Digital Terrestrial Television

Description

Digital Terrestrial Television (DTT) is a broadcast technology standard for delivering digital television content to home receivers (TVs) using terrestrial transmitters. While DTT itself is standardized by bodies like ITU-R, DVB, and ATSC, 3GPP has studied DTT extensively in the context of spectrum coexistence and possible technical convergence with mobile broadband services. Key 3GPP specifications, such as TS 36.104 (E-UTRA BS radio transmission) and TS 37.104 (Multi-RAT base station requirements), include emission masks and coexistence requirements for LTE/5G NR base stations operating adjacent to DTT broadcast channels, particularly in the 700 MHz and 600 MHz bands.

From an architectural perspective, DTT networks are fundamentally different from cellular networks. They employ a high-power, high-tower broadcast architecture where a single transmitter (or a network of synchronized transmitters in a Single Frequency Network - SFN) covers a wide geographic area. The signal is broadcast unidirectionally to all receivers within range. This contrasts with the cellular model of low-power, small-cell, bidirectional, and user-specific transmission. The primary DTT standards referenced in 3GPP studies are DVB-T/T2 (Digital Video Broadcasting - Terrestrial) and, in some regions, ISDB-T or ATSC.

How DTT works involves encoding audio, video, and data into an MPEG Transport Stream, which is then modulated using Orthogonal Frequency Division Multiplexing (OFDM)—a technique also used by LTE and 5G NR but with different parameters. The OFDM signal is transmitted over a designated UHF channel (e.g., 6, 7, or 8 MHz wide). Receivers within the coverage area tune to the channel, demodulate the OFDM signal, and decode the transport stream to present the selected program. The key technical parameters of concern for coexistence are the transmitter's high output power (up to tens of kW) and the receiver's sensitivity to interference from nearby mobile base stations, which operate at much lower power but on adjacent frequencies.

3GPP's role regarding DTT is not to define the broadcast standard but to ensure its mobile standards can operate harmoniously in shared or adjacent spectrum. This involves rigorous studies documented in Technical Reports (TRs) like 37.900, which evaluate interference scenarios. The work includes defining requirements for mobile base stations to limit their out-of-band emissions (spurious and adjacent channel leakage) to protect sensitive DTT receivers. Conversely, studies also examine the impact of high-power DTT transmissions on nearby cellular receivers. This coexistence analysis is critical for regulators planning spectrum re-farming, such as the digital dividend (repurposing UHF band from broadcast to mobile), enabling the introduction of services like LTE/5G in Band 28 (700 MHz) and n71/n28 (600/700 MHz) without degrading existing TV services.

Purpose & Motivation

The inclusion of DTT studies in 3GPP specifications is driven by the global phenomenon of spectrum re-farming and the need for coexistence between different radio services. Historically, the UHF band (470-862 MHz) was predominantly used for analog and later digital television broadcasting. This spectrum is highly valuable for mobile broadband due to its excellent propagation characteristics (good coverage and building penetration). As demand for mobile data exploded, regulators worldwide sought to repurpose portions of the UHF band for IMT technologies like LTE and 5G—a process known as the "digital dividend."

This repurposing created a direct technical problem: how to deploy high-density, low-power mobile networks in frequencies adjacent to high-power, wide-area broadcast towers without causing harmful interference to either service. The existing approaches before detailed coexistence studies were conservative guard bands, which wasted spectrum, or untested deployments that risked service disruption. 3GPP's work on DTT coexistence was motivated by the need to provide a solid technical foundation for spectrum policy. It aimed to define the precise technical conditions (e.g., required separation distances, base station emission limits) under which coexistence is feasible, thereby enabling efficient use of the spectrum.

Furthermore, there has been exploration of convergence, such as FeMBMS (Further evolved Multimedia Broadcast Multicast Service) in LTE and 5G Broadcast, which could theoretically offer broadcast-like services using cellular infrastructure. Understanding the incumbent DTT technology's performance and requirements is essential for evaluating such convergence scenarios. Thus, DTT in 3GPP context exists to solve the critical real-world problem of peaceful and efficient spectrum sharing between two vastly different radio service architectures, facilitating the rollout of mobile broadband in premium lower-band spectrum.