Performance Analysis and Improvements for the Future Aeronautical Mobile Airport Communications System. Candidate: Paola Pulini Advisor: Marco Chiani

Transcription

1 Performance Analysis and Improvements for the Future Aeronautical Mobile Airport Communications System (AeroMACS) Candidate: Paola Pulini Advisor: Marco Chiani

2 Outline Introduction and Motivations Thesis Summary Main Contributions Unequal Diversity Coding Fundamentals System Description Performance Results Conclusions

3 Introduction and Motivations (1/3) Current aeronautical communications system - air traffic control (ATC) and air traffic management (ATM) Voice based - Double-sideband amplitude modulation (DSB-AM) Data link based - VDL (VHF digital link) mode 2 Capacity of the system is already saturated Necessity of a new aeronautical communications infrastructure Robust, efficient, secure, flexible Able to cope with the future long-term increasing demands

4 Introduction and Motivations (2/3) New global aeronautical communications system Air-to-air communications Satellite-based communications Ground-based communications Airport communications (AeroMACS)

5 Introduction and Motivations (3/3) Airport Surface Communications High demand of capacity New frequencies assignments (ITU world radio conference 2007) C band ( MHz) IEEE standard (mobile WiMAX) has been chosen as the base technology for the future system (AeroMACS)

6 Thesis Summary Analysis and investigation of the performance of AeroMACS Evaluation of potential solutions for enhancing the performance of the system Improvement through the introduction of diversity techniques MIMO schemes es with multiple antennas as only on the control o tower (space diversity) Cooperative Communications with single relay (cooperative diversity) Packet level coding (time diversity)

7 Main Achievements (1/3) Preliminary Studies Development of a novel stochastic airport channel model Parameters based on measurement campaign at MUC Analysis of the performance of two WiMAX profiles in a realistic airport environment OFDM based waveform OFDMA based waveform Selection of the most suitable profile for AeroMACS

8 Main Achievements (2/3) Analysis of MIMO schemes and relative performance evaluation SIMO 1x2 with MRC (reverse link) MISO 2x1 STC (forward link) Introduction of a novel implementation of 2x1 STC for AeroMACS Investigation of the use of cooperative communications strategies in the airport context Study and performance analysis of single relay schemes Amplify and forward Decode and forward

9 Main Achievements (3/3) Introduction of the novel concept of unequal diversity (UD) coding for the relay channel (high efficiency) Development of a novel class of LDPC codes Analytical study of the codes Application of the proposed method to AeroMACS Packet level coding Development of two algorithms for the online design of LDPC codes Analytical study of the proposed method Application of the proposed scheme to AeroMACS

10 Fundamentals Cooperative Communications New paradigm based on the utilization of heterogeneous resources to improve the overall performance of the system Virtual antenna array by the combination of antennas of different users (also single antenna) Distributed MIMO network Spatial diversity (cooperative diversity) In a multi-user user system Each user represents a potential cooperative-partner No requirement of a dedicated infrastructure Potential cooperative- partners

11 Cooperative Communications Example: Single relay (cooperation between partner A and B) t 1 Source t 2 Partner

12 Cooperative Communications Basic Methods Amplify and Forward Decode and Forward Coded Cooperation Diversity gain of order 2 Reduction of the efficiency, overall coding rate 1/2 for achieving diversity-2

13 Cooperative Communications High-Rate Coded Cooperation We introduce a coded cooperation scheme which allows high code rates Diversity-2 is guaranteed for a part of the source message Unequal Diversity it (UD) Extension of unequal error protection ti Relevant for messages composed by parts having different priority/qos requirements e e Video streaming Aircraft communications (messages with different level of criticality) within the airport domain

14 Cooperative Communications High-Rate Coded Cooperation We propose a novel construction based on Low-Density Parity- Check codes which achieves the promised performance We provide an analysis on block-fading channels complemented by simulations

15 Low-Density Parity-Check Codes Basics Low-Density Parity-Check Codes (Gallager,1960) Near-Shannon limit error correcting codes with iterative (messagepassing) decoding Parity-check matrix: Check nodes H Tanner graph: Check nodes c c c c Parity-check equations: c c c c c c c c c c c c Variable nodes

16 Low-Density Parity-Check Codes Protograph Construction of the Tanner Graph Protograph: small bipartite graph describing the macroscopic structure of an LDPC code Tanner Graph: obtained by Q-fold replication of the protograph and by edge permutation among the protograph replicas

17 Low-Density Parity-Check Codes Protograph Construction of the Tanner Graph For the LDPC code associated with the Tanner graph, Minimum distance properties Iterative decoding threshold depend on the starting protograph only Code design reduces to protograph design! Additionally, protograph LDPC codes have structured parity-check Additionally, protograph LDPC codes have structured parity check matrices which facilitate the decoder implementation

18 Low-Density Parity-Check Codes Protograph Extrinsic Information Transfer (EXIT) Protograph EXIT analysis: track the evolution of the message probability densities over the protograph edges Allows to accurately predict the iterative decoding threshold, i.e. the signal-to-noise ratio (SNR) at which iterative decoding starts to converge EXIT analysis can be adapted to EXIT analysis can be adapted to block-fading channels

19 Low-Density Parity-Check Codes Protograph Analysis over Block Fading Channels (1/2) Block fading channel (BFC): The codeword is split into N blocks Each block is transmitted over a different flat fading channel Each block experiences a different SNR In each channel, the SNR follows an exponential distribution Accurate model for Frequency-hopping Relay communications

20 Low-Density Parity-Check Codes Protograph Analysis over Block Fading Channels (2/2) We introduced a modification of the protograph EXIT analysis in order to account for different SNRs for each variable node Protograph variable nodes = codeword blocks Feed each protograph variable node with a different SNR level Given a SNR profile, we determine whether iterative decoding converges or not (outage) Outage region of a protograph G, The definition of outage region can be extended to each single variable node (= block) We can characterize the UEP of the protograph nodes!

21 Protograph Analysis over the Relay Channel Block Fading Channel Approximation Conventional assumption: the Source-Relay (S-R) link is reliable Valid if the SNR over the S-R link is larger than the decoding threshold of the code employed at the Source Realistic assumption (relay selection protocol) Approximation with block fading channel with two independent channels (=two fading levels / SNRs), and

22 Conventional Approach for the Relay Channel Coded Cooperation The Source encodes the packet with a (n S,k) code C S and broadcasts (Time Frame 1) The Relay decodes and re-encodesencodes n R additional parity bits with a code C R,which are sent to the Destination (Time Frame 2) S Time frame 1 D R Time frame 2 The overall code has block length n S +n R the overall code rate is R=k/(n S +n R ) Diversity-2 can be achieved only if R 1/2 New solution: By re-encoding just a fraction of the information bits at the Relay, we can provide diversity-2 for certain codeword bits even if R>1/2

23 Unequal Diversity Coding A New Scheme for the Relay Channel The Source encodes the packet with an (n S,k) code C S and broadcasts (Time Frame 1) The Relay decodes and re-encodes n R additional parity bits with a code C R, out of k h<k information bits (Time Frame 2) u = information word (k bits)

24 Unequal Diversity Coding A New Scheme for the Relay Channel The Source encodes the packet with an (n S,k) code C S and broadcasts (Time Frame 1) The Relay decodes and re-encodes n R additional parity bits with a code C R, out of k h<k information bits (Time Frame 2) u h = high-priority fragment (k h bits)

25 Unequal Diversity Coding A New Scheme for the Relay Channel The Source encodes the packet with an (n S,k) code C S and broadcasts (Time Frame 1) The Relay decodes and re-encodes n R additional parity bits with a code C R, out of k h<k information bits (Time Frame 2) u l = low-priority fragment (k l bits)

26 Unequal Diversity Coding A New Scheme for the Relay Channel The Source encodes the packet with an (n S,k) code C S and broadcasts (Time Frame 1) The Relay decodes and re-encodes n R additional parity bits with a code C R, out of k h<k information bits (Time Frame 2) Time Frame 1: S encodes u (low+high priority fragments)

27 Unequal Diversity Coding A New Scheme for the Relay Channel The Source encodes the packet with an (n S,k) code C S and broadcasts (Time Frame 1) The Relay decodes and re-encodes n R additional parity bits with a code C R, out of k h<k information bits (Time Frame 2) Time Frame 2: R encodes u h (high priority fragment ONLY)

28 Unequal Diversity Coding Protograph Design Encoding at the Source with a high-rate LDPC code Encoding at the Relay with a short LDPC code Simple parallel concatenation of two LDPC codes S Time frame 1 D R Time frame 2

29 Unequal Diversity Coding Protograph Design Encoding at the Source with a high-rate LDPC code Encoding at the Relay with a short LDPC code Simple parallel concatenation of two LDPC codes At the Destination, joint decoding over the overall graph EXIT analysis: only two SNRs The channel profile is a 2-D vector

30 Unequal Diversity Coding Performance Overall code rate: 7/10 Block length k=1792 bits Probability Distribution of the channel profile Outage region for high-priority fragments Outage regions Outage / Block Error Probability

31 Unequal Diversity Coding Performance Overall code rate: 7/10 Block length k=1792 bits Outage region for low-priority i fragments Outage regions Outage / Block Error Probability

32 Unequal Diversity Coding Performance Overall code rate: 7/10 Excellent match between EXIT analysis and simulations Outage regions Outage / Block Error Probability

33 Unequal Diversity Coding Performance of AeroMACS Bandwidth 5 MHz, 512 subcarriers, f = KHz, OFDMA symbols Parking scenario Lack of diversity, low Rice factor (K = 0 db, no/limited mobility, low Doppler) The design may be tailored to the different PER requirements of the COCR messages Unequal diversity is achieved also over the aeronautical channel model

34 Summary Unequal Diversity Coding A coded cooperation scheme targeting high code rates (R>1/2) Diversity / coding rate trade-off by introducing an Unequal Diversity distributed coding scheme High priority fragments enjoy diversity, low priority fragments do not Accurate EXIT analysis for protograph LDPC codes over block fading channels Design of distributed protograph codes for Unequal Diversity achieving the target performance Potentially suitable for in-airport communications to protect messages with different priority levels

35 Conclusions We investigated the performance of the future system for the airport surface communications and we analyzed and proposed methods for improving its performance. We focused on techniques that increase the diversity of the system, and in particular space diversity (MIMO and cooperative communications) and time diversity (packet level coding) Generally, all the methods investigated may be suitable for AeroMACS

36 Publications Conference Proceedings P. Pulini, G. Liva, and M. Chiani Protograph EXIT Analysis over Block Fading Channels with Application to Relays, ICC 12, June 2012, Ottawa, CA P. Pulini, and M. Chiani, Improving the performance of AeroMACS by cooperative communications, DASC 30 th, October 2011, Seattle, US G. Liva, P. Pulini, and M. Chiani, Flexible on-line construction of IRA codes for packet erasure correction with application to aeronautical communications, ICC 11, June 2011, Kyoto, Japan P. Pulini, Forward Link Performance Analysis for the Future IEEE based Airport Data Link, ICC 2010, May 2010, Cape Town, South Africa P. Pulini, and M. Chiani, Improving the forward link of the future airport data link by space-time coding, InOWo 10, September 2010, Hamburg, Germany S. Gligorevic, and P. Pulini, Simplified airport surface channel model based on the WSSUS assumption, ICNS 10, May 2010, Washington, US P. Pulini, and S. Gligorevic, WiMAX performance in the airport environment, MCSS 09, May 2009, Hersching, Germany

37 Publications Submitted Journals: G. Liva, P. Pulini and M. Chiani, On-Line Construction of Irregular Repeat Accumulate Codes for Packet Erasure Channels, submitted to IEEE Transaction on wireless communications P. Pulini, G. Liva, and M. Chiani, Unequal Diversity LDPC Codes for Relay Channels, submitted to IEEE Transaction on communications Patents: G. Liva, P. Pulini, Method for flexible transmission with LDPC codes. (Sub. January 2011) P. Pulini, G. Liva, Method for relay transmission with UEP. (Sub. May 2011) P. Pulini, G. Liva, Coded cooperation with information appending (Sub. May 2011)

38 Seminars Future Airport Data Link Based on WiMAX Forward Link Performance, 22 October 2009, Oberpfaffenhoffen, Germany Improving the Performance of AeroMACS by Cooperative Communications, 11 November 2011, Oberpfaffenhoffen, Germany Unequal Diversity Coding for the Relay Channel, 14 December 2011, Oberpfaffenhoffen, Germany

39 Thanks for your attention!

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43 Introduction and Motivations (4/4) Airport Surface Communications WiMAX foresees a large number of profiles with different efficiency/robustness trade-offs The most suitable profiles should be selected, taking into account the airport environment peculiarities Analysis of the strengths/weaknesses of the selected profile may reveal the need for enhancements

44 Low-Density Parity-Check Codes Protograph Analysis over Block Fading Channels (3/4) Given an SNR profile, determine whether iterative decoding converges or not (outage) Outage region of a protograph G, = set of channel profiles for which iterative decoding does not converge SNR (0) SNR (1) SNR ( 2)

45 Low-Density Parity-Check Codes Protograph Analysis over Block Fading Channels (4/4) Block error probability (outage probability) = probability of having a channel profile in Probability Distribution of the channel profile The definition iti of outage region can be SNR (0) SNR (1) SNR ( 2) extended to each single variable node (= block) We can characterize the UEP of the protograph nodes!