© 2009 IBM Corporation

Unlocking Wireless Performance with Co-operation in Base-Station Pools

Parul Gupta, IBM Research – India

COMSNETS - Jan 8, 2010

© 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

© 2009 IBM Corporation3

Why Co-operate?

There is demand for supporting 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 work

shows 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 and

streaming 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 Transactions on 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 architecture with 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-peer BS interface (where exists) or a higher hierarchical element like RNC or ASN Gateway

© 2009 IBM Corporation5

BS

BS

BS

BS

Radio network controller

Radio network controller

Mobile switch center

Service support node Gateway

PSTNPSTN

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

InternetInternet

SMS/MMS

IMS

Content Service

Web Service

GS

M

GS

MW

iMA

XT

D-S

CD

MA

BS cluster

LTE

WiM

AX

WiM

AX

LTE

BS cluster

Edge gatewayManagementServer

BillingEdgegateway

© 2009 IBM Corporation6

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 become realizable

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

© 2009 IBM Corporation7

Interference Avoidance

Capacity of full frequency reuse systems gets limited due to interference, esp. for cell-edge users

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

Less complex, but takes a capacity hit

Each BS needs to share its power information with neighbors

Cell 1 Cell 2

Cell 3

Full Frequency Reuse System

Fractional Frequency Reuse System

© 2009 IBM Corporation8

Interference Avoidance – Example Communication Cost

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

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

Similarly for Uplink, Interference indicator messages restricted to once every 20 ms to avoid excess overhead

1 2 3 4 5 60

200

400

600

800

1000

1200

Number of neighboring eNodeBs

RN

TP

Sig

nalin

g O

verh

ead

(Kbp

s)

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 transmitters and receivers: K2

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

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

Note: Spectrum to feedback CSI to the transmitter potentially an issue. TDD systems can utilize channel reciprocity to estimate downlink-CSI

0

10

20

30

40

50

60

4 6 8

Quantization bits

Ba

ck

ha

ul

ov

erh

ea

d (

Mb

ps

)

2 Co-operating BS

3 Co-operating BS

4 Co-operating BS

[1] V. Cadambe and S. A. Jaffer, “Interference alignment and degrees of freedom for the K-users interference channel,” IEEE Transactions on Information Theory, vol. 54, no. 8, pp. 3425-3441, Aug 2008[2] H. Zhang et. Al., “Asynchronous Interference Mitigation in Co-operative Base-Station Systems”, IEEE Transactions on Wireless Communications, Vol. 7, No. 1, Jan 2008

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

© 2009 IBM Corporation10

Uplink Macro-Diversity

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

The extra gains due to Maximal Ratio Combining are untapped due to large amounts of data exchange and computation complexity

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

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

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

0

20

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60

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100

120

8 9 10 12 16

Quantization (bits/sample)

MR

C t

raff

ic o

verh

ead

(M

bp

s)

h1 h2

x

y1 y2

© 2009 IBM Corporation

MS ServingBS (#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/UCDRNG-REQ

AUTHENTICATION

Resume normal operation

MOB_SCN-REQ

MOB_SCN-RSP

RNG-RSP

Multiple iterations to adjust local parameters

REG-REQREG-RSP

Service interruption duration

…RNG-REQRNG-RSP

End Tx/Rx

Scan Channel

Scan Channel

RNG_REQ

RNG_REQ

MOB_ASC_REPORT

RNG_RSP

RNG_RSP

CONTEXT TRANSFER

Shorter ranging cycle

Resume normal operation

Faster Handovers with Co-operation

© 2009 IBM Corporation12

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 dynamic context (ARQ states, pending data)

BS1 BS2 BS3

Shared MS data

Co-located Base Station Pool

© 2009 IBM Corporation13

Summary and Future Work

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

– 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 potential cost-savings. Future work would be to demonstrate working schemes in a BS pool and solve associated issues.