There are several researches to overcome the issue of cross-slot interference on dynamic TDD mode in the recent years. They are briefly introduced as follows.:
3.1.1 Effect of Cross-Slot Interference in Different Wireless System
Interference Analysis and Resource Allocation for TDD-CDMA Systems to Support Asymmetric Services by Using Directional Antennas [5]
This paper explores the advantages of using directional antennas in TDD-CDMA system to support asymmetric traffic services and analyze the interference of the TDD-CDMA sys-tem with trisector cellular architecture, where three directional antennas are employed at each base station. The system architecture is shown in Fig.3.1. Each directional anten-nas can support different uplink and downlink ratios for the sector coverage to implement asymmetric traffic services, but the strong cross-slot interference is frequently generated.
A sample approach to avoid this interference is fixing same uplink and downlink ratio among all cells. The disadvantage of this sample approach is loss the flexibility of assigning uplink and downlink ratio as increasing new call blocking rate in the cell. Thus, the concept of virtual cell is introduced in the paper and virtual cell is composed of three sectors from the three adjacent cells. They can provide an additional degree of freedom for allocating radio resource. Fixing the same uplink and downlink ratio by considering requirement of the virtual cell to avoid the cross-slot interference. This approach can lower the new call blocking likes a dynamic TDD mode and improves the distribution of SINR better than other traditional approaches.
Figure 3.1: The architecture of virtual cell is composed of three sectors from the three adjacent cells.
Figure 3.2: The architecture of Red-TSA considers the MS location information to schedule time slots.
Time Slot Allocation Based on Region and Time Partitioning for Dynamic TDD-OFDM Systems [8]
In this paper, the MS location information is being considered to make properly schedul-ing to avoid the MS is locatschedul-ing at the cell boundary and transmittschedul-ing in cross time slot region in a TDMA system. The author provide a new time slot allocation (TSA) strategy, region-based decentralized time slot allocation (RED-TSA) strategy which utilizes partial location information of MS. The diagram of RED-TSA is shown in Fig. 3.2 and it can effective to reduce the effects of cross-slot interference than traditional strategies, Fix-TSA and Greedy-TSA. The Fix-TSA which fixes the same ratio of uplink and downlink time slots in all cells to mitigate the cross-slot interference and the Greedy-TSA is just like dynamic TDD mode which each cell can support different uplink and downlink ratios.
The new RED-TSA is a new scheduling approach to reduce the effect of cross-slot interference. Each cell is divided into inner and outer region. According to consider the effect of path loss, the inner region has better link quality than outer region. If Ms transmits or receives signal at inner region will get higher SINR than MS locates at outer region. Hence, the MS locates at inner region can tolerate stronger inter-cell interference
and due to this reason which the RED-TSA arranges thesis MS that locates at inner region transmit or receives signal on time slots which near the predefined boundary (likes the switch point introduced in last section). This scheduling method can reduce the effects of cross-slot interference as avoiding received the cross-slot interference when Ms locates at cell boundary.
Furthermore, the author also provides an analytical approach to find out the optimal predefined boundary and analyze system performance by using the Central Limit Theo-rem and Queueing TheoTheo-rem. The mathematical analysis can quickly compute the system performance and find out the optimal predefined boundary.
3.1.2 Inter-Cell Interference Reduction of Fractional Frequency Reuse (FFR) Multi-Cellular System
Optimal Fractional Frequency Reuse (FFR) in Multi cellular OFDMA System [14]
The fractional frequency reuse technique is a good interference management that intro-duced in last chapter. However, the advantages of fractional frequency reuse is introintro-duced in this paper. In addition, the author proposes an optimal analysis (Primal Dual Interior Point Method) [21] to find out the optimal design factors in FFR scheme based OFDMA system in downlink case. This analysis method will compute the optimal numbers of sub-channel can be used in each region. The simulation result shows the advantages on FFR scheme that can against the effects of inter-cell interference and improves system through-put by suitable design factors of FFR scheme. Hence, the fractional frequency reuse scheme is suitable to solve the effects of inter-cell interference.
Table 3.1: Comparison between propose work and recent research about effect of cross-slot interference.
Multiple Access Division Duplexing System Service Main Solution [4]− [5] CDMA Dynamic-TDD Asymmetric Directional antenna
[6]− [8] TDMA Dynamic-TDD Asymmetric Location-TSA
[9]− [11] CDMA/OFDM Dynamic-TDD Asymmetric Path gain-TSA
[12] OFDMA Dynamic-TDD Asymmetric RTSO-TSA
[13] TDMA Dynamic-TDD Asymmetric Power control
[14]− [19] OFDMA Fix-TDD Symmetric FFR
Propose work OFDMA Dynamic-TDD Asymmetric FFR