ADAPTIVE CODING AND MODULATION

WITH THE DVB-S2 STANDARDChristopher Payne

4/21/2014

Review: Shannon’s Law

• Variations in SNR– Channel Fading– Weather Variations– Propagation Losses

• Fixed-Function Radio designed to operate under SNR threshold

Comtech EF Data Corporation <http://www.comtechefdata.com/technologies/acm>

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

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

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

Requirements for ACM

3. The radio must support multiple waveforms that operate closer to capacity at different SNRs

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.)

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

Improvements over DVB-S