Jose F. Monserrat (Universitat Politècnica de València)
The specification of the 3GPP Release 14 began in September 2014 and marked the beginning of the New Radio (NR) specification, which will be the 3GPP candidate to 5G, as defined by the ITU-R for the IMT-2020 family of standards. The Release 14 is near its conclusion, as the definition of protocols (stage 3) is expected to be completed in June 2017, with the subsequent revisions to correct problems and bugs.
Definitely, the key input of Release 14 is the start of work on the specification of a new RAT non-backward compatible with LTE-A and with parallel evolution. This radio technology, which is known as New Radio, has started with a study item in Release 14 on scenarios and requirements, which began in December 2015. To date this study item has defined several important aspects of what shall be the proposal from 3GPP for 5G. In parallel, and from the second quarter of 2016, they are also studying the most appropriate technological solutions to meet the requirements marked. However, it is not going to be until the Release 15, when this NR is specified, expecting its realization in phase 1 in the second half of 2018.
However, Release 14 is much more than the beginning of the 5G, and there are more than 30 studies affecting aspects as diverse and as important as the following: V2X communications, improved location services, reducing latency in LTE, the separation of the user plane and control (so important for virtualization), improvements in the use of unlicensed spectrum, the extension relaying schemes for communication between machines, the carrier aggregation between bands, various improvements in broadcasting, or the extension in the number of antennas to more than 16. Because its special relevance, this post extends the description of three aspects, improved latency for LTE, the V2X communication aspects and Licensed-Assisted Access.
Improved latency for LTE
The item entitled “The study on techniques for LTE Latency reduction”, was finalized in June 2016 in the technical report 3GPP TR 36.881, giving terminate this study item from that point.
This improvement work mainly focused on improving semi-persistent scheduling (SPS), handover latency and reduction of the TTI length.
Regarding the former, it was interesting to enable SPS solution with 1 TTI period. This greatly reduced signalling for users with a high demand for resource availability. Regarding improving handover latency, the possibility to make a handover without a new RACH process was studied as well as maintaining connected the source cell throughout the handover period. Although both solutions were stressed as promising, it was not addressed the feasibility of these ideas. Finally, with regards to the reduction of TTI length, the simulation results were not promising for many services, and it was concluded not to reduce the TTI length below 1 slot, i.e. 0.5 ms.
Enabling direct communications between vehicles within the cellular system is a key to deal with the necessary security for the autonomous cars deployment. The standardization of V2X communications began in Release 13 with a study item on the requirements of ITS services. There are several specific aspects of this type of communication that make it particularly complex, including the relative lack of synchronism between the terminals and the high speed of transmitter and receiver, which requires a higher density of pilots to enable a proper coherent detection. In Release 14 these issues are addressed within the study item “Support for V2V services based on LTE sidelink”.
Although the study item has not yet been closed, the radio aspects are considered already completed being included in 3GPP technical report TR 36.785, while operational procedures are expected to be completed by March 2017.
The system is expected to operate with different bandwidths, including 10 MHz, using a dedicated carrier for V2X communications and the use of GNSS from satellites for time synchronization.
Two configurations have been defined. In configuration 1, the system is fully distributed, both for interference management and for scheduling, and it was defined a new way of scheduling, mode 4, which allows sensing and semi-persistent scheduling. Resource allocation also depends on the geographic information.
In configuration 2, mode 3 scheduling is used, which allows eNBs to assist in decision-making regarding interference management and scheduling, by using specific signalling over the Uu interface. In short, the eNB determines the set of resources that vehicles distribute dynamically.
3GPP already included in Release 13 the possibility of transmitting in downlink in secondary cells operating in unlicensed spectrum, with control of a main cell operating in licensed spectrum. It is what is known as Licensed-Assisted Access.
LAA improvements are included in Release 14 within the study item known as “Enhanced LAA for LTE”. In late 2016, major contributions have focused on the necessary changes within the core of the wireless protocols to support this functionality, with modifications especially in the RRC and MAC protocol and physical capabilities of the user equipment and base stations. Other aspects are under development, and it is estimated that enough progress on the LAA issue won’t be expected until mid-2017.
The main complexity of LAA lies in the coexistence with other protocols in unlicensed bands, such as the IEEE 802.11 family. Therefore, LAA should include procedures for listen-before-talk (LBT) and discontinuous transmission schemes to allow lower occupancy of possible channel. Furthermore, Release 14 will include the transmission in LAA for the uplink also, so that signalling must be highly compressed compared to conventional operation.