TITLE OF THE INVENTION A LOW COMPLEXITY BANDWIDTH EFFICIENT CARRIER FREQUENCY OFF SET ESTIMATION TECHNIQUE FOR OFDMA UPLINK TRANSMISSIONS FIELD OF THE INVENTION The present invention relates to a system and method for estimating low complexity bandwitdth efficient carrier frequency offset for orthogonal frequency division multiple access (OFDMA) uplink transmissions in communication systems. In orthogonal frequency division multiple communication system. BACKGROUND AND PRIOR ART Orthogonal frequency division multiple access (OFDMA) has gained acceptance recently as one of the prominent multiple access technique to provide orthogonal frequency division multiplexing (OFDM) services to a group of subscribers. The carrier frequency offset (CFO) estimation problem in OFDMA is more challenging than that in OFDM systems. In this work, we present a novel technique for the CFO estimation in the of uplink (from subscriber stations to the base station) OFDMA communication system. Here each active subscriber station has a different CFO relative to the common base station receiver which, if uncompensated, would results in self interference and multiple access interference. We propose a null sub carrier based CFO estimator where the estimation is done by minimizing a cost function which compute the total energy present in null sub carrier locations due to the CFO. The computational complexity and training overhead of our method are much lower as compared to some of the CFO estimation schemes available in the literature for OFDMA transmissions. All the CFO estimation schemes proposed for OFDMA, to the best of our knowledge, are carrier assignment scheme (CAS) specific. This introduces considerable in convenience in system design and bandwidth assignment. In an attempt to propose a fairly generalized yet efficient scheme, we propose a null sub carrier based CFO estimation technique for OFDMA which can be applied to subband CAS, generalized CAS and a modified form of interleaved CAS. The estimator performance is tested under the most adverse conditions like maximum FFO for all the users in the system. Being a null sub carrier based estimator, the proposed CFO estimation scheme do not require the knowledge of fading channel coefficients for deriving the frequency offset estimates. CARRIER ASSIGNMENT SCHEMES The main difference between OFDM and OFDMA is that in the case of later, in the downlink, each OFDM symbol conveys distinct sets of data to multiple subscribers simultaneously. Thus the available N subcarriers are usually evenly divided among the M users, by dividing N into R subchannels, each consisting of Q = NI R subcarriers. As the maximum number of users that an OFDMA system can support is decided by R, we assume that M