Joint Pilot Allocation and Robust Transmission Design for Ultra-dense User-centric TDD C-RAN with Imperfect CSI

摘要

This paper considers the unavailability of complete channel state information(CSI) in ultra-dense cloud radio access networks (C-RANs). The user-centriccluster is adopted to reduce the computational complexity, while the incompleteCSI is considered to reduce the heavy channel training overhead, where onlylarge-scale inter-cluster CSI is available. Channel estimation forintra-cluster CSI is also considered, where we formulate a joint pilotallocation and user equipment (UE) selection problem to maximize the number ofadmitted UEs with fixed number of pilots. A novel pilot allocation algorithm isproposed by considering the multi-UE pilot interference. Then, we considerrobust beam-vector optimization problem subject to UEs' data rate requirementsand fronthaul capacity constraints, where the channel estimation error andincomplete inter-cluster CSI are considered. The exact data rate is difficultto obtain in closed form, and instead we conservatively replace it with itslower-bound. The resulting problem is non-convex, combinatorial, and eveninfeasible. A practical algorithm, based on UE selection, successive convexapproximation (SCA) and semi-definite relaxation approach, is proposed to solvethis problem with guaranteed convergence. We strictly prove that semidefiniterelaxation is tight with probability 1. Finally, extensive simulation resultsare presented to show the fast convergence of our proposed algorithm anddemonstrate its superiority over the existing algorithms.