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8/9/2019 Unlocking Parul
1/13
2009 IBM Corporation
Unlocking Wireless Performance with Co-
operation in Base-Station Pools
Parul Gupta, IBM Research India
COMSNETS - Jan 8, 2010
8/9/2019 Unlocking Parul
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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
8/9/2019 Unlocking Parul
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2009 IBM Corporation3
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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2009 IBM Corporation4
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
8/9/2019 Unlocking Parul
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2009 IBM Corporation5
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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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 becomerealizable
In this work, we estimate the potential cost savings for a few such schemes
8/9/2019 Unlocking Parul
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2009 IBM Corporation7
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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2009 IBM Corporation8
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
1 2 3 4 5 60
20 0
40 0
60 0
80 0
1000
1200
Number of neighboring eNodeB s
RNTP
Signaling
O
ve
rhead
(Kbps)
Every Slot (0.5ms)
Every 2 slots (1ms)
Every 4 slots (2ms)
Every 8 slots (4ms)
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2009 IBM Corporation9
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
0
10
20
30
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Quantization bits
Backhauloverhead(Mbps)
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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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2009 IBM Corporation10
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
0
20
40
60
80
100
120
8 9 10 12 16
Quantizati(
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8/9/2019 Unlocking Parul
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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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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 dynamiccontext (ARQ states, pending data)
BS1 BS2 BS3
Shared MS data
Co-located Base Station Pool
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2009 IBM Corporation13
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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