Monday, 21 September 2015

TD-LTE-eNodeB-Analysis of the Key Technologies and Protocol Stacks for the VoLTE

Networking Environment

1.Definition: VoLTE(Voice over Long Term Evolution), which is defined by the 3GPP,  is the ultimate voice solution based on the IMS network. And it can provide high-definition voice service based on IP in the LTE network covered area.
2.VoLTE Bearers: From the angle of radio, For VoLTE, the bearer need to be built in the following way: 
(1) Bearer combination of voice: SRB1+SRB2+2xAM DRB+1xUM DRB, among them, QCI of UM DRB is 1; QCI of AM DRB is 5 and 8/9 respectively.
(2) Bearer combination of video: SRB1+SRB2+2xAM DRB+2xUM DRB, among them, QCI of UM DRB is 1 and 2 respectively; QCI of AM DRB is 5 and 8/9 respectively.
(3) Qos Bearers: QCI=1: voice;QCI=2: video;QCI=5: SIP/SDP transfers the IMS signaling;QCI=8/9: Internet.
(4) To achieve either the voice or the video service, the UE needs to establish three data bearers at the same time; the UE also needs to establish the RRC signaling link bearers: SRB1 and SRB2.  

3.VoLTE user registration: Before the user experiences the high quality calls, you must experience the process
of registration firstly.From the angle of radio, the registration can be divided into two steps: 
(1) LTE radio registration: Since VoLTE for the radio is essentially a kind of data business, the E-UTRAN network needs to provide a data transmission channel. That is, to establish the bearer of QCI = 8/9.
(2) IMS registration: The terminal which supports VoLTE to complete the registration of LTE, will be by the LTE bearer to register the IMS network (similar to the high-level application registration), That is, to establish the bearer of QCI = 5. After the registration is completed, the user can use the voice call. That is, the core network will establish a bearer of QCI = 1 used for the transmission of IMS voice package.
4.VoLTE protocol stacks:
(1) Overall protocol stacks are described as the below image: which is divided into the Control Plane and User Plane.

Problem Cause Analysis
(2) Control plane protocol stacks are described as the following image: the SIP/SDP flow established above on the UDP/IP is used to control the applications between terminals. The SIP flow is used for initialize a Session, and is responsible for the transmission of SDP package. And the SDP package is used to describe a Session containing what kind of media data and inviters, etc. SIP/SDP flow is transferring IMS signaling, which needs to establish a bearer of QCI = 5 with the highest priority. In order to guarantee its correctness, Radio Control Plane adopts RLC AM mode.


(3) User plane protocol stacks are described as the below image: the RTP/RTCP flow established above on the UDP/IP, When all the inviters need to be notified through their own terminal, the RTSP starts to control certain Media of communication, such as the control information of RTSP requires to start to play Audio/Video, then using the RTP to transmit real-time data, in the process of transmission, RTCP is responsible for the QoS. Voice and audio need to establish the bearer of QCI = 1 and QCI = 2. According to the request of transmission delay, the Radio User Plane adopts the RLC UM mode so as to ensure the real-time demand. 



5.Mapping of protocol stacks and key technologies are described as the following image:


(1) ECN (Explicit Congestion Notification): According to the hollow link, the eNodeB informs the UE to change the code rate of voice business in order to improve the capacity and coverage of system.
 Summary and Notes
(2) ROHC (Radio Overhead Compression): ROHC can compress the head of packets of the layers, effectively reducing the size of packets and improving the transmission efficiency.
(3) SPS (semi-persistent schedule): as the period of a voice business is 20 ms and it has certain regularity. using the SPS to allocate PDCCH resources can save CCE resources, and thus improving the access number of the system users.
(4) TTI Bundling: TTI Binding can be called sub frames binding. Designed for the edged cell users, it is one of the effective methods to improve the coverage of the edged cell users.

Budi Prasetyo

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