Micro cells

Description

Micro cells are a category of small cells in cellular networks, characterised by their relatively small coverage area compared to macro cells. The term is defined in 3GPP TS 21.905 simply as 'small cells', reflecting the broad and somewhat generic nature of the concept. In practice, micro cells typically cover a range of a few hundred metres to a kilometre, depending on the radio technology, frequency band, and deployment environment. They are deployed to provide additional capacity and coverage in areas where macro cell coverage is insufficient or where user density is high.

Micro cells are part of the broader small cell family, which also includes pico cells, femto cells, and metro cells. The distinction between these categories is primarily based on transmit power, coverage radius, and deployment scenario. Micro cells generally have higher transmit power than pico and femto cells but lower than macro cells. They are typically deployed outdoors on street furniture (e.g., lampposts, building walls) or indoors in large venues such as shopping malls and airports.

In terms of architecture, micro cells can be deployed in various configurations. In traditional distributed antenna systems (DAS), micro cells may use remote radio heads (RRHs) connected to a central baseband unit via fibre. In more modern deployments, micro cells may be integrated into small form-factor base stations (e.g., gNodeB or eNodeB) that connect to the core network via backhaul links (fibre, microwave, or DSL). In 5G networks, micro cells are often deployed as part of a heterogeneous network (HetNet) architecture, where they complement macro cells to provide seamless coverage and capacity.

Micro cells play a crucial role in network densification, which is a key strategy for meeting the capacity demands of modern mobile networks. By reducing the cell size, operators can reuse spectrum more efficiently, increase the number of simultaneous users per unit area, and improve the signal-to-interference-plus-noise ratio (SINR) for users. Micro cells also help extend coverage to areas that are difficult to reach with macro cells, such as deep indoor environments, underground stations, and dense urban canyons.

From a radio perspective, micro cells typically operate at lower transmit power than macro cells, which reduces interference and allows for tighter frequency reuse. They may use the same frequency bands as macro cells (co-channel deployment) or dedicated bands (dedicated deployment). In co-channel deployments, interference management techniques such as enhanced Inter-Cell Interference Coordination (eICIC) and Coordinated Multi-Point (CoMP) are used to mitigate interference between macro and micro layers. In 5G, micro cells can operate in both sub-6 GHz and mmWave bands, with mmWave micro cells providing very high capacity over short distances.

The term 'Micro cells' has been maintained in 3GPP TS 21.905 from Release 4 through Release 19. The definition text has remained essentially unchanged, although minor formatting variations (e.g., extra spaces) appear in some releases. These variations are not substantive and do not reflect any change in the technical meaning of the term. The stability of the definition reflects the fact that the concept of micro cells is well-established and does not require frequent revision.

Purpose & Motivation

Micro cells exist to address the fundamental limitations of macro cell networks in terms of capacity and coverage. Macro cells, which typically have a coverage radius of several kilometres, are designed to provide wide-area coverage with a limited number of base stations. However, as mobile data traffic has grown exponentially, macro cells alone cannot provide sufficient capacity in dense urban areas and indoor environments. Micro cells solve this problem by providing additional capacity through spatial reuse of spectrum and by extending coverage to areas that macro cells cannot reach effectively.

Historically, the concept of micro cells emerged in the 1990s as operators began to face capacity constraints in urban areas. Early micro cell deployments were primarily used to fill coverage holes and to offload traffic from congested macro cells. With the advent of 3G and 4G technologies, micro cells became an integral part of heterogeneous network (HetNet) architectures, where multiple layers of cells (macro, micro, pico, femto) work together to provide seamless coverage and capacity. In 5G, micro cells are even more important due to the use of higher frequency bands (e.g., mmWave) that have limited propagation range and require dense deployment.

The micro cell concept also addresses the need for flexible and cost-effective network deployment. Unlike macro cells, which require significant civil engineering work and large equipment, micro cells can be deployed quickly and at lower cost using small form-factor equipment. This makes them ideal for targeted capacity enhancements, temporary events (e.g., sports stadiums), and indoor coverage solutions. The ongoing evolution of micro cells towards higher levels of integration and automation (e.g., self-organising networks) continues to make them a vital tool for mobile operators.