Feature Article

August 19, 2008

Evolution Options from CDMA to LTE: The Benefits of eHRPD

By TMCnet Special Guest
Jonathan Morgan, Senior Director of Product Marketing, Starent Networks

The past decade has seen great changes in how people communicate, stay informed and are entertained. We’ve become the Internet generation, with access to video of breaking news, instant updates on global markets, the ability to easily send business documents, and communicate instantly with our friends both near and far. People expect this experience both at their desks and while on the go, giving rise to the mobile Internet. Central to the mobile Internet has been mobile operator deployments of broadband networks. Such networks are continually evolving, from 2G to 3G, and now with preparation for 4G networks, primarily Long Term Evolution (LTE).
 
For the past two decades, there have been primarily two standards-based technologies for mobile communications: 3GPP-based technologies (aka GSM/UMTS) and 3GPP2 (CDMA). Designated as a 4G, or fourth generation mobile specification, by 3GPP, LTE is designed to provide multi-megabit data rates, more efficient use of the radio network, latency reduction and improved mobility.
 
This combination aims to enhance the user’s interaction with the network and further drive demand for mobile multimedia services. Furthermore, the LTE evolution calls for a transition to a ‘flat,’ all-IP core network with a simplified architecture and open interfaces. This requirement is defined by the System Architecture Evolution (SAE) — also known as Evolved Packet Core (EPC) — the 3GPP specification for changes to the packet core network architecture.
 
While GSM/UMTS-based operators have a natural evolution to LTE, many CDMA-based mobile operators also have decided to evolve to the LTE specification. Changes in mobile communications have traditionally been evolutionary, and the deployment of LTE will be the same. The transition for CDMA operators from High Rate Packet Data (HRPD) to LTE will be over a period of several years, as is the case still with the transition from 1xRTT to HRPD. As a result, mobile operators must look for a migration path that will enhance their existing HRPD networks, while addressing LTE deployment requirements and will not require a ‘forklift’ upgrade.
 
The choice of migration path depends on many factors including radio access strategy, network resource strategy, services enabled, timing and cost. A key goal of LTE is to enhance service provisioning while simplifying interworking with non-3GPP mobile networks. This is essential for CDMA operators that have chosen to migrate to LTE. The following are three basic migration paths to LTE currently available for CDMA operators.
 
Overlay  — In this approach, a complete LTE network is deployed as a second network to the existing HRPD network.  However, this will be very expensive, and with an overlay, a subscriber roaming from the HRPD network to the LTE network will experience a loss of continuity for the IP session.
 
UMTS migration — A mobile operator can optionally migrate first to UMTS before going to LTE, requiring deployment of a new network and conversion of all their subscribers to UMTS. Again, this is very expensive and still lacks the IP session continuity between an HRPD network and the UMTS network.
 
eHRPD  — Evolved HRPD is a method that allows the mobile operator to upgrade their existing HRPD packet core network using elements of the SAE/EPC architecture. Additionally, eHRPD is a more evolutionary path to LTE while also allowing for seamless service mobility  — including seamless hand-offs  — between the eHRPD and LTE networks.
 
Starent Networks Diagram: CMDA upgrade paths to LTE
CDMA upgrade paths to LTE
 
 
A focus of 3GPP standardization has been the evolution and interworking among 3G UMTS networks, to 4G LTE networks, to non-3GPP access networks all using a single packet core called Enhanced Packet Core or EPC. Since several CDMA operators have committed to migrating to LTE, evolution becomes a key challenge. As currently defined, standard HRPD and LTE access networks will be complete overlay networks without seamless service mobility for customers, or sharing of common functions such as policy, charging and authentication.
 
Today’s mobile subscribers are used to a seamless service experience, which would require market wide deployment of LTE networks, or implement a solution for the LTE and HRPD networks to work together. 3GPP2 is defining such an inter-working method by evolving existing HRPD networks to be compatible with 3GPP EPC through a step called eHRPD. eHRPD is being standardized as a method of interworking multiple access networks (eHRPD, E-UTRAN) under a single packet switched core network, SAE/EPC.
 
By migrating existing HRPD networks to eHRPD as the first step towards SAE and then the introduction of LTE, the CDMA operators are able to provide a phased migration. The CDMA operators are able to upgrade the packet core network to EPC elements without an overlay, which instead they can phase in over time. The mobile operator gains features of SAE plus the benefit of being able to interwork the eHRPD network with the LTE network with a single packet core.
 
The introduction of eHRPD requires the introduction of an HRPD Serving Gateway (HSGW). The HSGW ensures converged mobility management between HRPD and LTE networks. The HSGW provides interworking between the HRPD access node and the Packet Data Network Gateway (PGW), a key element of the SAE/EPC network. In some network instances, the existing PDSN can be integrated with or upgraded to the HSGW while the existing HA can be integrated with or upgraded to the PGW (or provided as a separate node).
 
Starent Networks Diagram: Benefits of eHRPD
As CDMA operators evolve to LTE they will benefit from solutions that can evolve
the existing HRPD core to SAE using eHRPD.
 
After the migration to eHRPD is complete, the remaining step is to deploy the LTE RAN (E-UTRAN) and remaining EPC elements. Since the PGW is deployed during the upgrade to eHRPD, the final step to LTE in the core network is to deploy the Mobility Management Entity (MME) and upgrade the existing HSGW to a SGW. Since the eHRPD network works seamlessly with the new LTE radio elements, the LTE radio deployments can be phased in over time in the most economically appropriate manner.

Starent Networks Diagram: Integrated multi-access network core functionality 
Core functionality in an integrated multi-access network
 
One of the main advantages of eHRPD is the ability to provide seamless service mobility between HRPD and LTE access networks with a single mobility management protocol. Moreover, with eHRPD, the operator can leverage the benefit of optimized handover – no dropped sessions and reduced handover latency — between LTE and eHRPD. The benefit of seamless and optimized mobility for data is highly dependant on the mobile operator business model for adding new services. As new applications emerge, the requirement for seamless service mobility becomes greater. Since SAE is an all-IP network infrastructure, the network will quickly move to mobile VoIP for voice.  Moreover, with the introduction of eHRPD, the operator can leverage the benefit of optimized intra eHRPD handover when the user crosses the HSGW boundaries. This capability does not exist in current HRPD systems.
 
In addition, presence and location services are emerging in the market place. Most of these applications require seamless service mobility to remain viable with the customers. Examples of services include: messaging, push-to-talk, video, gaming and enterprise security applications such as Virtual Private Networks (VPNs). For example, there is nothing worse than a location-based service that is disrupted due to a handover that either takes too long to complete or results in a change in IP address – you may miss the right turn.
 
Starent Networks Diagram: Seamless migration applications
Applications for seamless migration

eHRPD provides a promising phased migration to LTE allowing reuse of elements within the existing 3G packet core network, interworking between the networks and a seamless service experience for mobile subscribers. Running a single packet core infrastructure will also simplify network deployment and management and ultimately improve mobile operator network performance and profitability.

Jonathan Morgan is senior director of product marketing at Starent Networks.
 

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