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Localisation Techniques

Localisation Techniques

Received Signal Strength Indicator (RSSI)

  • RSS: actual signal power strength recorded by the receiver, in decibel-milliwatts (dBm) or milliWatts(mW)
    • Inverse proportionality between RSSI and transmitter-receiver distance
  • RSSI: RSS indicated in arbitrary units
  • RSSI localisation uses trilateration with at least 3 transmitters
  • Advantages:
    • Simple and cost-efficient due to low hardware requirements
  • Disadvantages:
    • Poor localization accuracy, especially in non-Line-of-Sight situations
    • Additional signal attenuation from transmission through walls, obstacles, and multipath
    • Potentially high fluctuation over time

Channel State Information (CSI)

  • Captures amplitude and phase response across different frequencies
  • Higher granularity than RSSI, gives more information to reduce multipath and provide more consistent measurements
  • Overall higher localisation accuracy than RSSI

Fingerprinting/Scene Analysis

  • Discrete estimation of user location based on grid
    • Delicate balance between size of grid (and corresponding resolution), and the signal variation error
    • Eg: impossible to estimate the correct point if the difference in RSSI values between each grid cell is less than the uncertainty range of the signal strength
  • Offline phase: collect RSSI/CSI measurements to form a fingerprint map
  • Online phase: compare real-time measurements with fingerprint map using algorithms:
    • Probabilistic method: use histogram/kernel to calculate probability of receiver being in \(x\) grid of fingerprint map
    • Artificial Neural Network: train a neural network (eg: Multi-Layer Perceptron) and provide real-time input with weights
    • k-Nearest Neighbours: average nearest measurement patches to estimate the location
    • Support Vector Machine (SVM): machine learning method applicable to indoor localisation

Angle of Arrival (AoA)

  • Receiver's antennae array estimates the angle of transmitted signal by the time difference of arrival at individual parts of the array
  • Advantages:
    • Receiver location can be estimated with just 3 transmitters in a 3D environment
    • Accurate estimation when transmitter-receiver distances are small
  • Disadvantages:
    • Slight error in angle calculation translates to huge error in position determination
      • Thus not accurate at larger transmitter-receiver distances
    • Line of Sight for angle calculation is hard to obtain due to indoor multipath effects

Time of Flight (ToF)

  • Calculate physical transmitter-receiver distance using signal propagation time and speed of light
  • Trilateration is used similar to RSSI to estimate position
  • Requires strict time synchronisation between transmitter and receiver, such that timestamps may also be included in the signal
  • Advantages:
    • High sampling rate and large signal bandwidth increases the resolution of the position estimation
  • Disadvantages:
    • Highly sensitive to obstacles as they deflect the signals, resulting in a longer propagation time

Time Difference of Arrival (TDoA)

  • Create hyperbolas between each pair of transmitters
  • Intersection point of all hyperbolas represents the position estimate
    • Ie: solve the system of hyperbola equations for X,Y,Z
  • Difference between TDoA and ToF
    • ToF uses absolute signal propagation time between each transmitter and receiver
    • TDoA uses the difference between these propagation times
      • Synchronisation is thus only necessary between transmitters (receiver error is common between transmitter pairs)

Return Time of Flight (RToF)

  • Measures the round-trip signal propagation time from transmitter to receiver, and response signal from receiver to transmitter
  • Difference between RToF and ToF
    • Only moderate synchronisation between transmitter and receiver is required
    • All factors affecting ToF are worse in RToF due to twice the distance
    • Time delay in transmitting response signal (depending on the receiver's electronics) adds to position estimation error

Phase of Arrival (PoA)

  • Measures phase difference from transmission to reception
  • Typically used in conjunction with other methods (RSSI, ToF, TDoA) to enhance localisation accuracy
  • High accuracy is only achievable with direct line-of-sight

Localisation Technologies

Technology Range Advantages Disadvantages
Wifi 802.11 n/ac/ad 35m Widely available
High accuracy
Existing infrastructure		 Sensitive to noise
Complex processing algorithms needed
Wifi 802.11 ah (new) 1km High coverage range
Low power consumption		 New technology, not yet tested for indoor environments
Ultrawide Band (UWB) 10-20m Immune to interference
High accuracy		 Short Range
Additional hardware
High cost
Radio Frequency Indentification Device (RFID) 200m Low power consumption
High coverage range		 Low accuracy
Bluetooth 100m High throughput
High coverage range
Low power consumption		 Low accuracy
Sensitive to noise
Ultrasound <20m Less absorption from obstacles High dependence on sensor placement
Acoustics <2m High accuracy Niche applications
Sensitive to sound pollution
Additional hardware/anchor points
Visible Light (LEDs) 1.4km Widely available
High potential (with AoA technique)
Multipath-free		 Requires Line-of-Sight
Higher power consumption