Cellular 4G LTE
How LTE is built, from the eNB and Evolved Packet Core to the 10 ms frame, the resource block and the physical channels, with the 3GPP release in which each feature arrived.
Overview
LTE (Long Term Evolution) is the 4G cellular system standardised by 3GPP. Release 8 was the first LTE release, completed in 2009, and it defined both the radio access network (E-UTRAN) and the Evolved Packet Core (EPC), according to the 3GPP releases page. LTE kept evolving in later releases, and 3GPP still maintains its specifications. The copies of TS 36.211 and TS 36.300 in our reference library are Release 19 versions.
LTE matters to a 5G engineer for two reasons. The first NR specifications, in Release 15, were non-standalone: NR radio working inside an LTE network, according to 3GPP. And much of NR’s design, from OFDM with a cyclic prefix to HARQ and the split between control and shared channels, is easier to learn once you know the LTE version. This hub is for engineers who need to read LTE traces, run an open-source LTE cell or understand where NR came from.
Key ideas
A flat architecture: eNB and EPC
LTE has one kind of radio node, the eNB. TS 36.300 describes the eNBs as connected to each other over the X2 interface, and to the EPC over S1. S1 has two parts. S1-MME carries signalling to the Mobility Management Entity (MME), and S1-U carries user data to the Serving Gateway (S-GW). The S1 relation is many-to-many, so one eNB can talk to several MMEs and gateways. The PDN Gateway (P-GW) connects the core to external IP networks over SGi. The home subscriber server (HSS) holds subscription data. The core network itself is specified in TS 23.401, which 36.300 refers to.
The eNB is the only radio node in E-UTRAN. TS 36.300 places radio resource management, including the dynamic scheduling of uplink and downlink resources, in the eNB itself.
Frame structure and the resource grid
TS 36.211 (E-UTRA physical channels and modulation) organises time into 10 ms radio frames. In frame structure type 1, used for FDD, each frame holds ten 1 ms subframes, and each subframe holds two 0.5 ms slots. Frame structure type 2, for TDD, splits the frame into two 5 ms half-frames and assigns subframes to downlink or uplink by configuration. Type 3 serves Licensed-Assisted Access in unlicensed spectrum.
The downlink is OFDM with a cyclic prefix and 15 kHz subcarrier spacing. The uplink uses SC-FDMA, a DFT-spread form of OFDM. A resource block is 12 consecutive subcarriers for one slot, so 180 kHz wide. A slot carries seven symbols with the normal cyclic prefix and six with the extended one. TS 36.300 gives the downlink range as 6 to 110 resource blocks per component carrier.
| Quantity | Value (normal CP, 15 kHz) |
|---|---|
| Radio frame | 10 ms, 10 subframes |
| Subframe | 1 ms, 2 slots |
| Slot | 0.5 ms, 7 OFDM or SC-FDMA symbols |
| Resource block | 12 subcarriers × 1 slot (180 kHz) |
Physical channels and signals
TS 36.211 separates physical channels, which carry data from higher layers, from physical signals, which do not. In the downlink, the PDSCH carries user data, the PBCH carries the BCH transport block with the master information block, the PDCCH carries scheduling grants, the PCFICH says how many symbols the control region uses, and the PHICH carries HARQ acknowledgements for the uplink. In the uplink, the PUSCH carries data, the PUCCH carries control information and the PRACH carries random access preambles. Reference signals support channel estimation, and the synchronisation signals let a device find a cell and its timing.
Evolution by release
According to the 3GPP releases page, Release 10 (2011) brought LTE-Advanced with carrier aggregation and heterogeneous networks. Release 13 (2016) added LTE in unlicensed spectrum and created the LTE-Advanced Pro brand. Release 13 also introduced NB-IoT and LTE-M for low-power devices, covered in the LPWAN and IoT hub. A network or handset that “supports” a release rarely implements every feature in it. Check the device capability report, not the marketing.
Where to start
The radio side builds on OFDM from scratch and Fading channels. The OFDM and multicarrier hub explains why a cyclic prefix works. The 3GPP spec map shows which document covers what, and the OFDM link with channel estimation project builds a simplified LTE-like link in Python. When you are ready for NR, move on to the 5G NR hub.
Tools and platforms
| Tool | What for | Licence |
|---|---|---|
| srsRAN 4G | Software UE, eNB and a small EPC (MME, HSS, S/P-GW) on an SDR | AGPL-3.0 |
| OpenAirInterface | Open-source RAN and core code, including LTE | See the platform hub |
| Open5GS | Open-source EPC and 5G core | AGPL-3.0 |
| Wireshark | Decoding S1AP, NAS and GTP traces | GPL-2.0 |
Platform hubs: srsRAN, OpenAirInterface, Ericsson, Nokia and Qualcomm.
Common mistakes
- Assuming every LTE network or device supports every feature of the release it claims.
- Confusing a slot (0.5 ms) with a subframe (1 ms), which is the LTE scheduling interval.
- Reading resource block counts as bandwidth without allowing for guard bands.
- Running an open-source eNB over the air in licensed spectrum. Use a cable and attenuators, a shielded enclosure, or a test licence from your regulator.
- Quoting peak data rates from a UE category table as if they were what users see.
Going deeper
- 3GPP TS 36.211 (physical channels and modulation) and TS 36.300 (overall E-UTRAN description).
- Sesia, Toufik and Baker (eds), LTE: The UMTS Long Term Evolution, Wiley, 2011.
- The 3GPP releases timeline for what each release added.