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In this correspondence, we advocate the exploitation of direct links for downlink cooperative cellular networks, where one base station (source) communicates with one out of N mobile users (destinations) by using a half-duplex decode-and-forward (DF) relay. Motivated by the potential spectrum-efficiency enhancement, we propose two spectrally efficient schemes for the diversity exploitation of the considered systems in a homogeneous network topology. By scheduling the user with the best direct link to access the channel, an incremental DF relaying scheme is first presented, and its outage behavior is analyzed, revealing that this scheme can achieve full diversity order (N + 1). To further enhance transmission robustness against fading (in terms of outage probability), an improved scheme is also proposed, which substantially utilizes the opportunistic scheduling mechanism when the direct transmission fails. Outage analysis for this scheme shows that, aside from achieving full diversity order, it can further improve the transmission reliability to fading, compared with the preceding one. In addition, it is indicated that the expected spectral efficiency of the proposed schemes approaches that of direct transmission in the high-signal-to-noise-ratio (SNR) regime. Finally, adjustment of the proposed schemes for heterogeneous network topology is presented, and the corresponding outage behavior is characterized as well.
Date of Publication: Jan. 2012