TRANSPORTATION RESEARCH RECORD 1408 101 Communications Architecture for Early Implementation of Intelligent Vehicle Highway Systems D. J. CHADWICK, v. M. PATEL, AND L. G. SAXTON Communications-wireless communications in particular-is a cations. Given these spectrum constraints, there is a real in­ critical component of intelligent vehicle highway systems (IVHS). centive to overlay IVHS communication functions on existing It is costly when viewed from two different angles: first, its de­ services where possible. Two examples that have been dis­ pendence on using the scarce natural resources called radio fre­ cussed are digital cellular and FM subcarrier. quency (RF) spectrum, and second, the actual cost of imple­ menting the necessary infrastructure and the required (in-vehicle) IVHS is not a single system but rather a broad set of ap­ user equipment. On the other hand, it is imperative, for the plications with functions and technology that intersect in var­ success of the national IVHS program, to plan a near-term IVHS ious situations. IVHS also has a range of technology and architectural implementation to show positive first-user benefits. system availability spanning products that are available today Bearing in mind the above constraints, it is natural to think of to concepts not expected to be marketed until after the turn designing an IVHS communications architecture that makes use of the century. This range of both product types and market of existing infrastructures for other communications services. This strategy enables more efficient use of the RF spectrum while it timing poses a dilemma to IVHS. For the most efficient system reduces the total cost of services by sharing the communications and effective use of spectrum there is the need to establish infrastructure and end-user equipment. A communications ar­ an overall efficient architecture for IVHS. Efforts to make chitecture is proposed for IVHS called the Subsidiary Commu­ this analysis and to postulate the most preferred architecture nications Authority Traffic Information Channel (STIC), based are now being initiated but will require at least 2 to 3 years. on the widely available FM radio broadcast services' infrastruc­ However, there is an immediate need to bring to market and tures by making use of FM subcarrier technology. This prelimi­ nary design also shows that STIC has a higher data transmission thus realize the benefits of many IVHS applications that are capacity than any other existing FM subcarrier broadcast system near-term implementation candidates. A primary example is and that it has the potential to meet the one-way outbound the desire to market motorist information and route guidance­ (broadcast) data transmission capacity needs of IVHS for the next type systems. Thus, this early market need results in the ne­ few years. In addition, STIC architecture is capable of being cessity to establish communication resources and standards scaled up in the future. now for these current applications without benefit of knowing the form of the final IVHS architecture. The practical com­ The intelligent vehicle highway system (IVHS) program is promise is to attempt to establish a communication design to broadly described as applying modern communication and service these near-term needs that is easily available, mini­ control technology to the needs of highway transportation. In mizes any Federal Communication Commission (FCC) rule­ this regard, IVHS is yet another part of society that is in­ making needs, is economical, and is adaptable to future ar­ creasingly interwoven with and dependent on modern com­ chitecture designs (J). munications technology. At home, at the business place, or Given these objectives and constraints, FHWA has been in transit, currently available communications provides an considering various communications approaches to service additional dimension of information exchange to enhance one's motorist information and route guidance-type applications. business and personal life. These systems do not necessarily have a single or homoge­ The success and viability of IVHS is dependent on several neous set of communication needs as there is a strong need factors; however, perhaps none will be more central and fun­ to provide both low-cost, basic performance systems and higher­ damental than efficient, reliable, and affordable communi­ cost, high-performance systems. Low cost can be character­ cations. Although some IVHS functions will be serviced by ized as radio data system (RDS)-traffic management system "hard-wire"-type communications, the more advanced system (TMS) type systems, whereas the higher-performance systems concepts generally involve communication with a moving ve­ are of the TravTek type. hicle and thus require some form of wireless approach. At The use of the subcarrier part of each FM station's fre­ issue here is the substantial growing demand for RF spectrum quency assignment has some particular advantages-espe­ to support various perceived and developing markets such as cially for the low-end systems (2). However, there also ap­ high-definition television and advanced personal communi- pears to be the opportunity to reconfigure the subcarrier in a manner in which the FM subcarrier could also service high­ end systems. This potential has been the focus of recent ef­ D. 1. Chadwick and V. M. Patel, The MITRE Corporation, 600 forts, and this paper is part of an effort to share this promising Maryland Ave., S.W., Washington, D.C. 20024. L. G. Saxton, FHWA, communications approach with the IVHS community. If fur­ Turner Fairbank Highway Research Center, McLean, Va. 22101. ther analysis and preliminary field testing support the value 102 TRANSPORTATION RESEARCH RECORD 1408 of this approach the objective would be to work it into early services is under 300 bps. That rate will not begin to support operational tests. IVHS data requirements for traffic link time updates, for example. RDS is, however, a robust system. Its cost is also relatively low. FM broadcast stations in the United States COMMUNICATIONS OUTLETS FOR EARLY plan to adopt the system, if only to support a program-type IMPLEMENTATION function. Public policy analysts generally agree that new systems such as IVHS must provide their first increment of user services Highway Advisory Radio and Advanced Highway or public benefits within about 5 years of the inception of the Advisory Radio project. Further, the initial offerings to the public have to be "winners"; that is, they have to be perceived as providing the Highway advisory radio (HAR) and advanced highway ad­ benefits that they promised at their inception, at a cost com­ visory radio (AHAR) systems are widely used in the United mensurate with those benefits. The Interstate highway system States for broadcasting information to travelers in a limited is a good example of this incremental implementation strat­ area. Services include traffic information, parking availability egy. In this case, the new highways were opened to traffic at airports and national parks, scenic view alerts, and so forth. after the completion of segments only a few miles long, thus The systems use AM broadcast-band equipment, operating delivering their promised benefits (safer, higher-capacity, at low power levels. Until recently, the transmitters were higher-speed roadways) quickly and in ever-increasing measure. operated on fixed frequencies (530 and 1610 kHz), wjthin the For IVHS to operate in the same way, it is essential that frequency range available to standard car radios. In the future, initial implementations not depend on the design, develop­ HAR and AHAR stations will be assigned anywhere in the ment, and installation of large infrastructures. This is espe­ AM broadcast band that is available, considering the assign­ cially true in the case of communications systems because they ments to local commercial stations. Because they operate in have to be in place before any other subsystems can even be a band in which the maximum modulating frequency is 5 kHz, tested effectively. This drives the conclusion that early IVHS the capacity of the systems to handle data is limited and is projects will have to make use of existing communication meant to be a voice announcement medium. Because of their outlets to the maximum possible extent. limited range, HAR stations would have to be installed to Several available communications technologies are avail­ support IVHS services in nearly all cases, which goes against able for early IVHS projec.ts. Each will be discussed in turn, the strategy of limiting infrastructure installation initially. elaborating on issues such as capacity, coverage, and cost. FM/Subsidiary Communications Authorization RDS As part of its license, every FM broadcast station is granted RDS is widely used in Europe for a variety of services in­ the authority to broadcast other program material on sub­ cluding traffic information (3). Broadcast stations in the United carriers that cannot be detected by standard receivers (see States plan to install the system as well, but its applications Figure 1). These subsidiary communications authorization may well degenerate to the single function of program type (SCA) subcarriers are already used by some stations as sources identification (classical, rock, sports, news, etc.) because the of extra revenue and carry program material that ranges from system's capacity and hence level of detail for route guidance sports programs and paging data to background music and data transmission are very low. RDS uses an AM subcarrier stock quotes. Many stations do not currently use their SCA on the FM broadcast station that is located at 57 kHz on the subcarriers. FM broadcast stations provide a high grade of FM baseband. The total baseband bandwidth that RDS oc­ service to well over 90 percent of the area of the United States cupies is about 7 kHz or from just above the stereo L-R and to 100 percent of the area where early IVHS experiments spectrum limit at 53 kHz to 60 kHz. The gross data rate for are likely to be conducted.