3GPP RELEASE 8–15 · 4G STANDARD

LTE in a Nutshell
Engineer's Quick Reference

Architecture, air interface, protocol stack, KPIs, and procedures — everything you need to understand 4G LTE fast.

8
Sections
150
Mbps DL peak
3GPP
Rel-8 origin
OFDMA
Air interface
01 — OVERVIEW

What is LTE?

DEFINITION

Long Term Evolution (LTE) is the 4G mobile broadband standard developed by 3GPP. It uses OFDMA on the downlink and SC-FDMA on the uplink to deliver high-speed packet data.

KEY ADVANCEMENT

LTE is an all-IP, flat architecture. Voice (VoLTE) and data run over the same packet core. No circuit-switched domain — a major departure from UMTS/HSPA.

📶
DL Peak
150 Mbps
Cat-4, 2×2 MIMO
⬆️
UL Peak
50 Mbps
Cat-4, SC-FDMA
⏱️
Latency
< 10 ms
User plane
📡
Bandwidth
1.4–20 MHz
6 options
⚡
LTE-Advanced (Rel-10+)
Carrier Aggregation (up to 5 CCs → 100 MHz), 8×8 MIMO DL, 4×4 MIMO UL, CoMP, eICIC for HetNet. Peak DL: 1 Gbps. LTE-A Pro (Rel-13+) adds LAA, eMTC, NB-IoT.
02 — ARCHITECTURE

System Architecture

EVOLVED PACKET SYSTEM (EPS)
📱
UE
User Equipment
Uu
Air
🗼
eNB
eNodeB
S1-MME
S1-U
MME
Mobility Mgmt
S-GW
Serving GW
P-GW
PDN GW
HSS
Subscriber DB
PCRF
Policy / QoS
SGi
🌐
Internet
PDN
E-UTRAN (Radio Access)
eNBs connected via X2 interface. eNB handles scheduling, HARQ, RRC. No centralised BSC/RNC — intelligence pushed to the base station.
EPC (Core Network)
MME = control plane, S-GW = user plane anchor, P-GW = IP allocation + policy. HSS = subscriber database. PCRF = QoS rules.
03 — AIR INTERFACE

Air Interface Fundamentals

DOWNLINK — OFDMA
Orthogonal Frequency Division Multiple Access. Divides spectrum into narrow 15 kHz subcarriers, each independently modulated.
Subcarrier spacing 15 kHz
Resource Block 12 SC × 0.5 ms
TTI 1 ms
CP (Normal) 4.7 µs
UPLINK — SC-FDMA
Single Carrier FDMA. Lower PAPR than OFDMA — better power amplifier efficiency at the UE, extending battery life.
Access scheme SC-FDMA
PAPR advantage ~5 dB vs OFDMA
Sounding (SRS) Last symbol/SF
Power control Open + Closed loop
MODULATION SCHEMES & SPECTRAL EFFICIENCY
MODULATION BITS/SYMBOL SINR RANGE USE CASE
QPSK 2 < 5 dB Cell edge, weak signal
16-QAM 4 5–15 dB Mid-cell, typical users
64-QAM 6 15–25 dB Near cell, good SINR
256-QAM 8 > 25 dB LTE-A Pro Rel-12+, high SINR
04 — PROTOCOL STACK

User Plane Protocol Stack

RRC
Radio Resource Control · Connection setup/release · Measurement reporting · Handover · Paging · System information (SIBs)
PDCP
Header compression (ROHC) · Ciphering & integrity protection · Reordering & duplicate detection · In-order delivery
RLC
Segmentation & reassembly · ARQ retransmission · Modes: TM / UM / AM · SDU/PDU management
MAC
Scheduling (DL/UL) · HARQ (8-process) · Priority handling · Multiplexing logical channels · Random access
PHY
OFDMA / SC-FDMA modulation · CRC · Channel coding (Turbo) · MIMO · Beamforming · Resource mapping
HARQ
8 parallel stop-and-wait processes. Chase Combining or Incremental Redundancy. RTT = 8 ms.
ARQ (RLC AM)
Outer loop over HARQ. Window-based, handles residual errors. Status reports triggered on missing PDUs.
05 — PERFORMANCE

Key Performance Indicators

KPI TARGET THRESHOLD FORMULA / NOTE
RRC Setup SR > 98% < 95% RRC Setup Success / RRC Setup Attempts
E-RAB Setup SR > 98% < 95% E-RAB Success / E-RAB Attempts
Call Drop Rate < 0.5% > 2% Abnormal E-RAB Releases / Total E-RABs
HO Success Rate > 99% < 97% HO Success / HO Attempts (intra/inter)
DL Throughput > 30 Mbps < 10 Mbps Avg cell DL throughput (busy hour)
RSRP > -100 dBm < -110 dBm Reference Signal Received Power
RSRQ > -10 dB < -15 dB (N × RSRP) / RSSI · N = RBs in meas. BW
SINR / CQI CQI > 9 CQI < 6 UE reports 4-bit CQI (0–15) to eNB
PRB Utilisation < 70% > 85% Used PRBs / Available PRBs × 100
RSRP RANGES
Excellent> -80 dBm
Good-80 to -95 dBm
Fair-95 to -105 dBm
Poor< -110 dBm
CQI → MODULATION MAPPING
CQI 1–6QPSK (cell edge)
CQI 7–916-QAM (mid cell)
CQI 10–1564-QAM (near cell)
06 — PROCEDURES

Key Procedures

INITIAL ATTACH
1
RACH — Random Access (Msg1–4). UE sends preamble, eNB responds with RAR.
2
RRC Setup — eNB allocates resources, UE transitions to RRC_CONNECTED.
3
NAS Attach — MME authenticates UE via HSS (AKA), assigns GUTI.
4
Default Bearer — S-GW/P-GW allocate IP, E-RAB established end-to-end.
X2 HANDOVER (INTRA-LTE)
1
A3 Event — UE reports neighbour RSRP/RSRQ exceeds serving + offset.
2
HO Request — Source eNB sends HO Request to target via X2, resource check.
3
RRC Reconfig — UE sent RRCConnectionReconfiguration with target cell info.
4
Path Switch — MME/S-GW path updated. Source eNB releases UE context.
07 — CHANNELS

LTE Channel Structure

DOWNLINK CHANNELS
PDSCH Physical DL Shared Channel — user data
PDCCH DL Control — DCI scheduling grants
PBCH Broadcast — MIB (every 40 ms)
PCFICH Control Format Indicator — 1–3 OFDM symbols
PHICH HARQ ACK/NACK for UL transmissions
RS / CSI-RS Reference signals for channel estimation
UPLINK CHANNELS
PUSCH Physical UL Shared Channel — user data
PUCCH UL Control — CQI, HARQ ACK, SR
PRACH Random Access preamble (Zadoff-Chu)
DMRS Demodulation Reference Signal (UL)
SRS Sounding Reference Signal — UL channel quality
08 — QUICK REFERENCE

Values & Parameters Cheatsheet

BANDWIDTH OPTIONS
1.4 MHz6 RBs
3 MHz15 RBs
5 MHz25 RBs
10 MHz50 RBs
15 MHz75 RBs
20 MHz100 RBs
RRC STATES
RRC_IDLE — No data, paging monitored, DRX active
RRC_CONNECTED — Active session, scheduled by eNB
Transition: IDLE → CONNECTED takes ~100 ms (RACH + RRC Setup)
TIMING VALUES
TTI1 ms
Subframe1 ms = 2 slots
Frame10 ms = 10 SFs
HARQ RTT8 ms
SIB1 period80 ms
MIMO MODES (DL)
TM1 Single antenna port
TM2 Transmit diversity (SFBC)
TM3 Open-loop spatial mux
TM4 Closed-loop spatial mux
TM7/8 Beamforming (UE-specific RS)
HO EVENT TRIGGERS
A1 Serving becomes better than threshold
A2 Serving becomes worse than threshold
A3 Neighbour becomes offset better than serving
A4 Neighbour becomes better than threshold
A5 Serving < thresh1 AND neighbour > thresh2
COMMON FAILURE CAUSES
⚠RACH failure → coverage gap or high load
⚠HO failure → too-late / too-early / wrong cell
⚠E-RAB drop → S1 link issue, S-GW overload
⚠Low SINR → pilot pollution, interference
⚠High CDR → overshooting, weak DL coverage