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Cognitive Non-Orthogonal Multiple Access with Cooperative Relaying: A New Wireless Frontier for 5G Spectrum Sharing

Lu Lv, Jian Chen, Qiang Ni, Zhiguo Ding, Hai Jiang

arXiv:1801.04022v1cs.IT

TL;DR

The paper addresses how to integrate NOMA and CR for more intelligent spectrum sharing despite interference that can degrade cognitive NOMA reliability. It presents three architectures and cooperative relaying strategies, which show potential to significantly lower outage probabilities for each architecture.

  • Problem

    NOMA and CR offer efficient spectrum use but their interference-limited coexistence can degrade reception reliability in cognitive NOMA networks.

  • Method

    The paper integrates NOMA into CR concepts, presents underlay, overlay, and CR-inspired architectures, and proposes cooperative relaying for each.

  • Results

    The proposed cooperative relaying strategies show potential to significantly lower outage probabilities for each cognitive NOMA architecture.

  • Takeaways & Limitations

    Cognitive NOMA is presented as a framework for intelligent spectrum sharing aligned with high spectrum efficiency, massive connectivity, low latency, and better fairness.

Abstract

from arXiv · show

Two emerging technologies towards 5G wireless networks, namely non-orthogonal multiple access (NOMA) and cognitive radio (CR), will provide more efficient utilization of wireless spectrum in the future. In this article, we investigate the integration of NOMA with CR into a holistic system, namely cognitive NOMA network, for more intelligent spectrum sharing. Design principles of cognitive NOMA networks are perfectly aligned to functionality requirements of 5G wireless networks, such as high spectrum efficiency, massive connectivity, low latency, and better fairness. Three different cognitive NOMA architectures are presented, including underlay NOMA networks, overlay NOMA networks, and CR-inspired NOMA networks. To address inter- and intra-network interference which largely degrade the performance of cognitive NOMA networks, cooperative relaying strategies are proposed. For each cognitive NOMA architecture, our proposed cooperative relaying strategy shows its potential to significantly lower outage probabilities. Furthermore, we discuss open challenges and future research directions on implementation of cognitive NOMA networks.

I. INTRODUCTION

The paper integrates NOMA and CR into cognitive NOMA networks to enable intelligent spectrum sharing, while addressing interference that can degrade reception reliability. It presents three architectures and cooperative relaying strategies intended to enhance reliability.

  • NOMA and CR are both interference-limited, so their coexistence introduces inter-network and intra-network interference that degrades reception reliability.
  • Cognitive NOMA combines NOMA and CR to pursue more intelligent spectrum sharing for 5G wireless networks.
  • The paper surveys three cognitive NOMA architectures: underlay NOMA, overlay NOMA, and CR-inspired NOMA networks.
  • Cooperative relaying strategies are proposed for each architecture to enhance reliability in cognitive NOMA networks.
  • The article also outlines potential challenges and future research directions because cognitive NOMA studies remain at a nascent stage.

II. RATIONALES OF COGNITIVE NOMA NETWORKS

This section introduces NOMA and its downlink and uplink operation, emphasizing power-domain multiplexing and successive interference cancellation. The interference-free user differs between the two transmission directions.

  • NOMA multiplexes multiple users on one resource block through power-domain multiplexing and separates signals using successive interference cancellation.
  • Downlink NOMA: In downlink NOMA, the base station broadcasts a superposed signal to strong and weak users with different power allocation coefficients.
  • Downlink NOMA: In downlink NOMA, the strong user performs SIC and enjoys interference-free transmission, while the weak user treats the other signal as interference.
  • Uplink NOMA: In uplink NOMA, users transmit at different power levels and the base station decodes the stronger signal first before cancellation.
  • Uplink NOMA: In uplink NOMA, the weak user enjoys interference-free transmission, whereas the strong user observes interference.

2) CR Paradigms:

CR enables dynamic spectrum access through interweave, underlay, and overlay paradigms, while cognitive NOMA combines CR and NOMA to support spectrum efficiency, connectivity, latency, and fairness. Cooperative relaying is proposed to improve reliability.

  • 2) CR Paradigms:: CR adapts operating parameters to environmental conditions and supports interweave, underlay, and overlay spectrum-sharing paradigms.
  • Cognitive NOMA combines CR and NOMA spectrum sharing so multiple users can transmit simultaneously with differentiated power levels.
  • Massive connectivity: Cognitive NOMA can serve multiple primary and/or secondary users in one resource block, supporting massive connectivity.
  • Low latency: Cognitive NOMA may reduce secondary transmission delay by connecting multiple secondary users simultaneously and exploiting opportunistic secondary access.
  • Better fairness: Allocating more power to weak-channel secondary users can balance fairness and throughput in the secondary network.
  • Inter- and intra-network interference, path loss, and deep fading can considerably degrade outage performance in cognitive NOMA networks.
  • Cooperative relaying is proposed to improve reception reliability in cognitive NOMA networks.

III. COGNITIVE NOMA ARCHITECTURES AND COOPERATIVE RELAYING STRATEGIES

The paper surveys underlay, overlay, and CR-inspired NOMA architectures, then proposes cooperative relaying to improve reception quality and outage performance amid inter- and intra-network interference.

  • Architectures: Cognitive NOMA architectures comprise underlay NOMA, overlay NOMA, and CR-inspired NOMA networks.The section presents cooperative relaying as a way to improve reception quality across these architectures.
  • Underlay NOMA: Underlay NOMA provides simultaneous access for more secondary receivers, improving connectivity and secondary throughput compared with sequential underlay OMA access.In underlay OMA, only one secondary receiver transmits at a time while others wait.
  • Underlay NOMA: Underlay NOMA permits concurrent primary and secondary transmissions but requires strict interference management and power control.Secondary receivers experience both intra-network interference and interference from primary transmitters, while primary-receiver interference must remain below a preset threshold.
  • Underlay NOMA: Underlay NOMA coverage remains more constrained than traditional underlay CR because coexistence creates both inter- and intra-network interference, and suitable guard zones may not exist.Practical proximity between primary transmitters and secondary receivers can prevent deployment of an appropriate interference guard zone.
  • Underlay NOMA: Underlay NOMA can extend secondary coverage through an amplify-and-forward relay that increases receiver SINRs and supports SIC processing.Cooperative relaying can also lower secondary-transmitter and relay power through reduced path loss while better satisfying the primary-receiver interference constraint.
  • Underlay NOMA: Cooperative underlay NOMA significantly improves secondary outage performance and requires lower SNR than non-cooperative NOMA for a specific outage probability.Lower required SNR corresponds to less transmit power and energy consumption.

B. Overlay NOMA Networks

Overlay NOMA shares spectrum through primary relaying and simultaneous secondary transmission, while cooperative relay sharing improves reception reliability for both networks under poor channel conditions.

  • Overlay NOMA: Compared with traditional NOMA, overlay NOMA requires extra effort to handle inter-network interference.
  • Overlay NOMA: At a secondary receiver, maximal ratio combining first decodes the primary signal, followed by successive interference cancellation of secondary signals.Secondary signals are decoded sequentially until the receiver retrieves its own signal.
  • Overlay NOMA: Overlay NOMA uses simultaneous primary and secondary transmissions to improve spectrum utilization through two-time-slot communication.A secondary transmitter helps forward the primary signal while transmitting its own signals to multiple secondary receivers.
  • Overlay NOMA: Overlay NOMA cancels primary-to-secondary interference at secondary receivers and improves primary outage through diversity gain from secondary-transmitter relaying.The architecture therefore balances spectrum efficiency and reception reliability.
  • Cooperative overlay NOMA: The cooperative overlay architecture lets primary and secondary transmitters share the same relay through two-phase transmissions.The relay decodes primary and secondary signals in the first slot, then selects downlink NOMA or primary-only OMA according to its decoding result.
  • Evaluation setting: The overlay evaluation assumes one primary receiver, two NOMA secondary receivers, i.i.d. Rayleigh fading, and target rates of 0.8 bps/Hz and 0.5 bps/Hz.The target rate is 0.8 bps/Hz for the primary receiver and 0.5 bps/Hz for each secondary receiver.
  • Cooperative overlay NOMA: Cooperative relay sharing significantly improves reception reliability for both primary and secondary receivers compared with non-cooperative overlay NOMA.The reported comparison evaluates primary and secondary outage performance under the stated single-input setting.

C. CR-NOMA Networks

CR-inspired NOMA lets a weak-channel primary user retain the time slot while a strong-channel secondary user opportunistically accesses the spectrum. Cooperative relaying further improves primary and secondary outage performance.

  • CR-inspired NOMA assigns the time slot to the weak-channel primary user while a strong-channel secondary user accesses the same spectrum.
  • A QoS-guaranteed power allocation divides power between reliable primary reception and opportunistic secondary transmission.
  • The secondary relay decodes both unicast and multicast signals, then superimposes them for transmission to the primary user and other secondary users.
  • Cooperative CR-NOMA boosts primary and secondary outage performance, with curves decreasing faster than in non-cooperative CR-NOMA and OMA-TDMA.
  • A dynamic multiple access scheme can switch between cooperative NOMA and cooperative OMA to further improve the cooperative CR-NOMA architecture.

IV. OPEN CHALLENGES AND FUTURE RESEARCH DIRECTIONS

The paper identifies implementation challenges and future research directions for cognitive NOMA networks, including interference management, imperfect CSI, and energy efficiency.

  • Future cognitive NOMA research must address implementation challenges and directions relevant to cooperative relaying.

1) Interference Management:

Cognitive NOMA is highly interference-limited because secondary users experience both intra-network and inter-network interference. Interference management and careful power allocation are therefore central design requirements.

  • Interference management is important because cognitive NOMA networks remain highly interference-limited.
  • Underlay NOMA secondary users suffer intra-network interference from power-domain multiplexing and inter-network interference from primary transmission.
  • Total interference observed at a primary receiver should remain constrained by a controllable level.
  • Inter-network interference can be mitigated using interference alignment and joint transceiver beamforming.
  • Power allocation should be designed carefully to reduce intra-network interference in underlay NOMA networks.

2) Imperfect Channel State Information (CSI):

Imperfect CSI is a practical limitation for cognitive NOMA because estimation errors, mobility, and feedback delay can degrade performance. Future designs should be robust to CSI errors while also addressing unexplored energy-efficiency objectives.

  • Most existing cognitive NOMA research assumes perfect CSI, whereas estimation errors, mobility, and feedback delay create imperfect CSI in practice.
  • Imperfect CSI can cause inappropriate secondary-transmitter power allocation, additional primary interference, and error propagation at secondary receivers.
  • Delayed CSI may select the wrong relay in cooperative overlay NOMA and CR-NOMA, deteriorating outage performance through diversity-order loss.
  • Novel transmission designs robust to CSI errors are needed for cognitive NOMA networks.
  • Energy-efficiency maximization in cognitive NOMA remains unexplored, and spectrum-efficiency and energy-efficiency tradeoffs are important evaluation metrics.

4) Multi-Carrier Cognitive NOMA:

Multi-carrier and MIMO extensions broaden cognitive NOMA but create tightly coupled allocation, ordering, and reliability challenges. Cooperative relaying and coordinated relay selection can improve reception reliability, while future designs must jointly serve multiple users.

  • Multi-Carrier Cognitive NOMA: Interweave NOMA groups secondary users onto sensed-available resource blocks, using NOMA within groups and orthogonal blocks across groups.Resource allocation is central to improving network performance and user fairness.
  • Multi-Carrier Cognitive NOMA: Jointly optimizing spectrum sensing, user grouping, and subcarrier/power allocation is challenging because these problems are coupled.The coupled optimization motivates further research on corresponding resource allocation algorithms.
  • Cognitive MIMO-NOMA: Cognitive MIMO-NOMA preserves spatial-diversity benefits, but efficient design is difficult because channel and interference representations complicate user ordering.In downlink underlay MIMO-NOMA, channels and interference are represented by matrices or vectors rather than SISO scalars.
  • Cognitive MIMO-NOMA: Power allocation in underlay MIMO-NOMA must restrict resultant interference to the primary network.Users may experience different inter-network interference, which is also represented by matrices or vectors.
  • Relay Selection/User Scheduling: Cooperative relaying improves reception reliability in cognitive NOMA, but relay selection and user scheduling must jointly guarantee multiple NOMA-assisted users.Conventional strategies focused on a single receiver cannot be directly applied; advanced joint scheduling remains a future direction.

7) Physical Layer Security:

Physical-layer security in cognitive NOMA must address interference-based attacks and potentially compromised cooperative relays. Cooperative jamming is identified as a promising countermeasure, while full-duplex operation introduces additional interference and resource-allocation trade-offs.

  • Physical Layer Security: Cognitive NOMA is vulnerable to denial-of-service attacks that interfere with successive-interference-cancellation processing at NOMA-assisted secondary users.The attack operates by emitting harmful radio signals that create interference.
  • Physical Layer Security: Cooperative relays may be compromised and attempt to eavesdrop confidential information.This creates an information-leakage threat specific to relay-assisted cooperation.
  • Physical Layer Security: Cooperative jamming is proposed as a promising physical-layer-security solution for preventing information leakage.The approach is presented as a response to untrusted-relay threats.
  • Full-Duplex Cognitive NOMA: Full-duplex underlay NOMA incurs inter-network, intra-network, residual self-interference, and other-users’ uplink interference at each secondary user.Evaluating the benefits and costs of full-duplex cognitive NOMA and cooperative strategies remains an open research topic.
  • Full-Duplex Cognitive NOMA: Full-duplex cognitive NOMA can transmit downlink and uplink simultaneously on the same spectrum without an extra relaying time slot.Resource allocation must jointly consider user pairing, power allocation, and spectrum efficiency.
  • Conclusion: The paper surveys underlay, overlay, and CR-NOMA architectures, proposes cooperative relaying for improved reception reliability, and identifies relay installation cost and implementation challenges.The conclusion frames cooperative relaying as beneficial but not cost-free.
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