الرحيم الرحمن الله الرحيمبسم الرحمن الله الرحيمبسم الرحمن الله بسم

2010

ENG. SINAN M. A. DHEYAB

Esm012345@ gmail.com

12/1/2010

ORTHOGONAL FREQUNCYDIVITION MULTIPLEXING

الرحيم الرحمن الله الرحيمبسم الرحمن الله الرحيمبسم الرحمن الله بسم

Edited By: Eng. SINAN M. A. DHEYAB

ABSTRACTABSTRACTABSTRACTMulti-Carrier Modulation is a technique for data-transmission

by dividing a high-bit rate data stream to several parallel low bit ratedata streams and using these low bit-rate data streams to modulateseveral carriers.

Multi-Carrier Transmission has a lot of useful properties suchas delay-spread tolerance and spectrum efficiency that encourage theiruse in untethered broadband communications.

Orthogonal frequency division multiplexing OFDM:- is a multi-carrier modulation technique with densely spaced sub-carriers that hasgained a lot of popularity among the broadband community in the lastfew years. It has found immense applications in communication systems

OFDM is a special case of multi-carrier transmission, where asingle data stream is transmitted over a number of lower rate Sub-carriers.

In July 1998, the IEEE standardization group decided to selectOFDM as the basis for their new 5-GHz standard, targeting a range ofdata stream from 6 up to 54 Mbps. This new standard is the first one touse OFDM in packet-based communications, while the use of OFDM untilnow was limited to continuous transmission.

Two of the fundamental advantages of OFDM are its robustnessagainst channel dispersion and its ease of phase and channel estimationin a time-varying environment. With the advancement of powerfulsilicon DSP technology, OFDM has triumphed in a broad range ofapplications in the RF domain from digital audio/video broadcasting(DAB/DVB) to wireless local area networks (LANs). However, OFDM alsohas intrinsic disadvantages, such as high peak-to-average power ratio(PAPR) and sensitivity to frequency and phase noise.

OFDM is a special class of MCM system that has only recentlygained attention in the optical communication community, especiallyafter being proposed the attractive long-haul transmission format incoherent detection and direct detection.

chapter onechapter onechapter one

IntroductionIntroductionIntroduction

1. Introduction:-OFDM is becoming the chosen modulation technique for wireless

communications. OFDM can provide large data rates with sufficientrobustness to radio channel impairments. Many research centers in theworld have specialized teams working in the optimization of OFDM forcountless applications.

OFDM is of great interest by researchers, research universities andresearch laboratories all over the world. It has already been acceptedfor the new wireless local area network standards IEEE 802.11a, HighPerformance LAN type 2 (HIPERLAN/2) and Mobile Multimedia AccessCommunication (MMAC) Systems. Also, it is expected to be used forwireless broadband multimedia communications.

Data rate is really what broadband is about. The new standardsspecify bit rates of up to 54 Mbps. Such high rate imposes largebandwidth, thus pushing carriers for values higher than UHF band. Forinstance, IEEE802.11a has frequencies allocated in the 5- and 17- GHzbands.

OFDM can be seen as either a modulation technique or amultiplexing technique. One of the main reasons to use OFDM is toincrease the robustness against frequency selective fading ornarrowband interference.

OFDM divides the transmission bandwidth into many subchannels,each one occupying a narrow bandwidth. In this way, owing to theincrease in symbol duration, the effect of dispersion in time of thereflected signal on the receiver is minimized.

The effect of ISI is completely eliminated by inserting a guardband in the resulting composite OFDM symbol. Fast Fourier transform(FFT) is an efficient way to produce (in the digital domain) the requiredsubcarriers over which the information will be embedded.

In practice, OFDM is used in third-generation WLANs, WiMAX andDVB to eliminate ISI.

1.1-OFDM versus FDM:-OFDM is a special case of Frequency Division Multiplex (FDM). As

an analogy, a FDM channel is like water flow out of a faucet, in contrastthe OFDM signal is like a shower. In a faucet all water comes in one bigstream and cannot be sub-divided. OFDM shower is made up of a lot oflittle streams as shown in figure(1).

Think about what the advantage might be of one over the other? Oneobvious one is that if I put my thumb over the faucet hole, I can stop thewater flow but I cannot do the same for the shower. So although bothdo the same thing, they respond differently to interference.

Fig.(2) All cargo on one truck vs. splitting the shipment into more than one.Another way to see this intuitively is to use the analogy of making a

shipment via a truck. We have two options, one hire a big truck or abunch of smaller ones. Both methods carry the exact same amount ofdata. But in case of an accident, only 1/4 of data on the OFDM truckingwill suffer. These four smaller trucks when seen as signals are calledthe sub-carriers in an OFDM system and they must be orthogonal forthis idea to work.

OFDM is similar to FDM but much more spectrally efficient byspacing the sub-channels much closer together (until they are actuallyoverlapping). OFDM is a special case of FDM called orthogonal FDM. So,what the reason to establish OFDM?!!

As we describe above we conclude that:-••• Data may be lost in one or two sub-carriers, but we do not losethe whole stream.••• A clever way to combat frequency-selective channels (either wireline or wireless).

In older multi-channel systems using FDM, the total availablebandwidth is divided into N non-overlapping frequency sub-channels.Each sub-channel is modulated with a separate symbol stream and theN sub-channels are frequency multiplexed.

Even though the prevention of spectral overlapping of sub-carriersreduces (or eliminates) Inter channel Interference, this leads to aninefficient use of spectrum. The guard bands on either side of each sub-channel are a waste of precious bandwidth.

To overcome the problem of bandwidth wastage, we can insteaduse N overlapping (but orthogonal) ( orthogonal:- The word orthogonalindicates that there is a precise mathematical relationship between thefrequencies of the carriers in the system. In a normal frequency-division multiplex system, many carriers are spaced apart in such away that the signals can be received using conventional filters anddemodulators. In such receivers, guard bands are introduced betweenthe different carriers and in the frequency domain, which results in alowering of spectrum efficiency.) subcarriers, each carrying a baudrate of 1/T and spaced 1/T apart. Because of the frequency spacingselected, the sub-carriers are all mathematically orthogonal to eachother (i.e. to arrange the carriers in an OFDM signal so that thesidebands of the individual carriers overlap and the signals are stillreceived without adjacent carrier interference. To do this, the carriersmust be mathematically orthogonal. The receiver acts as a bank ofdemodulators, translating each carrier down to DC, with the resultingsignal integrated over a symbol period to recover the raw data. If theother carriers all beat down the frequencies that, in the time domain,have a whole number of cycles in the symbol period T, then theintegration process results in zero contribution from all these othercarriers. Thus, the carriers are linearly independent (i.e., orthogonal)if the carrier spacing is a multiple of 1/T.), to avoid the use of high-speed equalization and to combat impulsive noise and multipathdistortion, as well as to fully use the available bandwidth. This permitsthe proper demodulation of the symbol streams without therequirement of non overlapping spectra.

(a) (b)Figure(3) Spectra of (a) an OFDM sub channel and (b) and OFDM signal.Another way of specifying the sub-carrier orthogonality condition

is to require that each sub-carrier have exactly integer number ofcycles in the interval T. The idea in OFDM is to define a symbolsequence in the frequency domain, transmit it in the time domain, andmap the received samples back into the frequency domain.

In high speed data transfer, Quality of service is an importantcriterion. Therefore modulation techniques must be good enough forquality data transfer, which modulation can compromise allcontradicting requirements in the best manner. Using adaptiveequalization techniques at the receiver could be the solution, but thereare practical difficulties in operating this equalization in real time atseveral Mb/s with compact, low-cost hardware. A promising candidatethat eliminates a need for the complex equalizers is the OFDM.

Figure (4) illustrates the difference between the conventionalnonoverlapping multi-carrier technique and the overlapping multi-carrier modulation technique. As shown in Figure (4), by using theoverlapping multi-carrier modulation technique, we save almost 50%of bandwidth. To realize the overlapping multi-carrier technique,however we need to reduce crosstalk between sub-carriers, whichmeans that we want orthogonality between the different modulatedcarriers.

Figure (4) Concept of OFDM signal: orthogonal multi- carrier techniqueversus conventional multi- carrier technique

1.2-OFDM Basics:-

OFDM belongs to a family of transmission schemes called multi-carrier modulation, which is based on the idea of dividing a given high-bit-rate data stream into several parallel lower bit-rate streams andmodulating each stream on separate carriers—often called sub-carriers, or tones.

Multi-carrier modulation schemes eliminate or minimize inter-symbol interference (ISI) by making the symbol time large enough sothat the channel-induced delays—delay spread being a good measureof this in wireless channels are an insignificant (typically, <10 percent)fraction of the symbol duration. Therefore, in high-data-rate systems inwhich the symbol duration is small, being inversely proportional to thedata rate, splitting the data stream into many parallel streamsincreases the symbol duration of each stream such that the delayspread is only a small fraction of the symbol duration.

OFDM is a spectrally efficient version of multi-carrier modulation.Choosing the first sub-carrier to have a frequency such that it has aninteger number of cycles in a symbol period, and setting the spacingbetween adjacent sub-carriers (sub-carrier bandwidth) to be BSC =B/L, where B is the nominal bandwidth (equal to data rate), and L is thenumber of sub-carriers, ensures that all tones are orthogonal to oneanother over the symbol period. It can be shown that the OFDM signalis equivalent to the inverse discrete Fourier transform (IDFT) of thedata sequence block taken L at a time. This makes it extremely easy toimplement OFDM transmitters and receivers in discrete time usingIFFT (inverse fast Fourier) and FFT, respectively.

OFDM represents a different system design approach. It can bethought of as a combination of modulation and multiple accessschemes that segment a communications channel in such a way thatmany users can share it. OFDM segments according to frequency.

It is a technique that divides the spectrum into a number of equallyspaced tones, and carries a portion of a user’s information on each tone. Atone can be thought of as a frequency, much in the same way that each key ona piano represents a unique frequency.

OFDM can be viewed as a form of frequency division multiplexing(FDM). However, OFDM has an important special property that eachtone is orthogonal with every other tone. FDM typically requires there

to be frequency guard bands between the frequencies so that they donot interfere with each other. OFDM allows the spectrum of each toneto overlap, and since they are orthogonal, they do not interfere witheach other. By allowing the tones to overlap, the overall amount ofspectrum required is reduced.

The OFDM signal has a noise like amplitude with a very largedynamic range; therefore it requires RF power amplifiers with a highpeak to average power ratio.

In order to completely eliminate ISI, guard intervals are usedbetween OFDM symbols. By making the guard interval larger than theexpected multipath delay spread, ISI can be completely eliminated.Adding a guard interval, however, implies power wastage and adecrease in bandwidth efficiency. The amount of power wasteddepends on how large a fraction of the OFDM symbol duration theguard time is. Therefore, the larger the symbol period—for a givendata rate, this means more sub-carriers—the smaller the loss of powerand bandwidth efficiency.

Figure (5): time domain representation of guard band.It is more sensitive to carrier frequency offset and drift than single

carrier systems are due to leakage of the DFT or FFT. So the size of theFFT in an OFDM design should be chosen carefully as a balancebetween protection against multipath, Doppler shift, and designcost/complexity. For a given bandwidth, selecting a large FFT sizewould reduce the sub-carrier spacing and increase the symbol time.

This makes it easier to protect against multipath delay spread. Areduced sub-carrier spacing, however, also makes the system morevulnerable to inter-carrier interference owing to Doppler spread inmobile applications. The competing influences of delay and Dopplerspread in an OFDM design require careful balancing.

1.3-Single-carrier versuse multi-carrier:-

Multi-Carrier (MC) modulation has several advantages over single-carrier communication. The primary advantage of OFDM oversingle-carrier schemes is its ability to cope with severe channelconditions ——— for example, attenuation of high frequencies in a longcopper wire, narrowband interference and frequency-selective fadingdue to multipath——— without complex equalization filters. Channelequalization is simplified because OFDM may be viewed as using manyslowly modulated narrowband signals rather than one rapidly-modulated wideband signal.

When the channel is frequency selective is that, using a guardinterval whose duration exceeds the channel’s impulse responselength, it allows decomposing the channel in a set of Independent flatfading sub channels over which equalization amounts to a single phasecompensation. As a consequence, the guard protection also diminishesthe need for time domain equalization at the receiver, i.e. the Inter-Symbol Interference (ISI) is negligible.

In Single carrier system the signal representing each bit uses allof the available Spectrum. In Multi-carrier system the availablespectrum divided into many narrow bands and data is divided intoparallel data streams each transmitted on a separate band as shown infigure(6).

Figure (6) (a) single carrier (b) multicarrierIn a single carrier system, a single fade or interferer can cause the

entire link to fail, but in a multi-carrier system, only a small percentageof the sub-carriers will be affected. Error correction coding can then beused to correct for the few erroneous sub-carriers.

1.4- HISTORY OF OFDM:-

The concept of using parallel data transmission by means offrequency division multiplexing (FDM) was published in mid 60s. Someearly development can be traced back in the 50s. A U.S. patent wasfilled and issued in January, 1970.

In the 1980s, OFDM was studied for high-speed modems, digitalmobile communications, and high-density recording. One of thesystems realized the OFDM techniques for multiplexed QAM using DFT,and by using pilot tone, stabilizing carrier and clock frequency controland implementing trellis coding are also implemented. Moreover,various-speed modems were developed for telephone networks.

In the 1990s, OFDM was exploited for wideband datacommunications over mobile radio FM channels, high-bit-rate digitalsubscriber lines (HDSL; 1.6 Mbps), asymmetric digital subscriber lines(ADSL, 1,536 Mb/s), very-high-speed digital subscriber lines (VDSL;100 Mbps), digital audio broadcasting (DAB), and high-definitiontelevision (HDTV) terrestrial broadcasting.

In 2007 the first complete LTE air interface implementation wasdemonstrated, including OFDM-MIMO, SC-FDMA and multi-user MIMOuplink.

To date, 100 Gb/s CO-OFDM transmission over 1000 km standardsingle-mode fiber (SSMF) with high spectral efficiency of 2 bit/s/Hz hasbeen demonstrated by various groups.

Future, as the industry is embracing the imminent commercialrollout of 100 Gb/s Ethernet (100 GbE), the feasibility of 1 Tb/sEthernet is the next logical step. Also multimode fiber in conjunctionwith multiple-input multiple-output OFDM (MIMO-OFDM) is proposedas a technology to achieve 100 Tb/s per fiber that takes advantage ofmode multiplexing in the optical fiber.

1.5-OFDM advantages:-OFDM enjoys several advantages over other solutions for high-

speed transmission and possesses some inherent advantages forWireless Communications. These advantages are a few of the mostimportant reasons on why OFDM is becoming more popular in theWireless Industry today.•••Makes efficient use of the spectrum by allowing overlap.•••By dividing the channel into narrowband flat fading sub-channels, OFDM is more resistant to frequency selective fadingthan single carrier systems are.••• Eliminates ISI and IFI through use of a cyclic prefix.••• Using adequate channel coding and interleaving one can recoversymbols lost due to the frequency selectivity of the channel.•••Channel equalization becomes simpler than by using adaptiveequalization techniques with single carrier systems.•••It is possible to use maximum likelihood decoding withreasonable complexity, as discussed in OFDM is computationallyefficient by using FFT techniques to implement the modulationand demodulation functions.•••In conjunction with differential modulation there is no need toimplement a channel estimator.•••Is less sensitive to sample timing offsets than single carriersystems are.•••Provides good protection against cochannel interference andimpulsive parasitic noise.•••Use as a multi-access scheme.•••Suitable for coherent demodulation and direct detection.•••Multiple accesses using OFDM is highly flexible-TDMA: Scheduled access in time- FDMA: Scheduled access in frequency-CSMA: Random access using carrier sense- OFDMA: Assigning multiple users to different subcarriers

1.5.1-OFDM disadvantages:-••• Sensitive to Doppler shift.••• Sensitive to frequency synchronization problems.••• High peak-to-average-power ratio (PAPR), requiring lineartransmitter circuitry, which suffers from poor power efficiency.••• Loss of efficiency caused by Cyclic prefix/Guard interval.

1.6- OFDM Applications:-

The initial applications were in the military communications.In the telecommunications field, the terms of discrete multi-tone(DMT), multi-channel modulation and multi-carrier modulation(MCM) are widely used and sometimes they are interchangeablewith OFDM.

Due to their implementation complexity, OFDM applicationshave been scarce until quite recently. A lot of applications that useOFDM technology have spawned over the last few years. OFDM is thebasis for the global standard for asymmetric digital subscriber line(ADSL) and for digital audio broadcasting (DAB) in the Europeanmarket as well as for Terrestrial Digital Video Broadcasting (DVB-T)system.

It has already been accepted for the new wireless local areanetwork (LAN) standards IEEE 802.11a, High Performance LAN type2 (HIPERLAN/2), the IEEE 802.16a Metropolitan area network(MAN) standard and Mobile Multimedia Access Communication(MMAC) Systems. Also, it is expected to be used for wirelessbroadband multimedia communications. So, in the wireless networkspace, OFDM is at the heart of IEEE 802.11a and HiperLAN/2. OFDMused for Mobile Communications.

For fixed-wire applications, OFDM is employed in theAsynchronous Digital Subscriber Line (ADSL) and High bit-rateDigital Subscriber Line (HDSL) systems and it has also beensuggested for power-line communications systems due to itsresilience to time-dispersive channels and narrow-bandinterferers.

OFDM is also being pursued for dedicated short-rangecommunications (DSRC) for road side to vehicle communicationsand as a potential candidate for fourth-generation (4G) mobilewireless systems.

In optical communication, Optical OFDM which combines themerits of the coherent detection and OFDM technology has arrivedin time for the next-generation optical networks.

The following list is a summary of existing OFDM based standardsand products

Cable••• ADSL and VDSL broadband access via POTS copper wiring.••• Power line communication (PLC).•••Multimedia over Coax Alliance (MoCA) home networking.••• ITU-T G.hn, a standard which provides high-speed local areanetworking over existing home wiring (power lines, phone lines andcoaxial cables)

Wireless••• The wireless LAN radio interfaces IEEE 802.11a, g, n andHIPERLAN/2.••• The digital radio systems DAB/EUREKA 147, DAB+, Digital RadioMondiale, HD Radio, T-DMB and ISDB-TSB.••• The terrestrial digital TV system DVB-T.••• The terrestrial mobile TV systems DVB-H, T-DMB, ISDB-T andMediaFLO forward link.••• The cellular communication systems Flash-OFDM.••• The mobile broadband 3GPP Long Term Evolution air interfacenamed High Speed OFDM Packet Access (HSOPA).••• The Wireless MAN / Fixed broadband wireless access (BWA)standard IEEE 802.16 (or WiMAX).••• The Mobile Broadband Wireless Access (MBWA) standards IEEE802.20, IEEE 802.16e (Mobile WiMAX) and WiBro.••• The wireless Personal Area Network (PAN) Ultra wideband (UWB)IEEE 802.15.3a implementation suggested by WiMedia Alliance.