Blind High-resolution Uplink Synchronization of Ofdm-based Multiple Access Schemes

One of the major problems in OFDMbased multiple access schemes is uplink synchronization. In this paper, we propose an algorithm for the blind estimation of multi-user OFDM synchronization parameters. Our method assumes that all users employ the same pulse shaping filter and uses secondorder statistics only. The proposed approach provides high-resolution estimates in the sense of being able to resolve closely spaced time-frequency offsets. Finally, we provide simulation results demonstrating the performance of our algorithm. 1. MOTIVATION AND I N T R O D U C T I O N Orthogonal frequency division multiplexing (OFDM) [1]-[6] is part of the European digital audio broadcasting (DAB) standard. It is under investigation for digital video broadcasting (DVB) [7], and it is employed for high-bit-rate digital subscriber services on twisted-pair channels such as in the asymmetric digital subscriber line (ADSL) standard. To a lesser extent, OFDM has also been considered for mobile radio applications One of the major problems in OFDM-based multiple access schemes [6, 8, 9, 101 is synchronization on the system uplink (mobile to base station). On the downlink (base station to mobile) multi-user synchronization is easy to achieve, because the signals corresponding to the different mobiles all originate from the same source (base station). Methods for downlink synchronization have been proposed for example in [11][13]. If one of the mobiles is not synchronized on the downlink, the other users will not suffer from this failure. On the uplink, however, if one of the mobiles is not synchronized there will be interference between the users since the incoming signals are multiplexed in the base station. In [14] a random access protocol for uplink time-synchronization of multiple access OFDM has been proposed. A pilot symbol based approach for multi-user OFDM uplink synchronization has been provided in [8]. The use of pilot symbols or training 13, 6, 8, 9, 101. data, however, reduces the data rate. In this paper, we introduce an algorithm for the blind high-resolution estimation of time-frequency offsets in the uplink of multiple access OFDM systems. Unlike the methods proposed in [14,8], our approach does not make use of training sequences or pilot symbols. We assume that all users employ the same pulse shaping filter, which is known to the receiver (base station). The novel algorithm uses second-order statistics only, and provides high-resolution estimates in the sense of being able to resolve closely spaced time-frequency offsets. The paper is organized as follows. Section 2 briefly describes the OFDM multiple access scheme and provides our assumptions and the problem statement. Section 3 presents the estimation algorithm. Section 4 provides simulation results, and Section 5 concludes the paper. 2. MULTIPLE ACCESS OFDM The OFDM multiple access scheme considered in this paper has first been proposed in [8, 101. We assume that the total available bandwidth is divided into N subcarriers (which are allowed to overlap in frequency) and each of the K users is assigned one or several of the subcarriers during a specified transmission time. After this transmission time the frequency assignment can be changed. In the following we make two simplifying assumptions: e each user is assigned one subcarrier, i.e., K = N . e the frequency assignment is kept constant over time. The equivalent baseband signal of the i-th user is given by 166 0-7 803-5 599-71991% 10.00