 codebook-based beamforming for both links. There are up to four multi-user MIMO users in the downlink and up to two multi-user MIMO users in the uplink. A common confusion arises concerning the terminologies used for mobile and base stations compliant with different versions of the IEEE 802.16 standard and mobile WiMAX system profile. The IEEE 802.16-2009 standard specifies a large number of optional features and parameters that may define various mobile station and base station configurations. One of the possible implementation variants was selected and specified by the WiMAX Forum as Release 1.0 of the mobile WiMAX system profile. The latter configuration was chosen by the IEEE 802.16m as the reference for back- ward compatibility. Consequently, when referring to a mobile station and base station in different amendments of the IEEE 802.16 standard, as well as mobile WiMAX profiles, one must make sure that a consistent reference is made, and that backward compatibility and interoperability can be main- tained. Unlike the IEEE 802.16m specification that refers to the new IEEE 802.16 entities as "advanced mobile station," "advanced base station," and "advanced relay station" to differentiate them from their counterparts in the IEEE 802.16-2009 and IEEE 802.16j-2009 standards specifications, we refer to these entities as mobile station, base station, and relay station, assuming that the reference system is compliant with Release 1.0 of the mobile WiMAX system profile and that the extended functions and protocols corresponding to IEEE 802.16m can be distinguished from their legacy counterparts by the reader. Similar to the IEEE, the 3GPP initiated a project on the long-term evolution of UMTS radio interface in late 2004 to maintain 3GPP's competitive edge over other cellular technologies. The Table 2 Comparison of the Throughput of the Legacy Mobile WiMAX and IEEE 802.16m Systems Downlink Spectral Efficiency Uplink Spectral Efficiency (bits/s/Hz/cell) (bits/s/Hz/cell) IMT-Advanced IMT-Advanced IMT-Advanced IMT-Adva