Multi-Carrier and Spread Spectrum Systems: From OFDM and MC ...

WiMAX 267Frame n−1 Frame n Frame n+1DL-Sub-frameDL-PreambleFCHDL-burst# 1...DL-burst# nAAS-PreambleAAS-FCH AASDL-burst # 1..AASDL-burst # nBasicAAS ExtensionFigure 5-41Modified frame structure to support AASwill be done by sending corresponding signaling from the base station via a dedicatedmanagement channel.When comparing AAC with STC or the spatial diversity scheme, the main drawbackof the AAS implementation is its complexity, which resides mainly in the RF part, andis usually not an easy task. Furthermore, in the case of FDD the problem especially inthe mobile environment might be the adaptation of the weighting factor for the antennaadjustment, where the real instantaneous channel state information (CSI) is usually delayedat the transmission side (due to the closed loop, i.e. transmission of the measured CSIfrom each terminal station to the base station).– Adaptive combination of all transmit diversity schemes. In order to use the same hardwarecomponents (e.g. IFFT, D/A, RF, and antennas), to simplify the implementationof AAS and to adapt dynamically the transmit diversity on-the-fly depending on thechannel condition, a functional combination of all these transmit diversity schemesis depicted in Figure 5-42. Note that with up to four transmit antennas and withthe corresponding pre-coding scheme (STC, spatial multiplexing and beamformingweighting matrix) all these combinations can be realized and dynamically adaptedto the channel condition. In this scheme the AAS is realized in the digital domain;weighting of the antennas is done before the IFFT operation, which is used for eachantenna branch.5.3.6 WiMAX ProfilesThe IEEE 802.16x standard specifications are quite flexible and support a lot ofoptions [23, 24]. To reduce the number of options and make them implementable andtestable, a set of predefined profiles for both the MAC and PHY layer [34] has beenspecified.The implementation of these profiles is quite important, because based on this set ofprofiles the WiMAX certification tests will be done and the WiMAX conformance labelwill be agreed. The basic profiles for IEEE 802.16d are listed in Table 5-29. The favoriteduplex scheme for WiMAX is TDD. The initial release of WiMAX certification profileswill only include TDD. With ongoing releases, FDD profiles will be considered by theWiMAX forum.