[Starlink] Re: Starlink and Iran

[Ulrich Speidel <u.speidel@auckland.ac.nz>]

It's not that straightforward.

The EPFD limits under Article 21 apply as an aggregate limit, not on a 
per-beam basis. With Nco=1, a single beam can utilise this limit. This 
lets that beam project (for v2) 64QAM, and for v1 (16QAM) as the 
underlying modulation scheme of the OFDM. But only just - the fade 
margin there is ~1 dB. That gives the beam a theoretical capacity of 6 
bit/s / Hz for v2 and 4 bit/s / Hz for v1.

If you use both polarisations, each beam needs to be lowered in power by 
half (3 dB), which costs 1 bit/s / Hz. So in v2, using two co-frequency 
beams with opposing polarisation gives you 2 * (6 - 1) = 10 bit/s / Hz 
in total for v2, or 2 * (4 - 1) = 6 bit/s / Hz. That's what's been 
possible so far.

If you read 4. a. as "on the same frequency" rather than "in the same 
frequency band" and thus go to Nco=8, then that EPFD budget needs to be 
shared between the 8 beams. If that's done equally, this knocks back the 
received power per beam by 9 dB. Now, each 3 dB in power loss costs the 
beam 1 bit/s / Hz in capacity. So if these are v2 beams, you're now down 
to 3 bit/s / Hz and for v1 you're at 1 bit/s / Hz. So using just one 
polarisation gives you up to 8 * 3 = 24 bit/s Hz for v2. That's a factor 
of 2.4 in terms of capacity for v2 or 4/3 for v1. The latter is a bit 
academic since the grant is for v2 only.

So it's not nothing, but it's not a factor of 8. The main benefit here 
could be easier compliance with the requirements of Article 22 though, 
so the factor of 2.4 could translate into something a bit higher in some 
practical situations where Article 22 currently bites first.

Now we can't use the opposite polarisation of each beam here because 
that would remain a co-frequency beam and thus we'd exceed the limit of 
8. If Starlink were to use 4 co-frequency beams of each polarisation, 
we'd need to lop off another 3 dB or 1 bit/s / Hz per beam. That would 
give us 16 bit/s / Hz across the 2 * 4 beams.

Note that the satellite's beam shaping (different from beam steering) 
ensures that the footprint on the ground is more or less circular and 
covers the H3 hexagon pretty much exactly, with the beam contour leaking 
into the adjacent cells, thereby ensuring that if all 8 co-frequency 
beams point at a cell, then neither this frequency (nor beams with the 
opposite polarisation on the same frequency) can be used for the cells 
next door.

Also note that deploying that number of beams assumes that you have a 
sufficient number of spacecraft (very easy) in the right positions (much 
harder) to allow 8 beams to downlink without interfering with each other 
- you'll need significant angular separation here from the Dishy 
perspective.

David was asking what would happen if the EPFD limits were raised. In 
terms of Article 21, my answer would by "by how much?" Every 3 dB 
increase would translate into 1 bit/s / Hz extra per beam. But quite how 
many such steps are realistic before you get to power values that are 
difficult to provide in space is another question.

Sure, with Starship you could launch larger satellites. But larger 
satellites with larger panels also have a larger cross-section, and that 
has an impact on collision probabilities and so on.

Another notable point is the addition of multiple frequency bands for 
uplink and for what is clearly only suitable for downlink to gateway. 
This indicates that SpaceX are feeling the need for more uplink capacity 
as gateways now don't just supply bent pipes through a single satellite 
but are actually feed points for a much larger mesh.

On 28/01/2026 5:02 pm, Mike Puchol wrote:
> One clarification, nco = 8 means 8 co-frequency beams overlapping. If 
> you have 8 different channels in Ku band, that’s a total of 64 
> possible beams per cell. This is a huge upgrade from the original nco 
> = 1 for user/service beams (e.g. max 8 beams per cell).
>
> Best,
>
> Mike
> On Jan 27, 2026 at 19:30 -0800, David Lang via Starlink 
> <starlink@lists.bufferbloat.net>, wrote:
>> Ulrich Speidel wrote:
>>
>>> Either way, it's worth noting here that while the relationship between
>>> transmit power and and received power is linear, the relationship 
>>> between
>>> received power and achievable bit rate is logarithmic in nature. For 
>>> those
>>> not so mathematically inclined: Starlink is starting off with 64QAM 
>>> (6 bits
>>> per symbol) at current EPFD limits, and each extra bit beyond that 
>>> requires a
>>> doubling of the received power. Turning up the volume is not a game 
>>> you can
>>> stay ahead in for long, regardless of regulatory constraints.
>>
>> I think that the value in allowing higher radiated power at the 
>> receive end
>> isn't to increase the bit rate from a single satellite, but to allow 
>> multiple
>> satellites to cover the same area with the beam steering on the receiver
>> differentiating between the satellites.
>>
>> allowing two satellites with significantly different angles to cover 
>> the same
>> area allows for (close to) 2x the aggregate bandwidth to spread 
>> between all
>> receivers in that area.
>>
>>> Some interesting tidbits from the grant document though:
>>>
>>> * 4. a. No more than 8 satellite beams "in the same frequency" (I
>>> presume there's the word "band" missing here) into the same or
>>> overlapping areas at a time. That means at most 8 beams per cell.
>>
>> if that's a step up from a single beam, that will allow for close to 
>> 8x the
>> aggregate bandwidth.
>>
>> And if they can show (over time) that this doesn't cause problems, it 
>> will be
>> reasonable to expect raising this limit over time
>>
>> it may be band, or it may be frequency/channel (like wifi has 3 
>> bands, 2.4G, 5G,
>> 6G), but 5G has many non-overlapping channels, and operating on multiple
>> channels at the same time is quite reasonable for high-end APs
>>
>>> * 4. b. "SpaceX must maintain a minimum GSO arc exclusion zone of at
>>> least four (4) degrees with respect to operational GSO satellites".
>>> Finally a clear word?
>>
>> that geo exclusion has been in place for a while, but I thought it 
>> was a lot
>> wider than 8 degrees (4 degrees to each side) I seem to remember 
>> hearing 25
>> degrees being thrown around in discussions (I may very well be 
>> misremembering)
>>
>> David Lang
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-- 
****************************************************************
Dr. Ulrich Speidel

School of Computer Science

Room 303S.594 (City Campus)

The University of Auckland
u.speidel@auckland.ac.nz 
http://www.cs.auckland.ac.nz/~ulrich/
****************************************************************