Relaying access points and related business models for low cost mobile systems Klas Johansson, Jan Markendahl and Per Zetterberg Department of Signals, Sensors & Systems Royal Institute of Technology, Sweden E-mail:
[email protected],
[email protected],
[email protected] Abstract- In this paper we consider provisioning of high bit rate mobile services without the need of complex user terminals or deployment of a full coverage network with a very large number of traditional cellular base stations (access points) Existing broadband, Wireless Local Area Network (WLAN) and cellular solutions are combined with novel technologies such as ad-hoc network and Multiple-Input-Multiple-Output (MIMO) antenna systems in order to obtain high capacity links, highly flexible solutions and support for mobilityt. In the proposed solution we introduce an intermediate access point, or relay, that connects the end-user terminals to either a wireless or fixed network. Thus, a direct high bit rate connection between base stations and user terminals is not required, which would be beneficial both technically and cost wise. The proposed solution enables flexible rollout strategies and small-scale deployment for both traditional mobile operators and local access providers. The new type of equipment can also be owned and deployed by local actors like facility owners, hotels and private persons that imply possibilities for more flexible business models.
1 Introduction 1.1 Background, problem and paper objectives Providing cost effective, affordable wireless bandwidth (almost) everywhere is one of the key success factors for future wireless systems. However, it is highly unlikely that the consumers’ willingness to pay increase linearly with the bit rate required for new services. In the Swedish Foundation for Strategic Research (SSF) program “Affordable Wireless Services & Infrastructure” (AWSI), the Wireless@KTH research center in Stockholm has started the project “Low Cost Infrastructure” (LCI). The project objectives are to propose and analyze alternative wideband wireless infrastructures for wide-area coverage and mobile users. The network should have a significantly lower cost than what can be achieved by extrapolating today’s cellular technology. The target for the LCI project is to achieve a cost reduction in the order of 100 times per transmitted bit as compared to existing systems. This means e.g., 100 times higher capacity at the same cost as today, or 10 times more capacity at 10 % of the current cost. We believe that the hierarchical structure of cellular networks imply a limited potential for economies of scale, due to high costs for deployment of new sites, operations and maintenance. However, this problem area is not completely new. The last couple of years, WLANs have been presented as an alternative to cellular systems. Some debate regarding the choice between 3G and WLAN has also been going on. As a consequence,
WLAN systems with wide-area coverage are currently under development [1]. But is this really the best way forward, given that cellular systems such as 3G already are being deployed in many countries, and thus provide coverage and mobility, already at this point in time? No, the important question is how to combine the best from the two worlds, not the pointless “WLAN vs. 3G “ debate. Presented strategies to provide seamless mobility and “Always Best Connected” (ABC) include combinations of multiple radio access technologies [3],[4],[2]. Both types of systems, cellular and WLAN, are here, and have their respective strengths and weaknesses. Put simply, WLAN provides high bit rates with limited coverage, and thus little support for mobility, whereas 3G supports high coverage and mobility, but with more limited bit rates. Can those distinguishing properties of the two systems be united? The problems addressed in this paper can be summarized as (1) how can wide-area coverage be provided for high bit rates without a dense cellular network, and (2) how do we reduce the costs related to network deployment, maintenance and operation? This leads us to a concept based on high capacity links between the terminals and a relay, or intermediate access point, connected to either a cellular system or to the fixed network. For this purpose we propose a New Network Element with the working name: NNE.
1.2 Scenarios for future mobile communications In this paper we address three scenarios for wireless communications. . We are targeting the time frame beyond the year 2010 with the working assumptions that 1.Cellular systems are widely deployed using 3G and beyond 3G technology, and 2. Fixed broadband access networks are deployed in major urban areas. The first scenario is an urban environment with users moving at low velocity , such as found in shopping centers, offices and homes. The user density is assumed to be high, and the network therefore has to be dimensioned to provide capacity and not only coverage. The second class of scenarios is suburban and rural areas. Here the main requirement is to provide wide area coverage for high bit rates. The capacity requirements for individual users may be high, but the overall demand is smaller than in the first scenario. The third scenario is high mobility, i.e. users in cars, buses or trains. For this case the challenge is to provide high bit rate coverage for users traveling at e.g. 30-300 km/h. Hence, the network design should support mobility and local coverage within the vehicles
1.3 Paper outline The paper is organized as follows. In Section 2 the cost structure of cellular systems is summarized. The proposed network architecture with the NNE and related interfaces are described in Section 3. In Section 4 we discuss the potential benefits of the proposed solution with respect to different user scenarios, capacity and coverage requirements, terminal functionality, cost structures, operations and maintenance. We also comment on the relation to current standards and the evolution of legacy systems. Business models and possible new value chains are discussed in Section 5.
2 Cost structure of cellular systems To obtain a widespread demand for wireless services they have to be widely available, simple to purchase and access, and they must be affordable to large numbers of consumers. We expect that providing higher bandwidths that enable the use of truly new and innovative multimedia services is not sufficient. In addition, the users communication cost per month must be similar or even lower than in second and third generation cellular systems. The cost of the wireless infrastructure (Csystem) and the cost of an access point (cAP) can (for a given allocated spectrum) be broken down into the following factors [5][6]
Csystem ≈ c AP N AP ≈ c′N user Buser Aservice f (Q) Where NAP is the number of access points (base stations), Aservice is the covered service area, f(Q) is a function of the required Quality of Service, Nuser and Buser are the number of users and the average data rate of the users. We here assume that cost of the core network part is proportional to the number of access points (NAP.). The consequence is that M times higher capacity implies M times larger equipment cost, e.g. by deploying M times more base stations. Thus, the conventional cellular concept does not scale in bandwidth in an economical sense. Analyzing the overall network cost structure of wireless infrastructure reveals that the cost component representing the actual telecom equipment is a rather small fraction of the total cost of the current systems, see Figure 1.
Operator costs related to the network and the infrastructure 40 35 30 25 20 15 10 5 0
OPEX CAPEX for Equipment CAPEX for sites etc Interconnection costs
Figure 1. Network Cost structure for US cellular operators in the mid-90´s. [6]
Costs of the telecom equipment, the depreciation cost for the investment of base station and switching equipment, are only a small part (15%) of the total network cost. Costs related to marketing, billing and administration constitute around 50% of the total cost. The dominating cost factors of the network are related to the maintenance of the infrastructure (OPEX) and to the physical deployment of the infrastructure, such as planning, installation of antennas & towers, cabling and other site costs.
3 A layered architecture for low cost infrastructure In this section we briefly describe the proposed technical solution. Examples are given of how radio access technologies can be utilized in the different interfaces and how the NNE can be implemented to fulfill various requirements on connectivity, mobility, etc.
3.1 Solution description 3.1.1
Design principles
In summary, the main ideas behind the concept proposed in this paper are to • • • • •
Provide a scalable high capacity low cost solution. Enable incremental rollout of new infrastructure. Support more flexible business models and value chains using small-scale solutions. Utilize existing wireless technology as efficiently as possible; e.g. WLAN technology for user terminals, and cellular or fixed network connections for ‘last mile’ access. Keep terminals as cheap and simple as possible, and with a long battery lifetime.
3.1.2
Solution idea
We propose an intermediate relaying access point, called the New Network Element (NNE). This element resides between the terminal and the infrastructure, which may be a wireless or a fixed network; see Figure 2. The NNE provides local coverage for user equipment in its close vicinity. The local coverage would typically be based on evolved WLAN technology.
NNE MS
Internet
Fixed Network
Relay Air Interface
NNE controller
NNE
MS
Cellular Radio Access Network
MS
Cellular Core Network
MS Local Air Interface
NNE
Network Interface
Base station
Network control
Figure 2. The proposed low cost infrastructure with NNE and mobile stations (MS) and the proposed Network Interface (NI), the Local Air Interface (LAI) and the Relay Air Interface (RAI). A
The NNE can be connected to the fixed-part of the network via either wired or wireless interfaces. In order to provide high bit rates and mobility in the interface
between the NNEs and the fixed part of the network in the wireless case, base-stations and NNEs will employ smart antenna technology [12]. The NNE concept can be improved further by allowing the NNEs to connect directly to each other based on ad-hoc networking principles. This would allow an NNE without fixed connection that is out of the coverage of the cellular network to connect to another NNE that has such a connection, and thereby increase the coverage area and capacity. The NNE supports multiple standards both in the communication with the mobile stations and the fixed network. By employing multi-radio technology in the NNE, the mobile stations can be kept as simple as possible. However, to ensure compatibility with existing standards the mobile station should be able to connect to both to the base station directly and through the NNE. From the overall architecture in Figure 2, it is important to note that a specific control device, the NNE controller, controls the NNEs. This can be implemented in a base station, or in a separate server connected directly to the core network of the cellular network. The NNE controller should handle handovers of mobile stations between the NNE and the cellular network in addition to access control, traffic measurements, billing, and other resource management functionalities that cannot be managed locally in the NNE. Notice that the user plane data does not have to be routed the same way as the control plane traffic to the NNE controller. 3.1.3
Possible implementations of the New Network Element
The following implementations of the NNE have been identified for the scenarios described in Section 1.2: 1. A stationary relay point with fixed access to the Internet (for hot spots, offices, and homes). 2. A stationary relay point with wireless access to a cellular network (for wide area networks with coverage for high bit rates). 3. A mobile relay point with wireless access to a cellular network (for wide area networks with requirements on high mobility). Note that even though the NNE is employed as a stationary unit under 1) and 2) above it still possible for a layman to move it between different positions without any reconfiguring.
3.2 Related work Similar concepts have been proposed in the literature for the problem of organizing mobile networks with very small cells; see e.g. [7],[8],[9],[13], and [14]. The proposed solution in [13] and [14] is to have a cluster of “virtual base stations” controlled by and connected to an ordinary base station via a wireless connection. This solution should be scalable since the core network does not need to be aware of the “virtual base stations”. In [9] and [13] the formation of local ad-hoc networks were proposed, furthermore, a mobile router was introduced in [14]. This network element should connect a local, or personal, area wireless network to the wide-area cellular network. A solution with integration of 802.11 and CDMA 2000 was recently presented in [15], where fixed or mobile WLAN hotspots are connected with a 3G backhaul.
What makes the NNE concept different? • The use of fixed network as backbone when it is available, otherwise the cellular network is used • It enables simpler terminals by moving complexity to the NNE. • The same type of solution and concept can be used for both local area and wide area system and for both fixed and mobile wireless access. • The NNE can be deployed by users or network providers
3.3 Examples of radio access technologies for different interfaces Although the architecture with the NNE should be viewed as independent of radio access technologies, a few examples are given next to indicate how existing air interfaces could be utilized in the different interfaces described above (i.e., the NI, LAI, and RAI). For the NI, the basic idea is to support the same technology as in wide-area cellular networks in order to support mobile users and have a good coverage. Advanced antenna solutions, like Multiple-Input-Multiple-Output (MIMO) techniques may be used in this interface to improve the capacity and increase link bit rates. MIMO solutions look very promising since the NNEs should be large enough to carry a smart antenna (e.g. with a size of 20*20 cm), which has been one of the main doubts for implementing such in user terminals. Furthermore, simply having the option to use less attenuated wireless links can substantially enhance the wireless networks performance (as with external antennas on cars in, e.g., GSM). The NI can, e.g., be compatible with an evolved version of 3G. The LAI may utilize the same radio access as the NI, but it is also possible to use a different technology. E.g., DECT, Bluetooth, or an evolved version of WLAN supporting very high bit rates at short distances. The latter can also be used for the RAI, if it supports ad-hoc networking capabilities. The mobile stations are assumed to be highly personalized and may support various radio access technologies. Some terminals may support either a cellular system or WLAN, whereas other may have multi-mode capabilities and support both. Compare wireless local loops systems where mobility is supported in local area only. What radio access technologies that are supported by a specific mobile station of course depend on the applications supported and the size of the MS. Advanced terminals may also supports the RAI so that it can act as NNE. This may be the case, e.g., with laptops. It is then advantageous if the LAI and RAI are based on the same technology. The NI will use the spectrum allocated for wide area cellular access. The LAI can use un-licensed spectrum or specifically allocated frequency bands for “local area access”. To prevent the relay traffic to cause interference in the frequency bands used by the NI, the RAI will need separate spectrum allocation.
4 Discussion and analysis of the proposed solution In this section the benefits and drawbacks with the solution outlined in Section 3 will be analyzed. The performance will be investigated for the different user scenarios. Cost issues, compatibility and evolution of existing solutions will be discussed in general. The cost structure analysis will make use of the following entities: • • • • •
Deployment & Planning Operation & Maintenance Site costs Interconnection costs Terminal costs
4.1 Effects on the cost structure 4.1.1
Deployment & Planning
We expect no or low need for detailed radio network planning, provided that spectrum bands are allocated for use of the proposed new air interfaces LAI and RAI. The NNE concept requires dimensioning of the backbone and base station in order to support the traffic and also functionality to support self-configuration. 4.1.2
Operation & Maintenance
The concept implies capacity enhancements by “re-using” parts of existing radio access network, such as base station sites and broadband connections in homes and offices. Operation and Maintenance (O&M) costs will remain for the base station sites, but very low or no extra O&M costs are expected for the new equipment (NNE). Instead the NNEs will contain functionality to support self-diagnostics, failure reporting, reconfiguration and auto tuning. 4.1.3
Site CAPEX
No or low need for new sites imply low investments in new sites and non-telecom equipment, but more complex base stations are needed to support the higher capacity. 4.1.4
Interconnection and transmission
Low numbers of additional sites imply low need of new cellular transmission connections but capacity of each connection must be dimensioned to handle the increased traffic. . A large part of the high capacity traffic will use fixed broadband connections and stationary NNEs thus providing a low cost solution. High traffic will have an impact on the interconnection and roaming agreements, a new issue to address is the sharing of NNEs when using the relaying functionality. 4.1.5
Terminals
With the proposed concept the user terminals can be kept at the same complexity levels as today. A mobile station can communicate both with the traditional base stations
directly or indirectly via the NNE. The functionality driving the cost is therefore included in the base stations and NNEs, and not in the terminals. For the cases where mobile station requires high capacity connections to the NNE this will probably not be a major problem since in these cases the terminal will most likely be quite complex itself. Of course the overall costs for the user will increase when the NNE is included in the subscriber offer, in the 10 years time frame this is not considered to be an issue, compare today’s privately owned ADSL modems and receivers for satellite TV. 4.1.6
New Network Elements
An additional cost will be incurred by adding NNEs to the network. The costs associated with radio and base-band hardware are likely to be several-fold the cost of the same components in a mobile-station, considering that the NNE should host multiple antennas and multiple air interfaces. On the other hand the NNE is multi-purpose and may for instance serve the communication needs of a whole family who may have several mobile phones, PDAs and Laptops. Therefore all those devices should in some sense divide the cost. As a side effect of supporting many standards large production volumes may bring prices down. It also appears that this is a product that may be appreciated by the consumers in the same way as an expensive HIFI-systems or a satellite TV receiver. Last but not least, it should be remembered that number of NNEs obviously would grow with the user base – thus avoiding a heavy investment for the operator and still providing a large improvement for the users.
4.2
Performance and applicability
The performance and applicability will be discussed in terms of availability, coverage and capacity using the different user scenarios described in section 1.2. 4.2.1
High capacity, low mobility environments
For the hot spot environments and for offices and homes with high user densities the main driver is the capacity (not coverage or mobility). The main configuration of the proposed system will be a stationary NNE connected to e.g. a broad band connection at home or a corporate LAN. In cases where no fixed NNEs are deployed, or when too low capacity is offered by stationary NNEs, extra coverage and capacity can be provided by mobile NNEs. Using the fixed broadband infrastructure will fulfill capacity and coverage requirements in these environments. For the wireless part no major problems can be identified since the local coverage implies low transmission power and low mutual interference. Mobility support is not believed to be any problem due to low number of high mobility users; the wide area coverage parts of the system can handle these. The key research issues to address in this case are related to: •
How the cellular and WLAN type of technologies will be combined and how the different protocols will co-exist.
•
Performance of a layered radio access network
•
The functionality needed to enable self-deployment, automatic re-configuration and failure handling in an environment with many wireless systems.
4.2.2
Wide area coverage
In this case the main driver is the coverage for high data rates, scenarios include both urban and rural areas. The main part of the traffic will be carried by stationary or mobile NNEs with wireless connections to base stations. Capacity and coverage will be offered where NNEs are available. This means that the radio network planning and dimensioning primarily have to deal with the link performance between the base station and NNE (and not between base stations and the mobile stations). One benefit is that the concept in principle allows any terminal to connect to any NNE. Of course this feature raises both technical and trust issues.. In 4.2.1 it is implicitly assumed that a NNE is serving only terminals that belongs to the “own” company or family. Another benefit is that the main larger part of the power consumption related to transmission of data will be in the NNE and not in the user terminal. since most NNEs will be deployed where power supply is not an issue. Cooperating NNE´s can improve the performance further by using ad-hoc network techniques. Individual NNEs can relay to other NNEs, thus offering extended coverage using multi-hop and increased capacity by load sharing. The key research issues resulting from the discussion above are within three areas: • • •
The design of the high capacity, low cost wireless connection between base stations and NNEs. The strategies to form self-organizing ad-hoc networks between NNEs providing links that fulfill the capacity and delay requirements. The business agreements and technical solutions necessary to enable common and public use of privately owned equipment.
4.2.3
High mobility environments
To provide high capacity in cars, buses and trains the ability of the network to provide wide area coverage and mobility support is essential. The system configuration for this scenario is based on the Mobile NNE supporting terminals located in the same vehicle. The connections to the fixed network are based on wireless links to base stations or to other NNEs located in nearby vehicles. A Mobile NNE can either be owned by private persons, e.g. installed in a car, or by a public transportation company on a bus or in a train. Thus, the Mobile NNE should support a larger variety of mobile stations and also different types of subscription types. The key research issues in this case are similar to those in section 4.2.2: • • •
The design of the high capacity, low cost wireless connection between base stations and NNEs, with special focus on the support of high mobility. The business and technical solutions necessary to enable public use. The design of supporting wireless backbone between Mobile NNEs along roads.
4.3 Evolution and compatibility with existing solutions The proposed solution and the new interfaces can co-exist with the current 3GPP interfaces since both mobile stations and NNEs connect to base stations using the current air interface. The LAI can be designed quite independently from the other interfaces, since the protocols for other air interfaces are terminated in the NNE. And, by using WLAN technology with unlicensed spectrum for the LAI, the system can initially be implemented without major changes to the standards of existing cellular systems. The main issue concerning the air interfaces is believed to be the RAI connecting cooperating NNEs. Specific frequency allocation (in the same or other frequency band) is probably the most likely solution. In order to keep the mobile stations as simple a possible (avoiding multi-mode terminals), the NNE is instead designed for multi-mode air interfaces. Thus, different types of user equipment can be used to connect to the same NNE. A benefit with the concept for both users and operators is that users who do not want to have access to high data rates “everywhere” doesn’t have to buy complex and expensive equipment. On the other hand the operators can offer consumers with high capacity demands a package with one or several mobile stations and a NNE. The customer can then install the NNE, e.g., at home, in the office, in the car or in the summerhouse. For mobile operators the concept offers a high degree of flexibility. Investments can be made in small steps and can be focused to areas where a high market high demand exists. The concept also offers advantages in rollout phases and in areas and countries with a low degree of infrastructure.
5 Comments on applicable business models and value chains For mobile operators the main benefit with the NNE concept is the possibility to offer wide area coverage and high-speed services without large amounts of investments. Users such as private persons, facility owners or companies can also own the NNE equipment; see Figure 3. In this case the user still “belongs to” an operator through the subscription. Today
Possible types of ownership of the NNE
Fixed network
Fixed network
Fixed network
Base Station
Base Station
Base Station
NNE
NNE
Mobile station
Mobile station
Mobile station
Fixed k
Owned by traditional operator Owned by user Figure 3. Ownership of the NNE´s, using the traditional business model with mobile operators and subscribers.
However, the NNE concept can enable for small-scale solutions where market needs exists, thus making it easier for new market players to enter the market, see Figure 4. Local network providers can provide wireless access in specific areas, using e.g. some of the business cases and customer offers listed below: • • •
Provisioning of high speed access and to charge the users for the used capacity. Wireless access as a mean to attract customers to the “original business” (e.g. Starbucks and McDonalds). Provisioning of (and charging for) a special service in a specific location.
Service provider Services
Services
Services
Traditional operators Fixed network
Fixed network
Fixed network Base Station
Base Station Local access providers NNE
NNE
NNE
Mobile station
Mobile station
Mobile station
NNE owned by Service provider
Owned by traditional operator Owned by user Owned by local access provider and or service provider Figure 4. Ownership of NNE as part of emerging business models
In countries or regions with no available licenses or spectrum, the NNE solution also gives global operators a possibility to enter the market. The NNE concept can contribute to the creation of a market with both traditional mobile operators and new local access providers. The traditional operator provides the backbone network, the radio access network and broadband connections: Possible new roles also include capacity brokers and clearing houses, see e.g. [11][10] In order to make the best use of the resources, i.e. to provide high availability and capacity, the NNE concept requires a high degree of sharing of network resources. Thus we envision both a very high degree of co-operation, in terms of generalized roaming, clearing of interconnections costs and roaming agreements, and an increased competition on terms of services and “customer offerings” based on the common access resource pool.
6 Conclusions This paper has discussed how WLAN and cellular networks can be combined to form a wide-area wireless network that supports high bit rates and mobility. The idea is to introduce a relaying node that connects the end-user terminals with the fixed infrastructure, which may be a wireless or wired. Thus, a single-hop high bit rate connection between base stations and user terminals is avoided, which is beneficial both technically and economically. In addition already deployed infrastructure can in a large extent be re-used. This is achieved through the use of advanced radio access technology in the relay access point and not in the mobile terminal. The solution is also interesting from a business perspective since traditional operators, local network providers, or private persons can own the relaying access points. Therefore, we believe that this solution could enable a more efficient market for wireless infrastructure than what we see today.
Acknowledgements We would like to thank Prof. Bertil Thorngren (Stockholm School of Economics) and Dr. Tim Giles and Dr. Bo Karlsson (both with the Royal Institute of Technology) for valuable comments on this work. We also greatly appreciate the enthusiastic support from Prof. Jens Zander at the Royal Institute of Technology and Econ Lic Jonas Lind at the Stockholm School of Economics and for their contribution to the understanding of cost efficient designs of wireless systems. This work has been supported by the Swedish Strategic Research Foundation as part of the funding of the Affordable Wireless Services and Infrastructures program
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