Multicarrier modulation technique, also called Orthogonal Frequency Division Multiplexing (OFDM), is a modulation method that can be used for the high speed data communications. In this modulation scheme transmission is carried out in parallel on the different frequencies. The implement of OFDM system is very simple, the modulation and demodulation can be performed by the fast Fourier transform (FFT) algorithm which provides full digital implementation.
OFDM technique is desirable for the transmission of the digital data through the radio channel with multipath fading. Since by the parallel transmission the deleterious effect of fading is spread over many bits, therefore, it is more likely that several bits only be slightly affected by the channel. The other advantage of this technique is its spectral efficiency. In OFDM method the spectra of subchannels overlap each other while satisfying orthogonality, giving rise to the spectral efficiency. Because of the parallel transmission in the OFDM technique the symbol duration is increased. This has the added advantage of this technique to work in the channels having impulsive noise characteristics. And also bandwidth occupied by OFDM system is subdivided into very narrow subband and each subband is modeled as linear flat channel. Thus the OFDM system does not need the sophisticated equalization unit compared to single carrier system.
Recently it is widely used in the area of high speed digital radio communication because of the above advantages of it. However, the OFDM technique is known to be very sensitive to the carrier synchronization offset between the transmitter and the receiver. The timing period offset is difference between transmitter and receiver sampling period, and the frequency offset is caused by Doppler spread of the channel and/or local oscillator drift between the transmitter and the receiver. The resulting of these offset intercarrier interference (ICI) is introduced among the subchannels of the OFDM system, and severely degrades the bit error rate (BER) performance of the OFDM receiver.
This dissertation investigated carrier synchronization technique to improve the performance of OFDM system with the frequency and timing period offset. First, we derive the effects of the frequency and the timing period offset on OFDM system. There two deleterious effects caused by frequency and timing period offset; the first is the reduction of signal amplitude in the output of demodulator and the second is the introduction of ICI from the other carriers which are now no longer orthogonal to each subcarriers. Another effect of carrier synchronization offset is the phase rotation of demodulated data symbol. The amount of phase rotation is proportional to the frequency offset and is proportional to timing period offset times carrier index.
From this observation, we derive a new carrier synchronization algorithm for OFDM system which is suitable for burst mode transmission of digital data. The proposed algorithm can estimate not only the frequency offset, but also the timing period offset. The proposed scheme consists of two part; acquisition and tracking. In the acquisition process, the frequency and the timing period offset are reduced to small enough so that no decision error occurs due to intercarrier interference (ICI) by using only one pilot symbol. The pilot symbol uses only $P$ ($1$), and remainder of the other subcarriers remain unmodulated. And in the tracking process, during each symbol period the frequency and the timing period offset are estimated for compensation of the local oscillator drift. The simulation result for QPSK and 16-QAM show that the estimator is unbiased and the estimation error of the frequency and the timing period offset is small enough, which is nearly same as no SNR degradation due to residual ICI.
