As the saying goes: As an old driver, it is not only necessary to have good car skills, but also to know the working principle of the engine (in fact, Wu Laosi himself thinks out, is there a wood that feels particularly reasonable?).
The same reason can be proved, as a senior communication man, do not come to the underlying technical details of the 818, NB people really think that the brother is engaged in pyramid schemes.
So from the beginning of this article, we will go deep into the physical layer technology discussion of NB, so that everyone can not only eat melon, but also know how good watermelon is grown. (It seems that if you think pork is delicious, you must know how the pig is. Feed the same).
This article mainly discusses whether the bandwidth of NB is 200K or 180K?
1 LTE bandwidth analysis
As we all know, the goal of LTE design is to sum up the "three highs and one low": high speed, high bandwidth, high spectrum utilization, low latency. High bandwidth and high rate are coupled together. Dr. Shannon tells us such a general theorem:
Rmax=W*log2(1+S/N)
Where R is the capacity (rate), W is the bandwidth, and the visible rate is proportional to the bandwidth.
If you have seen the evolution of the LTE version, you will find a very interesting thing. The release version of LTE has undergone polarization. One is to continue the "high and handsome" route, and the other is to take the "short and poor" route. . As shown below:
Still based on Shannon's theorem, when we want to meet the low-speed IoT demand, it is easy to conclude that the bandwidth of NB will also be relatively reduced. Then the question is coming:
How much is the NB bandwidth reduced? why?
Before answering this, we may wish to take a closer look at the above picture, with LTE-M, Cat-0, Cat-M and other words in the middle, and finally NB. In fact, when we talked about the ins and outs of NB, we have already talked about it. 3GPP started the eMTC for the IoT. After the eMTC standard was quickly terminated for various reasons, we tried to hold another female X number. NB-IoT went (I think it seems like a good thing in the circle).
Let's look at the two basic characteristics of the eMTC physical layer:
â—¢System bandwidth = 1.4 Mhz, 6 RB
â—¢ Minimum scheduling time particle = 1 ms TTI (minimum transmission time interval)
Well, in fact, everyone knows that GPRS is not the same as GPS. It is a technology derived from GSM. Similarly, eMTC is derived from LTE. What is derivative? In the vernacular, it is repairing and repairing, and the second decoration is just three years of sewing.
However, in order to better adapt to the application scenario of IOT, based on factors such as cost and coverage, it is impossible to rely solely on patching (see the previous article on technical analysis of strong coverage, small power consumption, low cost, large connection, etc.), so 3GPP Determined to break the LTE frequency minimum bandwidth limit (which does not require such a large bandwidth and speed) and TTI limits (no need to transmit in time).
The next question is how much is the NB bandwidth down? Some small partners have begun to take care of her mother.
2 NB is 200K?
The bandwidth that Wu Laosi heard when he first started to contact NB was 200K. Let's make a proof.
It is known that GSM single frequency bandwidth is 200K (how familiar melody)
Proof: NB is so designed, you can turn over the frequency of GSM in the future, and take it directly after drying it (professional term is called refarming).
Conclusion: so far so good!
Wu Laosi comment
and many more! ! ! Say well, in order to reduce costs, try to implement NB based on the physical layer structure of LTE? Xiao Ming, please tell me where there is a 200K implementation in LTE?
3 NB is 180K?
In fact, for the sake of the industrial chain (as far as possible in the design of the bottom layer can use the point LTE to use points, just like although your home is renovated, but it will also be based on cost considerations, can be used in the original use) NB is designed to use the working bandwidth of one RB of LTE, that is, 180K. The following is a block diagram of an RB of LTE.
This is only explained for the vertical axis (LTE masters should skip automatically): the vertical axis represents the frequency axis, represented by subcarriers, each subcarrier is 15K, and each RB has 12 subcarriers, so the total bandwidth of one RB is 180K.
Ok, everyone can start to open the 3GPP chapter 36.211 Chapter 10 to testify, you will find that the entire agreement is talking about 180KHZ things.
4 Why are everyone talking about GSM frequency hopping to NB?
This simple, one-sentence thing: 200K bandwidth is returned from a GSM frequency point. You actually use 180K bandwidth, and others use it as a protection bandwidth. This is the same as buying a house. There are only three people living in the house, but you still try to buy a three-bedroom, one-bedroom house, so you can make a room (the brother of Beishenguang, such a luxury thing, you, lying down, there is wood ?).
Not coincidentally, we may wish to look at the bandwidth characteristics of LTE, the 20M bandwidth is configured, the actual subcarriers occupy 18M, and each side has 1M as the protection bandwidth, which is simply the same formula, the same taste!
Ok, the bandwidth analysis on NB is here.
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