Ziaul Hasan

Energy-efficient power allocation in OFDM-based cognitive radio systems: A risk-return model

Efficient and reliable subcarrier power allocation in orthogonal frequency-division multiplexing (OFDM)-based cognitive radio networks is a challenging problem. Traditional waterfilling approach is inefficient for such networks due to the strict requirements on the interference generated to the primary users (PUs). In this paper, we present a solution to an energy-efficient resource allocation problem which maximizes the cognitive radio (i.e., secondary) link capacity taking into account the availability of the subcarriers (and hence the reliability of transmission by cognitive radios) and the limits on total interference generated to the PUs. We consider an energy-aware capacity expression by taking into account another factor called subcarrier availability. Optimizing such an expression saves valuable resources such as battery life by selectively allocating power to underutilized subcarriers. Based on a risk-return model, we formulate a convex optimization problem which incorporates a linear average rate loss function in the optimization objective to include the effect of subcarrier availability. Due to the complex structure of the optimal solution, we propose three suboptimal schemes, namely, the step-ladder, nulling, and scaling schemes. We compare the performances of optimal and suboptimal algorithms with the performance of a classical waterfilling scheme. We conclude that waterfilling, unable to satisfy the interference criterion, performs the worst amongst all the schemes considered in this paper.

Subcarrier selection and power allocation for amplify-and-forward relaying over OFDDM links

We study the end-to-end capacity of a cooperative relaying scheme using OFDM modulation, under power constraints for both the base station and the relay station. The relay uses an amplify-and-forward cooperative relaying technique to retransmit messages on a subset of the available subcarriers. The power used in the base station and the relay station transmitters is allocated in such a manner that the overall system capacity is maximized. The subcarrier selection and power allocation are obtained based on convex optimization formulations and an iterative algorithm. The proposed technique outperforms nonselective relaying schemes over a range of relay power budgets.

Subcarrier and power adaptation for multiuser OFDM-based cognitive radio systems

This paper investigates the subcarrier and power allocation problem for an OFDM-based multiuser cognitive radio (CR) system. For CR systems it is important to keep the interference introduced to the primary user (PU) band within a tolerable range. Since the joint subcarrier and power allocation problem is a non-convex integer problem and a closed form solution is difficult to find, a suboptimal algorithm that separates subcarrier and power allocation is proposed. First, a suboptimal subcarrier algorithm is proposed that allocates subcarriers to CR users that not only increase the capacity, but also reduces the interference introduced to the primary user (PU) band. Further, for a given subcarrier allocation an optimal power loading algorithm is proposed that maximizes the capacity of CR users while keeping the interference introduced to the PU band and the total power below a threshold. Simulation results show that for CR systems the proposed algorithm is able to load power into the subcarriers while keeping the interference below a specified threshold.

Power Allocation for Cognitive Radios Based on Primary User Activity in an OFDM System

Efficient and reliable power allocation algorithm in cognitive radio (CR) networks is a challenging problem. Traditional water-filling algorithm is inefficient for CR networks due to the interaction with primary users. In this paper, we consider reliability/availability of subcarriers or primary user activity for power allocation. We model this aspect mathematically with a risk-return model by defining a general rate loss function. We then propose optimal and suboptimal algorithms to allocate power under a fixed power budget for such a system with linear rate loss. These algorithms as we will see allocate more power to more reliable subcarriers in a water-filling fashion with different water levels. We compare the performance of these algorithms for our model with respect to water-filling solutions. Simulations show that suboptimal schemes perform closer to optimal scheme although they could be implemented with same complexity as water-filling algorithm. Finally, we discuss the linearity of loss function and guidelines to choose its coefficients by obtaining upper bounds on them.

Relay Selection for OFDM Wireless Systems under Asymmetric Information: A Contract-Theory Based Approach

User cooperation although improves performance of wireless systems, it requires incentives for the potential cooperating nodes to spend their energy acting as relays. Moreover, these potential relays are better informed than the source about their transmission costs, which depend on the exact channel conditions on their relay-destination links. This results in asymmetry of available information between the source and the relays. In this paper, we use contract theory to tackle the problem of relay selection under asymmetric information in OFDM-based cooperative wireless system that employs decode-and-forward (DF) relaying. We first design incentive compatible offers/contracts, consisting of a menu of payments and desired signal-to-noise-ratios (SNR)s at the destination. The source then broadcasts this menu to nearby mobile nodes. The nearby mobile nodes which are willing to relay, notify back the source with the contracts they agree to accept in each subcarrier. We show that when the source is under a budget constraint, the problem of relay selection in each subcarrier with the goal of maximizing capacity is a nonlinear non-separable knapsack problem. We propose a heuristic relay selection scheme to solve this problem. We compare the performance of our overall mechanism and the heuristic solution with a simple relay selection scheme. Selected numerical results show that our solution performs better and is close to optimal. The benefits of the overall mechanism introduced in this thesis is that it is simple to implement, needs limited interaction with potential relays and hence it requires minimal signalling overhead.

Some research issues in cognitive radio networks

The cognitive radio (CR) technology will allow a group of potential users to identify and access available spectrum resources provided that the interference to the users for whom the band has been licensed is kept below a prescribed level. However, this research area is at a very immature stage because various research challenges have to be addressed and solved. In this paper our objective is to present an overview of some research issues for CR networks. Specifically, we present some research and development in CR networks with focus on i) information- theoretic aspects, ii) spectrum sensing, iii) link adaptation, iv) advanced transceiver design, and v) admission control. We also discuss some important research problems related to these specific topics that needs to be addressed before deployment of CR systems in practice.

Resource Allocation for Multiuser OFDMA-Based Amplify-and-Forward Relay Networks with Selective Relaying

We address the problem of designing efficient resource allocation schemes for an OFDMA based multiuser cooperative communication system that uses amplify and forward relaying. With a two-phase relaying protocol, we assume that both source and the relay have fixed power constraints and that the source employs a selective relaying mechanism. That is, the source adaptively decides on which frequency subcarriers relaying has to performed. We first formulate the problem as a capacity maximizing integer programming optimization problem and then propose a heuristic solution that is composed of several subproblems. The first part is to suboptimally allocate subcarriers to different users based on proportional rate fairness. The next part is a two step iterative approach to find relay decisions and power allocation at the source during the first phase and at the relay during the second phase. The last step is a water-filling algorithm to allocate power at the source during the second phase to all non-relaying subcarriers. Simulation results show that this selective relaying mechanism outperforms the always relay case while also providing an approximate proportional rate fairness.

Cooperative communication and relay selection under asymmetric information

Cooperative communication when other mobile nodes could act as potential relays, poses significant challenge in designing relay selection algorithms due to the information asymmetry between the source and the potential relays. The potential relays in this case are better informed about their channel conditions than the source. Therefore, the source must provide them with incentive to reveal their information and hence for their participation in relaying. In this paper, we study the problem of relay selection under a budget constraint when there is such an information asymmetry. By leaving the bargaining power only to the source, we propose a simple relay selection mechanism that requires limited interaction with potential relays and their participation is also voluntary. We will first use contract theory to formulate incentive compatible contracts that can be broadcasted to nearby mobile nodes which can act as potential relays. Once these mobile nodes notify the contracts they are willing to accept, source then optimally selects a set of relays based on this information. We analyze the performance of this mechanism by simulations and discuss the effect of information asymmetry.

ISI-free pulses for high-data-rate ultra-wideband wireless systems

Modified intersymbol interference (ISI)¿free pulses and ISI-free band-limited polynomial pulses for ultra-wideband (UWB) communication systems are proposed to provide the possibility of very high data rates not achievable by conventional pulses and the pulse design algorithms available in the literature. The power spectrum of the proposed pulses fits the Federal Communications Commission (FCC) spectral mask for very high data rates that have quantized values with various levels. Using this scheme and a particular choice of design variables, it is possible to design orthogonal pulses that could be used for multi-user data communication after they are truncated for the same time interval. Several possible improvements for these proposed pulses are considered for certain practical cases.

Design of a Family of ISI Free Pulses for Very High Data Rate UWB Wireless Systems

In this paper we propose modified raised cosine pulses and a family of modified ISI free and bandlimited polynomial pulses for UWB communication systems which could provide very high data rates not possible by the conventional pulses and other proposed pulse design algorithms. The power spectrum of these proposed pulses fits the FCC spectral mask even for very high data rates and data rates gets quantized for various levels depending upon the shift frequency. Using this scheme and with a certain choice of design variables it is possible to design orthogonal pulses which could be used for multiuser data communication after truncating them for the same time interval.