1

Millimeter Wave MIMO Prototype:
Measurements and Experimental Results
arXiv:1710.09449v1 [] 25 Oct 2017

Vasanthan Raghavan, Andrzej Partyka, Ashwin Sampath, Sundar Subramanian,
Ozge Hizir Koymen, Kobi Ravid, Juergen Cezanne, Kiran Mukkavilli, Junyi Li

Abstract
Millimeter-wave multi-input multi-output (mm-Wave MIMO) systems are one of the candidate schemes
for 5G wireless standardization efforts. In this context, the main contributions of this article are threefold. 1) We describe parallel sets of measurements at identical transmit-receive location pairs with
2.9, 29 and 61 GHz carrier frequencies in indoor office, shopping mall, and outdoor settings. These
measurements provide insights on propagation, blockage and material penetration losses, and the key
elements necessary in system design to make mm-Wave systems viable in practice. 2) One of these
elements is hybrid beamforming necessary for better link margins by reaping the array gain with large
antenna dimensions. From the class of fully-flexible hybrid beamformers, we describe a robust class
of directional beamformers towards meeting the high data-rate requirements of mm-Wave systems. 3)
Leveraging these design insights, we then describe an experimental prototype system at 28 GHz that
realizes high data-rates on both the downlink and uplink and robustly maintains these rates in outdoor
and indoor mobility scenarios. In addition to maintaining large signal constellation sizes in spite of
radio frequency challenges, this prototype leverages the directional nature of the mm-Wave channel to
perform seamless beam switching and handover across mm-Wave base-stations thereby overcoming the
path losses in non-line-of-sight links and blockages encountered at mm-Wave frequencies.

Index Terms
Millimeter-wave, experimental prototype, MIMO, channel measurements, beamforming, handover, RF.

I. I NTRODUCTION
Millimeter-wave multi-input multi-output (mm-Wave MIMO) systems are one of the candidates for the physical layer (PHY) in the currently ongoing standardization efforts for the Fifth
The authors are with Qualcomm Corporate R&D, Bridgewater, NJ 08807, USA and Qualcomm Corporate R&D, San Diego,
CA 92121, USA.

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Generation (5G) air link specifications. Over the last few years, there has been an exploding
interest on mm-Wave systems (see, e.g., [1]–[3] and references therein). Most of these works
either present expectations from mm-Wave systems, or use-case analysis, or channel measurements, or performance studies assuming certain PHY abstractions. With this backdrop, the focus
of this paper is on understanding the implementation of mm-Wave systems in practice and to
present a complete picture starting from channel measurements to system design implications to
prototype performance.
Towards this goal, we first study electromagnetic propagation in the mm-Wave regime using
a number of parallel measurements at the same transmit-receive location pairs in different usecases (indoor office, shopping mall and outdoor) at 2.9, 29 and 61 GHz. Such a parallel set of
measurements minimizes the number of confounding factors and allows a direct comparison of
propagation across different carrier frequencies. A limited number of such measurement studies
at the same location pairs are available in the literature. While our studies show that losses at
mm-Wave frequencies are typically higher than with sub-6 GHz systems, these losses are not
substantially worse. Nevertheless, additional losses due to hand/human blockages and material
penetration can be significantly detrimental to the link margins and are expected to play the role
of a serious differentiator for a mm-Wave chipset solution.
The above observations motivate the use of beamforming to overcome these losses. We then
briefly describe the radio frequency (RF) component challenges that impact the design of a
practical mm-Wave chipset. The use of near-optimal beamforming structures needed to improve
the link margin in both single- and multi-user contexts in a practical mm-Wave chipset is
not viable due to cost and complexity challenges of radio frequency (RF) components. Thus,
secondly, to overcome these constraints, we are motivated to use a certain subset of directional
beamformers for MIMO transmissions. In addition to low complexity, the proposed approaches
also enjoy advantages such as robustness to phase changes across paths (an issue of immense
importance for small wavelength systems such as mm-Wave) and a simpler system design for
initial user equipment (UE) discovery and subsequent beam refinement.
With this background, thirdly, we describe our prototype system operating at 28 GHz that
realizes a robust directional beamforming solution leading to high data-rates on both the downlink
and uplink. We describe various experiments performed with the prototype in both outdoor and
indoor scenarios and provide unique insights into the operations of a practical mm-Wave system.

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The key elements tested by these experiments include robustness of mm-Wave links in nonline-of-sight (NLOS) settings via beam switching in response to mobility and blockage, interbase-station handover, and interference management. Comparable prototypes in the literature
such as [4] mostly emphasize peak throughputs in line-of-sight (LOS) settings and do not
provide lessons applicable for practical deployments. Other prototypes of importance in practical
deployments include the CAP-MIMO architecture [5] and [6]–[8] that apply lens array techniques
for steering multiple beams from the base-station.
II. M ILLIMETER -WAVE C HANNEL M EASUREMENTS AND S YSTEM I MPLICATIONS
The focus of this section is on reporting mm-Wave channel measurements at 2.9, 29 and
61 GHz, which are representative of the three most-likely commercial offerings in the 2018-20
time-frame and likely to be compared against each other in terms of performance: sub-6 GHz
5G-NR, mm-Wave MIMO 5G-NR, and 802.11ad/ax/ay. While a number of mm-Wave channel
measurement campaigns have been reported in the literature, the novelty of this work is on
channel propagation comparisons across these three carrier frequencies at identical transmitreceive location pairs in different use-cases. Such studies are important as they eliminate most
confounding factors that prevent a direct comparison across frequencies.
Towards this goal, channel sounding is performed with both omni-directional antennas as well
as directional horn antennas. For directional measurements, an azimuthal scan (or a 360o view)
with a 10 dBi gain horn antenna and producing 39 directional slices, and a spherical scan (360o
azimuth view and −30o to 90o view in elevation) with a 20 dBi gain horn antenna and producing
331 directional slices are generated. The time-resolution of the channel sounder is approximately
5 ns. An Agilent E8267D signal generator is used to generate a pseudo-noise (PN) sequence at
a chip rate of 100 Mc/s, which is then used to sound the channel. At the receiver, an Agilent
N9030A signal analyzer is used for acquisition and the PN chip sequence is despread using a
sampler at 200 MHz and with 16 bit resolution.
These sounding measurements are obtained for the indoor office setting (two floors of the
Qualcomm building in Bridgewater, NJ), indoor shopping mall setting (Bridgewater Commons
Mall, Bridgewater, NJ), outdoor settings (open areas outside the Qualcomm building), including
the suburban setting (residential location in Bedminster, NJ) and the Urban Micro setting (New
Brunswick, NJ), etc., as well as emulation of stadium deployments. These measurement scenarios

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are representative of typical applications considered for future deployment efforts.
While macroscopic channel properties such as path loss are studied with omni-directional
scans, other properties such as delay spread, path diversity, etc., are studied with both omnidirectional and directional scans. Processing of these measurements lead to the following observations and implications on system design for mm-Wave channels. More technical details on
these studies can be found in [9].
Path Loss: Measurements in different deployments are used to study macroscopic properties of
LOS and NLOS links. We use a frequency-dependent path loss model with a close-in free space
reference distance of d0 = 1 m where the path loss (in dB) at a distance of d m is modeled as
PL(d) = PL(d0 ) + α · 10 log10 (d/d0 ) + X,

X ∼ N (0, σX2 ).

(1)

The path loss exponents (PLEs), denoted as α, and the shadowing factors (σX ) for different
types of links (LOS/NLOS) in different use-cases are learned with a least-squares fitting of the
model in (1) to the measured data. These parameters are listed in Table I and they show that
PLEs and shadowing factors for NLOS links generally increase with frequency. For LOS links,
PLEs are generally smaller than those for NLOS links and in indoor settings can be smaller
than the freespace PLE of 2. A plausible explanation for this observation is waveguide effect
where long enclosures such as walkways/corridors, dropped/false ceilings, etc., tend to propagate
electromagnetic energy via alternate modes/more reflective paths decreasing the PLE. Shadowing
factors show inconsistent behavior with frequency. From Table I, we conclude that while mmWave systems experience higher path losses than sub-6 GHz systems, the differential impact of
the PLEs and shadowing factors on link margin at higher carrier frequencies is not dramatic.
Delay Spread: The delay spread of the channel is an important metric to understand the
system overhead (in terms of the cyclic prefix length for a multi-carrier design). In this context,
frequency-dependent delay spreads are observed in NLOS settings both with omni-directional
and directional antennas. While omni-directional delay spreads are small in most scenarios (for
example, the essential spread is on the order of 30-50 ns in indoor office, 50-90 ns in indoor
shopping mall and 150-300 ns in outdoor street canyon settings), there are also scenarios where a
significantly large delay spread is seen (e.g., even up to 800 ns in outdoor open square settings).
These extreme scenarios can be explained with the radar cross-section effect, where seemingly
small objects that do not participate in electromagnetic propagation at lower frequencies show

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TABLE I
PATH L OSS M ODEL PARAMETERS IN D IFFERENT U SE - CASES

Parameter ↓
fc (in GHz) →

LOS
2.9

29

NLOS
61

2.9

LOS

29

61

2.9

Indoor office

29

NLOS
61

2.9

29

61

Indoor shopping mall

PLE (α)

1.62

1.46

1.59

3.08

3.46

4.17

1.93

1.98

2.05

2.61

2.76

2.98

σX (in dB)

5.49

4.25

4.81

6.60

8.31

13.83

5.32

3.56

4.29

9.08

9.47

12.86

Urban Micro street canyon

Outdoor open areas

PLE (α)

2.18

2.19

2.22

2.95

3.07

3.27

2.41

2.73

2.83

3.01

3.39

3.42

σX (in dB)

4.41

4.37

4.84

7.82

8.16

10.70

4.60

5.73

6.78

4.00

8.03

1.97

up at higher frequencies. Such behavior happens as the wavelength approaches the roughness
of surfaces (e.g., walls, light poles, etc.). Supporting these extremes without incurring a high
fixed system overhead is important. In most indoor scenarios, sparse scattering implies that the
beamformed delay spread is comparable with the omni-directional delay spread. While the same
trend holds for most scenarios in the outdoor setting, the beamformed delay spread can be
significantly smaller than the omni-directional delay spread for the tail values.
Blockage: An important feature that makes mm-Wave propagation significantly different from
propagation at sub-6 GHz frequencies is that a large area of the UE can be easily covered
and blocked by parts of the human body, other humans, vehicles, foliage, etc. Additional link
impairments due to these blockers (not seen at sub-6 GHz) are observed at mm-Wave frequencies
and the practical viability of mm-Wave systems are more dependent on blockage than the path
losses reported in Table I. For example, a typical UE design with multiple linear subarray units of
four antenna elements (on the Top and Long edges of the device) is presented for the Landscape
mode in Fig. 1(a). Corresponding to this UE antenna design, Figs. 1(b)-(d) show the received
gain in azimuth and elevation in Freespace, and with hand blocking in the Landscape and Portrait
modes, respectively. While almost the entire sphere is covered around the UE in the Freespace
mode, the presence of hand leads to an angular blockage region of 160o × 75o (blue areas) in the
Landscape mode. The region in blue stretches from behind the palm to the thumb. In this setting,
the Top edge subarray is not useful and the Long edge subarray allows good signal reception.

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Fig. 1.

(a)

(c)

(b)

(d)

(a) A typical UE design with multiple subarrays (Top and Long edges) in Landscape mode. Received gain as a function

of azimuth and elevation angles for the UE design at 28 GHz in (b) Freespace mode, and with hand blocking in (c) Landscape
and (d) Portrait modes.

Furthermore, in the Portrait mode, a blockage region of 120o × 80o (blue areas) is seen and the
Long edge subarray does not play an important role in signal reception as it is blocked with
the fingers resulting in significantly deteriorated antenna efficiencies. However, the Top edge
subarray is not affected much with the presence of the hand. These observations suggest that
subarray diversity in UE design is critical in overcoming near-field obstructions as well as to
ensure coverage at the UE side over the entire sphere.
Penetration Loss: For the outdoor-to-indoor coverage scenario, material measurements show
that penetration loss generally increases with frequency. Further, periodic notches that are several
GHz wide and often with more than 30 dB in loss are seen. These losses are attributed to
changing material properties with frequency due to which signals constructively/destructively
interfere from different surfaces that make the material. While a similar trend is observed across
these experiments for both polarizations and different choices of incidence angles, the precise
loss at a frequency and the depth of the notches depend on the material, incidence angle and

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polarization. This observation motivates the need for designs that support both frequency and
spatial diversity.
Path Diversity: Low pre-beamforming signal-to-noise ratios (SNRs) are typically the norm when
mm-Wave path losses and additional blockage/penetration losses are incorporated with typical
equivalent isotropically radiated power (EIRP) constraints. Thus, a viable system design has to
overcome these huge losses with beamforming array gains from the packing of a large number
of antennas within the same array aperture [3]–[5], [10]–[13]. In this context, a small number
of (at most 4-6) well-spread out (in direction) clusters/paths rendering multi-mode/multi-layer
signaling viable are typically observed. The viability of multiple modes suggests the use of both
single-user MIMO strategies for increasing the peak rate as well as multi-user MIMO strategies
for increasing the sum-rate [11], [14]. Such modes also offer robustness against blockages via
intra-base-station beam switching. In addition to the likelihood of multiple viable paths to a
certain base-station, there are also viable paths to multiple base-stations. These observations
suggest the criticality of a dense deployment of base-stations for robust mm-Wave operation and
inter-base-station handover to leverage these paths. Integrated access and backhaul operation is
highly desirable for small cell deployment, which also leads to the need to study inter-base-station
interference management issues more carefully.
III. E XPERIMENTAL P ROTOTYPE D ESCRIPTION
Motivated by the system design intuition developed in the previous section, we now describe
our experimental prototype system operating in a time-division duplexing framework at 28 GHz.
In this setup, baseband analog in-phase and quadrature (IQ) signals are routed to/from the modem
to an IQ modulator/demodulator at 2.75 GHz center frequency. The 2.75 GHz intermediate
frequency signal is translated to 28 GHz using a 25.25 GHz tunable local oscillator (with a 100
MHz step size). The bandwidth supported is 240 MHz at a sampling rate of 240 Msps. ADCs
with an effective number of bits (ENOB) resolution of 8 bits are used at both ends. At the basestation end, the 28 GHz signal is routed to an 16 × 8 element planar array (a waveguide design)
and analog beamforming is applied using tunable four bit phase shifters and gain controllers.
The prototype uses a transmit power dynamic range of 19 dB with a maximum EIRP of 55 dBm.
As motivated earlier, to overcome blockage, the UE end is made of four selectable subarrays,
each a four element phased array of either dipoles or patches as in Fig. 1(a).

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With beamforming being a central component in meeting the mm-Wave link budget, RF
component and architecture-driven challenges (e.g., cost, power, complexity, form factor, regulatory constraints, etc.) play a principal role in determining practically viable hybrid beamforming
solutions. In this context, the sparse and directional channel structure suggests the use of a certain
subset of directional beamforming strategies along the dominant clusters/paths at both ends [5],
[12]–[14] relative to optimal beamforming along the dominant eigen-modes/singular vectors of
the channel matrix. Directional beamforming structures offer robustness to small perturbations
in the channel matrix and also allow a tradeoff between peak beamforming gain and initial UE
discovery latency (with minimal loss relative to the optimal schemes) via the construction of a
hierarchy of directional codebooks [13]. The directional channel structure can be leveraged for
scheduling and can also be generalized to multi-user beamforming design with the following
solutions: i) beam steering to each UE (with complete agnosticism of the interference caused
to other users), ii) zeroforcing (where each user’s beamforming vector steers a beam null to the
other users), and iii) generalized eigenvector precoding (that performs a weighted combination
of beam steering and beam nulling). These solutions can result in substantial performance
improvement over single-user solutions. The readers are referred to [14] for technical details
on these constructions as well as performance studies in outdoor and indoor deployments.
Motivated by the robustness of directional beamformers, this solution is implemented in the
prototype by leveraging the beam broadening principles described in [13, Sec. IVB] to construct
static analog beam codebooks to be used at both the base-station and UE ends. The experimental
system implements mm-Wave beamforming by initially determining the best beam direction to be
used at either end. After this, the system continuously evaluates all possible beam directions from
all available transmitters and switches to the best beam and the best transmitter (handover) with
little to no performance degradation. In addition, the system adjusts its parameters to optimize
for the link type (LOS/NLOS) by SNR control allowing upto 64-QAM operation.
The seamless beam switching and capability to maintain high SNR enable the experimental
system to realize high rates (on both the downlink and uplink) and robustly maintain these rates
despite channel variations. That said, the main focus behind the experimental system is not the
optimization of data-rates, but to study the various fundamental difficulties in realizing mm-Wave
systems in practice, especially with NLOS links. In the next two sections, we describe some
experimental results illustrating the versatility of the prototype.

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(a)
Fig. 2.

(b)

(a) Elevated gNB and UE inside the testing vehicle used in outdoor testing. (b) Aerial layout of the testing range

including gNB locations, achieved rates and important features in rates as the UE is driven over the trajectory.

IV. O UTDOOR M OBILITY S TUDIES
An outdoor mobility testing experiment (see Fig. 2(a)) is conducted in the parking spaces
adjacent to the Qualcomm building (see aerial layout in Fig. 2(b)). One base-station is mounted
on a mast elevated 14 feet in a testing vehicle and located in the parking lot (marked gNB1 ),
and another base-station is mounted in the sixth floor of the Qualcomm building facing the
window and elevated 5 feet from the ground (marked gNB2 ). gNB1 and gNB2 have a 90o and
110o downtilt, respectively. The UE is mounted on the dashboard of another testing vehicle and
testing is done by driving through the parking spaces at 10-15 mph speeds. Fig. 2(b) plots the
achieved throughput (in Mbps) as a function of the driving trajectory. From this plot, we note that
a high throughput close to 600 Mbps is realized (Scenario 1) when gNB2 has an unobstructed
LOS path to the UE. As the UE moves over the trajectory (Scenario 2), seamless handover is
realized between gNB2 and gNB1 . Further, as the UE is driven on this trajectory, the LOS path
from gNB1 is obstructed and communication is realized through a reflected path first (Scenario
3) and other paths subsequently (Scenario 4) leading to a drop in throughput (gradings in the
heat map). This experiment illustrates the prototype’s capability to maintain mm-Wave links
robustly in outdoor scenarios with intra- and inter-base-station beam switching and handover.

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Fig. 3.

Left: Outdoor aerial layout of experiment including key geographical features. Right: Achieved rates as a function of

outdoor trajectory and time as well as beam indices at gNB and subarray indices at UE.

In a second study illustrated in Fig. 3, an outdoor mobility experiment around the Qualcomm
building is conducted. This environment is mostly a tree-lined open square-type setting with
some street canyon-type features. Specific points-of-interest include parking lots and structures
with bordering buildings having glass window panes, foliage (a mix of pine and spruce trees),
a large shopping mall in close vicinity (Bridgewater Commons Mall), highways (US Rt. 202),
etc. For the specific experiment reported here, a testing vehicle is driven for a period of ≈ 55
seconds through the exit lane of US Rt. 202 at a speed of 20-30 mph, onto the ramp and into a
side street enveloping the Qualcomm building (see trajectory in red in Fig. 3, Left side). In terms
of notable observations from this experiment, a base-station mounted on a raised platform at 24
feet with a 90o downtilt (marked gNB1 ) offers a LOS path to the UE as it starts exiting from Rt.
202 (throughput of 375 Mbps). However, as the UE traverses the exit lane, a small hillock-like
feature blocks the LOS path leading to a coverage hole that cannot bridged with any reasonable
NLOS path from this base-station and a significant deterioration in rate. This observation points
at the necessity of sufficient base-station density to enhance mm-Wave coverage under blockages.
For example, a base-station on the opposite side of the highway (Rt. 202) could have provided
coverage to the UE over this coverage hole. As the UE crosses this feature, a beam recovery
process recovers the LOS path albeit with a different subarray offering complementary coverage

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in the LOS direction leading to an improved throughput of 375 Mbps. Further, as the testing
vehicle enters the ramp, blockage loss due to foliage results in a throughput drop (125-250
Mbps) and two subarrays turn out to be useful over this period. As the UE exits the ramp onto
the adjoining street, the LOS path is recovered leading to a throughput of over 375 Mbps. The
distance between the gNB and UE varies from 50-100 m over the whole experiment.

(a)

(b)

(c)
Fig. 4.

(a) Typical UE testing with pedestrian mobility in an indoor scenario. (b) Indoor layout and building plan along with

two gNB locations and coverage areas. (c) Achieved rate along with key features in the rate trajectory over a certain indoor
segment.

V. I NDOOR M OBILITY S TUDIES
Complementary to the above discussion, an indoor mobility study (see Fig. 4(a)) in the third
floor of the Qualcomm building (see building layout in Fig. 4(b)) is now described. The floor
plan is mostly comprised of cubicles along the edge with walled offices and conference rooms
towards the center. Two base-stations (marked gNB1 and gNB2 ) are placed at the far corners of
the floor plan and the UE is moved at pedestrian speeds through the layout. From our studies,
these two base-stations are sufficient to guarantee adequate coverage with at least 1 bps/Hz

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spectral efficiency as the UE is traversed through the floor plan (coverage areas with each gNB
marked in red and blue of Fig. 4(b), respectively). Nevertheless, the coverage area corresponding
to each gNB does not lead to a well-defined cell boundary and is clearly dependent on the
environment, material properties, etc. This observation points to necessity of further system
coverage studies with irregular cell boundaries and base-station density to overcome coverage
holes in such scenarios. As a particular illustration of this study, the throughput achieved with
an ≈ 90 second trajectory is illustrated in Fig. 4(c) which illustrates both link drops due to
penetration loss through obstructions (concrete, elevator area, wall, metallic material, etc.) and
link variability due to changing material properties. Such link drops can be mitigated with
enhanced beamforming, fast subarray switching, network densification, etc.

(a)
Fig. 5.

(b)

(a) A successful handover from eNB1 to eNB2 in an indoor setup. (b) Layout of another indoor coverage experiment

with downlink/uplink rates using the proposed beamforming solutions.

More indoor mobility experiments can be seen in the video demonstration at [15]. In one
experiment, illustrated in Fig. 5(a) and seen in [15], a LOS link is initially established between
the transmitter (labeled eNB1 ) and receiver (labeled UE) by beam scanning at both ends. With a
high SNR from the LOS link, a high rate is established on both the downlink and uplink. As the
UE is moved across the long edge of the hallway (see relative positions of eNB1 , eNB2 and UE
in the bottom left inset of Fig. 5(a)), the link connecting eNB1 with the UE becomes NLOS with
increasing path loss as the distance between eNB1 and UE increases. As the UE turns the corner

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at the short edge, the NLOS1 link between eNB2 and UE becomes better than the NLOS link
between eNB1 and UE and a successful handover (illustrating robustness to blockage) happens
from eNB1 to eNB2 , as shown in Fig. 5(a).
Figure 5(b) illustrates the layout of yet another indoor experiment where the UE is moved from
an initial position (marked “1” in a white circle) towards its final destination (marked “1” in an
orange square) via the dashed white-line trajectory. As the UE is moved over the trajectory, the
achieved downlink and uplink rates show many disruptions as the connected path is blocked by
the pillars in the layout (marked as gray squares). For example, when the pillar blocks the LOS
path, connection is established to the dominant NLOS path (again through reflections) leading
to the first disruption in rate(s). Connection is re-established to the LOS path as the UE moves
past the pillar until the next pillar is reached where the second disruption happens. The third
disruption corresponds to the switch from the re-established LOS path to the dominant NLOS
path as the UE turns the corner. Thus, these examples illustrate the robustness of our proposed
beamforming solutions to blockages in real deployments.
VI. C ONCLUDING R EMARKS
A. Summary
This article provides a brief overview of mm-Wave channel measurements and what the
implications of these measurements are for system design. An immediate consequence of the
blockage and penetration losses inherent and specific to mm-Wave systems are the poor link
margins. These motivate the necessity to reap spatial array gains via the use of near-optimal
beamforming solutions over large antenna arrays. Prominent challenges in this goal include
the limited range and performance of mm-Wave components, as well as the robustness of the
beamforming solution to spatio-temporal channel variations and its impact on overall system
design. Further, cost considerations may allow only the use of a small number of RF chains
at either end and thus the beamforming solutions should be adaptive to changes in the RF
architecture(s). Towards this goal, directional beamforming approaches can be used as robust,
low-complexity, near-optimal solutions that help overcome the high propagation losses at mmWave frequencies. Such solutions are demonstrated with our experimental prototype, illustrating
1

Note that the LOS link between eNB2 and UE is blocked by a pillar in the layout, all marked in white squares.

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the viability of mm-Wave systems for high data-rate requirements. In particular, the prototype
system demonstrates: i) beamforming and beam scanning, ii) outdoor coverage and mobility,
iii) resilience to blockage of paths, iv) inter-base-station handover, v) indoor mobility, and vi)
interference management in both outdoor and indoor settings.
B. Future Research Directions
Important issues that require further study include: i) a more exhaustive study on realistic
channel modeling for mm-Wave propagation, ii) models for spatio-temporal channel variations,
iii) models for impairments such as hand/body/human blockages, phase noise, etc., iv) advanced
MIMO techniques for both single- and multi-user multi-carrier transmissions, v) impact of
mm-Wave channel properties on mm-Wave system/network design issues such as coverage
and network latency tradeoffs, mm-Wave handover, interworking with sub-6 GHz bands and
applications, integrated access-bachkaul solutions, vi) advanced MIMO RF architectures such
as [5]–[8] for prototype studies and real deployments, vii) RF tradeoffs in form factor UE
design, etc.
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