The purpose of the lecture: to study the characteristics of systems with feedback and consideration of the block diagram with the OS.
Content:
a) characteristics of feedback systems and their features;
b) block diagram of a system with information feedback (IFE) and decision feedback (DCF), characteristics and operating algorithms.
12.1 Characteristics of feedback systems and their features
In systems with OS, redundancy is introduced into the transmitted information taking into account the state of the discrete channel. As the channel condition deteriorates, the introduced redundancy increases, and, conversely, as the channel condition improves, it decreases.
Depending on the purpose of the OS, they are distinguished systems: with decisive feedback (DCF), information feedback (IOS) and combined feedback (COS).
Transfer from ROS is similar telephone conversation in conditions poor hearing when one of the interlocutors, having heard a word or phrase poorly, asks the other to repeat it again, and if audibility is good, either confirms the fact of receiving the information, or, in any case, does not ask for repetition.
The information (receipt) received via the OS channel is analyzed by the transmitter, and based on the results of the analysis, the transmitter makes a decision to transmit the next code combination or to repeat previously transmitted ones. After this, the transmitter transmits service signals about the decision taken, and then the corresponding code combinations. In accordance with the service signals received from the transmitter, the PKpr receiver either issues the accumulated code combination to the information recipient, or erases it and stores the newly transmitted one. In systems with a shortened IOS, naturally, there is less load on the reverse channel, but there is a greater likelihood of errors compared to a full IOS.
In systems with CBS, the decision to issue a code combination to the recipient of information or to retransmit it can be made both in the receiver and in the transmitter PDS systems, and the OS channel is used to transmit both receipts and decisions. OS systems are also divided into systems with a limited number of repetitions and with an unlimited number of repetitions. IN systems with a limited number of repetitions each code combination can be repeated no more than l times, and systems with an unlimited number of repetitions transmission of combinations is repeated until the receiver or transmitter decides to issue this combination to the consumer. With a limited number of repetitions, the probability of issuing the wrong combination to the recipient is greater, but there is less time wasted on transmission and the implementation of the equipment is simpler. Note that in systems with an OS, the message transmission time does not remain constant and depends on the state of the channel.
OS systems can discard or use the information contained in rejected code combinations in order to make a more correct decision. Systems of the first type are called systems without memory, and the second - systems with memory.
Feedback can cover various parts of the system (Figure 12.1):
1) communication channel, in which information about the received signal is transmitted via the OS channel before any decision is made;
2) discrete channel, while the decisions made by the first decision circuit PC 1 based on the analysis are transmitted via the OS channel single elements signal;
3) data transmission channel, while the decisions made by the second decision circuit PC 2 based on the analysis of code combinations are transmitted via the OS channel.
Figure 12.1 - Feedback in the PDS system
In systems with IOS, fidelity losses are also possible due to errors in OS channels. In shortened IOS, such errors arise for reasons similar to those stated above, when a receipt corresponding to a distorted signal in the OS channel is transformed into a receipt corresponding to an undistorted signal. As a result, the transmitter is unable to detect the fact of an erroneous reception. In full IOS, distortions are possible in the OS channel, completely compensating for distortions in the forward channel, as a result of which errors cannot be detected. Therefore, much attention is paid to the formation of OS channels in PDS systems. OS channels are usually formed in reverse communication channels using frequency or time separation methods from useful information transmission channels. FDM methods are usually used in systems with a relatively low specific transmission rate, for example, when transmitting data at a speed of 600... 1200 bit/s via PM channels. In many systems with POC, a structural separation method is used, when a special code combination is used for the interrogation signal, and any allowed code combination in the receiver is decrypted as an acknowledgment signal and any unauthorized combination as a reinterrogation signal. To protect against distorted signals transmitted via OS channels, the same methods are used as to increase the fidelity of useful information: correction codes, multiple and parallel transmissions.
Errors in channels are usually grouped; the state of the channel can be very different. Consequently, if you use a correction code in an ITS (information transmission system) without feedback, then with a significant error density it will be ineffective in terms of noise immunity, and with a low error density it will be ineffective in terms of transmission speed. Typically, the correcting code is designed for a constant noise density, so IPS without feedback is used in systems with a constant information delay time, and also if there is no reverse channel or its creation is impossible.
It is necessary to commensurate the redundancy introduced into the transmitted information with the state of the discrete channel at each moment in time. For example, an increase in the number of errors should be associated with an increase in redundancy. Redundancy is introduced in the transmitter, and the state of the channel can be judged by the results of receiving information. To regulate
redundancy, it is necessary for the receiver to inform the transmitter about the number of errors. Therefore, a feedback channel is established. SPI with a feedback channel are divided into systems with decisive feedback (DCF), systems with information feedback (IFE) and systems with combined feedback (CFC). In systems with POC, the receiver, having received the code combination and analyzed it for errors, makes the final decision either to issue the code combination to the consumer, or to erase it and send a resend signal via the reverse channel. Systems with POC are called systems with re-request or systems with automatic error request. If the code combination is received without errors, the receiver generates and sends a confirmation signal to the feedback channel. The transmitter, having received the confirmation signal, transmits the next code combination. The active role belongs to the receiver, and the decision signal generated by the receiver is transmitted through the feedback channel. In systems with IOS, information about code combinations (or their elements) arriving at the receiver is transmitted via a feedback channel before final processing and making a final decision. It is possible that the code combination is retransmitted from the receiver to the transmitter. Such systems are called relay systems. It is possible that the receiver produces special signals that have a smaller volume than helpful information, but characterizing the quality of its reception. These signals from the receiver are also sent through the feedback channel to the transmitter. If the amount of information transmitted via the feedback channel (receipt) is equal to the amount of information in the message transmitted via the forward channel, then the IOS is called complete. If the receipt information reflects only some of the characteristics of the message, then the IOS is called shortened. The receipt received via the feedback channel is analyzed by the transmitter. Based on the results of the analysis, the transmitter makes a decision to transmit the next code combination or to repeat previously transmitted combinations. After this, the transmitter transmits service signals about the decision made, and then the corresponding code combinations. In accordance with the service signals received from the transmitter, the receiver either issues the accumulated code combination to the recipient, or erases it and remembers it as newly transmitted. In systems with a shortened IOS, there is less load on the feedback channel, but there is a greater likelihood of errors compared to systems with a full IOS.
In CBS systems, the decision to issue a codeword to the recipient or to retransmit it can be made at both the receiver and the transmitter, and the OS channel can be used for both the receipt and the decision. OS systems are divided into limited and unlimited repetition systems. With a limited number of repetitions, the probability of error is higher, but the delay time is lower.
If an IPS with feedback discards information in rejected code combinations, then this system is without memory. Otherwise, the feedback SPI is called a memory system. OS systems are adaptive information transfer systems, because transmission over the channel is automatically adjusted to the specific signal conditions. Feedback channels are formed by methods of frequency or time separation from channels for transmitting useful information. To protect against distortion of signals transmitted over the OS channel, correction codes, multiple and parallel transmissions are used. Currently, numerous algorithms for operating OS systems are known. The most common systems among them are:
· ROS with waiting for OS signal;
· ROS with addressless repetition and receiver blocking;
· ROS with address repetition.
Systems with waiting, after transmitting a code combination, either wait for a feedback signal or transmit the same code combination, but begin transmitting the next code combination only after receiving confirmation of the previously transmitted combination.
Blocking systems transmit a continuous sequence of code combinations in the absence of OS signals for the previous n combinations. After detecting errors in the (n+1)th combination, the system output is blocked for the duration of receiving n combinations, n previously accepted combinations are erased in the memory device of the PDS system receiver and a resend signal is sent. The transmitter repeats the transmission of the n last transmitted code combinations.
, 33. Ensuring safety requirements and discipline.doc, Laboratory work for the discipline Introduction to specialty 14., work program for TX PM03 17.doc, 2-4 work program 2019-2020.docx.
Lecture No. 14. Characteristics of a feedback system and their features. Block diagram of a system with information feedback and decision feedback, characteristics and operating algorithm.
Main literature:
Transmission of discrete messages: Textbook for Universities / V. P. Shuvalov, N. V. Zakharchenko, V. O. Shvartsman, etc.; Ed. V. P. Shuvalova. – M.: Radio and communication, 1990 - 464 s.
Kupinov Yu.P. and others. Fundamentals of discrete message transmission - M.: Radio and Communications, 1992.
Digital communication. - M., Sank-P, Kyiv: Publishing house. house "William", 2003
Mirmanov A.B. Course of lectures on the discipline “Technology” digital communications" - Astana: KazATU, 2009. (electronic)
Keywords:
adaptive, decisive, informational, return channel, insertion, dropout, shift.
Issues covered:
Adaptation in traffic police systems
Feedback systems
Transmission systems with ROS.
Information transfer speed in systems with ROS and coolant
Methodology for calculating the probability of incorrect reception (without taking into account distortions in the OS channel)
Abstracts for the lecture
Adaptation in traffic police systems
Majority real channels connections are non-stationary. The condition and quality of such channels changes over time.
For best use channel, it is necessary to change the introduced redundancy (encoding, decoding algorithms, signals, etc.) depending on the state of the channel.
Systems in which the process of purposefully changing the parameters, structure or properties of the system is carried out depending on the conditions of message transmission, in order to achieve optimal functioning, are called adaptive.
Adaptive systems involve the use of feedback.
Feedback systems
Depending on the purpose of the OS, systems are distinguished:
with decisive OS (ROS);
with information (IOS).
Fundamental difference between POS and IOS systems is where decisions are made about the further behavior of the system. On systems with ROS the decision is made on reception, and in systems with IOS - on transmission.
To organize feedback in both systems it is used return channel.
Information transmitted over a channel from the OS is called receipt.
Systems with IOS in which complete transmission of received code combinations via the reverse channel is carried out are called relay.
More often, the receiver generates special signals that have a smaller volume than the useful information transmitted over the direct channel, i.e., the receipt is smaller - a shortened IOS.
Transmission systems with ROS.
The most common among systems with ROS are:
systems with waiting (ROS - OZH);
with continuous information transfer and blocking
with targeted questioning
In DOC-coolant systems there is always a delay for the waiting time t cool. This time consists of several intervals:
Where t p PC– signal propagation time in the forward channel; t en–– time to analyze the correctness of the reception; t oc– duration of the feedback signal; t p oc– OS signal propagation; t a oc– OS signal analysis.
In OS systems, specific distortions appear due to errors in the feedback channel. Such distortions are called "inserts" And "losses".
Causes and their occurrence:
if, as a result of interference in the OK, the “confirmation” signal was transformed into a “request” signal, then the already accepted CC is issued to the recipient, and the combination is sent to the channel again. Thus, the PS will receive two consecutive identical combinations - “insertion”.
if the transition “request” → “confirmation” occurs, then the erroneously accepted combination will be erased, but the next one will go into the channel. This means that the PS will not receive this combination - a “loss” will occur.
Combating the phenomenon of "shift" in systems with ROC - coolant
Increasing the noise immunity of the reverse channel.
Cyclic numbering of transmitted code combinations
Methodology for calculating the probability of incorrect reception (without taking into account distortions in the OS channel)
Taking each CC has three outcomes:
The QC was accepted correctly and there are no errors in it ( R ppr)
The CC was accepted and an error was detected in it ( R oo)
QC with error, but no error detected ( R npr)
Figure 14.1. State graph of the system under consideration with DOS - coolant
The probability of incorrect reception P * NP with an unlimited number of rounds of re-questioning will include the probability of NP in the first cycle, the probability of NP after the first, second, etc. re-questioning.
Information transfer speed in systems with ROS and coolant
The main disadvantages of DOC-coolant systems include a significant reduction in the R speed.
Reasons for slowing down:
introduction of redundant (checking) elements ( 1 );
Availability t cool– decision signal about reception quality ( 2 );
retransmissions KK ( 3 ).
R = B 1 2 3
Speed reduction factors due to the introduction of test elements
Considering both redundancy and expectation
3. If there is a possibility of detecting errors in the QC - P oo 
Analyzing
1
And
3
it follows that to increase the speed R (or reduce speed losses) it is necessary to increase the block length n. Increasing block lengthn:
reduces the relative number of redundant elements required to ensure a given fidelity;
reduces relative losses while waiting for a decision on the quality of reception.
Control questions
Errors in channels are usually grouped; the state of the channel can be very different. Consequently, if a correction code is used in the ITS without feedback, then with a significant error density it will be ineffective in terms of noise immunity, and with a low error density it will be ineffective in terms of transmission speed. Typically, the correcting code is designed for a constant noise density, so IPS without feedback is used in systems with a constant information delay time, and also if there is no reverse channel or its creation is impossible.
It is necessary to commensurate the redundancy introduced into the transmitted information with the state of the discrete channel at each moment in time. For example, an increase in the number of errors should be associated with an increase in redundancy.
Redundancy is introduced in the transmitter, and the state of the channel can be judged by the results of receiving information. To regulate redundancy, the receiver must inform the transmitter about the number of errors. Therefore, a feedback channel is established.
SPI with a feedback channel are divided into systems with decisive feedback (DCF), systems with information feedback (IFE) and systems with combined feedback (CFC).
In systems with POC, the receiver, having received the code combination and analyzed it for errors, makes the final decision either to issue the code combination to the consumer, or to erase it and send a resend signal via the reverse channel. Systems with POC are called systems with re-request or systems with automatic error request. If the code combination is received without errors, the receiver generates and sends a confirmation signal to the feedback channel. The transmitter, having received the confirmation signal, transmits the next code combination. The active role belongs to the receiver, and the decision signal generated by the receiver is transmitted through the feedback channel.
In systems with IOS, information about code combinations (or their elements) arriving at the receiver is transmitted via a feedback channel before final processing and making a final decision. It is possible that the code combination is retransmitted from the receiver to the transmitter. Such systems are called relay systems. It is possible that the receiver produces special signals that have a smaller volume than useful information, but characterize the quality of its reception. These signals from the receiver are also sent to the transmitter via the feedback channel. If the amount of information transmitted via the feedback channel (receipt) is equal to the amount of information in the message transmitted via the forward channel, then the IOS is called complete. If the receipt information reflects only some of the characteristics of the message, then the IOS is called shortened.
The receipt received via the feedback channel is analyzed by the transmitter. Based on the results of the analysis, the transmitter makes a decision to transmit the next code combination or to repeat previously transmitted combinations.
After this, the transmitter transmits service signals about the decision made, and then the corresponding code combinations. In accordance with the service signals received from the transmitter, the receiver either issues the accumulated code combination to the recipient, or erases it and remembers it as newly transmitted.
In systems with a shortened IOS, there is less load on the feedback channel, but there is a greater likelihood of errors compared to systems with a full IOS.
In CBS systems, the decision to issue a codeword to the recipient or to retransmit it can be made at both the receiver and the transmitter, and the OS channel can be used for both the receipt and the decision.
OS systems are divided into limited and unlimited repetition systems. With a limited number of repetitions, the probability of error is higher, but the delay time is lower.
If an IPS with feedback discards information in rejected code combinations, then this system is without memory. Otherwise, the feedback SPI is called a memory system. Figure 6.10 shows an illustration explaining the implementation of feedback in the SPI.
With option I information about the received signal is transmitted over the communication channel before any decision is made. With option II The feedback covers a discrete communication channel, and the decisions made by the first decision circuit are transmitted through the feedback channel. With option III The feedback covers the channel for transmitting discrete information, and the decisions of the second decision circuit, made on the basis of analysis of the code combination, are transmitted through the feedback channel.
OS systems are adaptive information transfer systems, because transmission over the channel is automatically adjusted to the specific signal conditions.
Feedback channels are formed by methods of frequency or time separation from channels for transmitting useful information.
To protect against distortion of signals transmitted over the OS channel, correction codes, multiple and parallel transmissions are used.
Instructions
