Feng-Tsun Chien

Network Selection in Cognitive Heterogeneous Networks Using Stochastic Learning

Coexistence of multiple radio access technologies (RATs) is a promising paradigm to improve spectral efficiency. This letter presents a game-theoretic network selection scheme in a cognitive heterogeneous networking environment with time-varying channel availability. We formulate the network selection problem as a noncooperative game with secondary users (SUs) as the players, and show that the game is an ordinal potential game (OPG). A decentralized, stochastic learning-based algorithm is proposed where each SUs strategy progressively evolves toward the Nash equilibrium (NE) based on its own action-reward history, without the need to know actions in other SUs. The convergence properties of the proposed algorithm toward an NE point are theoretically and numerically verified. The proposed algorithm demonstrates good throughput and fairness performances in various network scenarios.

Performance analysis of multicarrier CDMA systems with frequency offsets and random spreading under optimum combining

The carrier-frequency offset effect on the performance of asynchronous multicarrier direct-sequence code-division multiple-access (MC-DS-CDMA) systems with aperiodic random spreading and correlated Rayleigh fading is studied in this paper. We obtain the optimum combining filter that maximizes the signal-to-interference-plus-noise ratio (SINR) of the combined statistics and exploits correlated information among subchannels. A closed-form expression for the unconditional covariance matrix of the interference-plus-noise vector, which forms the basis of our theoretical analysis of the maximum SINR and the average bit error probability formula, is derived by averaging several random parameters including asynchronous delays, correlated Rayleigh fading, and signature sequences. The analytic results obtained are applicable to MC-CDMA with appropriate modifications. Furthermore, we show that the MC-CDMA system with a common random signature sequence over all subcarriers for a given user outperforms that with distinct sequences over different subcarriers. Finally, the performance of MC-CDMA systems using the optimum combining technique is compared with that of different combining filters in the simulation.

Blind Recursive Tracking of Carrier Frequency Offset (CFO) Vector in MC-CDMA Systems

A recursive algorithm for estimating and updating the effective carrier frequency offset (CFO) vector in a multicarrier code-division multiple-access (MC-CDMA) system is proposed in this work. The recursive relation is derived based on the expectation maximization (EM) algorithm with a quadratic constraint. This new approach enables the use of linear estimation theory to tackle the CFO estimation problem with or without training data, which leads to an analytic CFO estimate in closed form. Furthermore, the multiple access interference (MAI) is mitigated using the second order statistics of the interference-plus-noise vector, which is updated in a recursive manner under the EM formulation, too. When reaching a converged estimate, a fixed-norm quadratic constraint is imposed so that the final CFO estimate is robust to an imprecise covariance matrix estimate caused by insufficient data samples. It is demonstrated by computer simulation that the performance of an MC-CDMA system without the CFO information can be restored by the proposed scheme in the sense that its bit error probability (BEP) performance is close to that with perfect CFO knowledge

Opportunistic Access with Random Subchannel Backoff (OARSB) for OFDMA Uplink

A distributed medium access control (MAC) algorithm for uplink OFDMA networks under the IEEE 802.16 framework is proposed and analyzed in this work. We present a simple yet efficient algorithm to enhance the system throughput by integrating opportunistic medium access and collision resolution through random subchannel backoff. Consequently, the resulting algorithm is called the opportunistic access with random subchannel backoff (OARSB) scheme. OARSB not only achieves distributed coordination among users but also reduces the amount of information exchange between the base station and users. The throughput and delay performance analysis of OARSB is conducted using a Markov chain model. The superior performance of OARSB over an existing scheme is demonstrated by analysis as well as computer simulation.

Analysis of asynchronous long-code multicarrier CDMA systems with correlated fading

The effects of inter-carrier interference (ICI) and aperiodic random spreading sequences on the performance of asynchronous multicarrier code-division multiple-access (CDMA) systems with correlated fading between sub-carriers are investigated in this research. To conduct theoretical analysis for the maximal ratio combining (MRC) receiver, random parameters including asynchronous delays, correlated Rayleigh fading, and spreading sequences are averaged to find the unconditional covariance matrix of the interference-plus-noise vector. We demonstrate that the ICI in the system proposed by Kondo and Milstein (1996) can be mitigated by assigning a common random spreading sequence over all sub-carriers for each user, rather than using a set of distinct spreading sequences, where code sequences are assumed perfectly time-synchronized. Moreover, the analytic expression for the bit error probability (BEP) can be obtained with the Gaussian approximation. Simulation results are used to demonstrate the accuracy of our analysis. Various design tradeoffs including the number of sub-carriers, fading correlations, ICI and multipath effect are presented in simulation. We conclude that, when properly choosing a sufficiently large number of sub-carriers, the benefit of mitigating multipath interference and exploiting potential hidden frequency diversity shall prevail the impairment brought by increased ICI.

Spectrum sharing in multi-channel cooperative cognitive radio networks: a coalitional game approach

In this paper, we study a coalitional game approach to resource allocation in a multi-channel cooperative cognitive radio network with multiple primary users (PUs) and secondary users (SUs). We propose to form the grand coalition by grouping all PUs and SUs in a set, where each PU can lease its spectrum to all SUs in a time-division manner while the SUs in return assist PUs’ data transmission as relays. We use the solution concept of the core to analyze the stability of the grand coalition, and the solution concept of the Shapley value to fairly divide the payoffs among the users. Due to the convexity of the proposed game, the Shapley value is shown to be in the core. We derive the optimal strategy for the SU, i.e., transmitting its own data or serving as a relay, that maximizes the sum rate of all PUs and SUs. The payoff allocations according to the core and the Shapley value are illustrated by an example, which demonstrates the benefits of forming the grand coalition as compared with non-coalition and other coalition schemes.

Joint symbol detection and channel estimation for MC-CDMA systems in the presence of carrier frequency offset

The problem of carrier frequency offset in multicarrier code-division multiple-access (MC-CDMA) systems is studied in this research. We present an iterative approach based on the expectation-maximization (EM) algorithm to estimate system parameters, including the channel state information, the carrier frequency offset and the interference-plus-noise covariance matrix. Furthermore, the symbol detection is jointly achieved through the evaluation of the Kullback-Leibler information measure required for the EM iteration. A closed form expression of the modified Fisher information matrix (MFIM) is derived to obtain the the modified Cramer-Rao bound (MCRB) of the vector version. Finally, we verity the effectiveness of the proposed algorithm with computer simulation.

An Effective Routing Protocol with Guaranteed Route Preference for Mobile Ad Hoc Networks

Battery power and queue conservation are critical issues in mobile ad hoc network (MANET). These two factors not only affect the delivery ratio but also the lifetime of the network. In this work, we will propose a simple and effective routing protocol to extend the lifetime and to evenly distribute the traffic loads of the networks as possible. Furthermore, a concept of serving capacity is introduced to reflect the level of congestion around a node. In this way, the proposed routing protocols can avoid network congestion and achieve higher packet delivery ratios. The extensive computer simulation is conducted to compare the proposed protocol against many existing routing protocols. The results show that the proposed routing protocol can have better performance in terms of queueing length, lifetime, and packet delivery ratio and have comparable end-to-end delays.

Self-Organized Cognitive Sensor Networks: Distributed Channel Assignment for Pervasive Sensing

We study the channel assignment strategy in multichannel wireless sensor networks (WSNs) where macrocells and sensor nodes are overlaid. The WSNs dynamically access the licensed spectrum owned by the macrocells to provide pervasive sensing services. We formulate the channel assignment problem as a potential game which has at least one pure strategy Nash equilibrium (NE). To achieve the NE, we propose a stochastic learning-based algorithm which does not require the information of other players’ actions and the time-varying channel. Cluster heads as players in the game act as self-organized learning automata and adjust assignment strategies based on their own action-reward history. The convergence property of the proposed algorithm toward pure strategy NE points is shown theoretically and verified numerically. Simulation results demonstrate that the learning algorithm yields a 26% sensor node capacity improvement as compared to the random selection, and incurs less than 10% capacity loss compared to the exhaustive search.

An iterative approach to frequency offset estimation for multicarrier communication systems

An iterative approach to estimate the carrier frequency offset in multicarrier communication systems is proposed in this work. Based on the previous estimated channel coefficients and provided a sufficiently large number of subcarriers, an approximate maximum likelihood (ML) estimate of the normalized frequency offset, which in turn can be used to refine the previous obtained estimation of channel coefficients. Simulation results show that the proposed algorithm requires a reliable estimation of the channel fading coefficients. Through a fast acquisition of the frequency offset at the beginning symbol block of each frame, the proposed iterative process can be better initiated, and the possible drift of the carrier frequency offset can therefore be tracked accurately.