schemes

There are numerous techniques available to convert digital data into digital signals.

NRZ-L

NRZI

Manchester

Differential Manchester

Bipolar AMI

4B/5B Digital Encoding Scheme

Non return to Zero Digital Encoding Schemes

Nonreturn to zero-level (NRZ-L) transmits 1s as zero voltages and 0s as positive voltages.

Nonreturn to zero inverted (NRZI) has a voltage change at the beginning of a 1 and no voltage change at the beginning of a 0.

Fundamental difference exists between NRZ-L and NRZI

With NRZ-L, the receiver has to check the voltage level for each bit to determine whether the bit is a 0 or a 1.

With NRZI, the receiver has to check whether there is a change at the beginning of the bit to determine if it is a 0 or a 1.

Manchester Digital Encoding Schemes

Manchester encoding: a transition from low to high in the middle of the interval indicates 1 and a transition from high to low in the middle of the interval if the transmitted bit is 0.

Differential Manchester: a transition in the beginning of the interval to transmit 0. No transition in the beginning of the interval to transmit 1. The transition in the middle is always present.

Bipolar-AMI Encoding Scheme

The bipolar-AMI encoding scheme is unique among all the encoding schemes because it uses three voltage levels

When a device transmits a binary 0, a zero voltage is transmitted

When the device transmits a binary 1, either a positive voltage or a negative voltage is transmitted

Which of these is transmitted depends on the binary 1 value that was last transmitted.

4B/5B Digital Encoding Scheme

4B/5B code -yet another encoding technique that converts four bits of data into five-bit quantities

The five-bit quantities are unique in that no five-bit code has more than 2 consecutive zeroes

The five-bit code is then transmitted using an NRZI encoded signal.

Phase Shift Keying (PSK):

In phase shift keying, the carrier is modulated with a digital baseband signal. The effect of this modulation is represented in the form of phase shifts in the carrier signal.

figure 1.

In the figure.1(a),it is a digital message signal which is to be modulate with a carrier signal of high frequency.figure.1(b) shows the phase modulated signal which has the phase discontinuities due to the sudden changes in the message signal.

Phase discontinuity in = kp * md -------------------- > ( 1 )

Where kp is the modulation factor and md is the amount of discontinuity in m(t).

For example kp = pi/2 and md =1- (-1) =2.so we have phase discontinuity of pi rad in phase modulated signal. Which is also indicated in figure1 (b).[1]

Types of Phase Shift Keying (PSK):

There are six types of PSK schemes: BPSK, QPSK, OQPSK, MSK, GMSK, and QAM. All schemes are the basis of all wireless transmission schemes [2,3].

Binary Phase Shift Keying (BPSK) modulates a digital signal with 1bit encoded per signal element. and uses just two levels '0' or '1'.so it has the lower data rate.

QPSK is used to achieve high data rate with high frequency but it has several problems due to shared bandwidth. To solve these problems three new modulation schemes: OQPSK, MSK, and GMSK are developed. Moreover, to get broadband data transmission in the narrow bandwidth, the QAM technique is used [4].

Binary Phase Shift Keying (BPSK):

Binary phase shift keying is bases of any PSK scheme. So analyzing the BPSK

will give a better understanding of other PSK schemes.

Block Diagram:

Phase Modulator

kp

Carries Generator

c = Cos (2* π*1000000*t)

PM signal(Ç¿pm)

m(t)

Fig.2

Procedure:

The block diagram in fig.2 is of a Phase modulator system.

The inputs to phase modulator block are:

A digital message signal "m(t)".

A carrier signal generated through an oscillator.

The o/p of phase modulator block is given by:

ǿpm = Acos(wc.t+kp.m(t))  (1)

where kp is phase modulated variation factor.

Simulation results:

Carrier signal with fc = 1 MHZ.i.e;

C(t) = cos(2* π* fc*t )

Multiple Kp values.

Fig.3

Fig.4

Fig..5

Analysis:

Fig.3 is digital message signal m(t) of 8kHz. Fig.4 is carrier signal c(t) of 1MHz.The modulation result is shown in fig.5.It is showing the 3 phase changes because m(t) has 3 amplitude discontinuity.

Conclusion:

It is clear from the above simulation results and equ.(1) that with the change of message signal amplitude, the phase of modulated signal varies. Since we are having message signals, digital signals, the amount of phase change depends upon the amount of discontinuity in the message signal.

Amount of discontinuity = kp * md  (2)

Where md = positive peak value of m(t) + negative peak value of m(t) .

Since all the wireless communication modulation techniques depends upon Phase Modulation therefore other PSK based techniques are developed to achieve higher data rates and efficiency in wireless communication. The technique has multiple bits in each signal element. that requires use of multiple level signal elements

Code:

t=0:0.00025/200000:0.00025;

%Binary Phae Shift keying or PM

%message signal 'm(t)' generation of frequency 8kHz

m=[ones(1,50000) (-1+zeros(1,50001)) ones(1,50000) (-1+zeros(1,50000))];

figure,plot(t,m);grid;

title('Digital "m(t)" OF 8kHZ');

xlabel('------ t ------->');ylabel('------ amplitude ------->');

axis([0 0.00025 -2 2])

%Carrier Signal 'c(t)' generation of frequency 1MHz

c=cos(2*pi*1000000*t);

figure,plot(t,c);grid;

title('Carrier signal "c(t)" of 1MHZ');

xlabel('------ t ------->');ylabel('------ amplitude ------->');

axis([0 5/1000000 -1 1])

%effect of different kp

for n=1:8

kp=n*pi/4;

% Phase Modulating the carrier signal.

PM=cos(2*pi*1000000*t+kp*m);

figure,subplot(2,2,1)

plot(t,PM);

title('Phase modulated signal');

xlabel('------ t ------->');ylabel('------ amplitude ------->');

axis([0 150/1000000 -1 1])

subplot(2,2,2)

plot(t,PM);

title('IST phase chage in Phase modulated signal');

xlabel('------ t ------->');ylabel('------ amplitude ------->');

axis([60/1000000 65/1000000 -1 1])

subplot(2,2,3)

plot(t,PM);

title('2nd phase chage in Phase modulated signal');

xlabel('------ t ------->');ylabel('------ amplitude ------->');

axis([122/1000000 128/1000000 -1 1])

subplot(2,2,4)

plot(t,PM);

title('3rd phase chage in Phase modulated signal');

xlabel('------ t ------->');ylabel('------ amplitude ------->');

axis([185/1000000 190/1000000 -1 1])

end

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