User cooperation communication based on cognitive radio in DAS

Distributed antenna system (DAS) can improve the spectral efficiency of cellular systems, especially at the cell edge. In this paper we consider a cognitive radio technique to further improve the cell edge performance of DAS. We consider the following scenario: A secondary user (SU) shares the same band with a primary user (PU) at the cell edge of a DAS, and the SU relays the signal for the PU. The concept of the network coding is introduced to this scenario. A novel network coding scheme which is named joint modulation is presented for such system. We also propose an improved hybrid AF analogy network coding and DF complex field network coding for the case of decode failure of SU. At PU joint detection and decoding, i.e. turbo receiver is employed to improve the performance of the system. The performance of the schemes is evaluated in OFDM system with turbo receiver, and compared with the complex field network coding [1], XOR network coding [2], [3], and the amplified-and- forward (AF) based analogy network coding [4]. It is shown that the performance of the proposed scheme is better than that of [1] and [4].