THE IEEE FOURTH INTERNATIONAL CONFERENCE ON INTELLIGENT TRANSPORTATION SYSTEMS 1 Space Division Multiple Access (SDMA) for Robust Ad hoc Vehicle Communication Networks Soheila V. Bana, Pravin Varaiya Abstract| This paper proposes a novel robust and well-suited for V2V communication because they do not self-organizing architecture for mobile ad hoc networks. satisfy the following requirements. First, V2V communica- The proposed architecture, space division multiple access (SDMA), relies on user position information and provides tion for control needs to be in real-time and delay-bounded. users access to the communication channel based on their Second, the communication addresses for the current wire- spatial locations. SDMA divides the geographical space, less technologies are assumed to be resolved while in V2V where the users are located, into smaller spaces. The key communication addresses are not known a priori. Third, element of the design is a one-to-one map between the space divisions and the bandwidth divisions of time slots, the technology must be scalable and provide access to a frequency divisions, etc. Therefore, SDMA is compatible vast number of users. Fourth, the technology must take with any multiple access scheme such as TDMA, FDMA, efficient use of the bandwidth. Finally, initializing an ad CDMA, etc. The system requirement is the user informa- tion of its position and a priori knowledge of the one-to-one hoc network and maintaining its organization while differ- map between the space divisions and the bandwidth divi- ent users constantly join and depart the network are major sions. The scheme is self-starting and self-maintaining. It issues that need to be resolved. Initializing the network provides collision-free access to the communication medium for the users, and thereby, facilitates ad hoc communication organization must be independent of the number of users address resolution amongst the users. In addition, SDMA and the network topology. If arrival or departure of a user guarantees delay-bounded communication in real-time. The disturbs the communication network, or introduces addi- paper presents the SDMA architecture and discusses its ad- tional delay on existing users, it may cause problems with vantages and disadvantages. Moreover, enhanced SDMA is discussed for increased bandwidth efficiency. real-time communication. In the case of automated vehi- Keywords| Vehicle Communication, Ad hoc Network, cles in AHS, V2V communication must also be fair, i.e., Medium Access Control, Address Resolution provide access to all the users. We introduce space division multiple access (SDMA), an innovative scheme for medium access control with bounded I. Introduction delay for all users. It also provides address resolution. The HICLE-TO-VEHICLE (V2V) communication can pro- proposed architecture is scalable and is independent of the Vmote safety. Exchange of information regarding ve- number of users and network topology. SDMA makes effi- hicle dynamics and road conditions among vehicles could cient use of the bandwidth by minimizing the MAC and play a crucial role in driver and passenger safety. A driver, address resolution protocols. Finally, SDMA is a self- provided with information about road conditions and ve- organizing architecture. locities of the vehicles around it, is able to make better The paper is organized as follows. Section II explains the decisions with regards to vehicle control and travel path. specific characteristics of V2V communication networks. As an example, consider driving in a bad weather condi- Section III briefly overviews the major existing wireless ad tion such as heavy fog. When vehicles communicate their hoc technologies and explains why they are not appropriate real-time velocity values, a driver can avoid accidents by for V2V communication. Sections IV and V discuss SDMA adjusting her velocity according to neighboring vehicle ve- in detail and present an example of its application for auto- locities. mated vehicle networks, respectively. Section VI discusses V2V communication is useful not only in enhancing how SDMA can be further enhanced for more efficient use safety but also in vehicle automation. V2V communica- of the bandwidth. Section VII concludes the paper. tion is a key element in vehicle automation. A vehicle in automated highway systems (AHS) [1] communicates with II. Background neighboring vehicles to exchange vehicle control data [2], Two main challenges for wireless mobile ad hoc networks [3], [4]. As a result, automated vehicles interact safely. at the data link layer are medium access control and com- V2V communication is likely to be through ad hoc net- munication address resolution. The physical layer in wire- works that do not rely on infrastructure because of the less communication can take advantage of advanced RF high-speed mobility of roadway communications and their technology. The other layers [5] are not very different from dynamic topology. the conventional networks. Current wireless ad hoc network technologies are not A. Medium Access Control Soheila Bana is with 3Com Corporation. E-mails: So- heila [email protected] . Medium access control (MAC) refers to controlling user Pravin Varaiya is a professor at the Department of Electrical En- gineering and Computer Sciences at the University of California at access to the communication channel, i.e., sharing the Berkeley. Email: [email protected] . bandwidth amongst the users. Ideally, the bandwidth THE IEEE FOURTH INTERNATIONAL CONFERENCE ON INTELLIGENT TRANSPORTATION SYSTEMS 2 would be shared fairly with some quality of service con- A. Bluetooth siderations. When communication is used for real-time in- Bluetooth is a wireless technology optimized for short formation, there needs to be a bound on the delay that a range communication with low power. It is specially ap- user experiences before it can successfully access the chan- propriate for cable replacement and for use with portable nel. If users need to exchange MAC protocols to obtain devices in pervasive computing applications [8], [10]. access, then it is desired that MAC protocols use a mini- A Bluetooth ad hoc network, called a piconet, accom- mal portion of the bandwidth. modates up to seven users. Piconets that have common users can form a scatternet. However, the common user B. Communication Address Resolution can be active in one piconet at a time. In a piconet, an Communication address resolution in conventional net- arbitrary user plays the role of the \master" and the other works such as the Internet and telephony is the unique users act as \slaves". Initially, different users have different binding between a user's personal ID and the loca- clock times but in a piconet the slave clocks are synchro- tion of its communication device. An email address, nized with the master clock. A slave can be in the active [email protected], is an example of a binding communication or standby mode. The master controls the of a personal ID, soheila, and the Internet address, medium access. It polls the slaves for communication and eecs.berkeley.edu which specifies a host in the Berkeley schedules the transmission of the active users based on traf- campus. Similarly, the phone directory keeps a binding fic demands to and from the different slaves. In addition, it between a user's personal phone number and the location supports regular transmissions to keep slaves synchronized of her telephone. In the case of a cellular phone, the phone to the channel. company has a harder time to keep track of the location, i.e., mobility management [6]. However, the nature of the B. Wireless Local Area Network (WLAN) binding is the same. WLAN is based on the IEEE 802.11 standard. It pro- In the context of V2V communication, address resolu- vides two services, synchronous and asynchronous, that tion is defined differently [7]. Consider three vehicles on can be implemented with or without infrastructure, respec- the road as depicted in Figure 1. If the front vehicle wants tively. We focus on the asynchronous or ad hoc mode. The MAC protocol for the ad hoc mode is carrier sense multiple access with collision avoidance (CSMA/CA). A user senses the medium before attempting a transmission. If it senses the medium is idle for an interframe space, then it trans- C B A mits its data. If it senses the medium is busy, then it back- offs for a random interval between zero and a maximum Fig. 1. Vehicle A wants to communicate with its behind vehicle, B. contention window. The backoff interval is decremented when the channel is idle until it reaches zero and the user to warn the vehicle immediately behind it about its de- attempts to access the medium. The receiver sends an creasing velocity, it needs to address it by its relative po- ACK to the sender when the transmitted data is received. sition BEHIND. Here, the personal ID of the vehicle If the sender does not receive an ACK, it attempts another behind or its driver is not relevant. But address resolu- transmission [9], [11]. tion is defined as a binding between the relative position Currently, WLAN is mostly used with an access point of the addressee, i.e., BEHIND, and its receiver commu- in the synchronous mode where the access point polls the nication address. In Figure 1, call the vehicles from front users, similar to the master-slave scheme in Bluetooth. to back A, B, and C, respectively. The binding that A needs is < BEHIND; B-Receiver-ID >. Then A can C. Bluetooth and WLAN Shortcomings for V2V Commu- send a message to the vehicle behind it by addressing it nication to B-Receiver-ID. Note that if A sends a message that is addressed to BEHIND, then B cannot figure out that it Both Bluetooth and WLAN technologies support wire- is the addressee. less ad hoc networks. However, mobility needs to be con- sidered with more care in wireless technology.