LTE-U: LTE in Unlicensed Spectrum

LTE in Unlicensed Spectrum (LTE-U)

LTE in Unlicensed Spectrum (LTE-U), also known as LTE License Assisted Access (LTE-LAA), is an approach to expand the capacity of 4G LTE networks by utilising unlicensed spectrum, primarily the 5 GHz band used by 802.11a/ac WiFi equipment. Originally proposed by Qualcomm, LTE-U serves as an alternative to carrier-owned WiFi hotspots, enabling mobile operators to boost network coverage and capacity.

Background

With the exponential growth in mobile data demand, LTE networks face increasing pressure to deliver higher throughput and lower latency. While deploying smaller cells (e.g. small cells or femtocells) helps, it is not sufficient to meet capacity needs in high-traffic areas. LTE-U addresses this by leveraging unlicensed spectrum, particularly the 5 GHz Industrial, Scientific, and Medical (ISM) band, which offers hundreds of MHz of bandwidth globally. This band is already used by WiFi, Bluetooth, microwave ovens, and other devices, making coexistence a critical design consideration.

5 GHz Band for LTE-U / LTE-LAA

The 5 GHz band is the primary focus for LTE-U due to its wide bandwidth availability, though specific bands vary by region:

  • 5150–5350 MHz: Typically limited to 200 mW and restricted to indoor use in many countries.

  • 5350–5925 MHz: Higher power levels (up to 1 W) are often permitted, with fewer restrictions.

Regulatory requirements mandate coexistence mechanisms, such as Listen Before Talk (LBT) or Clear Channel Assessment (CCA), to ensure fair spectrum sharing. These mechanisms prevent LTE-U from monopolising the band, allowing other users like WiFi to operate effectively.

LTE-U / LTE-LAA Technical Overview

Introduced in 3GPP Release 13, LTE-U builds on LTE-Advanced’s carrier aggregation (CA) technology, which allows user equipment (UE) to combine multiple frequency bands for increased bandwidth. LTE-U integrates licensed and unlicensed spectrum, with the licensed band serving as the primary cell (PCell) for control signalling and the unlicensed band as the secondary cell (SCell) for data.

Deployment Modes

LTE-U supports multiple configurations:

  • Downlink Only: The simplest form, where the unlicensed 5 GHz band is used solely for downlink data. The primary cell in the licensed spectrum handles control signalling and uplink traffic, while the eNodeB ensures interference-free operation using LBT.

  • Uplink and Downlink: Full Time-Division Duplex (TDD) operation in the unlicensed band, enabling both uplink and downlink data transmission. This requires additional features to manage spectrum sharing and maintain reliability.

  • FDD/TDD Aggregation: Combines Frequency-Division Duplex (FDD) in licensed bands with TDD in unlicensed bands, offering flexibility in spectrum allocation and optimising performance for specific use cases.

Technical Requirements

  • Core Network Integration: LTE-U leverages the existing LTE core network for backhaul, security, and authentication, requiring no significant changes to the core infrastructure.

  • Base Station Modifications: eNodeBs must support unlicensed frequencies and implement LBT/CCA to comply with regulatory requirements.

  • User Equipment (UE): Smartphones and other devices need LTE-U/LAA-compatible chipsets to access the 5 GHz band.

LTE-U / WiFi coexistence

A primary concern with LTE-U is its potential to interfere with WiFi and other 5 GHz band users. To address this, LTE-U incorporates coexistence mechanisms:

  • Listen Before Talk (LBT): Ensures the channel is clear before transmitting, reducing interference with WiFi and other devices.

  • Channel Selection: LTE-U can operate on different 5 GHz channels than WiFi, minimising interference in many cases.

  • Fairness Algorithms: eNodeBs can implement algorithms to balance spectrum usage, ensuring WiFi maintains adequate throughput even when sharing the same channel.

Studies show that properly implemented LTE-U systems achieve effective coexistence, with WiFi throughput degradation limited to 10–20% in worst-case scenarios (shared channels with heavy LTE-U traffic). In practice, operators often select less congested channels to further reduce interference.

LTE in Unlicensed spectrum (LTE-U)

Benefits of LTE-U

  • Enhanced Capacity: By leveraging unlicensed spectrum, LTE-U significantly increases network capacity, particularly in dense urban areas or indoor environments.

  • Improved Data Speeds: LTE-U boosts data rates over short distances, offering a seamless alternative to Wi-Fi without requiring users to switch networks.

  • Consistent Performance: Unlike public Wi-Fi, which can suffer from congestion, LTE-U benefits from operator-managed quality of service (QoS).

  • Future-Proofing: LTE-U supports emerging applications like augmented reality, IoT, and real-time communications, which demand high throughput and low latency.

Industry Adoption

Since its introduction, LTE-U has seen adoption by major carriers like T-Mobile and Verizon, with deployments starting as early as 2016. LTE-U is now widely integrated into urban and indoor networks, particularly in regions with high data demand. Operators use LTE-U to complement licensed spectrum, offloading traffic to the 5 GHz band while maintaining control via licensed bands.

Comparison with WiFi Calling

Unlike WiFi calling, which routes voice calls over a WiFi network, LTE-U uses the unlicensed 5 GHz band for all data traffic (not just voice) while retaining LTE’s control channel. This allows for higher reliability and seamless integration with the cellular network, avoiding the need for users to connect to a separate WiFi network.

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