Resultados contrarios con Disilicato de litio

To prove DWC model regarding of timestamp resolution and accuracy for locating an SP based on various WAPs availability, an experiment is simulated in OPNET modeller network simulator as shown in Figure 3-17. In this experiment, DWC model and the reference model are used to generate the "local times" for four WAPs setup for different parameters.

Figure ‎3-17: Locating an SP (labelled SP1) scenario using 4 deployed WAP signals.

In this simulation experiment different parameters are configured, including:

1. For WAPs: The following functions and parameters are added and/or configured:

a. MAC Layer: The MAC layer for the SP localisation scenario was configured based on IEEE Standard 802.11b, where the beacon frame is transmitted at

X-Coordinate in meter

Y- Co or di na te in me te r

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every 100 millisecond. A single modified WAP-model within implemented DWC model is shown in Figure 3-18. Clock offset and clock drift parameters are initialised at this Layer for all WAP local-times in different values. For example, clock offset is (3, 6, 9 and 12 ppm for WAP1, WAP2, WAP3, and WAP4 respectively) while clock drift for all WAPs is based on the DWC model in ppb. These "local times" are used to timestamp the generated WAPs beacon frames during transmission, labelled “WT”. The timestamps of the received beacon frames at the SP are generated by the logical time, labelled

“ST”. The difference between WT and ST are used for measuring the distance between each WAP and the SP. But, to test the single scenario, two different generating local-times are used including WAPs “local time” is synchronised based on Network time (i.e. based reference model) and GNSS time separately.

a- Node-model level

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Figure ‎3-18: Modified OPNET WAP process model.

b. Clock Model: This new model represents as local time to make timestamp for each generated beacon frame by WAPs, near to the realistic behaviour. Since OPNET modeller supports time-event based to generate timestamps for beacons or for any events. The new timestamp values are based on the developed WAP clock model. The clock model produces the timestamps at every millisecond, continuously, during the simulated experiment. This assumption is referred to the fact that beacon frame generation interval is within 100 milliseconds. The c-language code of this model in the OPNET modeller is listed in Appendix A.1.

2. For SPs: The same WiFi-node functionalities have been used for: receiving beacon frame, generating timestamps for the received beacons based on GNSS time. However for localisation purposes SP_Move, GNSS_clock and Locate_WAPs models are added into the SP process model as shown in Figure 3-19, as follows:

b- Process-model level for WAPs (modified)

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Figure ‎3-19: Modified OPNET SP process model.

a. GNSS_Clck: The SP uses GNSS_Clck to generate timestamp at nanosecond resolution during receiving and transmitting frames. The c-language code of this model in the OPNET modeller is presented in Appendix A.2.

b. Locate SP: SP uses Locate_SP model to implement the procedure of Trilateration in an iterative LLS and then to calculate SPs location. As well as, this model has already implemented DOP calculation algorithm to indicate the HDOP value during location calculation. The c-language code of this model with the invoked functions for the OPNET modeller is presented in Appendix A.3.

c. Move model: A new trajectory mobility model was implemented at MAC layer, as shown in Figure 3-19, and it is used to model the SP moving around the WAPs. The speed for the SPs’ moving was configured to 1 meter/second.

Since, OPNET trajectory models either based on some pre-defined random waypoints or based on fixed/variable waypoints over the simulation time.

Thus, for test localisation algorithms moving node from one position to the

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next position will affected on location estimation accuracy if these models are being used. The new implemented trajectory mobility model avoids all these issue. Thus, the trajectory mobility is accurate and controllable (in terms of localisation) than the predefined OPNET trajectory models, since at every change of location the new model will precisely gives location information and putted into the localisation algorithms. The c-language code of this model in the OPNET modeller is listed in Appendix A.4.

The performance metric considered to evaluate the DWC model and compare it with the reference clock model in the localisation scheme was the location accuracy. In one scenario, the SP is located in three consecutive positions by calculating its distance from the four WAPs with known WAPs location.

The results show that the standard deviation of the accuracy in the SP location is 17.6 meters when using the reference model. When the "local times" generated by DWC model, the standard deviation of the accuracy of this SP location is reduced to 5.2 meters.

Therefore, the developed model is more accurate when compared to the reference model.

This is because when using the reference model, the accuracy of the measurement is un-stable, which is solved by using DWC model.

3.6 Summary

Available timing-function sources on SP cannot be used accurately to measure TOF measurements and then to estimate pseudorange between SPs and WAPs, since their accuracy and resolutions are within microseconds. This inaccuracy timing will produce large positioning error.

The novel SPs-WAPs clock synchronisation algorithm based on the DWC model, which can be used for accurate study localisation solution, in different simulated scenarios is developed. The accuracy of the clock model which represents the wireless device in the network is important to achieve dependable localisation. This is achieved because the accuracy of the developed DWC model is based on using accurate reference time, and

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including the clock offset and the clock drift in the model. The DWC uses GNSS time as a reference time and also models the clock offset & clock drift in a precise calculation based on actual clock noise sources. This is necessary to make the simulation model of the clock as accurate as the actual hardware clock inside WAPs.

It also observed that an efficient way to evaluate the performance of the enhanced clock model should be based on the quality of the clock synchronisation. Furthermore, precise clock synchronisation is a requirement for accurate localisation-algorithms, especially for algorithms which are based on the TOA technique.

OPNET simulation experiments prove that the quality/accuracy of DWC model is close to that of actual hardware clock synchronisation. The result of the single scenario to locate SP via WAPs signals using Trilateration, shows that our model achieves higher location accuracy of up to 41.33 nanosecond than existing approaches; resulting in average of 12.4 meter position error improvements. Note: the SP is located using the beacon frames from the four WAPs with known WAPs location to. I.e. through running the experiment, the SP uses pre-defined WAPs position. In order to overcome this limitation, a new scheme to locate WAPs, on-the-go via a single SP or multi-SPs, based on the WAPs-SPs cloak synchronisation algorithm is proposed in as well as the evaluation of the scheme is presented in next chapter.

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CHAPTER4. WAPS LOCALISATION SCHEME FOR SEAMLESS SMARTPHONES POSITIONING

4.1 Introduction

Providing the position of WAPs is one of the most viable requirements in WiFi-based localisation solutions, through which indoors-SPs can calculate their location based on this WAPs-positions information. Time based localisation technique could be utilised for locating SPs when the clock of WAPs is synched accurately and WAPs positions are defined. These WAPs requirements are necessary for SPs in order to be able to use these WAPs as reference positions to define their locations, while the SPs are moving indoors.

In the previous chapter, it has proved that WAPs clocks can be synched accurately with SPs clocks within few nanoseconds (i.e. 6 to 10 nanoseconds). This process has been conducted through using the obtained GNSS time on outdoors-SPs. However, on-the-go solution to define WAPs position information is remained unsolved to design a seamless outdoors-indoors SP positioning solution.

This chapter presents a proposed WAPs localisation scheme via any outdoor-available GNSS-enabled SPs in order to tackle the issue of on-the-go solutions to locate WAPs.

The scheme starts when an SP is outdoors and has already maintained GNSS position. As this SP approaches a building, it might receive WAPs signals from inside the building.

Through the received signals, all the WAPs pseudoranges could be measured. Then the SP can compute these WAPs locations through using Trilateration based on having the SP’s instantaneous geographical locations and the pseudoranges with WAPs. Note: the accuracy of pseudoranges measurement depends on the SPs-WAPs clocks synchronisation via using the obtained GNSS time on outdoors-SPs (as proved in section 3.5.2). After that, the located and synchronised WAPs can act as reference points for calculating the indoors-SPs location, particularly when GNSS signals are not available.

As a result, such scheme can offer seamless outdoors-indoors localisation, when it is installed on SPs at large scale via several SPs and WAPs. In addition, the fresh calculated WAP location can be transported into a central-server-database on the fly from any

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participating SPs. This transported WAP location could be shared as a beneficial aspect for saving cost and time-involved of using purpose-built-vehicles to survey WAPs location [117].

This chapter contains five main sections as follows: section 4.2, the main problem of WAPs location estimation through using various localisation techniques. Section 4.3,

“WiFi-Based Localisation Solutions” which describes how can Wi-Fi help indoors-SP localisations? And the sixth research question (as it is pointed out in section 1.4): “how can synchronised-WAPs be accurately located?” is addressed in. Section 4.4 includes related work and the design/prototype implementation details of the proposed WAPs localisation scheme. Section 4.5 demonstrates test and scenarios of the proposed scheme and the obtained results from OPNET. Section 4.6 summarises the achievements and advantages of the proposed WAPs localisation scheme that can be implemented on SPs.

4.2 Problem Statement

WAPs are usually available everywhere for accessing the Internet services. It means WAPs originally are not deployed to be used as a reference positions for SP localisation.

So, if the WAPs are not designed for localisation applications [118] (as using dedicated HW [50] and WAPs position accurately calibrated [117]), they cannot offer accurate position calculation and do not know their geographic position.

As it is concluded in section 2.4.2, most of the current indoors-SP localisation solutions to provide SPs position (based on WiFi technology) use pre-defined WAPs positions or obtained WAPs position information via costly offline training processes. WPS-Skyhook and RTLS-Ekahau are examples of indoors-SP localisation solutions [117]. However, the two assumptions are not applicable practically in many situations because there is no provision of automatic WAP position estimation.