Interference in LTE Small Cells:

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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

Mindspeed: Short history

A Legacy of Innovation

Semiconductor Systems

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Mindspeed Industry Firsts

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Optical PMDs FTTx VoIP Data Processors

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4G/LTE Base Station SoC

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M-D-YYYY

Mobile Broadband Architecture – Going Distributed

Supported Subscribers

1200 400 200 50 10

Business

Pico Femto

Residential

Enterprise Femto

BusinessMetro Business Business

Micro

Urban

Macro

Metro High-Density

Subs

Serving a broad range of basestations - from Macro to Femto

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Outline

• Source of interferences

• Solutions

• Spectrum management.

• eICIC.

• Receiver design

• SON

• Perspectives

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Source of interference (1/3)

Macro eNB

CSG HeNB

Relay Pico eNB

Open HeNB

Hybrid HeNB

Operator's Core

Network Internet

Even in simple small scenario, where small cell do not overlap much, there are multiple sources of interference

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Source of interference (2/3)

Macro eNB

CSG HeNB

Relay Pico eNB

Open HeNB

Hybrid HeNB

Operator's Core

Network Internet

Uplink Downlink

Small cells are beneficial to operators only if both UL and DL interference are properly managed

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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.

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Solutions to cope with interference

 1) Intelligent power and spectrum management

 2) ICIC

 3) Receiver design

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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.

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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

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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.

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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

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Macro eNB

Pico eNB

Picocell Range Expansion

Picocell range expansion (RE)

 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.

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Almost blank subframe (ABSF)

 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.

Macro

Small cell t

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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.

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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 2011.

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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).

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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.

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Perspectives

 Multi Rat small cells

 CoMP

 Carrier aggregation

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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.

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Hotspot 2.0

 HS 2 is based on IEEE 802.11u

– 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

UE from Operator A

Op B Wifi network

Op B AAA

Op A

AAA Connected

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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

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Carrier aggregation (CA)

Rel 8 F

20MHz

Continuous Non

continuous Multiple bands

CA is introduced in LTE Rel10 where bands up to 100 MHz can be used (larger bands are expected in future)

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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.

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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.

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Thanks !