Ruonan Zhang

A hybrid reservation/contention-based MAC for video streaming over wireless networks

To reserve or not for bursty video traffic over wireless access networks has been a long-debated issue. For uplink transmissions in infrastructure-based wireless networks and peer-to-peer transmissions in mesh or ad-hoc networks, reservation can ensure the Quality-of-Service (QoS) provisioning at the cost of a lower degree of resource utilization. Contention-based Medium Access Control (MAC) protocols are more flexible and efficient in sharing resources by bursty traffic to achieve a higher multiplexing gain, but the performance may degrade severely when the network is congested and collisions occur frequently. More and more wireless standards adopt a hybrid approach, which allows the coexistence of resource reservation and contention-based MAC protocols. However, how to cost-effectively support video traffic using hybrid MAC protocols is still an open issue. In this paper, we first propose how to use hybrid MAC protocols to support video streaming over wireless networks. Then, we quantify the performance of video traffic over wireless networks with contention-only, reservation-only, and hybrid MAC protocols, respectively. Admission regions for video streams with these three approaches are obtained. Using the standard WiMedia MAC protocols as an example, extensive simulations with a commonly-used network simulator (NS-2) and real video traces are conducted to verify the analysis. The analytical and simulation results reveal the tradeoff between reservation and contention-based medium access strategies, and demonstrate the effectiveness of the hybrid approach.

Performance Analysis of Distributed Reservation Protocol for UWB-Based WPAN

In this paper, we propose an analytical model for the distributed reservation protocol (DRP), which is defined in the WiMedia specification for ultra-wideband (UWB)-based wireless personal area networks (WPANs). We model the tagged user as a discrete-time queue with vacations, which captures the joint behavior of a queue length variation and a time-varying UWB channel due to shadowing under a given reservation pattern. Furthermore, we consider two reservation methods: hard reservation and soft reservation. With the hard reservation, a time slot is exclusively used by the owner, whereas the unused time slots can be accessed by other users using the soft reservation. Closed-form expressions of important performance metrics such as the mean service time, the waiting time, and the throughput are derived. Through numerical results, we validate the accuracy of the proposed analytical model and investigate the interaction between the DRP and various system parameters. This paper should provide insights into the performance of the DRP and useful guidelines to further improve the protocol to support isochronous applications with a tight delay requirement in UWB-based WPANs.

A packet-level model for UWB channel with people shadowing process based on angular spectrum analysis

Ultra-wideband (UWB) wireless communication technologies have been proposed to support high data rate multimedia services in office or residential environments. Due to the low transmission power of UWB, the shadowing effect by moving people can considerably reduce the received signal quality and thus significantly degrade the quality of service (QoS) of on-going transmissions. An open issue is to build a simple model which captures the temporal variation of UWB channels and the packet error rate (PER) due to the people shadowing effect (PSE), which will be a useful tool for upper layer protocol performance analysis and simulation. This paper presents an analytical study of the PSE and the temporal variation of UWB channels induced by the motion of a person. First, we derive the angular power spectral density (APSD) of the indoor UWB channel impulse response (CIR), and the PSE in terms of signal power attenuation. Second, based on a two-dimensional random walk mobility model, the PER variation due to people shadowing is modeled as a finite-state Markov chain (FSMC). The investigation of APSD provides important insights on the spatial propagation characteristics of UWB signals. The proposed packet-level channel model can be conveniently incorporated into analytical frameworks and simulation tools for evaluating upper-layer protocols of UWB networks.

Joint AMC and Packet Fragmentation for Error Control Over Fading Channels

Error control is critical for wireless networks to combat channel fading and ensure efficient resource utilization. Adaptive modulation and coding (AMC) in the physical (PHY) layer and packet fragmentation and automatic repeat request (ARQ) in the link layer are widely used error-control mechanisms. However, how to jointly optimize them in both layers for high-rate wireless networks is still open. In this paper, using the WiMedia ultrawideband (UWB) networks as an example, we first develop a general analytical framework to quantify the link delay and loss performance considering the channel fading, the joint error-control mechanisms, and the arbitrary reservation-based media access control (MAC) protocol. Second, we introduce a cross-layer design to optimize the PHY-layer AMC and the link-layer packet fragmentation and propose a joint-adaptation mechanism that is simple to implement and has near-optimal performance. Numerical results reveal that fragmentation has a greater impact than AMC on the delay and loss performance for marginal links and that the proposed joint-adaptation strategy is efficient for high-rate wireless networks.

Two-Dimensional DoA Estimation for Multipath Propagation Characterization Using the Array Response of PN-Sequences

Multipath propagation and power arrival profiles in three-dimensional (3-D) space determine the performance of the full-dimensional MIMO (FD-MIMO) systems. Field channel measurements are crucial in characterizing wireless channel properties. Nevertheless, in spatial channel measurements, estimating the direction-of-arrivals (DoAs) of multipath components (MPCs) is a challenging issue, because of the large number of propagation paths and the correlation among the multipath signals. The number of incidence angles and estimation precision in traditional methods is limited by the sensor array size and signal correlation. In this paper, we propose a scheme for measuring and estimating the 2-D DoAs of propagation paths called multipath angular estimation using the array response of PN-sequences (MAPS). By using a receiving planar antenna array (PAA), MAPS first extracts the complex path array response vector (PARV) for each propagation path and then estimates the DoAs of the paths individually and independently. The subspace-decomposition theory for MAPS is proved and extensive simulations are conducted to compare MAPS with other algorithms. Furthermore, a channel sounder using two PAAs and the probing signal of 2.6 GHz carrier modulated by PN-sequences has been developed. The simulation and field tests show that MAPS can estimate arbitrary number of resolved MPCs in a channel snapshot and effectively suppress the multipath interference.

An Adaptive Combination Query Tree Protocol for Tag Identification in RFID Systems

A reader must be able to identify tags as quickly as possible, so tag anti-collision is a significant issue for the RFID system. In this paper, we propose a novel tag anti-collision protocol (ACQT), which is suitable for a large mobile tags environment. Based on a 3-ary tree, it has the optimal capacity to identify for tree-based protocols. Using a combination query tree, it can solve the problem of not being able to generate a 3-ary tree when the length of a tags ID is not a multiple of 3. The joining and leaving strategies can efficiently be applied for tag mobility. The simulation results show that the protocol we present can achieve better performance than previous protocols by decreasing identification delay, collision cycles and idle cycles.

One Handshake Can Achieve More: An Energy-Efficient, Practical Pipelined Data Collection for Duty-Cycled Sensor Networks

To alleviate long sleep latency due to duty-cycled radio operations, existing collection protocols adopted pipelined scheduling techniques, which stagger the sleep-wakeup schedules of nodes along forwarding paths, requiring accurate time synchronization as underlying support. They either ignored the synchronization issue or just assumed that a local synchronization scheme over non-duty-cycled radios could meet the requirement, however, which may lead to a significant synchronization issue in practice. In this paper, we propose a practical pipelined data collection (PDC) protocol for duty-cycled sensor networks. PDC adopts an inter-layer incorporation of network and media access control layers. It only relies on an RTS/CTS-like handshake with a set of the proposed algorithms, not only for data transmission as commonly utilized, but also for pipelined scheduling and schedule synchronization, data-gathering tree establishment, and network topology control and maintenance, all of which are naturally and seamlessly incorporated together and able to support each other. PDC has been implemented in the Contiki operating system. The testbed evaluations based on two hardware platforms (Z1 and MicaZ) and the compared results with a de facto standard for data collection based on the fully emulated Z1 in Cooja have demonstrated its practicality and efficacy.

Angular spread measurement and modeling for 3D MIMO in urban macrocellular radio channels

When planar antenna arrays are introduced at the base stations (BSs) of cellular networks, accurate modeling of the spatial radio channel in there-dimension (3D) will become crucial. In this paper, a field measurement campaign was conducted to characterize the 3D spatial propagation to the BS in urban street canyon environments. Using a wideband channel sounder with two uniform planar antenna arrays, the angular spread (AS) in both azimuth and elevation domains at the BS was measured with the user equipment (UE) located at 100 different positions in LOS scenario and 95 positions in NLOS scenario. Contrary to the expected simple correlation between the large-scale fading (like path loss) and AS, our measurements have shown much more complicated relationship between AS and the surrounding environment. It is demonstrated that, in LOS scenario, the AS has positive correlation with the UE-to-BS distance in wide street, but negative correlation in narrow street. In NLOS scenario, the correlation between AS and the transceiver distance is positive when the UE is too close to a building to have “Quasi-LOS” path (diffracted over the building roof) propagation. However, if “Quasi-LOS” path is strengthened, the correlation is reversed. These observations lead to the need of more sophisticated models on the angular characteristics in urban radio channels. Based on the collected data, we also present the Laplacian distribution models of AS in both azimuth and elevation domains.

Delay Analysis of Distributed Reservation Protocol with UWB Shadowing Channel for WPAN

Ultra-wideband (UWB) technology is expected to provide high data rate services for future wireless personal area networks (WPANs). The WiMedia Alliance recently has launched its standard for UWB-based WPANs, where the distributed reservation protocol (DRP) is specified to allow the channel time being reserved in a distributed manner. In view of the urgent need of using DRP to support high data rate multimedia applications, we investigate the delay performance of DRP in this paper. Since the negotiation of channel time is fully distributed without centralized coordination, the reserved channel time may be non-evenly spaced. In addition, the channel dynamics due to shadowing that is notable in indoor environments can greatly affect the protocol performance. In this paper, we study the delay performance of DRP under different reservation patterns and take into account the dynamics of UWB shadowing channel. The system is modeled as a discrete-time single server queue with vacation, which can be represented by the quasi-birth and death (QBD) process and solved by the well-established matrix-geometric approach. We use numerical results to validate the accuracy of the mathematical modeling. The proposed analytical model can be useful to understand the actual performance of DRP, thereby further performance improvement can be guided.

Measurement and modeling of indoor channels in elevation domain for 3D MIMO applications

The 3-dimensional multi-input-multi-output (3D MIMO) scheme is one of the enabling technologies for next generation mobile communications and is expected to increase the capacity considerably by realizing the beamforming in the elevation domain. With the small cell architecture, 3D MIMO may be deployed in indoor environments. Thus 3D spatial measurement and characterization of the indoor channels are critical, which have been rarely conducted. In this paper, indoor channel measurements are performed at 2.6 GHz in typical hall and corridor environments including both line-of-sight (LOS) and non-line-of-sight (NLOS) scenarios. Using a sounder with two uniform planer antenna arrays, spatial channel responses were captured at about 200 positions. The multipath channel impulse responses (CIR) are extracted and the angle-of-arrival of each propagation path is estimated jointly in azimuth and elevation domains. Then the distributions and models of the incident angles and angular power distribution in the elevation domain are established. Our measurements and proposed models can support the design and simulation of 3D MIMO working in the indoor environment in the next generation cellular networks.