ength is 10 ms. Higher Data Rates Support: Another key enhancement of 3G systems is the support for higher data rates in both downlink and uplink. R-99 supports up to 2 Mbps in both directions in a 5 MHz channel. Handovers: The W-CDMA systems have the capability to handover to GSM/GPRS networks where UMTS coverage is not available. Core Network backward compatibility: The R99 inherits a lot from the GSM model on the core network side. The core network of R99 is an evolution of GSM core network which is based on MAP (Mobile Application Part). In addition to UTRAN, R99 also supports the legacy GSM BSS radio access network. This backward compatibility feature assists the legacy mobiles to operate in 3GPP R99 networks in a seamless fashion. 3.2.2 Key Aspects of Rel-4 Rel-4 is associated with the inception of TD-SCDMA (Time Division Synchronous CDMA) and EDGE (Enhanced Data rates for GSM Evolution) radio technologies. The details of 3GPP Rel-4 can be found in [3]. Support for EDGE: EDGE is a TDM-based technology that provides the evolution for 2G IS-136 and GSM systems. GERAN or GSM/EDGE Radio Access Network is capable of supporting 473.6 kbps with 8-PSK (Phase Shift Keying) modulation scheme. EDGE does not require a new spectrum and it is also known as EGPRS (Enhanced GPRS) since it can coexist with GPRS provided necessary upgrades are implemented. It uses the same network elements, interfaces, protocols and procedures as GPRS with minor differences primarily in the RAN and mobility management. The chip rate of a code is the number of pulses per second (chips per second) at which the code is transmitted (or received). Next Generation Mobile Communications Ecosystem Support for TD-SCDMA: TD-SCDMA or UTRA/UMTS-TDD 1.28 Mcps Low Chip Rate (LCR), is China's home grown 3G technology. The TD-SCDMA radio interface is integrated in 3GPP Rel-4 and is part of IMT-TC (IMT-Time Code) family of air interfaces. TD-SCDMA is based on CDMA, supports 2 Mbps in a 1.6 MHz channel bandwidth and uses UMTS core network. The details on TD-S