International Journal of Computer Applications (0975 – 8887) Volume 2 – No.3, May 2010 81 MIMO-OFDM Channel Estimation using Pilot Carries Kala Praveen Bagadi Ph.D Student Dept. of Electrical Engineering NIT Rourkela, India Prof. Susmita Das Associate Professor Dept. of Electrical Engineering NIT Rourkela, India ABSTRACT A multiple-input multiple-output (MIMO) communication system combined with the orthogonal frequency division multiplexing (OFDM) modulation technique can achieve reliable high data rate transmission over broadband wireless channels. Channel state information for both single-input single-output (SISO) and MIMO systems based on pilot aided arrangement is investigated in this paper. The estimation of channel at pilot frequencies with conventional Least Square (LS) and Minimum Mean Square (MMSE) estimation algorithms is carried out through Matlab simulation. The performance of MIMO OFDM and SISO OFDM are evaluated on the basis of Bit Error Rate (BER) and Mean Square Error (MSE) level. Further enhancement of performance can be achieved through maximum diversity Space Time Block Coding (STBC) and Maximum Likelihood Detection at transmission and reception ends respectively. MMSE estimation has been shown to perform much better than LS but is more complex than LS for the MIMO system using pilot carriers. Keywords Channel Estimation, MIMO-OFDM, Pilot carriers, Diversity, Spatial Multiplexing, Space time coding, Rayleigh fading. 1. INTRODUCTION OFDM (Orthogonal Frequency Division Multiplexing) is becoming a very popular multi-carrier modulation technique for transmission of signals over wireless channels. OFDM divides the high-rate stream into parallel lower rate data and hence prolongs the symbol duration, thus helping to eliminate Inter Symbol Interference (ISI). It also allows the bandwidth of subcarriers to overlap without Inter Carrier Interference (ICI) as long as the modulated carriers are orthogonal. OFDM therefore is considered as an efficient modulation technique for broadband access in a very dispersive environment. In this new information age, high data rate and strong reliability in wire-less communication systems are becoming the dominant factors for a successful exploitation of commercial networks. MIMO-OFDM (multiple input multiple output- orthogonal frequency division multiplexing), a new wireless broadband technology, has gained great popularity for its capability of high rate transmission and its robustness against multi-path fading and other channel impairments. The arrangement of multiple antennas at the transition end and reception end results increase in the diversity gain refers the quality of signal and multiplexing gain refers the transmission capacity. Space time block coding used in this paper to transmit multiple copies of a data stream across a number of antennas and to exploit the various received versions of data to improve reliability of data transfer. The major challenge faced in MIMO-OFDM systems is how to obtain the channel state information accurately and promptly for coherent detection of information symbols. The channel state information can be obtained through training based, blind and semi blind channel Estimation. The blind channel estimation is carried out by evaluating the statistical information of the channel and certain properties of the transmitted signals [3]. Blind Channel Estimation has its advantage in that it has no overhead loss; it is only applicable to slowly time-varying channels due to its need for a long data record. In training based channel estimation algorithms, training symbols or pilot tones that are known a priori to the receiver, are multiplexed along with the data stream for channel estimation [4]. Semi-blind channel technique is hybrid of blind and training technique, utilizing pilots and other natural constraints to perform channel estimation. The training-based method channel estimation can be performed by either block type pilots where pilot tones are inserted into all frequency bins within periodic intervals of OFDM blocks or by comb pilots where pilot tones are inserted into each OFDM symbol symbols with a specific period of frequency bins. The block type pilot channel estimation, has been developed under the assumption of slow fading channel; this assumes that the channel transfer function is not changing very rapidly it can be constant over transmission of few OFDM symbols. The comb-type pilot channel estimation has been introduced in case where the channel changes even in one OFDM block. The comb-type pilot channel estimation consists of algorithms to estimate the channel at pilot frequencies and interpolation is used to find the channel at signal frequencies [5]. The interpolation of the channel for comb-type based channel estimation can be depend on linear interpolation, low-pass interpolation and spline cubic interpolation. Further, this signal model is transformed into a linear form suitable for the LS (least- square) and MMSE (minimum mean square error) estimation algorithm. MMSE has been shown to perform much better than LS but more complex than LS. We can use optimal low rank MMSE estimator to reduce complexity. And finally we can conclude that MMSE is an optimal channel estimator in the sense of achieving the minimum MSE (mean squared error). The organization of the paper is as follows. Section II and III describe the system models of SISO & MIMO OFDM systems respectively. Section IV provided the performances of LS and LMMSE algorithms in both the systems and simulation results are presented in Section V Conclusion is given in Section VI.