Joint Resource Management of Cellular and Broadcasting Systems - Research Challenges Aurelian Bria Royal Institute of Technology - Stockholm
[email protected]
ABSTRACT One of the ways towards provision of low-cost mobile multimedia entertainment is to make possible the sharing of the wireless resources among multiple operators of different radio access technologies. Here we discuss opportunities for new system architectures enabled by sharing the resources belonging to the cellular and broadcasting systems, namely the spectrum and the infrastructure. The goal of this paper is to identify the bottlenecks of such architectures and the challenges that have to be addressed on their path towards a functional system able to deliver low-cost mobile multimedia services.
1. Introduction After the slow start of 3G networks and recent enforcements of property rights for multimedia content, the wireless industry has to seriously account that mass market demand for mobile multimedia entertainment will be conditioned by low cost provision of these services. That is, the cost of content delivery should be much less than the cost of the content itself. This is also supported by previous work on the vision about 4G system architectures and services performed by the author in two studies [1] and [2]. From these studies a number of working assumptions are also drawn. Among them, the future multimedia will mostly appear in the form of interactive applications that will generate asymmetric traffic, in the favor of much more retrieved bits by the users compared to bits sent. Also, much of the content is envisioned to be popular among the users and tolerance for delay is acceptable for the information that is not real-time critical. The limitations of the today’s most powerful actors in the wireless service provision, the traditional cellular and broadcasting operators, are evident. The present architecture of cellular systems is known to be limited in the broadband service offerings, as the cost per subscriber increases almost linearly with the bandwidth provided as far as no extra spectrum can be added to the system. This is more or less valid for any cellular network that offers wide area coverage and real-time services. The key reason for this cost increase is the investments necessary in new infrastructure (e.g. sites, transmission) as the demand for bandwidth and number of users increase. In addition the cellular architecture does not allow efficient multicasting/broadcasting of popular content. 3GPP is currently developing a cell broadcasting technology called MBMS (Mobile Multicast/Broadcast Service), but its performance in terms of bandwidth is poor compared to what broadcasting systems can offer.
Broadcasting systems benefit of significant spectrum allocation, especially in the UHF band, and purpose made infrastructure characterized by very tall towers and large transmission power. First problem is the difficulties encountered for offering economic feasible services (e.g. DVB-H at 10 Mb/s) for portable devices. Preliminary tests show that this task is very challenging if only existing infrastructure is considered. As a result, it seems that additional sites will be necessary to be patched to the existing broadcasting networks. In an industry specific terminology they are called gap-fillers, and their cost is not at all negligible. The second problem is the interactivity and transmission error control which cannot exist without a return channel, from the user to the system. For solving this problem the DVB-RCT standard was introduced, but it does not seem to reach wide acceptance among operators. The reason is not only the regulatory framework regarding public broadcasting, but also the necessary investment in infrastructure and equipment in order to support it. In conclusion, the provision of low cost multimedia for mobile users, both from cellular and broadcasting operator’s perspective, is problematic due to necessary investments in new infrastructure. Additionally, some important features have to be added to the present systems. Efficient multicasting and more spectrum is needed in the downlink of cellular systems. Support for portable terminals, interactivity and error control in broadcasting are required as well. This paper starts from the observation that the two systems are complementing each other: the features that one needs the other has already. We speculate that a marriage of resources and functionalities belonging to cellular and broadcasting systems is a better alternative for launching cheap mobile multimedia services with characteristics that do not map directly to the initial design of a traditional cellular or broadcasting network. Highly traffic asymmetric services, as file downloading, software distribution or TV in the mobile, are just some examples. A first insight of the author into how cellular and broadcasting systems may complement each other in order to offer broadband asymmetric multimedia can be found in [3]. In the following sections we describe and discuss some fundamental opportunities enabled by sharing cellular and broadcasting resources under assumption of both single and multi-operator business cases. Spectrum and existing infrastructure are targeted resources for sharing. The goal
of the study is to probe the feasibility of this approach, both from technical and business perspective. Several bottleneck issues are identified and proposed for further investigations.
2. Efforts on Concept Demonstration First proposals on the integration of cellular and broadcasting systems appeared in MEMO-ACTS project [4]. In this first proposal the targeted service was asymmetric Internet access (similar to a wireless version of ADSL). The broadcasting system (DAB/DVB-T) was supposed to provide high data rate downlink (2-10 Mb/s) while the uplink was implemented on circuit switched GSM at 9.6 kb/s. A following EU funded IST project MCP - Multimedia Car Platform demonstrated the feasibility of incar provisioning of multimedia services by combining GSM/UMTS and DAB/DVB-T. A demonstrator was presented at IFA2001 Congress in Berlin [5]. Another project dealing with similar issues was COMCAR [6]. This project focused on integration of broadcasting technologies as an additional downlink of UMTS cellular systems. A concept demonstrator was built. The IST-DRIVE [7] project started by considering the cellular system as a return and control channel, so implementing interactive broadcasting was now possible. The project also proposed a strategy to share the resources of both systems in a dynamic manner. However, the approach was limited to sharing the spectrum resources. The technique, called DSA (Dynamic Spectrum Allocation), was later perfected in the OverDRIVE project [8]. The main interest was the delivery of high quality vehicular multimedia services in a multi-operator environment. The projects addressed the interworking of cellular and broadcasting systems in a common frequency range, employing DSA. This approach is motivated by the appetite for generous amount of spectrum allocated for TV broadcasting in UHF band. Switching from analogue to digital broadcasting may free up some frequency channels that in turn can be assigned to cellular operation. The IST-MONASIDRE project [9] developed a platform for multi-radio resource management in heterogeneous systems (cellular, broadcasting and hot-spot). The goal was to design a joint access selection and resource allocation strategy that is able to assign in realtime the users to the best suited radio access for the service they require. The investigations were focused on the real-time and streaming services and how to allocate them to one of the managed radio accesses. These projects managed to show prototypes that outlined the benefit of higher spectrum efficiency achieved by combining cellular and broadcasting resources. The targeted services were mostly related to multimedia streaming or real-time voice or video calls. Regarding the resource sharing, the main approach was to reusing existing infrastructure and spectrum resources allocated to the present broadcasting systems by moving them to the cellular systems. There is no much attention payed on how the access capacity of one system can be employed for providing services specific to the other system.
3. Fundamental Concepts on Resource Sharing The resources we consider are the spectrum and the infrastructure belonging to cellular and broadcasting systems. The operators may consider the following ways of sharing these resources: • Spectrum sharing - this is the case treated by DRIVE and OverDRIVE. It assumes that a common pool of available frequencies can be dynamically shared between cellular and broadcasting networks, according to their instantaneous demand. • Infrastructure sharing - In this format the cellular operator considers for its system expansion the existing broadcasting sites instead of deploying new ones and viceversa. This approach can be also combined with spectrum sharing. • Access capacity sharing - this is a form of sharing the spectrum and infrastructure resources together, in a more or less transparent manner. For example, the cellular operator can use the existing broadcasting air interface for delivering its services, and viceversa. In a multi-operator environment sharing of any resource can be performed in a cooperative manner (the sharing partners cooperate for reaching agreements that fulfill common interests) or in a competitive manner (each actor compete with the others in the market for getting access to the required quantity of the resource). In the following sections, each of these resource sharing concepts is analyzed from both a technical and business perspective. 3.1. Spectrum Sharing Dynamic spectrum assignment (DSA) was introduced in the DRIVE project as a mean for increasing spectrum efficiency. The focus was on the co-operation of cellular and broadcast networks in a common frequency range by means of dynamic spectrum allocation. DSA has the key role of solving congestion situations by reallocating the available frequency carriers. DSA mechanism can be implemented in a centralized manner, when one entity (e.g. spectrum broker, operator) distribute the spectrum channels according to a certain policy, but can be also a free marked where operators buy and sell spectrum according to their demands. The investments that operators need to do concerns mainly new transceivers at each sites. From a regulatory perspective, current regulations in Europe specify a certain technology for each spectrum bands, and solutions as DSA are not presently possible. A summary of results of the DSA related investigations under the DRIVE project can be found in [10]. The spectrum efficiency improvement compared to fixed spectrum assignment (FSA) for a combination of UMTS and DVBT systems is shown to be around 30%, although it depends of the spatial and temporal correlation between the cellular and broadcasting traffic demands. 3.2. Infrastructure Sharing First, let us take a look at the coverage and service availability in traditional digital broadcasting systems.
Broadcasting sites were traditionally dimensioned for providing reception in a certain area, to fixed receivers employing rooftops antennas. Switching to digital technology the availability in time and location had to be increased from 50% to 90%, due to the sudden break-down of quality under a certain threshold in signal strength. To guarantee mobile reception the availability has to climb up to 99%. Due to Rayleigh fading, an extra margin, usually over 10 dB, has to be considered as well [11]. Under these conditions, if the broadcasting infrastructure is not dimensioned from the beginning for mobile or portable reception, a careful study has to be performed in order to assess the conditions existing for mobile and portable (even indoor) receivers. Recently, the efforts of adapting the digital broadcasting DVB-T standard to mobile and portable reception were concluded in a new standard called DVB-H [12]. DVB-H trials are performed in Berlin (Germany), Helsinki (Finland), Metz (France), and Pittsburg (USA). One interesting aspect is that mobile reception quality and field strength are strongly correlated. Forward error protection methods used in DVB are so powerful that they are able to reduce transmission errors to almost zero ( < 10−11 ) as long as a certain carrier-to-noise ratio in the physical transmission channel is exceeded. The effect of motion is a loss of the C/N ratio, which can be compensated, for example by increase in transmitted power. As such, the faster the receiver moves the more power is required [13]. In DVB-H the compensation for speed is also obtained by employing a technique called MPE-FEC (Multi Protocol Encapsulation - Forward Error Correction). Some field measurements for DVB-T show a need for more than 25 dB increase, compared to rooftop reception, in transmission power if portable indoor coverage is to be provided at 90% of locations [14]. In conclusion, this preliminary discussion show that by only reusing existing broadcasting sites is very difficult to achieve portable coverage in the same area as rooftop reception was assumed. An interesting insight into the dimensioning of a national DVB-H network in Finland can be found in [15]. One of the outcomes is that the investment and running costs of an IP based DVB-H network would be equivalent to about half of the costs of a cellular network (GSM) of the same coverage area. That is because more than half of the total number of transmitters are low power gap-fillers, and the costs associated to their deployment are comparable to GSM sites. Still, the study suggests that additional sites and deployment of gap-fillers is a must instead of an option, and reusing existing cellular sites for this purpose may avoid investments in new infrastructure. The feasibility of this approach has to be tested under the constraints of maximum transmission power acceptable at a GSM site (due to health concerns in some areas) and the number of sites needed. Evaluation of achieved coverage as a function of transmission power and number of additional sites will give a good indication on the deployment costs of such hybrid infrastructure networks. While for a small site the power do not represent a significant part of the costs, it is essential to note that cost of electrical energy at
traditional broadcasting sites may surpass the annualized cost of infrastructure for very high EIRP levels (e.g. close to 60 dBW). The second approach to share infrastructure relates to the cellular operator perspective to deploy its technology on the existing broadcasting infrastructure. Hierarchical cell structures and umbrella cells is not something new in the cellular world. This architecture is employed already in order to achieve good coverage in areas where traffic demand is low or to avoid excessive number of handovers for fast moving terminals. What is new is the fact that cellular operators can deploy on this high towers large umbrella cells with broadcasting bearers (e.g. MBMS) and let their network evolve towards heterogeneous type of access. Due to specific features as adaptive modulation and coding it will be possible to choose the assignment of users and services (to broadcasting or point-to-point bearers) so the capacity of the system is maximized. Such techniques falls into the category of multi-radio resource management (MRRM), a relatively new concept of coordinating, in a unified manner, the radio resources over a set of radio access technologies. The objective of MRRM can be maximization of profit, minimization of costs, maximization of satisfied number of users, minimizing the power consumption, maintaining delay in acceptable limits or others like these. The time scale of MRRM actions can be long term (days, months), short term (duration of the service) or at certain time instant (seconds). The main objective can also be a combination of several sub-objectives. Taking the assignment decision is not obvious and a set of input parameters should be taken into account by MRRM. They can be of different types: • Service characteristics: set of target receivers for each service, priority, packet size, expected session duration, tolerance for delay and jitter, asymmetry factor, etc. • User preferences: willingness to pay (user utility), terminal characteristics (power consumption, maximum rate), tolerance for delay, etc. • System state: congestion level (or remaining free capacity in each radio access), SINR on radio links, positions and mobility of users, etc. • Radio access characteristics: available set of data rates on the air interface, available RF power levels, delay figures in the transport network, cost of the airtime, etc. The result of the user-service-bearer assignment can be the allocation of a certain service to either p2p or broadcasting bearer, to both at the same time or to none of them. As well, all the users requesting a certain service can get it through one bearer or they can be split in subgroups, one for each bearer. One such MRRM technique having the cost minimization as objective was proposed in [16] and evaluated for a push-type of multimedia delivery service. The main idea was to consider a cost per unit time of bearer usage and find the optimum subgroups of users
(each subgroup assigned to one of the bearers) that minimizes the delivery cost of each item. The results of this work show that significant cost savings can be achieved by creating subgroups in some cases, but there are also situations when the benefits are negligible. This information can be used for designing algorithms that decide the optimum assignment by only using non-delay critical information (e.g. number of recipients for each item and costs of using each radio access). Algorithm design and performance evaluation of such techniques is subject of future work. First step would be to identify which input parameters impacts the results significantly and require most attention. This way make possible identification of situations when simplifications can be made without affecting the performance. The discussion on this topic will continue in the next subsection, where the reliability of the input information and flexibility of the air interfaces is actually depending on the business relationships among the operators involved in the resource sharing. 3.3. Access Sharing The basics of economic theories teach us that one way to decrease the production cost is to share the production facilities among several product lines. As we outlined in the end of the section 1, the broadcasting and cellular systems have resources and functionalities that complement each other when it comes to provision of multimedia services. However, the access to these resources and functionalities is strongly depending on the business agreement between the cooperating parties. The amount of shared control over existing resources offered by each radio access (RA) is an important dimension to consider. Three possible situations are discussed: • Interworking • Shared Control • Integration Interworking of the two radio accesses (RA) assumes that each RA is owned and managed by a different entity and it shares nothing more than a transparent access of users to services and viceversa. In this business scenario the major role is played by the service providers. They have to come with their own middleware platform that is able to interwork cellular and broadcasting accesses in order to provide the intended service. One example of such situation is the SABINA prototype in ACTS-MEMO project. Interworking will open ways for free competition among service providers for the shared capacity in broadcasting of cellular networks. Shared control is similar to interworking, but assumes virtual operation of the remote system. This means that a virtual operator can get access to limited control of the shared resources in broadcasting and cellular radio access. Competitive access for resources can take shape as well. The virtual operator may influence some parts of the resource management as rate selection, monitor the radio links quality and QoS and manage its own user database. Integration assumes that one operator fully control the shared resources in both RAs. This case is very close
to what we described when the cellular operator deploys its own technology for cell broadcasting on the existing broadcasting masts. The only difference is that now the broadcasting technology is already deployed and is a broadcasting specific technology, as DVB-H for example. Joint resource managements in the described situations will also perform the assignment of users and services, but this time to the best suited system or radio access. MONASIDRE project aimed towards a technique that optimizes an objective function which considers user utility, costs in each radio access and congestion situation in each system. They mathematically formulated the problem of optimal traffic distribution as an maximization problem with constraints. To find the solution of this problem a fairly complex real-time algorithm was implemented in a Network and Environment Simulator. This was used to validate a management decision prior to its application in the real network. The optimization problem is shown as requiring exhaustive search in the number of assignment alternatives, so the solution was to implement a greedy algorithm which provides a suboptimal solution. The investigations in MONASIDRE focused on real-time and streaming services in UMTS-WLAN environment, and little conclusions appear regarding UMTS and DVB-T combinations. Some other researchers have also looked at resource management for interworking and/or shared control case of UMTS and DVB-T [17]. The proposed framework consists of delivery of multimedia-on-demand by employing service scheduling, network selection and QoS adaptation. Service scheduling aggregates or batches user requests for the same content into a batching queue for a period known as batching duration. The idea behind is to aggregate requests for the same content and serve them through the broadcasting channel. An optimal batching duration is proposed in [18]. Network Selection assign each service to one radio access based on comparing a profit quote calculated for each of them. This profit quote is in fact an objective function of service QoS, number of cells, geographical area, number of interested users and load factor [19]. The technique is very suited for provisioning of multimedia streaming to fixed customers. An important aspect in a free market of access is the imminent competition for the shared resources. We may see in the future an open wireless access market where the resources are distributed in almost real-time according to the demand and supply paradigm. As a result, pricing the access becomes of great importance. Prices may vary according to the congestion level experienced by the each system or radio access (congestion pricing is a known method for maximizing producer revenues). How to adapt the resource management to the variation of prices for access is not obvious, but a first shot can be to extend the technique proposed in [16] to multi-operator environments. The considered costs become prices that service providers or virtual operators have to pay for using the radio resources offered by the network operators of cellular and broadcasting systems. In any competitive envi-
ronment time is money. The proposed scheme based on time of usage the radio channel can incorporate both extra revenues obtained when the demand for access is high, but also losses during the time when the radio access is not used. An illustrative example for high losses is the cost of keeping a high power broadcasting transmitter on even if there is no use for its capacity. In real life these transmitters cannot be switched on and off very often. A comparison of benefits obtained by resource sharing in these cases will lead to conclusions regarding the benefits of promoting more complex and integrated systems compared to the case of interworking.
4. Error Control in Hybrid Cellular-Broadcasting Systems
5. Multimedia Multicasting Gain : Achievable Capacity Regions To exemplify the benefits of resource sharing in single and multi-operator environment we perform a simple exercise of drawing hypothetical capacity regions. Their purpose is only illustrative. To outline the benefits arising from physical multicasting of popular multimedia content we consider as performance measure the achievable system throughput in the case of a two push services, one delivering personal and the other one popular multimedia items. A personal item is dedicated to only one recipient. By popular item we mean that it has to reach all receivers in the service area. Obviously, this exercise can be performed for other service combinations. Depending on how these services are mixed in the two RAs the capacity regions might look different.
Some multimedia services may require error-free delivery of data files. As the broadcasting radio access was not featured with such functionality, a challenging task would be to implement efficient error control schemes well suited for traffic asymmetric services and able to handle different radio accesses in uplink and downlink. Additional to forward error correction (FEC) or carrousels that are usually employed on broadcasting channels, now the presence of a return path from the user terminal allows introduction of retransmissions requests (ARQ). The main question is how much can be gained by this enhancement in terms of system throughput, especially in the case of multicast services. One good reference for the design of ARQ protocols for point-to-multipoint transmissions is [20]. The performance of ARQ family protocols is depending on the round-trip-time (RTT) of the acknowledgement packets in the system. As modularization and transparency of the systems lead to added latency, we may expect to experience a large RTT in the case of interworking, while an integrated cellular-broadcasting system may provide much lower RTT. However, in the particular case of large area broadcasting cells we suspect this advantage might not be existing for the users close to the cell border. To start with, let us take a look at a simple correlation model for shadow fading proposed by Gudmunson in [21]. From this model as well from the measurements supporting it is evident that correlation distance of shadow fading is increasing when distance from the transmitter increases. A good model taking this effect into account has not been found in the literature, but if this is true in reality then once in outage, depending on the speed and direction of movement, it might take a significant time for a mobile user to get back in coverage. This particularity of the propagation model may be successfully ignored in the case of cellular systems, but may become significant in the case of broadcasting cells with a coverage radius of more than 50 km. The issue is how the correlation distance of shadow fading can be used for avoiding unsuccessful re-transmissions for mobile users close to the border of service area. The results can be also useful when designing broadcast carrousels. Investigations of ARQ performance under these consideration deserve at least some attention.
Figure 1. Interworking vs. Evolved Cellular. Capacity regions for two services: popular and personal content delivery
The figure 1 shows an example of achievable capacity regions, for a certain setting of system parameters (number of cells, number of users per cell, etc.). On the vertical axis we have the achievable throughput of popular traffic and on the horizontal one the achievable throughput of personal traffic, under some QoS constraints (e.g. certain maximum accepted delivery delay is not exceeded). Regions noted with A represents the contribution of cellular resources to popular traffic throughput (the two upper areas) and contribution of broadcasting resources to the throughput of personal traffic (right area). The gain steams from the possibility to use free capacity in one radio access for delivering traffic that normally should be assigned to the other radio access. This happens because of other reasons, for example high congestion. The upper curve represents the capacity region achieved by the MRRM technique proposed in [16] for an evolved cellular operator that deploys its own multi-rate broadcasting bearer on the broadcasting site. In the interworking case the broadcasting radio access provides a single data rate in the coverage area. The capacity regions from previous example for the case when DSA is employed are shown in figure 2. The thicker lines (one continuous and one dotted) represent capacity regions for cellular and broadcasting for
Scenarios and Research Challenges, Special Edition IEEE Personal Communications, Dec.2001 [2] B. Karlson, A. Bria, J. Lind, P. Lonnquist, C.Norlin: Wireless Foresight, Wiley, 2003 [3] A. Bria: Digital Broadcasting and Mobile cellular Networks to provide Asymmetric Data Services - A Survey, Proceedings of RVK02 Conference, Stockholm, 2002 [4] ACTS MEMO Project http://MEMO.lboro.ac.uk, 1999 [5] IST - Multimedia Car Platform, http://mcp.fantastic.ch [6] COMCAR Project, http://www.comcar.de
Figure 2. DSA vs. Interworking. Capacity regions for two services: popular and personal content delivery
[7] IST-DRIVE Dynamic Radio for IP Services in Vehicular Environments, http://www.ist-drive.com [8] IST-OverDRIVE, http://www.ist-overdrive.org [9] IST-MONASIDRE, http://www.monasidre.com
the situation when the spectrum resources are completely moved from one to the other. Compared to the interworking, now the regions B are added to the achievable capacity region. However, the interesting regions are the ones called C (shadowed) that can be obtained both by DSA and interworking. From this examples at least two conclusions can be outlined. One is that the flexibility of data rate on the broadcasting bearer air-interface leads to significant gain in throughput of popular items. Second, in the case of multimedia services characterized by popular content, DSA can be avoided and replaced by clever access sharing techniques.
6. Conclusions The paper investigates the fundamental opportunities and concepts arising from sharing the resources of existing broadcasting and cellular systems, in an effort to provide low-cost multimedia services. Sharing of infrastructure and spectrum under different settings is discussed and few challenging issues are raised. The outcomes of the study show that significant gains are possible, even for low cooperation between the operators (interworking or infrastructure sharing). Flexibility of broadcasting air-interface may allow operators to address subgroups of users with different type of transmissions. The technique has the potential to approach similar performances with DSA. Future work is needed to propose and evaluate MRRM techniques that consider joint optimization of radio access selection and resource allocation, under different amount of shared control between cellular and broadcasting systems. The outcomes may then lead to new requirements on digital broadcasting technologies. This work was performed under the Swedish strategic research program Personal Computing and Communication (PCC).
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