Message length reduction method of reconfigured GPS CNAV-2 navigation message

Abstract

The invention relates to the message length reduction method of reconfigured GPS CNAV-2 navigation message by using the CRC-POLAR channel coding method instead of BCH and LDPC as the channel coding method to capture and correct errors caused by ambient noise.

Description

[0001] MESSAGE LENGTH REDUCTION METHOD OF RECONFIGURED GPS CNAV-2 NAVIGATION MESSAGE

[0002] Technical Field of the Invention:

[0003] The invention relates to the message length reduction method of reconfigured GPS CNAV-2 navigation message by using the CRC-POLAR channel coding method instead of BCH and LDPC as the channel coding method to capture and correct errors caused by ambient noise.

[0004] State of the art:

[0005] It is possible to define the navigation message broadcast by satellites such as GPS and global positioning systems (GNSS: Global Navigation Satellite System) as an information package that allows the user to calculate quantities such as location, time and speed. Since the receiver calculates its position, i.e. latitude, longitude and altitude, by analysing the data bits received from at least four different visible satellites, the navigation message has a direct impact on the receiver position determination. Any error in the navigation message, especially due to ambient noise, prevents the incoming data from being accurate and reliable. In order to minimize this situation and increase the reliability of the incoming navigation message and ensure more accurate analysis, channel coding methods are used in satellite positioning systems, as in many communication systems. In other words, receiving the navigation message, which is vital for receivers in GNSS, more accurately and safely is directly related to the error correction ability of the channel codes. Therefore, channel coding methods used to detect and correct errors in the navigation message are among the structures that need modernization to increase position accuracy in GNSS. Civil Navigation 2 (CNAV-2) message is the agreed upon navigation message format that will be broadcast not only from GPS but also from three other global positioning systems such as GLONASS, Galileo, and Beidou-3.

[0006] The invention subject to the application numbered "CN1 13541914A" in the known state of the art proposes a CNAV-2 message frame synchronisation method based on association rules, which is used to solve the poor error resistance performance of the existing CNAV-2 message frame synchronisation method.

[0007] The invention, which is the subject of the state of the art with application number "US2008228398A1", relates to the field of navigation using GNSS (e.g. GPS) and especially assisted GNSS (e.g. assisted GPS). It also relates to a module for use in a mobile station having a GPS receiver (or an A-GPS receiver) or a mobile Location Centre (SMLC) in service to determine an ITOW for a CNAV-1 signal.

[0008] When the state of the art is examined, the next generation CNAV-2 message to be broadcast from all global satellite navigation systems such as GPS, GLONASS, Galileo and Beidou-3 consists of 1800 bit, 18 second frames and three sub-frames of different lengths, each frame containing different information. BCH (Bose-Chaudhuri- Hochquenghem) and LDPC (Low Density Parity Check) methods are used as channel coding methods to capture and correct errors caused by ambient noise. Then, the second and third subframes are combined with the bit interleaving technique. CRC- Polar coding and QC-LDPC methods, which are especially popular with the 5G communication system in recent years, are more advanced and have better performance than other channel coding methods. A study evaluating the performance of these methods when adapted to the CNAV-2 message and improving the above- mentioned message structure is not included in the known state of the art. For this reason, it is important to develop a message structure that is more resistant to ambient noise and that will reach and be analysed by the user in a shorter time by changing only the channel coding method without interfering with the existing message content and structure.

[0009] As a result, due to the abovementioned disadvantages and the insufficiency of the current solutions regarding the subject matter, a development is required to be made in the relevant technical field.

[0010] Aim of the Invention:

[0011] The invention relates to the message length reduction method of reconfigured GPS CNAV-2 navigation message by using the CRC-POLAR channel coding method instead of BCH and LDPC as the channel coding method to capture and correct errors caused by ambient noise.

[0012] The most important aim of the invention is to reduce the frame length from 1800 bits to 1500 bits and the duration from 18 seconds to 15 seconds.

[0013] Another purpose of the invention is to ensure that the message is broadcast securely from the satellite in a shorter time and delivered to the receiver with a shorter resolution time by keeping the Bit Error Rate (BER) at the current level.

[0014] Another aim of the invention is to ensure that, as the message length decreases, the encoding and parsing time and analysis complexity are reduced to lower levels than the current situation.

[0015] Description of the Invention

[0016] The invention relates to the message length reduction method of reconfigured GPS CNAV-2 navigation message by using the CRC-POLAR channel coding method instead of BCH and LDPC as the channel coding method to capture and correct errors caused by ambient noise.

[0017] In the light of seven benchmarks, namely coding complexity, decoding complexity, coding time, decoding time, block length, code rate and Bit Error Rate (BER), a message structure has been developed for the new generation GPS CNAV-2 message structure depending on the ambient noise, without changing the existing message content and structure by using the CRC-Polar error capture and correction technique, which is especially preferred in innovative communication systems, with high accuracy and noise resistance.

[0018] As modernization, the reconfigured GPS CNAV-2 navigation message reducing method of the message length and duration; the message length of the current CNAV- 2 is reduced to 1500-bit and the message duration to 15 seconds by applying the CRC- Polar method, which is a combination of 8-bit and 24-bit CRC polynomial and Polar coding, which has the same BER performance as existing methods. In the message length and duration reduction method of reconfigured GPS CNAV-2 navigation message, in the first subframe (SF1 ), the CRC-Polar method was applied using 8-bit CRC polynomial with 9-bit TOI. Applying 1040-bit CRC-polar method using a 24-bit CRC polynomial with 576-bit orbit and clock parameters in the second subframe (SF2). In the third subframe (SF3), a 430-bit CRC-polar method is implemented using 250-bit variable data and a 24-bit CRC polynomial. Then, the second and third subframes are combined with the bit interleaving technique. When combined, a 30-bit TOI and a 1470-bit navigation message are created. In the message length and duration reduction method of reconfigured GPS CNAV-2 navigation message, in order to ensure that the performance of the CRC-Polar method and the current situation are close to each other, only the number of parity bits used has been reduced without making any changes to the navigation message parameters. Thus, for the CNAV-2 message with a length of 1800 bits and a duration of 18 seconds, a navigation message with a shorter message length and a shorter duration of 1500 bits and a duration of 15 seconds was obtained. At the same time, it has been observed that as the message length decreases, the encoding and parsing time and parsing complexity are lower than the current situation.

[0019] Message length and duration reduction method of reconfigured GPS CNAV-2 navigation message, characterized by comprising the processing steps of

[0020] - Generation of CNAV-2 navigation message to be broadcast from four global positioning systems,

[0021] - Applying CRC-Polar method using 8-bit CRC polynomial with 9-bit TOI in the first subframe (SF1 )

[0022] - Applying 1040-bit CRC-polar method using a 24-bit CRC polynomial with 576-bit orbit and clock parameters in the second subframe,

[0023] - Applying 430-bit CRC-polar method is implemented using 250-bit variable data and a 24-bit CRC polynomial in the third subframe,

[0024] - Combining the second and third subframes with the bit interleaving technique,

[0025] - When combined, creating 30-bit TOI and 1470-bit navigation message performed by the satellite and GNSS receiver. A modified message structure with reduced parity bits and thus lower message length and duration was developed by converging the performance results obtained with CRC-Polar to the results shown by the methods used in the current CNAV-2 message. Since reducing the parity bits means increasing the code rate, the code rates of said coding methods were changed and the performance of the methods was evaluated by taking the current situation of the current CNAV-2 message as a reference. The code rate was changed until the performance results of the relevant algorithm were worse than the current situation. Details of the code rate change analysis conducted for modernization purposes are given in Table 1 .

[0026] As the number of parity bits added to the message to be sent decreases, the code rate increases. A higher code rate means that the message bits sent will be less resistant to noise. Since the CRC polynomial in the CRC-Polar method varies depending on the code rate, the CRC polynomials used for each code rate are given in Table 2.

[0027] Table 2; CRC polynomials used by code rate for CRC-Polar

[0028] As the code rate increases for all three subframes, the performance of the CRC-Polar method naturally deteriorates, as expected in other channel coding methods. It can be said that performance close to the current situation or even slightly better is achieved at code rates of / ?=0.30 for SF1 , / ?=0.60 for SF2 and / ?=0.63 for SF3. The parity bit numbers corresponding to the above-mentioned code rates obtained from Table 1 clearly reveal that the CNAV-2 message may have lower bit length and lower transmission time. In the GPS system, each epoch time corresponds to 1.5 seconds. The current CNAV-2 message is 1800 bits long, 18 seconds long, and each frame requires 12 epochs to be transmitted. In this case, the message length and duration of the new CNAV-2 structure to be proposed must be selected in accordance with the epoch time. In other words, the epoch time corresponding to the CNAV-2 message must have an integer value, not a fractional value. In this context, even though the / ?=0.60 code rate has a lower bit length for sub-frame 2, it will not be a suitable choice for the CNAV-2 message structure due to the epoch time, TOI parameter and interleaver size. In case / ?=0.60 is taken, a CNAV-2 message with a total length of 1460-bit will be obtained, including sub-frame 1 30-bit, sub-frame 2 10OO-bit and subframe 3 430-bit. Considering that each epoch time is 1 .5 seconds, the required epoch time for 1460-bit is calculated as approximately 9.733. As mentioned before, epoch time should be expressed as an integer, not a fraction. This shows that the code rate / ?=0.60 will not be a suitable choice for sub-frame 2. In case R=Q.57 is taken, which has better BER performance, a CNAV-2 message with a total length of 1500-bit will be obtained, with sub-frame 1 being 30-bit, sub-frame 2 being 1040-bit, and sub-frame 3 being 430-bit. Considering that each epoch time is 1 .5 seconds, the required epoch time for 1500 bits is calculated as 10. The navigation message can be sent in 10 epochs instead of 12 epochs by changing the code rate and channel coding technique without making any changes to the content. The maximum value of the TOI parameter in subframe 1 , which shows the number of frames broadcast within a 2-hour period and ensures locking on the message, will be 480 (7200 / 15) in the new structure. Since this value can be expressed in the 9-bit section reserved for TOI, the allocated space will be sufficient. Details of the proposed CNAV-2 navigation message are summarized in Table 3.

[0029] Table 3; Message length reduction method of CNAV-2 navigation message properties

Claims

CLAIMS1. Message length and duration reduction method of reconfigured GPS CNAV-2 navigation message; characterized by comprising the process steps of:- Generation of CNAV-2 navigation message to be broadcast from four global positioning systems,- Applying CRC-Polar method using 8-bit CRC polynomial with 9-bit TOI in the first subframe (SF1)- Applying 104O-bit CRC-polar method using a 24-bit CRC polynomial with 576-bit orbit and virtual clock parameters in the second subframe,- Applying 430-bit CRC-polar method is implemented using 250-bit variable data and a 24-bit CRC polynomial in the third subframe,- Combining the second and third subframes with the bit interleaving technique,- When combined, creating 30-bit TOI and 1470-bit navigation message performed by satellite and GNSS receiver.

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