Reviewed Paper Volume 3 Issue 7 March 2016 International Journal of Informative & Futuristic Research Study Of Bit Error Rate Performance And CFO Estimation In OFDM Using QPSK Modulation Technique Paper ID IJIFR/ V3/ E7/ 092 Page No. 2599-2605 Subject Area Elec. & Comm. Engineering OFDM, QPSK, Error Rate Performance, Carrier Frequency Offset (CFO) Keywords Estimation, Bit Error Rate (BER), Inter Carrier Interference (ICI), Frequency Synchronization Errors 1 st Kavya Vinod 2 nd Suma Sekhar M.Tech. Scholar Department of Electronics & Communication Engg. LBS Institute of Technology for Women, Thiruvananthapuram, India Associate Professor Department of Electronics & Communication Engg. LBS Institute of Technology for Women, Thiruvananthapuram, India Abstract The main aim of this paper is to analyze bit error rate performance and CFO estimation in OFDM. In this paper, QPSK modulation scheme is used. Bit Error Rate (BER) specifies the number of bits that are error when a set of bits are transmitted. OFDM can be seen as either a modulation technique or a multiplexing technique. The OFDM systems are sensitive to the frequency synchronization errors in form of CFO. CFO can lead to the Inter Carrier Interference (ICI). So it is necessary to estimate CFO. One of the attractive reasons to use OFDM is to increase the robustness against frequency selective fading or narrowband interference. 1. INTRODUCTION The most frequently assumed model for transmission channel in digital communication theory is additive white Gaussian noise (AWGN) channel. However, for many communication systems the performance of AWGN channel is poor, hence more precise and accurate channel models are required. One basic type of non-gaussian channel is the Available online through - http://ijifr.com/searchjournal.aspx www.ijifr.com Published On: March 31, 2016 2599
fading channel. In a basic communication system, onto a single carrier frequency the data are modulated [3]. If the available bandwidth is completely occupied by each symbol, intersymbol-interference (ISI) may occur in frequency selective channels. The basic idea of OFDM is to split the available spectrum into several orthogonal sub channels so that each narrowband sub-channel experiences almost flat fading. Orthogonal frequency division multiplexing (OFDM) is becoming the chosen modulation technique for wireless communications. OFDM can provide large data rates with sufficient robustness to radio channel impairments. A lot of research works are currently going on the optimization of OFDM systems. In an OFDM scheme, a large number of orthogonal, overlapping, narrow band sub-carriers are transmitted in parallel divide the available transmission bandwidth. The separation of the sub-carriers is such that there is a very compact spectral utilization. With OFDM we can increase the transmission rate. The way of handling the multipath interference at the OFDM receiver gained a lot of attraction. Multipath phenomenon creates two effects (a) Frequency selective fading and (b) Inter symbol interference. The "flatness" understood by a narrowband channel overcomes the frequency selective fading. On the other hand, modulating symbols at a very low rate makes the symbols much longer than channel impulse response and hence reduces the ISI [7]. Use of suitable error correction codes provides more robustness against frequency selective fading. The insertion of an extra guard interval between consecutive OFDM symbols can reduce the effects of ISI more effectively. Modulation and demodulation functions implemented using FFT technique makes it computationally more efficient. During the last years, OFDM systems have gained an increased interest. More recently, OFDM has been implemented in mobile wideband data transmission, highbit-rate digital subcarrier lines 4 (HDSL), asymmetric digital subcarrier lines (ADSL), very high-speed digital subscriber lines (VHDSL), digital audio broadcasting (DAB), digital television and high-definition television (HDTV), IEEE 802.16 Wi-MAX standard and its predecessor multicarrier multipoint distribution service (MMDS) [8]. Despite the widespread acceptance of OFDM, it have many drawbacks. One drawback is that OFDM systems are not robust in carrier frequency estimation errors. Even small carrier frequency offsets destroy the orthogonality between the subcarriers and results in drastic error rate increase. The second drawback is that OFDM signals suffer from large envelope variations. Such variations are problematic because practical communication systems are peak power limited. Thus, envelope peaks require a system to accommodate an instantaneous signal power that require lager power efficiencies or power amplifier (PA) saturation. This problem is termed as Peak to Average Power Reduction. 2. ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING (OFDM) Orthogonal Frequency Division Multiplexing has grown to a popular communication technique for high speed communication in the last decades [7]. Being an important member of the multicarrier modulation technique, OFDM is also called as Discrete Multitude Modulation [8]. It splits a high rate data stream into a number of lower rate streams that are transmitted at the same time over a number of orthogonal subcarriers. Orthogonality is 2600
achieved by ensuring that the carriers are placed exactly at the null in the modulation spectra of each other. The increase of symbol duration for the lower rate parallel subcarriers decreases the relative amount of dispersion in time caused by multipath delay spread. Therefore OFDM is an advanced modulation technique which is suitable for highspeed data transmission due to its advantages in dealing with the multipath propagation problem, high data rate and bandwidth efficiency [8]. I. Understanding OFDM System Figure 1: Block diagram of OFDM The main blocks of OFDM systems are: i.) S/P converter: The input serial binary data stream is grouped into word size and word is transformed into parallel stream. Each stream is used to modulate one carrier out of group of orthogonal carriers. ii.) Modulator: Based on the modulation scheme used, data on each symbol is mapped to a particular phase. Usually each phase encodes an equal number of bits. iii.) Zero-padding and IFFT: By using IFFT technique, frequency domain data is converted into time domain signal. Prior to IFFT mapping zero-padding is performed to adjust the IFFT size. Zero padding is used because the number of subcarriers may be less then bit size. iv.) Cyclic Prefix: It is a cyclic extension of an OFDM symbol to eliminate ISI on original OFDM symbol. The length of cyclic prefix is chosen ¼ of the length of symbol.the cyclic prefix adds time over head decreasing the overall spectral efficiency of the system. Figure 2: Cyclic prefix and guard interval 2601
v) Channel Model: Additive white Gaussian Noise (AWGN) is a channel model in which the only impairment to communication is a linear addition of wideband or white noise with a constant spectral density (expressed as watts par hertz of bandwidth) and a Gaussian distribution of amplitude. The main advantage of this model is that does not account for fading, frequency-selectivity, interference, nonlinearity or dispersion. AWGN does not work well in most of the cases, so more specified models are used. Fading is deviation of the attenuation that a carried modulated telecommunication signal experiences over certain propagation media. A fading channel is mainly due to multipath reception. Actually, fading is statistical model for the effect of propagation environment on a radio signal such as issued by wireless devices. vi) Receiver: The receiver does the reverse of the transmitter. Firstly, the serial output is converted into parallel stream and then cyclic prefix bits are removed from it.then FFT of each symbol is performed. 3. DIGITAL MODULATION TECHNIQUE The process which facilitates the transfer of information over a medium is called mo. The transmission of sound in air has finite range for the amount of power your lungs can generate. To extend the range your voice can reach, we need to send it through a medium other than air, such as a phone line or radio. The process of converting data(voice in this case) so that it can be successfully sent through a medium (wire or radio waves) is called modulation [6]. QPSK The input binary bit stream is divided into two bit streams- even and odd bit streams (quadrature and in-phase streams) by the serial to parallel converter. Then, transmit alternating bits to I, Q channels: even bits to Q channel, odd bits to I channel [1]. Figure 3: QPSK generation The theoretical BER for QPSK modulation scheme is given by, 2602
= 0.5 2 4. CFO ESTIMATION The OFDM systems are vulnerable to the frequency synchronization errors in form of CFO. CFO can lead to the Inter Carrier Interference (ICI); therefore CFO plays a major role in Frequency synchronization [4]. Basically for getting a good performance of OFDM, the CFO should be estimated and corrected. Lack of the synchronization of the local oscillator signal (L.OSC); for down conversion in the receiver with the carrier signal contained in the received signal causes Carrier Frequency Offset (CFO) which can cause the following factors (i) Frequency mismatched in the transmitter and the receiver oscillator (ii) Inter Carrier Interference (ICI) (iii) Doppler Effect (DE) When CFO happens, it causes the receiver signal to be swapped in frequency (δf); this is shown in the figure 4. If the frequency error is an integer multiple I of subcarrier spacing δf, then the received frequency domain subcarriers are shifted by δf I. 5. SIMULATION Figure 4: Frequency offset (δf) (a) Generation of random sequence (b) QPSK/BPSK modulation i.e. bit 0 represented as -1 and bit 1 represented as +1 (c) Assigning to multiple OFDM symbols where data subcarriers from -26 to -1 and +1 to +26 are used, adding cyclic prefix (d) Convolving each OFDM symbol fading channel. The fading on each symbol is independent. The frequency response of fading channel on each symbol is computed and stored. (e) Concatenation of multiple symbols to form a long transmit sequence (f) Adding White Gaussian Noise (g) Grouping the received vector into multiple symbols, removing cyclic prefix (h) Transforming the time domain received symbol into frequency domain (i) Dividing the received symbol with the known frequency response of the channel 2603
(j) Taking the desired subcarriers (k) CFO estimation (l) Demodulation and conversion to bits (m) Counting the number of bit errors (n) Repeating for multiple values of Eb/No 6. RESULTS AND DISCUSSIONS The performance of BER of QPSK has been investigated using MATLAB R 2013a. The results presented show the BER performance as a function of the energy per bit to noise ratio and the output of CFO estimation. It can be seen that mean square error gradually decreases with increase in signal to noise ratio Figure 5: BER curve for QPSK modulation in AWGN channel Figure 6: BER curve for QPSK modulation in Rayleigh fading channel 2604
Figure 7: CFO estimation MSE curve 7. CONCLUSION The performance of Fourier transform based OFDM system in terms of bit error rate probability using QPSK was analysed. The paper analyses the BER performance and CFO estimation of the OFDM system using QPSK whereas the future work may include the implementation of other modulation schemes and different channels incorporating multiple antennas at transmitter and receiver for performance evaluation of any OFDM based system. 8. REFERENCES [1] Neha Mukul, Shailendra Singh Pawar, Mohd. Sarwar Raeen, BER &SER based performance analysis of bpsk and qpsk modulation scheme with OFDM in Rayleigh channel IJETST Volume-1, issue-8, ISSN 2348-9480 oct2014 [2] G.Tharakanatha, SK. Mahaboob kamal basha, Vijay Bhaskar chanda, I.Hemalatha Implementation and Bit Error Rate analysis of BPSK Modulation and Demodulation Technique using MATLAB IJETT Volume 4 Issue 9- Sep 2013 [3] John G. Proakis, Digital Communication (New York, McGraw Hill, 2001). [4] Kennedy, G.; Davis, B. Electronic Communication Systems (McGraw-Hill International, 1992). [5] Ms. Shruti Helonde, Prof. M. S. Pawar BPSK MODULATION TECHNIQUE FOR DIGITAL COMMUNICATION E-ICETT 2014, ISSN No: 2250-3536 [6] Simon Haykin, Communication Systems fourth edition Wiley-India edition 2001. [7] Orthogonal Frequency Division Multiplexing, U.S. Patent No. 3, 488,4555, filed November 14, 1966, issued Jan. 6, 1970. [8] Young Soo Cho, Jaekwon Kim, Won Young Yang, Chung Gu Kang MIMO-OFDM Wireless Communications with Matlab IEEE press, John Wiley & sons 2010. 2605