As the growing up of mobile device users and the demand of wireless multimedia services, the higher speed wireless system is expected to be developed. There are many standardization bodies and forums work for this issue, for example, Third Generation Partnership Project Long Term Evolution (3GPP-LTE) [1], Wireless World Initiative New Radio (WINNER) [2] and Worldwide Interoperability for Microwave Access (WiMAX) [3].
All of these standardization bodies and forums focus at high peak data rate, low latency, improved system capacity and coverage and flexible bandwidth operation.
In 3GPP-LTE, the peak rate requirement for downlink and uplink are set at 100Mbps and 50Mbps respectively. As the data rate increases, the latency needs to be reduced to show the practical improvement. Thus the latency requirement for LTE radio round trip time is set to be below 10ms and access delay below 300ms [4]. For the flexible bandwidth operation requirement, LTE support flexible bandwidth range from 1.4 MHz to 20 MHz [4]. The multiple access scheme in LTE adopts Orthogonal Frequency Division Multiple Access (OFDMA) for downlink and Single Carrier Frequency Division Multiple Access (SC-FDMA) for uplink. The primary advantage of OFDM based system is its resistance to the intra-cell interference. Intra-cell interference from user equipment (UE) can be ignored because of the orthogonality of subcarriers. But the neighboring cells may occupy the same subcarriers for their serving UE at the same time, it will cause significant inter-cell interference (ICI) or so-called co-channel interference (CCI), especially for users located at cell edge.
The performance of today’s cellular network is limited by interference problem more than by any other single effect [5]. Interference between cells will scale down the cell coverage and reduce the UE’s data rate. So interference mitigation is one of the key issues currently under investigation in different standardization bodies and forums. Interference
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mitigation techniques can be categorized into three main classes based on approaches adopted:
interference randomization, interference cancellation and interference coordination [6]. The
basic principle of these three approaches will be discussed in chapter 2.
1.1 E-UTRAN Architecture
3GPP proposes Evolved Universal Terrestrial Radio Access Network (E-UTRAN) as the air interface of LTE. The overall architecture of E-UTRAN is illustrated in Figure 1 [7]. In E-UTRAN, the macro base station is denoted as eNB (Evolved NodeB). The eNBs are interconnected with each other through X2 interface. The eNBs also connect to EPC (Evolved Packet Core) through S1 interface. For specifically description, EPC consist of MME (Mobility Management Entity) and S-GW (Serving Gateway). Main functions of MME in charging of are [4]
Authentication and security.
Mobility management.
Managing subscription profile and service connectivity.
and S-GW has the following main functions [4]
IP service mapping.
User Plane Tunnels for uplink and downlink data delivery.
Downlink data forwarding for handover.
Through Figure 1 we can see that each eNB must connect to at least one MME/S-GW which means that an MME/S-GW is in charge of several eNBs. The proposed method in this thesis needs a central controller of eNBs which plays an important role in the algorithm. We can take MME/S-GW as central controller of a set of eNBs.
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Figure 1 - Overall architecture of Evolved Universal Terrestrial Radio Access Network
1.2 Physical Resource and Frame Structure
Before talking about the resource allocation scheme, we need some background of LTE resource structure. The physical resource structure of LTE system for downlink and uplink is illustrated in Figure 2. The smallest resource unit in frequency domain is subcarrier which spacing is 15 kHz regardless of the total transmission bandwidth. The smallest time unit is so-called symbol which duration is 66.7 μs [4]. To put a subcarrier and a symbol together as a grid we call it resource element. But the minimum scheduling size is not resource element;
physical resource block (PRB or RB) is the basic scheduling unit both in uplink and downlink.
Physical resource block can be seen as a time- frequency grid which consists of 12 consecutive subcarriers in frequency domain. It means that the minimum bandwidth can be allocated is 180 kHz.
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Figure 2 - The concept of Resource Block in LTE system
There are two frame structure types that support by LTE system. One type is used in FDD system and another type is used in TDD system. Here we focus on FDD system. The frame structure of FDD LTE system is illustrated in Figure 3. One radio frame has 10 ms duration which consists of 10 subframes with 1 ms duration. One subframe is combined with two 0.5 ms time slots which are composed of 14 OFDM symbols.
Figure 3 - The frame structure of FDD LTE system
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Combine the knowledge of physical resource structure and frame structure mentioned above. We know that in time domain physical resource block refers to one time slot. But the Transmission Time Interval (TTI) is defined as a subframe in LTE system. TTI means the minimum time period in resource scheduling process. So, In general, one resource block have to be represented as resource elements grid where is number of symbols in a time slot and is number of consecutive subcarriers in a RB.
1.3 Organization
The remainder of this thesis is organized as follow. In chapter 2 we describe three categories inter-cell interference mitigation techniques, especially focus on interference coordination. Two well-known inter-cell interference coordination schemes, SFR and FFR, and some dynamic inter-cell interference coordination schemes will be discussed explicitly.
Based on the discussion of their advantage and drawback, a Two Phases Fractional F requency Reuse scheme has been proposed in chapter 3. The simulation results are discussed in chapter 4. Finally, the thesis is end with conclusion and future work in chapter 5.
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