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MuNinga
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Conversion between Mbps and MHz
18th Aug, 2017 at 1:09pm
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I am building a satellite-based WAN in Africa. The WAN will have 8,000 remote stations on VSAT and there will be a Teleport Hub in the host country.
In my conversations with vendors, I have come to know that the bandwidth  can be specified either in Mbps or MHz.
I have two questions:
1. How do I convert an Mbps specification to MHz and vice versa?
2. How do I convert MHz to Mbps(Download) and Mbps(Upload)?
  
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Re: Conversion between Mbps and MHz
Reply #1 - 18th Aug, 2017 at 2:49pm
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You need to talk to satellite operators about leasing satellite transponders for the outbound and return link traffic.

Ask about the coverage beam map and the area you want to serve.

Historically, C band beams tend to be large area and need at least 1.8m dishes. C band is reasonably reliable in rain.

Historically, Ku band beams are smaller, perhaps only part of Africa and need appox 1.2m dishes.

Ka band satellites are new and have multiple tiny spot beams. Dish sizes will be smaller. Transponders may not be for sale and the only way of getting service is to buy from the service provider who will own the whole satellite, teleport hubs and all transponders, like Eutelsat/Skylogic, HughesNet or Wildblue. So far, these new High Throughput Satellites (HTS) do not provide any connectivity within the spot beams. Connectivity is only provided to and from the telport hub. Things might change if someone out a IP router on the satellite!

Ask about transponder availability, timescales and costs. For the download to the customers, you need transponders connected from the teleport beam to the customer beam. A 15 year lease of 1 transponder will cost many $ millions.

Ask about dish sizes that customers would need to receive a carrier using whole transponder power and bandwidth.

Q2: Each VSAT customer needs 30 kbit/s down and 6 kbit/s up.

So for 8000 customers you need approx 240 Mbit/s download and 48 Mbit/s upload.

Q1: To convert MHz of transponder capacity to user information bit rate read here: http://www.satsig.net/symbol01.htm

Symbol rate is approx leased transponder bandwidth / 1.2   The ratio is approximate, 1.15 to 1.3 depending on particular equipment transmit filtering and guard band rules required by the satellite operator.
e.g. Transponder bandwidth purchased = 68 MHz
Approx Symbol rate = 52 Msps

Different modulation methods give different transmission rate bits per symbol.
BPSK = 1 only for ultra small dish or extreme rain
QPSK = 2 typical DVB-S2 in rain
8PSK = 3 typical DVB-S2 standard
16QAM = 4 typical DVB-S2 clear sky - high power

Higher order modulation methods need clear sky, more power from the satellite or large receive dishes. Lower order methods work with smaller dishes and rain.

For forward error correction (FEC), anything for 0.95 down to 0.25 is possible. FEC like 0.95 needs more power from the satellite or large receive dishes. FEC like 1/2 is needed for smaller dishes and rain.

A typical DVB-S2 carrier used for VSAT outlink, has an adaptive structure made up a blocks of different modulation/coding types each giving particular information bit rates, say 64, 50 and 21 Mbit/s.
The proportions of each type, say 8/9 8PSK, 3/4 8PSK, 1/2 QPSK, will vary according to the types of weather across the beam coverage.

In summary: One carrier of 52 Msps and FEC=0.95 and QPSK will give 64 Mbit/s information rate and be good for a downlink to 2100 customers in clear sky.

The customer uplinks will need perhaops 1/5 of the download bit rate but the bursty nature of the transmissions is inefficient and you will probably need to lease a third or more of the capacity of the customer uplinks. e.g. 20 MHz in the above axample.

I hope all this gives you some idea.

I would try to identify and contact companies planning new small spot beam satellites for Africa, including C, Ku and Ka band. They may be able for you to share in their infrastructure. A big satellite with 80-100 small spot beams over Africa might need 6-10 hubs, located spread across Africa, Middle East, Europe and Atlantic Islands.

Get different advice from many satellite companies.
« Last Edit: 16th Jun, 2018 at 1:38pm by Admin1 »  

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Re: Conversion between Mbps and MHz
Reply #2 - 18th Aug, 2017 at 3:05pm
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