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ASMS-TF MeetingASMS-TF MeetingBruxelles July 10Bruxelles July 10thth - 12 - 12thth 2001 2001

vernucci@space.itvernucci@space.it

ASMS-TF Technical Group AASMS_03_T14_0

Transmission Performance Comparison Among Different Access Schemes for

Streaming Broadcast Services

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BackgroundBackground Reference assumption: a European satellite

system providing broadcast streaming services (television, radio) to mobile users, at either S- or L-band (SIRIUS-like system)

Such system would require very high RF power (3,840 W in case of SIRIUS).

Issue: to select an access scheme allowing to best exploit available on-board RF power: a 1-dB saving means sparing 790W RF (SIRIUS case)

Comparison criterion: lowest RF power when the non-linear amplifier is at optimum operating point

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Considered Access SchemesConsidered Access Schemes

TDM:the most promising approach (low envelop variation)by adopting the very SIRIUS standard, user terminals would

already be available

DVB-T & DAB:OFDM (multicarrier) does not well match non-linear channel.

DAB also shows poor BER vs Eb/No (differential scheme)chip sets available. Service currently very limited (DVB-T) or

nearly non-existent (DAB)

CDMA:higher sensitivity to non-linearity (non-constant envelope)probably superior for multi-beam and multi-satellite caseslarge base of terminals available if UMTS standard adopted

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Schemes Retained For Schemes Retained For ComparisonComparison

TDM:chosen parameters are those of SIRIUS

Single-Access Wideband CDMA (SA/W-CDMA):this is the UMTS down-link standard (synchronous CDMA).

All channels are transmitted by a single feeder-link station

Multiple Single-Access Wideband CDMA (MA/W-CDMA):channels are transmitted by different feeder-link stationsSkyPlex-like conceptrequires clock / code synchronization among feedersaccess is non carrier-phase coherent (impact on envelope)

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Retained Access Schemes Retained Access Schemes Parameters Parameters SIRIUS-TDM SA/W-CDMA MA/W-CDMA

Modulation OQPSK QPSK QPSKCode RS(128,120) + conv.

rate 3/2, k=9conv. rate 1/2, k=9 conv. rate 1/2, k=9

Aggregate information rate (Mbit/s) ~4 Mbit/s ~3.4 Mbit/s ~3.4 Mbit/sBandwidth (MHz) ~4.2 MHz ~4.7 MHz ~4.7 MHzChannel filter roll-off ~0.32 (apparently) 0.22 0.22Apparent bandwidth efficiency for asingle-beam system (bit/s/Hz)

~0.95 ~0.72 ~0.72

Reference BER (SIRIUS datum) 10-5

10-5

10-5

Number of up-link stations 1 1 4 here assumedNumber of resources per carrier variable, depending on

TDM framesegmentation criterion(resource = time slot)

4 here assumed(resource = code)

4 here assumed(resource = code)

Number of user channels perresource

variable, depending ontime slots

segmentation criterion

variable, for amaximum aggregatetotal of ~850 Kbit/s

per code

variable, for amaximum aggregatetotal of ~850 Kbit/s

per codeTotal number of user channels variable, for a

maximum aggregatetotal of ~4 Mbit/s

variable, for amaximum aggregatetotal of ~3.4 Mbit/s

variable, for amaximum aggregatetotal of ~3.4 Mbit/s

Inter-code carrier-phase coherency n.a. yes no

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Performance ExpectationsPerformance Expectations

SIRIUS-TDM:good performance due to OQPSK and mild roll-off

SA/W-CDMA:somewhat worse performance with convolutional rate-1/2

code, but partially recoverable by improving the code scheme MA/W-CDMA:

relative up-link carrier frequency errors unavoidable in a multiple access context

frequency shifts cause loss of orthogonality among codeseffect of carrier phase shift on signal envelope statistic to be

assessed

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Computer SimulationsComputer Simulations Ad-hoc computer simulations were carried out in support

of the ASMS-TF activities

Adopted logic was, for each considered scheme;

to quickly simulate the uncoded access schemes using a semi-analytical model (acceptable if up-link noise is low). This permits to determine the optimum Input Back-Off

to assess the real (coded) access performance just at that back-off, using a more time-consuming error-counting technique

to determine degradations with respect to linear channel at the reference BER (10-5)

to determine the output back-off for the optimum Input Back-Off

to determine the overall loss (degradation + output back-off)

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SIRIUS TDM PerformanceSIRIUS TDM Performance

1.E-09

1.E-08

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1 1.5 2 2.5 3 3.5 4 4.5 5

Eb/No (dB)

BE

R

OQPSK (conv+RS)

OQPSK (Conv. Only)

Linear (Conv+RS)

Linear (Conv Only)

IBO = 2 dBOBO = 0.4 dB

OQPSKConv. Code r=2/3, k=9

RS(128,120)Infinite Interleaver

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UMTS W-CDMA PerformanceUMTS W-CDMA Performance

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1 1.5 2 2.5 3 3.5 4 4.5 5

Eb/No (dB)

IBO = 2 dB

Linear

use of convolutional rate-1/2 code assumed

no performance difference between single-access and multiple-access for inter-carrier frequency shift < 1.5 KHz

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Comparative PerformanceComparative Performance

SIRIUS-TDM requires 0.3 less Eb/No due to more powerful coding (concatenated)

Total advantage: 1.6 dB

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Results SummaryResults Summary

With the UMTS W-CDMA standard, it would be necessary to utilize an on-board RF amplifier having 1.6-dB higher power, should a plain rate-1/2 convolutional code be used

However, part of this gap (about half) could be recovered by adding an outer Reed Solomon code. Even better could instead be the adoption of the Turbo code option specified by 3GPP

If multiple feeder link stations are used in conjunction with W-CDMA, no change in performance occurs for small frequency errors among the various up-link carriers.

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ConclusionsConclusions

Obtained results should be regarded as tentative, but could be useful to stimulate discussion on the issues

W-CDMA looks an interesting alternative to TDM. Terminals availability is a very important driver

More simulations can be performed on other cases and assumptions, within reasonable limits of commitment