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2009 IBM Corporation

Unlocking Wireless Performance with Co-

operation in Base-Station Pools

Parul Gupta, IBM Research India

COMSNETS - Jan 8, 2010


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2009 IBM Corporation2

Overview

Why Co-operate?

Base Station co-operation in present network architecture

Pooled Base Station architecture

Potential cost savings through pooled BS model for a few scenarios

Interference Avoidance

Interference Alignment

Uplink Macro-Diversity

Efficient handovers

Summary and Future work


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Why Co-operate?

There is demand forsupporting many users with high data rates at high mobility.Challenges:

Spectrum is limited: Reuse desirable

For systems with spectrum reuse, capacity is fundamentally limited by interference

With the trend towards smaller cells for reducing transmit power and better reuse,

handovers become more frequent

Base Stations (BS) can co-operate to

Spatially multiplex many independent data streams on the same channel. Prior workshows increased channel rank for such virtual arrays [1]

Distributed Transmit Beamforming

Interference Avoidance and Interference Cancellation Load Balancing via joint-scheduling

Reduces latency during handoff, necessary for real-time applications like VoIP andstreaming video

[1] V. Jungnickel, S. Jaeckel, L. Thiele, L. Jiang, U. Krger, A. Brylka and C.V. Helmolt, Capacity measurements in a cooperative MIMO network,IEEE Transactionson Vehicular Technology, vol. 58, no. 5, pp. 2392-2405, Jun 2009.


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Co-operation in Distributed Network Architecture

Assumption of infinite backhaul not always true

US has 75% copper, 15% fiber and 10% microwave.

Companies like Clearwire are leasing T1 bundles for their new network deployment:

6 T1s per Wimax BS in Manhattan!

Cost increases with each extra T1-line leased: $400 p.m. for 1.54 Mbps

Some co-operation schemes might still be possible in the distributed network architecturewith limited backhaul

Schemes need to be designed appropriately for constraints, e.g. limited co-operation

There is a cost associated with communication over the backhaul: whether over a peer-peerBS interface (where exists) or a higher hierarchical element like RNC or ASN Gateway


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BS

BS

BS

BS

Radio networkcontroller

Radio networkcontroller

Mobile switch center

Service supportnode Gateway

PSTN

Access Network Core Network

Present 2G-3G Wireless Network architecture

Service Network

SMS/MMS

WAP GW

4G Wireless Network with Co-located Base-Station Pools

Internet

SMS/MMS

IMS

Content Service

Web Service

BS cluster

BS cluster

Edge gateway

ManagementSe

rver

BillingEdgegateway


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Base Station Pools eliminate communication costs in co-operation

Information resides in a common place, transparently accessible to all BSs

Make fine-grained communication possible

Co-operation schemes require exchanging high volumes of data in short times becomerealizable

In this work, we estimate the potential cost savings for a few such schemes


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Interference Avoidance

Capacity of full frequency reusesystems gets limited due tointerference, esp. for cell-edge users

Interference can be avoided with jointresource allocation and power control,e.g. Fractional Frequency Reuse

Less complex, but takes a capacity hit

Each BS needs to share its powerinformation with neighbors

Cell 1 Cell 2

Cell 3

Full Frequency Reuse System

Fractional Frequency Reuse System


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Interference Avoidance Example Communication Cost

Relative Narrowband Transmit Power (RNTP)messages specified in LTE specifications canindicate interference in the Downlink

Contain a bitmap for each Resource Block (100per slot in 20 MHz bandwidth)

Similarly for Uplink, Interference indicatormessages restricted to once every 20 ms to avoid

excess overhead

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Every Slot (0.5ms)

Every 2 slots (1ms)

Every 4 slots (2ms)

Every 8 slots (4ms)


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Interference Cancelation

Dimensionality of channel matrix with K transmittersand receivers: K2

For sharing this information with all co-operatingBSs, communication cost grows as K3

Example backhaul calculations are done assumingthe complex CSI for the 720 data subcarriers, 10MHz Wimax channel, fed back every 10 ms

Note: Spectrum to feedback CSI to the transmitterpotentially an issue. TDD systems can utilizechannel reciprocity to estimate downlink-CSI

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Rather than avoiding interference, co-operating BSs can pre-code the transmitted signals to minimizeinterference at the receiver

Interference alignment [1]

Asynchronous Interference mitigation [2]

More complex because of signal processing

Assumes all co-operating BSs have full Channel State Information (CSI) at the transmitter


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Uplink Macro-Diversity

Macro-Diversity schemes today (e.g. in Macro-Diversity Handover in Wimax) in the uplink relyon selection diversity

The extra gains due to Maximal Ratio Combiningare untapped due to large amounts of data

exchange and computation complexity

Example calculation shown for communicationcost for 10 MHz Wimax channel, 2:1 DL:UL ratio,5 ms frame, assuming 3 samples need to betransmitted per subcarrier

The amount of data to be transferred over thenetwork is large, even for few quantization bits

Base-Station Pools eliminate this communicationcost over the network, making MRC realizable

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2009 IBM Corporation

MSServingBS (#1)

TargetBS (#2)

TargetBS (#3)

MOB_NBR-ADV

MOB_MSHO-REQBS #2, BS #3

MOB_BSHO-RSP

Handover to BS # 2

MOB_HO-IND

DL/UL MAP,DCD/UCD

RNG-REQ

AUTHENTICATION

Res 9 @ eAB

C

malB

D eratiB

A

MOB_SCN-REQ

MOB_SCN-RSP

RNG-RSP

lti le iterations toadjust local parameters

REG-REQ

REG-RSP

Service interruptionduration

RNG-REQ

RNG-RSP

End Tx/Rx

Scan Channel

Scan Channel

RNG_REQ

RNG_REQ

MOB_ASC_REPORT

RNG_RSP

RNG_RSP

CONTEXT TRANSFER

Shorterra gi g y le

Res 9 meA ormal operation

Faster Handovers with Co-operation


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Faster Handovers with Co-operation

Handovers can be made faster by

Co-ordination between base stations for ranging

Transfer of static context (service flow,authentication & registration info) and dynamiccontext (ARQ states, pending data)

BS1 BS2 BS3

Shared MS data

Co-located Base Station Pool


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Summary and Future Work

Co-operation between Base Stations can improve wireless system performance in variousways

Interference Avoidance and Interference Cancellation

Load Balancing via joint-scheduling

Macro-Diversity Schemes

Faster Handovers

Fine-grained co-operation becomes possible due to transparent information sharing in Base-Station Pools

So far, we have set the motivation for co-operation in BS pools through estimating potentialcost-savings. Future work would be to demonstrate working schemes in a BS pool and solveassociated issues.

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