Changes between Version 12 and Version 13 of 802.11/wlan_exp/app_notes/tutorial_hop_mac/fast_hopping

Timestamp:

Aug 3, 2015, 4:18:36 PM (9 years ago)

Author:

chunter

Comment:

--

802.11/wlan_exp/app_notes/tutorial_hop_mac/fast_hopping

@@ -433 +433 @@
 == Characterization ==
 
+
+=== Hop Synchronization and Overhead ===
 To first characterize the design, we can use an oscilloscope to explicitly measure the frequency of frequency re-tune events as well as the duration of time it takes to re-tune the radio. In our implementation of {{{poll_hop()}}} above, we wrapped the call to the radio controller that tuned to a new frequency with a set and clear of the [wiki:802.11/Usage/UserIO#DebugHeaderSignals software debug GPIO] signals. The following oscilloscope captures were taken by probing the software debug pins on both the AP and STA WARP v3 hardware.
 
@@ -444 +446 @@
 1. The rise of the debug signal of the STA occurs within a couple µs of the rise of the debug signal at the AP. The jitter here is an artifact of the over-the-air synchronization occurring with the 802.11's TSF synchronization. The fact this value is so small informs us that our 96 µs of guard interval above is probably over provisioned and can be significantly reduced without incurring additional loss.
 
+=== Throughput Performance ===
+The previous oscilloscope plots tell us that our boards are able to stay synchronized across frequency hops and that the overhead of re-tuning is fairly small. We can test the real-world impact of this hopping structure by simply performing a throughput test while the nodes are hopping through the available channels. To do this, we use the WLAN Experiment Framework and run the existing [wiki:802.11/wlan_exp/examples/txrx_log_capture txrx_log_capture] and [wiki:802.11/wlan_exp/examples/txrx_log_analysis txrx_log_analysis] examples. We have not changed any of the parameters in these scripts, so the results are directly comparable to the stock non-hopping design presented on those pages.
+
 ||  [[Image(wiki:802.11/wlan_exp/app_notes/tutorial_hop_mac/figs:xput.png, width=800)]]  ||
 ||  '''Fast Hopping 802.11 Throughput vs. Time'''  ||
+
+Using the 18 Mbps PHY rate, we can see that the achieved unicast throughput is a little under 14 Mbps. This is consistent with a very slight reduction compared to the stock design's performance. As we expect, the overhead of implementing the frequency hopping structure is minimal.
+
+=== Spectrogram ===
+
+Finally, we can see the effect of this fast hopping strategy on the spectrum by capturing another spectrogram image while a backlogged communication link is taking place.
 
 ||  [[Image(wiki:802.11/wlan_exp/app_notes/tutorial_hop_mac/figs:standard_80211.jpg, width=800)]]  ||
@@ -453 +464 @@
 ||  [[Image(wiki:802.11/wlan_exp/app_notes/tutorial_hop_mac/figs:fast_hop_80211.jpg, width=800)]]  ||
 ||  '''Fast Frequency Hopping 802.11'''  ||
+
+For completion, the above screenshots include both the standard non-hopping 802.11 as well as the slow beacon-based approach presented in the prior section. The energy appears to be spread evenly across the spectrum, since the hopping frequency is much too fast to see individual hops with this spectrum analyzer.