Reliable Carrier Frequency Offset Estimation (CFO) Is Extremely Important In Multicar- Rier Communication Systems Where A Group Of Orthogonal Subcarriers Are Used For The Transmission. These Subcarriers Will Remain Perfectly Orthogonal Only If The Transmitter And Receiver Use Exactly The Same Frequencies. A CFO Can Be Present In The System Due To Oscillator Di®Erence, Drift And Doppler Frequency Shifts. As Low Cost Receivers Are Widely Being Deployed, Oscillator Instability Itself Will Be Very Signi¯Cant And The Resultant CFO Can Be Of The Order Of A Few Multiples Of Subcarrier Spacing. This Neces- Sitates Bandwidth And Computationally E±Cient CFO Estimation Techniques Which Can O®Er High Range Of Estimation. In This Thesis, We Address The Issue Of E±Cient Estimation Of CFO In Multicarrier Communication Systems Like Orthogonal Frequency Division Mul- Tiplexing (OFDM), Multi-Input Multi-Output (MIMO)-OFDM And Uplink Of Orthogonal Frequency Division Multiple Access (OFDMA). In The ¯Rst Part Of The Thesis Work, We Investigate The CFO Estimation In OFDM Systems And Propose Two Low Complexity Bandwidth E±Cient Hybrid CFO Estimation Techniques Based On Cyclic Pre¯X And Null Subcarriers. A Novel Null Subcarrier Allocation Technique Based On Fibonacci Series Is Also Proposed. Later We Develop A Low Complexity Maximum Likelihood CFO Estimation Technique Which Makes Exclusive Use Of Null Subcarriers. A Simpli¯Ed Expression For The Bit Error Probability Of An OFDM Receiver That Uses A Null Subcarrier Based CFO Estimator In The Presence Of A Rayleigh Fading Channel Is De- Rived. Also The Performances Are Evaluated By Deriving The CramEr- Rao Lower Bound (CRLB) Of The Estimators. The Work On OFDM Also Includes The Development Of A Low Complexity Integer Frequency O®Set Estimation Technique, Which Has A Signi¯Cantly Small Bandwidth Overhead. Later, The Two Estimation Schemes Developed For OFDM Systems Are Extended To The CFO Estimation In MIMO-OFDM Systems And Their Performances Are Analytically Evaluated In Terms Of The CRLB. The Last Part Of The Thesis Deals With The Investigation Of CFO Estimation Problem In The Uplink OFDMA System. We Came Out With A Fairly Generalized Method Which Can Be Applied To Most Of The Carrier Assign- Ment Schemes Used In OFDMA Systems. Key Words - OFDM, MIMO-OFDM, OFDMA, Carrier Frequency O®Set, Estimation, Null Subcarriers Xv