Association in 802.11ah
802.11ah should support more than 8000 stations. One of the most important challenges in 802.11ah is how to support the large number of stations efficiently. The problem become worst when network resets and all stations try to authentication/association simultaneously. Figure 1 shows the association procedure of 802.11ah. Figure 2 shows the simulation result of total number of successful association in first 300 seconds when number of stations contending varies. Qualnet network simulator is used to simulate Distributed coordination function (DCF) mode of standard 802.11. As it can be seen form the figure that as the number of station is increased, the total number of successful association decreased. The figure shows that when the number of total station is just 200, only 40 stations can successfully associate in first 300 seconds. The delay associated with authentication is due to collision. From the figure, we can speculate that how long it will take if 8000 stations try to authentication /association simultaneously. Thus, there is a need for a new fast and efficient association method for IEEE 802.11ah.
[ Authentication/Association Procedure ]
[ Performance Results for Authentication/Association of Stations within 300 Seconds Limit ]
Indoor Positioning with IR UWB radar
Recently, the researcher are very interested in indoor positioning systems (IPSs). Because the global position system (GPS) is not working at all. Therefore, to track the moving target in indoor environment, the variant tracking systems should be developed using other sensor methods such as ultrasound, Wi-Fi, infrared-ray, and impulse radio-ultra-wideband (IR-UWB) radar. In our laboratory, we have been developing and studying the IPS using the IR-UWB radar. This work is needed for the specific signal processing procedures such as background subtraction, detection, localization, and tracking procedure.
Pedestrian Dead Reckoning(PDR) Enables Indoor Navigation System
Inertial Measurement Unit(IMU) has advantage that can track the location of the pedestrian without any additional external device. Especially, MEMs IMU can be utilized at various locations on the body part as a low-cost and compact devices. However, It is required to design a variety of filters or algorithms to address the problem of measurement errors caused by low signal sampling rate. The system block diagram of a basic pedestrian tracking method is shown below. Generally, step detection and step length Estimation is measured by accelerometer, pedestrian heading is estimated by fused gyroscope and magnetic field sensor and pedestrian location is estimated by the trigonometric function about step length and heading calculated.
[ Procedure of message exchange using SDS-TWR ]
Two-way ranging with IEEE 802.15.4a UWB device
Ultra Wide Band 802.15.4a transceiver technology is emerging as an ideal fit for the requirements of the next generation wireless sensor network. IEEE has recognized the need to standardize UWB technology for use in personal area networks and has established the IEEE 802.15.4a standard specifying a new UWB physical layer for wireless sensor network. 802.15.4a has two formats of communication signal; the first one is impulse radio ultra wide-band (IR-UWB) signals and the second one is chirp spread spectrum (CSS) signals. The CSS is suitable for data communication purposes while the IR-UWB has the capability of precise ranging.
Two-way ranging for measuring the distance between UWB transceivers (tag and anchor) calculates time of arrival (TOA) by exchanging positioning messages. Specifically, IEEE 802.15.4a recommends applying symmetric double sided two way ranging (SDS-TWR).
[ Procedure of message exchange using SDS-TWR ]
Wireless Multimedia Transmission
A compressed video bitstream is extremely sensitive to errors when it is transmitted via networks with bursty packet loss such as 3G and mobile WiMAX. Error Resilience and error concealment techniques have been used to improve the recovered picture quality in cases in which parts of the coded picture are not available at the decoder for reconstruction. However, most conventional error resilient and concealment methods show low image reconstruction performance due to unavailable motion information about image blocks during burst packet loss, and require high energy consumption to find the best error concealment method in the receiver. We develop cross-layer designed channel-aware error resilient streaming video system over unreliable wideband wireless channel envrionment such as 3G, WLAN, mobile WiMax.
[ Concept of channel-aware video transmission over WiBro ]
Enerqy Saving in Low Duty Cycle MAC Protocols
Radio occupies the largest share of the energy consumption in most of the sensor nodes. The MAC protocols of WSN save energy by putting the nodes into sleep mode, i.e., turning the radio off as long as possible. This introduction of sleep/listen cycle motivates the introduction of synchronization. Therefore, these protocols locally manage synchronization and the synchronized schedule can be controlled by periodic SYNC packet broadcasted to their neighbors. Contention-based random access MAC protocols have separate parts for data and SYNC packets in their listen period. We develop intelligent network synchronization (INS), which lets the node bypass their SYNC period by exploiting the periodic nature of SYNC packet. INS allows the nodes to be in sleep state in their SYNC period when they recognize that nobody has SYNC packet queued in the current SYNC period.
[ Exchange of SYNC packets between two nodes in S-MAC and S-MAC with INS ]
Enerqy Efficient Routing Protocol over Wireless Sensor Network
Large amount of energy is consumed unnecessarily in the network because of large data message gathering at the sensor nodes, large distance of the nodes from the base station (BS) and redundant data transmission in the network. Cluster based routing protocols are proven to perform best in energy conservation in the network. Our research work is focused on developing the energy efficient routing protocols based on clustering schemes. The main work includes designing of algorithms for forming the clusters and routing path so as to reduce the total transmission distance for each sensor node to transmit the data to the cluster heads (CH) and to the BS, to reduce the size of data messages in the CHs, and to reduce the redundant data transmission in the network.
[ General topology of clustering scheme for WSN ]