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: "training and feedback optimization for multiuser mimo downlink"
  Abstract: We consider a MIMO fading broadcast channel where the fading channel coefficients are constant over time-frequency blocks that span a coherent time $\times$ a coherence bandwidth. In closed-loop systems, channel state information at transmitter (CSIT) is acquired by the downlink training sent by the base station and an explicit feedback from each user terminal. In open-loop systems, CSIT is obtained by exploiting uplink training and channel reciprocity. We use a tight closed-form lower bound on the ergodic achievable rate in the presence of CSIT errors in order to optimize the overall system throughput, by taking explicitly into account the overhead due to channel estimation and channel state feedback. Based on three time-frequency block models inspired by actual systems, we provide some useful guidelines for the overall system optimization. In particular, digital (quantized) feedback is found to offer a substantial advantage over analog (unquantized) feedback.

Connected through hyperedge HE-1 (induced by source paper "artificial noisy mimo systems under correlated scattering rayleigh fading a physical layer security approach"):
  with members:
    - "robust secure transmission of using main lobe integration based leakage beamforming in directional modulation mu mimo systems"
    - "enhancing the physical layer security of non orthogonal multiple access in large scale networks"
    - "artificial noise revisited"
    - "the secrecy capacity of the mimo wiretap channel"
    - "capacity and character expansions moment generating function and other exact results for mimo correlated channels"

Connected through hyperedge HE-2 (induced by source paper "fdd massive mimo via ul dl channel covariance extrapolation and active channel sparsification"):
  with members:
    - "multi band covariance interpolation with applications in massive mimo"
    - "fdd massive mimo efficient downlink probing and uplink feedback via active channel sparsification"
    - "channel estimation for tdd fdd massive mimo systems with channel covariance computing"
    - "millimeter wave beam selection using out of band spatial information"
    - "on the ergodic rate lower bounds with applications to massive mimo"
    - "dictionary learning based sparse channel representation and estimation for fdd massive mimo systems"
    - "low rank covariance assisted downlink training and channel estimation for fdd massive mimo systems"
    - "the world s first real time testbed for massive mimo design implementation and validation"

Connected through hyperedge HE-3 (induced by source paper "interleaving channel estimation and limited feedback for point to point systems with a large number of transmit antennas"):
  with members:
    - "interleaved training and training based transmission design for hybrid massive antenna downlink"
    - "compressed channel feedback for correlated massive mimo systems"
    - "spatially common sparsity based adaptive channel estimation and feedback for fdd massive mimo"
    - "antenna grouping based feedback compression for fdd based massive mimo systems"
    - "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"
    - "downlink training techniques for fdd massive mimo systems open loop and closed loop training with memory"

Connected through hyperedge HE-4 (induced by source paper "performance analysis of multi user massive mimo downlink under channel non reciprocity and imperfect csi"):
  with members:
    - "a novel hybrid beamforming algorithm with unified analog beamforming by subspace construction based on partial csi for massive mimo ofdm systems"
    - "analysis of massive mimo with hardware impairments and different channel models"
    - "massive mimo for next generation wireless systems"
    - "what will 5g be"
    - "massive mimo for maximal spectral efficiency how many users and pilots should be allocated"
    - "scalable synchronization and reciprocity calibration for distributed multiuser mimo"
    - "five disruptive technology directions for 5g"
    - "massive mu mimo downlink tdd systems with linear precoding and downlink pilots"

Connected through hyperedge HE-5 (induced by source paper "an overview on resource allocation techniques for multi user mimo systems"):
  with members:
    - "low complexity downlink user selection for massive mimo systems"
    - "alternating minimization algorithms for hybrid precoding in millimeter wave mimo systems"
    - "an overview of signal processing techniques for millimeter wave mimo systems"
    - "user association in 5g networks a survey and an outlook"
    - "cell free massive mimo versus small cells"
    - "mu mimo mac protocols for wireless local area networks a survey"
    - "on the number of rf chains and phase shifters and scheduling design with hybrid analog digital beamforming"
    - "hybrid digital and analog beamforming design for large scale antenna arrays"

Connected through hyperedge HE-6 (induced by source paper "robust regularized zf in cooperative broadcast channel under distributed csit"):
  with members:
    - "on the optimal fronthaul compression and decoding strategies for uplink cloud radio access networks"
    - "network mimo transmitters with no csi can still be very useful"
    - "regularized zf in cooperative broadcast channels under distributed csit a large system analysis"
    - "drone small cells in the clouds design deployment and performance analysis"
    - "linear precoding based on polynomial expansion reducing complexity in massive mimo"
    - "multihop backhaul compression for the uplink of cloud radio access networks"
    - "large system analysis of linear precoding in correlated miso broadcast channels under limited feedback"
    - "completely stale transmitter channel state information is still very useful"

Connected through hyperedge HE-7 (induced by source paper "low complexity downlink user selection for massive mimo systems"):
  with members:
    - "a novel scheme for downlink opportunistic interference alignment"
    - "an esprit based approach for 2 d localization of incoherently distributed sources in massive mimo systems"
    - "optimal design of energy efficient multi user mimo systems is massive mimo the answer"
    - "spectral and energy spectral efficiency optimization of joint transmit and receive beamforming based multi relay mimo ofdma cellular networks"
    - "resource allocation for cost minimization in limited feedback mu mimo systems with delay guarantee"
    - "scaling up mimo opportunities and challenges with very large arrays"
    - "on capacity of large scale mimo multiple access channels with distributed sets of correlated antennas"
    - "large system analysis of linear precoding in correlated miso broadcast channels under limited feedback"

Connected through hyperedge HE-8 (induced by source paper "transmit precoding for interference exploitation in the underlay cognitive radio z channel"):
  with members:
    - "energy efficient symbol level precoding in multiuser miso based on relaxed detection region"
    - "secure m psk communication via directional modulation"
    - "directional modulation via symbol level precoding a way to enhance security"
    - "constructive interference through symbol level precoding for multi level modulation"
    - "constructive multiuser interference in symbol level precoding for the miso downlink channel"
    - "exploiting known interference as green signal power for downlink beamforming optimization"
    - "rethinking the role of interference in wireless networks"
    - "dynamic resource allocation in cognitive radio networks"

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)