1. 1 ASMS-TF Meeting Bruxelles July 10 th - 12 th 2001 vernucci@space.it A ASMS-TF Technical Group A SMS_03_T14_0 Transmission Performance Comparison Among Different Access Schemes for Streaming Broadcast Services

2. 2 Background 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

3. 3 Considered 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 E b /N o (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 cases large base of terminals available if UMTS standard adopted

4. 4 Schemes Retained For Comparison 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 stations SkyPlex-like concept requires clock / code synchronization among feeders access is non carrier-phase coherent (impact on envelope)

5. 5 Retained Access Schemes Parameters

6. 6 Performance 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 codes effect of carrier phase shift on signal envelope statistic to be assessed

7. 7 Computer 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)

8. 8 SIRIUS TDM Performance

9. 9 UMTS W-CDMA Performance 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

10. 10 Comparative Performance SIRIUS-TDM requires 0.3 less E b /N o due to more powerful coding (concatenated) Total advantage: 1.6 dB

11. 11 Results 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.

12. 12 Conclusions 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