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: "downlink spectral efficiency of cell free massive mimo with full pilot zero forcing"
  Abstract: Cell-free Massive multiple-input multiple-output (MIMO) ensures ubiquitous communication at high spectral efficiency (SE) thanks to increased macro-diversity as compared cellular communications. However, system scalability and performance are limited by fronthauling traffic and interference. Unlike conventional precoding schemes that only suppress intra-cell interference, full-pilot zero-forcing (fpZF), introduced in [1], actively suppresses also inter-cell interference, without sharing channel state information (CSI) among the access points (APs). In this study, we derive a new closed-form expression for the downlink (DL) SE of a cell-free Massive MIMO system with multi-antenna APs and fpZF precoding, under imperfect CSI and pilot contamination. The analysis also includes max-min fairness DL power optimization. Numerical results show that fpZF significantly outperforms maximum ratio transmission scheme, without increasing the fronthauling overhead, as long as the system is sufficiently distributed.

Connected through hyperedge HE-1 (induced by source paper "joint power allocation and load balancing optimization for energy efficient cell free massive mimo networks"):
  with members:
    - "scalable cell free massive mimo systems"
    - "ubiquitous cell free massive mimo communications"
    - "a speculative study on 6g"
    - "optimal design of energy efficient cell free massive mimo joint power allocation and load balancing"
    - "making cell free massive mimo competitive with mmse processing and centralized implementation"
    - "distributed power control in downlink cellular massive mimo systems"
    - "energy efficiency maximization for c rans discrete monotonic optimization penalty and ell _ 0 approximation methods"
    - "on the total energy efficiency of cell free massive mimo"

Connected through hyperedge HE-2 (induced by source paper "rate analysis of cell free massive mimo noma with three linear precoders"):
  with members:
    - "what role can noma play in massive mimo"
    - "ubiquitous cell free massive mimo communications"
    - "interplay between noma and other emerging technologies a survey"
    - "signal processing for mimo noma present and future challenges"
    - "on the total energy efficiency of cell free massive mimo"
    - "power domain non orthogonal multiple access noma in 5g systems potentials and challenges"
    - "channel hardening and favorable propagation in cell free massive mimo with stochastic geometry"
    - "capacity comparison between mimo noma and mimo oma with multiple users in a cluster"

Connected through hyperedge HE-3 (induced by source paper "local partial zero forcing precoding for cell free massive mimo"):
  with members:
    - "scalability aspects of cell free massive mimo"
    - "ubiquitous cell free massive mimo communications"
    - "making cell free massive mimo competitive with mmse processing and centralized implementation"
    - "cell free massive mimo with limited backhaul"
    - "on the total energy efficiency of cell free massive mimo"
    - "cell free massive mimo versus small cells"
    - "reciprocity calibration for massive mimo proposal modeling and validation"
    - "are we approaching the fundamental limits of wireless network densification"

Connected through hyperedge HE-4 (induced by source paper "optimal design of energy efficient cell free massive mimo joint power allocation and load balancing"):
  with members:
    - "ubiquitous cell free massive mimo communications"
    - "making cell free massive mimo competitive with mmse processing and centralized implementation"
    - "enhanced max min sinr for uplink cell free massive mimo systems"
    - "performance of cell free massive mimo systems with mmse and lsfd receivers"
    - "on the total energy efficiency of cell free massive mimo"
    - "energy efficiency in cell free massive mimo with zero forcing precoding design"
    - "joint power allocation and user association optimization for massive mimo systems"
    - "cell free massive mimo versus small cells"

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)