With several advantages such as flexible downlink-to-uplink(DL-to-UL) ratio and flexible spectrum usage, Time Division Duplexing(TDD) is emerging as an alternate to Frequency Division Duplexing(FDD), especially in wireless broadband systems. We already have at least four different TDD systems in the industry: Time Division-Synchronous Code Division Multiple Access(TD-SCDMA), IEEE 802.16e-TDD, IEEE 802.16m-TDD, and Time Division-Long Term Evolution(TD-LTE). A disadvantage of TDD is that tight coordination such as time synchronization between adjacent operators is required to prevent interference between the adjacent TDD systems. In this paper, we investigate coexistence scenarios among the above four well-known TDD systems and calculate spectral efficiency(SE) loss in each scenario. Our findings are that SE loss can be significant if TDD ratios of the adjacent operators are considerably different. However, as long as the TDD ratios of the adjacent operators are similar, configurations in the systems permit perfect time synchronization between the two heterogeneous TDD systems, and the resulting SE loss is zero or reasonably low. We believe that the above findings and the configurations of the TDD systems recommended to minimize SE loss will be helpful for operators who deploy TDD systems in system parameter determination and cross-operator coordination.Key words : TDD, WiMAX, TD-LTE, TD-SCDMA, Coexistence.
Ⅰ. IntroductionFDD has been used as a duplexing scheme for the majority of cellular communication systems including IS-95, cdma2000, wideband code division multiple access (WCDMA), high speed downlink packet access(HS-DPA), high speed uplink packet access(HSUPA), and LTE-FDD. The only TDD-based system that has been meaningful footprint in the industry is TD-SCDMA.Compared with FDD, TDD has several advantages such as flexible DL-to-UL ratio(i.e., TDD ratio), simpler implementation of "smart" antenna techniques, and cheaper transceiver implementation. Consequently, TDD has emerged as another duplexing scheme, especially for wireless broadband systems such as IEEE 802.16e, IEEE 802.16m, and TD-LTE.A disadvantage of TDD is that TDD system performance is significantly degraded when DL and UL signals overlap in time. Therefore, TDD first of all requires time synchronization throughout the network within one operator to prevent performance degradation by this overlap. In addition, TDD requires a large guard band(GB) between the adjacent TDD operators and/or a high level of coordination between the adjacent TDD operators. Coordination to reduce interference between adjacent operators can be accomplished in several ways such as time synchronization, general radio network engineering, transmit power reduction, and tighter filtering. Among them, time synchronization can be an effective coordination method to reduce interference between adjacent operators [10] .Time synchronization between the same TDD systems (e.g., time synchronization between two adjacent IEEE 802.16e systems) is rath...