Australian Journal of Basic and Applied Sciences. Optimal PRCC Coded OFDM Transceiver Design for Fading Channels

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1 Australian Journal of Basic and Applied Sciences, 8(17) November 214, Pages: AENSI Journals Australian Journal of Basic and Applied Sciences ISSN: Journal home page: Optimal PCC Coded OFDM Transceiver Design for Fading Channels Dr. J. Jayaumari Professor & Head, Dept.of ECE, Noorul Islam University, Kumaracoil, Tamilnadu, India A T I C L E I N F O Article history: eceived 19 August 214 eceived in revised form 19 September 214 Accepted 29 September 214 Available online 8 November 214 Keywords: OFDM, Intercarrier Interferrence, Conjugate cancellatio adaptive phase rotation A B S T A C T In this paper, an adaptive receiver for efficient inter carrier interference (ICI) cancellation in OFDM systems using two-path conjugate transmission is proposed. This receiver design is based on the phase rotated conjugate cancellation (PCC) concept, where one common phase rotation is applied to the two transmit paths. An adaptive normalized bloc least mean-squared algorithm is used for the adaptive phase rotatio such that the requirement for feedbac of CFO information can be eliminated. The proposed adaptive receiver gives better bit error rate performance than the conventional OFDM systems. 214 AENSI Publisher All rights reserved. To Cite This Article: Dr. J. Jayaumari; Optimal PCC Coded OFDM Transceiver Design for Fading Channels. Aust. J. Basic & Appl. Sci., 8(17): , 214 INTODUCTION OFDM is a technique for transmitting data in parallel by using a large number of orthogonal sub-carriers. Eventhough OFDM is highly resistant to intersymbol interference, it is sensitive to time selective fading, which destroys the orthogonality among subcarriers in one OFDM symbol and thus causes inter-carrier interference (ICI). OFDM is also highly sensitive to the carrier frequency offset (CFO), which might arise from the doppler effect in mobile environments and/or from the mismatch between local oscillators at the transmitter and receiver. The way to solve the CFO problem is to synchronize the carrier frequency of the received signal with that of the transmitted signal. However, in this case, it is necessary to estimate the CFO and then to perform compensation according to the estimated result (J. van de Bee, M. Sandell, and P. O. Borjesso1997), (U. Tureli, D. Kivanc, and H. Liu, 21), (Y. Yao and G. B. Giannais, 25). Many researchers have proposed several techniques to reduce the effect of ICI. With the assumption of approximate linearity of channel time variations within one OFDM symbol, the ICI cancellation approaches employing pilot subcarriers or training symbols to realize channel estimation and frequency-omain equalization under fast fading multipath channels. However, this increases the transmitter power requirement.(mostofi and D. C. Cox,25), (W.S. Hou and B.-S. Che 25), (S. Lu and N. Al-Dhahir,28). Time domain partial response coding was proposed for reducing ICI in OFDM systems (Jayaumari.J and Sauntala S. Pillai, 29). This technique is very simple and highly efficient. However, since multilevel sequences are involved, as the order of partial response coder increases, the signal decoding becomes very difficult. Self ICI cancellation scheme has been proposed (Y Zhao and S. G. Haggma 21). Although this method can suppress the ICI significantly; it halves the bandwidth efficiency since it transmits each symbol over a pair of adjacent subcarriers with a 18 phase shift. Two alternatives to ICI self cancellation using conjugate cancellation (CC) (H.G. Yeh, Y.K. Chang, and B. Hassibi, 27) and phase rotated conjugate cancellation (PCC) (C.-L. Wang and Y.C. Huang, 21) has been proposed. In CC scheme, an OFDM symbol is transmitted along the first path, while its conjugate is transmitted along the second path. After combining the received signals from the two paths, the ICI in one path is suppressed by that in the other path. In PCC, an artificial phase rotation is introduced to the transmitter in CC such that the ICI induced from both paths can be mutually cancelled more effectively. Although CC and PCC wor well for time-invariant channels, they might not provide satisfactory performance for time-variant channels where the carrier frequency offset (CFO) changes with time. In this paper, an adaptive receiver for ICI self- cancellation is proposed. Unlie the PCC scheme, in which only one phase rotation is applied at the transmitter, here two phase rotations on both paths at the proposed receiver are used to cope with the fast fading channel problems. Furthermore, the optimal phase rotations that Corresponding Author: Dr.J.Jayaumari, Professor & Head, Dept.of ECE, Noorul Islam University, Kumaracoil, Tamilnadu, India Tel: jayaumari@niuniv.com

2 156 Dr. J. Jayaumari, 214 Australian Journal of Basic and Applied Sciences, 8(17) November 214, Pages: maximize the carrier-to-interference ratio (CI) is derived, thereby developing an adaptive process using the normalized bloc least mean-squared (BLMS) algorithm for the phase rotations, such that the requirement of CFO feedbac can be eliminated. Simulation results show that the proposed adaptive receiver significantly outperforms CC and PCC, especially when the CFOs at the two paths become fairly distinguished due to fast time-varying channels. Concept of CC and PCC Schemes: The CC scheme can achieve good ICI self-cancellation through two-path conjugate transmission. The transmitter of CC comprises a regular OFDM signal in one path and its conjugate replica in the other path, and these two-path signals can be transmitted using time- division multiplexing (TDM), frequency division multiplexing (FDM), code division multiplexing (CDM), or other transmission means; at the receiver, the two paths can be combined to form diversity reception and to achieve conjugate cancellation of ICI. The ey of the CC scheme is to reduce the ICI due to the loss of the orthogonality. Both the frequency offset(constant) and the doppler spread of multipath (time-varying channels) cause the ICI due to the loss of the orthogonality among the subcarriers.it has been shown that the CC scheme has a better CI than the standard OFDM system and the previous ICI self-cancellation schemes only at small CFOs. To have good ICI cancellation performance at both small and large CFOs, an appropriate artificial phase rotation was adopted in CC to form PCC. For the PCC scheme, the first path represents the standard OFDM signal with artificial phase rotation φ, while the second path is formed by the conjugate of the standard OFDM signal with artificial phase rotation φ. We can optimize φ by using the following expressio N 1 opt (1) N Where is the frequency offset and N, the number of carriers in the system. Since the optimal phase rotation in PCC is derived under the assumption that the transmitter nows the current CFO estimate, the CI performance might be sensitive to the CFO estimation error. Proposed System model: The CC scheme has inspired developments on a framewor for ICI self-cancellation that is bacward compatible with most existing OFDM systems and has potential for performance improvement over the previous schemes. Fig. 1: Bloc diagram of proposed transmitter. The transmitter section of the proposed system using single antenna with TDM is shown in Fig.1.The input bits are first serial to parallel converted. Then the parallel bits are modulated by using Binary Phase Shift Keying (BPSK) modulation technique. After modulation the orthogonal multicarrier modulation is performed using IFFT and are parallel to serial converted. In the 1st path the serial data is transmitted and its conjugate is transmitted along the second path. Both outputs of a regular OFDM system and a conjugate OFDM system can are combined coherently without interfering with each other by using Time Division Multiplexing technique (TDM). The coherency between these two paths can be achieved by employing preamble sequences or a phase loced loop. The proposed scheme is equivalent to PCC in terms of CI performance when φ=, which means the proposed scheme would be a more general approach for ICI self-cancellation among those based on conjugate transmission. The phase rotations used in the proposed scheme can be determined directly at the receiver, which eliminates the requirement to feed bac the CFO estimate to the transmitter, as required in PCC scheme.

3 157 Dr. J. Jayaumari, 214 Australian Journal of Basic and Applied Sciences, 8(17) November 214, Pages: The simplified bloc diagram of the receiver is shown in Fig.2. The received signal is first demultiplexed using TDM. The demultiplexed first path data and it s conjugate in the second path are converted to parallel form and is applied to the FFT bloc. Two phase rotations are applied for the FFT data. Maximal atio Combining (MC) is used to combine the phase rotations.channel Estimation and decision is done to calculate the estimate of the input data. To update the phase rotation value, an adaptive phase updater is connected between the phase rotation and channel estimation. CFO estimate is given to the adaptive phase updater for synchronizing the carrier frequency of the received signal with that of the transmitted signal. Channel estimation is used for increasing the capacity of OFDM system by improving the system performance in terms of bit error rate. The estimate of the transmitted signal is demodulated and the original signal will be recovered. Fig. 2: Bloc diagram of proposed receiver. To ensure that the information contained in every received signal is reflected uniformly in the stochastic gradient adaptationormalized BLMS algorithm is used. This alleviates the impact induced by the change of the received signal power due to unexpected factors such as channel fading. The bloc size is chosen to be equal to the FFT length. The received signal is given by, Y Y X H S W (2) n X H S W (3) n n The two individual phase rotations are applied using the following equatio 1 j n 1 1 e H Y (4) 2 j 2 2 e H Y n The MC output can be expressed as, 1 2 (6) The error term can be calculated as e d where d n, represents the output data. Simulation esults: The proposed system was simulated using MATLAB and BE performance is presented here. The modulation scheme used is BPSK (Binary Phase Shift Keying) modulation and channel used is multipath ayleigh fading channel with 5 taps and maximum Doppler spread 8Hz.. Number of input is 16 binary random bits and FFT length is also 16.The cyclic prefix is not considered in this paper. Fig.3 represents the input and BPSK modulated output. Fig.4 represents the IFFT output and TDM output. Fig.5 gives the BE performance comparison of the conventional OFDM and proposed PCC-OFDM with adaptive receiver. Simulation results have demonstrated that the proposed adaptive receiver is superior to conventional OFDM, particularly under time-varying channels. (5)

4 BE 158 Dr. J. Jayaumari, 214 Australian Journal of Basic and Applied Sciences, 8(17) November 214, Pages: INPUT BPSK modulation Fig. 3: Input and BPSK modulated output..4 IFFT data TDM data Fig. 4: IFFT output and TDM output. 1 BE vs SN Conventional OFDM OFDM with adaptive receiver SN,dB Fig. 5: Bit error rate comparison. Conclusion: In this paper, an adaptive receiver for efficient inter carrier interference (ICI) cancellation in OFDM systems using two-path conjugate transmission is proposed. This receiver design is based on the phase rotated conjugate cancellation (PCC) concept, where one common phase rotation is applied to the two transmit paths. Unlie PCC, which applies one common phase rotation for the two paths at the transmitter, the proposed scheme uses two artificial phase rotations on the two paths at the receiver to further address the problems encountered under fast time-varying channels. An adaptive normalized bloc least mean-squared algorithm is used for the adaptive phase rotatio such that the requirement for feedbac of CFO information can be eliminated. Simulation results show that the proposed adaptive receiver gives better bit error rate performance

5 159 Dr. J. Jayaumari, 214 Australian Journal of Basic and Applied Sciences, 8(17) November 214, Pages: than the conventional OFDM systems. This proposed system is feasible and can be applied to 4G communication systems. EFEENCES Hou, W.S. and B.S. Che 25. ICI cancellation for OFDM communication systems in time varying multipath fading channels, IEEE Trans. Wireless Commu 4(5): Jaya umari, J. and S. Sauntala Pillai, 29. A spectrally efficient method for reducing intercarrier interference in OFDM International Journal of Electronics & Communication Engineering, 2: 1-2. Lu, S. and N. Al-Dhahir, 28. Coherent and differential ICI cancellation for mobile OFDM with application to DVB-H, IEEE Trans. Wireless Commu 7(11): Mostofi, Y. and D.C. Cox, 25. ICI mitigation for pilot-aided OFDM mobile systems, IEEE Trans. Wireless Commu 4(2): Tureli, U., D. Kivanc and H. Liu, 21. Experimental and analytical studies on a high-resolution OFDM carrier frequency offset estimator, IEEE Trans., Veh. Technol., 5(2): Van de Bee, J., M. Sandell and P.O. Borjesso ML estimation of time and frequency offset in OFDM systems, IEEE Trans. Signal Process., 45(7): Wang, C.L. and Y.C. Huang, 21. Intercarrier interference cancellation using general phase rotated conjugate transmission for OFDM systems, IEEE Trans. Commun., 58(3): Yao, Y. and G.B. Giannais, 25. Blind carrier frequency offset estimation in SISO, MIMO, and multiuser OFDM systems, IEEE Trans. Commu 53(1): Yeh, H.G., Y.K. Chang, and B. Hassibi, 27. A scheme for cancelling intercarrier interference using conjugate transmission in multicarrier communication systems, IEEE Trans. Wireless Commu 6(1): 3-7.