The present invention relates generally to the management of mobile stations in a wireless communication system and, more particularly, to a system and method for providing improved channel selection procedures.
In a wireless communication system, the service area is typically divided into a plurality of cells, with each cell served by a base station. Mobile stations within a particular cell communicate over RF channels with the base station serving that cell. The base station may handle a plurality of simultaneous calls from mobile stations. The base stations are connected with one another and to the public switched telephone network by mobile services switching centers (MSC). The MSC coordinates the activities of all the associated base stations and connects the entire cellular system to the public switched telephone network. A typical MSC may handle 100,000 cellular subscribers and 5,000 simultaneous conversations at a time. The MSC also accommodates billing and system maintenance functions. In some higher density networks, several MSCs are used in a single network.
During the course of a call, the mobile station may move from one cell into another. A switching technique called a handoff enables the call to proceed uninterrupted when the user moves between cells. When the mobile station moves into a different cell while a call is in progress, the MSC automatically transfers the call from the current channel being used to a new channel belonging to the base station serving the new cell.
Processing handoffs is an important task in any wireless communication system. Handoffs ensure that acceptable signal quality standards are maintained as the mobile station moves out of range of one base station and into the radio coverage area of another base station. Handoffs should be performed successfully and be imperceptible to the user. Also, because of the signaling demand a handoff places on the system, handoffs should be performed as infrequently as possible and only as needed.
Handoff decisions are typically based on received radio signal strength and channel quality as monitored by the base station serving the mobile station. Received signal strengths are easily monitored and signal quality for any given channel is often determined by the bit error rate (BER) over a given channel. During any communication period, the cellular system will continually seek a better channel and cell for communications. A handoff from the current channel to another channel is initiated when the signal level or channel quality drops below acceptable levels and another channel is available capable of providing acceptable communications.
In digital systems, such as Global System for Mobile communications (GSM) and Time Division Multiple Access (TDMA) systems, the base stations enlist the assistance of the mobile station to determine when a handoff is required. In order to use a mobile station to assist handoff, the serving base station downloads a list of channels, commonly referred to as a neighbor list, at the start of a call or after a handoff. The neighbor list identifies channels in neighboring cells which are potential handover targets. In a mobile assisted handoff (MAHO), each mobile station measures the received power from surrounding base stations provided by the neighbor list, which is also referred to as a MAHO list. The mobile station continually reports the results of these measurements to the serving base station. These reports are often referred to as MAHO reports. The signal strength measurements are made by the mobile station in between periods of communication during a call. For example, in GSM systems, each radio frequency channel is divided into eight time slots. In TDMA systems, each radio frequency channel is divided into six slots. The mobile station is allocated one time slot for transmissions and another time slot to receive signals from the base station. During the remaining time slots, the mobile station is not communicating with the serving base station. The mobile station monitors channels belonging to neighboring base stations during these idle periods and then quickly returns to its assigned channels in time to transmit and receive signals in its allocated time slot. The measurements made by the mobile station of signals received from neighboring cells are reported back to the serving base station in a scheduled manner or on a separate channel so as not to interfere with voice or data transmissions. During multi-slot operation, the time slots normally used for communications are “borrowed” to allow the mobile station to report measurements to the base station. This concept is typically referred to as frame stealing and reduces transmission rates.
The measurement reports provided by the mobile station give the base station a list of the signal strength and possibly channel quality from adjacent cells, as measured by the mobile station at its present location. The network also knows which adjacent cells have unused radio channels that are available for allocation during a handoff. From the list of available channels, the network selects the cell which best will handle the call from a service quality and an overall interference point of view based on signal strength and bit error rate. A suitable traffic channel is assigned that cell as the target, and the mobile station is commanded to retune to the traffic channel in the target cell. At the same time, the call is switched by the MSC to the base station currently serving the mobile station to the base station in the target cell. The mobile station switches to the newly assigned channel during one of the idle periods so there is no interruption in transmission. Thus, from the user's perspective, the handover is seamless.
Cell reselection is the process of changing channels for service and registering with a new base station when the mobile station is not active. When the mobile station is between calls and not actively communicating with a base station, but is ready to receive or place calls, it monitors one or more control channels of proximate base stations to receive paging or other instructions. This state of operation is commonly referred to as the idle state or idle mode. Since there is no two-way communication in place while in idle mode, the cellular system does not know which base station the mobile station should monitor. Typically, the mobile station monitors signal strength along with other indicia indicative of channel quality in order to select a base station. In older analog systems, the selected base station was typically the one providing the strongest signal. In modern digital systems supporting hierarchical cell structures, the mobile station performs a much more elaborate evaluation of candidate cells.
As the mobile station moves throughout areas of coverage for various base stations, it acquires information about channels and their attributes used or associated with surrounding base stations by reading a list of channels sent on a broadcast channel for each cell from which it is able to receive information. This list is also commonly referred to as a neighbor list. The neighbor list identifies specific channels for the mobile station to monitor when idle. In essence, the mobile station monitors the listed channels and decides on the best channel for service. When the mobile station decides a new channel is required, it may send information to the new base station to register and effect cell reselection.
While idle, mobile stations make a tradeoff between accurate neighbor list measurements and energy consumption. U.S. Pat. No. 5,539,748 entitled Enhanced Sleep Mode in Radio Communication Systems discloses a few basic techniques to reduce the neighbor list measurements without sacrificing the quality of these measurements. In essence, the patent describes a very basic system-assisted and a mobile station-controlled method for reducing the number of neighbor list measurements. In the former, the cellular system sends the mobile station the minimum, nominal frequency and measurement for each entry in the neighbor list. The cellular system provides an indication that certain entries can be measured with a reduced frequency. This is applicable if there are many entries in the neighbor list or the entries are in a hierarchical cell layout. In the latter, the mobile allows a reduced measurement frequency from the nominal requirement if no cell reselection has been performed for a specified time. Alternatively, the measurement is reduced if changes of signal strength for the serving control channel and neighbor list entries are less than specified.
A significant amount of system resources and power are used while assisting a base station during handoffs and monitoring nearby stations for cell reselection. Given the premiums placed on getting the most use of allocated bandwidth and extending battery life, there is a need for an improved channel selection system and method to reduce processing time and energy consumption associated with channel selection during active and idle modes without degrading performance.
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