Non-contiguous spectrum

Description

Non-contiguous spectrum is a fundamental concept in advanced wireless systems, particularly in the context of carrier aggregation (CA) introduced in 3GPP Release 10 for LTE and extended in later releases for NR. It describes a spectrum arrangement where the total available bandwidth is not a single continuous block but is composed of multiple sub-blocks (also called component carriers) that are separated by gaps, known as sub-block gaps. These gaps may be due to spectrum fragmentation, where an operator owns non-adjacent frequency bands, or due to the need to avoid interference with other systems.

In carrier aggregation, non-contiguous spectrum allows the aggregation of multiple component carriers (CCs) that may be in different frequency bands (inter-band CA) or within the same band but non-adjacent (intra-band non-contiguous CA). Each component carrier functions as an independent cell, with its own synchronization, reference signals, and control channels. The UE aggregates these carriers to increase the effective bandwidth and thus the peak data rate. The sub-block gaps between carriers can vary in size, and the UE must be capable of tuning to each carrier independently while managing the gaps.

The implementation of non-contiguous spectrum poses several technical challenges. The UE must support multiple transceivers or a wideband transceiver with sufficient linearity to handle multiple carriers simultaneously. The gaps between sub-blocks may require the UE to filter out unwanted emissions and avoid intermodulation products. In the base station, the transmitter must ensure that the aggregated carriers meet spectrum emission requirements, including limits on out-of-band emissions and spurious emissions. 3GPP specifications such as TS 36.101 (LTE UE radio transmission and reception) and TS 38.101 (NR UE radio transmission and reception) define the requirements for non-contiguous spectrum operation, including allowed sub-block gaps and carrier aggregation configurations.

Non-contiguous spectrum is also relevant in other contexts, such as in the definition of operating bands for NR, where a band may be specified as non-contiguous to accommodate fragmented spectrum holdings. For example, in the 3.5 GHz band (n78), an operator might have two non-adjacent blocks; non-contiguous CA allows them to be used together. The concept is also applied in dual connectivity (DC) and supplementary uplink (SUL) scenarios, where different frequency ranges are combined.

The evolution of non-contiguous spectrum support has been progressive. Release 10 introduced intra-band non-contiguous CA for LTE, with later releases adding more band combinations and higher order CA. In NR, non-contiguous spectrum is supported from Release 15 onwards, with flexible numerology and wider bandwidths. The specifications continue to evolve to support new band combinations and deployment scenarios, including those for private networks and shared spectrum.

Overall, non-contiguous spectrum is a key enabler for operators to maximize the use of their fragmented spectrum assets, delivering higher data rates and better user experience. It requires careful design in both UE and network to handle the gaps and ensure coexistence with other systems.

Purpose & Motivation

Non-contiguous spectrum exists to address the reality that spectrum allocations are often fragmented, and operators may not have access to large contiguous blocks of spectrum. Before carrier aggregation, each operator could only use a single contiguous block of spectrum per carrier, limiting peak data rates. The introduction of carrier aggregation in Release 10 allowed the combination of multiple carriers, including non-contiguous ones, to form a larger effective bandwidth. This was motivated by the need to increase peak data rates to meet the requirements of IMT-Advanced and beyond.

The concept solves the problem of spectrum fragmentation by enabling the aggregation of non-adjacent sub-blocks, thereby allowing operators to utilize their entire spectrum portfolio efficiently. It also provides flexibility in network deployment, as operators can deploy carriers in different bands and aggregate them based on availability and load. This is particularly important in scenarios where spectrum is scarce or when an operator acquires spectrum in multiple auctions.

Furthermore, non-contiguous spectrum supports interference mitigation by allowing gaps to be placed between sub-blocks to avoid interfering with other systems or to meet regulatory requirements. It also facilitates spectrum sharing and refarming, as operators can gradually migrate spectrum to new technologies while maintaining service on legacy carriers. The ongoing evolution of non-contiguous spectrum support in 3GPP ensures that networks can continue to adapt to new spectrum bands and deployment scenarios, such as those in millimeter wave and sub-6 GHz bands.