Unlocking Wireless Performance with Co-operation in Base-Station Pools
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Transcript of Unlocking Wireless Performance with Co-operation in Base-Station Pools
© 2009 IBM Corporation
Unlocking Wireless Performance with Co-operation in Base-Station Pools
Parul Gupta, IBM Research – IndiaCOMSNETS - 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.
© 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 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
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
GS
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GS
MW
iMA
XTD
-SC
DM
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BS cluster
LTE
WiM
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WiM
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LTE
BS cluster
Edge gatewayManagementServer
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 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
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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
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Number of neighboring eNodeBs
RN
TP S
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bps)
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
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Quantization bits
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2 Co-operating BS
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[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
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Quantization (bits/sample)
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MS ServingBS (#1)
TargetBS (#2)
TargetBS (#3)
MOB_NBR-ADV
MOB_MSHO-REQBS #2, BS #3MOB_BSHO-RSPHandover 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_REPORTRNG_RSP
RNG_RSP
CONTEXT TRANSFER
Shorter ranging cycle
Resume normal 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 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.