Radio Technologies in 5G

Radio Technologies in 5G

This paper talks about how more throughput can be achieved for 5G networks, what are the new Radio technologies for 5G which are being considered and even being implemented by many large MNOs.

1. Introduction

5G is not just high speed, but a lot more, like low latency as well as more bandwidth. Combined usage of the latest RF technologies will overall help the 5G network to meet the expectations of eMBB, URLLC and mMTC. In fact these RF techniques are not new, but with the advancement of technologies, these techniques have become practically feasible.

2. Why 5G?

In the present age of technology, it is hard to believe that 30 years ago people lived without mobiles. No one would have imagined this Mobile revolution, being experienced today. It all started with 1G where people could experience their wired phone to become wireless, moving to 2G where even we could sent text messages and then 3G brought us online, that was when people really wanted to explore how a movie can be watched on the go on a cell phone, how a big size file can be downloaded in a short time. This was nevertheless not so smooth, hence we could see that between 3G and 4G there was not much gap and 4G is what we are enjoying currently.

Figure 1: From Wired to Wireless in 1G to 4G different advantages were achieved

But the reason to adapt to 5G is not the same as what was seen in case of moving from 3G to 4G mobile technology. Additionally there are few challenges which also need to be addressed to cater to the ever growing user base. Some of them are listed as follows:

  1. The existing frequency bands are almost saturated
  2. The speeds of 4G networks are also stagnated due to a large user base in the same spectrum range.
  3. Accommodation of Low latency use cases is almost impossible in the existing spectrum.

5G is more than just speed. We are talking about eMBB, URLLC, mMTC. To realize these services we must have something mystic on the RF side which can play a very significant role to realize these challenging services. In the next section we are going to see what are the radio technologies that have been materialized and how they are going to give us these services in practice.

3. Key RF Technologies

Though currently we are enjoying the high speed on current 4G networks, but at the same time the networks are choking up as more and more users are joining the same bandwagon of 4G. So you are back to square one. This situation, enforced to go beyond the current spectrum, being used for 4G and other wireless services. Listed below are the key RF technologies that are going to play an important role in the 5G network.

  1. Millimeter Waves
  2. Small Cell Networks
  3. Massive MIMO
  4. Beamforming
  5. Full Duplex Communications

3.1. Millimeter Waves (mm Waves)

Current smartphones and other wireless devices are using a specific Radio Frequency spectrum in the range of 300MHz to 6GHz. As we mentioned the users keep on increasing and the time comes when the whole wireless range becomes choked up and the real benefits of even 4G speed cannot be realized. There is a need to look beyond the current spectrum window. This whole new spectrum is called millimeter wave and seen as the next choice to meet this ever-growing need of spectrum. As the name states, mmWave is a new frequency band altogether that operates on ultra high frequency where the wavelength of the signals is in millimeter range. Below stated are the few primary reasons which entail the need of using millimeter wave as the next best option:

  1. The current range 300MHz to 6GHz is almost crowded and there is no further scope of expansion. With more and more devices coming online, there is more congestion and dropped connections.
  2. The new spectrum range above 6GHz, is being explored by researchers and since this new spectrum range was never used earlier, there is huge bandwidth available.

Figure 2: Various Radio bands ranging from sub-6GHz to mmWave

3.2. Small Cell Networks

Though there is an advantage of having technology available to use all new ranges of spectrum, that came with few problems as well. One of the biggest challenges is that mm waves cannot travel long distances, they cannot even penetrate walls, not only this but they are even absorbed by trees and get faded in rains. These issues can be overcome by another concept called Small Cell Networks. Till now the telecommunication has been dependent on using long waves (low frequency in the range of 100s of MHz to few sub 3GHz). The signals can travel longer distances giving good coverage in general. But to take full advantage of mmWave the cell size must be reduced so that the coverage is seamless.

Figure 3: Larger Coverage Area with Low frequency Spectrum

Figure 4: Obstacles enroute obstruct the mmWaves to reach to users

Figure 5: Reaching behind obstacles using Small Cell approach

In this way mmWaves combined with Small Cells can provide seamless connectivity to the users. Small Cells or Nano cells are practical in congested city scenarios, where the user can even get the seamless coverage behind the obstacles and can switch over to the next base station easily. Though the cell size is decided by the RF planner depending on type of users, service, transmit power, number of users, user QoS, and bandwidth. But as per few cell tower lease experts, the 5G network might be built on small cell site technology with antennas as close as 500 feet apart.

3.3. Massive MIMO (mMIMO)

Massive MIMO is an advanced version of MIMO where tests have been conducted with 128×128 (Transmitting and Receiving) antennae. This can be compared to having 128 ears to listen and 128 mouths to speak.

Below are the advantages of using Massive MIMO:

  1. Increased Signal to Noise ratio (S/N): The quality of signal improves with more number of Antennae. Also as the beams are focused per user, it significantly increases the S/N ratio.
  2. Increased capacity: With more antennae, there are more careers which can cater to a large number of subscribers in an area.
  3. Increased Coverage: Complemented with Beamforming techniques, users can experience consistent coverage at the center as well on the edge of the base station.

Figure 6: Massive MIMO depiction with increasing number of Radio Elements

Current 4G technology handles about a dozen ports for antennas to handle cellular traffic. Whereas massive MIMO base stations can support 100s of antennas. This can increase the capacity of today’s networks by a factor as high as 22.

3.4. Beamforming

Beamforming is a technique used to create focused beams in a specific direction, this helps to enhance network coverage in the cellular technologies. As the number of antennas increases, the interference of signals increases as well and beamforming helps to significantly reduce this.

Beamforming is a subset of massive MIMO, in other words Beamforming is achieved by appropriately tuning the magnitude and phase of individual antenna signals in an array of antennas. More the radiating elements, the narrower the beam will be. Hence massive MIMO plays a significant role here to achieve beamforming.

Figure 7: Beamforming – Tracking vs Switching the user
3.4.1. Beamforming Techniques: To facilitate Beamforming, the radiating elements are arranged in such a way that the beams in a specific direction are added and other beams are neglected. This can be accomplished using a finite impulse response (FIR) filter. There are two classifications of beam forming:
  1. Adaptive array systems
  2. Switched beam systems
To give seamless service experience, the beam is steered for each user, which means that the beam follows the user wherever they go. This is achieved by changing the phase of the signal at the radiating elements. The phase shifting allows the signal to be targeted at a specific user.
  1. Adaptive array systems have the arrangement to form a single beam for each user. In this system the phase and amplitude of the signal is adjusted using weight vectors that are applied by adaptive array processors.
  2. Switched beam systems, the beams are predefined as a result of fixed beam forming networks. It requires a switching network so that the user switchover can happen from a weaker beam to the stronger beam.In this arrangement there is more than one mobile which can be served by a single beam.
The diagram below depicts both types of beam forming systems.

Figure 8: Adaptive and Switched beamforming

3.5. Full Duplex Communications

The current systems are using Half Duplex mode of communication which is achieved by using one of the below techniques:

–  Use same frequency for both uplink and downlink and take turns to send and receive data

–  Use separate frequencies for uplink and downlink.

These techniques are not very efficient from a spectral efficiency point of view.

Full Duplex enables simultaneous transmission and reception of the signals over the same frequency band. This improves the spectral efficiency significantly. Solutions have been developed to use a kind of switches so that Tx and Rx can use the same channel at same time as depicted in the figure below.

Figure 9: Theoretical representation of a Full-duplex system

The Self Interference (SI) is a key challenge in Full-Duplex which may result in reduced capacity of Full Duplex gain. (SI is phenomena which can degrade the Full Duplex gain, due to huge power difference between the transmission power and the low power received from the device)

Passive suppression and Active suppression are the  SI cancellation techniques  which are helpful in mitigating the drawback of  Full Duplex and making it possible to be deployed at commercial level. Research is in progress to exploit the Full-duplex at its fullest.

4. Spectrum Needs

5G is expected to support significantly faster mobile broadband speeds and lower latencies in contrast to its predecessor technologies such as 4G and 3G. The success of 5G technology delivering its full potential is very much relying on the right amount of spectrum availability. Below are the types of spectrum being made available to 5G deployments to meet the coverage and data speed:

1. Frequency Range-1 (FR1):

  • Sub-1GHz Spectrum: This is the frequency range below 1GHz. This RF range supports larger coverage areas across urban, suburban and rural areas where the population density is low. This can be catering to mainly IoT connectivity as well as low bandwidth demanding use cases.
  • 1-6GHz Spectrum: This is the frequency range between 1GHz to 6GHz. This band offers a good mixture of coverage and capacity. Many initial 5G services can be offered within this band such as 3.3-3.8 GHz. It includes other RF ranges such as 1800MHz, 2.3GH and 2.6GHz.

2. Frequency Range-2 (FR2):

This is the frequency range beyond 6GHz. Beyond 6GHz is needed to meet the demand of real 5G services like eMBB and URLLC. 26GHz and 28GHz sub bands are the ones which are being worked out for 5G roll outs.

5. Modulation in 5G (Optional Read)

5.1. RF fundamentals

Before we explain about Modulation techniques in 5G, we will briefly discuss some fundamentals of Bandwidth, Channel and Career.
  • Bandwidth: The bandwidth of a signal is defined as the difference between upper and lower frequencies of the signal. As depicted in the below diagram the difference of f(H) and f(L) is called the bandwidth in Hertz (Hz).

Figure 10 : Bandwidth

  • Channel: It is the medium for transmitting the signal over the Electromagnetic waves. When we say that channel size ranging from 5MHz to 100MHz is needed for the spectrum below 6GHz, it means any frequencies between 5MHz to 100MHz can be used for the communication in the 6GHz spectrum band. Similarly when we say channel sizes from 50 MHz to 400 MHz in bands above 24 GHz spectrum, that means any frequencies within this range can be used for communication.
  • Career: It’s the frequency which actually modulates according to the input signal. Carrier frequency is basically a fixed frequency. This is generally high frequency. In academic terms the signals which are of low frequency are hard to travel longer distances. Hence the carrier is used to transmit the actual signal. A technique called modulation is used. The carrier is modulated according to the base signal. There are various types of modulation techniques like amplitude, frequency, phase modulation. We will discuss in a later section about the kind of modulation used for 5G.

Figure 11 : Frequency Modulation Generation

5.2. Modulation

There are a couple of considerations for using different modulation formats as stated below:

  1. Peak to average power ratio (PAPR): It’s the ratio of peak power to the average power of a signal. In a multi-carrier system the various sub-carriers are out of phase with each other. They have maximum values at certain times. So when the difference between their peak power and the average power is too high (sometimes to the tune of 10dB or so), then the Power amplifier in the base station should be capable to operate 10dB higher than its capacity. This significantly degrades the performance of the Power Amplifier in the base station. PAPR can be reduced using Crest Factor Reduction (CFR) algorithms.
  2. Spectral efficiency: Spectrum is a key resource which is available at a premium. So the modulation technique has to be chosen in such a way that the spectrum efficiency is high.

Below are the modulations used in the 5G radio networks.

  1. Phase Shift Keying (PSK): 5G implements QPSK type of modulation. In PSK or BPSK type of modulation, one bit is modulated by just shifting the phase of the career. In contrast, the QPSK type of modulation, two bits are modulated simultaneously separated by a phase shift. This doubles the rate as compared to normal PSK type of modulation.
  2. Quadrature Amplitude Modulation (QAM): In QAM, not only phase but amplitude of the signal is changed as well. By doing this there are more bit combinations which can be sent across. Higher the order of modulation, greater the throughput that can be achieved.

The modulation method can even go upto 256QAM for 5G.

6. Enhancements in Network Throughput

As we explained various methods in the radio technologies for 5G, using these techniques, the overall objectives of 5G can be achieved in terms of high speed, low latency and so on. Below formula Below formula for cellular capacity depicts how the throughput achieved in 5G is far better than the legacy technologies:

Figure 12 : Enhancement in Network ThroughPut

Capacity (bit/s per km2)  = Cell density (cells/km2) * Spectral efficiency (bit/s/Hz/cell) * Available spectrum (in Hz) This section concludes with the application of the RF technologies (discussed in earlier sections of this paper) to the formula for N/w capacity. 5G network is going to exploit each of the parameters in the above equation to achieve a higher throughput demand.
  • Cell density : Deploy more number of cells with reduced power, which means that the mobile will be much closer to the base station thereby spending less energy as well. This is what was explained in the Small Cell topic above.

Figure 13 : Higher Cell Density

  • Higher Spectral efficiency (bit/s/Hz): It is very challenging to improve spectral efficiency beyond a certain limit. We can increase transmit power, but that turns out to be very expensive, as if you want to double the spectral efficiency, we need to have 17 times more power ! Higher spectral efficiency can be achieved by having multiple sources of transmitting powers rather than a single source. This is exactly what MIMO can do.

Figure 14 : Higher spectral efficiency by more antennas

  • Available Spectrum: The existing spectrum range has already reached saturation and there is no more capacity available. As we explained earlier, the whole new spectrum range is identified that is not utilized till now. The same can be opened up for 5G. This concept was explored in mmWave section i.e to explore the spectrum range from 6GHz to even 300GHz.

Figure 15 :Available spectrum in different frequency bands

We can say that by using the latest RF technologies these 3 variables can be increased which is directly proportional to increase in throughput.

7. Conclusion

We saw in this paper that by the use of various technologies in the RF side, we can achieve the objectives laid down for 5G network deployments. These technologies are evolved with extensive research which is still on. Many MNOs have already used these solutions and are able to demonstrate speeds in the range of 1Gbps. These technologies promise to deliver the much-needed capacity enhancement, reliability and speed for the next generation of telecom networks.

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