The invention relates generally to wireless cellular communication systems and more particularly to techniques for automatic handover of an ongoing mobile communication from one cell to another cell within the system, using information transmitted by the base stations or other fixed part system transmitters.
Conventional wireless cellular communication systems include both xe2x80x9coutdoorxe2x80x9d and xe2x80x9cindoorxe2x80x9d systems. A typical outdoor system, such as the AMPS wireless cellular telephone system used in the United States, covers a relatively large geographical area. The system uses radio frequency (RF) signals to establish communication between system base stations and mobile radio units or xe2x80x9cmobiles.xe2x80x9d The mobiles are generally free to move throughout a coverage region of the system, while the base stations are in fixed locations, such as on towers positioned throughout the coverage region. Much of the communication in such an outdoor system takes place when the mobiles are located outside of any particular building or other structure. An indoor wireless system, in contrast, is generally set up within a given building, office complex or other structure or set of structures, and utilizes RF signals to support communications between mobiles and fixed base stations located throughout the interior of the structure(s).
A common characteristic of both outdoor and indoor wireless communication systems is the use of coverage cells. Each cell is designed to maintain favorable radio characteristics within its boundary. If a mobile engaged in an ongoing call crosses the boundary of a given cell and moves into another cell, the system must be capable of transferring the ongoing call to the other cell. This process is referred to as xe2x80x9chandover,xe2x80x9d and ideally should be implemented so as to cause substantially no disruption in the ongoing call. Wireless systems also use the handover process to overcome or avoid the effects of system disturbances, such as multi-path fading effects. The handover may be from one time or frequency channel to another, or from one cell to another. In many outdoor systems, such as AMPS, the handover process is centrally controlled and thus almost completely autonomous with respect to the mobile. In contrast, the handover process in many indoor systems, including systems based on standards such as DECT in Europe and PWT in the United States, is completely under the control of the mobile and is therefore based primarily on the mobile""s view of the radio spectrum in its own locale.
The radio spectrum environment in an indoor system can be particularly harsh, due to random propagation channel effects such as fading and shadowing. The distances between base stations in an indoor system are therefore generally much smaller than those of outdoor systems. From the perspective of the mobile, many base station signals or xe2x80x9cbeaconsxe2x80x9d from other cells can often be detected with excellent signal-to-noise (S/N) ratios at various times and locations as the mobile moves within a given cell. Since the mobile is often in control of any handover actions taken to improve signal quality in an indoor system, the mobile must keep track of all the beacons that it detects, regardless of which cell they originate from, in order to make an intelligent choice from among the available beacons.
The mobile in a system with conventional mobile-controlled handover therefore must examine beacons which may or may not be from xe2x80x9cnearest neighborxe2x80x9d or adjacent cells, since it has no way of determining which of the beacons that it detects are from the adjacent cells. Typically, many beacons from both adjacent and non-adjacent cells may be present. For example, it is possible in many indoor environments that a beacon from a non-adjacent cell may momentarily have a sufficient signal strength within the current cell to qualify as a potential handover candidate. An attempted handover to such a candidate will be likely to fail due to changes in multipath or other channel conditions. Even if the attempted handover is successful, changes in channel conditions will tend to necessitate an immediate additional handover. The result in either instance is a slowdown in the overall process of achieving a successful handover, with a corresponding degradation in voice quality. Therefore, despite the fact that many of the beacons that reach a given mobile are not useful in the handover decision, conventional mobile-controlled handover processes require the mobile to consider each beacon in turn in order to weed out the non-useful beacons. This takes time that often results in noticeable degradation of the voice quality in an ongoing call while a handover decision is being made.
A need therefore exists for improved handover techniques that can speed up the handover process, thereby avoiding the handover delays and corresponding degradation in voice quality associated with conventional techniques.
The invention provides methods and apparatus for implementing assisted handover in a wireless cellular communication system. The invention allows a mobile to maintain substantial control of the handover process, while utilizing cell adjacency information stored in the system infrastructure to provide information regarding the most relevant potential handover candidates for that mobile. The cell adjacency information is used to identify the cells adjacent to the cell in which the mobile is currently located, and the potential handover candidates will correspond to beacons generated by these adjacent cells. This allows the mobile to direct its handover attempts to beacons from adjacent cells, while ignoring beacons from non-adjacent cells. The invention thereby substantially reduces handover-related delays and corresponding voice quality degradation in a mobile-controlled handover process.
In an illustrative embodiment of the invention, when a mobile conducting an ongoing call in a xe2x80x9ccurrentxe2x80x9d cell determines that a handover to another cell may be needed, the mobile sends a handover assistance request to the base station of the current cell. The base station responds by transmitting to the mobile information regarding a set of potential handover candidates. The potential handover candidates are determined based on stored cell adjacency information which includes identifiers of each of the cells adjacent to the current cell. The adjacency information may be stored in the system base stations after initial system configuration, based on the actual physical layout of the cells, and may be altered during system operation to reflect the continual success or failure of particular attempted handovers.
The information regarding the set of potential handover candidates transmitted to the mobile may be in the form of a list of beacons associated with the cells adjacent to the current cell, and may also include corresponding channel identifying information such as frequency and time slot. This transmitted information enables the mobile to focus its handover attempts on handover candidates from adjacent cells, and therefore those candidates which are most likely to result in a successful handover. Alternative embodiments of the invention may transmit the potential handover candidate information from a fixed part transmitter other than a system base station, or may periodically broadcast this information throughout the system rather than transmit it upon receipt of a handover assistance request.