Dhadesugoor R. Vaman

Energy Efficient Adaptive Modulation in Wireless Cognitive Radio Sensor Networks

In this paper, we consider the lifetime maximization problem in a wireless cognitive radio sensor network, where the joint design of cognitive radio and multi-carrier modulation is proposed to achieve high power efficiency. Sensor nodes first sense the entire spectrum and locate the available subcarriers based on a pilot tone detection scheme. After each node locates the available subcarrier set, information is transmitted over the favorite channel that has the largest channel gain. Under this setting, an adaptive modulation strategy is proposed to maximize the network lifetime by selecting the optimal constellation size. Simulation results demonstrate the effectiveness of our approach. In addition, the impact of conflicting transmissions upon the network lifetime is also investigated.

Cognitive Radio Based Multi-User Resource Allocation in Mobile Ad Hoc Networks Using Multi-Carrier CDMA Modulation

In this paper, we propose a cognitive radio based multi-user resource allocation framework for mobile ad hoc networks using multi-carrier DS CDMA modulation over a frequency-selective fading channel. In particular, given preexisting communications in the spectrum where the system is operating, in addition to potential narrow-band interference, a channel sensing and estimation mechanism is provided to obtain information such as subcarrier availability, noise power and channel gain. Given this information, both frequency spectrum and power are allocated to emerging new users (i.e., cognitive radio users), based on a distributed multi-user resource allocation framework, in order to satisfy a target data rate and a power constraint of each cognitive radio user, while attempting to avoid interference to the existing communications as well as to minimize total power consumption of the cognitive radio users.

Unified analysis of energy detection of unknown signals over generalized fading channels

Reliable spectrum sensing is the very task upon which the entire operation of cognitive radio rests. Blind sensing of spectral-holes using a radiometer (energy detectors) is one of the solutions that have been proposed for enabling opportunistic spectrum access. This article revisits the problem of energy detection of an unknown deterministic signal over a myriad of fading environments. Specifically, a new approach (based on the canonical series representations of the generalized Marcum Q-function of real order in conjunction with the derivatives of the moment generating function of signal-to-noise ratio) is proposed to analyze the performance of maximal-ratio combining (MRC) and square-law combining (SLC) energy detectors with independent but non-identically distributed (i.n.d) fading statistics, including Rice and mixed-fading channels. Our analytical framework is also capable of treating the Nakagami-m channels with non-integer fading severity indices as well as halfodd integer values for the time-bandwidth product u. Many of these cases were either intractable with the classical probability density function/contour integral approaches, or that heretofore had resisted simple/computationally efficient solutions. Selected numerical results are also provided for the receiver operating characteristic (ROC) of MRC and SLC diversity energy detectors over Rice and Nakagami-m channels.

Distributed Energy Efficient Spectrum Access in Wireless Cognitive Radio Sensor Networks

In this paper, a wireless cognitive radio sensor network is considered, where each sensor node is equipped with cognitive radio and the network is a multi-carrier system operating on time slots. In each slot, the users with new traffic demand will sense the entire spectrum and locate the available subcarrier set. Given the required data rate and power bound, a fully distributed subcarrier selection and power allocation algorithm is proposed for each individual user to minimize the energy consumption per bit over all subcarriers, while avoid introducing harmful interference to the existing users. The multi-dimensional and non-quasi-convex/concave nature of the energy efficiency optimization problem in multi-carrier systems makes it more challenging than throughput/power optimization problems or the energy efficiency problem in the single carrier system. The optimal solution is derived by using a two-stage algorithm where the original problem is decoupled into an unconstrained problem and branch and bound method is applied thereafter to reduce the search space. In addition, a distributed power control is performed to manage the co-channel interference among new users when needed. Simulation results demonstrate that the proposed approach performs close to the centralized optimal solution, and it provides prolonged network lifetime.

Distributed energy efficient spectrum access in cognitive radio wireless ad hoc networks

In this paper, energy efficient spectrum access is considered for a wireless cognitive radio ad hoc network, where each node is equipped with cognitive radio, has limited energy, and the network is an OFDMA system operating on time slots. In each slot, the users with new traffic demand will sense the spectrum and locate the available subcarrier set. Given the data rate requirement and maximal power limit, a constrained optimization problem is formulated for each individual user to minimize the energy consumption per bit over all selected subcarriers, while avoid introducing harmful interference to the existing users. Because of the multi-dimensional and non-convex nature of the problem, a fully distributed subcarrier selection and power allocation algorithm is proposed by combining an unconstrained optimization method with a constrained partitioning procedure. Due to the non-cooperative behavior among new users, they will execute distributed power control to manage the co-channel interference when needed. Simulation results demonstrate that the proposed scheme performs tightly to the global optimal solution. In addition, the comparison between the proposed energy efficient allocation scheme and the well established rate or power efficient allocation algorithms is carried out to demonstrate the advantage of the proposed scheme in terms of network lifetime.

Cooperative and Constrained MIMO Communications in Wireless Ad Hoc/Sensor Networks

In this paper, we investigate the issue of cooperative node selection in MIMO communications for wireless ad hoc/sensor networks, where a source node is surrounded by multiple neighbors and all of them are equipped with a single antenna. Given energy, delay and data rate constraints, a source node dynamically chooses its cooperating nodes from its neighbors to form a virtual MIMO system with the destination node (which is assumed to have multiple antennas), as well as adaptively allocates the power level and adjusts the constellation size for each of the selected cooperative nodes. In order to optimize system performance, we jointly consider the optimization of all these parameters, given the aforementioned system constraints. We assume that the source node either has CSI, or has no CSI. Heuristic algorithms, such as maximal channel gain (MCG) and least channel correlation (LCC) algorithms are proposed in order to exploit available system information and to solve the constrained optimization problem.

Power control and proportional fair scheduling with minimum rate constraints in clustered multihop TD/CDMA wireless ad hoc networks

In order to achieve high end-to-end throughput in a multihop wireless ad hoc network, TD/CDMA has been chosen as the medium access control (MAC) scheme due to its support for high network throughput in a multihop environment. The associated power control and scheduling problem needs to be addressed to optimize the operations of TD/CDMA. In this paper, cluster based architecture is introduced to provide centralized control within clusters, and the corresponding power control and scheduling schemes are derived to maximize a network utility function and guarantee the minimum rate required by each traffic session. Because the resulted optimal power control reveals bang-bang characteristics, i.e., scheduled nodes transmit with full power while other nodes remain silent, the joint power control and scheduling problem is reduced to a scheduling problem. In order to achieve a balance between throughput and fairness, proportional fair scheduling is considered. The multi-link version of the proportional fair scheduling algorithms for multihop wireless ad hoc networks are proposed. In addition, a generic token counter mechanism is employed to satisfy the minimum rate requirements. Approximation algorithms are suggested to reduce the computational complexity. In networks that are lack of centralized control, distributed scheduling algorithms are also derived and fully distributed implementation is provided. Simulation results demonstrate the effectiveness of the proposed schemes

Distributed Power and Scheduling Management for Mobile Ad Hoc Networks with Delay Constraints

In this paper, we propose a cross-layer distributed power control and scheduling protocol for delay-constrained applications over mobile CDMA-based ad hoc wireless networks. Herein, we propose a novel scheme where power control is employed to combat delay occurring on multi-hop wireless ad hoc networks via cross-layer information exchange. Based on that, a distributed power control and scheduling protocol is proposed to control the incurred delay, as well as the multiple access interference (MAI). Unlike many previous works on power control and scheduling, we also investigate the impacts of Doppler spread and noisy channel estimates upon the system performance, and provide a robust system which employs a combination of power control and coding/interleaving to combat the effects of Doppler spread by exploiting the time diversity when the Doppler spread gets large. Hence, our proposed approach can function appropriately over a wide range of channel conditions

Distributed Cognitive Sensing for Time Varying Channels: Exploration and Exploitation

Spectrum under-utilization calls for the open and dynamic spectrum access mechanism, which allows the unlicensed user equipped with cognitive radios to opportunistically sense and access the spectrum that not occupied by primary users. In practice due to the hardware limitations, each cognitive radio user may be only able to sense a portion of the interested wide span spectrum. Hence, a hardware-constrained cognitive MAC to conduct efficient and intelligent spectrum sense decision is desired. In this paper, we formulate the cognitive radio spectrum sensing problem under time-varying channels as an adversarial bandit problem without any assumption of the channel statistics. A fully distributed strategy is proposed to address the fundamental tradeoff between spectrum exploration and spectrum exploitation during the sensing periods. Simulation results demonstrate that significant performance gain can be achieved by the proposed algorithm when the channels are time-varying on small time-scales. A coordination scheme for the multi-user case is also presented and the effectiveness is also demonstrated by the simulation results.

Cross-layer distributed joint power control and scheduling for delay-constrained applications over CDMA-based wireless ad-hoc networks

In this paper, we propose a cross-layer distributed power control and scheduling protocol for delay-constrained applications over mobile CDMA-based ad hoc wireless networks. Herein, we propose a novel scheme where power control is employed to combat delay occurring on multi-hop wireless ad hoc networks via cross-layer information exchange. Based on that, a distributed power control and scheduling protocol is proposed to control the incurred delay as well as the multiple access interference (MAI). Unlike other previous work on power control and scheduling, we also investigate the impact of Doppler spread upon the system performance, and provide a robust system which employs a combination of power control, and coding/interleaving to combat the effects of Doppler spread by exploiting the time diversity when the Doppler spread gets large. Hence, our proposed approach can function appropriately over a wide range of channel conditions.