gation, which simply means that any signal subject to multipath propagation will see the signal dispersed over time. When suffering multipath, some signals have a longer path than others and therefore take longer to propagate from transmitter to receiver. This means that the same information is received by the receiver at several different moments of time. These signals arriving over time can obviously interfere with each other, and this is known as inter-symbol interference (ISI). To retain the properties and integrity of the OFDM signal and to prevent ISI and ICI due to time- dispersive channel propagation, a guard interval known as a cyclic prefix (CP) is allocated to each OFDM symbol. To better understand CP, consult the multipath scenario shown in the Figure 10-10. LL LL Delay spread Path 1 ((cp))) Path 1 Path 2 Path 2 Path 3 Path 3 Path 1 Path 2 CP = duplicate initial part of the symbol, with Path 3 the end of the symbol Finally received signal Figure 10-10 ISI and ICI Avoidance with Cyclic Prefix The total delay between the first copy of the data received and the last copy of the data of the same signal is called the delay spread. If the length of the OFDM symbol that carries the useful data is smaller than the delay spread, there is interference between the symbols. But if the OFDM symbol size is increased, then the interference can be reduced. One technique to do this is to copy or duplic- ate the initial part of the symbol (the prefix) with the end of the symbol. Here, the CP length needs to be larger than the delay spread in the environment where the system is intended to operate. In LTE, the synchronization of the BS (known as the evolved node B [eNB] in LTE terminology) and UE is achieved at the physical layer. As shown in Figure 10-11, the LTE radio frame has a length of 10 ms and each frame is divided into ten equal-sized subframes of 1 ms. Each subframe consists of two equal-sized slots of 0.5 ms each. With a 15-kHz sub-carrier spacing, the useful LTE symbol time to transmit data is 66.7 us. Howev