ase 13, LTE got the new marketing name of LTE-Advanced Pro (LTE-A Pro). Figure 10-6 provides a quick view of some of the major use cases added by LTE, LTE-A, and LTE-A Pro. 3GPP R8 3GPP R9 3GPP R10 3GPP R11 3GPP R12 3GPP R13 3GPP R14 3GPP R15 VoLte Frequency OFDMA with flexible Orthogonal subcarriers Voice over LTE bandwidth assignment in OFDM Advanced MIMO loT & mMTC elCIC in HetNet Positioning Enhanced positioning algorithm Relaying ((c))) Traffic safety and control Multi-antenna support eMBMS MBSFN Carrier Aggregation Dual Connectivity FDD TDD Ultra-low latency communication Figure 10-6 LTE and Its Evolution This section focuses on covering some of these techniques and use cases in context to explain the change in time synchronization requirements for LTE and LTE-A. However, covering every use case and detail of these techniques and its implementation is beyond the scope of this book. One of the key differences between 3G and 4G LTE is the adoption of orthogonal frequency divi- sion multiplexing (OFDM) as the modulation technology for LTE. As Figure 10-7 illustrates, OFDM divides each channel into multiple narrower sub-carriers. The spacing is chosen in such a way that sub-carriers are orthogonal to each other, which means the crosstalk is eliminated and inter-carrier guard bands are not required. Guard band Frequency a) Frequency-division multiplexing Frequency b) Orthogonal sub-carriers in OFDM Figure 10-7 FDM Versus OFDM For downlink, LTE deploys orthogonal frequency division multiple access (OFDMA), which is a multiuser variant of OFDM. As shown in Figure 10-8, a subset of sub-carriers is assigned to differ- ent users SO that multiple users can transfer data simultaneously. Frequency User 5 User 2 Time slot User 3 User 1 a) OFDMA with flexible bandwidth assignment X [2] (((p))) Frequency X [3] Each sub-carrier in OFDM Transform to time domain Transmit signal Each sub-carrier is decoded separately b) OFDMA-based downlink transmission Figure 10-8 OFDMA as a Multiuser Access Scheme for Downlink For uplink, mo