You are here:

Satellite Communications/Link margin range ACM offers

Advertisement


Question
QUESTION: Sir,

A basic question. We know in DVB-S2 network Adaptive Coding and Modulation (ACM) can dynamically change MODCOD in transmission to remotes so as to ensure VSAT link stability even in heavy fade weather.
Assume the highest MODCOD a VSAT system supports is 16APSK 8/9 with threshold Eb/No as 8.3, and the lowest MODCOD supported is QPSK 1/4 with threshold Eb/No as 2.3, can we conclude the link margin provided by VSAT system to remotes is (8.3-2.3)=6 dB ?(If excluding the extra margins from external UPC or HPA).

Thanks,

Feng

ANSWER: Dear Mr. Feng
First the answer is No for the conclusion that 6 dB is the value of Link Margin.
The reason is explained below:

Normally the symbol rate of the link must be constant to ensure the allocated
bandwidth remains the same on the satellite.Standard duplex services assign
fixed modulation and FEC for Uplink and Downlink.

ACM technology is able to alter the modulation and FEC by implementing a
‘feed-back’ circuit containing link performance information. This circuit can be
combined with the service or provided on a separate public Network.

Accordingly the data rate is altered to compensate for any ACM changes in
modulation and FEC. Adaptive Coding and Modulation is a technology which can
automatically change the modulation and forward error correction or FEC of a link
(referred to as MODCOD) to compensate for changes in link conditions –
commonly weather induced (e.g. rain fade) but also due to changes in the
RF environment (e.g. level changes, interference).

The use of ACM makes it unnecessary for service operators and system
designers to trade off desired link availability and throughput. When compared
with links designed using fixed coding, ACM can increase the throughput of a
robust link by allowing it to dynamically adjust to a less robust modulation/coding
(MODCOD) resulting in higher throughput under clear sky conditions.
Conversely, when compared to a modestly robust fixed rate coded link, ACM can
provide increased link availability by dynamically adjusting to lower order
MODCOD under rain fade conditions.

ACM Link Design -When designing an ACM link, the following must be taken into consideration:

The rate of change in fade conditions versus the latency and processing
time of the chosen ACM system’s ‘feed back’ mechanism for MODCOD
adjustments.

Careful design and allocation of minimum MODCOD, maximum MODCOD
and MODCOD switching margins is required to insure the service is
operated within the required service availability.

The use of DVB-S2 ‘long block’ versus ‘short block’ mode can affect
system performance. This is especially true if the link is a low data rate
service.

Link budget analysis can be calculated for the best and worst case
MODCODs to compare throughput and availability which results in at least
a doubling of the number of link budgets required. ACM is one of various
techniques that the satellite industry is utilizing to help reduce
bandwidth costs for customers and improve network performance.
Cheers/GTRao

---------- FOLLOW-UP ----------

QUESTION: Thanks for so detailed explanations on ACM mechanism.

I should have expressed my question precisely.
I meant how much margin or "space" (in Eb/No) ACM can offer in downstream in order to maintain the link availability. Why it is NOT the difference between "threshold Eb/No" for Maximum MODCOD and "threshold Eb/No" for Minumum MODCOD ?

Feng

Answer
Dear Mr Feng, May be you are correct in the sense that if the ACM is designed for the tolerances in the extreme MODCOD, the available space to play with is 6dB. However this parameter is not used for manipulating margin in downlink alone because the variations are applied based on the feedback information and as I already explained in earlier mail:

The use of ACM makes it unnecessary for service operators and system
designers to trade off desired link availability and throughput. When compared
with links designed using fixed coding, ACM can increase the throughput of a
robust link by allowing it to dynamically adjust to a less robust modulation/coding
(MODCOD) resulting in higher throughput under clear sky conditions.
Conversely, when compared to a modestly robust fixed rate coded link, ACM can
provide increased link availability by dynamically adjusting to lower order
MODCOD under rain fade conditions.

As you can see this is not a tradeoff parameter but improves the QOS. So this should not be interpreteded to be threshold difference parameter but operational margin for QOS.
Cheers/GTRao

Satellite Communications

All Answers


Answers by Expert:


Ask Experts

Volunteer


TIRUMALA RAO G

Expertise

Satellite Communications Applications, Link Calculations, Teleports, Earth Stations,VSAT, Tracking, Mobile Communications, Next Generation Networks.

Experience

40 Years. Indian Space Research Organisation, PANAMSAT, Satellite Based VSAT Networks, IP Networks.

Organizations
ISRO, PANAMSAT, Engineering Institution for Higher Education (GVP College of Engineering), Consultancy.

Education/Credentials
Graduate in Electronics and Communication Engineering with Distinction. Guiding scholars for their Doctoral thesis in related areas in Academics.

©2016 About.com. All rights reserved.