Channel Estimation for MIMO-OFDM Systems Based on Data Nulling Superimposed Pilots Emad Farouk, Michael Ibrahim, Mona Z. Saleh, Salwa Elramly Ain Shams University Cairo, Egypt {emadfarouk, michael.ibrahim, mona.z.saleh, salwa_elramly}@eng.asu.edu.eg Abstract—This paper proposes a new channel estimation algorithm based on data nulling superimposed pilots for the spatial multiplexing multiple-input-multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems. In the proposed method each OFDM data symbol of each transmit antenna is spread over all subcarriers by using a spreading matrix then nulls are introduced at certain subcarriers to cancel the mutual interference between data symbols and superimposed pilots. At receiver accurate channel estimation can be easily acquired based on the superimposed pilots. Then the superimposed pilots are removed from the received signal and simple iterative data detection scheme is used to compensate the distortion which occurred in the data symbols. The simulation results of the proposed algorithm show improvement in the estimation accuracy, bit error rate (BER) and computational complexity compared to that of the conventional superimposed pilot technique. The simulation results also show that the performance of the proposed technique approaches that of the frequency division multiplexed pilots technique while having higher data rate and some excess in the receiver complexity. I. INTRODUCTION Orthogonal frequency division multiplexing (OFDM) offers high robustness to frequency selective-fading channels, high data rates, simple channel estimation and equalization methods [1]. Spatial multiplexing of multiple-input-multiple-output (MIMO) system offers high spectral efficiency and can be used to support high capacity demands [2-3]. These introduce spatial multiplexing MIMO-OFDM as an attractive scheme in nowadays standards [4-5]. However, the good performance of MIMO systems is conditioned on an accurate channel estimation at the receiver, so channel estimation is considered as a bottleneck for good performance MIMO systems. In MIMO-OFDM systems, channel estimation is commonly performed based on pilot-assisted techniques [6] where frequency division multiplexed (FDM) pilots are multiplexed with data symbols. This technique can easily acquire accurate channel estimates, but the inserted FDM pilots decrease the data rate and the spectral efficiency due to the large number of FDM pilots required [7], especially in fast varying channels in which the number of pilot symbols is usually increased to track the channel variations accurately. Another channel estimation approach is the blind techniques where no pilots are transmitted, and the estimation process is based on higher order statistics of the received signals [8]. The main drawback of this method is that the data sequence should be long enough in order to get an accurate channel estimation [7] which, may be impractical especially, in fast varying channels. On the other hand, semi- blind channel estimation methods [8] depend on both the transmission of pilot symbols and the statistical properties of the received signal which results in a smaller number of pilot symbols and hence increases the spectral efficiency. Another attractive technique which is introduced in single-input-single- output (SISO) systems is the channel estimation based on superimposed pilots, in which superimposed pilots sequence known to the receiver is algebraically added to the data symbols and then used for channel estimation [9].This technique increases the spectral efficiency, but yet it has a limited performance due to the mutual interference between data symbols and the superimposed pilots. Therefore, iterative methods are introduced [10] to mitigate the mutual interference between data symbols and the superimposed pilots. Hence, offer better performance at the expense of increased receiver complexity. So, channel estimation based on superimposed pilots is considered as an attractive method in MIMO systems as it will provide high data rates and high spectral efficiency [11-12]. A new channel estimation method based on superimposed pilots is introduced in [13] for the SISO-OFDM system named data nulling superimposed pilots (DNSP) where data symbols are pre-coded among all subcarriers, and then a nulling matrix is used to introduce nulls at certain subcarriers that correspond to the superimposed pilots' active subcarriers. This method showed a promising improvement in terms of channel estimation accuracy and BER. This paper proposes a channel estimation algorithm based on DNSP for the spatial multiplexing MIMO-OFDM system. The main objective of the proposed algorithm is to introduce an accurate channel estimation technique that increases the spectral efficiency compared to channel estimation techniques based on FDM pilots. In the proposed method each OFDM data symbol of each transmit antenna is pre-coded by using a spreading matrix that spreads data symbols on all subcarriers then nulls are introduced using a nulling matrix at certain subcarriers positions. Then orthogonal superimposed pilots are added in the frequency domain for each transmit antenna. This scheme eliminates the mutual interference between the data symbol and the superimposed pilots consequently it offers accurate channel estimation. After the superimposed pilots are removed from the frequency domain received signal and equalization process is performed, an iterative detection method is used to restore the OFDM data symbols. The rest of the paper is organized as follows: the system model of the proposed algorithm is described in Section II, and ______________________________________________________PROCEEDING OF THE 21ST CONFERENCE OF FRUCT ASSOCIATION ISSN 2305-7254