Abstract:
A method and apparatus for arranging transmission of overhead messages on the paging channel of a wireless communications system is described. Certain paging channel slots are established, based on a timing reference, that must contain an overhead message. In the intervening slots or slot fragments, the base station transmits mobile-directed messages, when they are available. When no mobile directed messages are available for transmission, the base station selects an overhead message from a queue for transmission on the intervening paging channel slot or slot fragment.

Description:
FIELD OF THE INVENTION 
     This invention relates to the field of wireless communications and more particularly to the field of overhead messaging in code division multiple access systems. 
     BACKGROUND OF THE INVENTION 
     A code division multiple access (CDMA) cellular telephone system uses spread spectrum technology to allow multiple users access to the same frequency band. The Telecommunication Industry Association and the Electronics Industry Association have adopted a standard, TIA/EIA IS-95 (IS-95), for digital cellular phone applications using CDMA techniques. Such IS-95 systems are disclosed in detail in Garg, Smolik and Wilkes, Applications of CDMA In Wireless/Personal Communications, Prentice Hall, 1997. 
     A CDMA system uses pseudorandom noise (PN) sequences to spread base station and mobile signals to the transmission bandwidth. Transmitted signals using different PN sequences can be received at the receiver, and the PN sequences allow the receiver to distinguish a desired signal from all the received signals. 
     The geographic coverage provided by a CDMA cellular telephone system is divided into coverage cells, where each cell corresponds to a base station. The cell may be further divided for certain types of base stations into multiple sectors, where each sector uses multiple carrier channels. 
     In an IS-95 system, the parameters a mobile uses to establish and maintain a call are transmitted as a set of over the air (OTA) messages, known as overhead messages, by every base station in the network. In a mobile originated call, the mobile determines if it has a current set of overhead messages before requesting traffic channel resources. 
     If the overhead messages are not up to date, the mobile monitors the paging channel to update the overhead messages. Once the mobile updates its overhead messages, it begins the access procedure. The likelihood of success of an origination attempt is related to the time taken to complete overhead message updates. The longer the process takes, the more likely the mobile is to leave the cell&#39;s coverage area, causing an access failure. 
     SUMMARY OF THE INVENTION 
     This invention provides a method for reducing the time for a mobile to update overhead messages prior to making an access attempt, thereby improving the call origination success rate. The method takes advantage of time slots on the paging channel which are typically filled with a null message. According to this invention, a base station transmits a selected overhead message, in lieu of a null message. Because overhead messages are thereby transmitted more frequently, the mobile cycles through required messages and updates its parameters in a reduced time. 
     In one embodiment, the base station runs a timer to determine the paging channel slots that must contain an overhead message. In the intervening slots or slot fragments, the base station transmits mobile-directed messages, when they are available. When no mobile directed messages are available for transmission and the timer has not signaled that an overhead message must be sent, the base station selects an overhead message from a queue for transmission on the paging channel slot or slot fragment. 
     The actual selection algorithm for the overhead message is not critical, although the various overhead messages should be selected with approximately the same frequency. A base station operating according to the method of the invention sends overhead message updates more frequently without consuming additional paging channel capacity, since the existing paging channel resources are used more efficiently. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete understanding of the present invention may be obtained from consideration of the following description in conjunction with the drawings in which: 
     FIG. 1 is an exemplary paging channel frame; 
     FIG. 2 is an exemplary paging channel slot; 
     FIG. 3 is an exemplary flow diagram according to this invention; 
     FIG. 4 is an exemplary paging channel slot according to this invention; and 
     FIG. 5 is an exemplary plurality of paging channel slots according to this invention. 
    
    
     DETAILED DESCRIPTION 
     This description discusses method and apparatus for increasing the frequency of transmission of overhead message updates in a CDMA cellular telephone system. In an IS-95 system, call processing for a mobile initiated call is divided into four states. During the mobile initialization state, the mobile seeks a suitable CDMA system (i.e. a serving base station), acquires a pilot channel, and obtains system configuration and timing information. Once the mobile completes this initialization, it enters an idle state where it begins monitoring the paging channel. 
     In an IS-95 system, a mobile only monitors a paging channel associated with one base station and soft hand-offs are not applicable while the mobile is in the idle state. The mobile, if it leaves a base station service area, may, however, execute idle handoffs, whereby it monitors a paging channel associated with a base station in the new service area. While monitoring the paging channel, the mobile receives overhead messages, including system parameter and access parameter messages. Before the mobile will attempt to access a base station, it monitors the paging channel until the access parameters have been updated. Additional call processing details can be found in Garg, Smolik and Wilkes, Applications of CDMA In Wireless/Personal Communications, Prentice Hall, 1997, p. 34 to p. 59. 
     Referring to FIG. 1, the CDMA message frame of the forward paging channel is illustrated. A paging channel message  200  has an 8 bit message length header  202 , a message body  204 , which is 2 bits to 1146 bits long, and a cyclic redundancy check  206  code of 30 bits. The padding  208  is not part of the paging channel message. When a paging channel message is formed, it is typically sent in a paging channel half-frame, consisting of a synchronization bit and a chunk of data. Eight paging channel half-frames combine to form a paging channel slot  300 , as shown in FIG.,  2 . An entire paging channel message is sent in N slots, and there are a finite number of slots a message can use. 
     The paging channel carries a variety of messages, including system parameter messages, access parameter messages, order messages, channel assignment messages and the null message. All mobiles in an area receive the system parameter messages, which specify the characteristics of the serving system. Similarly, all mobiles receive the access parameters, which specify the parameters the mobile is to use when originating a call. Other overhead messages in an IS-95 system include neighbor list messages, CDMA access channel list messages, channel assignment messages, extended system parameter messages and global service redirection messages. 
     On the other hand, mobile directed messages, such as order messages, are directed to a particular mobile and may direct an operation or confirm a request. A null paging message fills out unused slots and informs the mobile that no useful data is contained in the remainder of the slot. For an additional discussion of the paging channel and its message set, refer to Garg, Smolik and Wilkes, Applications of CDMA In Wireless/Personal Communications, Prentice Hall, 1997, p. 124-127. 
     In a preferred embodiment of this invention, the base station fills unused paging channel slots with overhead messages required for base station access, such as access parameter messages. In lieu of transmitting null messages, the base station selects an overhead message to fill available (otherwise unused) slots or slot fragments, resulting in near continuous transmission of overhead messages during periods of low traffic activity. 
     FIG. 3 illustrates in a flow chart, the steps performed by a base station  400  to implement this method. A first decision step  402  determines whether the paging channel is scheduled to transmit overhead messages. In a preferred embodiment, a base station timer determines the timing for a minimum number of overhead messages which must be sent. If overhead messages are scheduled, an overhead message is selected  404  from a queue and transmitted  412  on the paging channel. 
     If no overhead message is scheduled for transmission, a decision step  406  determines whether any mobile directed messages are available for transmission. If so, the base station selects the mobile directed message in a selection step  408  and transmits the message in a process step  412 . In other words, mobile directed messages typically preempt overhead messages for other than the minimum required overhead messages. 
     If no mobile directed message is available, an overhead message is selected from a queue in a selection step  410  and transmitted on the paging channel in process step  412 . Once the base station either transmits an available mobile directed message or an overhead message, process control is returned to the first decision step  402  and the procedure is repeated. A base station  400  operating according to this method uses substantially all available time slots on the paging channel for information bearing messages. 
     In FIG. 4, one exemplary paging channel slot  500  according to this invention is shown. The slot  500  consists of eight half frames  502 ,  504 ,  506 ,  508 ,  510 ,  512 ,  514 ,  516 . A base station operating according to the method of the invention transmits overhead messages according to a predetermined timing convention, as in the first half frame  502  of the paging slot  500 . In the next three half frames  504 ,  506 ,  508  the paging channel  500  carries mobile directed messages. Absent the method of the invention, however, the next four half frames  510 ,  512 ,  514 ,  516 , which precede the next scheduled overhead message, would contain null messages, as no mobile directed messages are available. It is the advantage of this invention that the remaining half frames  510 ,  512 ,  514 ,  516  carry overhead messages instead of null messages. A base station operating according to this invention rarely, if ever, transmits a null message on the paging channel. 
     A block diagram  600  of exemplary hardware for implementing the base station functions  400  of FIG. 3 is shown in FIG. 5. A programmable processor  604  controls the base station  400  paging channel function. The processor monitors a timer  602  which signals the processor  604  when the base station  400  must send overhead messages on the paging channel. Upon receipt of such a signal, the overhead message is selected from a queue  606 . During intervals when no overhead messages must be sent, the processor  604  determines whether any mobile directed messages are available for transmission, and, if so, it directs the transmitter  608  to send the message. In this exemplary embodiment, the mobile directed messages are stored in a memory  610 . If none are available, the processor  604  selects an overhead message from the queue  606  for transmission. 
     A CDMA system implementing the described invention will exhibit an improved overhead message update rate. The reduced time for a mobile to update overhead messages resulting from this invention will produce improved call origination success rates, without compromising paging channel capacity and without the need to modify existing mobiles.