Capacity and Load Sharing in Dual-Mode Mobile Networks

Author: Juha PeuraSupervisor: Prof. Patric ÖstergårdInstructor: M.Sc. Jarkko Laari, DNA Finland Oy

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Agenda

• Background• Objectives of the thesis• Capacity• Load sharing• Conclusions

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Background

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UMTS Downlink Data [MB]

• The amount packet data in mobile networks have increased dramatically • Operators have to guarantee quality of service• New solutions needed for traffic handling

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Objectives of the thesis

• What are the main performance bottlenecks in todays mobile networks?• Is it possible to ease the situation with load sharing algorithms?

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Capacity – channel elements

• Channel element is a measure of node B hardware resources• Separate CE pools for UL/DL, common to all sectors• One 12.2 kbps speech service uses one channel element• HSUPA takes up to 32 CEs, non-serving cells reserves also CEs• CEs are a capacity bottleneck in uplink direction • Many RAX-boards have 64 CEs• Suggested minimum is 128 CEs

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Capacity – Iub transmission

• Symmetric bit pipe between base station and RNC• 3GPP have specified two transport methods: ATM and IP• Implemented using so called E1s• Maximum throughput of one E1 is 2 Mbps• For speech traffic one E1 has been sufficient• Fast packet connections need multiple E1s• Transmission is a capacity bottleneck in downlink • Base station buffers data from Iub• Future choice – Ethernet/IP transmission

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Capacity – HSDPA

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Throughput[Mbps] = 0.0039 x SINR^2 + 0.0476 x SINR + 0.1421

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• HSDPA uses the power margin left over from R99 services• HSDPA throughput depends on achievable Signal-to- interference and noise ratio (SINR)• Power allocated for HSDPA effects the throughput largely

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Load sharing

• Enhances performance by pooling together resources • Inter Frequency Load Sharing

- Traffic sharing between WCDMA carriers• Directed Retry to GSM

- Speech traffic diverted from WCDMA to GSM

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Directed Retry to GSM

Why?

• 3G most beneficial for PS data users• 3G UEs are becoming more common and coverage improves constantly -> free capacity to GSM• More resources for data users• Balances load between networks• No additional investments

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Directed Retry to GSM - limitations

• Only applicable to speech traffic• Coverage of UMTS and GSM cells should be same• Overloading of GSM network possible• Should not be used if GSM -> UMTS handovers are in use (ping-pong effect)• GSM target cell quality not guaranteed (blind ISHO) -> call drops• Increased signaling, mobiles not reachable during LA updates• Configuration to entire network can be laborious• 3G users may wonder why they are in GSM

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Directed Retry to GSM - principles

Cell load (downlink carrier power)

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sharing threshold

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this load directed to GSM

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capacity reserved for HSDPA

• Redirection decission based on cell load (used downlink carrier power)• After cell load exeeds specified sharing threshold, speech calls are diverted to GSM network• Sharing fraction parameter specifies the percentage of directed calls while the cell load is above the sharing threshold• Released power can be allocated for PS users• Sharing parameters can be assigned independently to each cell

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Load sharing - traffic profiles

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Load sharing - measurements

• Functionality and different parameters were first tested in a single cell• Larger scale test in live network for a three week period• 20 most loaded cells were chosen for the measurement• Feature was tested with ”radical” parameters to really find out how load sharing performed• A set of key performance indicators (KPI) was defined to assess the effects of the feature

• DR-success ratio, Speech setup success rate, dropped calls,

admission number, lack of CEs, speech traffic (Erl), PS traffic ….

• Network counters were used to gather information about the functionality• Raw data was filtered and manipulated for final results

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Load sharing - results

• Directed Retry to GSM worked well in overall• Total of 93117 speech call redirection attempts, 86033 were succesful• DR-success rate was 92 %, with carefull cell selection > 95 % success rate possible

Directed Retry to GSM success rate

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Load sharing – results UMTS

KPI DR - FALSE DR - TRUE Difference

Speech setup success rate 99,63 29,29 -71 %

Dropped calls percentage 0,36 0,53 48 %

Speech traffic (Erl) 1,12 0,39 -65 %

PS R99 traffic (Erl) 1,86 2,06 11 %

PS R99 setup success rate 94,41 98,96 5 %

PS R99 retainability 94,14 97,97 4 %

HS traffic (Erl) 0,72 0,79 10 %

HS User Thu DL (kbps) 139,24 158,21 14 %

HS User Thu UL (kbps) 33,38 42,58 28 %

HS setup success rate 97,32 97,65 0 %

HS completion success rate 79,35 76,36 -4 %

CS speech payload (kbits) 240 706 657 99 813 954 -59 %

HSDPA RAB attempts 153 603 164 147 7 %

HS drop % 17,13 20,06 17 %

Admission number 2 949 1 504 -49 %

Failed after admission 3 013 2 789 -7 %

NG user down-switches 15 733 8 057 -49 %

UL hardware lack 2 351 817 -65 %

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Load sharing – results GSM

KPI DR - FALSE DR - TRUE Difference

TCH attempts 623 921 820 990 32 %

TCH H_Block % 0,06 0,02 -62 %

TCH T_Block % 0,73 1,66 129 %

TCH RF_Loss % 0,14 0,23 60 %

TCH traffic (Erl) 2,67 3,26 22 %

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Conclusions

• Transmission, CEs and HSDPA power allocation can form a capacity bottleneck• Load sharing between UMTS and GSM works reliably, if configured well• Performance of PS users can be enhanced with Directed Retry to GSM, at least a little• 3G traffic still relatively low, it is questionable if load sharing is needed at this point.

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Future research

• Load sharing between UMTS carriers• more sophisticated feature than Directed Retry to GSM• between UMTS2100 and UMTS900• two way directions taking into account cell load• applicable to all services

• Service based handover

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THANK YOU !