About multiplexing technology for wireless technology
Since the appearance of the first networks, the question of the simultaneous operation of several devices was acute. And now for decades there has been a struggle with this problem and the main task is to keep up with the needs of society.
The article proposes to quickly run through the main technologies used multiplexing. If where you lied, correct.
Actually, what are the possibilities for organizing comfortable simultaneous work of several users? There are only two of them for wired networks: the separation of data from different devices in space and in time.
It all started with the 10Base2 wired networks, where a common copper bus was used as the data transmission medium - coaxial cable. Briefly touch on this topic.
At that moment, when one of the devices is broadcasting, the other cannot start, because it will simply generate noise instead of data, since the electrical signal propagates along the entire length of the conductor. In such conditions, spatially, especially you will not scatter the data. It falls on time: the network card is waiting for silence on the network. If the channel is busy, then it tries to repeat after a random period of time. As soon as silence appears, the device begins to broadcast. This is the first multiplexing mechanism - CSMA / CD - Carrier Sense Multiple Access with Collision Detection .
This situation persisted with the advent of hubs, since they were essentially the same tires. But when the switches came to the network market, everything changed. They divided the network into several collision domains - in fact, each device is in a separate one, which meant the spatial separation of data from different devices.
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The situation is different with wireless technology. They are much more complex in terms of standards development and implementation. Here both inter-sector and inter-channel interference, signal attenuation, and more complex QoS support are added, and the most important thing is simultaneous operation of dozens of subscribers.
How can it be provided for wireless devices? There are two approaches: time diversity and frequency diversity.
TDMA - Time Division Multiple Access . This is one of the technologies we use daily to use - GSM is built on it. Here the data is spread over time. For each subscriber station a certain number of timeslots is allocated with full use of the allocated width of the radio channel. These timeslots are assigned to the device until the end of the session. Remember the dawn of GSM, when you were trying to dial a number for 5-10 minutes, and in response, you saw a message that the network was busy and that the call could not be made? At this point at the base station there were no free timeslots for you. Later TDMA received support in GPRS, EDGE. But it can hardly be called optimal and, there is hardly a place for him in our wireless future.
FDMA technology - Frequency Division Multiple Access - is practically not used in its pure form (it was implemented in 1G standards, for example, AMPS) For each subscriber station, a certain band is allocated from the used frequency range and is released only after the end of the session.
As in TDMA, this use of the radio channel is not optimal and requires a balance between the number of online users and their speed.
CDMA - Code Division Multiple Access . At the moment, it is one of the most promising multiplexing mechanisms. Used in CDMA (2000, EVDO) and UMTS standards.
Its essence is that each device registered on the network is given a unique code. On one side, the signal is modulated using such a unique code, on the other, the reverse process — demodulation. Thus, user data is not distributed either in time or from frequencies, but are mixed using the entire channel width.
CDMA is synchronous — using the properties of orthogonal vectors to choose a coding method — and asynchronous — using pseudo-random sequences. Each of the approaches has its advantages and disadvantages - this is the topic of an independent article and I will not dwell on this in detail.
DSSS - Direct Sequence Spread Spectrum . The technology has much in common with CDMA (pseudo-random sequence is used for modulation), is quite noise-resistant, due to eleven-fold redundancy, but it has low efficiency. Used in Wi-Fi at speeds up to 2 Mb / s.
Now let's move on to one of the most interesting, in my opinion, multiplexing technologies, which has a huge variety of applications, including Wi-Fi, WiMAX and LTE - OFDMA ( Ortho frequency frequency division ).
The OFDM itself, on which OFDMA is based, is also used in a number of standards and not only wireless: DVB-T, ADSL, VDSL.
Why is he so interesting? It's all about the high efficiency of the use of the radio channel. The secret is already revealed in the title: Orthogonal Frequency. For transmission, not one carrier with the entire selected channel width is used, but several subcarriers with intersecting subchannels.
OFDMA implies the allocation of time slots to each user, the size of which is nxm, where n is the number of subchannels, m is the number of OFDMA symbols !!!! .. Thus, this kind of multiplexing can be called a symbiosis of TDMA and FDMA, each of which is individually non-optimal.
The advantages of OFDMA include:
By cons:
I will be glad to your additions and notes.
The article proposes to quickly run through the main technologies used multiplexing. If where you lied, correct.
Actually, what are the possibilities for organizing comfortable simultaneous work of several users? There are only two of them for wired networks: the separation of data from different devices in space and in time.
It all started with the 10Base2 wired networks, where a common copper bus was used as the data transmission medium - coaxial cable. Briefly touch on this topic.
At that moment, when one of the devices is broadcasting, the other cannot start, because it will simply generate noise instead of data, since the electrical signal propagates along the entire length of the conductor. In such conditions, spatially, especially you will not scatter the data. It falls on time: the network card is waiting for silence on the network. If the channel is busy, then it tries to repeat after a random period of time. As soon as silence appears, the device begins to broadcast. This is the first multiplexing mechanism - CSMA / CD - Carrier Sense Multiple Access with Collision Detection .
This situation persisted with the advent of hubs, since they were essentially the same tires. But when the switches came to the network market, everything changed. They divided the network into several collision domains - in fact, each device is in a separate one, which meant the spatial separation of data from different devices.
')
The situation is different with wireless technology. They are much more complex in terms of standards development and implementation. Here both inter-sector and inter-channel interference, signal attenuation, and more complex QoS support are added, and the most important thing is simultaneous operation of dozens of subscribers.
How can it be provided for wireless devices? There are two approaches: time diversity and frequency diversity.
TDMA - Time Division Multiple Access . This is one of the technologies we use daily to use - GSM is built on it. Here the data is spread over time. For each subscriber station a certain number of timeslots is allocated with full use of the allocated width of the radio channel. These timeslots are assigned to the device until the end of the session. Remember the dawn of GSM, when you were trying to dial a number for 5-10 minutes, and in response, you saw a message that the network was busy and that the call could not be made? At this point at the base station there were no free timeslots for you. Later TDMA received support in GPRS, EDGE. But it can hardly be called optimal and, there is hardly a place for him in our wireless future.
FDMA technology - Frequency Division Multiple Access - is practically not used in its pure form (it was implemented in 1G standards, for example, AMPS) For each subscriber station, a certain band is allocated from the used frequency range and is released only after the end of the session.
As in TDMA, this use of the radio channel is not optimal and requires a balance between the number of online users and their speed.
CDMA - Code Division Multiple Access . At the moment, it is one of the most promising multiplexing mechanisms. Used in CDMA (2000, EVDO) and UMTS standards.
Its essence is that each device registered on the network is given a unique code. On one side, the signal is modulated using such a unique code, on the other, the reverse process — demodulation. Thus, user data is not distributed either in time or from frequencies, but are mixed using the entire channel width.
CDMA is synchronous — using the properties of orthogonal vectors to choose a coding method — and asynchronous — using pseudo-random sequences. Each of the approaches has its advantages and disadvantages - this is the topic of an independent article and I will not dwell on this in detail.
DSSS - Direct Sequence Spread Spectrum . The technology has much in common with CDMA (pseudo-random sequence is used for modulation), is quite noise-resistant, due to eleven-fold redundancy, but it has low efficiency. Used in Wi-Fi at speeds up to 2 Mb / s.
Now let's move on to one of the most interesting, in my opinion, multiplexing technologies, which has a huge variety of applications, including Wi-Fi, WiMAX and LTE - OFDMA ( Ortho frequency frequency division ).
The OFDM itself, on which OFDMA is based, is also used in a number of standards and not only wireless: DVB-T, ADSL, VDSL.
Why is he so interesting? It's all about the high efficiency of the use of the radio channel. The secret is already revealed in the title: Orthogonal Frequency. For transmission, not one carrier with the entire selected channel width is used, but several subcarriers with intersecting subchannels.
OFDMA implies the allocation of time slots to each user, the size of which is nxm, where n is the number of subchannels, m is the number of OFDMA symbols !!!! .. Thus, this kind of multiplexing can be called a symbiosis of TDMA and FDMA, each of which is individually non-optimal.
The advantages of OFDMA include:
By cons:
- Frequency Shift Sensitivity
- The FFT (Fast Fourier Transform) and FEC (Forward Error Correction) mechanisms operate continuously, regardless of the connection speed, which results in high power consumption.
I will be glad to your additions and notes.
Source: https://habr.com/ru/post/92980/
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