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LINKSTAR TX BW

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May 14th, 2010 at 3:43pm  
Hi,

How much Hz needed for each symbol rate (156.25/312.5/625/1250/2500 ksym/sec) in linkstar modems?
If you know, update me please.
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Reply #1 - May 15th, 2010 at 3:06pm  
Multiply the symbol rate by 1.4 to obtain the "allocated bandwidth", same as carrier centre to centre spacing for equal sized carriers.  Depending on the filtering at the transmit modem a lower figure of 1.35 may be applicable, but if you get unacceptable interference from the adjacent carriers move them apart. It helps if the levels of the adjacent signals are similar.  Be careful if some of your remote sites transmit at too high level or are causing spectrum regrowth due to overloading their BUCs.

Note the expression "occupied bandwidth" is used to refer to a bandwidth 1.2 times the symbol rate, approx -10 dB points.  If you put the carriers that close together you will get adjacent carrier interference degradation, particularly when the carriers either side are active simultaneously, or at higher level.

Best regards, Eric.
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Reply #2 - May 15th, 2010 at 9:16pm  
Hi Benjamin,

Eric is of course right - usually 1.4 times the Sym Rate (in kSym/sec) = amount of kHz.

Eventually, you can go down to 1.2 x Symbol rate, but his pretty much depends on your system ( I am not aware of HX carrier spacing requirements), and on top it depends on your satellite operator's policy.

Even if your modulator claims to support 1.2 spacing (instead of 1.4), the operator might force you to go for 1.4. This is usually indeed meant to avoid spectral regrowth and intermodulations.

regards
Richard
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Reply #3 - May 16th, 2010 at 6:03am  
Thanks ,
and do you know about max data rate in that symboles?
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Eric Johnston
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Reply #4 - May 16th, 2010 at 7:00pm  
Linkstar

I think this image answers everything.  
Channel spacing = 1.28 x symbol rate in all cases.
Maybe I should have read the sales brochure in the first place !

Best regards, Eric.
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Reply #5 - May 17th, 2010 at 6:42am  
Dear eric,
Thanks ,
I think it's "Transmission Rate" and "Throughput" is less than.
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Eric Johnston
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Reply #6 - May 17th, 2010 at 2:59pm  
The bit rates above, for two different FEC rates, refer to instants when the transmission is on. Data throughput on a return channel depends on the percentage of time that the channel is active. TDMA burst length, guard gap times between TDMA bursts, average TDMA burst length and burst headers and checksums all need to be taken into account. When there is random TDMA access you need to leave a lot of time space unused or you get excess collisions.

The most efficient operation is achieveable when the return link is configured as dedicated for one remote site only (same as SCPC).  The least efficient is when the traffic comprises just occasional random short burst mouse clicks from very many sites.  If the traffic comprises large files then the NMS will assign a proportion of TDMA time slots in pre-assigned mode, together with full length long bursts, which will improve the efficiency.  

Regarding transmission rate:

Symbol rate = 2500 ksps.
QPSK = 2 bits per symbol.
Transmission rate = 2 x 2500 = 5000 bit/s.
Using 6/7 FEC rate,
Information rate = 5000 x 6 / 7 = 4285 kbit/s

Best regards, Eric.
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Reply #7 - May 24th, 2010 at 8:52am  
Hi,

Can I use less than 200KHz upstream in linkstar nodes?
(for example: 64KHz)
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Eric Johnston
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Reply #8 - May 24th, 2010 at 9:34am  
The smallest LinkStar hub burst receiver works at 156ksps with allocated bandwidth of 200kHz (see above).

If you want to use less bandwidth on the satellite you might consider an iDirect MINI HUB which can be configured for 64ksps inroute (receive at the hub).
Outroute: 64 Ksps up to 11.5 Msps
Inroute: 64 Ksps up to 2.875 Msps
Model 10110 (up to 15 remotes)
Model 10130 (up to 30 remotes)

Best regards, Eric.
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Reply #9 - May 26th, 2010 at 3:06pm  
Hi,

I have a plan , guide me please about link budget for it. How many MHz I need in W3A for Up stream & Down stream?

Tx                              RX                        Nodes Number
128Kbps                   128Kbps                      25
512Kbps                   512Kbps                       7
768Kbps                   768Kbps                       3
1024Kbps                 64Kbps                         1

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Reply #10 - May 26th, 2010 at 4:12pm  
Tx                 RX        Nodes Number
128Kbps    128Kbps       25
512Kbps    512Kbps        7
768Kbps    768Kbps        3
1024Kbps     64Kbps       1

1.  DEDICATED ACCESS - Star network configuration

Assuming that you have a large dish hub teleport and  36 remote sites, all requiring dedicated symmetric service for very many simultaneous phone calls (except the last site above, used for continuous live action video transmit from CCTV).

Total outlink from the teleport = 128x25 + 512 x 7 + 768x3 + 64 = 9.153 Mbit/s one outlink carrier. Suggest 8PSK 7/8 turbo Bandwidth = 0.543x9.15=4.97 MHz.

Return links to the teleport:  128x25 + 512 x 7 + 768x3 + 1024 = 36 carriers of various bandwidths, total = 10.113 Mbit/s. Suggest 8PSK 7/8 turbo Bandwidth = 0.543x10.1=5.48 MHz.

Total bandwith needed = 4.97+5.48 MHz.  Cost approx $94,000 per month.

A TDMA LinkStar hub is not needed and not suitable. You would be better off with Comtech SCPC modems. The remote sites each need one tx/rx modem e.g. CDM570L. The hub need one tx/rx modem e.g CDM570 plus 9 quad receiver/demodulators e.g CDM564.

2. SHARED ACCESS  - Star network configuration

Assuming 36 remote sites, all requiring shared access (except the last site above).

Buy LinkStar service from some company like Skylogic. If technology is not a pre-determined issue then consider also a share of a big HX hub (Bentley Walker) or do your own thing with an iDirect hub / VNO.

Outlink carrier from the teleport probably something like 27500 ksps DVB-S2 type carrier.  Your sites get a share of that, with each site having a limited maximum bit rate that you specify.  Remember that may be many other sites sharing the came capacity. Expect a fair access policy and limitations on traffic amounts.

Your remote sites would have to adopt maximum transmit rates of say 208, 416, 833 or 1667 according to what TDMA burst receivers Skylogic actually has configured.  Your higher transmit rate sites will probably need larger power supplies, BUCs or dishes.

A typical LinkStar hub will have a full 36MHz transponder outlink carrier and about 12 MHz of return link carriers (say 20 x 156ksps and 20 x 312.5 ksps).  Cost  $432,000 per month.  Approx 4000 customer sites. Site spec: any downlink speed you like to specify, uplink speed max of 208 or 416 kbit/s.  All shared and traffic amounts limited for assure fair shares according to respective tariff paid.

Best regards, Eric.
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« Last Edit: May 31st, 2010 at 10:54am by Admin1 »  
 
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Reply #11 - Jun 2nd, 2010 at 10:04pm  
I don't understand the word "pupolatin"

Best regards, Eric.
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Reply #12 - Jun 3rd, 2010 at 4:51am  
Dear Eric,

Thanks to helps.
Please explain about "population" and its settings in linkstar.
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Reply #13 - Jun 3rd, 2010 at 11:11am  
Set the popid the same as what your hub tells you to do.

popid is a parameter specified in the DVB-S and DVB-S2 specification.

Brief explanation:
popId        OBJECT-TYPE
SYNTAX              Integer32
MAX-ACCESS          read-only
STATUS              current
DESCRIPTION  "Population identifier associated with the start-up forward link:.
More details: http://tools.ietf.org/html/draft-combes-ipdvb-mib-rcs-00

Best regards, Eric.
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Reply #14 - Jun 8th, 2010 at 10:55am  
Dear All,

Does use all of RCSTs in a population same TX freq or different freq to the HUB?
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Reply #15 - Jun 9th, 2010 at 1:14pm  
All RCSTs in a particular network need the same PID when they are commissioned.

I note that different networks have different PIDs, so if you are setting up a new network choose a PID that is different from other networks and satellites.  If your DVB-S -S2 outlink carrier has other traffic on it (e.g. TV) then consult with those service providers so your PID  differs from those other services.

Read more here: http://www.linuxtv.org/vdrwiki/index.php/Syntax_of_channels.conf

Also read the manuals for the LinkStar hub and the DVB-S -S3 encoder/modulator.

Once in service the hub will tell the RCSTs what transmit frequency to use (remote to hub direction).

Best regards, Eric.
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Reply #16 - Jun 9th, 2010 at 5:14pm  
Dear Eric,

Can you tell me about MF-TDMA (specially in linkstar)?
In SCPC/DVB I use different TX freq for each modem but in MF-TDMA ....?
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Reply #17 - Jun 9th, 2010 at 6:53pm  
It depends how many return links to the hub you have.

If you have one return link you have one TDMA receiver at the hub and one return link frequency. All your remote sites transmit on one frequency. Like a basic iDirect system with one line card.

In LinkStar you normally have many return links and many TDMA receivers, e.g. 10 TDMA receivers at 312ksps and 5 TDMA receivers at 625ksps.  Each TDMA receiver has a fixed and different frequency. Your remote sites may transmit on any compatible return link frequency as told by the hub. Traffic will jump about according to load and which TDMA receiver is free at any instant.

You could, rather expensively, assign a TDMA receiver to just one remote, in which case you are giving it SCPC like service. Cheaper to use Comtech CDM570 or iDirect 5100 series SCPC modems - and far less complex to learn, understand and manage.

Best regards, Eric.
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« Last Edit: Jun 10th, 2010 at 8:45am by Admin1 »  
 
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Reply #18 - Jun 10th, 2010 at 11:18am  
Dear Eric,

When I have one GCU for receive (one RX center freq , 2.5M symbol) , I have one return link. Should I set for all RCST That TX center freq & 2.5M symbol(but I want each RCST use defferent bit rate same 128,256,512,1024)?
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Reply #19 - Jun 12th, 2010 at 10:36am  
If your outlink (hub to remotes) is 2.5 Msps it will have a fixed frequency and all your remote sites will be tuned to that receive frequency.

If you want the remotes to be able to transmit at any of 128,256,512,1024 then you will need 4 receivers at the hub, each on different frequencies.  Set each remote to an appropriate return link frequency.

I don't quite understand what you are trying to do but I would suggest that if you have only one receiver at the hub that you choose some suitable bit rate such 512k and put all your remotes on that one TDMA frequency.

Best regards, Eric.
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