ofdm-orthogonal frequency division multiplexing

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ABSTRACT This thesis investigates the effectiveness of Orthogonal Frequency Division Multiplexing (OFDM) as a modulation technique for wireless radio applications. The main aim was to assess the suitability of OFDM as a modulation technique for a fixed wireless phone system for rural areas of Australia. However, its suitability for more general wireless applications is also assessed. Most third generation mobile phone systems are proposing to use Code Division Multiple Access (CDMA) as their modulation technique.. It was found that OFDM performs extremely well compared with CDMA, providing a very high tolerance to multipath delay spread, peak power clipping, and channel noise. In addition to this it provides a high spectral efficiency. Orthogonal FDM's (OFDM) spread spectrum technique distributes the data over a large number of carriers that are spaced apart at precise frequencies. This spacing provides the "orthogonality" in this technique, which prevents the demodulators from seeing frequencies other than their own. The benefits of OFDM are high spectral efficiency, resiliency to RF interference, and lower multi-path distortion. This is useful because in a typical terrestrial broadcasting scenario there are multipathchannels (i.e. the transmitted signal arrives at the receiver using various paths of different length). Since multiple versions of the signal interfere with each other (inter symbol interference (ISI)) it becomes very hard to extract the original information. Orthogonal FDM deals with this multipath problem by splitting carriers into smaller sub carriers, and then broadcasting those simultaneously. This reduces multipath distortion and reduces RF interference (a mathematical formula is used to ensure the sub carriers' specific frequencies are "orthogonal," or non-interfering, to each other), allowing for greater throughput. The only main weak point that was found with using OFDM, was that it is very sensitive to frequency, and phase errors between the transmitter and receiver. The main sources of these errors are frequency stability problems; phase noise of the transmitter; and any frequency offset errors between the transmitter and receiver. This problem can be mostly overcome by synchronizing the clocks between the transmitter and receiver, by designing the system appropriately

INDEX Page No. CHAPTER 1 INTRODUCTION

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CHAPTER 2 2.1 WHAT IS OFDM

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2.2 QUALITATIVE DESCRIPTION OF OFDM

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2.3 IMPORTANCE OF ORTHOGANILITY

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2.4 MATHEMATICAL DESCRIPTION OF OFDM

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2.5 IMPLEMENTATION OF OFDM

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2.6 BLOCK DIAGRAM

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CHAPTER 3: ADVANTAGES & DISADVANTAGES CHAPTER 4: FUTURE DEVELOPMENT CHAPTER 5: CONCLUSION REFERENCE

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CHAPTER 1 INTRODUCTION

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INTRODUCTION OFDM stands for Orthogonal Frequency Division Multiplexing and is an up and coming modulation technique for transmitting large amounts of digital data over a radio wave. W-OFDM stands for Wideband OFDM. OFDM is conceptually simple, but the devil is in the details! The implementation relies on very high speed digital signal processing. OFDM is conceptually simple, but the devil is in the details! The implementation relies on very high speed digital signal processing and this has only recently become available at a price that makes OFDM a competitive technology in the marketplace. OK, so what is the simple concept behind OFDM? Take one carrier and modulate it using Quadrature Phase Shift Keying (QPSK) where each symbol encodes 2 bits. Modulation theory tells us that the spectrum of such a modulated signal will have a sin (x)/x shape with nulls spaced by the bit rate. In OFDM, the carriers are spaced at the bit rate, so that the carriers fit in the fit in the nulls of the other carriers. Another view of Orthogonal Another view of Orthogonal is that each carrier has an integer number of sine wave cycles in one bit period The problem with the simple-minded approach is that it takes lots of local oscillators each locked to the others so that the frequencies are the exact multiples that they should be. This is difficult and expensive. DSP to the rescue! Each of the oscillators can be a digital representation of the sine carrier wave that can be modulated in the numerical domain. This can happen simultaneously for all of the carriers. The resulting output of each channel is added and then blocked. Since we have a representation of the signal in the frequency domain but need to modulate an actual carrier in the time domain, we just perform an Inverse Fast Fourier Transform (IFFT) to convert the block of frequency data to a block of time data that modulates the carrier. The receiver acquires the signal, digitizes it, and performs an FFT on it to get back to the frequency domain. From there, it is relatively easy to recover the modulation on each of the carriers.

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CHAPTER 2 SUBJECT DETALING

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2.1 WHAT IS OFDM OFDM (Orthogonal Frequency Division Multiplexing) is a method of using

many carrier waves instead of only one, and using each carrier wave for only part of the message. OFDM is also called multicarriermodulation (MCM) or DiscreteMulti-Tone (DMT). It is important to stress that OFDM is not really a modulation scheme since it does not conflict with other modulation schemes. It is more a coding scheme or a transportscheme. Orthogonal Frequency Division is where the spacing between carriers is equal to the speed (bit rate) of the message. A multiplex was primarily used to allow many users to share a communications medium like a phone trunk between two telephone central offices. In OFDM, it typical to assign all carriers to a single user; hence multiplexing is not used with its generic meaning. Orthogonal frequency division multiplexing is then the concept of typically establishing a communications link using a multitude of carriers each carrying an amount of information identical to the separation between the carriers. 2.2 QUALITATIVE DESCRIPTION OF OFDM Figure 0.4 shows structure of a multicarrier system.

th Figure 0.4: Basic structure of a multicarrier system

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The original data stream of rate R is multiplexed into N parallel data streams of rate

each of the data streams is modulated with a

different frequency and the resulting signals are transmitted together in the same band. Correspondingly the receiver consists of N parallel receiver paths. Due to the prolonged distance in between transmitted symbols the ISI for each sub system reduces to

In the case of DVB-T we have N=8192 leading to an ISI of

Such little ISI can often be tolerated and no extra counter measure such as an equalizer is needed. Alas as far as the complexity of a receiver is concerned a system with 8192 parallel paths still isn't feasible. This asks for a slight modification of the approach which leads us to the concept of OFDM. In OFDM, each carrier is orthogonal to all other carriers. However, this condition is not always maintained in MCM. OFDM is an optimal version of multicarrier transmission. In OFDM, each carrier is orthogonal to all other carriers. However, this condition is not always maintained in MCM. OFDM is an optimal version of multicarrier transmission Schemes.

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Fig. 3 The effect of adopting a multicarrier system. For a given overall data rate, increasing the number of carriers reduces the data rate that each individual carrier must convey, and hence (for a given modulation system) lengthens the symbol period. This means that the intersymbol interference affects a smaller percentage of each symbol as the

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In OFDM, the data is divided among large number of closely spaced carriers. This accounts for the “frequency division multiplex” part of the name. This is not a multiple access technique, since there is no common medium to be shared Instead of transmitting in serial way, data is transferred in a parallel way. Only a small amount of the data is carried on each carrier, and by this lowering of the bit rate per carrier (not the total bit rate), the influence of intersymbol interference is significantly reduced. In principle, many modulation schemes could be used to modulate the data at a low bit rate onto each carrier. Orthogonal Frequency Division Multiplexing: A method for multiplexing signals, which divides the available bandwidth into a series of frequencies known as tones. Modulation on each tone is usually quardature amplitude modulation. As shown in figure

Orthogonal tones do not interfere with each other because the bandwidth of a modulated carrier sinc shape (sinx/x) with nulls spaced by the bit rate. In OFDM, the carriers fit in the nulls of the other carriers. .

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All frequencies fade but the rapid switching, frequency-hopping technique is intended to allow more robust data service. Because of the orthogonal of the signals, establish overlap in frequency without interfering with each other, thus reducing the system bandwidth

Ofdm frequency domaine shown in figure

. SO OFDM can be simply defined as a form of multicarrier modulation where its carrier spacing is carefully selected so that each subcarrier is orthogonal to the other sub carriers

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OFDM can be simply defined as a form of multicarrier modulation where its carrier spacing is carefully selected so that each subcarrier is orthogonal to the other sub carriers

2.3 The importance of orthogonality The “orthogonal” part of the OFDM name indicates that there is a precise mathematical relationship between the frequencies of the carriers in the system. In a normal FDM system, the many carriers are spaced apart in such way that the signals can be received using conventional filters and demodulators. In such receivers, guard bands have to be introduced between the different carriers and the lowering of the spectrum Efficiency. It is possible, however, to arrange the carriers in an OFDM signal so that the sidebands of the individual carriers overlap and the signals can still be received without adjacent carrier interference. In order to do this the carriers must be mathematically orthogonal. The receiver acts as a bank of demodulators, translating each carrier down to DC, the resulting signal then being integrated over symbol period to recover the raw data. If the other carriers all beat down to frequencies which, in thtime domain, have a whole number of cycles in the symbol period (t), then the integration process results in zero contribution from all these carriers. Thus the carriers are linearly independent (i.e. orthogonal) if the carrier spacing is a multiple of 1/t. Mathematically, suppose we have a set of signals y , where y p is the pth element in the set

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2.4 Mathematical description of OFDM After the qualitative description of the system, it is valuable to discuss the mathematical definition of the modulation system. This allows us to see how the signal is generated and how receiver must operate, and transmission channel. As noted above, OFDM transmits a large number of narrowband carriers, closely spaced in the frequency domain. Mathematically, each carrier can be described as a complex wave:

The real signal is the real part of sc(t). Both Ac (t) and sc(t), the amplitude and phase of the carrier, can vary on a symbol by symbol basis. The values of the parameters are constant over the symbol duration period t. OFDM consists of many carriers. Thus the complex signals s (t)) is represented

This is of course a continuous signal. If we consider the waveforms of each component of the signal overcome symbol period, then the variables Ac (t) and fc(t) take on fixed values, which depend on the frequency of that particular carrier, and so can be rewritten:

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If the signal is sampled using a sampling frequency of 1/T, then the resulting signal is represented by:

At this point, we have restricted the time over which we analyse the signal to N samples. It is convenient to sample over the period of one data symbol. Thus we have a relationship=NT If we now simplify eqn. 3, without a loss of generality by letting w0=0, then the signal becomes:

Now Eq. 4 can be compared with the general form of the inverse Fourier transform: In eq. 4, the

is no more than a definition of the signal in the sampled frequency domain, and s (kT) is the time domain representation. Eqns. 4 and 5 are equivalent if:

This is the same condition that was required for orthogonality

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2.5 Implementation of ofdm If ofdm is implemented through multicarrier system Then the receiver and transmitter is as shown in figure

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Above figures shows that generation of a number of carriers using separate local oscillators. This was inefficient and costly (though increased the data rate). . DSP to the rescue 2.6 The use of the FFT in OFDM The main reason that the OFDM technique has taken a long time to become a prominence has been practical. It has been difficult to generate such signal, and even harder to receive and demodulate the demodulators, was somewhat impractical for use in the civil systems. The ability to define the signal in the frequency domain, in software onVLSI processors, and to generate the signal using the inverse Fourier transform is the key to its current popularity. The use of the reverse process in the receiver is essential if cheap and reliable. Although the original proposals were made a long time ago [5], it has taken at the transmitter; the signal is defined in the frequency domain. It is spectrum exists only at discrete frequencies. Each OFDM carrier corresponds to one element of this discrete Fourier spectrum. The amplitudes and phases of the carriers depend on the data to be transmitted. The data transitions are synchronized at the carriers, and can be processed together, symbol by symbol The definition of the (N-point) discrete Fourier transform (DFT) is: Dept. of Electronics & Communication

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and the (N-point) inverse discrete Fourier transform (IDFT):

A natural consequence of this method is that it allows us to generate carriers that are orthogonal. The members of an orthogonal set are linearly independent. Consider a data sequence (d0, d1, d2, … , dN-1), where each dn is a complex number dn=an+jbn. (an, bn=±1 for QPSK, an, bn=±1, ±3 for 16QAM,)

Where fn=n/(NDT), tk=kDt and Dt is an arbitrarily chosen symbol duration of the serial data sequence dn. The real part of the vector D has components

If these

components are applied to a low-pass filter at time intervals Dt, a signal is obtained that closely approximates the frequency division multiplexed signal

The incoming serial data is first converted form serial to parallel and grouped into x bits each to form a complex number. The number determines the signal constellation of the corresponding subcarrier, such as 16 QAM or Dept. of Electronics & Communication

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32QAM. The complex numbers are modulated in the base band by the inverse FFT (IFFT) and converted back to serial data for transmission. A guard interval is inserted between symbols to avoid intersymbol interference (ISI) caused by multipath distortion. The discrete symbols are converted to analog and low-pass filtered for RF up conversion. The receiver performs the inverse process of the transmitter. One-tap equalizer issued to correct channel distortion. The tap-coefficients of the filter are calculated based on the channel information.

Fig 4a shows the spectrum of an OFDM sub channel and Fig. 4b and Fig. 6 present compositeOFDM spectrum. By carefully selecting the carrier spacing, the OFDM signal spectrum can be made flat and the orthogonality among the sub channels can be guaranteed.

If the signal is passed through a time-dispersive channel, by appending a cyclic prefix at the front of every OFDM symbol. The cyclic prefix is a copy of the last part of the OFDM symbol of length equal to or greater than the maximum delay spread of the channel Dept. of Electronics & Communication

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2.7 Coded Orthogonal Frequency Division Multiplexing In practice, some of the carriers are used for channel estimation and there are extra bits added for error detection and correction. Doing this is called Coded Orthogonal Frequency Division Multiplexing (COFDM). Coding is now so common that many people drop the "C", as unnecessary, assuming that coding is used.

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2.8 When the radio signals travel from one location to another, they maybounceoffsurrounding objects (Figure 1), resulting in multiple paths between transmitter and receiver. This is analogous to echoes or reflections causing multiple copies of the message to arrive at the receiver at different times. The combination of all Modulated message signal to be distorted. A simple example is where there are only two paths, the line of sight path and reflected path from the ground. If message is sent at the right speed, then the second (reflected) copy of the Message may arrive exactly one bit time later than the first (direct) copy. The Receiver will then receive two different bits mixed together, thus distorting the Original message bit (Figure 1). Wireless

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communication systems have to be designed to cope with this socalledmultipath distortion. Figure 1

The main idea of using OFDM is to avoid problems caused by multipathreflections by sending the message bits slowly enough so that any delayed copies (reflections) are late by only a small fraction of a bit time. To maintain high bit rate, multiple carriers are used to send many low speed messages at the same time which can be combined at the receiver to make up one high speed message. In this way, we avoid the distortion caused by reflections.

2.8.2

ofdm act as a antinode for inter- symbol interference

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2.9 BLOCK DIAGRAM OF OFDM

3.1 Advantages of OFDM Spectral efficiency {The orthogonal sub channels are spaced 1/T Hz apartandoverlap in frequency) Simple implementation {IFFT/FFT pair ADC/DAC pair) Mitigation of ISI {Cyclic prefix/suffix guard

interval)

3.2 The disadvantages of the OFDM OFDM signal is contaminated by non-linear distortion of transmitter power amplifier, because it is a combined amplitude-frequency modulation (it is necessary to maintain linearity) OFDM is very sensitive to carrier frequency offset caused by the jitter of carrier wave and Doppler effect caused by moving of the mobile terminal. At the receiver, it is very difficult to decide the starting time of the FFT symbolOFDM

stands for Orthogonal Frequency Division Multiplexing and is an up and Dept. of Electronics & Communication

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coming modulation technique for transmitting large amounts of digital data over a radio wave OFDM is currently a very popular choice for future wireless applications, including wireless LANs, cellular and PCS data, and possibly 4G systems. Hopefully, inexpensive products that provide high-speed communications to individuals and appliances around the globe.

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Conclusions

OFDM/COFDM has long been studied and implemented to combat transmission channel impairments. Its applications have been extended from high frequency radio communications to telephone networks, digital audio broadcasting and terrestrial broadcasting of digital television. The advantages of COFDM, especially in the multipath propagation, interference and fading environment, make the technology a promising alternative in digital communications including mobile multimedia.

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6.REFERENCE [1] R. Prasad, “An overview of millimetre waves for future personal wireless communication systems”, Proc. IEEE First symposium. on communications and vehicular technology in the Benelux, K3, Delft, Netherlands, Oct. 27-28. 1993. [2] Ministerie van Verkeer and Waterstaat, Hoofddirectie Telecommunicatie en Post, Frequency allocations in the Netherlands, 2nd edition, Groningen, 1993. [3] R.W. Chang, ”Synthesis of Band-Limited Orthogonal Signals for Multichannel Data Transmission”, Bell Syst. Tech. J., vol.45, pp. 1775-1796, Dec. 1966. [4]B.R. Salzberg, “Performance of an efficient parallel data transmission system”, IEEE Trans. Commun. Technol., vol. COM-15, pp. 805-813, Dec. 1967. [5]S.B. Weinstein and P.M. Ebert, “Data transmission by frequency-division multiplexing using the discrete Fourier transform”, IEEE Trans. Commun. Technol., vol. COM-19, pp. 628-634, Oct. 1971. [6]A.W.M. van den Enden and N.A.M. Verhoeckx, Discrete-time signal processing: an introduction. London: Prentice Hall Int., 1989., ISBN 0-13-216763-8 [7]A.V. Oppenheim and R.W. Schaffer, Discrete -time signal processing, Prentice-Hall International, 1989., ISBN 0-13-216771-9