Method and apparatus for dropping and reacquiring a dispatch channel

[0001] This invention relates in general to the field of wireless communications. More specifically, this invention relates to a method and apparatus for automatically dropping and reacquiring a dispatch channel.

[0002] A wireless communication system such as the Motorola Integrated Digital Enhanced Network (IDEN®) combines the capabilities of a digital cellular phone with the benefits of a two-way radio with “push-to-talk” (PTT) dispatch feature. This combining of mobile communication technologies provides for state-of-the-art functions and benefits to mobile users while optimizing the available infrastructure resources such as the Radio Frequency (RF) spectral resources.

[0003] When operating in the dispatch mode, the typical cell-to-cell handoff procedure that occurs when a telephone user is on the move is known as “drag and drop” or “hard-hand-off” as compared to the “soft-handoff” that occurs when operating in the cellular mode. During a dispatch mode hand-off, there is usually a period of time during the transition between cells in which the audio may become warped. The dispatch link may also be dropped while a new channel is located by the system. If a person happens to be talking during the time the dispatch communication link is dropped by the system, the person will not know that the link has been dropped and that the other dispatch call participants will not have properly received some part of the conversation. This of course presents confusion to all parties involved in the dispatch call with regard to how much was heard and how much needs to be repeated, once the dispatch channel is reestablished. Therefore, a need exists for a method and apparatus for automatically dropping and reacquiring a dispatch channel that can help minimize some of the dispatch channel handoff problems previously mentioned.

[0008] While the specification concludes with claims defining the features of the invention that are regarded as novel, it is believed that the invention will be better understood from a consideration of the following description in conjunction with the drawing figures.

[0009] Referring now to the drawing and in particular to FIG. 1, there is shown a diagram of a communication system 100 in accordance with the invention. Communication system 100 supports dispatch mode of communication and can comprise a conventional IDEN® communication system that has been modified to support the present invention as described below, or other communication system having dispatch capability.

[0010] In communication system 100 a plurality of Enhanced Base Transceiver Systems (EBTS, hereinafter referred to as cell sites, cells or base stations) 102, 104 and 106 provide the RF link between the land network and the portable communication devices or Mobile Stations (MS) 110 and 112. Each cell site 102, 104, and 106 provides communication coverage over their respective cell coverage areas 116, 118 and 114.

[0011] Each cell site 102, 104 and 106 includes an Access Control Gateway (ACG, not shown) that acts as the site controller, and the communication gateway between the cell site and the system's central control 108. The ACG determines if a call is a dispatch, interconnect or packet data call and routes the traffic accordingly. Each cell site 102, 104 and 106 also includes one or more Base Radios (BR's, not shown), which are controlled by the ACG. System control 108 is comprised of several IDEN® communication system control components such as the Dispatch Application Processor (DAP) that is responsible for the overall coordination and control of dispatch and packet data services in the system, the Base Site Controller (BSC) which manages the interconnect call processing between the EBTS and other system devices, the Mobile Switching Center (MSC) which is the interface between the mobile network and other service provider's PST N's. A more detailed discussion of a conventional IDEN® communication system can be found in a publication entitled, IDEN® Technical Overview, Motorola publication number 68P81095E55-E, dated Aug. 8, 2000, which is hereby incorporated by reference.

[0012] When MS 110 and MS 112 are involved in a dispatch call, and MS 112 is moving from cell area 118 towards cell area 114, in accordance with the preferred embodiment of the invention, MS 112 and MS 114 monitor the signal quality, such as the signal strength of incoming signals while engaged in the dispatch call. Although signal strength is used in the preferred embodiment, other signal quality criteria can be used, for example, bit-error-rate, etc. Preferably, MS 112 and MS 114 monitor traffic flow on their respective Associated Control Channel (ACCH) while they are participating in the dispatch call. Once one or more of the dispatch channel participant's, such as MS 112 in this case, moves further from cell site 104, and determines that its signal strength has fallen below an acceptable threshold level, it sends a notice of the impending channel loss to EBTS 104 which forwards the information on to system control 108. In response to receiving the notice of the low signal quality at one or more of the dispatch call participants, the system control 108, primarily handled by the DAP (not shown), sends a notice to MS 110 and MS 112 informing them that the dispatch call between them will be temporarily disrupted.

[0013] The loss of the dispatch channel signal sent to MS 110 and MS 112 can cause an audible tone, or other type of alert (e.g., visual, vibratory, etc.) to be provided at MS 110 and MS 112. This alert signal lets MS 110 and MS 112 know that their dispatch call will be momentarily disrupted (suspended), and alerts the users that they should hold off momentarily with their conversations, since one or more of the dispatch call participants is having signal quality problems.

[0014] In the preferred embodiment, the ACCH is used to monitor the signal quality in this case, the signal strength of the received signals at each of the MSs 110, 112 involved in the dispatch call because the ACCH is the only active control channel available during voice communication, the ACCH is formed by taking bits from the Traffic Channel (TCH). The ACCH carries control and supervisory signaling for an MS while the user is engaged in voice communication. Although the ACCH is used in the preferred embodiment, respective MSs can also use other channel(s) that could provide the needed signal quality check.

[0015] After MS 110 and MS 112 have been alerted, the system 100 drops the previously established dispatch link between MS 110 and MS 112, and searches to reacquire a better channel for the dispatch call by continuously monitoring the signal strength levels from both MS 110 and MS 112. The system may monitor the signal strength of both MS 110 and MS 112 by preferably monitoring signal strength information sent by MS 110 and MS 112 to system control 108.

[0016] In an alternate embodiment, only those MS units, who had previously informed the system that their signal strength levels were below the predetermined threshold, need to transmit back their signal strength levels or other signal quality measurements. In order to save system overhead, these unit(s) would only need to inform the system of their signal quality level after it had gone back above the predetermined threshold level. When both signal strength levels are above a predetermined threshold, a dispatch channel is automatically re-established by the system control 108 and indication such as an audible alert is sent to both MS 110 and MS 112 that indicates to the users that their dispatch conversation may resume.

[0017] Referring now to FIG. 2, there is shown a flow diagram of the steps taken in accordance with the preferred embodiment. In step 202, a dispatch call is established between two or more MSs. In step 204, each of the MSs continuously monitors the signal quality of the ACCH while in the dispatch call. In step 206, one (or more) of the MS determines that its signal quality such as the signal strength of signals on the ACCH have fallen below a predetermined threshold, and sends a notification signal (signal quality information message) to its EBTS in step 208 which is forwarded to system control 108. In step 210, in response to receiving the signal quality information message, the system control 108 alerts all the dispatch call participants that the dispatch channel link will be dropped by sending an alert signal and the system tears down the dispatch communication link that had been established. The “bad channel” alert signal sent by the system can cause a distinct audible, visual, vibratory or other type of alert to be provided at each of the MS units participating in that particular dispatch call. The alert signal lets the dispatch call participants know that they should temporarily cease from continuing to talk.

[0018] In step 212, the system control 108 maintains information (e.g., participants involved, channel assignments, etc.) on the previously dropped dispatch call for a predetermined period of time referred to as the re-establishment time period, and continues to monitor the MSs to determine if the MSs involved in the dispatch call are all above their required signal strength levels. The amount of time the system control 108 maintains the information on the dropped dispatch channel can vary depending on system requirements. If the reestablishment time period (timer) expires prior to all of the dispatch call MS units having acceptable signal strength levels, the system control 108 in step 214, erases all the previously established dispatch call information stored in the system, requiring that a new dispatch call be set-up from scratch.

[0019] If the re-establish timer has not expired, in step 216, the system control 108 determines if the signal quality for all MS units is above the required threshold level. If it is determined that all of the dispatch call participants have acceptable signal quality, in step 218, the system control 108 automatically establishes a new dispatch call link and alerts all the dispatch call participants by transmitting a “good channel” alert signal. The system automatically sends any needed set-up information to all of the MSs involved in the dispatch call, given that one or more of the MSs may be located in new cell sites, etc. The system preferably sets up the required communication link and places each MS into the proper state so that the dispatch call can be re-established with a single PTT activation at any of the MS units. The indication that a new dispatch channel is ready can be given via an alert signal such as an audible or visual signal (e.g., light flashing) at each of the MSs, or other form of indication.

[0020] In the case where one or more of the original dispatch call participants is engaged in another activity (e.g., has established another dispatch call, is involved in an interconnect call, etc.) or has turned off the MS prior to receiving the alert signal that the dispatch call has been re-established, can be dealt with in different ways depending on the particular system design. For example, the good channel alert signal could override any present activity the MS may be involved in, or the system could wait until the MS(s) involved has finished with its current activity, and if the dispatch call is still ongoing, alert and re-establish the dispatch call for those unit(s) at that time.

[0021] In FIG. 3, there is shown a simplified diagram of the MS 112. MS 112 includes a controller such as a microprocessor and/or digital signal processor 302 that controls the functions and operations of the MS 112. A keypad and user controls 312 are coupled to the controller 302 as well as display 314. A microphone 306 is provided for converting voice from the user into electrical signals, while a speaker 308 provides audio signals to the user.

[0022] A Vector Sum Excited Linear Predicting (VSELP) voice codec (vocoder) and Analog-to-Digital (A to D) (and also Digital-to-Analog) block 304 provides all the necessary digital voice processing for converting analog voice into digital data ready for RF transmission and vice versa.

[0023] RF modulator/demodulator block 310 transmits and receives the RF signals via antenna 318. A signal strength block 316 measures the signal strength of the ACCH as required by the present invention. The controller 302 determines if the signal strength as measured by the signal strength block 316 falls below a predetermined threshold level that is preferably stored in memory 320. If the measured signal strength during a dispatch call falls below the predetermined level, controller 302 causes MS 112 to transmit a message to the system as previously described above.

[0024] By monitoring the signal quality of each MS unit participating in a dispatch call and automatically informing the MS units if one or more of the units is in a bad quality of signal situation, avoids the problem of users continuing on with their conversations even when one or more of the other dispatch group members is not in a good position to receive the conversation. Automatically setting the dispatch call back up when the participants are all experiencing good quality of signal reception makes the process very convenient for all of the dispatch call members. The system can drop the dispatch call session information if the quality of signal for each of the dispatch members does not reach an acceptable level within a predetermined period of time. If this time period expires, the system erases all dispatch session information, requiring the dispatch call to be set up from scratch by one of the dispatch call members. This avoids the system from having to store dispatch call set up information in situations where one or more of the dispatch call members may not be able to receive the dispatch call properly.

[0025] While the preferred embodiments of the invention have been illustrated and described, it will be clear that the invention is not so limited. Numerous modifications, changes, variations, substitutions and equivalents will occur to those skilled in the art without departing from the spirit and scope of the present invention as defined by the appended claims.