Residue number system coded differential space-time-frequency coding.

Thesis (Ph.D.)-University of KwaZulu-Natal, Durban, 2007.The rapidly growing need for fast and reliable transmission over a wireless channel motivates the development of communication systems that can support high data rates at low complexity. Achieving reliable communication over a wireless channel is a challenging task largely due to the possibility of multipaths which may lead to intersymbol interference (ISI). Diversity techniques such as time, frequency and space are commonly used to combat multipath fading. Classical diversity techniques use repetition codes such that the information is replicated and transmitted over several channels that are sufficiently spaced. In fading channels, the performance across some diversity branches may be excessively attenuated, making throughput unacceptably small. In principle, more powerful coding techniques can be used to maximize the diversity order. This leads to bandwidth expansion or increased transmission power to accommodate the redundant bits. Hence there is need for coding and modulation schemes that provide low error rate performance in a bandwidth efficient manner. If diversity schemes are combined, more independent dimensions become available for information transfer. The first part of the thesis addresses achieving temporal diversity through employing error correcting coding schemes combined with interleaving. Noncoherent differential modulation does not require explicit knowledge or estimate of the channel, instead the information is encoded in the transitions. This lends itself to the possibility of turbo-like serial concatenation of a standard outer channel encoder with an inner modulation code amenable to noncoherent detection through an interleaver. An iterative approach to joint decoding and demodulation can be realized by exchanging soft information between the decoder and the demodulator. This has been shown to be effective and hold hope for approaching capacity over fast fading channels. However most of these schemes employ low rate convolutional codes as their channel encoders. In this thesis we propose the use of redundant residue number system codes. It is shown that these codes can achieve comparable performance at minimal complexity and high data rates. The second part deals with the possibility of combining several diversity dimensions into a reliable bandwidth efficient communication scheme. Orthogonal frequency division multiplexing (OFDM) has been used to combat multipaths. Combining OFDM with multiple-input multiple-output (MIMO) systems to form MIMO-OFDM not only reduces the complexity by eliminating the need for equalization but also provides large channel capacity and a high diversity potential. Space-time coded OFDM was proposed and shown to be an effective transmission technique for MIMO systems. Spacefrequency coding and space-time-frequency coding were developed out of the need to exploit the frequency diversity due to multipaths. Most of the proposed schemes in the literature maximize frequency diversity predominantly from the frequency-selective nature of the fading channel. In this thesis we propose the use of residue number system as the frequency encoder. It is shown that the proposed space-time-frequency coding scheme can maximize the diversity gains over space, time and frequency domains. The gain of MIMO-OFDM comes at the expense of increased receiver complexity. Furthermore, most of the proposed space-time-frequency coding schemes assume frequency selective block fading channels which is not an ideal assumption for broadband wireless communications. Relatively high mobility in broadband wireless communications systems may result in high Doppler frequency, hence time-selective (rapid) fading. Rapidly changing channel characteristics impedes the channel estimation process and may result in incorrect estimates of the channel coefficients. The last part of the thesis deals with the performance of differential space-time-frequency coding in fast fading channels

Design and analysis of wireless diversity system

Ph.DDOCTOR OF PHILOSOPH

Coherent and Non-coherent Techniques for Cooperative Communications

Future wireless network may consist of a cluster of low-complexity battery-powered nodes or mobile stations (MS). Information is propagated from one location in the network to another by cooperation and relaying. Due to the channel fading or node failure, one or more relaying links could become unreliable during multiple-hop relaying. Inspired by conventional multiple-input multiple-output (MIMO) techniques exploiting multiple co-located transmit antennas to introduce temporal and spatial diversity, the error performance and robustness against channel fading of a multiple-hop cooperative network could be significantly improved by creating a virtual antenna array (VAA) with various distributed MIMO techniques. In this thesis, we concentrate on the low-complexity distributed MIMO designed for both coherent and non-coherent diversity signal reception at the destination node. Further improvement on the network throughput as well as spectral efficiency could be achieved by extending the concept of unidirectional relaying to bidirectional cooperative communication. Physical-layer network coding (PLNC) assisted distributed space-time block coding (STBC) scheme as well as non-coherent PLNC aided distributed differential STBC system are proposed. It is confirmed by the theoretical analysis that both approaches have the potential for offering full spatial diversity gain. 　　 Furthermore, differential encoding and non-coherent detection techniques are generally associated with performance degradation due to the doubled noise variance. More importantly, conventional differential schemes suffer from the incapability of recovering the source information in time-varying channels owing to the assumption of static channel model used in the derivation of non-coherent detection algorithm. Several low-complexity solutions are proposed and studied in this thesis, which are able to compensate the performance loss caused by non-coherent detection and guarantee the reliable recovery of information in applications with high mobility. A substantial amount of iteration gain is achieved by combining the differential encoding with error-correction code and sufficient interleaving, which allows iterative possessing at the receiver

Polarization-dependent nonlinear effects in coherent detection systems

In the last decades the demand for data capacity has increased exponentially. Optical Coherent Detection, firstly proposed at the end of the 1980s to improve receiver sensitivity, has proved as one of the most powerful techniques to increase the optical communication spectral efficiency and so the total per channel capacity. Indeed, thanks to the recent advances in digital signal processing (DSP) and high speed electronics, the DSP-based coherent detection in optical networks expedited the use of polarization division multiplexing (PDM) as a cost-effective way of doubling system capacity. Furthermore, coherent detection presents many others advantages with respect to direct detection such as the use of multilevel optical modulation formats like N-PSK and N-QAM and compensating linear propagation effects in the electrical domain as chromatic dispersion, polarization mode dispersion (PMD) and optical filtering. On the other hand, transmission reach of WDM systems is a major concern for the deployment of such a solution and is usually mainly limited by cross-nonlinear effects. In WDM transmission systems, the cross-nonlinearities make neighboring channels interact depending on their power and state of polarization (SOP). The last is of particular concern in PDM systems since they are more sensitive to a new kind of distortion that has been generally referred to as cross-polarization modulation (XPolM) as a way to distinguish it from the well known cross-phase modulation (XPM). At the beginning of our research activity in 2009, despite the growing interest and the number of publications on XPolM, many of its features were still unknown. For example, in Sept. 2009 Winter et al. provided a model that successfully measured the degree of polarization degradation in presence of XPolM, but it was still not clear when the bit error rate (BER) is dominated by XPolM and how XPolM relates to the other relevant nonlinear effects, such as XPM and self-phase modulation (SPM). With the investigations presented in this thesis we want to fill the gap, by providing a systematic simulation study of system performance where each nonlinear effect acts individually. Furthermore, thanks to the possibility in Optilux software to take into account separately the nonlinear terms of the propagation equation, we add some new piece of knowledge about XPolM. We quantify the XPolM-induced penalty as a function of transmission parameters such as the channel power, spacing and state of polarization (SOP). We also clarify the role of the Viterbi and Viterbi-based carrier phase estimator in mitigation of XPM and XPolM. We focused our investigation mainly on PDM-quadrature phase shift keying (QPSK) modulation format. The thesis is organized as follows. In the first chapter the principal impairments for long haul transmissions are briefly recalled. They are divided into linear and nonlinear effects, according to whether they are independent of the signal power or not. The first group is composed of fiber attenuation, chromatic dispersion and polarization mode dispersion. The second group is composed of nonlinear polarization-independent effects: such as SPM and XPM. Other linear effects such as polarization dependent loss and nonlinear effects as intra channel cross phase modulation, four wave mixing, nonlinear phase noise and non elastic scattering effects (stimulated Raman and Brillouin scattering) are not included in our discussion, while the XPolM is discussed at length in Ch. 3. The second chapter discusses the joint use of PDM and the coherent detection, as a solution to increase the transmission capacity. We also discuss a new technique, namely mode division multiplexing (MDM), to further increase the transmission capacity thanks to the joint use with PDM and coherent detection. In Ch.3, after the definition of the XPolM term in the propagation equation, we show the polarization rotation and the PDM-QPSK constellation distortion induced by XPolM as a function of the rotation axis orientation. We perform such analysis both mathematically and by simulation. In Ch. 4 we show when the bit error rate (BER) of a PDM-QPSK channel is dominated by XPolM, through a massive use of simulation in a wide range of system setups. We analyze different pulse shapes, transmission links and transmission fibers in both hybrid (PDM-QPSK -- OOK) and homogeneous systems (PDM-QPSK). Furthermore we clarify the role of channel power, spacing, state of polarization (SOP) and Viterbi and Viterbi-based carrier phase estimator on the XPM- and XPolM-induced penalty. In the last part of the chapter we quantify the nonlinear penalty in a PDM-BPSK transmission system, showing the average performance and its fluctuation induced by the transmission sequences and SOPs. In Ch. 5 we compare different optical methods to improve the resilience of coherent 112-Gb/s PDM-QPSK WDM transmissions against cross-channel nonlinearities. Such methods consist of increasing the line group velocity dispersion (GVD), or the line PMD, or inserting in-line XPM suppressors, which are passive devices that introduce different delays on adjacent channels at specific points of the line. In Ch. 6 we summarize the experimental results obtained during the research activity at Alcatel-Lucent Bell-Labs France on MDM. In such an activity we employ a mode converter based on a liquid-crystal on silicon (LCOS) spatial modulator and a prototype few mode fiber (FMF). Last but not least, in the Appendix we discuss three different rules to correctly simulate the cross-nonlinearities, showing also some artifacts that can arise with a non-correct setting of some numerical parameters, such as the nonlinear step of Split-Step Fourier method, the sequence length and the sequence type