Communication System With Automatic Handoff

This invention relates in general to the field of automatic interrogation and response of a multiplicity of remotes on a non-interfering basis and relaying response information to higher levels with fewer nodes, until at the highest level only a single node exists. Additionally, textual information can be sent downward from a higher level to a lower level and also in a corresponding method in an upward direction.

In tracking systems that include many portable, tracked remote units it is important coordinate the handoff between different monitors as the portable, tracked remote units move about and lose contact with one or another monitor. This problem is especially severe when communication requires line-of-sight (LOS) transmission of signals. In such systems, it is additionally desirable or essential to be able to assure that a message from a remote unit will be acknowledged by the monitoring sites in an organized fashion that assures reception of the acknowledgment by the remote unit that sent the initial message. This is particularly critical when the message that is being sent by the remote unit is an emergency (911) call which additionally provides descriptive information through a selectable canned message library or though a character-limited text message.

Thus, what is needed is a practical, economical method for providing seamless handoff between a multiplicity of monitors that are monitoring a (typically larger) number of remote units via a time division multiple access (TDMA) communications linkage. In such a system, it is particularly important to provide an acknowledging signal in an orderly fashion, especially when the initial communication is urgent.

FIG. 1 is a simplified plan view of a battlefield in accordance with a first preferred embodiment of the invention;

FIG. 2 is a sixty second TDMA slot assignment diagram for a twelve second emergency call scheme with twenty positions per minute reporting, in accordance with the first preferred embodiment of the present invention;

FIG. 3 is a sixty second TDMA slot assignment diagram for a six second emergency call scheme with thirty-seven positions per minute reporting, in accordance with the teachings of the present invention;

FIG. 4 is a simplified schematic diagram of a second preferred embodiment of a communication system using both line-of-sight and satellite communications links, in accordance with the teachings of the present invention in accordance with the teachings of the present invention;

FIG. 5 is a sixty second TDMA slot assignment diagram for a ten second emergency call scheme with thirty-seven positions per minute, in accordance with the teachings of the present invention; and

FIG. 6 is a thirty second SATCOM slot assignment diagram, in accordance with the teachings of the present invention.

.sctn. A. A first battlefield configuration.

FIG. 1 is a simplified plan view of a battlefield for deployment and operation of system 14 of the present invention. Communication system 14 described herein consists of a combined line-of-sight (LOS) and satellite communication (SATCOM) network used to keep track of up to thirty-seven movable remote units 10, operating on one frequency, by making use of a time division multiple access (TDMA) protocol which allows system 14 to operate in a half duplex mode in which only one remote unit 10 may radiate at a particular time. In other words, while point-to-point communication can occur in two directions at a given time, transmission can occur in only one direction.

Use of assigned time slots or intervals eliminates collisions between messages which may occur if two or more remote units 10 transmit at the same time when in line-of-sight of each other. System 14, using the Global Positioning System (GPS) for position determinations and time synchronization, can be used to monitor the position and status of ground, naval or air based remote units 10 (fixed or mobile). This is especially useful for early warning of: approach to a hostile border 12, approach to a "keep-out" zone which could contain enemy threats such as air-aircraft guns, land mines or missiles or as a search and rescue aid in the event of an emergency.

System 14 of the first preferred embodiment of the present invention includes observation posts 18, remote units 10, two command posts 17 and battalion command post or base station 16. In this embodiment, remote units 10 are all mobile and command posts 17 and battalion command 16 are fixed monitoring sites.

This invention allows for the expansion of the system by using a TDMA signaling protocol that is greater than sixty seconds in length. For example, a one hundred and twenty second TDMA interval provides for a network of up to seventy-four remote units.

.sctn. B. Sixty second TDMA LOS slot assignments for a twelve second emergency call scheme with up to twenty positions per minute reporting.

FIG. 2 is a sixty second TDMA LOS slot assignment diagram for a twelve second emergency call scheme with twenty positions per minute reporting, in accordance with the first preferred embodiment of the present invention. Slots in the first column correspond to GPS differential data that are sent from each observation post 18 to any or all remote unit(s) 10 that have an acceptable receive LOS communication link. Slots in the second column, in addition to the differential correction information, also contain up to six remote unit 10 addresses in what is called a Priority List (PL). Slots in the third column correspond to those reserved for 911 calls and acknowledgments interleaved or alternating with slots reserved for two BCTEXT slots, i.e., communications from battalion command to remote units 10 and/or fixed monitoring sites 17, 18, one slot for relay of remote position data from the nth monitoring site 18 to one or both command posts 17, and two slots for each of the two command posts 17 to relay remote positions to battalion command 16. Five time slots in the fourth column in the first thirty second sub-frame (upper half, FIG. 2) are reserved for text message transmission from the 20 remotes to one or more (if LOS communication links exist) monitoring sites (18). The second thirty second sub-frame (lower half, FIG. 2) contains two time slots reserved for communications to either battalion command post 16 (upward communications) or relaying messages from battalion command 16 or locally generated messages sent to monitoring site 18. Slots in the fifth column are reserved for text messages which may be addressed to a remote 10, another monitoring site 18 or command post 17 from fixed monitoring site 18, of which there are up to ten. Slots in the sixth and seventh columns are reserved for position reports from up to twenty different remote units 10.

.sctn. C. Sixty second TDMA slot assignments for a six second emergency call scheme with thirty-seven positions per minute reporting.

FIG. 3 is a sixty second TDMA slot assignment diagram for a six second emergency call scheme with thirty-seven positions per minute reporting, in accordance with the teachings of the present invention. Slots in the first two columns correspond to GPS differential data that are sent from each monitoring site 18 to each remote unit 10 via LOS communications, which may or may not spatially overlap with other LOS paths to other remote units 10. Slots in the first column also contain space for up to six remote unit 10 addresses or ID's but this TDMA slot configuration limits the number to a maximum of four for the first seven six second interval and three for the last three six second intervals. Slots in the third column correspond to those reserved for 911 calls and acknowledgments. Slots in the fourth column are reserved for text messages and relay of remote unit 10 position(s). The first four text intervals are used for relay of text messages from remote units 10 to monitoring site 18 to command post 17 and finally to battalion command 18. The fifth through seventh intervals are for text messages from battalion command 16 to be relayed to observation posts 18. The last three intervals, eight through ten, are used to relay the remote unit 10 positions received by the nth monitoring site 18 upward to the battalion post 16. Slots in the first seven positions of column five and all of the sixth and seventh columns are reserved for position reports from up to thirty-seven different remote units 10. Remote units 10 may be dynamically assigned to slot positions, i.e., slot assignments may be varied from one frame to the next. In this scenario, a new slot assignment list is generated and then is broadcast to the remote units 10.

.sctn. D. Second preferred embodiment.

A top level block diagram of system 14 in a second preferred embodiment is shown in FIG. 4. In system 14, up to forty-four remote units 10 operate on one frequency. Information is passed to and from remote units 10 to base station 16 through either a TDMA LOS UHF radio link via both fixed monitoring sites 18 and mobile monitoring sites 20, or through a TDMA LOS UHF and SATCOM radio link. In system 14, the TDMA LOS and SATCOM links operate independently of each other. Backup landline and microwave links (not illustrated) between fixed monitoring sites 18 and fixed base station 16 are also available for use in system 14. Position and normal messaging updates from each remote unit 10 occur every minute. 911 emergency calls from remote units 10 are transmitted and acknowledged six seconds later in the next, or second, priority window. GPS position accuracy is improved by the use of differential correction (pseudo range) data sent to remote units 10 from both fixed monitoring stations 18 and base station 16 once every thirty seconds.

Data are transmitted over the LOS and SATCOM network via a 1200/2400 baud minimal shift keying (MSK) modulated modem. The raw message data bits are further processed by: digital encryption standard (DES) encryption, Bose--Chadhuri--Hocquenghem (BCH) forward error correction encoding, bit interleaving and UART formatting. Then checksum bits are added.

As various remote units 10 move, some of fixed monitoring sites 18 will lose two-way communication capability (called "track") of a particular remote unit 10 due to propagation losses or physical topographical barriers and other different monitoring sites (either fixed 18 or mobile 20) must begin tracking that remote unit 10. Handover between monitoring sites 18 and/or 20 must be seamless, and, when a particular remote unit 10 issues a priority call, such as a 911 emergency call, it must be acknowledged in a timely and orderly method by at least one of monitoring sites 18 or 20.

All of remote units 10 and monitoring sites 18, 20 are configured such that they have a common TDMA protocol in which the frame time is divided into slots and transmissions by remote units 10 and monitoring sites 18 or 20 occur only during assigned time slots so that collisions between messages do not occur in a half duplex system. In order to accomplish this, all remote units 10 are given a semi-permanent unique ID number so that they may automatically transmit their position and any other information desired during their assigned time slot.

All monitoring sites 18, 20 having a LOS view and a sufficient signal-to-noise ratio (SNR) on the transmitted signal receive the position and message from remote unit 10. Monitoring sites 18, 20 can then `relay` or forward to higher reporting levels (e.g., to base station 16) information from a selected number of remote units 10 based on a variety of criteria such as minimal distance between remote unit 10 and monitoring sites 18, 20. Additionally, when a critical message such as a 911 emergency call is sent, acknowledgment from one or more of monitoring sites 18, 20 needs to be in an orderly fashion in order to prevent collisions of the messages due to multiple acknowledgments occurring during the same time slot. Even though a critical message is sent globally, a destination ID is added whose sole purpose is identifying the monitoring site 18, 20 that will send the acknowledgment message. Remote unit 10 determines the monitoring site 18, 20 ID for acknowledgment based on selection criteria from data contained in the differential correction and positions reported list. The monitoring site 18, 20 that acknowledged the critical message was selected based on the fact that recent communications have demonstrated that that monitoring site 18, 20 and remote unit 10 have established a successful two-way communications link based on monitoring sites 18, 20 having received a position update and remote unit 10 having received a position report list that indicates its position was received. Thus only one monitoring site 18, 20 transmits an acknowledgment message.

LOS system 14 communication protocol is TDMA. This approach was selected since the system must operate as a half-duplex system in which only one user may radiate at a particular time. Use of assigned time slots or intervals eliminates collisions between messages which may occur if two or more transmitters that use LOS communication with the same relay receiver radiate during the same time slot. Time is divided into a one minute frame containing two thirty second sub-frames: zero to thirty and thirty to sixty seconds. The thirty second sub-frame composition in terms of repetition period and number of each type of message per sub-frame is given below in Table I.

Every thirty second sub-frame, message slots are allocated as shown in Table II.

SATCOM system 14 uses a TDMA communication protocol. Use of assigned time slots or intervals eliminates collisions between messages which may occur if two or more transmitted messages use SATCOM communication with the same relay receiver during the same time slot. Time is divided into a one thirty second frame Remote unit 10 slot interval response numbers range from one to ten. Differential corrections occur every thirty seconds and clear 911 call and acknowledgment slot intervals occur every ten seconds. The thirty second frame composition in terms of repetition period and number of each type of message per frame is given below in Table III.

Every thirty second frame, message slots are allocated s shown below in Table IV.

The LOS system operation is described in terms of the messages used. The time sequence is specified by the TDMA protocol timing diagram shown in FIG. 5. Each sixty second frame is composed of two thirty-second sub-frames. This defines a minimum position location report rate of once per minute.

FIG. 5 shows the sixty second TDMA LOS slot assignment diagram. Each of the two thirty-second sub-frames has the same time slot time allocation except that remote unit 10 slot response number ranges from one to twenty-four in the first sub-frame, and from twenty-five to forty-eight in the second sub-frame, respectively. Differential corrections from all four fixed sites occur every thirty seconds and clear 911 call and acknowledgment slot intervals occur every ten seconds.

Differential correction data consist of "pseudo-range errors", measured at a fixed site with a known location by a co-located GPS receiver, that are derived based on knowing the location of the fixed site. Location of each fixed site 16, 18 can be determined based on:

1) Site survey data.

2) Self calibration--refinement of location by self measurement, typically over a period of hours. A software algorithm is required for this self-calibration procedure at a fixed site, e.g., at fixed monitoring units 18 and/or base station 16.

The Slot Assignment List (SAL) of IDs is generated at base station 16 by the operator using a computer keyboard or mouse to make the slot ID assignment(s). System 14 provides the capability of assigning up to forty-eight position location slots every sixty second frame, i.e., once per minute.

Since the Slot Assignment List can include the same remote unit 10 ID once or more than once, remote units 10 can provide position reports at intervals less than once per minute. For example, by assigning a remote unit 10 to slot #1 and slot #25, this remote unit 10 reports its location once every thirty seconds. Once a Slot Assignment List has been acknowledged, remote unit 10 continues to report at the assigned slot until it is reset with a subsequent Slot Assignment List. The reset command is contained in the canned message portion of remote unit 10 ID.

The format of each slot assignment ID includes sixteen bits for ID address, eight bits for canned message and eight bits for Status/Control. The eight bits used for the canned message means that there can be up to 256 different canned messages.

Every thirty second frame, a single sixty character text message (with thirty-two currently unused status/control bits) may be sent from base station 16 to all remote units 10. A different text message may be transmitted during the second thirty second sub-frame. Text messages are sent globally to all remote units 10.

Base station 16 (via airborne relay-mobile monitoring site 20) and the three fixed monitoring sites 18 each sequentially transmit differential correction data once every thirty second frame. For the three fixed monitoring sites 18 that are coupled to base station 16 via a microwave link, only the Slot Assignment List and text message is sent as clear data without: data interleaving, BCH error correction coding, encryption and checksum. At each fixed monitoring site 18, a co-located GPS receiver unit is used to append this "clear" data to the differential corrections measured at that fixed monitoring site 18. The differential correction data and the "clear" data (SAL and text message) are formatted into a single message and then interleaved, encrypted, BCH forward error correct encoded, appended with a checksum and transmitted at the beginning of the appropriate differential correction time slot interval (first, second or third fixed monitoring site 18). Thus the SAL/text data must be received at each fixed monitoring site 18 with sufficient lead time to allow message formatting prior to the specified transmission time.

The airborne relay link is slightly different in that: 1) it uses differential correction data based on base station 16 location since the precise position of the airborne relay 20 is not known and 2) it does not re-time the signal transmission at the airborne relay 20. Thus the entire message including differential correction, SAL and text message are formed at base station 16.

Each remote unit 10 acknowledges its slot assignment by automatically making a position report during its assigned slot interval. Once a remote unit 10 has received a slot interval assignment, it continues to report its location every time its allocated slot number occurs. This potentially allows other remote units 10 to continue to track a remote unit 10 that has moved out of the LOS path of airborne relay 20 and the three fixed monitoring sites 18.

Clear time slot intervals that are reserved for 911 calls, call acknowledgment and call clarifier occur every ten seconds.

A 911 call may be made by any remote unit 10. Once the emergency button is pressed, the 911 call is transmitted in the next 911 call/ack interval, which intervals occur every ten seconds, provided that remote unit 10 has been on and the GPS receiver has clock time. Once a 911 emergency call has been received at base station 16 via the mobile monitoring unit 20 and/or one to three fixed monitoring sites 18, the 911 slot interval is also used for 911 call acknowledgment.

Acknowledgment by base station 16 of a 911 call occurs in the next ten-second 911 clear interval. If the 911 call has been received via only one of the four possible monitoring sites 18, 20, then the 911 acknowledgment is returned via the same monitoring site 18, 20 during the next 911 clear slot, which occurs ten seconds later. When the 911 call was received by exactly two of the four monitoring sites 18, 20, acknowledgment is sent twice: (i) first over the closest fixed monitoring site 18 and then (ii) over the second monitoring site 18, 20 which may be either another fixed monitoring site 18 or mobile monitoring site 20. When the 911 call is received via three or four monitoring sites 18, 20, the acknowledgment is limited to two transmissions using the above criteria.

The computer software at base station 16 makes the selection of which monitoring sites 18, 20, one or two from the set of four, that the one or two acknowledgments are sent through. This is accomplished during the next one or two 911 call slot intervals which occur in the next ten or twenty seconds. The acknowledgment is repeated twice via different monitoring sites 18, 20 to provide a high assurance that the 911-calling remote unit 10 will receive an acknowledgment. The 911-calling remote unit 10 will automatically wait for twenty seconds (two 911 clear slots) to receive one or both of the 911 acknowledgments.

Twenty seconds after a 911 call has been transmitted, and 911-calling remote unit 10 has received either one or two 911 acknowledgments, a built-in clarifier message will automatically appear on the screen of remote unit 10. The 911 caller must use remote unit 10 to manually select one of the clarifier messages. After making the message selection and pressing the "SND" button, the clarifier message will be sent in the next 911 clear interval. The clarifier message is acknowledged in either the next one or possibly two 911 call intervals using the same strategy as was used for the 911 call acknowledgment.

Thus the process of a 911 call generates a sequence of four to six transmissions that occur during 911 clear slots. First is the 911 call, then ten seconds later the first acknowledgment, and twenty seconds later if it is transmitted is a second acknowledgment. Thirty seconds later, in the next 911 slot interval, the clarifier message is transmitted, and then, forty seconds later, the first clarifier acknowledgment is transmitted. Fifty seconds later, a second clarifier acknowledgment may be transmitted. Thus the complete 911 call sequence includes: 911 call, one or two call acknowledgments, 911 clarifier call, and one or two 911 clarifier acknowledgments and uses five to six 911 call intervals. Thus the total time required for completing a 911 call communication sequence varies between a minimum of forty seconds when 911 call is initiated just prior to the 911 clear slot interval and only one acknowledgment occurs for the 911 clarifier to a maximum of sixty seconds when the 911 call is initiated just after the 911 clear slot interval and two acknowledgments occur for the 911 clarifier.

SATCOM system 14 operation is described in terms of the messages used. The time sequence for the thirty second frame slot assignment is specified by the TDMA protocol timing diagram shown in FIG. 6. Remote unit 10 slot interval response numbers range from one to ten. Differential corrections occur every thirty seconds and clear 911 call and acknowledgment slot intervals occur every ten seconds. System protocol allows up to ten mobile monitoring sites 20 to be operational simultaneously, although only one is used in this application example.

Differential correction data consists of "pseudo-range errors", measured at base station 16 by a co-located GPS receiver, that are derived based on knowing the location of base station 16. Base station 16 transmits differential correction data from up to six tracked satellites every thirty seconds based on the location coordinates of base station 16 being accurately known.

The Slot Assignment List of mobile monitoring site 20 (helicopter 20) IDs is generated at base station 16 by the operator using a computer keyboard or mouse to make helicopter 20 slot ID assignment(s). System 14 provides capability for assigning up to ten helicopter 20 position location slots every thirty second frame which is the equivalent of up to twenty helicopter 20 position reports per minute (again, only one mobile monitoring site 20--helicopter 20--is used in this example).

By using a sub-frame technique like the one used in LOS system 14 in base station 16 computer software, it is possible to provide twenty position response slot intervals every minute.

Since the Slot Assignment List can include the same helicopter 20 ID once or more than once, base station 16 operator can cause system 14 to provide a limited number of helicopters 20 with position reports at intervals less than once per thirty seconds. For example, by assigning a helicopter 20 to slot #2 and slot #7, this helicopter 20 would report its location once every fifteen seconds. If the number of helicopters 20 needing position responses during a thirty second interval is less than ten, the unused slot intervals could be used to allow location updates twice every thirty second frame.

Once a Slot Assignment List has been acknowledged, helicopter 20 continues to report at the assigned slot(s) until it is reset with a subsequent differential correction/slot assignment list/text message, which is sent every thirty seconds. The reset command is contained in the canned message portion of remote unit 10 ID.

The format of each slot assignment helicopter 20 ID includes sixteen bits for helicopter 20 ID address, eight bits for canned message and eight bits for Status/Control. The eight bits used for the canned message means that every thirty seconds, every logged-on helicopter 20 could be sent a unique or common canned message from base station 16.

Once every thirty second frame, a single sixty character text message (with thirty-two currently unused status/control bits) may be sent from base station 16 to all of the logged-on helicopters 20.

System 14 provides for capability of assigning ten slots to particular helicopter 20 IDs by using a thirty second. The order in which helicopter 20 ID# appears (1-10) in the SAL determines the slot number in which that helicopter 20 will report its position. It then responds or transmits its location beginning at the time defined by its assigned TDMA slot number.

From this description, it is easy to see that by repeating a particular helicopter 20 ID# in the SAL of ten slots multiple times, the position of fast moving remote unit(s) 10, e.g., like helicopter 20, could respond more often than once every thirty seconds. Multiple responses can be accomplished by assigning a particular helicopter 20 ID# to more than one slot interval during a thirty second frame. For example, responses every fifteen seconds could be achieved by assigning the same helicopter 20 ID# to slot intervals 2 and 7 in a thirty second frame. The TDMA slot diagram in FIG. 5 shows that to provide response slot assignments every fifteen seconds, the assigned slots need to be 2 and 7 and not 1 and 6 as might be expected. If slots 1 and 6 are selected, the average response interval is still every fifteen seconds, but the approximate intervals alternate between twelve and eighteen seconds.

When any of helicopters 20 respond with their position, all other SATCOM logged-on helicopters 20 also receive the position. Thus the computer in each helicopter 20 can display the locations of all the other logged on SATCOM helicopters 20.

Messages from logged-on helicopters 20 to base station 16 have two text message slot intervals available every thirty seconds. The two message slot intervals are automatically assigned to all the logged-on helicopters 20 such that roughly half of helicopters 20 use each text message slot interval. The rule used is that helicopters 20 assigned odd-numbered slot intervals are always automatically assigned to text message slot interval number one. Similarly, helicopters 20 assigned even-numbered slot intervals are always automatically assigned to text message slot interval number two. Other rules are possible and may be usefully employed for splitting mobile monitoring units 20 between text message slot intervals.

Clear time slot intervals for 911 calls occur every ten seconds. Clear 911 slot intervals are also used for acknowledgments. Acknowledgment by base station 16 of a 911 call occurs in the next ten second 911 clear interval after receipt of a 911 call. Receipt of the 911 acknowledgment by calling helicopter 20 results in a clarifier message being displayed on the laptop computer. Then the operator uses the laptop computer to select the desired clarifier canned message which will automatically be transmitted in the next 911 slot. When the operator makes the clarifier selection within less than ten seconds of the clarifier message display, the selected clarifier message is automatically transmitted in the next 911 slot, ten seconds later. Acknowledgment to the clarifier message is then automatic, occurring in the next 911 slot ten seconds later. Thus the complete 911 call/acknowledgment/clarifier/clarifier acknowledgment cycle takes thirty to forty seconds (averaging thirty-five seconds), depending on the time the 911 call was initiated with respect to the 911 call slot interval that occurs every ten seconds

.sctn. E. Conclusion.

Thus, a communication system has been described which overcomes specific problems and accomplishes certain advantages relative to prior art methods and mechanisms. The improvements over known technology are significant. Several different messaging formats, all having common features, have been discussed to illustrate general advantages that these common features provide.

The foregoing description of the specific embodiments will so fully reveal the general nature of the invention that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and therefore such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments.

It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Accordingly, the invention is intended to embrace all such alternatives, modifications, equivalents and variations as fall within the spirit and broad scope of the appended claims.