Separation of data and control planes

Transcription

1 planes Senior scientist 5Green Summer School KTH, Sweden 28 August 2014

2 2 Table of content Introduction Concept of separating user and control planes Modelling of control signalling traffic Impact to the design of key network functions Q&A

3 3 Introduction

4 4 Background Energy costs of mobile networks are increasing with the increase of network capacity, and in some cases (e.g. in emerging markets) even account for ~30% of operators OPEX Energy saving may be achieved via various ways Improve hardware efficiency Green transmission techniques (e.g. massive MIMO) Network management (e.g. dynamic switching-off/on, large- and small-scale) Network deployment (e.g. network sharing, radio planning) Network architecture (e.g. HetNet, user/control-plane separation) Example results-> [1]

5 5 User plane vs. control plane User plane: capacity, throughput, various-sized packets with various QoS requirements. Control plane: reliability, signalling overhead, small packets. LTE user plane protocol (3GPP TS ) LTE control plane protocol

6 6 Limits of legacy mobile networks User plane and control plane are coupled (via the same cell) A significant part of the network (coverage layer) needs to be always on, even at very low traffic load. Network-based energy saving (switching off cells) is applicable, but at relatively large time scale.

7 7 Concept of separating user and control planes

8 8 Concept in brief Conventional cell Data cell Signalling cell User plane (data) data signalling Control plane (signalling) Conventional network: Data and control signalling are served by a single cell. Coverage and capacity are always available. User/control plane separation: Data and control signalling are served by different cells. Coverage is always available, but capacity is activated on-demand.

9 9 Status 5GrEEn GreenTouch Beyond Cellular Green Generation (BCG2) project 3GPP new-carrier type (NCT)- (not standardised yet) For lower overhead, interference and energy usage More options of system bandwidths. Stand-alone (new frequency band), or Macro-assisted Similar concepts: Phantom cell (NTT DoCoMo), Soft-cell (Ericsson), etc.

10 10 Realisation options-1 (example) Radio resource control (RRC) only via the macro-cell, data bears (DRBs) only via the small-cell. Source: Huawei s presentation at the GreenTouch Paris meeting.

11 11 Realisation options-2 (example) Radio resource control (RRC) only via the macro-cell, data bears (DRBs) via both the macro-cell and the small-cell. Source: Huawei s presentation at the GreenTouch Paris meeting.

12 12 Realisation options-3 (example) Radio resource control (RRC) and data bears (DRBs) via both the macro-cell and the small-cell. Idle UEs vs. active UEs Control signal for active UE mobility may be via the small-cell. Source: Huawei s presentation at the GreenTouch Paris meeting.

13 13 Essence of being more energy efficient Dynamic switching on/off radio resources according to varying traffic demands. Dynamically switching on (off) data cells at events of traffic arrivals (departures) data cell selection is inherently a part of session setup, while not in legacy networks. enabled by context-awareness. highly bursty load per data cell. The always on part (the signalling cell) is expected to be a relatively light-weight layer than that in legacy HetNet networks with energy saving feature. Dimensioned for control signalling plus eventual limited amount of data traffic.

14 14 Essence of being more energy efficient (cont.) The on-demand part (the data cells) is with lower signalling overhead, especially at low and medium load. Reduction of fixed signalling overhead, interference With lower traffic variation in the cells, a better operation point (especially of power amplifiers) might be realized with higher energyefficiency. Source: GreenTouch BCG2 project

15 15 Modelling of control signalling traffic

16 16 Modelling of control signalling traffic Control signalling overhead has two effects the net capacity available for user data is reduced additional energy is consumed for transmitting the signal traffic

17 17 Modelling of control signalling traffic User independent L1/L2 signals and common controls BCCH/PBCH: Broadcast control channel/physical broadcast channel; SCH/SS: Synchronisation channel/synchronisation signal CRS: Cell-specific reference signal (LTE Rel-8); CSI-RS: Channel state information-reference signal (LTE Rel-10) (LTE as example) PBCH+SCH CRS CSI-RS [2] (3GPP TS )

18 18 Modelling of control signalling traffic User independent L1/L2 signals and common controls (continued) modelled as fixed overhead (in percentage) of the total resources available Signals/common control channels 1 port 2 ports 4 ports 8 ports PBCH PBCH+SCH.14 / % 17 RB N.71/ N %.29/ N %.71/ N % 15 RB SCH/SS.14 / N % 17 RB 14 RB 15 RB CRS 4.76 % 9.52 % % % CSI-RS 0.015%~0.24 % 0.015%~0.24 % 0.03%~0.48 % 0.06%~0.95 %

19 19 Modelling of control signalling traffic User-related, but non-session related signalling e.g. for the purpose of idle-mode mobility (due to e.g. location update) and network acquisition and authentication. The assumption is that this part of signalling traffic is a very small portion of total userrelated signalling traffic, and thus is neglected.

20 20 Modelling of control signalling traffic Session-related L3 signals Mainly at session setup and (for mobile users) handover The total volume of signalling traffic is delivered by a sequence of signalling messages. These messages have different sizes, consuming different amount of radio resources. LTE session setup procedure for sessions without QoS requirements (default EPS bear) (according to 3GPP TR )

21 21 Modelling of control signalling traffic Session-related L3 signals (continued) The amount of signalling traffic depends on (1) With/wo QoS requirements; (2) network- or terminal- initiated (with/wo paging message) Amount of LTE session setup signalling traffic With QoS Without QoS Terminal-initiated 120 bytes 193 bytes Network-initiated 120+5=125 bytes 193+5=198 bytes Since the minimum granularity of LTE radio resource assignment is PRB, no matter how small the messages are each of them consumes at least one PRB. On average transmission of these manages consumes 11 segregated PRBs.

22 22 Modelling of control signalling traffic Session-related L1/L2 signals Downlink L1/L2 control signals (mainly PDCCH), for scheduling, power control, etc. [2] DM-RS, for demodulation of downlink user data. It is intended for a specific terminal and is only transmitted in the resource blocks assigned for transmission to that terminal. [2]

23 23 Modelling of control signalling traffic PDCCH PDCCH is required both for the delivery of the session setup messages and the delivery of data, noted as PDCCH_s and PDCCH_d, respectively. In LTE, PDCCH delivers Downlink Control Information (DCI) for active sessions. The size of DCI differs according to the transmission mode used by the session (up to 70 bits) In LTE, the DCI is delivered using various number of Control Channel Elements (CCEs) in the control region of each TTI, each consists of 36 resource elements of one OFDM symbol. The number of CCEs used by a specific user depends on radio conditions, the transmit mode of the user, etc. According to literature, in most cases 1 or 2 CCEs would be sufficient. We assume 2 CCEs (72 REs) per active user in a specific TTI.

24 24 Modelling of control signalling traffic Modelling approach (summary) Modelling approach Note PBCH Fixed overhead (in %) SCH/SS Fixed overhead (in %) CRS Fixed overhead (in %) LTE Release-8 CSI-RS Fixed overhead (in %) LTE Release-10 DM-RS Extension of user data LTE Release-10 L1/L2 control (PDCCH) Fixed overhead (in %) With reserved resource Extension of user data 72 REs/active user/tti Session setup messages (L3), including paging message Extension of user data On average 11 segregated PRBs in LTE Note (1) Fixed overhead (in %) is often dependent of bandwidth and BS antenna number. (2) Reserved resource may not be always active in transmission (impact to overhead in power consumption).

25 25 Modelling of control signalling traffic In the case of user and data plane separation Signalling cell Fixed overhead: PBCH, SCH/SS, CSI-RS, PDCCH_s Extension of user data: session setup messages, DM-RS Note: the capacity of PDCCH_s is limiting factor. So in order to have sufficient resource for PDCCH_s channels, we reserve e.g. M 1 =6 OFDM symbols within each TTI for PDCCH_s channels. It is beyond the current limit of LTE of M 1 <=4. Data cell Fixed overhead: SCH/SS, CSI-RS (?) Extension of user data: DM-RS, PDCCH_d Note: the necessity of SCH/SS and CSI-RS depends on (1) the requirements of UE-data cell synchronisation, and (2) the synchronisation accuracy between the signalling and data cells.

26 26 Impact to the design of key network functions

27 27 Impact to the design of key network functions Essential functions System information transmission Cell search (incl. synchronization) Paging Session setup procedure Mobility management Session management and termination Extended functions (not complete) Load balancing among cells Inter-cell interference coordination Advanced scheduling (exploiting e.g. CoMP and carrier aggregation) Broadcast services (MBMS) Emergency services Self-X Dynamic on/off radio resources

28 28 Impact to the design of key network functions Business-as-usual Cell search (of idle terminals) Paging Mobility management (idle terminals) (only signalling-cells are involved) Not business-as-usual System information transmission Session setup procedure Mobility management (active terminals) Session management and termination Dynamic on/off radio resources

29 29 Session setup procedure Selection of most suitable data-cells for individual sessions the cell where the terminal requests session setup (a signalling-cell) may not be the cell which at end provides data services to the terminal (a data-cell). Challenges Potentially inactive data-cells. Availability and accuracy of context information (e.g. UE location) Time consumed during the selection process. Potential solutions Best-server location-based methods. content/.../lte-guidelines-in- ICS-Designer-v1.3.pdf Measurement-based methods

30 30 Session setup procedure (random) Access procedure ( idle-to-active transit) the terminal may need to perform two consecutive random access procedures: RACH_s and RACH_d. Challenges Additional latency during idle-toactive transits. Potential solutions Contention-free RACH_d facilitated by RACH_s. Data-cell selection and RACH-d in parallel

31 31 Mobility management (active terminal handover) Handover of active terminals: scenarios See next slide

32 32 Mobility management (active terminal handover) Handover of active terminals: in-parallel handovers The terminal may enter handover regions of both layers simultaneously. Challenges The signalling messages associated with data-cell handover makes use of the signalling connection to the signalling-cell, while in the handover region of the signalling-cell, the signalling connection is less reliable. (however, in reality such occurrence may be very rare) Potential solutions Adjust handover parameters, when the terminal predicts a risk of entering handover regions of both layers, it may trigger an earlier data-layer or signalling-layer handover

33 33 Reference 1. R. Litjens, Y. Toh, H. Zhang, O. Blume, Assessment of the Energy Efficiency Enhancement of Future Mobile Networks, IEEE WCNC 2014 Conference, Istanbul, April E. Dahlman, S. Parkvall, and J. Skold, LTE/LTE-Advanced for Mobile Broadband, Chapter 10 & Chapter 14, C. Hoymann, D. Larsson, H. Koorapaty, et al., A Lean Carrier for LTE, IEEE Communication Magazine, vol. 51, no. 2, pp , Feb H. Ishii, Y. Kishiyama and H. Takahashi, A Novel Architecture for LTE-B: C- plane/u-plane Split and Phantom Cell Concept, IEEE Globecom Workshop: International Workshop on Emerging Technologies for LTE-Advanced and Beyond-4G, Anaheim, CA, USA, Dec

34 34 Questions?