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DVB-S2x Enabled Precoding for High Throughput Satellite Systems

Pantelis-Daniel Arapoglou, Alberto Ginesi, Stefano Cioni, Stefan Erl, Federico Clazzer, Stefano Andrenacci, Alessandro Vanelli-Coralli

arXiv:1504.03109v1cs.ITmath.IT

TL;DR

Aggressive frequency reuse in multibeam HTS systems makes practical precoding challenging, including under imperfect or outdated channel estimates. The paper examines DVB-S2x superframes, synchronization strategies, and a realistic system simulator; simulations report capacity gains over a four-colour benchmark, including with realistic CSI.

Abstract

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Multi-user Multiple-Input Multiple-Output (MU-MIMO) has allowed recent releases of terrestrial LTE standards to achieve significant improvements in terms of offered system capacity. The publications of the DVB-S2x standard and particularly of its novel superframe structure is a key enabler for applying similar interference management techniques -such as precoding- to multibeam High Throughput Satellite (HTS) systems. This paper presents results resulting from European Space Agency (ESA) funded R&D activities concerning the practical issues that arise when precoding is applied over an aggressive frequency re-use HTS network. In addressing these issues, the paper also proposes pragmatic solutions that have been developed in order to overcome these limitations. Through the application of a comprehensive system simulator, it is demonstrated that important capacity gains (beyond 40%) are to be expected from applying precoding even after introducing a number of significant practical impairments.

I. Introduction – The Need for Precoding

Terrestrial MU-MIMO demonstrates capacity gains, while limited Ka-band spectrum motivates more aggressive frequency reuse in multibeam HTS systems. Because that reuse increases co-channel interference, the paper examines precoding and its practical implementation challenges.

I. Introduction – The Need for Precoding

  • LTE releases developed MU-MIMO for up to eight antennas, in part because many user terminals cannot support many receive antennas and MU channels are less susceptible to spatial correlation.The paper uses terrestrial MU-MIMO as a precedent for interference management in satellite systems.Source: §I · p. 1
  • HTS systems use multiple ground spot beams and frequency-and-polarization reuse; the paper describes Viasat-1 as using a four-colour pattern and about 140 Gbit/s throughput.The available service spectrum must also be supported by the feeder link between gateway stations and the satellite.Source: §I · pp. 1–2, Figure 1 · p. 2
  • Aggressive frequency reuse can increase co-channel interference, lowering SNIR and spectral efficiency and eroding the capacity benefit of added spectrum.The paper presents precoding as an interference-management approach for this setting.Source: §I · pp. 2–3
  • The paper focuses on practical precoding challenges in DVB-S2-based HTS networks, noting that DVB-S2x’s superframe addresses many implementation difficulties.It also identifies multicast precoding and terminal synchronization as challenges addressed in later sections.Source: §I · p. 3

II. Implementation Challenges in Practical Systems

Practical DVB-S2 precoding faces constraints from frame structure, channel estimation, feedback latency, gateway distribution, user availability, and payload architecture.

Multicast PLFRAME

  • DVB-S2 interleaving mixes users’ bits within codewords, while headers and parity are multicast, motivating frame-based multicast precoding and user selection.The PLFRAME structure is the setting for these constraints.Source: Multicast PLFRAME · pp. 3–4, Figure 2 · p. 4

PLFRAME of Variable Length

  • ACM makes PLFRAME symbol lengths vary across beams, breaking alignment and misaligning pilots; precoder updates can approach symbol rate, while DVB-S2x Bundled PLFRAMEs address this.Figure 3 depicts misalignment across co-channel carriers when ACM is used.Source: PLFRAME of Variable Length · pp. 4–5, Figure 3 · p. 4

Imperfect Channel Estimation

  • Receiver CSI must cover every non-negligible carrier, including one 12 dB below the main carrier; residual estimation errors and inaccessible weak coefficients still reduce gains.The receiver must synchronize and estimate amplitude and phase even for a signal potentially a few dB below noise.Source: Imperfect Channel Estimation · p. 5

Outdated Phase Estimate

  • GEO feedback takes approximately 500 ms, allowing phase drift; payload-chain phase variation is most relevant, and a common stable oscillator can control its dynamics.The paper states that receiver LNB phase variation is common to a UT’s channel coefficients and does not affect received SNIR.Source: Outdated Phase Estimate · p. 6

Multiple Gateways

  • Multiple gateways lack CSI for all UTs, restricting pre-compensation to served beams and making precoding more suitable for regional systems with moderate beam counts.Aggressive frequency reuse aggravates the need for multiple gateways.Source: Multiple Gateways · p. 6

Impact on availability

  • Precoding can penalize unfavorable-channel UT availability; DVB-S2x adds BPSK and spreading at low SNIR, expected to preserve operator-required availability down to -10 dB.Source: Impact on availability · pp. 6–7

Payload mass implications

  • With passive Travelling Wave Tube Amplifier payloads, frequency reuse may require duplicated or quadruplicated transmitter chains; active or semi-active one-HPA-per-feed architectures avoid added payload mass.Source: Payload mass implications · p. 7

III. Summary of the DVB-S2x Superframe Option

DVB-S2x superframes combine SOSF and pilot sequences with bundled PLFRAMEs to support synchronization and reduce precoding-matrix computation, with scheduling and delay-jitter tradeoffs.

III. Summary of the DVB-S2x Superframe Option

  • Quasi-orthogonal SOSF and SF-Pilot sequences support synchronization and channel estimation under strong interference when co-channel superframes remain time-aligned.The SOSF is 270 symbols, and the alignment can remain virtually constant throughout service, neglecting small differential group-delay variation.Source: §III · p. 8
  • Constant-length Bundled PLFRAMEs let one multicast precoding matrix cover a bundle, reducing computation to the bundle rate but risking scheduler inefficiency and increased data-delay jitter.The bundled structures are arranged according to modulation format.Source: §III · p. 8
  • P2 pilots are precoded with the data matrix and carry the payload modulation format to assist main-carrier data detection and channel amplitude and phase detection.The modulation choice is useful under nonlinear distortions; the sequence gives the constellation points a fair share of time.Source: §III · p. 9

IV. Multicast Precoding Approaches

DVB-S2 framing motivates multicast precoding, which applies a shared matrix across multiple UT channels in a bundled frame. The section outlines three approaches and identifies the simple geographical-selection/MMSE option used in system evaluation.

IV. Multicast Precoding Approaches

  • Multicast precoding applies the same matrix over a bundled PLFRAME's symbols for multiple UT channels in each spot beam.This frame-based approach differs fundamentally from LTE and LTE-Advanced MU-MIMO.Source: §IV · p. 9
  • The three approaches pair user selection with matrix design: geographical selection plus MMSE, block SVD, and multicast-aware scheduling plus weighted max-min optimization.They differ in both user grouping and precoding-matrix calculation.Source: §IV · pp. 9–10
  • System evaluation implements geographical user selection with MMSE precoding for simplicity and encouraging results; all three approaches use linear precoding.The paper states linear precoding captures most of precoding's gain despite not achieving Dirty Paper Coding capacity.Source: §IV · p. 10

V. Terminal Synchronisation Strategy

Terminal synchronization and estimation in multicast-precoded HTS systems are challenging by design: precoding obscures auxiliary estimation on interferers, while non-precoded pilots face extreme interference. The UT synchronizes and estimates channels across selected waveform components.

V. Terminal Synchronisation Strategy

  • Synchronization and estimation are challenging by design because precoding suppresses interferer waveforms needed for auxiliary estimation, while non-precoded SOSF and pilots face extreme interference.The cited procedure addresses a precoded HTS system in which co-channel interference motivates precoding.Source: §V · p. 10
  • To return CSI to the gateway, a UT identifies frame boundaries, estimates each selected waveform's frequency, phase, and timing, then estimates its channel.The receiver first performs coarse frequency estimation, excludes waveforms without successful frame synchronization, and then performs fine tracking and channel estimation.Source: §V · pp. 10–11
  • The procedure has quasi-synchronous and nonsynchronous implementations, distinguished by whether relative waveform delays stay within one symbol time or may exceed it.The distinction assumes the received waveforms have the same baud rate.Source: §V · p. 11

VI. System level Performance Model & Results

The system-level evaluation compares precoding with classical four-colour frequency reuse under realistic conditions. Its simulation considers channel-estimation impairments, cluster-based scheduling, and multiple gateways.

VI. System level Performance Model & Results

  • The evaluation compares precoding against classical four-colour frequency reuse under ideal conditions and with imperfect, outdated, or erroneous channel estimates.These cases are intended to demonstrate precoding benefits under realistic conditions.Source: §VI · p. 12
  • The simulator includes traffic scheduling for precoding's user clusters rather than conventional user-per-user transmission, using a simple approach to inform future joint scheduling and precoding research.The paper characterizes the scheduling task as complex and potentially more cumbersome when precoding is used.Source: §VI · p. 12
  • The evaluation also considers multiple gateways, applying precoding among each gateway's served beams while disregarding interference from neighboring beams served by other gateways.Source: §VI · pp. 12–13

System level simulation framework

The simulator evaluates precoding across single- and multi-GW scenarios, incorporating scheduling and practical channel-estimation effects. Precoding exceeds the four-colour benchmark in the reported scenarios, with gains varying by CSI quality and gateway configuration.

System level simulation framework

  • The simulator has a physical-layer preprocessor and packet simulator; experiments use 63 beams, with one GW or nine GWs serving seven-beam clusters, and full frequency reuse for precoding.The scenarios use two polarizations per beam and appropriately scaled saturated transmit power; feeder links are ideal in both.Source: System level simulation framework · p. 13, System level simulation results · p. 14
  • Scheduling uses PLFRAME time granularity and a fair policy that prioritizes unserved traffic; five users are fixed per frame to limit scheduling complexity.Larger clusters may improve frame utilization but can increase channel-estimation error and scheduler complexity.Source: System level simulation framework · p. 13

System level simulation results

  • The simulations model channel-estimation limits, amplitude and phase errors, and outdated phase from a Ka-band converter with a 0.5-second RTT.Physical-layer impairments are based on the DVB-S2x ESA channel model.Source: System level simulation results · p. 15
  • 55 Gbit/s is the benchmark's full-service limit in the single-GW case; ideal-CSI precoding serves traffic to 120 Gbit/s, while realistic CSI lowers that region to 70 Gbit/s.The benchmark upper-bound capacity approaches 71 Gbit/s; ideal-CSI precoding has an upper bound around 170 Gbit/s, reduced to 120 Gbit/s with realistic CSI.Source: System level simulation results · p. 16, Figure 9 · p. 17
  • 113 Gbit/s is the multi-GW upper-bound capacity with ideal CSI, falling to 97 Gbit/s with realistic CSI; precoding capacity still exceeds the benchmark.Only signals for a gateway's own beams can be precoded in the multi-GW configuration, so throughput is generally below the single-GW case.Source: System level simulation results · pp. 16–17, Figure 10 · p. 18
  • 1.4 times higher upper-bound capacity with ideal CSI and 70% gain with realistic CSI in single-GW; multi-GW gains are 58% and 38%, respectively, versus the four-colour benchmark.These gains are for multicast precoding with five users per frame.Source: System level simulation results · p. 17, Figure 11 · p. 18, Table 3 · p. 19

VII. Conclusions

The paper examines practical precoding challenges in high-frequency-reuse multibeam HTS systems and reports capacity gains across its simulated scenarios. It identifies further optimization and payload-level comparisons as remaining work.

VII. Conclusions

  • Precoding gains range from 40% to 140% across system scenarios, using simple scheduling and multicast precoding with five users per frame.The gains are reported after accounting for the practical issues identified in the paper.Source: §VII · p. 19
  • The DVB-S2x superframe addresses many practical issues, while developed synchronization algorithms tackle imperfect and outdated channel-estimation challenges.Source: §VII · p. 19
  • Further work is needed to compare precoded networks with classical frequency reuse 4 at fixed maximum power and payload mass.The paper notes this comparison could inform use of less conventional payload architectures.Source: §VII · p. 19
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