Showing papers on "Data transmission published in 2010"

Capacity Limits of Optical Fiber Networks

Abstract: We describe a method to estimate the capacity limit of fiber-optic communication systems (or ?fiber channels?) based on information theory. This paper is divided into two parts. Part 1 reviews fundamental concepts of digital communications and information theory. We treat digitization and modulation followed by information theory for channels both without and with memory. We provide explicit relationships between the commonly used signal-to-noise ratio and the optical signal-to-noise ratio. We further evaluate the performance of modulation constellations such as quadrature-amplitude modulation, combinations of amplitude-shift keying and phase-shift keying, exotic constellations, and concentric rings for an additive white Gaussian noise channel using coherent detection. Part 2 is devoted specifically to the "fiber channel.'' We review the physical phenomena present in transmission over optical fiber networks, including sources of noise, the need for optical filtering in optically-routed networks, and, most critically, the presence of fiber Kerr nonlinearity. We describe various transmission scenarios and impairment mitigation techniques, and define a fiber channel deemed to be the most relevant for communication over optically-routed networks. We proceed to evaluate a capacity limit estimate for this fiber channel using ring constellations. Several scenarios are considered, including uniform and optimized ring constellations, different fiber dispersion maps, and varying transmission distances. We further present evidences that point to the physical origin of the fiber capacity limitations and provide a comparison of recent record experiments with our capacity limit estimation.

Networked Control Systems With Communication Constraints: Tradeoffs Between Transmission Intervals, Delays and Performance

Abstract: There are many communication imperfections in networked control systems (NCS) such as varying transmission delays, varying sampling/transmission intervals, packet loss, communication constraints and quantization effects. Most of the available literature on NCS focuses on only some of these aspects, while ignoring the others. In this paper we present a general framework that incorporates communication constraints, varying transmission intervals and varying delays. Based on a newly developed NCS model including all these network phenomena, we will provide an explicit construction of a continuum of Lyapunov functions. Based on this continuum of Lyapunov functions we will derive bounds on the maximally allowable transmission interval (MATI) and the maximally allowable delay (MAD) that guarantee stability of the NCS in the presence of communication constraints. The developed theory includes recently improved results for delay-free NCS as a special case. After considering stability, we also study semi-global practical stability (under weaker conditions) and performance of the NCS in terms of Lp gains from disturbance inputs to controlled outputs. The developed results lead to tradeoff curves between MATI, MAD and performance gains that depend on the used protocol. These tradeoff curves provide quantitative information that supports the network designer when selecting appropriate networks and protocols guaranteeing stability and a desirable level of performance, while being robust to specified variations in delays and transmission intervals. The complete design procedure will be illustrated using a benchmark example.

Magnetic Induction Communications for Wireless Underground Sensor Networks

Abstract: The main difference between the wireless underground sensor networks (WUSNs) and the terrestrial wireless sensor networks is the signal propagation medium. The underground is a challenging environment for wireless communications since the propagation medium is no longer air but soil, rock and water. The well established wireless signal propagation techniques using electromagnetic (EM) waves do not work well in this environment due to three problems: high path loss, dynamic channel condition and large antenna size. New techniques using magnetic induction (MI) create constant channel condition and can accomplish the communication with small size coils. In this paper, detailed analysis on the path loss and the bandwidth of the MI system in underground soil medium is provided. Based on the channel analysis, the MI waveguide technique for communication is developed in order to reduce the high path loss of the traditional EM wave system and the ordinary MI system. The performance of the EM wave system, the ordinary MI system and our improved MI waveguide system are quantitatively compared. The results reveal that the transmission range of the MI waveguide system is dramatically increased.

Carrier aggregation for LTE-advanced mobile communication systems

Abstract: In order to achieve up to 1 Gb/s peak data rate in future IMT-Advanced mobile systems, carrier aggregation technology is introduced by the 3GPP to support very-high-data-rate transmissions over wide frequency bandwidths (e.g., up to 100 MHz) in its new LTE-Advanced standards. This article first gives a brief review of continuous and non-continuous CA techniques, followed by two data aggregation schemes in physical and medium access control layers. Some technical challenges for implementing CA technique in LTE-Advanced systems, with the requirements of backward compatibility to LTE systems, are highlighted and discussed. Possible technical solutions for the asymmetric CA problem, control signaling design, handover control, and guard band setting are reviewed. Simulation results show Doppler frequency shift has only limited impact on data transmission performance over wide frequency bands in a high-speed mobile environment when the component carriers are time synchronized. The frequency aliasing will generate much more interference between adjacent component carriers and therefore greatly degrades the bit error rate performance of downlink data transmissions.

Signal Integrity Design for High-Speed Digital Circuits: Progress and Directions

Abstract: This paper reviews recent progress and future directions of signal integrity design for high-speed digital circuits, focusing on four areas: signal propagation on transmission lines, discontinuity modeling and characterization, measurement techniques, and link-path design and analysis.

Image display apparatus and method for operating the same

Abstract: An image display apparatus and a method for operating the same are disclosed The image display apparatus includes a display, a user input interface for receiving a control signal from a remote controller and processing the received control signal, a network interface for transmitting or receiving data over a network, a controller for controlling a pointer on the display according to the control signal received from the remote controller, and a platform for controlling data transmission or reception over the network according to the control signal received from the remote controller The platform includes an Operating System (OS) kernel and an application layer that runs on the OS kernel, and the application layer including an installable or deletable application downloaded over the network

Cooperative wireless power signal transmission method and device

Abstract: Disclosed are a method and device for cooperatively sending and receiving power wirelessly. Wireless power charging coverage at the level of data signal transmission can be ensured by establishing one or more modes for transmitting, to a specific terminal, a power transmission signal from any of a plurality of nodes which is (are) not engaged in data transmission, and by matching the phase synchronization of the power transmission signal between the one or more nodes and then transmitting, to the specific terminal, a power transmission signal of which the phase synchronization has been matched by means of the one or more modes.

Advanced Modulation Schemes for Short-Range Optical Communications

Abstract: The performance of advanced modulation schemes for spectrally efficient data transmission is reviewed, targeting short-range intensity-modulated optical channels with direct detection. Hereby, the focus lies on the performance of multilevel pulse-amplitude modulation combined with electronic equalization and, as an alternative modulation scheme, discrete multitone. A comprehensive statistical analysis of clipping noise is presented and exact expressions for the performance of symmetrically clipped discrete multitone are derived. It is shown that the clipping noise is impulsive and obeys a generalized Laplace distribution. The bit-error probability due to clipping is studied in detail, and it is found that the impact of clipping noise is reduced for an increasing number of subchannels. Finally, the optical link margins of multilevel pulse-amplitude modulation in combination with electronic equalization and that of discrete multitone in combination with margin-adaptive bit loading are compared. It is found that even symmetrically clipped discrete multitone suffers from its large crest factor in the peak-power-limited channel and that, in many instances, pulse-amplitude modulation provides higher link margins for the same target bit-error probability.

Signal Transmission by Galvanic Coupling Through the Human Body

Abstract: Galvanic coupling is a promising approach for wireless intrabody data transmission between sensors. Using the human body as a transmission medium for electrical signals becomes a novel data communication technique in biomedical monitoring systems. In this paper, special attention is given to the coupling of the current into the human body. Safety requirements have to be fulfilled, and optimal signal coupling is of essence. Therefore, different electrodes are compared. A test system offers up to 1 mA contact current modulated in the frequency range of 10 kHz to 1 MHz. The injected current is up to 20 times below the maximum allowed contact current. Such a low-current approach enables data communication that is more energy saving than other wireless technologies.

Method and apparatus for contention-based uplink data transmission

Abstract: A method and apparatus for a wireless transmit receive unit (WTRU) to use a contention-based uplink communications channel, applies a rule-based restriction of access to the contention-based uplink channel that attempts to use at least one contention-free uplink channel allocation for uplink transmissions on a condition that at least one contention-free uplink channel allocation has been granted.

InP Photonic Integrated Circuits

Abstract: InP is an ideal integration platform for optical generation, switching, and detection components operating in the range of 1.3-1.6 m wavelength, which is preferred for data transmission in the most prevalent silica-based optical fiber. We review the current state of the art in advanced InP photonic ICs.

Time-Critical data delivery in wireless sensor networks

Abstract: A number of wireless sensor network (WSN) applications demand timely data delivery. However, existing WSNs are designed to conserve energy and not to support timely data transmission. This paper shows how WSNs can be dimensioned, deployed and operated such that both reliable and timely data delivery is ensured while scarce energy is preserved. The presented solution employs a novel Medium Access Control (MAC) protocol that incorporates topology control mechanisms to ensure timely data delivery and reliability control mechanisms to deal with inherently fluctuating wireless links. An industrial process automation and control scenario at an oil refinery in Portugal is used to define protocol requirements. The paper details a TinyOS implementation of the protocol and its evaluation in a testbed. Under high traffic load, the protocol delivers 100% of data in time using a maximum node duty cycle as little as 2.48%. In an idle network a maximum node duty cycle of only 0.62% is achieved. This proposed protocol is thus an extremely energy efficient solution for time-critical data delivery.

Optical Communications for High-Altitude Platforms

Abstract: This paper contains a review of technologies, theoretical studies, and experimental field trials for optical communications from and to high-altitude platforms (HAPs). We discuss the pointing, acquisition, and tracking of laser terminals and describe how laser beams with low divergence can be used to transmit data at multi-Gigabits per second. Investigating the influence of the atmosphere, background light, and flight qualification requirements on system design, we explain why the data rates in free-space optical communications are still significantly below those possible in today's terrestrial fiber-based systems. Techniques like forward-error correction, adaptive optics, and diversity reception are discussed. Such measures help to increase the data rate or link distance while keeping the bit error ratio and outage probability of the optical HAP communication system low.

Management of mobile hotspot connections

Abstract: A system and method implemented at a communication device or mobile hotspot device for providing access to data services over a wireless communication network to one or more user devices The communication or hotspot device is adapted to provide router, mobile hotspot-type or ad hoc networking functions to user devices such as laptops, tablets, and other entertainment and productivity devices adapted to communicate over a wireless LAN and to provide access to broadband data services The data services provided over the network may be subject to one or more data allowances or data transfer limits The communication or hotspot device is adapted to manage the user device connections and data transfer to and from the communication device within the data allowances or according to predetermined prioritization

Millimeter-Wave Photonic Components for Broadband Wireless Systems

Abstract: We report on advanced millimeter-wave (mm-wave) photonic components for broadband radio transmission. We have developed self-pulsating 60-GHz range quantum-dash Fabry-Perot mode-locked laser diodes (MLLD) for passive, i.e., unlocked, photonic mm-wave generation with comparably low-phase noise level of -76 dBc/Hz @ 100-kHz offset from a 58.8-GHz carrier. We further report on high-frequency 1.55-μm waveguide photodiodes (PD) with partially p-doped absorber for broadband operation (f3dB ~70-110 GHz) and peak output power levels up to +4.5 dBm @ 110 GHz as well as wideband antenna integrated photomixers for operation within 30-300 GHz and peak output power levels of -11 dBm @ 100 GHz and 6-mA photocurrent. We further present compact 60-GHz wireless transmitter and receiver modules for wireless transmission of uncompressed 1080p (2.97 Gb/s) HDTV signals utilizing the developed MLLD and mm-wave PD. Error-free (BER = 10-9, 231 - 1 PRBS, NRZ) outdoor wireless transmission of 3 Gb/s over 25 m is demonstrated, as well as wireless transmission of uncompressed HDTV signals in the 60-GHz band. Finally, an advanced 60-GHz photonic wireless system offering record data throughputs and spectral efficiencies is presented. For the first time, we demonstrate photonic wireless transmission of data throughputs up to 27.04 Gb/s (EVM 17.6%) using a 16-QAM OFDM modulation format resulting in a spectral efficiency as high as 3.86 b/s/Hz. Wireless experiments were carried out within the regulated 57-64-GHz band in a lab environment with a maximum transmit power of - 1 dBm and 23 dBi gain antennas for a wireless span of 2.5 m. This span can be extended to some 100 m when using high-gain antennas and higher transmit power levels.

Transmitting uplink control information over a data channel or over a control channel

Abstract: Methods and apparatus are described for a User Equipment (UE) to concurrently transmit Uplink Control Information (UCI) and data information to a Node B. The Node B can configure the UE whether to transmit UCI together with data information in a Physical Uplink Shared CHannel (PUSCH) or, separate from data information, in a Physical Uplink Control CHannel (PUCCH). The UE may also determine whether to transmit UCI together with data information in the PUSCH or, separate from data information, in the PUCCH based on criteria involving either a ratio of PUSCH resources for UCI transmission over the total PUSCH resources, a Modulation and Coding Scheme (MCS) for the data information, an absolute power difference between PUCCH and PUSCH transmissions, whether the PUSCH transmission uses frequency hopping, or whether spatial multiplexing is used for the data transmission.

Effective Capacity Analysis of Cognitive Radio Channels for Quality of Service Provisioning

Abstract: In this paper, the performance of cognitive radio systems is studied when the secondary users operate under statistical quality of service (QoS) constraints. In the cognitive radio channel model, secondary users initially perform channel sensing, and then engage in data transmission at two different average power levels depending on the channel sensing results. A state transition model is constructed to model this cognitive transmission channel. Statistical QoS constraints are imposed as limitations on buffer violation probabilities. Effective capacity of the cognitive radio channel, which provides the maximum throughput under such QoS constraints, is determined. This analysis is conducted for fixed-power/fixed-rate, fixed-power/variable-rate, and variable-power/variable-rate transmission schemes under different assumptions on the availability of channel side information (CSI) at the transmitter. The interactions and tradeoffs between the throughput, QoS constraints, and channel sensing parameters (e.g., sensing duration and threshold, and detection and false alarm probabilities) are investigated. The performances of fixed-rate and variable-rate transmission methods are compared in the presence of QoS limitations. It is shown that variable schemes outperform fixed-rate transmission techniques if the detection probabilities are high. Performance gains through adapting the power and rate are quantified and it is shown that these gains diminish as the QoS limitations become more stringent.

Line-of-sight visible light communication system design and demonstration

Abstract: With the emergence of high lumen output white lighting LEDs, visible light communication has become an intriguing technology to realize low cost data transmission in conjunction with lighting. In this paper, we discuss some key issues in practical line-of-sight (LOS) visible light communication system design. They include a transmitter, channel model, and receiver front-end. A transmitter containing white LED arrays should be designed to achieve required lighting distribution and reliable data transmission at the same time. A basic LOS channel model to characterize the path loss and received optical power is also needed for optimization of the receiver under major noise sources. Finally we show a practical indoor LOS visible light communication system to demonstrate some of our analyses.

Driver risk assessment system and method having calibrating automatic event scoring

Abstract: A Driver Risk Assessment System and Method Having Calibrating Automatic Event Scoring is disclosed. The system and method provide robust and reliable event scoring and reporting, while also optimizing data transmission bandwidth. The system includes onboard vehicular driving event detectors that record data related to detected driving events and selectively store or transfer data related to said detected driving events. If elected, the onboard vehicular system will score a detected driving event, compare the local score to historical values previously stored within the onboard system, and upload selective data or data types to a remote server or user if the system concludes that a serious driving event has occurred. Importantly, the onboard event scoring system, if enabled, will continuously evolve and improve in its reliability by being periodically re-calibrated with the ongoing reliability results of manual human review of automated predictive event reports. The system may further respond to independent user requests by transferring select data to said user at a variety of locations and formats.

Wireless digital data transmission at 300 GHz

Abstract: Recently, analogue video signal transmission at 300 GHz has been demonstrated using a versatile Schottky mixer based measurement system designed for terahertz communication channel modelling and propagation studies. In this reported work, digital signal transmission at 300 GHz using this system is demonstrated and analysed. The performance of the digital transmission setup is characterised with respect to phase noise and modulation errors. For demonstration, high data rate digital video signals have been transmitted over a distance of up to 52 m.

Design of Learning-Based MIMO Cognitive Radio Systems

Abstract: This paper addresses the design issues of the multiantenna-based cognitive radio (CR) system that is able to concurrently operate with the licensed primary-radio (PR) system. We propose a practical CR transmission strategy consisting of three major stages, namely, environment learning, channel training, and data transmission. In the environment-learning stage, the CR transceivers both listen to the PR transmission and apply blind algorithms to estimate the spaces that are orthogonal to the channels from the PR. Assuming time-division duplex (TDD)-based transmission for the PR, cognitive beamforming is then designed and applied at CR transceivers to restrict the interference to/from the PR during the subsequent channel-training and data-transmission stages. In the channel-training stage, the CR transmitter sends training signals to the CR receiver, which applies the linear-minimum-mean-square-error (LMMSE)-based estimator to estimate the effective channel. Considering imperfect estimations in both learning and training stages, we derive a lower bound on the ergodic capacity that is achievable for the CR in the data-transmission stage. From this capacity lower bound, we observe a general learning/training/throughput tradeoff associated with the proposed scheme, pertinent to transmit power allocation between the training and transmission stages, as well as time allocation among the learning, training, and transmission stages. We characterize the aforementioned tradeoff by optimizing the associated power and time allocation to maximize the CR ergodic capacity.

Integration of false data detection with data aggregation and confidential transmission in wireless sensor networks

Abstract: In wireless sensor networks, compromised sensor nodes can inject false data during both data aggregation and data forwarding. The existing false data detection techniques consider false data injections during data forwarding only and do not allow any change on the data by data aggregation. However, this paper presents a data aggregation and authentication protocol, called DAA, to integrate false data detection with data aggregation and confidentiality. To support data aggregation along with false data detection, the monitoring nodes of every data aggregator also conduct data aggregation and compute the corresponding small-size message authentication codes for data verification at their pairmates. To support confidential data transmission, the sensor nodes between two consecutive data aggregators verify the data integrity on the encrypted data rather than the plain data. Performance analysis shows that DAA detects any false data injected by up to T compromised nodes, and that the detected false data are not forwarded beyond the next data aggregator on the path. Despite that false data detection and data confidentiality increase the communication overhead, simulation results show that DAA can still reduce the amount of transmitted data by up to 60% with the help of data aggregation and early detection of false data.

Performance—Complexity Comparison of Receivers for a LTE MIMO–OFDM System

Abstract: Implementation of receivers for spatial multiplexing multiple-input multiple-output (MIMO) orthogonal-frequency- division-multiplexing (OFDM) systems is considered. The linear minimum mean-square error (LMMSE) and the -best list sphere detector (LSD) are compared to the iterative successive interference cancellation (SIC) detector and the iterative -best LSD. The performance of the algorithms is evaluated in 3G long-term evolution (LTE) system. The SIC algorithm is found to perform worse than the -best LSD when the MIMO channels are highly correlated, while the performance difference diminishes when the correlation decreases. The receivers are designed for 22 and 4 4 antenna systems and three different modulation schemes. Complexity results for FPGA and ASIC implementations are found. A modification to the -best LSD which increases its detection rate is introduced. The ASIC receivers are designed to meet the decoding throughput requirements in LTE and the -best LSD is found to be the most complex receiver although it gives the best reliable data transmission throughput. The SIC receiver has the best performance-complexity tradeoff in the system but in the 4 4 case, the -best LSD is the most efficient. A receiver architecture which could be reconfigured to using a simple or a more complex detector as the channel conditions change would achieve the best performance while consuming the least amount of power in the receiver.

Energy Efficient State Estimation With Wireless Sensors Through the Use of Predictive Power Control and Coding

Abstract: We study state estimation via wireless sensors over fading channels. Packet loss probabilities depend upon time-varying channel gains, packet lengths and transmission power levels of the sensors. Measurements are coded into packets by using either independent coding or distributed zero-error coding. At the gateway, a time-varying Kalman filter uses the received packets to provide the state estimates. To trade sensor energy expenditure for state estimation accuracy, we develop a predictive control algorithm which, in an online fashion, determines the transmission power levels and codebooks to be used by the sensors. To further conserve sensor energy, the controller is located at the gateway and sends coarsely quantized power increment commands, only whenever deemed necessary. Simulations based on real channel measurements illustrate that the proposed method gives excellent results.

Sensor Network for Structural Health Monitoring of a Highway Bridge

Abstract: A bridge monitoring TestBed is developed as a research environment for sensor networks and related decision-support technologies. A continuous monitoring system, capable of handling a large number of sensor data channels and three video signals, is deployed on a four-span, 90-m long, reinforced concrete highway bridge. Of interest is the integration of the image and sensor data acquisition into a single computer, thereby providing accurate time synchronization between the response and corresponding traffic loads. Currently, video and acceleration records corresponding to traffic induced vibration are being recorded. All systems operate online via a high-speed wireless Internet network, allowing real-time data transmission. Elements of the above health monitoring framework are presented herein. Integration of these elements into an automated functional system is emphasized. The recorded data are currently being employed for structural system identification via a model-free technique. Effort is also underway to correlate the moving traffic loads with the recorded accelerations. Finally, the TestBed is available as a resource for verification of new sensor technologies, data acquisition/ transmission algorithms, data mining strategies, and for decision-support applications.

Terahertz wireless communication link at 300 GHz

Abstract: We present a terahertz wave wireless link operating at 300 GHz which has a potential for use in ultra fast future wireless services in short range. Terahertz wave was generated and modulated with photonic technologies in the transmitter, allowing us to use radio on fiber system concept as well. For the receiver, we used a Schottky barrier diode detector integrated with a planar antenna. With the link, error free data transmission at 12.5 Gbps was experimentally demonstrated. Taking the performance margin of the transmitter and receiver into consideration, we believe that even up to 20-Gbps data can be transmitted.

Transmission Impairments in DWDM Networks With Reconfigurable Optical Add-Drop Multiplexers

Abstract: Reconfigurable optical add/drop multiplexers (ROADMs) based on 1 X N wavelength-selective switches (WSS) are evolving to support DWDM networks with higher capacity and increased flexibility in wavelength routing. Different WSS technologies can be employed to provide colorless and steerable functionality for ring, or meshed architectures. Improvements in specifications of WSS modules operating on the 50 GHz wavelength grid have enabled 40 Gb/s transmission rates through extensive ROADM networks. The same ROADMs are also expected to support 100 Gb/s transmission in the near future. In parallel, development of lower-cost WSS technologies is allowing ROADMs to expand into edge networks. In all these network applications, propagation through multiple ROADMs generates transmission penalties for the DWDM channels, which need to be factored into the network design. Such OSNR or Q factor penalties can be induced by passband narrowing, imperfect isolation across the signal bandwidth, insertion loss, PDL, and other effects. The impact of these impairments depend on the transmitter and receiver types (e.g., data rate and modulation format), and on the WSS characteristics (e.g., insertion loss, passband width, shape, isolation magnitude and isolation stopband). Key transmission impairments such as bandpass narrowing, crosstalk, insertion loss, and PDL are estimated based on experiments and numerical simulations for common data rates and modulation formats. Implications of temporal fluctuations during power setting throughout a ROADM network are also discussed.

Buffer status reporting in a mobile communication system

Abstract: The invention relates to methods for transmitting a buffer status report (BSR) in a mobile communication system, more particularly to the definition of rules for triggering, generating and transmitting BSRs. The invention also relates to a data transmission method utilizing new rules to decide data of which radio bearers is transmitted within a given transmission time interval. Moreover, the invention relates to scheduling method for radio resources that is taking into account additional scheduling-relevant information from the buffer status reporting and/or data transmission method. To avoid unnecessary grants from the network and to suggest an advanced handling of data transmissions the invention suggests a buffer status reporting and data transmission schemes that take into account the scheduling mode of data of radio bearers pending for transmission to decide whether to report on it in a buffer status report, respectively, whether to multiplex the data to a transport block for transmission.

Chaos-on-a-chip secures data transmission in optical fiber links.

Abstract: Security in information exchange plays a central role in the deployment of modern communication systems. Besides algorithms, chaos is exploited as a real-time high-speed data encryption technique which enhances the security at the hardware level of optical networks. In this work, compact, fully controllable and stably operating monolithic photonic integrated circuits (PICs) that generate broadband chaotic optical signals are incorporated in chaos-encoded optical transmission systems. Data sequences with rates up to 2.5 Gb/s with small amplitudes are completely encrypted within these chaotic carriers. Only authorized counterparts, supplied with identical chaos generating PICs that are able to synchronize and reproduce the same carriers, can benefit from data exchange with bit-rates up to 2.5Gb/s with error rates below 10−12. Eavesdroppers with access to the communication link experience a 0.5 probability to detect correctly each bit by direct signal detection, while eavesdroppers supplied with even slightly unmatched hardware receivers are restricted to data extraction error rates well above 10−3.

Adaptive joint scheduling of spectrum sensing and data transmission in cognitive radio networks

Abstract: We consider a cognitive radio (CR) network that makes opportunistic use of a set of channels licensed to a primary network. During operation, the CR network is required to carry out spectrum sensing to detect active primary users, thereby avoiding interfering with them. However, spectrum sensing may cause negative effect on the performance of the CR network, as all CR communications has to be postponed during channel sensing. This paper focuses on adaptively scheduling spectrum sensing and data transmission so that negative impacts to the performance of the CR network are minimized. We first consider the case when CR nodes always have data to transmit and experience time-varying channels. Based on knowledge of channel conditions, the sensing periods are adaptively scheduled to maximize the spectrum efficiency of the CR operation. We show how optimal sensing/transmission scheduling policies can be obtained and prove some important structural properties of such optimal policies. We then consider the case when CR nodes experience both stochastic data arrival and time-varying channels. By treating each sensing period as a 'virtual sensing packet'', we convert the problem of joint spectrum-sensing/datatransmission scheduling into a standard queueing model. Based on that, an efficient scheduling algorithm that takes into account channel and queue conditions of the CR network is proposed.