Why spectrum sharing?
Commercial access and use of spectrum has traditionally been authorized in two ways: either through individual licenses or in accordance with license exempt (unlicensed or ‘commons’) rules. It is believed much of that spectrum is lightly used or even not used. At a time when most observers believe people, organizations and businesses will need vastly more Internet and communications capacity, that is a waste of scarce resources. To move incumbent users to a new frequency band is also a very costly and time consuming proposition. Thus, spectrum sharing offers a cheaper and quicker way to maximize use of scarce resources.
The long-neglected 3.5GHz spectrum band is coming into its own at last. The past week has seen Huawei launching the first LTE-Advanced device for the band, and T-Mobile joining the group of companies advocating it as suitable spectrum for a flexible licensing approach, and for the emerging LTE-LAA (Licensed Assisted Access) technology.
Many advocates see the unpaired 3.5GHz spectrum as ideal to be dedicated to small cells, which are well suited to the short range of high frequency base stations, and to create a near-global roaming platform and ecosystem. The current FCC consideration of plans for shared access in 3.5GHz may set interesting precedents for flexible licensing regimes which could encourage the uptake of new approaches, such as LTE-LAA.
While most work on this technology - which allows LTE to run in licence-exempt or shared bands to provide supplemental downlink for a licensed-band 4G network - has been focused on 5GHz, some believe 3.5GHz would be a better option as it does not involve potential conflicts with WiFi, and could be led by LTE. T-Mobile USA's influential CTO, Neville Ray, has supported this view, in a presentation called 'Promoting Innovation in the 3.5GHz Band'.
Although he still expects most short term LAA effort to be focused on 5GHz, the 3GPP has been specific that the standard should not be specific to one band. Ray said it was important that the FCC - which will set the pace for some other regulators - should remain flexible in its rules for the band, arguing that with respect to LAA, "no additional regulation is necessary" because it will be designed to be a good neighbour - better, in many cases, than other technologies which would be able to access the portion of the band that the FCC proposes to assign for 'General Authorized Access' (GAA). There will be two other tiers, one for protected incumbent government users, and one licensed to operators, possibly on a short term basis, under a Priority Access Licence (PAL) scheme.
Last week, lobby groups New America Open Technology Institute (OTI) and Public Knowledge (PK) met with the FCC to reiterate their support for keeping a majority of the 3550-3700MHz band reserved for GAA, and allowing opportunistic access to PAL frequencies until actual commercial services are operational from licensees. However, they do not want LTE-LAA to lead in this band in case it keeps WiFi out, and expressed concerns that Qualcomm, Verizon and others have been reportedly testing pre-standard LAA technologies in 3.5GHz. "Although Qualcomm has indicated this technology is most likely to be deployed by carriers in the 5GHz unlicensed bands, it could readily be used by licensed carriers in the 3.5GHz band," the filing said.
In other parts of the world, 3.5GHz has generally been assigned for fixed broadband wireless and has been used for proprietary or WiMAX networks in many areas. Some of those networks are being converted to TD-LTE, especially where regulators are allowing mobility. For instance, when UK regulator added mobile usage to the licence, 3.5GHz spectrum holder UK Broadband launched TD-LTE services in parts of London.
In Europe, CEPT has agreed to harmonize spectrum between 3400MHz and 3600MHz for 'TDD-preferred' allocation and 3600MHz to 3800MHz as TDD-only. And the three major Japanese cellcos recently gained 40MHz of spectrum in this band for TD-LTE. Other LTE-Advanced plans in this TDD spectrum have been announced in the Philippines, Bahrain, Canada, Argentina and Peru, said Huawei, and Australia's National Broadband Network company has been trialling the technology too.
Huawei claims to have launched the world's first 3.5GHz TDD LTE-A device, supporting two-carrier aggregation and maximum downlink speed of over 220Mbps.
Some companies are placing high hopes on a future flourishing of this spectrum, including small cell start-up Accelleran, which last year upgraded its M101 TD-LTE small cell to support the US spectrum, in the hope that the FCC's complex proposal will soon be finalized.
As first-wave LTE deployments, most of them in paired FDD spectrum such as 1.8GHz, start to reach capacity, many operators plan to 'densify' using small cells and TDD spectrum, which is particularly suited to downlink-heavy applications and for minimizing interference in multilayer HetNets.
"'At Accelleran we are very aware that one of the key requirements for operators is to have a small cell platform in 3.5GHz that can reduce the TCO,", said Jeff Land, director of business development at the start-up last year.