h MS in the serving cell is connected to a primary carrier. The system may also designate M > 0 partially-configured carriers, which can only be used as secondary carriers, along with a primary carrier for downlink-only data transmissions. The set of all component carriers used by a BS are called the available carriers. The available carriers may be in different parts of the contiguous spectrum block or in non-contiguous spectrum blocks. A sub-set of the available carriers are designated as assigned carriers, which can be activated for data transmission. A sub-set of assigned carriers are designated as active carriers, which are used for data transmission between the MS and the BS. In addition to the information about the primary carrier, the serving BS can also provide the MS with some configuration information about its available carriers through the primary carrier by the AAI_Global-Config and AAI_MC-ADV MAC control messages. 11.2 SUB-CARRIER ALIGNMENT AND USE OF GUARD SUB-CARRIERS If the operating frequency bands are physically contiguous and the sub-carriers of the adjacent frequency channels are perfectly aligned, the guard sub-carriers between two adjacent frequency bands can be utilized for data transmission. In the current IEEE 802.16-2009 standard [1]. the center frequency of each component carrier is an integer multiple of 250 kHz, also known as a frequency raster, thus the separation of any adjacent carriers will be an integer multiple of 250 kHz (e.g., 10 MHz frequency separation for 10 MHz channel bandwidth). In this case, the sampling frequency is 11.2 MHz and the sub-carrier spacing is 10.94 kHz. Given that 10 MHz is not an integer multiple of the sub- carrier spacing, the use of the guard sub-carriers will cause significant Inter-Carrier Interference (ICI) to the adjacent bands. Figure 11-2 illustrates the ICI problem which could be caused by using sub- carriers in guard bands between the adjacent component carriers. On the other hand, if the frequency raster is modified to make the frequency sep