Millimeter wave (also millimeter band) is the band of spectrum between 30 gigahertz (Ghz) and 300 GHz.
Millimeter wave, which is also known as extremely high
frequency (EHF) or very high frequency (VHF) by the International Telecommunications Union (ITU), can be used for high-speed wireless broadband communications.
Radio signals are measured by their wavelengths. The shorter
the wavelength, the higher the frequency. 5G signals will use wavelengths
(between 30 and 300 gigahertz) that are measured in millimeters. That's why 5G
is considered a millimeter wave technology
In NR, there are roughly two large frequency range specified
in 3GPP.
One is what we usually call (sub 6 GHz) and the other is what
we usually call millimeter wave.
Depending on the ranges, the maximum bandwidth and subcarrier
spacing varies. In sub 6 GHz, the maximum bandwidth is 100 MHz and in
millimeter wave range the maximum bandwidth is 400 MHz
Some subcarrier spacing (15, 30 KHz) which is used only in
Sub 6 Ghz and some subcarrier spacing (120 KHz) can be used in millimeter wave
range only, and some subcarrier spacing (60 KHz) can be used both in sub 6 GHz
and millimeter wave range.
As mentioned above, two types of frequency range is defined
in 3GPP. Sub 6 GHz range is called FR1 and millimeter wave range is called FR2.
The exact frequency range for FR1 (sub 6 GHz) and FR2 (millimeter wave) are defined as below.
< 38.101-1 Table 5.1-1: Definition of frequency ranges
>
Millimeter waves have
short wavelengths that range from 10 millimeters to 1 millimeter; they have
high atmospheric attenuation
(weakness) and
are absorbed by gases in the atmosphere, which reduces the range and strength
of the waves. Rain and humidity can impact performance and reduce signal strength,
a condition known rain fade. Due to its short
range of about a kilometer, millimeter wave travels by line of sight, so its
high-frequency wavelengths can be blocked by physical objects like buildings
and trees.
High frequencies have short range, and 5G may only be able to
span a few blocks of any given area. That means 5G may not project over long
distances. Smaller frequencies also don't penetrate obstacles very well, so
everything from concrete walls to tree leaves may disrupt signals. That makes
it a line-of-sight technology – your wireless modem or phone will need to be
close to a base station for best transfer speeds.
Table 5.3.3-1: Minimum GuardBand for each UE
channel bandwidth and SCS (kHz)
SCS (kHz)
|
5 MHz
|
10 MHz
|
15 MHz
|
20 MHz
|
25 MHz
|
30 MHz
|
40 MHz
|
50MHz
|
60 MHz
|
80 MHz
|
90 MHz
|
100 MHz
|
15
|
242.5
|
312.5
|
382.5
|
452.5
|
522.5
|
592.5
|
552.5
|
692.5
|
N/A
|
N/A
|
N/A
|
N/A
|
30
|
505
|
665
|
645
|
805
|
785
|
945
|
905
|
1045
|
825
|
925
|
885
|
845
|
60
|
N/A
|
1010
|
990
|
1330
|
1310
|
1290
|
1610
|
1570
|
1530
|
1450
|
1410
|
1370
|
Channel Bandwidth (CBW) is about [N_RB x NumOfSubcarrier
x SCS + GuardBand x 2]
Where N_RB = Maximum Transmission Bandwidth in 38.101-1 Table
5.3.2-1, 38.101-2 Table 5.3.2-1
Example >
In case of CBW = 60 MHz, SCS = 60 KHz, meaning the
second row in 38.101-1 Table 5.3.2-1
[N_RB x NumOfSubcarrier x SCS] = 79 x 12 x 60 = 56880 [kHz]
----- (a)
[GuardBand x 2] = 1530 x 2 = 3060
-------------------------------- (b)
(a) + (b) = 56880 + 3060 = 59940 kHz = 59.940 MHz which is
close to 60 MHz CBW
NOTE: GuardBand is defined as 'Minimum' value in the table,
so (a) + (b) may not be exactly same as CBW, but close enough to the CBW.
One possible approach to solve the blockage effect is through
a collection of non-line-of-sight (NLOS) communications. Although reflection
and diffraction reduce the range of mmWave LOS transmissions, it also
facilitates NLOS link communications. When a LOS link breakage happens, the
transmitter needs to
Quickly
search through different beam directions to bypass the obstacles such that the
receiver can collect some NLOS link signals to maintain the acceptable channel
quality. In other words, an adaptive beamforming prototype needs to be designed
to support the transformation from LOS links to NLOS links due to blockage
effect. However, the path loss of a NLOS link is much more severe than that of
a LOS link. We can look at an example of the Urban Micro (UMi) scenario [11]
defined in the IMT-Advanced radio interface, where the height of both the
antennas at the base station (BS) and those at the user devices are assumed to
be well below the tops of sur-rounding buildings, and all these antennas areas
summed to be outdoors in an area where streets are laid out in a Manhattan-like
grid.
The pathloss model for this UMi scenario is (2) where d is
the distance in meters and fc is the carrier frequency in GHz. The best NLOS
links can still be tens of dBs weaker than LOS signals.
Another possible approach to solving the blockage effect is
through a higher density infra-structure and/or relays [6]. The BSs and relays
are densely deployed in an outdoor urban area for mmWave communications, such
that when-ever a LOS beam blockage occurs and the collection of NLOS links does
not give satisfactory channel quality, the transmitter may steer the beam
direction to a target at a nearby BS or a relay which can have a LOS link with
the desti-nation.
Hence, the adaptive beamforming proto-type needs to be
designed to support the trans-formation from LOS links to NLOS links, and to
establish multihop LOS links to the destination. The two possible approaches
are illustrated in Fig. 3.
Other possible
approaches may include separation between control and data planes so that a
higher quality stringent control plan can be put into a reliable lower
frequency band, while a more bandwidth demanding data plane can be put into the
mmWave bands [12], and so on.
It should be noted
that the blockage issues in typical commercial deployment environments still
need extensive channel measurements and modeling at the targeted frequency
bands.
This is an important
area that requires further study. In addition, depending on the severeness and
the time varying nature of the channel blockage behavior, new multiple access
schemes and beamforming techniques may be required.
A possible solution is macro-assisted small cell, called the
phantom cell [12], or the booster cell in an anchor-booster architecture [15],
which establishes a new form of multicell cross-tier cooperation between macro
cells and small cells required for further network densification and spectrum
extension into higher frequency bands. This new radio access network
architecture is also referred to as the anchor-booster architecture.
It is a key enabling technology for efficient
utilization of high frequency bands through carrier aggregation techniques. In
such a system, lower frequency bands such as existing sub 3GHz cellular bands
are used in wide areas for macro cells to provide blanket Coverage and
mobility, while higher frequency bands such as mmWave bands are employed in the
small areas for small cells to provide high capacity. The control plane
(C-plane) and user data plane (U-plane) of phantom cells are separated: the
control information is sent by high-power nodes at lower frequencies, whereas
the payload data is conveyed by low-power nodes at mmWave frequencies [6]. The
phantom cell solution is illustrated in Fig. 5.
For this phantom cell approach with a C-plane/U-plane split, there are
no issues related to macro-to-small cell cross-tier interference. In the
meantime, the control signaling due to frequent handover between small cells
and macro cells, or among small cells, can be significantly reduced to enable
millimeter wave communications for 5G wireless systems.
Available from: https://www.researchgate.net/publication/273186522_Key_elements_to_enable_millimeter_wave_communications_for_5G_wireless_systems
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