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In radio technology
Radio is the technology of communicating using radio waves. Radio waves are electromagnetic waves of frequency between 3 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transmitter connected to ...
, symmetrical double-sided two-way ranging (SDS-TWR) is a ranging
Length measurement, distance measurement, or range measurement (ranging) all refer to the many ways in which length, distance, or range can be measured. The most commonly used approaches are the rulers, followed by transit-time methods and the ...
method that uses two delays that naturally occur in signal transmission
In telecommunications, transmission (sometimes abbreviated as "TX") is the process of sending or propagating an analog or digital signal via a medium that is wired, wireless, or fiber-optic.
Transmission system technologies typically refe ...
to determine the range between two stations:IEEE Standard 802.15.4a: Part 15.4: Wireless Medium Access Control (MAC) and Physical Layer (PHY) specifications for low-rate wireless personal area networks (WPANs) Amendment 1: Add Alternate PHYs.
* Signal propagation delay between two wireless devices
Wireless communication (or just wireless, when the context allows) is the transfer of information (''telecommunication'') between two or more points without the use of an electrical conductor, optical fiber or other continuous guided mediu ...
* Processing delay of acknowledgements within a wireless device
This method is called symmetrical double-sided two-way ranging because:
* It is symmetrical
Symmetry () in everyday life refers to a sense of harmonious and beautiful proportion and balance. In mathematics, the term has a more precise definition and is usually used to refer to an object that is invariant under some transformations ...
in that the measurements from station A to station B are a mirror-image of the measurements from station B to station A (ABA to BAB).
* It is double-sided in that only two stations are used for ranging measurement station A and station B.
* It is two-way in that a data packet
In telecommunications and computer networking, a network packet is a formatted unit of data carried by a packet-switched network. A packet consists of control information and user data; the latter is also known as the '' payload''. Control inform ...
(called a test packet) and an ACK packet is used.
Signal propagation delay
A special type of packet (test packets) is transmitted from station A (node A) to station B (node B). As time the packet travels through space per meter is known (from physical laws), the difference in time from when it was sent from thetransmitter
In electronics and telecommunications, a radio transmitter or just transmitter (often abbreviated as XMTR or TX in technical documents) is an electronic device which produces radio waves with an antenna (radio), antenna with the purpose of sig ...
and received at the receiver can be calculated. This time delay is known as the ''signal propagation delay''.
Processing delay
Station A now expects an acknowledgement from Station B. A station takes a known amount of time to process the incoming test packet, generate an acknowledgement (ack packet), and prepare it for transmission. The sum of time taken to process this acknowledgement is known asprocessing delay
In a network based on packet switching, processing delay is the time it takes routers to process the packet header. Processing delay is a key component in network delay.
During processing of a packet, routers may check for bit-level errors in t ...
.
Calculating the range
The acknowledgement sent back to station A includes in its header those two delay values – the signalpropagation delay
Propagation delay is the time duration taken for a signal to reach its destination, for example in the electromagnetic field, a wire, speed of sound, gas, fluid or seismic wave, solid body.
Physics
* An electromagnetic wave travelling through ...
and the processing delay. A further signal propagation delay can be calculated by Station A on the received acknowledgement, even as this delay was calculated on the test packet. These three values can then be used by an algorithm
In mathematics and computer science, an algorithm () is a finite sequence of Rigour#Mathematics, mathematically rigorous instructions, typically used to solve a class of specific Computational problem, problems or to perform a computation. Algo ...
to calculate the range between these two stations.
Verifying the range calculation
To verify that the range calculation was accurate, the same procedure is repeated by station B sending a test packet to station A and station A sending an acknowledgement to station B. At the end of this procedure, two range values are determined and an average of the two can be used to achieve a fairly accurate distance measurement between these two stations.See also
*Multilateration
Trilateration is the use of distances (or "ranges") for determining the unknown position coordinates of a point of interest, often around Earth ( geopositioning).
When more than three distances are involved, it may be called multilateration, f ...
* Real-time locating system
Real-time locating systems (RTLS), also known as real-time tracking systems, are used to automatically identify and track the location of objects or people in real time, usually within a building or other contained area. Wireless RTLS tags are ...
References
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