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A Survey of Physical Layer Security Techniques for 5G Wireless Networks and Challenges Ahead

Yongpeng Wu, Ashish Khisti, Chengshan Xiao, Giuseppe Caire, Kai-Kit Wong, Xiqi Gao

arXiv:1801.05227v1cs.IT

TL;DR

A comprehensive study of physical layer security techniques for 5G wireless networks was still missing. This paper surveys recent research on key 5G technologies, summarizes unresolved challenges, and reports that physical-layer security and conventional encryption can form an integrated solution.

  • Problem

    A comprehensive study of physical layer security techniques for 5G wireless networks was still missing.

  • Method

    The paper provides a comprehensive, detailed summarization of the latest physical layer security research results on key 5G wireless technologies.

  • Results

    The survey identifies unresolved security conditions, including cases where positive secrecy rates may not be achievable and pilot contamination can reduce massive MIMO secure degrees of freedom.

  • Takeaways & Limitations

    With careful management and implementation, physical layer security and conventional encryption can form an integrated solution that safeguards confidential information.

Abstract

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Physical layer security which safeguards data confidentiality based on the information-theoretic approaches has received significant research interest recently. The key idea behind physical layer security is to utilize the intrinsic randomness of the transmission channel to guarantee the security in physical layer. The evolution towards 5G wireless communications poses new challenges for physical layer security research. This paper provides a latest survey of the physical layer security research on various promising 5G technologies, including physical layer security coding, massive multiple-input multiple-output, millimeter wave communications, heterogeneous networks, non-orthogonal multiple access, full duplex technology, etc. Technical challenges which remain unresolved at the time of writing are summarized and the future trends of physical layer security in 5G and beyond are discussed.

I. INTRODUCTION

The paper surveys physical layer security research for key 5G technologies and organizes unresolved challenges and future directions. It presents physical layer security as complementary to conventional protections for safeguarding confidential communications.

  • Motivation: Unlike computation-based cryptography, physical layer security does not rely on computational complexity and can remain secure against computationally capable eavesdroppers.The passage contrasts this property with cryptography that may be compromised when eavesdroppers possess sufficient computational capacity.
  • Motivation: Physical layer security can provide secure data transmission or support cryptographic key distribution in 5G networks.With careful management and implementation, it can also add protection on top of existing security schemes.
  • Survey scope: The survey addresses physical layer security coding, massive MIMO, mmWave communications, heterogeneous networks, NOMA, and full duplex technology.It reviews LDPC, polar, and lattice codes; passive and active eavesdropper scenarios; point-to-point and network mmWave systems; multi-tier communications; and four full-duplex security categories.
  • 5G technologies: The paper reviews secure transmission designs for heterogeneous networks and identifies physical layer security for NOMA as a new and promising research frontier.The heterogeneous-network discussion concerns simultaneous secure multi-tier communications, while the NOMA discussion notes that only a few relevant results were available.
  • 5G technologies: Full-duplex physical layer security creates both an opportunity and a challenge because receivers can generate artificial noise while full-duplex eavesdroppers can actively attack communications.The survey categorizes full-duplex security communications by receiver, transmitter and receiver, base station, and eavesdropper roles.

II. PHYSICAL LAYER SECURITY CODING

The survey reviews practical physical-layer security codes for 5G, focusing on LDPC, polar, and lattice constructions that address limitations of non-constructive random-coding results.

  • Non-constructive random-coding arguments establish secrecy-capacity-achieving codes but offer limited practical usefulness, motivating practical code construction.
  • The survey reviews three practical code families for physical-layer security: LDPC, polar, and lattice codes.
  • A. LDPC Codes: Nested sparse-graph and coset-coded LDPC constructions achieve secrecy capacity under weak secrecy for selected wiretap channels.
  • A. LDPC Codes: Two-edge-type LDPC ensembles are numerically optimized to obtain good secrecy performance close to secrecy capacity.
  • A. LDPC Codes: Ramanuja-graph-based LDPC codes achieve strong secrecy with lower rates for a noiseless desired-user channel and a BEC eavesdropper.
  • A. LDPC Codes: Punctured LDPC codes can produce an eavesdropper BER close to 0.5 and reduce the security gap, but require higher transmit power than non-punctured codes.
  • A. LDPC Codes: Scrambling information bits in nonsystematic coded transmission achieves a comparable security gap without increasing transmit power.
  • A. LDPC Codes: Finite-length code design uses eavesdropper equivocation rate optimization to construct irregular LDPC codes approaching ultimate performance limits with small codeword lengths.

B. Polar Codes

The survey covers polar and lattice coding approaches for physical-layer security, including secrecy-capacity results, extensions to several network models, and secrecy-gain analysis.

  • B. Polar Codes: Polar coding schemes use bit channels favorable to both legitimate and eavesdropping receivers for random bits, while channels favorable only to the desired user carry information.
  • B. Polar Codes: The polar coding scheme achieves secrecy capacity and has been shown to achieve the entire rate-equivocation region.
  • B. Polar Codes: Polar-code constructions extend to key agreement, multiple-access wiretap, broadcast-confidential-message, interference-confidential-message, relay, and general wiretap channels.
  • B. Polar Codes: Additional polar schemes address deterministic wiretap channels, strong secrecy, discrete-memoryless broadcast channels, and concatenated polar or polar-LDPC designs.
  • C. Lattice Codes: Wiretap lattice codes are designed for Gaussian and fading channels using equivocation rate, secrecy gain, and reliability or secrecy criteria.
  • C. Lattice Codes: The secrecy gain of lattice constructions scales exponentially with lattice dimension, and symmetry-point methods identify secrecy gains for unimodular lattices.
  • C. Lattice Codes: The identified secrecy gains are used to determine the best wiretap lattice codes among the examined lattice constructions.

III. PHYSICAL LAYER SECURITY IN MASSIVE MIMO SYSTEMS

The survey reviews physical-layer security threats and countermeasures for massive MIMO under passive and active eavesdropper scenarios. It emphasizes pilot contamination as a serious TDD security threat and summarizes beamforming, artificial-noise, null-space, pilot, and key-based defenses.

  • Passive eavesdropper scenarios: Massive MIMO security research covers passive eavesdroppers, active attacks, finite-alphabet inputs, relay-aided systems, hardware limitations, and secrecy-outage analysis.The reviewed work also considers massive-MIMO eavesdroppers and distributed antennas.
  • Countermeasures: Matched-filter precoding with artificial noise, regularized channel inversion, and null-space designs are proposed to degrade the eavesdropper’s channel and counter pilot contamination.A unified design combines matched-filter precoding with artificial-noise generation and null-space design; simulations indicate reliable secure communication under the stated conditions.
  • Active eavesdropper scenarios: Pilot contamination lets an eavesdropper transmit the users’ pilots, increasing its eavesdropping capability during TDD channel training.The attack can cause the transmitter to beamform toward the eavesdropper instead of the desired user.
  • Active eavesdropper scenarios: If the eavesdropper’s pilot power is sufficiently large, a positive secrecy rate may not be achievable, and the maximum secure DoF could be zero.Without pilot contamination, the maximum secure DoF can equal the massive-MIMO maximum DoF when no eavesdropper exists.
  • Countermeasures: Pilot hiding, secret-key agreement, pilot retransmission, and game-theoretic transmission are additional defenses against active eavesdroppers.The secret-key approach adjusts secrecy-key length according to estimated information leakage.

IV. PHYSICAL LAYER SECURITY FOR MMWAVE COMMUNICATIONS

The survey examines physical-layer security in mmWave links, ad hoc networks, cellular networks, relaying, and vehicular communication. Directional transmission, antenna scaling, artificial noise, and blockage shape secrecy performance, while reflections can enable attacks.

  • System characteristics: MmWave security exploits large antenna arrays, short range, and highly directional transmissions, but also faces distinct propagation and blockage effects.MmWave systems commonly use many antennas with a limited number of RF chains.
  • Point-to-point and wiretap channels: With many transmit antennas, delay-tolerant mmWave transmission can achieve multi-gigabit per second secrecy rate.The result is reported for analog beamforming based on perfect CSI of the desired user.
  • Point-to-point and wiretap channels: The overlap between desired-user and eavesdropper spatially resolvable paths has an important impact on mmWave secrecy performance.The corresponding analysis considers matched-filter precoding and artificial-noise generation with perfect desired-user CSI and statistical eavesdropper CSI.
  • Cellular and ad hoc networks: Narrower directional beamwidth with more focused array gain benefits secrecy performance in mmWave cellular networks.The cellular analysis also investigates antenna-array pattern, base-station intensity, and artificial-noise generation.
  • Propagation and attacks: Blockages can decrease secrecy outage probability, whereas reflection signals from small physical objects can reduce achievable secrecy rate to zero.The latter attack can remain effective even with highly directional mmWaves.
  • Cellular and ad hoc networks: More transmit antennas help suppress eavesdropper-side array gains, while power allocation between the transmit signal and artificial noise is also analyzed.This finding comes from large-scale mmWave ad hoc network analysis using directional beamforming.

V. PHYSICAL LAYER SECURITY IN HETEROGENEOUS NETWORKS

The survey reviews physical-layer security in multi-tier heterogeneous networks, where node locations, cross-tier interference, and user association complicate secure transmission. Existing work derives secrecy metrics and designs association, coordination, allocation, and interference strategies.

  • Challenges: Multi-tier heterogeneous networks introduce security challenges because high- and low-power node locations affect design and user selection under security constraints.Connected devices also require protection against data leakage.
  • Challenges: Cross-tier interference and open-tier access make user-association policies coordinating quality of service and secrecy necessary.These issues arise alongside reliable and secure transmission design in heterogeneous networks.
  • Spectrum allocation: Orthogonal spectrum allocation removes intercell interference, whereas secrecy-oriented non-orthogonal allocation uses cooperating femtocells to generate interference near eavesdroppers while preserving femtocell-user QoS.The reviewed two-tier work considers single-antenna users and eavesdroppers.
  • Secrecy analysis: Closed-form secrecy-outage expressions are obtained for downlink K-tier networks with node locations modeled as independent Poisson Point Processes.The cited analysis assumes no intercell and intracell interference.
  • Secrecy analysis: A trade-off between secrecy and connection probabilities depends on association threshold, base-station density, and power allocation between useful signal and artificial noise.Network-wide and per-user secrecy throughput are analyzed under secrecy and connection probability constraints.
  • Secrecy analysis: Increasing base-station density benefits both secrecy and connection outage probability performance in the reviewed small-cell network.Other work derives exact successful-connection and secrecy-outage probabilities by approximating aggregate interference with two-dimensional Poisson Point Processes.

VI. PHYSICAL LAYER SECURITY OF NOMA

The survey treats NOMA security as important because scarce bandwidth and spectral-efficiency goals motivate its 5G use. Reviewed studies derive power-allocation and outage results, analyze pairing and multiple-antenna designs, and identify unresolved heterogeneous security requirements.

  • Motivation and challenges: NOMA supports high system throughput, reliability, coverage, low latency, and massive connectivity, while its spectral-efficiency benefit led to LTE-A inclusion.The survey therefore identifies physical-layer security for NOMA as a major design priority.
  • Motivation and challenges: Dissimilar transmit powers and heterogeneous user security requirements remain design challenges for secure NOMA.User cooperation is identified as an option for enhancing security.
  • Secure transmission designs: Under perfect user CSI, no eavesdropper CSI, and perfect SIC, optimal power allocation maximizing secrecy sum rate under users’ QoS constraints is derived.The setting uses a single-antenna transmitter, multiple users, and one eavesdropper.
  • Secure transmission designs: Pairing one internal-zone user with one external-zone user and establishing an eavesdropper-exclusion zone are used to reduce SIC complexity and analyze secrecy outage.For multiple-antenna base stations, matched-filter precoding and artificial-noise generation are employed to further increase secrecy performance.
  • Performance analysis: The weaker user in a NOMA pair determines the secrecy diversity order.The secrecy diversity order is obtained under the single-antenna base-station setting.
  • Performance analysis: In the high SNR regime, NOMA secrecy rate is no less than orthogonal multiple access secrecy rate for unicasting transmission.The same work also studies secrecy outage probability and considers multicasting and unicasting transmissions.

VII. PHYSICAL LAYER SECURITY FOR FULL DUPLEX TECHNOLOGY

Full-duplex physical layer security exploits simultaneous transmission and reception but introduces security designs spanning full-duplex receivers, transmitters, base stations, and eavesdroppers.

  • Full-duplex communication can theoretically double spectral efficiency by transmitting and receiving simultaneously in the same frequency band.
  • Research on full-duplex physical layer security is organized into four categories covering different transmitter, receiver, base-station, and eavesdropper configurations.
  • Full-Duplex Receivers: Full-duplex receivers transmit artificial noise while receiving data, and joint receiver and artificial-noise designs can maximize achievable secrecy rate.
  • Full-Duplex Receivers: With imperfect successive interference cancellation, a full-duplex receiver's secrecy rate no longer saturates at high signal-to-noise ratios, unlike the half-duplex case.
  • Full-Duplex Receivers: Multiple full-duplex receivers and eavesdroppers have been studied under perfect or imperfect channel-state information and successive interference cancellation assumptions.
  • Other Full-Duplex Settings: Existing studies also address multi-carrier, ad hoc, heterogeneous, and cognitive-wiretap settings through power allocation, deployment, and secrecy-outage analysis.

B. Full Duplex Transmitter and Receiver

Full-duplex secure communication research considers bidirectional nodes, full-duplex base stations, and active eavesdroppers using beamforming, artificial noise, and robust resource allocation.

  • Bidirectional Full-Duplex Communications: Bidirectional full-duplex systems design beamforming to minimize transmit power or maximize secrecy sum rate under secrecy, quality-of-service, or sum-power constraints.
  • Full-Duplex Base Stations: Full-duplex base-station studies address simultaneous uplink and downlink security through joint precoding and artificial-noise generation.
  • Full-Duplex Base Stations: For multiple uplink and downlink users with imperfect eavesdropper channel information, robust resource allocation minimizes total transmit power subject to rate constraints.
  • Full-Duplex Base Stations: A proposed full-duplex base-station design achieves significantly higher power efficiency than a zero-forcing beamforming baseline.
  • Full-Duplex Active Eavesdroppers: Active-eavesdropper studies design jamming or transmission strategies using KKT analysis, hierarchical games, and robust optimization under channel uncertainty.

VIII. OTHER IMPORTANT RESEARCH WORK

The paper surveys cross-layer security and practical test beds while identifying unresolved coding challenges for general wireless channels.

  • Cross-Layer Security: Cross-layer designs combine physical-layer channel coding with application-layer authentication and watermarking to use network resources more efficiently.
  • Cross-Layer Security: A joint coding, authentication, and watermarking scheme improves verification probability for both static and dynamic networks.
  • Other Applications: Physical-layer security is also applied to secure video transmission, routing in multi-hop ad hoc networks, and physical-layer authentication.
  • Practical Evaluation: WiFi and LTE test beds evaluate secrecy coding, secret-key generation, artificial noise, and beamforming in practical transmission environments.
  • Practical Evaluation: Artificial noise remains effective against eavesdroppers even when the desired user and eavesdropper are very close to each other.
  • Practical Evaluation: Channel realizations significantly affect secrecy-coding performance, motivating practical prototype evaluation.
  • Coding Challenges: Current LDPC and polar-code designs remain limited by channel models or eavesdropper knowledge, leaving general-channel strong-secrecy design unresolved.

B. Physical Layer Security in Massive MIMO Systems

The paper identifies unresolved physical-layer security problems across massive MIMO, millimeter-wave, heterogeneous, NOMA, and full-duplex systems.

  • Massive MIMO: Existing massive-MIMO defenses can require pilot lengths scaling with antenna count, reducing transmission efficiency, while active-eavesdropper approaches remain preliminary.
  • Massive MIMO: Massive-MIMO security research must address pilot contamination, including fundamental limits, unified schemes for general channels, and practical transmission designs.
  • Millimeter Wave: Millimeter-wave security remains challenged by active attacks on channel estimation and limited research on defenses for directional beamforming.
  • Millimeter Wave: Hybrid digital-analog precoding creates open security-design problems for point-to-point, network, and vehicle-to-vehicle millimeter-wave systems.
  • Heterogeneous Networks: Heterogeneous-network research could extend beyond secrecy analysis to user scheduling, precoding, and deliberate exploitation of inter-tier interference.
  • NOMA: NOMA security is important because users decode other users' information, but existing work largely studies idealized models and basic power allocation.
  • Full Duplex: Full-duplex security remains open when all nodes can transmit and receive, including adversarial artificial-noise generation and multiple-antenna base-station extensions.

G. Physical Layer Security for Other 5G Scenarios and Beyond

Physical layer security research for 5G and beyond extends to secret-key generation, IoT and MTC security, wireless power transfer, and emerging wireless technologies. The survey identifies unresolved challenges involving implementation conditions, resource constraints, channel models, and active attacks.

  • Secret-key generation: Physical-layer secret-key generation can be implemented on current wireless devices, and many prototypes have been reported.The approach exploits correlated observations of noisy phenomena exchanged over a public channel.
  • Secret-key generation: Research still lacks thorough analysis of how environmental conditions and channel parameters affect physical-layer secret-key generation, especially beyond passive-eavesdropper settings.Most key-agreement studies consider passive eavesdroppers.
  • IoT and MTC: IoT and MTC security must account for massive device populations, limited hardware and storage, low data rates, periodic traffic, and significant energy constraints.A theoretically grounded approach for precisely characterizing complexity and energy constraints in physical-layer security designs remains missing.
  • IoT and MTC: Open MTC problems include secrecy metrics for point-to-multipoint and multipoint-to-point systems and secure transmission over propagation channels unlike conventional Rayleigh and Rician models.These issues arise with very large numbers of downlink receivers and uplink transmitters.
  • Wireless power transfer: Wireless power transfer and physical layer security may be integrated because security jamming noise can also serve as an energy beam.The cited work indicates that jamming noise used for security is not always harmful.
  • Emerging 5G scenarios: Physical layer security supports emerging 5G scenarios including massive MIMO, millimeter-wave communications, machine-type communication, and the Internet of Things.These technologies introduce new security challenges and motivate secure transmission schemes that exploit radio-channel propagation properties.
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