Adaptive Coding and Modulation
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Transcript of Adaptive Coding and Modulation
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ADAPTIVE CODING AND MODULATION
WITH THE DVB-S2 STANDARDChristopher Payne
4/21/2014
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Review: Shannon’s Law
• Variations in SNR– Channel Fading– Weather Variations– Propagation Losses
• Fixed-Function Radio designed to operate under SNR threshold
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Comtech EF Data Corporation <http://www.comtechefdata.com/technologies/acm>
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Adaptive Coding and Modulation (ACM)
• “Link Adaption” or “Dynamic Coding Modulation”
• Functionality– Observes change in SNR of channel– Sends the Code and Modulation Information (CMI)
in the header of a packet– Changes modulation scheme to optimize throughput
• Dynamically changes throughput of SDR system
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Requirements for ACM
1. Current channel conditions must be known with reasonable accuracy– Open Loop Information• Weather Reports, GPS
– Closed Loop (Feedback) Information• Receiver send SNR Measurements to Transmitter• Requires a bidirectional link
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Requirements for ACM
2. Channel conditions must remain constant or change slowly relative to the adaption rate– Two Categories of Channel Fading Impairments• Fast Channel Fading (ex: Multi-Path)• Slow Channel Fading (ex: Shadow Fading)
– Goal is to adjust SNR update rates so that:• SNR updates slow enough to average fast fading effects• SNR updates fast enough to track slow fading effects
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Requirements for ACM
3. The radio must support multiple waveforms that operate closer to capacity at different SNRs
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DVB – S2
• Digital Video Broadcasting – Second Gen.– Implemented to replace DVB-S (end users) and
DVB-DSNG (Digital Sattelite News Gathering)• Current or Converting Providers– DirecTV (U.S.)– Dish Network (U.S.)
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DVB-S vs. DVB-S2• GSE: Allows DVB to carry
packet-oriented protocols (IPv4, IPv6, VLAN, etc)
• FEC: Low Density Parity Check concatenated with a BCH (class of cyclic error correcting code)
• 35% Roll off to 20% Roll off
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Improvements over DVB-S