Zohar Naor

Minimizing the wireless cost of tracking mobile users: an adaptive threshold scheme

Mobile user tracking is a major issue in wireless networks. Previous studies and traditional approaches dealt only with tracking algorithms which adapt themselves to the user activity. We propose a novel approach for user tracking, in which the tracking activity is adapted to both user and system activity. The basic idea is to make the user location update rate dependent not only on the user activity (such as the call profile and mobility pattern); rather, it is made dependent also on the signaling load, which reflects the actual cost of the update operation. Thus, in low signaling load locations, the users are to transmit location update messages more frequently. To carry out this approach we propose an adaptive threshold scheme (ATS). The network determines for each cell a registration threshold level (which depends on the cell load) and announces it, as a broadcast message, to the users. A user computes its own registration priority and then transmits a registration message only if its priority exceeds the announced threshold level. Thus, whenever the local load on the cell is low, the users are requested to update their locations more often, while in loaded cells the registration activity is minimised. Our analysis shows that the adaptive threshold scheme reduces the paging cost, in comparison with other dynamic methods, without increasing the wireless cost of registration.

Cell identification codes for tracking mobile users

The minimization of the wireless cost of tracking mobile users is a crucial issue in wireless networks. Some of the previous strategies addressing this issue leave an open gap, by requiring the use of information that is not generally available to the user (for example, the distance traveled by the user). For this reason, both the implementation of some of these strategies and the performance comparison to existing strategies is not clear. In this work we propose to close this gap by the use of Cell Identification Codes (CIC) for tracking mobile users. Each cell periodically broadcasts a short message which identifies the cell and its orientation relatively to other cells in the network. This information is used by the users to efficiently update their location. We propose several cell identification encoding schemes, which are used to implement different tracking strategies, and analyze the amount of information required by each tracking strategy. One of our major results is that there is no need to transmit a code which is unique for each cell. For example, a 3 bits CIC is sufficient to implement a distance-based tracking strategy in a two-dimensional system. In addition, we propose a combination of timer and movement tracking strategy, based on either a one-bit or a two-bit CIC, depending on system topology and user mobility. An important property of our framework is that the overall performance cost, and hence its comparison to existing methods, is evaluated for each tracking strategy. The CIC-based strategies are shown to outperform the geographic-based method currently used in existing networks, and the timer-based method, over a wide range of parameters. Moreover, this superiority increases as the number of users per cell increases.

Tracking mobile users with uncertain parameters

A method of reducing the wireless cost of tracking mobile users with uncertain parameters is developed in this paper. Such uncertainty arises naturally in wireless networks, since an efficient user tracking is based on a prediction of its future call and mobility parameters. The conventional approach based on dynamic tracking is not reliable in the sense that inaccurate prediction of the user mobility parameters may significantly reduce the tracking efficiency. Unfortunately, such uncertainty is unavoidable for mobile users, especially for bursty mobility pattern. In this study we propose a novel approach, which is efficient yet reliable. The basic idea is to incorporate a distributed scheme with a centralized scheme. The location update process incorporates a topology-independent distance based strategy and a load-sensitive timer. This combination forms a mechanism that bounds from above the total tracking cost. The registration activity is governed by the system, such that it increases at lowly loaded cells and decreases at heavily loaded cells. The expected wireless cost of tracking under the proposed method is significantly reduced, in comparison to the existing methods currently used in cellular networks. Furthermore, as opposed to other tracking methods, the worst case tracking cost is bounded from above and governed by the system. The proposed strategy can be easily implemented, and it does not require a significant computational power from the user.

LATS: a load-adaptive threshold scheme for tracking mobile users

Mobile user tracking is a major issue. We propose a novel approach for user tracking, in which the tracking activity is adapted to both user and system activity. The basic idea is to make the user-location update-rate dependent not only on the user activity (such as the call profile and mobility pattern). Rather, it is also made dependent on the signaling load, which reflects the actual cost of the update operation. Thus, at low-signaling load locations, the users are to transmit location update messages more frequently. To carry out this approach, we propose a load-adaptive threshold scheme (LATS): the network determines for each cell a registration threshold level (which depends on the cell load) and announces it, as a broadcast message, to the users. The user computes its own registration priority and then transmits a registration message only if its priority exceeds the announced threshold level. Thus, whenever the local load on the cell is low, the registration activity increases, while in loaded cells the registration activity decreases. Our analysis shows that the LATS reduces the paging cost, in comparison with other dynamic methods, without increasing the wireless cost of registration. Moreover, if higher user density is coupled with less mobility (e.g., consider vehicles), then the LATS strategy offers further performance improvement. The load-adaptive strategy can be used in addition to any other dynamic tracking strategy. Furthermore, the computational complexity imposed on the user is identical to that required by an equivalent load-insensitive scheme.

A centralized dynamic access probability protocol for next generation wireless networks

A multiple access protocol that is particularly suitable for cellular Internet access and satellite-based networks with on-board processing is developed. The basic idea is that when a user wishes to send a message, it transmits with a probability p/sub access/ that depends on the load on the channel. Under conditions of low load, the probability p/sub access/ approaches 1, while at high load p/sub access/ is relatively low. This media access control protocol guarantees high channel utilization at high load, as well as low delay at low load periods. Using the statistical usage of the shared channel, the load is estimated with certain uncertainty. Our analysis shows that using the statistical usage of the shared channel, the optimal access probability can be well estimated for a broad class of load distribution patterns. In addition, we propose to use a central station to broadcast the value of p/sub access/ in networks with poor collision detection capability, or long feedback delay. The proposed method is particularly suitable for shared channels with poor collision detection capability, under conditions of bursty traffic and a large number of users. Examples for such channels are the reservation channel in satellite-based networks with on-board processing, and the control channel in cellular networks. Hence, the proposed method can be used for cellular Internet access and for accessing public satellite-based networks. The broadcast mechanism that already exists in such networks can be used to inform the users the dynamic access probability.

Active tracking: locating mobile users in personal communication service networks

The problem of tracking mobile users in Personal Communication Service (PCS) networks is discussed. We propose a novel approach for reducing the wireless cost of tracking users. The basic idea is to use non‐utilized system resources for initiating queries about the location of mobile users, in addition to the process of user registration. Queries are applied at each cell, independently of the other cells, whenever the load on the local control channel drops below a pre‐defined threshold. Our study focuses on two issues: (1) proposing the initiated queries approach and an algorithm for its application, and (2) studying and quantifying the value of location information and evaluating the parameters affecting it. Our analysis shows that the expected benefit due to location knowledge in a Markovian motion model depends, among other things, on the determinant of the transition matrix and on the variability of the location distribution function. The active tracking approach, as opposed to other dynamic strategies, does not require any modification of user equipment. The importance of this property is in its practicality: An implementation of a new registration strategy in current systems would require a modification of the users equipment. Moreover, the proposed method can be easily implemented in addition to any known tracking strategy, to reduce further the tracking cost. The performance of the active tracking method is evaluated under two registration strategies: The geographic‐based strategy, currently used in cellular networks, and the profile‐based strategy, suggested elsewhere. Under both strategies, it significantly reduces the tracking cost.

Announced dynamic access probability protocol for next generation wireless networks

A multiple access protocol that is particularly suitable for cellular Internet access and satellite-based networks with on-board processing is developed in this paper. The basic idea is that when a user wishes to send a message, it transmits with probability paccess that depends on the load on the channel. Under conditions of low load, the probability paccess approaches 1, while at high load paccess is relatively low. This media access control protocol guarantees high channel utilization at high load, as well as low delay at low load periods. Using the statistical usage of the shared channel, the load is estimated with certain uncertainty. Our analysis shows that using the statistical usage of the shared channel, the optimal access probability can be well estimated for a broad class of load distribution patterns. In addition, we propose to use a central station to broadcast the value of Paccess in networks with poor collision detection capability, or long feedback delay. The proposed method is particularly suitable for shared channels with poor collision detection capability, under conditions of bursty traffic and a large number of users. Examples for such channels are the reservation channel in satellite-based networks with on-board processing, and the control channel in cellular networks. Hence, the proposed method can be used for cellular Internet access and for accessing public satellite-based networks. The broadcast mechanism that already exists in such networks can be used to inform the users the dynamic access probability.