The use of HAPs to deliver Internet connectivity on the ground has been discussed for almost two decades, with many research articles on various aspects of such a system, including the wireless link. Loon has developed the only stratospheric HAP that has provided connectivity to hundreds of thousands of people. To accomplish this, the Loon HAP is comprised of high-altitude balloons, floating between 17 and 21 km above the Earth, which beam standard compliant LTE signals to the ground. This is accomplished via an eNB that resides on the balloon which facilitates communication with standard UEs. For backhaul, the balloons are able to communicate over multiple balloon-to-balloon hops before landing the traffic on the ground with a balloon-to-ground hop, all using a proprietary high speed link. In this sense, one can consider the balloons as having formed a mesh network in the sky.
In prior work, channel models for HAP communications have been researched extensively (e.g., [
1‐
10]). In addition, chapter 10 in [
11] goes over recent work in stratospheric channel models. However, only a few papers (e.g., [
12‐
14]) discuss HAP-MIMO channel models and only a few ([
15,
16]) discuss the use of polarization to achieve diversity. For instance, [
13] discusses the advantages of having MIMO for HAP communication by evaluating the performance of downlink (DL) HAP channels. Through the use of simulation, this work shows that using 2 × 1 MIMO improves LTE performance by 1.4 to 12.3 dB and that using 2 × 2 MIMO can improve the performance by 7.7 to 15.7 dB. This research uses data from [
17] to model the channel as an elevation-dependent Ricean. The authors assume two independent, Rician faded channels in their simulation but it is unclear how this independence is achieved. Michailidis and Kanatas [
12] derive a three-dimensional, geometry-based, single bounce channel model for MIMO channels in Ricean fading environments. Using derived theoretical expressions, an evaluation of the HAP antenna inter-element spacing requirement for achieving uncorrelated responses in HAP MIMO channels is derived. However, using polarization to achieve the uncorrelated channels is not considered. In addition, [
9] measures building penetration loss (BPL) as a function of elevation angle and polarization. In order to simulate receive antennas with directional patterns, the authors place antennas in an orthogonal configuration (i.e., one antenna pointing vertically and another pointing horizontally). This configuration is used to measure the impact of polarization on BPL as a function of elevation. Figure 5 from [
9] shows a peak differential in BPL of 5 dB between vertical and horizontal polarization at 2 GHz with test receivers. However, typical UEs do not have nicely orthogonal antenna patterns below 1 GHz. Oestges [
10] discusses the effect of rain and ice depolarization on a HAP at 47 GHz, using dual-polarized antenna arrays. Dong et al. [
15] analyzes diversity performance from multiple HAP networks while also considering the single HAP use case. The authors show that due to the very close distance between the antennas in a single HAP, the use of traditional MIMO techniques cannot overcome large-scale fading. Due to the predominant line of sight (LOS) channel conditions in a HAP operating environment, propagation channels are highly correlated and most diversity techniques are not applicable. However, the authors also show that there may be exceptions such as using spatial diversity on the ground or using multiple HAPs. Also, using polarization to achieve diversity from HAPs is not explored. Michailidis et al. [
18] provides a mathematical model for polarization-based diversity from HAPs by calculating the XPD between orthogonal polarizations. Figure
2 from this paper is particularly interesting, as the computations show that XPD is expected to be low for an urban region (e.g., London) even when the HAP is directly overhead. This is somewhat counter intuitive, as a high XPD would be expected due to the strong LOS conditions. However, the analysis uses a Ricean K factor of 0 since it is for a dense urban area. This figure is not applicable to more rural areas, where the Ricean K factor is high. In addition, the authors assume an isotropic antenna pattern for the UE, which does not hold for typical UEs in real use. Nikolaidis et al. [
16] provide measurement data for XPD in LOS channels from airships using dual polarized antennas. Table
1 in this paper shows that an XPD of greater than 15 dB is expected for all conditions (e.g., LOS, non-LOS). Section III B, and the discussion around the Demmel condition number, leads to the conclusion that at high elevation angles, a large amount of multiplexing (MIMO) based transmission should be expected. However, using co-located HAP antennas at less than 1 GHz with real UEs, given their antenna limitations, has yet to be explored. Using polarization to provide diversity gain in terrestrial base stations has also been researched extensively (e.g., [
19‐
22]). For instance, [
19] demonstrates that polarization provides a means of realizing two independently fading signals with co-located antennas by relying on the ability of scatterers in the channel to depolarize and decorrelate the signals. In addition, [
21] demonstrates that polarization diversity is mostly preserved in LOS conditions for terrestrial applications. Finally, coexistence of transmissions from HAPs and terrestrial deployments using the same frequency has been researched extensively (e.g., [
23‐
28]). For instance, [
23] discusses coexistence of 3G in disaster scenarios, where some terrestrial towers are disabled due to an emergency. The terrestrial network is then overlapped by a HAP-based 3G network, and the impact of the HAP network to the terrestrial network is analyzed. Based on the parameters chosen, the authors are able to demonstrate that the simultaneous application of HAP and terrestrial networks impacts the terrestrial signal, particularly in suburban and urban macro cellular areas. Likitthanasate et al. [
24] discuss coexistence of WiMax at 5 GHz. The authors consider a single HAP with a single terrestrial base station located 10 km away from, but still within, the HAP coverage area. Based on the parameters chosen, the authors conclude that the HAP and terrestrial base station can coexist with low data rate modulation schemes. To get higher data rates, the authors expect that the UE antenna beamwidth would have to be narrow (e.g., less than 30
∘). A similar idea of exploiting antenna directionality in the UE is explored in [
25]. Here, the authors discuss coexistence among a constellation of HAPs, where interference from multiple HAPs is reduced by using a narrow antenna beamwidth at the UE. Park et al. and Park et al. [
26,
27] discuss coexistence of Code Division Multiple Access (CDMA) in terrestrial and HAP deployments. In this research, the minimum distance between terrestrial CDMA coverage regions and HAP CDMA coverage regions is computed to be between 2.5 and 9 km. However, coexistence for LTE at less than 1 GHz, with large HAP coverage, and omni-directional UE antennas has not been explored. While existing research has been done in many areas applicable to the Loon use case, many design challenges require more research. This paper intends to build on the existing research by measuring the impact of polarization diversity using co-located HAP antenna bands below 1 GHz, demonstrating why the HAP channel model in this use case is not sufficient to support MIMO communications with standard UEs and discussing LTE coexistence between HAPs and terrestrial cellular deployments for real UEs.
Table 1
Conducted power by loading per port (assuming 37 dBm per port at maximum loading) for 5 MHz channel bandwidth
0 | 0 | − 10.21 | 26.79 |
4 | 16 | − 6.2 | 30.8 |
12 | 48 | − 2.76 | 34.24 |
19 | 76 | − 1.06 | 35.94 |
25 | 100 | 0.00 | 37 |