SatSig topic:

Bandwidth Rate on Idirect and linkstar

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May 10^th, 2010 at 3:31pm

Hi,
Thanks for your support, please how would i know the bandwidth rate on idirect and linkstar modem when they are on network?
Caleb

May 10^th, 2010 at 5:08pm

Contact your service provider and ask. The maximum bit rates should be associated with the tariff you are paying.

Typically an VSAT site will be receiving an outlink (download) carrier with a bit rate anything from 256kbit/s to 40 Mbit/s. The peak bit rate allocated to your site (say per whole second) will be controlled by the hub router. The average bit rate per minute, hour, day, month may also be limited either by a deliberate fair sharing policy (so that the download amount is fairly related to what you pay) or by unmanaged congestion.
A small outlink carrier, say 256k might be shared by 12 sites. A 40 Mbit/s outlink carrier might be shared by 2000 sites.

Your uplink carrier bit rate may be something like 256k bit/s, a fixed speed. But the transmission is TDMA with your site only transmitting the occasional burst packet(s) of data, usually when a mouse if clicked or an email sent. Remember that there may be many other sites sharing the same 256k uplink carrier frequency.

There are hundreds of iDirect hub systems, typically with 5 - 100 customer sites. Linkstar, on the other hand, tend towards larger hubs with thousands of customers per hub, similarly Hughes HX hubs.

Aug 12^th, 2010 at 8:55am

Hello,
We have a VSAT network include 40 remotes stations, we use actually a bandwidth of 2 Mbps, 1 Mbps for Upstream and 1 Mbps for Downstream, the main station uses an antenna of 3.7 m and the remotes sites uses the 1.2 m antennas.

Question: we project to increase the satellite bandwidth from 2 Mbps to 4 Mbps, so, was it necessary to change some outdoor equipments, like dish (1.2 to 1.8 or 2.4), lnb, buc (4 watt)?
Technical information: IERP is 48 dbm, Carrier Data: IDirect Technologies

Best regards.

Aug 12^th, 2010 at 9:43am

Assuming you have one large carrier from the hub to all the remote sites. If you double the bit rate of this carrier you will need to double the hub BUC power. So if you are using a 4W BUC now and the carrier level is 3.5W you will need a 8W BUC and adjust the carrier level to 7W.

Regarding the remote sites you don't make it clear what bit rate they transmit. If they transmit at 1 Mbit/s and you want to increase this to 2 Mbit/s you will need to double the BUC power at the remote sites or use dishes of twice the area (1.42 x the diameter). If the remote sites transmit at lower speed, like 256kbit/s with 4 TDMA receivers at the hub and you are increasing the number of receivers to 8 and providing extra throughput by reducing the number of remotes per TDMA receiver from 10 to 5 then there will be no change in BUC power at the remotes, each remote will get twice the throughput as it will be allowed to transmit for more of the time, but at the same 256k bit rate as before.

Best regards, Eric.

Aug 12^th, 2010 at 11:00am

Eric,

Thank you for your quick reply, the remotes earth stations transmit at 430 kbps, with QPSK modulation and 0.66 T for FEC, however, changing the antenna is less dear than changing the BUCS.

Aug 12^th, 2010 at 1:23pm

If you have now: one outlink carrier 1 Mbit/s and two return link TDMA carriers each 430kbit/s, and each return link with 20 remote sites. Total 2 Mbit/s on the satellite.

With a total of 4 Mbit/s on the satellite; 2 Mbit/s for new outbound carrier and 2 Mbit/s for the total of the return links. Then you can double the return link capacity adding two further TDMA receivers at the hub and rearranging the remote sites so you have just 10 sites on each return link frequency. Each remote site remains at the same bit rate, 430kbit/s, but can transmit for 10% instead of 5% of the time thus doubling the throughput. There is then no need to increase the BUC power or dish size at the remote sites.

If you do upgrade dish size, particularly the smallest dishes in the system, this will benefit both the outlink and the return link.

Best regards, Eric.

Jun 9^th, 2011 at 6:54pm

Hello,
To resolve this situation, we think to another solution, in addition to the 2 existing mini-hubs, we want to add a third mini-hub, so, every mini - hub must employ 830 Kbit/ses in Downstream and 500 Kbit/s in Upstream, the total is 2,5 Mbit/s on Downstream and 1,5 Mbit/s on the Upstrem.

Actually, we use a BUC of 8 Watt in the hub side (antenna of 3.7m) designed for transmitting 1 Mb/s.

Question: Can we use a BUC of 8 Watt to transmit 2.5 Mb/s Downstream or it is necessary to change it by a new BUC of 16 watt?

I wait for your answer, it’s very urgent.

Best regards.

Jun 9^th, 2011 at 8:00pm

It is really important to transmit only one carrier. If you need more downstream (outlink) bit rate increase the bit rate of your single carrier.

Having one carrier only per BUC avoids causing intermodulation interference on the uplink into other nearby frequencies in the transponder.

If you have multiple carriers, you need to operate the BUC backed off by 3 or 4 dB and this means at least half the potential power is unuseable. If you insist on going down this route talk to BUC manufacturers about linearity and C/IM ratio at various output back-off values. Some manufacturers provide linearisers so you can operate nearer to saturation.

To transmit three 8 watt carriers through one BUC, the BUC power rating might well have to be around 48 watts. This assumes 3 dB output back off is sufficient to reduce the 3rd order intermods to an acceptable level - consult the satellite NOC. A high loss cross waveguide guide coupler or BUC output monitor point is essential plus a spectrum analyser to plot your BUC output spectrum.

To judge what BUC power you need first find out what power is actually being used with your present BUC and carrier bit rate. If you are using uplink power control the clear sky power will be several dB (typically 6 dB) below the higher value used during heavy rain. e.g. 2 watts (+3 dBW) in clear sky and 8 watts (+9 dBW) during worst rain.

Best regards, Eric

Jun 10^th, 2011 at 8:22am

Thank you for your reply, below the values of the Budget link:

18K QPSK 0.793T 425kbps 5dB HMD 3.7m to 45dBW 1.2m

CARRIER DATA : Type: iDirect, Info Rate: 425 kbps, Mod: QPSK, 0.793T BWo: 276kHz, BWa: 357kHz, C/N: 6.83dB, C/N_thresh: 6.83dB

What are the risks to incur if we use a BUC of least capacity, and what are the consequences?

Best regards.

Jun 10^th, 2011 at 1:21pm

A BUC has a rated power output, e.g. 8 watts. If you read the BUC specification you will see that this value is defined as the -1 dB gain compression point. If you plot the input/output transfer characteristic this point occurs when the output is 1 dB lower than would be the case if the gain remained linear (staight line).

It is safe to operate a BUC at its maximum rated power output with a single PSK modulated carrier or single CW carrier. Supply voltage should be correct and the temperature in the specified range. Be careful never to apply excess levels, even for a brief moment.

At a hub site you should have means of monitoring your BUC output power. This can be done using a high loss waveguide coupler across the output waveguide with a Agilent power meter or spectrum analyser. Some BUCs have an output monitor point. Some BUCs have an inbuilt power meter.

In simple non metered VSAT systems you can gradually increase the BUC drive (using a CW carrier) and observe when the gain becomes compressed. Then you know what is the drive level to achieve the rated power output. This -1 dB compression test is done when new VSATs are commissioned and the maximum allowed drive level is then recorded at the hub database.

If your normal operating power in clear sky and with threhold C/N, is the same as your BUC rated power then your service will fail in light rain. If you are operating several dB above threshold then you have that amount of rain fade margin. Automatic uplink power control will only work if your clear sky (threshold) operating point is well below the rated power of your BUC.

Do try to keep to one transmit carrier only per BUC, as multi carrier operation brings with it the problem of intermodulation interference.

Best regards, Eric

Jun 11^th, 2011 at 6:54pm

Eric,

Thank you very much for your help and for your attention.

Best regards.