| 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.
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
businesses. 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.
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 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 Communications Payload. The
HALO™ Aircraft would provide urban and rural coverage from a single platform to
provide service to:
- 100-750,000 subscribers
- 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 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 antenna 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.
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 below. It can be
used to connect physically separated Local Area Networks (LANs) within a corporate
intranet through frame relay adaptation or directly through LAN bridgers 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.
The HALO™ Network Accommodates a Variety of Interfaces
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 Internet backbone
through the HALO™ Gateway. On the subscriber side, the HALO™ Network provides
connectivity to local networks of various kinds.
The HALO™ Network Architecture
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 subscriber. 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 region, and through the HALO™ Gateway ("HG")
equipment directly connected to distant metropolitan areas via leased trunks. The HG is a
portal serving the entire network. It avails system-wide access to content providers and
it allows any subscriber to extend their communications beyond the HALO™ Network
service area by connecting them to dedicated long-distance lines such as inter-metro
optical fiber.
The HALO™ Network
The CPE, BPE and HG all perform the same functions: use a high-gain antenna that
automatically tracks the HALO™ Aircraft; extract modulated signals conveyed through
the air by millimeter waves; convert the extracted signals to digital data; provide
standards-based data communications interfaces; and route the digital data to information
appliances, personal computers, and workstations connected to the premise equipment. Thus,
some of the technologies and components, both hardware and software, will be common to the
designs of these three basic network elements.
The CPE, BPE and HG differ in size, complexity and cost, ranging from the CPE which is
the smallest, least complex, lowest priced and will be expressively built for the mass
market; followed by the BPE, engineered for a medium size business to provide access to
multiple telecommuters by extending the corporate data communications network; to the HG
which provides high bandwidth wireless data trunking to Wide Area Networks
("WANs") maintained and operated by the long distance carriers and content
handlers who wish to distribute their products widely.
In other words, the CPE is a personal gateway serving the consumer. The BPE is a
gateway for the business requiring higher data rates. The HG, as a major element of the
entire network, will be engineered to serve reliably as a critical network element. All of
these elements are being demonstrated in related forms by terrestrial 38 GHz and LMDS
vendors. Angel will solicit the participation of key component suppliers for adapting
their technologies to the HALO™ Network.
As with all wireless millimeter wave links, high rainfall rates can reduce the
effective data throughput of the link to a given subscriber. Angel plans to ensure maximum
data rates more than 99.7% of the time, reduced data rates above an acceptable minimum
more than 99.9% of the time, and to limit outages to small areas (due to the interception
of the signal path by very dense rain columns) less than 0.1% of the time. Angel plans to
locate the HG close to the HALO™ orbit center to reduce the slant range from its
high-gain antenna to the aircraft and hence its signal path length through heavy rainfall.
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 assume
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
antenna beams of the HALO™ and ground-based networks will be separated in angle far
enough to ensure a high degree of signal isolation.
HALO™ Aircraft Field of View
3.4 Frequency Plan
The frequency plan described in section 2.4.2 can be used to achieve a 5 to 1 reuse factor
throughout the metropolitan service area with the LMDS frequency allocation at 28 GHz.
Four of the sub-bands would be used for CPE and BPE links. The fifth sub-band would in
that case be used for the gateway links to the HG antenna and to dedicated users. Other
spectrum options like 38 GHz will require a different frequency plan, albeit similar in
approach.
3.5 Millimeter Wave Propagation
The abilities to transmit broadband wireless data to and from small cells on the ground
with small antennas are achieved by using MMW frequencies. The LMDS allocation provides
approximately 1 GHz of bandwidth at 28 GHz for local distribution of broadband services in
terrestrial systems. Here the paths are almost tangential to the earth and are restricted
to 5 km or less due to rain attenuation. For airborne systems, the look angle—the
angle between the horizon and the node platform—is 20 degrees or more. Since most of
the rain attenuation occurs in the lower atmosphere (within 3 km of the surface) the high
look angle reduces the portion of the path which traverses the volume of high rainfall
rate. Furthermore, in the airborne concept, high-gain antennas are used to form narrow
beams in a cellular pattern.
 The table
to the right summarizes the results of a typical path loss analysis. Here the slant range
is assumed to be 35 km, the gain of both the airborne and ground antennas are assumed to
be 34 dB and the transmitted power by both the airborne segment and the ground segment is
100 mW at 28 GHz. (Power levels up to 1 Watt are available at this frequency.) The system
analyzed uses QPSK modulation, a rate 7/8 convolutional code concatenated with a
Reed-Solomon (204,188) and an excess bandwidth factor of 0.21 with an assumed maximum BER
of 10-9 to achieve an information rate equivalent to OC-1 (i.e., 51.84 Mbps). The
calculations assume a rainfall rate (Dallas area) which permits a link availability of
99.9%. The results of this analysis indicate a margin of over 6 dB even after rain fade.
This margin could be increased by up to 10 dB if the transmitted power were increased to 1
Watt. The noise figure used is very conservative at 9 dB; another 3 dB of improvement is
expected by improving the receiver noise figure.
The propagation of MMW signals is strictly line-of-sight. Trees, as well as buildings,
vehicles and terrain, normally cause unacceptable path loss. The high look angle minimizes
such losses, and an unobstructed path between the transmitter and receiver can be assumed.
Mitigation techniques include increasing the height of the ground based terminals,
providing alternate nodes and removing blockage through proper site planning and
subscriber unit installation procedures.
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™
Network with the Communications Payload Pod suspended from the underbelly of its fuselage.
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™ paper.
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 geographic area,
e.g., 4 to 8 square miles. The wide bandwidths and narrow beamwidths within each beam or
cell are achieved by using MMW frequencies. Small aperture antennas can be used to achieve
small cells. For example, an antenna having a diameter of only one foot can provide a
beamwidth 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 lensed 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-duplex 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 dynamic bandwidth
allocation, this capacity can be shared by multiple users in an efficient manner. An
ATM-like packet switch on the HALO™ Aircraft provides the network 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.
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 individual antennas.
The network could initially have a small footprint and as demands on the HALO™
services increase, additional antennas could be added to the Communications Payload. This
results in a modular design, readily adaptable for growth.
Platform-fixed beams are simpler to construct generally, but require the
"handoffs" 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. This results in more electronic and physical complexity
for the antennas, but this may be a good trade-off to make since the burden on 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.
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 application 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 channel at any one time. The
particular channel and frequency is determined by the NIU.
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 modulator, multiplexers and
demultiplexers, and data, telephony and video interface electronics. 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 services. 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.
The electronics for the RF data signal would be identical if the same frequency band is
utilized.
The HALO™ Network is capable of providing high rate communications to users of
multimedia and broadband services. The feasibility of this approach is reasonably assured
due to the convergence of technological advancements. The key enabling 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 commercial
markets. The HALO™ Network seeks to integrate these various technologies into a
service of high utility to small and medium businesses and other mutlimedia consumers at a
reasonable cost.
Acknowledgments
The authors wish to thank J. Leland Langston for providing his valuable inputs and many
discussions on this topic.
REFERENCES
- N. Colella and J. Martin, "The Cone of Commerce™," Proc. of the SPIE
International 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.
BIOGRAPHIES
James Martin is the lead systems engineer and project manager for the HALO™
communications payload under development at Raytheon TI Systems for Angel Technologies. At
AT&T Bell Labs, he developed cellular wireless telecommunications equipment and
underwater fiber optic transmission systems. Mr. Martin has recently published a
"Systems Engineering Guidebook" with the CRC Press. His specialty is systems
engineering management, systems architecting and the total systems engineering process.
Dr. Nicholas J. Colella is the Chief Technology Officer of Angel
Technologies Corporation. In prior years, he held senior technical positions at Lawrence
Livermore National Laboratory. He invented the RAPTOR/TALON theater ballistic missile
defense concept and served as DOD's executing agent for pioneering low-cost,
high-altitude, long-endurance unmanned aircraft, high mass fraction kinetic kill
interceptors, electro-optics and communications systems. He co-created Brilliant Pebbles,
led LLNL's spacecraft design and survivability projects, and developed one-steradian wide
field of view (WFOV) cameras employing spherically concentric refractive optics for
tracking satellites and space objects. He is a founding partner of a multi-chip module
company and the National Robotics Engineering Consortium at Carnegie Mellon.
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