LINKSTAR TX BW

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.

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

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.

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.

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.

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.

Dear Eric,

Thanks to helps.
Please explain about "population" and its settings in linkstar.

Dear All,

Does use all of RCSTs in a population same TX freq or different freq to the HUB?

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 ....?

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)?