Robust Vehicular Communications for Traffic Safety---Channel Estimation and Multiantenna Schemes
Doctoral thesis, 2018
Vehicular wireless channels can be highly time- and/or frequency-selective due to high mobility of the vehicles and/or large delay spreads. IEEE 802.11p has been specified as the physical layer standard for vehicular communications, where the pilots are densely concentrated at the beginning of a frame. As a consequence, accurate channel estimation in later parts of the frame becomes a challenging task. In this thesis, a solution to overcome the ill-suited pilot pattern is studied; a cross-layered scheme to insert complementary pilots into an 802.11p frame is proposed. The scheme does not require modifications to the 802.11p standard and a modified receiver can utilize the complementary pilots for accurate channel estimation in vehicular channels.
The metallic components of present-day vehicles pose a challenge in designing antenna systems that satisfy a minimum required directive gain in the entire horizontal plane. Multiple antennas with contrasting directive gain patterns can be used to alleviate the problems due to low directive gains. A scheme that combines the output of L antennas to the input of a single-port receiver is proposed in the thesis. The combining scheme is designed to minimize the probability of a burst error, i.e., an unsuccessful decoding of K consecutive packets from a transmitter arriving in the direction of low directive gains of the individual antennas. To minimize complexity, the scheme does not estimate or use any channel state information. It is shown using measured and simulated directive gain patterns that the probability of burst errors for packets arriving in the direction of low directive gains of the individual antenna elements can be minimized.
The enhanced distributed channel access (EDCA) scheme is used in V2V communications to facilitate the sharing of allocated time-frequency resources. The packet success ratio (PSR) of the broadcast messages in the EDCA scheme depends on the number of vehicles and the packet transmission rate. The interference at a receiving vehicle increases due to multiple simultaneous transmissions when the number of vehicles grows beyond a limit, resulting in the decrease of the PSR. A receiver setup with sector antennas, where the output of each antenna can be processed separately to decode a packet, is described in the thesis with a detailed performance analysis. A significant increase in the PSR is shown in a dense vehicular scenario by using four partially overlapping sector antennas compared with a single omnidirectional antenna setup.
Cross-layer Pilot Scheme
Vehicular Communications (V2X)
Chalmers, Electrical Engineering, Communication and Antenna Systems, Communication Systems
K. K. Nagalapur, E. G. Ström, F. Brännström, J. Carlsson, and K. Karlsson, "Robust connectivity with multiple directional antennas for vehicular communications," submitted to IEEE Transactions on Intelligent Transportation Systems, May 2018
On Channel Estimation for 802.11p in Highly Time-Varying Vehicular Channels
IEEE International Conference on Communications (ICC), 10-14 June 2014,; (2014)p. 5659--5664-
Paper in proceedings
An 802.11p Cross-Layered Pilot Scheme for Time- and Frequency-Varying Channels and Its Hardware Implementation
IEEE Transactions on Vehicular Technology,; Vol. 65(2016)p. 3917-3928
K. K. Nagalapur, F. Brännström, and E. G. Ström, "Performance analysis of receivers using sector antennas for broadcast vehicular communications," submitted to IEEE Transactions on Communications, May 2018
Safety of road passengers depends to a large extent on drivers' attentiveness and their ability to overcome a dangerous situation. By enabling road vehicles to communicate with each other and sense their immediate surroundings, additional support can be provided to the drivers to prevent fatal accidents. The communication between the vehicles is achieved by allowing the vehicles to broadcast “Here I am” status messages containing their position, velocity, and other relevant information. The vehicles combine the information in the “Here I am” messages from other vehicles with their own to draw a picture of the current road traffic and anticipate any impending danger to prevent an accident. The exchange of the status messages also enables numerous road traffic applications apart from improving the traffic safety.
The advantages of enabling the vehicles to exchange status messages come with the challenges of designing wireless communication systems that support reliable communications for safety critical traffic applications. The high mobility of the vehicles, large number of vehicles sharing the same resources for communications, and the metallic nature of today's vehicles present several challenges in achieving reliable and robust communications between vehicles. In this thesis, a few solutions to overcome some of the challenges are presented.
Areas of Advance
Information and Communication Technology
Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4424
Chalmers University of Technology
Room EB, EDIT building, Hörsalsvägen 11, Chalmers University of Technology
Opponent: Prof. Christoph Mecklenbräuker, Institute of Telecommunications, TU Wien, Austria