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UMA and Femtocells: Making Fixed-to-Mobile-Conversion Happen

CHOUDHURY, PARTHO Principal Engineer, Hughes Systique

(www.hsc.com) (partho.choudhury@hsc.com)

DAHUJA, DEEPAK Senior Principal Engineer, Hughes Systique

(www.hsc.com) (deepak.dahuja@hsc.com)

This paper analyses two prominent concepts in the Fixed Mobile Convergence domain, namely Unlicensed Mobile Access (UMA) and Femtocells. Conventional cellular and wireless broadband access technologies (such as WIMAX and 3G) suffer from propagation loss issues in indoor and urban canyon environments. In an attempt to extend the reach of end-to-end IP-based wireless broadband, these two categories compete as well as complement each other in deployment scenarios across the world. We study various aspects like motivation, evolution, benefits, challenges, as well as deployment scenarios. We have also tried to capture current advancements in this area, and how these technologies can be put to use in emerging business models. What is Fixed-to-Mobile-Conversion? Fixed-Mobile Convergence (FMC) is a technological concept which allows subscribers to switch an active voice or data call session between fixed wireless (say, WLAN) and mobile (say, cellular) networks. The transition is assumed to be “seamless”, in that the handover from the fixed to mobile wireless networks (and back) happens without any loss of connectivity, disruption in end-user experience and/or user intervention. The basic idea behind a FMC solution is to extend all or a part of the services provided by the wireless Telecom Service Provider’s (TSP) Core Network (CN) to domestic as well as Small and Medium Enterprise (SME) subscribers through the public IP network (Internet). An intermediate Access Point (AP) (a “Micro” Base Station) connects a Mobile Station (MS), such as the subscriber’s 3G handset, to the CN via an existing “fixed” IP service (such as DSL or Cable broadband). When a mobile user is in the vicinity of a participating Microcell, all ongoing voice and data traffic is seamlessly handed over from the Macrocell to the “Micro” Base Station, which tunnels the traffic through a secure IP connection.

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Motivation for FMC [1][2]: The general motivation behind achieving FMC includes the following:

1. Improved indoor coverage – no deadspots: Most 3G networks operate in the L and S bands (for e.g., UMTS2100). These bands have comparatively poor propagation characteristics; specifically, they are not suitable for penetration through typical urban building materials such as glass, steel and concrete [11]. For e.g., if a macrocell Node B provides 98% coverage for voice calls outside, this figure drops to 70% in an urban canyon environment. The probability of a DSL class wireless broadband service (~384 kbps) being successfully received within an indoor environment is not more than 20%. In order to expand the reach of “high revenue” 3G services to such areas, an alternative that distributes the wireless signal from within the indoor environment provides a very compelling alternative.

2. Reduced Capital Expenditure (CapEx)/Operational Expenditure (OpEx) and CPE subsidy: Since the bulk of

the voice and/or data traffic in the “Fixed” component of the network shall be routed through the public Internet, a very minimal capital expenditure outlay needs to be planned for by the service provider. This is expected to lead to reduced billing rates (USD per bit) for customers, as well as rendering CPE subsidies largely redundant.

3. Reduced bandwidth load: By offloading voice and data traffic from the wireless spectrum on to the “Fixed”

part of the network (public Internet, for e.g.), the more valuable (and comparatively more expensive) wireless bandwidth resource may be conserved and used to not only provide additional Value Added Services (VAS) (such as a more granular Local Content Insertion (LCI) system to enable a more focused marketing/commercial advertising drive) to existing subscribers, but to also add to the overall “mobile only” subscriber base.

4. Reduced power requirements: 3G access technologies such as WCDMA operate on the principle of shared

power profiles. This means that the power transmitted from the Node B shall be shared by all users with a single macrocell. Because 3G signals from such transmitters would have to “work harder” to penetrate concrete, glass and steel walls and partitions to reach the end- user, their receiver handsets shall consume a larger proportion of the available power. Since a lower proportion of the total power from the Node B is now reaching mobile users outdoors, the overall QoS and capacity of the service degrades. By diverting some users to FMC solutions, the capacity and the QoS of the 3G service improves in the proportion of the number of users who have migrated towards an FMC solution.

5. Additional “high end” revenue streams: A higher per capita data capacity of the “fixed” backhaul through the

public Internet means that the “entry barrier” to high end revenue streams (such as video multicast/broadcast and interactive gaming) is now comparatively lower for wireless cellular service providers.

6. Several business and technological trends over the last few years have converged to provide traction to the

concept of FMC:

a. “Ubiquity” of the mobile phone: Analysts estimate that approximately 30-35% of all voice calls made over a mobile network are made by subscribers while within the confines of their residential properties. McKinsey reported in 2006 that 35% of video streaming and broadcasting services over cellular wireless networks occurred within home and office environments. Besides acting as personal communication devices, cellular handsets also function as general purpose utility devices for storing and managing contact lists, managing voice-mail inboxes, teleconferencing and a host of non-voice services such as GPS/LBS, Instant Messaging (IM) and wireless Internet.

b. Proliferation of Wireless Local Area Network/Wireless Personal Area Network (WLAN/WPAN) network devices: Since 1999 (the year the Apple’s iBook debuted in the Macworld Expo in New York with a WLAN port specifically for the purpose of accessing the wider Internet), wireless access devices in the WLAN (Wi-Fi) and WPAN (Bluetooth) categories have not only proliferated in actual numbers, but also become an increasingly affordable option for the average customer. The latest version of the Wi-Fi WLAN specification (IEEE 802.11n) is able to support time-sensitive data applications such as Voice-Over-IP (VoIP) and streaming video.

c. All-IP backbone: Traditionally, voice traffic has been transmitted over the classic “copper” backbone using Circuit-Switched (CS) networks. With the proliferation of the Internet and other private data networks, it

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has become progressively more economical to transport digital information (e.g., streaming/broadcast multimedia) over an all-IP network. Considering the fact that several Internet Service Providers (ISPs) around the globe are also invested heavily into traditional telephony businesses, a cost-effective technique for routing “voice” bits over an all-IP internet is slowly fructifying.

Besides this, some tangible benefits to the end-customer which may act as the USP for the concept of an FMC include [1][2][7]:

1. Single mode handset: A single handset for usage in both fixed as well as mobile environments means a lower per capita expenditure for the end user.

2. Single number: A subscriber may now be reachable on a single contact number, regardless of his actual

physical location. This also helps the customer in maintaining a single voice mail inbox and contact list, thereby simplifying his/her mobility experience.

3. Seamless handoff: A single call session (voice or data) shall continue without any discernable disruption or

end-user intervention while migrating between the mobile and fixed components of the network.

4. Consistency and Transparency of Service: All CS and PS based basic telecom and Value Added Services (VAS) available to the subscriber in the mobile environment shall also be available while in the fixed mode, and vice versa.

The challenges [1][2][3]: The problem of ubiquitous wireless coverage throws up several engineering and business challenges, as well as opportunities, particularly in adapting older core network technology to newer, more modern commercial needs and scenarios.

• Roaming and handover: One advantage of using micro- base station topologies over Wi-Fi based “dual mode” phones is the simplicity in the hardware platform, since there is no need for switching between the 3G and Wi-Fi RANs. However, additional software logic to provide the “intelligence” which detects the presence of co-located macro and femtocells, and switches to the latter based on various factors (such as signal strength, assured connectivity, etc.) poses yet an additional design challenge.

• Network and data security: Since the backhaul connectivity of micro- base station architectures (to the RNC) is through the wider (but unsecured) public Internet, latency and security of real-time voice and data traffic (e.g., streaming multimedia, video conferencing) shall continue be the focus of concern and continued research.

Procedures should be in place to allow the Node B to be secure-authenticated with the network it wishes to connect to once registration has been performed, in order to prevent service fraud. After authentication, authorization to provide 3G services to one or more 3G handsets should be performed.

• Billing: Service providers shall have to integrate their BOSS systems to keep track of all traffic that travels between the end-user’s CPE and the core network via the wireless network as well as the public IP network, so that revenue streams for all services provided are maintained, irrespective of the mode of communication (fixed or mobile).

• Consistent QoS: One of the biggest attractions of enabling FMC is the problem of inconsistent QoS of wireless services delivered in the 3G bands within home and small to medium office premises. Once 3G services are routed from the “macro” Node B to its “micro” base station through a secure and reliable backhaul, problems of deadspots, variable signal levels, shadow fading and other sources of external interference are completely mitigated. This is expected to be an additional attraction for data services, which rely heavily on secure and consistent services (for delivering high-end, high throughput services such as video conferencing and streaming multimedia).

• Distribution, support and subsidy of CPE: Most cellular service providers have experience in marketing “factory ready” handsets that do not require complex installation and configuration manuals. However, an additional change with CPE is the physical installation and configuration of the equipment, which needs to be provided by the service provider as part of its initial “assured service” contract. This is expected to pose a unique business process challenge for most traditional service providers.

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Hidden costs in the form of CPE subsidies shall also continue to be a challenge to most medium to large service providers. After accounting for miniaturization and other engineering costs, most factory ready femtocell CPE are expected to cost in the range of USD 150 to 200. Most cellular phone operators continue to sell their user end equipment (cellular handsets and accessories) at a comparatively lower price point (USD 50 to 100), and then recover their investments using a combination of “high end” data services and long term “lock in” subscription plans (with extremely tenuous exit clauses).

• Regulatory Compliance: The integration of the CPE with the core network should satisfy all regulatory concerns including emergency dialing services.

• Health concerns: Regulators such as FCC and Ofcom have consistently raised health concerns regarding the deployment of high energy sources in close vicinity of residential properties, hospitals and schools. Even with lower Tx power profiles, the prolonged proximity of a micro- base station transmitter to it’s target customer base shall continue to be an area of concern for potential customers.

Industry sources continue to downplay the potential health hazards that close proximity of femtocell base station transmitters may pose to customers. They claim that the smaller distances between base stations and receivers would decrease the necessary power levels at the receiver end, thereby reducing the potential hazards claimed against typical handheld receivers. Moreover, a handset connected to a micro- base station is expected to operate at the 1 mW range, which is about a thousand times less powerful than a regular GSM handset. In comparison, WiFi base stations operate at the ~100 mW range, while the older cordless phones operate in the ~10 mW range.

Possible Solutions: From the above discussion, we observe that the major obstacle to the adoption of 3G for “high end” data services is the inconsistent QoS within residential and office premises in urban and suburban areas. We also see that, irrespective of the underlying technology, wireless networks have been migrating towards smaller cell sizes with denser frequency re-use. In order to make this investment attractive for the service provider (who is expected to invest heavily in setting up ancillary customer support services) and the subscriber (who is loath to invest on additional CPE which may be difficult to use), a reduced form factor and low cost/low power are very attractive incentives. Two broad approaches have evolved over the last few years in an attempt to achieve the stated goal of FMC. Both methodologies rely heavily on two factors:

• The deep penetration of the wider public Internet and WLAN technologies (especially the IEEE 802.11x standard)

• The necessity of employing ever smaller cells within which subscribers are guaranteed a uniformly consistent level of QoS, even in conditions of heavy urban clutter.

In either case, the general architecture employs a combination of a handset and an indoor transceiver connected to the wider public Internet through the customer’s broadband subscription. The architecture described below is agnostic to the backhaul technology. Both approaches utilize special firmware to perform 2 basic tasks:

• Carrier detection: The firmware detects the presence (or lack thereof) of both the “macro” 3G network and the complementary indoor radio bearer, and chooses the appropriate mode of operation based on relative signal strength. The switch is meant to be completely seamless and transparent, with no manual intervention by the end subscriber or discernable disruption in service.

• Encapsulation: Once the firmware has detected an indoor radio bearer, it performs the encapsulation of the raw voice/data bits to be forwarded by the indoor access point through the public IP network to the edge server at the other end. The edge server (specific to the encapsulation scheme being employed) shall strip the data payload of its secure IP encapsulation and forward the datagrams consisting of raw voice/data bits to the core network of the wireless service provider for decoding.

The two approaches differ in the utilization of different wireless protocol stacks and air interfaces, though they appear to be significantly identical at the core network layer.

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1. UMA

This is the simplest (and by far, the most obvious) solution to the problem of achieving ubiquitous wireless coverage, including in areas where the problem of deadspots are endemic. The CPE comprises of a standard off-the-shelf IEEE 802.11x (Wi-Fi) Access Point (AP) and a “dual mode” handset device equipped with UMA enabled firmware and a Wi-Fi transceiver. The firmware is based on the Universal Mobile Access (UMA) protocol stack, also known as Generic Access Network (GAN), and ratified by the 3GPP. With a UMA-enabled dual-mode Wi-Fi handset, subscribers can automatically roam and handover between cellular and unlicensed spectrum based networks like Wi-Fi. To enable access to the mobile service core via IP-based networks, the UMA standard defines a new core network element (the UMA Network Controller or Generic Access Controller (GAC)) and associated protocols that provide for the secure transport of mobile signaling and user plane traffic over IP [4]. The UNC interfaces into the core network via existing 3GPP specified interfaces. UMA Deployments [5] – [12]: Many factors have lead to successful deployments based on UMA

1. IEEE 802.11x (Wi-Fi) APs are available at cheap prices, and there is already deep penetration of broadband

access based on local WiFi hot spots. 2. Support for WLAN is no longer limited to high-end expensive phones, but is becoming a standard feature

available in the most recently launched handsets.

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Technologies competing with UMA for the FMC slice:

a. Voice Call Continuity (VCC)

Voice Call Continuity, another 3GPP standard, is a SIP/IMS based FMC technology which allows seamless transfer of voice calls between CS and IP networks. Dual Mode handsets can use this technology as an alternative to UMA. VCC is capable of bringing in seamless voice, data or video service session continuity across multiple types of access networks (both wireline and wireless) and devices. Dependency on IMS enabled networks and handsets have lead to very few real deployments based on VCC.

b. Femtocells Femtocell allows mobile operators to extend core network services to subscribers’ home and office, but rather than using a standard Wi-Fi router, a “Micro” 3G/GSM Basestation is used to provide indoor coverage. Micro Basestations are connected to CN elements through the public Internet. Femtocells allow subscribers to use regular 3G handsets.

While Femtocells face the following issues:

- With large number of micro Basestations installed in vicinity of each other, interference and frequency re-

use issues become complex. - Need of a specialized micro Basestation increases cost of overall solution.

Future of UMA:

• UMA and IMS IMS enables new multimedia services to be developed and made accessible to multiple device types over various IP based access networks. UMA enables mobile handsets to transition between the macro cellular network and various IP based access networks and remain “always best connected” to the service provider’s CN. While I-WLAN standard (3GPP TS 22.234) allows mobiles (and portable laptops etc) to access CN Services through Wi-Fi, it does not provide for seamless mobility between networks. Hence, active sessions will be lost as subscribers attempt to transition between access networks. UMA can thus become an important standard in extending IMS based Service to Wi-Fi networks, and at the same time allowing seamless mobility across networks.

• UMA and Femtocells Cf. Section “UMA based RAN Gateway” below.

• Iu Interface support in UMA

While the UMA architecture supports A and Gb interfaces between the Generic Access Controller (GAC) and back-end CN elements, support for Iu interface between GAC and CN elements has been added to 3GPP TS 43.902 only fairly recently (Q2, 2007). This shall enable UMTS-only operators to use UMA in expanding their services.

2. Femtocells Considering the entry barrier that many potential start-ups may face due to the prohibitive cost of investing in (and possibly indirectly subsidizing) dual-mode handsets, a technology which transfers the burden of handling the encapsulation to a single “medium to low cost” CPE, and allows the use of regular 3G handsets is the direction in which the industry is expected to move. For example, end-user subscribers may prefer to continue using their regular 3G handsets that connect to a CPE, which is essentially a “micro” 3G Base Station (Node B). The CPE has been scaled down in chassis form factor, energy requirements, operational power levels and cost, although it still connects to the handset in much the same

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way as a regular “macro” Node B would do. Due to their lower transmit power levels, the reach of these transceiver base stations are limited to a typical radius of ~ 1 km, suitable for use within the interiors of most homes and small to medium sized offices. Due to the relatively miniscule size of the coverage area of these devices, they are generically called “Femtocells”. The essential idea is to utilize the traditional 3G radio bearer (Uu) to carry the voice/data bits to the “femtocell” CPE. Here, the 3G messages are either re-encapsulated using an additional protocol layer, or converted into a completely different format, before being tunneled through an IPSec data pipe. An end-to-end PS session is maintained between the femtocell CPE and an edge server at the edge of the service provider’s core network. This has given rise to three different solutions, which have their unique advantages and drawbacks.

a. “Iu-b over IP” [2]:

The earliest (and by far the simplest) solution suggested by the industry employs an “Iu-b over IP” approach, which involves encapsulating a traditional 3GPP-compliant Iu-b radio bearer over an IP packet scheme. In this topology, the Iu-b radio bearer generates a direct connection from the RNC to the femtocell “micro Node B”. However, the Iu-b messages are encapsulated within IPSec packets, which introduces an additional level of complexity, albeit minor. The encapsulation occurs behind the scenes, in that the operation, design and functionality of the user equipment are not affected in any way. Such an approach would be especially attractive to RNC OEMs, who would be able to leverage their existing core network architectures (through Iu-CS and Iu-PS). Besides ensuring full service transparency, this solution had lower initial deployments cost (since users shall not need to purchase new handsets and service providers can keep using their older RNC and other core network equipment with minimal reconfiguration) and minimal network disruption. The only additional expenditure shall be in the form of a subsidized femtocell CPE. However, scalability has turned out to be the nemesis of this approach. RNCs have been typically built with a focus on a very small number of connections to “extremely large capacity” Node B units. However, the femtocell architecture places the exact opposite design requirement on RNCs – an extremely large number of “low capacity” femtocell CPEs connected to each RNC.

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Moreover, while the 3GPP standardization body defines the Iu-b interface, the actual implementation is proprietary to each vendor. This has led to severe interoperability issues, which has acted as barrier to the entry of newer OEMs and vendors.

b. SIP/IMS [2]:

An alternative approach to using traditional wireless radio bearers is to route the entire voice/data traffic through a Session Initiation Protocol/IP Multimedia Subsystem (SIP/IMS). The wireless service provider shall have to set up the entire SIP based core network, running in parallel with their existing CS and PS core network. The 3G handheld device shall seamlessly switch between the macro cell and the femtocell in the background, and when connected to the femtocell, shall route all traffic through the SIP sub-system. All voice/data bits are transmitted through the traditional radio bearer (Uu) from the handheld to the femtocell AP. Here, the raw data and control bit-stream is encapsulated within SIP message blocks (or datagrams), and further wrapped within an IPSec layer. The IP packets are routed through the public Internet to the edge server sub-system comprising of a Security Gateway, SIP and IMS servers. The Security Gateway strips the IPSec protection, while the SIP/IMS sub-system converts the SIP messages into raw data, to be routed to the intended recipient either through the SIP/IMS core network, or through the traditional 3G core network of the same or different service provider. During the initial phase of SIP/IMS deployments, it was believed that all 3G service providers shall eventually migrate to SIP and IMS based infrastructure, rather than having to maintain parallel infrastructure which has been the norm till now. SIP based approaches also hold the promise of cost-effective scalability for large deployment scenarios. However, since the same handset has to effectively switch from the traditional core network to the parallel SIP/IMS core network when transiting from the outdoor to the indoor scenarios, service transparency continues to be a major challenge for this type of a solution. Service providers not only have to continue maintaining parallel networks, but also have to faithfully duplicate all VAS in both networks. This is especially untenable when the SIP/IMS and traditional back-end infrastructure are maintained and served by different service providers.

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c. UMA based RAN Gateway [2]:

This is the most recent approach to be employed by the industry, and utilizes a Radio Network Access (RAN) Gateway to aggregate voice/data traffic from a large number of femtocells within the coverage area, over a new “Iu over IP” interface. The RAN Gateway, similar to the traditional RNC, is intermediate between the existing core service network and the public Internet, and integrates the traffic into the traditional core network through standard Iu-CS/PS interfaces. The RAN Gateway approach leverages the existing core network elements of the service provider through standardized interfaces (Iu-CS and Iu-PS). Hence, it allows for full service transparency and a comparatively lower deployment cost, as well as overall minimal disruption to existing operations. In addition, the RAN Gateway approach employs a “flat IP” architecture, in which a number of functions traditionally assigned to the RNC at the edge of the core network are moved to the femtocell itself. This way, many of the scaling challenges of the “Iu-b over IP” approach are avoided. The basic linkages in the traffic path of the RAN Gateway approach include:

1. the radio bearer between the standard 3G handset and the femtocell AP (Uu)

2. an Iu interface between the femtocell and the RAN Gateway encapsulated within secure IP (“Iu over IP”)

3. the backend connectivity provided by Iu-CS and Iu-PS interfaces Of these three linkages, the Uu and Iu-CS/PS interfaces have been standardized, while the industry is considering either defining a new standard to specify the interface between the femtocell and the core network edge, or utilizing a current commercially and technologically proven specification available openly in the market. The basic requirements of the “Iu-over-IP” interface specification, irrespective of its origin, are:

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a. Compatibility with older technologies: Many service providers are in the process of migrating from 2G to 3G services, and would like to continue their 2G service track for as long as there is a viable customer base. Hence, the approach towards FMC that needs to be adopted by such service providers need to be agnostic to the precise nature of the Uu radio bearer.

b. Protected investment: Massive investments in CPE and core network elements (such as a RAN Gateways)

would compel service providers to look for multiple assured revenue streams through the deployment of additional applications to be accessed over their networks. The solution adopted by the service provider should allow multiple applications to be accessed by single/multiple subscribers connected to each CPE.

c. Ability to handle a variety of design challenges faced by Mobile IP networks: Since backhaul transmission is

through the public IP backbone, this mode of communication shall be unmanaged, unsecured and bandwidth constrained. The protocol used for encapsulation should be low-complexity, low bandwidth, and support a whole range of in-house as well as third party security paradigms (IPSec, IKE, EAP-SIM, EAP-AKA).

Besides security, speedy and automatic (meaning, with minimal user intervention) “authentication and registration” should be possible to deploy the technology to as many potential subscribers as possible within the least possible time. This also allows the service provider to ensure that only genuine subscribers are able to avail of the 3G services being offered through their networks, thereby mitigating service fraud to a large extent.

d. Scalability: The proposed solution should be able to scale upto several thousands of simultaneous secure

connections to the service provider’s core network over the public Internet.

e. Fast track to standardization: In order to reduce the Time-To-Market (TTM) for the CPE and accompanying support services and systems, a fast track standardization procedure needs to be put in place. Preferably, a technology which is already accepted widely by a section of the industry and has a considerable amount of deployment in the domestic as well as enterprise space serves as a suitable candidate. Moreover, an open standard, ratified by an international standards body or industry forum would be ideal since most vendors would not like to get ‘locked in’ to a vendor-specific, proprietary interface.

In other words, in order to achieve its stated goal of convergence between fixed broadband services (via the public Internet) and mobile wireless 3G services, we need a technology which is scalable, commercially deployed (and hence, proven), scalable with minimal disruption to existing services, and at the same time is provides a cost advantage to both the service provider as well as the potential subscriber. A combination of all these factors is found in good measure in UMA, also known by its 3GPP version, Generic Access Network (GAN). The GERAN Working Group (WG) within the 3GPP project is involved in the preparation of the GAN specification and architecture. The final version of the protocol is expected to be ratified and accepted as a standard within 3GPP by end of Q1 2008. Within the 3GPP, UMA/GAN specifies an edge server at the interface between the broadband backhaul and the mobile core network, which is known as the UMA Network Controller (UNC) (or Generic Access Network Controller (GANC)). The GANC essentially functions like a regular RAN Gateway. Since the UMA client (protocol stack) is built inside the femtocell CPE, regular 3G handsets with no additional software or hardware upgrades mean lower per capita investment for the end subscriber. Moreover, the regular subsidies borne by the service provider for handsets may now be passed onto the femtocell CPE. The UMA/GAN and IPSec protocol stacks are implemented on the CPE firmware.

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Current femtocell deployments [4][19]: Femtocell deployments have picked up since the initial interest shown in the early part of the decade. However, the major chunk of the market is restricted close to the OEMs active in this domain, which includes the United Kingdom and Western Europe, and the USA to a lesser extent. Current efforts by the OEMs include expanding to multimedia gateway hubs and femtocell network management firmware, which is believed to add value to the basic offerings of these companies.

Competing technologies:

• UMA

UMA with Dual-Mode Handsets (DMH) competes with Femtocells as a FMC technology. The single largest advantage of using femtocells to achieve FMC is the use of regular 3G handsets which do not require any expensive add-ons or customization. Since a femtocell is essentially a miniature version of a base station, the inherent “intelligence” of the node may be used by the operator to introduce a wide range of Value Added Services (VAS) such as Local Content Insertion (LCI) and targeted advertising. Such a business model is not possible with Wi-Fi based solutions which are not deployed by the service provider. However, as seen in section “UMA based RAN Gateway” UMA can also act as an enabling technology for Femtocells.

Outstanding challenges [18]:

• MIPS/Memory constraints: In order to enable the adaptation of ”macro” Node B technology to a “low cost/low power” chassis, severe MIPS and RAM constraints shall continue to pose significant engineering design challenges.

• Scalability: Classical FMC architectures and design methodologies need to be able to handle challenges in scalability. The traditional Radio Network Controller (RNC) (which forms the front-end of the core network) is designed to load from a couple of dozens to several hundreds of Node B equipment for the smooth operation of a typical 3G network. Each Node B manages the traffic load of several hundred subscribers on its own, without exposing traffic loading and network management issues to the core network. However, micro- and picocell topologies connect individual “micro” Node Bs directly to an RNC via the wider public Internet, and hence RNC architecture and design methodologies may need some form of major reconfiguration, in order to handle several thousands, if not millions of IP traffic sessions from individual CPE at any given moment.

• Ease of discovery and registration: To be a commercially viable consumer product, Node B should be able to be mass produced and “Plug And Play” (PnP) installed. In other words, the device should be able to discover all present networks on power up, and connect securely to the network of the subscriber’s choice (assuming that the subscriber has a paid subscription), with minimal provisioning overheads. Upon powering up, the base station AP firmware must have procedures in place to identify the appropriate connectivity protocol to the core network, determine the service bouquet it is allowed to service (and the subscribed has subscribed to), as well as register itself with the network.

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• Backhaul limitations: The overall reverse path speeds provided by FMC solutions shall be largely dependent on the data rates supported by the backhaul connectivity to the public IP network available to the end-user. While a DSL or cable modem connection to the Internet may be sufficient to support heavy duty browsing, IM and downloads, they may not be able to support the truly “high end” services (like video telephony and broadcast/multicast) which are supposed to be the USP of 3G.

Business Cases for FMC [18][19]: While FMC is a viable option for both mobile as well as fixed TSPs, the approach to realizing convergence may differ in some respects:

1. Mobile Service Providers: Mobile TSPs have the widest range of options available to them when it comes to FMC solution deployment. The ultimate goal of cellular service providers is to integrate all their offerings through a single back-end technology, such as IMS. Till such time that full integration is made financially and logistically feasible, interim solutions based on single or dual mode handsets driven by UMA may be utilized. Gradual migration from an all-CS to an all-IP based model is expected to last at least another five years.

Mobile operators hope to gain through this model by maintaining their branding and billing relationships with their traditional customers (whether they are in the mobile or home zone). This provides an incentive for mobile service providers compete with cable and “wireline” broadband service providers to (eventually) provide all-IP voice and data services to their subscribers in the homezone.

2. Fixed Service Providers: Since most fixed TSPs do not own back-end systems required for providing mobility

services (such as MSCs, RNCs etc.), initial CapEx could be prohibitive in many cases. Single mode solutions (such as those based on IMS) could be an attractive option to reduce the entry barriers for traditional fixed TSPs into the FMC domain.

An additional approach taken by many fixed TSPs is to morph into a Mobile Virtual Network Operator (MVNO). MVNOs do not require the prohibitive CapEx usually associated with wireless network deployments – this is taken care of by traditional wireless TSPs. MVNOs buy airtime (a virtual asset) from the service provider, and concentrate on the service branding and customer relationship aspects of the deployment scenario. In such a scenario too, the fixed service provider may initially depend on a UMA based solution, and eventually migrate to an all-IP based solution based on IMS.

Business case for end users and service providers: We present below a brief overview of the compelling factors behind the adoption of FMC solutions by both end consumers and wireless TSPs. Extended coverage for high end services, lower deployment costs, savings in “homezone” billing rates and ease of usage are only some of the more compelling factors which have driven FMC technology to fruition in the past few years. We make some basic assumptions about usage patterns and CapEx/OpEx of a typical subscriber. We assume 100% subsidy provided by the wireless TSP for the use of CPEs (the subsidy shall include installation costs, if any).The core network and integration costs shall amount to about 15% of the cost of the CPE unit. Along with technical support overheads, the monthly cost of a USD 200 CPE shall be in range of USD 15 to 20. We assume a 3G service based on a 2+2+2 HSDPA configuration, with a typical spectral usage efficiency of 2 Mbps/cell, and a maximum throughput of 650 GB per month, requiring 3 E1 lines for backhaul. 30% of all voice calls over the mobile network are made from the “homezone” (including home and enterprise premises). We assume the typical consumer to have a monthly bill of USD 60, with an 80-20 split between voice and data services. This means that the user may be expected to spend an additional USD 16. We assume a USD 0.04 subsidy for all voice calls made from the CPE (micro-cell). If the user makes an average of 50 minutes of voice calls from outside the homezone, but still within the same macrocell, this would amount to a saving of about USD 2 per month for the subscriber. For data services, we assume a typical heavy duty application such as video streaming (for e.g., watching YouTube on your cell phone). The business case is established by assuming a large suburban precinct (with 50000 subscribers), where data services are provided through the traditional macro network and a supplementary “micro-cell” network working in tandem.

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We assume that approximately 30% of all video services are accessed indoors, with a typical throughput of 2 Mbps. Peak load capacity of the network is experienced 20% of the time in any given day, and the acceptable success rate to establish a data connection is 99%. Mobile phone penetration in the community is assumed to be 90%, with the service provider having a 20% market share. The variables in the model used in calculating the cost savings in migrating to a FMC solution are:

1. Average video streaming usage per user per moth = 1 session 2. Video streaming bit rate = 128 kbps 3. Average duration of data session = 180 seconds 4. User penetration of “micro-cell” based FMC solutions = 20% 5. Average number of users per “micro-cell” based FMC node = 2 6. Price of each CPE = USD 200

This translates to the migration of (0.3 x 0.2) = 0.06 (6%) of the video streaming traffic from the traditional macro-cell network to the micro-cell based FMC network. A proportional number of users are removed from the macro network, thereby allowing an equal number of new subscribers (who may or may not use video streaming and other high-throughput services) to be added to the network. Moreover, it also frees up a proportional amount of bandwidth (and power budget, if the access technology used is WCDMA) for use by the macro network. Cell site spec 2+2+2 HSDPA configuration Spectral efficiency of 2 Mbps/cell Busy hour utilization is 80% Busy hour share of daily traffic is 20% Radio CapEx costs (includes TRX, RNC and core network) Cost per sector per carrier USD 23000 Number of sectors per cell 6 CapEx cost USD 138,000 Cost per month (with amortization over 6 years) USD 1916 Cell site costs Macro site acquisition USD 53,000 Macro site construction USD 127,000 Cost per month (with amortization over 15 years) USD 1000 OpEx Power (3 kW at USD 0.10 per kWh) USD 311 Cooling USD 100 Site rental USD 1000 Site maintenance (1 visit per annum) USD 60 OpEx per month USD 1471 Backhaul costs Cost of E1 backhaul line per month USD 865 E1 bandwidth (Gbps) 2 E1 full capacity (MB/month) 615,600 E1 actual max capacity (MB/month) 225,720 Number of E1 lines required (at full capacity) 3 Total backhaul cost per month USD 2600 Total monthly radio cost per cell site USD 4387 Total monthly backhaul cost per cell site USD 2600 Monthly cost of a cell site at full capacity (CapEx + OpEx inc backhaul) USD 6987

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Assuming no FMC deployments, the macro cell utilization is 2 x (1 – 0.3) = 1.4 Mbps (assuming that 30% of the users try to access the network from indoor environments, where the net bandwidth is much lower). Such a system would support 1.4 Mbps/128 kbps = 11 simultaneous video streaming sessions. At a 1% blocking rate, the traffic intensity is 4.46 Erlangs, corresponding to 446 users. Assuming a medium sized suburban town of size 50000, the number of macro sites required are 112. Assuming a population density of 10000 persons/km

2, the cell sites shall have to be

spaced ~ 300 m apart. Assuming 20% FMC deployment, 6% of all data sessions shall be diverted to the micro-cell sites (see above for details).The net throughput from each macro-cell site shall increase proportionately to 1.48 Mbps, which shall be able to support 12 simultaneous data streams. At a 1% blocking rate, this corresponds to 5.49 Erlangs, or 549 users per cell. Thus, each cell is now able to support 103 additional users, and the city would require only 91 (31 less number of) cell sites. Savings: Thus we see that a 20% FMC deployment leads to a saving of 31 macro-cell sites, which translates to a total monthly saving of about USD 217000 (see figures above). Assuming an average of 2 users per FMC CPE deployment, an average city of size 50000 would require (0.2*50000) = 5000 CPEs (at a 20% penetration rate), at a total investment of upto USD 100000 (the subsidized price of installment is assumed to range between USD 15 to 20 per month). This translates to a total CapEx saving of about USD 117000 per month per cell site, or about USD 1.5 million per year per cell site (USD 28 per year per user). Data presented by extrapolating the above model for various usage patterns and data rates show that the FMC investment can manage to break even with 1 3-minute data session per user every 2 days (a total data download of 56 MB per month at 128 kbps) or 1 3-minute data session every 3 days (a total download of 84 MB per month when subscribing to a 384 kbps service). Conclusion Fixed-Mobile Convergence has considerable business potential as well as technical motivation to provide benefits to both end consumers (lower monthly bills, better and seamless coverage within home and office environments) and operators (lower CapEx/OpEx overheads, flexible business models to provide a wider range of VAS). Since the target area for individual end-users is restricted to within their “homezone”, VAS such as LCI and Customized Advertising have become viable business propositions. As reported in this paper, UMA and Femtocells continue to be leading technological alternatives in the FMC stable. We have also observed that while being completing in nature, there exists enough synergies between these two approaches to be able to complement each other in the world-wide FMC market. References:

1. © Kineto Wireless, Inc., The Case for UMA-Enabled Femtocells, August 2006 2. © Kineto Wireless, Inc., UMA: The 3GPP Standard for Femtocell-to-Core Network Connectivity, August 2007 3. © Motorola, Inc., Femtocells – Opportunities and Challenges, Chand Gundecha, May 2007 4. 3GPP TS 43.318: Generic Access to A/Gb Interface 5. http://www.warsawvoice.pl/view/15974 6. http://www.engadget.com/2007/06/27/t-mobile-goes-national-with-hotspot-home-wifi-calling 7. http://www.dslreports.com/shownews/Cincinnati-Bell-Launches-UMA-84979 8. http://saunalahti.fi/tiedote/tiedote.php?index=2683 9. http://vowlan.wifinetnews.com/archives/2006/09/oranges_uma_lau.html 10. http://www.mobilemonday.net/news/teliasonera-to-launch-first-commercial-uma-service-in-denmark 11. http://www.telecomsitaly.com/2006/06/telecom_italia_ready_to_launch.html 12. http://www.btplc.com/news/articles/showarticle.cfm?articleid=80dd2a15-f1cb-48da-8160-4a67a7d35ec6 13. © Current Analysis, Inc., Femto Fray: Today’s Femtocell Vendors, Peter Jarich, December 2006 14. The Naked Femtocell, Anon 15. © Nikkei Business Publications, Inc., Femtocells to Expand Mobile Phone Coverage, Chikashi Horikiri 16. © Current Analysis, Inc., CTIA Wireless 2007: Femtocell Roundup, Peter Jarich 17. © Sonus Networks, Fixed Mobile Convergence: The Rationale, Characteristics and Technical Approaches to Integrating Fixed and

Mobile Wireless Networks, 2007 18. © GP Bullhound, Ltd., Femtocells: Hype or Reality, Christian Lagerling and Paul Rutherford, May 2007 19. © ip.access, Inc., The Case for Femtocells, Dr. Andy Tiller, May 2007 20. Wireless Communications, Theodore Rappaport 21. Modern Wireless Communications, Simon Haykin and Michael Moher