e 1.1: Backhaul options for the small cells [12]. Backhaul Type Backhaul Technology One-Way Latency (ms) Throughput (Mbps) Nonideal Fiber access 1 10-30 10-10,000 Fiber access 2 100-1000 Fiber access 3 50-10,000 DSL access 15-60 10-100 Cable 25-35 10-100 Wireless backhaul 10-100+ Ideal Fiber access 4 <2.5 us Up to 10,000 network. In this context, two scenarios can be considered: (1) UE is simultaneously in COV- erage of both the macro-cell and the small cell(s); and (2) UE is not simultaneously covered by the macro-cell and small cell(s). The small-cell nodes may be deployed indoors or out- doors, and in either case could provide service to indoor or outdoor UEs, respectively. For an indoor UE, only low UE speeds of 0-3 km/h are considered. For outdoor UEs, in addi- tion to low UE speeds, medium UE speeds up to 30 km/h were targeted. Throughput and mobility (seamless connectivity) were used as performance metrics for low- and medium- speed mobility scenarios. Cell-edge performance (e.g., fifth percentile of user throughput CDF) and network/UE power efficiency were also used as evaluation metrics. Backhaul is a critical component of heterogeneous networks, especially when the number of small nodes increases. Ideal backhaul, characterized by very high throughput and very low latency link such as dedicated point-to-point connection using optical fiber; and non-ideal backhaul (e.g., xDSL, 31 microwave backhaul, and relaying) for small cells were studied by 3GPP, and performance and cost trade-offs were made. A categorization of ideal and non- ideal backhaul based on operators' data is listed in Table 1.1. 3GPP conducted extensive studies to investigate the interfaces between macro and small cell, as well as between small cells considering the amount and type of information/signal- ing needed to be exchanged between the nodes in order to achieve the desired performance improvements; and whether a direct interface should be used between macro and small cells or between small cells. In those studies, LTE X2 interface (i