The Performance Of DAPSK Accompanied With OFDM Computer Science Essay

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In this project we will be analysing the performance of DAPSK accompanied with OFDM which is very effective technique for combating the multipath and other channel abnormalities without the use of complex channel estimation techniques in different channel environments to analyse the effects of channel conditions on these schemes.OFDM has proven very revolutionary in wireless communications due to its ability for combating the channel imperfections. Some of the advantages are it can easily adapt to severe channel conditions without complex equalization. This is due to fact that it decomposes the channel to several sub carriers and thus the multipath effect on the whole channel which  decomposes to several small channels is least of dangerous. In other words we can say that it reduces the frequency selective channel to flat fading channel and thus ease the process of equalization to great deal. Secondly it is very robust against narrow-band co-channel interference and intersymbol interference (ISI) which are major bottlenecks to the performance of the wireless communication systems. OFDM has high spectral efficiency as compared to conventional modulation schemes, spread spectrum, etc.One of the biggest advantage of OFDM is its very easy implementation using Fast Fourier Transform (FFT) and low sensitivity to time synchronization errors.

We will also compare the performance of DAPSK with that of QAM and DPSK modulation techniques. There will be the evaluation of BER in case of multipath channels which result in ISI(Inter symbol interference).

For evaluation and comparison of performance of DAPSK,QAM and DAPSK MATLAB will be used. There is also a full chapter especially to know about the basics of using this tool.

INTRODUCTION

1. Information source: It can be speech of a person or digital or analog signal from a machine eg.Computer.

2. Source encoder (A/D converter): This transform analog signal into digital signal along with it there is compression of signal too. So information sequence is generated, which will be transmitted after encoding and modulation.

3. Channel encoder

Transformation of information sequence into encoded sequence and adding of redundant information for error recognition and correction ie Code word.

4. Modulator:

In this step signal properties are changed in such a way that the signal is suitable for sending over a transmission medium. Some of the modulations are BPSK,QPSK,QAM etc which will be discussed in later part of this thesis.

5. Channel coder:

Channel coding refers to the class of signal transformations designed to [1]improve communications performance by enabling the transmitted signals to better withstand the effects of various channel impairments, such as noise, interference, and fading. To eliminate noise, interference and fading (or to reduce the channel impairment to a minimum level that allow maximum bandwidth utilization) we implement forward error correction method.

6. Demodulator: It extracts the original information bearing signal from the modulated career wave. It is generally a electronic circuit.

7. Channel decoder: It transforms received sequence into a binary sequence.

8. Source decoder (D/A converter):Transforms the digital signal ie binary sequence into continuous signal ie analog signal.

1.2 Modulation

What is modulation?

The processing of a signal to transpose its characteristics on another signal (the carrier) making it suitable for sending over a transmission medium is called modulation. In[1] the modulation process there are implied three signals: the carrier (a sine wave or a train of pulses), the modulating signal (the baseband signal containing the information to be transmitted) and the modulated signal that is the modulation process result.

Also we can say modulation is a technique used for encoding information into a RF channel. Typically the process of modulation combines an information signal with a carrier signal to create a new composite signal that can be transmitted over a wireless link. In theory a message signal can be directly sent into space to a receiver by simply powering an antenna with the message signal. However, message signals typically don't have a high enough bandwidth to make direct propagation an efficient transmission technique. In order to efficiently transmit data, the lower frequency data must be modulated onto a higher frequency wave. The high frequency wave acts as a carrier that transmits the data through space to the receiver where the composite wave is demodulated and the data is recovered.

Types of modulation:

1. Analog modulation: In this type modulation, the modulation is applied continuously in response to the analog information signal.

2. Digital modulation: In this type of modulation an analog carrier signal is modulated by a digital bit stream.

Types of Analog modultaion techniques:

Signals and Modulation

Here we are representing the by a sinusoid. In general of course, the signal will be a varying value obtained from a sensor, for example speech patterns, or room temperature. Approximating the signal to a sinusoid is acceptable; however, in order to transmit a signal, it is often necessary to superimpose it on a carrier signal of higher frequency. The [2] carrier is said to be modulated by the input signal. An obvious example of this is radio: Music requires a frequency range of around 100Hz to 15kHz, however radio waves are at a much higher frequency. A carrier signal at radio frequency is modulated with the audio signal to allow radio transmission. At the receiver, the carrier signal is removed and so the signal is shifted back down in frequency to the original audio range. The carrier signal may be represented as a sinusoid of the form:

(1)

From this, it can be seen that the carrier may be modulated by changing amplitude (amplitude modulation), frequency (frequency modulation) or phase (phase modulation).

There are mainly three types of analog modulation techniques:

1. Frequency Modulation (FM):For a frequency modulated (FM) system, the frequency of the carrier is changed in proportion to the amplitude[2] of the input signal, as shown below. Although this is more complex to implement than AM, it provides more protection against noise, as it is the amplitude rather than the frequency of the signal that tends to be altered by noise.

Frequency Modulated wave

In this figure you can see the low frequency signal, the high frequency carrier, and the FM modulated result.

2. Amplitude modulation (AM):In an amplitude modulated (AM) system, the amplitude of the carrier is changed in proportion to the amplitude of the input signal, as shown below. This is relatively simple to implement, but is more prone to noise.

In this figure you can see the low frequency signal, the high frequency carrier, and the AM modulated result.

2.Phase Modulation (PM):In an phase modulated (PM) system, the phase of the carrier is changed in proportion to the phase of the input signal, as shown below.

Digital modulations:

Baseband digital signals are suitable for transmission over a pair of wires or coaxial cables due to its sizable power at low frequencies. These[3] signals cannot be transmitted over a radio link because this would require impractically large antennas to efficiently radiate the low-frequency spectrum of the signal. Hence, for such purposes, we use analog modulation techniques in which the digital signal messages are used to modulate a high-frequency continuous-wave (CW) carrier. In binary modulation schemes, the modulation process corresponds to switching (or keying) the amplitude, frequency or phase of the CW carrier between either of two values corresponding to binary symbols "0" or "1". The three types of digital modulation are amplitude-shift keying (ASK), frequency-shift keying (FSK) and phase-shift keying (PSK).

There are mainly three types of digital modulation schemes:

1.Amplitude shift keying(ASK):

Amplitude Shift Keying (ASK) is widely used in wireless telegraphy (for example, Morse code) and in optical communication systems. Binary ASK[5] (ie the transmission of a single bit of data per symbol period) is also known as 'on-off keying', since if a binary signal is used to modulate the carrier, the carrier is effectively turned on and off in sympathy with the baseband data. Practical implementations are relatively straightforward as the carrier can be mixed with the data to produce the required signal.

In ASK, the amplitude of the carrier assumes one of the two amplitudes dependent on the logic states of the input bit stream. This modulated signal can be expressed as:

Demodulation follows the reverse of the process. The [6] RF carrier frequency is mixed with the incoming ASK signal to reproduce the original baseband input. A number of different amplitudes could be used to represent more than one bit of information in each symbol period. For example, if the transmitted amplitude could be any of the values 0, 1/3, 2/3 and 1, two bits of information could be carried in a single transmitted symbol. The generic name for an ASK system capable of transmitting log2m bits of data per symbol period is m-ASK and requires m transmitter power levels.

Note that the modulated signal is still an on-off signal.This type of modulation is very susceptible to noise. It is generally used to transmit digital data over optical fiber.

Types of Amplitude shift keying:

1. BASK (Binary amplitude shift keying)

2. ON/OFF keying

3. QAM (Quadrature amplitude modulation)

2. Phase shift keying (PSK):

Phase Shift Keying (PSK) is a modulation scheme whereby the phase of the transmitted signal is modulated by the data stream. In Binary PSK the phase of the carrier is shifted between to two positions that are 180 degrees apart. Figure 5-3 illustrates a typical BPSK modulator. Note the subtle difference from the ASK[4] modulator that the baseband signal takes the values of +ve and -ve instead of +ve and 0, thereby reversing the phase of the output signal for a 1 and a 0 rather than turning it on or off. In PSK, the phase of the carrier changes between different phases determined by the logic states of the input bit stream. In two-phase shift keying, the carrier assumes one of the two phases. A logic "1" produces no phase change and a logic "0" produces a 1800 phase changes This modulated signal can be expressed as:

As with FSK, PSK has the advantage (for use as a radio bearer) that the output signal has a constant carrier level. Demodulation of the signal can be the same as for ASK, that is that the BPSK signal is mixed with the original RF carrier to reconstruct the original baseband signal.

Types of Phase shift keying:

1. Quadrature PSK (QPSK), using M=4 symbols

2. 8PSK, using M=8 symbols

3. 16PSK, using M=16 symbols

4. Differential PSK (DPSK)

5. Differential QPSK (DQPSK)

6. Offset QPSK (OQPSK)

7. π/4-QPSK

3. Frequency shift keying (FSK):

Frequency Shift Keying (FSK) is a modulation scheme whereby the frequency of the transmitted signal is modulated with the data stream. It is widely used for data transmission over telephone connections in public switched telecommunication networks. Binary FSK is equivalent to applying ASK to two carrier frequencies, one being switched on when the baseband signal is 0 and the other when it is 1. Alternatively, the data signal can be fed to the control port of a voltage to frequency converter (such as a Voltage Controlled Oscillator or VCO). Figure 5-2 demonstrates both of these concepts.

In FSK, the frequency of the carrier is changed to two different frequencies depending on the logic state of the input bit stream. Usually, a logic high causes the centre frequency to increase to a maximum and a logic low causes the centre frequency to decrease to minimum. The modulated signal can be expressed as:

FSK [1] has the advantage (for use as a radio transmission modulation scheme) that it has a constant carrier level, hence the transmitter does not need to be linear and the receiver can limit (ie amplify until clipping is reached) the incoming signal in order to remove signal level fluctuations due to, for example, fading. Demodulation can take place in one of two ways: Two filters can be used to distinguish between the two transmitted frequencies, or a frequency locked loop can be used to convert the transmitted frequency back into two voltage levels (ie perform the inverse function of the VCO). To transmit more than one bit of information in each symbol period, a number of different frequencies could be used. The generic name for an FSK system capable of transmitting log2m bits of data per symbol period is m-FSK.

Types of frequency shift keying(FSK):

1. Audio frequency-shift keying (AFSK)

2. Multi-frequency shift keying (M-ary FSK or MFSK)

3. Dual-tone multi-frequency (DTMF)

4. Continuous-phase frequency-shift keying (CPFSK)

Figure illustrates the above digital modulation schemes for the case in which the data bits are represented by the polar NRZ waveform.

Comparison among the three:

Modulation Type

Resistivitiy to noise

Technical complexity

Amplitude Shift Keying

Less Resistivitiy to noise than other systems

Simple hardware

Frequency Shift Keying

More resistive to noise than ASK and less than PSK

Complex hardware with respect to ASK and simple with respect to PSK

Phase Shift Keying

The most resistive to noise with respect to the other systems

Complex hardware with respect to the other systems

Need of modulation:

1. Modulation is used to separate the different signals in a channel.For eg By using different carrier frequencies, many different radio stations, television stations, and cell phone users can each transmit and receive their signals simultaneously through the same medium.

2. Modulation can be used to minimize the effects of interference.

3. Modulation can also be used to place a signal in a frequency band where design requirements, such as filtering and amplification can be easily met.

4. The minimum length of the antenna required for a practical transmitter or receiver is one-tenth of the signal wavelength, given by , where f is the frequency and c is the speed of light. This means that the antenna needed to transmit a 10 kHz signal (e.g. music) would be 3 km in length.So with the use of modulation techniques we can reduce the size of antenna required.

Literature review:

OFDM

Orthogonal frequency division multiplexing [10] (OFDM) is based on the multicarrier communications technique. The idea of multicarrier communications is to divide the total signal bandwidth into number of subcarriers and information is transmitted on each of the subcarriers.Unlike the conventional multicarrier communication scheme in which spectrum of each subcarrier is non-overlapping and bandpass filtering is used to extract the frequency of interest, in OFDM the frequency spacing between subcarriers is selected such that the subcarriers are mathematically orthogonal to each other. The spectra of subcarriers overlap each other but individual subcarrier can be extracted by baseband processing. This overlapping [10] property makes OFDM more spectral efficient than the conventional multicarrier communication scheme.

QAM

QAM is a method[10] for sending two separate and uniquely different channels of information. The carrier is shifted to create two carriers namely the sine and cosine versions. The outputs of both modulators are algebraically summed and the results of which is a single signal to be transmitted, containing the In phase (I) and Quadrature−phase (Q) information.

The set of possible combinations of amplitudes, as shown on an x−y plot, is a pattern of dots known as a QAM constellation as shown in Figure

In-phase



A

Quadrature-phase

I value

Figure 4: IQ constellation diagram

Q value

M-ary QAM symbols:

si(t)= ai cos(2ft+ i) = xicos(2ft)+yisin(2ft) 0<=t<Ts

xi, yi correspond to the co-ordinates of the ith symbol in phasor constellation.QAM signals do not have constant envelope (even in absence of low pass filtering) making them sensitive to any amplitude nonlinearity or distortion. Consequently QAM is not often used in mobile communications.

FIGURE OF OFDM/QAM

For M-QAM the symbols are distributed over 2-D constellation space, while for M-PSK the symbols are distributed over a circle. Consequently, the symbols are more widely spaced within the constellation of M-QAM. This leads to M-QAM having a better PE (less required Eb/N0 for given Pe(bit)) than for M-PSK.

DPSK:

The digital[0] information is transposed, by modulation, in the sequence of phases or in the sequence of phase changes. The data transmission by PSK technique should be synchronous because the phase changes take place at equal intervals (T).

A local carrier, synchronous with the received carrier and having a known fixed reference phase, is necessary in the receiver to establish the phase sequence. Carrier recovery from the received signal itself by a synchronization loop leads to a multiple phase ambiguity, because there are M points on the interval around which the loop may perform the synchronization (M is the number of the distinct values for n).

To avoid [9]such difficulties and the necessity for a known fixed reference phase, the digital information can be coded in the phase changes but not in the absolute phases. This method is known as differential phase shift keying modulation (DPSK). In this way the reference phase in the receiver may have any value but with no sensible change in a symbol interval. Another advantage of DPSK technique is given by a low sensitivity to the slowly variation, compared with the symbol interval T, of the channel parameters.

As mentioned for BPSK and QPSK there is an ambiguity of phase if the constellation is rotated by some effect in the communications channel the signal passes through. This problem can be overcome by using the data to change rather than set the phase.

DIFFERENTIAL ENCODER

For example, in differentially-encoded BPSK a binary '1' may be transmitted by adding 180° to the current phase and a binary '0' by adding 0° to the current phase. In differentially-encoded QPSK, the phase-shifts are 0°, 90°, 180°, -90° corresponding to data '00', '01', '11', '10'. This kind of encoding may be demodulated in the same way as for non-differential PSK but the phase ambiguities can be ignored. Thus, each received symbol is demodulated to one of the points in the constellation and a comparator then computes the difference in phase between this received signal and the preceding one. The difference encodes the data as described above.

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