IEEE has called on 3GPP to be less ambiguous and more specific in its LAA specifications.
A March 2016 liaison notice from IEEE, giving notice of “change requests” made within 3GPP to LAA specifications, said that a number of statements made by 3GPP were either “underspecified” or “ambiguously specified”.
IEEE said that leaving things a little vague is not necessarily a problem when, like with WiFi and some licensed spectrum products, any glitches can be worked out in plugfests at a later stage. However, LAA, combining as it does WiFi and licensed spectrum equipment, does not have any such process.
Therefore, the IEEE rather politely reminded 3GPP that this “highlights the importance of doing a very good job ensuring that 3GPP LAA (and IEEE 802.11 from now on) are fully specified and unambiguous”.
Clip from IEEE liaison document submitted to 3GPP RAN-1 meeting.
Defining the ambiguities
The statement then went into IEEE members’ concerns in some detail, pulling out a number of segments.
One of these is a concern that LAA equipment may send out energy to stop other devices from attaching to unlicensed spectrum during the tiny time intervals that occur between an LAA cell gaining access to a channel and beginning to transmit.
IEEE said that, “The only way for the LAA system to stop another system from accessing the channel during this delay is to transmit energy of some sort in the channel. This energy represents a form of interference because its primary purpose is to stop other systems from accessing the channel.” This would be “contrary to the well accepted principle in unlicensed spectrum to not cause unnecessary interference to others”.
To get round this IEEE would like to see transmission of data start immediately after channel access is obtained – perhaps by allowing partial subframes to be transmitted immediately. Or equipment could delay sending energy in a channel until it is ready to transmit.
There were further concerns over how often an LAA eNodeB may send Discovery Reference Signals – too many may mean that WiFi devices see the messages as “load” on spectrum channels that in fact have no “load” in terms of user traffic. So IEEE would like to see limits on how often an LAA eNodeB can query a channel using the DRS mechanism.
There were also problems identified with how radios would detect neighbouring networks, with concerns over the use of energy detection for clear channel assessment, and of how timing of channel access “slots” are measured. There are also efficiency gains like stopping transmission once useful data has been sent – ie not transmitting unfulfilled sub frames. The IEEE would also like to see specifications ensure that only high priority data is sent, once channel access has been achieved by signalling the requirement to send high priority data. At the moment, it said, there seems to be the ability to send both high and low priority data within the same sub-frames, if there is spare space. This may increase the length of transmissions.
IEEE’s statement also said that the use of a back-off mechanism is also “ambiguous” about when the mechanism is used.
In summary then, although these may seem like esoteric points, and although everything is being carried out in a polite and co-operative manner, IEEE is calling for considerably more clarity in 3GPP LAA specifications in a range of areas.
Its statement said: “IEEE 802 believes this process of collaboration between IEEE 802 and 3GPP RAN will result in an LAA specification that better supports fair sharing of unlicensed spectrum by both 802.11 and LAA equipment. Further, it will also result in a better LAA specification that leverages the long experience of IEEE 802 in defining effective and fair sharing protocols in unlicensed spectrum.”
IEEE’s statement was distributed at 3GPP’s RAN-1 meeting held in South Korea this week, and a response is then expected from 3GPP.
