[Show abstract][Hide abstract] ABSTRACT: Block transmission with cyclic prefix is a promising technique to realize high-speed data rates in frequency-selective fading channels. Many popular linear precoding schemes, including orthogonal frequency-division multiplexing (OFDM), single-carrier (SC) block transmission, and time-reversal (TR), can be interpreted as such a block transmission. This paper presents a unified performance analysis that shows how the optimal precoding strategy depends on the optimization criterion such as capacity, mean-square error, and secrecy. We analyze three variants of TR methods (based on maximum-ratio combining, equal-gain combining and selective combining) and two-types of pre-equalization methods (zero-forcing and minimum mean-square error). As one application of our framework, we derive optimal precoding (i.e., OFDM with optimal power and phase control) in the presence of interference limitation for distributed antenna systems; we find that without power/phase control, OFDM does not have any capacity advantage over SC transmissions. When comparing SC and TR, we verify that for single-antenna systems in the high SNR regimes, SC has a capacity advantage; however, TR performs better in the low SNR regime. For distributed multiple-antenna systems, TR always provides higher capacity, and the capacity of TR can approach that of optimal precoders with a large number of distributed antennas. Furthermore, we make an analysis of secrecy capacity which shows how high-rate messages can be transmitted towards an intended user without being decoded by the other users from the viewpoint of information-theoretic security. We demonstrate that TR precoding can be the best candidate among the non-optimal precoders for achieving high secrecy capacity, while the optimal precoder offers a significant gain over those non-optimal precoders.
[Show abstract][Hide abstract] ABSTRACT: Energy efficiency becomes evergreen than ever before. Over the last few years a significant research in wireless communications, aiming to enhance communications efficiency subject to constraint of power consumption, has given rise to optimization techniques of power control and management in wireless networks. Although the initial target was the utilization of resources in order to exploit communications services, the benefits of this effort, underlying the green perspective, are now opening eyes out to research community for a more green matter of conscience in use of communication equipment and devices. Towards this direction and affected by a green sensitivity in terms of wasting useful resources, we propose a system model for infrastructure WiFi networks aiming to reduce the consumed power, by introducing a scheme for channel allocation to Wireless LANs under energy conserving criteria.
[Show abstract][Hide abstract] ABSTRACT: With emerging of electric vehicles (EV), portable charging stations (PCSs) will play a key role to manage EV charging operations off-grid. Otherwise, charging hundreds of EVs at random locations and time instants would create major burden on a power supply network. This work addresses an outstanding issue in PCS networks: development of dynamic pricing strategies between buyer EVs and energy sellers to optimize deployment of PCSs. Constraints are formulated, and a realistic and yet simplistic energy incentive model is developed. A method for optimum PCS deployment to maximize profit for PCS service providers is developed for single-buyer-single-seller and multiple-buyers-single-seller cases, conditioned on a given pricing strategy.
[Show abstract][Hide abstract] ABSTRACT: In this paper, we present a routing protocol design and implementation for the Advanced Metering Infrastructure (AMI) in Smart Grid. The proposed protocol implementation is based on the framework of the IPv6 Routing Protocol for Low Power and Lossy Networks (RPL), which is proposed by IETF and currently still in its design phase. RPL is based on the idea of maintaining a directed acyclic graph (DAG) structure for the network. We provide a practical implementation of RPL with a number of proper modifications so as to fit into the AMI structure and meet stringent requirements enforced by the AMI. In particular, we propose a novel DAG rank computation method and a reverse path recording mechanism, which enables real-time automated meter reading and real-time remote utility management in the AMI. Our proposed routing protocol design for AMI networks is validated through extensive simulations.
[Show abstract][Hide abstract] ABSTRACT: As mobile stations (MSs) in the next generation cellular wireless systems will more frequently operate on multiple applications, such as web browsing, VoIP, online video etc., energy saving becomes more critical and face new challenges from the quality of service (QoS) requirements. A special operation state, called sleep mode, is designed for energy saving of MS, in which MS operates on continuous sleep cycles, where every sleep cycle is the sum of a listening window and a sleep window. This paper proposes an energy saving method that adaptively determines sleep cycles and shifts listening window. When MS is in sleep mode, the sleep cycles are extended by probabilistic decisions related to the traffic statistic attributes. Different from conventional methods, our method is also able to deal with mixed traffic pattern by shifting listening window. Extensive simulation results validate the advantages of our method both in terms of energy saving and shorter response time.
[Show abstract][Hide abstract] ABSTRACT: Fractional frequency reuse (FFR), using different frequency reuse factors for cell center and edge regions, is able to effectively improve spectrum efficiency in multi-cell OFDMA networks. However, optimal performance is hard to achieve in practice as the efficiency of resource allocation could drop drastically due to the constraint from the frequency partitions formed by FFR. Since the radio resource is pre-partitioned for cell edge and center, fair resource allocation in a cell is also difficult to implement. Conventional frequency partition adjustment either has high complexity due to global optimization or suffers from heavy performance degradation due to absence of effective control on inter-cell interference (ICI). To solve this issue, we create models for analyzing geographical distribution of interference in multi-cell networks. Based on the observed non-uniform distributed ICI, we redefine the zones for fractional reuse and propose clustering based FFR, which offers resource allocation higher flexibility and better fairness with additional spatial dimension. Extensive simulation has been performed to validate practicality and effectiveness of our proposed scheme.
[Show abstract][Hide abstract] ABSTRACT: The performance unfairness problem in a single cell IEEE 802.11 wireless local area network (WLAN) is considered. While existing research is based on the assumption that all nodes have the same transmission success probability and per-node throughput, this fairness exists only if all nodes within range of the access point can sense each other. Recent measurements suggest that this is not necessarily true and terminals can be hidden from each other. In this paper, the impact of hidden terminals on the performance unfairness among individual nodes is investigated via analysis, simulation and experimental measurements in a real network. In the presence of hidden terminals, it is observed that the widely accepted conclusion of equal performance among nodes does not hold any more. Instead, nodes far from the access point (AP) see more hidden terminals than those close to the AP, so they get more packet losses and lower throughput. This phenomenon is not due to inter-cell interference, or channel disparities among nodes, and is significant even when the RTS/CTS mechanism designed to mitigate the impact of hidden terminals is turned on. The simulation results show that for a 16-node WLAN with a fixed data rate of 6 Mbps, the throughput of a node close to the AP is more than twice that of an edge node, due to hidden terminals.
[Show abstract][Hide abstract] ABSTRACT: To mitigate inter-cell interference (ICI) and achieve higher spectrum efficiency, fractional frequency reuse (FFR) has been widely adopted by the next generation wireless systems, wherein different frequency reuse factors are applied to cell center and cell edge zones. In such conventional FFR, a contiguous radio spectrum is partitioned in a fixed fashion across all cells for edge and cell center zones. This approach evidently lacks the flexibility of inter-cell and/or intra-cell resource allocation adjustment and the capability of dealing with traffic load fluctuation and quality of service (QoS) requirements variations. This paper models the implementation of FFR in a multi-cell network environment and proposes a scheme called D-FFR that can adaptively partition radio spectrum in a distributed manner to achieve different FFR configuration among different cells. Resource demands and various inter-cell/intracell allocation constraints are accounted in D-FFR to enable differentiable inter-cell and/or intra-cell throughput and deliver higher spectrum efficiency. Analytical and extensive simulation results are provided in the paper to validate the effectiveness of our proposed scheme.
[Show abstract][Hide abstract] ABSTRACT: Next generation wireless system such as advanced WiMAX (i.e., IEEE802.16m) and LTE advanced will fully embrace multi-hop relay architecture. The conventional Automatic Repeat reQuest (ARQ) and the more recent Hybrid ARQ (HARQ) are two simple yet highly effective error control techniques designed for single hop system. Nevertheless, extending them in a synergistic manner to support multi-hop relay networks is by no means a trivial undertaking. This paper explores a variety of multi-hop error control techniques such as hop-by-hop ARQ, 2-link ARQ and end-to-end ARQ, and various possible combinations with HARQ. We further establish an analytical framework for each of these key techniques and evaluate the performance. Based on the analysis and comparison, we propose a low complexity error control mechanism tailored for the multihop transmission features. Extensive simulation results compare the performance and validate our analytical framework.
[Show abstract][Hide abstract] ABSTRACT: We address the problem of collaborative sensing in cognitive radios. In a cognitive radio network, all the nodes may sense the spectrum simultaneously. They should then exchange their sensing results in order to improve the reliability of the detection. This exchange of information has to be done efficiently to improve on the bandwidth efficiency of the network. We propose a medium access control (MAC) signaling protocol and study its performance behavior. For the case of a single-band channel, we present a thorough analysis of the proposed protocol and use the results to pick the protocol parameters that minimizes the signaling time for a given probability of detection. Analysis of the proposed protocol for multiband channels is solved by introducing a matrix formulation of the proposed protocol that allows its evaluation numerically.
[Show abstract][Hide abstract] ABSTRACT: A novel concept of portable charging station networks to serve Electric Vehicles is described. An optimum charging station deployment method is explained, and its performance has been simulated for single highway, two intersecting highways, and Manhattan-like grid traffic models. Outage probability and service waiting delay performances are evaluated. Impact of the number PCSs and the ratio of EVs to PCSs in the service area on outage probability and waiting delay are studied. The gained insights will be used in extending this pioneer step to a stochastic framework with more realistic traffic models.
[Show abstract][Hide abstract] ABSTRACT: Block transmission with cyclic prefix is a promising technique to realize high-speed data rates in frequency selective fading channels. Many popular linear precoding schemes, including orthogonal frequency-division multiplexing (OFDM), single-carrier (SC) block transmission, and time-reversal (TR), can be interpreted as such a block transmission. This paper presents a unified performance analysis which shows how the optimal precoding strategy depends on the receiver type and the optimization criterion (capacity and mean-square error). We analyze three variants of TR methods (based on maximum-ratio combining, equal-gain combining and selective combining) and two-types of pre-equalization methods (zero-forcing and minimum mean-square error). As one application of our framework, we derive optimal power control for OFDM in the presence of interference limitation for distributed antenna systems; we find that without power control, OFDM does not have any capacity advantage over SC transmissions. When comparing SC and TR, we verify that for single-antenna systems at high SNRs, SC has a capacity advantage; however, TR performs better in the low SNR regime. For multiple-antenna systems, TR always provides higher capacity, and the capacity of TR can approach that of optimal precoders with a number of distributed antennas.
[Show abstract][Hide abstract] ABSTRACT: A novel practical low-complexity multicell orthogonal frequency-division multiple access (OFDMA) downlink channel-assignment method that uses a graphic framework is proposed in this paper. Our solution consists of two phases: 1) a coarse-scale intercell interference (ICI) management scheme and 2) a fine-scale channel-aware resource-allocation scheme. In the first phase, state-of-the-art ICI management techniques such as ICI coordination (ICIC) and base-station cooperation (BSC) are incorporated in our framework. In particular, the ICI information is acquired through inference from the diversity set of mobile stations and is presented by an interference graph. Then, ICIC or BSC is mapped to the MAX k -CUT problem in graph theory and is solved in the first phase. In the second phase, channel assignment is accomplished by taking instantaneous channel conditions into account. Heuristic algorithms are proposed to efficiently solve both phases of the problem. Extensive simulation is conducted for various practical scenarios to demonstrate the superior performance of the proposed solution compared with the conventional OFDMA allocation scheme. The proposed scheme can be used in next-generation cellular systems such as the 3GPP Long-Term Evolution and IEEE 802.16 m.
No preview · Article · Oct 2009 · IEEE Transactions on Vehicular Technology
[Show abstract][Hide abstract] ABSTRACT: A graph-based framework for dynamic fractional frequency reuse (FFR) in multi-cell OFDMA networks is proposed in this work. FFR is a promising resource allocation technique that can effectively mitigate inter-cell interference (ICI) in OFDMA networks. The proposed scheme enhances the conventional FFR by enabling adaptive spectral sharing per cell load conditions. Such adaptation has significant benefits in a practical environment where traffic load in different cells may be asymmetric and time-varying. The dynamic feature is accomplished via a graph approach in which the resource allocation problem is translated to a graph coloring problem. Specifically, in order to incorporate various versions of FFR in our framework, we construct a graph that matches the specific version of FFR and then color the graph using the corresponding graph algorithm. The performance improvement enabled by the proposed dynamic FFR scheme is further demonstrated by computer simulation for a 19-cell network with asymmetric cell load. For instance, the proposed dynamic FFR scheme can achieve a 12% and 33% gain in cell throughput and service rate over conventional FFR, and render a 70% and 107% gain in cell throughput and service rate with respect to the reuse-3 system.
[Show abstract][Hide abstract] ABSTRACT: A novel, practical and low-complexity multi-cell OFDMA downlink channel assignment method using a graph- based approach is proposed in this work. The inter-cell interference (ICI) information is obtained through inference from the diversity set of mobile stations (MSs) and presented in the form of an interference graph. The proposed downlink channel assignment method consists of two phases. The task of ICI reduction is mapped to the MAX fe-CUT problem in graph theory and solved in the first phase. Then, channel assignment is conducted by taking into account instantaneous channel conditions in the second phase. State-of-the-art ICI management techniques such as ICI coordination (ICIC) and base station cooperation (BSC) are incorporated in our framework. Heuristic algorithms are proposed to solve both phases of the problem efficiently. Simulation is conducted to demonstrate the effectiveness of the proposed solution, where the SINR improvement can be as high as 4.5 dB. The proposed solution can be used in next generation cellular systems such as 3 GPP long term evolution (LTE) and IEEE 802.16 m.
[Show abstract][Hide abstract] ABSTRACT: Cooperative communication is a buzz word in research community today. It enables nodes to achieve spatial extensive investigations have been directed to closely examine its performance by means of both analysis and simulation. However, the study of this new technology in an imple- mentation based system is very limited. In this paper, we present two implementation approaches to demonstrate the viability of realizing cooperation at the MAC layer in a real environment. The paper describes the technical challenges encountered in each of the approaches, details the cor- responding solution proposed, and compare the limitations and benefits of the approaches. The experimental measurements are reported, which not only help developing a deeper understand- ing of the protocol behavior but also confirm that the cooperative communication is a promising realistic technology for boosting the performance of next generation wireless networks.
Preview · Article · Jan 2009 · EURASIP Journal on Wireless Communications and Networking
[Show abstract][Hide abstract] ABSTRACT: The paper provides a method for combining HARQ along with Self-Interference Cancellation Coding (SICC), so that the reliability of spatial multiplexing MIMO transmissions can be improved. The simulation results show that significant gain is achieved over the traditional Chase Combing.