Saturday, November 14, 2009

LBS positioning technologies & TDOA

LBS positioning technologies:
  • cell of origin (nearest cell)
  • distance (lateration)
  • angle (angulation)
  • location patterning (pattern recognition)
 RF fingerprinting – calibrating RF model to the particular environment
location precision – repeatability/confidence level of successful detection 10m~90% of the time
inter-access point recommended separation 50-70ft

TDOA formula: D=C(t)

Distance in Meters = propagation speed of ~300m/microsecond (time in microseconds)

TDOA uses relative time measurements at each receiving sensor in place of absolute time measurements. It does not require a synchronized time source – the TDOA receivers require a time sync.

TDOA is rooted in hyperbolic lateration, also referred to as Enhanced Observed Time Difference

Path loss model for indoor propagation:

PL = PL1meter + 10log (dn) + s

PL = total path loss between receiver and sender in dB
PL1meter = reference path loss for desired frequency when receiver to transmitter distance is one meter
d = distance between receiver and transmitter in meters
n = path loss exponent for the environment
s = standard deviation associated with the degree of shadow fading present in the environment in dB

Pure RSS based lateration techniques that do not take additional steps to account for attenuation and multipath in the environment rarely produce acceptable results except in very controlled situations

Angle of arrival – (direction of arrival) locates the mobile station by determining the angle of incidence at which signals arrive at the receiving sensor.
     Well known implementation – VOR VHF Omnidirectional Range – system used for aircraft navigation from 108.1 – 117.95 MHz

Location Patterning – may be implemented totally in software, which can reduce complexity and cost significantly compared to angulation or purely time based lateration systems.

Location Patterning Positioning Algorithms:
     deterministic – attempt to find minimum statistical signal distance between a detected RSSI location vector and the location vectors of the various calibration sample points
     probabilistic – use probability inferences to determine the likelihood of a particular location given that a particular location vector array has already been detected.
     other techniques – neural networks, support vector modeling outside boundaries of deterministic and probabilistic approaches.

It is not possible to use calibration data in more than one area of a building. Despite any similarities, the probability that the location vectors collected at the same positions on each floor being identical is significantly low. *Cisco RF fingerprinting can share RF calibration models

Active logistics shipping & receiving area:
     Accuracy degradation of 20% can reasonably be expected in a 30 day period
It is not unreasonable to expect to re-verify calibration data accuracy quarterly and to plan for a complete recalibration semi-annually.

Client probing

Unassociated clients generate probe requests quite regularly. Associated clients scan less often and may active scan or passive scan, or a mix of the two. Passive scanning does little to improve location fidelity.

CCXv2 clients support S36 Radio Measurement Requests and will perform active scanning and probe all configured SSIDs on command.

CCX Location Measurement in WLC can be adjusted from 60 seconds (default) to 32,400 seconds

WCS WLAN clients – first 250 are displayed, to view location data of those beyond the first 250, client filtering must be enabled

Off-channel scans occur for ~500ms out of every 180 seconds of operation.
     - or ~50ms per non-primary channel per 180 second interval

APs configured as WGB do not respond to broadcast Radio Measurement Requests. use IOS CLI command mobile station period threshold

- this command causes the WGB to perform an active scan when it detects low AP RSSI, excessive interference or a high % of frame loss

Tag battery state info can be pushed via asynchronous NB notifications from the location appliance (MSE), tag telemetry is made available to location clients only via the SOAP/XML API

Location appliance/MSE disk space command:
df –H

The dual on-board Ethernet controllers on the location appliance (MSE) are NOT intended for redundant/simultaneous connections to the same IP network. This configuration is not recommended at this time

#/etc/init.d/loc serverd stop
#shutdown –h now

The remote SSH session will be terminated before power down will be visible. Wait 2 minutes after disconnect before powering off the location appliance/MSE

Insufficient inter-Access Point distance can expose the system to short range antenna pattern anomalies, which may also be non-conducive to good location accuracy.
     distances of 40~70ft between each AP is best, they should be spaced greater than 28ft apart

7921g, -67dBm, 20% overlap, 25dB SNR = 24 ft cell size
Inter-Access Point spacing = 33 ft

APs that are placed into the design solely for location purposes should not be included in either RRM TX PWR CTRL or coverage hole remediation processes. Configure a static channel/power to remove them from the RRM process.


No comments:

Post a Comment