Intellectual Merit of the Proposed Activity. The proposed research is creative and original because it addresses two important challenges in wireless communications, namely energy eciency and channel uncertainty, in combination and oers to establish the important tradeos when energy eciency is the ultimate goal. The expected outcomes of the proposed research and educational activities include an understanding of the impact upon the ultimate performance limits, such as channel capacity, of operating under channel uncertainty; proposals of novel energy ecient signaling schemes using the fundamental bandwidth-power tradeo as a benchmark; an understanding of the eect of cooperation and cross-layer designs on the energy-eciency; creation of excitement in the discipline among undergraduate and graduate students using the attractive wireless communication ideas and using this excitement to have the students participate in the advancement of knowledge by actively engaging in research.
Broader Impacts. Integration of this research project with education will be accomplished through revised and newly introduced courses, involving undergraduates in the PI's research program, and the secondary school outreach. The participation of underrepresented groups will be achieved by the PI's eorts to recruit female undergraduate and graduate students, and activities in the Nebraska MESA Program, which target Nebraska's minority and underrepresented youth. Broad dissemination of results will be accomplished by publishing the results of this research project in high quality, peer-reviewed journals and international conferences and creating a web site for the PI's research group. It is ex- pected that the outcomes of the research will improve wireless systems to achieve ecient and robust performance even under severe physical impediments, leading to the realization of the wireless vision of ubiquitous communications. Especially the energy eciency analysis is expected to have an impact upon the design of next generation wireless networks, including sensor networks that have various ap- plication areas in health, the military, and the home. Along with these research outcomes, society will also benefit from the outreach programs proposed as part of the educational plan of the project.
In this project, we have worked on the ultimate performance limits and energy efficiency of wireless communications. The contributions are listed below: 1) Capacity and Energy Efficiency of Training-Based Transmissions over Fading Channels In this project, we have characterized the capacity-achieving input structure when the input is subject to peak power limitations. Subsequently, we have obtained the capacity and identified the optimal resource allocation strategies. We have also characterized the optimal energy-bandwidth tradeoff in imperfectly-known fading channels where the channel is estimated using pilot symbols and input is subject to peak power limitations. 2) To Cooperate or Not To Cooperate in Imperfectly-Known Fading Channels We have analyzed the performance of cooperative communications in imperfectly-known fading relay channels. We have considered network training schemes. We have obtained achievable rate expressions and identified optimal resource allocation strategies. 3) Error Rate Analysis of Peaky Signaling in Fading Channels We have analyzed the error performance of signaling techniques with high peak-to-average power ratio. In particular, we have considered on-off phase-shift keying and on-off frequency-shift keying modulations. We have shown that increasing the peakedness of the signals results in lower error probabilities at fixed bit energy values and hence equivalently improves the energy efficiency for fixed error probabilities. 4) Energy Efficiency of Orthogonal Signaling We have analyzed the non-asymptotic energy efficiency of orthogonal signaling when the receiver quantizes the received signal through hard-decision detection. We have shown that bit energy requirements grow without bound as SNR vanishes. 5) Energy-Aware and Secure Wireless Communications We have considered the energy efficiency and security of wireless transmissions jointly. We have studied a scenario in which a wireless transmitter intends to send confidential messages to a receiver in the presence of an eavesdropper, and at the same time operate energy efficiently. We have identified low-SNR secrecy capacity and determined the minimum bit energy required for both reliable and secure wireless communication. We have described the transmission strategies that achieve the minimum bit energy. 6) Collaborative Relay Beamforming for Secrecy We have investigated the collaborative use of relays to form a beamforming system and provide physical-layer security. In particular, we studied decode-and-forward and amplify-and-forward relay beamforming designs under total and individual relay power constraints with the goal of maximizing the secrecy rates when perfect channel state information is available. The beamforming design under individual relay power constraints is formulated as an optimization problem which is shown to be easily solved using two different approaches, namely semidefinite programming and second-order cone programming. 7) Energy Efficiency in the Presence of Quality of Service (QoS) Constraints We have combined tools from both information theory and queueing theory to identify the fundamental limits on the energy efficiency when the wireless system is operating under quality of service (QoS) constraints. Fundamental limits on the energy efficiency are identified by obtaining the minimum bit energy required for reliable communications under QoS constraints. Overall, the impact upon the energy efficiency of QoS constraints, channel knowledge, and various transmission strategies are quantified. 8) Performance Analysis of Cognitive Radio Transmissions under QoS Constraints We have studied the performance of cognitive radios under QoS limitations. We have formulated the channel sensing performed by the cognitive radios as a hypothesis testing problem. We have determined the correct detection and false-alarm probabilities. We have constructed a state-transition model for cognitive transmission, in which the states depend on activities of primary users and the results of channel sensing. We have characterized the maximum throughput of cognitive transmissions in the presence of queueing limitations by finding the effective capacity of the state-transition model. We have determined the impact of QoS constraints, sensing duration and threshold, correct-detection and false-alarm probabilities on the performance. 9) MIMO Wireless Communications under Statistical Queueing Constraints We have investigated the performance of multiple-input multiple-output wireless systems in the presence of statistical queueing constraints. Queuing constraints are imposed as limitations on buffer violation probabilities. A detailed analysis of the effective capacity is carried out in the low-power, wideband, and high--signal-to-noise ratio (SNR) regimes. In the low-power analysis, expressions for the first and second derivatives of the effective capacity with respect to SNR at SNR= 0 are obtained under various assumptions on the degree of channel state information at the transmitter. 10) Energy Efficiency Analysis in Wireless Networks We have studied the energy efficiency of cooperative networks operating in either the fixed Amplify-and-Forward (AF) or the selective Decode-and-Forward (DF) mode. We have considered the optimization of the M-ary quadrature amplitude modulation (MQAM) constellation size to minimize the bit energy consumption under given bit error rate (BER) constraints. In the computation of the energy expenditure, the circuit, transmission, and retransmission energies are taken into account. It is seen that while large constellations are preferred at small transmission distances, constellation size should be decreased as the distance increases; the cooperative gain is computed to compare direct transmission and cooperative transmission. A new protocol termed CoopXLM that integrates cooperative communication and Cross-Layer Module (XLM) is created and examined via simulation. CoopXLM modifies XLM by allowing multiple cooperative nodes to participate in receiver-based contention. Although there is increased energy consumption for processing, simulation results indicate that across all duty cycles there is an average energy savings of 38% with CoopXLM in comparison to XLM.
