h large 15 But TDCDFDSDMA would be a bit of a mouthful. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Worldwide CDMA Standards Worldwide CDMA Standards downloads of data. It is easier to find narrower radio spectrum allocations of 1.6 MHz than the broadband 5-MHz carriers require, especially in their preferred FDD mode, where they require 10 MHz. The seven time slots of TD-SCDMA allow the system to vary its forward to reverse ratio from 6-to-1 to 1-to-6 dynamically as demands change. 10.4.4 TD-SCDMA channel coding The TD-SCDMA channel is both time division and code division multiple access and di- vides its resources as required by its user community. With time slots of 10 ms, it has 100 opportunities per second to adapt to its service requirements. The TDMA mode is the dominant mode because it can operate at full rate. The CDMA mode of TD-SCDMA can handle spreading gains of up to 16. Like the TDD mode of W-CDMA, TD-SCDMA uses a midamble in its data bursts for coherent demodulation because a steady pilot is impractical without a steady carrier in each direction. TD-SCDMA user terminals compensate for the distance from their serv- ing base stations by advancing their timing. This eliminates collisions between adja- cent time slots that would occur due to propagation delay at the speed of light. The tim- ing advance allows the system to use very short time gaps between adjacent time slots so that air-interface time is not wasted. 10.4.5 TD-SCDMA space division with smart antennas A central design point of TD-SCDMA is the use of adaptive antenna arrays, also called smart antennas. Since the radio frequency is the same in both forward and reverse di- rections, we can rely on measurement of the reverse signal to determine the phase re- lationships in the forward direction. An array of eight antennas measures each channel 200 times per second an