Keywords: Cyclic Prefix, Guard Interval, OFDM, PAPR

Transcription

1 Volume 3, Issue 6, June 013 ISS: 77 18X International Journal of Advanced Research in Comuter Science and Software Engineering Research Paer Available online at: Performance Analysis of Synchronization on Orthogonal Frequency Division Multilexing Communication System amisha Rani *, Mohit Mehta Deartment of Electronics & Communication Engineering Punjab college of Engineering & echnology, Lalru, India Absract - Orthogonal frequency division multilexing signal suffer from high eak to average ower ratio. his high eak to average ower ratio results in the oeration of the ower amlifier in saturation region. Orthogonal frequency division multilexing signals may also result in out band distortion due to overlaing of side lobes. hus guard band is inserted to reduce the out band radiation. Cyclic refix and zero adding are methods to insert guard interval Orthogonal frequency division multilexing signal. In this aer these guard interval techniques has been studied and analyzed in detail and bit error erformance of Orthogonal frequency division multilexing system with 64 oint Fast Fourier ransform has been analyzed for different values of cyclic refix. From the study it has been concluded that bit error rate erformance in an Additive White Gaussian oise channel is less as comare to the Raleigh fading channel as the length of cyclic refix is increased. Crest factor is an imortant arameter for calculation of eak to average ower ratio which is equal to square root of eak to average ower ratio. In this Paer, simulation have also been carried out for the measurement of crest factor for clied, clied & filtered Orthogonal frequency division multilexing signal. Cliing is most widely technique for eak to average ower ratio reduction. But it results in cliing distortion which can be filtered out by using ulse shaing filter. his Paer also resents the bit error rate erformance of the clied and clied & filtered techniques. Results shows that cliing ratio has significant effect on eak to average ower ratio, smaller the cliing ratio the greater is the eak to average ower reduction effect. Results also show that bit error rate erformance decreases as cliing ratio decreases. Keywords: Cyclic Prefix, Guard Interval, OFDM, PAPR I. IRODUCIO Wireless communications is regarded as the most imortant develoment with wide range of alications. Advances and develoment in this field aim for fast and reliable communication. Multi Inut Multi Outut- Orthogonal frequency division multilexing (MIMO-OFDM) system rovides high data streams of radio links. MIMO system uses orthogonal frequency division multilexing (OFDM) as modulation technique [1]. OFDM is a multicarrier transmission scheme which facilitates high data rate transmission and rovides with sectral efficiency. OFDM has been roosed in various wireless communication standard such as IEEE 80-11a standard for wireless Local Area etwork (WLA), IEEE-16a Standard for wireless Metro olitician Area etwork (WMA), digital audio/video broadcasting systems in Euroe. Single carrier transmission scheme is not useful for a high data rate wireless transmission because such transmission requires a high comlexity equalizer to deal with the inter-bol interference roblem. Multile carriers can be used for high rate data transmission to overcome the frequency selectivity of the wideband channel exerienced by single-carrier transmission. An OFDM signal can have a high eak-to-average ower ratio (PAPR) at the transmitter, which causes signal distortion such as in-band distortion and out-of band radiation due to the nonlinearity of the high ower amlifier (HPA) and a worse bit error rate (BER) []. Advantages of OFDM system has many advantages such as High sectral efficiency, Simle imlementation by Fast Fourier transform (FF), Low receiver comlexity, Robust ability for high data rate transmission over multiath fading channel, High flexibility in terms of link adatation, Low comlexity multile access schemes such as orthogonal frequency multile access (OFDMA). Disadvantages of OFDM system like Relatively higher eak to average ower ratio (PAPR) comared to single carrier system, which tends to reduce the ower efficiency of the RF amlifier, Sensitive to frequency offsets, timing errors and hase noise [3]. his aer is organized in the following way:section gave the overview of OFDM. Section 3 exlain the detail of OFDM Parameters. In this section we also exlained the different arameters like guard interval,. Comlementary Cumulative Distribution Function (CCDF), measurement of crest factor using CCDF lots. In next Section exlained the simulation result shown that BER erformance analysis for OFDM system with 16-QAM and varying CP. Finally, in section 5 exlained the conclusion of best results for better understanding of the mentioned system. II. BASICS OF OFDM OFDM is a method of digital modulation in which a signal is slit into several narrowband channels at different frequencies. he technology was first conceived in the 1960s and 1970s during research into minimizing interference among channels near each other in frequency. 013, IJARCSSE All Rights Reserved Page 896

2 amisha et al., International Journal of Advanced Research in Comuter Science and Software Engineering 3(6), June - 013, A. OFDM ransmission Scheme: Orthogonal frequency division multilexing (OFDM) transmission scheme is similar to the FM transmission scheme only in the sense that it emloys multile subcarriers. Fig 1 shows the overview of OFDM transmission scheme. OFDM does not use individual bandlimited filters and oscillators for each subchannel and the sectra of subcarriers are overlaed for bandwidth efficiency. On the contrary in FM scheme the wideband is full divided into orthogonal narrowband subchannels. he multile orthogonal subcarrier signals, which are overlaed in sectrum, can be roduced by generalizing the single-carrier yquist criterion in equation (1) into the multi-carrier criterion. i G f (1) i (a) Overview of OFDM ransmission scheme (b) IDF/ DF imlementation of OFDM scheme (c) he sectrum of OFDM signal Fig 1 Structural and sectral characteristics of OFDM ransmission scheme Practically, discrete Fourier transforms (DF) and inverse DF (IDF) rocesses are useful for imlementing these orthogonal signals. Fig 1(b) shows the IDF/DF imlementation of OFDM scheme. he DF and IDF used can be imlemented efficiently by using fast Fourier transform (FF) and Inverse Fast Fourier ransforms (IFF). Fig 1 (c) shows the sectrum of OFDM signal. he sectrum of the OFDM signal can be considered as the sum of the frequency shifted sinc functions in the frequency domain, all subcarriers being of the finite duration. III. OFDM PARAMEERS A. Guard Interval: Orthogonality can be defined for two signals if the integral of the roducts for their common (fundamental) eriod is zero. Mathematically it can be defined as: 013, IJARCSSE All Rights Reserved Page 897

3 amisha et al., International Journal of Advanced Research in Comuter Science and Software Engineering 3(6), June - 013, k i j t j t j fkt j f it e e dt e e dt 1 1 ( ki) j t 1 e dt 0 0 In equation () the time-limited comlex exonential signals 1, integer k=i () 0, otherwise { } j f k t 1 e k 0 is considered which reresent the different subcarriers at fk k / in the OFDM signal, where 0 t. Equation () can be written in the discrete time domain taking the discrete samles with the samling instances at t n n / s, n = 0, 1, as: k k n i n 1 j n i s 1 j j j n s 1 1 e e e e ( ki) 1 j n 1 e n0 n0 n0 Orthogonality is an essential condition for the OFDM signal to be ICI-free. 1, integer k=i (3) 0, otherwise B. OFDM Guard Interval: An OFDM signal may incur out-of-band radiation, which causes non-negligible adjacent channel interference (ACI). It is clearly seen from Fig that the first side lobe is not so small as comared to the main lobe in the sectra. herefore, OFDM scheme laces a guard band at outer subcarriers, called virtual carriers (VCs), around the frequency band to reduce the out-of band radiation [3]. he OFDM scheme also inserts a guard interval in the time domain, called cyclic refix (CP), which mitigates the inter-bol interference (ISI) between OFDM bols [4]. Fig Power sectrum of OFDM signal (db) he OFDM guard interval can be inserted in two different ways. One is the zero adding (ZP), that ads the guard interval with zeros. he other is the cyclic extensions of the OFDM bol with cyclic refix (CP) or cyclic suffix (CS) [5]. C. BER of OFDM Scheme: he analytical BER exressions for M-ary QAM signaling in AWG and Rayleigh channels are resectively given as ( M 1) 6Eb log M Pe Q. M log M o M 1 (4) For AWG channel and for Rayleigh fading channel its M 1 3 log M / ( M 1) Pe 1 M log M 3 log M / ( M 1) 1 Where and M denote Eb and the modulation order, o resectively [6]. Q(.) is the standard Q function defined as (5) 013, IJARCSSE All Rights Reserved Page 898

4 amisha et al., International Journal of Advanced Research in Comuter Science and Software Engineering 3(6), June - 013, t / Q( x) e dt (6) x If used subcarriers out of total (FF size) subcarriers (excet vc used carrying data, the time-domain SR, SR t, differs from the frequency-domain SR, virtual subcarriers) are used for SR f,[] as: used SRt SR f 10log [ db] (7) D. CCDF Measurement Of OFDM Signals: Comared with single-carrier systems, OFDM systems are known to have a high PAPR (Peak-to-Average Power Ratio) because the transmit signals in an OFDM system can have high eak values in the time domain since many subcarrier comonents are added via an IFF oeration. Decreasing the SQR (Signal-to-Quantization oise Ratio) of ADC (Analog-to-Digital converter) and DAC (Digital-to-Analog Converter) while degrading the efficiency of the ower amlifier in the transmitter the high PAPR is considered one of the most detrimental asects in the OFDM system. In the ulink the efficiency of ower amlifier is critical due to the limited battery ower in a mobile terminal hence the PAPR roblem is more imortant. E. Peak-to-Average Power Ratio (PAPR): PAPR is the ratio between the maximum ower and the average ower of the comlex ass band signal st ()[7], that is, j f ct si t j f ct E{ si ( t) } max Re( si ( t) e max ( ) PAPR{ si ( t)} E{Re( s ( t) e } I he above ower characteristics can also be described in terms of their magnitudes (not ower) by defining the crest factor (CF) as (8) Passbandcondition: CF Basebandcondition: CF PAPR PMEPR F. CCDF Plots: A CCDF curve shows how much time the signal sends at or above a given ower level. he ower level is exressed in db relative to the average ower. A CCDF curve is basically a lot of relative ower levels versus robability. Mathematically CCDF can be exlained with a set of data having the robability density function (PDF). o obtain the Cumulative Distribution Function (CDF), the integral of the PDF is comuted. hen inverting the CDF results in the CCDF. It concludes that the CCDF is the comlement of the CDF or CCDF = 1 CDF. G. Measurement Of Crest Factor Using CCDF Plots: An OFDM signal with subcarriers exhibits the maximum ower when every subcarrier comonent coincidently has the largest amlitude with identical hases. he maximum ower becomes larger as increases and the robability that maximum-ower signal occurs decreases as increases. Cliing And Filtering: In PAPR reduction schemes cliing, being considered the simlest, limits the maximum of transmit signal to a resecified level. Cliing has some disadvantages which are listed as follows: BER erformance degradation occurs because of the in-band signal distortion caused by Cliing. Out-of-band radiation is caused by cliing imosing out-of-band interference signals to adjacent channels. he out-of-band signals caused by cliing can be reduced by filtering it may affect high-frequency comonents of in-band signal (aliasing) when the cliing is erformed with the nyquist samling rate in the discrete-time domain. But if cliing is erformed for the sufficiently-oversamled OFDM signals (e.g., L 4 ) in the discrete-time domain before a low-ass filter (LPF) and the signal asses through a band-ass filter (BPF), the BER erformance will be less degraded [8]. Peak re-growth is the roblem faced by the system when filtering the clied signal is used to reduce out-of-band radiation. he signal after filtering oeration may exceed the cliing level secified for the cliing oeration [9]. A PAPR reduction scheme using cliing and filtering is shown in Figure 4.9 where L is the oversamling factor and is the number of subcarriers. In this scheme, the L-times oversamled discrete-time signal x'[ m ] is generated from the IFF ( X '[ k] with ( L 1) zero-adding in the frequency domain) and is then modulated with carrier frequency f to yield a ass band signal x [ m ]. Let x [ m ] denote the clied version of x [ m ], which is exressed as: c 013, IJARCSSE All Rights Reserved Page 899 c

5 amisha et al., International Journal of Advanced Research in Comuter Science and Software Engineering 3(6), June - 013, A x [ m] A xc [ m] x [ m] x [ m] A (9) A x [ m] A Or x [ m] if x [ m] A x [ ] c m x [ m] A otherwise x [ m] (10) where A is the re-secified cliing level. ote that Equation (10) can be alied to both baseband comlex-valued signals and assband real-valued signals, while Equation (9) can be alied only to the assband signals. Let us define the cliing ratio (CR) as the cliing level normalized by the RMS value s of OFDM signal, such that A CR (11) Fig. 3 Block diagram of a PAPR reduction scheme using cliing and filtering he values of arameters used in the QPSK/OFDM system for analyzing the erformance of cliing and filtering technique are shown in able 1. able 1 Parameters used for simulation of cliing and filtering Parameters Value Bandwidth, BW 1 MHz Samling frequency, f BW. L with 8 MHz Oversamling factor, L = 8 Carrier frequency, f c s MHz FF size, 18 umber of guard interval samles (CP) 3 Modulation Order QPSK Cliing ratio (CR) 0.8, 1.0, 1., 1.4,1.6 and / in the baseband and ass band OFDM signals with subcarriers, It has been known that resectively. In general, the erformance of PAPR reduction schemes can be evaluated in the following three asects [10]: In-band rile and out-of-band radiation that can be observed via the ower sectral density (PSD). Distribution of the crest factor (CF) or PAPR, which is given by the corresonding CCDF. Coded and uncoded BER erformance. IV. RESULS A. Simulation results of Guard Interval he effect of ISI (inter bol interference) can be simulated as the length of a guard interval (CP or ZP) varies. he BER erformance of an OFDM system with 64-oint FF (=64) and varying guard interval in the AWG or a multiath Rayleigh fading channel is resented in the below simulation results. he BER erformance with CP of length 16 samles, as shown in Fig 4(d), is consistent with that of the analytic result in the Rayleigh fading channel. his imlies that the OFDM system is subjected to a flat fading channel as long as CP or ZP is large enough. able shows 013, IJARCSSE All Rights Reserved Page 900

6 amisha et al., International Journal of Advanced Research in Comuter Science and Software Engineering 3(6), June - 013, the effect of ISI on BER erformance at different SR with different length of GI in AWG channel. he effect becomes significant as the length of GI decreases. (a) Cyclic refix = (b) Cyclic refix = 4 (c) Cyclic refix = 8 013, IJARCSSE All Rights Reserved Page 901

7 amisha et al., International Journal of Advanced Research in Comuter Science and Software Engineering 3(6), June - 013, (d) Cyclic refix = 16 Fig 4 BER erformance for OFDM system with 16-QAM and varying CP able BER at different SR with different CP for AWG channel Cyclic refix length Signal ower CP- CP-4 CP-8 CP e e e e- 00 BER at BER at BER at BER at B. Simulation Results For Measurements Of Crest Factor: Simulation has been done using MALAB in order to obtain the CCDFs of Crest Factor for the clied OFDM signal and Clied & Filtered OFDM signal. Crest factor being the square root of PAPR facilitates in determining the PAPR distribution. he PAPR distribution and the BER erformance are shown with Fig 5 and Fig 6 resectively. Performance of OFDM signal is analyzed using different values of Crest ratio. he erformance of signal after cliing and after cliing & filtering are resented. Fig 5 PAPR Distribution of OFDM signal 013, IJARCSSE All Rights Reserved Page 90

8 amisha et al., International Journal of Advanced Research in Comuter Science and Software Engineering 3(6), June - 013, Fig 6 BER Performance of OFDM signal V. COCLUSIO In this aer investigations are done on different arameters of Orthogonal frequency division multilexing system with different exerimental values in order to conclude the best results for better understanding of the mentioned system. Bit Error Rate erformance of the system is analyzed by varying the guard interval in different fading environments. he effect of Inter Symbol Interference is significant in Rayleigh as comared to Additive White Gaussian oise and becomes more significant with the decreasing Guard Interval. Crest Factor being the square root of eak to average ower ratio of an Orthogonal frequency division multilexing signal can be used to define eak to average ower ratio distribution. Cliing and filtering are the techniques used to reduce eak to average ower ratio in a Orthogonal frequency division multilexing system. It is shown by the Comlementary Cumulative Distribution Functions of crest factor that the eak to average ower ratio values decreases after cliing whereas increases slightly after filtering. he significance of Cliing Ratio is also shown. Variation in the cliing ratio affects the bit error rate and the eak to average ower ratio distribution of the Orthogonal frequency division multilexing signal. REFERECES [1] Evegenii Krouk, Sergei Semnov, Modulation and coding techniques in wireless communications, 011. [] E. Costa, M. Midro, and S. Puolin, Imact of amlifier nonlinearities on OFDM transmission system erformance, IEEE Communication Lett., vol. 3, , Feb [3] Hongwei Yang, A road to future broadband wireless access: MIMO-OFDM- Based air interface." IEEE Communications Magazine, vol 43, 53-60,Jan [4] Peled, A. Frequency domain data transmission using reduced comutational comlexity algorithms. IEEE ICASSP, 5, , [5] Xiaojing Huang, Effect of DC offset on OFDM system with zero added suffix. IEEE, 006. [6] Proakis, J.G. Digital Communications, fourth edition, McGraw-Hill, ew York, 001. [7] Palicot, J. and Lou et,y. Power ratio definitions and analysis in single carrier modulations. EUSIPCO, Antalya, urkey, Se [8] Ochiai, H. and Imai, K. On cliing for eak ower reduction of OFDM signals. IEEE GC, vol., , Dec , IJARCSSE All Rights Reserved Page 903

9 amisha et al., International Journal of Advanced Research in Comuter Science and Software Engineering 3(6), June - 013, [9] Han, S.H. and Lee, J.H. An overview of eak-to-average ower ratio reduction techniques for multicarrier transmission. IEEE Wireless Communication, 1(), 56 65, 005. [10] Li, X. and Cimini, L.J. Effects of cliing and filtering on the erformance of OFDM. IEEE Communication.Letter, (0), , [11] Richard Van ee and Ramji Prasad OFDM for Wireless Communication, Artech House,. [1] Simon Litsyn, Gerhard Wunder, Generalized bounds on the crest factor distribution of OFDM signals with alications to code design., IEEE osium of Information theory, , March 004. [13] Wonhoon Jang, uno Borges Carvalho, Crest factor reduction through scaling and recovering by frame in OFDM communications systems. IEEE, , IJARCSSE All Rights Reserved Page 904