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The global race is now on to develop 5G, short for 5th generation mobile networking or 5th generation wireless systems which will change the face of the mobile communication. Most global 5G deployments kicked off using mid-band spectrum in the 3-4 GHz range, and now the second wave of 5G spectrum allocations is happening in the millimeter-wave range. The spectrum for 5G services not only covers bands below 6 GHz, including bands currently used for 4G LTE networks but also extends into much higher frequency bands. It is the use of frequency bands in the 24 GHz to 100 GHz range, known as millimeter wave (mmWave), that provide new challenges and benefits for 5G networks.
5G technologies will need to be capable of delivering fiber-like 10 Gb/s speeds will depend on ultra-wide bandwidth with sub-millisecond latencies. The scarcity of conventional microwave band has led to the exploration of mmWave realm. The multi-gigahertz bandwidth available in mmWave range has a strong potential in addressing the capacity demands of 5G and beyond wireless networks.
MmWave spectrum operates in high frequencies found between 30 GHz and 300 GHz, and is attractive for a number of reasons. One advantage in these high frequency bands is the availability of wider bandwidth channels. Additional radio spectrum would be available at millimetre wave frequencies from 30 GHz to 300 GHz will provide 10 times more band¬width than the 4G cellular-bands.
Second, there is more mmWave bandwidth available, which improves data transfer speed, making it possible to deliver gigabit wireless services. It also avoids the congestion that exists in lower spectrum bands (prior to researching potential 5G uses of mmWave frequencies, the only major operators in that area of the spectrum were radar and satellite traffic). The shorter wavelengths of mmWave create narrower beams, which in turn provide better resolution and security for the data transmission and can carry large amounts of data at increased speeds with minimal latency. Finally, mmWave components are smaller than components for lower bands of the spectrum, allowing for more compact deployment on wireless devices.
We now have commercial 5G mmWave networks deployed in the U.S. and other regions of the world that support extreme capacity and blazing-fast download speeds, and there is a growing selection of 5G mmWave devices — smartphones, laptops, and more.
Mmwave challenges
However, mmWave has its share of challenges. While its short wavelengths and narrowness of its beam allow for improved resolution and security of data transfer, these qualities can also restrict the distance at which mmWaves can propagate.
Pathloss is the attenuation or reduction in power density of an electromagnetic wave as it propagates. MmWaves suffer higher path loss relative to lower frequencies thereby limiting the range. This is a consequence of the decrease in receive aperture of a unity gain antenna, which is proportional to the square of the wavelength. Relative to 1 GHz, the free space path loss at 28 GHz is 29 dB higher for the same distance. At 38 GHz, the free space path loss is 31.6 dB higher.
The line of sight path loss is further reduced by transmitting and receiving antenna gains scattering, diffraction, multipath, penetration losses, and atmospheric absorption. GHz signals Because the mmWave wavelength is less than a centimeter, so most objects in the environment
appear relatively larger. When in contact with these objects, mmWave signals may experience full or partial signal absorption, reflection, scattering, and/or diffraction.
This challenge is further aggravated by the fact that mmWaves can be easily blocked by obstacles like walls, foliage, and the human body itself. The large-scale variation in signal strength caused by objects in the physical environment between the mmWave transmitter and receiver is called blockage or shadowing. Since shadowing results in a large scale (typically on the order 10 meters to 100 meters) variation in signal strength, it is typically modeled as a large-scale variation around the path loss. The higher reflection also causes reflected rays to bounce off objects in the environment, resulting in propagating multipath signals, and each multipath signal can then be further attenuated by the obstructions.
The aggregate attenuation caused by these obstructions is typically quantified as penetration loss,
which is measured as the difference in power levels between the unobstructed and the obstructed path. The major factors that impact penetration loss include signal frequency, material permittivity, material thickness, and surface roughness, the incident angle, and the polarization of the mmWave signal. The typical indoor and outdoor building materials result in typical outdoor penetration loss to vary from 28 dB to 40 dB and indoor losses from 3.6 dB to 24.5 dB.
Close to ground level mmWave links are very likely to also encounter the presence of foliage
and more broadly, vegetation clutter. mmWave propagation is impacted more by foliage compared to sub6 GHz frequency band signals. Fading due to moving foliage at 29 GHz can be up to 10 dB, while for 5 GHz it is around 2 dB.
For the mmWave frequency range, the dominant sources of atmospheric loss arise from
oxygen (O2) and water vapor (H2O). These atmospheric absorptions can result in measurable attenuation of the radio signal, leading to reduced propagation range. For example, the 60 GHz peak is particularly strong – roughly 13 dB/km at sea level and 20°C. The peaks at ~180 and ~315 GHz are also strong, with attenuations of several dB/km. The spectral regions between these absorption peaks provide transmission windows that are roughly centered near 35 GHz, 94 GHz, 140 GHz and 220 GHz.
To alleviate shadowing severity, mmWave communication systems tend to leverage high gain and narrow beamforming antenna arrays.
Spectrum
While most 5G networks have so far been deployed in a variety of spectrum bands, including below 1 GHz and between 1 and 6 GHz, 5G New Radio (NR) is the first mobile technology generation to make use of the mmWave spectrum. 5G NR in unlicensed spectrum (NR-U) was standardized in Release 16 and it is a key enabler for the 5G expansion to new use cases and verticals, providing expanded spectrum access to mobile operators, service providers, and industry players.
Millimeter wave spectrum falls in the range of 30 GHz to 300 GHz. The 5G industry is also using spectrum that is a little longer than mmWaves, such as 24 GHz and 28 GHz –but these frequencies share a lot of the same operating characteristics. These bands, along with 39 GHz frequencies and higher, are referred to as millimeter wave. n257, the global 28 GHz band, and n260, the global 39 GHz band each have 3 GHz of spectrum standardized.
n257 28 GHz (26500 MHz – 29500 MHz )
n258 26 GHz ( 24250 MHz – 27500 MHz)
n260 39 GHz (37000 MHz – 40000 MHz)
More spectrum equals more capacity, higher peak throughputs, and opportunities for new use cases that were not possible with the limited spectrum in the sub-6 GHz range. To that end, much larger carrier sizes have been standardized for mmWave: 50 MHz, 100 MHz, 200 MHz and 400 MHz. 50 and 100 MHz are already widely supported in the mmWave ecosystem, while support
is slow to build for 200 MHz (and 400 MHz support is for the time being optional). These larger SCSs are necessary for mmWave to combat inter-symbol interference and phase noise.
These bigger carrier sizes make it easier to aggregate large allocations of contiguous spectrum. There are however tradeoffs to be considered when deciding to deploy larger carriers or smaller across contiguous spectrum. For instance, spreading the Tx power both from the cell site (5G NodeB or gNodeB, a name carried over from UMTS) and the device (User Equipment, or UE) across a 200 MHz carrier will effectively reduce the power spectral density of that carrier, thereby inhibiting its coverage, when compared to a smaller carrier.
At the same time, we are starting to push the mmWave boundary to even higher bands toward the sub-Terahertz (i.e., >100 GHz) range. Expected in Release 17, 5G NR will support spectrum bands up to 71 GHz, leveraging the 5G NR Release 15 scalable numerology and flexible framework. This opens up 5G to operate in the globally unlicensed 60 GHz band, which can fuel a broad range of new applications and deployments.
5G mmWave potential Applications
With peak throughput speeds of 10 Gbps or more and the ability to support a huge number of devices, 5G mmWave has performance targets that will deliver a transformation in how wireless communications are utilized.
When outlining the requirements for 5G services, the International Telecommunication Union (ITU) identified three main categories for the 5G NR architecture; Enhanced Mobile Broadband (eMBB) for greater mobile capacity, Ultra-reliable and Low-latency Communications (uRLLC) for mission-critical services, and Massive Machine Type Communications (mMTC) for vast numbers of low-cost, low-energy devices (Internet of Things). These broad areas provide plenty of early deployment possibilities for 5G mmWave, such as the following:
Fixed wireless Internet access. The Gigabit data rates of 5G mmWave could completely replace a number of Internet access technologies with hybrid fiber and wireless networks connecting subscriber homes. Although not truly a mobile system, it could provide competition to existing Wi-Fi systems that provide this type of fixed wireless access.
Outdoor urban/suburban small cells. An expected deployment scenario for 5G mmWave would be to provide increased capacity in high-demand public spaces and venues. With cell sizes around 100m, small 5G mmWave access points can be placed on poles or buildings to provide the required coverage.
Mission-critical control applications. Autonomous vehicles, vehicle-to-vehicle communications, drone communications, and other latency-sensitive, high-reliability applications provide other possible deployment scenarios for 5G mmWave with a projected network latency of less than a millisecond.
Indoor hotspot cells. Shopping malls, offices, and other indoor areas require a high-density of 5G mmWave micro cells. These small cells will potentially support download speeds of up to 20 Gbps, providing seamless access to cloud data and the ability to support multiple applications, as well as various forms of entertainment and multimedia.
Internet of Things. The general connectivity of objects, sensors, appliances and other devices for data collection, control, and analysis. Potentially could cover smart home applications, security, energy management, logistics and tracking, healthcare, and a multitude of other industrial operations.
Continued technology evolution is rapidly expanding 5G’s reach, and mmWave brings important benefits to a wide range of new use case opportunities. For instance, Release 16 delivers enhanced capabilities in mmWave and sub-7 GHz to meet more stringent latency and reliability requirements (i.e., up to 99.9999%), allowing 5G to address high-performance industrial IoT use cases such as factory automation.
In addition, mmWave’s ultra-wide bandwidth can enable high-precision positioning, which is beneficial for industrial and broader vertical use cases. For the lower complexity IoT, 5G NR-Light (aka Reduced Capability), defined in Release 17, will expand the reach of mmWave to lower-tier devices (e.g., wearables and industrial sensors) using narrower bandwidth operations (e.g., 50/100 MHz). Also in Release 17 is a Study Item on extended reality (XR), including virtual reality and augmented reality, that will further optimize 5G networks for XR experiences.
5G mmWave technology
Even though large antenna arrays can be leveraged to provide beamforming gain to recover some of the losses, the resultant transmission range in mmWave networks is still significantly less that of the sub-6 GHz networks. As a result, given a coverage area where mmWave coverage is desired, higher density of mmWave cells would be expected in that area compared to the density of sub-6 GHz cells if used to provide coverage for the same area.
The mmWave cell sizes will, therefore, be smaller and higher in density. With a higher cell
density, mmWave band can deliver on the higher data rates and capacity promise while providing
the needed coverage.
Higher mmWave densification also comes with certain challenges, such as co-channel interference, increased network mobility and higher deployment cost. The use of highly directional antennas typically used in the mmWave networks, the co-channel interference is significantly mitigated.
This creates a high infrastructure cost, as a mmWave network would require densely populated base stations throughout a geographic area to ensure uninterrupted connectivity.
The smaller cell sizes of 5G mmWave not only provides high throughput, but also allows for efficient use of spectrum as frequencies can be reused over relatively small distances. It is projected that outdoor cell sizes will be typically 100m to 200m and indoor high-density deployments might be as small as 10m. An important part of 5G mmWave performance is therefore dependent on line-of-sight (LOS) and non-line-of-sight (NLOS) propagation of signals and antenna design.
The high penetration losses and blocking mean that mmWave deployments will cover outdoor or indoor environments, but not provide outdoor to indoor connectivity.
To address the higher cost of mmWave deployments, most mmWave operators in the early phase of their deployment efforts have co-sited mmWave base stations with existing sub-6 GHz base stations, and later increase the mmWave densification by adding more mmWave base stations in areas with coverage holes. In this way, the existing base station sites and backhaul infrastructures can be leveraged, reducing the mmWave deployment cost.
5G mmWave uses Massive MIMO Antennas
mmWave frequency signals with extremely short wavelengths create opportunities to design
antenna arrays that are several times greater than the wavelength itself, while remaining physically small. This brings about critical advantages to such antenna systems where cost reduction is realized through inexpensive fabrication technology, and manufacturing scale is achieved from compact integration.
Massive MIMO ( Multiple Input Multiple Output ) is the new wireless access technology in 5G, in both sub-6 GHz and mmWave bands. The main idea is to use multiple antennas at a transmitter and receiver to improve the performance of wireless communication systems. Massive MIMO ( or very large ), is an extension of MIMO and uses more than 100 antennas. Whereas MIMO antennas for under 6 GHz wireless may support eight elements, at mmWave frequencies the number of massive MIMO elements might be 128, 256, or higher.
These “phased arrays” perform the beam-forming, beam-steering, and beam-tracking techniques that enable a 5G mmWave network to deliver such high capacity and efficiency. A massive MIMO technique can increase 10 times or more channel capacity and improve 100 times or more energy efficiency.
The computational and switching capacity available enables “massive multiple-input-multiple-output (massive MIMO)” antennas to create highly directional beams that focus transmitted energy in ways that can overcome path losses and NLOS conditions. The resulting antenna arrays can achieve higher gains of up to 29 dBi (with a total of 256 cross-polarized elements) to help overcome increased propagation and radio hardware impairment losses. Their adaptive capabilities enhance performance as well.
Fortunately, great advancements have been made in RF silicon that allow a large number RF chains to be supported in large antenna arrays. The use of beam-steering and beam-tracking techniques leverages massive MIMO antenna arrays to address this issue by creating highly directional beams where the transmitted energy is focused to improve system performance on both uplink and downlink.
Usage of a larger number of antenna elements and RF chains into cost-effective phased array RFICs is considered to be a key mechanism to address this challenge. There has been significant progress in the miniaturization of phase-array antenna systems for low-power, cost effective 5G mmWave devices.
A fundamental characteristic of mmWave, the short wavelengths, means that even massive MIMO antennas can be relatively compact and small effective antennas can be easily integrated into user devices. . An array with an equal number of half-wavelength total length cross-polarized dipole elements stacked vertically and horizontally, and with half λ separation, would be almost twelve times less in either length or height when constructed for 28 GHz frequency as compared to one for 2.4 GHz.
OFDM/OFDMA
Underlying the basic 5G mmWave technology is a new air interface based on time-division duplexing and robust orthogonal frequency division multiplexing (OFDM) methods similar to those as used in LTE and Wi-Fi networks. One of the key technical characteristics of OFDM/OFDMA is its orthogonality between adjacent subcarriers. This orthogonality comes from the choice that the subcarrier spacing (SCS) and symbol duration (ds ) satisfy the condition that SCS = m/ds where m is a positive integer (1, 2. …). The specific values for SCS and ds for an OFDM-based technology such as 5G NR are then based upon other considerations such as multipath, Doppler and latency.
The novelty of 5G is the integration of multiple networks serving diverse sectors, domains and applications, such as multimedia, virtual reality (VR) and augmented reality (AR), machine to machine (M2M) and internet of things (IoT), automotive applications, smart city, etc. The
diversity of the 5G applications and their related service requirements in terms of data rate, latency, reliability, and other parameters leads to the necessity for operators to provide a diverse set of 5G networks.
A key ingredient for 5G is to enable applications in mmWave spectrum is mobile edge computing
(MEC), which is expected to bring information and processing closer to the mobile users and enable ultra-high speed and low latency communications
Fast adaptation to changing channel conditions will enable switching within and across cells to maintain performance and coverage. In addition, there will almost certainly be a key role for Software-defined networking (SDN) and network functions virtualization (NFV) in how networks operate and provide seamless connectivity for users.
Integrated access and backhaul (IAB)
One key challenge to broadly expand 5G NR mmWave network coverage is the cost of deploying additional mmWave base stations, which usually requires new fiber optics backhaul installations. To make mmWave densification more cost-efficient, Release 16 introduces integrated access and backhaul that allows a base station to provide both wireless access for devices and wireless backhaul connectivity via neighboring base stations using the same mmWave spectrum. This eliminated the need for wired backhaul opening doors to a more flexible densification strategy, allowing operators to quickly add new base stations dynamically, before having to install additional fibers to increase backhaul capacity.
Release 16 established foundational IAB capabilities, such as dynamic topology adaptation for load balancing and blockage mitigation, and Release 17+ will further enhance IAB by bringing new features like full-duplex operation, topology redundancy, and ML-based network management.
Enhanced beam management for better mmWave performance and robustness
5G mmWave beam management is an area of research that will continue to take place for the foreseeable future. Release 16+ beam management will bring enhanced reliability and performance, especially for mobility use cases. For instance, Release 16 added support for multiple antenna panels (i.e., multiple transmission and reception points — multi-TRP) that improves throughput and diversity. More flexible beam selection (e.g., decoupling of uplink and downlink) can also optimize performance in both directions. In addition, multi-beam repetitions and enhanced beam failure recovery procedures further improve mmWave system robustness. Release 17, which is currently in progress, will further reduce beam management signaling overhead and support expanded beam selection to bring improved intra- and inter-cell mobility.
New power saving techniques for longer battery life
While 5G mmWave unlocks the ultra-wide bandwidth opportunity (i.e., delivering multi-Gbps at much lower cost-per-bit), mmWave-enabled devices face additional power and thermal performance challenges as they operate at much wider bandwidths (e.g., 400/800 MHz) than traditional cellular systems. Release 16 and 17 introduce a broad range of power-saving features that can further improve device power efficiency, such as reduced signaling overhead, faster link feedback, and maximized device sleep duration.
References and Resources also include:
https://www.rcrwireless.com/20210204/5g/whats-in-the-future-of-5g-millimeter-wave
