Integrating Femtocells with Existing Wireless Infrastructure


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Integrating Femtocells with Existing Wireless Infrastructure

  1. 1. Integrating Femtocells with Existing Wireless Infrastructure By Manish Singh, Vice President of Product Line Management, Continuous Computing Unlike UMA, femtocells operate in strictly regulated licensed spectrum sparking the challenge of radio frequency (RF) interference with the existing macro-cellular network. How can interference be minimized? Wikipedia describes “femto” as an SI prefix in the International System of Units denoting a factor of 10-15, or one quadrillionth. In sharp contrast to a macro-cell, a femtocell is a very small wireless base station residing in a consumer’s home. Femtocells are designed to provide excellent coverage in indoor environments and to work seamlessly with any existing 3G handset, thereby eliminating the need for power-hungry dual-mode handsets. Existing Internet Protocol (IP) broadband links (such as DSL or cable) are leveraged to backhaul the mobile voice, video, SMS, and data traffic from the home and integrate with an existing 3G Wireless Core Network. Today’s Wireless Network Wireless network build-outs are created by strategically placing hundreds of wireless base stations (Node Bs) across a city and along freeways. These macro-cells provide wireless coverage enabling seamless mobility in the wireless networks. Carriers also strategically place micro-cells and pico-cells in dense urban population areas like Multi-Dwelling Units (MDUs) and airports to address both coverage and capacity challenges. Unlike femtocells, which leverage existing IP broadband links for backhaul, the macro-cells, micro-cells, and pico-cells are all connected to Radio Network Controllers (RNCs) over dedicated ATM links such as fractional E1/T1. Figure 1. Simplified 3G Network Architecture RNCs aggregate the traffic from Node Bs and deliver the aggregated traffic to the mobile core network. RNCs deliver Circuit Switched (CS) traffic – mostly voice – to the Mobile Switching Center (MSC) over the Iu-CS interface, while the Packet Switched (PS) traffic – mostly data – is sent to the Serving GPRS Support Node (SGSN) over the Iu-PS interface. Another key element of the core is the HLR/HSS, which holds customer provisioning data including subscriber profiles and location information for routing incoming calls.
  2. 2. 2 Femtocell Architecture Options Femtocells extend carriers’ wireless networks directly into the homes. With this extension comes the challenge of integrating potentially millions of wireless base stations with the existing mobile infrastructure. There are many different ways of achieving this integration, and in the December 2007 Femto Forum meeting in Dallas, some 13 different network architectures were proposed. For simplicity and ease of understanding we broadly classify these 13 architectures into the following three categories: UMTS-centric architectures, UMA-based architectures, and SIP/IMS-based architectures. Figure 2. Different Femtocell Architectures UMTS-Centric Architectures In principle, Universal Mobile Telecommunications System (UMTS)-centric architectures leverage the existing UMTS core and integrate femtocells via an Iu-concentrator. In this case, RNC and Node B functions are built into the femtocell and the femtocell provides the standard 3G Iu interface over IP. An Iu concentrator aggregates the traffic from tens of thousands of femtocells and delivers the aggregated traffic to the MSC over an Iu-CS interface and to the SGSN over an Iu-PS interface. The Iu concentrator mirrors the Core Network (CN) functions toward femtocells and the RNC functions toward the CN, thereby making the integration seamless and not requiring any changes in the CN infrastructure. Because traffic from femtocells can potentially traverse public IP networks, for security reasons IPSec is used to encrypt the traffic. An Iu concentrator or a separate Security Gateway terminates the IPSec tunnels from the femtocells.
  3. 3. 3 Figure 3. Tunneled Iu and corresponding Femtocell Signaling Interfaces The UMTS-centric architecture essentially leverages the CN handoff functions as the mobile call is anchored at the MSC and/or SGSN and the handoff is supported by these network elements. The drawback of these architectures is that they do not offload the CN, as all the traffic from femtocells goes via the CN. Because each femtocell could potentially support 7.2Mbps HSDPA traffic, as volume rollouts start the aggregate mobile traffic could quickly multiply – potentially leading to the need for a CN infrastructure upgrade. The UMTS- centric architecture is the easiest and simplest way of integrating the femtocells with the existing mobile CN and enables carriers to leverage their existing CN assets. Nokia Siemens Networks in July 2007 announced 3G Femto Gateway which is essentially based on this architecture. UMA-Based Architectures Fixed-Mobile Convergence (FMC) led to the development of the Unlicensed Mobile Access (UMA) architecture, which was later standardized by 3GPP for Generic Access Networks (GAN). To integrate the mobile traffic via a Generic IP network (such as 802.11) to the mobile core network, a Generic Access Network Controller (GANC) was defined. The GANC provides the Up interface toward mobile handsets and a standard A/Gb interface toward the mobile core network. In principle, UMA-based architectures extend the 3GPP GAN architecture to integrate femtocells in a similar way as UMA mobile handsets. The GANC, also commonly known as a UMA Network Controller (UNC), aggregates the traffic from femtocells and delivers it to the existing mobile core over a standard Iu-interface. The UNC also provides support for security functions and terminates IPSec tunnels from femtocells. Figure 4. UMA Based Architecture and corresponding Femtocell Signaling Interfaces
  4. 4. 4 Similar to UMTS-centric architectures, UMA-based architectures leverage the mobile CN to anchor mobile calls and support handoffs. Traffic generated from femtocells traverses through the CN rather than being offloaded. UMA-based architectures bring strategic advantage to operators who already have commercialized UMA services and can leverage their existing UNC assets to quickly and efficiently integrate femtocells into their CN. For example, T-Mobile currently offers HotSpot@Home services in the United States and recently announced femtocell trials in the UK, Germany, and the Netherlands . SIP / IMS-Based Architectures Session Initiation Protocol/IP Multimedia Subsystem-based architectures integrate femtocells directly with the IP Multimedia Subsystem (IMS) core. Alternative architectures under this category include softswitch-based implementations where the femtocell is integrated to a softswitch via a SIP interface. Integrating femtocells directly with the IMS core offers distinct advantages. First, the mobile core network is completely offloaded, and as femtocell traffic scales the operator avoids the need for CN infrastructure upgrades. Second, traffic latency challenges are mitigated as the number of hops a packet must traverse is greatly reduced. Third, this architecture is forward-looking as it simultaneously solves the near-term 3G coverage challenge while also providing long-term options for delivering rich innovative services via an IMS core that leverages femtocells. Figure 5. SIP/IMS-Based Architecture and corresponding Femtocell Signaling Interfaces The biggest challenge that needs to be addressed with SIP/IMS-based architectures is supporting seamless handoff between the femtocell and the macro-cellular network. As the mobile CN is completely bypassed, there is the need for a Voice-Call-Continuity (VCC)-like application to support handoff functions. This architecture has the strongest appeal to carriers who have a strong IMS core and have both fixed and wireless assets. Carriers who are deploying FTTx (fiber to the home/curb/etc.) for delivering IPTV services can leverage this architecture to deliver “triple-screen play” services by efficiently place-shifting content across the TV, PC, and mobile handset via the femtocell. Conclusions on Architecture Options As discussed above, it is clear that there are many different ways of integrating millions of femtocells with existing wireless core network infrastructure. Each of these architectures provides distinct advantages and has strong appeal to a subset of operators – but there is no single choice of architecture that is best for all. It should also be noted that the underlying femtocell hardware does not necessarily change with each variation of network architecture; what essentially changes is the software and the signaling interfaces that the femtocell
  5. 5. 5 must support. As a result, ensuring remote software upgradeability within femtocells is critical to ensuring that the widest range of near-term and long-term opportunities is addressable. Too many architecture choices can also lead to challenges pertaining to interoperability and market fragmentation. For femtocells to be widely deployed, it is essential to achieve low per-unit price and simultaneously reduce the CAPEX and OPEX savings for carriers. Architecture harmonization and phased evolution is essential for the industry to achieve the economies of scale which will drive overall market success, and the good news is that the Femto Forum has recently decided to make this a priority. Additional Challenges While this article primarily focused on the core network integration options and challenges, it must be noted that there are other significant integration challenges that need to be resolved. Unlike UMA, femtocells operate in strictly regulated licensed spectrum. While this provides distinct advantages to deliver high quality voice, it also brings the challenge of radio frequency (RF) interference with the existing macro-cellular network. Femto-to-macro signals, macro-to-femto signals, and femto-to-femto signals all can potentially interfere, creating “dead-spots” in the network. Solutions to these challenges will require dynamic scanning of the RF environment and rapidly configuring femtocells with appropriate operating frequencies and code to minimize interference. Quality of service (QoS) and traffic prioritization are other issues to consider as well. While for data the bottleneck might still be the radio as the maximum HSDPA downlink rate supported is 7.2Mbps, carrying real- time voice on a shared IP link is altogether a different matter. As real-time voice competes with Internet data and peer-to-peer (P2P) traffic on a shared IP link to a consumer’s home, for QoS it is essential that real-time traffic be prioritized right from the home network all the way into the ISP’s network. Because mobile operators do not necessarily control the ISP networks, service level agreements and business arrangements need to be addressed. Femtocells are thus a disruptive force that could even lead to further industry consolidation, thereby accelerating FMC. As with any new disruptive technology, challenges abound for femtocells – such as customer premise equipment (CPE) device provisioning and management; potential consumer concerns around health, security, and access control; regulatory hurdles; and so on. (Note: Many of these challenges and their potential solutions will be addressed in a future installment of this series). However, none of these challenges is insurmountable. With the amount of investment that is pouring into the femtocell market, successfully addressing these challenges is just a matter of when – not if. About Continuous Computing Continuous Computing® is the global source of integrated platform solutions that enable network equipment providers to overcome the mobile broadband capacity challenge quickly and cost effectively. Leveraging more than 20 years of telecom innovation, the company empowers customers to increase Return on Investment (ROI) by focusing internal resources on differentiation for 3G, Long Term Evolution (LTE), Femtocell and Deep Packet Inspection (DPI) applications. Expertise and responsiveness set the company apart: only Continuous Computing combines best-in-class AdvancedTCA (ATCA) platforms with world-famous Trillium® protocol software to create highly-optimized, field-proven wireless and packet processing products. Continuous Computing is an active member of 3GPP, CP-TA, eNsemble Multi-Core Alliance, ETSI, Femto Forum, Intel Embedded Alliance, PICMG and the SCOPE Alliance. Continuous Computing, the Continuous Computing logo and Trillium are trademarks or registered trademarks of Continuous Computing Corporation. Other names and brands may be claimed as the property of others.