Pico Cells

Description

Pico cells are a type of small cell in cellular networks, defined in 3GPP specification 21.905 as 'cells, mainly indoor cells, with a radius typically less than 50 metres.' They are used to provide localized coverage and capacity, particularly in areas with high user density such as shopping malls, airports, offices, and train stations. Pico cells are a fundamental component of heterogeneous networks (HetNets), where they are deployed alongside larger macro cells to improve overall network performance.

In terms of architecture, a pico cell is typically served by a low-power base station, often referred to as a pico base station or pico NodeB (in UMTS) or pico eNodeB (in LTE). These base stations transmit at significantly lower power than macro base stations, which limits their coverage to the intended small area. The backhaul connection for pico cells can be via fiber, copper, or wireless links, depending on the deployment scenario. In UMTS, pico cells are integrated into the radio access network (UTRAN) and can be managed as part of the same network as macro cells, with mobility support for UEs moving between them.

The primary role of pico cells is to offload traffic from macro cells and to provide better signal quality indoors, where macro signals may be weak due to building penetration losses. By deploying pico cells, operators can increase network capacity in hotspots without the need for additional macro sites, which are costly and difficult to acquire. Pico cells also enable higher data rates because the shorter distance between the UE and the base station reduces path loss and allows for higher-order modulation and coding schemes.

In 3GPP standards, pico cells are supported through features like soft handover (in UMTS) and carrier aggregation (in LTE and 5G). In UMTS, pico cells can be included in the active set of a UE, allowing seamless handover between macro and pico layers. The concept of pico cells has evolved with later releases, where they are often deployed as part of self-organizing networks (SON) to automate configuration and optimization. In LTE, pico cells are typically deployed with a bias in cell selection to encourage UEs to connect to them, a technique known as cell range expansion (CRE).

Pico cells are distinct from other small cell types like femtocells and microcells. Femtocells are typically user-deployed and operate in closed subscriber groups, while pico cells are operator-deployed and open to all subscribers. Microcells are larger than pico cells, with a radius of a few hundred metres. The definition in 21.905 specifically characterizes pico cells as mainly indoor with a radius under 50 metres, though in practice they can also be deployed outdoors in dense urban areas.

The introduction of pico cells in Release 4 was part of the ongoing evolution of UMTS to support higher data rates and better coverage. Since then, the concept has been extended to LTE and 5G, where small cells are even more integral to network densification strategies. In 5G, pico cells are part of the ultra-dense network (UDN) vision, supporting millimeter wave frequencies and massive MIMO. The fundamental idea remains the same: to provide high-capacity, low-power coverage in localized areas.

Purpose & Motivation

Pico cells were introduced to address the growing demand for mobile data capacity and coverage, particularly in indoor and hotspot environments. Macro cells alone often struggle to provide adequate signal strength and capacity in areas with high user density or significant building penetration losses. Pico cells solve this by bringing the base station closer to the user, reducing path loss and enabling higher data rates. They also offload traffic from macro cells, which can become congested during peak hours.

The motivation for pico cells stems from the limitations of macro-only networks. Macro cells are expensive to deploy and operate, and their coverage is often imperfect indoors. Additionally, the available spectrum is limited, so improving spectral efficiency through smaller cells is a key strategy. Pico cells allow frequency reuse over smaller areas, increasing overall network capacity. They also support the deployment of new services that require high throughput and low latency, such as video streaming and interactive applications.

Historically, pico cells emerged in Release 4 as part of the UMTS evolution, but their importance grew with the advent of HSPA and LTE, where higher data rates and more sophisticated interference management techniques made small cell deployments more viable. In 5G, pico cells are a cornerstone of network densification, enabling the use of higher frequency bands and supporting massive connectivity. The definition in 21.905 has remained stable, reflecting the enduring nature of the concept.