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Hello Rice,
I've almost finished reading your entire OFDM design. The sample period, sample rate and symbol period are confusing me a lot. Could you please answer the following questions for me so that I can fully understand your entire design?
1) For the Assert block, I think the sample rate is the same thing as the sample period. To verify this idea, I connected the Assert block directly after Gateway In block. In the Gateway In, I set the sample period to be 2(second); Under this situation, only when I set the sample rate of the Assert block to be 2 can I pass the simulation. So I think the sample rate and the sample period is the same thing. Is that correct?
2) Is the OFDM subcarrier sample period the minimum sample period in your design? I think the minimum sample period is 1(sec) according to the System Generator.
3) In OFDM Tx MIMO/TxControl, why did you attach a down sample block in front of the Preamble Index Counter and an Up Sampler after it? What is the sample period before downsampling? If each OFDM subcarrier sample occupies one system sample period, why is the Preamble period twice as long as each OFDM sample (modulated on each subcarrier later)?
4) In TxControl/FFT Start Gen, the base-rate symbol is only modulated(transmitted later) when the LSB of the symbol number is 0; will this cause half of the base-rate symbol lost? I know in this situation, we are dealing with symbol period instead of sample period. However, what is the difference between an output symbol period and an OFDM symbol period? Is output symbol period twice as long as an OFDM symbol period?
5) In OFDM Tx MIMO/Training_Data/Ant Selector, there is an Assert block in the oscillator and the sample rate is 2. In Multiplexing mode, when transmitting full-rate symbols, does the antenna switch (Sel changes twice) during each OFDM symbol period? Are the two OFDM symbols assigned to both antenna at the same time? For the base-rate symbols, the antenna will not switch. Thus, both antenna will transmit the same OFDM symbol during each symbol period, right?
6) In the Equalizer&Packetizer/Packet_Constructor/Modulation RAM, I found FullRateMasks. I think FullRateMasks can be used to mask off one antenna modulation mask(either for antenna A or antenna B), hence realizing SISO mode. Is this correct? If this is correct, the full-rate symbols work exactly the same as the base-rate symbols in SISO. Only one antenna (instead of 2) transmits one symbol during each OFDM symbol period, right? Could you point out all the control logic related to configuring the SISO mode? because I plan to implement a novel algorithm under SISO mode first.
Thanks a lot.
Tan
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1) Yes. But the assert block should only be used to resolve ambiguity about sample periods or datatypes in feedback loops. In every other case, rates/types should be configured at gateways, arithmetic blocks, etc.
2) The system sample period of the PHY is 1 in Simulink. This corresponds to 40MHz in hardware. The OFDM subcarrier period is 100ns (10MHz), corresponding to a Simulink period of 4.
3-5) I'd suggest not trying to infer symbol-level timing from the timing of individual control signals. Every OFDM symbol (training, base-rate or full-rate) occupies the same amount of time (8µs = 80 samples * 100ns/sample) in SISO, Alamouti and multiplexing modes.
6) The full-rate modulation can be configured from software using warpphy_set_modulation(). You can also edit the mask definitions in that function to enable/disable subcarriers. This does not control SISO vs. MIMO mode, which are selected via configuration bits in other registers. The Rx SISO mode bit is bit 4 of the Rx ControlBits register (/Memory-mapped Registers/Rx Registers/ControlBits Slices). You can follow that bit's GOTO to see each block which uses the configuration bit. A similar bit is in the Tx ControlBits register.
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Thank you so much for all your timely replies. They are really helpful to me! One more question, what is the period for short training sequence(160 samples) and long training sequence(160 samples) in preamble? Is each sample period also 100ns?
Tan
Last edited by tan (2010-Jun-09 13:46:24)
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Yes- every sample is 100ns (resulting in the 10MHz occupied bandwidth at RF).
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Hello tan and murphpo
i am on the way in understanding reference design its very complex for me
although i have deep conceptual knowledge of ofdm system. i want to build my own simple siso ofdm tx and rx. if you have a simple ofdm siso reference design it will make my jouny easy
thanx
Last edited by imran (2010-Aug-08 05:30:49)
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