Semi-blind multiple-frequency-offset estimation for decode-and-forward OFDM cooperative networks

Abstract

Abstract-Cooperative networks recently became attractive because they can achieve spatial diversity. Multiple decode-andforward (DF) relay nodes result in multiple carrier frequency offsets (CFOs) because each relay node owns its local oscillator. This paper presents a novel semi-blind multiple-CFO estimator for DF OFDM cooperative networks. A procedure is designed to effectively derive the semi-blind multiple-CFO estimator for relay nodes. Each relay node occupies its own subchannels, and its CFO falls in a nonoverlapped spectrum. The CFO mapping can simply match the corresponding relay nodes. In addition, the proposed method can reduce hardware and computational complexity. Semi-blind random sequences are derived from the characteristics of the signal matrix in the multiple signal characterization (MUSIC) algorithm. Comprehensive simulations show that random sequences can replace periodic training sequences. Furthermore, some information can be conveyed by random sequences to increase spectral efficiency. I. INTRODUCTIONCooperative network systems using multiple relay nodes can achieve spatial diversity, increase capacity and enhance coverage [1] [2]. Cooperative networks create a virtual multiple antenna structure by combining multiple relay nodes. However, the process of synchronization might be very intricate due to the numerous nodes in this type of cooperative networks. Most past research on cooperative networks assumes perfect synchronization to simplify the model. Some research papers [3] [4] verify that the performance is degraded singularly if the synchronization errors are large. The presence of multiple carrier frequency offsets (CFOs) in cooperative networks arises from multiple distributed relay nodes. Each relay node owns its local oscillator and brings mismatch between with the destination respectively. Additionally, the OFDM communication is very sensitive to the CFO. The CFO could introduce intercarrier interference and thus dramatically degrade bit error rate performance. Therefore, handling the multiple frequency synchronization problem is a key issue for the successful deployment of a cooperative network.Conventional CFO estimation methods in OFDM systems can be categorized into two types: cyclic-prefix-based methods [5] [6] and pilot-based methods [7]. These conventional CFO estimations only handle a single CFO. Multiple-CFO