The role of AHS road-to-vehicle communication systems is to provide vehicles with information on constantly changing obstacles and other such phenomena that are detected. AHS requires broadcast transmission to provide the same information (standing vehicles, slow vehicles, presence of pedestrians, road surface conditions, road shape information) to all the vehicles in the communication area. It also requires individual communication that provides information to individual vehicles. AHS, which requires real-time operation, has the lag time for road-to-vehicle communications set at 0.1 second or less. It is known that human beings have a simple reaction time of 0.3 seconds from recognizing an obstacle to initiating braking. In order for AHS to achieve faster response performance than human beings, 0.1 second is allocated as the lag time for each function, from the information acquisition sensors, to roadside processing facility, to road-to-vehicle communication. This 0.1 second represents the information provision per movement time for one vehicle (when traveling at 120 km/h, movement of 3.3 m in 0.1 second).

  Road-to-vehicle communication systems are configured from wireless facilities (base stations and aerial wires) installed along the road and on-board unit installed on vehicles. The base stations and on-board unit conduct two-way radio communications. The radio signals are limited to a range of several tens of meters, and information is exchanged instantly within that area. This type of road-to-vehicle communication is referred to as Dedicated Short Range Communication (DSRC).

  Road-to-vehicle communication system development began with continuous communications covering an area of several hundred meters using multiple DSRCs, a capability that conventional systems did not have. Proving tests of AHS services by means of continuous communications were conducted at Demo2000 in fiscal 2000. However, circumstances changed with the adoption of new DSRC standards, and from the latter part of fiscal 2000, the focus of research shifted instead to the development of DSRC for early practical application using spot communications.

  The continuous communication system forms a radio zone of continuous communication by means of multiple roadside units in order to periodically provide information on standing vehicles and other such constantly changing conditions to vehicles in zones up to the one immediately before the one where a phenomenon is detected. The spot communication system, on the other hand, forms a radio zone with a single roadside unit.

  The Telecommunications Technology Council, which is an advisory body to the Ministry of Posts and Telecommunications, issued a report on Technical Conditions for Radio Facilities, Etc., for DSRC Systems in October 2000 that suggested moving toward multipurpose application of the DSRC standard, which is used for ETC. AHS responded to the report by adopting the same DSRC standard (ARIB STD-T75) as ETC and planning common uses for the ETC on-board equipment.

  Estimates have confirmed that information provision by spot communication have nearly 50% the effect of information provision by continuous communication. From study of the Quadrature Phase Shift Keying (QPSK) method of modulation, future research on continuous-communication DSRC will take up the issues of such modulation methods as Orthogonal Frequency Division Multiplexing (OFDM) and Phase Shift Keying-Varied Phase (PSK-VP).

  AHS in the early phase of practical application of spot communication will be configured with two road-to-vehicle communication functions. One is the starting beacon, which provides AHS service reference points and starting information, and the other is the information beacon, which provides cruise-assist information.

  The vehicle uses position information obtained from the starting beacon together with cruise-assist information sent from the information beacon to judge the content and timing of services, and provides that information to the driver. The combination of information from the two beacons the starting beacon and information beacon allows the vehicle to find out the direction in which services are provided and judge whether to accept the services. This arrangement makes that possible, and also serves as a countermeasure against radio wave leakage to oncoming lanes. In addition, the information beacon and starting beacon both use one-way broadcast transmission from roadside units to vehicles in order to provide information to all vehicles within the communication area.

  Services for road sections of uninterrupted flow field transmit cruise-assist information repeatedly in order to be certain of providing the information. Moving vehicles take the position where they received starting information as the relative AHS service distance origin point, after which they receive information on phenomena such as the presence of a standing vehicle from the information beacon. In services for intersections, moving vehicles receive starting information, then every 0.1 second they receive information on the position of vehicles entering the intersection from the information beacon, and assist drivers with their driving.

  The purpose of road-to-vehicle communication in AHS services is to support driving safety. It is therefore required not only to provide information without any errors, but also to assure safety in communications, so that information is securely conveyed to vehicles. AHS refers to the index of such security as safety integrity level, and assigns a share of 99.1% or more of it to road-to-vehicle communications for services in road sections of uninterrupted flow field. Factors that interfere with this safety include changes in the environment, radio wave leakage, overlook code error, code error, shadowing, radio interference, crosstalk, equipment malfunction, power failure, and so on. There is also starting point position accuracy. Of these factors, those that cannot be limited to an ignorable level by measures relating to installation, and so on, are shadowing (0.68% or less) by other vehicles that blocks radio waves, code errors (0.12% or less) caused by multipath signals occurring when radio waves reflect from other vehicles, and equipment malfunction (0.1% or less) in roadside unit. The figures in parentheses are the allocated values for allowable safety integrity level.

  Equipment malfunction is considered a matter to be addressed by equipment design and operational measures. Code errors caused by shadowing and multipath signals were subjected to testing in order to verify their probability of occurrence.

  Shadowing was measured by installing cameras at test locations on actual roads. In the testing on National Highway No. 25 (Maitani District) in Nara Prefecture, the approximately 20,000 vehicles that passed through during the three-day test period (14 hours) were analyzed. The results confirmed that shadowing will occur with a probability of 0.16% or less per single DSRC installed at a height of 8 m. Consequently, it was verified that the figure for a configuration of two DSRC will be 0.32% or less, which falls below the allocated value.

  Coding errors due to multipath signals were tested by creating an environment that generates multipath signals on the expressway facility of a test course. After 550 test runs, it was confirmed that the results achieved safety integrity level at an allowable value of 0.12% or better.

  In conclusion, the features of DSRC used in AHS are shown here:

Future issues for used in AHS are shown below:

1. Conceptual approach of safety of information provision for intersection services where shadowing occurs frequently due to oncoming vehicles, etc.
   
2. Problem of provision of multiple applications and standardization of communications protocols
   
3. Problem of deterioration of signal reception characteristics due to oblique reception when on-board unit is installed indoors