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Architectural and technical aspects for Ad Hoc Networks based on UTRA TDD for Inter-Vehicle Communication Luca Coletti1, Nicola Riato1, Antonio Capone2, Luigi Fratta2 1

Siemens Mobile Communications S.p.A., Milan, Italy 1 email:{Luca.Coletti | Nicola.Riato}@siemens.com 2 Dipartimento Elettronica e Informazione, Politecnico di Milano, Italy 2 email: {capone | fratta}@elet.polimi.it

ABSTRACT The CarTALK 2000 [1] project aims at the development of an Ad Hoc wireless network, which can be classified as fast moving outdoor network. Main characteristics of such a kind of network are the large and variable number of mobile terminals, an extremely dynamic network topology and stringent requirements related to broadcast warning messages. In order to satisfy the basic requirements of such a radio interface, the framework of the UMTS Terrestrial Radio Access Time Division Duplex (UTRA-TDD) with modifications has been adopted as target system in the CarTALK 2000 project. This paper introduces the architectural reference model of the communication system and discusses the main technical issues that arise when passing from a centralised network architecture (as the current UTRA-TDD communication system architecture) to a totally decentralised Ad Hoc mode. I. INTRODUCTION The CarTALK 2000 project aims at the development of new driver assistance systems, which are based upon intervehicle communication. For this purpose, a mobile wireless Ad Hoc network that supports the requirements of the targeted co-operative driver assistance applications has to be developed and specified. This paper presents the fundamental technology, the protocols and the architectural concepts proposed for advanced driver assistance systems. The proposed concepts represent a significant and valuable conceptual base to start the process leading to a future standardised communication system. The main technological challenge consists in designing a mobile Ad Hoc communication system tailored to the demands and the requirements of various driver assistance applications. In particular three different CarTALK 2000 application clusters have been defined: 1) Information and Warning Functions (IWF): this kind of signals transmit information regarding stopped vehicles, traffic incidents, detected traffic density, congestion, or road surface conditions allowing an early warning for the vehicles following on the same road. The transmission of short, aperiodic, broadcast, with high priority and low latency messages is required for the IWF service. To guarantee a fast warning in a larger zone the one-hop transmission range should be up to 1000 meters, but multihop transmissions should be possible to cover a wider area.

2) Communication-Based Longitudinal Control (CBLC): this application can allow anticipating an early braking manoeuvre when an invisible vehicle in front is braking. Medium sized messages have to be transmitted for the CBLC service. It requires normal priority in short periodic intervals. The one-hop transmission range should be up to 500 meters and multihop forwarding is desirable. 3) Co-operative Driver Assistance (CODA): this application can be typically helpful in highway entry and merging scenarios. A direct addressing of neighbouring vehicles and a reliable unicast communication link, with low latency, are needed for CODA applications. Information may be only sent upon request as for instance initiated by a vehicle on the ramp. The transmission range should be up to 500 meters and medium-sized messages are exchanged in a short interval, as long as the merging manoeuvre lasts. Broadcast functionality might be useful for the CODA functions as well, e.g. to send a gap request to all nearby vehicles. The CODA functions will use different type of messages, which can be easily separated in emergency, assistance and information messages of different priorities. The philosophy of the CarTALK 2000 communication system is to modify an existing air-interface to minimise costs and to benefit from mass market. Requirements best suited to support the demands of the target system and applications are: • High velocity (< 500 Km/h); • Large radio range (>500 m); • Adaptive data rate (30 kbps-1 Mbps); • Availability of unlicensed spectrum; On the other hand, services to be provided include: • Multicasting of periodic data traffic to propagate cruising information to the surrounding vehicles for traffic control; • Broadcast of alarm messages with different priorities; • Point-to-point communication between terminals with QoS features; Finally, due to the fact that terminals are installed in cars, no power consumption limitation (energy-saving) exist. II. CarTALK 2000 PROTOCOL ARCHITECTURE The simplified protocol architecture of CarTALK 2000 communication system, derived from the OSI model, is illustrated in Figure 1. The radio interface is functionally split into three layers: the physical layer (PHY), the Data Link Layer (DLL) and the Network Layer (NL).

without a fixed base station that can manage the power and GPS Location. At this level we will use only the GPS clock as base time signal for coarse time synchronisation. Since GPS positions will be also used in other layers, e.g. for the position-aware routing protocol in the network layer, this function is implemented in the system management plane. C. CT Network Layer:

Figure 1: Protocol Architecture A short explanation of the characteristics functionalities of each block follows.

and

A. CT Physical Layer: At the physical layer, everything is concerned with how messages are transmitted/received. The main functionalities provided by this block are: data modulation, which encodes information from the source in a way suitable for communication; bit synchronization in which the decision instant is brought into alignment with the received bit; channel coding which protects digital data from errors by selectively introducing redundancies in the transmitted data. The main function of the Physical Layer Management are frequency selection, i.e. the selection of a specific band of frequencies in the Table of Frequency Allocations assigned to the system and the Timing, that is related with the overall synchronisation of the system over one or both forward and backward links. B. CT Data Link Layer: The Data Link Layer is further divided into two sub-layers: Medium Access Control (MAC) and Logical Link Control (LLC). The main LLC functions are: fragmentation/reassembly, buffer management allowing the control of datagram flows and scheduling; error control providing error correction by retransmission in acknowledge data transfer mode (ARQ). The MAC scheme has to cope with the common requirements of Ad Hoc networks and the main functions are: frame synchronisation, i.e. distinctive time slots and bit sequences identification, scheduling and channel access strategy in order to avoid or reduce collisions. The MAC is also responsible for priority and capacity reservation mechanisms. The main function of DLL Control are: channel allocation in a dynamic mode and neighbour discovery in a system that works without fixed base stations. Identification of other stations in the communication range and link establishment are very important and critical issues in an Ad Hoc radio system, especially when the topology changes very frequently, like in CarTALK 2000 scenarios. The main functions of DLL Management are: power control that is a critical issue in a spread spectrum system, especially in Ad Hoc mode

The third layer in CarTALK 2000 architecture determines the routing of data packets from sender to receiver via the DLL and it is used by the upper layer. Its main functions are: routing able to manage multihop paths in high dynamic topology, IP adaptation in order to adapt the packets from the IP format when necessary and service classes. MAC expects two different classes of priority levels (high and normal). At Network Layer it is possible to define other service classes or priorities. The main functions of NL Control are: neighbour discovery, location service and routing decision. The neighbour discovery in the NL is different from that in the DLL. In NL it depends on geographical information (digital map). In the DLL it depends on the radio link and communication range. Since a position-based routing protocol will be used on the NL, each data packet will be forwarded in the geographical direction of its destination. To this goal, the sender and intermediate nodes on the path from the sender to the receiver need to have a mechanism to determine the location of the receiver or the next hop. The location service, implemented in NL, allows to determine the position. Routing decision between different routing strategies such as multicast, broadcast or unicast, will be made according to the type of the packet. Main functions of NL Management are: GPS positioning and Map information. The precise position information (coordinates) needs to be associated to the environment context information. Digital maps provide this information. In order to fulfil the CarTALK 2000 requirements, both, positioning and maps are needed in a suitable quality. D. CT Higher Layers: The CT Transport Layer is optional and we can consider it like an adaptation layer. It will be inserted only if required by applications. It is also possible that the applications, especially urgent messages, can communicate directly with the network layer, without this block in between. The TCP/UDP IP Layer are part of the Internet protocol suite. The presence of these protocols in our architecture guarantees the compatibility with already existing standards and applications based on the Internet protocol. The CT Application Layer describes an interface that can be used by advanced driver assistance applications that use inter-vehicle communications based on the CarTALK 2000 communication system. The application interface (API) facilitates the development of driver assistance applications, because it abstracts from the details of the communication systems and provides simple

communication classes with corresponding Service Access Points (SAPs). A detailed technical specification will be provided in a future technical report and it will contain the definitions of the different messages structures and contents, which are exchanged by peer applications. III. CarTALK 2000 RADIO TECHNOLOGY With respect to the communication requirements expected, the communication technology for the selected target system is UTRA-TDD. In particular the low chip rate option (1.28 Mcps) called UTRA-TDD-LCR has been adopted, since a earlier market introduction can be foreseen. The UTRA-TDD-LCR proposal coming from the CWTS (Chinese Wireless Telecommunication Standardisation) has been admitted between the 3GPP systems starting from the Release 4 of 3GPP specification. One argument for using UTRA-TDD in the CarTALK 2000 communication target system is the availability of the UMTS frequency range of 2010 to 2020 MHz, which is allocated as a license-free band for UTRA-TDD mode. Looking at the evolution of UMTS, it is furthermore expected that low cost components will be available. In addition, UTRA-TDD offers high flexibility with respect to asymmetric data flows and it is extremely well-suited for packet data transfer and for tomorrow’s IP traffic. Finally it supports high speed mobility, sufficient user bit rates and quality of service (QoS). It is appropriate for operation in an Ad Hoc network and it allows communication over large distances and operation in an multipath environment. IV. AD HOC MODE OF UTRA-TDD UTRA-TDD system has been designed to operate in a centralised network architecture and, in order to adapt it to the Ad Hoc mode operation, it is necessary to evaluate, review and solve technical problems related to: synchronisation, power control, medium access (MAC) and radio resource management (RRM). A. Synchronisation: In the centralised system, each communication link involves basically a mobile User Equipment and the fixed Base Station (Node B). Furthermore, in UTRA-TDD-LCR, both Uplink and Downlink synchronisations are required. The BS itself determines the time reference for each link. On the other side, in the Ad Hoc mode operation, the network structure is distributed and subject to frequent topology changes. A basic consequence is that, in the Ad Hoc mode, the distinction between Uplink and Downlink has no meaning. Nevertheless, the use of the same time slotted radio frame structure of UTRA-TDD LCR implies that synchronisation between all mobile stations (MS) transmissions have to be maintained. This is necessary in order to avoid the improper overlap of the respective user bursts and to align all stations to the commonly used frame structure. Within this approach the allocation of more than

one user per time slot, in the CDMA sense, becomes really a difficult task to implement because users inside the same time slot have to be perfectly synchronised in order to be correctly demodulated (either using standard detection techniques, e.g. Rake Receiver, or using Joint Detection). Also power impairment problems, as will be shown later, arise in this case and a precise power control scheme is needed. Clearly, synchronisation is a fundamental issue in the implementation of the CarTALK 2000 system.

Figure 2: Transmitter time offset, observed at UE2 In order to overcome these problems maintaining an acceptable level of complexity, a solution is to allow only one user to transmit in each time slot. In such a way complex equalisation techniques, such as Joint Detection or joint channel estimation (as implemented in the centralised UTRA TDD LCR) are no more needed and the required synchronisation level can be kept limited at a time slot level and not at a chip level. The decentralised synchronisation scheme, accounting for the lack of a central instance does not need the Special time slots for synchronisation which could be used for different purposes. A synchronisation scheme that can be proposed for the Ad Hoc mode can contain two levels of synchronisation. First, a coarse synchronisation achieved by using the Global Positioning System (GPS) in order to globally know the frame and slot boundaries and the system frame number (SFN) which can be used by all stations for e.g. resource allocation. Second, a fine synchronisation can be carried out to estimate and compensate residual time and frequency offsets of a received burst [2]. The accuracy of the GPS based time signal depends on the number of satellites that the GPS receiver is tracking simultaneously. Even in the case of only one measured satellite at a time, the value of the absolute time error is significantly below 1 µs and could be sufficient for CarTALK 2000 requirements. Different scheme without GPS are under investigation. B. Power Control: One of the basic problems affecting CDMA systems is the so-called ‘near-far’ problem. Users differentiated by means of spreading codes should arrive at the receiver with differences in power as low as possible. In a communication system like the CarTALK 2000 system, this problem cannot be solved in a simple way. In fact, in an inter-vehicle communication, all users signals will undergo different propagation conditions and will all be received at each receiver with different and no controllable power levels. The lack of a centralised reference usually provided by the base station (BS) in the centralised systems, or a master in this communication scenario

prevents the implementation of usual power control schemes. A solution that gives the possibility to overcome the power impairment problem and considers also the constraints posed by synchronisation, is to allow each MS to transmit in separated time slots. In this case, different users transmit over time intervals kept distinct by means of synchronisation. Different and varying distances are no more a problem once the signal is received with a sufficient power level. Finally, this approach grants the simplicity of the communication system. With this approach, the decision on the actual transmission power to be used by each terminal, is determined by the terminal itself considering constrains deduced from information coming, for example, from the MAC layer. In fact, another basic assumption of the CarTALK 2000 communication system is that each car communicates with the cluster of cars inside a communication range of 1-Km radius maximum.

Figure 3: Communication clusters The power control should influence the communication radius both for minimising interference to vehicles outside the range of interest (cluster size) and to grant reliable connections inside the cluster regarding fading impairments. Furthermore an extension of the cluster could be determined at the MAC layer through an evaluation of the congestion level. In case of a high congestion situation, the cluster should be reduced, while at a low traffic level, the cluster can be made larger. These considerations are strictly related to the MAC scheme specifications, which are currently under discussion. C. Medium Access Control: In order to satisfy the CarTALK 2000 communication system requirements, the multiple access mechanism that has been considered in literature is the Dynamic TDMA, mainly because of its ability to deliver QoS and because it corresponds to the UTRA-TDD physical layer approach. Some proposals suggest to set up a broadcast signalling channel that is used to set up and tear down traffic channels in a dynamic way. Although the signalling channel can be based on the same approach, i.e. TDMA, two problems remain unsolved: 1) Since Ad Hoc networks are composed of several broadcast segments, it is not immediately clear whether the signalling channel must broadcast over the entire network or not. The solution depends on the information that the signalling channel must convey, i.e., on the type of channels that must be set-up, such as, local broadcast

channels, point-to-point channels with slot reuse, global broadcast channels and so on. 2) The signalling channel must be attained in some dynamic way. Many proposals suggest to use Reservation ALOHA (R-ALOHA). In R-ALOHA, random access is used to try transmission in an available slot, say the k-th slot of a frame of N slots. If the transmission is successful the slot is reserved to that terminal in subsequent frames and is no longer accessed by other terminals until the channel is released. However, to correctly operate RALOHA requires a broadcast environment that enables all terminals to receive all the transmitted signals and, most important, to get the same slot status information, e.g., busy, free, or collided. In this way a terminal can avoid collisions with ongoing transmissions and can discover possible collisions on access. However, since in Ad Hoc networks no central repeater is present, a terminal is not guaranteed to hear all the transmissions (hidden terminal) and, therefore, destructive interference with already established channels can occur when trying to access a slot. Furthermore, the accessing terminal does not know the outcome of its transmission, which might be different at different terminals. So, the use of R-Aloha in its original form could be a problem in absence of a central repeater that allows each terminal to "see" the activity of all terminals on the channel. To overcome these problems and exploiting the suitability of the TDMA, a new protocol, named Reliable R-ALOHA (RR-ALOHA) protocol, is currently under study [3]. This protocol, by transmitting some additional information, let all terminals know the status (AVAILABLE or RESERVED) of each slot, thus safely allowing in the Ad Hoc environment the same procedure of R-ALOHA. This protocol becomes the base of a new MAC architecture which using this signalling channel, that is reliably received by all terminals within the broadcast segment, allows to completely solve the hidden terminal problem. Using the information that RR-ALOHA makes available to terminals, additional reliable channels, either for local broadcast or point to point can be set-up . Furthermore, this MAC solves the network broadcast problem in an efficient way. In fact, for any packet that must be broadcast to the entire network, a limited set of terminals is chosen to relay the packet. This choice is dynamically made on a frame by frame basis, so that, even in presence of great mobility of terminals, optimal choices can always be made. The information propagated by RR-ALOHA causes an overhead that increases with the size of broadcast segments in the network and that is approximately 20-30% for 50100 terminals broadcast segments. Work is in progress to specify the operation of the entire MAC, to verify its correct behaviour and to evaluate different implementation alternatives. D. Radio Resource Management: The radio resource management is a major challenge for the Ad Hoc mode. It comprises the reservation of capacity, the operation of channels and the release of reserved

code

capacity. For QoS, the ability to guarantee the reservation of transmission capacity is a basic requirement. In the Ad Hoc mode of UTRA-TDD the physical structure of time slots is maintained but the proposed solution, i.e. only one station is allowed to transmit in a slot, brings substantial differences in the resource allocation with respect to what happens in the centralised approach [4].

structure. In such a way, if further resources are needed for user data, they can be found in the overall superframe. This approach might reduce the waste of capacity. The number of frames in a superframe is a system parameter that has to be properly determined depending on applications bit rate and delay, distribution of vehicles and the MAC scheme used. V. CONCLUSIONS

14 TIME SLOT (TS)

3 SPECIAL TS

time

RADIOFRAME (10 ms)

Figure 4: Time/Code matrix In the proposed approach, transmission capacity can be flexibly assigned by using different spreading codes (up to 16) in parallel for different connection from one station to all others. This exploits the advantage of fine granularity of capacity offered by the CDMA component over a pure TDMA system. As in UTRA-TDD, physical channels identified by the spreading code, the timeslots (TS) and the carrier frequency can be a unicast, anycast, multicast or broadcast connection. Since only one station transmits at the same time it can adapt the right transmission power to all codes. The high priority services have high QoS requirement, in particular very low acceptable delays. We define two main priority classes: low and high. High priority services are used only for urgent messages, e.g. to inform other stations of an accident or traffic jam, etc., through the transmission of an emergency broadcast. In order to satisfy these requirements, two possibilities have been devised. The first approach (fixed time slot reservation) requires a fixed reserved bandwidth for high priority service even when these messages are not actually present. The second one, instead, foresees a dynamic allocation of physical resources according to the priority level of the message. With this approach an ongoing low priority service can be interrupted by a higher one, and consequently will suffer a decrease in quality of service. Both these approaches are under considerations. Once a user terminal has accessed the network, it is proposed to permanently reserve a small amount of transmission capacity, even if no user data packets have to be transmitted. This capacity is primarily used for signalling purposes (e.g. frame occupancy, request for more resources, etc.) and it allows to avoid the high number of reservation attempts in contention mode, as it happens in R-Aloha that is used for the initial reservation. The resource permanently reserved to each station for signalling can be assigned periodically with a period greater than the UTRA frame, e.g., one slot every second, fourth, or eight frame, resulting in a larger superframe

This paper describes the architectural reference model and the main tasks involved in adapting the UTRA-TDD system to the CarTALK 2000 Ad Hoc mode, and represents the starting point for the detailed analysis of the CarTALK 2000 target communication system. The highly dynamic changing topology and the lack of a centralised reference, characteristics of the Ad Hoc mode, require new Synchronisation, Power Control, MAC and RRM procedure. Since there are some degrees of freedom and some open issues, the detailed specification of the data link layer (e.g. initial reservation, addressing, increasing transmit transmission capacity, details of MAC scheme, release of connections) need to be further refined and detailed. To solve the open problems and to exploit the suitability of the TDMA, a new protocol, based on an evolution of the R-ALOHA protocol, is currently under study. To reach the final system design, detailed analysis together with supporting simulation investigations are in progress. VI. ACKNOWLEDGEMENTS This work has been done within the CarTALK 2000 project started in August 2001 as a three-years project which is funded within the IST Cluster of the 5th Framework Program of the European Commission. The CarTALK 2000 consortium consists of the following partners: DaimlerChrysler AG (D) as project co-ordinator, Centro Ricerche Fiat (I), Siemens Mobile Communications S.p.A. (I), TNO (NL), Robert Bosch GmbH (D), the Universities of Stuttgart and Cologne (D). REFERENCES [1] CarTALK 2000 Home page: www.cartalk2000.net [2] A. Ebner, H. Rohling, R. Halfmann, M. Lott, Synchronization in Ad Hoc networks based on UTRATDD, in Proc. of IEEE PIMRC, Lisboa, Portugal, Sept. 15-18, 2002 [3] F. Borgonovo, A. Capone, M. Cesana, L. Fratta, ADHOC MAC: a new, flexible and reliable MAC architecture for Ad Hoc networks, in Proc. of WCNC 2003, New Orleans March 2003. [4] M. Lott, R. Halfmann, E. Schultz, M. Radimirsch, Medium access and radio resource management for Ad hoc Networks based on UTRA TDD, 2001 ACM MobiHoc, New York, NY, 76-86.