Umts on the road: broadcasting intelligent road safety information via mbms

UMTS on the Road: Broadcasting Intelligent Road Danilo Valerio, Fabio Ricciato∗†, Pavle Belanovi´c, Thomas Zemen Telecommunications Research Center Vienna (ftw.) A-1220 Vienna, Austria Universit`a del Salento, Lecce, Italy Email: {valerio,ricciato,belanovic,zemen} Abstract—In this work we explore the feasibility of imple-
we quantify this limit in a realistic application scenario and menting infrastructure-to-vehicle (I2V) communication through
propose to overcome the problem by means of the multimedia the UMTS infrastructure. We identify the problems that arise
broadcast/multicast services (MBMS) as introduced in Release when providing I2V services on top of legacy UMTS networks
as conceived in 3GPP Release 5 and consider an I2V architecture

6 [6]. In order to quantify the benefit of MBMS, we extended that exploits a new feature introduced by 3GPP Release 6, namely
the analytical model presented in [7] by taking into account the multimedia broadcast/multicast services (MBMS). By means
the MBMS channel overhead. Finally, we used the extended of an analytical model, we quantify the performance achievable
model to analyze the coexistence between dedicated channels with both releases in some realistic scenarios. We show that
(DCH) and MBMS channels. To the best of our knowledge MBMS is able to provide I2V services efficiently on top of the
UMTS network.

this is the first work that addresses explicitly the technicaland architectural aspects related to the adoption of MBMS for Traffic telematics has recently gained considerable interest The remainder of this paper is organized as follows: In in the research community and is now among the hot topics Section II we describe the analytical model. This is used for industry-driven research. Several public co-funded projects as the basis for the analysis of ITS-over-UMTS performance and associations (e.g. [1]–[5]) are working on architectures, presented in Section III. In Section IV we introduce MBMS services, and application scenarios for intelligent transporta- and define a parametric model for system-level capacity tion systems (ITS). Typical ITS services include high speed requirement, taking into account the most critical system and safe distance warning, lane keeping support, intersection variables on the sides of the network (e.g. cell configuration, safety, road congestion warning, accident warning, etc. All available bearer services), road (e.g. number of lanes and these services aim at increasing road safety by providing segments, road congestion), and applications. From this model timely information directly to the car and/or to the driver we derive a worst case reference scenario used for exploring so as to prevent accidents. The problem of ensuring timely and reliable communication amongst all the involved entities (vehicles and infrastructure elements) remains a central issuein the development of any ITS. Most of the running projects The UMTS cell capacity in the downlink is limited by the (e.g. [1]–[3]) pursue the development of new standards and total available power at node B. A well-known method for ad-hoc technologies for infrastructure-to-vehicle (I2V) and estimating the required node B’s transmission power (and thus vehicle-to-vehicle (V2V) communication (e.g. 802.11p), while its capacity) has been presented in [7]. The model is based on few others (e.g. [4] and [5]) focus also on the possibility to the assumption that user equipments (UE) in the DCH are able provide such services by reusing existing technologies (e.g.
to get exactly the required Eb/N0, i.e. the fast power control The traffic characteristics of ITS applications differ in general from the voice/data traffic patterns found nowadays in operational UMTS networks. Typical ITS traffic flows consist of small packets, sparse in time and directed to many different users. On the other hand, the mechanisms of dynamic radio channel assignment/release that are commonly in place in current UMTS networks are designed and optimized for is the expected transmission power at node B relatively dense flows directed to a single user. In other words to UE i, W is the chiprate, Ri is the bitrate for the selected there is a mismatch between the requirements of most ITS ap- service, Lm,i is the path loss from the serving node B to the plications and the connectivity services delivered by currently UE i, Ln,i is the path loss from another node B n to the UE deployed networks based on Release 5. Such mismatch leads i, N is the number of relevant neighboring nodes B, Ior is the to an inefficient usage of the available resources imposing a total transmission power of the nodes B (assumed to be equal severe limit to the sustainable number of users. In this work for all the surrounding nodes B), αi is the codes orthogonality factor, and PN is the thermal noise power. The first and the second terms in the denominator represent respectively the intra-cell and inter-cell interference. Solving (1) for Ec,i we obtain the transmission power required at node B to serve i +fDL)Ior + PN Lm,i] , factor. The maximum number of users in DCH would be As a starting point we explore the performance that can be achieved when ITS services are offered through conventional dedicated channels. In other words, MBMS functionalities are that the communication performance is affected by severaloperator-dependent parameters in the radio network: inactivity is satisfied. Equation (3) is valid when only DCH are provided timers and thresholds, handover and cell-reselection parame- within the cell. In the following we extend this model to take ters all have a strong impact on the global system efficiency.
into account the power overhead caused by the introduction Moreover, the dynamic nature of the traffic conditions makes of MBMS. In fact, a fraction of the Ior needs to be assigned it very difficult to find an optimal static setting. The critical statically to the high bitrate FACH channel used to carry requirements change dramatically in the two extreme road situations: congestion and fluent high-speed traffic. In the caseof congestion a larger cell capacity is needed, while for high speed traffic the support of mobility becomes critical. Notably different radio channel models have to be considered in the analysis of different road conditions. We evaluate the cell capacity both for the stationary and the high speed mobility MBMS is the power overhead due to the MBMS case. For the latter we use the channel model defined as “case OH is the power overhead due to the other 3” in the UMTS standard [8] and described in Table I. Note MBMS quality requirements and the MBMS bearer bitrate.
that Eb/N0 is mostly a function of the mobility channel and By restricting the model to the traffic telematic services, it service requirements as defined in [8].
is reasonable to assume that all connections will present the We analyze the performance of a UMTS Release 5 configu- same bitrate and quality requirement. Equation (4) would lead ration in terms of the number of users that use the ITS services.
The unicast nature of the DCH places a strict limitation on themaximum number of users-per-cell that can be served with ITS: a separate DCH must be setup for each user, causing multiple content duplications, hence wastage of the overall capacity. The spreading factor (SF) of the DCH plays an important role in the calculation of the maximum offered N L. (5) cell capacity. Table II summarizes the results considering that SF=256 is sufficient to meet the considered ITS service re-quirements, i.e. each user being able to receive simultaneously The estimated maximum number of users on the dedicated up to 8 service updates of 50 bytes. Note that the calculation in (6) does not consider the additional delay overhead caused by the channel assignment/release procedures. Hence, the values in Table II represent a theoretical performance bound.
This model forms the basis of the following discussion, where we analyze the offered cell capacity for specific traffic 1Note that we do not consider here the channelization code limit as we verified that in our scenarios it was much looser than the power limit ITS-over-UMTS Architecture with Release 6 functionalities Broadcast Mode: Data is transmitted to all the MBMS
Capacity Utilization Factor vs. Number of packets per burst capable UEs in a broadcast service area without anysubscription procedure; These additional delay components are much larger than the Multicast Mode: Data is delivered only to the UEs in a
transmission time of a few packets and reduce the achievable multicast service area that have joined a multicast group capacity utilization. The latter depends on the number of service updates that are sent in the same transmission burst.
Both streaming and file download services are provided by In other words, the achievable cell capacity is obtained by MBMS [10]. In the streaming mode, a continuous data flow multiplying the results in Table II with a capacity utilization provides a stream of media with optional additional text and/or still images. In the file download mode, the MBMS bearer distributes binary files and utilizes file repair procedures for providing reliability. The bitrate of a single MBMS bearer can assignment + Trelease + Tburst transmission Figure 1 depicts the value of γ as a function of the number of packets per burst. We have considered an allocation delayof 900 ms (following [9, page 281]) and an inactivity timer In our vision each car is equipped with a full 3GPP Release of 100 ms. Although such a small inactivity timer is atypical 6 compliant on board unit (OBU). All the authorized OBUs in operational UMTS networks optimized for world wide web join the multicast group where ITS messages are distributed.
(www) and wireless application protocol (wap), it represents Each message is sent in the form of binary data by the road an optimal choice for ITS, where the minimization of resource side units (RSU) through the file download service provided by consumption is of primary importance and the interval between the MBMS architecture. We assume that the UMTS network covers the road regions served by the ITS completely (the From the values in Table II it can be seen that when 8 “multicast service area” in [6]). The road is divided into road messages are sent in the same transmission burst (γ = 0.29) segments. Each segment forms a “local multicast area”, where the effective achievable cell capacity would reach only 450 the same content is delivered. Different local multicast areas Kbps in the stationary scenario and 250 Kbps in the high speed case. In summary the “multiple unicast” transmission The proposed architecture is depicted in Figure 2. Note offered by DCHs is shown to be inefficient and the average that some new entities are introduced in comparison to capacity in terms of transferred data per second is inadequate the conventional UMTS architecture: the broadcast/multicast- service center (BM-SC) and the road operator content provider(ROCP). The BM-SC belongs logically to the UMTS operator and can be implemented in the gateway GPRS support node(GGSN) or as an independent entity. It serves as an entry point for content delivery services, which are provided by an The lack of a real broadcast/multicast service in the UMTS external content provider, i.e. the ROCP. In other words, the network has been addressed in 3GPP Release 6 with the ROCP delivers binary data through the UMTS network in a introduction of MBMS [6]. The latter introduces a new point- real multicast mode to registered OBUs, where a higher layer to-multipoint transmission bearer by using shared network client application decodes the information.
resources in the service layer, in the core network, andin the radio access network. MBMS data uses a high rate C. Model for System Capacity Requirements forward access channel (FACH) that in turn is carried by The use of a real multicast transmission reduces the require- the secondary common control physical channel (S-CCPCH).
ments in terms of downlink capacity: as the same content is distributed to different users the required capacity scales with the number of services and is independent on the number of period of each service, etc. The fourth group of parameters receivers. In order to estimate the downlink capacity require- deals with the upper bound on the allowable update period ments of an I2V system based on any type of broadcast tech- for each segment and hence includes parameters such as the nology, a system-level capacity model is required, taking into maximum allowable speed in the system and the minimum consideration both the infrastructure (length of road, number of retention time for each segment. Finally, the fifth group of segments, number of lanes, speed limits, offered services etc.) parameters describes the data rates present in the system, such and present traffic conditions (number of vehicles, average as the data rates for each service in each segment, as well as speed, weather conditions, traffic jams, etc.). Our model [11] the total data rate for the entire system, which is ultimately is entropy-based and takes into consideration all of the above the output of the whole system capacity model.
One of the basic propositions of our model is that there exists a varying amount of overlap in the information being We considered as worst case a traffic jam scenario. Input transmitted to various users in the broadcast system. This parameters for the system capacity model are listed in Table overlap can be exploited to compress the downlink data stream, III. The efficiency of an ITS-over-MBMS depends on the effectively reducing the required downlink capacity. We use relative fraction of dedicated content over the total volume the concept of information entropy, as introduced by Claude of ITS Traffic, captured by the variable η defined above. In Shannon [12], to describe the magnitude of this overlap.
fact, in some cases the ROCP is required to send dedicated Two independent entropy parameters exist in our system ca- content to single users (e.g. keep-lane support and high speed pacity model, being the user entropy, η, and segment entropy, warning). This feature complicates the analysis of the ITS φ. User entropy expresses the variety of information content capacity requirements.
for any given service across the various users in each segment, We propose two different methods for handling the trans- while segment entropy expresses the variety of information mission of dedicated content. The first strategy consists in content for that same service across the various road segments using MBMS for multicast messages and the remaining DCHs that the I2V system comprises. Both entropy parameters are for unicast transmissions of dedicated contents. In this case normalized, such that η ∈ [0, 1] and φ ∈ [0, 1]. When ηj = 0, the limiting resource is again the available power at the base this signifies that during the current update all the users in station. As MBMS consumes part of the available cell capacity, the given segment are receiving the same information for the it limits the number of available DCHs that can be established.
particular service j. Conversely, if ηj = 1, this means that Recall that MBMS is typically carried by the FACH channel, each individual user is receiving information content different in which no fast power control is implemented. Therefore to that of all the other users. An analogous interpretation holds the Base Station is required to pre-assign a certain amount of power to the MBMS services. Clearly the MBMS power Along with the two forms of entropy, the model considers overhead depends on the expected MBMS bearer bit rate (see a number of other parameters, divided into five groups. The [13] for more details). Once µMBMS is defined, equation (6) first set of parameters describes the construction of the road can be used for estimating the residual cell capacity on the network which is served by the I2V system and includes DCH. The results for the combined MBMS/DCH scenario are parameters such as the number of segments the road network is divided into, the lengths of individual segments, and the The alternative approach makes use of a so called “carousel number of lanes in each segment. The second set collects all transmission” inside the MBMS session. With this approach parameters that describe traffic flow, such as the average speed dedicated contents are broadcast in the whole area and an in each segment, the following distance between vehicles, the application layer addressing mechanism at the OBUs is in number of users (i.e. vehicles) in each segment, the probability charge to selectively identify the messages to be processed or that a given segment is in the state of a traffic jam, etc. The discarded. Two advantages can be achieved by using this ap- third group of parameters models the set of services offered proach: a single bi-sectorial macro-cell can meet the capacity by the system, and hence includes the number of services requirements and coexistence with voice users is eased since in the system, the activity factor, data volume, and update the remaining DCHs remain untouched.
The choice between the two approaches depends on the In this work we have provided a quantitative assessment of the system performance of ITS over UMTS, with and withoutMBMS. We have highlighted the dependency between the required system capacity and the characteristics of the ITScontent (dedicated vs. shared) and proposed an entropy-based metric to capture such dependency. Finally we have analyzed ahybrid bi-modal scheme that can handle the dynamic condition The new capabilities introduced in UMTS Release 6 enable the efficient support of ITS services over legacy UMTS networks. The lower cost barrier due to the reuse of existing infrastructure is likely a critical advantage over alternative MBMS capacity requirements in case of Road Congestion (semi-log schemes requiring the deployment of new technologies, paving the ground for the concrete introduction of ITS applicationsin the near future.
value of η, which in turn determines the total required data rate. Figure 3 depicts the data rate requirements for different This work was supported by the EU Project COOPERS values of η in our reference scenario. At one extreme point and by the strategic projects N0 and I0 of ftw. The authors η = 1, i.e. when only dedicated content is transferred to all would like to acknowledge the kind cooperation of Alexander users, up to ca. 1 Mbps is required. As expected, a typical ITS Fr¨otscher and Thomas Scheider of AustriaTech.
scenario, where 0 ≥ η ≤ 0.3, requires only 300 Kbps.
We evaluate a hybrid bi-modal scheme that switches auto- matically between DCH and carousel transmission modes for [1] VII, “Vehicular Infrastructure Integration”,
dedicated content. The switching logic is based on thresholds [2] Safespot, “Cooperative Vehicles and Road Infrastructure for Road on the number of registered OBUs present in the cell and on Safety”,
[3] C2C, “Car-to-Car Consortium”,
[6] 3GPP TS 23.246, “Multimedia Broadcast/Multicast Services (MBMS): In case 1 only few cars are on the road (e.g. during the Architecture and functional description”.
[7] K. Sipilae, Y. Honkasalo, J. Steffens, A. Wacker “Estimation of Capacity night). The allocation of the MBMS multicast radio bearer and Required Transmission Power of WCDMA Downlink Based on is unjustified since the power that would be required to serve a Downlink Pole Equation”, IEEE Vehicular Technology Conference(VTC 2000), Tokyo, Japan, May 2000.
the users directly on the DCHs is less than the power required [8] 3GPP TS 25.101, “User Equipment (UE) Radio Transmission and to setup an MBMS FACH transmission. Therefore, the users should be served via normal DCHs. When the number of cars [9] Harri Holma and Antti Toskala “WCDMA for UMTS: Radio Access for Third Generation Mobile Communications, third edition”, John Wiley with registered OBUs exceeds a certain threshold (case 2) the network switches automatically to the MBMS multicast [10] 3GPP TS 22.246, “Multimedia Broadcast/Multicast Services (MBMS): transmission. In this case all the OBUs are served through the [11] P. Belanovi´c and T. Zemen “An Entropy Based Model for System- MBMS channel and dedicated contents are sent via “carousel Level Downlink Capacity requirement in V2R Telematic Systems”, transmissions”. If ROCP needs to send several dedicated mes- IEEE Workshop on Automotive Networking and Applications (AutoNet sages, i.e. high η (case 3), the remaining dedicated channels 2007), Washington, DC, USA, November 2007.
[12] C.E. Shannon “A Mathematical Theory of Communication”, Bell System should be used in support of the MBMS multicast transmis- Technical Journal, Vol. 27, pages 379-423 and 623-656, July and October sion. The feasibility of this approach is guaranteed by the “MBMS counting” functionality introduced in [6], which lets [13] 3GPP TR 25.803, “S-CCPCH performance for MBMS” the radio access network identify the number of UEs withactivated MBMS services within a cell.


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