What is the LTE physical layer?

LTE downlink uses OFDMA (Orthogonal Frequency Division Multiple Access) with 15 kHz subcarrier spacing. 12 subcarriers form one Resource Block (RB) = 180 kHz. A 20 MHz channel has 100 RBs (1200 subcarriers). The uplink uses SC-FDMA (Single Carrier FDMA), which is a DFT-precoded version of OFDMA that has lower peak-to-average power ratio (2 to 4 dB versus 8 to 10 dB), enabling more efficient power amplifiers in the handset and better battery life. Modulation: QPSK, 16QAM, 64QAM (Release 8), 256QAM (Release 12). MIMO: 2x2 standard, 4x4 optional in Release 8, 8x8 in Release 10. Frame structure: 10 ms frame, 1 ms subframe (TTI), 0.5 ms slot, 7 OFDM symbols per slot (normal CP). Scheduling: 1 ms granularity. The eNodeB scheduler allocates RBs to users based on channel quality (CQI), buffer status, and QoS requirements. Channel-dependent scheduling exploits frequency-selective fading by assigning each user the RBs where their channel is strongest.

What is LTE-Advanced?

LTE-Advanced (LTE-A, Release 10 and beyond) meets the IMT-Advanced requirements for true 4G. Key features: Carrier Aggregation (CA): combines up to 5 component carriers (CCs) of up to 20 MHz each for a maximum of 100 MHz aggregate bandwidth. CCs can be contiguous or non-contiguous, in the same band (intra-band) or different bands (inter-band). Practical peak: 3 Gbps DL with 5x20 MHz, 256QAM, 8x8 MIMO. Enhanced MIMO: up to 8 layers DL (8x8 MIMO) and 4 layers UL (4x4). Transmission mode 9 (TM9) uses CSI-RS for per-user beamforming. Relay nodes: extend coverage by relaying signals from the donor eNodeB to cell-edge users. Uses the LTE air interface for the backhaul link. Coordinated Multi-Point (CoMP): multiple eNodeBs coordinate their transmissions to reduce inter-cell interference at cell edges. Joint processing CoMP can provide 50 to 100 percent cell-edge throughput improvement. LTE-A Pro (Release 13 to 14): adds LAA (License Assisted Access for unlicensed spectrum), LWA (LTE-WLAN Aggregation), and narrowband IoT (NB-IoT).

How does LTE compare to 5G NR?

LTE and 5G NR share the same OFDM foundation but differ in several key aspects. Numerology: LTE has fixed 15 kHz subcarrier spacing. 5G NR has flexible numerology with SCS of 15, 30, 60, 120, or 240 kHz, enabling operation from sub-1 GHz to 52.6 GHz (and beyond in Release 17). Latency: LTE has 1 ms TTI (minimum 4 ms round-trip). NR has mini-slots as short as 2 OFDM symbols (approximately 36 microseconds at 120 kHz SCS), enabling sub-1 ms latency for URLLC. Bandwidth: LTE maximum 20 MHz per carrier (100 MHz aggregated). NR supports up to 100 MHz per carrier in FR1 and 400 MHz in FR2 (up to 1.6 GHz aggregated). MIMO: LTE maximum 8x8 MIMO (8 layers). NR supports massive MIMO with up to 256 antenna ports and 12 layers MU-MIMO. NR also adds analog and hybrid beamforming for mmWave. Architecture: LTE uses a flat architecture with eNodeB and EPC. NR separates the control plane (AMF) and user plane (UPF) for network slicing. NR supports standalone (SA) and non-standalone (NSA, using LTE as anchor) deployment.

Mobile Standard

LTE

/ell-tee-ee/ — Long Term Evolution

4G OFDMA DL / SC-FDMA UL. 15 kHz SCS, 12 SC/RB, 100 RB/20MHz. Peak: 300 Mbps (R8), 3 Gbps (LTE-A CA 5×20 256QAM 8×8). TTI: 1 ms. Latency: 10-20 ms. SC-FDMA: low PAPR (2-4 dB) for UE PA efficiency. Flat IP: eNodeB + EPC. LTE-A: CA, CoMP, relay. Bands: 700-2600 MHz. Release 8-16.

BW: 1.4-20 MHz

Peak: 3 Gbps

TTI: 1 ms

Understanding LTE

LTE fundamentally changed mobile communications by adopting OFDMA for the downlink, eliminating the CDMA-based approaches of 3G. This shift enabled much higher spectral efficiency, simpler frequency-domain equalization, and natural support for MIMO. Combined with the flat all-IP architecture (no circuit-switched core), LTE delivered a 10x improvement in spectral efficiency and 5x latency reduction over 3G.

LTE-Advanced (Release 10+) introduced carrier aggregation, combining multiple frequency bands to multiply bandwidth, and advanced MIMO with up to 8 layers. These features bridge LTE to 5G, with many LTE-A Pro features later adopted by 5G NR.

LTE Parameters

Physical layer:
Subcarrier: 15 kHz, RB: 12 SC = 180 kHz
20 MHz: 100 RB (1200 subcarriers)
Slot: 0.5 ms, 7 symbols (normal CP)
Subframe (TTI): 1 ms = 2 slots

Peak throughput:
R8: 75 Mbps (2×2, 64QAM, 20MHz)
R10: 300 Mbps (4×4, 64QAM, 20MHz)
LTE-A: 3 Gbps (8×8, 256QAM, 100MHz)

SC-FDMA PAPR:
PAPR ≈ 2-4 dB (vs OFDMA 8-10 dB)
DFT precoding before IFFT

LTE Evolution

Release	Feature	Peak DL	MIMO	BW
R8 (LTE)	OFDMA/SC-FDMA	300 Mbps	4×4	20 MHz
R10 (LTE-A)	CA, 8-layer	3 Gbps	8×8	100 MHz
R12	256QAM, D2D	4 Gbps	8×8	100 MHz
R13 (Pro)	LAA, NB-IoT	4+ Gbps	32 ports	640 MHz
R14	V2X, eMTC	25 Gbps	32 ports	640 MHz

Common Questions

Frequently Asked Questions

Physical layer?

DL: OFDMA, 15kHz SCS, 12 SC/RB, 100 RB/20MHz. UL: SC-FDMA (DFT+IFFT), PAPR 2-4 dB (PA efficiency). Modulation: QPSK/16/64/256QAM. MIMO: 2×2-8×8. Frame: 10ms, TTI 1ms. CQI-based scheduling assigns best RBs per user. Normal CP: 4.69μs.

LTE-A?

CA: 5×20MHz = 100MHz (intra/inter-band). 8-layer MIMO (TM9, CSI-RS). Relay nodes. CoMP: multi-cell coordination, 50-100% cell-edge gain. 256QAM (R12). LTE-A Pro: LAA (unlicensed), NB-IoT, LWA. Bridge to 5G NR. 3+ Gbps peak.

LTE vs 5G?

LTE: fixed 15kHz SCS, 1ms TTI, 20MHz/carrier, 8×8 MIMO. NR: flexible SCS (15-240kHz), mini-slot (36μs @120kHz), 400MHz/carrier FR2, massive MIMO 256 ports. NR: sub-1ms URLLC, network slicing. NSA: LTE anchor + NR. SA: standalone NR. Same OFDM foundation.

Related Terms

← LSB LTE-A →

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