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We are trying to implement cognitive radio network using WARP. We are planning to use OFDM for the same. Hence, we would like to experiment a little bit on OFDM. We would like to do some modifications like selectively shutting of certain subcarriers. Since we might need to implement spectrum sensing, i would like to know if it is possible to dynamically change the sub-carriers used for transmission after sensing the available spectrum using some standard algorithms. Also i would like to know if there's any detailed documentation as to the exact frequencies used for transmission and if these can be selected dynamically. Looking forward to a reply soon as i'm in a bit of a hurry...
Last edited by Abhinandan (2009-May-18 00:24:22)
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Our OFDM model support arbitrary mappings of modulation rates to subcarriers. In the current version, the mapping must be known by both Tx and Rx nodes ahead of time (it's not communicated in the header). It would be possible to extend the PHY so the mapping would be included in each packet's header.
All of our documentation is on the WARP repository site; we're working on improving all the docs this summer.
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Hi Patrick,
I have few more queries...
1) Can sub-carriers be selectively enabled/disabled as per our needs? I found a post on the forums saying that this is possible by modifying parameters in the PHY layer C-code... I'm using the OFDM reference design posted on your site in the 2.4GHz band... Suppose i want to perform this experiment on this OFDM reference design, i would like to know if the C-code i must modify is warpphy.c...
2) Also, you have mentioned there are 64 sub-carriers... Does this mean there are totally 64 sub-carriers in the entire band or there are 64 sub-carriers in each of the 14 sub-channels??
3) Just clarify if i'm right here... Suppose we want to change the modulation scheme on an active sub-carrier during transmission, it would only be possible if we transmit the header again with the full rate details of the sub-carrier and also, i will be modifying the scheme on all other sub-carriers too right?
Thank you in advance...
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1) Look at the code for the warpphy_set_modulation() function in warpphy.c. The current code uses a hard-coded map of 48 data-bearing subcarriers, but you could customize the mapping by altering that code.
2) The PHY has baseband I/Q I/O; it doesn't care what carrier frequency the radio tunes to. The PHY operates using 64 subcarriers in 10MHz bandwidth, centered wherever the RF transceiver is tuned.
3) Changing the modulation scheme in a subcarrier mid-packet is quite a bit harder. You would definitely need to customize the PHY Sysgen model so the transition between schemes is timed correctly relative to the PHY pipeline. It's certainly doable, but will require more design work than modifying C code.
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Hey,
With reference to OFDM reference design, could you tell me what is the number of points used in the FFT block?
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The PHY uses a 64-point FFT, one for each subcarrier.
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Thank you,
Few more queries, hope i'm not troubling you with too many questions:
1) Can this FFT length be modified?
2) With reference to OFDM, can you just give me a brief outline on which part of the board implements which module and what is left to the processor? For example, i would like to know where the blocks of FFT and cyclic prefixing are done... I'm mainly interested in what exactly is happening on the FPGA...
3) Can the length of the packet you want to transmit be modified or is it fixed? If fixed, what is the value of the length of each packet?
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The PHY model is available in the repository; you can change its behavior any way you like. You would need to modify the Sysgen model to change the FFT length. The model implements the full packet<->waveform transceiver. The C code implements the MAC protocol and basic PHY control (setting run-time parameters, configuring modulation rates, packet lengths, etc.); none of the OFDM signal processing is done in C. The packet lengths are totally flexible.
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Please any body knows how to implent Spectrum sensing of CR in MATLAB .please explain me steps .
Thank u .
setshri@gmail.com
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