The CBRS 3.5 GHz framework in the US is an innovative take on spectrum sharing. Regulators and the rest of the wireless industry worldwide are watching the adoption of CBRS in the US because it has the potential to increase spectrum utilization and to create entirely new business models, and to strengthen existing ones (e.g., small cells or neutral host).
The CBRS regime creates many opportunities to inject capacity into existing networks, to deploy new networks (e.g., MSOs), to make existing models financially sustainable (e.g., small cells), and to create new types of networks (e.g., LTE private networks). From this perspective, it is more difficult to see how CBRS will not succeed, rather than whether it will succeed at all.
But there is definitely uncertainty as to how it will succeed – or what aspects of CBRS the market will embrace. The CBRS band was created by combining the 3550–3650 MHz band, reserved for military radars and fixed satellite services (FSS) receive stations, and the 3650–3700 MHz band, available to the same users and to WISPs. The spectrum covered by the CBRS band in the US in LTE Band 42 (3550–3660 MHz) and the ITU LTE Band 43 (3660–3700 MHz). Work is under way to create a CBRS-specific LTE band – ITU Band 48 (3550–3700 MHz).
The CBRS framework has been designed to ensure fair access to all users, based on their tier and on the near-real-time demands on band access. In an environment where demand outstrips available capacity, some users will be granted access, and some will not – and this will change with time as demand and availability change.
For PAL and GAA users, access is opportunistic, not guaranteed, although PAL access rights are nearly guaranteed in areas where there is no incumbent activity. At the core of CBRS, and what makes it unprecedented, is the mechanism for picking the users who will be able to transmit. It is centered around two new functions – the ESC and the SAS – which enable spectrum coordination.
The ESC monitors the activity of military incumbents and transmits information on CBRS use to the SAS. The SAS takes into account ESC information and zone-exclusion requirements to allocate spectrum access fairly to tier 2 (PAL) and tier 3 (GAA) users in a way that maximizes spectrum utilization and provides interference protection.
During the initial phase of CBRS deployments, the SAS will be required; the ESC will be introduced later. To protect incumbents during this interim period, CBRS access for PAL and GAA users will be blocked in areas where incumbents are present. This will restrict availability of spectrum, but also ensure a smooth transition to the shared-spectrum regime. It will also encourage the rollout of private networks ahead of public access ones; private networks typically need less area and often are located indoors, so they are less likely to request access in an area with ongoing incumbent activity.
The SAS communicates the authorization to use a specific spectrum band to the CBSDs – the RAN elements in CBRS networks. The FCC defines three types of CBSD: a low-power one for indoor/outdoor use, and two outdoor CBSDs – one of them higher-powered for rural environments. In turn, the CBSDs transmit to the EUDs, the end-user devices, which may establish a connection with the CBSD only when authorized to do so.
For more information, download the report “Learning to share. CBRS in the 3.5 GHz band changes how we use spectrum”
