Novel multiuser detection and multi-rate schemes for multi-carrier CDMA

A large variety of services is [sic] expected for wireless systems, in particular, high data rate services, such as wireless Internet access. Users with different data rates and quality of service (QoS) requirements must be accommodated. A suitable multiple access scheme is key to enabling wireless systems to support both the high data rate and the integrated multiple data rate transmissions with satisfactory performance and flexibility. A multi-carrier code division multiple access (MC-CDMA) scheme is a promising candidate for emerging broadband wireless systems. MC-CDMA is a hybrid of orthogonal frequency division multiplexing (OFDM) and code division multiple access (CDMA). The most salient feature of MC-CDMA is that the rate of transmission is not limited by the wireless channel\u27s frequency-selective fading effects caused by multipath propagation. In MC-CDMA, each chip of the desired user\u27s spreading code, multiplied by the current data bit, is modulated onto a separate subcarrier. Therefore, each subcarrier has a narrow bandwidth and undergoes frequency-flat fading. Two important issues for an MC-CDMA wireless system, multiuser detection and multi-rate access, are discussed in this dissertation. Several advanced receiver structures capable of suppressing multiuser interference in an uplink MC-CDMA system, operating in a frequency-selective fading channel, are studied in this dissertation. One receiver is based on a so-called multishot structure, in which the interference introduced by the asynchronous reception of different users is successfully suppressed by a receiver based on the minimum mean-square error (MMSE) criterion with a built-in de-biasing feature. Like many other multiuser schemes, this receiver is very sensitive to a delay estimation error. A blind adaptive two-stage decorrelating receiver based on the bootstrap algorithm is developed to combat severe performance degradation due to a delay estimation error. It is observed that in the presence of a delay estimation error the blind adaptive bootstrap receiver is more near-far resistant than the MMSE receiver. Furthermore, a differential bootstrap receiver is proposed to extend the limited operating range of the two-stage bootstrap receiver which suffers from a phase ambiguity problem. Another receiver is based on a partial sampling (PS) demodulation structure, which further reduces the sensitivity to unknown user delays in an uplink scenario. Using this partial sampling structure, it is no longer necessary to synchronize the receiver with the desired user. Following the partial sampling demodulator, a minimum mean-square error combining (MMSEC) detector is applied. The partial sampling MMSEC (PS-MMSEC) receiver is shown to have strong interference suppression and timing acquisition capabilities. The complexity of this receiver can be reduced significantly, with negligible performance loss, by choosing a suitable partial sampling rate and using a structure called reduced complexity PS-MMSEC (RPS-MMSEC). The adaptive implementation of these receivers yields a superior rate of convergence and symbol error rate performance in comparison to a conventional MMSEC receiver with known timing. All the above receiver structures are for a single-rate MC-CDMA. Three novel multi-rate access schemes for multi-rate MC-CDMA, fixed spreading length (FSL), coded FSL (CFSL) and variable spreading length (VSL), have been developed. These multi-rate access schemes enable users to transmit information at different data rates in one MC-CDMA system. Hence, voice, data, image and video can be transmitted seamlessly through a wireless infrastructure. The bit error rate performance of these schemes is investigated for both low-rate and high-rate users

High Speed Optical Wireless Communication Systems.

Visible light communication (VLC) systems have become promising candidates to complement conventional radio frequency (RF) systems due to the increasingly saturated RF spectrum and the potentially high data rates that can be achieved by VLC systems. Over the last decade, significant research efforts have been directed towards the development of VLC systems due to their numerous advantages over RF communication systems, such as the availability of simple transmitters (light emitting diodes, (LEDs)) and receivers (silicon photo detectors), better security at the physical layer, improved energy efficiency due to the dual functionally (i.e., illumination and communication) and hundreds of THz of license-free bandwidth. However, several challenges face VLC systems to achieve high data rates (multi gigabits per second). These challenges include the low modulation bandwidth of the LEDs, inter symbol interference (ISI) due to multipath propagation, co-channel interference (CCI) and the light unit (i.e., VLC transmitter) should be ‘‘ON’’ all the time to ensure continuous communication. This thesis investigates a number of techniques to design robust highspeed indoor VLC systems that support a single user and multi-users. Light engines composed of RYGB laser diodes (LDs) are used for communication and illumination. The main goal of using LDs is to enable the VLC system to achieve high data rates while using simple modulation techniques (such as on off keying (OOK)), which adds simplicity to VLC systems. Three VLC systems based on the computer generated holograms (CGHs) are proposed in this thesis, which are single beam static CGH-VLC system, static CGH-VLC system and adaptive CGH-VLC system. Whereas in the first and the second systems a single photodetector is used (added simplicity), an imaging receiver is used in the third one to obtain spatial multiplexing. We consider the lighting constraints where illumination should be at an acceptable level and should consider diffuse reflections (up to second order) to find the maximum data rate that can be offered by each system. In the first system, the CGH is used to produce one fixed broad beam from the best light unit and focus it to a specific area on the communication floor. In the second system, the CGH generates 100 beams (all these beams carry same data) from the best transmitter and directs these beams to an area of 2 m × 2 m on the communication floor. In the third system, the CGH is used to generate eight beams from the best transmitter and steer these beams to the receiver’s location. In addition, each one of these eight beams carries a different data stream. This thesis also presents an indoor VLC system in conjunction with an imaging receiver with parallel data transmission (spatial multiplexing) to reduce the effects of inter-symbol-interference (ISI). To distinguish between light units (transmitters) and to match the light units used (to convey the data) with the pixels of the imaging receiver, we proposed the use of subcarrier multiplexing (SCM) tones. Each light unit transmission is multiplexed with a unique tone. At the receiver, a SCM tone decision system is utilised to measure the power level of each SCM tone and consequently associate each pixel with a light unit. We proposed a high data rate single user VLC system based on wavelength division multiplexing (WDM) in this thesis. An imaging diversity receiver (IMDR) is used as an optical receiver. Based on the location of the IMDR, each colour of the RYGB LDs sends a different data stream at a different rate where the variable data rates are attributed mainly to the different power levels assigned to the colours to yield the desired white colour. Each pixel of the IMDR is covered by a specific colour optical filter. WDM in conjunction with the SCM tones are used to realise a high data rate multi-user VLC system in this thesis. The SCM tones are used to allocate an optimum transmitter to each user and to calculate the co-channel interference (CCI). Two novel optical receivers are used to evaluate the performance of the VLC systems: an array of non-imaging receivers (NI-R) and an array of non-imaging angle diversity receivers (NI-ADR). This thesis proposes a multi-branch transmitter (MBT) as a solution that can improve the VLC system performance over an indoor channel and support multi-user operation. The MBT has many transmitter branches (TBs) where each branch is directed to a specific area. By reducing the semi angle of each TB, the effect of multipath propagations is reduced and the received optical power is improved. The performance of the MBT is evaluated with a single user VLC system using a wide field of view (W-FOV) receiver and then with an angle diversity receiver (ADR). The results show that this system can provide a data rate of 4 Gb/s and 10 Gb/s when using wide FOV receiver and ADR, respectively. In addition, the performance of the MBT is evaluated in a multi-user scenario. We used the MBT with WDM and SCM tones to realise a high data rate multiuser indoor VLC system. Four colour laser diodes (RYGB LDs) are used as sources of illumination and data communication. One colour of these four colours is used to convey the SCM tones at the beginning of the connection to set up the connection. When the connection is set up, the data is transmitted in parallel through the RYGB LDs

Interference Mitigation in Wireless Communications

The primary objective of this thesis is to design advanced interference resilient schemes for asynchronous slow frequency hopping wireless personal area networks (FH-WPAN) and time division multiple access (TDMA) cellular systems in interference dominant environments. We also propose an interference-resilient power allocation method for multiple-input-multiple-output (MIMO) systems. For asynchronous FH-WPANs in the presence of frequent packet collisions, we propose a single antenna interference canceling dual decision feedback (IC-DDF) receiver based on joint maximum likelihood (ML) detection and recursive least squares (RLS) channel estimation. For the system level performance evaluation, we propose a novel geometric method that combines bit error rate (BER) and the spatial distribution of the traffic load of CCI for the computation of packet error rate (PER). We also derived the probabilities of packet collision in multiple asynchronous FH-WPANs with uniform and nonuniform traffic patterns. For the design of TDMA receivers resilient to CCI in frequency selective channels, we propose a soft output joint detection interference rejection combining delayed decision feedback sequence estimation (JD IRC-DDFSE) scheme. In the proposed scheme, IRC suppresses the CCI, while DDFSE equalizes ISI with reduced complexity. Also, the soft outputs are generated from IRC-DDFSE decision metric to improve the performance of iterative or non-iterative type soft-input outer code decoders. For the design of interference resilient power allocation scheme in MIMO systems, we investigate an adaptive power allocation method using subset antenna transmission (SAT) techniques. Motivated by the observation of capacity imbalance among the multiple parallel sub-channels, the SAT method achieves high spectral efficiency by allocating power on a selected transmit antenna subset. For 4 x 4 V-BLAST MIMO systems, the proposed scheme with SAT showed analogous results. Adaptive modulation schemes combined with the proposed method increase the capacity gains. From a feasibility viewpoint, the proposed method is a practical solution to CCI-limited MIMO systems since it does not require the channel state information (CSI) of CCI.Ph.D.Committee Chair: Professor Gordon L. StBe

Optically Switched Quantum Key Distribution Network

Encrypted data transmission is becoming increasingly more important as information security is vital to modern communication networks. Quantum Key Distribution (QKD) is a promising method based on the quantum properties of light to generate and distribute unconditionally secure keys for use in classical data encryption. Significant progress has been achieved in the performance of QKD point-to-point transmission over a fibre link between two users. The transmission distance has exceeded several hundred kilometres of optical fibre in recent years, and the secure bit rate achievable has reached megabits per second, making QKD applicable for metro networks. To realize quantum encrypted data transmission over metro networks, quantum keys need to be regularly distributed and shared between multiple end users. Optical switching has been shown to be a promising technique for cost-effective QKD networking, enabling the dynamic reconfiguration of transmission paths with low insertion loss. In this thesis, the performance of optically switched multi-user QKD systems are studied using a mathematical model in terms of transmission distance and secure key rates. The crosstalk and loss limitations are first investigated theoretically and then experimentally. The experiment and simulation both show that negligible system penalties are observed with crosstalk of -20 dB or below. A practical quantum-safe metro network solution is then reported, integrating optically-switched QKD systems with high speed reconfigurability to protect classical network traffic. Quantum signals are routed by rapid optical switches between any two endpoints or network nodes via reconfigurable connections. Proof-of-concept experiments with commercial QKD systems are conducted. Secure keys are continuously shared between virtualised Alice-Bob pairs over effective transmission distances of 30 km, 31.7 km, 33.1 km and 44.6 km. The quantum bit error rates (QBER) for the four paths are proportional to the channel losses with values between 2.6% and 4.1%. Optimising the reconciliation and clock distribution architecture is predicted to result in an estimated maximum system reconfiguration time of 20 s, far shorter than previously demonstrated. In addition, Continuous Variable (CV) QKD has attracted much research interest in recent years, due to its compatibility with standard telecommunication techniques and relatively low cost in practical implementation. A wide band balanced homodyne detection system built from modified off-the-shelf components is experimentally demonstrated. Practical limits and benefits for high speed CVQKD key transmission are demonstrated based on an analysis of noise performance. The feasibility of an optically switched CV-QKD is also experimentally demonstrated using two virtualised Alice-Bob pairs for the first time. This work represents significant advances towards the deployment of CVQKD in a practical quantum-safe metro network. A method of using the classical equalization technique for Inter-symbol-interference mitigation in CVQKD detection is also presented and investigated. This will encourage further research to explore the applications of classical communication tools in quantum communications