Performance Analysis of MIMO SFBC CI-COFDM System against the Nonlinear Distortion and Narrowband Interference

Carrier Interferometry Coded Orthogonal Frequency Division Multiplexing (CI-COFDM) system has been widely studied in multi-carrier communication system. The CI-COFDM system spreads each coded information symbol across all N sub-carriers using orthogonal CI spreading codes. The CI-COFDM system shows the advantages of Peak to Average Power Ratio (PAPR) reduction, frequency diversity and coding gain without any loss of communication throughput. On the other side, a great attention has been devoted to Multi Input Multi Output (MIMO) antenna systems and space-time-frequency processing. In this paper, we focus on two Transmit (Tx)/one Receive (Rx) antennas configuration and evaluate the performance of MIMO OFDM, MIMO CIOFDM and MIMO CI-COFDM systems. Space Frequency Block Coding (SFBC) is applied to MIMO OFDM, MIMO CI-ODFM and MIMO CI-COFDM systems. For CI-COFDM realization, digital implemented CI-COFDM is used in which information conventional is encoded, CI code spreading operation and carrier allocation are processed by IFFT type operation. From simulation results, it is shown that MIMO SFBC CI-COFDM reduces PAPR significantly as compared with that of MIMO SFBC CI-OFDM and MIMO SFBC OFDM systems. In Narrow Band Interference (NBI) channel MIMO SFBC CI-COFDM systems achieve considerable Bit Error Rate (BER) improvement compared with MMO SFBC CI-OFDM and MIMO SFBC OFDM system. KeywordsCOFDM, CI, PAPR, MIMO, SFBC. I.INTRODUCTION Orthogonal Frequency Division Multiplexing (OFDM) technique has been adopted as the standards in the several high data rate applications, for example, European DAB/DVB (Digital Audio and Video Broadcasting) system and high-rate WLAN (Wireless Local Area Networks) such as IEEE802.11x and HIPERLAN II. OFDM system transmits information data by many sub-carriers, where sub-carriers are orthogonal to each other and sub-channels are overlapped so that the spectrum efficiency may be enhanced. OFDM can easily be implemented by the Inverse Fast Fourier Transform (IFFT) and Fast Fourier Transform(FFT) process in digital domain, and has the property of high-speed broadband transmission and robustness to multi-path interference, frequency selective fading. However, OFDM signal has high PAPR because of the superimposition of multicarrier signals with large number of sub-carriers. The high PAPR makes the signal more sensitive to the nonlinearities of the HPA and result in signal distortion when the peak power exceeds the dynamic range of the amplifier. To transmit the high PAPR signal without distortion requires more expensive. Recently, a new kind of technique called CI-COFDM has been widely studied [1-4, 9]. In the CI-COFDM technique, each coded information symbol is sent simultaneously over all carriers and the each carrier for the symbol is assigned a corresponding orthogonal CI spreading code. This CI/COFDM system not only can reduce PAPR significantly problem but also achieve coding gain and frequency diversity gains without any loss in throughput. Besides, a great deal attention has been devoted to MIMO antenna array systems and space-time-frequency processing [5-7]. MIMO diversity technique which exists diversity gain and coding gain can resolve the high link budget problem in the high data rate transmission, especially in the multi-path fading channel Besides, the space-time-frequency processing, especially Alamouti’s diversity technique offers significant increase in performance at a low decoding complexity. Alamouti’s Space-Time Block Coding (STBC) method is very Performance Analysis of MIMO SFBC CI-COFDM System against the Nonlinear Distortion and Narrowband Interference _______________________________________________________________________________________________________________ __________________________________________________________________________________________________________________ International Journal of Electronics Signals and Systems (IJESS), ISSN No. 22315969, Volume-1, Issue-2, 2012 14 efficient when delay spread is big or channel’s time variation is very small during the coded continuous OFDM symbols and OFDM sub carrier number is small. On the other hand, when Doppler spread is big or channel’s time variation is large and channel is non frequency selective, the inter-sub carrier’s channel frequency response is nearly constant in the OFDM system with many sub carriers, the SFBC method is more efficient for the high quality transmission. In recent times, some studies in the area of CIOFDM system with MIMO have been performed [8-10]. There, STBC method is applied in some algorithms, and, MIMO CI-COFDM system is utilized to compensate the BER penalty caused by Doppler frequency or frequency selective fading. In this paper, we focus on two Tx / one Rx antenna and two Tx/ two Rx antenna configurations, we evaluate the performance of MIMO OFDM, MIMO CI-OFDM and MIMO CI-COFDM system on the basis of MIMO technique theoretical analysis when HPA nonlinearity or NBI are included. SFBC coding is applied in MIMO OFDM, MIMO CI-OFDM and MIMO CI-COFDM systems. For CI-COFDM realization, digital implemented CI-COFDM is used in which information is coded by conventional encoder and CI code spreading operation and carrier allocation are separately processed by simple IFFT type operation [11]. As a result, MIMO CI-COFDM system outperforms MIMO SFBC OFDM and MIMO SFBC CI-COFDM systems significantly in the existence of both HPA nonlinearity and NBI. II.SYSTEM DESCRIPTION In this paper, SFBC transmit diversity technique is applied into the OFDM system. Simply, the 2Tx/1Rx and 2Tx/2Rx antenna configuration are considered to compare the system performance of the MIMO OFDM and MIMO CI-COFDM system. First, we discuss the traditional MIMO SFBC OFDM structure with 2Tx/1Rx and 2Tx/2Rx antenna. a) 2Tx/1Rx SFBC OFDM system Fig.1. MIMO SFBC OFDM transceiver diagram with 2x1 diversity. When 2 Tx antennas and 1 Rx antenna are considered, assuming the system transmits data symbols X0, X1,...,Xk,Xk+1,...,XN-1on carriers 0,1,...,k,k+1,...,N-1, respectively, the encoding algorithm is ⎥ ⎥ ⎦ ⎤ + ⎢ ⎢ ⎣ ⎡ + − + * 1 * 1 1 2 1 k X k X k X k X k f k f Tx Tx Channel coefficients between the Tx1, Tx2 antenna and Rx1 antenna is H, H respectively Received signals at the Rx1antenna is Rk ,Rk+1 for k, (k+1) carriers respectively. So, received signal in frequency domain is as follows (1) Let’s assume that adjacent two carriers have same channel characteristic, such as . 2 2 1 2 , 1 1 1 1 H k H k H H k H k H = + = = + = Then, decoding algorithm is as follows. ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎣ ⎡ + ⎥ ⎥ ⎦ ⎤ − ⎢ ⎢ ⎢