[Namaste-dev] Re: Physical Downlink Paper Feedback

Matt Ettus matt at ettus.com
Sun Jun 15 18:03:52 PDT 2008

I tried to parse this but the lack of formatting makes it hard to figure 
out who said what.  Is this just a summary of feedback, or are you 
making a point or question you need answered?.  You've quoted large 
quantities of text written by different people and it isn't clear who 
wrote what.

To answer you statement in the last paragraph, the reason TDMA cellular 
needed linear amps was that it used RRC-filtering on its Pi/4-DQPSK to 
narrow the bandwidth.  The constellation does not even go through the 
origin, which is the reason to use Pi/4-DQPSK in the first place.  Since 
we will not need to RRC filter, this is not an issue for us.  In any 
case, I have done simulations for BPSK, Pi/2-BPSK, OQPSK, and plain QPSK 
since it was suggested.  In the unlikely event that we ever get a wiki 
and/or version control system set up, I can upload the files to it.  I 
tried to put them in OpenKM, but it won't take Matlab files, and 
wouldn't be the right place for them anyway.


Michelle wrote:
> This feedback is for the paper at:
> http://www.delmarnorth.com/namaste/downlink_phy.pdf
> The document needs a name, revision, and date to better help identify who wrote it and when.
> There seems, to me, to be a conflict between the document and an emailed explanation to the list.
> In the document, 
> "One of the key aspects which defines the ACP is that there is a single downlink stream which will be...
> required to service users with widely varying capabilities, the most important of which for this
> discussion is antenna gain. Initial discussions have suggested a spread of as much as 20dB between
> well-equipped fixed stations with a moderately sized dish, and smaller mobile ones with something
> more like a patch or helix. It would be very easy, but also very wasteful, to simply send all bits with
> enough energy such that the weakest stations could always receive them. Instead, we need to find a
> way to impart as much as 20dB per bit of extra energy to those bits which are destined for the weaker
> stations, and this represents most of the challenge for the downlink PHY design.
> This (varying EbN0) allows the link can be designed for the highest capability users, and throttled back for the lower
> rate users. By allowing for a variable number of repetitions of each block of data, we can smoothly
> vary the rate in accordance with the actual capabilities of the weaker stations. We could even take into
> account the current utilization level of the satellite to smoothly trade off robustness vs. capacity. When
> the system is lightly loaded, everything might be repeated. When utilization is heavy, we could reduce
> the repetition factor to accommodate more users. The only real cost to this system is that synchronization 
> and the information about what is a repetition of previous data need to be robust enough for the weaker 
> stations to be able to track."
> Later on, Matt wrote an email (May 29th)
> "When I talked about a continuum of data rates depending on the size of 
> the receiver, I didn't really mean that anybody could use any antenna 
> and the satellite would send at the appropriate rate.  What I really 
> meant was that we could set that repetition rate to whatever was 
> necessary for the standard small antennas that you guys shipped, and 
> this was a variable we didn't need to decide on early in the process."
> Ok, I think I need some help understanding. When I read the document, and then read the email, the two descriptions don't seem to agree. "In accordance to the actual capabilities of the weaker stations", as well as the repeated term "weaker stations", to me, means (first) that there are weaker microwave stations (which I think were ruled out at the 2006 San Diego meeting?), and (second) that the link changes based on the capabilities of the stations in the system in real time, in real use. 
> Are the classes of stations therefore only a development strategy? Is there no plan to dynamically allocate bandwidth and/or coding gain to support a variety of types of stations? There's an a priori problemo in here that I think Paul KB5MU already brought up, as well as the problem of when a station identified as weaker gets a new antenna plugged in and is suddenly in the black, so to speak, with respect to the link. Station capability isn't fixed, in other words, since many people upgrade whenever and whatever they can. 
> Continuing with other feedback received from others:
> "I'd expect usage to follow local time, so the peakedness of usage would
> likely follow geographical user density. Practically, this likely means peak
> usage will move across the United States.
> In peak usage times, both low capability and high capability stations will
> be in use, probably in a mix following the percentage of the type stations
> in use.  If low capability stations are less expensive, they may form a
> serious majority.
> Ignoring admission controls, the possibility that low capability stations
> will fully dominate the satellite's capacity might should be modeled and
> analyzed."
> How does the system assign resources to a mix of low and high capability stations in congestion?
> Some comments about congestion control can be read here:
> http://www.delmarnorth.com/namaste/Namaste_Link_Discussion.pdf
> These are Namaste discussions from March and assumes that we intend to support stations of varying capability.
> Feedback continues:
> "Maybe this can be done with some sort of variable "frame
> boundary," where some quanity of timeslots are reserved for different
> capability users?
> It's difficult to analyze the downlink without considering the uplink
> structures.  I guess this comes later. 
> If this is a true "router in the sky," then some interesting things may
> happen with a mix of user capabilities and retransmissions.  Some smart
> person could probably model this.  Most data transmission systems seem to
> start falling apart at about 66% of maximum capacity.  The added latency due
> to hop time on retransmissions may be an issue.
> I feel some control capability may be necessary to handle variable traffic
> levels, control admission, and so on.
> I recall, and my references are at the office, that some of the "XPSK"
> modulations are not true constant envelope as some of the state transitions
> pass thru zero amplitude. E.g., TDMA cellular amps were linear amps."
> more soon!
> -Michelle W5NYV
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