Carrier Frequency Offset Compensation for Uplink of Ofdm-Fdma Systems

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  • Topic: Frequency-division multiplexing, Multiplexing, Complexity
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IEEE COMMUNICATIONS LETTERS, VOL. 4, NO. 12, DECEMBER 2000

Carrier Frequency Offset Compensation for Uplink of OFDM-FDMA Systems Jihoon Choi, Changoo Lee, Hae Won Jung, and Yong Hoon Lee
Abstract—A frequency offset compensation method for OFDMFDMA, that can correct offsets after the DFT via circular convolution, is proposed as an alternative to the direct method that corrects frequency offsets by multiplying the complex exponents of the offset estimates before the DFT. In contrast to the direct method whose complexity increases proportional to the number of users, the computational load of the proposed scheme decreases as the number of users increases. It is shown that the proposed method is simpler to implement than the direct method when the number of users is greater than two. Furthermore, the former can outperform the latter, because a frequency offset compensation for one user after the DFT does not affect the data of other users. Computer simulation results demonstrate the advantage of the proposed method. Index Terms—Carrier synchronization, frequency offset correction, OFDM, OFDM-FDMA.

I. INTRODUCTION UE TO the fact that orthogonal frequency division multiplexing (OFDM) is based on using a number of subcarriers, the frequency division multiple access (FDMA) method that assigns clusters of subcarriers to different users has been recognized as a basic multiple access scheme for OFDM [1]–[6]. OFDM using such a FDMA method, herein referred to as OFDM-FDMA, is conceptually simple, however, its implementation needs some caution: in particular, OFDM-FDMA not only requires accurate receiver timing and a carrier frequency for each user, but also requires precise synchronization among all users in order to avoid any intercarrier interference (ICI). Synchronization between users is difficult in the uplink communication (from mobile stations to a base station). One approach to overcoming this difficulty is to use a downlink control channel which conveys the estimated synchronization information to each user [2], [3]. An alternative is to achieve synchronization via signal processing at the uplink receiver without the help of a control channel. This paper will be focused on the latter: in particular, an efficient carrier frequency compensation method at the uplink receiver will be developed. Carrier frequency synchronization at the uplink receiver is achieved via frequency offset estimation and correction. Manuscript received March 27, 2000. The associate editor coordinating the review of this letter and approving it for publication was Dr. N. Van Straleen. J. Choi and Y. H. Lee are with the Department of Electrical Engineering, Korea Advanced Institute of Science and Technology, Taejon, 305-701, Korea (e-mail: yohlee@ee.kaist.ac.kr). C. Lee and H. W. Jung are with the Wireless ATM LAN Team, Electronics and Telecommunications Research Institute, Taejon, 305-350, Korea. Publisher Item Identifier S 1089-7798(00)11509-1.

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Estimating the frequency offsets for OFDM-FDMA is rather straightforward: the method in [7], which was developed for a single user OFDM, can be applied, as shown in Section II below. In contrast, the direct correction of a frequency offset by multiplying the complex exponents of the estimated offsets beforethediscrete Fourier transform(DFT)tendstorequireheavy computation and may cause some performance degradation. Since the frequency offsets associated with different users are different to each other, an offset correction followed by the DFT should be performed separately for each user. Therefore, the direct frequency compensation for OFDM-FDMA with users requires DFT blocks. Performance degradation in the direct method is caused by the fact that a frequency compensation for one user before the DFT can increase the frequency offsets in the data of other users within the DFT block. In this letter, an alternative signal processing method for frequency offset compensation is developed. This...
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