April 1987 

(NASA-CB- 179bG5) SBTELLIZE AbALCG EDBA/FH 

!IC DIGI'IAL TDBA CCYVEESXZCL Einal IjlepOrt, 

bay - Dec, 19EE (Ecrd AerorFace and 

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G3/33 45167 

NASA 

NASA LEWIS RESEARCH CENTER 

Contract No: NAS3-24890 
1 Repor! No 

CR 179605 

April, 1987 

6. Performing Organization Code Satellite Analog FDMA/FM to Digital 

TDMA Conversion 650-60-26 

~ 

2 Goveroment Accession No 3 Recipient's Catalog No 

4 Title and Subtitle 5 Report Date 

16. Abstract 

This Final Report presents the results of a Ford Aerospace and 

Communications Corporation study which investigated design issues 

regarding the use of analog to digital (A/D) conversion on board a 

satellite. The study was conducted for NASA Lewis Research Center under 

contract No. NAS3-24890. 

7 Authorb) 

T. Driggers, T. Nguyen, V. Kolavennu 

~ 

9 Performing Organization Name and Address 

The need for A/D, and of course D/A as well, conversion arose from a 

satellite design which required routing analog FDMA/FM up and down links 

to/from a digitally modulated (i.e., QPSK) inter-satellite link. There 

are also some advantages when one must interconnect a large number of 

various spot beams which are using analog, and therefore cannot take 

advantage of SS/TDMA switching among the beams, thus resulting in low 

fill factors. 

8 Performing Organization Report No 

WDL- TR11068 

10. Work Unit No. 

Various tradeoffs were performed regarding the implementation of on-board 

A/D processing, including mass, power, and costs. The various 

technologies which were considered included flash ADCs, surface acoustic 

wave (SAW) devices, and digital signal processing (DSP) chips. Impact 

analyses were also performed to determine the effect on ground stations 

to convert to digital if the A/D approach were not implemented. 

Ford Aerospace & Communications Corp 

3939 Fabian Way 

Palo Alto, CA 94303 

12. Sponsoring Agency Name and Address 

NASA Lewis Research Center 

21000 Brookpark Road 

- Cleveland, Qhio 44135 

11. Contract or Grant No. 

NAS3-24890 

13. Type of Report and Period Covered 

Final 

Mav 1986-n~r 1986 

14. Sgonsoring Agency Code 

17. Key Words (Suggested by Author(s)l 

Analog/Digital Conversion 

Digital switching. 

~ 

' For sale by the National Technical Information Service Sprinefield Virginia 22161 

18. Distribution Statement 

General Release 

NASA-C-168 (Rev. 10-75) 

19. Security Classif. (of thir report) 

Unclassified 

20. Security Classif. (of this page) 21. NO. of pager 22. Price' 

Unclassified 70 
1.0 OVERVIEW 

1.1 Purpose 

This study was performed by Ford Aerospace and Communications Corporation to investigate 

design issues regarding tpe utilization of analog to digital (A/D) conversion on board a satellite. 

The concept originated as part of the NASA Communications Platform Payload Definition (CPPD) 

Study, Ref [l], conducted by FACC. One of the payload concepts developed in that study required 

routing lntelsat traffic from Latin AmericdCaribbean countries via an intersatellite link (ISL) 

to Europe. It was assumed that many of these countries would continue to use FDMA/FM, 

whereas the ISL used digital modulation: hence A/D (and of course D/A) conversion would be 

required under these assumptions. 

Under the FDMA/FM approach, it was highly desirable that different accesses within a (36 

MHz) transponder be independently routed or switched to the ISL or other beams, since the 

lntelsat gateway stations were covered by several small spot beams. If an entire 36 MHz 

transponder were the smallest switchable unit, then very poor fill factors would have resulted. 

The fill factor issue also applied to U.S. domestic coverage as well. The issue of fills has also 

been considered in this study. 

The objectives of the study were as follows: 

Perform assesment of applications where A/D and D/A conversion could enhance 

system architectures 

Analyze technical Performance of various alternative systems 

Perform economic comparisons involving selected system(s) 

1.2 Approach 

The study was organized into the following tasks: 

Task 1: Develop a data base of cost and performance of various potential hardware 

components, numbers of ground station users, and traffic estimates. 

Task 2: Develop candidate concepts to the block diagram level, with emphasis on A/D 

conversion, but including D/A. 

Task 3: Refine the approach(es) from Task 2 selected by NASA and generate bottom line 

economic comparisons with a no on-board conversion architecture; determine 

other advantages/disadvantages of the selected system(s). 

1.3 Selected Concept 

The basic approaches considered were: 

(1) Analog filtering prior to A/D 

- Switchable filter banks 

- Programmable filters 

1 
(2) Digital filtering after ND 

- Frequency domain processing 

- Time domain processing 

Based on cost, mass, and power considerations, the analog filtering with switchable filter banks 

was deemed to be the prefered approach. Although the use of 1993 technologies showed digital 

closing the gap somewhat, the selected approach still maintained its advantage. 

1.4 Economic Analysis 

A detailed analysis was performed for the selected approach, comparing the use of on-board A/D 

conversion with other approaches, such as converting earth stations to digital modulation, with 

on-board demodulation for switchinghouting of the accesses. An overall summary of the costs 

for the two approaches (with and without ND) is presented in Tables 1 and 2 for the lntelsat 

and U.S. domestic situations respectively. 

If no on-board A/D conversion is provided in the international situation, then it must be 

performed on the ground, because of the assumption that an ISL uses digital modulation. There is 

no forcing requirement to convert to digital in the U.S. domestic situation, and so earth station 

costs are not included in Table 2. Sections 4.1.3 and 4.2.3 provide additional details. 

1.5 Major Findings 

The use of A/D and D/A conversion on board a satellite can provide flexible routing between 

analog users and digital inter-satellite links, as well as improved fill factors of static switched 

transponders. The provision of this capability appears to have significant cost advantages over 

ground-based solutions in the former case, and also in the latter case under some circumstances. 

In the arena of technology development, the prefered approach for A/D conversion used hardware 

which exists today. There would need to be some space qualification of the hardware, but no 

significant problems are seen there. The major development would be the high speed digital 

switch required to handle the data rates output by the A/D converters. 

2.0 Component Cost and Performance 

As a preliminary to the technical analysis of various approaches and their economic impact, data 

bases of appropriate hardware, traffic affected, and ground station users were developed. 

2.1 Data Base Description 

This subsection provides the summary descriptions of the various current state-of-the-art 

product data-bases. The first part provides the cosVperformance comparisons of the flash A/D 

converter products. The second part compares the costlperformance trade offs of the high-speed 

D/A converter products. The third part provides the information on the digital signal processing 

(DSP) products. The last part of this subsection provides the performance capabilities of 

sixface acoustic wave (SAW) filters with the currently available, and projected 1993 

state-of-the-art parts. 

2 
2.1.1 Flash Analog-to-Digital-Converters: 

The price/performance comparison of the flash A/D converters is shown in Tables 3 and 4. 

They show that the lower the resolution, sampling frequency and linearity requirements, the 

lower the price of the A/D converters. Also, it is easier to implement with CMOS type 

low-power technologies at lower sampling frequencies, whereas the higher frequency operations 

may require ECL or bipolar logic implementations, which have much higher power 

requirements. Sony currently provides an A/D converter (model: CX20016) with 8 

bit-resolution and with acceptable sampling frequencies of up to 100 MHz and input signal 

bandwidths of up to 40 MHz. A/D parts offered by TRW (TDC1029), Analog Devices 

(AD501OKD) and Siemens (SDA5200) can also accept sampling frequencies of up to 100 MHz, 

but provide only 6 bits of resolution. All are ECL implementations, requiring a minimum power 

of 1W. In comparison, the A/D parts for sampling rates of 10 MHz or below, are available 

from many more vendors and provide better overall performance to price ratio (i.e., better 

resolution, lower power-requirements and much lower prices 

2.1.2 High-speed Digital-to-Analog Converters: 

Table 5 presents the price-performance features of the currently available D/A converters. 

From this table, it is easy to infer that the current state of technology allows implementation of 

D/A converters at much higher speeds than the A/D converters and at a much lower cost. For 

example Honeywell offers a D/A converter (model: HDAC-9700), which can accept 8-bit input 

digital signals at rates of up to 200 MHz. Sony offers a D/A converter (model: CX20201) which 

accepts input digital signals of resolutions of upto 10 bits and at rates of up to 100 MHz. 

2.1.3 Digital Signal Processing Integrated Circuits: 

Table 6 lists a few Digital Signal Processing (DSP) chips, currently available from TRW and 

Zoran. This table shows that the current state-of-art of technology limits the maximum rates to 

20 MHz for these signal-chip implementations of the DSP functions. The maximum number of 

cells per chip is 8 and the maximum number of taps implementable with multiple chips is 

limited to 1,032. 

2.1.4 Surface Acoustic Wave (SAW) Analog Filters: 

Table 7 shows the capabilities of SAW analog filter implementations with the current 

state-of-the-art parts. This table clearly shows that the filter implementation for the 

on-board A/D processing with analog filtering approach, is completely feasible with the current 

state-of-the-art part. We shall elaborate on this issue later. By 1993, the SAW technology is 

expected to be much more mature and the required entire filter-banks are expected to be 

implemented on a small (2"x4") PC Board. 

2.2 Derivation of A/D Traffic 

2.2.1 Traffic Definition 

The quantity of analog traffic which is subject to the satellite on-board A/D and D/A conversion 

is the Fixed Satellite Service traffic of the two areas described in the following subsections. 

2.2.1.1 Latin AmericalCarribbean Traffic Area. 

The traffic is made up of all Region 2 international FSS analog traffic: (1) between the countries 

3 
within the area of Latin America and the Caribbean; (2) between the area of Latin 

AmericdCarribbean and the continental US; and (3) between Region 2 and all other regions 

through the intersatellite links. 

This traffic is routed through international gateway earth stations of the countries of this area. 

2.2.1 -2 US Domestic. 

The traffic considered for U.S. domestic areas which had potential for A/D application is that 

traffic assigned to the 12 MHz or 18 MHz transponders in the CPPD study. These "narrow band" 

transponders were included to improve fill and provide a little more flexibility in 

routinghwitching. With 9 beams and only 12 transponders, flexibility would have been 

minimal. The use of A/D on-board conversion provides even greater flexibility than the use of 

narrow-band transponders. 

2.2.2. Traffic Derivation 

For the Latin AmericaEarribbean area, the most likely (or "medium") estimate is the 

INTELSAT traffic forecast used in the CPPD study. This forecast is based on the August, 1984 

INTELSAT Traffic Model. The nominal growth rate for Atlantic Ocean Region (AOR) traffic in 

that forecast is 8%. The growth rates are adjusted for low and high level estimates based on the 

following growth rates: 

LOW Medium High 

4% 8% 12% 

For US area, the most likely estimate is the NASA U.S. domestic traffic forecast used in the 

CPPD study. This forecast is based on the NASA FSS Traffic Model, Ref [2], plus FACC estimates 

of growth rates beyond the year 2000. 

The assumed growth rates are: 

Low Medium High 

1985-2000 10% 14% 14% 

2000-2008 6% 10.5% 14% 

Based on the above assumptions, Table 8 summarizes the total traffic where A/D conversion 

could be used. 

2.3 Derivation of Access Sizes 

A trade-off exists between the number of accesses within a 36 MHz transponder and the 

resulting fill factors; the smaller the access size (the larger the number of accesses), the 

higher the resulting fills. However, one reaches a point of diminishing returns regarding the 

fills, and so there is an "optimal" size, at least in an overall average sense. 

4 
A simple program was developed to process the traffic matrices developed for Latin 

AmericdCaribbean in the CPPD study. This program considered a three level hierarchy, which 

is quite similar to the multiplexing hierarchy used in terrestrial communications systems - 

e.g., channel groups, supergroups, and rnastergroups. This allows the flexibility to match the 

size of accesses to the actual traffic in an access. To illustrate, consider two access sizes of 20 

and 100 circuits. If a particular traffic bundle (access) were 80 circuits, then one could use 

either a single "large" group at 80% fill, or four "small" groups at 100% fill. The latter 

provides higher fills, but at the cost of more equipment. 

The program mentioned in the previous paragraph simply iterates on various sizes for the level 

1 (smallest) and level 2 groups. Level 3 is fixed at the entire 36 MHz transponder. One other 

input to the program is the minimum fill required to establish a level 2 or 3 group. To use the 

above example, if the minimum fill were 75%, then an access of size 100 would be selected; if 

the minimum fill were set to 90% (greater than the 80% fill actually attainable) then the 

traffic bundle would be "overflowed" to the next lower level, and four groups of 20 would be 

used. Appendix A contains sample outputs of the program, and the results are summarized in 

Figure 1. Three minimum fills were used: 85%. 70%, and 50%. At the two higher fills, the 

choice of level 2 size had a negligible effect on the results, so only a single (solid) curve for fill 

is indicated. For the 50% case, two curves are shown, one through the set of minimum fills 

(given the level 1 size) and one through the set of maximum fills for each level 1 value. 

The dotted lines represent the total number of groups required, and are a rough measure of the 

amount of hardware required. The trade-off point chosen is circled on the graph, and 

corresponds to level sizes of 96, 480, and 2400 circuits, or in terms of MHz (ignoring guard 

bands), 1.44, 7.2, and 36 MHz respectively. 

To provide a flexible capability within each transponder, a switchable filter scheme, shown in 

Figure 2, was developed. The ratios of the number of groups at the two lower levels are based on 

the total number of groups at each level. Thus, one can have a single 36 MHz access, or three to 

five 7.2 MHz accesses, or up to ten 1.44 MHz accesses. The selection is made using switchable 

filter banks, as described in Section 3.2.1. 

2.4 DERIVATION OF A/D GROUND STATION USERS 

2.4.1 Number of ground stations in Latin AmericaKarribbean area. 

international traffic in the Latin AmericdCarribbean countries are routed through ground 

stations functioning as international gateways. By their function and due to regulatory issues 

(one PTT per country), the number of international gateways is limited to 1 or 2 at the most 

(Brazil) for each country. The number of those stations is quite stable and independent of the 

traffic. At most, a fluctuation of 5 countries about the 1984 number (27) is assumed. 

2.4.2 Estimation of Number of Ground Stations in US. Area 

In terms of Fixed Satellite Service, earth stations in the US can be categorized into (1) trunking 

stations handling voice trunking, data trunking and video conferencing trunking and (2) 

customer premise service (CPS) stations. The on-premise CPS station carries a mixed and 

relatively light traffic volume of voice, data and video conferencing, such traffic being processed 

beforehand into digital formats for ease of use and efficient transmission. Therefore only 

trunking stations operated by the common carriers for shared use will carry analog traffic by 

year 2008. 

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The number of carrier (shared use) trunking stations for year 1984 was obtained in the CPPD 

study based on a survey of FCC filings for a total of 575 stations. Because these stations are 

operated by common carriers, US international traffic may be routed through any of those 

stations. The methodology for estimating the increase in number of stations carrying analog 

traffic from year 1984 to 2008 is based on following assumptions: 

The growth rates for total earth stations correspond to traffic growth 

(Section 2.2.2) 

Due to assumed emergence of Ka-band in the time frames involved, the number of 

earth stations for analog is proportional to the bandwidth of (C+Ku)/(C+Ku+Ka) 

To reflect potential emergence of ISDN, the "low" case assumes no analog, and the 

"medium" case is biased somewhat toward fewer analog users 

2.4.3 Summary of Ground Station Users 

Table 9 summarizes the projected number of ground station users for low, medium, and high 

estimates. 

3.0 CANDIDATE CONCEPTS 

Task 2 of this study addressed the various concepts for the implementation of the 

Analog-to-Digital (ND) conversion of the uplink analog signals and the associated processing 

on-board the satellite system. It also addressed the concepts of implementation of the associated 

digital-to-analog conversion of the analog down-links. 

The following subsections describe the approach taken. The main objectives of Task 2 are first 

derived. Then, the various candidate concepts for the implementations of analog-to-digital 

conversion and the associated on-board processing are developed. Implementation alternatives 

and the associated issues are discussed. For each of the concepts, the timing and control 

requirements are identified. The possible component configurations are also identified for each 

approach, following which, the approach that is best suited, is recommended. Then, the concepts 

of implementations of Digital-to-Analog (D/A) conversion are described. 

3.1 Objectives 

The main objectives of task 2 are: 

- to develop alternative concepts for implementing the analog-to-digital conversion of 

the uplink analog signals 

- to investigate the associated on-board processing required 

- to analyze the timing and control requirements for each of the concepts 

- to provide the cost vs performance comparisons 

- to rank the alternatives and recommend the approach best suited for the specific 

application 

- to develop concepts for the implementation of the associated down-link digital-to-analog 

(D/A) conversion 

6 
3.2 Concepts for Analog-to-Digital (A/D) Conversion 

In this subtask, various approaches for implementation of the A/D conversion of the uplink 

analog signals are discussed. 

There are two fundamental approaches that are distinguished from each other by the nature of 

implementing the A/D conversion either prior to or after separating the sub-channel accesses . 

3.2.1 Concept 1 - Analog Filtering Approach 

The first approach separates the sub-channel accesses prior to the A/D conversion of the signals 

and is called the "Analog Filtering Approach". A simplified block diagram of this implementation 

concept is shown in Figure 3. 

The uplink received analog signals are passed through a Low Noise Amplifier (LNA) and the first 

stage of receive RF section, which down converts the received uplink signals. It consists of a 

mixer, a low-pass filter and, an RF de-multiplexer, which separates the individual 

transponder-channels. 

The second stage of receiver's RF section down-converts the individual transponder-channel 

outputs of the first stage further to an Intermediate Frequency (IF), where the individual 

sub-channel accesses are separated by means of an IF demutiplexer. Each output of the IF 

demultiplexer passes through a switchable demodulator, which may be required for 

accommodating incompatible modulation-demodulation formats on various links. 

The output of each switchable demodulator is then passed through the AID converter. The output 

of the A/D converter is passed through the switching and routing system, which switches the 

signal to the appropriate downlink, Inter-Satellite Link (ISL) or the CONtinental United States 

(CON US) processor. 

3.2.1 .l. Implementation of IF Demultiplexer 

The IF demultiplexer shown in Figure 4 is comprised of a bank of band-pass filters (BPF) and a 

filter-selector which selects and switches the appropriate configuration of the BPFs for a given 

set of transponder sub-channel access configurations. 

The bank of BPFs can be implemented by means of a set of BPFs with remotely tunable passband, 

programmable BPFs or by means of a set of fixed passband switchable BPFs. 

For the specific case, when the Intermediate Center Frequency of 70 MHz of the transponder 

channel of bandwidth 36 MHz, with a minimum subchannel access size of 1 MHz and a maximum 

access-size of 6 MHz, a possible implementation with a set of fixed passband Surface Acoustic 

Wave (SAW) Band Pass Filters is illustrated in Figure 5. Current state of the art of technology 

allows the entire filter bank with the filter-selector, the associated timing and control 

circuitry included, to be placed on one small Printed Circuit Board (PCB). The programmable 

passband remotely tunable filter approach, however is still not feasible with the current state 

of the art devices for applications requiring more than 1 to 2 MHz access-sizes. 

3.2.1.2 Implementation of Switchable Demodulator and A/D 

During Task 2, a switchable demodulator was included to provide a capability to interface with 

different modulation types or the Ka-band baseband processor. Upon further study and 

refinement during Task 3, it was deemed that this approach was not viable, as it would require 

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demultiplexing to the circuit level or an equivalent transmux function. This amount of 

processing would be very large, and the capability provided was beyond the scope of this study. 

This meant that U.S. - Latin AmericanKaribbean traffic could not be routed to the baseband 

processor, as originally envisioned in the CPPD study. Rather, this traffic was handled by C. and 

Ku band transponders using FDMNFM. 

3.2.1.3 Timing and Control Requirements for Concept 1 

Timing and control for this analog filtering approach are required, for at least the following 

functional implementations: 

i. For filter configuration, selection and switching 

ii. For programmable passband filter control for the case when the IF demux is 

implemented with tunable passband programmable filter bank 

iii For the demodulator switching and its control 

iv. Sampling frequency generation for A/D 

V. For providing the data and control interface for switching and routing system 

3.2.2 Concept 2 - Digital Filtering Approach 

This approach differs from the previous approach in the order of ND conversion and 

access-separation. In this approach, the A/D conversion is performed prior to the digital 

filtering of the IF input-signal, for separating the different accesses of the transponder channel, 

as shown in Figure 6. 

The IF input-signal passes through the ND, where the analog signal is digitized and then passes 

through the Digital Filter Processor (DFP) bank in order for the subchannel accesses to be 

separated. The outputs of the DFPs are passed through the switchable digital-demodulators, 

which may be required for transmission on different links with incompatible modulation 

formats. The outputs of the switchable demodulators are then fed to the switching routing system 

for further processing and routing to the appropriate downlink or ISL-link processor. 

3.2.2.1 Implementations of Digital Filter Bank 

The DFP bank can be implemented in two basic approaches: 

i. Time Domain Processing 

ii Frequency Domain Processing 

3.2.2.1.1 Time Domain Processing 

There are at least two ways of implementing the DFPs in time domain -- the so called Finite 

Impulse Response (FIR) filter processors using Very High Speed Integrated Circuits (VHSICs), 

or the convolvers using either VHSlCs or SAW devices. 

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3.2.2.1.2 Frequency Domain Processing 

DSP implementations using Frequency Domain Processing approach can again be performed in at 

least two ways -- the so called the Fast Fourier Transform (FFT) implementations with VHSICs, 

or the Chirp 2-Transform (CZT) implementations, using the SAW devices. 

3.2.2.1.3 State of the Art of DSP Implementations 

Tables 10 and 11 show the current (1986) and projected (1993) state of the art of signal 

processing functions that can be embodied in a single chip. The projections for 1993 state of the 

art are needed because of the assumptions that the launch of the payload is going to be in 1998 

and that a five year lead time is needed for the availability of fully tested and flight-qualified 

parts. 

It should be first mentioned that a 36 MHz transponder channel requires a minimum of 72 MHz 

sampling rate. Typically, a sampling rate of 90 to 100 MHz is desirable. From these two tables, 

it is easy to see that single chip implementations of both ND and D/A are currently available 

that meet this requirement. No information is available at this point on the the implementation 

of DSPs with CZT approaches. The FFT approaches of the DFP implementations meeting the 

requirement of 100 MHz rates are expected to be available in 1993. The current 

state-of-the-art single chip implementations can support only up to rates of 10 MHz, which is 

an order of magnitude below the 1993 requirement. The time-domain approaches of DFP 

implementations, supporting 100 MHz sampling rates, may not be available even in 1993. 

3.2.2.1.4 Implementation of DFPs with FFT Aproach 

From the discussion of the previous section, it can be concluded that the only viable DFP 

implementation for this application seems to be via the FFT approach. An illustration of the 

implementation of the DSP bank with the FFTs is shown in Figure 7. 

In the FFT approach, the digital signal at the output of the A/D is buffered and passed to an FFT 

machine, where the spectrum of the input signal is computed. The spectral output of the FFT 

machine is then passed to the Spectral Sample Memory (SSM), from which the various 

transponder subchannel accesses are separated by selecting the appropriate parts of the SSM. 

The spectral samples of the subchannel accesses are then passed through the bank of Inverse 

Discrete Fourier Transform (IDFT) computers, providing the subchannel access signals in the 

time-domain, which are then forwarded to the Switching and Routing System. 

3.2.2.2 Timing and Control Requirements for DFP Approach 

The timing and control requirements for the illustrated implementation of the DFP Approach are 

given below: 

i. Sampling frequency generation 

ii. DFP timing and control, which includes the buffer memory and FFT timing and 

con t ro I functions. 

i ii. Spectral sample memory timing, window-selection, processing and control 

functions. 

iv. IDFT configuration, selection, timing and control 

9 
v. Demodulator’s switching, timing and control 

vi. Timing, control and data interface for interfac..ig with the switching and routing 

sys tem. 

3.2.3 Switching and Routing System 

The digital Switching and Routing System (Figure 8) facilitates the communication paths 

between the uplink and the appropriate downlink accesses. The digital subchannel access signals 

are inputted into the appropriate access input memory buffers. The digital switch, operating 

under the control of the digital routing controller, facilitates the communication paths between 

the input and the appropriate output access-memory-buffers. The outputs of the 

access-output-memories are passed to the appropriate transmit subsystems. 

3.3 Comparison of A/D Approaches and Recommendations 

The two main concepts of implementations of A/D conversion schemes -- the analog filtering 

approach and the digital filtering approach -- are compared in this subsection both qualitatively 

and quantitatively, leading directly to the recommended approach. 

3.3.1 Qua I i ta t ive Com pa rison 

Table 12 shows a qualitative comparison between the analog and digital filtering approaches. The 

complexity of implementation of each of the functions listed is rated low, moderate or high with 

the current(l986) and the projected 1993 state-of-the-art parts. 

This table shows that the analog filtering approach with switchable fixed passband filter bank is 

implementable with low complexity, even with the current state-of-the-art parts. The 

remotely programmable passband filter bank, on the other hand, is very difficult to implement 

with the current state-of-the-art parts, and is not expected to be easily implementable even 

with 1993 state-of-the-art parts. 

The digital filtering approach requires timing and control functions with very high complexity 

of implementation using currently available parts. Even in 1993, this function does not seem to 

be very easily implementable. 

3.3.2 Quantitative Comparison 

Tables 13 and 14 show the power, size, weight and cost estimates with the current and 1993 

state-of-the-art parts for the analog filtering and digital filtering approaches respectively. 

This comparison is on the basis of one 36 MHz transponder channel. The costing estimates are 

based on the off-the-shelf component availability. No flight-hardware cerification or other 

Non-Recurring-Expenditures (NRE) are considered. 

These tables clearly show that the analog filtering approach with switchable fixed passband SAW 

filter-bank has lower power requirements, size, weight and costs compared with the digital 

filtering approach even with the projected 1993 state-of-the-art techonologies (such as 

Gallium Arsenide) and parts. 

10 
3.3.3 Recommendations 

From the qualitative and quantitative comparisons presented here, it is clear that the analog 

filtering approach with switchable fixed passband SAW bandpass filterbank is superior to the 

other implementation approaches. This implementation utilizes parts (such as SAW filters and 

relatively lower speed A/D converter devices) with proven and mature technology. This 

approach is overall the simplest of all the implementations considered here, provides compact 

size, and has both low power and timing & control requirements. 

Since this approach provides overall superior performance and utilizes parts with proven 

mature technology, the analog filtering approach using switchable fixed passband SAW bandpass 

filter bank is recommended for implementation of the AID on-board the satellite system. 

3.4 Concepts for Digital to Analog (D/A) Conversion 

The analog downlinks require Digital-to-Analog (D/A) conversion of the digital subchannel 

access-signals. This is accomplished by the introduction of a D/A converter prior to analog 

modulation and downlink upconversion as shown in the block-diagram, Figure 9. 

The digital output stream of the downlink transponder signals from the switching and routing 

system passes through the high speed D/A converter, whose output analog signal is 

demultiplexed for forwarding to the switchable analog subchannel access modulators and, to the 

downlink upconverters 

The high-speed D/A (up to 200MHz rates) converters are implementable with the current 

state-of-the-art single chip converters quite easily. 

3.5 Summary of Task 2 

Various alternative concepts of implementation of onboard ND conversion of the analog uplink 

signals and the associated onboard processing, timing and control requirements are investigated 

in this task. The component identification for each concept is also performed in this task. The 

analog filtering approach with switchable fixed passband SAW filter banks and high-speed flash 

A/D converters is recommended because of the overall superior performance of this approach. 

Concepts of implementation of the associated D/A conversion for the analog downlink processing 

are also presented. 

4.0 IMPACT ANALYSIS 

The architecture selected from the candidate concepts discussed in Section 3 was the analog, 

switched filter approach. In this section, economic comparisons are made between this concept 

and the situation where no on-board AID processing is used and changes are required on the 

ground as well as the spacecraft. 

4.1 International 

4.1.1 Options 

Figures 10 and 11 present the two basic architectures considered - with and without A/D 

conversion. In the A/D case, no changes are required on the ground. On the spacecraft the 

following hardware is added between the usual input receiver/MUX (Le., at a 36 MHz level) and 

the output MUX: 

11 
- 

- Switched filter bank 

- A/D converter 

- Digital switch 

- D/A converter 

- 

- Upconverter to RF 

Local oscillator to bring signal to IF 

MUX to 36 MHz level 

In Figure 11, the ground station must be modified to convert incoming analog circuits to digital 

by means of back-to-back analog and digital channel banks, or by a transmux. Also, digital 

multiplexing equipment (M12 and M13) is required, as well as a change of the modulator to a 

digital mode such as QPSK. The satellite also requires modification similar to the above list, 

except the A/D and D/A equipment is replaced by a QPSK modem. 

Because the demod output in Figure 11 is at baseband, the data rates for the different access 

sizes are lower than in Figure 10, where the A/D conversion is performed on the modulated 

signal, thus requiring a higher sampling rate. The digital switch is therefore smaller in the no 

A/D case, but other costs, particularly for the ground stations, are much higher as described 

subsequently. 

4.1.2 Satellite Cost Assumptions 

4.1.2.1 Space Qualification 

Individual parts used in satellite applications must be subjected to certain testing to ensure high 

reliability. These tests include inspecton of construction quality, centrifuge tests, high 

temperature burn-in , etc. Assembled items, such as circuit cards also require testing. We 

have assumed the following costs: 

$20,000 per part 

$1 5,000 per assembly 

4.1.2.2 Development Costs 

All development costs, except for the digital switch, are based on packaging design. These costs 

vary from 0 to $40,000, as indicated in Table 15. 

There are currently digital switches under development for satellite use, i.e., the ACTS BBP and 

MILSTAR. However, the former is designed for individual circuits at 64 Kbs, and the latter uses 

even lower data rates. Counterbalancing the higher total throughput required by our application 

is the fact that terminal synchronization is not required: all synchronization, timing, and 

framing is done on board. Therefore, data frames can be much shorter than when terminal to 

satellite timing must be considered. This reduces the "size" (buffer requirements) of the 

switch, but also increases the processing throughput required. A parallel architechture would 

probably be the best approach. 

As pcinted out in Section 4.1.1, the data rates input to the switch depend on whether A/D is 

performed on board or not. The former case requires a larger switch for the same number of 

circuits. The following assumptions were used for the large switch : 

12 
D eve lopm e n t effort 10 man years 

Hou rs/y e ar 1760 

Loaded labor/hour 

Total development cost 

$70 

$1,230,000 

The "small" switch is assumed to require 7 man years, so the cost is $862,000. 

4.1.2.3 Payload Mass, Power, and Recurring Costs 

See Section 3.3.2. 

4.1.2.4 Total Number of Transponders Requiring AID 

The CPPD study developed the number of transponders required for Latin ArnericdCaribbean, 

i.e., twenty-seven 36 MHz transponders. In order to extrapolate these results to include the 

U.S. as well, the number of transponders was increased to allow for the U.S. international 

requirements (147K circuits) as well: 

93 transponders = 27 transponders x (60.2 + 147)/60.2 

4.1 -2.5 Integration and Test 

These costs were assumed to be 8% of the recurring costs. The CPPD study showed a range of 

6% to 10% for the integration and test, so an average of 8% was applied. 

4.1.2.6 Power Effects 

The additional power requirements result in not only a direct cost for solar arrays and 

batteries, but an indirect cost for launch (see next section) for the increased mass of power 

equipment. The direct costs are assumed to $1M per KW, and the additional mass is obtained 

from Figure 12, using the "total power for 100% eclipse" curve. This curve is developed in the 

Ford Aerospace Platform Bus Study, Ref [3]. 

4.1 -2.7 Launch Costs 

Launch Costs were calculated on an incremental basis, allowing for additional mass for payload, 

fuel, and power equipment. The basic formula is from the Rockwell study on launch costs, 

Ref 141, which is 

($M) = 44.6 x In(B0L-lbs) - 293.2 

This can be converted to an incremental increase in launch costs by assuming an initial satellite 

mass of BOL and an increase of "d, i.e., 

($Mi = 44.6 x ln((BOL+djiEOij 

13 
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The baseline mass used is 21404 Ibs, derived as follows from the CPPD study: 

- International BBP deleted (184 Kg) 

- 3200 Kg payload mass 

- 31% of BOL is payload 

Thus, the baseline BOL value is 

21404 Ibs = 2.2 X (3200 - 184)/.31 

Other values for BOL will be considered in Sections 4.2.2.5 and 4.3. 

4.1.2.8 Bandwidth Efficiency 

A penalty has been assigned to the use of digital modulation, reflected as an increased number of 

transponders required on the satellite. A level 1 analog access (1.44 MHz) contains 96 

equivalent voice grade circuits; for a digital signal, it is assumed only 90 circuits can be 

carried in the same bandwidth, based on the following: 

- 1.5 bits/Hz 

- 24 Kb/s per voice channel (see Ref 121) 

The total number of transponders is thus 99 = 93 x (96/90). 

4.1.3 Earth Station Cost Assumptions 

If on-board satellite A/D and D/A conversion is not available, then analog traffic to/from earth 

stations must be converted into digital traffic because of the assumption that any ISL will utilize 

digital modulation. This applies to a laser ISL as well as a W-band ISL. This conversion is to be 

realized (1) at the terrestrial interface between the earth station and terrestrial lines, and (2) 

at the interface of the earth station and the RF link. 

Terrestrial interface is required only if the terrestrial lines are analog. By year 2008 it is 

assumed that only 50% of those lines are analog and the terrestrial interface will be costed 

accordingly. Conversion analog/digital for voice is acheived either with the channel bank or the 

transmux, converting 24 voice channels into standard digital format DS-1 of 1.544 Mbits. The 

DS-1 signal is then multiplexed into DS-2 signal of 6.312 Mbits (96 channels) and DS-2 to 

DS-3 of 46.304 Mbits (672 channels). The 96 channel DS-2 capacity is equivalent to level 1 

(1.44 MHz) and the 672 channel DS-3 to about 2 times level 2 (7.2MHz) or 25% of level 3 

(36 MHz) of the solution using on-board AID. 

RF link interface at the earth station is acheived by PSK modem driven either by DS-2, DS-3 or 

multiple DS-3 signals with FDMA or TDMA access mode. TDMA mode seems not practicable due to 

the high cost of adding TDMA operation to an FDM operated earth station and the difficulty in 

synchronization of great number of earth stations specially through intersatellite link. Access 

mode such as QPSWFDMA seems the most feasible and the cost of RF interface at the station is 

limited to the QPSK modem (with or without FEC). The FDMA access mode requires that to each 

terresiriai PSK modem should correspond the same functional equivalent modem on the satellite. 

On the satellite, the modem demodulates the uplink signal to DS-2 or DS-3 formats; such 

formats are then not demultiplexed to DS-1 or 64 Kbits but routed as is through International 

Routing Switch #2 for PSK modulation to the downlink. 

I 

I 

14 
The channel bank or transmux and the digital multiplex (DS-1 to DS-2 and DS-2 to DS-3) are 

well established technology, extensively used in terrestrial network with very stable price. The 

QPSK modem for standard DS-2/DS-3 signal is currently available and expected to be well 

establised by year 2000. 

The following are estimated recurrent cost per installed circuit for one end (Le. originating + 

terminating in the earth station at one end of the communication link). Price is quoted by NEC 

for year 2000. 

Channel bank or transmux $1 6O/circuit 

DS-2 or DS-3 mux $69/circuit 

QPSK modem with FEC $7/ci rc u i t 

4.1.4 Summary of International 

Tables 15 through 17 summarize the various items listed above and provide a final bottom line 

comparison. The satellite costs are very close in both architectures, but the earth station costs 

dominate in the design with no A/D conversion on-board. The basic reason these costs are so 

large is that when A/D is done at the terrestrial interface, the conversion is at the individual 

circuit level, either explicitly by use of back-to-back channel banks, or implicitly by means of 

a transmux, which is itself a fairly large, complex equipment. 

In contrast to the recommended design which uses a few SAW filters and ADC chips per 36 MHz 

transponder, the advantage for on-board converstion is obvious. The disadvantage is that one 

must pay a penalty in both switch size and ISL capacity due to the higher bit rates involved. 

4.1.5 Crossover Analysis 

The crossover analysis of Figure 13 shows three curves, each representing the total network 

cost as a function of the number of 36 MHz transponders for the international case; further 

parametric analyses are contained in Section 4.3. The curves are of the form NRE + REC N, 

where N is the number of 36 MHz transponders shown on the abscissa of Figure 13. 

1. Curve A/D: The earth stations are carrying analog traffic with on-board satellite A/D 

and D/A conversion. There is no modification required at the earth station and the cost is 

limited to satellite A/D and D/A conversion. The NRE cost used is obtained from columns C 

and D of Table 15, less the switch 2 row, for a total of $1495K. The REC cost per unit 

(36 MHz) is derived from the A/D total of Table 17, less the NRE, and divided by 93, or 

$45.7K per unit. 

2. Curve NO A/D, NO US COST: There is no on-board A/D and D/A conversion. 

Analog traffic is converted into digital traffic under standard digital format DS-1, DS-2 

and DS-3. This requires analog/digital conversion at the earth station. The NRE is 

obtained from columns C and D of Table 15 for those items applicable to the domestic 

design, for a total of $977K. The REC is calculated from the NO ND column of Table 17, 

less NRE and earth station costs, and divided by 99 to obtain $47.6K per unit. 

3. Curve NO ND, WITH US COST: This is the same as curve 2 above, except that the 

earth station costs in Table 17 have been included, for a total REC of $391.3K per unit. 

15 
4.2 U.S. Domestic Impacts 

4.2.1 Options 

Figure 10 still applies for the use of on-board A/D. The other option, used in the CPPD study, is 

depicted in Figure 14, and represents the additional transponders required when splitting a 36 

MHz bandwidth into 2 or 3 channels of 18 or 12 MHz respectively. 

4.2.2 Satellite Cost Assumptions 

Most of the assumptions used are identical to those in Section 4.1.2; only differences will be 

covered in the following subsections. 

4.2.2.1 Total Number of Transponders Requiring A/D 

In this case, there are two 36MHz transponders in each of 9 beams on a total of seven satellites 

(see CPPD study), for a total of 126 transponders. 

4.2.2.2 Switch Costs 

Development costs for the small switch described in Section 4.1.2.2 were used, but recurring 

costs were taken from the large switch design. The reason these assumptions were used is 

because any one switch on a satellite in the domestic application interfaces with only 18 

transponders (considerably less than the international case), but the data rate ouput from each 

36 MHz transponder after ND is the same as the international situation. Since we have chosen 

to base recurring costs on a 36 MHz transponder basis, this approach is the most reasonable. 

4.2.2.3 Fill Factor Penalties 

The fill obtained when using A/D is 90% (see Section 2.4). An analysis of the fill obtained 

within the 12 and 18 MHz transponders in the CPPD study showed that only an 80% fill was 

obtained. This is to be expected, as the ND approach provides smaller access sizes. The number 

of additional transponders required is calculated as follows: 

15 transponders = 126 x (.9/.8 - 1) 

4.2.2.4 Additional Transponders Required for Baseline 

In the baseline approach, two 36 MHz channels were converted to a total of two 18 MHt and 

three 12 MHz channels, each with its own transponder. Hence, there are three additional 

transponders required in each of nine beams on seven satellites, for a total of 189 which can be 

eliminated if ND is used. These are charged against the "no ND" approach, at a cost of $172.1 K 

each. 

4.2.2.5 Launch Costs 

The base BOL weight in this case is based on 

ni - ueletion of 27 transponders @ 3.43 Kg 

- 2260 Kg payload mass 

- .31% of BOL mass is payload 

16 
The BOL is then: 

15381 Ibs = 2.2 x (2260 - 27 x 3.43)/.31 

4.2.3 Earth Station Costs 

In the U.S. domestic case, there is no forcing requirement for conversion to digital. The CPPD 

design of providing some narrow band (12 and 18 MHz) transponders was simply one way to 

provide additional routing flexibility which was essentially transparent to the earth stations. 

The use of on-board A/D conversion is another way to accomplish the same objective. 

4.2.4 Summary of U.S. Domestic 

Tables 18 through 20 summarize the US. domestic analysis. Here, the dominant cost is the 

additional transponders required by the baseline design. 

4.3 Parametric Analysis 

The results presented in Sections 4.1 and 4.2 were derived from large platform designs. In 

order to investigate the use of A/D conversion in a more conventional situation, two other 

satellite designs were considered - the INTELSAT-V and INTELSAT-VI. These were chosen 

because it would make no sense to use A/D for the purpose of improving fill in a design with 

only CONUS coverage (such as WESTAR or RCA Satcom). Thus, satellites with some degree of 

spatial reuse were chosen. The improvement in fill was the primary issue in these analyses. 

4.3.1 Satellite Cost Assumptions 

4.3.1.1 Total Number of Transponders 

In order to simplify the analysis relative to the previous sections, it is assumed that the I-V and 

I-VI use 36 MHz transponders. Although some channels are so configured, the majority are 72 

MHz in bandwidth. Also, only the Hemi/Zone beams were considered; the global coverage beam 

would not use A/D for the same reason as a CONUS beam, as discussed above. 

I-v I-VI 

Number of beams 

#36 MHz xpdrs/beam 

#xpdrs/beam using A/D 

Number of satellites 

4 6 

9 9 

2 2 

12 5 

4.3.1.2 Switch Costs 

The same figures as for U.S. Domestic were used (see Section 4.2.2.2). 

4.3.1.3 Fill Factor Penalties 

An assumed fill of 65% was used. The penalty Is a total (for aii satellites) of 36 and 25 

transponders for the I-V and I-VI respectively. A cost of $140K per transponder was assumed. 

17 
4.3.1.4 Power and Mass 

The assumptions used here are as follows (all masses in Ibs): 

I-v I-VI 

BOL 2460 5060 

Xpdr mass 13.43 11.61 

Xpdr pwr 24.6 24.1 

Switch mass 4.8 7.2 

Fuel (% of BOL) 21.5 16 

Switch pwr 7.8 11.7 

Lbslwatt for solar/battery .18 .2 

4.3.1 -5 Summary of Parametric Analysis 

Tables 21 through 23 show the results for the I-V, and Tables 24 through 26 cover the I-VI. As 

can be seen, the A/D approach is not the preferred approach. The reason for this is that in each 

of the previous two analyses, there was a major cost driver which is is not present here, namely 

the terrestrial interface costs in the international case, and the large number of additional 

narrow band transponders (12 and 18 MHz) used in the U.S. Domestic case. 

5.0 CONCLUSIONS AND RECOMMENDATIONS 

It appears that ND processing on board a satellite provides advantages when digital routing on 

board is absolutely required, as in the case of a digital ISL. The use of AID would also provide 

advantages in improving fills in some situations. For example, several beam-to-beam demands 

among the small spot beams used in the CPPD study in Latin America only had 19 circuits, which 

would have resulted in near-zero fills for a 36 MHz transponder. However, the approach does 

not seem to offer any advantages for satellite coverage designs with a lower reuse, plus 

overlapping beams, i.e., the INTELSAT Hemi beams cover one or more zone beams, thus offering 

improved fills relative to independent, non-overlapping spot beams. 

It appears that ND conversion can be obtained with technology that either exists today, or would 

need only modest development. The area of greatest uncertainty is the high speed digital switch, 

which is different than the ACTS baseband processor. 

Also, a more complete analysis of how the A/D conversion would affect performance relative to 

CCITT or other standards would need to be performed before any final implementation. 

18 
REFERENCES 

[ 1 ] "Communication Platform Payload Definition Study Final Report," Vols 1-3, E.M. Hunter, 

T.F. Driggers, and R. Jorasch, Ford Aerospace and Communications Corporation, March, 

1986 

[ 21 "Demand For Satellite-Provided Domestic Communications Services to the Year 2000," 

NASA TM 86894, S. Stevenson, W. Poley, J Lekan, and J. Salzman, Lewis Research 

Center, November, 1984 

[3] "Geostationary Platform Bus Study Final Report, " Vol II, Ford Aerospace and 

Communications Corporation, September, 1986 

[ 41 "Space Platform Expendables Resupply Concept Definition Study," Vols I & 11, Rockwell 

International, NASA STS 85-01 74, March-December, 1984 

.. 

19 
Appendix A 

ACCESS SIZE SAMPLE RESULTS 

The printouts included in this appendix are a subset from the program used to derive the curves 

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mpendix B 

BIBLIOGRAPHY 

Papers/Art icles; 

1. E. Stern, "Comparison of New Analog Device Technologies for Signal Processing," IEEE 

1983 Ultrasonics Symposium Record, pp. 129-136. 

2. J. Roberts, P. Simpson and B. Merrifield, "Applying Digital VLSl Technology to Radar 

Signal Processing," Microwave Journal, January 1986, pp. 129-144. 

3. R. Cushman, "Digitization is on the way for FFT designs," EDN August 5 1981, pp. 

99-1 06. 

4. P. Greiling, " The Future Impact of GaAs Digital IC's," IEEE Journal on Selected Areas 

in Communications, vol. SAC-3,no. 2, March 1985, pp. 384-393. 

5. DSP Associates "DSP-Chip comparison Chart" prepared for EE Times 

6. W. Sullivan, "Ultra-Wideband IFM Receiver Fulfills Design Requirements," MSN &CT 

April 1985, pp. 105-105. 

7. M. Jack, P. Grant and J. Collins, "The Theory, Design, and Applications of Surface 

Acoustic Wave Fourier-Transform Processors," Proceedings of the IEEE, vol. 68, no. 4, 

April 1980,pp. 450-468. 

8. J. Maines and E. Paige, "Surface-Acoustic-Wave Devices for Signal Processing 

Applications," Proceedings of the IEEE, vol. 64, no. 5, May 1976, pp.639-652. 

9. R. Hays and C. Hartmann, "Surface-Acoustic-Wave Devices for Communications," 

Proceedings of the IEEE, vol. 64, no. 5, May 1976, pp. 652-671. 

A/D Devices: 

1. Analog Devices AD501 OKD Data Sheet 

2. Siemens SDA5200 Data Sheet 

3. Sony CX20116 Data Sheet 

4. TRW TDC1029 Data Sheet 

D/A Devices; 

1. Analog Devices AD9700 Data Sheet 

2. Honeywell HDAC-97000 Data Sheet 

28 
I 

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3. lntech VDAC-0805 Data Sheet 

4. Sony CX20201 Data Sheet 

5. TRW TDC1018 Data Sheet 

DSP VHSlC Devices; 

1. TRW TDC1028 Data Sheet 

2. Zoran ZR33481 Data Sheet 

3. Zoran ZR33881 Data Sheet 

4. Zoran DSP Products Catalog 

Diaital IFM Receivers: 

1. NSL-1050/1052/1054 Modular Digital IF Receivers- General Instrument 

NSL1050 Series IFM Receiver Product Bulletin. 

2. NSL-1040/1042/1044 Digital Frequency Discriminators -- General Instrument 

NSL1040 Series DFDs Product Bulletin. 

Tunable (Programmable Paswd) An&a Filters 

1. RF Products Inc. Remotely Tuned/ Manual Tuned/ Frequency Agile Communication 

Filters Brochure. 

2. Tektron Micro Eloctronics Inc. Remotely Programmed Filter Set Brochure. 

Fixed Passba nd Analoa Filter Banks 

1. CTI-260 Surface Acoustic Wave Filter Bank -- Crystal Technology Data Sheet and 

application notes. 

2. Raytheon SAW Components Catalog 

3. Anderson Laboratories Signal Processing Devices & Components Catalog 

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