A channel coding mechanism added in order to increase the reliability of communication in wireless communication.
Generally use three parameters to set (n, k, m).
Among them, n is the number of bits output at a time, k is the number of bits input at a time, and m is the number of memorized inputs.
Since k inputs become n outputs, it is called Rate k/n. It is equivalent to that only k/n is valid data in the transmitted bits, and the other bits are used to increase the redundancy of the signal and thereby reduce the bit error rate of the transmission (improve reliability).
It actually performs a convolution operation on the current input and m previous inputs, and n indicates how many convolutions there are.
For the situation in the figure below, we see that the unit impulse response of the first convolution operation is (1 0 1 1), and the unit impulse response of the second convolution operation is (1 1 0 1).
From left to right correspond to the latest input and the oldest input saved, 1 means there is a tap, and 0 means no tap.
As shown in the figure below, it is a BCC diagram of (2,1,3).
Taking the input sequence 10110 as an example, let's take a look at how to get the output sequence 11, 01, 01, 01, 11, 01, 11.
Time 0,
The input node is 1000 from left to right, and the output is 11. The three bits of 0 are the default state.
Time 1,
Input is 0100, output is 01
Moment 2,
Input is 1010, output is 01
Time 3,
Input is 1101, output is 01
Time 4,
Input is 0110, output is 11
Time 5,
Input is 0011, output is 01
Time 6,
The input is 0001, the output is 11,
Moment 7,
The input is 0000 and the output is 00.
What are the benefits of turning k-bit data into n-bit output data ?
It actually helps to solve the transmitted data more accurately under noise interference.
At the receiving end, you can use Soft Decesion instead of Hard Decesion, which can have better anti-interference performance.
The basic meaning is that single-bit data becomes multi-bit data.
With a single bit, the decision depends on the setting of the threshold.
With multiple bits, the threshold has changed from one to more, which is more anti-interference ?
If more complex modulation methods are used, the anti-interference performance will be reduced, but the effective data transmission rate may be maintained, and only sufficient modulation methods must be selected.
If the same modulation method is used under a single bit, the effective data rate will decrease, but the anti-interference ability will increase.
For example, for a single bit, the threshold is set to 0.6, higher than 0.6 is 1, and lower than 0.6 is 0
In the case of interference, 0 becomes 0.7, then it will be judged as 1
Similarly, if it becomes a transmission 01, the former is judged as 1, and the latter is judged as 1. But what does 11 correspond to? This can be solved using probabilistic methods.