Summary

Chapter 24 Summary: 4G LTE and 5G NR

Key Points

• 1.

  LTE physical layer established the OFDM/MIMO foundation for modern cellular: 15 kHz SCS, 1 ms TTI, OFDMA downlink, SC-FDMA uplink, and up to 8-layer MIMO. The resource grid of 12 subcarriers $\times$ 14 symbols per RB remains the fundamental scheduling unit in both LTE and NR. CRS-based channel estimation with CQI feedback and HARQ enable adaptive link operation.

• 2.

  5G NR flexible numerology ($\Delta f = 2^\mu \times 15$ kHz, $\mu = 0, \ldots, 4$) is the most consequential architectural change from LTE. Wider SCS at mmWave mitigates phase noise ICI ($\text{SIR} \propto \Delta f$) and suits shorter delay spreads, while shorter slots ($T_{\text{slot}} = 1/2^\mu$ ms) reduce latency. Bandwidth Parts (BWP) enable dynamic UE bandwidth adaptation for power saving.

• 3.

  NR beam management (P1/P2/P3) solves the initial access and beam tracking challenge for massive MIMO and mmWave. SSB beam sweeping (up to 64 beams in FR2) provides coarse alignment; CSI-RS refinement and UE Rx beam sweeping complete the alignment. Type II CSI feedback with $L$-beam linear combinations enables high-resolution MU-MIMO precoding.

• 4.

  URLLC targets $10^{-5}$ BLER at 1 ms latency through mini-slot scheduling (2--7 symbols), preemptive eMBB puncturing, configured grants (grant-free access), and conservative MCS selection. The finite-blocklength penalty for URLLC reliability is 4--6 dB compared to eMBB operation.

• 5.

  NR achieves $\sim$25$\times$ throughput over LTE in sub-6 GHz by combining $5\times$ bandwidth (100 vs. 20 MHz) with $\sim$$5\times$ spectral efficiency from massive MIMO. The peak rate of a single 400 MHz FR2 carrier with 8 layers reaches $\sim$13 Gbps. Carrier aggregation and dual connectivity (EN-DC, NR-DC) further extend capabilities.

• 6.

  The path to 6G builds on NR's flexible framework: 5G-Advanced (Release 18+) introduces AI/ML-aided CSI, full-duplex, reconfigurable intelligent surfaces (RIS), and non-terrestrial networks (NTN). 6G research targets sub-THz bands (100--300 GHz), joint communication and sensing (JCAS), and semantic/goal-oriented communications.