human: Given a node-centered hypergraph: <hypergraph>, where nodes represent papers and hyperedges represent sets of papers co-cited by a source paper.

Details of the hypergraph:
Center node: "two stage beamformer design for massive mimo downlink by trace quotient formulation"
  Abstract: In this paper, the problem of outer beamformer design based only on channel statistic information is considered for two-stage beamforming for multi-user massive MIMO downlink, and the problem is approached based on signal-to-leakage-plus-noise ratio (SLNR). To eliminate the dependence on the instantaneous channel state information, a lower bound on the average SLNR is derived by assuming zero-forcing (ZF) inner beamforming, and an outer beamformer design method that maximizes the lower bound on the average SLNR is proposed. It is shown that the proposed SLNR-based outer beamformer design problem reduces to a trace quotient problem (TQP), which is often encountered in the field of machine learning. An iterative algorithm is presented to obtain an optimal solution to the proposed TQP. The proposed method has the capability of optimally controlling the weighting factor between the signal power to the desired user and the interference leakage power to undesired users according to different channel statistics. Numerical results show that the proposed outer beamformer design method yields significant performance gain over existing methods.

Connected through hyperedge HE-1 (induced by source paper "interference alignment and soft space reuse based cooperative transmission for multi cell massive mimo networks"):
  with members:
    - "structured compressive sensing based spatio temporal joint channel estimation for fdd massive mimo"
    - "on interference alignment with imperfect csi characterizations of outage probability ergodic rate and ser"
    - "two stage subspace constrained precoding in massive mimo cellular systems"
    - "training sequence design for feedback assisted hybrid beamforming in massive mimo systems"
    - "two tier precoding for fdd multi cell massive mimo time varying interference networks"
    - "downlink training techniques for fdd massive mimo systems open loop and closed loop training with memory"
    - "five disruptive technology directions for 5g"
    - "scaling up mimo opportunities and challenges with very large arrays"

Connected through hyperedge HE-2 (induced by source paper "robust transceiver design based on interference alignment for multi user multi cell mimo networks with channel uncertainty"):
  with members:
    - "exploiting known interference as green signal power for downlink beamforming optimization"
    - "optimal multiuser transmit beamforming a difficult problem with a simple solution structure"
    - "rethinking the role of interference in wireless networks"
    - "two tier precoding for fdd multi cell massive mimo time varying interference networks"
    - "spatially sparse precoding in millimeter wave mimo systems"
    - "grouping based interference alignment with ia cell assignment in multi cell mimo mac under limited feedback"
    - "multi cell random beamforming achievable rate and degrees of freedom region"
    - "outage probability and outage based robust beamforming for mimo interference channels with imperfect channel state information"

Connected through hyperedge HE-3 (induced by source paper "beamspace aware adaptive channel estimation for single carrier time varying massive mimo channels"):
  with members:
    - "channel acquisition for massive mimo ofdm with adjustable phase shift pilots"
    - "a generalized framework on beamformer design and csi acquisition for single carrier massive mimo systems in millimeter wave channels"
    - "pilot reuse for massive mimo transmission over spatially correlated rayleigh fading channels"
    - "pilot beam pattern design for channel estimation in massive mimo systems"
    - "what will 5g be"
    - "joint spatial division and multiplexing for mm wave channels"
    - "pilot contamination and precoding in multi cell tdd systems"

Connected through hyperedge HE-4 (induced by source paper "base station selection for massive mimo networks with two stage precoding"):
  with members:
    - "joint power allocation and user association optimization for massive mimo systems"
    - "training sequence design for feedback assisted hybrid beamforming in massive mimo systems"
    - "two tier precoding for fdd multi cell massive mimo time varying interference networks"
    - "scaling up mimo opportunities and challenges with very large arrays"

Connected through hyperedge HE-5 (induced by source paper "a generalized framework on beamformer design and csi acquisition for single carrier massive mimo systems in millimeter wave channels"):
  with members:
    - "enhancing the estimation of mm wave large array channels by exploiting spatio temporal correlation and sparse scattering"
    - "an overview of signal processing techniques for millimeter wave mimo systems"
    - "channel acquisition for massive mimo ofdm with adjustable phase shift pilots"
    - "spatially common sparsity based adaptive channel estimation and feedback for fdd massive mimo"
    - "pilot reuse for massive mimo transmission over spatially correlated rayleigh fading channels"
    - "channel estimation and hybrid precoding for millimeter wave cellular systems"
    - "pilot beam pattern design for channel estimation in massive mimo systems"
    - "distributed compressive csit estimation and feedback for fdd multi user massive mimo systems"

Connected through hyperedge HE-6 (induced by source paper "hybrid beamforming for massive mimo a survey"):
  with members:
    - "on the number of rf chains and phase shifters and scheduling design with hybrid analog digital beamforming"
    - "limited feedback hybrid precoding for multi user millimeter wave systems"
    - "two stage subspace constrained precoding in massive mimo cellular systems"
    - "near optimal hybrid processing for massive mimo systems via matrix decomposition"
    - "hybrid block diagonalization for massive multiuser mimo systems"
    - "hybrid mimo architectures for millimeter wave communications phase shifters or switches"
    - "channel estimation and hybrid precoding for millimeter wave cellular systems"
    - "massive mimo for next generation wireless systems"

Connected through hyperedge HE-7 (induced by source paper "a rate splitting strategy for massive mimo with imperfect csit"):
  with members:
    - "a hierarchical rate splitting strategy for fdd massive mimo under imperfect csit"
    - "sum rate maximization for mu miso with partial csit using joint multicasting and broadcasting"
    - "rate analysis of two receiver miso broadcast channel with finite rate feedback a rate splitting approach"
    - "optimal multiuser transmit beamforming a difficult problem with a simple solution structure"
    - "massive mimo multicasting in noncooperative cellular networks"
    - "massive mimo for next generation wireless systems"
    - "two tier precoding for fdd multi cell massive mimo time varying interference networks"
    - "what will 5g be"

Connected through hyperedge HE-8 (induced by source paper "efficient feedback mechanisms for fdd massive mimo under user level cooperation"):
  with members:
    - "spatially common sparsity based adaptive channel estimation and feedback for fdd massive mimo"
    - "distributed compressive csit estimation and feedback for fdd multi user massive mimo systems"
    - "training sequence design for feedback assisted hybrid beamforming in massive mimo systems"
    - "massive mimo for next generation wireless systems"
    - "two tier precoding for fdd multi cell massive mimo time varying interference networks"
    - "downlink training techniques for fdd massive mimo systems open loop and closed loop training with memory"
    - "trellis extended codebooks and successive phase adjustment a path from lte advanced to fdd massive mimo systems"
    - "noncoherent trellis coded quantization a practical limited feedback technique for massive mimo systems"

Question: Please tell me which class the center node belongs to. The 40 classes are: cs.NA(Numerical Analysis), cs.MM(Multimedia), cs.LO(Logic in Computer Science), cs.CY(Computers and Society), cs.CR(Cryptography and Security), cs.DC(Distributed, Parallel, and Cluster Computing), cs.HC(Human-Computer Interaction), cs.CE(Computational Engineering, Finance, and Science), cs.NI(Networking and Internet Architecture), cs.CC(Computational Complexity), cs.AI(Artificial Intelligence), cs.MA(Multiagent Systems), cs.GL(General Literature), cs.NE(Neural and Evolutionary Computing), cs.SC(Symbolic Computation), cs.AR(Hardware Architecture), cs.CV(Computer Vision and Pattern Recognition), cs.GR(Graphics), cs.ET(Emerging Technologies), cs.SY(Systems and Control), cs.CG(Computational Geometry), cs.OH(Other Computer Science), cs.PL(Programming Languages), cs.SE(Software Engineering), cs.LG(Machine Learning), cs.SD(Sound), cs.SI(Social and Information Networks), cs.RO(Robotics), cs.IT(Information Theory), cs.PF(Performance), cs.CL(Computation and Language), cs.IR(Information Retrieval), cs.MS(Mathematical Software), cs.FL(Formal Languages and Automata Theory), cs.DS(Data Structures and Algorithms), cs.OS(Operating Systems), cs.GT(Computer Science and Game Theory), cs.DB(Databases), cs.DL(Digital Libraries), cs.DM(Discrete Mathematics). Directly output the class name.

gpt: cs.IT(Information Theory)