LTE: Where are my 100 Mbit / s?
Good day, dear readers!
I want to share my thoughts on speed in LTE networks. I think that many of you have seen colorful advertisements by operators about how cool it is to use modern mobile Internet at a huge speed - 100 Mbps. Only this number 100 should be with an asterisk.
Why with an asterisk? Let's see. So, how do you calculate the maximum speed in an LTE network? Let's first dig a little inland. How is the data transfer in the network? In the time domain, the LTE structure consists of radio frames of 10 ms. Each frame consists of 10 subframes, each 1 ms long. Each frame consists of two slots (0.5 ms). Each slot can contain either 7 (more often) or 6 (less often) OFDM symbols.
We now turn to the frequency domain. It's all easier. One OFDM symbol is 15 kHz. Thus, the number of these symbols is limited only by the size of the frequency band allocated to the operator.
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So, how to calculate the maximum speed in the LTE network? Yes, very simple. One OFDM symbol (or one resource element, ER) may contain 2, 4 or 6 information bits, depending on the type of modulation. We already know that in one time slot (0.5 ms) there are 7 OMs, respectively, in one millisecond of such elements 14, in a second - 14000.
Go ahead. In the frequency domain, the OM occupies 15 kHz. Suppose we assume that the operator was lucky and he was given the maximum possible frequency range - 20 MHz. Considering that 10% of this band must be left at guard intervals, it has 18,000 kHz useful, in which it is possible to fit 1200 OFDM symbols. Multiply 1200 * 14000 and get 16.8 Megasimvolov per second. Each of these characters is modulated with either two (QPSK), or four (QAM-16), or six (QAM-64) bits. Thus, with the best scenario, applying QAM-64 modulation, we get: 16.8 M-characters * 6 = 100.8 Mbit / s. Here they are, the same advertising, 100 megabits. However, what do we have in practice?
The Big Three operators have two frequency bands: 10 MHz in the 2.5-2.6 GHz region and 7.5 MHz in the 800 MHz band. As far as I know, the band 7.5 MHz is now practically not used by our operators. What is the conclusion? This condition alone cuts the maximum speed in the LTE network twice, up to 50 Mbit / s. To obtain it, you need ideal conditions: the base station antenna is in line of sight, there is no interference, a smartphone capable of operating at that speed. In addition, besides you at this moment no one should use the Internet. Unfortunately, even if all these conditions are met, it will not be possible to achieve a speed of 50 Mbit / s, because, in addition to information useful to you, the base station also exchanges service signals (signaling).
Let us now descend from heaven and imagine that you are not sitting near the base station, but in your living room or office. On the line of sight of the antenna speech can not be, respectively, the level of the signal coming to the smartphone, low. In such conditions, part of the received data will come with errors and, to avoid this, the LTE network will have to reduce the modulation scheme to QAM-16 or even to QPSK. Also, we should not forget that due to interference in the radio channel, redundant coding is used.
So what is the speed you can expect on your smartphone? The situation is quite real when, sitting at home on the couch, a KAM-16 modulation scheme will work for you (let me remind you, under this condition, one resource element contains 4, not 6 information bits, as in QAM-64) and the code rate R will value 1 \ 2. Let's calculate: due to robust coding, the maximum speed will drop from 50 to 25 Mbit / s. Given that the modulation scheme has changed to QAM-16, the speed will drop another 2 \ 3 times, to 16.6 Mbit \ s. And it provided that the base station serves only one subscriber - you. Of course, for mobile devices, these speeds are more than enough, but this is far from those 100 Megabits per second, which adorn the posters.
I want to share my thoughts on speed in LTE networks. I think that many of you have seen colorful advertisements by operators about how cool it is to use modern mobile Internet at a huge speed - 100 Mbps. Only this number 100 should be with an asterisk.
Why with an asterisk? Let's see. So, how do you calculate the maximum speed in an LTE network? Let's first dig a little inland. How is the data transfer in the network? In the time domain, the LTE structure consists of radio frames of 10 ms. Each frame consists of 10 subframes, each 1 ms long. Each frame consists of two slots (0.5 ms). Each slot can contain either 7 (more often) or 6 (less often) OFDM symbols.
We now turn to the frequency domain. It's all easier. One OFDM symbol is 15 kHz. Thus, the number of these symbols is limited only by the size of the frequency band allocated to the operator.
')
So, how to calculate the maximum speed in the LTE network? Yes, very simple. One OFDM symbol (or one resource element, ER) may contain 2, 4 or 6 information bits, depending on the type of modulation. We already know that in one time slot (0.5 ms) there are 7 OMs, respectively, in one millisecond of such elements 14, in a second - 14000.
Go ahead. In the frequency domain, the OM occupies 15 kHz. Suppose we assume that the operator was lucky and he was given the maximum possible frequency range - 20 MHz. Considering that 10% of this band must be left at guard intervals, it has 18,000 kHz useful, in which it is possible to fit 1200 OFDM symbols. Multiply 1200 * 14000 and get 16.8 Megasimvolov per second. Each of these characters is modulated with either two (QPSK), or four (QAM-16), or six (QAM-64) bits. Thus, with the best scenario, applying QAM-64 modulation, we get: 16.8 M-characters * 6 = 100.8 Mbit / s. Here they are, the same advertising, 100 megabits. However, what do we have in practice?
The Big Three operators have two frequency bands: 10 MHz in the 2.5-2.6 GHz region and 7.5 MHz in the 800 MHz band. As far as I know, the band 7.5 MHz is now practically not used by our operators. What is the conclusion? This condition alone cuts the maximum speed in the LTE network twice, up to 50 Mbit / s. To obtain it, you need ideal conditions: the base station antenna is in line of sight, there is no interference, a smartphone capable of operating at that speed. In addition, besides you at this moment no one should use the Internet. Unfortunately, even if all these conditions are met, it will not be possible to achieve a speed of 50 Mbit / s, because, in addition to information useful to you, the base station also exchanges service signals (signaling).
Let us now descend from heaven and imagine that you are not sitting near the base station, but in your living room or office. On the line of sight of the antenna speech can not be, respectively, the level of the signal coming to the smartphone, low. In such conditions, part of the received data will come with errors and, to avoid this, the LTE network will have to reduce the modulation scheme to QAM-16 or even to QPSK. Also, we should not forget that due to interference in the radio channel, redundant coding is used.
So what is the speed you can expect on your smartphone? The situation is quite real when, sitting at home on the couch, a KAM-16 modulation scheme will work for you (let me remind you, under this condition, one resource element contains 4, not 6 information bits, as in QAM-64) and the code rate R will value 1 \ 2. Let's calculate: due to robust coding, the maximum speed will drop from 50 to 25 Mbit / s. Given that the modulation scheme has changed to QAM-16, the speed will drop another 2 \ 3 times, to 16.6 Mbit \ s. And it provided that the base station serves only one subscriber - you. Of course, for mobile devices, these speeds are more than enough, but this is far from those 100 Megabits per second, which adorn the posters.
Source: https://habr.com/ru/post/239309/
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