Interference in LTE Small Cells:

Transcription

1 Interference in LTE Small Cells: Status, Solutions, Perspectives. Forum on small cells, 2012, December. IEEE Globecom 2012 Presenter: Dr Guillaume de la Roche Mindspeed France 1

2 Mindspeed: Short history A Legacy of Innovation Semiconductor Systems 2

3 Mindspeed Industry Firsts #1 Market Position First to Market FTTx VoIP Data Processors Multicore ARM-based Broadband CPE High-Density Crosspoint Switches 4G/LTE Base Station SoC GPON/GEPON Optical PMDs Video/VoIP Convergence Processor - Dual mode DFF TDD LTE SoC already deployed and leadership for LTE in Korea. - Common lab with main chinese operator for TD-SCDMA/TD-LTE 3

4 Mobile Broadband Architecture Going Distributed Macro Micro Pico Business Enterprise Femto Femto Metro Urban High-Density Subs Business Metro Residential Supported Subscribers Serving a broad range of basestations - from Macro to Femto 4 M-D-YYYY

5 Outline Source of interferences Solutions Spectrum management. eicic. Receiver design SON Perspectives 5

6 Source of interference (1/3) Pico enb Macro enb CSG HeNB Hybrid HeNB Operator's Core Network Relay Open HeNB Internet Even in simple small scenario, where small cell do not overlap much, there are multiple sources of interference 6

7 Source of interference (2/3) Uplink Downlink Pico enb Macro enb CSG HeNB Hybrid HeNB Operator's Core Network Relay Open HeNB Internet Small cells are beneficial to operators only if both UL and DL interference are properly managed 7

8 Source of interference (3/3) Co-layer interference Uplink: due to uplink signal from UEs reaching each small cell Downlink: Due to small cells signals. Cross layer interference Uplink: due to all Ues Downlink: Due to all cells Main difference introduced by small cells: In traditional cellular networks, macrocells are installed and maintained by operator, and frequency planning was quite easy. With small cells, number of cells may be so huge that there is more chance of overlap between cells. Moreover some small cells may be deployed by customers at uncontrolled positions.

9 Solutions to cope with interference 1) Intelligent power and spectrum management 2) ICIC 3) Receiver design

10 Power and spectrum management (1/3) Spectrum management Cross channel deployment different carriers are used for small cells and macrocells Macro-to-small-cell interference is avoided Co-channel deployment All cells share the whole spectrum Higher performance can be reached, if resources are correctly allocated Hybrid channel deployment Combination of both previous techniques can be implemented. 10

11 Power and spectrum management (2/3) How to allocate the spectrum between cells and users? In LTE, sets of resource blocks (Rbs) are scheduled to the users Frequency reuse schemes (FRS) are commonly used Exemple: Many combinations can be implemented: - depending on number of sectors - based on distance from macrocells - based on known positions - based on traffic and mobility f 11

12 Power and spectrum management (3/3) Power management: the shape of the best server area of each small can be optimized depending on conditions, so that overlap between neighboring small cells is minimized. Solutions: Power control: Based on estimation of power from neighboring cells (can be known network monitoring mode or CQI reported by Ues) Idle mode: based on presence of UEs, radiated power can be switched on only when necessary. Optimized antennas: small cells can use combination of different antennas (sector antennas, patch antennas, MIMO) in order to change direction of signal where needed. 12

13 Intercell interference Coordination (ICIC) When number of small cells is too high, power and frequency management (interference avoidance techniques) as described before is not always easy to implement. ICIC are techniques to improve coordination In LTE advances it is known as eicic. Relies on use of two major techniques Picocell range expansion Almost blank subframes

14 Picocell range expansion (RE) Pico enb Macro enb Picocell Range Expansion In RE, UEs are able to connect to cells which have not the strongest signal It is a virtual expansion of the small cell range. With RE, macrocells can be eficiently offloaded. This technique requires a very good synchronization between cells (X2 in LTE) therefore used for picocells only.

15 Almost blank subframe (ABSF) Macro Small cell t With this techniques some subframes can be left empty (except pilots so that the network keeps synchonized). The amount of ABSF can be adapted. ABSF can be allocated to small cells or macrocells. In example of this figure: When ABSF is used at macrocell layer there is no interference with small cells. During that time, users at the edge of the macrocell can used RE to the small cells.

16 Small cell design (1/2) When designing small cells, useful techniques can help to manage interference. Network Monitor mode (sniffing) is used to listen to neighboring cells and know their parameters GPS can be used to know position of cells Successive interference cancellation (SIC) where the decoded signal is successively removed from remaining one, so that other signals can be decoded. Advanced RF to support more bands and more antennas.

17 Small cell design (2/2) HetNet (Heterogeneous Network) is not only combination of small and big... but can also be sen as combination of standards... Hardware constraints. Advantage of system on chips Cost constraints. Energy consumption. Radio interface. Complexity of algorithms. Portability and reuse [4] G. de la Roche and A. Taylor. A new wave in wireless: Small cells for a heterogeneous network. EE Times, November

18 Self Organizing Network (1/2) SON in 3GPP [8] Network Monitor and measurement reports Self-configuration Self-organization Self-healing SON is one of the ways to handle interference (centralized vs distributed).

19 Self Organizing Network (2/2) With SON interference optimization can be performed in real time Power control and frequency parameters can be changed depending on: Statistics from UEs. Mobility, traffic. Errors in the network.

20 Perspectives Multi Rat small cells CoMP Carrier aggregation

21 Multi-RAT small cells In the future small cells will combine different technologies such as 3G, 4G, Wifi. Therefore in order to reduce interference, users need to be connected to the optimal RAT. Changing the user from a RAT to another should be smooth. Example: news small cells have hotspot 2 capabilities, which can be included in the same small cells.

22 Hotspot 2.0 HS 2 is based on IEEE u Subscription Service Provider (SSP) is responsible for managing the user s subscription and credentials Homogeneous Extended Service Set ID (HESSID) which identifies BS that belong to same network Access Network Query Protocol (ANQP) which allows UE to request information Op B Wifi network Op B AAA Op A AAA Connected UE from Operator A

23 Coordinated multi point (CoMP) CoMP require X2 interface but is very efficient was to reduce interference. With CoMP, neighboring cells can perform beam forming in a synchronized way. Coordinated Multi-Points Joint scheduling/beamforming

24 Carrier aggregation (CA) 20MHz Rel 8 Continuous Non continuous Multiple bands F CA is introduced in LTE Rel10 where bands up to 100 MHz can be used (larger bands are expected in future)

25 Carrier aggregation (2/2) CA brings a new dimension to all interference avoidance and mitigation techniques. Larger spectrum makes it easier to avoid overlap between small cells Larger spectrum makes it more challenging: RF design is an issue. Synchronization is an issue.

26 Conclusion Interference is an important issue in small cells. Current solutions to mitigate interference are Power management Spectrum and frequency management Receiver design and SON ICIC In future small cells will also use: More RATs (e.g., combines with Wifi) More coordination (CoMP) due to X2 interface More spectrum (CA) This will give to chances to fight against interference but new techniques will have to be proposed.

27 Thanks!