Wavelet-based Orthogonal Frequency Division Multiplexing (WOFDM) represents a modern orthogonal modulation technique. In WOFDM, the bits to be transmitted are split into multiple streams, which are simultaneously transmitted at different rates, using wavelet carriers. Every stream is mapped to a different transmission scale and is affected differently by the time-varying radio channel. This influence is studied in the present work by computing some errors per scale measures. These statistics show what are the best transmission scales in the flat, time-variant channel and open the path towards the coded WOFDM, where the coding rate can be adapted on a per scale basis.

Errors per scale statistics for a wavelet OFDM transmission in flat fading channels

Wavelet modulation sends multiple copies of data at multiple rates through an unknown channel. We present new methods that result in dramatic bit error rate performance improvements when multiple rates are available and utilized for demodulation. Our results indicate significantly better performance than binary phase-shift keying in fading channels. We also introduce and illustrate new algorithms that identify the channel type and mitigate the effects of intersymbol interference in frequency selective fading channels.

Wavelet modulation in Rayleigh fading channels: Improved performance and channel identification

Due to the ever increasing demand on bandwidth, and the demonstrated success of wireless optical communications in providing broadband communications, interest has grown into further expanding the deployment of wireless optical links, specifically for ground-to-air and air-to-air communications. However, the weather dependent reliability of wireless optical links makes it necessary to develop system architectures capable of providing reliable communications in various channel conditions. It has been shown that through diversity, performance of communication systems can be greatly improved. Third generation networks will be exploiting multi-rate diversity to enhance the provided quality of service. 
In this dissertation, we investigate cloud obscured wireless optical links, and examine system performance enhancement through such channels using multi-rate communications, and provide two system architectures that provide multi-rate communications for wireless optical systems.
Channel models for cloud obscured wireless optical links through Monte Carlo Ray Tracing simulations based on Mie Scattering formulation are developed. Based on the simulation results, empirical formulas for temporal channel models are obtained; which in conjunction with statistical cloud coverage data, show that considerable bandwidth is available; with an average of 32.5 GHz, 160 and 240 GHz in the low, medium and high altitude clouds, respectively.
Using the developed channel models, performance of fractal modulation in cloud obscured channels is investigated. It is shown that fractal systems provide higher reliability that conventional single rate square root raised cosine (SRRC) systems, achieving bit error rates as low as 10-9 in 1 Km length channels, with optical thicknesses as high 7.0.
Optical pulse shaping is also investigated, where an optical pulse shaping technique that can provide arbitrary waveform generation is proposed, capable of eliminating space-time coupling without resorting to the conventionally used spatial filtering.
The results of the optical pulse shaping technique are used in developing all optical transmitter and receiver architectures, capable of multi-rate multi-wavelength communications. It is shown that the performance of short-pulsed multi-rate multi-carrier communications substantially out performs conventional fractal modulation techniques, achieving bit error rates as low as 10-9 1 Km length channels with optical thicknesses as high 10.0 for systems using SRRC pulse shaping, and optical thicknesses as high 18.0 for systems using Meyer wavelet pulse shaping.
These results combined, show that huge communication speeds at low bit error rates are possible through cloud-obscured channels. Multi-rate communications provide a simplified system architecture that is capable of providing reliable communications in adverse channel conditions.

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Multi-rate wireless optical communications in cloud obscured channels

Multi carrier modulation (MCM) is an attractive approach for high-speed power line communication networks. In a real communication system, the choice of the modulation is one of the most important design steps. In this paper, we analyze and compare the performance obtained in power line networks when using orthogonal frequency division multiplexing (OFDM) or discrete wavelet multi tone (DWMT) modulations. We show that, due to the multi-path, OFDM performance is better than DWMT performance in power line communications.

OFDM and wavelets performance comparison in power line channels

Problem statement: In the modern speech communication technology, the speech coding
with bitrate scalability was needed. However, various types of noises in the communication channels
cause damages in the transmitted information especially speech data. Tonal-language speech was also
affected by this situation. Approach: Based on the high pitch delay resolution technique, the MPCELP
speech coding was proposed over the environment with CDMA AWGN and Rayleigh fading
channels for tonal language. The proposed coder supports multiple bitrates and also has the
functionality of bitrate scalability. Results: Through performance analysis and computer simulation,
the quality of the proposed coding was presented with an improvement from conventional scalable
MP-CELP in the specific-noise environments. The HPDR technique was applied to the MP-CELP to
use for tonal language, meanwhile it can support the core coding rate of 5.6, 8.2, 12.2 kbps and
additional scaled bitrates. Conclusion: By applying the high pitch delay resolution technique with the
MP-CELP speech coding, we can improve the quality of tonal encoded speech. Moreover, the coding
quality of the proposed coder was better than the conventional coder for Thai language over both
AWGN channel and Rayleigh fading channel.

Performance Evaluation of Multi-Pulse Based Code Excited Linear Predictive Speech Coder with Bitrate Scalable Tool over Additive White Gaussian Noise and Rayleigh Fading Channels

Wavelet modulation simultaneously sends data at multiple rates through an unknown channel. Previous research has demonstrated that wavelet modulation bit error rate performance in the additive white Gaussian noise channel is comparable to theoretical binary phase shift keying. We extend the investigation to the performance of wavelet modulation in several time varying channels: Rayleigh, flat, slow fading channels and frequency selective, slow fading channels.

M.J. Manglani

Papers

Wavelet modulation performance in Gaussian and Rayleigh fading channels