Wireless Communication

Broadband wireless millimeter wavelength services provided from a High Altitude Long Operation (HALO™) Aircraft are now feasible. Our talk will emphasize the conceptual design of a "bandwidth-on-demand" wireless network whose data rates to and from the subscriber will measure in the multi-megabit per second range. A variety of metropolitan area spectrum bands offer the needed bandwidth. An attractive choice is the LMDS band near 28 GHz and system characteristics at this frequency will be described.

The HALO™ Aircraft fuselage will house packet switching circuitry and fast digital network functions. The communications antenna and related components will be located in a pod suspended below the aircraft fuselage. To offer "ubiquitous" service throughout a large region, the HALO™ antenna will utilize multiple beams arranged in a typical cellular pattern. Broadband channels to subscribers in adjacent cells will be separated in frequency. As the beams traverse over a user location, the virtual path through the packet switch will be changed to perform a beam-to-beam handoff.

Overviews of the system architecture and the network elements will be presented along with descriptions of the frequency plan and equipment. The utilization of components under development for terrestrial LMDS products will be described.

Keywords: HALO™ Aircraft, HALO™ Network, Cone of Commerce™, broadband wireless services, metropolitan area network, switched broadband, megabit data services, packet switching, wireless multimedia

1.1 Introduction

Passage of the 1996 Telecommunications Act and the slow growth of infrastructure for transacting multimedia messages (those integrating voice, text, sound, images, and video) have stimulated an intense race to deploy non-traditional infrastructure 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, atmospheric, 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 reductions in cost and large increases of effective data rates. An era of inexpensive bandwidth has begun which will transform the nature of commerce.

The convergence of innovative technologies and manufacturing capabilities affecting aviation, millimeter wave wireless, and multi-media communications industries enables Angel Technologies Corporation and its partners to pursue new wireless broadband communications services. The HALO™ Network will offer ubiquitous access 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 communications technology world-wide. The HALO™ Network will be a universal wireless 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 service will be evolutionary in nature, not revolutionary. Most of the technology already 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 maintained and calibrated. This also allows for equipment upgrades as technology advances 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 communications 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 demand 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.

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 will circle at high altitudes for extended periods of time and it will serve as a stable platform from which broadband communications services will be offered. The High Altitude 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 encompassing 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) frequencies (20 GHz or higher). Operation at MMW frequencies enables broadband systems to be realized, i.e., from spectrum bandwidths of 1 to 6 GHz. MMW systems also permit very narrow beamwidths to be realized with small aperture antennas. Furthermore, since the aircraft is above most of the earth's oxygen, links to satellite constellations can be implemented using the frequencies overlapping the 60 GHz absorption 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 network or dedicated users). Each cell will cover an area of a few square miles. The entire 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 network 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 broadband 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 opportunity to serve the growing wireless communications market by using a HALO™ Aircraft 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.

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 milesThis 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:

1. 100-750,000 subscribers
2. 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.

The authors wish to thank J. Leland Langston for providing his valuable inputs and many discussions on this topic.

1. 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.
2. 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.

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.

Opinions are divided.

I personally don't see anything wrong with them havingthem at school, though I do feel they should not be allowed to use them in school outside of break periods (lunch or possibly between classes), bona fide emergencies ("We're being evacuated because the school is on fire, I need someone to come pick me up"), or with the express knowledge and consent of a teacher or administrator ("Hey mom, I left my homework on the counter and Mr. Garrison told me to call and see if you could bring it to me").