Patent Publication Number: US-2009219916-A1

Title: Method to scan for critical transmissions while transmitting on a conventional time division multiple access channel

Description:
FIELD OF THE DISCLOSURE 
     This disclosure relates generally to communication systems, and more particularly, to a method to allow communication devices to scan for critical transmissions while transmitting on a conventional time division multiple access (TDMA) channel. 
     BACKGROUND OF THE DISCLOSURE 
     Public organizations such as first responders, as well as other organizations, often rely on conventional two-way, wireless mobile and portable radios to convey voice and data in an efficient, reliable manner. Often these conventional communication systems have no centralized controlling infrastructure, only dispatch consoles or base stations. The communication devices wirelessly communicate with each other and the base sites, typically using frequency division multiple access (FDMA) technology with half-duplex communication devices. 
     When a communication device begins a transmission on a conventional channel, the transmission occurs on a preprogrammed channel. The communication device transmits on the preprogrammed channel until the communication device is dekeyed. Using FDMA technology, however, when communication devices operate in half-duplex mode, the communication device is typically not capable of receiving any signals or commands from a base station or another communication device while it is transmitting. 
     Consequently, if a communication device is transmitting while a critical transmission is being broadcast of which the communication device should be aware, the communication device misses the critical transmission. In order to scan for critical transmissions, the communication device needs to dekey which would otherwise adversely affect the quality of its own transmission. As features, such as Automatic Vehicle Location (AVL), are introduced that require frequent transmissions, there is a greater likelihood of missing the critical transmission. Moreover, missing the critical transmission also becomes concerning when there is a need to respond quickly. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The benefits and advantages of the present invention will become more readily apparent to those of ordinary skill in the relevant art after reviewing the following detailed description and accompanying drawings, wherein: 
         FIG. 1  illustrates an exemplary block diagram of a wireless communication system that may be used for implementing the present disclosure; 
         FIG. 2  illustrates a flow chart for a communication device to periodically scan for critical transmissions in an alternate slot of a conventional TDMA channel while transmitting in its own slot on the TDMA channel in accordance with the present disclosure; and 
         FIG. 3  illustrates a block diagram of an embodiment of an embedded link control signal in a conventional TDMA channel in accordance with the present disclosure. 
     
    
    
     Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. Also, common, but well-understood elements that are useful or necessary in a commercially feasible embodiment are not often depicted in order to facilitate a less obstructed view of these various embodiments of the present disclosure. It will be further appreciated that certain actions and/or steps may be described or depicted in a particular order of occurrence while those skilled in the art will understand that such specificity with respect to sequence is not actually required. It will be understood that the terms and expressions are used with respect to their corresponding respective areas of inquiry and study except where specific meaning have otherwise been set forth herein. 
     DETAILED DESCRIPTION OF THE DISCLOSURE 
     A method allowing a wireless communications device operating in a half-duplex mode to periodically scan for critical transmissions in an alternate slot of a conventional TDMA channel while transmitting in its own slot on the channel is disclosed. In accordance with the present disclosure, a communication device may be provisioned to consider a transmission to be a “critical transmission” if it is an emergency transmission, a transmission that has a high priority, a transmission that has a higher priority than its own transmission, a transmission from a particular communication device (e.g., a supervisor), and/or the like. Each communication device is configured to operate in half-duplex mode, in which each communication device is capable of either transmitting or receiving at a given instant. A wireless communication device transmits a transmission using TDMA technology on a conventional channel which is divided into slots. In the present disclosure, the channel is divided into at least two slots, a first slot and a second slot, each capable of carrying audio, video, control, and/or data transmissions. A first slot is used by a first communication device while a second slot is used by a second communication device. While the first communication device is transmitting a first transmission in the first slot of the TDMA channel, the second communication device is transmitting a second transmission in the second slot of the TDMA channel. While the first communication device is transmitting the first transmission, it also periodically scans the second slot for signaling information segments embedded in the second transmission being transmitted by the second communication device in the second slot of the TDMA channel. Once the first communication device reads all of the embedded signaling information segments from the second transmission, that are needed to form a link control message, the first communication device determines whether to terminate its own transmission being transmitted in the first slot in order to receive the second transmission being transmitted in the second slot by the second communication device, or whether to ignore the second transmission being transmitted in the second slot by the second communication device and continue transmitting its own transmission in the first slot. It is important to note that the communication device makes the decision to terminate its own transmission being transmitted in the first slot in order to receive the second transmission being transmitted in the second slot, or to ignore the second transmission being transmitted in the second slot and continue transmitting its own transmission in the first slot is made independently (i.e., the first communication device does not receive direction or instructions from the second communication device, the base station/radio, or any other device in the wireless communication system) based on the embedded signaling information segments and/or link control message read from the second transmission, for example, the status of the emergency and priority bits in the service options field of the link control information embedded throughout the second transmission. 
     Let us now refer to the figures to describe the present disclosure in greater detail.  FIG. 1  illustrates an exemplary block diagram of a wireless communication system  100  that may be used for implementing the present disclosure. Wireless communication system  100  comprises communication devices  110  and  120 , which may, for example, be a mobile or portable radio, a cellular radio and/or telephone, a video terminal, a portable computer with a wireless modem, a personal digital assistant, or any other type of wireless communication device. It should be noted that the communication devices  110  and  120  are also referred to in the art as mobile stations, mobile equipment, handsets, subscribers, or the like. 
     In this exemplary embodiment, communication devices  110  and  120  communicate over a communication access network  130 . Of course, those of ordinary skill in the art will realize that any type of network is within the scope of the teachings herein. Thus, the communication access network  130  may comprise infrastructure devices, such as, but not limited to, base stations (with a single base station  140  shown for clarity) to facilitate the communications between the communication devices  110  and  120  having access to the communication access network  130 . 
     For example, communication device  110  and communication device  120  may communicate with each other by communication device  110  establishing a wireless link or radio connection  150  with the base station  140  over an available radio frequency (RF) channel, and communication device  120  establishing a wireless link  160  with the base station  140  over an available RF channel. As is well understood in the art, the base station  140  generally comprises one or more repeaters that can receive a signal from communication device  110  over link  150  and retransmit the signal to communication device  120  over link  160 , or can receive a signal from communication device  120  over link  160  and retransmit the signal to communication device  110  over link  150 . For ease of illustration, only two communication devices  110 ,  120  and one base station  140  are shown. Those skilled in the art, however, will realize that in a typical system, a much larger number of communication devices are supported by the communication access network, which has many more base stations than is shown in  FIG. 1 . Moreover, although in this embodiment, communications between communication devices  110  and  120  are illustrated as being facilitated by the base station  140 , communication devices  110  and  120  may communicate using a direct mode of operation without the base station. The teachings herein are equally applicable to direct mode operation between the two communication devices  110  and  120 . 
     Since the communication access network  130  is a wireless network, meaning that it supports a wireless or air interface protocol for signal transmission, both of the communication devices  110  and  120 , and the base station  140  comprise a transmitter and a receiver (or a transceiver) for transmitting and receiving RF signals, respectively. Communication devices  110  and  120 , and the base station  140  further comprise one or more processing devices (such as, a microprocessor, a digital signal processor, a customized processor, a field programmable gate array (FPGA), unique stored program instructions (including both software and firmware), state machines, etc.) and typically some type of conventional memory element for performing (among other functionality) the air interface protocol and channel access scheme supported by network  130 . Using these protocols, communication devices  110  and  120  generate RF signals containing one or more transmissions comprising a plurality of fields for organizing the continuous bits of information and/or signaling for transmission to another communication device. 
     Of course, while one embodiment of the wireless communication system  100  is described with regards to  FIG. 1 , those skilled in the art will recognize and appreciate that the specifics of this illustrative example are not specifics of the disclosure itself and that the teachings set forth herein are applicable in a variety of alternative settings. For example, while one embodiment is in compliance with the European Telecommunications Standards Institute (ETSI) Technical Specification (TS) for Digital Mobile Radios (DMR), other embodiments compatible with the present disclosure are contemplated. Further, while the air interface protocol for a DMR using a TDMA channel access scheme is defined in ETSI TS 102 361-1, the present disclosure is compatible with other protocols utilizing a TDMA-type air interface. 
     Referring now to  FIG. 2 , a flow chart  200  exemplifying the present disclosure for periodically scanning for critical transmissions in an alternate slot of a conventional TDMA channel while transmitting in its own slot on the TDMA channel is shown. In accordance with the present disclosure, a communication device  110  transmits a burst of information from a first transmission (e.g., audio, video, control, and/or data transmission) in slot A of a two-slot TDMA channel having a slot A and a slot B at step  210 . As well understood in the art, a transmission comprises at least one burst of information. While the present disclosure uses a two-slot TDMA channel, it is to be understood that the present disclosure can also be utilized on three, four or more slotted TDMA channels. For simplicity, a two-slot TDMA channel is shown and described. The TDMA structure utilizes a measure of information to be condensed into a smaller packet of information for transmission. For example, sixty (60) milliseconds of audio can be condensed into a thirty (30) millisecond burst of information on the inbound channel. A communication device  120 , however, that receives the 30 millisecond burst of information receives the full 60 millisecond of audio. This technology is well known in the art and is not described in greater detail in the present disclosure. 
     After transmitting the burst of information at step  210 , the communication device  110  switches to slot B of the two-slot TDMA channel, scans slot B, and reads signaling information embedded in a second transmission being transmitted in slot B at step  220 . In practice, for example, the communication device  110  transmits a burst of information in slot A (the communication device  110  condenses 60 milliseconds of audio into a 30 millisecond burst) and utilizes the gap in time between transmission of bursts to scan the alternate channel, slot B, for critical transmissions (during the remaining 30 milliseconds, the communication device switches to the alternate slot, slot B, scans for critical transmissions, and then switches back to its own transmission in slot A). 
     When scanning for critical transmissions, the communication device  110  reads signaling information segments embedded within the bursts of the second transmission being transmitted in slot B; at least a portion of the signaling information segments embedded in the second transmission is required to assemble an entire embedded link control message. The communication device  110  determines if all the embedded signaling information segments that is required to assemble the embedded link control message have been read from the second transmission at step  230 . If the communication device  110  has not read all the embedded signaling information segments from the second transmission that is required to assemble the embedded link control message, the communication device  110  switches back to slot A at step  240 , and transmits another burst of information from the first transmission (i.e., its own transmission) in slot A. This loop continues until the communication device  110  has read all of the embedded signaling information segments required to assemble an entire embedded link control message from the second transmission. 
     When the communication device  100  has read all of the embedded signaling information segments from the second transmission that is required to assemble the embedded link control message at step  230 , the communication device  100  determines whether the second transmission is considered a critical transmission at step  250 . If the second transmission being transmitted in slot B is not a critical transmission (e.g., low priority, a lower priority than the first transmission, or non-emergency), the communication device  100  continues with its own transmission by switching back to slot A at step  240 , and transmitting another burst of information from the first transmission (i.e., its own transmission) in slot A. If, however, the second transmission being transmitted in slot B is critical (e.g., high priority, a higher priority than the first transmission or emergency), the communication device  100  independently determines whether it should terminate the first transmission (i.e., its own transmission) being transmitted in slot A at step  260 . If the communication device  100  does not decide to terminate the first transmission at step  260 , the communication device  100  switches back to slot A at step  240 , and transmits another burst of information from the first transmission (i.e., its own transmission) in slot A. If, however, the communication device  110  independently decides to terminate the first transmission at step  260 , the communication device  100  remains on slot B and receives the second transmission being transmitted in slot B at step  270 . Thus, the present disclosure enables the communication device  100  to detect critical transmissions in progress by other communication devices and respond quickly, if necessary. 
     Let us now describe the present disclosure pictorially with reference to  FIG. 3 .  FIG. 3  illustrates an embodiment of a two-slot TDMA channel having a slot A and a slot B. The TDMA channel is used by communication device  110  and communication device  120 . The TDMA channel has an inbound channel and an outbound channel, each of which is divided into slots. The inbound channel is represented by directional arrows from communication device  110  and communication device  120  to the outbound channel. Communication device  110  transmits bursts in slot A and communication device  120  transmits bursts in slot B. “Burst” in  FIG. 2  refers to portions of an audio, a video, a control, and/or a data transmission, as mentioned above. 
     The outbound channel illustrates the outbound transmissions occurring on the two-slot TDMA outbound channel. Slot A and slot B are separated in time and each slot is capable of carrying an independent transmission. As communication device  110  and communication device  120  transmit bursts on their respective inbound channels, the bursts are repeated in the outbound channel. Each burst is repeated on the outbound channel, for example in a present embodiment, approximately sixty (60) milliseconds, after the initial transmission. For example, communication device  110  transmits Burst  2  to the base station  140  on its inbound channel. Burst  2  is repeated later in time on the outbound channel. Similarly, communication device  120  transmits a header burst to the base station  140 , and the header burst is repeated in the outbound channel. Various signal transmission speeds and configurations are also contemplated. 
     Communication device  110  is able to receive from both slots of the TDMA channel when it is not transmitting. While transmitting, communication device  110  is able to detect at least a portion of the burst being transmitted in the alternate slot, slot B. Likewise, communication device  120  is able to receive on both channels, and while transmitting, it is able to detect at least a portion of the burst being transmitted in the alternate slot, slot A. For simplicity, the present embodiment refers to the point of view of communication device  110 ; however, it is important to note that each communication device is capable of scanning the alternate slot. 
     A first transmission being transmitted in slot A, and a second transmission being transmitted in slot B is shown in  FIG. 2 . As noted above, either transmission may be audio, video, control, or data transmission. The transmission being transmitted by communication device  120  in slot B comprises at least the following bursts: Burst A, Burst B, Burst C, Burst D, Burst E and Burst F. One (1) burst contains an embedded synchronization pattern while the remaining bursts contain embedded signaling information, indicated by a, b, c, d, e, and f. In this particular example, of these bursts containing embedded signaling information, four (4) bursts contain embedded signaling information segments (i.e., b, c, d, and e) that together form the embedded link control message, Embedded LC. The embedded signaling information segments are positioned within the bursts in slot B such that the communication device  110  has sufficient time to transition from its own transmission in slot A to read at least one of the embedded signaling information segments in slot B, and back to slot A, without disrupting its own transmission in slot A. 
     The embedded link control message contains information regarding the origin, destination, transmission type and relative importance of the transmission. The relative importance of the transmission may be indicated, for example, by a service options field found embedded within at least one of the embedded signaling information segments. In the present embodiment, the service options field is an eight-bit field having one of the bits indicating whether the transmission is an emergency or not and a set of bits indicating the relative priority of the transmission. Communication device  110  determines if the transmission in slot B is more critical than its own transmission in slot A by comparing the emergency and priority bits to its own. It is contemplated, however, that other methods indicating the relative importance of a transmission may also be compatible with the present disclosure. 
     The embedded signaling information segments is positioned within the bursts to allow time for communication device  110  to transition to the outbound channel of slot B, read the embedded signaling information segment or embedded synchronization information, and transition back to slot A to continue transmitting its own transmission without interruption. Positioning the embedded signaling information segment in this manner allows the communication device to quickly respond to critical transmissions. In order to allow the communication device  110  to continuously transition between transmitting in slot A and scanning slot B, the communication device  110  synchronizes its timing to the outbound channel timing. 
     Further, the embedded link control message allows the communication device  110  to have all the necessary information required, including the relative criticality of the transmission, to independently determine if it will terminate its own transmission and receive the transmission being transmitted in the alternate slot or if it will continue with its own transmission in slot A. It is important to note that, unlike reverse channel messaging in which the communication device  110  is sent a specific message from another communication device or base station  140  to halt its transmission, in the present disclosure, communication device  110  is monitoring normal activity being transmitted in slot B of the same frequency pair while transmitting in its own transmission in slot A. The signaling information segments embedded in a transmission being transmitted in slot B do not instruct the communication device  110  to terminate its own transmission being transmitted in slot A. Rather, the communication device  110  continuously scans the transmission being transmitted in slot B for embedded signaling information segments, and independently determines whether to terminate its own transmission and receive the second transmission being transmitted in slot B, or whether to continue transmitting its own transmission in slot A. 
     In conclusion, the present disclosure allows a communication device operating in a half-duplex mode to periodically scan for critical transmissions in an alternate slot of a conventional TDMA channel while transmitting in its own slot on the channel without interruption. Thus, the present disclosure allows a communication device to transmit its own transmission, as well as be aware of critical transmissions being transmitted in the alternate slot. 
     While the present invention is susceptible of embodiment in various forms, there is shown in the drawings and is described a presently preferred embodiment with the understanding that the present disclosure is to be considered an exemplification of the invention and is not intended to limit the invention to the specific embodiment illustrated. In addition, in the present disclosure, the words “a” or “an” are to be taken to include both the singular and the plural. Conversely, any reference to plural items shall, where appropriate, include the singular. 
     It should be further understood that the title of this section of this specification, namely, “Detailed Description of the Disclosure”, relates to a requirement of the United States Patent Office, and does not imply, nor should be inferred to limit the subject matter disclosed herein. 
     All patents referred to herein, are incorporated herein by reference, whether or not specifically done so within the text of this disclosure. 
     Further advantages and modifications of the above described method will readily occur to those skilled in the art. The disclosure, in its broader aspects, is therefore not limited to the specific details, representative system and methods, and illustrative examples shown and described above. Various modifications and variations can be made to the above specification without departing from the scope or spirit of the present disclosure, and it is intended that the present disclosure cover all such modifications and variations provided they come within the scope of the following claims and their equivalents.