MULTI-CHANNEL ACCESS CONTROL METHOD AND APPARATUS FOR AIS BASED ON DULTIPLE TRANSMITTERS

The present invention relates to a method and apparatus in which an Automatic Identification System (AIS) accesses a radio channel. The present invention includes selecting a slot on which a packet is to be transmitted on at least one radio channel, temporarily sending a first carrier, notifying other AISs that the selected slot has been selected by the AIS, on the selected slot in transmission mode, and after temporarily sending the first carrier, detecting a second carrier transmitted by another device on the selected slot in reception mode. In accordance with the present invention, the AIS temporarily sends a first carrier, notifying other AISs that a selected slot has been selected by the AIS, on the selected slot prior to the detection of a second carrier, and another AIS accessing the same slot detects the first carrier temporarily transmitted. Accordingly, a collision between channels can be avoided.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Hereinafter, in this specification, the contents of the present invention will be described in detail in connection with some exemplary embodiments, with reference to the accompanying drawings. It is to be noted that in assigning reference numerals to elements in the drawings, the same reference numerals denote the same elements throughout the drawings even in cases where the elements are shown in different drawings. Furthermore, in describing the embodiments of the present invention, a detailed description of the known functions and constitutions will be omitted if it is deemed to make the gist of the present invention unnecessarily vague.

Furthermore, in describing the elements of this specification, when it is said that one element is ‘connected’, ‘combined’, or ‘coupled’ with the other element, the one element may be directly connected or coupled with the other element, but it should also be understood that a third element may be ‘connected’, ‘combined’, or ‘coupled’ between the two elements.

FIG. 1shows a channel access method based on SOTDMA in a Class A type AIS.

Referring toFIG. 1, in the case where communication is performed based on SOTDMA in the Class A type AIS, if the transmission of a packet is requested or necessary, the AIS selects (or allocates, the same hereinafter) a slot according to an SOTDMA algorithm in order to send the packet at step S100. In this case, the AIS can check a cycle in which another AIS sends a packet, write a frame map based on the cycle, search for a slot on which another AIS does not send a packet, and select the retrieved slot. Next, the AIS enters transmission mode at step S110, turns on Radio Frequency (RF) power at step S120, and sends a packet on the selected slot through RF communication at step S130.

Steps S110and S120may be omitted depending on embodiments. For example, the AIS may explicitly enter transmission mode or may directly send the packet on the selected slot without turning on the RF power. This process is performed as follows.

FIG. 2shows a channel access method based on CSTDMA in a Class B type AIS.

Referring toFIG. 2, if communication is performed based on CSTDMA in a Class B type AIS, the AIS selects a slot according to a CSTDMA algorithm at step S200. Next, the AIS enters reception mode at step S210and performs carrier detection at step S220. The AIS determines whether the selected slot is used or not based on a result of the carrier detection at step S230. If a signal level greater than a carrier detection threshold is detected in the selected slot, that is, the selected slot is being used, the AIS selects a slot again according to the CSTDMA algorithm at step S200and repeats the subsequent procedures. If a signal level greater than the carrier detection threshold is not detected in the selected slot, that is, the selected slot is not being used, the AIS enters transmission mode at step S240, turns on RF power at step S250, and sends a packet at step S260. In this case, the AIS allocates the packet to the selected slot and sends the packet. Step S210may be omitted according to an embodiment. For example, the AIS may directly perform carrier detection without entering reception mode explicitly. This process is performed as follows.

FIG. 3shows an example in which collision avoidance is performed when a channel access method based on SOTDMA in a Class A type AIS and a channel access method based on CSTDMA in a Class B type AIS are performed at the same time.

Referring toFIG. 3, a detailed operation of an AIS1 of the Class A type based on SOTDMA according to steps S300-1to S330-1is the same as the process of steps S100to S130, and a detailed operation of an AIS2 of the Class B type based on CSTDMA according to steps S300-2to S360-2is the same as the process of steps S200to S260, and thus a detailed description thereof is omitted. The AIS1 turns on RF power at step S320-1and sends a packet at step S330-1. Thus, if the AIS2 performs carrier detection at step S320-2, the AIS2 detects a carrier transmitted by the AIS1. If a signal level greater than a carrier detection threshold is detected at step S330-2, the AIS2 performs step S300-2, selects another slot according to a CSTDMA algorithm, and then performs steps S310-2and S330-2again. As a result, if a carrier having a signal level greater than a carrier detection threshold is not detected, the AIS2 performs steps S340-2to S360-2. Accordingly, the AIS2 can send a packet to be transmitted without a collision.

FIG. 4shows timing at which a packet is transmitted in the case of the AIS1 of the Class A type based on SOTDMA and timing at which a packet is transmitted in the case of the AIS2 of the Class B type based on CSTDMA. InFIG. 4, a unit in a horizontal axis indicates ‘ms’.

Referring toFIG. 4, the AIS1 using SOTDMA turns on the RF power and starts transmission right after a slot is selected. In contrast, the AIS2 using CSTDMA performs carrier detection in reception mode, for example, from 833 μs to 1979 μs after selecting a slot and then (only when a signal level greater than a carrier detection threshold is not detected) turns on the RF power and starts transmission. Accordingly, if the AIS1 using SOTDMA and the AIS2 using CSTDMA attempt to send packets through the same slot at the same time, the AIS1 is already sending a carrier when the AIS2 performs carrier detection. As a result, the AIS2 can detect the carrier without a collision.

Formerly, in an ITU-R communication sector, two VHF channels were allocated for AIS purposes. Recently, four channels are additionally allocated to an AIS, and thus a scheme capable of sending pieces of additional information in addition to location information is being sought. Two of the additional four channels are expected to be used for satellite detection, and the remaining two of the additional four channels are expected to be used for the exchange of common data between vessels. In an existing AIS, two receivers and one transmitter are used in order to use two VHF channels. In order to efficiently use the additional four channels, an AIS needs to be improved to have a minimum of two transmitters, and a channel access scheme for efficiently using a plurality of channels using a plurality of transmitters is necessary. In particular, when an AIS sends common data in a specific time zone, there is a good possibility that a collision between packets can occur in any one channel (particularly, a slot). Accordingly, a method for preventing or avoiding the collision is necessary.

In a current AIS, a primary object of the AIS is to periodically send information about the location of a vessel, and the information occupies most of data. Accordingly, an AIS based on SOTDMA in which an unused slot is detected based on a frame map and used can efficiently operate in a channel without a collision between packets. If common data is aperiodically transmitted in a newly added channel, however, it is difficult to use SOTDMA because efficiency of a frame map is low. Meanwhile, if the CSTMDA scheme is used, an AIS can determine whether slots are used or not by detecting a carrier when sending a packet so that a collision is prevented in a channel (particularly, slot). If a plurality of AISs uses CSTDMA and selects the same slot, however, a problem in that a carrier is not detected in a carrier detection step can occur.

FIG. 5shows a collision avoidance operation through carrier detection between an AIS1 and an AIS2 of the Class B type based on CSTDMA.FIG. 5shows a problem of collision avoidance through carrier detection when both the AIS1 and the AIS2 use CSTDMA.

Referring toFIG. 5, a detailed operation of steps S500-1to S560-1performed by the AIS1 and a detailed operation of steps S500-2to S560-2performed by the AIS are the same as the process of steps S200to S260shown inFIG. 2, and thus a detailed description thereof is omitted. If both the AIS1 and the AIS2 use CSTDMA, use the same slot allocated thereto, and send packets at the same time, the AIS1 and the AIS2 enter reception mode at the same time or at a similar time at steps S510-1and S510-2and wait in order to detect carriers at steps S520-1and S520-2. In this case, a signal level greater than a carrier detection threshold is not detected because both the AIS1 and the AIS2 are in standby state in reception mode. Thus, results of the carrier detection at steps S530-1and S530-2are commonly “No”. As a result, there is a problem in that a collision is generated because the AIS1 and the AIS2 send the packets through the same slot at steps S540-1to S560-1and steps S540-2to S560-2.

In order to solve the above problem, a method and apparatus for preventing a collision when an AIS accesses a channel using CSTDMA according to the present invention are described below.

FIG. 6is a flowchart illustrating a channel access method for preventing a collision between channels in a Class B type AIS based on CSTDMA in accordance with an embodiment of the present invention.

Referring toFIG. 6, the AIS selects a slot on which a packet will be transmitted on at least one radio channel at step S600. The AIS can select a slot on which a packet will be transmitted according to a CSTDMA algorithm.

Next, the AIS enters transmission mode at step S602, turns on RF power at step S604, temporarily sends a first carrier, notifying other AISs that the selected slot has been selected by the AIS, on the selected slot, and then turns off the RF power at step S606. In this case, another AIS can detect the first carrier when another AIS using CSTDMA accesses the same slot. That is, in accordance with the present invention, although CSTDMA is used, an AIS can enter transmission mode without entering reception mode after selecting a slot and temporarily send a first carrier in order to notify other AISs that the slot is being used.

In this case, the first carrier may carry a predetermined and specific flag packet (or signal) indicating that the slot is being used or may carry a null packet (or signal). To send the first carrier temporarily may include that the RF power is turned on and the first carrier is transmitted for a predetermined time or may include that the first carrier is transmitted through an impulse signal.

Next, the AIS enters reception mode at step S610and detects a second carrier transmitted by another device on the selected slot in reception mode at step S620. The AIS determines whether or not the second carrier has been detected at step S630. If, as a result of the determination at step S630, a signal level greater than a carrier detection threshold is detected, the AIS repeats the steps following step S600. In this case, the AIS can select (or reselect) another slot according to the CSTDMA algorithm.

If, as a result of the determination at step S630, a signal level greater than a carrier detection threshold is not detected, the AIS enters transmission mode at step S640, turns on the RF power at step S650, and starts sending a packet at step S660. In this case, the AIS can allocate the packet to the selected slot and send the packet.

Some of steps S602, S604, S606, S610, S640, and S650may be omitted depending on an embodiment of the present invention. For example, the AIS may enter transmission mode explicitly, may send the first carrier temporarily without turning on the RF power, or may perform carrier detection directly without entering reception mode explicitly. For another example, the AIS may send the packet on the selected slot without entering transmission mode explicitly or turning on the RF power.

FIG. 7shows timing at which a Class B type AIS using CSTDMA and performing collision avoidance sends a packet in accordance with an embodiment of the present invention. InFIG. 7, a unit in a horizontal axis is ‘ms’.

Referring toFIG. 7, the AIS according to the present invention selects the slot, turns on the RF power in transmission mode, temporarily sends the first carrier, and then turns off the RF power. Next, the AIS enters reception mode and detects the second carrier. That is, the AIS temporarily sends the first carrier prior to the detection of the second carrier. In this case, a collision between channels (particularly, slots) can be avoided because another AIS accessing the same slot can detect the first carrier temporarily transmitted.

FIG. 8is a detailed block diagram of an AIS in accordance with an embodiment of the present invention. In the present embodiment, the AIS includes two VHF antenna modules in order to operate in six VHF channels. It is assumed that each of the VHF antenna modules includes two TDMA receiver (RX) modules and one TDMA transmitter (TX) module for transmission and reception and a TDMA encoding module and a TDMA decoding module for a TDMA message. It is to be noted that the number of channels, the number of antenna modules, and the number of RX modules and RX modules forming the antenna module are illustrative. The AIS may operate in a channel other than a VHF, may operate in a larger or smaller number of channels, and may include a larger or smaller number of antenna modules, and one antenna module may include a larger or smaller number of TDMA TX modules or TDMA RX modules without departing from an intrinsic characteristic of the present invention.

Referring toFIG. 8, the AIS includes a control module800, a Global Navigation Satellite System (GNSS) module805, TDMA encoding modules810-1and810-2, TDMA TX modules815-1and815-2, antenna modules820-1and820-2, TDMA RX modules825-1,825-2,825-3, and825-4, TDMA decoding modules830-1,830-2,830-3, and830-4, a Digital Selective Calling (DSC) encoding module835, a DSC TX module840, a DSC RX module845, and a DSC decoding module850.

The control module800controls the modules forming the AIS. The GNSS module605receives information about the location and time of the AIS (or a vessel equipped with the AIS) from satellites and transfers the information to the control module800.

The TDMA encoding modules810-1and810-2receive messages from the control module800and produce the messages into TDMA message by encoding the received message. The TDMA TX modules815-1and815-2receive the TDMA messages from the TDMA encoding modules810-1and810-2, perform radio signal processing on the TDMA messages, and send the processing results through the antenna modules820-1and820-2.

The antenna modules820-1and820-2can send and receive signals having a VHF band.

The TDMA RX modules825-1,825-2,825-3, and825-4receive TDMA messages from the outside through the antenna modules820-1and820-2. The TDMA decoding modules830-1,830-2,830-3, and830-4decode the TDMA messages received by the TDMA RX modules825-1,825-2,825-3, and825-4.

The DSC encoding module835encodes a DSC message. DSC is a standard regarding the sending of a pre-defined digital message, and a distress signal, etc. can be transmitted rapidly in accordance with the DSC standard.

The DSC TX module840performs radio signal processing on the encoded DSC message and sends the processing result. The DSC RX module845receives a DSC message from the outside. The DSC decoding module850decodes the received DSC message.

The TDMA RX modules825-1825-2,825-3, and825-4can operate in different channels, and at least one of the TDMA TX modules815-1and815-2can access a channel using the channel access method described with reference toFIG. 6and send a packet. For example, the TDMA TX module815-1can access a channel using the channel access method described with reference toFIG. 2, and the TDMA TX module815-2can access a channel using the channel access method described with reference toFIG. 2.

More particularly, the control module800selects a slot on which a packet will be transmitted on at least one radio channel. The control module800may select the slot according to a CSTDMA algorithm. The TDMA TX module815-2temporarily sends a first carrier, notifying other AISs that the slot has been selected by the AIS, on the selected slot. After the TDMA TX module815-2temporarily sends the first carrier, the TDMA RX module825-4detects a second carrier transmitted by another device on the selected slot. The control module800may perform control so that the TDMA TX module815-2temporarily sends the first carrier notifying other AISs that the slot has been selected by the AIS and perform control so that the TDMA RX module825-4detects the second carrier transmitted by another device.

The control module800determines whether or not the intensity of the second carrier detected on the slot is greater than a threshold (or carrier detection threshold). If, as a result of the determination, the intensity of the second carrier is smaller than the carrier detection threshold, the TDMA TX module815-2can send a packet on the slot. The control module800can perform control so that the TDMA TX module815-2sends the packet on the slot when the intensity of the second carrier is smaller than the carrier detection threshold.

Meanwhile, if, as a result of the determination, the intensity of the second carrier is greater than the carrier detection threshold, the control module800can reselect a slot on which the packet will be transmitted. In this case, the control module800can select the slot according to the CSTDMA algorithm. The TDMA TX module815-2can temporarily send a third carrier, notifying other AISs that the reselected slot has been selected by the AIS, on the reselected slot. After the TDMA TX module815-2temporarily sends the third carrier, the TDMA RX module825-4can detect a fourth carrier transmitted by another device on the reselected slot. The control module800can perform control so that the TDMA TX module815-2temporarily sends the third carrier notifying other AISs that the reselected slot has been selected by the AIS and then perform control so that the TDMA RX module825-4detects the fourth carrier transmitted by another device.

Next, the control module800determines whether or not the intensity of the fourth carrier detected on the reselected slot is greater than a carrier detection threshold. If, as a result of the determination, the intensity of the fourth carrier is smaller than the carrier detection threshold, the TDMA TX module815-2can send a packet on the reselected slot. The control module800can perform control so that the TDMA TX module815-2sends the packet on the reselected slot when the intensity of the fourth carrier is smaller than the carrier detection threshold.

In accordance with the present invention, if the AIS uses a channel access method based on CSTDMA, the AIS temporarily sends a carrier, notifying other AISs that a selected slot has been selected by the AIS, on the selected slot prior to carrier detection, and another AIS accessing the same slot detects the temporarily transmitted carrier. Accordingly, a collision between channels, particularly, slots can be avoided.

While some exemplary embodiments of the present invention have been described with reference to the accompanying drawings, those skilled in the art may change and modify the present invention in various ways without departing from the essential characteristic of the present invention. Accordingly, the disclosed embodiments should not be construed as limiting the technical spirit of the present invention, but should be construed as illustrating the technical spirit of the present invention. The scope of the technical spirit of the present invention is not restricted by the embodiments, and the scope of the present invention should be interpreted based on the following appended claims. Accordingly, the present invention should be construed as covering all modifications or variations derived from the meaning and scope of the appended claims and their equivalents.