1. Field of the Invention
The present invention relates generally to communication systems. More specifically, the invention relates to a system and method for controlling handoff between individual cells for a mobile subscriber in a multicellular communication environment.
2. Description of the Prior Art
Handoff is a general term describing the function of transferring a communication signal for a mobile subscriber from one base station transceiver to another base station transceiver as the subscriber negotiates the cellular territory. There are two general methods of conducting handoff in a cellular communication system, soft and hard handoff. Quality may suffer using either handoff method as the mobile subscriber switches base stations between individual cells.
Hard handoff typically occurs near the boundaries between cells. Through continuous measurements of received signal power from a mobile subscriber, the base station where the subscriber has established communication determines if the power is reduced below a nominal value near the cell boundary. The hard handoff to a candidate cell""s base station occurs instantaneously without disrupting the call in progress. The received power at the candidate cell""s base station is much greater than required to ensure an ideal handoff. The difference between the current cell""s power and the candidate cell""s power significantly reduces capacity by interfering with other users.
A soft handoff occurs throughout a given range of distances from the current and candidate cells"" base stations. In the soft handoff method, the user is connected to both base stations as he travels near the common cell boundary. The decision to switch is made depending upon the reception of the mobile subscriber""s pilot signal. A central switching center decides at what point one of the base stations should be dropped. Shared communication is performed for a finite period of time, during which transmission from the current and candidate cells is required.
In a typical CDMA communication system, the plurality of signals are transmitted within the same frequency band. Frequency reuse not only applies to users in the same cell, but also to those in all other cells. Since the same frequencies are used, the transmitted power levels from the mobile subscriber and the base station must be monitored closely. If power control is not strictly adhered to, the overall transmission interference and the total number of usable channels is adversely affected. Therefore, the number of signals which can be successfully transmitted and received is associated with the total power of all users.
Both soft and hard handoff methods have shortcomings. Hard handoff can potentially suffer from high drop-out rates. The soft handoff method requires a duplication of transmission resources from the current base station and a base station from at least one candidate cell. The mobile subscriber must establish two concurrent communication links, thereby requiring twice the transmission power which would otherwise be required. The increase in transmission power wastes power and air capacity and contributes to total system interference while the handoff is taking place. The power or energy transmitted in a CDMA system by each user must be kept at the minimum necessary to convey information and to minimize interference with the other users. Careful control of transmission power also contributes to extended use of portable devices relying on battery power. Furthermore, diversity combining of the duplicate signals at the central switching center is cumbersome unless the delay from both cell base stations is nearly identical.
Accordingly, there exists a need for an efficient, fast and reliable handoff method.
The present invention relates to a multicellular communication system where transmission between a roving subscriber and multiple base station transceivers is maintained. Each mobile subscriber unit has global code seeds for the entire communications system and continuously searches to access available cell base stations while maintaining a communication link with one base station. Candidate cell base stations interrogate and communicate with the, mobile subscriber unit. The candidate base station that requires the least transmit power from the mobile subscriber unit is closely monitored. When the mobile subscriber unit can successfully communicate with less transmit power than currently required by the current base station, the mobile subscriber unit renders the decision to be handed off to the candidate base station transceiver.
Accordingly, it is an object of the present invention to provide an efficient system and method for handoff between individual cells in a multicellular communications environment.
It is a further object of the invention to provide a system and method permitting the mobile subscriber unit to handoff between cells in a multicellular transmission system with reduced complexity and interference.
Other objects and advantages of the invention will become apparent to those skilled in the art after reading the detailed description of the preferred embodiment.