 adaptation is performed at the same rate as precoding adaptation. It simply means that the switch- ing across different ranks does not require higher layer signaling. Dynamic switching can be seen as the adaptation of a certain multiantenna configu- ration to the current channel conditions. Semistatic switching implies higher layer signaling and may occur across different modes of operation (e.g., between open-loop and closed-loop operations). 3.4 Physical Layer Procedures In the previous sections, a general overview of the LTE physical layer has been pre- sented, according to the parameters and procedures defined by the 3GPP (3GPP 36.211; 3GPP 36.212; 3GPP 36.213). However, from the receiver point of view, © 2009 by Taylor & Francis Group, LLC Long Term Evolution: 3GPP LTE Radio and Cellular Technology some mechanisms are not explicitly specified in the standard because their imple- mentation is vendor specific. This section is focused on those procedures that allow the reception and decoding of the frame under a certain BER level: synchroniza- tion and channel estimation. Furthermore, this section also presents the random access (RA) structure and procedure, which is necessary for initial synchronization in the uplink prior to UE data transmission. 3.4.1 Synchronization Although the use of OFDM in current and future communication systems is widely extended due to its appealing properties, from a practical point of view, some draw- backs exist. One of the major issues is related to the strict requisites on time and frequency synchronization; this section is thus dedicated to analyzing these aspects on OFDM-based systems. There are two main points related to synchronization in OFDM-based systems, as depicted in Figure 3.28. The first is symbol synchronization or time synchroniza- tion, which consists of how the receiver is able to determine the exact instant at which the OFDM symbol starts-that is, the correct position of the DFT window. The second is frequency synchronization, which tries to eliminate the carri