Rapid decoding of control channel to decrease handoff time

A portion of a traffic channel message is detected and decoded to determine if a rapid handover procedure may be used. A first portion of the traffic channel includes information data and CRC data. The CRC data is decoded, and if the CRC determines the message is valid, the mobile station may proceed directly to handover.

TECHNICAL FIELD

This invention relates to wireless communication systems, and more particularly to decreasing handoff time between traffic channels.

BACKGROUND

The use of wireless communication systems is growing with users now numbering well into the millions. One of the popular wireless communications systems is the cellular telephone, having a mobile station (or handset) and a base station. Cellular telephones allow a user to talk over the telephone without having to remain in a fixed location. This allows users to, for example, move freely about the community while talking on the phone.

In a radiotelephone communication system, a communication link via an RF channel is established between a mobile station, or subscriber unit, and a source base station. As a mobile station moves out of range of the source base station, the signal quality will degrade until the communication link would ultimately be broken, or the call “dropped”. To avoid loss of the communication link resulting from a dropped call, the communication link is shifted from the source base station to a target base station. This process of making the shift is commonly referred to in the radiotelephone communication area, or cellular communication area as a handover process.

A handover can be defined as a change of channel during a call, either because of degradation of the quality of the RF channel which includes, power level or communication link quality below a certain threshold, or because of the availability of another channel which can allow communication at a lower transmit power, or to prevent a mobile station from grossly exceeding the planned base station boundaries. A handover may occur during a call in progress (e.g. from a traffic channel to a traffic channel), or during the initial signaling during call set-up. The handover may be either from a channel on the source base site to another channel on a target base site or between channels on the source base site.

DETAILED DESCRIPTION

FIG. 1illustrates components of a wireless communication system. A mobile switching center102communicates with a base station104. The base station104broadcasts data to and receives data from mobile stations106within a cell108. The cell108is a geographic region, roughly hexagonal, having a radius of up to 35 kilometers or possibly more.

The mobile station106is capable of receiving data from and transmitting data to a base station104. Additional cells adjacent to the cell108permit mobile stations106to cross cell boundaries without interrupting communications. This is because base stations104in adjacent cells assume the task of transmitting and receiving data for the mobile stations106. The mobile switching center102coordinates all communication to and from mobile stations106in a multi-cell region, thus the mobile switching center102may communicate with many base stations104.

The mobile stations106may move about freely within the cell108while communicating either voice or data. The mobile stations106not in active communication with other telephone system users may, nevertheless, scan base station104transmissions in the cell108to detect any telephone calls or paging messages directed to the mobile station106.

One example of such a mobile station106is a cellular telephone used by a pedestrian who, expecting a telephone call, powers on the cellular telephone while walking in the cell108. The cellular telephone synchronizes communication with the base station104. The cellular telephone then registers with the mobile switching center102to make itself known as an active user within the wireless network.

The mobile station106scans data frames broadcast by the base station104to detect any telephone calls or paging messages directed to the cellular telephone. In this call detection mode, the mobile station106receives, stores and examines paging message data, and determines whether the data contains an identifier matching an identifier of the mobile station106. If a match is detected, the mobile station106establishes a call with the mobile switching center102via the base station104. If no match is detected, the mobile station106enters an idle state for a predetermined period of time, then exits the idle state to receive another transmission of paging message data.

FIG. 2illustrates one example of a series of cells108a-108kin a wireless communication system. The cells108a-108kare generally hexagonal, although they may be other shapes including circular, square, oval, oblong, or any other polygon. The size of each cell108a-108kmay vary depending on location. For example, in densely packed urban areas, a cell108fmay be small but in a more rural area the size of a cell108bincreases. Each of the cells108a-108khas a corresponding base station104a-104k.

InFIG. 2, the mobile station106bis located in the cell108b. While the mobile station106bis in cell108b, it is likely being served by the base station104b, although due to loading and other requirements, it may be served by any base station104providing a useable signal. While in one cell108, the mobile station106periodically checks the signal strength of the base stations104in each neighboring cell108. For example, while the mobile station106bis in the cell108b, the mobile station106bmonitors the signal strength of base stations104a,104c,104d, and104e. If the mobile station106btravels into cell108e, the mobile switching center102may cause the mobile station106bto handover to base station104e. In this circumstance, the mobile station106then periodically monitors the signal strength of base stations104b,104c,104d,104g, and104h. To travel between the cells108a-108k, the mobile stations106may detect a traffic channel message from neighboring base stations104. Once the traffic channel message is confirmed, the mobile station106may initiate a handover procedure to switch base stations104.

FIG. 3illustrates a traffic channel message assembly process300according to one embodiment. In a Personal Digital Communication (PDC) system, the traffic channel includes a Fast Associated Control Channel (FACCH). The handover time can be decreased by decreasing the decoding time of the FACCH. Handover time is the period from when the mobile station106switches from the current traffic channel until synchronization is established with the newly assigned traffic channel. Thus, if the new traffic channel can be decoded quickly, the mobile station106can decrease the time require until synchronization, and therefore decrease handoff time.

In the PDC system, the FACCH contains a number of slots305,310. Each of these slots305,310includes information that make up the entire FACCH. In the FACCH, each slot305,310has 112 bits of data. The first slot305includes information bits315and Cyclic Redundancy Check (CRC) bits320. The second slot310includes Forward Error Control (FEC) bits325. Forward error control (FEC) bits provide the ability to detect and correct digital messages even in the presence of transmission errors. However, if the CRC bits indicate the message was received without error, the FEC bits may be redundant. If the CRC bits do not indicate the message was received without error, the FEC bits may be used to further check the status of the message. Currently, both the CRC bits and the FEC bits and received, extracted, and stored prior to processing any message from the traffic channel.

FIG. 4illustrates a process400to decode the traffic channel message according to one embodiment of the present invention. The process400begins at a start block405. Proceeding to block410, the first slot305of the traffic channel message is extracted and stored in bytes. Proceeding to block415, CRC decoding is applied to the CRC bits320in the first slot320.

Proceeding to block420, the results of the CRC decoding of block415are examined. If the data in the first slot305was successfully received, the CRC bits320would be intact and the CRC decoding would indicate the decoding was successful. If the CRC decoding is successful, the process400proceeds along the YES branch to block445. In good channel conditions when the bit error rate (BER) is low, the error probability is low. Thus, the chances the CRC decoding will be successful is increased. If the CRC decoding is unsuccessful, the process proceeds along the NO branch to block425.

In block425, the second slot310of the traffic channel message including the FEC bits325is extracted and stored in bytes. Proceeding to block430, the traffic channel message may be decoded using Bose-Chaudhuri-Hocquengh (BCH) coding. BCH codes are cyclic block codes that are rooted in linear algebra and the properties of those equations. The design of BCH codes may be selected by defining desired coding parameters that may be related directly to overhead and performance. The BCH codes are powerful linear codes for a significant range of block lengths.

Proceeding to block435, after the traffic channel message is decoded, the CRC decoding is performed a second time. The process400then proceeds to block440to check the results of the CRC decoding. If the decoding indicates that the traffic channel was received successfully, the process proceeds along the YES branch to block445. In block445, the handover process to the new traffic channel may be initiated. The handover process may be directed by the wireless communication network.

Returning to block440, if the CRC decoding indicates the traffic channel was not successfully received, the process400proceeds along the NO branch to state450. In state450, the traffic channel message is discarded as unreliable. Following both block445and450, the process terminates in end block455.

The principles of the present invention which apply to a cellular-based digital communication system also apply to other types of communication systems, including but not limited to personal communication systems, trunked systems, satellite systems and data networks. Likewise, the principles of the present invention which apply to all types of digital radio frequency channels also apply to other types of communication channels, such as electronic data buses, wireline channels, optical fiber links and satellite links.

Numerous variations and modifications of the invention will become readily apparent to those skilled in the art. Accordingly, the invention may be embodied in other specific forms without departing from its spirit or essential characteristics.