IEICE TRANS. COMMUN., VOL.E89–B, NO.1 JANUARY 2006 235 LETTER A Synchronization and Cell Searching Technique Using Pilot Tones for OFDM Cellular Systems Dong Han KIM † , Kwang Soon KIM †† , Jae Young AHN ††† , Nonmembers, and Yong Soo CHO † , Member SUMMARY In this letter, a pilot structure and an efficient algorithm for downlink synchronization and cell searching in OFDM-based cellular sys- tems are proposed. The pilots, randomly allocated in the frequency domain, allow us to minimize inter-cell interference (ICI) as well as to increase cell searching capability, estimation range of integer carrier frequency off- set (CFO), and estimation accuracy of symbol timing offset (STO). The proposed low-complexity joint algorithm for integer CFO estimation, cell searching, and downlink detection is robust to ICI, multipath channel, STO and fine CFO. key words: OFDM, synchronization, cell searching, pilot, downlink detec- tion 1. Introduction Recently, orthogonal frequency division multiplexing (OFDM) has been widely accepted as the most promising radio transmission technology for the next generation wire- less systems. Unlike the conventional OFDM-based wire- less systems such as digital audio broadcasting, digital video broadcasting, IEEE 802.11a, and Hiperlan/2 [1], OFDM- based cellular systems require robust synchronization and cell searching capability even in a bad cellular environment. However, these conventional OFDM systems are inappro- priate for the cellular environment with a frequency reuse factor equal to 1 since they cannot discriminate signals from different cells. In [2], a downlink synchronization and cell searching technique using the preamble in the time domain was proposed for OFDM-based cellular systems. In this let- ter, a new pilot structure and an efficient algorithm for syn- chronization and cell searching in the frequency domain are proposed. The proposed pilots are randomly inserted in the frequency domain and allocated to each cell so that we can avoid or minimize inter-cell interference (ICI) as well as to increase cell searching capability, estimation range of inte- ger carrier frequency offset (CFO), and estimation accuracy of symbol timing offset (STO). After describing the pro- posed pilot allocation scheme in Sect. 2, we propose a low- complexity joint algorithm for integer CFO estimation, cell searching and downlink detection using the pilots in Sect. 3. In Sect. 4, the performances of the proposed techniques are verified via simulation. Conclusions are made in Sect. 5. Manuscript received May 30, 2005. Manuscript revised August 1, 2005. † The authors are with the School of Electrical and Electronic Engineering, Chung-Ang Univ., Seoul 156-756, Korea. †† The author is with Yonsei Univ., Seoul 120-749, Korea. ††† The author is with ETRI (Electronics and Telecommunication Research Institute), Daejeon 305-700, Korea. DOI: 10.1093/ietcom/e89–b.1.235 2. A Pilot Allocation Scheme for OFDM(A)-Based Cel- lular Systems Frame structure and pilot structure for OFDM(A)/TDD- based cellular systems are shown in Fig. 1. The pilot lo- cation index set (PLIS) assigned to the k-th cell is defined as P k =  p k,m :0 ≤ m ≤ N p - 1, 0 ≤ p k,m ≤ N u - 1  , where N u and N p denote the number of used subcarriers and pilots, respectively. The PLIS is constructed such that the spacing between adjacent pilots are random and P k 1 ∩ P k 2 = ∅ for different cell ID numbers of k 1 and k 2 . In this letter, the pilots required for channel estimation are not considered for simplicity. By employing the random pilot pattern instead of the conventional pilot pattern placed on equidistance subcar- riers, we can significantly increase the estimation range of integer CFO and the number of cells to be searched. Also, better performance can be achieved for fine STO estimation when the random pilot pattern is used. An IFFT output sig- nal of a base station at the k-th cell is given by x k (n) = 1 N             N FFT -1  l=0 lP k D(l) · e j2π ln /N FFT +  l∈P k S k (l) · e j2π ln /N FFT             (1) where D (l) denotes the complex symbol such as transmit Fig. 1 Frame and pilot structure for OFDM(A)/TDD-based cellular sys- tems. Copyright c  2006 The Institute of Electronics, Information and Communication Engineers