New M.E. Thesis Submitted from ECE Student


Orthogonal frequency division Multiplexing (OFDM) is a special form of multicarrier modulation, uses DSP algorithms like Inverse Fast Fourier Transform (IFFT) to generate waveform that are mutually orthogonal. After more than thirty years of research and development carried out in different places, OFDM is now being widely implemented in high speed digital communications. OFDM is used in the European digital broadcast radio system and is being investigated for broadband indoor wireless communications. Standards such as HIPERLAN2 (High Performance Local Area Network) and IEEE802.11a have emerged to support IP-based services and provide some quality of service (QOS) support such systems are based on OFDM and are designed to operate in the 5GHz band.
The main advantage of OFDM transmission is its robustness to frequency-selective fading characteristics of a mobile radio channel. In OFDM, the entire spectrum is divided into subcarriers which are narrow-banded with respect to the coherence bandwidth of the channel. This provides resistance against frequency-selectivity of the channel for wideband data transmission.
This study investigates the Bit Error Rate (BER) performance of link adaptation techniques in OFDM system. Under the first adaptive scheme for OFDM, two methods are considered. In the first method, the fixed number of weak sub-carriers which are highly attenuated are left out to improve the BER performance. In the second method, depending on the channel quality of each sub-carrier, the modulation level on each sub-carrier is varied to adapt to the link condition to improve the system BER performance.
In the above scheme, it is necessary to transmit allocation information of modulation mode of each sub-carrier (according to its channel state information) from transmitter to receiver and this leads to degradation in the performance. So in this study, an adaptive modulation scheme for OFDM is considered that makes possible less allocation information to be transmitted from the transmitter to the receiver by using data spreading. Moreover, sub-carrier power control is combined in order to improve transmission performance further. The bit error rate performance of the given scheme is examined by computer simulations and better BER performance is confirmed as compared to the conventional scheme.
Subsequently to improve the BER performance of OFDM system error correcting codes (convolutional codes) are used, hence making simple OFDM system to COFDM system. Error correction codes are usually applied to protect digital source data across the hostile mobile wireless channel. Most of the error correction codes are designed to correct random channel errors. However, channel errors caused by the mobile wireless channel are bursty in nature. Hence, interleaving is usually employed to randomize the bursty channel errors so that error correction codes could be more effective.
In this study, a novel interleaving technique called adaptive interleaving for OFDM systems is implemented. This scheme could better exploit the instantaneous channel variation by rearranging the transmitted symbols according to the Channel State information (CSI) of the frame. The objective is to break up a long burst of the bad CSI sequence so that the overall bit error probability is reduced or minimized. The simulation for above mentioned schemes are done in MATLAB Toolbox. In the contest of time varying channels, the channel is assumed to be Rayleigh multipath fading channel. The channel noise is assumed to be AWGN.

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