4G (LTE) Basics: Duplex Modes to Network Architecture

I. The Two Duplex Modes of LTE: FDD vs. TDD

LTE systems primarily use two duplexing methods, which differ in frequency usage and frame structure:

Feature	LTE FDD (Frequency Division Duplex)	LTE TDD (Time Division Duplex)
Frequency Usage	Uses paired frequencies for separate downlink and uplink transmission	Uses a single frequency, allocating different time slots for uplink and downlink
Frame Structure	1ms subframe, 0.5ms slot	1ms subframe, 0.5ms slot
Suitable Scenarios	Symmetrical spectrum resources	Asymmetrical spectrum resources

Both duplex modes use a subframe duration of 1ms and a slot duration of 0.5ms.

II. LTE Resource Allocation Unit: Resource Block (RB)

In LTE, the Resource Block (RB) is the smallest unit of radio resource allocation. Its structure is as follows:

Time Domain: 0.5ms slot
Frequency Domain: 12 subcarriers
Symbols: 7 OFDM symbols under Normal Cyclic Prefix (CP); 6 OFDM symbols under Extended CP
Resource Elements (RE): One resource block contains a total of 84 (12x7) time-frequency units

The Resource Element is the smallest physical resource unit in LTE, corresponding to the resource of one subcarrier over one OFDM symbol period.

III. The Three Types of LTE Channels

LTE uses three types of channels to support the establishment, maintenance, and termination of connections between user equipment (UE) and the base station (eNodeB):

Channel Type	Function	Classification
Logical Channels	Classified based on the type of information transmitted	Control channels, traffic channels
Transport Channels	Classified based on how data is transmitted over the radio interface	—
Physical Channels	Correspond to a set of resource elements used by the physical layer	—

These three types of channels define the LTE communication mechanism from three perspectives: "what content is transmitted," "how it is transmitted," and "how it is mapped to physical resources."

IV. Reference Signals vs. Synchronization Signals

Signal Type	Function	Classification
Reference Signal (RS)	Acts as pilot subcarriers for channel estimation and tracking	Demodulation RS (DRS): For synchronization and channel estimation (uplink/downlink); Sounding RS (SRS): For channel quality estimation (uplink only)
Synchronization Signal (SS)	Acts as a preamble sequence for synchronization	Primary SS (P-SS): For initial synchronization; Secondary SS (S-SS): For determining frame boundaries

V. Cell Search and Broadcast Channel

Cell Search: The UE finds and camps on an LTE cell (eNodeB) through the cell search process. This is the first step for a UE to access the network.

Physical Broadcast Channel (PBCH): After initial cell synchronization, the UE reads the Master Information Block (MIB) on the PBCH. The MIB contains:

Downlink system bandwidth (in RBs)
PHICH duration
PHICH resources
System Frame Number (SFN)

At the transport layer, this broadcast channel is called BCH; at the logical layer, it is called BCCH.

VI. Technical Advantage of SC-FDMA

LTE uplink uses SC-FDMA (Single-Carrier Frequency Division Multiple Access). Its key advantage is a lower PAPR (Peak-to-Average Power Ratio) compared to the OFDMA used in the downlink. This enables higher power amplifier efficiency in terminal devices, thus extending battery life.

VII. RSSI and Cell Handover

RSSI (Received Signal Strength Indication) measures the power received by the terminal from the cell. It is used in nearly all radio access technologies. Its core functions include:

Helping the UE maintain a connection to the best available cell
Triggering handover or cell reselection when signal strength drops

RSRP and RSRQ are also key parameters for measuring signal quality in LTE.

VIII. Circuit Switched Fallback (CSFB)

CSFB is a transitional technology that allows a UE in an LTE network to utilize existing GSM circuit-switched infrastructure for voice services. When a voice call needs to be initiated or received, the system triggers a RAT (Radio Access Technology) handover from LTE to GSM to enable the voice call.

IX. LTE Network Architecture and Interfaces

The LTE network architecture consists of the User Equipment (UE), the Base Station (eNB), and the Evolved Packet Core (EPC). Key interfaces include:

Interface	Connecting Entities	Function
Uu	UE and eNB	Radio interface between the terminal and the base station
X2	Between eNBs	Interconnect between base stations, supporting handover and load balancing
S1	eNB and EPC	Interface between the base station and the core network

X. SRVCC and LTE-Advanced

SRVCC (Single Radio Voice Call Continuity): Supports handover from E-UTRAN (LTE) to UTRAN/GERAN (WCDMA/GSM), transferring an ongoing IMS-based PS (Packet Switched) voice call in LTE to a CS (Circuit Switched) voice call in legacy networks, ensuring voice call continuity.

Differences between LTE and LTE-Advanced:

Feature	LTE	LTE-Advanced
3GPP Release	Release 8/9	Release 10
Core New Technologies	Basic OFDMA/SC-FDMA	Carrier Aggregation (CA) introduced
MIMO Support	Basic multi-antenna	Support for up to 8 antennas
Peak Data Rate	100 Mbps downlink (20 MHz)	Up to 1 Gbps (with carrier aggregation)

Carrier Aggregation (CA) aggregates multiple dispersed LTE frequency bands (component carriers) to form a wider effective bandwidth (up to 100 MHz), significantly increasing peak data rates. Additionally, LTE-Advanced introduced enhanced MIMO technology, supporting up to 8 antennas.

XI. Conclusion

As the critical generation transitioning mobile communications from circuit-switched to packet-switched networks, the fundamental principles of 4G (LTE)—including band division, resource allocation, channel structure, and network architecture—remain important building blocks for 5G and future communication technologies. For industrial IoT devices like routers and CPEs, considerations such as support for LTE FDD/TDD bands, carrier aggregation capabilities, and signal quality measurements directly impact their applicability and stability across different operator networks.