Abstract:
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.

Description:
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. 
    
    
     DESCRIPTION OF DRAWINGS 
     Features and advantages of the invention will become more apparent upon reading the following detailed description and upon reference to the accompanying drawings. 
     FIG. 1 illustrates components of a wireless communication system appropriate for use with an embodiment of the invention. 
     FIG. 2 illustrates a series of cells in a wireless communication system. 
     FIG. 3 illustrates a traffic channel message assembly process according to one embodiment of the present invention. 
     FIG. 4 illustrates a procedure to decode the traffic channel message according to one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 illustrates components of a wireless communication system. A mobile switching center  102  communicates with a base station  104 . The base station  104  broadcasts data to and receives data from mobile stations  106  within a cell  108 . The cell  108  is a geographic region, roughly hexagonal, having a radius of up to 35 kilometers or possibly more. 
     The mobile station  106  is capable of receiving data from and transmitting data to a base station  104 . Additional cells adjacent to the cell  108  permit mobile stations  106  to cross cell boundaries without interrupting communications. 
     This is because base stations  104  in adjacent cells assume the task of transmitting and receiving data for the mobile stations  106 . The mobile switching center  102  coordinates all communication to and from mobile stations  106  in a multi-cell region, thus the mobile switching center  102  may communicate with many base stations  104 . 
     The mobile stations  106  may move about freely within the cell  108  while communicating either voice or data. The mobile stations  106  not in active communication with other telephone system users may, nevertheless, scan base station  104  transmissions in the cell  108  to detect any telephone calls or paging messages directed to the mobile station  106 . 
     One example of such a mobile station  106  is a cellular telephone used by a pedestrian who, expecting a telephone call, powers on the cellular telephone while walking in the cell  108 . The cellular telephone synchronizes communication with the base station  104 . The cellular telephone then registers with the mobile switching center  102  to make itself known as an active user within the wireless network. 
     The mobile station  106  scans data frames broadcast by the base station  104  to detect any telephone calls or paging messages directed to the cellular telephone. In this call detection mode, the mobile station  106  receives, stores and examines paging message data, and determines whether the data contains an identifier matching an identifier of the mobile station  106 . If a match is detected, the mobile station  106  establishes a call with the mobile switching center  102  via the base station  104 . If no match is detected, the mobile station  106  enters an idle state for a predetermined period of time, then exits the idle state to receive another transmission of paging message data. 
     FIG. 2 illustrates one example of a series of cells  108   a - 108   k  in a wireless communication system. The cells  108   a - 108   k  are generally hexagonal, although they may be other shapes including circular, square, oval, oblong, or any other polygon. The size of each cell  108   a - 108   k  may vary depending on location. For example, in densely packed urban areas, a cell  108   f  may be small but in a more rural area the size of a cell  108   b  increases. Each of the cells  108   a - 108   k  has a corresponding base station  104   a - 104   k.    
     In FIG. 2, the mobile station  106   b  is located in the cell  108   b . While the mobile station  106   b  is in cell  108   b , it is likely being served by the base station  104   b , although due to loading and other requirements, it may be served by any base station  104  providing a useable signal. While in one cell  108 , the mobile station  106  periodically checks the signal strength of the base stations  104  in each neighboring cell  108 . For example, while the mobile station  106   b  is in the cell  108   b , the mobile station  106   b  monitors the signal strength of base stations  104   a ,  104   c ,  104   d , and  104   e . If the mobile station  106   b  travels into cell  108   e , the mobile switching center  102  may cause the mobile station  106   b  to handover to base station  104   e . In this circumstance, the mobile station  106  then periodically monitors the signal strength of base stations  104   b ,  104   c ,  104   d ,  104   g , and  104   h.    
     To travel between the cells  108   a - 108   k , the mobile stations  106  may detect a traffic channel message from neighboring base stations  104 . Once the traffic channel message is confirmed, the mobile station  106  may initiate a handover procedure to switch base stations  104 . 
     FIG. 3 illustrates a traffic channel message assembly process  300  according 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 station  106  switches 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 station  106  can decrease the time require until synchronization, and therefore decrease handoff time. 
     In the PDC system, the FACCH contains a number of slots  305 ,  310 . Each of these slots  305 ,  310  includes information that make up the entire FACCH. In the FACCH, each slot  305 ,  310  has 112 bits of data. The first slot  305  includes information bits  315  and Cyclic Redundancy Check (CRC) bits  320 . The second slot  310  includes Forward Error Control (FEC) bits  325 . 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. 4 illustrates a process  400  to decode the traffic channel message according to one embodiment of the present invention. The process  400  begins at a start block  405 . Proceeding to block  410 , the first slot  305  of the traffic channel message is extracted and stored in bytes. Proceeding to block  415 , CRC decoding is applied to the CRC bits  320  in the first slot  320 . 
     Proceeding to block  420 , the results of the CRC decoding of block  415  are examined. If the data in the first slot  305  was successfully received, the CRC bits  320  would be intact and the CRC decoding would indicate the decoding was successful. If the CRC decoding is successful, the process  400  proceeds along the YES branch to block  445 . 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 block  425 . 
     In block  425 , the second slot  310  of the traffic channel message including the FEC bits  325  is extracted and stored in bytes. Proceeding to block  430 , 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 block  435 , after the traffic channel message is decoded, the CRC decoding is performed a second time. The process  400  then proceeds to block  440  to 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 block  445 . In block  445 , the handover process to the new traffic channel may be initiated. The handover process may be directed by the wireless communication network. 
     Returning to block  440 , if the CRC decoding indicates the traffic channel was not successfully received, the process  400  proceeds along the NO branch to state  450 . In state  450 , the traffic channel message is discarded as unreliable. Following both block  445  and  450 , the process terminates in end block  455 . 
     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.