Load Estimation Based Dynamic Access Protocol for Satellite Internet of Things
Round 1
Reviewer 1 Report
This paper studies the satellite Internet of Things scenario with aggregation nodes, and proposes a time slot random access protocol based on Walsh codes, which increases the number of users that can be accessed in one frame. Moreover, the article proposes a dynamic Walsh code time slot random access protocol based on load estimation algorithm. The introduction of load estimation can effectively reduce the resource occupancy rate of the system, and at the same time ensure that the performance of the random access protocol based on Walsh code is not degraded.
But the following comments should be considered:
1. Introduction does not cover all the latest literature, some important papers are missing, and a more in-depth literature review should be done. The contributions section should be rewritten as the description is not very clear.
2. I recommend to check the presentation of the paper. The writing style could be improved. Please check carefully the form, the grammar and the construction of the phrases.
3. The time slot random access protocol based on Walsh code proposed in this paper breaks the restriction that one time slot can only receive one user data packet correctly, and increases the number of users that can be accessed in one frame. However, the overhead of using Walsh codes should be explained.
4. The dynamic Walsh code time slot random access protocol based on the load estimation algorithm proposed in this paper uses the load estimation algorithm to analyze the load situation of the network, and then selects the appropriate Walsh code length and frame length. The author should explain the impact of introducing load estimation on system resource occupancy, and the occupancy of resource overhead under high load.
5. The paper did not indicate what kind of satellite，GEO satellite or LEO satellite? In addition, the load estimation algorithm in the article should be introduced in detail.
6. The handover jitter in Figure 14 in the case of low number of users needs to be explained in detail, the phenomenon of handover jitter is a big problem for dynamic access protocols.
Author Response
一、Reviewer 1：
- Introduction does not cover all the latest literature, some important papers are missing, and a more in-depth literature review should be done. The contributions section should be rewritten as the description is not very clear.
Answer：The introduction has been revised according to the comments of the reviewers, and the latest literature has been supplemented. The contribution section has also been revised as shown below.
- I recommend to check the presentation of the paper. The writing style could be improved. Please check carefully the form, the grammar and the construction of the phrases.
Answer：According to the opinions of reviewers, the content of the article has been polished.
- The time slot random access protocol based on Walsh code proposed in this paper breaks the restriction that one time slot can only receive one user data packet correctly, and increases the number of users that can be accessed in one frame. However, the overhead of using Walsh codes should be explained.
Answer：The larger the number of time slots in a frame, the higher the order of the shift register of the Walsh sequence. Moreover, the access performance of the protocol is better, but it will cause greater overhead. Therefore, in order to effectively utilize the advantages of the Walsh protocol, this paper introduces a load estimation algorithm to reduce overhead.
- The dynamic Walsh code time slot random access protocol based on the load estimation algorithm proposed in this paper uses the load estimation algorithm to analyze the load situation of the network, and then selects the appropriate Walsh code length and frame length. The author should explain the impact of introducing load estimation on system resource occupancy, and the occupancy of resource overhead under high load.
Answer：In the case of medium and low load, the introduction of load estimation can effectively reduce the resource occupancy rate of the system, and at the same time ensure that the performance of the random access protocol based on Walsh codes is not degraded. However, under high load conditions, a large resource overhead is still required to ensure the access performance of the system.
- The paper did not indicate what kind of satellite，GEO satellite or LEO satellite? In addition, the load estimation algorithm in the article should be introduced in detail.
Answer：Thanks to the comments of the reviewers, this article uses a low-Earth orbit satellite based on a sun-synchronous orbit. Load estimation is to estimate the number of nodes sending data packets in the frame, that is, the size of the collision set, so as to design an effective channel access strategy.
- The handover jitter in Figure 14 in the case of low number of users needs to be explained in detail, the phenomenon of handover jitter is a big problem for dynamic access protocols.
Answer：Due to the existence of load estimation error, the performance of throughput and packet loss rate has obvious jitter problems near the load threshold. Moreover, when the load value reaches about 200 users, the jitter range of the packet loss rate is the largest. Therefore, handover jitter is the biggest problem in current dynamic access protocols.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
"Load Estimation Algorithm Based Dynamic Satellite Internet of Things Access Protocol" - Title is hard to follow, please rephrase to make it more clear
Line 27 - "The Internet of Things (IoT) has become the third wave of the development of the
information industry after computers and the Internet" - reference needed.
Line 29 - "As an extension and extension" - please rephrase
Lines 33, 205 - "sat-ellite" - please correct splitting of the word in sylabus. Same for line 391 "ap-peared" and 459 "pack-ets."
Line 38 - "Iridium Communications announced a partnership with Amazon Web Services" - reference needed
Lines 56-76 - I recommend to briefly explain in the context the solutioning of the conflict part of the ALOHA protocol in the case of simultanous broadcasts (please consult Reference Bruce R. Elbert, "Introduction to Satellite Communication", 6th edition, Artech House)
Lines 116-118 "According to the research status of satellite IoT, there are mainly two application scenarios of satellite IoT, namely, scenarios of satellite IoT with sink nodes and scenarios of
satellite IoT without sink nodes." - reference needed
Lines 162, 164, 189, 256, 317- the size of the text for Figures is inconsistent (different from Figures 1, 2)
Line 217 - Ts - s should be subscripted, just like in formula below
Line 264, 269 - same for Gp, Mmax, Nslots etc, p, slot and max should be subscripted, just like in formulas below
Line 349 - "requires a packet loss rate of no less than ." sentence incomplete?
Figures 13, 14 - please provide more comments on what is presented in the charts and how it improves/benefits the alghoritm
Overall I would not recommend using a Walsh Code as a training sequence since it does not weigh all the frequencies in the channel equally (in contrast to a PRN sequence)
It would be interesting also to discuss the proposed algorithm in the context of QoS for Satellite IP Networks (see Ref. https://ieeexplore.ieee.org/document/8034905 for an overview)
Author Response
二、Reviewer 2：
- "Load Estimation Algorithm Based Dynamic Satellite Internet of Things Access Protocol" - Title is hard to follow, please rephrase to make it more clear
Answer：Thanks to the reviewers for their comments, the title of the article has been changed to Load Estimation Based Dynamic Access Protocol for Satellite Internet of Things
- Line 27 - "The Internet of Things (IoT) has become the third wave of the development of the information industry after computers and the Internet" - reference needed.
Answer：Thanks to the comments of the reviewers, the references have been supplemented.
- C. -Q. Dai, M. Zhang, C. Li, J. Zhao and Q. Chen, "QoE-Aware Intelligent Satellite Constellation Design in Satellite Internet of Things," in IEEE Internet of Things Journal, vol. 8, no. 6, pp. 4855-4867, 15 March15, 2021, doi: 10.1109/JIOT.2020.3030263.
- Line 29 - "As an extension and extension" - please rephrase.
Answer：According to the opinions of reviewers, the content of the sentence has been polished.
As an extension of the satellite Internet, the satellite Internet of Things is further developed under the promotion of the new generation of information technology and commercial market.
- Lines 33, 205 - "sat-ellite" - please correct splitting of the word in sylabus. Same for line 391 "ap-peared" and 459 "pack-ets."
Answer：Thanks to the comments of the reviewers, the word separators in the article are automatically generated due to system settings.
Lines 33:Many satellite communication companies are developing low-orbit IoT small satellite constellations to provide global users with low-cost, wide coverage, and low-latency IoT services.
Lines 205: This paper proposes an effective access scheme for satellite IoT scenarios with sink nodes.
Lines 391: When the number of users increased to 100 appeared more obvious performance difference
Lines 459: The protocol breaks the limitation that only one user data packet can be correctly received in a time slot, greatly increases the number of accessible users in a frame, and at the same time enables the protocol to achieve high performance without relying on repeated data packets.
- Line 38 - "Iridium Communications announced a partnership with Amazon Web Services" - reference needed
Answer：Thanks to the comments of the reviewers, the references have been supplemented.
[1] Elbehiery K, Elbehiery H. Cloud Technology: Conquest of Commercial Space Business[M]//Internet of Things—Applications and Future. Springer, Singapore, 2020: 53-72.
- Lines 56-76 - I recommend to briefly explain in the context the solutioning of the conflict part of the ALOHA protocol in the case of simultanous broadcasts (please consult Reference Bruce R. Elbert, "Introduction to Satellite Communication", 6th edition, Artech House)
Answer：According to the opinions of reviewers, the article has explained the reasons for the conflicts of the ALOHA protocol, and introduced the load estimation algorithm to perceive the conflicts of the system.
The pure ALOHA protocol also has a big problem, that is, its maximum normalized throughput is too low, only 0.184. Its low throughput performance is mainly caused by the generation of “collisions” and new “collisions” caused by packet retransmission. Especially in the case of excessive load, the existence of “collision” almost makes the system unable to work, resulting in the instability of the system.
In a scenario without a aggregate node, the data packets transmitted by a single node are very small, so time synchronization or resource application operations will greatly in-crease overhead, causing a sharp drop in transmission efficiency. Therefore, the ALOHA protocol, which is asynchronous and does not need to apply for resources, is suitable for the application scenario of the satellite Internet of Things, but it needs to solve the problem of low throughput performance.
[16] Elbert B R. Introduction to satellite communication[M]. Artech house, 2008.
- Lines 116-118 "According to the research status of satellite IoT, there are mainly two application scenarios of satellite IoT, namely, scenarios of satellite IoT with sink nodes and scenarios of satellite IoT without sink nodes." - reference needed
- Answer：Thanks to the comments of the reviewers, the references have been supplemented.
[16] Elbert B R. Introduction to satellite communication[M]. Artech house, 2008.
[17] Abramson N. THE ALOHA SYSTEM: another alternative for computer communications[C]. //national computer conference, 1970:281-285.
- Lines 162, 164, 189, 256, 317- the size of the text for Figures is inconsistent (different from Figures 1, 2)
Answer：According to the opinions of the reviewers, the size of the pictures in the text has been modified.
- Line 217 - Ts - s should be subscripted, just like in formula below
Answer：According to the opinions of the reviewers, the subscript in the text has been modified.
- Line 264, 269 - same for Gp, Mmax, Nslots etc, p, slot and max should be subscripted, just like in formulas below
Answer：According to the opinions of the reviewers, the subscript of the variable in the text has been modified.
- Line 349 - "requires a packet loss rate of no less than ." sentence incomplete?
Answer：According to the opinions of the reviewers, the incompleted sentence in the text has been modified.
This section assumes that the system requires a packet loss rate of no less than .
- Figures 13, 14 - please provide more comments on what is presented in the charts and how it improves/benefits the alghoritm
Answer：The throughput performance and packet loss rate performance are shown in Figures 13 and Figures 14. As can be seen from the figure, thanks to the load estimation, the protocol realizes the dynamic switching of frame length and codebook, and meets the correspond-ing throughput and packet loss rate requirements in different load ranges. However, it should also be noted that due to the existence of load estimation error, there is an obvious jitter problem in the performance of throughput and packet loss rate near the load thresh-old, in which when the load value reaches about 200 users, the jitter range of packet loss rate is the largest.
- Overall I would not recommend using a Walsh Code as a training sequence since it does not weigh all the frequencies in the channel equally (in contrast to a PRN sequence).It would be interesting also to discuss the proposed algorithm in the context of QoS for Satellite IP Networks (see Ref. https://ieeexplore.ieee.org/document/8034905 for an overview)
Answer：Although most of the Walsh codes are completely orthogonal, the calculation results of the cross-correlation function show that not all the Walsh codes have a cross-correlation value of 0 after shifting. This means that the performance of Walsh codes is directly related to the accuracy of time synchronization. However, the time slot random access protocol used in the article can guarantee a certain time synchronization accuracy, so the Walsh code is suitable for the background of the satellite Internet of Things.
Author Response File: Author Response.pdf
Reviewer 3 Report
The authors studied the satellite IoT scenario with sink nodes. To solve the existing problems in the Contention Resolution Diversity Slotted ALOHA (CRDSA) protocol, they proposed a time slot random access protocol based on the Walsh code. To select the appropriate access protocol according to the load condition, the authors presented a load estimation algorithm. The simulation results show that the introduction of load estimation under medium and low load conditions can effectively reduce the resource occupation of the system and ensure that the performance of the proposed protocol is not degraded.
I consider that the proposed protocol has merit. However, I do have some concerns with the current version that need correction during the revision. I believe the following comments can further enhance this paper's quality.
(1) More technical analysis is required in subsection 3.3 Dynamic WALSH Code Time Slot Random Access Protocol Based on Load Estimation Algorithm.
(2) The authors must discuss in more detail the advanced maximum likelihood estimation (MLE) algorithm used.
(3) They must write the implications of their work. For example, what is the key benefit of the proposed protocol? They must also present the limitations of the proposed protocol.
(4) They must improve the “flow” of the ideas presented in the paper.
(5) They must improve the quality of Fig. 1.
(6) English must be improved.
Author Response
三、Reviewer 3：
- More technical analysis is required in subsection 3.3 Thanks to the reviewer's comments, the technical details of the dynamic Walsh code slotted random access protocol based on the load estimation algorithm have been added.
Answer：Thanks to the reviewers for their opinions, the text has been uniformly corrected to be a integrated satellite terrestrial system.
The workflow of the load estimation module is described in detail below:
- set the load threshold before using it for the first time.
- read the data from the cache and get the load estimate according to the time slot state statistics in a frame of data.
- the appropriate frame length and codebook information are selected according to the comparison between the load estimate and the load threshold.
- The authors must discuss in more detail the advanced maximum likelihood estimation (MLE) algorithm used.
Answer：1. Traditional energy detection generally assumes that the noise power is definite. However, there is Gaussian white noise and other interferences in the satellite-ground integrated cognitive system. Moreover, noise power changes with time and relative position within a certain range, and the instability of this noise is noise uncertainty. The sensing system will deteriorate the spectrum sensing performance due to the increase of noise uncertainty.
- According to Figure 2, when the energy detected by the system is between the dual thresholds , it is difficult for the perception system to judge whether it is an authorized user or noise, thus creating an fuzzy state.
- They must write the implications of their work. For example, what is the key benefit of the proposed protocol? They must also present the limitations of the proposed protocol.
Answer：Thanks to the reviewers for their comments, the main benefits and limitations of the protocol have been supplemented.
The dynamic Walsh code time slot random access protocol based on the load estimation algorithm uses the load estimation algorithm to estimate the load situation of the current network, and then selects the appropriate Walsh code length and frame length. The simulation results show that in the case of medium and low load, the introduction of load estimation can effectively reduce the resource occupancy rate of the system, and at the same time ensure that the performance of the random access protocol based on Walsh codes does not decrease. However, under high load conditions, a large resource overhead is still required to ensure the access performance of the system.
- They must improve the “flow” of the ideas presented in the paper.
Answer：Thanks to the reviewer's comments, the writing style of the Dynamic Walsh Code Slotted Random Access Protocol Based on Load Estimation Algorithm has been improved.
The architecture of the client side and the on-board payload are shown in Figure 7 and Figure 8. After the MAC packet segment is generated by the user's data, according to the frame length and codebook information in the instructions broadcast by the satellite, the packet segment is spread spectrum operation, and enters the cache, and the appropri-ate time slot is selected to send. When the satellite receives the demodulation data, it firstly caches the data by frame, and then the correlator carries out relevant operation on the data by time slot. If there is any data packet that can be solved, it will be unexpanded, and the data packet will be reconstructed while decoding the output. The reconstructed data pack-et will be deleted from the cache data of the current time slot, and relevant operations will continue in the next step. At the same time, the load estimation module also reads the data of a frame from the cache, counts the number of time slots in different states, and com-pletes the load estimation. The appropriate frame length and codebook are selected according to the estimated results, and the corresponding information is broadcast to each user.
- They must improve the quality of Fig. 1.
Answer：The quality of Figure 1 has been improved and a replot was performed.
Figure 1. Satellite IoT scenario model with sink nodes.
- English must be improved.
Answer: According to the opinions of reviewers, the content of the article has been polished.
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
The text size for Figures 1 and 2 still remains different from the other Figures in the text (that is smaller than Fig 3, 4, etc).
The overall text still requires some serious brushing to properly be fit for publishing, especially at formulas and simbols (probably a better equation editor would do the trick).
The discussion of the proposed algorithm in the context of QoS for Satellite IP Networks has not been approached as suggested, however this is the decision of the authors and the editors.
Author Response
1.The text size for Figures 1 and 2 still remains different from the other Figures in the text (that is smaller than Fig 3, 4, etc).
Answer：Thanks to the reviewer's comments, the size of Figure 1 and Figure 2 has been adjusted.
2. The overall text still requires some serious brushing to properly be fit for publishing, especially at formulas and simbols (probably a better equation editor would do the trick).
Answer：Thanks to the comments of the reviewers, the formulas and symbols in the article have been edited and revised using MathType.
3. It would be interesting also to discuss the proposed algorithm in the context of QoS for Satellite IP Networks (see Ref. https://ieeexplore.ieee.org/document/8034905 for an overview)
Answer：Satellite IP network is the application scenario of the load estimation based dynamic access protocol proposed in this paper. The simulation results show that the algorithm can increase the number of access users in the same time slot, reduce the packet loss rate, and has certain robustness, which can improve the service quality (QoS) of the satellite network.
Reviewer 3 Report
The revised version of the paper can be accepted for publication.
Author Response
According to the opinions of reviewers, the content of the article has been polished.
