Channel Coding

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Channel Coding

• NCU-EE VLSI/DSP Lab.
• Freshman Training Course
• Speaker ???
• Advisor ??? ??

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Outline

• Overview of Channel coding
• Digital Communication System
• Types of Error Control
• Types of Channel Coding
• Turbo Codes
• Introduction
• System model
• Log-MAP vs SOVA
• Simulation
• SW Memory Architectures
• Conclusions

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Digital Communication System
JPEG, MPEG, etc.
RS code, Turbo code,
QPSK, QAM, BPSK, etc.
rb
rc
Information
Source
Channel
Modulator
Source
Encoder
Encoder
rs
Channel
Demodu-
Channel
Source
Data Sink
lator
Decoder
Decoder
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Channel Coding

• Channel coding refers to the class of signal
  transformation designed to improve communication
  performance by enabling the transmitted signals
  to better withstand the effects of various
  channel impairments.
• Channel coding can be partitioned into two areas,
  waveform (or signal design) coding and structured
  sequences (or structured redundancy.)
• Waveform coding deals with transforming waveforms
  into better waveforms, to make the detection
  process less subject to errors.
• Structured sequence deals with transforming data
  sequences into better sequences, having
  structured redundancy.

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Types of Error Control

• Before we discuss the detail of structured
  redundancy, let us describe the two basic ways
  such redundancy is used for controlling errors.
• Error detection and retransmission, utilizes
  parity bits (redundant bits added to data) to
  detect that an error has been made and requires
  two-way link for dialogue between the transmitter
  and receiver.
• Forward error correction (FEC), requires a one
  way link only, since in this case the parity bit
  are designed for both the detection and
  correction of errors.

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Why Use Error-Correction Coding

• Trade-off
• Error Performance verse Bandwidth
• Power verse Bandwidth
• Data Rate verse Bandwidth
• Capacity verse Bandwidth
• Coded verse Uncoded Performance
• Coding Gains
• For a given bit-error probabilities, coding gain
  is defined as the reduction in Eb/N0 that can be
  realized through the use of code.

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Types of Channel Coding

• Block codes
• Extended Golay code
• Hamming code
• BCH code
• Convolutional codes
• Recursive or Nonrecursive
• Systematic or Nonsystematic
• Reed-Solomon Codes
• Interleaving and Concatenated Codes
• Turbo Codes

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Block Codes

• (n,k) Block Codes
• message k-tuple u(u1,u2,,uk)
• code word n-tuple v(v1,v2,,vn)
• code rate Rk/n

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Convolustional Codes

• (n,k,m) Convolutional Codes
• message k-tuple u(u1,u2,,uk)
• code word n-tuple v(v1,v2,,vn)
• code rate Rk/n
• memory order m
• Constraint length Km1
• Generator polynomials g1(x) 1xx2
  g2(x)1x2

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Outline

• Overview of Channel coding
• Digital Communication System
• Types of Error Control
• Types of Channel Coding
• Turbo Codes
• Introduction
• System model
• Log-MAP vs SOVA
• Simulation
• SW Memory Architectures
• Conclusions

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Turbo Codes

• Shannons channel coding theorem guarantees the
  existence of codes that can achieve arbitrary
  small probability of error if the data
  transmission rate is smaller than the channel
  capacity.

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Applications

• Turbo code is currently adopted as the channel
  coding schemes in many next-generation
  communication systems
• WCDMA, CDMA2000
• CCSDS in space communications
• Baseband Signal compensation in Fiber
  transmission systems

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Specifications in WCDMA
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Specification in CDMA2000
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Turbo Code v.s. Convolutional Code

• Convolutional Code
• Non-recursive
• Non-systematic
• Without Interleaver

• Turbo Code
• Recursive
• Systematic
• Parallel structure
• Use Interleaver

NSC
RSC
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Design Flow
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System Model
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Log-MAP vs SOVA
G7 5, Unpunctured(1/3), frame
size1024, Iteration8.
Iteration increment..
SOVA
Iteration increment..
Log_MAP
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The SOVA algorithm
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Log-MAP Algorithms
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Log-MAP Algorithms
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Sliding Window Memory Issue

• The extrinsic and APP value are made with a
  delay, which is equal to received sequence
  length.
• But the decoder decisions length can be reduced
  to about six times the encoder memory because of
  reliable decoding decision.

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Simulation Results (1/3)
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Simulation Results (2/3)
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Simulation Results (3/3)
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SW - Memory Architectures

• Advantage
• 1 Less memory size. 2. Might be Lower
  latency.
• Disadvantage
• 1.Read-modify-write access required for the
  memory.
• 2.Address is hard to be controlled.

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Timing Diagram
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Position VS Time
A
DO
B0
B1
G
Decode output without LIFO
Decode output with LIFO
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ACS Unit
Add-compare-select (ACS) Unit
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Forward / Backward Processor
Trellis States
Forward Processor (A) / Backward Processor (B)
Block Diagram
Block Diagram based on Trellis States
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Novel Turbo Code Iteration Issues

• In high-quality channel environments, a large of
  decoding iterations are not required to obtain
  the target BER, and it is possible to terminate
  the process after a few numbers of decoding
  iterations.

Denterlever
SISO (MAP1) Decoder
SISO (MAP2) Decoder
Interlever
Threshold Detection
Hard Decision
Denterlever
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Outline

• Overview of Channel coding
• Digital Communication System
• Types of Error Control
• Types of Channel Coding
• Turbo Codes
• Introduction
• System model
• Log-MAP vs SOVA
• Simulation
• SW Memory Architectures
• Conclusions

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Conclusions

• We discuss some fundamentals of channel coding.
• We discuss some basic implementation issues for
  turbo codes.
• This study can be exploited in development of
  high performance receiver with different
  constraints of cost and throughput.
• The novel turbo decoder can practically have
  lower iteration with the adaptive SNR channel
  estimation.