If there’s one thing we know, it’s a ruckus. And there’s one going on in the world of telecommunications around the use of the unlicensed spectrum for LTE services. With Mobile World Congress just a week away, there will undoubtedly be a barrage of announcements on the topic.
This will prompt, from various constituents, strong reaction ranging from extreme antipathy and fear, to statements of expectant support.
Problem is, much of what’s being reported doesn’t paint a complete picture of what’s really going on. So here’s some context to help (with more posts to come).
Basic Background Required
In the fall of 2013, Qualcomm proposed an innovative use of unlicensed spectrum to carry LTE traffic. They referred to their proposal as LTE Advanced in unlicensed Spectrum, or LTE-U for short.
Conceptually, over the air LTE consists of a control connection between the e-Node B (LTE radio access node) and the User Equipment (UE), aka client. There is an uplink path for data traffic from the UE to the eNB, and a downlink path for data traffic from the eNB to the UE.
LTE supports two deployments models in licensed spectrum: Frequency Division Duplexing or FDD-LTE and Time Division Duplexing or TDD-LTE.
FDD-LTE is when the control and uplink are typically deployed in one band, while the downlink is deployed in a separate, paired band.
TDD-LTE, in contrast is where the control, uplink and downlink are deployed within a single band.
LTE is a full duty cycle technology. This means it is assured full use of the band(s) it is operating in. The control path is used to coordinate the airtime on the traffic channels between the eNB and its connected UEs (in TDD mode, the control path is also used to coordinate the amount of airtime which will be used for uplink and downlink operation).
LTE Advanced (LTE-A) introduced the concept of Carrier Aggregation (CA). [In licensed terms, “carrier” equates to what the Wi-Fi industry calls a “channel”.] CA allows an operator to effectively bond multiple portions of spectrum for the downlink and/or uplink to achieve greater capacity.
The original Qualcomm proposals for LTE-U were rather wide ranging, including a possibility of implementing all of the LTE paths (control, UL, and DL) in unlicensed spectrum.
3GPP (the organization responsible for LTE standardization) began looking at LTE in Unlicensed Spectrum in early 2014. A first 3GPP Workshop was held in January 2014, and a second in June 2014 which established some initial priorities for 3GPP’s activities. It was at this second meeting that 3GPP adopted the term Licensed-Assisted Access (LAA) to denote their proposed use of unlicensed spectrum for LTE. A few other outcomes from this second Workshop included decisions to focus on the 5 GHz unlicensed bands and a goal for a single global solution. At the Radio Access Network Specification Group (TSG-RAN) meeting in September 2014, a formal LAA Study Item was approved. This meant that LAA was now officially a study item for 3GPP Release 13. The main goal of the study item is to “study the LTE enhancements needed to operate in unlicensed spectrum and to ensure fair coexistence with Wi-Fi”.
3GPP members are now conducting coexistence testing and will be reporting their findings to TSG-RAN. The timeline below shows some of the milestones and expectations.
LTE-U versus LAA
There’s a lot of confusion at this point about the terms LTE-U and LAA. Some are using them interchangeably, while others have distinctly different things in mind when they mention them. For instance, Ericsson recently announced that it will introduce LAA support on some of its small cell platforms by the end of 2015.
However, LAA, as a 3GPP standard, isn’t expected to be finalized until March of 2016. So Ericsson is really referring to a pre-standard/non-standard technology, which has distinctive features from what is most likely to be standardized as 3GPP LAA.
While Qualcomm and 3GPP seem to be using consistent terminology at this point, given the growing alphabet soup, here are some basic definitions and distinctions that the industry should consider adopting as standard nomenclature to keep things clear:
Pre-standard/non-standard LTE in unlicensed spectrum implementations use 3GPP Release 10-12 CA features to provide Supplemental Downlink (SDL) service over Wi-Fi. Coexistence with Wi-Fi is provided via initial channel sensing/selection, and in the presence of Wi-Fi operating co-channel the use of on/off duty cycle mechanisms controlled by algorithms to determine the allocation of channel airtime for LTE and Wi-Fi (e.g. Qualcomm’s Carrier Sensing Adaptive Transmission – CSAT). Due to the lack of Listen Before Talk (LBT) support, this solution would only be deployable in regulatory regimes that do not require LBT such as the US, China, Korea, and India.
The proposed 3GPP Release 13 standard implementing LTE within the Unlicensed Spectrum is designed to opportunistically boost data rates. LAA can be used as a secondary carrier (channel) for the downlink only, uplink only, or both. The initial focus is on SDL – downlink only]. Coexistence with Wi-Fi operating co-channel would be provided via LBT mechanisms. Because of the LBT support, LAA would be deployable virtually worldwide.
Subsequent posts on the topic will use the terms with these definitions in mind. [Notice that neither of these current initiatives include proposals to implement the entire LTE system (control, UL, and DL) in unlicensed spectrum].
Next up, we’ll compare proposals for LTE in Unlicensed with Wi-Fi, and finally look more closely at the coexistence issues. So stay tuned.