November 2014 White Paper APPLICATION OF EQUALINK TO INCREASE PERFORMANCE OF DTH AND DISTRIBUTION LINKS By Dirk Breynaert CTO, Introduction This document describes the performance of Equalink, a technique used on the MDM6100 Broadcast Satellite Modem, for linear and non-linear predistortion. Results are shown for specific use cases with DVB-S2 QPSK 5/6 and 8PSK 5/6 MODOCDs. The performance increase is assessed for STBs both with and without receiver equalizers (EQLZR). The gain for a QPSK 5/6 DVB-S2 MODCOD is 0.4 db with receiver equalizer and 0.6 db without receiver equalizer, and can be applied to increase the link availability or to increase symbol rate, while reducing the modulator roll-off. In the latter case the additional link margin is used to compensate for the decrease of the power spectral density of the carrier, driven by the increase of the symbol rate under fixed uplink power. Typical results show an increase of bandwidth efficiency of 7% (in case of a receive equalizer) and 10% (in case of a receiver without equalizer). Note that the gain is larger for higher MODCODs, e.g. 15% in case of a 8PSK 5/6 MODOCD (with Equalizer). The performance depends on the non-linear characteristics of the transponder, its IMUX and OMUX filters characteristics and the MODCOD used. www.newtec.eu This paper first describes the link model used in the performance assessment. The next section elaborates on the parameters used as performance criterium. It gives the results in the case of STBs without on-board Equalizer. It also covers the automated procedure used for the initial calibration of Equalink. The last section summarizes the results for the case of STBs with on-board Equalizer. Rev.2 02/2015 1
DTH Link Model In this example a DTH link consisting of a large uplink earth station, a transponder consisting of IMUX, TWTA and OMUX, and a large test downlink earth station is considered. Like most DTH links, the transponder is operated in ALC mode with low modulated output backoff (MOBO). In fact, single-rim MODCODs (8PSK & QPSK) which are principally applied by DTH links can be driven into saturation with relatively low increase of the non-linear distortion. The conclusions of this white paper can be very well extrapolated to other transponders, as long as their AM/PM characteristics are not equalized. For equalized AM/PM transponders the NLPD gains are significantly less. Considering a strong protected carrier DVB-S2 with QPSK5/6, Pilots ON, roll-off 20% and 21 MBaud; the performance gain of linear predistortion (LPD) and nonlinear predistortion (NLPD) is described hereafter. The link margin (LM) of STB with and without equalizer (EQLZR) has been measured before and after predistortion. Before LPD+NLPD is MER=14.55dB Performance for STB without Equalizer MER - Performance An important parameter of DTH links is the modulation error ratio (MER) which indicates the variance of the error. When the non-linear performance has to be evaluated, the uplink and downlink noise should be negligible compared to the MER. Also other elements like C/IM from the uplink station HPA and C/XPOL should be negligible for correct non-linearity measurements. The reason is because in this way any increase or decrease of the distortion will reflect to the MER since it is dominant over the noise component. These conditions are fulfilled by the use of a large uplink station and a large downlink test station. In the next figure, the MER plots before and after predistortion are shown. After LPD+NLPD is MER=18.9 db As indicated, applying LPD+NLPD improves the MER by 4.35 db. This does not allow the user to predict the link margin improvement, since the non-linear distribution is not Gaussian and MER itself cannot be used to calculate the non-linear EsNo degradation. Hence the MDM6100 uses the C/D parameter, described hereafter. A list of abbreviations is available on page 6. 2
C/D - Performance The distortion is added to the Gaussian noise parameter (normally downlink noise of the small consumer DTH terminals). The demodulator can be described as seeing a virtual Gaussian distribution which corresponds with the actual EsNo degradation. The critical point is when the demodulator is operated at threshold whereby for the same drive level (which was optimized for LPD+NLPD OFF) the C/D values improved from 12.75 db to 16.5 db as shown in the table below. LPD+NLPD MER (db) C/D (db) OFF 14.55 12.75 ON 18.9 16.5 Without EQLZR & MOBO: 0.3 db The diagrams hereafter show that D is in fact larger than MER. Note also that a MER distribution with the same value but in a different orientation would cause a different C/D value and different degradation. So MER on its own is not a sufficient parameter for the link performance. AFTER LPD+NLPD ( Without EQLZR & MOBO:0.3dB) Link Margin - Performance The final objective is to provide the maximum fading margin. During downlink fading, the C and D values go down by the same value, as they are both attenuated in the same way, while the N remains more or less constant or even goes up a little as the antenna sees also the cloud temperature. This is not simple to simulate, but since C/D remains constant, this is equivalent to keeping C&D constant and increasing the N value. The solution which results in highest N for a certain threshold (eg PER 1E- 3) is the best solution. In fact, during clear weather Nref maximum is equivalent to LM=C/N (operational)-c/nref (PER=1E-3) maximum. When noise is increased until Nref (PER=1E-3), then the LM is 0 db. BEFORE LPD+NLPD ( Without EQLZR & MOBO:0.3dB) 3
Automated Equalink The Automated Equalink procedure gives as an output the optimum linear and non-linear predistortion parameters. The procedure can be applied without interruption of the operational services. It also shows the gain obtained by applying Equalink. The noise added in the course of the procedure is added digitally in the Equalink calibration receiver and does not influence the uplink. The AUTOMATED EQUALINK PROCEDURE is as follows: 1. Start with the existing ALC setting of the transponder (which is assumed to be optimal already for that mode); LPD+NLPD = OFF 2. Increase the noise until PER=1E-3 and note down Nref for that point. The Nref is normalized to 0 and for the next measurements we will use the delta values. A positive delta indicates a higher N value to arrive at a certain threshold (eg PER=1E-3), which is translated to an increase of the available link margin. Step 5 resulted in an optimized ALC which gave 1.6 db more drive level to the TWTA input. The modulated OBO of the carrier decreased by 0.2 db, but the C/D decreased from 16.5 to 15.3 db. The net gain at the end was a noise increase of 0.6 db. CONCLUSION: For STB in DVB-S2 mode without equalizer LPD+NLPD can improve the link margin of those specific QPSK 5/6 DTH links by 0.6 db or 15%. This would allow for the baud rate to be increased while reducing modulator roll-off in the order of 0.45 db or 10%. In a second case on another satellite transponder of a DVB-S2 carrier with 8PSK5/6, Pilots ON, rolloff 20% and 22 MBaud, Equalink improved the link margin with 3.27 db. 3. Perform auto linear Equalink (ALEQ) on the modulator and confirm LPD settings. Activate LPD mode on the modulator. 4. Perform auto non-linear Equalink ANLEQ on the modulator and confirm NLPD settings. Activate NLPD mode in the modulator. 5. Request the satellite operator to activate several ALC settings, which normally results in higher drive levels of the TWTA. For each ALC setting the corresponding Nref point is measured. The optimum ALC setting results in the highest Nref being selected. Test results over ALC the satellite MOBO are shown LPD+ in the C/D following Noise table: setting (db) NLPD (db) (db) 1E-3 Step 2 Original 0.3 OFF 12.75 0 Step 4 Original 0.3 ON 16.5 0.5 Step 5 Original +1.6 db Overview test results when equalizer is OFF 0.1 ON 15.3 0.6 4
Performance for STB with a Good Equalizer However, compared to the above use case without equalizer, the optimum ALC drive level must be increased. The optimum drive level without NLPD corresponds to MOBO=0.25 db. The corresponding C/D is therefore the same as the above use case and 12.8 db. Test results over the satellite are shown hereafter. BEFORE LPD+NLPD ( With EQLZR & MOBO:0.3dB) ALC setting MOBO (db) NLPD C/D (db) Noise (db) 1E-3 Step 2 Orig+0.3 db 0.25 OFF 12.8 0.0 Step 4 Orig+0.3 db 0.25 ON 16.5 0.3 Step 5 Orig+1.6 db 0.10 ON 15.3 0.4 Overview test results when EQLZR is ON CONCLUSION: For STB with equalizer, NLPD can improve the DVB-S2 link margin of those specific DTH links (QPSK 5/6) by 0.4 db or 10%. This would allow for the baud rate to be increased, while reducing modulator roll-off in the order of 0.3 db or 7%. AFTER LPD+NLPD ( With EQLZR & MOBO:0.3dB) The above plots show the effect of an STB equalizer on the MER. Note that with the equalizer ON the MER is improved from 14.55 db to 17.32 db when NLPD is OFF. NLPD will improve MER further to 19.06 db. The relationship with the corresponding C/D is shown in the table hereafter. Considering the second case of a carrier DVB-S2 with 8PSK5/6, Pilots ON, roll-off 20% and 22 MBaud, Equalink improved the link margin with 0.63 db or 15%. LPD+NLPD MER (db) C/D (db) OFF 17.32 13.9 ON 19.06 16.5 With EQLZR & MOBO: 0.3 db 5
Abbreviations ALC: automatic level control EsNo: energy per symbol to noise density ratio ALEQ: automated linear Equalink HPA: high power amplifier AM/PM: amplitude modulation/phase modulation IMUX: transponder input multiplexer filter C/D: carrier to distortion ratio LM: link margin C/IM: carrier to intermodulation ratio LPD: linear predistortion C/N: carrier to noise ratio MER: modulation error rate C/Nref: carrier to reference noise ratio MOBO: modulated output back-off C/XPOL: carrier to cross polar ratio D: distortion DTH: direct-to-home DVB-S: ETSI EN 301 210 standard DVB-S2: ETSI EN 302 307 standard EQLZR: equalizer MODCODs: modulations and coding rates as defined in the DVB standards NLPD: non-linear predistortion OMUX: transponder output multiplexer filter STB: set-top-box TWTA: travelling wave tube amplifier 6
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