ong University). The FuTURE 4G TDD system adopts advanced link transmission technologies such as MIMO and OFDM, which provide the peak data rate of 122 Mb/s on the 3.45-GHz carrier frequency, with the bandwidth of 17.27 MHz and the spectrum efficiency of about 7 b/Hz. It supports IPv6, simultaneous multichannel VOD (video on demand), HDTV (high- definition television), high-speed data download (file transfer protocol, FTP), Internet browsing, instant messaging, and VoIP (voice over Internet protocol) with data rates up to 1,000 times higher and lower power consumption than before. 2.2.1 Main Features of FuTURE TDD System The FuTURE TDD system employs time division duplex mode, where the uplink and downlink work on the same frequency but at different time slots, and then realizes bidirectional communication. Compared with the FDD system, the TDD system will bring advantages in many aspects: More flexible spectrum usage. The TDD system does not require paired frequency bands, SO it has better flexibility without the constraint of spec- trum usage that FDD systems have. Better support for unsymmetrical services. The TDD system is more suit- able for unsymmetrical service by allocating the time slots for uplink and downlink adaptively according to the traffic. Reciprocity between the uplink and downlink. In the TDD system, the uplink and downlink work on the same frequency; hence, the electromagnetic wave propagation characteristics of uplink and downlink are almost the same, SO as the channel parameters. This reciprocity makes it possible that the channel estimation of uplink can be directly used by the downlink and vice versa, and it brings many advantages to TDD systems in the aspects of power control, application of transmission preprocessing, smart antennas, and transmission diversity. The system offers better support for the technologies of multihop, relay, and self-organization. © 2009 by Taylor & Francis Group, LLC Evolution from TD-SCDMA to FuTURE There are also some challenges to the TDD system. For example