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5GNOW: Challenging the LTE Design Paradigms of Orthogonality and Synchronicity

Gerhard Wunder, Martin Kasparick, Stephan ten Brink, Frank Schaich, Thorsten Wild, Ivan Gaspar, Eckhard Ohlmer, Stefan Krone, Nicola Michailow, Ainoa Navarro, Gerhard Fettweis, Dimitri Ktenas, Vincent Berg, Marcin Dryjanski, Slawomir Pietrzyk, Bertalan Eged

arXiv:1212.4034v1cs.ITcs.NImath.IT

TL;DR

MTC, collaborative and heterogeneous networks, and fragmented spectrum expose limits of LTE's strict synchronism and orthogonality design. 5GNOW proposes asynchronous, non-orthogonal PHY concepts and expects a more robust architecture supporting differentiated services, with outcomes intended to inform 5G standardization.

Abstract

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LTE and LTE-Advanced have been optimized to deliver high bandwidth pipes to wireless users. The transport mechanisms have been tailored to maximize single cell performance by enforcing strict synchronism and orthogonality within a single cell and within a single contiguous frequency band. Various emerging trends reveal major shortcomings of those design criteria: 1) The fraction of machine-type-communications (MTC) is growing fast. Transmissions of this kind are suffering from the bulky procedures necessary to ensure strict synchronism. 2) Collaborative schemes have been introduced to boost capacity and coverage (CoMP), and wireless networks are becoming more and more heterogeneous following the non-uniform distribution of users. Tremendous efforts must be spent to collect the gains and to manage such systems under the premise of strict synchronism and orthogonality. 3) The advent of the Digital Agenda and the introduction of carrier aggregation are forcing the transmission systems to deal with fragmented spectrum. 5GNOW is an European research project supported by the European Commission within FP7 ICT Call 8. It will question the design targets of LTE and LTE-Advanced having these shortcomings in mind and the obedience to strict synchronism and orthogonality will be challenged. It will develop new PHY and MAC layer concepts being better suited to meet the upcoming needs with respect to service variety and heterogeneous transmission setups. Wireless transmission networks following the outcomes of 5GNOW will be better suited to meet the manifoldness of services, device classes and transmission setups present in envisioned future scenarios like smart cities. The integration of systems relying heavily on MTC into the communication network will be eased. The per-user experience will be more uniform and satisfying. To ensure this 5GNOW will contribute to upcoming 5G standardization.

I. KILLER APPLICATIONS: THE 5G DRIVERS

Wireless applications have progressed from telephony and messaging to smartphone data, while machine connectivity is presented as a coming killer application. Their varied traffic patterns and demand for higher rates challenge mobile networks.

I. KILLER APPLICATIONS: THE 5G DRIVERS

  • Wireless adoption progressed through telephony, SMS, and data connectivity to smartphones, with M2M or IoT identified as the next foreseen killer application.The passage describes M2M as massive wireless connectivity between machines.Source: §I · p. 2

II. APPLICATION CHALLENGES

  • Smartphone services span bandwidth-hungry streaming, latency-sensitive gaming, and sporadic updates, complicating uniform service experience in heterogeneous networks.The passage also describes operators facing higher per-user rates and overall bandwidth needs.Source: §II · p. 2
  • More than 50% of cellular-network data volume is attributed to streaming, while future wireless connectivity is projected to require 10–100 Gb/s.The passage relates the projection to high-definition 3D streaming and rapid downloads of multiple streams.Source: §II · p. 3

B. Fast dormancy and sporadic access

Fast dormancy saves handset battery by allowing a return to idle after data delivery, but each subsequent transmission restarts synchronization. Repeated sporadic access raises signaling load, and asynchronous MTC traffic may amplify the challenge.

B. Fast dormancy and sporadic access

  • After delivering data, a fast-dormancy handset can move from active to idle, requiring the full synchronization procedure again for later data.The passage says this can happen several hundred times a day.Source: B. Fast dormancy and sporadic access · p. 3
  • Repeated synchronization can produce significant control-signaling growth and threaten network congestion.The passage describes this as a consequence of sporadic access under fast dormancy.Source: B. Fast dormancy and sporadic access · p. 3
  • Asynchronous access by a multitude of potentially billions of MTC devices is expected to dramatically amplify the sporadic-access problem.Figure 2 is captioned as a comparison of data and signalling traffic.Source: B. Fast dormancy and sporadic access · p. 3, Figure 2 · p. 3

C. Fragmented Spectrum and Carrier aggregation

Carrier aggregation combines non-contiguous bands to pursue higher rates, while fragmented-spectrum use also raises spectral-mask and filtering challenges. The passage identifies TV white spaces as a potential niche market subject to stringent out-of-band-radiation requirements.

C. Fragmented Spectrum and Carrier aggregation

  • Carrier aggregation uses separate RF front ends to access aggregated non-contiguous channels and can provide higher downlink data rates.The passage describes it as a response to the need for higher rates.Source: C. Fragmented Spectrum and Carrier… · p. 3
  • TV white-space exploration could form a niche market if spectrum agility meets rigorous low out-of-band-radiation requirements for protecting legacy systems.The passage notes preparation of a regulatory framework in Europe.Source: C. Fragmented Spectrum and Carrier… · p. 3
  • Fitting traditional OFDM with cyclic prefix to an emission spectrum mask loses efficiency because of its non-negligible sidelobes.The passage says partial use of available spectrum can reduce the need for additional filtering and, in analog transmission, its output insertion loss.Source: C. Fragmented Spectrum and Carrier… · p. 3

A. Toward higher spectral efficiency

LTE advances higher rates through antenna expansion and carrier aggregation, with inter-cell coordination and CoMP also described as routes to greater spectral efficiency. The section additionally outlines unresolved MTC requirements, including low-power and sporadic traffic needs.

A. Toward higher spectral efficiency

  • LTE R8/R9 ideally offer 300Mb/s downlink and 75Mb/s uplink; carrier aggregation may enable over 1Gb/s downlink and over 500Mb/s uplink.R10 increases downlink transmit antennas from 4 to 8 and uplink antennas from 1 to 4.Source: A. Toward higher spectral efficiency · p. 3
  • ICIC and CoMP are described as further increasing spectral efficiency, while the passage says rates on the order of 10Gb/s must be addressed.The passage characterizes these schemes as evolutionary approaches within a synchronous, orthogonal PHY framework.Source: A. Toward higher spectral efficiency · pp. 3–4

B. Incorporating MTC traffic

  • MTC requirements include low power, low data transmission, bursty or sporadic traffic, and possible periodic or delay-tolerant services.The passage says physical-layer techniques are outside the described 3GPP activities, while some MAC amendments may be considered.Source: A. Toward higher spectral efficiency · p. 4

C. Fragmented spectrum

Fragmented spectrum spans multiple bands, and the lack of harmonization pushes vendors toward compromises in globally compatible LTE devices. 5GNOW’s approach is introduced as a response to these challenges.

C. Fragmented spectrum

  • LTE already spans multiple frequency bands, including 700 MHz and AWS in the USA, 800, 1800, and 2600 MHz in Europe, and 2100 and 2600 MHz in Asia.The passage expects 2600, 1800, and 800 MHz to be the most widely used bands in Western Europe for 4G deployments.Source: C. Fragmented spectrum · p. 4
  • Lack of spectrum harmonization forces vendors to find compromises when delivering globally compatible LTE chipsets and devices.The passage also says OFDM spectrum properties do little to ease analog front-end signal-processing challenges.Source: C. Fragmented spectrum · p. 4

IV. 5GNOW APPROACH

  • 5GNOW is a European collaborative research project supported by the European Commission under FP7 ICT Call 8 from September 2012 to February 2015.The passage introduces the project's scope after identifying it.Source: §IV · p. 4

A. Questioning the common design priciples

5GNOW hypothesizes that synchronism and orthogonality hinder envisioned service architectures. It explores alternatives for MTC, CoMP/HetNet, and carrier aggregation using non-orthogonal waveforms for asynchronous signalling.

A. Questioning the common design priciples

  • 5GNOW’s main hypothesis is that synchronism and orthogonality in current mobile-radio PHY layers are major obstacles to envisioned service architectures.The project motivates an innovative, partly disruptive PHY-layer redesign.Source: A. Questioning the common design… · p. 4
  • 5GNOW examines its hypothesis across MTC, CoMP/HetNet, and carrier-aggregation scenarios.The MTC example is developed as asynchronous uplink signalling, particularly for RACH.Source: A. Questioning the common design… · pp. 4–5
  • For MTC, asynchronous non-orthogonal uplink waveforms, including for RACH, are intended to let devices transmit with only coarse synchronization and reduce signalling overhead.The passage describes this as a way to avoid the full synchronization procedure designed to meet orthogonal constraints.Source: A. Questioning the common design… · p. 5
  • CoMP/HetNet coordination entails message sharing, base-station synchronization, channel-state feedback, and control forwarding, while uncoordinated overlapping cells make required information difficult to provide to every entity.The passage argues that unattainable coordination requirements support asynchronous, non-orthogonal signalling and improved downlink robustness.Source: A. Questioning the common design… · p. 5
  • For carrier aggregation, 5GNOW proposes non-orthogonal waveforms with sharp frequency notches to limit interference with legacy systems and meet tight spectral masks.The passage identifies generous guard bands as a current response that can reduce spectral efficiency or prevent band use.Source: A. Questioning the common design… · p. 5

B. Enabler for new PHY

5GNOW proposes a joint system view of control-signalling effort and waveform choice, relaxing synchronism and orthogonality while managing the resulting impairments. The project argues that increased receiver complexity is feasible given current mobile power and processing budgets.

B. Enabler for new PHY

  • Current LTE baseband processing uses less than 20% of overall mobile-phone power, while the inner receiver uses less than 10% of baseband operations.Figure 4 presents workload for current mobiles; its outer receiver consists of the channel decoder and de-interleaver.Source: B. Enabler for new PHY · p. 5, Figure 4 · p. 6
  • 5GNOW jointly considers required control-signalling effort and waveform choice in a broader non-orthogonal robustness concept.The project proposes admitting some crosstalk or interference and controlling it through transceiver structure and transmission technique.Source: B. Enabler for new PHY · p. 5
  • The project investigates tradeoffs between relaxing orthogonality and synchronism, network performance and user experience, and required signal-processing capabilities.Its scenarios are MTC, CoMP/HetNet, and carrier aggregation.Source: B. Enabler for new PHY · p. 5
  • 5GNOW contrasts its comprehensive system view with approaches that increase bandwidth or antenna count while overlooking the network cost of maintaining orthogonality and synchronism.The passage says those costs can consume much of the promised gains and argues for a system view to enable more scalable architectures.Source: B. Enabler for new PHY · p. 5

V. NOVEL IDEAS AND KEY RESEARCH AREAS

The project frames the shift from synchronous, orthogonal systems to asynchronous, non-orthogonal systems as a PHY-design challenge. Its proposed PHY uses non-orthogonal waveforms to ease synchronism requirements and support future access-network services.

V. NOVEL IDEAS AND KEY RESEARCH AREAS

  • The project treats the shift from synchronous, orthogonal systems to asynchronous, non-orthogonal systems with increased robustness as a research challenge.This paradigm shift motivates the design work in the section.Source: §V · p. 6
  • 5GNOW aims to design a 5G PHY using non-orthogonal waveforms that alleviate the synchronism requirements of existing systems.Source: §V · p. 6
  • The proposed PHY techniques are intended to support all service requirements for future mobile access networks.Source: §V · p. 6

A. Physical Layer

5GNOW develops a non-orthogonal PHY layer for asynchronous signalling and robust transmission across MTC, CoMP/HetNet, and carrier aggregation scenarios. Its objectives include adapting waveforms to scenarios and measuring gains against synchronization, signalling, and spectrum-efficiency criteria.

A. Physical Layer

  • A non-orthogonal RACH channel using sparse MIMO processing at the base station targets asynchronous MTC access, including sporadic and concurrent access by many nodes.The objective is to measure outcomes including congestion probability, MTC-node lifetime, and infrastructure costs.Source: A. Physical Layer · p. 6
  • CoMP/HetNet concepts combine non-orthogonal MIMO PHY and transceiver designs with relaxed synchronization and imperfect channel-state information.Evaluation targets reduced signalling and backhaul traffic while maintaining benchmark performance, scalability, and support for sporadic low-rate and high-rate users.Source: A. Physical Layer · p. 6
  • Carrier-aggregation techniques use non-orthogonal waveforms with relaxed or omitted synchronization, with increased bandwidth efficiency in white spaces as the objective.Source: A. Physical Layer · p. 6
  • Scenario-dependent waveform adaptation is intended to support seamless system operation, with performance measured against a static setup.Source: A. Physical Layer · p. 6

B. MAC layer

5GNOW adapts MAC and networking functions for efficient, scalable multi-cell operation over the proposed PHY. It also plans a hardware demonstrator and over-the-air experiments to assess selected concepts against LTE-A.

B. MAC layer

  • MAC control signalling is designed around the structure of each non-orthogonal PHY waveform, unlike approaches where transmitted signals do not affect control-channel design.The project also targets a non-orthogonal scheduler, evaluated through sum throughput and standard fairness metrics.Source: B. MAC layer · p. 7

C. Demonstrator

  • A hardware demonstrator and over-the-air experiments are planned as proof-of-concept tests for selected non-orthogonal PHY techniques and 5G concepts.Source: C. Demonstrator · p. 7
  • The demonstrator will compare performance with the LTE-A air interface using indicators including rate, synchronization and orthogonality relaxation, overhead, energy efficiency, and receiver complexity.The comparison also considers hardware accuracy and MTC-device-lifetime metrics; receiver complexity is explicitly a trade-off against potential performance gains.Source: C. Demonstrator · p. 7

VI. CONCLUDING REMARKS

5GNOW expects a robust transmission technique that uses the design degrees of freedom of heterogeneous networks and supports differentiated services from MTC to high-rate video. It aims to consolidate its results into a proposal for a non-orthogonal, asynchronous 5G architecture.

VI. CONCLUDING REMARKS

  • 5GNOW expects a robust transmission technique for heterogeneous networks that supports differentiated services from MTC traffic to super-high-rate video streaming.The project also aims to develop a fundamental understanding of the potential benefits of a non-orthogonal, asynchronous air interface and condense its results into a single 5G architecture proposal.Source: §VI · p. 7
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