In contrast to voice traffic, wireless data traffic is mostly asymmetric and time-variant with a requirement for a dynamically adjusting technique to divide the uplink (UL) and downlink (DL) resource. In typical cellular systems, the length of UL resource and the length of DL resource are predetermined. In a typical frequency-division duplex (FDD) system, the UL and DL transmission use distinctive frequency bands, which is especially efficient in cases of symmetric traffic due to the avoidance of possible interference between UL and DL transmission. However the FDD system has difficulty in adjusting its UL and DL resource in asymmetric traffic since the resource division is operated by the duplexer in the hardware. A typical time-division duplex (TDD) system is capable of adjusting the UL and DL transmission in time domain. However, due to the requirement of synchronization in order to eliminate the interference, the UL and DL resource is still fixed. To support asymmetric and time-variant traffic, LTE provides small cell base stations (BSs) with dynamic TDD by supporting seven TDD UL/DL configurations, enabling the BSs dynamically to change the ratio of UP and DL resource to handle the time-variant traffic. Nevertheless, such a scheme also induces two type of interference: BS–BS interference and MS–MS interference. In this chapter the interference issues and several interference mitigation methods will be extensively discussed.
Dynamic TDD system overview
To divide the UL and DL traffic resource, some typical communication systems apply FDD, where different frequency bands are used for transmitting and receiving, the benefit of which is that no interference will be incurred between UL and DL signals. For the symmetrical traffic on UL and DL (e.g., voice service), the FDD system is suitable since the BS is assigned the same amount of radio resource in the UL and DL. Whereas for wireless data services, FDD is not flexible enough to handle this type of dynamic UL/DL traffic due to the character of the UL and DL traffic being asymmetric and time-variant in these cases.
Compared to FDD, TDD is different in that the UL and DL resource is divided in time domain and can be easily adjusted. It possesses an advantage of greater flexibility in handling the dynamic UL/DL traffic. In the TDD system, the boundary between the UL and DL duty cycle is adaptively adjustable according to service requirements.