ch CQI - ABS or normal - shall be used for the link adaptation and packet scheduling decisions for each subframe depending on whether the macro is using ABS or normal transmission. In case of centralized architecture, all the necessary information is available at the macro cell, including CQI for all UEs in the cluster, instantaneous load information for all cells in Small Cell Interference Management Slow ABS adaptation Macro BTS Macro cell UE Small cell edge UE Small cell center UE Fast ABS adaptation Macro BTS Macro cell UE = Normal subframe Small cell edge UE = Mandatory ABS subframe Small cell center UE = Optional ABS Figure 7.16 Slow and fast ABS adaptation the cluster and scheduling decision and related metrics. The fast decisions in the centralized architecture are made shortly before each optional ABS on whether to configure as ABS or normal transmission. We will show that the algorithms can also be decomposed to be applicable for the distributed architecture. The macro cell acts as the master for fast ABS decisions based on the information exchange with pico cells over the X2 interface. The macro cell acquires knowledge about the load in the pico cell. The rate of fast ABS adaptation for the distributed architecture is therefore dependent on how frequent the aforementioned information is exchanged between cells as well as the X2 signalling delays. One option is to have the pico cells periodically reporting the required information to the macro cell every N Transmission Time Interval (TTI). Another option is event-triggered reporting where the pico cells inform the macro whenever there is a significant change in the load or in proportional fair scheduling metric. The periodic reporting is shown in Figure 7.18. The pico cell informs the macro cell of the number of RE users in every N TTIs. Macro cell can then decide the ABS configuration and provide the information to the pico cell. Notice that the X2 reporting is independent of the number of optional ABS per period and there is no signalling overhead in 