Contiguous Carriers

Description

Contiguous carriers is a term used in 3GPP radio specifications to describe a scenario where multiple carriers are placed adjacent to each other within a single spectrum block, and the RF requirements for co-existence between un-coordinated operators do not apply within that block. This concept is fundamental to carrier aggregation (CA) in LTE and 5G NR, where multiple component carriers are aggregated to increase bandwidth and user throughput. The definition is found in 3GPP TS 36.101 (E-UTRA UE radio transmission and reception), TS 36.104 (E-UTRA BS radio transmission and reception), TS 38.104 (NR BS radio transmission and reception), and others.

In a contiguous carrier scenario, the carriers are typically owned by the same operator, and the spectrum block is managed as a single entity. This allows for more relaxed RF requirements because the operator can coordinate the transmission and reception across the carriers, avoiding interference. The opposite is non-contiguous carriers, where carriers are separated by other spectrum, possibly belonging to different operators, requiring stricter RF requirements to ensure co-existence. The concept of contiguous carriers is important for defining test cases and RF performance requirements for base stations and user equipment.

The technical implications of contiguous carriers are significant. For example, in carrier aggregation, when carriers are contiguous, the UE can use a single RF chain to transmit and receive across the aggregated bandwidth, reducing complexity and power consumption. The base station can also benefit from simplified filtering and amplification. The 3GPP specifications define specific RF requirements for contiguous carrier aggregation, such as adjacent channel leakage ratio (ACLR) and spurious emissions, which are less stringent than for non-contiguous scenarios because the operator has control over the entire block.

The definition has evolved slightly over releases. In Rel-10, it was defined as 'a set of two or more carriers configured in a spectrum block where there are no RF requirements based on co-existence for un-coordinated operation within the spectrum block.' In Rel-12, the wording changed to 'set of two or more carriers...' (dropping 'a'). In Rel-17 and Rel-18, the definition starts with 'A set of two or more carriers...' (capital A). These are minor editorial changes, but they reflect the ongoing maintenance of the specifications. The core concept remains the same: contiguous carriers enable efficient spectrum use within a controlled block.

Contiguous carriers are relevant for both LTE and 5G NR. In 5G, the concept extends to wider bandwidths and higher frequency bands, including millimeter wave. The specifications TS 38.104 and TS 38.141 define RF requirements for NR base stations, including contiguous carrier scenarios. The term is also used in the context of spectrum sharing and dynamic spectrum access, where operators may coordinate to create contiguous blocks. Overall, contiguous carriers are a key enabler for high-capacity mobile broadband, allowing operators to maximize spectral efficiency and deliver higher data rates.

Purpose & Motivation

The concept of contiguous carriers exists to define a spectrum configuration where multiple carriers are adjacent and managed by a single operator, allowing for relaxed RF requirements and more efficient use of spectrum. It solves the problem of co-existence interference that arises when carriers from different operators are adjacent. By identifying contiguous carriers, 3GPP can specify appropriate RF requirements that balance performance and cost.

Historically, as mobile networks moved to carrier aggregation to increase bandwidth, the need to distinguish between contiguous and non-contiguous aggregation became clear. Non-contiguous aggregation requires more complex RF front-ends and stricter filtering to avoid interference with other operators' spectrum. Contiguous aggregation, on the other hand, allows for simpler and more power-efficient implementations. The definition of contiguous carriers provides a clear boundary for when these relaxed requirements can be applied.

The motivation for this concept is to enable operators to deploy wide channels (e.g., 20 MHz or more) by aggregating smaller carriers within their own spectrum. This is particularly important in fragmented spectrum scenarios, where an operator may own multiple non-adjacent blocks. By aggregating contiguous carriers, the operator can achieve higher peak rates and better spectral efficiency. The 3GPP specifications ensure that equipment can be tested and certified for these scenarios, promoting interoperability and global roaming. The concept continues to evolve with 5G, where wider bandwidths and new frequency ranges are being introduced.