TELEDYNE PARADISE DATACOM APPLICATION NOTE

SAVING SATELLITE BANDWIDTH BY OPTIMISING SPECTRAL ROLL-OFF

AN_035

2

1. INTRODUCTION Teledyne Paradise Datacom has introduced 5%, 10% and 15% spectral roll-off options (using root-raised-cosine filtering) on all Evolution, Quantum and Q-Series (Q-Flex and Q-Lite) satellite modems with immediate effect. These new options significantly reduce the required allocated satellite bandwidth thereby directly reducing the operational expenditure associated with leasing transponder bandwidth.

• 5% roll-off provides a 20% bandwidth saving compared to 35% roll-off. • 5% roll-off provides a 10% bandwidth saving compared to 20% roll-off. • When using the new roll-off factors no increase in Eb/No level is required to

achieve the same BER performance. The new roll-off factors are available for all satellite services other than DVB-S2 and SmartLink (which will be supported at a later date). Support for the new roll-off factors is available for all existing Evolution and Quantum modems as a software-only upgrade (from software version V2.0.27 onwards). 2. SPECTRAL CHARACTERISTICS The spectral characteristics as a function of transmit symbol rate are detailed in Table 1.

BANDWIDTH NORMALISED TO SYMBOL RATE

RELATIVE LEVEL

35% Roll-off

25% Roll-off

20% Roll-off

15% Roll-off

10% Roll-off

5% Roll-off

-3dB

0.9882 1.0019 0.9946 0.9982 1.0019 0.9982

-25dB

1.3249 1.2378 1.1942 1.1471 1.1071 1.0672

-26dB

1.3294 1.2406 1.1967 1.1490 1.1089 1.0695

-30dB

1.3394 1.2451 1.2015 1.1579 1.1144 1.0745

Table 1 - Measured Modem Tx Output Bandwidth Normalised to Baud Rate

Definitions of terms such as occupied and allocated bandwidth vary. For that reason, Table 1 shows the carrier bandwidth at specific points that allow link budget calculations to use whichever roll-off factor is relevant. For example, to calculate the bandwidth (in Hz) of a carrier with a 5% roll-off at the -30dB point, the symbol rate of the carrier should be multiplied by a factor of 1.0745. In order to calculate the difference in required bandwidth between two spectral roll-off factors, use the following equation:

Bandwidth saving (%) = 100 – ((roll-off A / roll-off B) * 100)

3

where A is the lower of the two roll-off factors. For example, comparing a 5% roll-off to a 35% roll-off at the -30dB point:

Bandwidth saving = 100 – ((1.0745 / 1.3394) * 100) = 19.7%

3. BANDWIDTH SAVING EXAMPLE The following real-world example demonstrates the bandwidth saving that was made by switching a symmetric bi-directional 10Mbps FastLink low-latency LDPC link from a 20% spectral roll-off to a 5% roll-off. The link uses two Quantum PD60 modems in the following configuration:

• FEC: FastLink LDPC rate 0.726.

• Modulation: 16APSK.

• Terrestrial data rate: 10Mbps.

• Symbol rate = 3,442,383 symbols = 3.442Msps.

• Spectral efficiency: 2.9.

• Spectral roll-off factor: 20%.

• Carrier bandwidth at -3dB point = 0.9946 * 3.442383 = 3.424MHz.

• Carrier bandwidth at -30dB point = 1.2015 * 3.442383 = 4.136MHz. Now changing the spectral roll-off factor from 20% to 5%:

• Symbol rate = 3,442,383 symbols = 3.442Msps.

• Spectral efficiency: 2.9.

• Spectral roll-off factor: 5%.

• Carrier bandwidth at -3dB point = 0.9982 * 3.442383 = 3.436MHz.

• Carrier bandwidth at -30dB point = 1.0745 * 3.442383 = 3.699MHz. The overall bandwidth saving at the -30dB point from switching from 20% roll-off to 5% roll-off in this case is therefore:

Bandwidth saving = 4.136MHz – 3.699MHz = 0.437MHz per carrier Total bandwidth saving = 0.437MHz x 2 = 0.874MHz

As a percentage of the original bandwidth of 4.136MHz per carrier, this represents a saving of 10.5%. For this C-band service, the bandwidth cost is $3000 per megahertz per month. Consequently the total cost of the bandwidth for both carriers reduced from $24,816 per month to $22,194 per month, i.e. a saving of $2,622 per month.

4

4. IMPACT ON PEAK-TO-AVERAGE-POWER RATION The relationship between transmit Peak-to-Average-Power Ratio (PAPR) is detailed in Table 2.

PAPR (dB) WITH MODULATION SCHEME

ROLL OFF

BPSK

QPSK OQPSK 8PSK 8QAM 16QAM 16APSK 32APSK 64QAM

35%

4.1dB 3.9dB 3.8dB 5.3dB 6.0dB 4.8dB 5.6dB 6.5dB

25%

5.1dB 4.9dB 4.7dB 6.0dB 6.6dB 5.7dB 6.3dB 7.1dB

20%

5.7dB 5.4dB 5.2dB 6.4dB 6.9dB 6.1dB 6.7dB 7.3dB

15%

6.4dB 5.9dB 5.7dB 6.7dB 7.2dB 6.6dB 7.1dB 7.6dB

10%

7.2dB 6.4dB 6.1dB 7.2dB 7.6dB 7.0dB 7.5dB 7.8dB

5%

7.9dB 6.8dB

2.2dB

6.6dB 7.6dB 7.9dB 7.4dB 7.9dB 8.1dB

Table 2 - Theoretical Modem Tx PAPR As Function of Modulation Scheme and Roll-Off The table shows that extra back-off may be required in the uplink HPA when using lower roll-off factors. The transmit spectral masks for the various roll-off factors are shown in the following appendix.

5

APPENDIX A – TRANSMIT OUTPUT SPECTRA FOR PARADISE ROLL-OFF FACTORS

Figure 1 - Spectrum For 35% RRC For 1MBaud With -25dB BW Measurement

6

Figure 2 - Spectrum For 25% RRC For 1MBaud With -25dB BW Measurement

7

Figure 3 - Spectrum For 20% RRC For 1MBaud With -25dB BW Measurement

8

Figure 4 - Spectrum For 15% RRC For 1MBaud With -25dB BW Measurement

9

Figure 5 - Spectrum For 10% RRC For 1MBaud With -25dB BW Measurement

10

Figure 6 - Spectrum For 5% RRC For 1MBaud With -25dB BW Measurement

Paradise Datacom Limited 2&3 The Matchyns, London Road, Rivenhall End, Witham, Essex, CM8 3HA, England Telephone +44 (0)1376 515 636 Facsimile +44 (0)1376 533 764 email: http://www.paradisedata.com/support.php www.paradisedata.com

Paradise Datacom LLC 328 Innovation Blvd., State College, PA 16803, U.S.A. Telephone +1 814 238 3450 Facsimile +1 814 238 3829 email: http://www.paradisedata.com/support.php www.paradisedata.com

29 August 2012