3G UMTS

3G cellular systems, most notably UMTS, are currently the most widely deployed mobile broadband technology with a huge established presence in terms of operators, customer base, brand, deployed base station sites, and backhaul capacity. Standardized by 3GPP in its Release 5, HSDPA is a tremendous performance upgrade for UMTS packet data, enabling peak data rates up to 14.4 Mbit/s, although the initial limit is 1.8 Mbit/s.

Latency is also reduced, and spectral efficiency is improved as well. These improvements are achieved through improved modulation and coding, and implementing fast scheduling and retransmissions at base station level.

Although most WCDMA/HSDPA deployments are based on FDD where different radio bands are used to separate downlink and uplink transmission, 3GPP specifications also include a TDD version of UMTS where both transmit and receive functions alternate in time on the same radio channel. This can be beneficial for the many asymmetric data applications that consume more bandwidth in the downlink than in the uplink.

A TDD radio interface can dynamically adjust the downlink to uplink ratio accordingly, and thus can balance both forward link and reverse link capacity. Spectral allocation is also more straightforward, as TDD requires only one band instead of two bands and a further guard band in FDD. UMTS TDD is also known as TD-CDMA and has been commercialized by the vendor IP Wireless. (Rysavy 2005)