PAPR Reduction Technique Using Tone                Reservation Scheme along with Gradient                              Alg...
The PAPR of an OFDM symbol is defined as theratio of the maximum to the average power of the OFDM                         ...
scheme for mobile users sharing an OFDM uplink. In thismethod of Tone reservation of OFDM, I propose to allocate         F...
reduction. In fig. 3 and 4, CDF shows at one of its point,probability 0.8, PAPR is reduced to 7 dB instead of 7.9 dBunder ...
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Papr Reduction Technique Using Tine Reservation

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Papr Reduction Technique Using Tine Reservation

  1. 1. PAPR Reduction Technique Using Tone Reservation Scheme along with Gradient Algorithm Sunjeev Kumar Gupta Telecommunication Department of Nepal Telecom (NTC), URL: www.ntc.net.np, Kathmandu, Nepal PH: +977-9855068555, E-mail: sk.gupta@ntc.net.np /4u.gupta@gmail.com components such as power amplifiers (PAs). The nonlinear Abstract-As we know multi carrier signal can support effects on the transmitted OFDM symbols are spectralhigh transmission data rate in wireless channel environments spreading, intermodulation, and changing the signalbut meanwhile, large number of subchannels in the multi constellation. Some recent work have investigate OFDMcarrier signals cause large Peak to Average Power Ratio reduction technique via selected mapping(SLM), Partial(PAPR) when using multi carrier modulation technique like Transmit Sequence(PTS), Repeated Clipping and FilteringOFDM which substantially reduces efficiency of transmit highpower amplifier. In this Paper, we are greatly concerned with (RCF), Block coding, Interleaving, Active ConstellationTone Reservation technique (TR) applied for complex Extension (ACE), Nonlinear Companding Transformbaseband signals of OFDM systems with N subcarriers, L (NCT), Tone Injection and Tone Reservation.reserved subcarriers don’t carry any data information, areonly used for PAPR reduction and further elaborates gradient Though any of the methods is implemented, theyalgorithm after N-point IFFT computation iteratively for phase have the same disadvantages in common that additionalappraisal simultaneously by using subset of the sub-carriers power is needed. So it is obvious that this additional powercalled the peak reduction tones (PRT). must be weakest possible so that the method is viable in practice. I this paper, a set of tones in each sub band of 1. INTRODUCTION system OFDM is allocated in order of improve reduction of Orthogonal Frequency Division Multiple(OFDM), PAPR without any degradation of BER and data bit ratewhich is a multi-carrier communication technique, has degradation and without sending any SI. For this I will usebecome widely accepted primarily because of its robustness unused carriers to generate the corrective signal [2].against frequency selection fading channels which are Among the schemes using PRT, the most widelycommon in broadband mobile wireless communications [1]. used Tone Reservation scheme which is one of good PAPRIn OFDMA, closely spaced and overlapped subcarriers are reduction performance when operating with optimized PRTdivided into groups and assigned to multiple users for positions. In this, I present complementary cumulativesimultaneously transmissions. distribution function (CCDF) for measurement of PAPR Comparing to frequency division multiple access reduction.(FDMA), where any overlapping of the frequency spectrum 2. PAPR DERIVATION PATTERNof different users introduces multiple-access interference(MAI), Orthogonality of subcarriers guarantees that there is Let there is N subcarriers in which there is Lno intercarrier interference (ICI), which prevents MAI reserved unused subcarriers computed under N-point IFFTamong users in OFDM systems. In OFDM, closely spaced and FFT for modulation and demodulation. The OFDMmultiple subcarriers are assigned to different users for samples dn at the output of the IFFT are given by:parallel signal transmissions. Generally concerned hereabout the interleaved subcarriers assignment scheme ispreferred because it provides maximum frequency diversityand increases the capacity in frequency-selective fadingchannels. While OFDM has great advantage of having simple For n= 0,1….N-1, where Dk is the quadratureequalization, it has inherent drawbacks of PAPR which phase shift keying (QPSK) or quadrature amplitudedistorts the signal if the transmitter contains nonlinear modulation (QAM) data symbol transmitted through the Kth sub-carriers.
  2. 2. The PAPR of an OFDM symbol is defined as theratio of the maximum to the average power of the OFDM Memorysymbol and can be expressed as the ratio: Controller Reserved Tone (L) Where PAPR () represents PAPR calculation, d=(d0, d1, d2…..dN-1) and E[] denoted the expectation. Transmitted Tone Signal N point S/P Gradient It has assumed that Dk implies {1,-1, j,-j} (i.e Dk is Allocation IFFT Converter Algorithmto be a QPSK data symbol). From the central limit theory, (N) Informatthe real and imaginary values of dn become Gaussian (N-L) Data iondistributed for N>>1, each with mean zero and variance½.Thus the amplitude of dn has a Rayleigh distributionwhile the power distribution becomes a central chi-squaredistribution with two degree of freedom [3]. Figure 1. Structure of OFDM transmitter using Tone Reservation. 3. PROPOSED TRANSMITTERFOR TONE RESERVATION (TR) SCHEME NO In the structure of OFDM system transmitter,tones are reserved for PAPR reduction and are assigned fordata information. All tones are allocated according to YESpredetermined tone locations and after that IFFT is executed PAPAR Contrand the gradient algorithm is operated. IFFT Adder Calculat Output oller ion The structure of TR scheme is composed of three Transmittedparts: PRT reservation, IFFT, and IGA. First, is the PRT Signalreservation, it is considered to have L sub-carriers out of Nsub-carriers are reserved for PAPR reduction. Subsequently,the data and null symbols are modulated by IFFT on (N-L) Scalingand L sub-carriers respectively. Finally the transmitted Peak Circul &portion is x (tr) with PAPR reduced via Gradient Algorithm Detect ar Phase(IGA). ion Shift Rotatio n Details operation of IGA are as follow: First, theindex of the OFDM sample with the maximum absoluteamplitude (i.e. arg[max[dn]] is detected, and then apredetermined peak reduction kernel (PRK), denoted by P, Figure 2. Procedure of Gradient Algorithm.is an impulse like time domain signal and can be obtainedby setting all the PRTs to a constant. The circularly shifted If the number of Iteration reaches predeterminedPRK is scaled by a given step size and by the difference maximum iteration number, control escapes the process andbetween the maximum absolute amplitude if the OFDM resulting signal is transmitted. If not clipping operation issample and a desired threshold. The PRK is then phase- executed iteratively [4].rotated in such a way that the phase of the PRK is theopposite to that of the OFDM sample with the maximum 4. MATHEMATICAL DERIVATIONabsolute amplitude. Finally, the modified PRK c is added to FOR PAPR REDUCTIONthe OFDM symbol d and the PAPR of the modified OFDM The idea behind in this paper is to add an artificialsymbol d+c is calculated. If the PAPR is smaller than the signal to unused carriers. The signal addition is frequencydesired threshold, xth is transmitted; otherwise the above domain. Thus, the PAPR of the resulting signal is given by:procedure is repeated iteratively. Where x (m) is multi-carrier signal of user m and c (m) is an additive corrective signa and E [] denoted the expectation l. In this portion, I introduce a PAPR reduction
  3. 3. scheme for mobile users sharing an OFDM uplink. In thismethod of Tone reservation of OFDM, I propose to allocate Figure 4. Simulation Parametersa set of tones in each of interleaved subbands. All users andthe base station (BS) know the positions of the reserved IFFT size (nFFT.): 128tones. The set of tones reserved, no data is transmitted and Symbols (s): 10000the reserved tones do not interface with data transmission. Modulation: BPSKEach user can design its compensating signal Cm without Total subcarriers used (N): 100knowledge of other users’ compensating signals [5]. Total unused carriers (L): 28However, adding a signal Cm to Xm to reduce the PAPR Figure 5. Simulation Parametersincreases, the transmit power. The relative increase meanpower delta E due to PAPR reduction as [8]: IFFT size (nFFT.): 128 Symbols (s): 10000 Modulation: BPSK and QPSK Total subcarriers used (N): 128 Total unused carriers (L): 0 The parameter must be as small as possible in Figure 6. Simulation Parametersorder to be compatible with current power amplifiers [7].Indeed, it is easy to understand that if one increases IFFT size (nFFT.): 128indefinitely the average power of the signal Xm+Cm one Symbols (s): 10000would have obviously PAPR=0dB but a signal which can’t Modulation: BPSK and QPSKbe transmitted, Thus the relative mean power must be upper Total subcarriers used (N): 100bounded and can be written as: Total unused carriers (L): 28 is a constant closely related to the characteristic of the amplifier. 5. SIMULATION RESULTS For evaluating the performance of the proposedmethod, CDF curves of OFDM signal is presented. In mysimulation, the bandwidth is 10MHz is divided into N=128subcarriers and purposed unused subcarriers L=28 as thereserved tone for PAPR reduction. To evaluate the PAPRreduction performance, I concentrate on the complementarycumulative distribution function of the user’s PAPR, in allthe cases the maximum computation IFFT size is 128 andgradient algorithm used 30 iteration for computation of eachsimulation result given below.Matlab simulation for this purpose is presented sequentiallyas below [6]: Figure 3. PAPR Performance measurement with BPSK modulation without using unused carriers L=0.Figure 3. Simulation Parameters As seen to the simulated Figures, It is concluded that, for BPSK and QPSK modulation pattern, ToneIFFT size (nFFT.): 128 Reservation offers remarkable PAPR reduction whenSymbols (s): 10000 reserving reasonable amount subcarriers (L). The amount ofModulation: BPSK reserved Tones resembles the tones don’t contain “Actual”Total subcarriers used (N): 128 information data but this gives us enhancement of PAPRTotal unused carriers (L): 0
  4. 4. reduction. In fig. 3 and 4, CDF shows at one of its point,probability 0.8, PAPR is reduced to 7 dB instead of 7.9 dBunder BPSK modulation when L=28 unused subcarriers isused. Likewise, in fig 5 and 6, CDF shows at one of itspoint, probability 0.5, PAPR is reduced to 6.6 db instead of7.4 db under QPSK modulation when L=28 unusedsubcarriers is used. 6. CONCLUSIONS In this paper. I have proposed PAPR reductionusing Tone Reservation (TR) along with GradientAlgorithm which is one of the promising techniques interms of Performance and Complexity enhancement. Takinginto account the importance of total subcarriers used andsubstantially taking care towards to the unused reserved tone(L) which plays vital role for PAPR reduction as clearlyjustified in Simulation results in both case of Modulationtechniques BPSK and QPSK. 7. REFERENCES Figure 4. Performance of Tone Reservation with BPSK modulation using unused carriers L=28. [1]S. Zid, and R. Bouallegue, “Reducing PAPR of OFDM signal using Partial Transmit Signal (PTS) and Selected Mapping (SLM)”, IEEE ISIVC Bilbao, Spain, July 2008. [2]Seungsoo Yoo and Sun Yong,”PAPR reducing scheme for OFDM system: selecting mapping of partial tones (SMOPT)”, IEEE Transactions, Feb 2006. [3]Kelvin Huang, SUID,”Reducing the Peak to Average Power Ratio in OFDM”, Dec 5th, 2003. [4]Sung Eun Park,Jae YoelKim and Suwon-si,”Tone Reservation Methodfor PAPR Reduction Scheme”,IEEE 802.16 Broadband Wireless Access Working Group,31th,2003. [5] Zid and Bouallegue,”SOCP approach for reducing PAPR for different kind of allocations OFDM in Uplink via Tone Reservation”, SUP’ COM, 6’Tel Laboratory. [6] Google Search on www.dsplog.com and use of Language of Technical Computation “MATLAB”. Figure. 5 PAPR Performance measurement with BPSK and QPSK modulation without using unused carriers. Figure 6. Performance of Tone Reservation with BPSK and QPSK modulation using unused carriers L=28.

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