Round Trip Propagation Time Estimation Computer Science Essay

Published Date: 02 Nov 2017

User localization in a wireless network has been the focus of researchers since decades. Many techniques have already been developed to locate a user in indoor and outdoor environment. Different techniques offer varied precision and accuracy. Global Positioning System has been a breakthrough in user localization techniques, but it offers limited precision and its expensive transceivers make this technique less usable. On the other hands, researchers have been working on developing simple and cost effective solutions to address this problem, they have been attempting to use the information exchanged between hand-held device and Base Transceiver Station to locate the user. Many such cost effective techniques have been developed and this dissertation also presents such a simple technique but attempts to introduce enhancement in already developed techniques, which offer limited accuracy which in turn reduces their usability.

This chapter is devoted to present background knowledge available in literature about this area of research and let the reader know about pros and cons of already developed algorithms for user localization. Earlier part of this chapter investigates the background of this project, attempting to explain different localization schemes and later part of this chapter focuses on different localization techniques based on Received Signal Strength and final section gives a brief overview of research presented in the area of subject.

3.1 Background

It is absolutely infeasible to do localization without knowledge of the physical world [1]. Different techniques using a reference to locate a user has been developed, there are many distance based, angle and hop-count based techniques. There are many techniques for mobile user localization but most powerful among them is that offer higher accuracy and less computation [1]. Many methods have been introduced for indoor localization but most commonly used methods are based on Radio Signal Strength (RSS) and Time of Arrival (TOA) [2]. The positioning technique which takes into account the time taken by a signal to arrive a receiver is called "Time of Arrival" technique [3]. "Radio Signal Strength" technique rely on the signal strength received at the receiver [4]. "Angle of Arrival" technique aims to find the angle of transmitted signal to receiver whose location is to be identified [5]. RSS and TOA techniques are distance based techniques, both of them depends upon Transmitter – Receiver separation distance [1]. On the other hand AOA technique is based on angle. Other techniques takes are based on area, hop-count and neighborhood [1].

Localization is a two-step procedure, in first step geographic area needs to be identified in which target is located, next step involves locating the target in specific geographic area. First step is trivial, but second step may be complex depending upon the accuracy and precision requirements, computational capability of devices and geographic area. Localization algorithms are designed while taking into account the available resources, accuracy & precision requirements, terrain and geographic area characteristics. An algorithm may be best in a particular situation but may be worse in other scenario, so no algorithm is absolutely favorable in every situation [1].

3.1.1 Distance Based Techniques

Many physical quantities of a signal depends upon distance characteristics, such as received signal strength and propagation time. Researchers in this field have been working to find out a direct relation between distance and such characteristics of signal, so they can be successful in converting these acquired physical quantities into distance. This section is devoted to distance based localization techniques: time of arrival (TOA), radio signal strength (RSS), and time difference of arrival (TDOA) [1].

Time of Arrival (TOA)

For an acoustic signal with known velocity, propagation time can be directly converted to distance covered. Main problem in this mechanism is to measure the propagation time with precision and accuracy [1]. TOA-based systems require high clock synchronization between all system components in order for a beacon to know the exact time when a signal was transmitted by a source which is quite difficult, due to hardware costs and system limitations.

Constrains:

Poor synchronization.

Low SNR.

Typical TOA-based range estimation methods may involve the transmission of a wideband pseudo noise (PN) sequence and subsequent correlation of the received signal with this sequence; we need high bandwidth for better results.

Several signal-processing techniques can be applied to achieve better resolution without increasing signal BW. Like maximum likelihood delay estimation which can provide greeter delay resolution [6].

TOA based techniques can be characterized into two main categories: one-way propagation time estimation and round-trip propagation time estimation [1] .

One-way Propagation Estimation

Propagation time of a signal is known to be time lag between transmission time from transmitter and receiving time at receiver. Simply, it is the travel time for a signal from a transmitter to the receiving unit [1]. Propagation delay or Time of Arrival can be mathematically described as:

………... (3.1)

Here denotes propagation delay, denotes signal arriving time and signal transmission time.

Assuming the signal speed to be v, the Transmitter-Receiver separation ‘d’ can be calculated by following relation:

………. (3.2)

Using this basic mechanism, receiver must know the transmission time for the arrived signal to compute the propagation delay. Time synchronization requirement can only can be exempted in case of combined transmission of signals with different speeds like acoustic and radio signals.

Round-trip propagation time estimation

One-way propagation time estimation necessitate the synchronization mechanism between transmitting and receiving nodes, because of propagation delay computation demands. This synchronization mechanism can be avoided by using round trip propagation time (RTT). For RTT nodes should only account for the local time duration rather than timestamps.

RTT estimation for two nodes labeled as, A and B, happens as follows.

Node A transmits a data packet to node B.

Node B receives this packet and after a delay of tdelay seconds, replies node A in form of acknowledgement packet.

The Node A determines the RTT by following relation

……… (3.3)

Here shows the distance-induced propagation time of signal.

Node B reports the tdelay , then Node A is able to calculate the signal propagation time by following relation:

……… (3.4)

This technique also faces minimal error during RTT calculation due to clock drift between both nodes [1].

Radio Signal Strength (RSS)

Radio signal suffers different type of distortions and attenuation during transmission which causes Radio signal strength to drop down as it passes more distance and suffers from more attenuation. RSS based technique takes benefit of this phenomenon, an understanding of radio signal strength attenuation is mandatory to map the radio signal strength to distance [1].

The idea behind using RSS techniques for localization problem is to demonstrate a one to one relationship between received RSS and transmitter-receiver separation distance [7].

It is important to note that RSS value and distance does not always have a linear relationship, particularly in an indoor environment and in different terrain characteristics. In an indoor environment RSS level may drop down due to reflection through many obstacles and while relating RSS levels to distance, receiver would appear to be at a far distant place than actual place [8].

RSS level detection is a simple process and it doesn’t require extra hardware, as it only require power detectors and they are already available in almost all wireless devices like UWB, Wi-fi, Zigbee, Bluetooth and infrared devices. So RSS measurement is quite easy in wireless network, due to already available hardware. It makes this technique favorable.

The Ultra Wide Band (UWB) based technologies offer better results than Zig-bee based technologies, as they provide better accuracy. Although their installation cost is relatively high [9].

Another positive factor or RSS based techniques is that they do not need timing information, which in turn does not require the nodes to be synchronized, this property make this technique robust to multipath propagation effects. RSS based technique is more useful where transmitter-receiver separation is low, in such areas it is quite successful technique but this technique lacks when it comes to accuracy, it offers less accurate localization than TOA based systems [6].

Now looking at other side of RSS based techniques, they RSS systems need to be trained and they require complex algorithms to perform localization [10]. RSS is also sensitive to non-line of sight propagation, low Signal to Noise Ratio and shadowing. Shadowing and reflection can be dealt with log-normal random variable, so RSS as a linear parameter cannot correctly characterize in case of shadowing and reflection.

For Wireless Sensor Network (WSN) it is advantageous to use RSS technique for localization because of its continuous monitoring, low cost and capacity to work for years unattended. Although some problems may be caused due to other devices operating on 2.4 GHz band, such interference increases the probability of error [8].