Frame synchronization

Basic information

The frame synchronization code is specified as 0011011 in the TS0 time slot of even-numbered frames. Frame synchronization is to search for and identify the synchronization codeword from the received data stream, and use the time slot as the head of a frame to make the frame structure of the receiving end and the sending end completely consistent, so as to ensure that the two switches can work synchronously. Only in this way can the correct reception and exchange of digital information be realized.

In the digital relay circuit, the function of frame synchronization is realized by the frame synchronization detector. The frame synchronization detector has two basic states: the frame synchronization state and the frame out-of-synchronization state. A known frame synchronization code word is detected on a given frame synchronization code bit, which is called the frame synchronization state. When the codeword detected three times in a row does not match the frame synchronization codeword, it is judged as the frame out-of-synchronization state, and then the out-of-synchronization alarm code is sent in the TS0 of the odd-numbered frame to notify the opposite office. This method of determining the frame out-of-synchronization only when the frame synchronization codeword is not detected for three consecutive times is called forward protection. This is to prevent the system from being out of synchronization due to occasional errors or interference, which may cause too frequent system error adjustments and take protective measures. When the frame is out of synchronization, the system adopts backward protection, which stipulates that only when the frame synchronization codeword is detected in two consecutive frames and the second bit of the intermediate odd-numbered frame Tso is "1", it is judged to be restored to the frame synchronization state.

Classification

There are two types of frame synchronization: start-stop synchronization method and insert special synchronization code group method.

Start-stop synchronization method: It uses special code element coding rules at the sending end to make the code group itself carry grouping information.

Insert special synchronization code group method: refers to inserting several synchronization codes for frame synchronization in the transmitted symbol sequence. There are two main types: centralized insertion and interval insertion.

Centralized insertion, also called continuous insertion, requires the frame synchronization special code group to have excellent autocorrelation characteristics. The commonly used frame synchronization code group is Barker code.

Interval insertion. Also known as distributed insertion, a simple periodic cyclic sequence is usually used as the frame synchronization code, and it is inserted evenly into the information code stream.

Setup time

When transmitting data, the requirements are very strict. Even if the frame synchronization recovery time is 2 ms, a lot of data will be lost.

Protection

In digital communication systems, due to the influence of noise and interference, there will be errors in the frame synchronization code group, which will cause synchronization problems; in addition, due to The information code may also have the same symbol combination as the frame synchronization code group, which will also cause the problem of false synchronization. The existence of false synchronization and missing synchronization will make the frame synchronization system unstable and unreliable.

In an actual communication system, if the system finds a frame synchronization code group, it is considered to have been synchronized, or when there is a missing synchronization, the system is considered out of synchronization, then such a system may always be synchronized Switching back and forth between the gait state and the out-of-gait state cannot work normally. Therefore, some measures must be taken to protect the frame synchronization system to improve the performance of the frame synchronization. The protective measure often adopted for the frame synchronization system is to divide the working state of the frame synchronization into two states, namely the capture state and the maintenance state. That is, when the system starts or is in the capture state (search state, also called agait state), when the frame synchronization signal is detected multiple times by a certain rule, the system will switch from the capture state to the maintenance state (synchronization state); In the state, the system will not switch to the working state due to an accidental non-frame synchronization signal, but after multiple regular inspections, it is indeed found that the system has lost synchronization before switching to the capture state and re-capture.

1. Frame synchronization protection in the continuous insertion method

In the above introduction of the principle of the continuous insertion method to achieve frame synchronization, from the requirement that the probability of missing synchronization P1 is low and false If the synchronization probability P2 is low, the choice of the decision threshold level of the Barker code recognizer is contradictory. Of course, considering the reliability of the system, I hope:

①In the capture state, increase the decision threshold level of the recognizer (reduction in chat) to make the false synchronization probability P. Reduce.

②When the state is maintained, reduce the discriminator's decision threshold level (increase in research), so that the probability of missing synchronization P is reduced.

The principle block of frame synchronization protection in the continuous insertion method, which is mainly composed of RS flip-flops, counters, and and or OR circuits.

2. Frame synchronization protection in the intermittent insertion method

When using the code-by-code shift method to achieve frame synchronization in the intermittent insertion method, the key to the protection of the frame synchronization system is to increase two counters. The working principle will be introduced below from two working states.

System requirements

The frame synchronization system should generally meet the following requirements:

(1) The frame synchronization establishment time is short, and the device should be able to be quickly turned on To establish synchronization. Once the system loses synchronization, synchronization can be quickly restored.

(2) The work of the synchronization system should be stable and reliable. The synchronization system should have the ability to identify false out-of-synchronization and avoid false synchronization, and have strong anti-interference ability.

(3) The length of the synchronization code word should be as short as possible on the premise that the synchronization establishment time and the premise are satisfied.

System stability

Channel errors may cause frame synchronization codes to generate errors, resulting in false out-of-synchronization. In the case of normal frame synchronization, if the adjustment is made according to the false out-of-synchronization, the state that is already in frame synchronization will become the out-of-frame state, that is, misadjustment will be caused. The wrong adjustment result will interrupt normal communication. Therefore, the frame synchronization system should have a certain degree of anti-interference ability, for which the frame synchronization system should be equipped with protective measures.