Channel Estimation Algorithms in OFDM, A Review

I nternational Journal of Application or I nnovation in E ngineering & M anagement (I JAI E M ) Web Site: www.ijaiem.org Email: [email protected], [email protected] Volume 2, Issue 6, June 2013 ISSN 2319 - 4847 Channel Estimation Algorithms in OFDM, A Review Navdeep Bansal1 Sukhjeet Singh 2 Pardeep Kumar Jindal 3 1 ECE Department, GTBKIET Chhapianwali ECE Department, GTBKIET Chhapianwali 3 ECE Department GGSCET Talwandi Sabo 2 Abstract Main objectives of this paper is to review the work already done related to channel estimation in OFDM (Orthogonal Frequency Division Multiplexing) using QAM (Quadrature Amplitude multiplexing) & PSK (Phase Shift Keying) modulation techniques used in OFDM for wireless communications. To estimate the channels three algorithms are used i.e. LS, LMMSE & Modified MMSE. These three algorithms have basically played key role in channel estimation. Keywords: OFDM, PSK, QAM 1. INTRODUCTION In this paper we will review the work done by the different research fellows and professors related to Channel Estimation by using different techniques. Chi-Hsiao Yih1In this paper, we study the effects of channel estimation error on the bit-error-rate (BER) of orthogonal frequency division multiplexing (OFDM) systems in frequency-selective slowly fading channels of Rayleigh fading. Due to the additive white Gaussian noise (AWGN) and the inter-carrier interference (ICI) caused by the residual carrier frequency offset (CFO), the channel estimation based on the training symbols is not perfect. The author characterized the performance degradation resulting from imperfect channel state information (CSI) by deriving the BER formulas for BPSK, QPSK, 16-QAM, and 64-QAM modulation schemes. The derived BER formulas contain no numerical integrals and can be evaluated easily and accurately. Simulation results validate the correctness of our theoretical analysis.The results given in this paper are: Fig 1: Effect of channel estimation error on the BER of BPSK modulated OFDM signals in multipath Rayleigh fading channels. Fig 3:Effect of channel estimation error on the BER QAM of 16-QAMmodulated OFDM signals in multipath channels Rayleigh fading channels Volume 2, Issue 6, June 2013 Fig 2: Effect of channel estimation error on the BER of QPSK modulated OFDM signals in multipath Rayleigh fading channels. Fig 4: Effect of Channel estimation error on the BER of 64-modulated OFDM signals in multipath Rayleigh fading Page 290 I nternational Journal of Application or I nnovation in E ngineering & M anagement (I JAI E M ) Web Site: www.ijaiem.org Email: [email protected], [email protected] Volume 2, Issue 6, June 2013 ISSN 2319 - 4847 Fig 5: BER performance of BPSK, QPSK, 16QAM, 64- QAM modulated OFDM signals with different numbers of training symbols. Fig 6: BER performance of BPSK and 16QAM modulated. OFDM signal with three different training sequences. We investigated the effects of channel estimation error on the BER performance of OFDM systems in multipath fading channels. For BPSK, QPSK, 16-QAM, and 64- QAM modulated OFDM signals, we derived the BER formula characterizing the performance degradation due to imperfect channel estimation. Computer simulations were conducted to verify the accuracy of our theoretical analyses. The BER analysis can be extended to frequency selective Rician fading channels by generalizing the Lemma 1 to the case of nonzero mean complex-valued Gaussian random variables. From the BER expression, we learn the BER depends on the patterns of training sequences. The design of optimal training sequence in the sense of minimizing the average BER subject to the PAPR constraint is left for future studies Sajjad Ahmed Ghauri, Sheraz Alam, M. Farhan Sohail, Asad Ali, Faizan Saleem7 During the last few years, the development in digital communication are rapidly increasing to meet the ever increasing demand of high data rates. OFDM has an edge over other frequency multiplexing techniques by using highly densely packed carriers which helps in achieving higher data rates using similar channels. In this paper author discussed the channel estimation in OFDM and its implementation in MATLAB using pilot based block type channel estimation techniques by using LS and MMSE algorithms. This paper includes the comparisons of OFDM using BPSK and QPSK on different channels, followed by designing the LS and MMSE estimators on MATLAB. In the end, the results of different simulations are compared to conclude that LS algorithm gives less complexity but MMSE algorithm provides comparatively better results. Results are based on simulation parameters: Table No. 1 Parameters Specifications FFT Size 64 No. of Subcarriers 52 Cyclic Prefix 16 No. of OFDM symbols 100 Constellation BPSK/QPSK Channel Model AQWGN, FNS, Multipath No. of taps/multipath 8 Fig 7: Comparison of BER for BPSK in AWGN/FNS/Rayleigh channel Volume 2, Issue 6, June 2013 Fig: 8 : Comparison of BER for QPSK in AWGN/FNS/Rayleigh Page 291 I nternational Journal of Application or I nnovation in E ngineering & M anagement (I JAI E M ) Web Site: www.ijaiem.org Email: [email protected], [email protected] Volume 2, Issue 6, June 2013 ISSN 2319 - 4847 Fig 9: Comparison of BER of QPSK for different no of taps Fig 10: BER for MMSE/LS Estimator based receiver for 3taps Channel Estimation Parameters: Table No.2 Parameters No. of Subcarriers No. of OFDM Symbols Channel K γm Constellation Specifications 64 100 Slow fading 0……..N-1 Value of Taps BPSK The paper highlights the channel estimation technique based on pilot aided block type training symbols using LS and MMSE algorithm. The Channel estimation is one of the fundamental issues of OFDM system design. The transmitted signal under goes many effects such reflection, refraction and diffraction. Also due to the mobility, the channel response can change rapidly over time. At the receiver these channel effects must be canceled to recover the original signal. The BER of AWGN channel is approximately 10-4which is better than Rayleigh fading and flat fading channel at SNR=10dB using BPSK & QPSK on different number of taps. The MMSE is compared with LS and the MMSE performs better than the LS using 3 taps where the performance metric is mean square and symbol error rate. Jan-Jaap van de Beek, Ove Edfors, Magnus Sandell, Sarah Kate Wilson and Per Ola B.rjesson4 The use of multi-amplitude signaling schemes in wireless OFDM systems requires the tracking of the fading RF channel. This paper includes channel estimation based on time-domain channel statistics. Using a basic m o d e l fo r a slowly fading channel, author explained the MMSE and LS estimators and a method for modifications compromising between complexity and performance. The SER for a 16-QAM system is presented by means of simulation results. Depending upon estimator complexity, in the LS estimator 4 dB SNR can be gained. Table No: 3 Estimator MMSE LS Modified MMSE Modified LS Volume 2, Issue 6, June 2013 Notation MMSE LS MMSE-0 MMSE-5 MMSE-10 LS-0 LS-5 LS-10 Taps Used 0-63 0-63 0-4 0-9, 59-63 0-14, 54-63 0-4 0-9, 59-63 0-14, 54-63 Size Q’ 64X64 NA 5X5 15X15 25X25 5X5 15X15 25X25 Page 292 I nternational Journal of Application or I nnovation in E ngineering & M anagement (I JAI E M ) Web Site: www.ijaiem.org Email: [email protected], [email protected] Volume 2, Issue 6, June 2013 ISSN 2319 - 4847 Fig 11: Mean Square Error for three modified MMSE Estimators Fig 13: Symbol Error Rate for three modified MMSE Estimators Fig 12: Mean Square Error for three LS Estimators Fig 14: Symbol Error Rate for LS Estimators Fig 15: Comparision between three modified MMSE & LS Estimator. The Estimators in this study can be used to efficiently estimate the channel in an OFDM system given a certain knowledge about the channel statistics. The MMSE estimators assume a priori knowledge of noise variance and channel covariance. Moreover its complexity is large compared to LS estimators. For high SNRs the LSE estimators are both simple and adequate. However for the Low SNRs the presented modifications of MMSE and LS estimators will allow compromise between estimator complexity and performance. For 16-QAM, signaling constellation, up to 4 dB gain in SNR over the LS estimator was obtained, depending upon estimator complexity. Even relatively low complex modified estimator, however, perform significantly better than the LS estimator for a range of SNR. REFERENCES: [1] Chi-Hsiao Yih, “Effects of Channel Estimation Error in the Presence of CFO on OFDM BER in FrequencySelective Rayleigh Fading Channels,” JOURNAL OF COMMUNICATIONS, VOL. 3, NO. 3, JULY 2008 [2] J. Torrance and L, Hanzo, Multicarrier Modulation for Data Transmission: An idea whose time has come IEEE comun. Magazine, pp.5-14, May 1990. [3] J.V. de Beek, O. Edfors, M. Sandell, S.K.Wilson and P.O Borjesson, “On Channel Estimation In OFDM” Vehicular Technology Conference, vol. 2 pp. 815-819, Chicago, USA, September 1995 [4] Jan-Jaap van de Beek, Ove Edfors, Magnus Sandell, Sarah Kate Wilson and Per Ola B.rjesson” On Channel Estimation In OFDM Systems”, In Proceedings of Vehicular Technology Conference (VTC Ô95), vol. 2, pp. 815819, Chicago, USA, September 1995. [5] K. Fazel and G. Fettwis, “Performance of an Efficient Parallel Data Transmission System,” IEEE Trans. Commun. Tech., pp. 805-813, December1967. [6] M. Okada, S. Hara and N. Morinaga, “Bit Error Performances of Orthogonal Multicarrier modulation radio transmission schemes.” IEICE Trans. Commun, Vol.E76-B, pp. 113-119, Fed. 1993 Volume 2, Issue 6, June 2013 Page 293 I nternational Journal of Application or I nnovation in E ngineering & M anagement (I JAI E M ) Web Site: www.ijaiem.org Email: [email protected], [email protected] Volume 2, Issue 6, June 2013 ISSN 2319 - 4847 [7] Sajjad Ahmed Ghauri, Sheraz Alam, M. Farhan Sohail, Asad Ali, Faizan Saleem, i m p l e m e n t a t i o n o f O F D M a n d c h a n n e l e s t i m a t i o n u s i n g L S a n d M M S E E s t i m a t o r s . International Journal of Computer and Electronics Research [Volume 2, Issue 1, February 2013]. [8] Sinem Coleri, Mustafa Ergen, AnujPuri, and Ahmad Bahai, “Channel Estimation Techniques Based on Pilot Arrangement in OFDM Systems,” IEEE transactions on Broadcasting, Vol. 48, No. 3, September 2002. [9] Sinem Coleri, Mustafa Ergen, Anuj Puri, and Ahmad Bahai “Channel Estimation Techniques Based on PilotArrangement in OFDM Systems” IEEE TRANSACTIONS ON BROADCASTING, VOL. 48, NO. 3, SEPTEMBER 2002. [10] Yuping Zhao, Aiping Huang, “A novel channel estimation method for OFDM mobile communication systems based on pilot signals and transform-domain processing ,” IEEE VTC , Vol. 3, May 1997 AUTHOR Navdeep Bansal received B.Tech Degree in Electronics & Communication from Adesh Institute of Engineering & Technology, Faridkot in 2007.He is pursuing M.Tech Degree from Guru Teg Bhadur Khalsa Institute of Engineering & Technology (Regional Center) in under Punjab Technical University, Jalandhar.I am on my thesis work. Volume 2, Issue 6, June 2013 Page 294