LTE Technology: Basics, Frame Structure, and Key Features

This tutorial covers the fundamentals of LTE technology, including:

• Main features
• Terminology
• Frame structure (TDD/FDD)
• Channel types
• Physical layer stack
• Throughput
• VoLTE
• Carrier Aggregation (CA)
• Cell search
• Network entry procedures
• Timers
• Primary Synchronization Signal (PSS) vs. Secondary Synchronization Signal (SSS)
• Security
• LTE Bands
• Evolved Absolute Radio Frequency Channel Number (EARFCN)
• Hotspot routers

This guide offers a deep dive into LTE concepts.

Introduction to LTE

LTE, or Long Term Evolution, represents the next generation of mobile technology. It maintains backward compatibility with earlier cellular technologies like HSPA, GSM, and CDMA. LTE, often branded as 4G technology, is specified in Releases 8 and 9 of the 3GPP standard. Release 10 is known as LTE-Advanced.

The technical specifications for LTE radio transmission and reception are detailed in:

• TS 36.101: User Equipment (UE)
• TS 36.104: Evolved Node B (eNB)

LTE employs multiple access technologies for downlink and uplink transmissions:

• Downlink: Orthogonal Frequency Division Multiple Access (OFDMA)
• Uplink: Single-Carrier Frequency Division Multiple Access (SC-FDMA)

The LTE specifications are constantly evolving. Updated versions of the 36-series documents are released quarterly and can be found at: https://www.3gpp.org/ftp/specs/archive/36_series/

LTE Physical Layer

The physical layer of LTE is described in TS36.211 and TS36.212. TS36.211 defines physical channels and modulation techniques, while TS36.212 covers multiplexing and channel coding.

Basic Parameters and Frame Structure

• Frame Size: 10ms
• Number of Slots: 20
• Slots per Subframe: 2
• Slot Duration: 0.5 ms
• Subframe Duration: 1 ms
• Basic Time Unit (Ts) for 20MHz Bandwidth: (1/15000)*2048 seconds = 32.55ns

LTE supports two frame types: Frequency Division Duplexing (FDD) and Time Division Duplexing (TDD).

Type 1: FDD

• Total of 20 slots, each 0.5ms long
• 2 slots per subframe
• Total frame duration: 10ms

Type 2: TDD

• 10 subframes, each 1 ms long
• Subframes 0 and 5 are always dedicated to downlink
• Subframes 1 and 6 are dedicated for control frames
• Subframes 2, 3, 4 and 7, 8, 9 depend on the Uplink/Downlink configuration table as defined in the standard.
• Frame has a switch-point periodicity of 5 ms.

Key Features of LTE Physical Layer

3GPP documents for LTE and LTE-Advanced are available at: https://www.3gpp.org

References

1. TS 36.201- Evolved Universal Terrestrial Radio Access (E-UTRA); LTE physical layer; General description
2. TS 36.211- Evolved Universal Terrestrial Radio Access (E-UTRA); Physical channels and modulation
3. TS 36.212- Evolved Universal Terrestrial Radio Access (E-UTRA); Multiplexing and channel coding