DSSS remains the low rate modes for 802.11 chip sets at 1 and 2 Mbps operation.? ? It might be good to review the issues around using DSSS in 802.11, as this is very well understood in terms of packet recovery, protocol and other issues.?
As mentioned below, multipath remains the primary challenge for DSSS demodulation.? Early Wifi DSSS radios used various architectures to mitigate multipath, the primary one being the rake receiver.? ?Several mentions were made to summing the different multipath rays for better detection.? This does not work on a single antenna, as the singles get summed at the input!? ?OFDM MIMO radios (802.11n, ac, and ax) can use signals from different receive chains and combine them into a single packet. a technique called Maximum Ratio Combining (MRC).?
Going back to DSSS modulation, early Wifi systems sometimes used diversity antennas to select the best path with the lowest amount of multipath and or flat fade.? ?The best / easiest algorithm determined the transmit antenna with the highest packet success rate (or even last success packet) and used that antenna.? ?The use of diversity antennas can still be seen with many IOT radio modules.?
The use of multiple receive or transmit antennas is being enabled by todays SDR technology even at HF.? ? Some of the available SDR boards support 2x2 transceivers.? ?I was looking at a SDR board that showed up on ebay yesterday that had to receiver inputs and A/D converters.? ? Signal processing could be done in either the FPGA, or in the backend computer, allowing for a lot of experiments in signal processing and signal combining techniques.? ? If you had all the time in the world (Wifi is time bound by it's protocol) to process a packet, you could attempt to demodulate it at various time slots until you found a time slot that correctly decoded the packet (this is usually determined by a valid CRC-32).? ? Something similar could also be done if you had two diversity signals.? ?Decode them individually to see if one decodes properly, and add them together (OK - cross correlate) to see if that helps!? You might also experiment with Forward Error Correction, another technique used by OFDM.
73's Jim N8CAH
