WiFi Distance Determination via Time-of-Propagation
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Solution Overview
Problem
Existing methods for determining the location of a non-AP station, such as a cellular telephone handset, in a WiFi IEEE 802.11 network are unreliable due to environmental and obstructions affecting signal strength, leading to substantial errors in distance calculation.
Innovation Solution
A time-of-propagation distance-determining method is employed, where a probe request is sent from the handset to an AP, and the round-trip propagation time is measured to determine distance, with the lowest overall transmit energy setting being determined to optimize energy consumption and ensure reliable reception, using techniques like channel loss compensation, sequential reduction, or binary search to find the optimal bit rate and transmit power settings.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If signal-strength-based distance determination is used, then distance can be estimated from RSSI and transmit power, but substantial errors occur due to environmental conditions and obstructions affecting channel loss
Solution Approach 1:
The patent replaces the signal-strength-based (RSSI) distance determination method with a time-of-propagation-based method. Instead of using electromagnetic signal strength which is affected by environmental factors, the system measures the actual time it takes for a probe request to travel from the non-AP station to the AP and for the ACK to return. This time measurement is then converted to distance using the speed of light, providing a more reliable and accurate distance determination that is not affected by obstructions or environmental conditions.
2Reliability
If transmit power is increased to ensure reliable reception of probe requests, then reception reliability improves, but energy consumption increases
Solution Approach 1:
The patent implements dynamic adjustment of transmit power based on measured channel loss. The non-AP station measures the channel loss by comparing the transmit power of sent probe requests with the received signal strength of returned ACKs. Based on this measured channel loss, the system dynamically adjusts the transmit power for subsequent probe requests to the minimum level required for reliable reception, rather than using a fixed high power level. This dynamic adaptation ensures reliable communication while minimizing energy consumption.
Solution Approach 2:
The system uses feedback from the ACK packets to continuously monitor and adjust transmit power. Each ACK provides information about the actual received signal strength, which is fed back to the non-AP station to recalculate the channel loss and adjust the transmit power for the next probe request. This closed-loop feedback mechanism ensures that transmit power is always optimized for current channel conditions, balancing reliability and energy efficiency.
3Use of energy by moving object
If bit rate is increased to reduce transmission time, then energy consumption per bit decreases, but reception reliability may deteriorate in challenging channel conditions
Solution Approach 1:
The patent dynamically adjusts the bit rate based on measured channel loss conditions. The system calculates the channel loss from the difference between transmitted probe request power and received ACK signal strength, then uses this information to select an appropriate bit rate for subsequent transmissions. In channels with higher loss, the system selects lower bit rates to ensure reliable reception, while in better channels, it can use higher bit rates for more efficient energy consumption per bit transmitted.
Data Source
AI summary
A time-of-propagation distance-determining method involves sending a probe request from a handset to an AP and receiving a return ACK. The time it takes for the probe request to propagate to the AP, the turnaround time, and the time it takes the ACK to propagate back to the handset is measured on the handset. The turnaround time is subtracted from the measured time, and the result is used to make a distance determination. In order to reduce power consumption, the “lowest overall transmit energy” setting that will give acceptable reception of a probe request sent by the handset to the AP station is determined. The lowest overall transmit energy setting involves a bit rate setting and a transmit power setting. The lowest overall transmit energy setting is used to send probe requests when probe requests are sent from the handset in carrying out a time-of-propagation distance-determining transaction.