CARRIER FREQUENCY OFFSET ESTIMATION IN UPLINK OFDMA

Abstract

Orthogonal frequency division multiplexing (OFDM) is a transmission method that can achieve high data rates by multicarrier modulation. Orthogonal Frequency Division Multiple Access (OFDMA) is a multi-user version of OFDM which provides a unique multiple-access capability, in which multiple subcarriers are assigned to different users for simultaneous transmission. In OFDMA, signals from different users overlap in the frequency domain but occupy different subcarriers. However. OFDM has some disadvantages; one of them being its sensitivity to carrier frequency offset. The Carrier Frequency Offset (CFO) causes Inter-Carrier Interference (IC!) and a reduction in the amplitude of the desired subcarrier that results in loss of orthogonality OFDMA inherits from OFDM the sensitivity to CFOs, resulting in IC!, which subsequently results in Multi-User Interference (N4UI). In this thesis, the effects of frequency offset are discussed in terms of loss of orthogonality, which is proved through simulation results suggesting the BER degradation by carrier frequency offset, followed by the CFO estimation technique; one of them being called as Moose's approach for OFDM system. Extended Moose approach is proposed in this thesis which estimates CFO for each user in the uplink using Maximum Likelihood (ML) approach. The simulation results showing the estimator performance are furnished later in this report, thus accomplishing the desired estimation of carrier frequency offsets encountered by each user in the uplink of OFDMA. Once these estimates become available, then they can be used for compensating for the effect (MUI) CFOs' had caused, following which the BER performance of uplink OFDMA can be assessed by simulation results obtained in this thesis.