Pseudo-Handover Based Power and Subchannel Adaptation for Two-tier Femtocell Networks

Hongjia Li, Xiaodong Xu, Dan Hu, Xin Chen, Xiaofeng Tao and Ping Zhang

IEEE WCNC 2011

Report: 吳承祐

Outline

Introduction Many Problem System Model and Problem Formulation Pseudo-Handover Based Power and Subchannel Adaptation

Scheme Pseudo-Handover based Scheduling Information Exchange Subchannel and Power Adaptation Scheme Subchannel Allocation Phase Power Control Phase

Performance Evaluation Conclusion

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Introduction

The co-tier co-channel interference among femtocells and the cross-tier co-channel interference between the femtocell tier and the macrocell tier is one of the most urgent challenges.

In the universal frequency reuse scheme, how to avoid interference through effective resource management and interference management schemes are critical requirements for the femtocell configuration.

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Many Problem

But how to implement the information exchanging procedure, considering the cost of overhead, the cross-tier and co-tier synchronization and compatibility with LTE and LTE-A systems, is not well discussed in prior works.

But also of the specific non-CSG MUEs which are in the femtocell coverage.

Since Macrocell is the infrastructure of mobile communications, the service quality of MUEs should be guaranteed as a priority, which means that femtocell should adjust its resources.

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System Model and Problem Formulation

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Therefore, considering minimizing the interference to neighboring co-channel UEs (including MUEs and FUEs) and satisfying the camped FUEs’ data rate requirements

Cont.

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Pseudo-Handover Based Power and Subchannel Adaptation Scheme

If FBS1 allocates subchannels occupied by FUE (2,2) or MUE1, collision interference can jam the communication of them.

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Pseudo-Handover based Scheduling Information Exchange

As same as the regular handover procedure, UEs start searching for femtocells when they are in their vicinity. However, because femtocells only provide services to their CSG users, performing a handover in the two-tier femtocell networks is not always a possible option.

This feature can be utilized to execute a control plane handover for co-tier and cross-tier scheduling information exchanges.

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Cont.

In the procedure of pseudo-handover, every FBS sets up and maintains the table containing the IDs of non-CSG interferers (hereafter, we call them pseudo-handover users) and their scheduling messages, which are obtained in the process of Pseudo-Handover.

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Subchannel and Power Adaptation Scheme

Having received the scheduling message, the FBS can find out subchannels which are being used by pseudo-handover users, and delete these subchannels from its candidate subchannel set to construct a new candidate subchannel set .

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Subchannel Allocation Phase

1. Given transmit power , FBS k calculates the service rate gains for for all FUE i (in ) on subchannel n:

2. FBS k calculates the sum of the current service rate gain of for all FUEs (in

) on subchannel n:

3. Assign subchannel n to the FUE according to the rule:

4. Update the FUE i∗’s subchannel set as

5. Exclude subchannel n from as as

6. Consider the next subchannel in , and go back to Step (1) until is empty

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Power Control Phase

The FBS minimizes the transmit power of the allocate subchannels according to FUEs’ data rate requirements.

FBS k solves this problem by each FUE i independently. And are referred to as Lagrange multipliers.

Then, the necessary and sufficient conditions for optimality are given by the Karush-Kuhn-Tucker (KKT) conditions:

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Cont.

1. Calculate the transmit power for all items in

2. If any item in is less than zero, set

3. According to current , current FUE i’s data rate is calculated As

4. If , decrease a the predefined step size and return to Step (1). Or else, the power allocation of FUE i is completed.

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Performance Evaluation

Comparison scheme 1-the proposed scheme without PHO:The proposed scheme does not operate the PHO based collision

interference avoidance. Comparison scheme 2-the random allocation scheme :Each femtocell randomly selects a subset of subchannel for

transmission. The fraction of radio resources per transmission interval accessible by each femtocell is set as 60%.

Comparison scheme 3-the scheme based on reuse 3: The system frequency reuse factor is 3, and the femtocell tier can

only use 2/3 frequency spectrum that is not allocated to its local macrocell. The transmit power of each femtocell is controlled according to its FUEs’ data rate requirements.

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CDF of MUEs’ SINR

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CDF of FBSs’ Transmit Power

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Reciprocal of co−tier intereference power received by FUE

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Average throughput per FUE varying with the number of femtocells

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Conclusion

Performance evaluation results show that minimizing the interference radiated by each FBS is in turn beneficial to reduce every FBS’s own required transmit power and improve its FUEs’ throughput.

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每週一句 It is noticed from the power allocation algorithm that more

transmission power is assigned to the subchannel which experiences less interference or better link gain, while no transmission power is allocated to the subchannel, (i.e., the subchannel is turned off), which experiences heavy interference caused by cross-tier or co-tier interference from MBSs or neighboring FBSs.

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