A Software Simulation Testbed for CDMA Wireless Systems

1
A Software Simulation Testbed for CDMA Wireless
Systems

• Vishwas Sundaramurthy

Electrical and Computer Engineering Department and
April 26th 1999
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Wireless cellular systems

• Tremendous growth in coverage

• More economical than wired-line systems --
  infrastructure

• Mobility

• Capacity increases

• Code Division Multiple Access(CDMA)
• Efficient usage of channel capacity
• Robust against defects in the transmission and
  channel

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Challenges in designing CDMA systems

• Communications research improvements in
• performance
• usage of capacity

• Advanced signal processing techniques
• channel estimation
• multiuser detection
• coding

• A large number of design choices
• complex environment (channel models)
• different algorithms for blocks in the system

4
Motivation for a simulation testbed

• What are the channel properties or
  environmental factors to deal with?

• multi-path delays

• attenuation

5
Motivation for a testbed (contd.)

• What is the best combination of algorithms ?

• How do these algorithms behave with one another?

• Need for performance data before investing in
  infrastructure

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The concept of a CDMA Testbed

• Algorithms
• evaluation
• tradeoffs

• System parameters
• channel models
• user dynamics

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The concept of a CDMA Testbed (contd.)
Mobile phone

Base station

• Algorithms
• Uplink
• Downlink

Prototyping

• System parameters

8
Contributions

• Flexible wireless CDMA multiuser link

• Developed in the Simulink and Matlab Environment

• Algorithms
• channel estimation
• multiuser detection

• System configuration
• channel models
• different spreading methods

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The CDMA concept
Spreading waveforms
chip
1
2
3
bit period
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CDMA works!!

• Spreading

-1 -1 1 1 1 -1 1
-1 -1 -1 -1 1 1 1 1
-1 1 -1 1 1 1
Nc7
(spreading matrix)
3 users

• Detection

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CDMA Wireless Link
Users data bits
multiple users
SOURCE CODING
CHANNEL CODING
MODULATION
SPREADING
TRANSMITTER
noise
Detected bits of all the users
DECODER
DETECTOR
DEMODULATION
CHANNEL ESTIMATOR
RECEIVER
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W-CDMA Simulation Testbed in Software

• Matlab environment
• Simulink block diagrams

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CDMA Wireless Link
Users data bits
multiple users
SOURCE CODING
CHANNEL CODING
MODULATION
SPREADING
TRANSMITTER
noise
Detected bits of all the users
DECODER
DETECTOR
DEMODULATION
In Simulink
CHANNEL ESTIMATOR
RECEIVER
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CDMA Wireless System Testbed Simulink Version
Multiuser Detection
Chip matched filter
AWGN Channel
Error Rate Calculation
Chip MF
Wireless Channel
User_Data
Multiuser Detector
Decorrelating
Error Counter
Detector
Channel Estimation
Channel Estimation
User Data
Max. Likelihood
Channel Est.
Show Stats
Update Parameters
Parameters
Statistics
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Features

• Flexible wireless CDMA multiuser link
• better modeling of a real system

• Clean user interface
• vary parameters of the system

• Modular
• library of different algorithms

• State-of-the-art algorithms
• multiuser detection
• channel estimation

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Development Methodology
C - Code
MATLAB Code
Simulink Library
Algorithms
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System Development using Simulink
SIMULINK Library
C - MEX S - function
MATLAB S - function Block
BLOCK B
BLOCK A
data in
BLOCK C
data out
BLOCK D
BLOCK E

• Simulation framework discrete time-steps
• Data-flow based simulation

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Algorithms

• Spreading methods
• Gold Codes
• M-Sequences
• Random spreading codes

• Channel estimation
• maximum likelihood
• Multiuser detection
• matched filter
• decorrelator
• differencing multistage

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Maximum Likelihood Algorithm

• ri ( U Z) bi ni

Received chip matched filter output
Noise
Preamble bits
Shifted spreading matrix and channel impulse
response
Estimate by Maximum likelihood method
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Multiuser detection

• Code matched filter

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Modularity
CDMA library
Channels
Detectors
Associated components
Multiuser Matched-Filter Detector
x
Matched-Filter
b

• Library of different algorithms
• Plug and play!

Detector
Multipath Channel
cdma-tx-gen
x
Multiuser Decorrelating Detector
Multipath Wireless Channel
b
cdmatx
Decorrelating
Detector
Rd wksp
Single user receiver
preamb_wksp
Multiuser Multistage Detector
Multistage
chan i/p
code MF o/p
Detector
bitconvert
sync delay
chip MF o/p
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Graphical User Interface

• Control parameters
• SNR
• spreading gain
• users
• bit length

• Change modules / algorithms
• spreading
• multiuser detection

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Automated Net-List Generation

• The Simulink model is reconfigured based on
  parameters
• add/remove user data blocks depending on number
  of users
• change algorithms

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The segment under consideration
Mobile Transmitters K Asynchronous Users
data bits
Spreading and modulation
Detected bits of K users
Demodulation and chip matched filtering
detection
AWGN noise
synchronization
Multi-User (Base-station) Receiver
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Spreading, Channel and Chip Matched Filtering
spreading
channel
chip matched filter vector
. . . .
data bits
x
spreading
spreading
Chip waveform
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Spreading, Channel and Chip Matched Filtering
Spread bit sequences
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Traditional method of simulation modeling

• Represents baseband signals explicitly
• superimposed chips
• AWGN noise
• baseband signal

• Problem
• baseband signals are continuous time signals
• a simulation system models them as a discrete
  time signal
• finite number of samples leads to an error
• slow simulation speed

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Traditional Method
Spreading, Channel modeling and Chip-matched
filtering
sprd
bits
Sample
Tx-delay-1
bits-1
and Hold
Spreading
Function-1
T
AWGN
S/H

z-1
AWGN
channel
Discrete-Time
Integrator
sprd
bits
Tx-delay-2
bits-2
Spreading
Chip Matched FIlter
Pulse
Function-2
Generator
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The efficient method of simulation modeling

• Does not represent intermediate signals explicitly

• All the blocks in the baseband segment
  collapsed into one entity
• spreading, channel model and matched filtering
• input
• bit streams
• spreading, channel parameters
• provides discretized channel waveform (matched
  filter output)

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Efficient Method
Spreading, Channel modeling and Chip-matched
filtering
Unit Delay
bits
1
bits-1
z
sprd_cmf
bits
spreading
channel
and
bits-2
chip matched filter
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Simulation time speed-up
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Analysis of simulation time from profiling
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Simulink Test-bed Demo
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Simulation in Software and DSP hardware
Simulink Block Diagrams
Real-Time Workshop (RTW)
TI C67 EVM (Evaluation Module)
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Prototyping Methodology
C - Code
Matlab Code
Block Diagram Libraries
Algorithms
Simulink
Display and Analysis of Data
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RTW support for C67 EVM
Simulink Model

• Create template make file
• Change references to cross-compiler (TI C6x
  code generation tools)
• Generate code
• Download to DSP board
• Analyze data in Simulink / Workstation

C67 DSP hardware
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Future Work - CDMA Library of blocks

• Fading channel and supporting estimation and
  detection algorithms
• Channel parameter estimation and tracking
• Maximum Likelihood
• Subspace
• Source and Channel coding
• Power control

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Future Work - DSP support

• Execution of software test-bed on DSP hardware
• Real-Time Workshop (RTW) support for C67 boards
• Use multiprocessing on DSPs to execute the
  Simulink based system

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Summary

• A Multi-user Wireless CDMA Link
• backbone to grow a bigger system
• Flexible system
• vary parameters
• state-of-the-art algorithms from the library
• Simulink/Matlab Environment
• GUI for changing parameters and algorithms
• data analysis and profiling
• RTW for DSP code generation