Table 2 Wi-Fi based indoor positioning techniques

Name

ToA [22,23,24]

TDoA [25, 28]

RSS-based method [2, 21]

RToF [41]

AoA [26, 27]/AoD [28]

Method

The distances are directly measured with respect to the propagation time

Examine the difference in time at which the signal arrives at multiple reference access points [17]

Calculate signal path loss due to propagation

Based on the round-trip time-of-flight of the signal

Find the intersection of several pairs of angle direction lines [17]

Advantages

It can be deployed with various signals, such as the direct sequence spread-spectrum (DSSS) [22] or ultrawide band (UWB) signal [23]

The cross correlation between the signals received at a pair of reference access points can be used to estimate an accurate location [25]

An RSS-based method is used to estimate the range of the difference between the transmitted signal strength and the received signal strength [42]

An observational error, which is the difference between a measured quantity and its true value, of only a few meters can be achieved [41]

It does not require time synchronization between reference access points [26, 27]

Drawbacks

Requires respect to signal from at least three reference points

A timestamp must be labeled in the transmitting signal [17]

The multiple reference access points must share a precise time reference and reference signals

Path-loss models will be changed owing to the different indoor environment conditions, such as severe multipath fading and shadowing

It is difficult to model the exact delay or processing time in a real environment

The angle direction is affected by shadowing or multipath reflections [28]