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|Title:||C2x Communication measurement system|
|Project:||Car transport safety|
The test method is designed to measure the delivered packet ratio for a given location. In each test the vehicle moves around a test area and for every GPS position update sends a group of test packets to the base unit. The base unit records the location and the ratio of received data packets for further processing. The test equipment consists of two systems, one is mounted on board a vehicle and the other is acting as an infrastructure point, being stationary during one experiment. Each system consists of a computer and communication modules for Wi-Fi and IEEE 802.15.4, the system on vehicle includes in addition two GPS receivers. The Wi-Fi test is built on IP stack utilizing UDP to transport the test packets. The UDP/IP protocol introduces an extra overhead that is not corresponding to the IEEE 802.11p architecture. The IEEE 802.15.4 test uses a custom protocol built on top of the IEEE 802.15.4 MAC layer. The test equipment is built to support two independent measurements; one set of measurement tools utilizes IP protocol and is designed for use with IEEE 802.11 based systems, the second set of tools is designed for evaluation of low rate wireless communications based on IEEE 802.15.4 standard.
Measurement system prior to installation
The purpose of the described measurements is to evaluate the area coverage of C2X communications in urban areas. The test method is designed to measure the packet delivery ratio (PDR) for a given location. In each test one vehicle moves around a given area and for every GPS position update sends a group of test packets to a base unit (a second vehicle). The base unit records the location and the ratio of the received data packets for further processing.
- Designed for area coverage
- GPS-based vehicle positioning
- Test packets sent regularly
- Logging of direct and round-trip data
The IP based test algorithm is divided into two parts, one for the base unit and one for the mobile unit. The mobile unit algorithm is acting as a UDP client, sending a sequence of test packets to the base unit when a new position update from the GPS receiver is received. The base unit is acting as a UDP server, listening on a dedicated port. Upon reception, each packet is examined if it belongs to a new sequence or is a part of an active sequence. After all packets of a sequence have been received or a timeout has occurred, the base unit records the ratio of successfully received packets for the given location. All packets received by the base unit are forwarded back to the mobile unit to allow round trip calculations and two-way communication evaluation. The algorithm runs in three separate threads – for reading updates from the GPS receiver, for sending the series of test packets and for receiving the loop-back packets from the base unit.
The result of each driving test is a set M of 4-tuples containing sequence number, location information and a ratio of the received data packets for the given location. To interpret the measurements, a mapping is proposed to transform the measured data set into a reception probability matrix R. The test area is divided into an equally spaced grid of tiles and for each tile the value of packet reception probability is assumed to be constant. The elements of the matrix R correspond to the tiles of the test area, creating a map of signal coverage for a given location. For each tile the average packet delivery ratio is calculated for all the measurements that fall into the area of the tile. After completing the measurements, a set of maps Rp for each infrastructure point location is obtained and can be used as an input for a traffic simulator.
- For each position a series of test packets is sent = one sample
- Data processed to a grid for further use in simulation
- All samples in one location (one grid cell) are combined
Processing of measurement samples into the coverage grid
Detail of processed data (grid size 50 m)
Grid values R are calculated based on the following equation:
kde R is the matrix of packet delivery ratios, r is the packet delivery ratio of sample m, u is a function defining if the sample m falls to a given grid cell: