1 Multi-Mode High Altitude Platform Stations HAPS for Next Generation Wireless Networks

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Multi-Mode High Altitude Platform Stations

(HAPS) for Next Generation Wireless Networks

Safwan Alfattani, Wael Jaafar, Halim Yanikomeroglu, and Abbas Yongaçoglu

Abstract—The high altitude platform station (HAPS) concept

has recently received notable attention from both industry and

academia to support future wireless networks. A HAPS can

be equipped with 5th generation (5G) and beyond technologies

such as massive multiple-input multiple-output (MIMO) and

reconﬁgurable intelligent surface (RIS). Hence, it is expected

that HAPS will support numerous applications in both rural

and urban areas. However, this comes at the expense of high

energy consumption and thus shorter loitering time. To tackle

this issue, we envision the use of a multi-mode HAPS that can

adaptively switch between different modes so as to reduce energy

consumption and extend the HAPS loitering time. These modes

comprise a HAPS super macro base station (HAPS-SMBS) mode

for enhanced computing, caching, and communication services, a

HAPS relay station (HAPS-RS) mode for active communication,

and a HAPS-RIS mode for passive communication. This multi-

mode HAPS ensures that operations rely mostly on the passive

communication payload, while switching to an energy-greedy

active mode only when necessary. In this article, we begin with a

brief review of HAPS features compared to other non-terrestrial

systems, followed by an exposition of the different HAPS modes

proposed. Subsequently, we illustrate the envisioned multi-mode

HAPS, and discuss its beneﬁts and challenges. Finally, we validate

the multi-mode efﬁciency through a case study.

I. INTRODUCTION

Beyond ﬁfth-generation (B5G) and sixth-generation (6G)

technologies are expected to support novel use cases thanks

to their anticipated ubiquitous and reliable connectivity with

a massive numbers of devices with high data rates and low

latency. But it has been shown to be unfeasible and cost-

inefﬁcient to attempt to fulﬁll the requirements of future

networks by relying solely on terrestrial networks. To com-

plement terrestrial systems, non-terrestrial networks (NTN)

are envisioned as a key enabler for next-generation networks.

More speciﬁcally, given the intrinsic features of NTN, in-

cluding ﬂexible deployment, strong channel links, and wide

coverage footprints, the latter can support terrestrial networks

by enhancing aspects such as communication, computing, and

caching capabilities.

Typically, three types of NTN systems are proposed to

support future networks, namely unmanned aerial vehicles

(UAVs) [1], high altitude platform station (HAPS1) systems

[2], and low-Earth-orbit (LEO) satellites [3]. These NTN

systems have different features such as operating altitude,

size, payload, ﬂight duration, and communication capabilities.

This work is funded by a scholarship from King Abdulaziz University,

Saudi Arabia and the Natural Science and Engineering Research Council of

Canada (NSERC).

1In line with the convention in recent ITU (International Telecommuni-

cations Union) documents, in this paper the abbreviation HAPS is used to

denote both the singular and plural usage.

While UAVs are limited by energy consumption and ﬂight

time, LEO satellites suffer from signiﬁcant path-loss, high

mobility, and long communication delays. By contrast, HAPS

systems have the largest platform size and payload, less path-

loss and delay than LEO satellites, and they can sustain

longer missions than UAVs. Accordingly, the development of

HAPS technologies has attracted signiﬁcant interest from both

academia and the industry [2]. Examples of current HAPS

projects include X-station by StratXX, Zephyr by Airbus,

Stratobus by Thales, Hawk30 by HAPSMobile, and Phasa-35

by BAE Systems.

One of the main issues in HAPS research is the design of

the communication payload subsystem, as it impacts the range

of supported applications, energy consumption, ﬂight duration,

and deployment costs. Traditionally, HAPS were developed to

serve rural and hard-to-reach areas, and the communication

payload was designed in a single mode to operate either

as a base station (HAPS-BS) or as a relay station (HAPS-

RS). An advanced HAPS-BS, referred to as HAPS super

macro base station (HAPS-SMBS), was recently proposed in

[4], where it was used in urban areas for novel applications

beyond connectivity, such as computing, storage, and sensing.

Similarly, an energy-efﬁcient version of a HAPS-RS was

recently proposed in [5], where a HAPS is equipped with

a reconﬁgurable intelligent surface (RIS), aiming to provide

relaying functions. The latter is called HAPS-RIS.

Nevertheless, designing a HAPS with a single payload

mode either increases its energy consumption or limits its

communication capabilities. Hence, to cope with the user

trafﬁc and service demand dynamics in the most efﬁcient

and cost-effective manner, we propose here the design of a

multi-mode HAPS payload, where the HAPS can adaptively

switch between different operating modes, i.e., HAPS-SMBS,

HAPS-RS, and HAPS-RIS, based on the received demands.

Consequently, the usage of active components on the HAPS

will be minimized and a more energy-efﬁcient operation will

thus be achieved.

II. HAPS VERSUS OTHER NTN SYSTEMS

We discuss here the characteristics of HAPS and its dif-

ference from other NTN systems such as UAVs and LEO

satellites, in terms of communication payload capabilities,

operations, and suitable applications.

A. HAPS versus LEO Satellites

HAPS systems have unique properties. First, HAPS are

typically located at an altitude of 20 km, against altitudes

between 350 and 2000 km for LEO satellites. Hence, a

arXiv:2210.11423v2 [eess.SP] 22 Jun 2023

HAPS would experience less path-loss and stronger line-of-

sight (LoS) links. The high quality of HAPS communication

links to the ground allows it to connect directly to the user

equipment (UE) without requiring a special receiver design.

This is in contrast to current LEO systems, where sophisticated

receivers with high antenna gain are required2. Also, due

to its low altitude, a HAPS is more appealing for delay-

sensitive and critical applications than a LEO satellite. Second,

HAPS systems are quasi-stationary either through ﬁxed-wing

HAPS circular trajectories or airships loitering, whereas LEO

satellites orbit the Earth at high speeds. Thus, unlike HAPS,

satellites suffer from signiﬁcant Doppler effects, frequent

handover, and wasted capacity, due to orbiting under-populated

areas. Moreover, given the continuous movements of LEO

satellites, a tracking system in current receivers is required.

Third, HAPS are giant platforms, e.g., aerostatic HAPS have

lengths between 100 and 200 m, and aerodynamic HAPS

have wingspans between 35 and 80 m. This is up to 20

times the size of a standard LEO satellite. Such HAPS sizes

allow to accommodate several communication technologies,

including massive MIMO and large RIS. Moreover, HAPS can

host heavier payloads, e.g., storage and computing equipment.

Finally, the lifetime of HAPS is estimated to be between a

few months and several years, depending on the nature of its

mission. Although this is lower than the LEO satellite 10-

year life, HAPS will be recoverable at the end of its lifetime.

Moreover, HAPS can be maintained either in the sky or by

bringing it back to Earth, which makes it possible to extend

its lifetime.

B. HAPS versus UAVs

UAVs can achieve reliable and low-latency communica-

tions with ground UEs over small distances of up to a few

hundred meters. In contrast, HAPS enjoys a wider footprint

radius ranging between 40 and 100 km for high throughput

communications, and this can be extended to 500 km (ITU-

R F.1500). To achieve an equivalent footprint to HAPS, the

deployment of costly UAV swarms is needed. Also, a better

LoS probability can be realized with HAPS, while UAV links

are sensitive to blockages and high-rise buildings. Given that

HAPS can be powered by renewable energy sources, e.g.,

solar panels, or hydrocarbon fuel (backed up by batteries

and fuel cells) [2], they can sustain longer missions than

the battery/hydrocarbon fuel-limited UAVs3. Finally, the small

size of UAVs limits their communication payload and potential

applications. For instance, authors of [6] showed that RIS-

equipped UAVs perform worse than RIS-equipped HAPS. For

the same reason, high storage and computation power cannot

be deployed on UAVs, in contrast to HAPS.

III. SINGLE-MODE HAPS COMMUNICATION PAYLOAD

A HAPS consists of three onboard subsystems: an energy

management subsystem, a ﬂight subsystem, and a communi-

2Recently, satellite direct-to-device solutions are being tested and validated

with standard UE. However, until today, the only validated services are limited

to emergency messaging and localization.

3Note that the consumed energy by the communication payload is signiﬁ-

cantly lower than that required by the ﬂying system.

cation payload subsystem [2]. The energy management sub-

system is responsible for power generation using photovoltaic

(PV) panels and/or hydrocarbon fuel and for energy storage

through Lithium-ion batteries or fuel cells. Moreover, this

subsystem controls the energy consumption required by the

other subsystems. The ﬂight subsystem controls the mobility

and stabilization of the HAPS, whereas the communication

payload subsystem mainly manages the communications be-

tween the HAPS and other aerial or terrestrial nodes, while

also processing and storing other required data. Based on the

capabilities of the HAPS in terms of communication, comput-

ing, and storage, its power requirements and applications may

vary. Typically, three types of HAPS communication payload

have been deﬁned, namely HAPS-SMBS [4], HAPS-RS, and

HAPS-RIS [5]. The type of communication payload impacts

the potential applications supported, onboard consumed en-

ergy, and thus the ﬂight duration of the HAPS. In what follows,

we discuss the properties and potential use cases of each

HAPS-equipped communication payload type.

A. HAPS-SMBS

The main role of the HAPS-SMBS communication payload

involves radio frequency (RF) ﬁltering, frequency conversion,

and signal ampliﬁcation. Its multiple antennas transceivers

can also encode/decode, precode, and modulate/demodulate

signals, as well as switch and route data. The communication

payload of the HAPS-SMBS used exclusively for communica-

tions is called a “regenerative payload” by the 3rd Generation

Partnership Project (3GPP) standards (TR 38.811), and it sup-

ports similar tasks to a ground base station or a Node B (gNB).

A HAPS-SMBS’s “regenerative payload” can fully process

signals and serve users directly, unlike other communication

payload types. When the communication payload of a HAPS-

SMBS integrates computation and caching capabilities, its

role can be extended beyond simple data transmission and

reception for users in rural and underserved areas. Indeed,

HAPS-SMBS can work in tandem with terrestrial networks

in dense urban areas to provide numerous applications and

novel services for 5G and beyond networks. We discuss some

unique HAPS-SMBS use cases below.

1) Increasing network capacity: To cope with the increas-

ing demands in metropolitan areas, network operators have

traditionally relied on densiﬁcation with macro and small

gNBs. However, this might not be a cost-effective solution in

dynamic and highly mobile environments. In addition, small-

cell densiﬁcation might not be sufﬁcient to absorb the ever-

increasing trafﬁc of connected devices. Moreover, the ﬁxed

deployment of terrestrial networks is unable to handle unpre-

dictable congestion caused by temporary events. To tackle this

issue, a HAPS-SMBS can complement a terrestrial network by

providing wide coverage, continuous, and agile connectivity to

the terrestrial network’s cell-edge and high-trafﬁc demanding

UEs.

2) Supporting aerial networks and aerial users: The de-

ployment of UAVs as BSs or relays is seen as a key enabler

of future networks, given the ﬂexibility and mobility of UAVs.

However, their processing and computation powers are limited,

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1Multi-ModeHighAltitudePlatformStations(HAPS)forNextGenerationWirelessNetworksSafwanAlfattani,WaelJaafar,HalimYanikomeroglu,andAbbasYongaçogluAbstract—Thehighaltitudeplatformstation(HAPS)concepthasrecentlyreceivednotableattentionfrombothindustryandacademiatosupportfuturewirelessnetworks.AHAPScanbeeq...

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