Chung-Wei Wang

Modeling and Analysis for Spectrum Handoffs in Cognitive Radio Networks

In this paper, we present an analytical framework to evaluate the latency performance of connection-based spectrum handoffs in cognitive radio (CR) networks. During the transmission period of a secondary connection, multiple interruptions from the primary users result in multiple spectrum handoffs and the need of predetermining a set of target channels for spectrum handoffs. To quantify the effects of channel obsolete issue on the target channel predetermination, we should consider the three key design features: 1) general service time distribution of the primary and secondary connections; 2) different operating channels in multiple handoffs; and 3) queuing delay due to channel contention from multiple secondary connections. To this end, we propose the preemptive resume priority (PRP) M/G/1 queuing network model to characterize the spectrum usage behaviors with all the three design features. This model aims to analyze the extended data delivery time of the secondary connections with proactively designed target channel sequences under various traffic arrival rates and service time distributions. These analytical results are applied to evaluate the latency performance of the connection-based spectrum handoff based on the target channel sequences mentioned in the IEEE 802.22 wireless regional area networks standard. Then, to reduce the extended data delivery time, a traffic-adaptive spectrum handoff is proposed, which changes the target channel sequence of spectrum handoffs based on traffic conditions. Compared to the existing target channel selection methods, this traffic-adaptive target channel selection approach can reduce the extended data transmission time by 35 percent, especially for the heavy traffic loads of the primary users.

Load-Balancing Spectrum Decision for Cognitive Radio Networks

In this paper, we present an analytical framework to design system parameters for load-balancing multiuser spectrum decision schemes in cognitive radio (CR) networks. Unlike the non-load-balancing methods that multiple secondary users may contend for the same channel, the considered load-balancing schemes can distribute the traffic loads of secondary users to multiple channels. Based on the preemptive resume priority (PRP) M/G/1 queueing theory, a spectrum decision analytical model is proposed to evaluate the effects of multiple interruptions from the primary user during each link connection, the sensing errors (i.e., missed detection and false alarm) of the secondary users, and the heterogeneous channel capacity. With the objective of minimizing the overall system time of the secondary users, we derive the optimal number of candidate channels and the optimal channel selection probability for the sensing-based and the probability-based spectrum decision schemes, respectively. We find that the probability-based scheme can yield a shorter overall system time compared to the sensing-based scheme when the traffic loads of the secondary users is light, whereas the sensing-based scheme performs better in the condition of heavy traffic loads. If the secondary users can intelligently adopt the best spectrum decision scheme according to sensing time and traffic conditions, the overall system time can be improved by 50% compared to the existing methods.

Studies of OFDM signal for broadband optical access networks

Using orthogonal-frequency-division-multiplexing (OFDM) format for passive optical network (PON) is a subject of great interest for recent research works. OFDM signal has high spectral efficiency, high tolerance to the fiber chromatic dispersion and the high flexibility on both multiple services provisioning and dynamic bandwidth allocation. In this paper we study the use of OFDM signals for optical access networks, including carrier distributed PON, heterogeneous optical wired/wireless network and a 100 Gb/s OFDM-PON. We first quantify the performance of the OFDM signal when subjected to the noise produced by two different Rayleigh backscattering (RB) components that are present in the carrier-distributed PON. Then, based on these results, we also study the possibility of using OFDM for signal remodulation in a long-reach (LR)-PON. We propose carrier-distributed OFDM PONs using a dual-feeder fiber architecture and wavelength-shifting to mitigate the RB noise. Afterwards, we propose heterogeneous optical wired/wireless access networks, combining 10 Gb/s PON and 10 Gb/s OFDM radio-over-fiber (ROF) network. Finally, we propose a 100 Gb/s OFDM-PON using subcarrier multiplexing (SCM).

A queueing-theoretical framework for QoS-enhanced spectrum management in cognitive radio networks

This article outlines the fundamental modeling issues of opportunistic spectrum access in cognitive radio networks. In particular, we identify the effects of connection-based channel usage on the QoS performance of spectrum management techniques. During the transmission period of a secondary users connection, the phenomenon of multiple spectrum handoffs due to interruptions of primary users arises quite often. In addition to multiple interruptions, the connection-based channel usage behaviors are also affected by spectrum sensing time, switching between different channels, generally distributed service time, and channel contention between multiple secondary users. An analytical framework based on the preemptive resumption priority M/G/1 queueing theory is introduced to characterize the effects of the above factors simultaneously. The proposed generalized analytical framework can incorporate various system parameters into the design of very broad spectrum management techniques, including spectrum sensing, spectrum decision, spectrum sharing, and spectrum mobility. The applications of this analytical framework on spectrum decision as well as spectrum mobility are discussed, and some open issues using this framework are suggested in this article.

100 GHz ultra-wideband (UWB) fiber-to-the-antenna (FTTA) system for in-building and in-home networks

Fiber-to-the-antenna (FTTA) system can be a cost-effective technique for distributing high frequency signals from the head-end office to a number of remote antenna units via passive optical splitter and propagating through low-loss and low-cost optical fibers. Here, we experimentally demonstrate an optical ultra-wideband (UWB) - impulse radio (IR) FTTA system for in-building and in-home applications. The optical UWB-IR wireless link is operated in the W-band (75 GHz - 110 GHz) using our developed near-ballistic unitraveling-carrier photodiode based photonic transmitter (PT) and a 10 GHz mode-locked laser. 2.5 Gb/s UWB-IR FTTA systems with 1,024 high split-ratio and transmission over 300 m optical fiber are demonstrated using direct PT modulation.

Analysis of Reactive Spectrum Handoff in Cognitive Radio Networks

In this paper, we present an analytical framework to evaluate the effects of multiple spectrum handoffs on channel utilization and latency performances in cognitive radio (CR) networks. During the transmission period of a secondary connection, multiple interruptions from the primary users result in multiple spectrum handoffs. In order to decide the target channel for each spectrum handoff and resume the unfinished transmission, wideband sensing is performed in an on-demand reactive manner. Although spectrum handoff procedure can enhance channel utilization, transmission latency of the secondary users is prolonged due to multiple handoffs. Thus, two fundamental issues in CR networks with multiple spectrum handoffs arise: (1) to what extent the channel utilization can be improved; and (2) how long the transmission latency will be extended for the secondary users. To solve the first problem, we introduce the preemptive resume priority (PRP) M/G/1 queueing network to characterize the channel usage behaviors of CR networks. Based on this queueing network, channel utilization under various traffic arrival rates and service time distributions can be evaluated. Furthermore, on top of the proposed queueing network, a state diagram is developed to characterize the effects of multiple handoff delay on the transmission latency of the secondary users. The analytical results can provide a helpful insight to study the effects of traffic arrival rates and service time on channel utilization and transmission latency and then facilitate the designs of admission control rules for the secondary users subject to their performance requirements.

Optimal Number of Clusters in Dense Wireless Sensor Networks: A Cross-Layer Approach

Cluster-based sensor networks have the advantages of reducing energy consumption and link-maintenance cost. One fundamental issue in cluster-based sensor networks is determining the optimal number of clusters. In this paper, we suggest a physical (PHY)/medium access control (MAC)/network (NET) cross-layer analytical approach for determining the optimal number of clusters, with the objective of minimizing energy consumption in a high-density sensor network. Our cross-layer design can incorporate many effects, including lognormal shadowing and a two-slope path loss model in the PHY layer, various MAC scheduling, and multihop routing schemes. Compared with the base-line case with one cluster per observation area (OA), a sensor network with the proposed optimal number of clusters can reduce the energy consumption by more than 80% in some cases. We also verify by simulations that the analytical optimal cluster number can still effectively function, regardless of the different densities of sensors in various OAs.

Rayleigh Backscattering Performance of OFDM-QAM in Carrier Distributed Passive Optical Networks

The passive optical network (PON) using an orthogonal frequency-division-multiplexing (OFDM) format is a subject of many research works recently. Due to the high spectral efficiency, low bandwidth optical components can be used in this OFDM-PON. We quantify, for the first time, the performance of the OFDM signal when subjected to the noise generated by the two components of the Rayleigh backscattering (RB): carrier generated RB (carrier-RB) and signal generated RB (signal-RB) that are present in the carrier-distributed PONs. An RB comparison between OFDM quadrature amplitude modulation and nonreturn-to-zero is preformed.

Latency Analysis for Dynamic Spectrum Access in Cognitive Radio: Dedicated or Embedded Control Channel?

Dynamic spectrum access (DSA) is the key feature of cognitive radio (CR) networks, but it also poses many new challenges on the medium access control (MAC) design. One of key challenges is the fact that the secondary CR users can only borrow the licensed spectrum from the primary users for a short period of time. Hence, unlike many available multi-channel MAC protocols for ad hoc networks where throughput is the main performance issue, the DSA protocols in CR networks shall place more emphases on the access latency. Hence, one fundamental issue arises: how can the spectrum be dimensioned for control channels in order to minimize the access delay of DSA protocol in CR networks? In this paper, we provide a comparative study in an analytical manner on the latency performance of two DSA protocols: 1) dedicated control channel, and 2) embedded dedicated control channel approaches. Our results show that an optimal ratio of the control channel bandwidth over the total channel bandwidth can be found to minimize the latency of DSA with dedicated control channels. However, the delay performance of DSA with dedicated control channels is more sensitive to the variations of the data lengths than that of DSA with embedded control channels. Hence, we conclude that the way of dimensioning the spectrum for control frames for DSA in CR networks should be adaptive to the variations of the traffic characteristics and the number of users.

Modeling and analysis of multi-user spectrum selection schemes in cognitive radio networks

In this paper, we study the spectrum selection problem in cognitive radio network with emphasis on resolving the channel contention and the spectrum sharing issues of multiple secondary users. For the traditional channel selection methods, the secondary users select their operating channels based on various criteria. However, these methods neglect the effect that multiple secondary users may content for the same channel if they have the same consensus on one particular good channel. Compared to the existing spectrum selection methods, we consider the sensing-based and the probability-based spectrum selection schemes which can prevent too many secondary users from contending the same channel. An analytical model integrated with the preemptive resume priority M/G/1 queuing network theory is developed to evaluate the overall transmission time of the both schemes. Based on this model, we discuss how to find the optimal selection probability for the probability-based scheme. Furthermore, we also analyze in which condition dependent of sensing time and traffic parameters that the sensing- or the probability-based scheme should be used. Based on the analytical results, we provide a principle to guide system operators which scheme should be used in CR networks. Then, we conclude that channel selection scheme should be adaptive to the variations of the traffic characteristics.1