Browsing by Subject "Broadband communication systems"
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Item Adaptive antenna systems for mobile broadband communications(2002) Dong, Liang; Ling, Hao; Xu, GuanghanMobile broadband communication is experiencing rapid growth in technology, markets and range of services. This rapid growth has driven the recent surge of research and development activities for high-data-rate/high-mobility wireless systems, with improved network performance and enhanced economics. One technology thrust in wireless communications is the use of adaptive antennas at the transceivers, along with the associated advanced array signal processing, to improve cell coverage, link quality and system capacity. This dissertation first provides a framework of adaptive antenna systems for wireless communications, and characterizes the multipath fading channels of mobile broadband systems. In particular, we consider antenna systems for the base station, as well as multiple-input multiple-output systems where antenna systems are utilized at both the base station and the mobile unit. It is proved that the channel fading can be modeled and predicted using linear models of low order. The correlation of fading at multiple antennas or over the wideband is exploited to perfect channel modeling and prediction. Secondly, this dissertation develops the theory of adaptive antenna arrays with applications to mobile broadband systems. Through analysis of the propagation pattern and the channel structure, new techniques of uplink power control and downlink beamforming are derived to adapt to the rapid variation of the vector channel. The low variability of the channel subspace and the negligible distance between uplink and downlink channel subspaces are exploited to enhance the performance of adaptive transmission. Constraints are put on the model structure, which leads to a reduction of computational complexity of the channel estimation. Finally, this dissertation describes the correlation of sub-channels embedded in the multiple-input multiple-output antenna systems, and discusses its effect on the channel capacity. Multiple antennas with dissimilar radiation patterns are employed to introduce decorrelation of the sub-channels, thus increasing channel capacity. Specifically, a prototype of compact antenna array at the mobile unit is proposed that exploits antenna pattern diversity. In summary, this dissertation investigates the modeling and prediction of the time-varying multipath channels of antenna systems, while developing new techniques for mobile broadband communications that are based on the channel characterization. The general feasibility of the algorithms developed in this dissertation is demonstrated through a ray tracing simulator in various scattering environments.Item Architecture and implementation of intelligent transceivers for ultra-wideband communications(2008-05) Hsieh, Tien-ling, 1975-; Gharpurey, RanjitThe wide bandwidth employed in the UWB system allows for high data-rate communications, while its broadband nature requires it to coexist with other systems. For instance, several communication systems, such as digital TV, wireless LANs, WiMAX, and satellite receivers, utilize spectrum that is in the UWB band. According to Federal Communications Commission (FCC) regulations, the power spectral density (PSD) of UWB devices for communication applications is limited to less than -41.25dBm/MHz in the 3.1-10.6GHz frequency band, to minimize the impact of UWB on other systems. The impact of narrowband signals on UWB systems can also be significant, even though these signals may occupy a small part of the UWB spectrum, due to their much larger power. The performance and capacity of UWB systems can be significantly degraded by these narrowband interferers. In-band interference can be tolerated by increasing the dynamic-range of the receiver such that the interferers are accommodated within the linear range of the receiver. Alternatively, if the interferers can be avoided altogether, the excessive linearity requirements imposed by the interferers can be relaxed. Such an avoidance mechanism requires the ability to detect interferers. This work presents a low-power and low-cost detector for this purpose that can be employed in multi-band approaches to UWB, including pulse-based schemes, and those employing OFDM. The UWB band is divided into narrower sub-bands in these schemes. During transmission, the carrier hops to a new sub-band every symbol. The detector is designed to provide a profile of interference over the entire UWB spectrum, during each symbol period. This information would be available to the main-path UWB receiver to decide a frequency sequence of sub-band hopping, in order to avoid sub-bands occupied by large interferers. This relaxes the dynamic-range requirement, and hence the power dissipation of the main-path receiver, thus compensating for the extra power dissipation of the detector. The detector is based on a cascade of image-reject downconverter stages. An implementation of the architecture is demonstrated in a 0.13[mu]m CMOS process.Item Energy efficient transmission in wireless communication networks(2008-08) Lee, Chulhan; Vishwanath, SriramIn this dissertation, we study energy efficient transmission in wireless communication networks. The general problem of energy efficient transmission over wireless networks is formulated into optimization problems for the following distinct (but inter-related) settings: Problem Setting 1: The minimization of energy (power) consumption given a system throughput and other constraints, and Problem Setting 2: The maximization of system throughput given energy (power) constraints. Under Problem Setting 1, we focus on energy efficient transmission problems over wideband channels. The first result we obtain is as follows: We consider a two user multiple access channel. In this multiple access channel, previous research shows that cooperation with respect to block error rate is only possible if two transmitters share their sources completely. However, we find that a modified pulse position modulation with synchronization enables cooperation without complete sharing of their sources between two transmitters if we replace a block error rate requirement with a normalized error rate constraint. Normalized error rate, a quantity that resembles bit error rate, is developed in this work as an error metric that is of value in practical communication systems. We show full cooperation between two transmitters without sharing their sources by deriving that the minimum energy per bit required for reliable transmission reduces by quarter compared with the minimum energy per bit required for point-to-point channels. Next, we generalize this analysis to a cognitive communication framework with a wideband cognitive transmitter, which can causally sense signal levels over multiple frequency bands, and a cognitive receiver. We assume that multiple legitimate users already exist in the system and each one transmits in its own non-overlapping frequency band. In this setting, from order statistical analysis, we show that the wideband cognitive transmit-receive pair is able to communicate reliably with minimum energy as if the legitimate users were absent from the system, while causing negligible interference to bandlimited legitimate users. The wideband cognitive transmit-receive pair employs a strategy defined as opportunistic group orthogonal signaling to achieve the minimum energy per bit. Under Problem Setting 2, we investigate the impact of correlation and transmit and receive strategies on the throughput of multiple antenna broadcast channels in cellular networks. With perfect channel state information at the transmitter, it is well known that dirty paper coding (DPC) is the optimal multi-user broadcast transmission method. However, with partial channel state information at the transmitter, the picture changes significantly. Specifically, since multi-user transmission is unable to employ DPC perfectly, singleuser transmission strategies can have a better performance than multi-user transmission strategies when we have a small number of users and correlated antenna gains. We explore the trade-offs between the single-user and multiuser MIMO transmission strategies. Order statistical analysis provides us with both analytical expressions and insights about these trade-offs. We verify that the analytical framework that we develop is accurate by checking the values obtained against numerical results. From this analysis, we confirm that 'mode switching' between single-user and multi-user MIMO transmission schemes is necessary for maximizing throughput for emerging MIMO solutions. Finally, we suggest an adaptive mode switching algorithm between single-user and multi-user MIMO transmission strategies based on this analytical framework.Item An experimental investigation of wideband MIMO channels for wireless communications(2006) Yang, Yaoqing; Ling, Hao; Xu, GuanghanItem Wideband direction finding via shielded gradient beamspace techniques(2003-05) Brudner, Terry J.; Powers, Edward J.; Henderson, Terry LeeMonopulse techniques have been used for over 50 years in the radar community to estimate the direction of arrival (DOA) of incoming echoes. In recent years, a variant of the monopulse technique has been developed, termed the shielded gradient technique which allows DOA estimation for signals of arbitrary bandwidth. In its simplest from, the technique involves the formation of two beam outputs which may be combined to produce a single DOA estimate. The technique may be extended to generate multiple beam outputs, any pair of which may be used for DOA estimation. These multiple beam outputs may also be combined using a model similar to the signal subspace model underlying the narrowband MUSIC algorithm, in order to estimate the DOA of multiple echoes arriving simultaneously. The work presented here describes the shielded gradient beamspace model, and develops wideband DOA estimation algorithms analogous to the narrowband MUSIC, root-MUSIC, and ESPRIT algorithms. The performance of these newalgorithms is studied through simulation and application to measured sonar data, and they are compared to existing wideband DOA estimation algorithms.