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TECHNICAL REPORT STANDARD TITLE PAGE

1. Report No. 88-12 2. Government Accession No.

4. Title and Subtitle

MSAT-X: A Technical Introduction and

Status Report

7. Authors) Khaled oessouky and Miles Sue

9. Performing Organization Name and Address

JET PROPULSION LABORATORY

California Institute of Technology

4800 Oak Grove Drive

Pasadena, California 91109

12. Sponsoring Agency Nome and Address

NATIONAL AERONAUTICS AND SPACE ADMINISTRATION

Washington, D.C. 20546

3. Recipient's Catalog No.

5. Report Date

April 15, 1988

6. Performing Organization Code

8. Performing Organization Report No.

10. Work Unit No.

11. Contract or Grant No.

. NAS7-918

13. Type of Report and Period Covered

External Report

JPL Publication

14. Sponsoring Agency Code

RE4 BP 650-60-15-01-00

15. Supplementary Notes

16. Abstract

NASA initiated the Mobile Satellite Experiment (MSAT-X) program at

the Jet Propulsion Laboratory (JPL) to accelerate the introduction of a

U.S. commercial Mobile Satellite System (MSS). Through MSAT-X, the

identification, development, and demonstration of critical, high-risk

MSS technologies have been performed.

This document is intended as a technical introduction and status report for MSAT-X. It first introduces the concepts of an MSS and its

unique challenges. It then delineates MSAT-X's role and objectives and

subsequently focuses on its achievements. After outlining the MSS design

philosophy adopted, it presents and analyzes the MSAT-X results and casts

them in the broader context of an MSS.

The current phase of MSAT-X has focused notably on the ground segment of MSS. The accomplishments in the four critical technology areas

of vehicle antennas, modem and mobile terminal design, speech coding,

and networking are presented. A concise evolutionary trace is incorporated in each area to elucidate the rationale leading to the current

design choices. The findings in the area of propagation channel modeling are also summarized and their impact on system design discussed.

To facilitate the assessment of the -MSAT-X results, technology and

subsystem recommendations are also included and integrated with a quantitative first-generation MSS design.

17. Key Words (Selected by Author(s))

Communications; Electronics and

Electrical Engineering

18. Distribution Statement

Unclassified; unlimited

19. Security Clossif. (of this report)

Unclassified

20. Security Clossif. (of this page)

Unclassified

21. No. of Pages 22. Price

JPL 0184 R 9183
JPL Publication 88-12 MSAT-X Report ua

MSAT-X: A Technical Introduction

and Status Report

Khaled Dessouky

Miles Sue

April 15,1988

NASA

National Aeronautics and

Space Administration

Jet Propulsion Laboratory

California Institute of Technology

Pasadena. California
The research described in this publication was carried out by the Jet Propulsion

Laboratory, California Institute of Technology, under a contract with the National

Aeronautics and Space Administration.

Reference herein to any specific commercial product, process, or service by trade

name, trademark, manufacturer, or otherwise, does not constitute or imply its

endorsement by the United States Government or the Jet Propulsion Laboratory,

California Institute of Technology.
ABSTRACT

NASA initiated the Mobile Satellite Experiment (MSAT-X) program at

the Jet Propulsion Laboratory (JPL) to accelerate the introduction of a U.S.

commercial Mobile Satellite System (MSS). Through MSAT-X, the identification,

development, and demonstration of critical, high-risk MSS technologies have

been performed.

This document is intended as a technical introduction and status

report for MSAT-X. It first introduces the concepts of an MSS and its unique

challenges. It then delineates MSAT-X1s role and objectives and subsequently

focuses on its achievements. After outlining the MSS design philosophy

adopted, it presents and analyzes the MSAT-X results and casts them in the

broader context of an MSS.

The current phase of MSAT-X has focused notably on the ground

segment of MSS. The accomplishments in the four critical technology areas of

vehicle antennas, modem and mobile terminal design, speech coding, and

networking are presented. A concise evolutionary trace is incorporated in

each area to elucidate the rationale leading to the current design choices.

The findings in the area of propagation channel modeling are also summarized

and their impact on system design discussed.

To facilitate the assessment of the MSAT-X results, technology and

subsystem recommendations are also included and integrated with a quantitative

first-generation MSS design.

111
ACKNOWLEDGMENT

The work presented in this report is the result of the effort of many

individuals involved in MSAT-X, both at JPL's and contractors' facilities.

Without their dedication the progress reported would not have been possible.

It is through the generous assistance of many colleagues on the MSAT-X team

that this report itself has been made feasible. The authors would like to

thank Dave Bell, Jeff Berner, Faramaz Davarian, Richard Emerson, Dariush

Divsalar, John Huang, Vahraz Jamnejad, Tom Jedrey, Jim Parkyn, Ramin Sadr,

Charles Wang, Robin Winkelstein, Ken Woo, and Tsun-Yee Yan, all at JPL.

Special thanks go to William Rafferty, who fostered the concept of an MSAT-X

document and vigorously supported this effort. The many fruitful discussions

the authors had with him have enhanced the presentation in this report.

IV
CONTENTS

1. INTRODUCTION 1-1

2. MSS CONCEPT 2-1

3. MSS SCARCE RESOURCES 3-1

3.1 EIRP 3-1

3.2 SPECTRUM 3-1

3.3 GEOSYNCHRONOUS ORBIT 3-2

4. NASA'S APPROACH TO OVERCOMING MSS SCARCE RESOURCES 4-1

4.1 NASA'S CONCEPTUAL MSS DESIGN PHILOSOPHY 4-1

4.2 MSS SPACE SEGMENT EVOLUTION 4-3

4.3 MSAT-X, ITS OBJECTIVES, AND ITS RELATIONSHIP TO INDUSTRY . . 4-4

5. TECHNOLOGY AREAS RESEARCHED UNDER MSAT-X 5-1

5.1 VEHICLE ANTENNAS 5-1

5.1.1 Challenging Objectives 5-1

5.1.2 Antenna Design Considerations 5-1

5.1.3 Salient Vehicle Antenna Requirements 5-4

5.1.4 Vehicle Antenna Designs 5-5

5.1.4.1 Mechanically Steered, Tilted Linear Array . . . 5-5

5.1.4.2 Electronically Steered Phased Array Antenna . . 5-8

5.1.4.3 Planar Mechanically Steered Array 5-12

5.2 MODULATION, CODING, AND MOBILE TERMINAL DESIGN 5-12

5.2.1 Modulation/Coding 5-12

5.2.1.1 Minimum Shift Keying (MSK) and

Gaussian MSK (GMSK) 5-13

5.2.1.2 Trellis-Coded Modulation/8-Phase

Shift Keying (TCM/8PSK) 5-14

5.2.2 Mobile Terminal Design 5-18
5.3 SPEECH CODING 5-19

5.3.1 Background 5-20

5.3.2 Speech Compression Techniques Developed for MSAT-X . . 5-22

5.3.2.1 LPC-Based Compression Techniques 5-22

5.3.2.2 Speech Coding Techniques Developed at UCSB . . 5-23

5.3.2.3 Speech Coding Technique Developed by GIT . . . 5-24

5.4 NETWORKING 5-25

5.4.1 Perspective 5-25

5.4.2 I-AMAP 5-26

5.4.2.1 Channel Partitioning 5-27

5.4.2.2 Error Control, Packet Replication, and

Packet Size 5-27

5.4.2.3 Request Channel Simulator 5-28

5.5 MOBILE SATELLITE CHANNEL MODELING 5-29

6. RESULTS OF MSAT-X: RECOMMENDED TECHNOLOGY UTILIZATION FOR MSS . . 6-1

6.1 INTRODUCTION 6-1

6.2 BASIC MSS ARCHITECTURE 6-1

6.3 ON THE SPACE SEGMENT TO COMPLEMENT MSAT-X 6-3

6.4 TECHNOLOGY RECOMMENDATIONS FOR MSS 6-4

6.4.1 Mobile Terminal Technologies 6-5

6.4.1.1 Mobile Terminal Architecture 6-5

6.4.1.2 Speech Coding 6-6

6.4.1.3 Modulation/Coding 6-8

6.4.1.4 Transceiver Capabilities 6-9

6.4.1.5 Vehicle Antennas . 6-10

6.4.2 Link Budgets 6-13

6.4.3 System Capacity 6-14

7. REFERENCES 7-1

VI
Tables

3.2.1 WARC 1987 Frequency Spectrum Allocations for

Mobile Services 3-3

5.1.1 Estimated Cost Breakdown and Per Unit Cost

for the MSAT-X Phased Array Mobile Antennas 5-32

5.1.2 Salient Characteristics of MGA Breadboards 5-33

6.4.1 Comparison of the Performances of TCM/D8PSK,

DQPSK, and Differential GMSK (DGMSK) Under

Typical MSS Link Conditions 6-16

6.4.2 Forward (Ground-to-Mobile) Link Budget for

BER = 1 x 10~3 6-17

6.4.3 Return (Mobile-to-Ground) Link Budget for

BER = 1 x 10~3 . 6-18

Figures

2.1 Mobile Satellite Network Concept 2-2

3.2.1 Frequency Spectrum Allocations by the FCC

for Mobile Services 3-4

4.2.1 Example of Evolution of Multibeam Coverage 4-6

4.3.1 Areas of Concentration for MSAT-X Technology

Development 4-6

5.1.1 A Typical Radiation Pattern Demonstrating

the Adjacent Satellite Isolation Achievable

by a Low-Gain Antenna 5-34

5.1.2 A Typical Radiation Pattern Demonstrating

the Adjacent Satellite Isolation Achievable

by a Medium-Gain Antenna With Taper 5-34

5.1.3 Mechanically Steered Linear Array

Antenna Assembly 5-35

5.1.4 Schematic of Vehicle Antenna Pointing System

Based on Single-Channel Monopulse Tracking 5-36

5.1.5 Elevation Pattern of Mechanically

Steered Antenna 5-37

vxi
Figures (Contd)

5.1.6 Azimuth Pattern for Mechanically Steered

Linear Array Antenna 5-38

5.1.7 Cross-Sectional View of the Phased

Array Antenna 5-38

5.1.8 Triangular Lattice Pattern of Phased

Array Antennas 5-39

5.1.9 Stacked Dual Resonant Microstrip Element

Developed by Ball Aerospace Corporation . . 5-39

5.1.10 Stripline-Fed Crossed-Slot Radiator Developed

by Teledyne Ryan Electronics 5-40

5.1.11 Typical Elevation Radiation Pattern of TRE

Phased Array Antenna 5-41

5.1.12 Typical Elevation Pattern Showing Axial

Ratios for TRE Phased Array Antenna 5-41

5.1.13 Typical Azimuth Radiation Pattern of TRE

Phased Array Antenna 5-42

5.1.14 Typical Azimuth Pattern Showing Axial

Ratios for TRE Phased Array Antenna 5-42

5.2.1 A Worst-Case Interference Scenario in MSS 5-43

5.2.2 Adjacent Channel Interference for GMSK 5-44

5.2.3 System Block Diagram for Coherent TCM/8PSK 5-45

5.2.4 Performance of Rate 2/3, 16-State TrellisCoded 8PSK Modulation (TCM/8PSK) Over Rician

Fading Channel With CSI (Ideal Coherent

Detection) and Interleaving 5-46

5.2.5 Two Versions of the Tone Calibrated Technique (TCT):

(a) TCT Spectrum With Single Tone 5-46

(b) DTCT Spectrum 5-46

5.2.6 Block Diagram of Trellis-Coded MDPSK System 5-47

5.2.7 Simulation Results for Bit Error Rate

Performance of Rate 2/3, 16-State TrellisCoded 8DPSK (TCM/D8PSK) 5-48

5.2.8 Comparison of TCM/D8PSK With Coherent

TCM/8PSK (Using DTCT With Noisy Pilot) at

UHF and L-Band 5-48

viii
Figures (Contd)

5.2.9 Eye Diagram of Root Raised-Cosine Pulse

Shaping With 100% Excess Bandwidth 5-49

5.2.10 Mobile Terminal Block Diagram 5-49

5.3.1 Model of LPC Speech Coders 5-50

5.3.2 VAPC Functional Block Diagram 5-50

5.3.3 Block Diagram of VXC Speech Coder 5-51

5.3.4 LSP Quantization System Concept Developed

by GIT for the SEV 5-51

5.4.1 An Illustration of Channel Partitioning at

Two Instants of Time:

(a) This Partition Supports 25 Voice Calls per Minute

and 3600 Data Messages per Minute 5-52

(b) This Partition Supports 4 Voice Calls per Minute

and 6600 Messages per Minute 5-52

5.4.2 Number of Request Channels (N ) and Data

Channels (N ) Versus Data Message

Arrival Rate 5-52

5.4.3 Number of Voice Channels (N ) and Total

Number of Channels (N) Versus Message

Arrival Rate (QJ 5-53

d

5.4.4 Average End-to-End Delay Versus Packet Size

When Arrival Rate = 65 Messages/s 5-54

5.4.5 Average End-to-End Delay Versus Closed-End

Traffic for Various Packet Sizes (Total

Message Length = 4096 bits) 5-54

5.4.6 Block Diagram of the Simulator for the

Request Channel of MSAT-X 5-55

5.5.1 Cumulative Signal Level Distributions for Various

Geographic Locations and Elevation Angles 5-56

5.5.2 Predicted Peak-to-Peak Power Variation as a

Function of Channel Bandwidth 5-57

6.3.1 Multibeam Coverage of CONUS by FirstGeneration MSS Satellite 6-19

IX
SECTION 1

INTRODUCTION

This document is intended as a technical introduction and status

report for the Mobile Satellite Experiment (MSAT-X). It introduces the reader

to the Mobile Satellite System (MSS) concept, introduces MSAT-X and identifies

its role, briefly presents the accomplishments of MSAT-X, and reports on the

results and their impact on future mobile satellite systems.

These goals are achieved in this document through the performance

of four tasks. First, the concepts of an MSS are introduced. Second, after

the unique challenges of such a system are identified, NASA's approach to

overcoming these challenges is presented, and thereupon the MSAT-X effort is

placed in the proper perspective. Third, the different components of the

MSAT-X effort undertaken under the leadership of the Jet Propulsion Laboratory

(JPL) are presented. This then sets the stage for performing the fourth and

final task" of assessing the system concepts and technology developed in a

first-generation MSS context. Recommendations derived from that assessment

highlight the contributions of NASA and JPL to achieving the goal of bringing

about an early realization of a technically sound and economically viable

MSS. This is the ultimate goal of MSAT-X and, more generally, of NASA's

communications program, which aims to preserve the U.S.'s leadership in space

communications.

Because of the high-level nature of this document, the emphasis in

the presentation is not on fine detail—rather, the intent is to reveal as

many as possible of the technical issues in MSAT-X, report the salient

results, and identify their impact on MSS. In a parallel effort, a more

detailed and technically exact document is currently being prepared. Many of

the significant points that are not addressed here are addressed in that

document, which is referred to in the text as SYSDOC [Jet Propulsion

Laboratory, in preparation]. Further information can be found also in the

many references cited in the present document. It is worth noting that in

choosing the references, the emphasis has been on those articles that provide

the reader with overviews of the technical issues addressed. These sources,

in turn, provide more references that guide the specialist to the fully

detailed technical information.

1-1
This section is followed by the introduction of the MSS concept in

Section 2. Section 3 contains a clear identification of the challenges that

have to be conquered before an MSS becomes a reality. These challenges are to

overcome the scarce resources peculiar to the difficult environment in which

the MSS will operate. Section 4 then explains NASA's approach to overcoming

these challenges, and from there introduces NASA's MSAT-X program, its

objectives, and the premise and philosophy upon which it is based. Section 4

also points out the relationship between MSAT-X and the MSS industry, embodied

at present in the consortium of MSS applicants. This perspective is needed

for the proper appreciation of the implications of Sections 5 and 6. Section

5 contains five subsections, each introducing one of the MSS ground segment

technology areas researched under MSAT-X. This sets the stage for Section 6,

which draws upon the results described in Section 5 to present JPL's

recommendations for MSS ground segment technology utilization. The

recommendations are directed not only at each technology area, but also at

creating a cohesive effective ground segment. Inasmuch as the ground segment

is a part of an overall MSS system architecture, consideration of the space

segment and overall system issues is necessary. Realistic space segment

assumptions are made, and the ground segment is quantitatively tied to the

space segment through link budgets and system capacity results.

1-2
SECTION 2

MSS CONCEPT

An MSS is a satellite-based communication network that provides

voice and data communications to mobile users over a vast geographical area.

The relay satellite, or satellites, in geosynchronous orbit at a height of

22,000 miles permits an extremely large coverage array. The type of service

provided augments, and complements, the land mobile or cellular terrestrial

systems, which are generally more economical in localized urban areas.

A variety of services have been proposed for MSS by its potential

commercial operators. The services envisioned include two-way voice, two-way

interactive data communication, and one-way data as in dispatch operations,

position determination, or data base query.

The MSS network concept is depicted in Figure 2.1. At the heart of

the system is the Network Management Center (NMC). It performs such functions

as answering requests for channel assignment, setting up call routes, billing,

and generally overseeing the network's operation. Other entities on the

ground include the gateways and base stations. The gateways are terminals

that provide an interface between the MSS and other networks such as the

Public Switched Telephone Networks (PSTN). The base stations are not

necessarily connected to other networks and are normally centers for dispatch

operations.

The two other key elements in MSS are the mobile terminal and the

satellite. The mobile unit operates in a hostile environment where multipath

from terrain exists together with shadowing from trees or obstacles. A

minimum mobile terminal consists of the radio, antenna, and user interface (as

will be explained later). This equipment is constrained to being low in cost

and small in size; both are necessary features if the system is to have the

wide customer acceptance needed for its economic viability. The satellite, on

the other hand, by its dedicated design, is limited by the feasible technology

and evolves in steps or generations. As new technology becomes available, and

the spacecraft evolves accordingly, the spacecraft becomes capable of

supporting an MSS network with growing needs.

2-1
NETWORK

MANAGEMENT

CENTER

PUBLIC SWITCHED

TELEPHONE NETWORK

BASE

STATION Ku-BAND LINK

L-BAND LINK

TERRESTRIAL LINK

Figure 2.1. Mobile Satellite Network Concept

2-2
SECTION 3

MSS SCARCE RESOURCES

A number of studies undertaken by NASA have revealed that a growing

MSS will have to overcome scarcity in three critical areas, viz., spacecraft

EIRP, spectrum, and orbital slots. Although this is not unique to MSS, some

of the problems and solutions are unique because of the specific nature of MSS.

3.1 EIRP

A critical MSS communications link is the forward link between the

satellite and the mobile terminal. The small, possibly unsteered mobile

antenna on the vehicle has relatively low directivity and can be susceptible

to multipath. In addition, shadowing by trees and other obstructions can

severely attenuate the signal. Hence, the space segment will have to make up

part of the deficiencies in the link power budget by transmitting higher EIRP.

A mobile satellite system generally favors a single-channelper-carrier (SCPC) architecture. As will be explained in Section 4.1,

frequency division multiple access (FDMA) is the multi-access scheme of choice

under the severe spectrum limitations described below and the requirement for

simple mobile terminals operating in a hostile fading environment. This,

coupled with the large geographical distances between the users, leads to an

SCPC design. The SCPC operation exacerbates the spacecraft-transmitted radio

frequency (RF) power problem. To limit intermodulation distortion in the

spacecraft transmitter, the high-power amplifier will have to operate in the

linear region at a reduced efficiency. For a fully operational MSS with

thousands of users, the power requirement on the spacecraft would be very

large. Thus, it is of great importance to devise power-efficient technologies

to minimize the power-per-channel requirement in the MSS.

3.2 SPECTRUM

The potential frequency spectrum available for the MSS in the U.S.

is severely limited. The Federal Communications Commission (FCC) has

allocated the frequency spectrum shown in Figure 3.2.1 to a combination of

3-1
mobile satellite services. MSS will operate as co-primary with the

Aeronautical Mobile Satellite Service (AMSS-R) in the bands 1549.5 - 1558.5

and 1651 - 1660 MHz, and as secondary to the aeronautical mobile satellite

service in the bands 1545 - 1549.5 and 1646.5 - 1651 MHz. On the

international level, the frequency assignment issue was recently addressed by

the 1987 World Radio Administrative Conference (WARC). The allocations are

shown in Table 3.2.1. Although the United States and Canada took reservations

on the WARC provisions, it appears that, on the international level, the

spectrum allocation for MSS will be even more restrictive than within North

America. It is therefore evident that efficient utilization of the limited

available spectrum, within the constraints placed by the other scarce

resources, is paramount.

3.3 GEOSYNCHRONOUS ORBIT

Due to the requirement that the mobile antenna be small and

relatively inexpensive, high directivity, comparable to fixed satellite

systems, is not possible. An ornni antenna would illuminate the.entire

geosynchronous arc over the contiguous United States (CONUS). This, unless

spread spectrum is used, would preclude the possibility of employing orbit

reuse (using more than one satellite operating in the same frequency band to

increase system capacity). For either low- or medium-gain antennas, antennas

that rely only on low axial ratios to provide polarization isolation as a

primary means of orbit reuse are not possible. Hence, pattern discrimination

and polarization isolation are needed to permit orbit reuse. One of the

design goals for a medium-gain antenna would then be to provide sufficient

inter-satellite isolation for multiple satellite operation. The inherent

directivity creates a challenging design problem to meet the required tracking

of the satellite while the vehicle moves.

3-2
Table 3.2.1. WARC 1987 Frequency Spectrum Allocations for Mobile Services

Downlink (MHz) Allocation

1530 - 1533 Coequal primary MSS and maritime mobile satellite

services (MMSS)

1533 - 1544 MSS secondary to MMSS (limited to nonspeech low-bit-rate

data)

1545 - 1555 Aircraft-to-ground mobile telephone with restrictions

1555 - 1559 Exclusive primary MSS

Uplink (MHz) Allocation

1626.5 - 1631.5 MSS secondary to MMSS

1631.5 - 1634.5 Coequal primary MSS and MMSS

1634.5 - 1645.5 MSS secondary to MMSS

1646.5 - 1656.5 Ground-to-aircraft mobile telephone with restrictions

1656.5 - 1660.0 Exclusive primary MSS

1660.0 - 1660.5 MSS coequal primary with radio astronomy

3-3
UHF

CONVENTIONAL PROPOSED

ANDTRUNKED PUBLIC

PRIVATE^ SAFETY

MOBILE TRANSMIT

PROPOSED

CELLULAR

CELLULAR

SYSTEMS

PROPOSED

CELLULAR

PROPOSED

RESERVE

(CANADIAN

CtLLULAR^. IVIbb.'J- MSS?)- y

\ i \ i n

MOBILE RECEIVE

t

D6 82

^

n 82

.

f

4 !

^

325 8'

•

^ 1 rl

IS 849 851

i i

i i 1 i i i

i i i i

1 i i i

851 866 869870 890 894 896

L-BAND

SPACE-TO-EARTH

AERONAUTICAL MOBILE

SATELLITE (R) PRIMARY

MOBILE SATELLITE SERVICE

(MSS) SECONDARYMOBILE SATELLITE

SERVICE (MSS)

AERONAUTICAL MOBILE

SATELLITE (R) CO-PRIMARY

AERONAUTICAL

MOBILE

SATELLITE (R)

—V tAULUblVt— EXCLUSIVE- j

\ i Fi

1545 1549.5 1558.5 1559

EARTH-TO-SPACE

1646.5 1651 1660 1660.5

Figure 3.2.1. Frequency Spectrum Allocations by the FCC for Mobile Services

3-4
SECTION 4

NASA'S APPROACH TO OVERCOMING MSS SCARCE RESOURCES

Since the 1970s, the U.S.'s national policy, as evidenced by

presidential directives such as PD42, has aimed at bolstering the role played

by NASA in the field of satellite communications. In response, NASA's

communications program has been directed at maintaining the U.S.'s position in

the increasingly competitive satellite communication market. In this spirit,

NASA launched the MSAT-X program to support the U.S. industry in developing

new concepts and the high-risk technologies that would enable the realization

of a commercial MSS.

Since the MSS can be conveniently divided into a space segment and

a ground segment, NASA structured different program phases to develop the two

segments. It was also clear that the ground segment requirements would have a

significant impact on all aspects of the MSS. Consequently, the ground

segment has received the greatest amount of research and development to date.

The development strategy reflected a broad conceptual MSS design philosophy

that NASA had formulated in the early eighties. This design philosophy

influenced the ensuing development through implicit assumptions about how an ~

efficient MSS could be put together to overcome its scarce resources. The

envisioned system concept will be explained in the next subsection. This is

followed by two subsections to outline the roles of the space and ground

segments, and their development effort, in the overall MSS picture.

4.1 NASA'S CONCEPTUAL MSS DESIGN PHILOSOPHY

In this document, "system design philosophy" means the broad

approach to creating an MSS concept that overcomes the scarce resources of

bandwidth, power, and orbit. The system concept envisioned by NASA is based

on an FDMA scheme combined with bent-pipe spacecraft that utilize multibeam

technology to effect frequency reuse. The FDMA efficiently packs as many

channels as possible in the limited spectrum of Figure 3.2.1, or any given

part thereof (consistent with the isolation requirements of the multibeam

scenario). Since, even with efficient modulation and speech representation,

the ratio of the overall available bandwidth to a single channel bandwidth is

4-1
too small to support the number of users expected in a full system, frequency

reuse is essential to complement the FDMA scheme.

The choice of FDMA and frequency reuse through satellite multibeam

technology evolved from JPL in-house and NASA-contractor studies that

evaluated a variety of preliminary MSS concepts and foreseen market trends,

demands, and requirements [Naderi, 1982; TRW, 1983; and General Electric

Corporation, 1983]. The merits of such a choice may not be immediately

obvious when compared with other alternative concepts based on code division

multiple access (CDMA) or time division multiple access (TDMA). There are,

however, strong arguments to be made against those alternate choices,

particularly under the circumstances that have been associated with the

evolution of MSS throughout the eighties (such as the regulatory climate).

A CDMA system would, firstly, be hampered by the limited bandwidth

available. Its key design parameters and performance relate directly to the

spreading (contiguous) bandwidth available. Thus, in an uncertain regulatory

climate, with the possibility of being allowed less than the expected

bandwidth, predicted CDMA performance would be questionable. Although adding

users to the CDMA system would be possible within a certain ceiling, it is

contended that CDMA is not as flexible as FDMA in accommodating unexpected

growth in the future. This is because of the high impact of the CDMA

parameters on the user equipment. For example, a change in the basic

spreading parameters may be unfeasible because of the need to upgrade the user

terminals.

The above factors have steered NASA away from CDMA. Nevertheless,

discounting CDMA completely as a basis for a network architecture should be

based on a thorough system trade-off study.

TDMA is arguably a less desirable choice. First, TDMA is not

well-suited to the low burst rates envisioned for MSS user terminals. If MSS

were configured for high burst rates, this would lead to high peak power

requirements and most likely to more expensive terminal hardware—a strong

drawback. Low burst rates, on the other hand, require long frames and lead to

long delays. Equally important, in a fading and shadowing environment, TDMA

4-2
would be vulnerable to frequent loss of synchronization. Moreover, the higher

bit rates of TDMA would create even more demanding synchronization

requirements, with resulting network and user terminal complications.

Although the technology developed for MSS is strongly influenced by

the FDMA/raultibeam scheme, critical technologies such as vehicle antennas and

speech coding are largely independent, and essential in any conceivable MSS

architecture. It should also be kept in mind that NASA's goal has been to

assist the industry in developing new concepts and high-risk technologies

rather than in actually implementing a specific system. This philosophy has

been reflected in NASA's efforts, as will become clearer in the following two

subsections.

4.2 MSS SPACE SEGMENT EVOLUTION

As outlined above, the MSS envisioned by NASA relies on multibeam

spacecraft technology. The reliance, however, is in response to the potential

need to expand the capacity of future systems. Hence, it allows for the time

necessary to develop generations of more advanced spacecraft multibeam

antennas. As larger spacecraft antennas with more beams become available, an

MSS network with increasing capacity can be supported.

A first-generation MSS spacecraft would be based on the current

technology of communication satellites. Such a first-generation satellite

would probably have an antenna diameter of less than 5 meters. This would

allow launch via the space shuttle without the need for new, high-risk

deployable antenna technology. Such an antenna would only have a limited

multibeam capacity. Later-generation satellites would have larger antenna

diameters and a considerably higher multibeam capacity. These antennas would

be large enough to resolve the EIRP problem, and have enough beams to enable

sufficient frequency reuse to resolve the bandwidth problem. A schematic

illustration giving an example for the evolution of the multibeam coverage is

given in Figure 4.2.1.

Early studies have been performed to investigate the feasibility of

the MSS using multibeam technology. Unfortunately, the studies were performed

for an MSS using UHF rather than L-band, as now is the case. These early

4-3
results have been presented in [Naderi, 1982] and later in [Sue et al.,

1985]. Because the MSS concept has evolved since those early days, the

numbers quoted are no longer relevant, but the analysis serves to illustrate

the scope of the problem. The sizes of the antennas would be smaller at

L-band and consequently easier to build, at least from a structural standpoint.

A fair amount of work under the guidance of NASA and JPL has

already been accomplished in the area of large antennas. A summary of these

activities is given in [Naderi et al., 1984], together with the appropriate

references. Looking towards the future, a later-generation antenna will

probably be a large, flexible, and asymmetric structure posing challenging

problems to control system designers. NASA, with its Space Station capability,

will have a unique facility to experiment with the assembly of large

structures in space.

4.3 MSAT-X, ITS OBJECTIVES, AND ITS RELATIONSHIP TO INDUSTRY

The mobile satellite experiment (MSAT-X) is the framework within

which NASA has been developing critical ground segment technologies, and it

has been organized as a research and development task at JPL. Its present

goals are the development and demonstration of advanced ground segment

technologies for first-generation mobile satellite systems, the objective

being the acceleration of the implementation of MSS through timely infusion to

the industry of critical high-risk technologies. The ground segment

technologies all aim at overcoming the scarce resources of power, bandwidth,

and orbit. Figure 4.3.1 depicts the technology areas related to the

resolution of the scarcity of each of the three resources. Validation of the

developed technology by a set of experiments is also a goal for MSAT-X. In

addition to validating the newly developed equipment, these tests will aid

NASA's and JPL's effort to assist government agencies in determining the

usefulness of MSS services to their mobile operations.

Since the MSAT-X task has been defined as a technology development

task (as opposed to the development of a specific end-to-end network), system

requirements have basically taken the form of requirements for the individual

technology development areas. Of course, the different requirements reflect a

4-4
consistent end-to-end definition that utilizes FDMA/multibeam spacecraft

architecture. Yet, because the detailed network architecture in an

operational MSS of the future depends on the networking philosophy of the

network operator, the requirements at this stage have been maintained as

requirements for the technology areas, each contributing its share to the

overall efficiency and viability of the overall system.

The FCC has determined that only one MSS is feasible for the first

generation of mobile satellite service. This is a reflection of the scarcity

of resources that a successful MSS has to conquer and the present

state-of-the-art in mobile communications. As a result of the FCC's ruling, a

single U.S. consortium (the American Mobile Satellite Consortium [AMSC]) has

been formed of all MSS industry applicants. At present there are eight

members in the consortium. The members have differing opinions on a variety

of technical issues pertaining to the MSS architecture. These views reflect

differing technical opinions or business orientations. Differences

notwithstanding, the members will have to agree on a single system

architecture. That architecture may then contain a variety of technically

feasible options whose ultimate viability will probably depend on market

forces and technology maturity.

One of MSAT-X's goals is to make available the system concepts and

technology needed to steer the consortium towards making the decisions that,

it is hoped, will ultimately lead to early realization of the most viable

MSS. JPL, as an arm of NASA, an impartial government agency, and as a leading

technology research center, is in a unique position to assume this

responsibility. Through concrete and demonstrated results of MSAT-X, this

positive and highly visible role of NASA/JPL should be a major factor in

achieving the ultimate goal of maintaining the U.S.'s leadership in space

communications.

Concrete and demonstrated results rest on calculated, executed, and

disseminated work. The following sections will attempt to provide a concise

presentation of this work. It should be kept in mind that the presentation is

mainly concerned with the MSAT-X effort; it will not deal with decisions made

within the consortium so far.

4-5
Figure 4.2.1. Example of Evolution of Multibeam Coverage

ORBIT

CONSERVATION

DEVELOP:

STEERABLE

VEHICLE

ANTENNAS FOR

MULTI-SATELLITE

OPERATION

SPECTRUM

CONSERVATION

DEVELOP:

• EFFICIENT

MODULATION

• EFFICIENT

SPEECH

ENCODING

• EFFICIENT

MULTIPLE

ACCESS

TECHNIQUES

POWER

CONSERVATION

DEVELOP:

• MEDIUM-GAIN

VEHICLE

ANTENNA

• POWER

EFFICIENT

MODULATION

• CODING

Figure 4.3.1. Areas of Concentration for MSAT-X Technology Development

4-6
SECTION 5

TECHNOLOGY AREAS RESEARCHED UNDER MSAT-X

In the following sections the different research performed and

technologies developed under MSAT-X are described. The design principles,

salient characteristics, and performances obtained are presented for each

technology area. It is not the purpose here to be exact—rather, the intent

is to give a presentation that touches on the different technical issues,

which relate to the reader the work undertaken within MSAT-X, and which form

the background needed for MSS system-level decision making. (A more complete

presentation is given in [SYSDOC].) The results reported here (in Section 5)

are considered and compared from a broader MSS system-level point of view in

Section 6.

5.1 VEHICLE ANTENNAS

5.1.1 Challenging Objectives

As was illustrated in Figure 4.3.1, the vehicle antenna is a major

consideration in the areas of power and orbit conservation. It was realized

from the outset that the vehicle antenna will be a very critical technology.

The reasons are essentially the small-size and low-cost requirements that have

been placed on the vehicle antenna and that are crucial for the economic

viability of an MSS. The challenges are to simultaneously: (1) provide as

high an antenna gain as possible, (2) provide sufficient directivity to enable

orbit reuse, (3) minimize the physical dimensions and preferably make the

antenna conformal to the vehicle, (4) reduce the effects of multipath at low

elevation angles, and (5) be capable of rapidly acquiring and then tracking

the satellite despite the adverse operating environment (vehicle motion,

multipath, and shadowing). All of the foregoing requirements have to be

achieved through a low-cost, reliable antenna.

5.1.2 Antenna Design Considerations

Two broad categories of antennas have been considered: very-lowcost, low-gain (omni-type) antennas (LGAs) and directive medium-gain antennas

5-1
(MGAs). The LGAs may provide up to about k dB of gain at 20° elevation from

the horizon. Invariably, they have very wide beams covering essentially all

the orbit above CONUS. MGAs, on the other hand, have higher gains (8 to 12 dB

at 20° elevation) and considerably more directivity than LGAs. Because of the

MGAs1 directivity, they need a satellite acquisition and tracking capability.

This contributes significantly to their cost and causes them to be

considerably more expensive than LGAs.

Because of MSS capacity considerations, one of the most important

factors affecting the mobile antenna design is the need for orbit reuse. To

enable orbit reuse, the mobile antenna must provide sufficient adjacent

satellite discrimination. Apart from frequency discrimination, which is

wasteful of precious MSS bandwidth, adjacent satellite discrimination can be

obtained through spatial (pattern) discrimination and/or polarization

discrimination. Either one or a combination of both could be used, depending

on the antenna characteristics and the amount of discrimination required.

Generally, 20 to 25 dB discrimination will be sufficient for mobile satellite

systems employing digital modulations. To achieve the needed discrimination,

polarization discrimination alone is not sufficient in MSS. This is in part

due to the inability to achieve good circular polarization at low elevation

angles with antennas mounted on conducting vehicle rooftops. Moreover, since

the elevation angle varies from about 20° to 60° over CONUS, at or below the

Brewster angle the sense of polarization reverses upon reflection from the

ground or obstacles, making polarization discrimination very limited. Figures

5.1.1 and 5.1.2 [Huang, 1987a] show a typical cross-polarized pattern in

relation to a typical co-polarized pattern for an LGA and an MGA.

Polarization discrimination, if employed, must be combined with pattern

discrimination.

The amount of pattern discrimination is directly related to the

size of the mobile antenna. An LGA is compact in size, but obviously offers

little or no spatial discrimination. On the other extreme, a high-gain

antenna (HGA) would be large and require a pointing capability that would make

it very expensive. Hence, because of their size and cost, HGAs are unsuitable

for the mobile application. Motivated by the need to effect orbit reuse as

well as to alleviate the spacecraft power burden, and to work within the cost

5-2
and size constraints of MSS, the research and development effort in MSAT-X has

recently concentrated on MGAs.

The concentration on MGAs in MSAT-X does not mean that LGAs have

been discarded. In spite of their major drawbacks (having low gain, having

low directivity, and offering less immunity to multipath, particularly at low

elevation angles, because of their wide azimuthal beamwidths), LGAs are cheap

and simple, and therefore continue to be of interest to MSS designers. LGAs

have been studied at JPL during the early stages of MSAT-X, and several

breadboard units have been developed. The results can be found in [Woo, 1982;

Woo, 1984; and Naderi et al., 1984] or [SYSDOC, Section 2]. The problems

stemming from the utilization of LGAs on the vehicle are addressed from an MSS

overall system perspective in Section 6.4.1.

A major effort to study, develop, and demonstrate MGAs was

undertaken under the leadership of JPL. After detailed conceptual studies by

contractors with experience in antenna manufacturing [Ball Aerospace

Corporation, 1984; Cubic Corporation, 1984; and SYSDOC, Section 2.2], three

types of MGAs were selected for development: a low-profile mechanically

steered linear array, a conformal electronically steered phased array, and a

planar mechanically steered array. The intent was basically twofold, to

provide the maximum reasonable number of options to the industry and

eventually the system user, and to maintain a certain amount of

diversification, since the technology involved is high risk and may lead to

disappointing results when implemented. The mechanically steered, tilted

linear array breadboard was developed mostly in-lab at JPL. The others are

being developed by contractors. All the breadboards are currently undergoing

testing in realistic conditions to determine their full characteristics. The

detailed requirements, characteristics, and performance of the different

antennas are discussed in [SYSDOC, Section 2]. In what follows we summarize

the key technical features and performance characteristics. This will then

set the stage for the technology recommendations for MSS given in Section 6.

5-3
5.1.3 Salient Vehicle Antenna Requirements

Detailed requirements for the different medium-gain vehicle

antennas are given in [SYSDOC, Section 2], The references therein give the

complete specifications. Here we mention the most important requirements to

prepare for the antenna descriptions to follow and for subsequent comparisons.

The RF requirements for the mechanically steered, tilted linear

array and electronic phased array are basically the same. The design goals

call for a. minimum gain of 10 dBic over elevation angles of 20° to 60° from

the horizon (the real engineering challenge is to achieve it at 20°); a

minimum of 20-dB adjacent satellite isolation for 30° to 40° satellite

spacing; a. 6-dB roll-off from 20° to 0° elevation to reject multipath fading,

which can be severe near the horizon. For the planar mechanical array the

requirements are the same except that a 12-dBic minimum gain is specified.

There are also detailed requirements on insertion losses (see, for

example, [SYSDOC] and the references cited therein). From a system

standpoint, the contribution of these losses to the ratio of the antenna's

gain to its effective noise temperature (G/T) is the major concern. The G/T

is a critical antenna performance measure. It determines the vehicle

antenna's receiving performance, which is on the most critically power-starved

and vulnerable link from the satellite to the mobile terminal. G/T values for

MGAs that are significantly higher (by 5 dB or more) than for LGAs are

imperative for MGAs1 viability, and important for MSS as a whole. Simple

LGAs, like the drooping dipole, can readily achieve G/T values of -20 dB/K at

20° elevation (with a receiving system of 1.8-dB noise figure [NF]). Hence,

G/T values of -15 dB/K or higher (at 20°) need to be demonstrated with the

MGAs.

As for the pointing subsystem, requirements are somewhat different

for the mechanical and electronic antennas. The acquisition time is 10 s for

the phased array and 15 s for the mechanical arrays. Both antenna types are

required to track the satellite with 2° or less error despite possible vehicle

rotation at a rate of up to 45°/s.

5-4
5.1.4 Vehicle Antenna Designs

In the following sections the different antenna types are

described. Their design principles, characteristics, and performance are

presented. The results are compared in Section 6 from an MSS standpoint.

5.1.4.1 Mechanically Steered, Tilted Linear Array

5.1.4.1.1 Antenna System Design. Tilted linear arrays containing 4, 6, and 8

elements have a broad elevation beamwidth (typically 65°) and a narrow azimuth

beamwidth about 25°, 19°, and 13°, respectively. Because of the narrow

azimuthal beam, tracking in the azimuthal direction is required. To point the

beam at the desired range of elevation angles, the array is tilted

approximately 45°. Preliminary investigations indicated that the minimum gain

and adjacent satellite isolation requirements can be met by an array

containing only 4 elements. Consequently, a proof-of-concept breadboard

tilted linear array antenna system using 4 elements and including the pointing

subsystem was developed by JPL [Jamnejad, 1988a and 1988b]. It was

demonstrated successfully in the first of a sequence of Pilot Field

Experiments (PiFEx) [Jet Propulsion Laboratory, 1988]. The principal

objective of this experiment was to evaluate the antenna's performance,

particularly the performance of its pointing system developed at JPL. This

objective was met, as discussed further in the following subsection.

The antenna contains the radiating elements, which are four square

microstrip patches, stripline combiners, and an antenna pointing system, which

includes an angular rate sensor, stepper motor, drive assembly, motor

controller, hybrid coupler, phase modulator, and monopulse receiver. The

whole assembly is covered by a radome. The antenna assembly is shown in

Figure 5.1.3. The rate sensor, motor controller, motor, and drive assembly

are placed on the fixed base-plate of the antenna, while the antenna array,

sum/difference hybrid, coupler, and phase modulator are placed on a rotating

circular platform, which is stacked on top of the fixed platform. The

breadboard has a 20-in. diameter, and a 9-in. height, which will be reduced to

5 in. in later versions. This may involve the use of a "pancake" motor

5-5
assembly or the relocation of the pointing electronics behind, rather than

under, the array.

5.1.4.1.2 Pointing Subsystem. Antenna pointing is accomplished using a

closed-loop pseudo-monopulse system aided by an open-loop system. The

open-loop system includes an angular rate sensor which enables initial antenna

pointing and maintains pointing during signal fades by providing an inertial ;

reference [Bell and Naderi, 1986; Jamnejad, 1988a and 1988b; and Berner and

Winkelstein, 1988].

The closed-loop system requires a satellite-transmitted signal or

pilot as a reference for tracking. Signals from two subarrays (adjacent

halves) are passed through a hybrid coupler to form the sum and difference

signals. The difference signal is then modulated by a squarewave signal and

added to the sum channel to form a composite signal. The composite signal is

then routed to the receiver, where the error signal is detected. After

filtering, the error signal is fed to a stepper motor, closing the servo

tracking loop. The motor edge-drives the rotating antenna platform. Figure

5.1.4 gives a schematic block diagram of the pointing system [Jamnejad, 1988a].

Field tests have demonstrated that the antenna/pointing system can

successfully acquire and track a satellite-transmitted signal in an actual

mobile environment, i.e., in the presence of fading, shadowing, and vehicle

motion [Berner, 1987, and Berner and Winkelstein, 1988]. Acquisition is

performed within the specified 15 s, and the pointing system tracks the

satellite within 2° of error. This requirement is met when the vehicle is

turning at 45°/s [Jamnejad, 1988a, and Berner and Winkelstein, 1988].

The pointing algorithm is currently implemented on a personal

computer for the field tests. The entire algorithm is suitable for

implementation on a single circuit board.

In a multibeam, frequency-reuse mobile satellite system, the

satellite transmits a number of pilot signals, each having a distinct

frequency, to facilitate antenna pointing and perform other functions. The

tracking system must perform spatial (in the azimuth direction) and frequency

5-6
acquisitions in order to lock onto the desired pilot signal. The task of

pilot frequency acquisition is assigned to the transceiver, which interacts

with the antenna under the supervision of the mobile terminal processor. For

the mechanical antennas the rotation is sufficiently slow that it allows for

frequency scanning during a single rotation. Frequency scanning could be

repeated at a number of angular positions determined by the dwell time

necessary for pilot acquisition. The dwell time per pilot has been shown

through analysis to be 40 ms [Winkelstein, 1987]. Since the antenna takes

9.6 s per rotation, frequency scanning can be repeated at up to 60 positions

in an MSS with four pilot frequencies.

5.1.4.1.3 Antenna Performance and Cost. The antenna is tilted with respect

to the ground plane to provide elevation coverage from 20° to 60°. The

minimum gain demonstrated in the coverage range is 10 dBic and the maximum

gain is about 12 dBic. Elevation and azimuth radiation patterns are shown in

Figures 5.1.5 and 5.1.6 [Jamnejad, 1988b]. Preliminary results indicate that

the adjacent satellite requirement is met, as are the pointing requirements as

outlined above.

As mentioned earlier, a critical performance measure that determines

the efficacy of this type of antenna is G/T, the ratio of its gain to effective

noise temperature. G/T plays a major role in determining the performance of

the critical forward downlink (the link from the satellite to the mobile

terminal), and as such, it has a significant impact on the design of the

satellite communications payload. The contributors to the antenna temperature

are the sky temperature, which is basically fixed; the ground temperature as

seen by the antenna, which is generally lower for a more directive antenna;

and finally, the internal temperature due to the circuitry inside the antenna.

For MGAs a major contributor to the effective antenna temperature is the

internal component. Preliminary tests have been performed recently at JPL to

obtain values for the G/T (and its components) for the proof-of-concept

antenna (and other antennas as well). G/T values around -15.3 dB/K have been

obtained [Bell et al., 1988]. It should be kept in mind that the proof-ofconcept antenna is not a refined production prototype and therefore yields a

conservatively low G/T.

5-7
The mechanically steered linear array antenna concept with its good

performance, but nonconformal shape, is expected to become available for a

manufacturing cost of $600 at a production level of 10,000 units per year over

a five-year period.

5.1.4.2 Electronically Steered Phased Array Antenna. Phased array antenna

development was initiated in 1984 through a number of study contracts. In the

early phase of the development, various concepts and designs were examined.

These have evolved into the breadboard designs presently under consideration.

The evolution up to the present status is traced in [SYSDOC, Section 2.2].

Here we present and compare the breadboards recently delivered by Ball

Aerospace and Teledyne Ryan Electronics (TRE). (Each contractor has delivered

two breadboard units.) A more detailed presentation is given in [SYSDOC].

5.1.4.2.1 Array Design. Figure 5.1.7 [Huang, 1987a] depicts a

cross-sectional view of the antenna. The planar array contains a radiator

layer, a hybrid layer, a phase shifter and power divider layer, and a

supporting structure layer.

In the two designs the radiator layer contains 19 radiating

elements, which are arranged in a triangular lattice to maximize the resulting

gain and avoid grating lobe formulation (Figure 5.1.8). The radiating

elements are implemented using microstrip or stripline technologies. In the

Ball Aerospace design, the radiating elements are dual resonant, stacked,

0.5-in.-thick circular microstrip elements. The 3-dB beamwidth of a single

element is about 90°. In the Teledyne design, a stripline, cross-slot

radiator is employed, with a 3-dB beamwidth of approximately 140° [Huang,

1987a]. The microstrip element, due to its narrower beamwidth, may have a

lower gain at a low elevation angle than the stripline radiator. However, the

microstrip element is mostly composed of low-cost foam material, in contrast

to the stripline radiator, which requires higher-cost dielectric material.

Figures 5.1.9 and 5.1.10 [Huang, 1987a] show the microstrip and stripline

designs, respectively.

5-8
The hybrid layer contains the hybrid circuits that generate the

orthogonal phases to create circular polarization. The hybrids of the TRE

antenna are of stripline design. In Ball's design, the hybrid is a microstrip

design etched on the same layer as the radiator.

The phase shifter and power divider layer contains 18 3-bit phase

shifters and a 19-way power divider. The two contractors employ different

designs for the phase shifters. Ball Aerospace uses a hybrid reflection/

loaded-line design for the phase shifters, while Teledyne employs switched-line

diode phase shifters. Ball's hybrid design requires 6 diodes per phase

shifter, has about a 1.5-dB insertion loss, and purportedly has a 20° maximum

phase error (see Section 5.1.4.2.3). TRE's switched-line design requires 12

diodes per phase shifter, achieves a 1.3-dB insertion loss, and an 11° maximum

phase error. The phase shifters and the 19-way power dividers are implemented

on a microstrip board [Huang, 1987a].

5.1.4.2.2 Beam Pointing Technique. The mobile operating environment and the

directive beam of the 19-element phased array antenna necessitate the use of a

beam-pointing mechanism. This pointing mechanism performs the tasks of

satellite acquisition and tracking in the presence of multipath fading and

changing vehicle orientation. Tracking and acquisition are accomplished using

an open-loop angular rate sensor in combination with a closed-loop sequential

lobing technique. Initial acquisition of the satellite is accomplished by a

full azimuth and limited elevation search for maximum signal strength of a

transmitted pilot. (Two elevation positions are used due to the narrower

beamwidth of the phased array in the elevation plane.) The open loop is used

to establish the initial reference in azimuth. Once the satellite is

acquired, the closed-loop system begins to perform azimuth tracking with an

occasional elevation search. An occasional elevation search is sufficient

because the change of elevation angle is slow. During fading outages, the

beam pointing system relies on information derived from the open-loop system

to maintain its pointing at the satellite [Huang, 1987a].

The open-loop subsystem is basically the same as for the mechanical

antenna. It is equipped with a crystal or quartz-type angular rate sensor

5-9
[Bell and Naderi, 1986]. Designs using a mechanical gyro and compass have

been investigated and determined unsuitable for mobile applications because of

accuracy, cost, or reliability considerations.

Because azimuthal scanning is fast for the phased array, faster

than what would allow for simultaneous pilot acquisition, in a frequency reuse

system, in which several pilots are used, the entire azimuth is scanned at one

frequency, then repeated for others. The acquisition time, then, is a

function of the product of the number of pilot signals and the number of beam

positions. The number of pilot signals is determined by the number of the

frequency reuse factor, and the number of beam positions is the number of

positions that the beam must be rotated through in order to complete one 360°

rotation in azimuth. Analysis has shown that for a system using four pilot

signals and 50 beam positions, the pointing system can acquire the pilot in 8

s with a probability greater than 99% at a 35 dB-Hz carrier-to-noise ratio

[Winkelstein, 1987].

5.1.4.2.3 Antenna Characteristics and Cost. The phased array antenna

breadboards delivered by both contractors are conformal and can be easily

mounted on the top of a passenger car. The TRE antenna, including a radome,

is 0.7 in. in height and 20.7 in. in diameter. Ball's breadboard, on the

other hand, is 1.3 in. by 24 in. The maximum gain is 12 to 13 dB and occurs

at close to 60° elevation. The minimum gain for both antennas occurs at a 20°

elevation angle. For Teledyne's antenna, the average gain (over eight azimuth

positions) is 9.9 dBic at 1545 MHz (receive) and 8.2 dBic at 1660 MHz

(transmit). For Ball's antenna, the average gain at 20° is 8.2 dBic at 1545

MHz and 8.1 dBic at 1660 MHz. Complete patterns covering the three-dimensional

field of view are being obtained. Typical radiation and axial ratio patterns

in elevation and azimuth planes are shown in Figures 5.1.11 through 5.1.14.

The 3-dB beamwidth is about 25° in azimuth and about 35° in elevation.

As with the mechanical antenna, the G/T for the phased array is a

critical parameter. This is even more true here because of the potentially

high resistive internal loss resulting from the numerous diodes, power

dividers, etc. Preliminary G/T tests for the phased arrays have yielded G/T

values of about -15.4 dB/K for TRE's and -16.7 dB/K for Ball's.

5-10
From the preliminary data available, the 20-dB adjacent satellite

isolation is expected to be met 81% of the time by the Teledyne antenna, but

only 38%, on the average, by the Ball antenna. Multipath rejection,

represented by the 6-dB roll-off, is, in general, achieved. However, backlobe

levels that are higher than specification have been observed at certain

points. These high levels can be detrimental to inter-satellite isolation,

multipath rejection, and equivalent antenna temperature.

As for the performance of the pointing subsystem, acquisition for a

single pilot frequency takes about 2.5 s. The estimated tracking error is 5°

on either side of the boresight, of which the 3-bit phase shifter quantization

accounts for 2° and the remainder is due to pattern asymmetry, receiver noise,

etc. During tracking, the sequential lobing pointing algorithm imparts

unwanted amplitude modulation (AM) and phase modulation (PM) on the received

data channel signal. The specification calls for the peak-to-peak AM to not

exceed 1 dB and the PM to not exceed 10°. Ongoing tests indicate that both

antennas exhibit about 1.5 dB of AM. TRE's antenna also exhibits about 15° of

PM, while Ball's antenna creates an obviously unacceptable 37° of PM. It is

not clear at this time whether Ball's poor phase performance is due to design

limitations or implementation problems.

Last, and certainly not least, is the cost consideration for this

type of antenna. Unfortunately, the cost for conformity is not small. The

manufacturing costs have been estimated by Teledyne and Ball to be $1848 and

$1610 for their respective units, based on a production level of 10,000 units

per year over a five-year period. The cost breakdown for the two antennas is

given in Table 5.1.1 [Huang, 1987b]. It can be concluded that further

research is needed to reduce the cost of the dielectric material.

The salient characteristics of the phased array antennas (as well

as of the mechanically steered, tilted linear array) are summarized in Table

5.1.2.

Due to its superior RF performance and smaller size, the Teledyne

antenna has been preferred by JPL and has been viewed as the more likely

candidate for further phased array development. A final decision will be made

5-11
after more comprehensive field testing is performed by JPL. Antenna G/T,

reliability, and performance repeatability (from one unit to the second) will

be taken into account.

5.1.4.3 Planar Mechanically Steered Array. This antenna type is a

compromise between the conformal, yet expensive, phased array and the cheapest

MGA, the nonconformal mechanical tilted array. Due to its expected moderate

cost ($900 to $1000 manufacturing cost) and its nearly conformal shape

(1.5-in. thickness), it remains a very viable MGA candidate. JPL has recently

awarded a one-year contract to TRE to develop two breadboard units.

A minimum gain of 12 dBic is specified. At 12 dB all intersatellite and multipath rejection requirements are expected to be met. The

biggest challenge for this antenna is in the drive mechanism, particularly the

low-profile motor and rotary joints.

Progress for the planar mechanical array will be reported in later

versions of this document.

5.2 MODULATION, CODING, AND MOBILE TERMINAL DESIGN

5.2.1 Modulation/Coding

Modulation and coding play a key role in meeting the challenging

power and spectrum requirements in MSS. A considerable amount of research and

development effort has been performed at JPL with the aim of identifying,

implementing, and demonstrating a modulation/coding scheme well-suited to the

MSS application. The mobile environment requires a scheme that is bandwidthand power-efficient and robust (to fading, shadowing, frequency uncertainties

due to motion and hardware, etc.), and that lends itself to easy

implementation at a reasonable cost.

The modulation and coding candidates regarded as most promising

have changed with the evolution and maturity of the MSS concept and the other

technologies developed for it. The choices have also been strongly influenced

by the regulatory decisions affecting MSS. The FDM structure of the network,

5-12
combined with the desire to transmit 4800-bps near-toll-quality voice, has led

to the selection of 5 kHz as the basic channel bandwidth. This has been a

major factor influencing the choice of a modulation/coding scheme. The

evolution of the modulation/coding in MSAT-X is discussed in detail in

[SYSDOC]. In what follows we outline the progression of modulation schemes

considered to put the present candidate in the proper perspective and prepare

for the recommendations given in Section 6.

5.2.1.1 Minimum Shift Keying (MSK) and Gaussian MSK (GMSK). In the early

stages of MSAT-X, when a data rate of 2400 bps in a 5-kHz channel was

envisioned, MSK and then GMSK were considered. MSK is a form of continuous

phase frequency shift keying (CPFSK) that has a modulation index of 0.5.

Because MSK maintains a phase continuity during bit transitions, its spectral

sidelobes fall off more rapidly than in phase shifted keying (PSK) or quadriphase shifted keying (QPSK). This is, of course, of major importance in

controlling adjacent channel interference in the FDMA network. MSK was considered for mobile applications in the early phase of the research because.of

its relative spectral and power efficiencies. The focus was later shifted to

baseband filtered MSK to reduce adjacent channel interference.

Interference in the mobile satellite environment can be severe. A

mobile satellite system is likely to employ multibeam frequency reuse

technology and orbit reuse in order to maximize the spectral utility. Such a

system is often interference limited. A number of potential interference

sources are identified in Figure 5.2.1 [Sue et al., 1985]. Gaussian baseband

filtered minimum shift keying (GMSK) is spectrally more efficient than MSK,

but at the expense of a somewhat higher power requirement (received E /N

for the same data rate). By varying the baseband filter parameter, adjacent

channel interference can be significantly reduced. Figure 5.2.2 [Murota and

Hirade, 1981] shows the adjacent channel interference as a function of the

normalized channel separation (f T) for selected baseband filter parameter

values (B,T). As shown in the figure, with B,T = 0.5 and f T = 1.5, the

b b s

adjacent interference can be 10 dB lower than that for MSK. A separation

equal to 1.5 times the bit rate, which, at 2400 bps, corresponds to 3600 Hz,

would reduce adjacent channel interference to about 40 dB. With 5-kHz channel

5-13
spacing, this would still leave ample margin to accommodate other frequency

errors.

To demonstrate the practicality of MSK-type modulation in the

mobile environment, a practical two-bit differential MSK (DMSK) receiver was

built [Bavarian et al., 1985]. This receiver could also test GMSK by including

the appropriate Gaussian filtering along the chain. The receiver demonstrated

robust operation in the Rician fading environment and exhibited a lower error

floor than its coherent counterpart [Bavarian et al., 1985].

5.2.1.2 Trellis-Coded Modulation/8-Phase Shift Keying (TCM/8PSK). As the

voice coding requirement was reshaped to 4800 bps, a more efficient modulation/

coding was sought. The scheme had to provide the needed bandwidth containment

without sacrificing power efficiency. Research at JPL arrived at a combined

modulation/coding scheme as the solution to this challenging task of squeezing

4800 bps in a 5-kHz bandwidth while maintaining power efficiency. Trelliscoded modulation (TCM), combined with MPSK, was investigated, and a 16-state

rate 2/3 TCM with an 8PSK scheme was adopted [Divsalar and Simon, 1986]. (A

comparison of the relevant aspects of the different modulation schemes is

provided later, in the recommendations of Section 6.)

TCM is an approach that combines modulation and coding. Tradi-^

tionally, coding and modulation have been treated separately and independently

optimized. In TCM, the difference is that modulation and coding are combined

as a single entity in system optimization. In so doing, a significant improve

ment is obtained. In the combination, the 8PSK signaling offers the needed

spectral efficiency, while trellis coding matched to it provides performance

improvement, which offsets the power penalty that would be paid in a more

traditional scheme such as MSK.

Since a form of TCM/MPSK is the present candidate, we will present

some of its salient features in the following subsections. Again, a detailed

technical presentation will be found in [SYSDOC].

5-14
Research focused first on coherently detected TCM/MPSK, in which

phase information was assumed to be extracted by using some form of pilotaided technique (more on that shortly). Figure 5.2.3 shows a general block

diagram for a coherent end-to-end TCM system using MPSK [Divsalar and Simon,

1986]. Input bits representing data or the coded speech are first encoded by

a rate n/(n+l) trellis encoder. The encoded output symbols are block interleaved by an interleaver to ameliorate the effects of the bursty multipath

channel. The symbols at the output of the interleaver are then mapped into

the MPSK signal space using the set partitioning method [Ungerboeck, 1982].

To limit spectral occupancy and minimize intersymbol interference, the

in-phase and quadrature components of the MPSK signal are pulse-shaped. To

allow coherent detection, one or two pilot tones are inserted into a data-free

portion of the allocated transmit channel spectrum. At the receiver, the

in-phase and quadrature signal components are coherently demodulated using the

extracted pilot tone (or tones), quantized, block deinterleaved, and decoded

by a trellis decoder.

Various TCM schemes were investigated with symmetrical and nonsymmetrical signal sets. The most suitable candidate, as stated earlier, was

found to be rate 2/3, 16-state TCM with symmetric 8PSK modulation and interleaving .

Interleaving is essential to randomize burst errors induced by the

fading channel. Software simulation was used to obtain the optimum interleaver parameters, subject to the maximum processing delay requirement for

voice [Divsalar and Simon, 1986]. This delay requirement was placed at 60 ms

or less. At 4800 bps, and for rate 2/3 TCM/8PSK (2400 sps), the optimized

interleaver length was found to be 128 symbols (of 8PSK) with a depth of 16

and span of 8 symbols. This interleaver length corresponds to about 53 ms.

The bit error performance of this scheme, assuming ideal coherent detection,

is shown in Figure 5.2.4 [Divsalar and Simon, 1986]. As the figure indicates,

TCM provides about 3-dB improvement over uncoded QPSK, while maintaining the

same spectral efficiency. (Further comparisons, such as for the case of a

noisy reference, will be given shortly and in Section 6.)

5-15
To perform coherent demodulation, for TCM or otherwise, carrier

phase information is needed. Due to their feedback configuration, Costas loop

type schemes for carrier recovery are well-known to be unsuitable for use on

fading channels. Hence, research has considered a variety of schemes belonging

to the class of tone calibration techniques (TCTs) to effect carrier phase

recovery. In one such technique, a pilot signal is inserted into the data

spectrum. The problem with this is that the power in the data spectrum near

the pilot constitutes self-noise and consequently degrades its detection.

One way to avoid this problem is to create a notch in the data spectrum for

the pilot tone. There are basically two variations of TCT: single- and

dual-tone TCT (Figure 5.2.5 [Rafferty and Divsalar, 1988]). The single-tone

TCT involves creating a notch in the spectrum (which may expand the bandwidth,

if some form of spectral coding, such as Manchester coding, is used) and

inserting a tone in the gap in the middle. On the other hand, the dual-tone

TCT (DTCT) is generally simpler to implement and less wasteful of bandwidth.

Yet the performance of the DTCT is inferior to that of the single-tone TCT by

more than 3 dB, because of the squaring loss implicit in the carrier recovery

process. Another variation is a scheme referred to as transparent tone in

band (TTIB) [McGeehan and Bateman, 1984]. In this scheme, bandwidth is saved

relative to single-tone TCT by splitting the data spectrum rather than

expanding it. Unfortunately, to date no practical means of reconstituting the

split spectrum without distorting the data has been found (see, for example,

[Rensselaer Polytechnic Institute, 1988]). Therefore, DTCT is the prime

candidate if coherent detection is used.

Although TCT or DTCT can mitigate the effects of multipath fading,

they cause a decrease in the allowed data bandwidth in addition to the power

penalty. At UHF, 200 Hz are needed to accommodate each pilot. The 200 Hz are

needed to account for Doppler spread as well as for other frequency uncertainties. (Doppler spread is a phenomenon associated with the mobile satellite

channel caused by multipath fading. A carrier transmitted over this channel

is spread by an amount equal to the Doppler frequency of the mobile vehicle.)

The required pilot bandwidth became a critical issue as the regulatory process

forced the abandonment of UHF in favor of L-band. At L-band, the bandwidth

needed to handle a pilot would be about 400 Hz, due to the roughly doubled

Doppler spread. This bandwidth not only reduces the bandwidth available for

5-16
data transmission, but also increases the noise power in the notch(es) and

degrades pilot detection. To compensate for the increased noise bandwidth,

one would have to more than double the power in the pilot signal, making TCT

unattractive.

Differential detection is well-suited to the fading channel because

it requires phase stability over a short time interval (basically two

symbols). Figure 5.2.6 shows the block diagram for a multilevel trellis-coded

differential phase-shift keying (MDPSK) system [Simon and Divsalar, 1987].

The performance of the TCM 8PSK with differential detection (TCM/D8PSK) is

shown in Figure 5.2.7 for the cases with 10-dB fading (i.e., k = 10 dB, where

k is the direct line-of-sight to multipath ratio), no fading, with

interleaving, and without interleaving. As indicated, an average E /N

^ D O

of 9.5 dB is sufficient to achieve a bit error rate of 1.0 x 10 for k = 10 dB

with interleaving. In comparison with the performance of idealized coherent

TCM/8PSK in Figure 5.2.4 (where no degradation is assumed due to the implicit

DTCT), TCM/D8PSK requires about 2.5 dB more signal power for the same BER.

However, it is very important to note that DTCT detection is inferior to ideal

detection. This is a natural consequence of the noise surrounding the pilots,

specifically that observed within the band needed to accommodate the possible

Doppler spreading. To place the actual performance of DTCT in perspective, a

comparison of differential versus coherent DTCT detection of TCM/8PSK in the

fading environment is shown in Figure 5.2.8. As can be easily seen, DTCT is

slightly better than differential detection at UHF, but worse by about 2 dB at

L-band.

From the results just presented it is concluded that differential

detection is the viable option for the present setting in L-band.

Consequently, differential detection of 16-state rate 2/3 TCM/8PSK (TCM/D8PSK)

with interleaving is the baseline at present.

One of the important features of the modem in MSAT-X is the

built-in ability to estimate phase errors induced by Doppler or residual

frequency uncertainties. The large components of frequency errors in the

network are corrected at the NMC (see Section 6.4.1.4). The precompensated

pilot is tracked by the transceiver and used to set the local oscillator. The

5-17
remaining Doppler due to vehicle motion and residual frequency errors can be

up to 200 Hz. Consistent with differential detection, an open-loop Doppler

estimation and correction scheme was developed for the D8PSK demodulator

[Simon and Divsalar, 1988]. The scheme exploits the fact that the phase shift

observed over half a channel symbol is due to Doppler and not to data

modulation. Further, with the proper processing, the unwanted phase shift can

be mostly removed. For the modulation in MSAT-X, the presence of the two ISI

free points at T /4 and 3T /4 in the 100% root raised-cosine case (Figure

s s

5.2.9) is crucial to the operation of the Doppler estimation. (Samples with

independent noises can be extracted there. See [Simon and Divsalar, 1988] for

the theoretical details.)

Modem breadboard implementation at JPL is in its final stages of

development [Jedrey and Lay, 1987]. Lab testing using the channel simulator

(Sections 6 and 5.6) has been recently accomplished. Modem performance within

1 dB of theory was demonstrated for both the AWGN and fading channels [Jedrey

et al., 1988]. This is a significant result in favor of the present

modulation/coding baseline design. Field testing of the mobile terminal

breadboard will take place soon.

5.2.2 Mobile Terminal Design

Although a considerable amount of effort went into furthering the

state of the art in modulation/coding research, the modem is but one of the

ingredients of the mobile terminal. Research and development effort was also

undertaken to arrive at the best architecture for the mobile terminal. For

MSAT-X the goal has been to develop a structure that satisfies two objectives:

First, the architecture had to be modular and flexible to support the

technology validation experiments, and second, the breadboard transceiver

should utilize techniques that a final user mobile terminal design could take

advantage of.

The mobile terminal structure given in the block diagram of Figure

5.2.10 was arrived at [Cheetham, 1987]. The modular design is evident. The

terminal processor plays a central role in controlling the operation of the

mobile terminal. It controls the interfaces among the different subsystems,

5-18
such as the speech coder, modem, and transceiver, and also contains the logic

to control the execution of the antenna pointing algorithms and implement the

networking protocol. Details pertaining to the terminal processor can be

found in [Cheetham, 1987, and SYSDOC]. The terminal processor was developed

at JPL and is currently part of the experimental setup for technology

validation.

Proper design of the transceiver is crucial in the MSS link. The

performance of the receiver is inseparable from that of the receiving vehicle

antenna. The overall G/T is a critical factor in selecting a particular

terminal/antenna design option. This has major implications for the overall

MSS architecture. In particular, it strongly influences the satellite design

through the higher EIRP required to offset a lower receiving terminal G/T. A

higher EIRP implies a larger satellite and possibly high-risk satellite

development.

The transceiver/antenna interface has been validated in the experiments conducted by JPL. Effort to refine the transceiver design is also under

way at JPL to demonstrate the desired G/T in the neighborhood of -15 dB/.K.

The transceiver performs other functions that are also vital to the proper

operation of the mobile link. The functions of Doppler estimation and correction, and frequency calibration of the inexpensive local oscillator to a

network reference, are integral components in the operation of the ground

terminal and the network. Research and development effort in this area is

continuing.

5.3 SPEECH CODING

Voice communication is an essential ingredient of MSS. The challenge in MSAT-X has been to devise and develop voice compression algorithms

that operate at a low rate commensurate with the narrow channel bandwidth

requirement of MSS. For the purposes of MSAT-X, this bandwidth is taken to be

5 kHz for all channels.

5-19
5.3.1 Background

Both analog voice and digital voice have been considered for mobile

applications. Analog voice includes those techniques using frequency modulation (FM), single sideband modulation (SSB), and amplitude companded single

sideband modulation (ACSB). FM voice has been widely employed by terrestrial

systems, but it is spectrally inefficient compared to ACSB and SSB. Digital

voice is compatible with digital modulation techniques, and, with the introduction of advanced digital signal processing hardware, it has emerged as an

approach with considerable promise in the MSS application. The technology

development program has focused on digital voice.

The objective in MSAT-X has been to develop a "near-toll-quality,"

low-rate, real-time speech coding system, which, when coupled with power- and

bandwidth-efficient modulation/coding, can provide a quality voice service

within the tight bandwidth constraints of MSS. "Near-toll-quality" here is

intended to mean a natural-sounding, intelligible voice with good speaker

identification, that is free of processor-induced anomalies (such as bleeps).

As the name implies, it is a quality that approaches that of.a long-distance

telephone call. The goal is to achieve this quality with a real-time coder

and handset-type of user equipment.

Speech compression techniques can be divided into two broad

categories: the techniques of waveform coding, and those of source coding

[Flanagan et al., 1979]. Hybrids have also emerged. Waveform coding aims at

representing an analog waveform with a digital representation that, when

decoded, would produce a replica as close as possible to the original

waveform. Source coding techniques, on the other hand, aim at modeling the

human voice-producing mechanism. Waveform coding results in high-quality,

natural-sounding speech with robustness to background noise. Unfortunately,

it requires high data rates (16 kbps - 64 kbps). In 1984, high-quality

(broadcast commentary quality) speech was attained at 64 kbps using companded

PCM. At present, the CCIR is adopting a version of differential PCM (DPCM) at

32 kbps as the standard for toll-quality speech. Currently it is also

possible to get high-quality speech at 16 kbps.

5-20
A prominent source coding approach is Linear Predictive Coding

(LPC), which uses linear filtering theory to model the vocal tract. LPC has

been used in the past to produce fairly low quality speech at low data rates.

For example, the Department of Defense standard at 2400 bps is the LPC-10

vocoder. Its voice suffers from a lack of naturalness, has a synthetic

quality, and results in some loss of speaker recognizability. It is not

considered suitable for general public use. LPC-based techniques have also

tended to be more vulnerable to background environment noise than waveform

coding techniques are.

Since synthetic quality results at 2400 bps, it is evident that a

higher data rate will be needed in MSS. Since, on the other hand, the channel

bandwidth is limited to 5 kHz, and multiple bps/Hz modulation schemes are

inconsistent with power, complexity, and cost constraints of MSS, the goal for

the speech rate has been set at 4800 bps. The challenge in MSAT-X, therefore,

has since been to develop a speech coding system that operates at 4800 bps,

meets the quality objective, is simple enough to be implemented in a mobile

terminal for real-time operations (with less than 50-ms delay), and is robust

in the mobile environment, where the speech is corrupted by vehicle noise, and

the communication channel is impaired by multipath fading and shadowing.

In late 1984, JPL awarded a three-year contract to each of two

academic institutions to research and develop the speech coder breadboards.

The contracts had three phases: concept identification, algorithm development,

and hardware implementation.

The University of California, Santa Barbara (UCSB), developed two

algorithms, called Vector Excitation Coding (VXC) and Vector Adaptive

Predictive Coding (VAPC). The Georgia Institute of Technology (GIT) developed

a Self-Excited Vocoder (SEV). Recently both contracts were amended to include

the development of three robust pre-prototype units that can be used in field

testing.

In what follows we briefly present the different speech coding

techniques and equipment under development.

5-21
5.3.2 Speech Compression Techniques Developed for MSAT-X

The basic framework within which the different algorithms operate

is as follows. The input speech is sampled at 8 kHz with 12-bit resolution.

The sampled speech is divided into frames of approximately 20 ms. A

compression algorithm is used so that the number of bits of information

transmitted per frame may not exceed the aggregate rate of 4800 bps. In

addition, it is necessary to allocate some of the bits in each frame for

functions such as frame synchronization and error detection.

The speech compression techniques developed by both contractors are

based on the classical LPC compression technique and are generalizations of it

wherein waveform coding is used in combination with LPC to achieve

natural-sounding speech.

5.3.2.1 LPC-Based Compression Techniques. LPC speech coders can be modeled

as in Figure 5.3.1. The core of these coders is the so-called analysissynthesis model. The speech analyzer consists of a short-term predictor and a

long-term predictor. The parameters of the long- and short-term predictors

can be determined by LPC analysis of the speech waveform. The long-term

predictor is concerned with modeling the pitch in the voice, while the

short-term predictor is for tracking the short-term waveform variations in the

speech. The speech synthesizer in the encoder also contains the long-term and

short-term predictors. A coded excitation signal or residual is injected into

the speech synthesizer to generate the coded speech. The optimal code is

obtained using an analysis-by-synthesis procedure, which chooses an excitation

sequence from an ensemble of possible excitation sequences, subject to the

criterion of minimizing the resultant perceptually weighted distance between

the coded speech and the original speech. Different LPC-based techniques vary

in the means by which the excitation sequence is selected. The address of the

selected codevector is transmitted to the synthesizer in the decoder, which

reconstructs an approximation of the original vector by mapping the codevector

from its stored codebook or by generating the output sequence based on a fixed

model.

5-22
5.3.2.2 Speech Coding Techniques Developed at UCSB. Using the vector

quantization (VQ) technique, UCSB has developed two speech coding schemes:

vector adaptive predictive coding (VAPC) and vector excitation coding with

pulse excitation (VXC).

5.3.2.2.1 VAPC. In principle, the VAPC scheme works as follows. A sampled

speech waveform is first filtered by a long-terra predictor (pitch predictor)

to remove the long-terra redundancy due to pitch quasi-periodicity. It is also

filtered by a short-term predictor, which is, in effect, a whitening filter,

to produce the so-called "residual" sequence. The predictor parameters are

computed, vector-quantized, and transmitted to the decoder once every voice

frame, which is 22.5 ms. The residual is then divided into nine vectors, each

representing a 2.5-ms segment of the residual waveform. These vectors are

quantized by a gain-adaptive vector quantizer. The gain and the VQ index of

the codevectors, which is selected from the VQ codebook (which contains 128

codevectors, each of dimension 20), are then sent to the receiver. The speech

is reconstructed at the receiver by using the short-term and long-term predictors, which are excited by the quantized residual. The speech signal is

further enhanced by postfiltering. Figure 5.3.2 shows the VAPC functional

block diagram.

5.3.2.2.2 VXC. The second coder developed by UCSB employs a codebook excitation scheme. The VXC functional block diagram is shown in Figure 5.3.3 [Chen

et al., 1987]. This technique employs the classical LPC model. Notable

contributions to this technique have been the development of algorithms for

small codebooks used in modeling the excitation sequence, and the reduction of

computational complexity for real-time coder operation.

5.3.2.2.3 VAPC and VXC Predicted Performance. The performance of both VAPC

and VXC operating in the presence of white noise and truck noise has been

predicted by simulation. Results have indicated that the two techniques can

reproduce speech with fairly good intelligibility and quality even in the

presence of truck noise and. at low SNR (6 dB) [Chen et al., 1987]. UCSB

reports that, compared to the original speech, the decoded speech is slightly

muffled, but the overall quality and intelligibility are improved due to the

noise suppression effect. Laboratory versions of the vocoders have been

5-23
demonstrated by UCSB in March 1987. Speech intelligibility and naturalness

were achieved, but the overall speech quality could not be considered

near-toll-quality. The speech quality is expected to improve with recent and

continuing refinements.

UCSB has recently delivered a stand-alone single-board VAPC/VXC

Voice Compression Unit (VCU), which is based on three AT&T DSP-32 digital

signal processors. (A change in EPROMs effects the switch from one scheme to

another.) A critical evaluation will include laboratory listening tests,

testing of the vocoder with the hardware simulator, and testing of the vocoder

as a part of the end-to-end link to be demonstrated in the field.

5.3.2.3 Speech Coding Technique Developed by GIT. After considering a wide

variety of algorithms, GIT selected a form of extended Self-Excited Vocoder

(SEV) for real-time implementation. An SEV can be considered to be similar to

a Code-Excited Linear Predictive Coder (CELPC) or a Multi-Pulse Linear Predictive Coder (MPLPC) with the exception that the current excitation signal for

the SEV is also determined from the excitation signal's own past history.

Actually, the excitation sequence in this technique is a hybrid of codebook

excitation and the excitation used in previous speech frames.

Because the SEV determines the current excitation signal by utilizing the excitation signal's past history, fewer bits are needed for the coding

of the excitation function. To encode and quantize the short-term filter

coefficients, SEV employs the quantized line spectrum pair (LSP) for bit rate

reduction. The concept is depicted in Figure 5.3.4 [GIT, 1987]. The details

can be found in the reference cited and [SYSDOC]. Extensive simulation has

indicated that the relatively simple SEV can achieve speech quality similar to

i>^

that of more complex vocoders.

GIT has delivered a VCU that consists of a PC/AT chassis with the

Intel IPAX/286 mother board, a DSP-32 board, a TMS 320C25 board, and an I/O

board. The I/O board is used mainly to interface with the terminal processor

and the handset. The speech decoding and analog-to-digital (A/D) and D/A

conversion is performed on the TMS 320C25 board. The speech encoding is

performed on the DSP-32 board, which has three DSP-32 processors. The

5-24
remainder of the hardware is used as a development system. This hardware is

obviously considerably larger than that of the UCSB vocoder; this is

attributed to the fact that it is a developmental system that is not intended

by GIT to be a stand-alone vocoder. This is more convenient for the development work performed by GIT, but makes the hardware inconvenient to integrate

with the remainder of the mobile terminal.

Preliminary listening tests have indicated that the speech obtained

is generally intelligible and natural-sounding, but of only fair quality. As

with the UCSB vocoder, thorough testing will be performed by JPL, both in the

lab and in the field. These tests will determine the exact qualities of the

speech obtained.

5.4 NETWORKING

5.4.1 Perspective

Networking concepts in MSAT-X reflect the uniqueness of its requirements and operating environment. The networking concepts are also directly

coupled to the basic system design philosophy arrived at by NASA and discussed

in Section 4.1.

It became evident early in the MSAT-X program that novel networking

approaches had to be developed. The challenge in the protocol design arises

from the integrated voice and data services that a mobile satellite system

provides. Voice and data services have different characteristics in terms of

message duration, message arrival rate, acceptable delay, and blocking probability. The available networking protocols were not suitable or adaptable to

MSAT-X [Signatron, 1985]. The protocols that integrated voice and data

utilized TDMA as a general rule. Broadcast-type protocols would have been

impractical in an FDM system. Other schemes were simply not applicable due to

the low-cost constraint on the mobile terminal.

In 1983 an adaptive multiple access protocol (AMAP) was developed

at JPL for the data messages in MSAT-X. From this evolved, in 1984, the

5-25
integrated adaptive multiple access protocol (I-AMAP) that integrated voice

and data services.

5.4.2 I-AMAP

In I-AMAP the channels (of equal bandwidth or capacity) are divided

into information and request channels. The information channels are further

divided into open-end and closed-end channels. The open-end channels (open

connection duration) are used for voice or long file transfers, while the

closed-end channels (specified connection duration) are used for usual data

channels. I-AMAP adaptively varies the mix of request, open-end, and closedend channels to suit the demands placed on the network. The protocol treats

the open-end links according to a blocked calls cleared strategy with the main

objective of minimizing the blocking probability. For closed-end data links,

the objective is to minimize the end-to-end delay.

In the 1984 time frame, I-AMAP was designed to use pure ALOHA for

reservations. Subsequently, a feasibility and software sizing contract was

awarded to Signatron Corporation. Improvements to I-AMAP that involve minor

or moderate extra cost were suggested in a study by Signatron. These included

using modified slotted ALOHA, integrating data into available voice channels,

and prioritizing the voice and data messages. The first recommendation, viz.,

slotted ALOHA, relies on slot-time definition, a task for which inexpensive

solutions were found [Signatron, 1985, and Emerson, 1985]. Slotted ALOHA

became (and still is) the baseline in I-AMAP. The second suggestion, i.e.,

integrating data into available voice channels, was not pursued. The third

suggestion, i.e., prioritizing voice and data messages, will be incorporated

into I-AMAP in the future. The Signatron study gave detailed signaling

handshakes, software builds with time and cost estimates, and estimates of the

needed computation power in the NMC. Because of the predicted cost ($3.6

million over 24 months) and NASA's budget constraints, detailed software

development was relegated to future MSS operators, each with his specific

system architecture.

Concurrent with and subsequent to the Signatron study, several of

the key design details for I-AMAP were established by JPL, e.g., the

5-26
recommendation of slotted ALOHA with binary exponential backoff. The

technical details of I-AMAP are given in [SYSDOC] and the references cited

therein. Here we only highlight some of the salient features and results.

5.4.2.1 Channel Partitioning. The adaptive partitioning of the channels in

I-AMAP is illustrated in Figure 5.4.1: (a) shows the channel partition at one

instant of time, and (b) shows the channel partition at another instant of

time as the traffic mix changes [Yan and Naderi, 1986]. Figures 5.4.2 and

5.4.3 show the partitioning of channels as a function of the message arrival

rate for a fixed average message delay (1.75 s) and blocking probability (2%)

[Yan and Wang, 1985]. The required number of request channels and the number

of closed-end channels as a function of the data message arrival rate are

shown in the first figure, while the total number of channels and the number

of open-end channels are shown in the second figure. For example, at a

message arrival rate of 100 messages per second for data and 0.1 for voice,

the channels of a 97-channel system will be partitioned as follows: 35 request

£•

channels, 46 data channels, and 16 voice channels. By monitoring the

message/call arrival rates, the utilization of resources can be maximized by

dynamically adjusting the channel partitioning.

5.4.2.2 Error Control, Packet Replication, and Packet Size. Analysis at

JPL also arrived at optimal error control design, packet replication strategy,

and packet length (all in the sense of minimizing the end-to-end delay).

Error control is effected by means of an inner forward error correction (FEC)

code, and an outer cyclic redundancy check (CRC) for packet error detection.

The FEC in the baseline is the rate 2/3 16-state trellis code combined with

the D8PSK and interleaving. (See Section 5.2.) The CRC is used in conjunction with a selective automatic retransmission request (ARQ) algorithm [Wang

and Yan, 1987a]. All successfully received packets, excluding voice packets,

will be acknowledged within a predetermined period of time. Packets received

with errors will be negatively acknowledged and subsequently retransmitted.

The method of packet retransmission varies according to the type of channel

connection. In an open-end connection, in which the channel is assigned to

the user until it is relinquished, packet retransmission takes place over the

same assigned channel and immediately after the receipt of the negative

acknowledgment. In a closed-end connection, retransmission occurs after the

5-27
completion of the transaction that is in progress, and generally over a

different channel, for which the user must make a separate connection request

and successfully obtain an assignment.

Request packets that are not positively acknowledged within a

specific time period will be assumed to have been lost or received with errors

and will be treated as negatively acknowledged packets.

Multiple transmission of the acknowledgment and assignment packets

reduces the probability of their retransmissions and can minimize message

delay. Computer simulation has shown that packet replication (two copies

inclusive) significantly lowers the average end-to-end delay, as indicated in

Figure 5.4.4 [Wang and Yan, 1987a]. This figure shows the average delay as a

function of the packet sizes with and without packet replication. As

indicated, for the range of packet sizes considered, packet replication

reduces the average delay significantly. Figure 5.4.4 is based on a message

length of 4096 bits and an arrival rate of 65 messages per second. From this

figure it is also evident that for a given message size, message arrival rate,

and channel performance, the end-to-end message delay is affected by the

packet size. Figure 5.4.5 provides additional information on the message

delay as a function of the message arrival rate. As seen from these figures,

a packet size between 256 and 512 bits is optimal in the sense of minimizing

the delay during busy traffic hours [Wang and Yan, 1987a].

5.4.2.3 Request Channel Simulator. Recently, with the parameters of I-AMAP

established mostly through analysis, the focus has shifted to developing and

implementing a simulator for networking purposes. The simulator is concerned

particularly with the request channel, because of its vulnerability, and is

currently close to being complete. Its objectives are to validate the results

derived through analysis (which always contains some idealized assumptions) in

a realistic simulated fading channel environment, to refine the the parameters

of the request channel, and to test new potentially useful protocols. The

simulator is shown in the block diagram of Figure 5.4.6 [Wang and Yan, 1987b]

and is discussed also in [SYSDOC]. Along the line of investigating other

protocols, an in-house effort at present is investigating tree algorithms that

have been developed in the eighties and that promise higher capacities than

5-28
slotted ALOHA, while being stable, unlike slotted ALOHA. Tree algorithms will

also be tested on the request channel simulator.

5.5 MOBILE SATELLITE CHANNEL MODELING

In the strict sense of the phrase, propagation modeling is not a

technology development. Nevertheless, the development of new and effective

techniques to model the MSS propagation environment is essential to the

prediction of the MSS's performance. This, in turn, has a significant impact

on the development of the MSS subsystems. Since the MSAT-X program's

inception, a joint effort under NASA's MSAT-X and propagation programs has

been dedicated to developing the necessary propagation models and an

indispensable channel simulation capability.

Important propagation parameters for MSAT-X include signal

attenuation, fading rate, depth and duration, and Doppler spread. The

statistics of these parameters are needed for a variety of MSS terrains, e.g.,

unobstructed flatland, forested areas, hilly areas, and areas with buildings

and other man-made structures. Some estimates of these parameters can be

obtained through analysis based on simplified assumptions. Yet the most

effective method to obtain reliable data is through propagation experiments,

since the propagation characteristics are a function of the mobile environment,

including antenna patterns, elevation angles, and vehicle speed. Propagation

experiments were performed that utilized stratospheric balloons, helicopters,

remotely piloted drones, and high, fixed towers. These tests were funded by

MSAT-X and the propagation program in support of MSAT-X. A large body of data

has been gathered and is being analyzed.

The experiments, the data, and the results are discussed in detail

in [SYSDOC] and the references cited therein. For the system-level presentation, it suffices to mention some of the most salient results. The signal

attenuation statistics are given in the curves of Figure 5.5.1 [Vogel and

Goldhirsh, 1987]. (The references cited in the figure can be found in

[Bostian, 1987]. It should be noted that curves A and B were results of

experiments funded by another program.) In summary, they show that at L-band,

5-29
the following fade depths (attenuations) are exceeded 1% of the time: (a) 1.4

dB for an unobstructed path, (b) 5.5 dB at a 45° elevation in multipath

(canyon) terrain, (c) 7.6 dB at a 30° elevation in multipath terrain, and (d)

at least 8 dB worse in wooded flat areas than in multipath terrain! These

results are extremely significant. First, they indicate that foliage

shadowing is the dominant cause of degradation. Second, they indicate that

serious attenuation can result from multipath alone, without even shadowing.

In general, these results indicate worse effects due to propagation than had

been thought initially. As a result they have a significant impact on the

development of the technologies in MSAT-X. This will be addressed again in

Section 6.

Computer simulation and analysis to augment and explain the results

of the propagation experiment have also been performed. Although the

measurements are yet to be fully analyzed, some results have been applied to

software channel simulators that model the propagation of radio waves,

simulate terrain blockage effects, and estimate link performance [Divsalar,

1985, and Bostian, 1987]. The analytical effort has included modeling

scatterers, the effects of the distance to the scatterer, and tree complexity

modeling. Tree complexity modeling, for example, aims at furnishing a tree

attenuation model that assesses the effect of tree complexity on signal

attenuation. A tree can be modeled as a two-dimensional figure consisting of

a number of finite opaque strips that are canted together to approximate the

trunk and branches of a real tree [Vogel and Hong, 1987]. By varying the

number of branches, different attenuations levels are predicted. Results of

the correct order of magnitude have been obtained.

A significant result that has come out of the scattering modeling

effort pertains to MSS in general, and predicts the channel bandwidth

restriction that results from propagation in the MSS environment [Vogel and

Hong, 1987, and SYSDOC]. As can be inferred from Figure 5.5.2, the bandwidth

of the channel, as measured by a specified amount of envelope peak-to-peak

variation, is restricted to few tens of kilohertz (depending on the average

fading depth). This result supports the choice of an FDMA system by making

the broad assertion that wide band signals, such as TDMA and CDMA, are less

suitable for transmission on MSS channels than signals in an FDMA scheme are.

5-30
To facilitate the development of the MSAT-X hardware, improve the

efficiency of testing the new technology, and facilitate in general the study

of the effects of propagation anomalies on communication system performance, a

flexible hardware channel simulator was developed at JPL [Salmasi et al.,

1984, and Bavarian, 1987]. The results of the propagation experiments, e.g.,

fade statistics, are being included in the channel simulator for realistic

in-lab testing. This will precede the full-scale field experiments of the

end-to-end MSAT-X breadboard system.

5-31
Table 5.1.1. Estimated Cost Breakdown and Per Unit Cost for the MSAT Phased

Array Mobile Antennas3

Percentage of Total Cost (A + B)

Ball Teledyne

A. Material/Parts

Dielectric Circuit Boards

- Radiator and Hybrid Layers

- Phase Shifter Layer

Phase Shifter Components

- Pin Diodes

- Resistors

- Capacitors, Chokes, Etc.

Open-Loop Pointing Sensor

Electronics

C. Total Manufacturer's Cost

(A + B)

18.9

3.4

26.9

0.1

11.5

19.9

2.5

11.5

0.8

7.5

13.7

- Microprocessor 1

- Peripheries

Mechanical Structures

and Radome

Cables, Connectors, Etc.

Subtotal

B. Assembly /Labor

Radiators and Hybrids ]

'

1

Phase Shifter Layer '

Electronics

Mechanical Structures

and Radome

Antenna Integration

Subtotal

| 4.3

23.0

14.8

3.1 1.6

2.3 1.1

90.6 77.7

12.5

6.2

4.6

1.7 1.7

2.8

1.5

0.7

9.4 22.3

$1610 $1848

aThe production quantity is 10,000 units annually for a five-year period.

5-32
Table 5.1.2. Salient Characteristics of MGA Breadboards

JPL Mechanically TRE Ball Aerospace

Tilted Linear Array Phased Array Phased Array

Averagea Received Gain

(1545 MHz), dBi

20° Elevation

40° Elevation

60° Elevation

Averagea Transmit Gain

(1660 MHz), dBi

20° Elevation

40° Elevation

60° Elevation

10.0

11.5

10.0

10.0

12.0

10.0

9.9

11.3

12.8

7.8

10.1

12.2

8.2

11.6

11.5

8.1

11.4

12.4

Average3 Receive System

G/T at 20° (including NF =

0.86 Mobile Terminal [MT]

Receiver), dB/K

Acquisition Time, s

Interference to Data

Channel During Tracking

AM, dB (peak-to-peak)

PM, ° (peak-to-peak

maximum)

Size (height x diameter),

in.

Manufacturing Cost

(estimated), $

-15.2

15

Negligible

Negligible

9.0b x 20.0

600

-15.4

1848

-16.7

1.5 1.5

15 37

0.7 x 20.7 1.3 x 24.0

1610

aAveraging gains over eight azimuth beam positions.

^Height of mechanical linear array is to be reduced to 4.5 in.

5-33
03

T3

0

-10

-20

-30

-40

_

r— bAlbLLHb SAT b

(RHCP) (LHCF

s. ^—^ x-— -» ,

LHCP LEFT-HAND CIRCULARLY

POLARIZED

RHCP RIGHT-HAND CIRCULARLY

POLARIZED

I I I

90 -60 -30 0 30

LLIIb —

)

'

B

^«s^— ^

L^'~"^

~

I

60 90

~"1

ISOLATION = WdB

\

LOW-GAIN

CROSSED DROOPING

ANTENNA

AZIMUTH ANGLE (<£), deg

Figure 5.1.1. A Typical Radiation Pattern Demonstrating the Adjacent Satellite

Isolation Achievable by a Low-Gain Antenna

DESIRED

SATELLITE

(RHCP)

UNDESIRED

SATELLITE

(LHCP)

MEDIUM-GAIN

19-ELEMENT PHASED

ARRAY ANTENNA

-40

60 90

AZIMUTH ANGLE (<#>), deg

Figure 5.1.2. A Typical Radiation Pattern Demonstrating the Adjacent Satellite

Isolation Achievable by a Medium-Gain Antenna With Taper

5-34
QUARTZ

RATE

SENSOR

GROUND PLANE

WATSON

RATE

SENSOR

APPLE COMPUTER

EQUIPPED WITH

A/D AND

DIGITAL I/O

STEPPER

MOTOR

STEPPER MOTOR

CONTROLLER

RACK MOUNTED

12-volt POWER SUPPLY

SPECIAL INTERFACE

CIRCUITRY

TRANSCEIVER

ANTENNA SUBSYSTEMS

RF ARRAY AND FEED

STEERABLE PLATFORM

CONTROL/SUPPORT ELECTRONICS

A CHANNEL DEMODULATION/FADING CORRECTION CIRCUITRY

APPLE POINTING CONTROL COMPUTER

Figure 5.1.3. Mechanically Steered Linear Array Antenna Assembly

5-35
180° HYBRID

COUPLER

ROTARY

MOTOR

ANTENNA AND

TRACKING

CONTROL

JOINT

SEQUENTIAL

PHASE SHIFTER

SERVO

SCAN CODE

GENERATOR

TRANSCEIVER

TRANSMITTER DIPLEXER RECEIVER

AGC SIGNAL

ERROR ANGLE SIGNAL

Figure 5.1.4. Schematic of Vehicle Antenna Pointing System Based on SingleChannel Monopulse Tracking

5-36
320°

40°

350°

340° 10o

330°

 20°

30°

0°

10°

350° 20°

340°

30°

330°

40°

320°

310°

50°

50°

310°

60°

300°

70°

290°

80°

280°

260°

100°

250°

110°

240°

120°

230°

130°

130°

230°

220°

140°

140°

220°

210°

150°

200°

160° 190°

170°

180°

160°

170° 200°

190°

150°

210°

CO-POLARIZATION

CROSS-POLARIZATION

5 in.

,

-^ iy in. ^

i i ^

-0.5 in.

-3.35 in.

ARRAY DIMENSIONS

•FOAM

ELEVATION PATTERN

AT 1600MHz

MAXIMUM GAIN

* 12dBic

GROUND PLANE

DIMENSIONS: 48 in. x 56 in.

ARRAY TILT ANGLE: 45°

Figure 5.1.5. Elevation Pattern of Mechanically Steered Antenna

5-37
to

•a

01

>

01

<E

CO-POLARIZATION

CROSS-POLARIZATION

30 -

90

AZIMUTH, deg

Figure 5.1.6. Azimuth Pattern for Mechanically Steered Linear Array Antenna

AIR

-RADIATOR

LAYER

HYBRID

LAYER

-PHASE SHIFTER

AND POWER

DIVIDER LAYER

•STRUCTURE

SUPPORT

Figure 5.1.7. Cross-Sectional View of the Phased Array Antenna

5-38
Figure 5.1.8. Triangular Lattice Pattern of Phased Array Antennas

TOP VIEW

FOAM

w//////////////,

CIRCULAR

DISKS

•DIELECTRIC ORTHOGONAL

FEED PROBES

Figure 5.1.9. Stacked Dual Resonant Microstrip Element Developed by Ball

Aerospace Corporation

5-39
TOP VIEW

270° *-

;;^>; o°

90°

•SLOT

•STRIPLINE

FEED

METALLIC

ENCLOSURE

DIELECTRIC

SUBSTRATE

FEED PROBES

Figure 5.1.10. Stripline-Fed Crossed-Slot Radiator Developed by Teledyne Ryan

Electronics

5-40
m•o

0

4

8

12

£ 16

O

| 20

O

uj 24

28

32

36

40

144 108 72 36 36

ANGLE, deg

72 108 144

Figure 5.1.11. Typical Elevation Radiation Pattern of TRE Phased Array

Antenna. 9S = 30° elevation, <J>S = 0° azimuth.

mT3

UJ

z

o

0

4

8

12

16

20

I 24

OL

LU

> 28

ui , 32

cc

36

40

144

Figure 5.1.12.

108 72 36 0

ANGLE, deg

36 72 108 144

Typical Elevation Pattern Showing Axial Ratios for TRE Phased

Array Antenna. 9S = 30° elevation, <j>s = 0° azimuth.

5-41
0

4

8

m

> 12

LU 16

O

cc 20

LU

O 24

LU

^ 28

<

cc

 32

36

40

144 108 72 36 0

ANGLE, deg

36 72 108 144

Figure 5.1.13. Typical Azimuth Radiation Pattern of TRE Phased Array

Antenna. 0S = 30° elevation, 4>s = 0° azimuth.

144 108 36 0 36

ANGLE, deg

72 108 144

Figure 5.1.14. Typical Azimuth Pattern Showing Axial Ratios for TRE Phased

Array Antenna. 9S = 30° elevation, <j)s = 0° azimuth.

5-42
DESIRED SIGNAL

INTERFERENCE

UPLINK INTERFERENCE

TURNED AROUND UPLINK INTERFERENCE

INTERBEAM INTERFERENCE

INTERMODULATION PRODUCTS

INTERSATELLITE INTERFERENCE

UPLINK SIGNAL

DOWNLINK SIGNAL

Figure 5.2.1. A Worst-Case Interference Scenario in MSS. Mobile-to-mobile

links will pass through a gateway, but that does not generally

alter the interference at L-band.

5-43
-20

mTJ

o

cc

LU

CC

Ul

H-

-40

-J -60

LU

z

•z.

CJ

Iz

o -80

Q

-100

-120

0.4

•BbT= oo (MSK)

-1.0

-0.7

I I

0 0.5 1.0 1.5 2.0

NORMALIZED CHANNEL SEPARATION

Figure 5.2.2. Adjacent Channel Interference for GMSK

2.5

5-44
a

:o

Zp

a.IUJ

1/12 Q Q <Da

LO1a: LUa:<

IU

Of

LUUUJa:

u.

1

CARRIER—*-

LL.

a:

LJADRATURE aMODULATOR LUQ

-

LUZ2i—o1 a.

XTRACTOR LU

OCK

ER- ER DEIN LEA

o

•H4JO

Ui cCD JJ

rt JJCO

(1)

CO

.dUenoenPHOO80) .cobO(0oo(U

4-1

CO>* CDLO(Uu00

5-45
2

10

-1

10

,-2

10

10"

T

| SIMULATION POINTS

. TCM

. (DOPPLER

FREQUENCY = 100 Hz)

• RICIAN K = 10 dB

• INTERLEAVE = 128 SYMBOLS

• DECODER BUFFER = 32 SYMBOLS

• PERFECT SYNC (PHASE, TIME)

• NO ISI

TCM

(DOPPLER

FREQUENCY = 20 Hz)

BITSNR

10

Figure 5.2.4. Performance of Rate 2/3, 16-State Trellis-Coded 8PSK Modulation

(TCM/8PSK) Over Rician Fading Channel With CSI (Ideal Coherent

Detection) and Interleaving

(a)

DATA

(b)

PILOT

EBANDS —

f\

s. I

<\ 1A.

"o

CO, rads/s *. CO , rads/s

Figure 5.2.5. Two Versions of the Tone Calibrated Technique (TCT): (a) TCT

Spectrum With Single Tone, and (b) DTCT Spectrum

5-46
tr

LU

E

o

LL

Q£

<

h

2

u

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0

c

<

c

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tf

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]

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tgv< 3 LU (

X

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ffio

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1

BLOC

K

IMTPR.

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I

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t

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to

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tgv/

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BIT

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fy

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L.

D

0

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>-*

^H"

LU

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0}

4-1

to

en

en

P-I

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<u

TJ

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|

tfl

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i-l

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n (4-1

Q

g

(0

00

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£

60

b

— 1

J

5-47
10

-1

d? 10-2

10

-3

10

-4

I I

DOPPLER SPREAD = 40 Hz

128-SYMBOL

INTERLEAVER

k=10dB

I I

9 10

Eb/NQ,dB

NO INTERLEAVING

k= 10 dB

4DPSK

11 12 13

Figure 5.2.7. Simulation Results for Bit Error Rate Performance of Rate 2/3,

16-State Trellis-Coded 8DPSK (TCM/D8PSK)

10"

10

-2

tr

tr

O

tr

GC

LLJ

K

CD

10

-3

10

-4

| SIMULATION

POINTS

U = UHF

L= L-BAND

k = 10

DOPPLER = 20 Hz (UHF) OR

40 Hz (L-BAND)

128-SYMBOL BLOCK

INTERLEAVER

•DIFFERENTIAL

TCM/8PSK

DQPSK

DTCT

COHERENT

TCM/8PSK

10 11 12

Eb/N0, dB

Figure 5.2.8. Comparison of TCM/D8PSK With Coherent TCM/8PSK (Using DTCT With

Noisy Pilot) at UHF and L-Band.

5-48
ORIGINAL PAGE IS

OF POOR QUALITY

Figure 5.2.9. Eye Diagram of Root Raised-Cosine Pulse Shaping With 100%

Excess Bandwidth

VOCODER

TERMINAL

FRONT

PANEL

TERMINAL

PROCESSOR

DATA *

ACQUISITION

SYSTEM

MODEM

*

DATA

XMIT

*

DATA

RCV

4

PILOT

RCV

J

ANTENNA

POINTING

SYSTEM

Figure 5.2.10. Mobile Terminal Block Diagram

5-49
SPEECH ANALYZER

I

SAMPLED

SPEECH

) —

LONG-TERM

PREDICTOR

-vz

I RESIDUAL

I

D(z)

SPEECH MODEL

CODED |~

RESIDUAL

INPUT

SPEECH

L

Figure 5.3.1. Model of LPC Speech Coders

OUTPUT

SPEECH

CODED

SPEECH

TRANSMITTER

Figure 5.3.2. VAPC Functional Block Diagram

5-50
ENCODER ORIGINAL

SPEECH

INPUT

LONG-TERM

SYNTHESIZER

SHORT-TERM

WEIGHTED

SYNTHESIZER

Hs(z) = Hs(z) W(z)

DECODER RESP.

FROM PREVIOUS

FRAME

W(z)

EUCLIDEAN

NORM

CHANNEL

PULSE

EXCITATION

CODEBOOK

C — c>-~

LONG-TERM

SYNTHESIZER

' H,(z)

SHORT-TERM

SYNTHESIZER

Hs(z)

s

sn SYNTHETIC

f SPEECH

OUTPUT

DECODER

Figure 5.3.3. Block Diagram of VXC Speech Coder

INPUT

SPEECH

QUANTIZED

LPC

COEFFICIENTS

OUTPUT

SPEECH

Figure 5.3.4. LSP Quantization System Concept Developed by GIT for the SEV

5-51
22

REQUEST

CHANNELS

(b)

38 REQUEST

^CHANNELS;

12 VOICE,

CHANNELS/

Xxxxx

Figure 5.4.1. An Illustration of Channel Partitioning at Two Instants of

Time: (a) This Partition Supports 25 Voice Calls per Minute

and 3600 Data Messages per Minute, and (b) This Partition

Supports 4 Voice Calls per Minute and 6600 Messages per Minute

100

90 F

;' ' ' ' I ' ' ' ' I ' ' ' I ' ' ' ' I ' ' ' ' I ' ' ' ' I ' I ' I I ' '

: SLOTTED ALOHA

0 10 20 30 40 50 60 70 80 90 100

AVERAGE DATA MESSAGE ARRIVAL RATE (messages/s)

Figure 5.4.2. Number of Request Channels (Nr) and Data Channels

Versus Data Message Arrival Rate

5-52
100

90

80

70

in

w 60

<

x

o

Uo

en

LU

m

50

40 -

30 ~t?

20

10 r

I I i I I I I I I | I i I I |

SLOTTED ALOHA

N(nv = 0.2)

. N (nv = o>

: VOICE MESSAGE

ARRIVAL RATE

t

Nv(nv = 0.2)

NV(S2V=0.0)

1 1 , i , i , 11, , i 11,T . 11 . . 1111.i.1 1 .

0 10 20 30 40 50 60 70 80 90 100

fij, messages/s

i-*. v.*..-;.!*^lna»v^..»x->*?t*gt'.'fi»«;«w-»y».^^y,,-.. . -^.-*v^^.'.*"i.'^S5;*

 lV^.¥V^v

TS;'-"5^<:i~w:lV

Tv;-^- *•'""':'''"""•••^""*•—-•;•*,-•••*-. - —* • •. -. . . --.--»-- -

Figure 5'.4v3.t: Number, of Voic.e.Chjannels (Nv) and Total Number of Channels

?";:;%:& v/'(N) Versus •• Message-'Arrival Rate (Qd)

5-53
Q

Q

2

LU

6

Q

2

LU

7.0

6.5

6.0

5.5

<r 5.0

LU

4.5

4.0

WITHOUT

PACKET

REPLICATION

MESSAGE ARRIVAL RATE = 65 messages/s

TOTAL MESSAGE LENGTH = 4096 bits

I _ I _ I

WITH

PACKET

REPLICATION

I

128 256 512 1024

PACKET SIZE, bits/packet

2048

Figure 5.4.4. Average End-to-End Delay Versus Packet Size When Arrival Rate

= 65 Messages/s

6.0

> 5.5

5.0

LU

I O

9 4.5

LU

o

c 4.0

LU

3.5

3.0

I

 r I ' I ' I

E

S/N0 = 12dB

PREAMBLE = 104 symbols

PACKET SEQUENCE NUMBER = 16 bits

TOTAL MESSAGE LENGTH = 4096 bits

256

I I I

10 20 30 40 50

ARRIVAL RATE, messages/s

60 70

Figure 5.4.5. Average End-to-End Delay Versus Closed-End Traffic for Various

Packet Sizes (Total Message Length = 4096 bits)

5-54
1 —

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0.2 0.5 1 2 5 10 1520 30 40 50 60 70 808590 95 98 9999.599.8

PERCENTAGE OF TIME RECEIVED SIGNAL LEVEL EXCEEDS ORDINATE

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Locations and Elevation Angles. See [Bostian, 1987] for

references cited.

5-56
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= 0.003 xBW(5-dB FADE)

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100 200 300

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400 500

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Bandwidth. (Bandwidth = 5 kHz for MSAT-X.)

5-57
SECTION 6

RESULTS OF MSAT-X: RECOMMENDED TECHNOLOGY UTILIZATION FOR MSS

6.1 INTRODUCTION

MSAT-X has been concerned with the development of ground segment

technology to promote the realization of mobile satellite systems. In line

with this, the focus here will be on arriving at the recommended manner of

utilizing the developed ground segment technology. Inasmuch as an MSS is an

integration of space and ground segments, fitting within an overall system

architecture, it is necessary to deal with the space segment to ensure that

the ground segment results are consistent with a realistic space segment.

The technology recommendations given below will be for an MSS based

on the fundamental system concept arrived at by NASA, JPL, and their

contractors as discussed in Section 4.1. At the outset, Section 6.2 will

reiterate and elucidate this system concept. This is followed by a discussion

of the spacecraft. The rationale behind the spacecraft assumptions is given,

and how these spacecraft attributes relate to or impact the ground segment

recommendations is clarified. This sets the stage for the main thrust of this

section: an exposition of JPL's recommendations on how to utilize the ground

segment technologies developed under MSAT-X to achieve the most efficient and

viable MSS.

6.2 BASIC MSS ARCHITECTURE

The MSS proposed by JPL is a single-channel-per-carrier (SCPC)

demand-assigned FDMA system. The perceived system will provide service to the

contiguous United States (CONUS). Its space segment will consist of two

geosynchronous satellites, separated by 35° orbital spacing. Each satellite

will illuminate the entire CONUS, providing overlapped coverage. When mediumgain vehicle antennas are used, the 35° orbital spacing will be consistent

*

with the inter-satellite isolation that these antennas can offer. MGAs could

then enable the desired orbit reuse. The two-satellite architecture also

provides for continued coverage when one of the satellites is eclipsed.

6-1
As discussed earlier, in Section 4.1, the proposed architecture

relies on satellite antenna multibeam technology to effect frequency reuse.

Although FDMA efficiently packs as many channels as possible in the limited

spectrum of Figure 3.2.1 as permitted by interference considerations, frequency reuse is necessary to achieve the necessary MSS capacity. (This topic

will be revisited in the following sections.)

To optimize the utilization of the very limited frequency spectrum

at L-band (Figure 3.2.1), only the links between the satellites and the mobile

terminals (in either direction) will be at L-band frequencies. The links

between the satellites and gateway stations, base stations, or the NMC,

collectively referred to as the backhaul links, will be at Ku-band frequencies.

The satellites will be bent-pipe repeaters transponding from L-band to Ku-band

or vice versa. There will be no direct L-band to L-band link. To avoid

unnecessary congestion of the precious L-band spectrum, miscellaneous

information exchanged between the NMC and fixed earth stations, e.g., a base

station, could utilize a Ku-band to Ku-band cross-strap. Communications

between mobile and fixed terminals will require one hop, while mobile-tomobile communications will require two hops with a gateway station in the

middle.

At the heart of the MSS architecture is the NMC. It oversees the

network, controls its operation, and ensures the proper and efficient utilization of its resources. The NMC monitors the traffic and assigns channels

according to the I-AMAP protocol, which dynamically partitions the channels

between request, open-end, and closed-end links. The NMC also handles

operational functions, such as billing.

Each link between a satellite and a mobile user will contain a data

channel plus a shared pilot channel. The pilot is used as a reference by the

antenna pointing system to aid the functions of satellite acquisition and

tracking. In MSAT-X, all the channels are assigned a 5-kHz bandwidth. The

challenge has been to transmit the highest quality voice possible, with the

least power requirements attainable with a relatively inexpensive ground terminal and realistic present-day spacecraft, within this tight channel

bandwidth. The challenges for any MSS will basically be the same, although

6-2
marketing considerations may demand the use of more than one bandwidth to

support a wide variety of services. Whatever the eventual bandwidth(s) that

will be used by an MSS operator, all technologies developed under MSAT-X are

immediately applicable to an eventual MSS.

6.3 ON THE SPACE SEGMENT TO COMPLEMENT MSAT-X

Uncertainty relating to the space segment arises from the fact that

the MSAT satellite(s) will depend on decisions made by the consortium of

companies that will be developing the operational MSS. As mentioned earlier,

agreement within the consortium on a variety of issues has not yet been

achieved. Nevertheless, for MSAT-X purposes it is possible to make generic

assumptions about the satellite(s) that should be valid for reasonable and

expected choices of spacecraft. This is achieved by relying on present-day

spacecraft technology and using designs that are consistent with a realistic

state-of-the-art ground segment. This reflects the goal of obtaining the most

efficient utilization of proven concepts while minimizing risk.

In a first-generation MSS, the satellite (or two satellites, if a

two-satellite configuration is assumed for the first generation) would be

designed using an existing commercial high-power communication satellite bus.

A satellite would employ two deployable L-band communication antennas with a

4.2-m (14-ft) diameter. Although existing satellite buses are capable of

deploying a larger antenna [Sue et al., 1985; Ford Aerospace, 1985; and RCA

Astro Electronics, 1985], the selected antenna size represents approximately

the upper limit that can be launched by the Shuttle without requiring unfurlable reflector technology. The antennas would be stowed during launch and

deployed in orbit. One of these antennas would be used for reception of the

L-band signals, while the other would be used for transmission. Separate

transmit and receive antennas alleviate the problem of passive intermodulation

which often exists in high-power communications satellites using a shared

antenna for transmission and reception [Ford Aerospace, 1985, and Hoeber et

al., 1986]. The gain of the L-band antenna is about 33.7 dB at 1.5 GHz and

34.3 dB at 1.6 GHz, and the corresponding 3-dB beamwidths are about 3.2°. The

G/T of the satellite receiving system at L-band is 4 dB/K. (These figures

6-3
will be used later, in the link budgets of Section 6.4.2.) Four beams would

be needed to illuminate the entire CONUS, as illustrated in Figure 6.3.1.

The satellite would be equipped also with a 0.4 m antenna to

support the Ku-band communication link. This antenna, which has a 31.2-dB

transmit gain and a 32.3-dB receive gain, produces a single beam covering the

service area. The satellite receiving system at Ku-band has a corresponding

G/T of 21 dB/K.

The MSS satellite would likely employ a high-power and a low-power

transmitter in each beam in order to provide services to users with different

mobile terminals. The high-power transmitter would serve the users with lowgain terminals, while the low-power transmitter would serve the users with

medium-gain terminals. The separate-transmitter design alleviates the intermodulation and power-robbing problems associated with unequal carrier power

amplification. The Ku-band and L-band power amplifiers could be implemented

using solid-state devices and designed to operate at 5-dB backoff to reduce

intermodulation products. The overall efficiency of the transmitters would be

about 25%.

The realistic assumptions outlined in the above paragraphs will be

reflected in the link power budgets to be given in Section 6.4.2. These budgets will tie together the space and ground segments for a conceived firstgeneration MSS. They will demonstrate the feasibility of the links, and lead

into the system capacity results presented in Section 6.4.3.

6.4 TECHNOLOGY RECOMMENDATIONS FOR MSS

The technology recommendations presented in this section are based

on the MSAT-X research performed at JPL. The evolution and results of the

technology research have been summarized in Section 5. As mentioned above, in

Section 6.2, the results of the technology research will be immediately

applicable to any eventual MSS, particularly to an MSS with an architecture

based on an SCPC FDMA concept as explained and advocated in Sections 4.1 and

6.2.

6-4
Because a consortium of U.S. companies will be developing the

operational MSS, it is likely that, to accommodate the desires of the different participants, the eventual system may contain a mixing and matching of a

variety of technologies. It is JPL's aim at this point to put forth what it

believes is a consistent set of recommended ground segment technologies that,

when integrated in the manner identified by JPL, would create a union more

effective than the individual parts, with the result being an MSS that would

most efficiently utilize its scarce resources and hence be most viable.

Since the thrust of MSAT-X has been ground segment technologies,

the mobile terminal technologies are treated at some length in the following

subsection. Again, since the mobile terminal is a part of an overall system,

the mobile terminal technologies are quantitatively tied to the remainder of

the MSS through the link budgets of Subsection 6.4.2 and the system capacity

results of Subsection 6.4.3.

6.4.1 Mobile Terminal Technologies

In the five subsections to follow, the recommendations are given

starting with the overall architecture for the mobile terminal. The architecture organizes the subsystems around a central processor. This is elucidated

first. In the subsections that follow, the recommendations for the subsystems

are given starting at the beginning of the link with the speech coder, then

the modem, the transceiver, and finally the antenna. The issue of networking

is addressed later, in Section 6.4.3.

6.4.1.1 Mobile Terminal Architecture. The mobile terminal consists of

several subsystems linked together under the management of a central

controller called the terminal processor. This is identified clearly in

Figure 5.2.10. The figure depicts a modular design built around the terminal

processor. Two strong reasons have made this architecture ideally suited to

MSAT-X, and also make it the recommended approach for a mobile terminal in an

operational MSS.

6-5
The first attractive feature is that the modular design easily

supports the evolutionary development and upgrade of the different subsystem

components such as the vocoder, modem, and transceiver. This modularity would

easily allow for the substitution of subsystem options (different antennas,

transceivers, vocoders, etc.) either to upgrade the system as technologies

mature, or simply to fulfill the needs of the particular user. The change in

interfaces would be handled mostly by modifying the software in the terminal

processor (assuming that the appropriate controls on the hardware were exercised).

The second attractive feature in the architecture is the presence

of the terminal processor acting, in essence, as the brain of the terminal,

capable of combining information from the different subsystems it supervises

and making global decisions. For example, if the modem senses the loss of

signal while the pilot is still present, the terminal processor would interpret the event as a fade in the data channel and subsequently take appropriate

action according to the length of the fade. By virtue of being the mobile

terminal's master controller, the terminal processor becomes a central authority that coordinates diverse yet related functions such as pilot acquisition

sequences and burst transmission timing for the slotted ALOHA scheme. It also

serves as the primary human interface and carries out the requirements of the

networking protocol.

6.4.1.2 Speech Coding. In a variety of mobile applications, both analog

and digital voice have been suggested. Analog voice uses either FM or amplitude companded single sideband (ACSB). FM is spectrally inefficient and

consequently unsuitable for MSS. ACSB needs a higher signal-to-noise ratio,

but may generally degrade more gracefully than digital voice under reduced

signal conditions. As pointed out in Section 5.3.1, digital voice has been

viewed as a promising approach with major future potential for the MSS

application. It has therefore been the choice adopted in MSAT-X.

The speech coders developed for MSAT-X and recently delivered have

been designed with the fading channel in mind. Special frame synchronization

features to handle outages are inherent in the vocoder designs. To date, the

best speech quality has been obtained from the vector adaptive predictive

6-6
coding (VAPC) vocoder. This technique was developed at UCSB and implemented

in a compact form using a customized DSP-32 board. (See Section 5.3, and for

further detail see [SYSDOC, Section 4].) Preliminary results indicate that

the speech quality, robustness, and suitability to the mobile environment is

superior to that obtained from the self-excited vocoder developed at GIT.

Preliminary subjective testing has shown that the speech quality of

the VAPC is "fair to good." The voice is natural-sounding, intelligible, and

free of processor-induced anomalies such as bleeps. Although it does not yet

meet the desired near-toll-quality, it is much superior to the 2400-bps

"communication quality" speech obtainable by the standard LPC-10 vocoder, which

actually sounds synthetic and suffers from loss of speaker identification.

A recommendation as to which speech coding scheme is definitely

superior will be made at the end of the contracts. This will occur in late

summer/early fall of 1988. As was previously noted, the preliminary results

indicate that the VAPC algorithm developed by UCSB currently produces the best

speech quality. With further refinement of the algorithm, true near-tollquality speech may be attainable. Speech quality improvement is indeed an

ongoing process. The refinement process, however, will eventually reach a

point of diminishing return. Such a point has not yet been reached.

A development that is relevant to the MSAT-X speech coding effort

is the study under way at the National Security Agency (NSA) which aims to

evaluate a host of vocoders that have become available recently. Fourteen

speech coders, which operate at 4800 bps and have been developed by institutions as prominent and diverse as AT&T, Motorola, Entropies, UCSB, and GIT,

are being considered. A military-standard 4800-bps vocoder should soon emerge

from this evaluation. The result will undoubtedly influence the broader

recommendation that JPL will reach for a speech coding algorithm (or

algorithms) most suited for the MSS environment. It is expected, however,

that a speech coder such as the VAPC will exhibit overall performance superior

to vocoders not specifically designed to handle fading outages.

6-7
6.4.1.3 Modulation/Coding. As discussed in Section 5.2, the choice for

modulation and coding in MSAT-X had to evolve with MSAT-X. The necessity of

using 4800-bps speech for near-toll-quality and the requirement to squeeze it

through a 5-kHz bandwidth resulted in the abandonment of DMSK-type schemes

suited for 2400 bps in 5 kHz for the more efficient combined modulation/

coding. Extensive analytical research, aided by real-time simulation, showed

that 16-state, rate 2/3 TCM 8PSK with interleaving is the scheme that can

attain the required spectral efficiency (by using eight phasors) and power

efficiency (by using trellis coding matched to the modulation). Moreover,

with the obligatory shift to L-band from UHF, it has been concluded (see

Section 5.2) that differential detection is superior to coherent detection

(using some form of TCT). This was illustrated in Figure 5.2.8. The present

baseline in MSAT-X has therefore become the 16-state, rate 2/3 trellis-coded

differentially detected 8PSK (TCM/D8PSK for short). The best interleaving

constrained by a 60-ms delay requirement (stemming from voice transmission)

was found to be 128 8PSK symbols, 16 depth x 8 span, with a 32-symbol decoder

buffer.

Since TCM/D8PSK is an innovative technique, a convincing argument

comparing it to more established techniques becomes helpful in advocating it

to the industry. This is done by first comparing the performance of TCM/D8PSK

to that of DQPSK and GMSK. The comparison with DQPSK is fair because it has

the same bandwidth as TCM/D8PSK for the same data rate. GMSK is included for

historical reference. Table 6.4.1 gives this simple comparison of performance

under typical MSS link conditions, but with no timing jitter or intersymbol

interference (ISI) assumed. A 2.5-dB or more improvement is achieved by

TCM/D8PSK over DQPSK under the stated conditions, and about 1 additional dB

relative to DGMSK.

Obviously, on the real channel there will be ISI and timing

jitter. This has been investigated at JPL on the hardware channel simulator

and will also be tested in the field. Test results demonstrate a deviation

within 1 dB from analysis/simulation for TCM/D8PSK on the Gaussian (AWGN)

channel and the fading channel with k = 10 [Jedrey et al., 1988]. This is a

remarkable result that demonstrates that TCM/D8PSK is not particularly

sensitive to ISI and timing jitter, a common concern for 8-ary modulation

6-8
schemes. The square-root raised-cosine with 100% excess bandwidth pulse

shaping discovered at JPL (see, for example, [Rafferty and Bivsalar, 1988])

and used to achieve the combined features of ISI-free points and narrow

spectrum is of critical importance here.

From all of the above, it is evident that the performance improvement over DQPSK and GMSK in the fading environment more than overshadows the

slight increase in modem hardware complexity, which should not be of concern

anyway, with today's integrated circuit and digital signal processor technologies. It therefore follows that the TCM/D8PSK baselined in MSAT-X (with its

key design details) is JPL's recommendation for the modulation/coding scheme

in MSS. It is also recommended that TCM/D8PSK be further demonstrated in the

industry so that it will make the transition from a research environment to

where it becomes commercially well established and readily implementable in an

operational MSS.

6.4.1.4 Transceiver Capabilities. The transceiver requirements for an MSS

represent a departure from the usual requirements for voice systems used in

the communication industry. Instead of the customary use of wide bandwidth FM

for voice, narrow-band digital voice is the recommended approach for MSS, as

mentioned in Section 6.4.1.3. The transceiver for MSS also requires unique

features; most notably, in addition to receiving the data channel, it has to

receive and process the shared pilot signal transmitted from the satellite.

The data and shared pilot channels occupy 5 kHz each, and the transceiver

should be frequency agile, i.e., capable of selecting or tuning to the desired

frequencies within the L-band frequencies of Figure 3.2.1. The receiver should

also provide the pilot I and Q channel outputs (received pilot amplitude and

phase information) to the steerable vehicle antenna.

The transceiver should also be capable of tracking the Doppler of

the received pilot and preferably be capable of measuring it and correcting

its transmitted pilot frequency. The modem in the mobile terminal requires a

signal within 200 Hz of the channel center. The NMC plays the key role in

correcting the major Doppler components. The NMC transmits precompensated

pilot signals derived from a highly stable reference. These are used to

calibrate the transponder, fixed stations, and mobile reference oscillators.

6-9
The result is correction of satellite motion and transponder oscillator drift,

as well as correction of fixed station and mobile reference oscillator

drifts. What remains is the contribution due to the motion of the vehicle,

which can be up to 160 Hz. Depending on the implementation of the

transceiver, the transmitted Doppler (on the return from the mobile terminal)

may be double this amount, which is obviously undesirable.

A brass board transceiver with received Doppler tracking has been

designed, implemented, and used in MSAT-X tests at JPL. Future plans call for

including transmit Doppler correction and full frequency control by the terminal processor as per the architecture given above in Section 6.4.1.1 (Figure

5.2.10).

6.4.1.5 Vehicle Antennas. The vehicle antenna is a critical element in the

MSS link. This is because it will have a significant impact on the economics

of the operational MSS. Because of the inherent antenna performance/cost

trade-off, the vehicle antenna will play a major role in determining the

approach through which the capital investor will realize the return on his

investment. Several key system issues are related to the vehicle antenna

through the trade-off of the mobile terminal contribution versus the satellite

contribution to the overall link performance. Some in the industry consortium,

for example, feel that a very low cost vehicle antenna should be used in conjunction with spacecraft with higher EIRP. (Unfortunately, there is more than

simple engineering performance to influence that choice of approach.) On the

other hand, others feel that a more sound and less risky approach would utilize

medium-gain vehicle antennas. This would basically make the user pay more in

advance rather than per minute. The point to be accentuated here is the complexity of the vehicle antenna issue. With this in mind, JPL has adopted an

approach that maintains as many viable options as possible. In the following

paragraphs, JPL's impartial position based on technology research and

engineering assessment is summarized.

As mentioned earlier (see, for example, Section 5.1), to alleviate

the spacecraft power burden (and permit the use of a lower-risk spacecraft

design), to enable orbit reuse, and to reduce the deleterious effects of

6-10
multipath, JPL views a medium-gain vehicle antenna as a necessity. It should

be noted, however, that this does not preclude supplementary use of low-gain

antennas, as will be pointed out next.

A user with a low-gain antenna (LGA) would have to pay more per

minute of service and perhaps per month in service charges to account for the

higher-power, and certainly more expensive, spacecraft. The extra cost is

related to the differences in gain and G/T between the LGAs and MGAs. The

difference in minimum gain is expected to be at least 4 to 5 dBic, and the

difference in G/T at least 5 dB. This does not take into account the effects

of multipath interference or sensitivity to background blackbody radiation,

which could, in effect, reduce the G/T of an LGA significantly more than that

of an MGA [Bell et al., 1988]. Ongoing testing will accurately determine

these figures.

There are, in any event, two fundamental problems with using LGAs.

The first is that an omni antenna illuminates all the geosynchronous arc over

CONUS; hence, multi-satellite operation would not be possible in any frequency

band used by the LGAs. (It would be very difficult to achieve the axial ratio

j

required in that case to effect inter-satellite isolation.) The second

problem, which is less severe, pertains to the performance in a multipath

environment. In the following we will elaborate on these two issues.

An MSS based only on one spacecraft, i.e., with no orbit reuse,

would probably not have sufficient capacity to ensure a viable system. (See

Section 6.4.3.) It is unlikely that an MSS can be be based solely on the use

of LGAs—some hybrid with MGAs would be needed. This mixed mode of operation

has its disadvantages, unfortunately. Separate spacecraft transmitters would

likely be needed. A high-power transmitter would service the links with LGAs

exclusively. This would avoid the problems of intermodulation and power

robbing that would be experienced when different-level signals share the same

high-power amplification. Even with separate transmitters, interference in a

full-scale MSS would generally be increased due to a combination of two

factors: a lack of directivity on the ornni antennas, and increased intermodulation due to the likely need for higher, nonlinear amplification.

6-11
The second problem pertaining to LGAs is their performance in a

multipath environment. Propagation results suggest that the link performance

can be severely degraded due to multipath. By virtue of the LGAs' inherent

radiation patterns, multipath would have a stronger degrading effect on LGAs

than on MGAs. (Although the drop-off is similar by design on both antenna

types, the LGAs have broader azimuthal beamwidths and thereby collect more

energy from low elevation angles.) Actual voice link performance with an LGA

may therefore be unacceptably poor in multipath environments. It should be

noted, however, that the extent of this effect has not yet been established.

Voice link performance will therefore need to be solidly established through

field testing before LGAs become a real option.

Following the arguments of the above paragraphs, JPL recommends

that medium-gain vehicle antenna technology be the dominant technology

utilized in the MSS. Along with the choice of the MGA approach comes the

choice of what kind of MGA to use. JPL believes that the MSS operator should

offer all of the three options developed. The benefits of this are explored

in what follows.

The mechanically steered, tilted linear array and the electronic

phased array (TRE's design) have demonstrated performances that generally meet

the MSS requirements (see Table 5.1.2). The phased array has a somewhat lower

performance at low elevation angles and slightly less multipath rejection, but

faster reacquisition. In addition, the planar mechanical array is expected to

perform at least as well as the phased array of similar size, but at a cost

about half that of the phased array (see Section 5.1). The trade-off is,

therefore, mostly a cost-versus-conformity or appearance trade-off. For

utility-type vehicles, the cheaper linear array is the obvious choice. For

trucks, the linear array is the choice unless a conformal design would

significantly enhance fuel efficiency for long distances. If the conformal

design is desired, the planar mechanical array would probably be the choice.

For passenger vehicles, the mechanical planar array may be a good alternative

to the very attractive but quite expensive phased array. The phased array

would probably be the choice when concealing the antenna becomes of primary

importance, as with such agencies as the FBI and the CIA. It should be noted

that reliability data with long-term testing may also have an impact on the

6-12
choice of the antenna and may depend on the user's environment. In

conclusion, it is JPL's view that maintaining all the MGA options available

has the potential of increasing the user base for the MSS.

6.4.2 Link Budgets

The link budgets given below serve the following purposes. First,

they demonstrate that with reasonable space segment assumptions, the

recommended system architecture and mobile terminal technologies lead to the

desired link performance. Second, they provide insight into the contributions

of MSS' different segments so that no part will be overdesigned. Third, they

provide information needed for obtaining estimates of system capacity. The

system capacity, in turn, leads to further insight into the efficacy of the

ground segment technologies.

The budgets reflect the assumptions about the space segment

outlined in Section 6.3. They also reflect the ground segment performance

that has been demonstrated as explained throughout Section 5. For example,

the 9.9-dBi vehicle antenna gain and the receiver G/T are consistent with the

numbers provided in Table 5.1.2. Also, the required E, /N is consistent

with the results of Figure 5.2.7 and accounts for the actual hardware losses

observed in the experiments reported in Section 5.2.

It should be noted that the budgets of Tables 6.4.2 and 6.4.3 are

still preliminary in nature. They are not intended to provide precise results

or to reflect a firm straw-man design for the MSS. In that spirit, a simple

noise contribution analysis is used to assess channel interference, and no

preliminary tolerance values are given.

The most prominent observation to be made regards the margin available on the L-band links. The budgets indicate the availability of about 3 dB

of margin. This number, although not intentionally provided as a fading margin, is adequate to provide protection in the event of moderate multipath

fading (see Figure 5.5.1, curve H). It cannot, however, overcome deep fades

or the effects of shadowing. This observation is in line with the

well-accepted premise that the MSS is a line-of-sight system.

6-13
6.4.3 System Capacity

It is interesting, and, in fact, enlightening, to have at least a

rough estimate of the system capacity based on the link budgets given above.

This places in perspective the utilization of the MSS resources that result

from the design and technology recommendations of MSAT-X.

Using the figure provided in the link budgets for the transmitted

power per channel, with 25% HPA efficiency, and allowing for the power

consumption by the K-band and other spacecraft subsystems, a rough estimate of

the spacecraft power and weight requirements can be obtained [Sue et al.,

1985]. A satellite of 5000-lb geostationary transfer orbit (GTO) mass and

with 3 kW of spacecraft power can support a number of channels, in the range

of 2000. The exact number depends on several factors, viz., the traffic mix,

channel duty cycle, and service quality. For example, assuming that the

service quality has a 2% blocking probability and a 3.2-s message delay, that

the user traffic is one 90-s voice call or one 4096-bit data message per hour,

and that 40% of the traffic is attributed to voice, the satellite would

support 1929 channels. According to I-AMAP, these would be partitioned into

204 voice channels, 260 data channels, and 1465 reservation channels. (The

seemingly large number of reservation channels can be reduced if the message

delay requirement can be relaxed.) With this partitioning, a total of about

89,000 users could be accommodated by the satellite, corresponding to about

50 users per channel.

With about 2000 5-kHz channels per satellite, the required spectrum

would be 10 MHz. Frequency reuse can be applied to achieve a reduction in the

spectrum. However, only limited reuse is possible with the first-generation

satellite beam layout shown in Figure 6.3.1. The outer (east and west) beams

can use the same frequencies to reduce the overall spectral occupancy by a

factor of 4/3. A look back at the spectrum allocations of Figure 3.2.1 and

Table 3.2.1 then reveals that a first-generation MSS, based on the technologies

of MSAT-X, would probably be either close to or not quite spectrum-limited.

This indicates that the full utilization of the MSAT-X technologies will not

occur until second-generation MSS satellites become available. These advanced

spacecraft would utilize large multibeam antennas (say, 15 m at L-band). Those

6-14
antennas would alleviate any remaining power deficiency and concurrently permit

a considerably larger number of channels per satellite. Hence, because they

are both power- and bandwidth-efficient, the technologies developed under

MSAT-X are well-suited to alleviating the power deficiencies of a firstgeneration MSS, as well as to resolving the spectrum limitations of latergeneration mobile satellite systems.

6-15
Table 6.4.1. Comparison of the Performances of TCM/D8PSK, DQPSK, and

Differential GMSK (DGMSK) Under Typical MSS Link Conditions

(No ISI or Timing Jitter)

EL/N at BER=1 x 10

(k = 10; ~40-Hz

Doppler Spread)

-3

Relative Bandwidth (BW)

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DQPSK 12.2

DGMSK

(BT = 0.5)

13 1.6

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(R = 2/3; 16-state

128 8PSK Symbol

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9.5

6-16
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Figure 6.3.1. Multibeam Coverage of CONUS by First-Generation MSS Satellite

6-19
SECTION 7

REFERENCES

Ball Aerospace Corporation, "Concepts and Cost Trade-Off for Land Vehicle

Antenna in Satellite Mobile Communications," Boulder, Colorado, July 1984.

D. Bell and F. M. Naderi, "Mobile Satellite Communications—Vehicle Antenna

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D. Bell et al., "MSAT-X Antennas Noise Temperature and System G/T," to appear

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Pasadena, California, July 1988.

J. Berner, "PiFEx Tower 1 Results," Proceedings of NAPEX XI, held on June 19,

1987, at the Virginia Polytechnic Institute and State University, Blacksburg,

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C. Bostian, "Propagation Studies Related to LMSS," Proceedings of NAPEX XI,

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Laboratory, Pasadena, California, August 31, 1987.

C. Cheetham, "The Terminal Processor: The Heart of the Mobile Terminal,"

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California, October 1987.

J. H. Chen et al., "Speech Coding for the Mobile Satellite Experiment," ICC

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7-1
Cubic Corporation, "Trade-Off Between Land Vehicle Antenna Cost and Gain for

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Laboratory, Pasadena, California, August 1984.

F. Bavarian, "Channel Simulation to Facilitate Mobile Satellite Communications

Research," IEEE Transactions on Communications, Vol. COM-35, January 1987.

F. Bavarian et al., "BMSK: A Practical 2400-bps Receiver for the Mobile

Satellite Service," JPL Publication 85-51 (MSAT-X Report No. Ill), Jet

Propulsion Laboratory, Pasadena, California, June 15, 1985.

B. Bivsalar, "JPL's Mobile Communication Channel Software Simulator,"

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Bocument B-2208, Jet Propulsion Laboratory, Pasadena, California, January 1985.

B. Bivsalar and M. K. Simon, "Trellis Coded Modulation for 4800 to 9600 bps

Transmission Over a Fading Satellite Channel," JPL Publication 86-8, Jet

Propulsion Laboratory, Pasadena, California, June 1, 1986.

R. F. Emerson, "Clock Synchronization Requirement for Mobile Terminals," JPL

Interoffice Memorandum, MSAT-X: 331-85 (JPL internal document), Jet

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J. Flanagan et al., "Speech Coding," IEEE Transactions on Communications, Vol.

COM-27, April 1979.

Ford Aerospace and Communications Corporation, "Spacecraft Configuration Study

for the Second Generation Mobile Satellite System," Final Report, prepared

under contract to the Jet Propulsion Laboratory, Pasadena, California, January

1985.

General Electric Corporation (GE), "Mobile Radio Alternative Systems

Study—Vol. 3: Satellite/Terrestrial (Hybrid) Systems Concepts," GE Technical

Report, prepared for NASA Lewis Research Center, Cleveland, Ohio, June 1983.

7-2
Georgia Institute of Technology, "Development, Design, Fabrication, and

Evaluation of a Breadboard Speech Compression System at 4800 BPS," ninth

quarterly progress report submitted to the Jet Propulsion Laboratory,

Pasadena, California, by the School of Engineering, Georgia Institute of

Technology, Atlanta, Georgia, June 5, 1987.

C. Hoeber et al., "Passive Intermodulation Product Generation in High Power

Communications Satellites," AIAA llth Communication Satellite Systems

Conference, San Diego, California, March 1986.

J. Huang, "L-Band Phased Array Antennas for Mobile Satellite Communications,"

37th IEEE Vehicular Technology Conference, Tampa, Florida, June 1987a.

J. Huang, "Production Costs of MSAT Conformal Phased Array Antennas," MSAT-X

Quarterly No. 11 (JPL 410-13-11), Jet Propulsion Laboratory, Pasadena,

California, July 1987b.

V. Jamnejad, "A Mechanically Steered Monopulse Tracking Antenna for PiFEx:

Overview," MSAT-X Quarterly No. 13 (JPL 410-13-13), Jet Propulsion Laboratory,

Pasadena, California, January 1988a.

V. Jamnejad, "A Mechanically Steered Monopulse Tracking Antenna for PiFEx: RF

System Design," MSAT-X Quarterly No. 13 (JPL 410-13-13), Jet Propulsion

Laboratory, Pasadena, California, January 1988b.

T. C. Jedrey and N. E. Lay, "The Implementation of a 4800 bps Trellis Encoded

8-DPSK Modem," MSAT-X Quarterly No. 12 (JPL 410-13-12), Jet Propulsion

Laboratory, Pasadena, California, October 1987.

T. C. Jedrey et al., "An All-Digital 8-DPSK TCM Modem for Land Mobile

Satellite Communications," International Conference on Acoustics, Speech,' and

Signal Processing (ICASSP), New York City, New York, April 1988.

Jet Propulsion Laboratory, MSAT-X Quarterly No. 13 (JPL 410-13-13), Special

Issue on PiFEx Tower-1 Experiments, Pasadena, California, January 1988.

7-3
Jet Propulsion Laboratory, "SYSDOC (MSAT-X Technical Documentation)," JPL

Internal Document (in preparation), Pasadena, California.

J. P. McGeehan and A. J. Batetnan, "Phase-Locked Transparent Tone-in-Band

(TTIB): A New Spectrum Configuration Particularly Suited to the Transmission

of Data Over SSB Mobile Radio Networks," IEEE Transactions on Communications,

Vol. COM-32, January 1984.

K. Murota and K. Hirade, "OMSK Modulation for Digital Mobile Radio Telephony,"

IEEE Transactions on Communications, Vol. COM-29, No. 7, July 1981.

F. Naderi (editor), "Land Mobile Satellite Service (LMSS): A Conceptual

System Design and Identification of the Critical Technologies," JPL

Publication 82-19, Part II, Jet Propulsion Laboratory, Pasadena, California,

February 15, 1982.

F. Naderi et al., "NASA's Mobile Satellite Communications Program: Ground and

Space Segment Technologies," Proceedings of the 35th Annual International

Astronautics Federation Congress, Lausanne, Switzerland, October 1984.

W. Rafferty and D. Divsalar, "Modulation and Coding With Doppler Estimation

for Land Mobile Satellite Channels," IEEE International Conference on

Communications, Philadelphia, Pennsylvania, June 1988.

RCA Astro Electronics, "Spacecraft (Mobile Satellite) Configuration Design

Study," Final Report, prepared under contract to the Jet Propulsion

Laboratory, Pasadena, California, June 1985.

Rensselaer Polytechnic Institute (RPI), "A Study of Transparent Tone in Band

Modulation for Binary PSK Signalling," Interim Report, Troy, New York,

February 1, 1988.

A. Salmasi et al., "Land Mobile Satellite Service (LMSS) Channel Simulator:

An End-to-End Hardware Simulation and Study of the LMSS Communications Links,"

JPL Publication 84-12, Jet Propulsion Laboratory, Pasadena, California, May 1,

1984.

7-4
Signatron, Inc., "MSAT-X Networking System Study Final Report—Vol. 1: A

Feasibility Assessment of I-AMAP," JPL Document 9950-1198, Jet Propulsion

Laboratory, Pasadena, California, June 8, 1985.

M. K. Simon and D. Divsalar, "The Performance of Trellis Coded Multilevel DPSK

on a Fading Mobile Satellite Channel," JPL Publication 87-8, Jet Propulsion

Laboratory, Pasadena, California, June 1, 1987.

M. K. Simon and D. Divsalar, "Open Loop Frequency Synchronization of MDPSK

With Doppler," IEEE International Conference on Communications, Seattle,

Washington, June 1988.

M. K. Sue et al., "Second-Generation Mobile Satellite System: A Conceptual

Design and Trade-Off Study," JPL Publication 85-58, Jet Propulsion Laboratory,

Pasadena, California, June 1, 1985.

TRW, "Mobile Communications Satellite System," TRW Technical Report, Vol. 2,

prepared for NASA Lewis Research Center, Cleveland, Ohio, April 11, 1983.

G. Ungerboeck, "Channel Coding With Multilevel/Phase Signals," IEEE

Transactions on Information Theory, Vol. IT-28, No. 1, January 1982.

W. Vogel and J. Goldhirsh, "Multipath Measurements at L-Band and UHF in

Mountainous Terrain for Land-Mobile Satellite System," Proceedings of NAPEX

XI, held on June 19, 1987, at the Virginia Polytechnic Institute and State

University, Blacksburg, Virginia, JPL Internal Document D-4647, Jet Propulsion

Laboratory, Pasadena, California, August 31, 1987.

W. Vogel and U. S. Hong, "Measurement and Modeling of Land Mobile Satellite

Propagation at UHF and L-Band," Proceedings of NAPEX XI, held on June 19,

1987, at the Virginia Polytechnic Institute and State University, Blacksburg,

Virginia, JPL Internal Document D-4647, Jet Propulsion Laboratory, Pasadena,

California, August 31, 1987.

7-5
C. C. Wang and T. Y. Yan, "Performance Analysis of an Optimal File Transfer

Protocol for Integrated Mobile Satellite Services," Globecom 1987, Tokyo,

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