From CDMA to LTE via eHRPD

Jonathan A. Morgan
12 Nov 2008

In ten short years people have significantly changed the way they communicate, stay informed and seek entertainment. With access to breaking-news, video updates and instant global communications, we have become the Internet generation. And now it's going mobile. The mobile Internet of course relies on wireless broadband network deployments, which have also come a long way-from 2G to 3G, and now from 3G to Long Term Evolution (LTE).

For the last 20 years, there have been primarily two standards-based technologies for mobile communications - 3GPP-based technologies, or GSM/UMTS technologies, and 3GPP2-based technologies, or CDMA technologies. 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.

Migration options

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.

First is the overlay approach, a complete LTE network is deployed as a second network to the existing HRPD network.  However, this will be very expensive and a subscriber roaming from the HRPD network to the LTE network will experience a loss of continuity for the IP session.

Second, an 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. This is also very expensive and still lacks the IP session continuity between an HRPD network and the UMTS network.

Third is eHRPD (evolved HRPD), a method that allows the mobile operator to upgrade their existing HRPD packet core network using elements of the SAE/EPC architecture. 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.

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.

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