The 3dimensional (3D) channel model gives a better understanding of statistical characteristics for practical channels than the 2dimensional (2D) channel model, by taking the elevation domain into consideration. As different organizations and researchers have agreed to a standard 3D channel model, we attempt to measure the 3D channel and determine the parameters of the standard model. In this paper, we present the statistical propagation results of the 3D multipleinput and multipleoutput (MIMO) channel measurement campaign performed in China and New Zealand (NZ). The measurements are done for an outdoortoindoor (O2I) urban scenario. The dense indoor terminals at different floors in a building form a typical 3D propagation environment. The key parameters of the channel are estimated from the measured channel impulse response (CIR) using the spatialalternating generalized expectationmaximization (SAGE) algorithm. Till now there is abundant research performed on the azimuth domain; this paper mainly considers the statistical characteristics of the elevation domain. A statistical analysis of 3D MIMO channel results for both China and NZ measurements is presented for the following parameters: power delay profile (PDP), root mean square (rms), delay spread (DS), elevation angleofarrival (EAoA) distribution, elevation angleofdeparture (EAoD) distribution, elevation angular spread (AS), and crosspolarization discrimination (XPD).
The mobile traffic is experiencing an explosive growth due to the flourishing spread of mobile internet and smart phones. It is predicted that the mobile traffic will grow more than 1000 times in the next few years [
Conventional multipleinput and multipleoutput (MIMO) technology only utilizes the horizontal plane of the spatial domain in the spatial signal processing. Usually, a linear antenna array with vertical polarization or crosspolarization is deployed in the horizontal plane at both ends of the communication link, while a fixed downtilting angle is configured at the antenna array of the base station (BS). Although the signal propagates through a 3D space, the fading channel is still modeled as 2dimensional (2D) channel models. With the advent of the adaptive antenna array, the realization of 3D MIMO is more practical. Thus an extensive interest has been motivated to exploit the elevation domain in 3D MIMO system [
An early research on the elevation angle can be traced back to 1979 by Aulin, who extended Clark’s scattering model to 3D space for the first time [
In this paper, we present efforts to the study of the statistical characteristics of the 3D MIMO channel for outdoortoindoor (O2I) scenario. Using the same measurement equipment, measurements are repeated for two locations with very diverse surroundings in China and NZ, respectively. This helps to compare the similarities and differences between the results. We report on measurements of a 3D MIMO CIR and give results for the following channel parameters:
power delay profile (PDP),
distribution of the delay spread (DS),
distributions of elevation angleofarrival (EAoA) and elevation angleofdeparture (EAoD),
distribution of the angle spread (AS) in the elevation domain,
crosspolarization discrimination (XPD).
In Section
Measurements are done by using the Elektrobit Propsound Sounder described in [
To capture the propagation rays in the 3D environment efficiently, fully dimensional antenna arrays were equipped at both sides of the measurement link. The layout of the antenna arrays at Tx and Rx side are illustrated in Figures
Antenna configuration and parameters used in measurements.
Parameter  Value  

Antenna type  ODA  UPA 
Element number  56  32 
Element number used  16  16 
Polarized  ±45°  ±45° 
Distribution of antenna elements  Cylinder  Planar 
Angle range  
Azimuth  [−180°, 180°]  [−70°, 70°] 
Elevation  [−70°, 90°]  [−70°, 70°] 
Carrier  3.5 GHz (China), 2.35 GHz (NZ)  
Bandwidth  100 MHz (China), 10 MHz (NZ)  
Tx power  33 dBm (China), 26 dBm (NZ)  
PN sequence  63 (China), 31 (NZ) 
Antenna layouts used in the measurements.
UPA at Tx
ODA at Rx
Utilizing both the horizontal and vertical dimensions, 3D MIMO is particularly suitable for scenarios with vertical user location distributions. Transmissions from outdoor BSs to users located at different floors can be well separated in their elevation angles.
We carried out the field measurements for 3D CIR in Beijing, China, and Auckland, New Zealand, for the O2I scenario. In China measurement, the receive antenna array was positioned at 5 different floors of a 60 m high building, while the transmit antenna was fixed at the top of a nearby 11.5 m high building at a distance of 21.5 m. This typical O2I scenario is shown in Figure
A view of measurement areas in China (a) and NZ (b).
Measurement in China
Measurement in NZ
The NZ measurement, shown in Figure
The field measurements described above provided numerous snapshots of the impulse response of the timevarying radio channel. The collected channel impulse responses were fed to a highresolution algorithm to estimate the channel parameters for each snapshot. Maximum likelihood estimation (MLE) provides an optimum unbiased estimation from a statistical perspective; however, it is computationally prohibitive due to the multidimensional searches required. Thus a lowcomplexity approximation of MLE, spatialalternating generalized expectationmaximization (SAGE) algorithm [
complex polarization (vertical (
delays
azimuth angleofdeparture (AoD)
elevation AoD (EAoD)
Doppler frequency
A single link of the 3D fading channel, between the base station and the mobile user, is shown in Figure
Generic 3D fading channel model.
A similar expression is used to obtain spherical unit vector
The distribution results along with the 3GPP reference value are summarized in Table
MIMO channel model parameters.
Parameters  China  NZ  3GPP [ 

RMS delay, 


−7.06  −6.45  −6.62 

0.14  0.30  0.32 
Elevation AoD, 


1.28  1.13  — 

0.22  0.28  — 
Elevation AoA, 


1.39  1.56  1.01 

0.31  0.24  0.43 
Azimuth AoD, 


1.09  1.25  1.25 

0.14  0.16  0.42 
Azimuth AoA, 


1.76  1.92  1.76 

0.10  0.11  0.16 
Crosspolarization discrimination (XPD) 


4.5  5.2  9 

6.4  9.1  11 
Delay distribution  EXP  
Azimuth AoA and AoD distribution  Gaussian  
Elevation AoA and AoD distribution  Laplacian 
Power delay profiles (PDP), for both China and NZ measurements, are presented for an intuitive glance over multipaths, dynamic power range and power fading with delays, and so forth. With a bandwidth of being 100 MHz for China and 10 MHz for NZ measurement, the delay resolution is 10 ns and 100 ns, respectively. Based on the CIR exactly, the averaged PDP over all snapshots is shown in Figure
PDP and delay spread in China and NZ.
Average PDP of measurements in China and NZ
RMS delay spread in China and NZ
For each snapshot, we also calculate the delay spread and present the cumulative distribution function (CDF) of all rms DS in Figure
The EAoA and EAoD dispersion at different floors are presented for China measurement. Taking the 1st and 3rd floor for example, the dispersions of EAoAEAoD for the snapshot near the window (red star in Figure
EAoA and EAoD dispersion and 2D versus 3D clustering for the snapshot near the window (red star) in China measurement.
EAoA versus EAoD at Floor 1
EAoA vversus EAoD at Floor 3
2D clustering example
3D clustering example
As multipath components (MPCs) are dispersive for both 1st and 3rd floor in the 3D environment, an interesting question arises whether the cluster number will change compared to that in 2D channel? To identify clusters [
Cluster number increase from 2D to 3D.
Floor  1st  2nd  3rd  5th  7th 

CluNum 2D  10.2  8.4  9.3  9.5  12.2 
CluNum 3D  15.2  12.5  13.4  15.4  17.4 
The distribution of EAoD and EAoA for China and NZ measurements is shown in Figures
EAoA and EAoD distributions and angular spread.
EAoD and EAoA distribution in China measurement
EAoD and EAoA distribution in NZ measurement
EASD and EASA CDFs in NZ
The CDF of elevation angular spread for AoD (EASD) and AoA (EASA) is shown in Figure
Angular spread for different floors in China measurement.
Floor  EASD (°)  EASA (°)  AASD (°)  AASA (°) 

1st  20.8  27.4  20.4  45.5 
2nd  20.6  20.9  12.5  42.2 
3rd  19.5  22.3  19.3  43.3 
5th  20.8  22.1  18.0  40.9 
7th  26.0  28.3  17.9  43.1 
As XPD is being defined in Table
XPD measurements for China and NZ.
PDF of XPD in China
PDF of XPD in NZ
In this paper, we studied the statistical characteristics of 3D channel impulse response and presented comparative results for measurements performed in China and NZ. We have focused our study on the elevation domain as these measurements are rare. Simulation results show that the PDP in 3D propagation fades exponentially both in China and in NZ measurements. The cluster can be further resolved when the elevation domain is taken into consideration. A lognormal distribution fits well with the AS while the ESA in NZ is larger than that of China due to a larger scattering room size in NZ measurement. Because of a more dispersed distribution of measured locations in the elevation domain, the XPD in China is smaller than that of NZ and the XPD varies with the height difference between Tx and Rx, and so forth. All these results of the elevation domain provide a better understanding of 3D channel and contribute to 3D MIMO system design, including 3D antenna arrays for future 5G where the elevation domain cannot be ignored. Therefore, these measurements are a precursor to 5G systems capability studies.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The China measurements are supported in part by the 863 Program of China under Grant no. 2015AA01A703 and by National Science and Technology Major Project of the Ministry of Science and Technology (project name is “IMT2002 Candidate Band Analysis and Evaluation”) with 2015ZX03002008 and by National Science and Technology Major Project of the Ministry of Science and Technology (project name is “Wireless mobile spectrum research and verification for WRC15”) with 2014ZX03003013004 and by National Science and Technology Major Project of the Ministry of Science and Technology (project name is “Research and Development of MIMO Channel Emulator”) with 2013ZX03001020002 and by Huawei Company for Massive MIMO study.