LDPC vs. Convolutional Codes for 802.11n Applications by 52S4R6

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									 January 2004                                   doc.: IEEE 802.11-04/0071r1




     LDPC vs. Convolutional Codes
       for 802.11n Applications:
       Performance Comparison
                            January 2004

Aleksandar Purkovic, Nina Burns, Sergey Sukobok, Levent Demirekler

                           Nortel Networks

                (contact: apurkovi@nortelnetworks.com)
Submission                      Slide 1        Aleksandar Purkovic, Nortel Networks
  January 2004                                        doc.: IEEE 802.11-04/0071r1


 Outline
• Background
• Simulation Methodology
• Simulation Results
   –   Packet Error Rate (PER) vs. SNR
   –   Throughput vs. SNR
   –   PHY data rate vs. distance
   –   LDPC – Convolutional coding gain difference vs. Block size
   –   Demonstration of embedded interleaving capability of LDPC codes
• Summary and Conclusions
• References




 Submission                          Slide 2          Aleksandar Purkovic, Nortel Networks
   January 2004                                           doc.: IEEE 802.11-04/0071r1


 Background
• Advanced coding has been identified as one of techniques to be considered in the
  process of 802.11n standard development (among other considerations, such as
  MIMO, higher order modulations, MAC efficiency improvement, etc.)
• Advanced coding candidates: Turbo coding, LDPC, Trellis Coded Modulation,
  more powerful convolutional codes, etc.
• Contribution IEEE 802.11-03/865 (Intel, Albuquerque meeting), [1] introduced
  Low-Density Parity-Check (LDPC) codes as candidate codes for 802.11n
  applications. It showed potential advantages of those codes over existing
  convolutional codes used currently (802.11a/g).
• This submission compares performance of example LDPC codes and existing
  (802.11a/g) convolutional codes in a systematic fashion, with:
    – Various frame lengths
    – Various code rates
    – Various line conditions (channel models)
• At this time performance comparison is addressed only, in order to justify further
  consideration of LDPC codes. In the next related submission (planned for March
  2004 meeting) emphasis will be on performance/complexity tradeoffs for both:
  existing convolutional codes and candidate LDPC codes.

  Submission                             Slide 3         Aleksandar Purkovic, Nortel Networks
  January 2004                                                           doc.: IEEE 802.11-04/0071r1


 Simulation Methodology - General
• PHY model based on the 802.11a spec [2] expanded with 256-QAM
  constellation. Simulation included:
 Data Rate (Mbits/s)    6      9      12     18          24    36      48       54         64          72

 Modulation            BPSK   BPSK   QPSK   QPSK      16QAM   16QAM   64QAM   64QAM      256QAM     256QAM

 Coding Rate (R)       1/2    3/4    1/2    3/4         1/2    3/4     2/3      3/4        2/3         3/4




• Channels simulated:
   – AWGN channel
   – Fading Channel Model D with power delay profile as defined in [3], NLOS,
     without simulation of Doppler spectrum. This implementation utilized the
     reference MATLAB code [4].

• Simulation scenario assumed:
   – Ideal channel estimation
   – All packets detected, ideal synchronization, no frequency offset
   – Ideal front end, Nyquist sampling frequency
 Submission                                        Slide 4              Aleksandar Purkovic, Nortel Networks
    January 2004                                         doc.: IEEE 802.11-04/0071r1


  Simulation Methodology - FEC
• General FEC:
   – Packet lengths: 40, 200, 600, 1500 bytes, chosen based on distributions in [1]
     and [5]
   – Code rates: 1/2, 2/3, 3/4 (as in 802.11a)
   – Uniform bit loading

• Convolutional codes:
   – Viterbi decoding algorithm

• LDPC codes:
   – Iterative Sum-Product decoding algorithm with 50 iterations
   – Concatenated codewords for longer packets




   Submission                          Slide 5           Aleksandar Purkovic, Nortel Networks
 January 2004                      doc.: IEEE 802.11-04/0071r1

Simulation Results: PER vs. SNR




     AWGN




    Channel
    Model D


Submission               Slide 6   Aleksandar Purkovic, Nortel Networks
   January 2004                            doc.: IEEE 802.11-04/0071r1

  Simulation Results: Throughput vs. SNR
Throughput =
PHY_data_rate (1 - PER)




       AWGN




     Channel
     Model D


  Submission               Slide 7         Aleksandar Purkovic, Nortel Networks
       January 2004                         doc.: IEEE 802.11-04/0071r1

      Simulation Results: PHY Data Rate vs. Distance
Channel Model D path loss
Tx power: 23dBm
Noise figure: 10dB
Implementation margin: 5dB
PER: 10-1




      Submission               Slide 8      Aleksandar Purkovic, Nortel Networks
   January 2004                                             doc.: IEEE 802.11-04/0071r1

  Simulation Results:
  LDPC – Convolutional Coding Gain Difference vs. Block Size
Modulation: BPSK
Code rate: 1/2
Coding gain difference measured at PER of 10 -2




 Submission                                       Slide 9   Aleksandar Purkovic, Nortel Networks
   January 2004                         doc.: IEEE 802.11-04/0071r1

  Simulation Results:
  Embedded Interleaving Capability Demonstration
Channel Model D
Code rate: 1/2




      Block size:
      40 bytes




      Block size:
      200 bytes


  Submission               Slide 10     Aleksandar Purkovic, Nortel Networks
    January 2004                                    doc.: IEEE 802.11-04/0071r1


  Summary and Conclusions

• LDPC codes offer considerable performance advantages over the
  existing convolutional codes.
• With the proper design LDPC codes can be made flexible enough to
  satisfy demands of 802.11n applications.
• LDPC codes have an inherent feature which eliminates need for the
  channel interleaver (this was already pointed out in [1]).
• Preliminary complexity analysis showed that reasonable solution is
  feasible. A submission on the performance/complexity analysis and
  potential real system design tradeoffs is planned for the March meeting.




   Submission                      Slide 11         Aleksandar Purkovic, Nortel Networks
 January 2004                                         doc.: IEEE 802.11-04/0071r1


References
[1] IEEE 802.11-03/865r1, “LDPC FEC for IEEE 802.11n Applications”, Eric
    Jacobson, Intel, November 2003.
[2] IEEE Std 802.11a-1999, Part 11: Wireless LAN Medium Access Control
    (MAC) and Physical Layer (PHY) Specifications, High-speed Physical Layer
    in the 5 GHz Band
[3] IEEE 802.11-03/940r1, “TGn Channel Models”, TGn Channel Models Special
    Committee, November 2003.
[4] Laurent Schumacher, “WLAN MIMO Channel Matlab program,” October
    2003, version 2.1.
[5] Packet length distribution at NASA Ames Internet Exchange (AIX),
    http://www.caida.org/analysis/AIX/plen_hist/.




Submission                          Slide 12          Aleksandar Purkovic, Nortel Networks

								
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