[Part 1 begins with an introduction and a look at IEEE 802.16M mobile station (MS) state diagrams. Part 2 continues with a look at the mobile station Access
State, Connected State, and Idle State, and their respective operating modes. Part 3 describes network entry/re-entry procedures and the handover process. Part 4 looks at the relay station state diagram, which consists of three states: Initialization, Access and Operational.]
4.5 OPERATIONAL STATES OF FEMTO BASE STATIONS
A femto BS or femto access node is a base station with a low transmit power, typically installed by a subscriber in home or small office/home office environment to provide access to a closed or open group of users as configured by the subscriber and/or the access provider. A femto BS is connected to the service provider's network via a broadband wired (or wireless) connection and typically operates in licensed spectrum, and may use the same or different frequency as macro base stations. Its coverage may overlap with that of a macro BS.
A femto BS may be intended to serve open subscriber groups or closed subscriber groups . Figure 4-22 illustrates the femto BS state diagram. The state diagram consists of the Initialization and Operational States.
FIGURE 4-22 State transition diagram of a femto base station
The Initialization State includes procedures such as configuration of radio interface parameters and time synchronization. After successful attachment to the network, a femto BS enters the Operational State. The Operational State consists of two modes: Normal Mode and Low-Duty Mode. In Low-Duty Mode, the femto BS will be periodically unavailable to reduce the adjacent/co-channel interference to neighboring femto or macro base stations.
Femto base stations are required to synchronize with the network timing to the extent that synchronization error does not disrupt network operations. They may use different schemes to achieve synchronization with the network depending on the deployment scenario. A femto BS may obtain time and frequency synchronization by using schemes such as GPS,xi IEEE1588,xii etc.
Femto base stations are plug-and-play devices that, once connected to the operator's core network, are automatically configured and are self-optimized and self-organized while in operation. Since femto BS has limited coverage, it is typically used as an overlay with the macro BS (Although it may operate in remote and non-overlay cases, as well). The femto BS and macro BS may operate on the same or different frequency assignment (FA) or frequency band. Femto BS may belong to an open (accessible to all subscribers) or closed (accessible to authorized subscribers) group.
Since the femto base stations are usually densely deployed, the use of the typical cell search and cell selection methods, which are utilized in initial network entry/re-entry or handover in macro base stations, would be cumbersome and would result in extreme overhead in system operation, MS complexity, and power consumption. Furthermore, private femto base stations may be overloaded with signaling with unauthorized mobile stations trying to select them as target base stations for handover or system entry/re-entry.
As mentioned earlier, the femto base stations may belong to closed subscriber groups (CSG) or open subscriber groups (OSG). In CSG, the access and services are restricted to authorized mobile stations. The credentials or electronic certificates may be provided to the mobile station by the cellular system operator at the time of subscription. The serving macro BS may have the knowledge (through MS context generated following session set-up) of the CSGs where the MS may have access. The OSGs, on the other hand, are publicly accessible and no specific authorization is required.
xiThe Global Positioning System (GPS) is a global navigation satellite system developed by the United States Department of Defense and managed by the United States Air Force. It is the only fully functional satellite navigation system in the world. It can be used freely by anyone, unless the system is technically restricted. These restrictions can be applied to specific regions by the US Department of Defense. The GPS can be used almost anywhere near the earth, and is often used by civilians for navigation purposes. An unobstructed line of sight to four satellites is required for non-degraded performance. The GPS location determination accuracy is about 15 meters (50 ft). The GPS uses a constellation of between 24 and 32 medium Earth orbit satellites that transmit precise radio signals, which allow GPS receivers to determine their current location, the time, and their velocity .
xiiThe Precision Time Protocol is a time-transfer protocol defined in the IEEE 1588-2002 standard that allows precise synchronization of networks (e.g., Ethernet). Accuracy within the nanosecond range can be achieved with this protocol when using hardware generated timestamps .