FIELD

The present disclosure generally relates to the technical field of communications. More specifically, the present disclosure is directed to systems and methods for improving receiver performance in a Digital Selective Calling (“DSC”) system.

BACKGROUND

Systems that monitor a situation or location for occurrence of an event, and which provide an alarm in response to the event, are well-known and are embodied in many different designs. In various traditional alert systems, devices are utilized for alerting potential rescuers in the event of a maritime overboard emergency. These devices, once activated, send a signal indicating that an emergency event has occurred. For example, some devices issue a simple alarm signal, such as a flashing light or audible signal. Activation of these devices may be automatic, such as by contact with water, or manual. The alert issued by different devices varies, and devices that issue such alerts, known as emergency position-indicating radio beacons (“EPIRBs”), are organized into several classes. Some classes of EPIRBs are capable of transmitting data, which may include the name or location of the vessel in trouble. These signals may be of the type that are monitored by a constellation of satellites, in which case the alarms are relayed to local rescue resources. Some of these devices generate special-purpose alarms, such as man overboard alarms, which can warn the crew in the event that a person falls overboard from a vessel. A transceiver on the vessel can relay the alarm to other systems, such as EPIRBs or marine radios, for rescue backup.

In the early 1970's an international committee developed and standardized the DSC. The DSC was developed to standardize a means of handling maritime communications traffic by facilitating more efficient calling and providing an automated standard distress and safety system. The motivation for developing the DSC was to relieve congestion in the very high frequency (VHF) band on the voice distress and calling channel, channel 16. DSC solves this problem because it uses channel 70, VHF frequency 156.525 Mhz, for its routine calling. DSC permits mariners to instantly send an automatically formatted distress alert to the Coast Guard or other rescue authority anywhere in the world. DSC also allows a mariner to initiate or receive distress, urgency, safety, and routine radiotelephone calls to or from any similarly-equipped vessel or shore station. DSC allows for addressing of specific other radios when transmitting the alert, and allows others to respond specifically to the caller. This capability is available because DSC may be used to designate a telephone as the terminal equipment in the DSC message, and provide the PSTN address as selection information in the data. Thus, a point-to-point distress call may be completed, even when the receiving entity is not connected to a radio loudspeaker. DSC allows for storage records of failed attempts to reach another radio, at that radio. According to the DSC standard, the distress alert has the following parameters: (a) Transmit frequency: 156.525 MHz; (b) Channel Bandwidth: 25 kHz; (c) Transmit power: ≥0.5 W EiRP; (d) Baseband modulation: Frequency-shift keying (FSK); and (e) Carrier modulation: PM.

Distress alerts can be relayed to other ships within radio range (usually up to about 30 miles) or directly to a shore station, which can forward the alert to U.S. Coast Guard rescue coordination centers (RCC). Presently, the United States Coast Guard is implementing the National Distress Response System Modernization Project (NDRSMP—also known as the ‘Rescue 21’ System). This emergency monitoring system relies on the DSC protocol to expand any marine distress call transmissions to both VHF channels 16 and 70. Using DSC enabled VHF marine radios, the ‘Rescue 21’ system provides enhanced emergency radio coverage up to 20 nautical miles from the coastline of the United States. However, the ‘Rescue 21’ system has coverage limitations; particularly for ‘man overboard’ or other emergency incidents occurring beyond the 20 nautical mile range limit of the United States coastline. Significant coverage gaps in the Rescue 21 distress system limit the United States Coast Guard's ability to rescue mariners in distress.

As discussed above, the VHF DSC transceiver facilitates distress related communications over terrestrial marine radio systems. In the event of an emergency, the VHF transmitter is used to send an automatically formatted distress alert signal to surrounding vessels and/or coastal stations of rescue authorities. The distress alert signal can include a designation specifying the nature and priority level of the call, a vessel's unique identifier (e.g., Maritime Mobile Service Identity (“MMSI”)), and the vessel's position. In response to a reception of a distress alert signal, vessels and/or coastal station(s) send a DSC acknowledgement message to the vessel that transmitted the distress alert signal. The DSC acknowledgement message is received by the VHF DSC receiver.

U.S. Pat. No. 9,743,264 to Harris Corporation discloses a spaced-based DSC system that collects and processes DSC emergency messages transmitted from DSC transmitters of terrestrial radios using satellites to position validate the DSC emergency messages and determining first positions of the DSC transmitters in transit based on geolocation data and time location data respectively assigned by the satellites to the DSC emergency messages. In this system, information contained in a transmitted DSC message is used to establish a return channel communication with a transmitter using a non-maritime VHF electro-magnetic spectrum (e.g., Satcom services (Iridium) and/or free space optic services).

When developing space-based systems of communication, such as the one described above, the first step in determining feasibility of decoding the distress alerts is link budget development. Link budget is a design aid, calculated during the design of a communication system, to determine the received power and ensure that the information is received intelligibly with an adequate signal-to-noise (“SNR”) ratio. An example of a simplified link budget equation is: Received Power (dB)=Transmitted Power (dB)+Gains (dB)−Losses (dB). The goal of any link budget is to determine the SNR required for “error free” decoding at the transceiver. To determine SNR required for “error free” decoding, you must determine the (a) decoder input error rate as a function of the received SNR; and (b) the decoder input error rate yielding “error free” decoding. In other words, the key to link budget is the Carrier-to-Noise Ration (CNR) required for “error free” decoding. CNR required may be computed using the decoder input error rate as a function of the CNR received and the decoder input error rate yielding “error free” decoding.

According to the International Electrotechnical Commission (“IEC”), a DSC decoder is evaluated by determination of the symbol error rate (“SER”). The decoded DSC call sequence applied with forward error correction, interleaving techniques, and check-sum information is divided into blocks each corresponding to one information symbol in the received signal. The ratio between the numbers of incorrect blocks to the total number of blocks is determined as the symbol error rate. Presently, the standard describes that forward error correction may utilize time-diversity. Furthermore, the DSC standard only mandates that for the VHF call sequence, the receiver decoder should provide maximum utilization of the received signal, including use of the error-check character and by using an iterative decoding process with adequate memory provision.

It would be advantageous to alleviate the limitations in the current terrestrial system using space-based VHF. More specifically, it would be advantageous to provide an improved decoding technique that maximizes performance, by enabling reception of weaker signals, which improves the chances of distress alert reception and subsequent rescue of mariners in distress.

SUMMARY

According to various embodiments, a method is disclosed. The method comprises receiving at least one signal via a receiver. The at least one signal is extracted for data via a processor coupled to the receiver, wherein the data includes at least one message and a set of parameters related to the message. A signal output is generated using the at least one message and the set of parameters such that the signal output includes a first portion and a second portion. At least one error is identified in the signal output and corrected using the first portion and the second portion. An output is generated that is used to perform at least one task related to the at least one signal.

According to various embodiments, a system is disclosed. The system comprises receiving at least on signal receiver and extracting data from the at least one signal via a processor coupled to the receiver. The data includes at least one message and a set of parameters related to the message. The system further comprises generating a signal output using the at least one message and the set of parameters such that the signal output includes a first portion and a second portion, identifying at least one error in the signal output, and correcting the identified at least one error using the first portion and the second portion. The system generates an output that is used to perform at least one task related to the at least one signal.

According to various embodiments, a non-transitory computer readable medium having instructions stored thereon is disclosed. Wherein the instructions, when executed by at least one processor, cause a device to perform operations that comprise receiving at least one signal and extracting data from the at least one signal. The data includes at least one message and a set of parameters related to the message. A signal output is generated using the at least one message and the set of parameters such that the signal output includes a first portion and a second portion. Operations further comprise identifying at least one error in the signal output, correcting the identified at least one error using the first portion and the second portion, and generating an output that is used to perform at least one task related to the at least one signal.

