A study of multilevel partial response signalling for transmission in a basic supergroup bandwidth

Includes bibliographical references.The work in this thesis is primarily directed toward the design, construction and testing of an experimental multilevel partial response signalling baseband system. The system will find practical application in existing frequency division multiplexed-frequency modulated microwave links. The basic supergroup bandwidth of these links is 240 kHz. The design requires a transmission rate of 1.024 Mb/s in this bandwidth. Class-4 15 partial response signalling is the coding technique suitable to achieve this. A pilot tone scheme is used to facilitate symbol timing recovery at the demodulator. A sixth order Butterworth low pass filter approximates the ideal raised-cosine Nyquist channel. A theoretical discussion on impairments caused by deviation from this channel is given. Since the experimental system was non-ideal, it produced a degradation in the channel signal to noise ratio. This degradation, coupled with other factors, showed that further development was necessary for the system to be suitable for connection into an existing microwave link

Performance issues in hybrid fiber radio communication systems due to nonlinear distortion effects in laser transmitters

With the increasing demand for broadband services, it is expected that hybrid fiber radio systems may be employed to provide high capacity access networks for both mobile and fixed users. In these systems, the radio frequency data signals are modulated onto an optical carrier at a mobile switching centre and then sent over fiber to a number of base stations, before being transmitted over air to the users. A possible method of generating the optical radio frequency data signals for distribution over fiber is to directly modulate the electrical signal onto an optical carrier using a laser diode. The major problem with this technique is that nonlinearities in electncal-to-optical conversion may seriously degrade the system performance. In this work we initially examined the distribution of a wideband code division multiple access signal (centered around 6 GHz) through an optically fed microwave system. Our results show that the adjacent channel leakage ratio is degraded from -52 to -32 dBc after passing through the optical system. We then examined the technique of externally injecting light into the directly modulated laser, to extend the bandwidth of the laser diode and hence, increase it’s linear region to beyond the frequency of interest With this technique an improvement of over 10 dB in the adjacent channel leakage ratio of the signal was achieved. We subsequently went on to examine the distribution of a 5-channel radio frequency signal (each channel carrying 10 Mbit/s) through a hybrid fiber system As in the previous work, we examined how external light injection into the directly modulated laser could be used to improve system performance, and our results show an improvement of up to 5 dB. Finally a model was designed using Matlab, which simulated the 5-channel system mentioned above. It used the laser rate equations to mimic the nonlinear effects of the laser diode Good correlation was observed between experimental and simulated results

Digital signal processing for fiber-optic communication systems

As the available bandwidth of optical fibers has been almost fully exploited, Digital Signal Processing (DSP) comes to rescue and is a critical technology underpinning the next generation advanced fiber-optic systems. Literally, it contributes two principal enforcements with respect to information communication. One is the implementation of spectrally-efficient modulation schemes, and the other is the guarantee of the recovery of information from the spectrally-efficient optical signals after channel transmission. The dissertation is dedicated to DSP techniques for the advanced fiber-optic systems. It consists of two main research topics. The first topic is about Fast-orthogonal frequency-division multiplexing (OFDM) — a variant OFDM scheme whose subcarrier spacing is half of that of conventional OFDM. The second one is about Fresnel transform with the derivation of an interesting discrete Fresnel transform (DFnT), and the proposal of orthogonal chirp-division multiplexing (OCDM), which is fundamentally underlain by the Fresnel transform. In the first part, equalization and signal recovery problems result from the halved subcarrier spacing in both double-sideband (DSB) and single-sideband (SSB) modulated Fast-OFDM systems are studied, respectively. By exploiting the relation between the multiplexing kernels of Fast-OFDM systems and Fourier transform, equalization algorithms are proposed for respective Fast-OFDM systems for information recovery. Detailed analysis is also provided. With the proposed algorithms, the DSB Fast-OFDM was experimentally implemented by intensity-modulation and direct detection in the conventional 1.55-μm and the emerging 2-μm fiber-optic systems, and the SSB Fast-OFDM was first implemented in coherent fiber-optic system with a spectral efficiency of 6 bit/s/Hz at 36 Gbps, for the first time. In the second part, Fresnel transform from optical Fresnel diffraction is studied. The discrete Fresnel transform (DFnT) is derived, as an interesting transformation that would be potentially useful for DSP. Its properties are proved. One of the attractive properties, the convolution-preservation property states that the DFnT of a circular convolution of two sequences is equal to the DFnT of either one convolving with the other. One application of DFnT is practically utilized in the proposal of OCDM. In the OCDM system, a large number of orthogonal chirped waveforms are multiplexed for high-speed communication, achieving the maximum spectral efficiency of chirp spread spectrum systems, in the same way as OFDM attains the maximum spectral efficiency of frequency-division multiplexing. Owing to the unique time-frequency properties of chirped waveforms, OCDM outperforms OFDM and single-carrier systems, and is more resilient against the noise effect, especially, when time-domain and frequency-domain distortions are severe. Experiments were carried out to validate the feasibility and advantages of the proposed OCDM systems