ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING AND ITS APPLICATIONS TO WIMAX

BYP.G.RAMYA (11691A0480)

ABSTRACT

Orthogonal Frequency Division Multiplexing (OFDM) is a multi-carrier modulation technique which is very much popular in new wireless networks of IEEE standard, digital television, audio broadcasting and 4G mobile communications. The main benefit of OFDM over single-carrier schemes is its ability to cope with severe channel conditions without complex equalization filters. It has improved quality of long-distance communication by eliminating Inter Symbol Interference and improving the signal to noise ratio. This paper gives the over view about OFDM and its applications in Wimax whose IEEE standard 802.16.

CONTENTS:

Chapter Page No

1. INTRODUCTION 1-4

1.1. HISTORY OF OFDM

1.2. BASICS

2 .OFDM-OVERVIEW 5-15

2.1. ARCHITECTURE OF OFDM

2.2. WORKING

2.3. IMPLEMENTATION OF OFDM

3 .WIRELESS TECHNOLOGY 16-19

3.1. WIRELESS TECHNOLOGY THAT USES OFDM

3.2. OFDM BASED MULTIPLE ACCESS

4 .OFDM-FUNDAMEMENTALS 20-21

4.1. OFDM VERSUS FDM

4.2. COMPARING TDMA, CDMA, OFDM

5 .WIMAX PHYSICAL LAYER 22-30

5.1 .WIMAX SYSTEMS

5.2. IEEE 802.16: EVAUTION AND ARCHITECTURE

5.3. WOKING OF WIMAX

5.4. APPLICATIONS OF WIMAX IN OFDM

6. CONCLUSION 31-31

REFERENCES 32-32

1. INTRODUCTION

1.1. HISTORY OF OFDM:OFDM stands for orthogonal frequency division multiplexing. OFDM has

chosen for several current and communications system all over the world. It has recently been gaining interest from telecommunications systems all over the world .OFDM had a long history of existence shown in below table. The first multichannel modulation systems appeared in the 1950s as frequency division multiplexed military systems, such as KINEPLEX, ANDEFT and KATHRYN.

In December 1966, Robert W. Chang outlined first OFDM Scheme. This was a theoretical way to transmit simultaneous data stream through linear band limited channel without Inter Symbol Interference (ISI) and Inter Carrier Interference (ICI).Chang obtained the first US patent on OFDM in 1970. Around the same time, Saltzberg performed an analysis of the performance of the OFDM system and concluded that the strategy should concentrate more on the reducing cross talk between adjacent channels than on perfecting the channels. Until this time, we needed a large number of sub-carrier oscillators to perform parallel modulations and demodulations . This was the main reason why the OFDM technique has taken a long time to become a prominence. It was difficult to generate such a signal, and even harder to receive and demodulate the signal. The hardware solution, which makes use of multiple modulators and demodulators, was some-what impractical for use in the civil systems.

In the year 1971, Weinstein and Ebert used Discrete Fourier Transform (DFT) to perform baseband modulation and demodulation. The use of DFT eliminated the need for bank of subcarrier oscillators. These efforts paved the way for the way for easier, more useful and efficient implementation of the system. The availability of this technique, and the technology that allows it to be implemented on integrated circuits at a reasonable price, has permitted OFDM to be developed as far as it has. The process of transforming from the time domain representation to the frequency domain representation uses the Fourier transform itself, whereas the reverse process uses the inverse Fourier transform. All the proposals until this moment in time used guard spaces in frequency domain and a raised cosine windowing in time domain to combat ISI and ICI. Another milestone for OFDM history was when Peled and Ruiz introduced Cyclic Prefix (CP) or cyclic extension in 1980. This solved the problem of maintaining orthogonal characteristics of the transmitted signals at severe transmission conditions. The generic idea that they placed was to use cyclic extension of OFDM symbols instead of using empty guard spaces in frequency domain. This effectively turns the channel as performing cyclic convolution, which provides orthogonality over dispersive channels when CP is longer than the channel impulse response. It is obvious that introducing CP causes loss of signal energy proportional to length of CP compared to symbol length but, in turn, it facilitates a zero ICI advantage which pays off. By this time, inclusion of FFT and CP in OFDM system and substantial advancements in Digital Signal Processing (DSP) technology made it an important part of telecommunications

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landscape. In the 1990s, OFDM was exploited for wideband data communications over mobile radio FM channels, High-bit- rate Digital Subscriber Lines (HDSL at 1.6 Mbps), Asymmetric Digital Sub- scriber Lines (ADSL up to 6 Mbps) and Very-high-speed Digital Subscriber Lines (VDSL at 100 Mbps). The first commercial use of OFDM technology was made in Digital Audio Broadcasting (DAB).The development of DAB started in 1987 and was standardized in 1994. DAB services started in 1995 in UK and Sweden. The development of Digital Video Broadcasting (DVB) was started in 1993. DVB along with High-Definition Television (HDTV) terrestrial broadcasting standard was published in 1995. At the dawn of the twentieth century, several Wireless Local Area Network (WLAN) standards adopted OFDM on their physical layers. Development of European WLAN standard Hiper LAN started in 1995.Hiper LAN/2 was defined in June 1999 which adopts OFDM in physical layer. OFDM technology is also well positioned to meet future needs for mobile packet data traffics. It is emerging as a popular solution for wireless LAN, and also for fixed broad-band access. OFDM has successfully replaced DSSS for 802.11a and 802.11g. Perhaps of even greater importance is the emergence of this technology as a competitor for future fourth Generations (4G) wireless systems. These systems, expected to emerge by the year 2010, promise to at last deliver on the wireless ‘Nirvana’ of anywhere, anytime, anything communications. It is expected that OFDM will become the chosen technology in most wireless links world wide and it will certainly be implemented in 4G radio mobile systems.

TABLE.1: A BRIEF HISTORY OF OFDM:

DATES OFDM LANDMARK ACHIEVED1996 Chang postulated the principle of transmitting messages simultaneously

through a linear band limited channel without ICI and ISI. This is considered as the first multicarrier modulation. Earlier work on the OFDM was Holsinger’s 1964 MIT dissertation.

1967 Saltzberg observed that, in order to increase efficiency of parallel system, cross talk between adjacent channels should be reduced

1971 Weinstein and Ebert show that multicarrier modulation can be accomplished by using a DFT

1980 Peled and Ruiz introduced use cyclic prefix1985 Cimini at Bell Labs identifies many of the key issues in OFDM transmission

and does a proof-of-concept design

1990-1995

OFDM was exploited for wideband data communications over mobile radio FM radio, DSL, HDSL, ADSI and VDSL. First commercial use of OFDM in DAB and DVB.

1999

20022003

2004

The IEEE 802.11 used OFDM at the physical layer. Hiper LAN and Hiper LAN/2 also adopted OFDM at the physical layer.The IEEE 802.16 committee released WMAN standard 802.16 based on OFDM.The IEEE 802.11 committee releases the 802.11g standard for operation in the 2.4GHZ band.

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200520072008

OFDM is candidate for IEEE 802.15.3a standard for wireless PAN.OFDMA is candidate for the 3GPP long term Evolution.The first complete LTE air interface implementation was demonstratedMobile WIMAX base stations and subscriber devices were first certified by WIMAX Forum.

1.2. BASICS:

Orthogonal Frequency-Division Multiplexing Basics:

The best way to learn about OFDM basic facts is to look at a good OFDM video tutorial (Explore Gate has the best ones). However, in the interim, this short article will teach you some information about basic OFDM and how it is used. OFDM is an acronym that stands for Orthogonal Frequency-Division Multiplexing - it is a way to transfer data at high speeds using broadband multicarrier modulation methods.

The rate at which data is transferred has a direct effect on how good the quality of the transmission. The higher the speed, the more likely there will be line noise and interference, and the data may become corrupted. There will usually be much less interference when data is sent at a slow rate. A multicarrier modulation method, as used by OFDM, allows for many small pieces of data to be sent and then later combined to arive as a single unit.

With OFDM the data is split into up to 52 sub streams, and later made into a single mutiplexed stream. This allows for the data to be sent more slowly, and therefore line noise and interference is decreased; although the receiver will receive far more data in the same time period than using other data delivery methods.

What may surprise you, is this is not a recent concept. Research into the possibility of this type of data transfer was known more than 50 years ago. The method was originally referred to as multicarrier modulation, but needless to say, the technique would have been extremely hard if not impossible to implement as the hardware for such technology was simply not available at the time. The computer technology we have today finally makes this type of data transfer possible.

In the last few years, interest in OFDM has greatly increased along with the demand for new wireless technology and high speed data transfers. Until now, OFDM is probably the most efficient technology we have for transferring data, because it eliminates the problem of multipath broadcasting that previously resulted in huge data errors and loss of signal.

The main advantage of using OFDM is the increase in bandwidth efficiency it creates. What this actually means is that it is possible to transmit more data faster in a given bandwidth without the presence of noise. The measure of bandwidth efficiency available is measured at bps/Hz or bits per second per Hertz.

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Wireless high-frequency data transfer usually has multipath issues, but these are eliminated with OFDM. In theory, short-wavelength signals should be easily received over short distances, as long as the receiving and transmitting antennas are within line of sight. However, in reality buildings, hills, trees and even people, effect the transmission by reflecting parts of the signal. Unfortunately, this means a copy of the original signal is also sent and received by the antenna. This often results in data cancellations and other issues. OFDM divides the data into small pieces, and transmits it at low rates. As a result there is far less line noise and very few problems with large data transmission.

Fig 1: signal process of OFDM

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2. OFDM—OVERVIEW

2.1 .ARCHITECTURE OF OFDM:

Practically, OFDM modulation for standard IEEE 802.20 is used by both the forward and reverse links. IEEE 802.20, also referred to as Mobile-wifi, is optimized for IP and roaming in high-speed mobile environments. This standard is ready to fully mobilize IP, opening up major new data markets beyond the more circuit-centric 2.5G and 3G cellular standards. Its main operation is to develop the specification for an efficient packet-based air interface optimized for the transport of IP-based services.

For IEEE 802.20, transmission on the forward link is divided into super frames, where each super frame consists of a preamble followed by a sequence of 25 Forward Link Physical Layer (NFLPHY) frames [4]. Transmission on the reverse link is also divided into units of super frames, with each super frame consisting of a sequence of 25 reverse link PHY frames. In order to support cell sizes of macro, micro, and pico IEEE 802.20 should operate in a traditional cellular environment. To increase the availability of coverage area, increase throughput available to the users, and enable a higher overall spectral efficiency, advanced antenna technologies such as multi antenna at the base station should be employed. To avoid ISI, a guard interval of length Tg is inserted before OFDM block. During this interval, a cyclic prefix is transmitted. The signal with cyclic prefix is thus given as, The Fast Fourier Transform (FFT) is a computationally efficient implementation of DFT. Inverse Fast Fourier Transform (IFFT) and FFT are main modulation and demodulation techniques used in OFDM.

2.2. WORKING OF OFDM:

OFDM is based on the concept of frequency-division multiplexing (FDD), the method of transmitting multiple data streams over a common broadband medium. That medium could be radio spectrum, coaxial cable, twisted pair, or fiber-optic cable. Each data stream is modulated onto multiple adjacent carriers within the bandwidth of the medium, and all are transmitted simultaneously. A good example of such a system is cable TV, which transmits many parallel channels of video and audio over a single fiber-optic cable and coaxial cable.

OFDM is a form of multicarrier modulation. An OFDM signal consists of a number of closely spaced modulated carriers. When modulation of any form - voice, data, etc. is applied to a carrier, then sidebands spread out either side. It is necessary for a receiver to be able to receive the whole signal to be able to successfully demodulate the data. As a result when signals are transmitted close to one another they must be spaced so that the receiver can separate them using a filter and there must be a guard band between them. This is not the case with OFDM. Although the sidebands from each carrier overlap, they can still be received without the interference that might be expected because they are orthogonal to each another. This is achieved by having the carrier spacing equal to the symbol period.

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fig2: Traditional view of receiving signals carrying modulation

To see how OFDM works, it is necessary to look at the receiver. This acts as a bank of demodulators, translating each carrier down to DC. The resulting signal is integrated over the symbol period to regenerate the data from that carrier. The same demodulator also demodulates the other carriers. As the carrier spacing equal to the reciprocal of the symbol period means that they will have a whole number of cycles in the symbol period and their contribution will sum to zero - in other words there is no interference contribution.

fig3: OFDM Spectrum

One requirement of the OFDM transmitting and receiving systems is that they must be linear. Any non-linearity will cause interference between the carriers as a result of inter-modulation distortion. This will introduce unwanted signals that would cause interference and impair the orthogonality of the transmission.

In terms of the equipment to be used the high peak to average ratio of multi-carrier systems such as OFDM requires the RF final amplifier on the output of the transmitter to be able to handle the peaks whilst the average power is much lower and this leads to inefficiency. In some systems the peaks are limited. Although this introduces distortion that results in a higher level of data errors, the system can rely on the error correction to remove them.

OFDM WORKING NOW-A-DAYS:

The FDD technique is typically wasteful of bandwidth or spectrum because to keep the parallel modulated carriers from interfering with one another, you have to space them with some guard bands or extra space between them. Even then, very selective filters at the receiving end have to be able to separate the signals from one another. What researchers discovered is that with digital transmissions, the carriers could be more

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closely spaced to one another and still separate. That meant less spectrum and bandwidth waste.

Fig4: How does OFDM works.

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WHY WE ARE USING OFDM METHOD?

High spectral efficiency –provides more data services.

Resiliency to RF interference –good performance in unregulated and regulated frequency bands.

Lower multi-path distortion –works in complex indoor environments as well as at speed in vehicles.

Fig5:why we need OFDM.

OFDM is accomplished with digital signal processing (DSP). You can program the IFFT and FFT math functions on any fast PC, but it is usually done with a DSP IC or an appropriately programmed FPGA or some hardwired digital logic. With today’s super-fast chips, even complex math routines like FFT are relatively easy to implement. In brief, you can put it all on a single chip.

ORTHOGONALITY IN TIME DOMAIN:

In time domain the OFDM works under the condition of orthogonality which is explained the below figure. In this all the carrier signals are orthogonal to each other meaning that cross-talk. Between the sub-channels is eliminated and inter carrier guard bands that simplify the design for both the transmitter and the receiver.

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The orthogonality requires that the sub-carrier spacing is

∆f =K/Tu HZ

Fig 6: orthogonality in time domain

2.3. IMPLEMENTATION PROCESS OF OFDM:

The principle of OFDM was already around in the 50s’ and 60s’ as an efficient MCM technique. But, the system implementation was delayed due to technological difficulties like digital implementation of FFT/IFFT, which were not possible to solve on that time. In 1965, Cooley and Tukey presented the algorithm for FFT calculation [22] and later its efficient implementation on chip makes the OFDM into application.

The digital implementation of OFDM system is achieved through the mathematical operations called Discrete Fourier Transform (DFT) and its counterpart Inverse Discrete Fourier Transform (IDFT). These two operations are extensively used for transforming data between the time-domain and frequency-domain. In case of OFDM, these transforms can be seen as mapping data onto orthogonal subcarriers. In order to perform frequency-domain data into time domain-data, IDFT correlates the frequency domain input data with its orthogonal basis functions, which are sinusoids at certain frequencies. In other ways, this correlation is equivalent to mapping the input data onto the sinusoidal basis functions. In practice, OFDM systems employ combination of fast fourier transform (FFT) and Inverse fast fourier transform (IFFT) blocks which are mathematical equivalent version of the DFT and IDFT.

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At the transmitter side, an OFDM system treats the source symbols as though they are in the frequency-domain. These symbols are feed to an IFFT block which brings the signal into the time-domain. If the N numbers of subcarriers are chosen for the system, the basis functions for the IFFT are N orthogonal sinusoids of distinct frequency and IFFT receive N symbols at a time. Each of N complex valued input symbols determines both the amplitude and phase of the sinusoid for that subcarrier. The output of the IFFT is the summation of all N sinusoids and makes up a single OFDM symbol. The length of the OFDM symbol is NT where T is the IFFT input symbol period. In this way, IFFT block provides a simple way to modulate data onto N orthogonal subcarriers.

Fig 7: OFDM transmitter and receiver

Serial to parallel converter:

Data to be transmitted is typically in the form of a serial data stream. Serial to parallel conversion block is needed to convert the input serial bit stream to the data to be transmitted in each OFDM symbol. The data allocated to each symbol depends on the modulation scheme used and the number of subcarriers.

For example, in case a subcarrier modulation of 16-QAM each subcarrier carries 4 bits of data, and so for a transmission using 100 subcarriers the number of bits per symbol would be 400. During symbol mapping the input data is converted into complex value constellation points, according to a given constellation. Typical constellations for wireless applications are, BPSK, QAM, and 16 QAM. The amount of data transmitted on each subcarrier depends on the constellation. Channel condition is the deciding factor for the type of constellation to be used. In a channel with high interference a small constellation like BPSK is favourable as the required signal-to-noise-ratio (SNR) in the receiver is low. For interference free channel a larger constellation is more beneficial due to the higher bit rate. Known pilot symbols mapped with known mapping schemes can be inserted at this moment. Cyclic prefix is inserted in every block of data according

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to the system specification and the data is multiplexed to a serial fashion. At this point of time, the data is OFDM modulated and ready to be transmitted. A Digital-to-Analogue Converter (DAC) is used to transform the time domain digital data to time domain analogue data. RF modulation is performed and the signal is up-converted to transmission frequency. After the transmission of OFDM signal from the transmitter antenna, the signals go through all the anomaly and hostility of wireless channel. After the receiving the signal, the receiver down- converts the signal; and converts to digital domain using Analogue-to-Digital Converter (ADC). At the time of down-conversion of received signal, carrier frequency synchronization is performed. After ADC conversion, symbol timing synchronization is achieved. An FFT block is used to demodulate the OFDM signal. After that, channel estimation is performed using the demodulated pilots. Using the estimations, the complex received data is obtained which are de-mapped according to the transmission constellation diagram. At this moment, FEC decoding and de- interleaving are used to recover the originally transmitted bit stream. OFDM is tolerant to multi path interference. A high peak data rate can be achieved by using higher order modulations, such as 16 QAM and 64 QAM, which improve the spectral efficiency of the system.

Fig 8: OFDM serial to parallel converter circuit

OFDM TRANSMISSION:

The OFDM based communication systems transmit multiple data symbols simulta- neously using orthogonal subcarriers .The principle behind the OFDM system is to decompose the high data stream of bandwidth W into N lower rate data streams and then to transmit them simultaneously over a large number of subcarriers. Value of N is kept sufficiently high to make the individual bandwidth (W/N) of subcarriers narrower than the coherence bandwidth (Bc) of the channel. The flat fading experienced by the

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individual subcarriers is compensated using single tap equalizers. These subcarriers are orthogonal to each other which allows for the overlapping of the subcarriers. The orthogonality ensures the separation of subcarriers at the receiver side. As compared to FDMA systems, which do not allow spectral over- lapping of carriers, OFDM systems are more spectrally efficient. OFDM transmitter and receiver systems are described in Figs. 2.5 and 2.6. At the transmitter, the signal is defined in the frequency domain. Forward Error Control/Correction (FEC) coding and interleaving block is used to obtain the robustness needed to protect against burst errors. The modulator transforms the encoded blocks of bits into a vector of complex values, Fig. 2.7. Group of bits are mapped onto a modulation constellation producing a complex value and representing a modulated carrier. The amplitudes and phases of the carriers depend on the data to be transmitted. The data transitions are synchronized at the carriers, and may be processed together, symbol by symbol.

Fig 9: OFDM transmitter

Fig 10: OFDM receiver

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Fig11: OFDM modulator.

As the OFDM carriers are spread over a frequency range, chances are there that some frequency selective attenuation occurs on a time varying basis. A deep fade on a particular frequency may cause the loss of data on that frequency for that given time, thus some of the subcarriers can be strongly attenuated and that will cause burst errors. In these situations, FEC in COFDM can fix the errors. An OFDM system with addition of channel coding and interleaving is referred to as Coded OFDM (COFDM). An efficient FEC coding in flat fading situations leads to a very high coding gain. In a single carrier modulation, if such a deep fade occurs, too many consecutive symbols may be lost and FEC may not be too effective in recovering the lost data. In a digital domain, binary input data is collected and FEC coded with schemes such as convolutional codes. The coded bit stream is interleaved to obtain diversity gain. Afterwards, a group of channel coded bits are gathered together (1forBPSK, 2 for QPSK, 4 for QPSK, etc.) and mapped to the corresponding constellation points.

OFDM SYSTEM CONSIDERATIONS:OFDM system design issues aim to decrease the data rate at the subcarriers, hence, the symbol duration increases and as a result, the multipath effects are reduced effectively. The insertion of higher valued CP will bring good results against combating multipath effects but at the same time it will increase loss of energy. Thus, a trade off between these two parameters must be done to obtain a reasonable system design.

System design requirements :

OFDM system depends on the following four requirements:

Available bandwidth: The bandwidth limit will play a significant role in the selection of number of sub-carriers. Large amount of bandwidth will allow obtaining a large number of subcarriers with reasonable CP length.

Required bit rate: The system should be able to provide the data rate required for the specific purpose.

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Tolerable delay spread: An user environment specific maximum tolerable delay spread should be known beforehand in determining the CP length.

Doppler values: The effect of Doppler shift due to user movement should be taken into account.

SYSTEM DESIGN PARAMETERS:

The design parameters are derived according to the system requirements. The design parameters for an OFDM system are as follows

Number of subcarriers: We stated earlier that the selection of large number of subcarriers will help to combat multipath effects. But, at the same time, this will increase the synchronization complexity at the receiver side.

Symbol duration and CP length: A perfect choice of ratio between the CP length and symbol duration should be selected, so that multipath effects are combated and not significant amount bandwidth is lost due to CP.

Subcarrier spacing: Subcarrier spacing will depend on available bandwidth and number of subcarriers used. But, this must be chosen at a level so that synchronization is achievable.

Modulation type per subcarrier: The performance requirement will decide the selection of modulation scheme. Adaptive modulation can be used to support the performance requirements in changing environment.

FEC coding: A suitable selection of FEC coding will make sure the robustness channel to the random errors.

OFDM SYMBOL PARAMETER:There are two types of OFDM parameters (primitive and derived) that

characterize OFDM symbol completely. The later one can be derived from the former one because fixed relation between them. In our MATLAB implementation of the physical layer, the primitive parameters are specified as ´OFDM params´ and primitive

parameters are calculated as ´IEEE802.16 paparams´ which can be accessed globally.

The used OFDM parameters are listed in below table:

Table no:2 :Table showing the parameters required:

PARAMETERS: VALUE:Nominal Channel Bandwidth, BW 1.75MHzNumber of used subcarrier, N used 200Sampling Factor 8/7Ratio of Guard time to useful symbol time, G

1/4, 1/8, 1/16, 1/32

N fft(smallest power of 2 greater than N 255

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usedSampling frequency, Fs Floor(n.BW/8000) X 8000Subcarrier spacing, ∆f Fs/N fftUseful symbol time, Tb 1/∆fCP time, Tg G.TbOFDM symbol time, Ts Tb+TgSampling time Tb/N fft

The Orthogonal frequency division multiplexing in FDM can be implemented with the help of the following figure:

fig 12 : figure showing OFDM signal

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3. WIRELESS TECHNOLOGY:

3.1. WIRELESS TECHNOLOGY THAT USES OFDM:

The list is long and impressive. First, it is used for digital radio broadcasting specifically Europe’s DAB and Digital Radio Mon dial. It is used in the U.S.’s HD Radio. It is used in TV broadcasting like Europe’s DVB-T and DVB-H. You will also find it in wireless local-area networks (LANs) like Wi-Fi. The IEEE 802.11a/g/n standards are based on OFDM. The wideband wireless metro-area network (MAN) technology WiMAX uses OFDM. And, the almost completed 4G cellular technology standard Long-Term Evolution (LTE) uses OFDM. The high-speed short-range technology known as Ultra-Wideband (UWB) uses an OFDM standard set by the WiMedia Alliance. OFDM is also used in wired communications like power-line networking technology. One of the first successful and most widespread uses of OFDM was in data modems connected to telephone lines. ADSL and VDSL used for Internet access use a form of OFDM known as discrete multi-tone (DMT). And, there are other less well known examples in the military and satellite worlds.

The serial digital data stream to be transmitted is split into multiple slower data streams, and each is modulated onto a separate carrier in the allotted spectrum. These carriers are called subcarriers or tones. The modulation can be any form of modulation used with digital data, but the most common are binary phase-shift keying (BPSK), quadrature phase-shift keying (QPSK), and quadrature amplitude modulation (QAM). The outputs of all the modulators are linearly summed, and the result is the signal to be transmitted. It could be up converted and amplified if needed.

The FFT is a variation of the discrete Fourier transform (DFT). Fourier, as you may remember from your college math days, was the Frenchman who discovered that any complex signal could be represented by a series of harmonically related sine waves all added together. He also developed the math to prove it. The math is difficult, and even early computers couldn’t perform it quickly. Cooley/Turkey developed the fast Fourier transform in the 1960s as a way to greatly speed up the math to make Fourier analysis more practical. In general, you can take any analog signal, digitize it in an analog-to-digital converter (ADC), and then take the resulting samples and put them through the FFT process. The result is essentially a digital version of a spectrum analysis of the signal. The FFT sorts all the signal components out into the individual sine-wave elements of specific frequencies and amplitudes—a mathematical spectrum analyzer of a sort. That makes the FFT a good way to separate out all the carriers of an OFDM signal.

The IFFT just reverses the FFT process. All the individual carriers with modulation are in digital form and then subjected to an IFFT mathematical process, creating a single composite signal that can be transmitted. The FFT at the receiver sorts all the signals to recreate the original data stream.

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This is where the term “orthogonal” comes in. Orthogonal really means at a right angle to. The signals are created so they are orthogonal to one another, thereby producing little or no interference to one another despite the close spacing. In more practical terms, it means that if you space the subcarriers from one another by any amount equal to the reciprocal of the symbol period of the data signals, the resulting sine (sin x/x) frequency response curve of the signals is such that the first nulls occur at the subcarrier frequencies on the adjacent channels. Orthogonal subcarriers all have an integer number of cycles within the symbol period. With this arrangement, the modulation on one channel won’t produce inter symbol interference (ISI) in the adjacent channels.

OFDM implemented in real world:

OFDM is accomplished with digital signal processing (DSP). You can program the IFFT and FFT math functions on any fast PC, but it is usually done with a DSP IC or an appropriately programmed FPGA or some hardwired digital logic. With today’s super-fast chips, even complex math routines like FFT are relatively easy to implement. In brief, you can put it all on a single chip.

Fig13: one of the IP-Based wimax network architecture

3.2. OFDM BASED MUTIPLE ACCESS:

It is a multi-user version of the popular orthogonal frequency-division multiplexing (OFDM) digital modulation scheme. Multiple access is achieved in OFDMA by assigning subsets of subcarriers to individual users as shown in the illustration below. This allows simultaneous low data rate transmission from several users.

Based on feedback information about the channel conditions, adaptive user-to-subcarrier

assignment can be achieved. If the assignment is done sufficiently fast, this further

improves the OFDM robustness to fast fading and narrow-band co-channel interference,

and makes it possible to achieve even better system spectral efficiency.

Different numbers of sub-carriers can be assigned to different users, in view to support

differentiated Quality of Service (QOS), i.e. to control the data rate and error probability

individually for each user.17

OFDMA can be seen as an alternative to combining OFDM with time division multiple

access(TDMA) or time-domain statistical multiplexing communication. Low-data-rate

users can send continuously with low transmission power instead of using a "pulsed"

high-power carrier. Constant delay, and shorter delay, can be achieved.

OFDMA can also be described as a combination of frequency domain and time domain

multiple access, where the resources are partitioned in the time-frequency space, and

slots are assigned along the OFDM symbol index as well as OFDM sub-carrier index.

OFDMA is considered as highly suitable for broadband wireless networks, due to

advantages including scalability and use of multiple antennas (MIMO)-friendliness, and

ability to take advantage of channel frequency selectivity.

In spectrum sensing cognitive radio, OFDMA is a possible approach to filling free radio

frequency bands adaptively. Timo A. Weiss and Friedrich K. Jondral of the University

of Karlsruhe proposed a spectrum pooling system in which free bands sensed by nodes

were immediately filled by OFDMA sub-bands.

Fig 14: serial to parallel converter circuit

Multiple access schemes are used to allow many users to share

simultaneously a finite amount of spectrum

There are three fundamental multi-carrier based multiple access techniques for OFDM systems:

• OFDM-FDMA

• OFDM-TDMA

• OFDM-CDMA18

Fig15: OFDM in FDMA

OFDM-FDMA:

Each user is allocated a pre-determined band of sub-carriers

• First proposed for CATV systems (used to send upstream data from subscriber to cable head-end)

• Later adopted for wireless communications systems

• Supports a number of identical down-streams, or different user data rates (different no. sub-carriers)

• Allows adaptive techniques per sub-carrier, based on sub-channel condition

OFDM—FDMA VARIATIONS: Based on the allocation of sub-carriers to users, we can have the following variations: • Random allocation (adaptive frequency hopping)

• Interleaved FDMA (comb spread carriers)

FIG16: OFDM SUBCARRIER CHANNELS

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4 .OFDM-FUNDAMEMENTALS

4.1. OFDM VERSUS FDM:Orthogonal Frequency Division Multiplexing is an advanced form of Frequency

Division Multiplexing (FDM) where the frequencies multiplexed are orthogonal to each other and their spectra overlap with the neighboring carriers. In contrast to FDM, OFDM is based on the principle of overlapping orthogonal sub carriers. The overlapping multicarrier technique can achieve superior bandwidth utilization. There is a huge difference between the conventional non-overlapping multicarrier techniques such as FDMA and the overlapping multicarrier technique such as OFDM. In frequency division multiplex system, many carriers are spaced apart.

The signals are received using conventional filters and demodulators. In these receivers guard bands are introduced between each subcarriers resulting into reduced spectral efficiency. But in an OFDM system it is possible to arrange the carriers in such fashion that the sidebands of the individual subcarriers overlap and the signals are still received without adjacent carrier interference. The main advantage of this concept is that each of the radio streams experiences almost flat fading channel. In slowly fading channels the inter-symbol interference (ISI) and inter-carrier interference (ICI) is avoided with a small loss of energy using cyclic prefix. In order to assure a high spectral efficiency the sub-channel waveforms must have overlapping transmit spectra. But to have overlapping spectra, sub-channels need to be orthogonal. Orthogonality is a property that allows the signals to be perfectly transmitted over a common channel and detected without interference. Loss of orthogonality results in blurring between the transmitted signals and loss of information. For OFDM signals, the peak of one sub carrier coincides with the nulls of the other sub carriers. Thus there is no interference from other sub carriers at the peak of a desired sub carrier even though the sub carrier spectrums overlap. OFDM system avoids the loss in bandwidth efficiency prevalent in system using non orthogonal carrier set.

FIG 17: OFDM IN MULTIPLE ACCESS

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4.2.COMPARING TDMA, CDMA , OFDM:The comparision of the CDMA, TDMA, OFDM is explained in the below table.

Table 3: Comparision of TDMA,CDMA,OFDM

The time syncronizatinon is very elegant when compared to TDMA and CDMA. The frequency synchronization is easy but it will be fine of sync. Timing and Frequency Tracking is very complex in TDMA and CDMA. Channel equalization is also easy. In OFDM the Analog Front-end is complex and the cost is also high where as in TDMA and CDMA it is easy.

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5 .WIMAX PHYSICAL LAYER

5.1 .WIMAX SYSTEMS:

A WiMAX system consists of two parts:

A WiMAX tower - similar in concept to a cell-phone tower- A single WiMAX tower can provide coverage to a very large area as big as 3,000 square miles (~8,000 square km).

A WiMAX receiver – The receiver and antenna could be a small box or PCMCIA card, or they could be built into a laptop the way WiFi access is today. A WiMAX tower station can connect directly to the Internet using a high bandwidth, wired connection (for example, a T3 line). It can also connect to another WiMAX tower using a line-of-sight, microwave link. This connection to a second tower (often referred to as a backhaul), along with the ability of a single tower to cover up to 3,000 square miles, is what allows WiMAX to provide coverage to remote rural areas.

The WiMAX base station (BS) and WiMAX subscriber station (SS), also referred to as customer premise equipments (CPE). Therefore, it can be built quickly at a low cost. Ultimately, WiMAX is also considered as the next step in the mobile technology evolution path. The potential combination of WiMAX and CDMA standards is referred to as 4G.

System Model:

IEEE 802.16 supports two modes of operation: PTP and PMP.

Point-to-point (PTP):

The PTP link refers to a dedicated link that connects only two nodes: BS and subscriber terminal. It utilizes resources in an inefficient way and substantially causes high operation costs. It is usually only used to serve high-value customers who need extremely high bandwidth, such as business high-rises, video postproduction houses, or

Scientific research organizations. In these cases, a single connection contains all the available bandwidth to generate high throughput. A highly directional and high-gain antenna is also necessary to minimize interference and maximize security.

Point-to-multipoint (PMP):

The PMP topology, where a group of subscriber terminals are connected to a better choice for users who do not need to use the entire bandwidth. Under PMP topology, sectoral antennas with highly directional parabolic dishes (each dish refers to a sector) are used for frequency reuse. The available bandwidth now is shared between a group of users, and the cost for each subscriber is reduced.

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Fig 18: Wimax point to point

5.2. IEEE 802.16: EVAUTION AND ARCHITECTURE:

Standards Associated With Wimax:

Fig 19: standards of Wimax

IEEE 802 refers to a family of IEEE standards dealing with local area networks and

metropolitan area networks. More specifically, the IEEE 802 standards are restricted

to networks carrying variable-size packets. (By contrast, in cell-based networks data

is transmitted in short, uniformly sized units called cells. Isochronous networks,

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where data is transmitted as a steady stream of octets, or groups of octets, at regular

time intervals, are also out of the scope of this standard.) The number 802 was

simply the next free number IEEE could assign, though “802” is sometimes

associated with the date the first meeting was held — February 1980.

IEEE 802.16 : The IEEE 802.16 Working Group on Broadband Wireless

Access Standards, which was established by IEEE Standards Board in 1999, aims

to prepare formal specifications for the global deployment of broadband Wireless

Metropolitan Area Networks. The Workgroup is a unit of the IEEE 802 LAN/MAN

Standards Committee. A related future technology Mobile Broadband Wireless

Access (MBWA) is under development in IEEE 802.20.

Although the 802.16 family of standards is officially called

Wireless MAN, it has been dubbed “WiMAX” (from "Worldwide Interoperability

for Microwave Access") by an industry group called the WiMAX Forum. The

mission of the Forum is to promote and certify compatibility and

interoperability of broadband wireless products.

Fig 20: Types of 802.16

In January 2003, the IEEE approved 802.16a as an amendment to IEEE 802.16-2001, defining (Near) Line-Of- Sight capability.

In July 2004, IEEE 802.16REVd, now published under the name IEEE 802.16-2004,introduces support for indoor CPE (NLOS) through additional radio capabilities such as antenna beam forming and OFDM sub-channeling.

Early 2005, an IEEE 802.16e variant will introduce support for mobility. From above fig for the applications associated with each of these standards .The WiMAX Forum intends to do for 802.16 what the Wi-Fi Alliance did for 802.11

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Harmonize standards and certify interoperability between equipment from different

vendors. Standardized interoperable solutions will result in mass volume and bring

down cost. Promote and establish a brand for the technology

WiMAX, the reality beyond the hype

As mentioned above, WiMAX can offer very high data rates and extended

coverage.

•75Mbit/s capacity for the base station is achievable with a 20 MHz channel in best

propagation conditions. But regulators will often allow only smaller channels

(10 MHz or less) reducing the maximum bandwidth.

• Even though 50 km is achievable under optimal conditions and with a reduced data

rate (a few Mbit/s), the typical coverage will be around 5 km with indoor CPE (NLOS)

and around 15 km with a CPE connected to an external antenna(LOS) Networks. The

Workgroup is a unit of the IEEE 802 LAN/MAN.

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5.3. WOKING OF WIMAX:

The working of the WIMAX is explained by below fig:

Fig 21: working of Wimax

WIMAX is short for Worldwide Interoperability for Microwave Access, and it also stands for the IEEE name 802.16. WiMAX has the potential to do to broadband Internet access what cell phones have done to the traditional wire phone industry. WiMAX will replace cable & DSL services, providing Internet access anywhere you go. WiMAX will also be as painless as WiFi — turning your computer on will automatically connect you to the closest available WiMAX antenna. In the next five years, Wimax will have an enormous impact on the cellular markets, particularly that of third-world countries, as well as that of the United States (See the Wimax Map). The cost-effectiveness of WiMax to that of pre existing systems is much higher. One application that can be used by cellular companies is WiMax’s ability to serve as a high bandwidth “backhaul” for internet or cellular phone traffic from remote areas back to an Internet backbone; WiMax may be an answer to reducing the cost of T1/E1 backhaul as well.

TYPES OF WIMAX:

"[WiMAX] could potentially be the bigest thing since the Internet itself", as Sri ram Viswanathan, general manager of Intel's Worldwide Interoperability for Microwave Access (WiMAX) program office. That is quite a bold statement and underscores the sentiment of many WiMAX proponents. But as the WiMAX industry has grown, so has WiMAX hype and confusion.

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Fixed Wimax : The IEEE originally formed the IEEE 802.16 working group in 1998 to provide a standard for wireless metropolitan area networks. The primary application was for high-speed fiber access solutions using high-frequency line-of-sight (LOS) fixed wireless connections. The original standard was referred to as 802.16 and evolved to support fixed broadband wireless access over lower-frequency non-line-of-sight (NLOS) wireless connections. The evolved standard, 802.16-2004, is often referred to as fixed WiMAX.

Mobile Wimax: Once the 802.16-2004 standard was complete, the IEEE committee began work to further evolve the standard to support mobile applications. Mobile communication is more complex than fixed communication. The technology must be able to hand off a wireless connection from one base station to another while the user is moving, without dropping the connection. The new 802.16e-2005 standard was completed in December 2005 and not only supports mobile applications but also nomadic and fixed applications. 802.16e-2005 is often referred to as mobile WiMAX.

Physical layer: Fixed/mobile WiMAX PHY layer uses a technology called orthogonal

frequency division multiplexing (OFDM). OFDM techniques have been around for

decades. The technology is now commonplace in wireless systems such as Wi-Fi.

OFDM evolved from earlier single-carrier modulation systems and frequency division

multiplexing systems. OFDM is a type of frequency division multiplexing system that

provides better channel throughput because all of the underlying sub-carriers are

orthogonal to one another.

Medium Access Control Layer: The primary job of the MAC layer is to provide an

interface between the PHY layer and upper layer protocols. Each instance of the MAC

layer in a fixed/mobile WiMAX station has a 48-bit address as defined by the IEEE Std

802.

Unlike the distributed and connectionless 802.11 MAC, the WiMAX MAC is

centralized and connection-oriented. A 16-bit connection identifier (CID) identifies each

WiMAX connection. Each WiMAX MAC layer protocol data unit uses the CID, instead

of the MAC address, to identify source and destination. WiMAX has a rich quality of

service mechanism that is based on the Data Over Cable Service Interface Specifications

(DOCSIS).

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Fig 22:Fixed and Mobile Wimax

The WiMAX technology relates to WiMAX applications. Over time, 802.16e-2005 is likely to become the dominant standard for fixed, nomadic and mobile applications, thus limiting the use of 802.16-2004.

Table 4: WiMAX applications and technologies

5.4 .APPLICATIONS OF WIMAX IN OFDM:

WiMAX stands for wireless inter operatibility for microwave access. WiMAX is expected to do more for Metropolitan Area Networks (MANs) and WiMAX is not projected to replace Wi-Fi, but to complement it by connecting Wi-Fi networks to each other or the Internet through high-speed wireless links. You can therefore use WiMAX technology to extend the power and range of Wi-Fi and cellular networks. However, in developing countries, WiMAX may become the only wireless technology because Wi-Fi and cellular have not penetrated areas that can be reached with WiMAX technology.

Range:

The wide range of the WiMAX technology depends on the height of the antennas

Data Rate : The technology used for WiMAX is Orthogonal Frequency Division

Multiplexing (OFDM), it is not appreciably more supernaturally efficient then the Technology commonly used for 3G that is Wideband Code Division Multiple Access (WCDMA). However OFDM is coupled with a high channel bandwidth, that allows greater data rates. So, on average, for an equivalent spectrum allocation, users

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will see similar data rates. In specific simulations, where there are few users, it is possible that WiMAX will provide a higher data rate than 3G.However, in commercial systems, such simulations are likely rare.

Fig 23: graph of wimax with speed and mobility

COST: The network costs of WiMAX will be likely to be higher than for 3G because of the reduced range and hence the necessity to build more cells. The subscriber subside costs may be lower if WiMAX is built into processor chips, although this may not apply if users wish to have WiMAX handsets.

Quality of Service (QOS)

Excellent Quality Of service management donates from variety of WiMAX features.

Just as on a Wi-Fi network, WiMAX users share a data pipe and QOS can degrade as

more users are added to the network.

ADVANTAGES:

Advantages of OFDM are listed as follows:

OFDM makes resourceful utilization of the spectrum by overlapping. By dividing the channel into narrowband flat fading sub channels, OFDM is more resistant to frequency selective fading than single carrier systems.

It can easily adapt to severe channel conditions without complex time-domain equalization.

It reduces ISI and IFI through use of a cyclic prefix and fading caused by multipath propagation.

Using sufficient channel coding and interleaving lost symbols can be recovered.

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Channel equalization becomes simpler than by using adaptive equalization techniques with single carrier systems.

OFDM is computationally capable by using FFT techniques to implement the modulation and demodulation functions.

It is less sensitive to sample timing offsets than single carrier systems are. It is robust against narrow-band co-channel interference.

DISADVATAGES:

The disadvantages are as follows:

The OFDM signal has a noise like amplitude with a very large dynamic range; hence it requires RF power amplifiers with a high peak to average power ratio.

It is more sensitive to carrier frequency offset and drift than single carrier systems are due to leakage of the DFT.

It is sensitive to Doppler shift. It requires linear transmitter circuitry, which suffers from poor power efficiency. I t suffers loss of efficiency caused by cyclic prefix.

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6. CONCLUTION

OFDM has promising future in wireless networks and mobile communications. Growth in number of worldwide customers for wireless networks and ever-increasing demand for large Band width has given birth to this technology. OFDM is already playing an important role in WLAN and will be part of MAN too. In coming years, it will surely dominate the communication industry. Also, Wimax and 802.20 use OFDM-MIMO, which is emerging as the main technology for future cellular packet data networks, including 3GPP long-term evolution and 3GPP2 air interface evolution as well. Although OFDM has proven itself with packet-based data, it is not yet clear whether the technology can either handle large numbers of voice customers or work with voice and data as well as CDMA.

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REFERENCES:1) www.ewh.ieee.org/r4/chicago/Yu-WiMAX.pdf

2) http://computer.howstuffworks.com/wimax.htm

3) www.wimaxforum.org

4) http://standards.ieee.org/catalog/olis/lanman.html

5) IEEE 802.16e: IEEE 802.16e Task Group (Mobile Wireless MAN) http://www.ieee802.org/16/tge/.

6) IEEE Std. 802.16: IEEE Standard for local and metropolitan area networks-Part 16: Air Interface for Fixed Broadband Wireless Access Systems, 2004.

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