LTE Terminology Explained

This tutorial section covers LTE basics, including: main features, terminologies, frame (TDD/FDD), channel types, PHY stack, throughput, VoLTE, CA, cell search, network entry, timers (PSS vs SSS), security, LTE bands, and EARFCN.

This page focuses on defining LTE and LTE-Advanced technology-related terms.

It includes terms like eNB, eNodeB, UE, OFDMA, SC-FDMA, LTE frame, Resource block (RB), Resource Element (RE), Slot, sub frame, reference signal, synchronization signal, S-GW, MME, X2 interface, S1 interface, Uu interface, Control channel, data channel, LTE channel types, logical channel, transport channel, physical channel, P-SS, S-SS, PBCH, PDSCH, PDCCH, PCFICH, PCH, RS, SRS, DMRS, PRACH, PUSCH, PUCCH, carrier aggregation, voice over LTE, etc.

Key LTE Terminology

• eNB or eNodeB: Similar to the Base station used in GSM networks. Also referred to as eNodeB.

• UE: User Equipment; similar to a mobile subscriber.

• OFDMA: Orthogonal Frequency Division Multiple Access, used in the physical layer of LTE Downlink.

• SC-FDMA: Single Carrier Frequency Division Multiple Access, used in the physical layer of LTE Uplink.

• LTE Frame: LTE frames come in two types: TDD (Time Division Duplexing) and FDD (Frequency Division Duplexing). In both cases, a frame is composed of 10 subframes, and each subframe is made up of 2 slots. The frame size is 10ms.

• Resource Block (RB): The smallest block of resource that can be allocated to a UE by the eNB; it consists of 12 subcarriers for 7 symbols.

• Resource Element (RE): The smallest unit of radio resources, consisting of one subcarrier for one symbol.

• Slot: 7 consecutive symbols for a short Cyclic Prefix, or 6 symbols for a long cyclic prefix.

• Subframe: 2 consecutive timeslots.

• Reference Signal: Similar to a pilot carrier; used for channel estimation at the receiver.

• Synchronization Signal: There are two synchronization signals: Primary (P-SS) and Secondary (S-SS). Both are transmitted in slot 0 and slot 10 in all frames. This is similar to the preamble used in earlier systems and is used for time and frequency synchronization.

• S-GW: Serving Gateway.

• MME: Mobility Management Entity.

• X2 interface: The interface used between eNodeB and eNodeB.

• S1 interface: The interface used between eNodeB and the core network interface (MME/S-GW).

• Uu interface: The air interface used between eNodeB and UE.

• Control Channel: This channel carries control information used to establish, maintain, and terminate the connection. It’s used for the transfer of control plane information in LTE.

• Data Channel: This channel carries traffic information and is used for the transfer of user plane information.

Channel Structure in LTE

LTE adopts a hierarchical channel structure, defining three channel types: logical, transport, and physical channels. Each is associated with a service access point (SAP) between different layers. These channels are used by lower layers to provide services to the upper layers.

• Logical Channels: Define what to transmit. They’re used by the MAC layer to provide services to the RLC layer. Each logical channel is defined based on the type of information it carries. In LTE, there are two main categories of logical channels, depending on the service they provide: control channels and traffic channels. The new LTE categories M1 and NB1 were added in LTE 3GPP Release 13.

• Transport Channels: Define how to transmit. The PHY layer uses transport channels to offer services to the MAC layer. It is characterized by how, and with what characteristics, data is transferred over the air.

• Physical Channels: Define the actual transmission. Each physical channel maps to a set of resource elements in the time-frequency grid that carries information from upper layers. The basic entities that make up a physical channel are REs and RBs. A resource element is one subcarrier by one OFDM symbol and typically this could carry one (or two with spatial multiplexing) modulated symbol(s). A resource block is a collection of resource elements and, in the frequency domain, represents the smallest quanta of resources that can be allocated.

Specific Physical Channels

• P-SS: Primary Synchronization Signal.

• S-SS: Secondary Synchronization Signal.

• PBCH: Physical Broadcast Channel.

• PDSCH: Physical Downlink Shared Channel.

• PDCCH: Physical Downlink Control Channel.

• PCFICH: Physical Control Format Indicator Channel.

• PHICH: Physical Hybrid ARQ Indication Channel.

• PCH: Paging Channel.

• RS: Reference Signal, used both in uplink and downlink.

• SRS: Sounding Reference Signal, used in uplink.

• DMRS: Demodulation Reference Signal.

• PRACH: Physical Random Access Channel used in uplink.

• PUSCH: Physical Uplink Shared Channel.

• PUCCH: Physical Uplink Control Channel.