This paper proposes a time and frequency synchronization technique for a ¡ transmit and ¢ receive ( ¡ ¤ £ ¢ ) , MIMO OFDM system. The synchronization is achieved using training symbols which are simultaneously transmitted from ¡ transmit antennas. The training symbols are directly modulatable orthogonal polyphase sequences.The synchronization algorithm shows satisfactory performance even at a low SNR and in a frequency selective channel. The training sequence structure is specialized such that channel parameters in terms of channel coefficients and noise variance can be estimated once synchronization is achieved. I. . INTRODUCTIONOrthogonal Frequency Division Multiplexing (OFDM) has become popular for wireless communications [1]. A multicarrier system can be efficiently implemented in discrete time using Inverse Discrete Fourier Transform (IDFT) to act as a modulator. The actual data to be transmitted, now reperesent "frequency" domain coefficients of the signal and the samples at the output of the IDFT stage are in the "time" domain. OFDM requires synchronization in both the time and frequency. Time synchronization involves finding the best possible time instant for the start of received and down converted OFDM frame. Frequency synchronization deals with finding an estimate of the difference in the frequencies between the transmitter and receiver local oscillators. Frequency offset estimation in OFDM is critical since any frequency offset causes a loss of subchannel orthogonality which results in Inter Carrier Interference (ICI) and hence performance degradation. Synchronization schemes based on training symbols must be efficient in terms of performance and overhead. Also they must be able to operate in both burst and continuous modes. The structure should also aide in channel estimation since channel estimation is an integral part of the OFDM demodulation. Numerous techniques have been suggested in the literature for OFDM time and frequency synchronization [3, 4] however none of them can be applied without major modifications to Multi Input Multi Output (MIMO) systems. The techniques that are described in the literature suggest ¥ The authors wish to thank the Yamacraw Mission, of the State of Georgia, U. S. A. http://www.yamacraw.org for supporting this research.training symbol structure specialized for synchronization but do not take into account the channel estimation that also needs to be done. Hence additional training symbols are needed for channel estimation. This paper proposes time and frequency synchronization for a MIMO OFDM system using training symbols. The training symbols are directly modulatable orthogonal polyphase sequences. The algorithm performs well at low SNR and in frequency selective channels. The training symbol structure is optimized for channel estimation that follows the synchronization phase hence reducing the overhead [6]. II. ANALYSISA block of ¦ samples at the output of the OFDM modulator represents an OFDM symbol and the net time required to transmit one symbol is called the symb...
Abslra+lnthis paper, we present preamble and receiver design for a Q transmit L receive multi input multi output OFDM system. Tasks performed by the receiver are synchronization in time and frequency domains, followed by channel estimation and noise variance estimation. An algorithm is developed such that only Q OFDM symbols of a generalid length are required to perform functions of the preamble. Coarse time synchronization is performed using inherent periodicity in the preamble. Frqueocy offset estimation is then performed in the time domain, followed by residual frequency offset estimation in the frequency domain followed by fine time synchronization. The channel estimntion algorithm is developed nsing least squares principle. Simulations are carried out for a broadband fixed wireless access scenario usinga preamble with and Withont zeropadding of toner in the frequency domain.
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