December 23, 2009 at 7:17 WiMAX Base Station
WiMAX base station.
My past article “WiMAX. How it works ” I devoted to the description of the technology, general words about the mechanisms used, and showed an approximate network structure. Since this topic is interesting to the community, I continue.
This time, you will come close to the WiMAX Network Base Station, find out how it works and be able to ask your questions.
In fact, a WiMAX network is an ordinary IP network, on one of the segments of which radio waves are used as a data transmission medium. On the other hand, at the physical level, WiMAX is very similar to GSM, CDMA and any other wireless networks.
Under the cut you can learn about the elements of the BS, the principle of operation and the mechanisms used.
Habrakat is a wonderful thing, it helps to separate the grains from the chaff for each specific article. However, I digress, sorry.
The BS circuit does not fundamentally differ from that used in other wireless technologies: the main internal module, amplifiers and antennas. The only difference: the presence (sorry for the pun) of the GPS antenna.
Before starting the story, I want to say that we will talk about the implementation of Huawei, so some points may be specific to this company.
So. The DBS3900 (Distributed Base Station) consists of an internal processing unit (BBU in Huawei terminology), amplifiers (RRU), and antennas.
Of course, the "brain" of the BS is BBU - BaseBand Unit. Almost the entire burden of data processing, synchronization, management, and statistics collection rests on this node.
In the BBU from the ASN, physical layer PDUs are received. The data of the channel level are processed using various coding schemes, modulation corresponding to the signal level, undergo the inverse Fourier transform, Cycle Prefix is added to them and then sent to the amplifier.
DMS39000 consists of the following boards.
The first 4 slots (from 0 to 3) are reserved for BBBI (BWA BaseBand processing and radio Interface unit) boards. Each board has three optical interfaces (CPRI-Common Public Radio Interface) for connecting amplifiers. One or two boards can be installed, each of which occupies two slots.
Slots 4 and 5 are reserved for the USCU (Universal Satellite card and Clock Unit). This board also occupies two slots and is used for synchronization. For WiMAX using time-duplex (TDD) synchronization is a very important issue, since the transmit / receive time should be synchronized both for different sectors of the same BS and for other base stations that are adjacent to the one under consideration. Generally speaking, GPS is used as the easiest and most universal way. But, as I said somewhere in the comments to other articles, in Russia they oblige to install, in addition to GPS, the GLONASS receiver board - K161, for which the manufacturer has provided mounting legs and a small jumper on the USCU. The main connector on this board is the SMA for the dzhmper from the GPS antenna.
The sixth and seventh slot is used for the “main” BBU board - BMPT (BWA Main Processing & Transmission unit). Actually, this is BS MCC, and it manages all the processes. It has an Ethernet port for control, two RG-45 FE / GE ports and two optical LC ports for communication with the ASN network (only two ports can be logically configured). In addition, a logical interface is configured on it for communication with the control network.
Two more boards are located on the right side of the front panel - UPEU (Universal Power and Environment interface Unit), which operate in Hot Standby mode. They have a switch, a power connector, dry contacts and connectors for connecting external monitoring units (RJ-45).
What happens next with data processed on the BBU? Optical interfaces connected to the CPRI ports transmit data to the amplifier unit (RRU - Radio Remote Unit). On the amplifier, digital-to-analog (analog-to-digital) conversions, radiation power control, superposition of an analog signal on a high-frequency signal and multiplexing of reception and transmission signals occur.
Next, the signal is fed through a feeder to the antenna. To implement MIMO, essentially two antennas are installed in each antenna, having cross-polarization to reduce interference. The antennas themselves are just tiny when compared to GSM. This, of course, is related to the frequencies used.
DBS3900 supports up to 1024 subscribers per sector (both active users and those who are in IDLE mode are considered). Thus, more than 3000 subscribers can be “planted” on the base station in this way. Unfortunately, I cannot say how realistic this is - such values have not yet been achieved.
When using the 10 MHz bandwidth on the downlink, you can get 30 Mbit / s per sector (at 5 MHz - 15 Mbit / s). This is possible if you configure MIMO 2T2R SM on the BS (two antennas for reception, two for transmission, transmitting and receiving various information), PUSC with All SC (this will be discussed later) and the downlink to uplink 32:15 ratio (we are talking about the ratio of the number of OFDMA symbols in the frame allocated for DL / UL, respectively). Under the same conditions, uplink you will have 6 Mbit / s per sector.
Now you can focus on various specific things.
Signal quality is determined by two main parameters: RSSI and CINR.
RSSI = Recieve Signal Strength Indicator. Measured in dBm and means signal strength. The farther from the BS, the lower this value. Any obstacles worsen it: buildings, trees.
CINR = Carrier to Interference and Noise Ratio. An objective indicator of the quality of the received signal, measured in dB. The lower the value, the worse the signal. This may be due to either remoteness from the BS or interference.
These two parameters are defined both for downlink (sometimes reflected on subscriber devices) and uplink (you can request this information only from BS equipment). The uplink RSSI may be much lower due to weak transmitting antennas of mobile devices, but this is offset by powerful BS receiving antennas.
What is MIMO? Technology with this modest name allows the use of several antennas for reception and transmission. Why might this be needed? There are two possible answers to this and two corresponding implementations:
MIMO Matrix A (Diversity mode): the same data is transmitted through different antennas. This circuit is used to improve the quality of the transmitted signal and to improve reception.
MIMO Matrix B (Spatial Multiplexing (SM) mode): Different data is transmitted through different antennas. This allows you to increase the data transfer rate.
Huawei is currently testing the MIMO 4T4R in the United States.
The data rate for each particular device is determined, roughly speaking, by the modulation used, the encoding scheme and the Repetition parameter. Repetition means the number of retransmissions (2, 4, 6) with a bad signal. All these parameters are selected automatically, for which the AMC (Adaptive Modulation and Coding) mechanism is responsible.
In the comments to the previous topic, I was asked to touch on the topic of the “breathing cell”.
Breathing hundredth refers to the ability of a BS to change its power, and therefore the coverage area, in accordance with the load on the BS itself and on neighboring ones. The number of subscribers and the consumed traffic are taken into account. It turns out that the radius of action of a lonely standing BS will be much larger than that of a BS, near which there are neighboring ones, under otherwise equal conditions. This is in theory. In practice, unfortunately, we did not have to deal with this.
One of the major advantages of WiMAX is the possibility of using an adaptive antenna system that adjusts its radiation pattern in accordance with the movement of the subscriber station, that is, it picks it up and tracks it. But the rationality of its use is another question. This will most likely work perfectly in rural areas with low development and a small number of subscribers. How it will work in an urban environment, I can’t presume.
Now about what is PUSC with all sub-channels, and at the same time PUSC 1/3 sub-channels and FFR.
It's about reusing frequencies.
PUSC = Partially Usage of Sub-Channels. PUSC 1/3 uses one single frequency for all BSs. Each sector uses 1/3 of all subchannels (hence the name. You can see a little more details, for example, here ).
PUSC with all sub-channels (aka FUSC - Full Usage of Sub-Channels) uses all sub-channels on one sector. That is, for one BS three different frequencies are needed.
FFR (Fractional Frequency Re-Use) - roughly, smart PUSC 1/3. All BSs use the same frequency. But at the same time, in the field of confident coverage of a particular sector, PUSC with all sub-channels is used, and 1/3 at the borders of the sectors. This allows subscribers located near the BS to maximize the capabilities of the network.
So, now you know that if you have permission for only one frequency, you can use PUSC 1/3 or FFR. Of course, if the equipment supports FFR, then it is better to use it. And if you have at least three frequencies, then it is better to use PUSC with all sub-channels.
<In this article, I did not go into details so as not to produce essence beyond necessity (exhaustive information on most issues can be found on Wikipedia), but only touched on their meaning and significance for us users>
My past article “WiMAX. How it works ” I devoted to the description of the technology, general words about the mechanisms used, and showed an approximate network structure. Since this topic is interesting to the community, I continue.
This time, you will come close to the WiMAX Network Base Station, find out how it works and be able to ask your questions.
In fact, a WiMAX network is an ordinary IP network, on one of the segments of which radio waves are used as a data transmission medium. On the other hand, at the physical level, WiMAX is very similar to GSM, CDMA and any other wireless networks.
Under the cut you can learn about the elements of the BS, the principle of operation and the mechanisms used.
Habrakat is a wonderful thing, it helps to separate the grains from the chaff for each specific article. However, I digress, sorry.
Scheme
The BS circuit does not fundamentally differ from that used in other wireless technologies: the main internal module, amplifiers and antennas. The only difference: the presence (sorry for the pun) of the GPS antenna.
DBS3900
Before starting the story, I want to say that we will talk about the implementation of Huawei, so some points may be specific to this company.
So. The DBS3900 (Distributed Base Station) consists of an internal processing unit (BBU in Huawei terminology), amplifiers (RRU), and antennas.
Of course, the "brain" of the BS is BBU - BaseBand Unit. Almost the entire burden of data processing, synchronization, management, and statistics collection rests on this node.
In the BBU from the ASN, physical layer PDUs are received. The data of the channel level are processed using various coding schemes, modulation corresponding to the signal level, undergo the inverse Fourier transform, Cycle Prefix is added to them and then sent to the amplifier.
DMS39000 consists of the following boards.
The first 4 slots (from 0 to 3) are reserved for BBBI (BWA BaseBand processing and radio Interface unit) boards. Each board has three optical interfaces (CPRI-Common Public Radio Interface) for connecting amplifiers. One or two boards can be installed, each of which occupies two slots.
Slots 4 and 5 are reserved for the USCU (Universal Satellite card and Clock Unit). This board also occupies two slots and is used for synchronization. For WiMAX using time-duplex (TDD) synchronization is a very important issue, since the transmit / receive time should be synchronized both for different sectors of the same BS and for other base stations that are adjacent to the one under consideration. Generally speaking, GPS is used as the easiest and most universal way. But, as I said somewhere in the comments to other articles, in Russia they oblige to install, in addition to GPS, the GLONASS receiver board - K161, for which the manufacturer has provided mounting legs and a small jumper on the USCU. The main connector on this board is the SMA for the dzhmper from the GPS antenna.
The sixth and seventh slot is used for the “main” BBU board - BMPT (BWA Main Processing & Transmission unit). Actually, this is BS MCC, and it manages all the processes. It has an Ethernet port for control, two RG-45 FE / GE ports and two optical LC ports for communication with the ASN network (only two ports can be logically configured). In addition, a logical interface is configured on it for communication with the control network.
Two more boards are located on the right side of the front panel - UPEU (Universal Power and Environment interface Unit), which operate in Hot Standby mode. They have a switch, a power connector, dry contacts and connectors for connecting external monitoring units (RJ-45).
Amplifier
What happens next with data processed on the BBU? Optical interfaces connected to the CPRI ports transmit data to the amplifier unit (RRU - Radio Remote Unit). On the amplifier, digital-to-analog (analog-to-digital) conversions, radiation power control, superposition of an analog signal on a high-frequency signal and multiplexing of reception and transmission signals occur.
Antennas
Next, the signal is fed through a feeder to the antenna. To implement MIMO, essentially two antennas are installed in each antenna, having cross-polarization to reduce interference. The antennas themselves are just tiny when compared to GSM. This, of course, is related to the frequencies used.
Technical details
DBS3900 supports up to 1024 subscribers per sector (both active users and those who are in IDLE mode are considered). Thus, more than 3000 subscribers can be “planted” on the base station in this way. Unfortunately, I cannot say how realistic this is - such values have not yet been achieved.
When using the 10 MHz bandwidth on the downlink, you can get 30 Mbit / s per sector (at 5 MHz - 15 Mbit / s). This is possible if you configure MIMO 2T2R SM on the BS (two antennas for reception, two for transmission, transmitting and receiving various information), PUSC with All SC (this will be discussed later) and the downlink to uplink 32:15 ratio (we are talking about the ratio of the number of OFDMA symbols in the frame allocated for DL / UL, respectively). Under the same conditions, uplink you will have 6 Mbit / s per sector.
Now you can focus on various specific things.
Signal quality is determined by two main parameters: RSSI and CINR.
RSSI = Recieve Signal Strength Indicator. Measured in dBm and means signal strength. The farther from the BS, the lower this value. Any obstacles worsen it: buildings, trees.
CINR = Carrier to Interference and Noise Ratio. An objective indicator of the quality of the received signal, measured in dB. The lower the value, the worse the signal. This may be due to either remoteness from the BS or interference.
These two parameters are defined both for downlink (sometimes reflected on subscriber devices) and uplink (you can request this information only from BS equipment). The uplink RSSI may be much lower due to weak transmitting antennas of mobile devices, but this is offset by powerful BS receiving antennas.
What is MIMO? Technology with this modest name allows the use of several antennas for reception and transmission. Why might this be needed? There are two possible answers to this and two corresponding implementations:
MIMO Matrix A (Diversity mode): the same data is transmitted through different antennas. This circuit is used to improve the quality of the transmitted signal and to improve reception.
MIMO Matrix B (Spatial Multiplexing (SM) mode): Different data is transmitted through different antennas. This allows you to increase the data transfer rate.
Huawei is currently testing the MIMO 4T4R in the United States.
The data rate for each particular device is determined, roughly speaking, by the modulation used, the encoding scheme and the Repetition parameter. Repetition means the number of retransmissions (2, 4, 6) with a bad signal. All these parameters are selected automatically, for which the AMC (Adaptive Modulation and Coding) mechanism is responsible.
In the comments to the previous topic, I was asked to touch on the topic of the “breathing cell”.
Breathing hundredth refers to the ability of a BS to change its power, and therefore the coverage area, in accordance with the load on the BS itself and on neighboring ones. The number of subscribers and the consumed traffic are taken into account. It turns out that the radius of action of a lonely standing BS will be much larger than that of a BS, near which there are neighboring ones, under otherwise equal conditions. This is in theory. In practice, unfortunately, we did not have to deal with this.
One of the major advantages of WiMAX is the possibility of using an adaptive antenna system that adjusts its radiation pattern in accordance with the movement of the subscriber station, that is, it picks it up and tracks it. But the rationality of its use is another question. This will most likely work perfectly in rural areas with low development and a small number of subscribers. How it will work in an urban environment, I can’t presume.
Now about what is PUSC with all sub-channels, and at the same time PUSC 1/3 sub-channels and FFR.
It's about reusing frequencies.
PUSC = Partially Usage of Sub-Channels. PUSC 1/3 uses one single frequency for all BSs. Each sector uses 1/3 of all subchannels (hence the name. You can see a little more details, for example, here ).
PUSC with all sub-channels (aka FUSC - Full Usage of Sub-Channels) uses all sub-channels on one sector. That is, for one BS three different frequencies are needed.
FFR (Fractional Frequency Re-Use) - roughly, smart PUSC 1/3. All BSs use the same frequency. But at the same time, in the field of confident coverage of a particular sector, PUSC with all sub-channels is used, and 1/3 at the borders of the sectors. This allows subscribers located near the BS to maximize the capabilities of the network.
So, now you know that if you have permission for only one frequency, you can use PUSC 1/3 or FFR. Of course, if the equipment supports FFR, then it is better to use it. And if you have at least three frequencies, then it is better to use PUSC with all sub-channels.
<In this article, I did not go into details so as not to produce essence beyond necessity (exhaustive information on most issues can be found on Wikipedia), but only touched on their meaning and significance for us users>