Both 3rd Generation Partnership Project (3GPP) and Worldwide Interoperability for Microwave Access (WiMAX) forum are developing the next generation communication systems that are called Long Term Evolution (LTE) and WiMAX respectively. Both 3GPP and WiMAX forum also propose their own femto-cell system recently. And Femto-cells are named as Home eNode Bs (HeNBs) in 3GPP.
Telecom service providers must keep installing macro base stations (BS) in order to deal with the increasing number of 3G and 4G users. There are many costs if the operator needs to setup a new macro BS, such as BS itself, housing, electricity and backhaul to the core network. Although there are many macro BSs, users still get poor signal quality inside buildings since macro BSs must be placed outside buildings. Adopting femto-cells can reduce costs and improve signal quality inside buildings.
Femto-cells are very small and low cost BSs, but femto-cells still need to provide the same functionalities as macro BSs do. Note that femto-cells are approximately of the same size as the current Wi-Fi access points. Femto-cells are deployed at customers’ premises, such as home and office. In addition, femto-cells are configured by the core network automatically, so customers can install femto-cells by themselves without professional knowledge. Femto-cells are powered from the customers’ electricity sockets and the customers’ internet connections are used as backhaul connections.
For operators, deploying femto-cells can reduce costs such as housing and electricity bill that are necessary for macro BSs. Furthermore, many value-added services will be integrated into femto-cells, and customers that have installed femto-cells are unlikely to change their subscription if they are satisfied with the services. Femto-cells also enhance the indoor coverage. As mentioned above, lack of indoor coverage is the weakness when
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deploying macro BSs only.
For customers, in addition to enhanced indoor coverage, short communication rage between femto-cells and UEs means that using time of mobile devices can be prolonged.
And operators may give discounts to those who installed femto-cells.
Although femto-cells, or named as HeNBs for LTE in 3GPP, have many benefits for operators and users, the development of femto-cells still encounter many challenges. The basic requirement of HeNBs is that users can place HeNBs in anywhere without the aid of operators, and it may raise many problems.
Because that HeNBs could be placed in anywhere, HeNBs need to negotiate with the core network to setup all necessary parameters. After the installation of HeNBs, owners of the HeNBs may want to restrict unknown users from accessing their own HeNBs, so 3GPP defines the Closed Subscriber Group (CSG). Obviously, HeNBs do not have the information of the CSG after installation; HeNBs must negotiate with the core network to acquire the necessary information.
The paging message is needed when a connection request to the mobile station is raised. In the cellular environment, the paging message is sent to a group of macro BSs. The set of those macro BSs are called a paging group. Because the positions of those macro BSs are known by the operator, the design of paging group can be done easily. In the HeNB environment, the locations of HeNBs cannot be known in advance, the setup of paging group must be dynamic. And the paging group must be carefully designed in order to avoid the flooding of paging messages.
The traffic between a HeNB and the core network goes through the internet, so the security of both data and control flow are important. There must be a security tunnel between a HeNB and the core network to protect the messages from eavesdropping and altering.
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Different kinds of HeNBs may have different functionalities in the future. Thus, the authentication between a HeNB and the core network is very important. In addition, each operator has their own operation frequency. HeNBs must acquire the frequency parameter from the core network and operate in the pre-defined frequency. Optionally, if a connection is established between two HeNBs, data packets are not necessarily transmitted from the source HeNB to the core network and then from the core network to the destination HeNB.
If the data packets can be transmitted directly from one HeNB to another without going through the core network, the loading in core network can be released.
The motivation and objectives of this thesis are described as follows. As mentioned before, the number of concurrently connecting users is restricted by the limited data rate of femto-cells, and there will be many HeNB users in buildings. This means that UEs’
connections would be blocked because of the lack of resources. In the mean time, the number of connecting users would be further restricted if HeNBs provide QoS support. If connections of the overloaded HeNB can be transferred to the non-overloaded one by handover, the number of concurrently connecting users can be increased.
We consider QoS requirement and load balance index between HeNBs to achieve our goal. In addition to QoS requirement and load balance between HeNBs, we also consider the movement of UEs. In other words, we would choose the UE to the appropriate HeNB based on the movement and the UE can acquire the minimum QoS requirement at least.
Hence, the proposed method can balance the loading between femto-cells while guarantee the minimum QoS requirement.
The reset of this thesis is organized as follows. We give an overview of 3GPP HeNB system in chapter 2. And we introduce some related works about load balance in chapter 3.
Then the proposed load balance method is discussed in chapter 4. And we give the performance evaluations in chapter 5.
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