nd tracking. The acquisition range is used in the initial phase while accuracy and stability is more important during OFDM and OFDMA the tracking stage. A frequency offset at the OFDM receiver can cause losses in subcarrier orthogonality, and thus introduce inter-channel interference (ICI). In Fourier transformation theory this phenomenon is known as DFT Leakage. The two common frequency offset estimation methodologies are pilot-based and non-pilot based. To ensure an adequate acquisition range, many practical OFDM systems employ concentrated pilot symbols while continuous pilot sub-carriers are also available for frequency tracking purposes. Pilot- based approaches are more reliable and accurate than non-pilot (blind) based techniques. Whereas non- pilot techniques eliminate the overhead of the system which is associated with pilot-based approaches. 2.5.2 Phase Offset The changes in the phase also cause offsets and loss of orthogonality at the receiver. Phase changes mainly occur due to multipath fading over the radio interface. The minor phase shifts can be corrected by an equalizer while larger ones can cause ambiguity in bit interpretations. 2.5.3 Sampling Offset Sampling offset can occur in both time and frequency domains. A sampling time offset occurs if the transmitter and receiver are slightly out of sync. Due to this offset, the sampling of the received signal takes place at different times. This means that the samples taken at the receiver could not be perfectly matched to an OFDM symbol. The OFDM symbol boundaries can be distinguished by using a cyclic prefix. As long as the OFDM symbol boundaries are maintained, a sampling time offset is equivalent to a linear phase shift, which in most cases can be handled by the receiver. A sampling frequency offset occurs when sampling takes place less frequently or more frequently than expected. In other words it occurs when the A/D converter output is sampled either too fast or too slow. In an OFDM system with many parallel sub-carriers, a sampling frequency offs