Preferential Access

Description

Preferential Access is a concept defined in 3GPP specifications 22.043 and 42.043, primarily associated with Licensed Shared Access (LSA). LSA is a regulatory approach that allows new users (LSA licensees) to share spectrum with incumbent users (e.g., government or military) under a set of conditions. Preferential Access ensures that LSA users receive priority treatment when accessing the resources of LSA cells, which are cells deployed specifically for LSA operation. This means that in scenarios where both LSA and non-LSA users are present, the LSA users are given precedence in resource allocation, ensuring they receive the service quality they are entitled to.

The technical implementation of Preferential Access involves several network elements and procedures. When an LSA user attempts to access an LSA cell, the network must identify the user as an LSA subscriber. This is typically done through subscription information stored in the HSS/UDM and conveyed to the access network. The base station (eNodeB or gNB) then applies scheduling policies that prioritize LSA users over non-LSA users. This can be achieved through differentiated admission control, resource reservation, or weighted scheduling algorithms. For example, the scheduler might allocate a higher proportion of resource blocks to LSA users or preempt non-LSA users if necessary to maintain the LSA service level.

Preferential Access is closely tied to the LSA architecture, which includes the LSA Repository, LSA Controller, and the LSA1/LSA2 interfaces. The LSA Repository holds information about spectrum availability and usage conditions. The LSA Controller manages the spectrum resources and communicates with the network's Operations, Administration, and Maintenance (OAM) system to configure the LSA cells. When an LSA cell is activated, the network must ensure that LSA users can access it preferentially. This may involve broadcasting specific cell access barring parameters or using dedicated signaling to indicate the cell's LSA status.

In practice, Preferential Access is essential for meeting regulatory obligations. Incumbent users often require assurances that their spectrum usage will not be adversely affected by LSA users. By giving LSA users preferential access, the network can guarantee that LSA services are delivered with the agreed quality, even in the presence of other traffic. This is particularly important in scenarios where LSA spectrum is shared with mobile network operators (MNOs) who also serve non-LSA users on the same cell. The preferential treatment ensures that the LSA licensee's users are not starved of resources.

The concept has been maintained across 3GPP releases from Rel-4 to Rel-19, although its application has evolved with technology. In early releases, it was primarily relevant to GSM and UMTS networks. With the advent of LTE and 5G, the principles have been adapted to new radio access technologies. The core idea remains the same: LSA users get priority access to LSA cell resources. This ensures that the sharing arrangement is viable and that all parties adhere to the agreed terms.

Purpose & Motivation

Preferential Access was created to support the Licensed Shared Access (LSA) regulatory framework, which allows spectrum sharing between incumbent users and new LSA licensees. The primary motivation was to enable more efficient use of spectrum, which is a scarce resource, while protecting the rights of incumbent users. Without preferential access, LSA users might not receive the guaranteed performance they need, making the sharing arrangement unattractive and potentially violating regulatory conditions. Thus, Preferential Access ensures that LSA users can rely on the network to prioritize their traffic.

Before the introduction of LSA and Preferential Access, spectrum sharing was limited, and new entrants often faced challenges in obtaining dedicated spectrum. The traditional approach was to allocate exclusive spectrum licenses, which led to spectrum fragmentation and underutilization. LSA introduced a new paradigm where spectrum could be shared under a set of rules. However, for this to work, the network had to be able to differentiate between LSA and non-LSA users and treat them accordingly. Preferential Access solves this by providing a mechanism to give priority to LSA users, thereby making spectrum sharing feasible.

Historically, the concept was introduced in Release 4, aligning with early discussions on spectrum sharing in Europe. It has since been refined and adapted to new technologies. In 5G, similar concepts are applied in the context of spectrum sharing frameworks like CBRS (Citizens Broadband Radio Service) in the US, where Priority Access Licenses (PAL) users get preferential treatment over General Authorized Access (GAA) users. The 3GPP specifications continue to define Preferential Access as a key requirement for LSA, ensuring interoperability and consistent implementation across vendors.