The scrambling code should be considered relatively easy to understand in communication technology, but there are still many stereotypes in learning, and many misunderstandings have been formed, such as:
- Scrambling can only be used to process digital signals;
- The scrambling code can only be the same as the code rate of the original signal;
- Scrambling will not change the frequency distribution of the signal;
Of course, only my misunderstandings are listed here. Others may have other misunderstandings. The following will elaborate on these differences.
第一个问题The source is that we see in the reference materials, scrambling is a way of digital signal processing, or according to the current formulation, it is a way of transformation. The example cited is the binary signal of "0" and "1". A kind of mindset is created invisibly, and scrambling is used to process digital signals. It is further extended that scrambling can only be used to process digital signals.
In fact, scrambling is just a transformation, and the essence of this transformation is to use simple operations to transform and restore signals. If there is no scrambling code; the signal is encoded, and if there is a scrambling code, it is scrambling. There are two simple operations, addition or multiplication. In addition, the feature of scrambling is to facilitate the implementation, and the signal conversion and restoration adopt the same set of algorithms.
First look at the addition, suppose the original signal A, scrambling code B, get A+B after scrambling, and get A+B+B after restoration, which is A+2B. If you want to get A, obviously B=0, which means it can’t be introduced. The scrambling code, so the scrambling cannot use addition. Aside from the topic, if the reduction uses subtraction, it is also feasible. B is equivalent to interference, which is the interference cancellation technology.
Looking at the multiplication again, suppose the original signal A, scrambling code B, scrambled to get A B, and restored to get A B B, which is A (the square of B). Obviously, if you want to get A, you need (the square of B)=1, that is, the value of B is 1 or "-1". This condition can be met, so multiplication is selected for scrambling, and the sequence corresponding to B is called the scrambling code, which is a binary signal, that is, a digital signal.
It can be seen that whether A can be restored has nothing to do with the value of A, so any value of A can be restored after scrambling and descrambling. The key is to use the same binary sequence-scrambling code.
In this way, we have solved the first misunderstanding: scrambling can be applied to any signal, as long as the scrambling code is a digital signal.
第2个误区It is a way of thinking that the scrambling code used for scrambling must be the same as the code rate of the original signal. Indeed, many communication books say so, but it does not mean that this is a conclusive conclusion. My understanding is that if you look at scrambling very narrowly, the scrambling code used for scrambling must be the same as the code rate of the original signal. But if we broaden our horizons and think that scrambling is a transformation, then according to the information mentioned on the previous page, the scrambling code does not have to be the same as the code rate of the original signal. In this sense, the orthogonal modulation (Walsh) in CDMA is also a kind of scrambling.
第3个误区It is relatively simple, mainly because many WCDMA data believe that the signal code rate after spreading (Walsh) is 3.84M, and then scrambling, the scrambling code still uses the code rate of 3.84M, so the scrambled output does not change the frequency distribution of the signal . Of course, now we know that the spread spectrum in WCDMA is based on scrambling. After scrambling, the spectrum of the original signal is expanded, so this misunderstanding is naturally cracked.
The third misunderstanding also raises a question. Why is the code rate of the signal before and after scrambling the same, but the spectrum of the signal is extended? The reason is that the code rate of the signal is not equal to the frequency of the signal. It does not mean that the higher the code rate of the signal, the greater the width of the spectrum.
Reprinted from: http://www.readhere.cn/page.php?id=1821
Other information:
https://blog.csdn.net/qq_33668920/article/details/79661454
https://www.sharetechnote.com/html/Handbook_UMTS_ScramblingCode.html