Re: [Discuss-gnuradio] Split-function implementation of 802.11g OFDM PHY and MAC on USRP2

[Nick Foster]

On Fri, 2011-08-12 at 23:26 +0100, Nemanja Trecakov wrote:
> 
> Hi list, 
> 
> 
> 
> 
> My name
> is Nemanja and I am working on my master thesis trying to implement
> 802.11g OFDM standard on USRP2 with XCVR2450. In order to meet the
> timing requirements of the standard, I want to split the
> functionality between the host and the FPGA, very similar to 
> Split-functionality MACs project
> (https://www.cgran.org/wiki/CMUmacs).
> Since 802.11g TX PHY has been developed by FTW
> (https://www.cgran.org/wiki/ftw80211ofdmtx)
> and further extended with MAC layer by Uwicore
> (http://www.uwicore.umh.es/mhop-software.html),
> both for use on the host, my approach would be to  use these and 
> buffer the “completely baked”  TX  packets from the host on the
> unused 1MB SRAM on the USRP2. These can then easily be sent with low
> latency when the channel is sensed free. Moreover, the Carrier
> Sensing with Collision Avoidance will be implemented on FGPA and 
> based on RSSI from the daughter card. In the RX mode FPGA  should be
> able to check the preamble in order to decide if the packet is for
> this host or not, and accordingly proceed it to the host or discard.
> However, since this standard is using OFDM, an FFT is needed. Maybe
> FFT can be avoided by implementing a Look Up Table, which could be
> used in closed networks where all senders are known. I will wait a
> while with implementation of ACK and  RTS/CTS.   
> 
> 
> 
> 
> 
> I am using the following:
> 
> 
> 
> - Ubuntu 10.04
> - GNUradio 3.3.0 with
> UHD_003.001.002-ba0e3c8
> 
> 
> 
> 
> I am a newbie to all of this and I have
> a few questions:
> 
> 
> 
> 
> 
> 1) Did anybody use this external SRAM
> on USRP2 before? According to the FAQ page the usual FPGA build is
> not using it.
> (http://gnuradio.org/redmine/projects/gnuradio/wiki/USRP2GenFAQ)
> However, my experience with these pages is that many of them are
> outdated. I also found the folder called extramfifo in 
> uhd/fpga/usrp2 folder for which I believe has something to do with
> external RAM.True?

The external SRAM is currently used for buffering TX packets.

> 
> 
> 
> 2) How to build the new FPGA UHD image?
> I have access to Xilinx 12.4 on a Windows XP machine for building the
> image. At first I will try only to add more code without omitting any
> of the present code. I have read the discussion at
> (http://old.nabble.com/Finally-compiled-USRP2-code-works-fine-with-UDP-image-...but-not-with-compiled-Raw-Ethernet-Image-td30653029.html)
> , where it is written that no project file should be made. However,
> the process described is not fully clear to me.

The existing makefile is really for unix installations. You will have a
much easier time compiling your custom image with a Linux ISE
installation.

> 
> 
> 
> 3) Does firmware image also have to be
> changed after modification with using the SRAM, or with any FPGA modification?
> 

Generally no. Use a firmware image version that corresponds to your FPGA
image though.

> 
> 
> 
> 
> 4) How to read RSSI? I have read at
> (http://old.nabble.com/measuring-RSSI---N210-with-RFX2400-td31773654.html#a31773654)
> that this could be used:
> 
> from the uhd api:
> 
> usrp->get_rx_sensor("rssi") 
> 
> or in
> python w/ gr-uhd 
> usrp.get_dboard_sensor("rssi")
> 
> 
> 
> 
> But what is uhd api?  

UHD API refers to the host (PC side) interface. If you're reading RSSI
from the FPGA it doesn't apply to you.

--n

> 
> BTW, I am aware that using RSSI from
> XCVR2450 is the signal strength over wider bandwidth than used in the
> software.
> 
> 
> 
> 
> 
> 
> Please feel free to come with any
> comment.
> 
> 
> 
> Thank you in advance!
> 
> 
> 
> Regards,
> Nemanja Trecakov
>                                         
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