The guard time between uplink and downlink is 106 us, and it is possible to support a cellular radius as large as 15.9 km. If a multiantenna technique is adopted, it is possible to enlarge the cellular radius. There are two types of time slots: short and long. The unequal length for downlink and uplink not only can decrease the cost for guard time, but also can guarantee the flexibility of resource allocation. The alterable switch point can flexibly support the service requirements. The change of data ratio after changing switching point can be seen clearly in Figure 2.3. Regarding the asymmetric tendency of future services, the time slot ratio between the uplink and the downlink is about 1:4. This is a default mode. 2.2.2.2.2 Block Structure for Link Layer In Figure 2.4, the block structure for uplink design is plotted. Here, MIMO, OFDM, and LA are adopted for FuTURE TDD radio transmission, in which P/S is the parallel-to-serial transform module and S/P is the converse operation. CP represents the adding of cyclic prefix (CP). © 2009 by Taylor & Francis Group, LLC Long Term Evolution: 3GPP LTE Radio and Cellular Technology Preamble Frame Adaptive modulation and coding User k Coding Interleaver Space-time processing Conv or Frame Channel Frame sync Preamble AMC signalling frame Space-time Decoder interleaver processing Frame sync Channel estimation frame Figure 2.4 The block structure of FuTURE TDD uplink. 2.2.2.2.3 Multiple Access Techniques Multicarrier transmission, such as OFDM, should be exploited in the FuTURE TDD system to support high data rate transmission. A group of subcarriers is employed as the basic resource unit in an OFDMA system. A subcarrier group is not necessarily composed of adjacent subcarriers; the interleaved subcarriers in fre- quency domain are also supported, as shown in Figure 2.5. In addition, OFDMA can be combined with time division multiple access (TDMA) and spatial division medium access (SDMA) in order to provide flexible multiple access and resource allocation, as shown in Figure