Re: [Discuss-gnuradio] Project enquiry/interest

[Marcus Müller]

Hi Sajeev,

On 02/22/2015 05:37 PM, Sajeev Manikkoth wrote:
> Hi Marcus,
>
> Thanks for your input and as suggested I can check on the GNU radio
> presentations. As far as I know none of the current wireless access
> technologies allow simultaneous transmissions on same channel. 
Um, being access technologies, that's actually exactly what they do, and
why they're existing.

"Simultaneous" is kind of a difficult term, and of course TDMA systems
like GSM allow different people to use the same channel "at the same
time" only from a higher-up point of view; functionally, the same
channel is shared simultaneously. "Same Channel" is yet another
difficult term, but for example in the context of GSM, the same channel
estimate applies to successive access by different users to the same
subchannel, which is, from a information theoretical perspective, what
I'd call "accessing the same channel".

There are various different FDMA schemes, which allow parallel access to
the spectrum (think the OFDM-based LTE, where you as a user are assigned
resource blocks), and of course, CDMA systems share the same
time-frequency ranges without interfering; that's what they use codes for.
> And we do all
> form of frequency planning and interference avoidance schemes offline and
> deploy wireless solutions.
And that is a must, no matter what you'll do technically, because being
absolutely synchronous and knowing the channel exactly would be the only
alternative to that. You might want to read up on stuff like
inter-symbol-interference and why you can't allow unlimited interference
power in any channel, no matter how good your system becomes at handling
that (channel capacity).
> I was looking for research interests on new PHY
> and MAC layer techniques which can allow 2 or more transmitters at the same
> location to use same frequency, but receivers being capable of turning to
> the desired transmitters.  
What you describe applies to things like CDMA, and basically any form of
MIMO.
These are not new research, though, since CDMA has at least been in
use/development since the mid-1950s (cold war) and is used in many
mobile phone standards (2G: IS-95, 3G: UMTS, CDMA2000 etc), and GPS.
MIMO is a umbrella term for systems that use a single channel with
multiple "observers" and "signal producers" (e.g., antennas): It's
wide-spread in many access technologies (WiFi, WiMax, LTE, but even
stuff like home-network powerline standards) and allows for the
"creation" of multiple let's say "virtual" channels over one medium.


Best regards,
Marcus Müller