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The suitability of OFDM as a modulation technique for wireless telecommunications,
with a CDMA comparison.
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Revised: 16/10/2001 (Fixed up several mistakes in the simulations and fixed up spelling and grammar).
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This is a PDF of the complete thesis, with no Matlab Code
Original
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Thesis submitted by Eric Lawrey in October 1997
in partial fulfilment of the requirements for the Degree of Bachelor of Engineering with Honours in
Computer Systems Engineering at James Cook University.
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This thesis investigates the effectiveness
of Orthogonal Frequency Division Multiplexing (OFDM) as a modulation technique
for wireless radio applications. The main aim was to assess the suitability
of OFDM as a modulation technique for a fixed wireless phone system for
rural areas of Australia. However, its suitability for more general wireless
applications is also assessed.
Several of the main factors affecting the
performance of a OFDM system were measured, including multipath delay spread,
channel noise, distortion (clipping), and timing requirements. The performance
of OFDM was assessed by using computer simulations performed using Matlab,
and practical measurements. These measurements were performed by recording
a low bandwidth (audio) OFDM signal, generated using Matlab, on to a tape
player. This recorded signal was then played back and recorded using the
sound card of a PC. This was then decoded using a Matlab script.
Most third generation mobile phone systems
are proposing to use Code Division Multiple Access (CDMA) as their modulation
technique. For this reason, CDMA was also investigated so that the performance
of CDMA could be compared with OFDM.
It was found that OFDM performs extremely
well compared with CDMA, providing a very high tolerance to multipath delay
spread, peak power clipping, and channel noise. In addition to this it
provides a high spectral efficiency.
OFDM was found to have total immunity to
multipath delay spread provided the reflection time is less than the guard
period used in the OFDM signal. In fact, multipath signals lead to a strengthening
of the received signal, improving the performance. In a typical system
a delay spread of up to 100 msec could be tolerated, corresponding to multipath
reflections of 30 km. The only problem caused by multipath is frequency
selective fading, which can result in carriers being heavily attenuated
due to destructive interference at the receiver. This can result in the
carriers being lost in the noise.
For the modulation schemes investigated
(BPSK, QPSK and 16 QAM), clipping of the OFDM signal was found to have
little effect on the performance of the system, allowing the peak power
of the signal to be clipped by up to 6 - 9dB before the symbol error rate
became significant. This tolerance to clipping reduces the dynamic range
overhead required in output stages of OFDM transmitters.
The noise performance of OFDM was found
to depend solely on the modulation technique used for modulating each carrier
of the signal. The performance of the OFDM signal was found to be the same
as for a single carrier system, using the same modulation technique. The
minimum signal to noise ratio (SNR) required for BPSK was ~7 dB, where
as it was ~12 dB for QPSK and ~25 dB for 16PSK.
CDMA was found to perform poorly in a single
cellular system, with each cell only allowing 7-16 simultaneous users in
a cell, compared with 128 for OFDM. This was for a 1.25 MHz bandwidth and
19.5 kbps user data rate. This low cell capacity of CDMA was attributed
to the use of non-orthogonal codes used in the reverse transmission link,
leading to a high level of inter-user interference.
The only main weak point that was found
with using OFDM, was that it is very sensitive to frequency, and phase
errors between the transmitter and receiver. The main sources of these
errors are frequency stability problems; phase noise of the transmitter;
and any frequency offset errors between the transmitter and receiver. This
problem can be mostly overcome by synchronizing the clocks between the
transmitter and receiver, by designing the system appropriately, or by
reducing the number of carriers used.
Download
OFDM Matlab Code
This is the code that was used in the
thesis.
Audio
Data Demonstration
Shows the effect of noise and signal clipping
on audio data sent using OFDM
1. Introduction
1.1
Third Generation Wireless Networks
1.1.1
Evolution of Telecommunication Systems.
1.1.2
Overall Aims of Universal Mobile Telecommunications System
1.1.3
Teleservices
1.1.4
UMTS Environments
1.1.5
Cell types
1.1.6
Radio Interface
1.1.7
Satellite Networking
1.1.8
Timetable for System Implementation
1.1.9
Conclusion
1.2
Propagation Characteristics of mobile radio channels
1.2.1
Attenuation
1.2.2
Multipath Effects
1.2.3
Doppler Shift
1.3
Multiple Access Techniques
1.3.1
Frequency Division Multiple Access
1.3.2
Time Division Multiple Access
1.3.3
Code Division Multiple Access
1.3.4
CDMA Process Gain
1.3.5
CDMA Generation
1.3.6
CDMA Forward Link Encoding
1.3.7
CDMA Reverse Link Encoding
1.3.8
Orthogonal Frequency Division Multiplexing
1.3.9
OFDM generation
1.3.10
Adding a Guard Period to OFDM
2. OFDM Results
2.1
OFDM Model Used
2.1.1
Serial to Parallel Conversion
2.1.2
Modulation of Data
2.1.3
Inverse Fourier Transform
2.1.4
Guard Period
2.1.5
Channel
2.1.6
Receiver
2.1.7
OFDM Simulation Parameters
2.2
OFDM Simulated Results
2.2.1
Multipath Delay Spread Immunity
2.2.2
Peak Power Clipping
2.2.3
Gaussian Noise Tolerance of OFDM
2.2.4
Timing Requirements
2.3
Practical Measurements
2.3.1
Extended Model
2.3.2
Transmission Protocol
2.3.3
Video Recorder
2.3.4
Peak OFDM Performance for the VCR link
2.3.5
Audio Tape Player
2.4
Picture quality verse signal to noise ratio
2.4.1
Experimental comparison between QPSK and power averaged 256PSK
2.4.2
Results
2.5
Mathematical Model for OFDM performance
2.5.1
RMS Demodulated Phase Error
2.5.2
BER verses Channel Noise
2.6
OFDM system implementation
2.6.1
Using general purpose DSPs
2.6.2
Future DSP Processing Power
2.6.3
Hardware FFT Implementation
3.
CDMA Results
3.1
Simulated Model
3.1.1
Forward Link
3.1.2
Reverse Path
3.2
Simulation Results
3.2.1
BER verses the number of users in a cell
3.3
Mathematical Model for Reverse Link
3.3.1
Cell Capacity for a CDMA system
3.3.2
Capacity of a single CDMA cell
3.3.3
Capacity of CDMA and OFDM with Multiple Cells
4.
Conclusion
Bibliography
Appendix
I. Acronyms
Appendix
II. OFDM Guassian Noise Performance Prediction
Appendix
III. BER verses Eb/No for a CDMA system
