4G LTE Tutorial : Basics, Architecture, Channels and More

4G LTE (Long-Term Evolution) technology represents a significant advancement in mobile communications, providing users with high-speed data and seamless connectivity. LTE is often referred to as a 4G technology. It evolved from previous generations such as 2G (GSM) and 3G (UMTS/HSPA) to support the increasing demand for higher data rates and improved Quality of Service (QoS). LTE network offers about 100 Mbps data rate in the downlink and about 50 Mbps in the uplink. It has now become the backbone for most mobile networks worldwide, enabling services like HD video streaming, VoIP (Voice over IP), and advanced web applications. With the introduction of LTE Advanced and LTE Pro, additional features such as carrier aggregation, MIMO (Multiple Input, Multiple Output) and improved spectral efficiency have been incorporated. This 4G tutorial covers LTE basic fundamentals, its network architecture, channels, frequency bands, QoS, protocol stack, comparison with 2G/3G, advantages and disadvantages.

LTE Terminology

• Evolved NodeB (eNodeB): LTE base station that handles radio communication with user equipment (UE).
• User Equipment (UE): Any device that communicates with the LTE network (e.g., smartphones, tablets).
• Evolved Packet Core (EPC): Core network that provides connectivity and controls data routing.
• Carrier Aggregation: Technique used to increase bandwidth by combining multiple carriers.
• MIMO (Multiple Input, Multiple Output): Antenna technology to enhance data throughput and reliability.
• PDN (Packet Data Network): External networks (e.g., Internet) that the EPC provides access to.
• QoS (Quality of Service): Framework ensuring consistent service delivery based on application requirements.

LTE Parameters

• Frequency Range: LTE operates on different frequency bands depending on the region, typically between 700 MHz and 2600 MHz.
• Bandwidth: LTE supports variable bandwidths of 1.4, 3, 5, 10, 15, and 20 MHz. The flexibility in bandwidth allocation makes LTE adaptable to diverse spectrum availability.
• Modulation: LTE uses OFDMA (Orthogonal Frequency Division Multiple Access) for downlink and SC-FDMA (Single Carrier-Frequency Division Multiple Access) for uplink. Modulation schemes such as QPSK, 16-QAM, and 64-QAM are employed to optimize spectral efficiency.
• Coding: Turbo coding is used as the channel coding scheme in LTE, ensuring error correction and data integrity.
• Data Rate: LTE provides peak data rates up to 100 Mbps (DL) and 50 Mbps (UL) for 20 MHz bandwidth.

LTE QoS

QoS in LTE is managed through a concept known as bearers, which are logical data pipes that carry traffic between the UE and the EPC. Each bearer is associated with a QoS Class Identifier (QCI) that defines the priority and performance characteristics of the data flow. The following are key QoS parameters.
1. Latency: Time taken for data to travel between endpoints. LTE targets a latency of less than 10 ms for user-plane traffic.
2. Jitter: Variation in packet delay that impacts real-time services like voice and video.
3. Packet Loss: Percentage of data packets lost during transmission.

E-UTRA Frequency Bands

LTE operates on various frequency bands globally, identified by the E-UTRA band numbering scheme. Below are some prominent bands used worldwide. The Air interface between LTE network and UE uses these frequencies for communication. OFDMA modulation is used from network to UE air interface and SC-FDMA is used from UE to network air interface.

LTE Band Number Frequency Range (MHz) Region
Band 1 1920 - 1980 / 2110 - 2170 Asia, Europe
Band 3 1710 - 1785 / 1805 - 1880 Asia, Europe
Band 7 2500 - 2570 / 2620 - 2690 Worldwide
Band 20 832 - 862 / 791 - 821 Europe
Band 40 2300 - 2400 India, Asia

LTE Architecture

As shown in the figure LTE SAE(System Architecture Evolution) consists UE,eNodeB and EPC(evolved packet core). Various interfaces are designed between these entities which include Uu between UE and eNodeB, X2 between two eNodeB, S1 between EPC and eNodeB. eNodeB has functionalities of both RNC and NodeB as per previous UMTS architecture.LTE is completely IP based network.

The LTE architecture contains the following network elements.
1. LTE EUTRAN (Evolved Universal Terrestrial Radio)
2. LTE Evolved Packet Core.

LTE tutorial -LTE SAE

LTE EUTRAN : It is a radio access network standard meant to be a replacement of the UMTS, HSDPA and HSUPA . Unlike HSPA, LTE's E-UTRA is an entirely new air interface system. It provides higher data rates, lower latency and is optimized for packet data. EUTRAN (Evolved Universal Terrestrial Radio) consists of eNB (Base station). EUTRAN is responsible for complete radio management in LTE. When UE powered is on, eNB is responsible for Radio Resource Management, i.e. it shall do the radio bearer control, radio admission control, allocation of uplink and downlink to UE etc. When a packet from UE arrives to eNB, eNB shall compress the IP header and encrypt the data stream. It is also responsible for adding a GTP-U header to the payload and sending it to the SGW. Before the data is actually transmitted the control plane has to be established. eNB is responsible for choosing a MME using MME selection function. The QoS is taken care by eNB as the eNB is only entity on radio. Other functionalities include scheduling and transmission of paging messages, broadcast messages, and bearer level rate enforcements also done by eNB.

LTE Evolved Packet Core (EPC) : The LTE EPC consists of MME, SGW, PGW, HSS and PCRF.
• MME (Mobility Management Entity): Manages session states, UE tracking, and authentication.
• SGW (Serving Gateway): Routes data between eNodeBs and the PDN.
• PGW (Packet Gateway): Manages IP allocation and connects the LTE network to external PDNs.
• HSS (Home Subscriber Server): Stores user profiles and performs authentication.

LTE Advanced Roaming Architecture

LTE Advanced E-UTRAN Architecture

LTE Advanced architecture for E-UTRAN consists of P-GW, S-GW, MME, S1-MME, eNB, HeNB, HeNB-GW, Relay Node etc. LTE Advanced protocol stack consists of user plane and control plane for AS and NAS.
Refer LTE Advanced Architecture and Stack➤.

LTE Channels

The channels in LTE system are mainly categorized into logical, transport and physical channels based on their functions.
1. Logical Channels: Define the type of information transmitted. Examples include BCCH (Broadcast Control Channel) and DCCH (Dedicated Control Channel).
2. Transport Channels: Specify how data is transferred. Examples include PCH (Paging Channel) and DL-SCH (Downlink Shared Channel).
3. Physical Channels: Define the actual radio frequency resources used. Examples include PDSCH (Physical Downlink Shared Channel) and PUSCH (Physical Uplink Shared Channel).

LTE logical transport physical channels mapping

The downlink (eNodeB to UE) channels are PBCH, PDSCH, PDCCH, PMCH, PCH etc. The uplink (UE to eNodeB) channels are PRACH, PUSCH and PUCCH.

LTE Frame structure

LTE frame is 10 ms in duration and consists of 10 subframes. Each subframe consists of two slots. LTE (Long-Term Evolution) supports two duplexing modes for radio communication viz. Frequency Division Duplex (FDD) and Time Division Duplex (TDD). The frame structure in LTE differs for each mode, depending on how uplink and downlink transmissions are separated.

LTE frame structure FDD LTE frame structure TDD

➨In FDD mode, the uplink and downlink transmissions occur simultaneously on separate frequency bands. This means that the uplink and downlink communication is continuous and does not interfere with each other.
➨In TDD mode, the uplink and downlink transmissions share the same frequency band but are separated in time. The frame is divided into time slots dedicated to either uplink or downlink transmission.
Refer LTE Frame >>.

LTE Protocol Stack Layers

The LTE protocol stack is divided into two planes viz. user and control plane.
1. User Plane: Handles the transfer of user data and is composed of PDCP (Packet Data Convergence Protocol), RLC (Radio Link Control), and MAC (Medium Access Control) layers.
2. Control Plane: Manages control signaling and includes protocols such as RRC (Radio Resource Control), NAS (Non-Access Stratum), and S1AP (S1 Application Protocol).

LTE

The LTE stack consists of different layers viz. Physical, MAC, RLC, PDCP and RRC as shown in the diagram.
➨Refer LTE stack >> and LTE Physical Layer >>.

LTE Advantages and Disadvantages

Following are advantages of LTE technology :
1. High data rates and low latency.
2. Efficient use of bandwidth with support for carrier aggregation.
3. Enhanced spectral efficiency and capacity.
4. Seamless support for voice and multimedia services over IP (VoLTE).
Following are disadvantages of LTE :
1. Higher deployment and infrastructure costs.
2. Battery consumption is higher compared to 2G/3G.
3. Spectrum fragmentation issues due to varied frequency bands.

Comparison with 2G/3G

Following table compares 4G LTE with its predecessor cellular technologies (2G GSM and 3G UMTS).

Feature 2G GSM 3G UMTS 4G LTE
Frequency Band 900/1800 MHz 2100 MHz 700 MHz - 2600 MHz
Data Rate Up to 384 kbps Up to 42 Mbps Up to 100 Mbps (DL)
Latency >100 ms 50 - 100 ms < 10 ms
Modulation GMSK QPSK, 16-QAM QPSK, 16-QAM, 64-QAM
Core Network Circuit-Switched Circuit/Packet-Switched Packet-Switched
Voice Technology Circuit-Switched Circuit/VoIP VoIP (VoLTE)
Handover Hard Handover Hard/Soft Handover Seamless Handover

Conclusion

4G LTE has revolutionized mobile communications by delivering faster data rates, low latency, and improved QoS. Its efficient use of spectrum and support for IP-based services have made it the preferred technology for network operators worldwide. While the evolution to 5G is underway, LTE remains a critical component of mobile networks, providing reliable connectivity and supporting advanced applications such as IoT and smart cities. This LTE tutorial is very useful for beginners who would like to start learning LTE fundamentals.

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