[Starlink] Re: Starlink and Iran

Ulrich Speidel
2026-01-28T09:05:39Z

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/
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