Summary This thesis is concerned with the study of space-time and space-frequency block-coded orthogonal frequency division multiplexing(OFDM)transmitter diversity techniques for combating the detrimental effects of frequency-selective fading channels. first, by combining the space-time block coding and OFDM modulation tech niques, the space-time block-coded OFDM (STBC-OFDM) transmitter diversity sys- tem is proposed. It has been shown that the STBC-OFDM system is capable of achieving nearly optimum spatial diversity performance in slow fading frequency selective channels. By applying space-time block codes across the subcarriers of OFDM, the space-frequency block-coded OFDM(SFBC-OFDM) system is devel oped. Simulation results have shown that SFBC-OFDM can achieve the same diver sity performance as STBC-OFDM in slow fading frequency-selective channels, but SFBC-OFDM significantly outperforms STBC-OFDM in fast fading channels Both the STBC-OFDM and SFBC-OFDM systems require the use of a sufficiently long cyclic prefix, which can cause a significant reduction in bandwidth efficiency. To improve the bandwidth efficiency, the iterative STBC-OFDM (ISTBC-OFDM) and iterative SFBC-OFDM(ISFBC-OFDM) systems, which do not require a cyclic prefix, have been developed. The proposed ISTBC-OFDM and ISFBC-OFDM algorithms are the only known algorithms applicable to OFDM transmitter diversity systems without a cyclic prefix Finally, channel parameters are required for diversity combining and decoding in
the proposed OFDM transmitter diversity systems. Multirate pilot-symbol-assisted (PSA)channel estimators have been developed for the STBC-OFDM, SFBC-OFDM ISTBC-OFDM, and ISFBC-OFDM systems. The proposed PSA channel estimators have been shown to have good performance while requiring a relatively low compu tational load when compared to existing channel estimators
Chapter 1 Introduction The last decade has witnessed an explosive growth of wireless communications, espe- cially in mobile communications and personal communications services(PCS). The tremendous growth of the wireless consumer market has in large part been driven by advances in digital communications, digital signal processing(DSP), and integrated circuit(Ic)technologies. New modulation and multiple access schemes and improved source and channel coding techniques allow for more reliable and spectrally eficient wireless services, which in turn lower the average cost of usage. a key factor in this growth is that the density of large scale integrated circuits, and similarly the computa- tional power per unit cost, continues to follow the amazing rate modelled by Moore's Law, i.e., doubling every 18 months [1, 2. This exponential decrease in the cost of computations enables implementation of the powerful signal processing algorithms required by the new modulation, multiple access, and coding schemes, while still keeping the equipment cost at an affordable level for the general consumer market. It is anticipated that advances in digital communications, DSP, and ic technologies will continue to be the driving forces behind future growth in wireless communications With the continuing expansion in both existing and new markets, and the in- troduction of exciting new services such as wireless internet access and multimedia applications, the rapid growth of wireless communications is expected to continue 1
well into the next decade The large wireless communications user base and the eve increasing demand for faster and more reliable services to support new applications have created a strong interest in developing high data rate wireless communications systems. With existing and emerging wireless applications competing for a limited radio spectrum, for this new generation of high data rate wireless communications systems, the development of techniques that are spectrally efficient is especially im- ortant The main challenge in developing reliable high data rate mobile communications systems is to overcome the detrimental effects of the unforgiving mobile communi- cations channeL. Unlike the well-behaved additive white Gaussian noise (AWGn) channel, the mobile communications channel is subject to multipath where the trans. mitted signal arrives at the receiver via several propagation paths. the two dominant impairments caused by multipath propagation are multipath fading and frequency se lectivity in the channel response, i.e., frequency-selective fading Multipath fading is caused by the destructive combining of replicas of the infor- mation-bearing signal that have traveled over diferent propagation paths. Multipath fading can severely attenuate the instantaneous power of the received signal and ignificantly degrade the performance of mobile communications systems. The most effective means of combating multipath fading are the various diversity techniques. Of particular interest is a form of spatial diversity technique that employs multiple antennas at the transmitter and is known as transmit antenna diversity, or simply transmitter diversity. The recently proposed space-time block coding technique has been shown to be an efficient means of achieving nearly optimum spatial diversity gain in multiple transmit antenna communications systems [3-6. Space-time block-coded
transmitter diversity will be a primary focus of this the When the difference of the propagation delays among the multiple paths, known as delay spread, is a significant fraction or a multiple of the symbol duration, the im- pulse response of the channel is dispersed in time and the channel becomes frequency selective. The dispersive impulse response, or, equivalently, the frequency selectivity of the channel, causes intersymbol interference(Isi) at the detector. Because of the time-varying nature of the mobile communications channel and the long channel memory relative to the symbol rate, reducing iSI distortion in high data rate mobile communications systems is especially challenging. Because of their computational demands, techniques commonly used for combating ISI, such as maximum-likelihood sequence estimation MLSE)detectors or linear and nonlinear equalizers, are less at tractive for high data rate mobile communications systems. Although MLSE is the ptimal detection algorithm in the presence of isI [7 its computational complexity becomes prohibitive for a channel with a large number of memory states. Because of this long channel memory, equalizer complexity will also be high, and equalizer convergence time becomes a concern. On the other hand, properly implemented multicarrier modulation is known to be more resilient to isi than single carrier mod- ulation[8, 9. In particular, a discrete Fourier transform(DFT) based multicarrier modulation technique known as orthogonal frequency division multiplexing(OFDM) is both spectrally and computationally efficient. With OFDM, ISI can be eliminated easily by using a guard interval of sufficient length. Therefore, OFDM is an attractive modulation scheme for high data rate wireless communications systems This thesis is concerned with the study of transmitter diversity techniques for mitigating frequency-selective fading in mobile communication systems. In particular