Kab Seok Ko

A Novel Random Access for Fixed-Location Machine-to-Machine Communications in OFDMA Based Systems

Machine-to-machine (M2M) communications typically exhibit features such as a large number of devices, low data-rates, small-sized packets, and low or no mobility, while human-to-human (H2H) communications typically support a small number of users, high data-rates, large-sized packets, and high mobility. To support M2M communications in future cellular systems, one of the most challenging problems is to resolve a collision problem in random access because of access attempts from a large number of devices. For a large class of fixed-location M2M services such as smart metering and remote sensing, each machine device has fixed uplink timing alignment (TA) due to a fixed distance between the machine device and its eNodeB. We propose a novel random access scheme based on its fixed TA information for M2M communications at a large number of fixed-location machine devices in future orthogonal frequency division multiple access (OFDMA)-based cellular systems like Long Term Evolution (LTE) system. The proposed random access scheme yields significantly lower collision probability, shorter access delay, and higher energy-efficiency, compared with the conventional random access scheme.

Spatial Group Based Random Access for M2M Communications

We expect that the number of machine-to-machine (M2M) devices will rapidly increase in the near future due to a growing demand for a wide range of M2M applications such as e-health, public safety, surveillance, remote maintenance and control, and smart metering. Therefore, the future cellular networks should accommodate a large number of M2M devices and their random access (RA) requests at a specific time instant. In this letter, we propose a novel RA scheme to effectively increase the number of available preambles for the future M2M communication environment. The proposed scheme provides additional preambles by spatially partitioning a cell coverage into multiple group regions and reducing cyclic shift size in RA preambles. As a result, the proposed RA scheme can accommodate a significantly larger number of machine nodes with much lower collision probability and shorter random access delay, compared to a conventional RA scheme.

Prioritized random access for machine-to-machine communications in OFDMA based systems

In machine-to-machine (M2M) communications, machine nodes perform a random access (RA) to synchronize with the eNodeB in order to transmit their new packets. However, the conventional RA cannot support prioritized accesses during the RA procedure, even though there are a variety of classes with different quality-of-service (QoS). In this paper, we propose a prioritized random access (PRA) scheme, which enables machine nodes to indicate their access priority during the access phase by differentiating their transmission power level. Moreover, we thoroughly analyze the PRA scheme in terms of collision probability and access delay. The performance evaluation shows that the PRA scheme enables the nodes to indicate the access priority according to their QoS during the RA procedure as well as to achieve much lower collision probabilities and shorter access delay.

A group-based communication scheme based on the location information of MTC devices in cellular networks

Recent standardization efforts in the 3rd Generation Partnership Project (3GPP) have made the Long Term Evolution (LTE) system a very attractive radio access technology for various types of devices and applications. However, the system has not yet been optimized to support diverse traffic profiles. In this paper, we propose a group-based communication scheme to alleviate inefficiencies in the radio access network. The proposed scheme groups multiple connections triggered by different User Equipments (UEs). For the efficient grouping of the UEs, the location information of the Machine-Type Communication (MTC) devices is utilized to identify each group. A four-phase system model is presented to analyze each segment of the connection establishment procedure in the access network. We found that the random access procedure is the main variable factor for the communication delay, and we, therefore, carefully design the procedure with details. Simulations are conducted on our LTE testbeds, and the results show that the proposed scheme is very effective in ensuring the system performance of legacy services and in utilizing radio resources within a given delay bound.

A random access scheme based on a special preamble for supporting emergency alarms

In emergency situations such as a disaster and a blackout, machine nodes should report emergency alarms, including the disaster type and disaster area to an M2M server. When many related machine nodes attempt a random access (RA) to report emergency alarms, and this may cause a physical random access channel (PRACH) overload. In this case, the machine nodes hardly access the network within a given delay requirement of emergency alarms. In this paper, we propose a novel random access scheme based on a special preamble for emergency indication in an emergency situation and adaptively reserve more PRACH resources in proportion to the number of machine nodes using the special preamble. The proposed RA scheme outperforms the conventional RA scheme in terms of access delay and can accommodate much more machine nodes in emergency situations, compared with the conventional RA scheme.

Performance-Based Settlement of Frequency Regulation for Electric Vehicle Aggregators

Vehicle-to-grid (V2G) aggregators may have an uncertain capacity submission problem due to random behaviors of pertaining EVs. From the market perspective, we investigate whether the market settlement rules are appropriate enough to induce V2G aggregators to submit an accurate capacity rather than an exaggerated over-capacity. We first review the frequency regulation settlement rules of most representative ISOs in North America such as PJM, MISO, CAISO, and NYISO and several European TSOs. Based on the settlement rules, we analyze how the uncertain submission of V2G aggregators affects the settlements in terms of mileage and accuracy, and perform simulations to evaluate the effect of the exaggerated over-capacity submission on mileage, accuracy, and reward. Through simulations, we show that the conventional frequency regulation settlements fail to induce V2G aggregators to estimate accurate achievable power capacity. To solve this problem, we propose to enhance the accuracy of performance in settlement. We deal with the effect of delay and stochastic properties on payments.

Delays-dependent region partitioning approach for stability criterion of linear systems with multiple time-varying delays

Abstract This paper considers a delay-dependent stability criterion for linear systems with multiple time-varying delays. To exploit all possible information for the relationships among the marginally delayed states ( x ( t − τ i M ) , x ( t − τ i + 1 M ) ), the exactly delayed states ( x ( t − τ i ( t ) ) , x ( t − τ i + 1 ( t ) ) ), and the current state x ( t ) for each pair ( i , i + 1 ) of time-varying delays, a delays-dependent region partitioning approach in double integral terms is proposed. By applying the Wirtinger-based integral inequality and the reciprocally convex approach to terms resulted from the region partitioning, a stability criterion is derived in terms of linear matrix inequalities. Numerical examples show that the resulting criterion outperforms the existing one in literature.

Transmit power optimization for prioritized random access in OFDMA based systems

We have proposed a prioritized random access (PRA) scheme, where each node indicates its access priority by differentiating the transmit power of preamble signatures and the eNodeB utilizes multiple detection thresholds to detect the prioritized preamble signatures. In this paper, in order to make the PRA scheme more robust, we enhance our previously proposed PRA scheme by investigating the effect of wireless channels on the detection performance such as false alarms and mis-detections. We find the optimal detection thresholds and the optimal preamble transmit power levels. Moreover, we analyze the energy consumption during the RA procedure by applying the optimal transmit power levels to the PRA scheme. The performance evaluation shows that the PRA scheme enables each node to achieve differentiated collision probability and access delay according to the access priority with reduced energy consumption, compared to the conventional RA scheme.

Prioritized Random Access for Accommodating M2M and H2H Communications in Cellular Networks

In cellular machine-to-machine (M2M) communications, one of the most challenging issues is to accommodate random access (RA) requests of a massive number of machine nodes while not severely degrading the RA performance of human-to-human (H2H) communications. As the number of RA requests from a massive number of machine nodes increases, human nodes may hardly access the network because the current cellular network cannot support the access priority during the RA procedure. In this paper, we propose a prioritized random access (PRA) scheme, which provides a higher access priority to the human nodes in a co-existing environment of both M2M and H2H by differentiating the transmit power of preamble sequences at the first step of the RA procedure. We mathematically analyze the proposed PRA scheme in terms of collision probability and access delay. The performance evaluation shows that the proposed PRA scheme enables the human nodes to achieve much lower collision probabilities and shorter access delays than the machine nodes, while accommodating a massive number of machine nodes.

A dynamic point of preferred operation (PPO) scheme for charging electric vehicles in a residential area

Recently, the penetration of electric vehicles (EVs) has been expected to increase gradually. From the power grid point of view, a group of EVs could act as a load as well as energy storage. In order to maintain the stability of electric power grid, we need to properly manage an effective charging mechanism for EVs. In this paper, we investigate the benefits of managing EV charging in order to flatten the total electricity load in a residential area. We model a distribution grid in a residential area including homes with several penetration levels of EVs. We proposed a dynamic point of preferred operation (PPO) as a reference value to flatten the fluctuation of the total electricity load. We investigate the total electricity load for uncontrolled charging, static PPO and the proposed dynamic PPO schemes for charging-only, discharging, and bidirectional energy flow scenarios. Results show that the proposed dynamic PPO scheme can significantly shave the peak and alleviate the fluctuation of the total electricity load in a residential area.