An equalization technique for high rate OFDM systems

In a typical orthogonal frequency division multiplexing (OFDM) broadband wireless communication system, a guard interval using cyclic prefix is inserted to avoid the inter-symbol interference and the inter-carrier interference. This guard interval is required to be at least equal to, or longer than the maximum channel delay spread. This method is very simple, but it reduces the transmission efficiency. This efficiency is very low in the communication systems, which inhibit a long channel delay spread with a small number of sub-carriers such as the IEEE 802.11a wireless LAN (WLAN). To increase the transmission efficiency, it is usual that a time domain equalizer (TEQ) is included in an OFDM system to shorten the effective channel impulse response within the guard interval. There are many TEQ algorithms developed for the low rate OFDM applications such as asymmetrical digital subscriber line (ADSL). The drawback of these algorithms is a high computational load. Most of the popular TEQ algorithms are not suitable for the IEEE 802.11a system, a high data rate wireless LAN based on the OFDM technique. In this thesis, a TEQ algorithm based on the minimum mean square error criterion is investigated for the high rate IEEE 802.11a system. This algorithm has a comparatively reduced computational complexity for practical use in the high data rate OFDM systems. In forming the model to design the TEQ, a reduced convolution matrix is exploited to lower the computational complexity. Mathematical analysis and simulation results are provided to show the validity and the advantages of the algorithm. In particular, it is shown that a high performance gain at a data rate of 54Mbps can be obtained with a moderate order of TEQ finite impulse response (FIR) filter. The algorithm is implemented in a field programmable gate array (FPGA). The characteristics and regularities between the elements in matrices are further exploited to reduce the hardware complexity in the matrix multiplication implementation. The optimum TEQ coefficients can be found in less than 4µs for the 7th order of the TEQ FIR filter. This time is the interval of an OFDM symbol in the IEEE 802.11a system. To compensate for the effective channel impulse response, a function block of 64-point radix-4 pipeline fast Fourier transform is implemented in FPGA to perform zero forcing equalization in frequency domain. The offsets between the hardware implementations and the mathematical calculations are provided and analyzed. The system performance loss introduced by the hardware implementation is also tested. Hardware implementation output and simulation results verify that the chips function properly and satisfy the requirements of the system running at a data rate of 54 Mbps