 estimated quality of the currently used frequency (LTE) falls below a certain threshold. Essentially, this event triggers the start of UE mea- surements on UTRAN or GERAN. The threshold value that defines this event should be sufficiently low in order to avoid unnecessary inter-RAT measurements. The estimated quality of the currently used frequency (LTE) is above a cer- tain threshold. This event should stop inter-RAT measurements. © 2009 by Taylor & Francis Group, LLC Long Term Evolution: 3GPP LTE Radio and Cellular Technology Inter-RAT Cell Re-selection Camping on WCDMA Camping on LTE Initiate Call/Session Initiate Call/Session on WCDMA on LTE Inter-RAT Redirection upon Session (RB) Setup, e.g., "Directed Retry" Change (in RAB Assignment Response) Change RAT ? or at RRC Connection Setup RAT ? Call/Session Call/Session Established in WCDMA Established in LTE Change Change RAT ? Inter-RAT RAT ? Handover Call/Session Call/Session in WCDMA in LTE Inter-RAT Redirection upon RRC Release Call/Session Call/Session Release Release Figure 4.22 Radio resource management techniques for multi-RAT networks. Changing the serving RAT can happen at different phases during the lifetime of a connection. The figure illustrates this by indicating when inter-RAT cell reselec- tion, redirection upon connection establishment, handover, and redirection upon connection release happen. The estimated quality of the other system (e.g., UTRAN) is above or below a certain threshold. This event allows the other RAT to be selectable. The estimated quality of the currently used system (LTE) is below a certain threshold and the estimated quality of the other system is above a certain threshold. When this event occurs, it should start the inter-RAT handover procedure. The exact specification of these events will be part of 3GPP TS 36.331 [21]. © 2009 by Taylor & Francis Group, LLC Architecture and Protocol Support for RRM 4.6 Summary In this chapter, we have seen that a number of advanced RRM functions are needed in today's wireless systems, whi