Mimo Ofdm Papr Precoding

8/13/2019 Mimo Ofdm Papr Precoding

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Degree project

Precoding in MIMO, OFDM to reduce PAPR

(Peak to Average Power Ratio)

Authors: Muhammad Irfan Ishaq.Yasir Ali Khan.Muhammad Talha Gul.Supervisor: Prof. Sven Nordebo

Date: 2012-04-18 Subject: Electrical Engineering

Level: Master Course code:5ED06E


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ACKNOWLEDGEMENT

All praises to Almighty Allah; Whose enormous blessings give us strengthand make us able to complete this thesis. We would like to thanks our super-visor. Professor Sven Nordebo for his kind support, assistance and guidancethroughout our thesis. His motivation brought the source of inspiration toour thesis. We are grateful to Swedish government for such a wonderfuleducation system, who welcomed us to complete our master education inSweden. Finally, all blessings upon our beloved parents and families whoseprayers and moral support always motivated us to complete our studies.


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Abstract

One of the critical issues of systems utilizing Orthogonal FrequencyDivision Multiplexing (OFDM) is the high peak to the average powerratio of OFDM signals. We have used Precoding as a way to mitigatethe PAPR problem. Furthermore the performance of Precoded OFDMin fading multi-path channels has been studied. This thesis is based onan efficient technique for reducing the PAPR of OFDM signals. Theproposed technique is data-independent and thus, does not requirenew processing and optimization for each transmitted OFDM block.The reduction in PAPR of the OFDM signal is obtained through aproper selection of a Precoding scheme that distributes the power of

each modulated symbol over the OFDM block. The obtained resultsshow that this Precoding scheme is an attractive solution to the PAPRproblem of OFDM signals.


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Contents1 Introduction 7

1.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71.2 Advantages . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

1.2.1 Lower implementation complexity . . . . . . . . . . . . 81.2.2 Elimination of ISI . . . . . . . . . . . . . . . . . . . . . 81.2.3 Robustness against narrowband interference . . . . . . 9

1.3 Disadvantages . . . . . . . . . . . . . . . . . . . . . . . . . . . 91.3.1 Carrier phase noise and Frequency offset . . . . . . . . 91.3.2 Peak to Average Power Ratio Problem . . . . . . . . . 9

1.4 Previous techniques for PAPR reduction . . . . . . . . . . . . 102 Orthogonal Frequency Division Multiplexing 11

2.1 Single Carrier and Multicarrier System . . . . . . . . . . . . . 112.1.1 single carrier system . . . . . . . . . . . . . . . . . . . 112.1.2 Multicarrier system . . . . . . . . . . . . . . . . . . . . 11

2.2 FDMA and OFDM . . . . . . . . . . . . . . . . . . . . . . . . 122.3 Basic structure of OFDM system . . . . . . . . . . . . . . . . 132.4 Mathematical Representation . . . . . . . . . . . . . . . . . . 162.5 Fourier Transform . . . . . . . . . . . . . . . . . . . . . . . . . 172.6 Guard Band . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182.7 Interleaving . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182.8 Windowing . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192.9 Cyclic Prex of OFDM System . . . . . . . . . . . . . . . . . 202.10 OFDM Design Issues . . . . . . . . . . . . . . . . . . . . . . . 20

2.10.1 Useful symbol duration . . . . . . . . . . . . . . . . . . 202.10.2 Number of carriers . . . . . . . . . . . . . . . . . . . . 212.10.3 Modulation scheme . . . . . . . . . . . . . . . . . . . . 21

2.11 Advantages and Drawbacks of OFDM . . . . . . . . . . . . . . 212.11.1 Advantages of OFDM . . . . . . . . . . . . . . . . . . 21

2.12 Disadvantages of OFDM . . . . . . . . . . . . . . . . . . . . . 22

2.12.1 Strict Synchronization Requirement . . . . . . . . . . . 222.12.2 Peak-to-Average Power Ratio . . . . . . . . . . . . . . 232.12.3 Co-channel Interference Mitigation in Cellular OFDM . 23

3 Investigation of SISO PAPR Reduction 243.1 Precoding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243.2 Transmitter Model for a Precoded OFDM System (SISO) . . . 253.3 PAPR of precoded OFDM . . . . . . . . . . . . . . . . . . . . 273.4 Selection Criteria Of Precoding Matrix . . . . . . . . . . . . . 28

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3.5 Theorem 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303.6 Examples of precoding schemes . . . . . . . . . . . . . . . . . 333.7 Simulation Results . . . . . . . . . . . . . . . . . . . . . . . . 34

4 System Performance In Fading-Multipath Channels 374.1 Channel Model . . . . . . . . . . . . . . . . . . . . . . . . . . 374.2 Receiver Structure . . . . . . . . . . . . . . . . . . . . . . . . 374.3 CSI(channel-state information) . . . . . . . . . . . . . . . . . 384.4 MMSE detector . . . . . . . . . . . . . . . . . . . . . . . . . . 394.5 Simulation Parameters and Results . . . . . . . . . . . . . . . 40

5 Multiple Input Multiple Output (MIMO) 425.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 425.2 Basic Structure of MIMO System . . . . . . . . . . . . . . . . 42

5.2.1 SISO System . . . . . . . . . . . . . . . . . . . . . . . 425.2.2 SIMO System . . . . . . . . . . . . . . . . . . . . . . . 435.2.3 MISO System . . . . . . . . . . . . . . . . . . . . . . . 435.2.4 MIMO System . . . . . . . . . . . . . . . . . . . . . . 43

5.3 Channel Capacity . . . . . . . . . . . . . . . . . . . . . . . . . 445.3.1 Channel Capacity for SISO . . . . . . . . . . . . . . . 445.3.2 Channel capacity for SIMO . . . . . . . . . . . . . . . 455.3.3 Channel Capacity for MISO . . . . . . . . . . . . . . . 455.3.4 Channel Capacity for MIMO . . . . . . . . . . . . . . . 46

5.4 Diversity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 475.4.1 Types of Diversity . . . . . . . . . . . . . . . . . . . . 47

5.5 Advantages of MIMO . . . . . . . . . . . . . . . . . . . . . . . 48

6 MIMO-OFDM 496.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 496.2 Basic Structure of MIMO-OFDM System . . . . . . . . . . . . 506.3 Advantages of MIMO-OFDM . . . . . . . . . . . . . . . . . . 52

7 PAPR Reduction in MIMO-OFDM 537.1 System Model for Precoding MIMO OFDM . . . . . . . . . . 537.2 Precoder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 537.3 Modulator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 547.4 Almouthi Space Time Block Codes . . . . . . . . . . . . . . . 547.5 Simulation and results . . . . . . . . . . . . . . . . . . . . . . 56

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8 Conclusion and Future work Suggestions 578.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 578.2 Future Work Suggestions . . . . . . . . . . . . . . . . . . . . . 57

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Chapter 1

1 IntroductionWorld is moving very fast, and every day, there will be new deployments willoccur no one have idea. What will be the next technology will emerge inthe eld of wireless communication, if we go back to our passed there wasonly one way of a medium to people connect to each other is post Letter.Now a days world moving towards on fourth generation of wireless tech-nology. Orthogonal Frequency Division Multiplexing (OFDM) is promising

technique to perform Multicarrier modulation with maximum utilization of bandwidth and high mitigation characteristics prole against fading in Mul-tipath. MIMO in combination with other schemes can increase capacity, reli-ability, support to internet services and multimedia application. MIMO withOFDM reduces the equalization complexities by transmitting data on differ-ent frequency levels to gain spectral efficiency and error recovery features,it offers high spatial rate by sending data on multiple antennas and trans-mission in NLOS. Thus, MIMO-OFDM to achieve diversity it utilize threeparameters frequency(OFDM), time(STC) ,spatial(MIMO). MIMO-OFDMis reproductive and famous for services of the Wireless broad band access.Combination of MIMO and OFDM accumulates benets of each scheme,provides high throughput.

1.1 BackgroundThe basic idea of multicarrier modulation is quite simple and follows natu-rally from the competing desires for high data rates and ISI-free channels.Multicarrier Modulation (MCM) is the principle of transmitting data by di-viding the input data stream into some lower rate parallel bit streams andusing these substreams to modulate a number of carriers. The individual sig-nals are called sub-carriers, and the technique is coined as Frequency Division

Multiplexing (FDM). Multicarrier modulation divides the high-rate transmitbit stream into L lower-rate substreams, each of which has TS/L >> and ishence effectively ISI free. These individual substreams can then be sent overL parallel subchannels, maintaining the total desired data rate. OrthogonalFrequency Division Multiplexing (OFDM) is a unique case of multicarriertransmission orthogonal is a unique case of multicarrier transmission. InOFDM, the carriers are arranged such that the frequency spectrum of theindividual carriers overlap and the signals are still received without adja-cent carrier interference. In order to achieve this, the carriers are chosen

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to be mathematically orthogonal. The data rate on each of the subchan-nels is much less than the full data rate, so the corresponding subchannelbandwidth is much less than the total system bandwidth. The number of substreams is chosen to ensure that each subchannel has a bandwidth lessthan the coherence bandwidth of the channel, so the subchannels experiencerelatively at fading. Thus, the ISI on each subchannel is small. More-over, in the digital implementation of OFDM, the ISI can be completelyeliminated through the use of a cyclic prex. For a long time, usage of OFDM in practical systems was limited. Main reasons for this limitationwere the complexity of real time Fourier Transform and the linearity requiredin RF power ampliers. However, with the advent of Fourier transform elimi-

nated the initial complexity of OFDM where harmonically related frequenciesgenerated by Fourier and inverse Fourier transforms are used to implementOFDM systems [1]. Since 1990s, OFDM is used for wideband data commu-nications over mobile radio FM channels, High-bit-rate Digital SubscriberLines (HDSL, 1.6Mbps), Asymmetric Digital Subscriber Lines (ADSL, up to6Mbps), Very high-speed Digital Subscriber Lines (VDSL, 100Mbps), DigitalAudio Broadcasting (DAB), and High-Denition Television (HDTV) terres-trial broadcasting.

1.2 AdvantagesOFDM has many advantages over single carrier systems.

1.2.1 Lower implementation complexity

The implementation complexity of OFDM is signicantly lower than that of a single carrier system with equalizer. When the transmission bandwidthexceeds the coherence bandwidth of the channel, resultant distortion maycause inter symbol interference (ISI). Single carrier systems solve this prob-lem by using a linear or nonlinear equalization. The problem with this ap-proach is the complexity of effective equalization algorithms. OFDM systems

divide available channel bandwidth into a number of subchannels. By select-ing the subchannel bandwidth smaller than the coherence bandwidth of thefrequency selective channel, the channel appears to be nearly at and noequalization is needed.

1.2.2 Elimination of ISI

Furthermore, by inserting a guard time at the beginning of OFDM symbolduring which the symbol is cyclically extended, inter symbol interference

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pendently modulated subcarriers, which can give a large Peak-to-AveragePower Ratio (PAPR) when added up coherently. When N signals are addedwith the same phase, they produce a peak power that is N times the averagepower. In order to avoid nonlinear distortion, highly linear ampliers are re-quired, which cause a severe reduction in power efficiency. Several methodsare explained in the literature in order to solve this problem.

1.4 Previous techniques for PAPR reductionMost widely used methods are clipping and peak windowing the OFDMsignal when a high PAPR is encountered. However, these methods distort

the original OFDM signal resulting in an increase in the bit error probability.There are other methods that do not distort the signal. Two of these methodscan be listed as Partial Transmit Sequences [2][3] and Selected Mapping [4][5].The principle behind these methods is to transmit the OFDM signal with thelowest PAPR value among a number of candidates all of which represent thesame information. Coding is another commonly used method. In this case,the information bits are coded in a way that no high peaks are generated.Mainly, Golay codes, Reed Muller codes and linear block codes are used [6][7].

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Figure 2: Basic structure of a multicarrier system

2.2 FDMA and OFDMThe main reason to chose the FDM (Frequency division multiplexing) toequalize the conict with complexities and impulsive noise. It was rst pub-lished in 60s [9][10] FDM requires a bank of subcarrier oscillator, coherentdemodulator and bank of lters for each sub channel (parallel system). Be-tween subcarriers guard band is introduced, which lowers the efficiency of thespectrum. Weinstein and Ebert [11] apply DFT and IDFT to parallel datastream instead of subcarrier oscillators and demodulators in FDM, which giverise to OFDM. Similarly, in MCM (multi carrier modulation) bandwidth di-vided into many non-overlapping sub channels (parallel subcarriers) [12] andnot essential that all is sub carriers are orthogonal to each other [13]. Figure 3shows the different view of spectrums normal, FDM and OFDM respectively,

Figure 3: different view of spectrums

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2.3 Basic structure of OFDM systemA basic OFDM system architecture is shown in gure 4. Which is consistedof two parts rst one transmitting side other one is receiving side.

At the transmitting end, rst of all, input binary serial data stream isrst processed by serial to parallel converter. A single signal is divided into N parallel routes after N-point InverseFast Fourier transform (IFFT). Each orthogonal sub-carrier is modu-

lated by one of the N data routes independently. By denition the Nprocessed points constitute one OFDM symbol.

Then copies the last L samples of one symbol to the front as cyclicprex (CP) then converts modulated parallel data to serial sequence. After the process of digital to analog (D/A) conversion and radio fre-quency (RF) modulation the signal will be transmitted. Reception process is converse and self-explanatory to recover the infor-mation in OFDM system, demodulated signals are fed into an analog

to digital (A/D) converter, sample output and take timing estimationto nd initial position of OFDM symbol the digital down conversion is

carried out.

Removed the cyclic prex and convert the data into the parallel toserial. Then applies the Fast Fourier Transform (FFT) and transformationwill be conducted on the left sample points to recover the data in the

frequency domain.

The output of demodulation is passed to a decoder, which eventuallyrecover the original data after passing through the parallel to the serialconverter.

Orthogonal Frequency Division Multiplexing (OFDM) is deriving fromtechnique based on multi carrier modulation (MCM) and frequency divisionmultiplexing (FDM). It seems to be an optimal form of multicarrier modu-lation. It emerges modern digital modulation technique FFT (fast Fouriertransform), which inherently avoids demodulators, lters and bank of oscil-lators. It supports smart antennas, directional and advance antenna tech-niques. OFDM is a good technique against interferences and multi pathfading. OFDM uses subcarrier, which is mathematically orthogonal in which

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Figure 4: Basic structure diagram of OFDM

information can be sent parallel overlapping sub channels inversely relatedto data rate for each individual subcarrier, in return increases the time lapsof symbol, from which information or data can be extracted individually.Which solve the problem of delayed version of signals in multipath environ-

ment [13]. Each sub channel is orthogonal to each other and faces at fading[13]. Orthogonality depends up on carrier spacing. Carrier space is to choosethat it must be reciprocal of a symbol period. This is helpful in reducinginter-symbol interference (ISI) and good spectral efficiency of the receiverby correlation technique. In the digital domain, OFDM symbol is createdbefore transmission so data is arranged through common methods i.e. BPSKor 16-QAM. The data is converted into N parallel streams, which are to beconverted into an OFDM symbol. An OFDM symbol generated by an N subcarrier. In OFDM system, symbol consists of N samples and then theOFDM symbol is

xk =N 1n =0

an ej 2 kn

N . (1)

where an is the data symbol on the n th subcarrier,

1T T x(t)y(t)dt = 0. (2)

Mathematically, where x(t) and y(t) are two different independent signals[14]. OFDM is an attractive transmission scheme, its wide support to high

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data rate, and compatibility with multi path environment, makes it useful inhigh-speed modems.

Figure 5: OFDM bandwidth divided in many overlapping narrow strips (subcarrier)

In the simple narrow band scheme data transmitted sequentially in aserial form, each symbol occupies the whole bandwidth. While in paralleldata transmission many symbols transmitted simultaneously on each subchannel.

In OFDM, whole bandwidth divided into many overlapping narrow strips(sub carrier) as shown in gure 5 above. Which are orthogonal to each other.Lower data rate provided to each narrow strip (total bit rate still high) whichis useful to reduce the ISI (inter symbol interference) [15], and accuratedata information can be extracted from each sub carrier [14]. Spectrumof OFDM contains many subcarriers. Where the wireless channel possesdifferent frequency response to each subcarrier at a different time.

As data information distributed to each subcarrier. In case of deep fad-ing or selective fading cause, some part of information received with errorand other without error. By adding extra bits to transmitted informationas error correcting code, there is high probability of correcting information.Because code related to corrupted information might be transmitted in adifferent sub carrier who might not suffer to fade. In OFDM transmission, aseach subcarrier contains part of information so only particular, part of infor-mation destroys. While adjacent subcarrier suffer nearly at fading becauseit occupies little space in bandwidth. Which makes equalization at the re-

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ceiver simpler or by introducing coding equalization and its complexities canbe complete removed from OFDM receiver [15]. Encoded OFDM is calledCOFDM.

2.4 Mathematical RepresentationThe OFDM transmitted signal s(t) is sum of N OFDM symbols. EachOFDM symbol is multiplied by carrier ck . All these modulated symbols aremultiplexed and nally multiplexed signal p(t) is multiplied by carrier signal.

s(t) =N 1

k=1xk(t)e j 2 (f k + F )t (3)

Where,

f k = kNT

. (4)

e j 2F tN 1

k=0xk(t)e j 2f k t (5)

Where,ck = e j 2f k t = e j 2

kNT t . (6)

= e j 2F tN 1

k=0xk(t)e j 2

kNT t (7)

= e j 2F t p(t). (8)

Assuming amplitude of carrier signal is equal to 1 and phase is representedby C k , for symbol period C k cannot change (amplitude and phase), for everysymbol values of C k would be different. Total no of subcarriers available isN subcarrier. In order to main orthogonality sinc-shaped pulses are use todene subcarriers in frequency domain. sinc-shaped pulses chooses so as it

zero crossing occurs at the 1/T and multiple of 1/T . Equation (9) below is centre of carrier frequency and fc is main carrier frequency. Maximumvalue of each sub carrier spectra occurs at its own frequency and zero on thecentre of adjacent subcarrier frequencies [14].

f i = f c + iT

(9)

Where,

i = N 2

....N 2

.

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Figure 6: OFDM bandwidth divided in many overlapping narrow strips (subcarrier)

OFDM system is mainly used the division of the frequency selective chan-nel into smaller subchannels. These subchannels can be equal to coherencebandwidth, in which the channel is behaved like at fading channel, system

has been correctly designed then OFDM symbol frequency selective fadingchannel and the subcarrier signals at fading channel as shown in 7 [14].

2.5 Fourier TransformFourier transform is a mathematical method to convert a signal from the timedomain to frequency domain, or from the frequency domain to time domain,Fourier transforms of practical and famous technique is DFT(discrete Fouriertransform), which samples the signal in both temporal and frequency domain.FFT (Fast Fourier transform) is fast and efficient method of DFT used bycomputer application for analysis and signal manipulation. In OFDM, theincoming data is reshaping in order to parallel from serial bits of information,from serial and group of data bits in appropriate size according to design of OFDM and convert into complex numbers. Then these complex numbers ismodulated by using IFFT (inverse fast Fourier transform) in a base band toreshape again from parallel to serial for transmission [14][15]. Zeros pad at theend and start of a composite spectrum of subcarriers, to avoid interferencesbetween a next and previous composite spectrum.

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Figure 7: Orthogonality of subcarriers

2.6 Guard BandMultipath propagation cause copies of a symbol to be a delay in differenttime and attenuation. For that reason, the result in inter symbol interfer-ence(ISI) and creates another problem is ICI (inter carrier interference), itscause the energy spread of one sub carrier into another sub carrier due toDoppler Effect, so creates the Cross Talk. This problem resolved by insert-ing a guard band to symbol in OFDM. Guard band insertion to symbol timeincrease the temporal period of symbol .i.e. T t = T + T gb. T t is total symbolduration and T is original symbol time, while T gb is guard band extensiontime to symbol. T gb depends up on scenario and king of application. Usuallyit is < T/ 4. To reduce ISI guard band time T gb must be greater than chan-nel impulse response and delay spread by multipath [14][15]. T gb could bekept adaptive with channel situation. Increasing the time of a symbol whichis attained by using larger number of carrier, there is tradeoff between the

number of carriers and FFT size, Doppler shift, latency, carrier instabilityetc [15]. Guard time inserted between consecutive symbols of OFDM [14]Another way of extending symbol is to place end part of signal to start of signal, shown in gure 8.

2.7 InterleavingInterleaving is a simple technique to get rid off errors in the burst. In OFDMsymbol, information is distributed to many sub carrier frequencies. When

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Figure 8: guard band insertion and cyclic prex

frequency selective fading occurs at different points. OFDM symbol whilepropagation through channel and this channel cause deep fades data loss inthe burst. There is not good enough scheme to handle and recover data inburst error scenario. Interleaving technique rearranging data makes burst

error as random error, which could be recovered at the receiver by simplecoding. Interleaving is a method of rearranging bits in certain way, at receiverreverse of rearranging is performed to original shape, which makes error toappear in random. Interleaving method is also called block code, in whichdata is written in row by row and retrieve in column by column [14][15].

2.8 WindowingFFT of square wave is sinc-function. Sharp transition of bits shapes causespreading of signal into neighbor spectrum and leakage of energy. Windowingis a technique which causes a spectrum decreases rapidly to go down. Win-dowing technique is applied on each individual OFDM symbol. Time waveform is truncated by windowing scheme to make individual OFDM symbol.A periodic waveform is truncated to a single OFDM symbol length by ap-plying windowing in the time domain optimum windowing scheme is raisedcosine windowing technique. It accommodates channel bandwidth from cer-tain minimum value( R/ 2) to certain maximum value( R) [14].

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2.9 Cyclic Prex of OFDM SystemCyclic Prex (CP) is the use in OFDM system, can guarantee orthogonalityof signals even when the moving through multi-path channels [8]. To avoidISI, the condition; T G > T max should be satised, where T G is the length of CP and T max is the maximum delay spread [16]. As gure 9 shows that, aCP is a copy of the last part of an OFDM symbol moved to the front of a symbol. Assuming that T G is the number of the extended OFDM, thenthe time period of a OFDM symbol is T + T G , where T is cycled for theFFT transform, T G is the length of the guard interval, which is inserted tosuppress ISI caused by multipath distortion. An OFDM symbol includingCP can expresses as follows:

sn = sn (t) |t= nt x | =N 1

t=0die j. 2.

inN , n = N G , ..., 1, 0,...,N 1 (10)

Operation between the signal and channel changes from linear convolutionto cyclic convolution when CP is used with OFDM. In the frequency domain,linear weighing will be used. These changes avoid inter-symbol interference,while ensuring orthogonality among the sub-carriers all the time.

Figure 9: OFDM symbols with cyclic prex

2.10 OFDM Design IssuesThere are certain key factors needed to be taken under serious considerationwhen developing and designing OFDM system.

2.10.1 Useful symbol duration

The length of the useful symbol with respect of time affect the number of carriers and spacing between them. It is helpful in measuring. Larger symbol

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duration is helpful in accommodation delay prole of channel and cause in-crement number of subcarrier, reduces subcarrier spacing, higher FFT size.In practice there may become issue of subcarrier offset and instability of OFDM symbol. Subcarrier spacing and number of carriers depend up onapplication and requirement. In mobile environment due to Doppler shiftsubcarrier spacing is chosen to be large [17].

2.10.2 Number of carriers

Number of Subcarrier selection depends on the channel bandwidth, datarate, through put requirements depends up to case (ruler, urban, etc). If the

number of carriers is N then it would be reciprocal of duration of a symbolin time T i.e.N =

1T

.

Number of carrier selection depends up FFT size, which is supported by FFTmodule. For higher the number of carrier there would be the higher numberof complex point processing by FFT [17].

2.10.3 Modulation scheme

OFDM system having a capacity to the different modulation scheme can be

applied to each sub channel depends up on channel condition, data rate,robustness, throughput, channel and bandwidth. There could be differentmodulation scheme applied i.e. QPSK, 16 QAM, 64 QAM [13]. Modulationto each sub channel can be made adaptive after getting information andestimation of channel at the transmitter [17].

2.11 Advantages and Drawbacks of OFDMIn this section try to summarize the strengths and weaknesses of orthogonalfrequency division multiplexing (OFDM).

2.11.1 Advantages of OFDM

The following advantages of OFDM may be identied:

As OFDM is a parallel transmission system which converts the prob-lem of frequency selective fading to at fading by distributing datato sub channels, OFDM good to ght again multipath fading and torandomizing the errors in the burst [15].

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As compared to Single-Carrier systems OFDM equalization is very sim-ple.

The OFDM transmitter is low cost as the design is simple becausethe modulation technique is simpler implementation based on a highlyoptimized FFT/IFFT block. Also OFDM transmitters poss the abilityto implement the mapping of bits to unique carriers via the use of theInverse Fast Fourier Transform (IFFT) [18].

In a relatively slow time-varying channel, it is possible to signicantlyenhance the capacity by adapting the data rate per subcarrier accordingto the value of SNR for that particular subcarrier [19].

OFDM can be used for high-speed multimedia applications with lowerservice cost. OFDM can also support dynamic packet access. It is attractive for broadcast applications by using single frequency [19]. Subcarrier spacing could be adjustable according to the requirement of applications and data rate; it supports different modulation schemes

for different sub channels [20].

Smart antennas can be integrated with OFDM. MIMO systems andspace-time coding can be realized on OFDM, and all the benets of MIMO systems can be obtained easily [21].

2.12 Disadvantages of OFDM2.12.1 Strict Synchronization Requirement

OFDM is highly sensitive to time and frequency synchronization errors. De-modulation of an OFDM signal with an offset in the frequency can lead to

a high bit error rate. These are two sources of synchronization errors. Oneis caused by the difference between local oscillator frequencies in transmitterand receiver, while the other is due to the relative motion between the trans-mitter and receiver who gives Doppler spread. Local oscillator frequenciesat both points must match as closely as they can. For the higher num-ber of subchannels, the matching should be even more perfect. Motion of transmitter and receiver causes the other frequency error. So, OFDM mayshow signicant performance degradation at high-speed moving vehicles [22].To optimize the performance of an OFDM link, accurate synchronization is

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therefore, of prime importance. Synchronization needs to be done into threeaspects: symbol, carrier frequency and sampling frequency synchronization.A description of synchronization procedures is given in [19].

2.12.2 Peak-to-Average Power Ratio

Peak to Average Power Ratio (PAPR) is proportional to the number of sub-carriers used for OFDM systems. The PAPR for an OFDM system is givenby 10log(N ) where N is the number of subcarriers. For example, for a 48 sub-carrier system, such as 802.11a where 48 out of 64 subcarriers are active, thePAPR is approximately 17 dB. Therefore, OFDM system with a large num-

ber of sub-carriers will thus have a very large PAPR when the sub-carriersadd up coherently. A large PAPR of a system makes the implementation of Digital-to-Analog Converter (DAC) and Analog-to-Digital Converter (ADC)to be extremely difficult. The design of RF amplier also becomes increas-ingly difficult as the PAPR increases. To moderate the effect of such a largePAPRs on performance degradation of the OFDM system, the design of theOFDM system needs to incorporate costly RF hardware, such as efficientand large linear dynamic range power ampliers. Incorporating costly RFhardware, however, increases the cost of the OFDM system. There are ba-sically three techniques that are used at present to reduce PAPR; they areSignal Distortion Techniques, Coding Techniques and nally, the ScramblingTechnique which will be discussed later [21].

2.12.3 Co-channel Interference Mitigation in Cellular OFDM

OFDM system exhibit performance degradation due to frequency coherenceof the channel. Coherence bandwidth is required to closer the spacing be-tween the adjacent subcarriers. In many channels, adjacent subcarriers willfall within the coherence bandwidth and will thereby experience at fading.In cellular communications systems, co-channel interference (CCI) is com-bining adaptive antenna techniques, such as factorization, directive antenna,

antenna arrays, etc. Some are just prevention techniques but others may betruly interference cancellation methodologies. Using OFDM in cellular sys-tems will give rise to CCI. Similarly, with the traditional techniques, with theaid of beam steering, it is possible to focus the receivers antenna beam on theserved user, while attenuating the co-channel interferers. This is signicantsince OFDM is sensitive to CCI [21].

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Chapter 3

3 Investigation of SISO PAPR Reduction

3.1 PrecodingAs described in the previous chapter, most of PAPR reduction methods tryto exploit the subcarrier symbols of the OFDM block by creating some cor-relation between them. As a result, the reduction in PAPR achieved bythese techniques is relative and is obtained at the expense of either an addi-tional complexity to the OFDM transceiver, a high coding overhead, and/orthe need of some kind of transmitter/receiver symbol handshake. Signalcompanding is another method that has been proposed and studied in theliterature [23][24]. Companding is a nonlinear transformation applied to theOFDM signal giving quite low PAPR values. However, such a nonlinearlyoperation destroys the OFDM orthogonality property and degrades its per-formance, especially in fading-multipath channels. Other possible alternativesolutions are then to try to exploit other parameters of the OFDM signal.Exploiting the subcarrier waveforms of the OFDM signal appears as an at-tractive solution for reducing the PAPR of OFDM signals. This approach hasthe potential of reducing the PAPR of the OFDM signal without affecting thebandwidth efficiency of the system and, thus, leaves the chance to use codingfor channel protection. Such an approach has been adopted in [25], where aset of subcarrier waveforms was proposed. It has been shown that the PAPRof OFDM signals can be reduced if the subcarrier waveforms have differentshapes. It has also been shown in [25] that subcarrier waveform shaping inOFDM is a form of precoding scheme, where each OFDM block is linearlytransformed by a shaping matrix before modulation and transmission. Inthe literature, precoding has been considered as a way of maximizing thediversity gain of OFDM signals and of trying to take advantage of the fre-quency selectivity of the multipath-fading channel [26]-[30]. For instance, in

[26], linear constellation precoding (LCP), together with subcarrier grouping,has been designed to maximize both diversity and coding gains. It has beenshown that subcarrier grouping can reduce the complexity of the receiverwithout affecting the maximum possible diversity and coding gains. How-ever, Liu et al. [26] did not investigate the implication of LCP on the signalvariations of the OFDM signal. Clearly, regardless of the LCP scheme used,the PAPR of the OFDM will increase as the number of subgroup increases.This has been shown in [28], where the numerical results showed that thelowest PAPR ratio is obtained when only one group is used. However, using

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LCP with only one group will make the receiver too complex and limits thenumber of subcarriers that can be used. In fact, the precoder proposed in[27] was designed based on maximizing the diversity gain and minimizing thePAPR of the OFDM signal simultaneously. We consider precoding as a wayof reducing the PAPR of OFDM transmitted signals. Precoding in OFDMsystems consists of multiplying the modulated data of each OFDM block bya precoding matrix before OFDM modulation [inverse discrete Fourier trans-form (IDFT)] and transmission. A predened precoding matrix is used in theOFDM system, and thus, no handshake is needed between the transmitterand the receiver. Having the same precoding matrix for all OFDM blocks willalso avoid all the processing needed in block-based optimization methods. A

design procedure for good precoding schemes is proposed and analyzed inthis paper. We will show that it is possible to reduce the PAPR of OFDMsignals through precoding without destroying the delectability property of the different symbols of the OFDM block. The obtained results show that,with a good precoding matrix, the PAPR of OFDM modulated signals canbe made very close to that of single carrier signals.

3.2 Transmitter Model for a Precoded OFDM System(SISO)

An OFDM system with multiple phase-shift-keying (MPSK) modulation anda total of N baseband-modulated symbol per OFDM block is considered inthis paper. As shown in gure 10, the transmitter consists of a baseband(complex) modulator followed by a precoder and the conventional OFDMmodulator (IDFT). The incoming information data is rst modulated in base-band using a bandwidth-efficient modulation (MPSK-type modulation). Thebaseband-modulated stream, with data rate 1/Ts, is grouped into blocks of length N symbols each.

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Figure 10: Transmitter of precoded OFDM system

Each block of symbols is then precoded by an L N precoding matrix,denoted P , and dened as

P =

p0,0 p0,1 p0,N 1 p1,0 p1,1 p1,N 1... ... . . . ... pL1,0 pL1,1 pL1,N 1

(11)

Where pi,j s are the entries (complex numbers) of this precoding matrix, L =N + N p is the total number of subcarriers, and N p is the extra subcarriers(overhead) used with 0 N p < N . When no precoding is used, the matrixP reduces to an N N identity matrix, and no overhead is used.

X = [X 0, X 1, X 2 . . . . . . . . . X N 1] (12)

Which after serial to parallel conversion becomes

X =

X0X1...XN1

(13)

Where

X i = E S /T e j (0 + d2/M ) d {0, 1, ......., M 1} (14)And 0 is some initial phase.

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The precoding process (matrix) transforms this vector into a new vector of length L with

Y = PX =

Y 0Y 1...Y L1

(15)

Where

Yi = N 1m =0

P i,m Xm [i = 0, 1, 2, . . . . . . . L 1] (16)

These precoded symbols are then transmitted over the different subcar-riers of the OFDM-modulation scheme. In this case, the equivalent low passof the OFDM transmitted signal can be written as follows:

x(t) =L1

i=0Y ie j2 it / T T g t < T (17)

where T = NTs is the duration of the OFDM block, and Tg = GTs is a timeguard interval introduced between consecutive OFDM blocks at the trans-mitter to prevent the possible problem of intersymbol interference, which canbe caused by the communication channel, and to preserve the orthogonality

between the OFDM subcarriers. This guard interval is, in general, ignoredat the receiver before demodulation and signal detection.The OFDM-band pass signal is related to its equivalent low pass by thefollowing expression:

s(t) = Real {x(t) e j 2f c t} (18)

Where f cis the carrier frequency.In the linear case (when no power amplier is used) and for large values of thenumber of subcarriers, the spectrum of conventional OFDM signals goes to anideal band-limited rectangular spectrum. This means that the OFDM signalwithin each block appears as Gaussian with very high variations from onesample to the next. Thus, the power spectral density of the modulated signalwill be broadened by the nonlinear distortions of a high-power amplier. ThePAPR is one way to measure such variation of the transmitted signal.

3.3 PAPR of precoded OFDMThe PAPR of the precoded OFDM transmitted signal of equation (17) canbe dened as follows:

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PAPR = max |x(t)|2E { |x(t)|2}(19)

The value of the above expression will depend on the kind of precoderused at the transmitter. Our objective in this paper is to select a precoderthat minimizes the above expression. Combining equations (16) and (17),the equivalent low pass of the OFDM transmitted signal can be rewritten asfollows:

x(t) =L1

i=0Y ie j 2it/T (20)

=N 1

m =0X m (

L1

i=0P i,m e j 2it/T ) 0 t < T (21)

where we have ignored the guard interval, for now, since it is just anextension by periodicity. Using the above equation in (19), we can relate thePAPR of the OFDM signal to the different entries of the precoding matrix.For OFDM systems with MPSK modulation schemes and uncorrelated sym-bols within each OFDM block, the PAPR of the OFDM signal at a giventime instant t can be upper bounded as follows:

PAPR (t) 1N N 1m =0 |(

L1i=0

pi,m e j 2it/T )|2

(22)

and the maximum PAPR is then obtained as

PAPR max = max0t < T PAPR (t)= 1N

max0t < T

N 1m =0 |( L1i=0 pi,m e j 2it/T )|2 (23)

Where we have assumed that |X m |2 = E sWhere, m = 0,1,. . . . . . . . . ..,N-1, which represents the average-energy-per-transmitted symbol.

3.4 Selection Criteria Of Precoding MatrixWe notice that the PAPR of the OFDM signal is a function of the size of theOFDM block and the entries of the precoding matrix. Since the size of theOFDM block is xed, one can reduce the PAPR of the signal by a properselection of the precoding matrix P . However, selecting the proper matrixis not an easy task because its entries are complex numbers and they cantake any value, which makes computer-search methods very difficult to use.

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Thus, before doing any precoding design, we should take a closer look at thePAPR expression given in equation (22).From equation (22), we dene a set of time limited (complex) function

{pm (t )} as follows:P m (t) =

L1i=0 pi,m e j 2it/T , 0 t < T 0, otherwise (24)

for m = 0, 1, . . . , N - 1. With the above denition, the PAPR of theprecoded OFDM signal can be rewritten in terms of pm (t ) as

PAPR (t) 1N

N 1

m =0 |( pm (t))|2

(25)We notice that the PAPR ratio is now related to the sum of N positive

functions within the time interval 0 t < T . This gives some hint on how toselect the entries of the precoding matrix P . A possible solution is to makesure that the peak amplitudes of the N functions | pm (t)|, m = 0, 1, . . .,N -1 do not occur at the same time instant within the interval of denition. Byensuring that, the peak power of the OFDM signal can be reduced withoutaltering the average signal power.

A possible set of functions that avoids having the peak amplitudes tooccur at the same time instant can be obtained by selecting the differententries such that the different functions are cyclic shifts of each other withinthe time interval 0 t < T . In other words, we can impose the followingrelation between the different functions:

pm (t) = p0(t mT s + T ), 0 t < mT s p0(t mT s ), mT s t < T

(26)

and solve for the entries of the precoding matrix P .By letting p(t ) = p0(t ), we can relate all the different functions to this motherfunction p(t ) with

pm (t) = L1i=0 pi, 0e j 2im/N e j 2it/T , 0 t < T 0, otherwise (27)where p i,m are the entries of the precoding matrix P in equation (11).

It is clear that if p0(t) has only one amplitude peak, then all the otherfunctions will also have one amplitude peak and all the amplitude peaks willnot occur at the same time instant. Hence, this format will certainly reducethe peak-to-average ratio of the precoded OFDM scheme.

We also notice from the above expression that the entries of the precodingmatrix are related to each other. Furthermore, all the entries of the different

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columns of the precoding matrix are directly obtained from the entries of the rst column. This indicates that we only need to nd the rst columnof the matrix P , which will of course simplify the design of the precoderconsiderably. In fact, from equation (27), we notice that having a motherfunction p(t ), we can extract all the entries of the precoding matrix fromp(t ) as

pi,m = pi, 0e j 2im/N = e j 2im/N T

0 p(t)e j 2it/T dt (28)

With p(t ) as any of the complex functions dened within the time interval0 t < T .

3.5 Theorem 1With a precoding matrix designed according to equation (28), the maximumPAPR of uncoded OFDM transmitted signals is upper bounded as

PAPR max N (29)with equality when the precoding matrix is an N N identity matrix (noprecoding).

Proof:From equation (25), the maximum PAPR of the precoded OFDM transmittedsignal is upper bounded as

PAPR max = 1N

max0tT

N 1

m =0 | pm (t)|2

(30)

Using the relation given in equation (27), the maximum PAPR of the pre-coded OFDM signal becomes

PAPR max = 1N max0tT N 1m =0 | p( (t, m ))|

2

1N

1T s

T 0

| p(t)

| dt

2 (31)

where (t, m ) = (t mT s + T ), 0 t < mT s(t mT s ), mT s t < T

(32)

Using Schwarzs inequality in equation (31) and replacing T s by T/N , theupper bound of equation (29) is obtained.

Note that with this precoding scheme, the PAPR of the precoded OFDMsignal is always reduced in comparison with that of conventional OFDM.This improvement in PAPR is valid for any number of subcarriers N . The

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design of such a precoding scheme is also simple since all the entries of theprecoding matrix are obtained from its rst column.

A very important property of OFDM modulation is the separability of the N transmitted symbols of each OFDM block at the receiver for detection.For regular OFDM, each symbol is transmitted over a different subcarrier,and hence, symbol separability at the receiver is ensured by the orthogonalityproperty between the different subcarriers. In precoded OFDM, each symbolis spread by the precoding matrix over more than one subcarrier, as shownin equation (16). Hence, the orthogonality property between the differentsubcarriers alone is not enough to separate the different symbols of the blockin precoded OFDM. As a result of precoding, crosstalk between the different

symbols of the block will exist at the receiver, and symbol-by-symbol detec-tion is no longer optimum. However, with a proper selection of the precodingmatrix, we can eliminate this possible crosstalk and allow simple symbol-by-symbol detection for precoded OFDM signals. This can be done by selectingthe precoding matrix as an orthogonal matrix, i.e., a precoding matrix thatsatises the following relation:

P P = I (33)where I is the N N identity matrix, and P represents the Hermitiantranspose of the matrix P .Using equation (11), the above condition can be written as follows:L1

i=0 pi,m pi,k =

1, m = k0, m = k

(34)

Replacing pi,m by its expression given in equation (28) and with some ma-nipulations, the symbol-separability condition of the precoded OFDM signalbecomes

1T s

L1

i=0P (

iT

)2e j 2 (mk)i/N = T, m = k0, m = k (35)

WhereP (f ) =

T

0 p(t) e j 2f t dt (36)

is the Fourier transform of the function p(t ).For large number of subcarriers ( N 1), the above condition can be approx-imated by an integral as

(1+ )/T s

0P (f )2 e j 2 (mk)T s f dt = T, m = k0, m = k

(37)

with = Np/N , which is just the Fourier inverse of |P ( f )|2 at time instantt = ( m - k )T s .Denoting this function by q (t )

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q (t) =

(1+ )/T s

0P (f )2e j 2it/T dt (38)

we conclude that the symbol-separability property of OFDM is preservedwhen q (t ) satises the following criterion:

q (mT s ) = T, m = k0, m = k

(39)

which is simply the Nyquist criterion [31].The above conclusion indicates that in order to preserve the symbol-

separability property of the OFDM scheme, the function p(t ) should have abandwidth of at least 1 /Ts . A bandwidth comparable to the bandwidth of the OFDM signal.To summarize, a precoding matrix designed according to the following twocriteria.(1)

pi,m = pi, 0e j 2im/N = e j 2im/N T

0 p(t)e j 2it/T dt

Withi {0,1,........,L-1} and m {0,1,... . . . ,N-1}(2)

(1+ )/T s

0P (f )2 e j 2 (mk)T s f dt = T, m = k0, m

= k

can reduce the PAPR of the OFDM signal without altering the symbol-separability property of the OFDM scheme.

Note that by selecting a precoding matrix satisfying equation (39), webasically obtain an OFDM signal similar to a single-carrier signal due to thewide bandwidth of the set of functions {pm (t )} and the cyclic shifting intime. However, this precoding scheme does not remove any of the propertiesof OFDM, where as discussed in the following sections, simple (single tap)equalization is still possible. It should also be mentioned that this schemeis also different from single-carrier signals with cycle prex and frequencyequalization [25]. In the latter, a cyclic prex is added at the transmitter,and both fast Fourier transform and inverse fast Fourier transform (IFFT)operators are done at the receiver. The proposed precoded scheme in thispaper is an OFDM scheme and does not alter any of the structure andproperties of OFDM systems.

In the following example, we illustrate a possible precoding scheme de-signed based on the technique just described above and evaluate the PAPRof the corresponding precoded OFDM signal.

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3.6 Examples of precoding schemesA well-known function that satises the Nyquist criterion is the raised cosinefunction [31]. Consider the square root of a raised cosine function, which isdenoted psrc(t ), and having a Fourier transform [31]

P src (f ) =

T s sin( f T s2 ), 0


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Compute the entries of the precoding matrix P . The parameter L is a designparameter and will depend on the extra subcarriers used, since L = N +Np. It is easy to verify that this precoding matrix satises the orthogonalitycondition given in equation (33).

3.7 Simulation ResultsUsing equation (45), we plot the precoder matrix in the matlab and get thefollowing results.

Figure 11: Precoder

It is clear from the simulation result that precoding matrix is a sinc func-tion.the rst graph shows the all column results and second one show the onlyrst column of precoding matrix. It is also clear from the graph that pre-coding matrix distribute the whole power uniformly during the transmissiontime, Therefore it is better to redue the peak to average power ratio. Usingequation (23), we can now compute the maximum PAPR of the precodedOFDM signal and compare it to that of the conventional scheme.When the number of subcarriers N is large, the maximum of the PAPRwill occur only very seldom, and thus, the measure of such a parametermay not give the whole picture about the dynamic variations of the OFDMsignal. As shown in equation (19), the PAPR is a random variable and takesthe values between zero and PAPRmax. A better measure of the PAPRof communication signals is then to consider the complementary cumulativedistribution function dened as.P PAPR = P r (PAPR PAPR 0) (cumulative distribution Function)P r (PAPR PAPR 0)= 1- (P PAPR ) (complementary cumulative distribution)where PAPR 0 is the PAPR threshold.

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Figure 12: CCDF of OFDM signal using precoder

(Complementary distribution function of the PAPR of the OFDM signal withand without precoding. Precoder 1 is used, and N = 64.)Figure 12 illustrates the complementary cumulative distribution function of the PAPR of the precoded OFDM signal for the case of N = 64 subcarriersand Precoder . It is observed that the proposed precoding scheme providesconsiderable gain in PAPR for the OFDM signal when compared to thatof conventional OFDM. We also notice here similar results where the pre-coded OFDM scheme outperforms conventional OFDM in terms of PAPR.It compared to the case of the DFT matrix [27], we notice that with littleoverhead, the PAPR of the OFDM signal can be reduced with a reductionthat depends on the amount of overheads. For instance, the PAPR can bereduced by about 1.5 dB with an overhead of 10% and by about 3 dB withan overhead of 20% at a complementary CDF value of 10 3.

To assess the effects of the number of subcarriers on the signal variationsof the precoded OFDM signal, we have looked at the complementary cumu-lative distribution function of the precoded OFDM signal for the differentnumber of subcarriers. This is illustrated in gure 13 as a function of thePAPR threshold. Similar to the results of the maximum PAPR, we noticethat the complementary distribution function of the PAPR of the precodedscheme is not very sensitive to the number of subcarriers and especially athigh values of the PAPR threshold.

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Figure 13: CCDF for different block sizes

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Chapter 4

4 System Performance In Fading-MultipathChannels

As already shown in the previous chapter, it is possible to design precod-ing schemes that reduce the PAPR of OFDM signals without altering theproperties of symbol reparability of the OFDM block at the receiver. Thus,when the communication channel is ideal (additive interference only), theperformance of the precoded OFDM system will be identical to that of con-ventional OFDM. However, any frequency variation of the communicationchannel will destroy this Orthogonality property and can degrade the systemperformance. We will treat this case in this section and see what kind of reliable low complexity detector(s) can be used.

4.1 Channel ModelThe fading channel considered in this paper is a fading multipath channelwith coherence bandwidth smaller than the total bandwidth of the OFDMsystem and, thus, seen as frequency-selective fading. The fading process

is assumed to be stationary and slowly varying compared with the blockduration of the OFDM signal, such that it is approximately constant duringat least one block length. The complex baseband representation of the fadingmultipath channel impulse response can be described by [31]

h( ) =P 1

l=0h l ( l) (46)

where h l is a complex random-variable tap weight with variance pl , l is thetime delay of the l th path, and P is the total number of received paths. Thetap weights are assumed independent of different paths.

4.2 Receiver StructurePassed through the channel, the equivalent lowpass of the received precodedOFDM signal during the zeroth block interval is given by

r (t) =P 1

l=0h lx(t l) + z (t) T g t < T (47)

where z (t ) is complex Gaussian random process with zero mean and powerspectral density N 0, and T g is the time guard interval.

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Assuming a guard interval larger than the maximum delay spread of thefading-multipath channel, the output sample of subcarrier i , after demodu-lation, is obtained as

D i = 1 T T 0 r(t)e j 2it/T dt

= T H iY i + Z i i= 0, 1, . . . . . . . . . . . , L 1 (48)where Y i is as given in (16), Z i is a complex Gaussian random variable withzero-mean and variance N 0, and H i is the channel frequency response atsubcarrier i with

H i =P 1

l=0h le j 2i i /T (49)

which is a complex Gaussian random variable with variance

22 =P 1

l=0 pl (50)

Figure 14: Precoded OFDM tranciever

4.3 CSI(channel-state information)To allow coherent detection at the receiver, channel-state information (CSI)needs to be known at the receiver. For wireless systems employing coher-ent OFDM modulation techniques such as HIPERLAN/2 and IEEE 802.11a

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systems, one or two complete OFDM symbols are provided in the preamblein order to support channel estimation. For the proposed precoded OFDMscheme, the pilot symbols are precoded the same way as the informationdata symbols. As shown in equation (48), a priori knowledge of the trans-mitted precoded pilot signal facilitates the generation of the CSI vector atthe receiver. Denoting the pilot OFDM symbol by

[C0, C1, . . . , CN2, CN1] ,

the CSI at subcarrier i can be obtained from equation (48) as

H i = Di

T Y i

= H i + Zi

T Y i

i= 0, 1, . . . . . . . . . . . , L

1 (51)

Where

Y i =N 1

m =0 pi,m C m i= 0, 1, . . . . . . . . . . . , L 1 (52)

which is known at the receiver since it is completely dened by the pilotOFDM symbol and the precoding matrix. Considering all the received sub-carrier samples during the zeroth OFDM block, the expression in equation(48) can be rewritten in a matrix form as follows:

D = THPX + Z (53)where X is as given in equation (13), Y is as given in equation (15)

Z = [Z0, Z1, .................., ZL1]T (54)

is the noise vector, and H is an L L diagonal matrix representing thechannel coefficients of the different subcarriers withH = diag{H 0, H 1, H L1} (55)

4.4 MMSE detectorHaving the demodulated OFDM signal, the next step is to separate the mod-ulated symbols X m s using the received vector given in equation (53) and makea decision on the transmitted symbols. It is observed that when the channelis frequency selective, a direct multiplication of the vector D by the precod-ing matrix P does not give a perfect separation, and crosstalk between thedifferent modulated symbols will remain. Several detection techniques thatdeal with this type of signal separation have appeared in the literature. Theoptimum detector is of course the maximum-likelihood sequence-estimationdetector where all the N symbols are detected jointly. However, such adetector has a complexity that increases exponentially with N and is not

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practical even for moderate number of subcarriers. Fortunately, much sim-pler detection schemes do exist and can provide very good performance infading-multipath channels.

Here, we consider the minimum-mean-square-error (MMSE) detector,which is basically a one-tap equalizer per subcarrier. In this detector, eachelement of the received vector is rst weighted using the following weightingparameter:

Gi =H i

H i 2 + 2z /2s, i= 0, 1, . . . . . . . . . . . , L 1 (56)

where 2z is the variance of the additive white noise, and 2S is the varianceof the data-transmitted symbol. The parameter Gi is used to compensatefor the channel phase and to minimize interference between the modulatedsymbols of the OFDM block. The obtained vector is then multiplied by theprecoding matrix P .

The block diagram of the OFDM system using this receiver structure isillustrated in gure 15 In matrix form, the received vector after weightingand multiplication by the matrix P is written as follows:

V = P GD= TP GHPX + Z (57)

where G is an L

L diagonal matrix with

G = {G0, G1, . . . , GL1} (58)and Z is the complex Gaussian noise vector. Decisions on the transmitted

symbols of the OFDM block are then carried out using the vector V.

4.5 Simulation Parameters and ResultsIn this section, the performance of the precoded OFDM scheme is evaluatedthrough computer simulations for different multipath radio channels. The

system parameters were adjusted according to the HIPERLAN/2 standard[32] with an OFDM modulation using coherent QPSK and having a total of N = 64 subcarriers. The entries of the precoding matrix are obtained fromequation (40) with an overhead of = Np/N = 10%. This gives a total of L = 70 subcarriers for the OFDM system. The MMSE detector discussedin the previous section is employed at the receiver, where we have assumedthat perfect CSI is available. The multipath radio channel considered is asspecied in [33]. It contains different channel models, representing differentenvironments, and with tapped delay lines with a total of 18 taps. Each tap

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suffers independent Rayleigh or Rician fading with a mean corresponding toan exponentially average power-delay prole.

Figure 15: Performace in fading multipath channels

It is clear from the simulation results that bit error probability for precodingsystem goes to zero much earlier as compared to the conventional system.In terms of bit to noise ratio the precoding system gives the gains of morethan 25 dB as compared to the conventional system. Preocoding is goodtechnique to increase the performance (in terms of bit error probability andBit to Noise ratio) of the system in multipath fading channel.

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5.2.2 SIMO System

SIMO stands for single input and multiple outputs. It system model uses onlythe single antenna at the transmitter side and multiple receiving antennas.For uplink purpose SIMO system are preferably, as shown in gure 17 [17].

Figure 17: SIMO System

5.2.3 MISO System

MISO stands for multiple input and single output. It system model hasbeen only multiple transmitting antennas and one receiving antennas. Fordownlink purpose MISO, systems are preferably, as shown in gure 18.

Figure 18: MISO System

5.2.4 MIMO System

MIMO stands for multiple inputs multiple outputs. It system model usesmultiple antennas both for transmission and reception. A number of anten-nas are placed at the transmitting and receiving ends. Their distances areseparated far enough. For both uplink and downlink purpose, MIMO systems

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are effective. In MIMO system, multiple transmitting and receiving anten-nas will achieve antenna diversity without reducing the spectral efficiency, asshown in gure 19.

Figure 19: MIMO System

5.3 Channel CapacityChannel capacity can be dened as the number of bits per channel per Hz.According to recent research on Shannons capacity has shown that there ishuge channel capacity could be attained from the MIMO systems relativelysingle antenna system. Which is also depended up on different scenarios,fading, channel knowledge, impulse response of channel quality, correlationgain of channel on both antenna elements and transmitter receiver channelquantity knowledge [37].Channel capacity for different antenna system canbe analyses as.

5.3.1 Channel Capacity for SISO

According to Shannon for SISO system, the channel capacity as shown ingure 20 [38]

Figure 20: Channel Capacity for SISO system

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5.3.2 Channel capacity for SIMO

According to Shannon for SIMO system, the channel capacity as shown ingure 21 [38], where M shows the multiple receiver, In which we have asingle antenna on transmitting side and multiple antennas on receiving sidethe channel capacity of SIMO systems provides receiver diversity because of multiple antennas at the receiver side and capacity is slow logarithmic risingwith increasing number of antennas. SIMO system model it is a kind of smartantenna in which there is Single Input at the Transmitter and on the receiverside. There are having multiple Outputs, it is used for different purpose asshortwave radio operators, military, amateur and at frequency below 30 MHzsince the rst world war [39].

Figure 21: Channel Capacity for SIMO system

5.3.3 Channel Capacity for MISO

gure 22 shows the channel capacity of MISO system. In which N showsthe multiple transmitter antennas, The MISO channel model provides trans-mits diversity because of multiple numbers of antennas at transmitter side,and slow logarithmic rise of capacity with increasing number of antenna.MISO is used for the improvement of transmission at a distance; the multi-ple transmitters are used to be combined and minimize errors and optimizedata speed. It is also kind of smart antenna technology that uses multipletransmitters and single receiver. MISO technology is used widely in digitaltelevision, wireless local-area networks (WLANs) etc [40].

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Figure 22: Channel Capacity for MISO system

5.3.4 Channel Capacity for MIMO

In gure 23, shows the channel capacity of a MIMO system, in which wehave multiple antennas on both sides either on receiving or transmittingside, M and N shows multiple transmitter and receiver antennas, and it isalso in the category of smart antenna, and it improves the communicationperformance, MIMO technology is used in modern wireless communicationWi-Fi, WiMAX, HSPA, 4G and 3GPP LTE (Long Term Evolution).

Figure 23: Channel Capacity for MIMO system

In MIMO multiple antennas channel faces multiple input, and outputare used, and its capacity is also determined by extended Shannons capacity[38]. Antenna with N input from transmitter and M output in a receiverchannel is expressed as N M matrix of channel H as shown in belowequation (60):

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C = log2det(I + 12nHR xH H ) (59)

Where H is N x M channel matrix, Rx is covariance of input signal x,H H transpose conjugate of H matrix and is the variance of the uncorrelatedand Gaussian noise [41]. Since equation (3.1) is obtained by large theoreticalcalculations, but practically it has never been achieved yet. To get preciseresults linear transformation at both transmitter and receiver can be per-formed by converting MIMO channel ( N M ) to a SISO sub channel min(N M ). According to singular value decomposition (SVD) every matrixcan be decomposed. Suppose the channel matrix H transformation is given

by

H = U DV H (60)

Where the matrix U is N x N matrix, V is N c x N c matrix and D isa non-negative diagonal matrix of N x M Therefore, capacity of N SISOsub channels is some of individual capacity and results from the total MIMOcapacity [42].

5.4 Diversity

Diversity scheme is a technique which is used to improve the performance of the communication system by effectively transmitting the same informationmultiple times to improve the signal to noise ratio such that transmittedsignal is detected correctly.

5.4.1 Types of Diversity

To increase the capacity of MIMO system one of the methods is diversitytechnique. There are several types of diversities.

1. Frequency Diversity: In this type of diversity, the information is trans-mitted different frequencies. To achieve the uncorrelated diversity, thecarrier frequency should be separated suitable.

2. Time Diversity: Time diversity can be explained like, different signalare transmitted on the different time interval which is depending onfading range. If the time interval between time slots is longer, then thefading becomes much low [43].

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3. Space Diversity: It is a type of diversity in which there are two ormore signals is sent over different propagation paths by using multipleantennas at both side transmitter and receiver. The spacing is carefullychosen to ensure the independence of possible fading events occurringin the channel [44].

4. Transmit Diversity: In transmit diversity multiple antennas are placedat a transmitter side, and data are sent through multiple channels.

5. Receive Diversity: In receive diversity of multiple antennas are placedin a receiver to pick the independent copies of transmit signal [45].

5.5 Advantages of MIMOMIMO had some signicant advantages over SISO, SIMO, MISO and othertechniques:

MIMO enhanced the result of QoS and coverage area due to array gain. Higher the multiplexing gain which in the result increase spectral effi-ciency.

MIMO increase QoS service, higher diversity gain and fewer chance tolose information.

Co-channel interference is minimized which is helpful in increasing cel-lular capacity [46].

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Chapter 6

6 MIMO-OFDM

6.1 IntroductionIn advance broad-band access in LAN and MAN OFDM (stands for frequencydivision multiplexing) is used in different combinations and techniques, e.g.MIMO. This can combat better against multipath fading (deep fading) andalso supports high data rate. Over a radio link like HDTV, it supports mul-timedia applications. MIMO-OFDM reduces the receiver complexities andmanipulations as they distribute over multiple sub carriers the data informa-tion and transmit at different frequency levels, which are helpful in spectralefficiency and error control transmission. All individual functions of OFDMsystem such as IDFT/DFT and CP are applied to individual transmit anten-nas and receiver antennas (MIMO), and then this makes the combination of MIMO-OFDM. Also for error-free transmission it supports Alamouti schemeand with the maximum degree of diversity. MIMO-OFDM sends the streamof independent data information to increase a spatial rate over different an-tennas [47].

In OFDM the bandwidth is divided into narrow band at fading channelsand data are transmitted on each channel. Thus, we can say that it is thetechnique which converts frequency selective channels to many at fadingchannels and to each of sub channels the MIMO is applied [48].

Lipson wireless rst introduced the MIMO-OFDM scheme. In NLOS, itallows transmission and successful communication. It performs communica-tion on NLOS paths, like base station using MIMO-OFDM utilizes multipathscenario. Three techniques are used by MIMO-OFDM to achieve diversityof time, frequency and spatial. Consider MIMO-OFDM system having Ntransmitter antennas and M receiver antennas as in MIMO technique, spa-tial multiplexing is applied. Encoding can be performed collectively or per

antenna. Individual encoding on each antenna branch of a transmitter sys-tem is called per antenna coding (PAC) [49].In high-speed wireless communication, combining MIMO and OFDM

technology, OFDM can be applied to transform frequency-selective into par-allel at MIMO channel to reducing the complexity of the receiver, throughmultipath fading environment can also achieve high data rate robust trans-mission. Therefore, MIMO-OFDM systems obtain diversity gain and codinggain by space-time coding, at the same time. The OFDM system can be re-alized with simple structure. Therefore, MIMO-OFDM system has become

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a welcome proposal for 4G mobile communication systems.

6.2 Basic Structure of MIMO-OFDM SystemAt the transmitting end, a number of transmission antennas are used. Aninput data bit stream is supplied into space-time coding, then modulatedby OFDM and nally fed to antennas for sending out (radiation). At thereceiving end, in-coming signals are fed into a signal detector and processedbefore recovery of the original signal is made. Figure 24 shows the basicstructure of a MIMO-OFDM system.

Figure 24: Basic structure of MIMO-OFDM system

Presently, most of the companies and research institutions developedMIMO OFDM experimental system. Airbust production of Iospan Com-pany that rst used MIMO and OFDM technology in the physical layer atthe same time for wireless communication systems [50]. In MIMO OFDMsystem, the frequency response of kth subcarrier can be expressed as follows:

H (q,p)k (n) =

L1

l=1h

(q,p)l (n)W klk (61)

Where,K = 0, . . . , K 1, h

(q,p)l (n) (62)

is the impulse response, that is from pth transmitter antenna to lth channelof q th receiver antenna. n is sequence number of the symbol and K is thetotal number of subcarriers. Assuming that W K = exp 2 j/K , while M andN is the total number of transmitter and receiver antennas. The outputresponse for the q th receiver antenna can be written as:

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yqk(n) =M

p1H (q,p)k (n)X

pk (n) + k(n) (63)

q = 1, . . . , N ; k = 0, . . . , K 1 and k(n) is Gaussian noise with variance 2n .For N transmit antenna, there would be N OFDM transmitter or N

parallel branches of OFDM system for N antennas. Raw digital bits aremultiplexed in to N branches. For each antenna, there is individual OFDMtransmitter performing encoding interleaving, bit mapping (QPSK 16 QAM),IFFT, Guard interval or cycle prex to each symbol and nally up convertthe OFDM symbol to radio frequency then transmit over a radio link [49].

The receiver should have information and estimation about channels forreliable wireless channel transmission. The transmitter for this purposetransmits a training sequence periodically or with every packet to a receiver.So that according to channels variations the receiver could update. Thetraining sequence depends upon applications and requirements sent by thetransmitter to the receiver. In order to keep track of phase drift and ampli-tude variation the pilot symbols are used to insert it into OFDM modulator.

At the receiver multiple antennas receive information and on the basisof training sequence it performs estimation and correction in the pream-ble or performs forward error correction, which depends upon the encodingtechnique and equalization stages. In nding phase drift the Estimation orforward error correction, frequency offset and symbol timings. Guard band isremoved, and information is presented to IFFT. Then per OFDM sub chan-nel MIMO detection is performed. Received signal of each sub channel is sentto the MIMO detector to retrieve N signal transmitted on a particular subcarrier. De mapping, de inter leaving and decoding is done per transmittersymbol, a resultant combined value is raw digital data, which was originallysent by transmitter and all these operations are performed over each indi-vidual branch of receiver antennas. To the fourth-generation communicationsystem, the MIMO OFDM technology is the door step [49].

With MIMO OFDM, there are some certain limitations such as extra RFcost, antenna sizes and complexities at receivers [50]. For cell mobiles be-cause of mutual coupling and power limitation it is still big issue to designand manufacture multiple antennas on cellular mobiles. MIMO-OFDM isa technique which arises for mobile and wireless broad band access in com-binations of OFDM and MIMO. Due to its support to high-speed wirelessbroad-band access, low complexities (in respect of equalization at a receiver)and spectral efficiency and exibilities, it is considered to be the prominentand promising candidate for further wireless technologies. Examples are LTE,

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4G, IEEE 802.16 (WiMAX), and IEEE 802.11n.

6.3 Advantages of MIMO-OFDM

Less interference. Diversity Gain. Increase data capacity. Power efficiency.

Bandwidth gain.

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Chapter 7

7 PAPR Reduction in MIMO-OFDM

7.1 System Model for Precoding MIMO OFDMThere are three Major parts in our Precoding MIMO OFDM system model.

Precoder Modulator Almouthi codes.As shown in the gure 25 below,

Figure 25: MIMO OFDM System Model

7.2 PrecoderAs discussed in the chapter 3, Precoder is an L N precoding matrix,denoted P , and as dened in equation (12)

P =

p0,0 p0,1 p0,N 1 p1,0 p1,1 p1,N 1... ... . . . ... pL1,0 pL1,1 pL1,N 1

(64)

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Where pi, j s are the entries (complex numbers) of this precoding matrix, L =N + N p is the total number of subcarriers, and N p is the extra subcarriers(overhead) used with 0 N p < N . When no precoding is used, the matrixP reduces to an N N identity matrix, and no overhead is used.

7.3 ModulatorWe use QPSK modulator and Demodulator for our system. Digital data istransferred in an OFDM link by using a modulation scheme on each subcar-rier. A modulation scheme is a mapping of data words to a real (In phase)and imaginary (Quadrature) constellation, also known as an IQ constella-

tion. For example, 256-PSK (Phase Shift Keying) has 256 IQ points in theconstellation, constructed in a square with 16 evenly spaced columns in thereal axis and 16 rows in the imaginary axis. The number of bits that can betransferred using a single symbol corresponds to log2 (M), where M is the or-der of a modulation scheme used. Thus 256-PSK transfers 8 bits per symbol.Each data word is mapped to one unique IQ location in the constellation.

7.4 Almouthi Space Time Block CodesAlmouthi space time block coding is useful for 2x2 MIMO systems. Thereare two transmit and receive antennas for Almouthi codes. We assume inour whole discussion that transmitter does not have channeled knowledge,but receiver knows about the channel H.

Suppose that we wish to send a message m = 0,1,2,3 (2 bits) using QPSKmodulation. The modulated baseband signal at each antenna at time n isthen given by

s (n) = Ae j4 e j

m2 , m = 0; 1; 2; 3 (65)

where A is the transmitted amplitude, and A 2 the transmitted power. Thetotally transmittede power by the two antennas is then

P tot = 2A2 (66)

We consider the following channel model for two consecutive channel uses attime n and n+1

y(n) = Hc(n) + v(n)y(n + 1) = Hc(n + 1) + v(n + 1) (67)

where c(n) are code words, and v(n) are zero mean uncorrelated additivenoise with variance 2v .The signal to noise ratio is dened by

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SN R = 10log10 P tot

2v= 10 log10 2A

2

2v. (68)

The two code words c(n ) and c(n + 1) in the Alamouti code are givenby

c(n) = s(n )

s(n + 1) , c(n + 1) = s (n )

s(n + 1) . (69)

By inserting the code words equation(69) in the signal model equation(67)we get the following signal model in explicit component form

y1 (n) = H 11s (n) + H 12s (n + 1) + v1 (n)y2 (n) = H 21s (n) + H 22s (n + 1) + v2 (n)y1 (n + 1) = H 11s(n + 1) + H 12s(n) + v1 (n + 1)y2 (n + 1) = H 21s(n + 1) + H 22s(n) + v2 (n + 1)

(70)

Now, the Alamouti decoding is given by

s(n) = H 11y1(n) + H 12y1(n + 1) + H 21y2(n) + H 22y2(n + 1)s(n + 1) = H 12y1(n) + H 11y1(n + 1) + H 22y2(n) + H 21y2(n + 1)

(71)

By inserting the signal model equation(70) into the decoding equations (70)we obtain nally

s(n) = ( |H 11|2 + |H 12|2 + |H 21|2 + |H 22|2)s(n) + v (n)s(n + 1) = ( |H 11|2 + |H 12|2 + |H 21|2 + |H 22|2)s(n + 1) + v (n + 1)

(72)

Or

s (n) = tr {H H H}s (n) + v (n)s (n + 1) = tr {H H H}s (n + 1) + v (n + 1) (73)

where v (n) and v (n + 1) are noise terms which are linear combinations of the original noise terms v1 (n), v2 (n), v1 (n + 1) and v2 (n + 1) .

Equations (72) and (73) show the action of the Alamouti coder/decoder.Note that the two equations are decoupled , i.e. the estimate s (n) is propor-tional to s (n) (except for the noise) and is independent of the other mes-sage s (n + 1), etc. The gain factor for each decoded symbol is tr H H H =

|H 11|2 + |H 12|2 + |H 21|2 + |H 22|2which displays the spatial diversity obtained.Eventhogh there is strong fading, it is very unlikely that all channel compo-nents should fade at the same time.The channel capacity in this case when the receiver knows the channel butthe transmitter [51].

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7.5 Simulation and resultsUsing above parameters and equations, we simulate whole system in Matlaband obtained the following results.

Figure 26: SNR Vs BER of MIMO OFDM System

The above simulation result shows that the precoded MIMO system gives2dB SNR(signal to noise ratio) gain as compared to conventional MIMO sys-

tem.We simulate whole MIMO system in a frequency domain at transmitterand receiver. Therefore, we do not have so appreciable difference betweenconventional MIMO system and precoded MIMP system. But still it has 2dBsignal to noise ration advantage as compared to conventional MIMO system.If all the processing done in frequency domain and transmission and recep-tion will be done in time domain then we get hopefully remarkable differencein SNR (signal to noise ratio) and BER (Bit Error Rate) between precodedMIMO system and conventional MIMO syatem.

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8 Conclusion and Future work Suggestions

8.1 ConclusionsThis thesis utilizes a newer technique to reduce the PAPR for OFDM trans-mission. The method is based on signal precoding, where each data block ismultiplied by a precoding matrix prior to OFDM modulation and trans-mission. This method is data-independent and, thus, avoids blockbasedoptimization. It also works with an arbitrary number of subcarriers andany type of baseband modulation used. The PAPR distribution function of the OFDM transmitted signal was investigated for two suggested precodingschemes. The obtained results showed that precoding can reduce the PAPRof OFDM signals considerably. In terms of BER performance, it has alsobeen shown that the proposed precoding scheme is similar to the previouslyproposed precoding schemes for OFDM as it takes advantage of the frequencyvariation of the fading-multipath channel and improves the BER of OFDMsignals. It compared to conventional OFDM (no precoding), relative powergains of up to 15 dB for a bit-error probability of 10 4 can be obtained,depending on the frequency selectivity of the channel. The implementationcomplexity of the proposed technique is acceptable, since it does not requireany optimization from one OFDM block to the next. The technique is alsovery exible, and it can be used for OFDM and discrete multitone transmis-sions. It is proposed in the thesis that the implementation complexity canfurther be reduced by common subexpression elimination in the OFDM sys-tem employing precoding as a way of a PAPR reduction scheme. A precodedOFDM system has been used whereby constant multiplications involved inthe precoding are decomposed into shifts and additions and then commonsubexpression elimination involving multiple variables is employed to reducethe number of operations required. It has been found that with commonsubexpression elimination almost 35% to 36% fewer adders can be used toimplement the same precoding scheme. Which would result in improvedperformance, less area coverage and reduced power consumption.

8.2 Future Work Suggestions1. The implementation of the OFDM system employing precoding as a

way to reduce PAPR along with the common subexpression eliminationto reduce the system complexity on hard ware is intended.

2. The thesis focused on the PAPR reduction of Single input single outputOFDM system. Investigating the usage of the precdoing scheme on

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Multiple input multiple output system in intended.

3. Another possibility to combined pre-FFT smart antennas with pre-coded MIMO OFDM system to analyze bit error rate, signal to noiseratio and channel capacity of the system.

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[2] L.J. Cimini, N.R. Sollenberger, P eak-to-Average Power Ratio Reductionof an OFDM Signal Using Partial Transmit Sequences, IEEE Communi-cations Letters, Vol.4, No.3, March 2000, S.H. Mller.

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