Browsing by Subject "MIMO systems"
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Item Antenna and algorithm design in MIMO communication systems: exploiting the spatial selectivity of wireless channels(2006) Forenza, Antonio; Heath, Robert W., Ph. D.,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 Feedback methods for multiple-input multiple-output wireless systems(2004) Love, David James; Heath, Robert W.Item Grassmann quantization for precoded MIMO systems(2006) Mondal, Bishwarup; Heath, Robert W., Ph. D.It is projected that future mobile cellular networks will carry traffic that is Internet intensive and capacity hungry. A bottleneck in providing such capacity is the limited availability of spectrum and power along with the random fluctuations in the propagation medium. Using antenna arrays at the transmitter and at the receiver and creating a multiple-input multiple-output (MIMO) wireless channel for data transmission has emerged as a candidate for improving the performance of wireless networks. The wireless propagation medium for a signal transmitted from an antenna array may be modelled as a matrix, called the channel matrix. The knowledge of the channel matrix may be used at the transmitter to signifi- cantly improve system performance. Unfortunately, in many wireless systems, the transmitter may not have access to this channel knowledge and will rely on feedback of quantized channel information from the receiver. This feedback consumes a part of the capacity available for data transmission from the receiver, thereby assigning a cost to the system performance. The objective of this dissertation is to analytically quantify the system performance as a function of this feedback cost. This dissertation formulates the problem of quantization of channel information in a non-Euclidean space called the complex Grassmann manifold. This formulation is novel and traditional signal processing tools and techniques do not extend naturally to the Grassmann manifold since it is not a vector space. The fidelity of channel information is then characterized as a function of the number of quantization levels. Using these results, the achievable signal-to-noise ratio and the outage probability of a MIMO beamforming system are expressed in terms of the feedback rate. In the general case of a precoded spatial multiplexing system or a space-time block coded system, the received signal strength is quantified as a function of the feedback rate. The bounds and approximations derived herein are validated to be tight under practical circumstances using simulation results, thus providing a performance benchmark. A sufficient condition is derived that will guarantee no loss in the diversity performance of precoded MIMO systems due to quantization of channel information.Item MIMO networking with imperfect channel state information(2008-05) Huang, Kaibin; Andrews, Jeffrey G.; Heath, Robert W., Ph. D.The shortage of radio spectrum has become the bottleneck of achieving broadband wire-less access. Overcoming this bottleneck in next-generation wireless networks hinges on successful implementation of multiple-input-multiple-output (MIMO) technologies, which use antenna arrays rather than additional bandwidth for multiplying data rates. The most efficient MIMO techniques require channel state information (CSI). In practice, such information is usually inaccurate due to overhead constraints on CSI acquisition as well as mobility and delay. CSI inaccuracy can potentially reduce the performance gains provided by MIMO. This dissertation investigates the impact of CSI inaccuracy on the performance of increasing complex MIMO networks, starting with a point-to-point link, continuing to a multiuser MIMO system, and ending at a mobile ad hoc network. Furthermore, this dissertation contributes algorithms for efficient CSI acquisition, and its integration with beamforming and scheduling in multiuser MIMO, and with interference cancelation in ad hoc networks. First, this dissertation presents a design of a finite-rate CSI feedback link for point-to-point beamforming over a temporally correlated channel. We address various important design issues omitted in prior work, including the feedback delay, protocol, bit rate, and compression in time. System parameters such as the feedback bit rate are derived as functions of channel coherence time based on Markov chain theory. In particular, the capacity gain due to beamforming is proved to decrease with feedback delay at least at an exponential rate, which depends on channel coherence time. This work provides an efficient way of implementing beamforming in practice for increasing transmission range and throughput. Second, several algorithms for multiuser MIMO systems are proposed, including CSI quantization, joint beamforming and scheduling, and distributed feedback scheduling. These algorithms enable spatial multiple access and multiuser diversity in a cellular system under the practical constraint of finite-rate multiuser CSI feedback. Moreover, this dissertation shows analytically that the throughput of the MIMO uplink and downlink using the proposed algorithms scales optimally as the number of users increases. Finally, the transmission capacity of a MIMO ad hoc network is analyzed for the case where spatial interference cancelation is applied at receivers. Most important, this dissertation shows that this MIMO technique contributes significant network capacity gains even if the required CSI is inaccurate. In addition, opportunistic CSI estimation is shown to provide a tradeoff between channel training overhead and CSI accuracy.Item Multiple antenna communications in an interference-limited environment(2006) Choi, Wan; Andrews, Jeffrey G.Item Multiple antenna downlink: feedback reduction, interference suppression and relay transmission(2006) Tang, Taiwen; Heath, Robert W., Jr.Item Multiuser MIMO systems in single-cell and multi-cell wireless communication(2007) Chen, Runhua; Andrews, Jeffrey G.; Heath, Robert W., Ph. D.MIMO technology improves the capacity and link robustness of wireless communication by deploying multiple transmit and receive antennas. A multiuser MIMO communication system involves multiple mobile stations (MS) and potentially multiple base transceiver stations (BTS). These systems are fundamentally limited by interference, and require new treatment of both the capacity characteristics and physical layer algorithm design. In this dissertation, multiuser MIMO systems in both single-cell and multi-cell environments are studied. A single-cell MIMO broadcast channel is defined by a central BTS transmitting to multiple MSs simultaneously over the same spectrum. A multi-cell MIMO system consists of multiple BTSs transmitting to MSs in different cells. For a single-cell MIMO broadcast channel, block diagonalization is a transmit precoding technique that multiplexes multiple users in the spatial domain and pre-cancels inter-user interference. Precoder can be adaptively designed based on the size of transmit/receive antenna arrays and the number of users. In the case where the BTS has more antennas or radio frequency (RF) units than strictly required for interference cancellation, this dissertation proposes novel downlink precoder with enhanced transmit selection diversity. Eigenmode selection and transmit antenna selection are derived to optimize a symbol error rate upper bound and improve the diversity performance. When there are a large number of users in the system, a subset of users and receive antennas may be selected to maximize the sum capacity under the block diagonalization signaling. The optimum joint user and antenna selection involves brute-force search, therefore is prohibitively complicated. In this dissertation, two low-complexity sub-optimal selection algorithms are proposed to significantly reduce the selection complexity. Conventional single-user MIMO techniques suffer significant performance loss in an interference-limited multi-cell network. Interference on a MIMO system is more severe than in a single-antenna cellular network, as each antenna element acts as a unique interferer. In this dissertation, power control is investigated as an interference management tool to properly determine the transmit power of MIMO array under a pre-determined SNR constraint. Two uplink MIMO power control techniques are proposed. The first equal allocation algorithm enforces each antenna element of a MIMO array to transmit at the same power, resulting in a closed-form but suboptimal solution. The second algorithm adaptively distributes power on a MIMO antenna array to exploit the channel selectivity, hence substantially reduces the transmit power and interference, and creates far better cell coverage. Finally, block diagonalization precoding in the single-cell scenario is generalized to the multi-cell environment as a coordinated MIMO transmission technique. Multiple BTSs cooperate with each other to design the downlink signal, thereby eliminating interference and improving the spectral efficiency. An improved precoding scheme is proposed to address the per base station power constraint in the cellular environment. Future research topics for cellular block diagonalization precoding are discussed.Item Multiuser MIMO systems in single-cell and multi-cell wireless communication(2007-05) Chen, Runhua; Heath, Robert W., Ph. D.; Andrews, Jeffrey G.Item Rate-robustness tradeoffs in multicarrier wireless communications(2006) Kim, Tae Yoon; Andrews, Jeffrey G.Emerging wireless communication systems exploit various resources to increase their robustness and data rate. Since these resources are limited, there is a tradeoff between the need for robust communication and the desire for high throughput. The aim of this dissertation is to study and optimally balance this tradeoff for a few important cases in multicarrier communications. First, multi-code code division multiple access (CDMA) techniques tradeoff the number of supportable subscribers with the per subscriber data rate. However, the interference scales linearly with the data rate of each user since they use multiple codes. To resolve this interference problem, a novel multi-code multicarrier CDMA system is proposed, and this system clearly outperforms previous systems in terms of bit error probability and user capacity. This shows that flexible data rates can be successfully balanced with robustness in a multiuser multi-rate CDMA system by carefully choosing the data rates number of subcarriers. Second, in multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM), pilots are used to estimate the channel, but in addition to consuming bandwidth, they reduce the transmitted energy for data symbols under a fixed transmit power constraint. This suggests a tradeoff between the power allowed to data symbols and the accuracy of channel estimation. The optimal pilot-to-data power ratio (PDPR) for maximizing a capacity lower bound is formulated and derived for four likely pilot patterns and two different channel conditions. The optimal PDPR shows about 10%∼30% higher capacity lower bound than equal power allocation. Third, and closely related to the second contribution, adaptive M-QAM, spectral efficiency, and symbol error rate (SER) are considered since these are respectively the dominant modulation type and quality metrics in emerging standards. The effect of the system structure on the PDPR is analytically shown, and the optimal PDPR for minimizing the average SER and maximizing the spectral efficiency is derived for two well-known linear receivers; zero-forcing and minimum meansquare error. The distributions of the SNR (including channel estimation error) for these receivers are derived and used to find the optimal PDPR. Exact guidelines are provided for the power allocation between data and pilot symbols for these cases.