Virtual Soft Hand-off in IP-Centric Wireless CDMA Networks

We present a novel method, Virtual Soft Hand-off, for wireless IP-centric CDMA networks whose routers, hosts and mobile stations have small group multicast (SGM) capability. The underlying ideas of the virtual soft hand-off method are that it • views soft hand-off support as setting up an explicit synchronous multicast session among a small group of participants with dynamic group membership, and • recovers multiple packet flows from the multiple received signals concurrently, and combines them to create a single packet flow at higher layers. The virtual soft hand-off approach uses the Session Initiation Protocol (SIP) for setting up necessary multicast sessions. 1. Purpose and Scope The growing demand for high-speed wireless access to the Internet is the driving force behind the current trend towards the design of IP-centric wireless networks comprising a large number of Pico-cells, each covered by an autonomous IP router base station. The CDMA spread spectrum communication technique seems to be more suitable for design of IP-centric wireless networks because it can achieve considerably higher capacity than other multiple access techniques. A key feature of CDMA that improves its capacity and performance is its soft hand-off capability that allows a mobile station to communicate with multiple base stations concurrently. However, accurate realization and use of soft hand-off in IP-centric wireless environments is too complex. This complexity has prompted some debate on not using the CDMA soft hand-off in IPcentric wireless networks [1]. Without any discussion on the merits of these suggestions, this paper presents a novel method, virtual soft hand-off, for IP-centric Wireless CDMA environments whose routers, hosts and mobile stations are SGM (small group multicast) capable [2] [3]. The underpinnings of virtual soft hand-off are that it • views soft hand-off support as setting up an explicit synchronous multicast session among a small group of participants with dynamic group membership, • recovers multiple IP packet flows from the multiple received signals, • synchronizes contents of these packets using a delay equalization method, and • combines them to synthesize a single packet flow for higher layers. This paper is organized as follows: Section 2 provides an overview of the soft hand-off process and explains how current CDMA networks support it. In section 3, we describe the architecture of an IP-centric wireless CDMA network and highlight issues that hinder proper operation of current soft hand-off mechanisms in such an environment. Section 4 includes the description of proposed virtual soft hand-off mechanism. Finally, Section 5 concludes the paper with a summary of the scheme and a list of open issues 2. An Overview of Soft Hand-off Figure 1 depicts the basic architecture of today’s wireless CDMA networks, and illustrates the soft hand-off process schematically. Besides mobile stations/terminals, a wireless access network also comprises a radio access network (RAN), and an edge router and controller (ERC). Before delving into the soft hand-off process, let us briefly describe