Hi Glen,
I've got some exciting news, I just tried cross polarity (H and V) and
discovered that it seems to do almost as good as frequency diversity.
[image: Inline image 1]
The horizontal polarity (bottom) is attenuated by about 10db, this is
partly because the h-pol antenna is not entirely omni-directional and also
my receiving antenna is vertical.
In any case this method allows both channels to be aligned very closely
frequency wise, even to the point that they are orthogonal to each other..
*Downlink* *Uplink* *Freq A* Frame 1 Frame 1 *Freq B* Frame 1
Freq A and B could be like 2khz apart or even less, uplink it doesn't
matter because you only need one uplink channel and the two RX antennas for
each pol.
I think this method has big gains for mobile devices as they are not locked
as vertical or horizontal.
A few thoughts on TDD, there's no reason why the repeaters can't be GPS
synced so all the repeaters share a common TX slot, you could have may
repeaters at one site without any affect. This could be done at very
minimal cost. A $10 dollar module can give a pulse signal that is tied to
the UTC.
One thing that I like about TDD is the ability for the CE to go full duplex
voice. If you start having to play with duplexers 600khz becomes useless
for mobiles, and even 1.2mhz would be a pain to work with. FDD is great in
the sense that you have a 3db advantage over TDD, but it comes at a big
cost.
One last thought on this, if we can get low cost hardware out there that
WAY out dose existing FM. Even the "Old boy's" will want to switch.
We stand to win a few ways:
- Low Cost Repeaters - even a handheld could be used
- More services - messaging etc
- Longer range - way longer
- Way better quality - as soon as the SNR supports it we can have HD
voice.
- Smart routing.
- Full-duplex voice
When people start realizing how great it really is, we should be able to
get a lot more people on board.
We just need some more bright people to get involved to help out with
getting this GMSK code all finished up. It would be really nice to
actually see a BER for each method to see what it's doing.
Daniel Mundall VA7DRM
On Tue, Feb 17, 2015 at 2:29 PM, glen english <g...@cortexrf.com.au> wrote:
> Hi Daniel
> I'll be looking forward to your single antenna split result- just a T
> piece on a BNC will do the job (costs 6dB rather than 3dB) I think you
> will find a higher diversity gain compared with two antennas.
>
> Now, you need to set up a test course etc, that you always drive in
> order to compare apples with apples day after day.
> Adding another carrier is a nice demonstration but likely to provide the
> great gain for the effort.
> It is pretty obvious by visual inspection how good it is.
>
> I think that TDMA duplexorless repeaters should be avoided because they
> do not enforce good filtering . But maybe that is like bad government
> policy from an old guy like me.
>
> Now, the next thing is, how to do two TX frequency , two RX frequency 2m
> repeater duplexor !
> hmmm that is harder. I wouldnt recommend it at 600kHz offsets. 5 megs
> 70cm is a bit easier.
>
> Let's put aside the 2nd diversity channel for the moment.
>
> The issues with single frequency TDD (time division duplex) are a
> sharing the frequency with other users. All current comms hardware is
> two frequency duplex, that is there is a transmit band and a receive
> band, and they are separated enough to make filtering easy.
>
> Therefore operating single frequency TDD on a comms site is very
> unfriendly , and therefore because you will NOT be able to be in any
> existing repeater frequency plan, a new bandplan will be required that
> gets you out on your own away from TX and RX repeater pairs, and far
> enough to have practical filtering.
> At 2m, for same antenna, for 600kHz offset, about 100dB rejection is
> needed of the TX noise. You will need to have the TDD operation at
> least 600-1000 kHz away from any other repeater input/output (otherwise
> you will not get the rejection from an affordable set of filters)
>
> So, sounds easy in theory, not in practice. Filtering is not that hard,
> so I would suggest sticking with two frequency duplex.
>
> How would you integrate the diversity system into a standard 2m
> bandplan. Not easily and it would require extra cavity filters. ef the
> existing was RX 146.3, TX 146.9, might become RXA146.1, RXB 146.3, TXA
> 146.9, TXB 147.1
> but... whatever you come up with is going to have to fit in the
> bandplane, and with other users, and their egacy expectations.
> certainly 70cm there is more room and scope, and also 70cm benefits even
> more form diversity because the nulls are much much deeper than 2m.
>
> Compatibility with existing bandplans and users etc is harder than the
> technology.
>
>
> with TDD, then there is the problem of multiple users in the one area.
> Because multiple users may be transmitting while others are receiveing
> just a few tens of kHz apart, the system performance will be heavily
> degraded by desense between the users.
> At least with two frequency duplex, there is a decent frequency sep
> between those trying to listen and those talking....
>
>
> glen.
>
>
>
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