00:00Hi, in the last session, we talked about different 5G deployment options.
00:05And I hope you have some background and overview on 5G now.
00:09Now let's focus on the 5G Radio Access Network or 5G RAN.
00:14Compared to the LTE AIR interface, the 5G AIR interface should meet main two requirements.
00:21First, it must efficiently utilize the new spectrum which also lies in the millimeter
00:27wave band.
00:28Second, it needs to support the various new use cases introduced in 5G.
00:34This leads to the development of a new wireless access called NR or New Radio.
00:40In the 5G ecosystem, NR will not be the only wireless technology.
00:44Instead, it will work together with other wireless technologies such as LTE.
00:49In the new 5G designs, there are many enhancements that cater to the different goals of 5G.
00:56For enhanced mobile broadband, 5G also introduced some new concepts such as massive MIMO, beamforming,
01:03wideband carriers, and carrier aggregations, etc.
01:07Also to support the ultra-reliable low-latency use cases, 5G included new features like flexible
01:14numerology, preemptive scheduling, and grant-free uplink access.
01:18To improve the cost efficiency and flexibility, there are new concepts like virtualizations.
01:25So starting from this session and in next few sessions, we will explore these defining
01:30features of the 5G radio access network and we will know how they function.
01:36Let's start with MIMO.
01:38So the question is, how can we use the advancements in the antenna technologies to improve the
01:44capacity and the data rates in 5G?
01:47Let's begin that by exploring the evolutions of MIMO technology first.
01:52So at very beginning, omnidirectional antennas were used.
01:56They offer broad coverage, but that is uniform coverage.
02:00You can see in the top left picture, we have antenna which is providing uniform coverage
02:05in that radius.
02:07Then it was realized that the capacity of the network can be improved by using sectors
02:12which brought the coverage like this, like at the bottom right section here.
02:17So you can see that this approach creates three different cells from a single base station.
02:24Now we can further enhance this by directional beams, more precisely to a specific user equipment
02:30or UAE or some directive points.
02:33This is where beamforming comes in picture.
02:36But before learning about massive MIMO, we have to understand the basics of MIMO.
02:41In a single input and single output system, which is generally called a SISO system, we
02:47have one transmit antenna.
02:49The signal passes through a fading channel and reaches to the single receiving antennas.
02:55Now in this step and according to the Shannon equation, the capacity of this system can
03:01be enhanced by either increasing the channel bandwidth or by improving the signal to noise
03:06ratio or SNR at the receiver side.
03:10Now if you have multiple transmit antennas and a single receive antenna, then this configuration
03:16is known as multiple input, single output or MISO system.
03:21And in this setup, if the antennas are placed at least at the half of the wavelength, then
03:27each transmit antenna creates an independent fading channel to the receive antenna.
03:33This phenomena is known as transmit diversity.
03:37Now when the same data sent through these multiple channels, then they are affected
03:41by their independent channels.
03:44And by combining these received signals at the receiver, the signal to noise ratio or
03:48SNR gets improved and that enhances the channel capacity.
03:54This approach is especially beneficial for downlink because the multiple antenna setup
03:59requires more space.
04:01So they can easily locate it at the base stations.
04:04In a similar fashion, if there is a single transmit antenna and multiple receive antennas,
04:10then we also achieve independent fading channels.
04:13This is called receive diversity.
04:16And it also enhances the received signal to noise ratio or SNR by combining the effects
04:22of different fading channels.
04:25In this setup, the signal processing occurs at the receiving end.
04:29Now when the multiple antennas are used at the both end, means at the transmitting end
04:34and at the receiving end, then this is referred as MIMO or multiple input, multiple output
04:41setup.
04:42And when there are at least 16 transmit antennas and 16 receiving antennas, then it is often
04:48called as massive MIMO.
04:50However, some says that the minimum number of antennas for massive MIMO should be 32
04:56antennas or more for each side.
04:59Now MIMO offers three main methods to enhance the data transfer rate.
05:04First when signal to noise ratio is low, then the diversity techniques can be applied to
05:09improve the signal to noise ratio, just like before.
05:13However, if signal to noise ratio is high, then special multiplexing can be used to increase
05:19the data rates and system throughput.
05:22In special multiplexing technique, throughput is improved by transmitting multiple data
05:27streams simultaneously over the channel.
05:31Now there are two variants of special multiplexing.
05:35First is open-loop special multiplexing and second is closed-loop special multiplexing.
05:41In open-loop special multiplexing, the transmitter does not require channel state information.
05:47However, it is less efficient than the closed-loop special multiplexing.
05:51In the closed-loop special multiplexing, the transmitter receives the channel state information
05:56through the feedback from the receiver.
05:59And with the knowledge of the transmitter, appropriate pre-coding is done in the transmitter
06:05and processing is done in the receiver side.
06:08This allows the data streams to be separated effectively.
06:12The maximum number of streams in such MIMO systems is limited by the minimum number of
06:17either transmit antennas or receiving antennas.
06:22Now if all the received antennas belong to the same UE, it's known as single-user MIMO.
06:29Like you see in the top section of this picture, where all the receiving antennas belong to
06:34the same UE.
06:35So it is called single-user MIMO.
06:38Same way if multiple streams belong to the multiple UE at receiving end and these simultaneous
06:44streams received by different UEs at the same time, then this setup is called multi-user
06:50MIMO.
06:51Like you see in the bottom section of this picture.
06:55Now this is the third MIMO technique, where the same signal is transmitted from each transmitting
07:01antenna.
07:02But these signals are phase shifted.
07:05And the phase adjustment is done in such a way that the signal power is maximized to
07:10some specific point, where the UE is receiving the data.
07:15So this process is known as beamforming, which can be implemented in three different ways.
07:22The first method is analog beamforming.
07:25In analog beamforming, the phase of individual antenna signals are adjusted in the radio
07:30frequency domain.
07:32This adjustment influences the radiation pattern and the gain of the antenna array to provide
07:38the enhanced coverage.
07:40In digital beamforming, which is also called as baseband beamforming or precoding, this
07:46signal is precoded during the baseband processing, before the RF transmissions, means the processing
07:53occurs in the frequency domain.
07:55As a result, different signals are transmitted from each antenna.
08:00These are represented by different colors in this picture here.
08:04And the third is hybrid beamforming, when both analog beamforming and digital beamformings
08:10are combined.
08:12This technique is referred as hybrid beamforming.
08:16Hybrid beam enables the concentration of power towards a specific spatial region or
08:21a specific UE.
08:23So you see in this picture here.
08:25As a result, now the scheduler has an additional responsibility, because now it has to plan
08:31not only the time domain and frequency domain, but also the spatial domain.
08:36So this is how the beamforming works in 5G.
08:39LTE and its evolutions support different forms of MIMO, like transmit diversity, spatial
08:46multiplexing, multi-user MIMO, etc.
08:50But in 5G NR, since there are spectrums in the millimeter wavebands, and as a result,
08:56the antenna size reduces significantly with such increasing frequencies.
09:02So it is easy to place many numbers of antennas in some form of array and paste them together
09:08in one small package.
09:10So this huge array of antennas transmits narrow beams, and when the power from the different
09:15antennas combine in one direction, it acts like a strong beam.
09:21So this is the concept of beamforming.
09:24So in beamforming, the phase of the transmitting signals is adjusted in such a way that it
09:30adds a power in some specific direction to form a beam and eliminates each other power
09:36in the other directions.
09:38So that was one thing.
09:40But we also consider that millimeter waves have poor propagation characteristics.
09:45So for millimeter frequency bands, beamforming is important to serve 5G users.
09:51So here is a quick summary on what are the key benefits of using massive MIMO in 5G NR.
09:58Number one, massive MIMO systems have a larger number of active antennas elements.
10:04Number two, massive MIMO allows for more focused energy to improve signal strength.
10:10Number three, high intensity beam signals are less impacted by interference.
10:15So it has better signal quality.
10:18Number four, in good SNR conditions, more special multiplexing layers are possible.
10:24So you can enhance the data transmissions.
10:27Number five, massive MIMO improves the cellular network throughput and coverage.
10:33Number six, massive MIMO can serve more users simultaneously.
10:38Number seven, massive MIMO supports two-dimensional beamforming.
10:42So it offers better signal precisions in both horizontal and vertical planes.
10:48Okay, so that's it for today.
10:50In the next session, we will be talking about 5G spectrum.
10:54So stay tuned for the updates.
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