nfigured carriers. The required feedback for link adaptation and information for closed-loop MIMO operation can be sent through the primary carrier. The HARQ feedback corresponding to the PDUs transmitted over the primary and secondary carriers can be sent via the primary carrier. The HARQ feedback CHAPTER 11 Multi-Carrier Operation related to the PDUs transmitted on the secondary carrier is sent over the secondary carrier, if the secondary carrier configuration permits. The initial ranging for a non-synchronized MS is conducted on a fully-configured carrier. The periodic ranging for a synchronized MS is performed on the primary carrier, but may also be per- formed on a secondary carrier, depending on the secondary carrier configuration. The serving BS transmits the ranging response on the same carrier that received the initial/periodic ranging message. The uplink sounding is conducted on the primary and secondary carriers. The bandwidth request is transmitted only on the primary carrier. Depending on the correlation between different component carriers, separate uplink power control for active carriers is necessary. 11.3 CARRIER AGGREGATION AND SPECTRAL MASK CONSIDERATIONS Following the introduction of carrier aggregation and support of wider bandwidths, we should also consider the effects of bandwidth extension and the use of guard sub-carriers on the design of the spectral mask. The studies suggest that the spectral mask of the aggregated spectrum is different from that of the non-aggregated spectrum [32]. Figure 11-7 illustrates the difference between the two cases. Note that the spectrum emission mask is scaled in proportion with the channel bandwidth and the relationship between the single-carrier and aggregated-carrier spectral masks. An example spectral mask specification is shown in Figure 11-7. The contiguous/non-contiguous carrier aggregation is a new concept for the BS and the MS RF specifications, and requires the appropriate extension of transmission (e.g., Adjacent Channel Leakage Power Ratio [ACLR