aration an integer multiple of sub-carrier spacing, backward compatibility and interoperability with legacy system is compromised, but the guard-band sub-carriers can be used for data transmission. The guard sub-carrier can then be utilized for data transmission, if the information of the available guard sub-carriers suitable for data transmission is communicated to multi-carrier-capable mobile stations. This information includes the number of available sub-carriers in the upper-side and the lower-side relative to the DC sub-carrier. As a result, when two physically contiguous carriers are used to deploy the IEEE 802.16m system in a multi-carrier operation, the overlapped sub-carriers should be aligned in the frequency-domain. In order to align the overlapped sub-carriers of the OFDM symbols transmitted over the adjacent carriers, a permanent frequency offset Afoffset is applied to one of the carriers. The basic principle is illustrated in Figure 11-3 where the overlapping of guard sub-carriers before and after application of frequency offset Joffset is shown. A carrier CHAPTER 11 Multi-Carrier Operation fc2 #n Additional Sub-carriers Guard Band from Guard Bands ICI to FA1 ICI to FA2 FIGURE 11-2 An illustration of adjacent channel ICI due to use of guard bands Iseparation Overlapped Guard Sub-carriers before Applying Offset Afseparation + Afoffset FA2 + Afoffset Overlapped Guard Sub-carriers after Applying Offset FIGURE 11-3 An illustration of a sub-carrier alignment scheme [2] group is a group of contiguous carriers whose sub-carriers are aligned with a sub-carrier spacing Af (see Table 9-2). When one component carrier is used for co-deployment of the IEEE 802.16m and the legacy system, and the adjacent component carrier is exclusively dedicated to the IEEE 802.16m system, the overlapped sub-carriers should also be aligned. The center frequency of the mixed-mode component carrier will be located exactly on the channel raster grid. Different base stations may have different multi-carrier configurations according 