High Speed Communication Channel Modelling

1
High Speed Communication Channel Modelling

• ADSL DMT Modem Operating Environment Simulation

2
Contents

• Introduction ADSL System
• DMT model
• Channel model
• Our Simulations
• Results and Analysis
• Future Prospects

3
ADSL Twisted Pair access to broadband network
4
ADSL Modulation Method DMT Discrete Multitone
Channel Frequency Response
Response

Frequency
5
Broadband access alternatives and ADSL

• Broadband access alternatives
• HFC
• Sattelite (direct and non direct)
• FTTC (Fibre To The Curb)

ADSL Advantages No new line installation Always
on-line while still connected to POTS
Disadvantages Require new sampler at exchange
and ADSL modem at remote site Distance and loop
characteristic limitations
6
ADSL System Overview
Broadband Network
Central Office
7
ADSL Applications
xDSL Performance Standards
HDSL 784 kbits/sec (symmetrical up/down) ADSL 1
1.5 Mbits/sec (18000 feet) ADSL 2 3 Mbits/sec
(12000 feet) ADSL 3 6 Mbits/sec (6000
feet) VDSL 50 Mbits/sec (1000 feet)
8
ADSL OperatingEnvironments

• Channel Characteristics
• Frequency Response

• Noise Characteristics

• AWGN

• Radio Frequency Interference for high bit rate
  systems

• Impulse Noise

9
Construction- system architecture
Simulator implemented using C/C and Matlab.
10
Transmitter
...101011...
scrambler
011010...
101 001001 1110
FEC Coder Interleaver
011010...
101 001001 1110
13j -75j 5-3j
Constellation Encoder with Gain Scaling
13j -75j 5-3j
IFFT Modulator
Appending Cyclic Prefix
Appending Cyclic Prefix
11
Transmitter - Interleaver
1
2
3
4
15
0
1
2
1
1
0
3
1
1
0
1
1
4
1
1
0
5
1
0
6
1
1
1
0
255
0
1
Channels
12
Transmitter - Constellation
. . .
Input
1
0
1
1
1
0
1
1
0
1
1
1
13
Transmitter - IFFT Modulator
Inputs
1 2 3 . . . . . 256 . . . . . 512
14
Transmitter - IFFT Modulator
512-pt Inverse Fast Fourier Transform
15
Channel - models

• Perfect Termination Model

• Frequency response
• Different Line lengths and Gauges
• Impulse response

• Schmucking Model

• LINEMOD

16
Bit allocation scheme
17
AWGN, Impulse Noise, Xtalk

• Normal Distribution Random Number Generator -
  AWGN
• Time domain Impulse Noise Samples (from sources)
• Crosstalk models (FEXT and NEXT)

18
Receiver
Equalized samples with guard period removed
voltage samples
TEQ Time Domain Equalizer
FFT Demodulator
Complex constellation points
FEQ Frequency Domain Equalizer
Constellation points with common decision
boundaries
Decision/Demap
Tone block
Deinterleaver/Decoder/Descramber
19
Receiver - TEQ

• Feedforward FIR filter that compresses the
  impulse response into the first 32 samples of
  each frame
• Issues
• Filter taps to be determined during Connection
  Initialization (before simulation)
• Complex algorithm involves inversion of large
  matrix

20
Receiver - Decision
Transmitted Point 1
Similar to QAM detectors
21
Implementation Issues

• What we implemented?
• uni directional simplex model central office
  to remote site
• Assumptions
• Sychronised clocks on both transmitter and
  receiver.

22
Analysis - Impulse Noise
Impulse 5
Impulse noise are random in duration and magnitude
Impulse 3
Impulse 1
23
Analysis

• Different impulses have different effect on the
  bit error rate
• AWGN effects similar to other communications
  systems
• Crosstalks are stationary, they effectively
  modify the frequency response of the channel

24
Analysis - ISI
25
Future prospect

• Integration with hardware
• Implement Trellis Coding and Veterbi Decoding
• Accurate Modem Initialization and Training
• Uses 2 terminals and shows communciation between
  them
• Real Time System Concurrent processes for
  transmitter, channel and receivers, uses accurate
  (realistic) timing
• Simulate network and/or higher layers

26
Conclusion

• ADSL Standard T1.413
• Modem model with DMT implementation
• Channel model
• Analysed ISI, AWGN, Impulse Noise and Crosstalk
  Noise.

27
A Comparison
28
Simulation Frequency model.
29
Effect of line length and Gauge.
30
Impulse response
31
Impulse Noise.
32
Crosstalk.