High Altitude Long Operation Network

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ABSTRACT

The High Altitude Long Operation Network TM is a broadband

wireless metropolitan area network, with a star topology, whose solitary

hub is located in the atmosphere above the service area at an altitude

higher than commercial airline traffic. The HALO/Proteus airplane is the

central node of this network. It will fly at altitudes higher than 51,000 ft.

The signal footprint of the network, its "Cone of Commerce," will have a

diameter on the scale of 100 km. The initial capacity of the network will

 be on the scale of 10 Gb/s, with growth beyond 100 Gb/s. The network 

will serve the communications needs of each subscriber with bit rates in

the multimegabit per second range. A variety of spectrum bands licensed

 by the FCC for commercial wireless services could provide the needed

millimeter wavelength carrier bandwidth. An attractive choice for the

subscriber links is the LMDS band.

The airplane's fuselage can house switching circuitry and fastdigital network functions. An MMW antenna array and its related

components will be located in a pod suspended below the aircraft

fuselage. The antenna array will produce many beams, typically more

than 100. Adjacent beams will be separated in frequency. Electronic

 beamforming techniques can be used to stabilize the beams on the

ground, as the airplane flies within its station keeping volume. For the

alternative of aircraft-fixed beams, the beams will traverse over a user 

location, while the airplane maintains station overhead, and the virtual

 path will be changed to accomplish the beam-to-beam handoff. For each

isolated city to be served, a fleet of three aircraft will be operated in shifts

to achieve around-the-clock service. In deployments where multiple cities

will be served from a common primary flight base, the fleet will be sized

for allocating, on average, two aircraft per city to be served. Flight

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operational tactics will be steadily evolved and refined to achieve

continuous presence of the node above each city. Many services will be

 provided, including but not limited to T1 access, ISDN access, Web

 browsing, high-resolution videoconferencing, large file transfers, and

Ethernet LAN bridging.

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Chapter-1

Introduction

1.1 Introduction

Passage of the 1996 Telecommunications Act and the slow growth of infra-

structure for transacting multimedia messages (those integrating voice, text, sound,

images, and video) have stimulated an intense race to deploy non-traditional infra-

structure to serve businesses and consumers at affordable prices. The game is new and

the playing field is more level than ever before. Opportunities exist for entrepreneurs

to challenge the market dominance enjoyed for years by incumbents. New types of 

service providers will emerge.

An electronic "information fabric" of a quilted character—including space, at-

mospheric, and terrestrial data communications layers—will emerge that promises to

someday link every digital information device on the planet. Packet-switched data

networks will meld with connection-oriented telephony networks. Communications

infrastructures will be shared more efficiently among users to offer dramatic reduc-

tions in cost and large increases of effective data rates. An era of inexpensive band-

width has begun which will transform the nature of commerce.

The convergence of innovative technologies and manufacturing capabilities af-

fecting aviation, millimeter wave wireless, and multi-media communications indus-

tries enables Angel Technologies Corporation and its partners to pursue new wireless

 broadband communications services. The HALO™ Network will offer ubiquitous ac-

cess to any subscriber within a "super metropolitan area" from an aircraft operating at

high altitude. The aircraft will serve as the hub of the HALO™ Network serving tens

to hundreds of thousands of subscribers. Each subscriber will be able to communicate

at multi-megabit per second data rates through a simple-to-install subscriber unit. The

HALO™ Network will be steadily evolved at a pace with the emergence of data com-

munications technology world-wide. The HALO™ Network will be a universal wire-

less communications network solution. It will be deployed globally on a city-by-city

 basis.

The equipment needed to perform the functions of this broadband wireless ser-

vice will be evolutionary in nature, not revolutionary. Most of the technology already

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exists. The engineering effort will be focused primarily at adapting and integrating the

existing components and subsystems from terrestrial markets into a complete network 

solution. Proven technology will be used to the maximum extent. Since the HALO™

Aircraft are operated from regional airports, the equipment will be routinely main-

tained and calibrated. This also allows for equipment upgrades as technology ad-

vances yield lower cost and weight and provide increased performance.

1.2 Wireless Broadband Communications Market

There are various facts that show the strong interest in wireless communica-

tions in the United States:

• 50 million subscribers to wireless telephone service

• 28 million dollars annual revenue for wireless services

• 38,000 cell sites with 37 billion dollars cumulative capital investment

• 40% annual growth in customers

• 25 million personal computers sold each year 

• 50 million PC users with Internet access

"The demand for Internet services is exploding and this creates a strong de-

mand for broadband, high data rate service. It is expected that there will soon be a

worldwide demand for Internet service in the hundreds of millions". (Lou Gerstner,

IBM, April 1997) The growth in use of the World Wide Web and electronic commerce

will stimulate demand for broadband services.

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1.3 A Broadband Wireless Metropolitan Area Network 

HALO™ Aircraft Provides Wireless Broadband Services over Metropolitan Centers

An airplane specially designed for high altitude flight with a payload capacity

of approximately one ton is being developed for commercial wireless services. It willcircle at high altitudes for extended periods of time and it will serve as a stable plat-

form from which broadband communications services will be offered. The High Alti-

tude Long Operation (HALO™) Aircraft will maintain station at an altitude of 52 to

60 thousand feet by flying in a circle with a diameter of about 5 to 8 nautical miles.

Three successive shifts on station of 8 hours each can provide continuous coverage of 

an area for 24 hours per day, 7 days per week. Such a system can provide broadband

multimedia communications to the general public.

One such platform will cover an area of approximately 2800 square miles en-

compassing a typical metropolitan area. A viewing angle of 20 degrees or higher will

 be chosen to facilitate good line-of-sight coverage at millimeter wave (MMW) fre-

quencies (20 GHz or higher). Operation at MMW frequencies enables broadband sys-

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tems to be realized, i.e., from spectrum bandwidths of 1 to 6 GHz. MMW systems

also permit very narrow beam widths to be realized with small aperture antennas. Fur-

thermore, since the aircraft is above most of the earth's oxygen, links to satellite con-

stellations can be implemented using the frequencies overlapping the 60 GHz absorp-

tion band for good immunity from ground-based interference and good isolation from

inter-satellite links.

The HALO™ Network can utilize a cellular pattern on the ground so that each

cell uses one of four frequency sub-bands, each having a bandwidth up to 60 MHz

each way. A fifth sub-band can be used for gateways (connections to the public net-

work or dedicated users). Each cell will cover an area of a few square miles. The en-

tire bandwidth will be reused many times to achieve total coverage throughout the

2800 square mile area served by the airborne platform. The total capacity of the net-

work supported by a single airborne platform can be greater than 100 Gbps. This is

comparable to terrestrial fiber-optic (FO) networks and can provide two-way broad-

 band multimedia services normally available only via FO networks.

The HALO™ Network provides an alternative to satellite- and ground-based

systems. Unlike satellite systems, however, the airborne system concentrates all of the

spectrum usage in certain geographic areas, which minimizes frequency coordination

 problems and permits sharing of frequency with ground-based systems. Enough

 power is available from the aircraft power generator to allow broadband data access

from small user terminals.

1.4 A New Layer in the Wireless Infrastructure

Raytheon TI Systems and Angel Technologies Corporation have the opportun-

ity to serve the growing wireless communications market by using a HALO™ Air-

craft that transmits high-speed data traffic throughout a metropolitan region. The goal

is to interconnect more than 100,000 subscribers within a metropolitan center and its

surrounding communities through a star topology network. This HALO™ Network has the benefits of low cost, high flexibility, and high quality of service.

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HALO™ Aircraft provide a new layer in the traditional hierarchy of wireless

communications. The HALO™ Network can be thought of as a "tall tower" approach

that provides better line of sight to customers without the high cost of deploying and

operating a satellite constellation.

Terrestrial Towers

HALO™ Aircraft

Low Earth Orbit (LEO) Satellites

Geostationary Earth Orbit (GEO) Satellites

<200 feet

10 miles

400 miles

22,300 miles

This paper will describe the architecture and the concept of operations of the

HALO™ Network. It will also describe key characteristics of the HALO™ Aircraft

and the communications payload and subscriber units. A companion paper1 entitled

"The Cone of Commerce™" covers the business and market aspects of the HALO™

 Network. The paper by Djuknic2 provides an overview of the various options and

highlights the unique advantages of stratospheric platforms for providing wireless

communications services.

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  Chapter-2

THE HALO NETWORK CONCEPT

2.1 Overall Concept

The attributes of the HALO™ Network are illustrated in the figure below. Many

types of subscribers will benefit from the low price of HALO™ Network broadband

services—schools, families, hospitals, doctors' offices, and small to medium size busi-

nesses. The equipment will connect to existing networks and telecommunications

equipment using standard broadband protocols such as ATM and SONET. The

HALO™ Gateway provides access to the Public Switched Telephone Network 

(PSTN) and to the Internet backbone for such services as the World Wide Web and

electronic commerce.

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High-Speed Data Links Transmitted Over Millimeter Wave Frequencies Provide

Broadband Data Services to Various End-Users

2.2 Key Features

The key features of the HALO™ Network are summarized below.

• Seamless ubiquitous multimedia services

• Adaptation to end user environments

• Enhanced user connectivity globally

• Rapidly deployable to sites of opportunity

• Secure and reliable information transactions

• Bandwidth on demand provides efficient use of available spectrum

2.3 Service Area

Most metropolitan areas will fit within a signal "footprint" of 40-60 miles

diameter. The following figure shows the coverage of a 50-mile HALO™ Network 

service-area footprint for the New York City metropolitan area. Notice that "double

coverage" of certain areas occurs due to overlapping adjacent footprints. This

 provides higher reliability links and reduces blocking factors on requests for service.

The footprint over Manhattan covers 4.8 million households or 12.5 million people.

2.4 Service Attributes

There are various classes of service to be provided. A consumer service would

 provide 1-5 Mbps communication links. A business service would provide 5-12.5

Mbps links. Since the links would be "bandwidth-on-demand," the total available

spectrum would be time-shared between the various active sessions. The nominal data

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rates would be low while the peak rates would expand to a specified level. A gateway

service can be provided for "dedicated" links of 25-155 Mbps.

Based on the LMDS spectrum and 5-fold reuse, the service capacity would be

10,000 to 75,000 simultaneous, symmetrical T1 circuits (1.5 Mbps) per Communica-

tions Payload. The HALO™ Aircraft would provide urban and rural coverage from a

single platform to provide service to:

a. 100-750,000 subscribers

 b. 40-60 mile diameter service area (1,250 to 2,800 square miles)

Coverage of the New York City Metropolitan Area by HALO™ Aircraft

2.4.1 Spectrum Options

There are various options for spectrum utilization, the main options being

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spectrum at 28 GHz for the Local Multipoint Distribution Service (LMDS) and the

microwave point-to-point allocations at 38 GHz. The FCC is expected to allocate 850

MHz of spectrum between 27.5 and 28.35 GHz for the LMDS service. The system

characteristics described in this paper are for the LMDS frequency.

2.4.2 Network Access

Various methods for providing access to the users on the ground are feasible.

The figure below shows one approach where each spot beam from the payload an-

tenna serves a single "cell" on the ground in a frequency-division multiplex fashion

with 5 to 1 frequency reuse, four for subscriber units and the fifth for gateways to the

 public network and to high-rate subscribers. Other reuse factors such as 7:1 and 9:1

are possible. Various network access approaches are being explored.

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Cell Coverage by Frequency Division Multiplexing using Spot Beams

2.4.3 Network Services

The HALO™ node provides a multitude of connectivity options as shown be-

low. It can be used to connect physically separated Local Area Networks (LANs)

within a corporate intranet through frame relay adaptation or directly through LAN

 bridges and routers. Or it can provide videoconference links through standard ISDN

or T1 interface hardware. The HALO™ Network may use standard SONET and ATM

 protocols and equipment to minimize the cost of the equipment and to take advantage

of the wide availability of these components.

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The HALO™ Network Accommodates a Variety of Interfaces

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  Chapter-3

HALO NETWORK ARCHITECTURE

3.1 Network Elements

The major elements of the HALO™ Network are shown below. The HALO™

 Network interfaces to the Public Switched Telephone Network (PSTN) and to the In-

ternet backbone through the HALO™ Gateway. On the subscriber side, the HALO™

 Network provides connectivity to local networks of various kinds.

The HALO™ Network Architecture

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3.2 Network Architecture

At the apex of a wireless Cone of Commerce™, the payload of the HALO™Aircraft becomes the hub of a star topology network for routing data packets between

any two subscribers possessing premise equipment within the service coverage area. A

single hop with only two links is required, each link connecting the payload to a sub-

scriber. The links are wireless, broadband and line of sight.

Information created outside the service area is delivered to the subscriber's

consumer premise equipment ("CPE") through business premise equipment ("BPE")

operated by Internet Service Providers ("ISPs") or content providers within that re-

gion, and through the HALO™ Gateway ("HG") equipment directly connected to dis-

tant metropolitan areas via leased trunks. The HG is a portal serving the entire net-

work. It avails system-wide access to content providers and it allows any subscriber to

extend their communications beyond the HALO™ Network service area by connect-

ing them to dedicated long-

3.3 Field of View

Angel assumes the "minimum look angle" (i.e., the elevation angle above the

local horizon to the furthest point on the orbit as seen by the antenna of the premise

equipment) is generally higher than 20 degrees. This value corresponds to subscribers

at the perimeter of the service footprint. In contrast, cellular telephone designers as-

sume that the line of sight from a customer to the antenna on the nearest base station

is less than 1 degree. Angel chose such a high look angle to ensure that the antenna of 

each subscriber's premise equipment will very likely have access to a solid angle

swept by the circling HALO™ Aircraft free of dense objects, and to ensure high

availability of the service during heavy rainfall to all subscribers.

The high look angle also allows the sharing of this spectrum with ground-

 based wireless networks since usually high-gain, narrow beams are used and the an-tenna beams of the HALO™ and ground-based networks will be separated in angle

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far enough to ensure a high degree of signal isolation.

Service area

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Chapter-4

HALO AIRCRAFT

The HALO™ Aircraft is under development and flight testing is expected to

occur by mid-1998. The aircraft has been specially designed for the HALO™ Net-

work with the Communications Payload Pod suspended from the underbelly of its fu-

selage.

HALO™ Aircraft with Suspended Communications Payload

The HALO™ Aircraft will fly above the metropolitan center in a circular orbit

of five to eight nautical miles diameter. The Communications Payload Pod is mounted

to a pylon under the fuselage. As the aircraft varies its roll angle to fly in the circular 

orbit, the Communications Payload Pod will pivot on the pylon to remain level with

the ground. Other details on the aircraft can be found in the Cone of Commerce™ pa-

 per.

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  Chapter-5

COMMUNICATIONS PAYLOAD

The HALO™ Network will use an array of narrow beam antennas on the

HALO™ Aircraft to form multiple cells on the ground. Each cell covers a small geo-

graphic area, e.g., 4 to 8 square miles. The wide bandwidths and narrow beam widths

within each beam or cell are achieved by using MMW frequencies. Small aperture an-

tennas can be used to achieve small cells. For example, an antenna having a diameter 

of only one foot can provide a beam width of less than three degrees. One hundred

dish antennas can be easily carried by the HALO™ Aircraft to create one hundred or 

more cells throughout the service area. If lenses antennas are utilized, wider beams

can be created by combining beams through each lens aperture, and with multiple

feeds behind each lens multiple beams can be formed by each compound lens.

If 850 MHz of spectrum is assumed, then a minimum capacity of one full-du-

 plex OC-1 (51.84 Mbps) channel is available per cell. For example, a single platform

reusing 850 MHz of spectrum in 100 cells would provide the equivalent of two,

OC-48 fiber optic rings. Higher capacities are possible by increasing the number of 

cells. By using Asynchronous Transfer Mode (ATM) technology with over-the-air dy-

namic bandwidth allocation, this capacity can be shared by multiple users in an effi-

cient manner. An ATM-like packet switch on the HALO™ Aircraft provides the net-

work switching capability to cross-connect all users within the coverage area as well

as connections to other users through gateways. The elements in the communications

 payload are shown below. It consists of MMW transceivers, pilot tone transmitter,

high-speed modems, SONET multiplexers, packet switch hardware and software, and

associated ancillary hardware such as power supplies, processors, etc.

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Functional Block Diagram of the Communications Payload

The major design options for antennas in the Communications Payload are to

utilize either platform-fixed beams or earth-fixed beams. For the case of platform-

fixed beams, each antenna would have a fixed field of view. The total field of view for 

the entire HALO™ Network would be the sum of these fields of view of the individu-

al antennas. The network could initially have a small footprint and as demands on the

HALO™ services increase, additional antennas could be added to the Communica-

tions Payload. This results in a modular design, readily adaptable for growth.

Platform-fixed beams are simpler to construct generally, but require the "han-

doffs" between beams to be accomplished by the packet switching equipment as the beams "sweep" across the ground with the movement of the aircraft. However, the

cost and performance penalties for frequently changing the virtual path through the

 packet switch may be appreciable.

An alternative is to electronically steer the beams so they remain "fixed" on

the ground as the aircraft moves. These results in more electronic and physical com-

 plexity for the antennas, but this may be a good trade-off to make since the burden on

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the packet switch and its network management software would be greatly reduced.

These trade-offs are still being assessed.

For the case of earth-fixed beams, each antenna would have a wider field of 

view than the sum of the beams in that antenna since each beam can be steered in all

directions. Each beam could be capable of steering throughout the HALO™ footprint,

or could be assigned a smaller portion. If there are "gaps" in the required coverage

due to such things as rivers, hills, or forests, then the earth-fixed beams can be steered

away from these undesirable coverage zones and more efficient usage of the antennas

might result compared to the case of platform-fixed beams.

.

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  Chapter-6

SUBSCRIBER UNITS

A block diagram describing the CPE (and BPE) is shown below. It entails

three major sub-groups of hardware: The RF Unit (RU) which contains the MMW

Antenna and MMW Transceiver; the Network Interface Unit (NIU); and the applica-

tion terminals such as PCs, telephones, video servers, video terminals, etc. The RU

consist of a small dual-feed antenna and MMW transmitter and receiver which is

mounted to the antenna. An antenna tracking unit uses a pilot tone transmitted from

the Communications Payload to point the antenna toward the airborne platform.

The MMW transmitter accepts an L-band (950 - 1950 MHz) IF input signal

from the NIU, translates it to MMW frequencies, amplifies the signal using a power 

amplifier to a transmit power level of 100 - 500 mW of power and feeds the antenna.

The MMW receiver couples the received signal from the antenna to a Low Noise

Amplifier (LNA), down converts the signal to an L-band IF and provides subsequent

amplification and processing before outputting the signal to the NIU. Although the

MMW transceiver is broadband, it typically will only process a single 40 MHz chan-

nel at any one time. The particular channel and frequency is determined by the NIU.

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Functional Block Diagram of the Subscriber Equipment

The NIU interfaces to the RU via a coax pair which transmits the L-band TX

and RX signals between the NIU and the RU. The NIU comprises an L-band tuner 

and down converter, a high-speed (up to 60 Mbps) demodulator, a high-speed modu-

lator, multiplexers and de multiplexers, and data, telephony and video interface elec-

tronics. Each user terminal will provide access to data at rates up to 51.84 Mbps each

way. In some applications, some of this bandwidth may be used to incorporate spread

spectrum coding to improve performance against interference (in this case, the user 

information rate would be reduced).

The NIU equipment can be identical to that already developed for LMDS and

other broadband services. This reduces the cost of the HALO™ Network services to

the consumer since there would be minimal cost to adapt the LMDS equipment to this

application and we could take advantage of the high volume expected in the other ser-

vices. Also, the HALO™ RU can be very close in functionality to the RU in the other 

services (like LMDS) since the primary difference is the need for a tracking function

for the antenna

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Chapter-7

SUMMARY

The HALO™ Network is capable of providing high rate communications to

users of multimedia and broadband services. The feasibility of this approach is reas-

onably assured due to the convergence of technological advancements. The key en-

abling technologies at hand include:

• GaAs RF devices which operate at MMW frequencies

• ATM/SONET Technology and Components

• Digital Signal Processing for Wideband Signals

• Video Compression

• Very Dense Memory Capacity

• Aircraft Technology

These technologies are individually available, to a great extent, from commer-

cial markets. The HALO™ Network seeks to integrate these various technologies into

a service of high utility to small and medium businesses and other multimedia con-

sumers at a reasonable cost.

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REFERENCE

•  N. Colella and J. Martin, ” The cone of Commerce ” ,Proc. Of the SPIE Inter-

national Symposium on Voice, Video, and Data Communications : Broadband

Engineering for Multimedia Markets,1997.

• G. Djuknic , J. Freidenfelds, et al. “Establishing Wireless Communications

Services via High Altitude Aeronautical Platforms: A concept Whose Time

Has Come? “ IEEE Communications Magazine , September 1997.

• www.angeltechnologies.com/techpapers

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