Universal Mobile Telecommunication System (UMTS)
Standardization and UMTS evolution

The evolution of 3G networks had been initiated in 1999 by issuing the very first UMTS release known as Release 99. Since then several releases have been already approved as suggested in the following.

## Release 99

This release is based on GSM network, thus UMTS is backward compatible with GSM and interoperation between those two is possible. In comparison to 2G networks, Release 99 brings completely new type of radio access network, known as UTRAN (UMTS Universal Radio Access Networks). The theoretical data bit rates achieved by this release are 2 Mbps in downlink and 384 kbps in uplink.

## Release 4

Release approved in 2001 introduces several major changes to core network and GERAN. The main features are separation of transport bearer and control bearers in the CS (Core Switched) network, introduction of new interfaces in CS network, enabling of low chip rates and mainly, introduction of IMS (IP Multimedia Subsystem).

## Release 5

The main improvement of Release 5 over former releases rests in introduction of HSDPA (High Speed Downlink Packet Access). The HSDPA increases the bit rates in downlink direction up to approximately 14 Mbps.

## Release 6

Similarly as previous release, Release 6 brings significant improvement in data transmission as it specifies HSUPA (High Speed Uplink Packet Access). The HSUPA enables bit rates in uplink direction up to 5.76 Mbps. In addition, significant work has been accomplished regarding IMS subsystem.

## Release 7

In this release, HSPA+, also called as Evolved High Speed Packet Access, has been introduced. It further increases achievable bit rates both in uplink and downlink direction. The improvement is enabled by introduction of more effective modulation (64 QAM) and MIMO (Multiple Input Multiple Output) technique where more antennas at the receiver and the transmitter can be employed. Consequently, maximal theoretical bit rates are up to 42 Mbit/s in downlink and up 11.5 Mbit/s in uplink.