Int. Journal of Electrical & Electronics Engg.
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A Review Paper on: The PAPR Analysis of MIMO-OFDM Systems 1
Raj Lakshmi Shukla , 2Garima Saini
12
1
ME Scholar, Assistant Professor, ECE Department 1,2 NITTTR Chandigarh
rajlakshmi.shukla@gmail.com,2garimasaini_18@rediffmail.com
ABSTRACT:Orthogonal frequency division Multiplexing (OFDM) is one of the most popular multicarrier or multiplexing modulation techniques which transmits many signals over a single path in high speed wireless communication. OFDM convert high data rate stream in to smaller data rate stream. Due to this high data rate and ability to combat frequency selective fading, OFDM has a strong candidate for 4G wireless networks. Because of OFDM-MIMO advantages in multipath fading channel e.g. stout against ISI, ICI and some other advantages like best QoS for multiple users, efficient convention of bandwidth it is suggested to be the modulation technique for next generation 4G networks e.g. LTE. OFDM combined with multiple-input multiple-output (MIMO) to increase system capacity over the time variant frequency-selective channels and the diversity gain. The radio transmitter stations for covering and getting enough transmitted power in their desired area has to use High Power Amplifier (HPA) operable near to the saturation region or else, an aspect memory-less nonlinear distortion will affect the communication path. But along with all its advantages there are some disadvantages also. e.g. High PAPR (Peak to Average Power Ratio) at the transmitter end and BER (Bit Error Rate) at the receiving end. As OFDM is only used in the downlink of 4G networks. To reduce the problems of OFDM-MIMO systems some procedures as SLM, PTS, Clipping, Coding, & Pre-coding etc are suggested but none of them is proficient enough to reduce the PAPR and BER up to standard value. This Paper will discuss some techniques of PAPR reduction, and both their advantages and disadvantages.
and this indirectly leads to very inefficient amplification and increases in transmitter power. There are several reduction technique that have been proposed to reduce the PAPR problem in MIMO-OFDM systems such as Clipping And Filtering[2], Partial Transmit Sequences (PTS)[3], Selected Mapping Technique[4], tone reservation (TR) and tone injection (TI)[5]. And some coding techniques. SLM and PTS are scrambling techniques which uses probabilistic methods [4]. The principle of probabilistic method depends on reducing the probability of high PAPR by generating several OFDM symbols (multiple candidates) carrying the same information and selecting the one having the lowest PAPR [5]. The absence of the PAPR reduction techniques will cause the increase in the transmit power, the in Bit error rate at the receiver, the loss in data rate, and the computational complexity. [6] II SYSTEM MODEL
Keywords: Orthogonal Frequency Division Multiplexing multiple in multiple out (OFDM-MIMO), Peak–to-Average Power Ratio (PAPR), High Power Amplifier (HPA), selective mapping (SLM) Transmit sequence (PTS), Probability Distribution Function (PDF).
I. INTRODUCTION MIMO-OFDM is an attractive technique for high data rate wireless communication systems, but it exhibits a large peak-to-average power ratio (PAPR), due to the superposition of the individual nonlinear region of High Power Amplifier (HPA) [1-3], and the signal distortion induces the degradation of Bit Error Rate (BER). High PAPR causes signal distortions such as the in-band deformation and out-of-band emission and induces the degradation of bit error rate performance due to the nonlinearity effects [1]. Therefore, RF power amplifiers should operate in a very large linear region to avoid the signal peaks from getting into the non-linear region of the power amplifier causing in-band distortion. i.e., inter modulation among the subcarriers and out of band radiation. To surmount this, the power amplifiers should be operated with a large power back-offs 123
As in Fig 1 in a MIMO-OFDM system with N subcarriers (or tones) the individual data streams are first passed through OFDM modulators which perform an IFFT on blocks of length N followed by a parallel-to-serial conversion. A cyclic prefix (CP) of length Lcp _ L containing a copy of the last Lcp samples of the parallelto-serial converted output of the N-point IFFT is then prepended. The resulting OFDM symbols of length N + Lcp are launched simultaneously from the individual transmit antennas. The Cyclic Prefix is essentially a security and a guard interval which serves to eliminate interference between OFDM symbols and turns linear convolution into circular convolution such that the channel is diagonalized by the FFT. In the receiver the individual signals are passed through OFDM demodulators which NITTTR, Chandigarh
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first discard the CP and then perform an N-point FFT. The outputs of the OFDM demodulators are finally separated and decoded. III: PEAK TO AVERAGE POWER RATIO Peak-to-Average Power Ratio (PAPR) is defined as the relation between the maximum power divided by the average power of the OFDM system [6]. The PAPR is defined as the ratio of the peak power of the signal to its average power, which is in other words a measure of the fluctuations or amplitude ebb and flow of the signal. The transmit signals in an orthogonal frequency-division multiplexing (OFDM) system can have high peak values in the time domain since many subcarrier components are added via an inverse fast Fourier transformation (IFFT) operation. High PAPR results from the nature of the modulation itself where multiple subcarriers /sinusoids are added together to form the signal to be transmitted. When total of N sinusoids add, the peak magnitude would have a value of N, where the (rms) average might be quite low due to disparaging interference between the sinusoids. In fact, the high PAPR is one of the most detrimental aspects in an OFDM system as it decreases the signal-toquantization noise ratio (SQNR) of the analog-digital convertor (ADC) and digital-analog convertor (DAC) while deteriorating the efficiency of the power amplifier in the transmitter.
(1) IV. DECISIVE FACTOR FOR PAPR REDUCTION In this section a decisive factor is characterised for the techniques used for both PAPR and BER reduction. There are many different techniques and some hybrid techniques used for PAPR and BER reduction. But still none gave acceptable results. The technique must reduce the PAPR and BER largely plus the following performance factors must be considered for OFDM based system in order to be acceptable. a) Wherewithal of PAPR Reduction The primary factor of selecting PAPR reduction technique is the wherewithal and capability of PAPR reduction. A technique is believed to be finest if it reduces PAPR largely. OOB radiation and IB distortion are two of the many considerable factors for selecting a technique. b) Low Average Power A procedure must not only reduce PAPR but also the average power of the signal which should not be increased from an acceptable region or else it will require a large linear region for operation in HPA, which will enlarge the BER rate of the OFDM system. c) Low Complexity The technique should also not increase the complexity of the overall system. Complexity and intricacy includes both time and hardware requirements for implementation and thereby the functioning of the system.
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Some techniques like scrambling techniques needs side information, which increase the use of the bandwidth. Some coding techniques also enlarge the bandwidth due to code rate generation. A technique must not be such which enhances and increases the bandwidth to value which ultimately causes degradation and filth in the throughput and hence efficiency is reduced.. e) Less BER Performance Degradation The main objective of the PAPR reduction technique is is to achieve better performance including BER as compared to usual and conventional OFDM system. f) Less Additional Power Need The technique must require no need of additional power for PAPR reduction, as it will degrade BER performance of the system plus power efficiency is the main goal of wireless based systems. h) Other Factors Several previous factors like nonlinear devices such as ADC/ DAC convertors and cost should be considered. V. PEAK TO AVERAGE POWER REDUCTION TECHNIQUE Peak to Average Power ratio (PAPR) is the major problem in the Orthogonal Frequency Division Multiplexing (OFDM) [7]. For reduction or minimization of PAPR various techniques are used [8]. Some of the techniques used for the PAPR reduction like Clipping, Scrambling Techniques, Tone Reservation and Injection techniques, Active Constellation Extension, Adaptive Pre Distortion Techniques, coding techniques are the foundation of PAPR reduction. These techniques must change hardware of transmitter and receiver according to the techniques which is used for PAPR reduction. Also some techniques which does not change hardware, are based on the software programming and are called the channel coding. 1. CLIPPING AND FILTERING This is simplest technique used for PAPR reduction of OFDM signal. A Clip is also called as non linear saturation which is employed around the peaks to reduce the peaks before HPA to reduce PAPR and so is called Clipping Technique. This is simple technique but it yet introduces Out Of Band Radiation and In Band Distortion in OFDM Signal. It also destroys the Orthogonality of OFDM subcarriers. The part of the signal which are above the allowed region are clipped in a simple clipping technique. Joint flitering and clipping technique reduce the OOB radiation but IB distortion are still there since this method degrades OFDM system performance e.g. spectral efficiency and BER. Peak Windowing improves spectral efficiency but it increase OOB radiation and BER Modulation. Envelop scaling is used for PAPR reduction due to equality envelop properties of all subcarriers input [9] Clipping means the amplitude of the signal is clipped at the predefined values which limit the peak value of the input signal to a predetermined or threshold value [10]. Let X[n] denote the input signal and Xc [n] denote the clipped signal of X[n], which can be represented as,
d) Less Bandwidth Expansion [ ] = NITTTR, Chandigarh
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−Y
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[ ] Y
[ ] <Y [ ] â&#x2030;ĽY
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(2)
Where Y is the threshold or predetermines value of clipping level. Clipping is simple but yet it has some drawback. Clipping cause signal distortion which increase bit-error-rate performance. After the filtering operation performed on the clipped signal clipping level may exceed to the specified for the clipping operation. 2. CODING Coding is the technique of reduction of PAPR by selecting such a codeword that reduced the PAPR. Coding does not cause any distortion and out of band radiation. But still it has the drawback of bandwidth efficiency as the code rate is reduced. It also suffer from the complexity of the algorithm to find the optimal code and store large tables for encoding and decoding for a multi-carrier system Precoding based techniques are combined with scrambling techniques, clipping techniques, ACE etc to get best performance. Scrambling Techniques uses the concept of phase rotation. These techniques (Selective Mapping Technique) & PTS (Partial Transmit Technique) are very popular technique for PAPR reduction . As number of phase rotation increases these techniques simply augment complexity. Also the side information is needed for receiver to decipher signal in these techniques
4. PARTIAL TRANSMIT SEQUENCE (PTS) In this technique first step is the phase shifting of the subblock and then multiplication of data structure by random vector [12]. The main idea behind this technique is that input data block is divided into non-overlapping sub-block and each sub-block is phase shifted by a constant factor to reduce PAPR as can be seen from Fig 3. PTS technique is modified version of the SLM technique. PTS method has a capability that minimizes the PAPR more then SLM and main advantage of the PTS technique is no need to send side information. This advantage reduces the complexity of the PTS technique .
3. SELECTED MAPPIMG (SLM) Selected Mapping (SLM) technique reduces PAPR of the system by selected mapping so it is call Selected Mapping technique [11]. A set of generated signal has the same information in selected mapping, and the most favourable signal is selected for transmitted. In the SLM the input data stream is multiply by random weighted factor and the lowest PAPR is chosen for transmission as can be seen from Fig 2. To recover the original data at the receiver side multiplying sequence can be sent as side information. SLM method for reduction of the PAPR does not eliminate the peak. The drawback of this technique is the side information that requires to be transmitted to the receiver of the system in order to recover the original data stream. After IFFT operation select one with low PAPR for transmits. For measurement the PAPR commutative distributed function (CDF) and Complementary commutative distributed function (CCDF) is used. CCDF is measure the probability that the PAPR of a data block exceed the given threshold value.
5. TONE RESERVATION (TR) Tone reservation technique is proposed for PAPR reduction. In this technique small set of tone is used for PAPR reduction. This can be originated as convex problem. The amount of PAPR reduction is dependent on the number of reserved tones, their location, quantity of complexity and allowed power on them [13]. Tone reservation shows that reserving a small fraction of tones give larger minimization in PAPR even using simple algorithm. The advantages of tone reservation method is that it is less complex, no side information is sent, and also no additional operation can be performed at the receiver side. 6. TONE INJECTION (TI) This technique is based on the increased size of the constellation. Each point of the original constellation is
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mapped into several equivalent points in the extended constellation [13]. So that all the information can be mapped into several equidistant constellation point. In Tone injection the new point with an extended constellation is replaced with a place of basic constellation point which injects a tone with a proper phase and frequency in multi carrier system. The drawback of the tone reservation is need to decode side information at the receiver side and so that needed extra IFFT operation which increases the complexity of the system. 7. ACTIVE CONSTELLATION EXTENSION (ACE) Here some of the outer signal constellation points in the data blocks are extended towards the outside of the original constellation such that PAPR of system is reduced.[14]. 8. ADAPTIVE PRE-DISTORTION TECHNIQUE These techniques can recompense for the non linear consequences of HPA (High Power Amplifier). The non linear deviation of the amplifier can be handled by the usual adjustment of the input constellation with the help of minimum usage of hardware. The convergence time of predistorter and the MSE (Mean Square Error) can be lessened via screening out and broadcasting techniques by the training design assistance. All these techniques are comparatively poles apart and entail different restriction like multifaceted optimization, (out-of-band) emission, IB (in-band) deformation, side efficiency diminishing, computational complexity, bandwidth expansion, BER (bit error rate) degradation and data rate loss. Relative
VI CONCLUSION This paper investigates one of the bottleneck problems that exist in OFDM wireless communication systems. High Peak average power ratio (PAPR of OFDM signal), and discuss how to reduce it by different effective algorithms. Both scrambling and non scrambling techniques have been discussed. Each technique of PAPR reduction in OFDM based system are different from each other and impact different constraints as in bandwidth expansion, OOB radiation, IB distortion, reduction of spectral efficiency radiation, BER reduction, high peak and average power, and taken as a whole system’s intricacy and complexity. Precoding based techniques results good with no need of side information and works with less complexity. Clipping cause signal distortion which increase bit-error-rate performance. Coding increases bandwidth efficiency as the code rate is reduced. It also suffer from the complexity of the algorithm. SLM method for reduction of the PAPR does not eliminate the peak. The drawback of this technique is the side information that requires to be transmitted to the receiver of the system in order to recover the original data stream. Though PTS technique does not need side information but still the implementation complexity is increased due to number of IFFT operations and due to complex multiplication and summation. The drawback of the tone reservation is the need to decode side information at the receiver side and so that extra IFFT operation are needed which increases the complexity of the NITTTR, Chandigarh
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system. Adaptive Predistortion technique can be dependent upon the layout circuitry of hardware. Less is the hardware the deviation can be handled of HPA. Precoding based techniques results good with no need of side information and works with less complexity Pre-coding plus other techniques gives best result for PAPR reduction. Also hybrid SLM and PTS allows further PAPR reduction even though it requires too many calculations But it reduces the efficiency of the use of the side bits. VII FUTURE WORK Though all the techniques in individual and hybrid effects gives good results but still every method is having certain drawbacks. So the future work will be discussing some hybrid techniques amongst scrambling and non scrambling and checking for The Cumulative Distribution Function (CDF) which is the most important parameters of the OFDM-MIMO systems to measure the PAPR. VIII ACKNOWLEDGEMENT Authors would like to thank to Dr.M.P.Puniya, Director NITTTR,Chandigarh, Dr.S.B.L.Sachan,HOD-ECE, NITTTR Dr.Prabal Chakravorty, Director VGI, Dadri, without whose support and valuable guidance things would not have been practically implemented.Last we would thank our colleagues for giving their endless support. IX REFERENCES [1] A. Alabadelah, T. Fernandez, A. Media villa, B Nauwelaers, A. Santarelli,D. Schreurs, A. Tazn,P.A.Traverso. Nonlinear, “Models of Nonlinear Microwave Power Devices and Circuits", In 12th GAAS Symposium, Amsterdam, the Netherlands, 2004 [2] H. Saeedi, M. Sharif, and F. Marvasti, “Clipping noise cancellation in OFDM systems using oversampled signal reconstruction”, IEEE Communications Letters, vol. 6, no. 2, pp.73–75, 2002. [3] H. Saeedi, M. Sharif, and F. Marvasti, “Clipping noise cancellation in OFDM systems using oversampled signal reconstruction”, IEEE Communications Letters, vol. 6, no. 2, pp.73–75, 2002. [4] N. Alon and J. H. Spencer. The probabilistic method. Wiley InterScience, 2nd edition, 2000. [5] R.F.H. Fischer." Peak-to-Average Power Ratio (PAR) Reduction in OFDM based on Lattice Decoding", In h Int. OFDM Workshop, Hamburg, Germany, August 2006. [6] D. Guel and J. Palicot, “FFT/IFFT Pair based Digital Filtering for the Transformation of Adding Signal PAPR Reduction Techniques in Tone Reservation Techniques”, Fifth International Conference on Wireless and Mobile communications, pp. 200-204, August 2009. [7] I. Baig and V.Jeoti,” PAPR Reduction in OFDM Systems: Zadoff-Chu Matrix Transform Based Pre/Post-Coding Techniques”, IEEE Trans. on Communication Systems and Networks, pp. 373-377, July 2010. [8] S. H. Han and J. H. Lee, “An Overview of Peak –to- Average Power Ratio Reduction Techniques for Multicarrier Transmission”, IEEE Transaction on Wireless Communication, vol. 12, no.2, pp. 56-65, April 2005. [9] T. Wattanasuwakull and W. Benjapolakul, “PAPR Reduction for OFDM Transmission by using a method of Tone Reservation and Tone Injection”, IEEE ICSC, pp 273-277 2005. [10] R.W. Bäuml, R. F. H. Fischer, and J.B. Huber, “Reducing the Peakto Average Power Ratio of Multicarrier Modulation by Selective Mapping”, IEEE Trans. on Electronics Letters., vol. 32, no. 22, pp. 2056– 2057, October 1996. [11] L. Sahraoui, D. Messadeg and N. Doghmane,” Analyses and Performance of Techniques PAPR reduction for STBC MIMO-OFDM System in (4G) Wireless Communication”, International Journal of Wireless & Mobile Networks, vol. 5, no. 5, pp. 35-48, October 2013. [12] S. Singh, J. Malhotra and M. Singh,” A Novel SLM based PAPR reduction Technique in OFDM-MIMO System”, International Journal of Computers & Technology, vol. 2, no. 3, pp. 41-46, June, 2012. [13] J. Gao, J. Wang and Z. Xie.” A Novel Concurrent PAPR Reduction Algorithm for STBC MIMO-OFDM Systems”, International Journal of
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Science Information and Systems Sciences, vol. 4, no 3, pp. 449-461, December 2007. [14] B. Kang, H-G. Ryu, and saangburm Ryu, “A PAPR Reduction Method Using New ACE (Active Constellation Extension) With Higher Level Constellation”, IEEE International Conference on Signal Processing and Communications, November 2007, pp 24-27.
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