A New Handover Mechanism for Femtocell-to-Femtocell

Adviser: Frank , Yeong - Sung Lin

Presented by Li Wen Fang

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Agenda• Introduction • The Hidden FAP Scenario • Proposed Handover Mechanism• Performance Analysis• Conclusion

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Agenda• Introduction • The Hidden FAP Scenario • Proposed Handover Mechanism• Performance Analysis• Conclusion

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Introduction• It is estimated that nearly 23% of calls and over 90%

of data services occur indoors.

• A poor coverage problem

• The femtocell networks which use FAP and existing networks as backhaul connectivity can both fulfill the upcoming demand of high data rate for wireless communication system and extend the coverage area.

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Introduction• Femtocell is so important for the following reasons :

1. Femtocell can provide indoor coverage for places where macrocells cannot.

2. Femtocell will significantly improve the total network capacity by reusing radio spectrum indoors.

3. Femtocell is a good solution for FMC (Fixed Mobile Convergence).

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Introduction• Femtocell is so important for the following reasons :

4. Femtocell can offload traffic from the macrocell and improve macrocell capacity.

5. Femtocell can provide significant power saving.

6. Users will pay to install femtocells.

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Introduction• Three access modes for FAP:

– CSG (Closed Subscriber Group) mode

– OSG (Open Subscriber Group) mode

– Hybrid access mode

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Introduction• CSG (Closed Subscriber Group) mode

– only for CSG users

– The membership of the CSG can be modified by the service level agreement between the subscriber and the access provider.

• OSG (Open subscriber Group) mode– open to any user

• Hybrid access mode– non-CSG users can only get limited services

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Introduction• Three types of handovers in femtocell networks:

– femtocell-to-macrocell handover

– macrocell-to-femtocell handover

– femtocell-to-femtocell handover

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Introduction• In [7], a new handover algorithm based on the MS’s

speed and Quality of Service (QoS) is proposed.

– better performance than traditional handover algorithm

• Dense femtocells deployment is the ultimate goal of the femtocell networks.

– So handover between femtocells is also very important.

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Introduction• In this paper, we try to build a neighbor femtocell list

which considers several factors such as received signal level, the hidden FAP problem and the access mode of FAP.

• And then we propose a new handover mechanism for femtocell-to-femtocell.

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Agenda• Introduction • The Hidden FAP Scenario • Proposed Handover Mechanism• Performance Analysis• Conclusion

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A. Basic concepts of femtocell

B. Hidden FAP Scenario

The Hidden FAP Scenario

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Basic concepts of femtocell• Femtocell is a low-power and small-capacity base

station which can provide high quality and high speed transmission of wireless communication services to balance the loading of macrocell.

• It also contains RNC (Radio Network Controller) and all the core network elements.

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Basic concepts of femtocell

Figure 1. The basic structure of femtocell access

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Hidden FAP Scenario• Whenever a mobile station realizes that the received

signal strength from the serving FAP is going down, it will receive many signals from several neighbor FAPs for handover.

• neighbor femtocell list

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Hidden FAP Scenario• The hidden FAP problem is the case when a

neighbor FAP is very close to the MS but the MS cannot receive a strong enough signal level from the hidden FAP because of some barriers between them.

• The hidden FAP will not be included in the neighbor femtocell list if the neighbor femtocell list is based on RSSI (received signal strength indicator) only.

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Hidden FAP Scenario

Figure 2. The hidden FAP scenario

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Agenda• Introduction • The Hidden FAP Scenario • Proposed Handover Mechanism• Performance Analysis• Conclusion

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Proposed Handover Mechanism

• The handover from femtocell to femtocell is demanding and complicated since there are hundreds of possible target FAPs when MS moves out of its serving FAP’s coverage.

• In traditional scheme for femtocell-to-femtocell handover, handover decision is made according to received signal level only.

– It is not enough to consider RSSI level only

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Proposed Handover Mechanism

• The access mode of FAP may bring a result that MS may not be permitted to get access to the target FAP which is selected by the serving FAP and it will lead to a handover failure.

• The lack of radio resources in the selected FAP will also cause a handover failure.

• Hidden FAPs in the neighbor femtocell list will cause a handover failure.

– We take received signal strength level, the access mode and location of neighbor femtocells into consideration.

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Proposed Handover Mechanism• When the serving FAP coordinate with the neighbor

FAPs, it will acquire those FAPs’ access mode and current load.

• A priority setting

– CSG mode femtocell who’s allowed CSG list includes MS’s ID.

– Hybrid mode femtocell who’s allowed CSG list includes MS’s ID.

– OSG mode femtocell.

– Hybrid mode femtocell who’s allowed CSG list dose not include MS’s ID.

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Proposed Handover Mechanism

• Once FAP receives the handover requests containing the RSSI information of a neighbor FAP whose RSSI level is stronger than a threshold , it will set a priority to this neighbor FAP.

• The neighbor FAP within a range of distance whose RSSI level is less than , but stronger than is regarded as hidden FAP and it will also get a certain level of priority.

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Proposed Handover Mechanism

• “If at least one highest priority FAP has spare bandwidth resource, it will be put into the neighbor femtocell list.”

• We may face a problem that multiple FAPs with the same priority exist in the neighbor femtocell list.

– comparing their RSSI level and current load

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Proposed Handover Mechanism

• Our proposed scheme considers initially the received RSSI level, and then the hidden FAP problem. Finally, we set priority to certain FAPs and make a neighbor femtocell list according to the rule.

• The serving FAP can choose a appropriate neighbor FAP from the list. If there is no FAP meet the requirements, the MS keeps detecting signals from neighbor femtocells.

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Proposed Handover Mechanism

Figure 3. Flow chart of the proposed mechanism

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Agenda• Introduction • The Hidden FAP Scenario • Proposed Handover Mechanism• Performance Analysis• Conclusion

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Performance Analysis• We randomly distribute femtocells. The MS is assumed

to be at the edge of the reference femtocell.

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Performance Analysis

Figure 4. A comparison of handover failure rate

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Performance Analysis• The result shows that our scheme has a lower

handover failure possibility than the traditional scheme.

• It is possible that the target femtocell is not included in the neighbor femtocell list.

– cause a handover failure to the target femtocell

• The increasing number of deployed femtocells increases the possibility that neighbor FAPs coordinate with the reference FAP and keep informed about the location of the hidden FAPs.

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Performance Analysis

Figure 5. A comparison of the number of femtocells in neighbor femtocell list

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Performance Analysis• The result shows that the neighbor femtocell list of

our proposed scheme contains a smaller number of femtocells than the traditional scheme.

• However, the reduced number of neighbor femtocells in the neighbor femtocell list does not make the handover failure probability rise.

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Performance Analysis• Our proposed scheme has some other benefits:

– Some unnecessary handovers can be avoided and the QoS can be ensured through setting priority of FAPs.

– It could keep a load balance in some extent.

– It can lighten the burden of the resources of open mode femtocell.

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Agenda• Introduction • The Hidden FAP Scenario • Proposed Handover Mechanism• Performance Analysis• Conclusion

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Conclusion • The dense femtocell scenario is the ultimate goal

of the femtocell networks.

• We propose a new handover mechanism which considers many factors such as access mode, hidden femtocell and signal strength.

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Thanks for listening !