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Hello, RICE,
I have read through your description about the packet structure and also the questions on the forum. However, I still have some doubt about the packet and frame structure.
1) According to http://warp.rice.edu/forums/viewtopic.php?id=214, you mentioned that there are 24-byte mac header added to the base-rate symbols. So are there two headers in base-rate symbols, one for PHY and on for MAC? In that case, the number of base-rate symbols should be 50 (24byte*2+2 byte CRC). Is this correct?
2) Is there a fixed modulation scheme assigned for base-rate symbols in advance just like training symbols? If so, is it BPSK or QPSK?
3) According to http://warp.rice.edu/forums/viewtopic.php?id=457, you mentioned that among 80 samples, 64 samples are actually fed into FFT block and default offset is 8. I'm assuming that the first 16 samples are cyclic prefix. Thus, by setting the default offset value as 8, will some of the data carriers get lost? Considering that 8 cyclic prefix samples are fed into FFT, this will cause 8 data samples unable to pass into the FFT. Will it cause a problem?
Thanks.
Tan
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First, see http://warp.rice.edu/trac/wiki/OFDM/MIM … rameFormat.
One clarification- the term "symbols" gets overloaded in OFDM. A data symbol is one point from the constellation (BPSK, QPSK, etc.) and conveys a few bits (BPSK=1, QPSK=2, 16-QAM=4, etc.). An OFDM symbol is the group of samples in/out of the IFFT/FFT. Our OFDM symbols are built from 64 subcarriers; 48 of those subcarriers contain data symbols. Base-rate and full-rate refer to the modulation scheme applied to the header and payload. A base-rate or full-rate symbol is an OFDM symbol (i.e. with 48 data-bearing subcarriers).
1) The 24-byte header contains both MAC and PHY fields, all modulated at the base rate. By default, we use QPSK for the base rate, so the 24-byte header occupies 2 OFDM symbols.
2) The base-rate modulation can be changed at runtime. However, both the Tx and Rx nodes must know the rate; it can't be communicated over-the-air. The full-rate can be communicated wirelessly (it's one of the header fields).
3) No data is lost, as long as the sum of the channel delay spread (in sample periods) and the number of cyclic prefix samples used in the FFT is less than 16 (the cyclic prefix length). This is standard practice in OFDM systems; it's why the guard-period is filled with cyclic prefix and not zeros. Lots of OFDM tutorials are available that describe this.
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Hi,
Thank you very much for your reply! I have understood the first question and last question now. But for the second question, you said that:
murphpo wrote:
2) The base-rate modulation can be changed at runtime. However, both the Tx and Rx nodes must know the rate; it can't be communicated over-the-air. The full-rate can be communicated wirelessly (it's one of the header fields).
What do you mean by it can't be communicated over-the-air? If I didn't change the default modulation scheme for the base-rate symbols, I can transmit them over the air, right?
Also, "it's one of the header fields". What do you mean by this?
Thanks.
Tan
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Nodes have to use the same base-rate modulation scheme for headers to be received, so the base-rate has to be set a-priori at every node. The full-rate modulation scheme (the scheme used for the packet payload) can be set per-packet. The full-rate scheme is communicated via the fullRate field in each packet header. The receiver uses this field to configure its demodulator per-packet.
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Ok, I understand that now. However,
murphpo wrote:
1) The 24-byte header contains both MAC and PHY fields, all modulated at the base rate. By default, we use QPSK for the base rate, so the 24-byte header occupies 2 OFDM symbols.
According to your frame format, 2 byte cyclic prefix will be added to the header, right? In that case, is there a third OFDM symbol used to transmit the CRC code or the CRC code is merged into the payload and transmitted with the payload in full-rate?
Tan
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The cyclic prefix is not the same as the cyclic redundancy check (CRC). The cyclic prefix is composed of repeated samples with each OFDM symbol to guard against multipath in the wireless channel. CRC is the method used to calculate checksums in the packet header and payload. The 24-byte header has 22 user-supplied bytes followed by an automatically inserted 2-byte checksum. The payload is similar, with a 4-byte checksum.
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Ok, I fully understand that now. Thanks again.
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