With MIMO, Wi-Fi led the way to the adoption of antenna array signal processing techniques for fine-grained localization using commodity hardware. MIMO techniques, previously exclusive to specific domains of applications for instance radar systems, allow to consider estimating the orientation in space of a device equipped with COTS Wi-Fi chip. In fact, the availability channel state information measurement in recent Wi-Fi chips, makes it a candidate readily available for experimentation. Accordingly, we propose the ORION system to estimate the orientation (heading and yaw) of a MIMO Wi-Fi equipped object, relying on a joint estimation of the angle of arrival and the angle of departure.
We have implemented the system using the Intel Wi-Fi Link 5300 NICs. Indeed, we have used the 533AN_MMW (Full) model with the following SPS references: 480986-0001 0E, 506679-001 0B, 480986-0010A. The firmware of this COTS Wi-Fi card was modified in order to extract the CSI matrices through the Intel CSI tool for 802.11n HT packets. In case of OFDM systems, as for instance Wi-Fi, we are able to extract a CSI matrix for each sub-carrier. In our case the Wireless NIC offers up to 30 subcarriers. We use the 5 GHz band with 20MHz of bandwidth. We set up our wireless cards in the injection mode, which avoids the need of association with an AP and allows raw Wi-Fi packets transmission.
We setup two uniform linear antenna arrays (ULAs). We use a classical 5dBi omnidirectional compatible with 2.4 GHz as well as the 5GHz band. As we are using the 5.32 GHz band for our system, we respect a 2.8cm of inter-antenna spacing, which corresponds to half the wavelength. These ULAs are connected to the terminal through extension cables in order to have liberty of movement, which is essential for our experimentation. The cables are 3 meters long and compatible with both the Wi-Fi bands with 2dB signal attenuation for 2.4 GHz band.
The transmission is controlled by disabling the antenna selection algorithm and by specifying the desired number of antennas, streams and transmission technique which is in our case the Spatial Multiplexing (SM). The rotation of the antenna array is performed using extension cables that connect the coplanar transmitter and receiver antenna arrays to the access points. In order to specify correctly the antenna mapping as well the number of streams used for the transmission, we change the monitor_tx_rate file, after making sure that all the previous steps for collecting CSI were respected, we could for example launch the following command:
sudo echo 0x4101 |sudo tee /sys/kernel/debug/ieee80211/phy0/iwlwifi/iwldvm/debug/monitor_tx_rate
This command is meant for sending 1 stream on the first antenna. In fact there the number of streams that are sent is dependent on the MCS index used .
If we want to build a rate index we need to choose and appropriate MCS index corresponding to the number of streams. For example, for sending two streams we choose the MCS index number 9 (make sure that the index is in hexadecimal). For choosing the antenna involved in the transmission we need to apply the appropriate mask:
0x04000 for the first antenna
0x08000 for the second antenna
0x10000 for the third antennaWe use the HT leagacy packet format by using the flag 0x100.
And thus if we want to send 2 streams on the second and third antenna using HT leagacy packets, the appropriate rate would be:
0x18109=0x08000+0x10000+0x100+0x9And thus the command used is as follows:
sudo echo 0x18109 |sudo tee /sys/kernel/debug/ieee80211/phy0/iwlwifi/iwldvm/debug/monitor_tx_rate.
For further details check: https://github.com/dhalperi/linux-80211n-csitool/blob/csitool-3.13/drivers/net/wireless/iwlwifi/dvm/commands.h#L245-L334
For the transmission we choose the injection mode based on lorcon-old. Lorcon, which stands for Loss Of Radio CONnectivity is library for injecting 802.11 frames https://github.com/dhalperi/lorcon-old.
If you choose to use nepi-ng for running your experiment, you should apply an appropriate rate index in https://github.com/parmentelat/r2lab/blob/public/demos/jobs-angle-measure/angle-measure.sh#L128-L134
We jointly estimate AoA and AoD using a 2D MUSIC algorithm on phase corrected CSI matrices collected through out the experiment for a subcarrier.
The figure shown below represents the values computed for the following pseudo-spectrum:
$$ \begin{equation} \textbf{P}(\theta_i,\phi_i)=\frac{1}{(\textbf{a}_r(\theta_i) \otimes \textbf{a}_t(\phi_i))^H.\textbf{Q}_n.\textbf{Q}_n^H(\textbf{a}_r(\theta_i) \otimes \textbf{a}_t(\phi_i))} \end{equation} $$with $\theta_i $ is the angle of arrival and $\phi_i$ is the angle of departure. The peak represents the AoA and AoD estimates.
In [6]:
import matplotlib.pyplot as plt
%matplotlib inline
from music import *
csi_corr='./log/csi_corr.dat'
csi_target='./log/csi_target.dat'
music(csi_corr, csi_target, 2, 3, 0.5, 0.5,640)
#music_pl(s, 2, 3, 0.5, 0.5,1)
Out[6]:
In [2]:
%matplotlib inline
from music_pl import *
from IPython import display
import time
x = []
y = []
z = []
s = phase_correction(csi_corr, csi_target)
for i in range(1900):
x = np.append(x, i)
o=music_pl(s, 2, 3, 0.5, 0.5,i)
y = np.append(y, o[0])
z = np.append(z, o[1])
plt.gca().cla()
plt.plot(x,y,label='DOA')
plt.plot(x,z,label='DOD')
plt.ylabel('Angle in degrees')
plt.xlabel('packet number')
plt.legend()
plt.ylim(-90, 90)
display.clear_output(wait=True)
display.display(plt.gcf())
In [4]:
angle=np.array(np.squeeze([[y],[z]]))
angle.shape
Out[4]:
In [5]:
import pandas as pd
df = pd.DataFrame(angle)
df.to_csv("exp_meas3.csv",header=None)
As shown in the figure above, the estimates are jittery with a strong presence of outliers. Thus, in order to smoothen our estimates, we apply an outlier detector on the measurement data before using a Kalman filter to mitigate the statistical noise.
In [6]:
plt.figure()
plt.grid(True)
plt.ylabel('Angle in degrees')
plt.xlabel('packet number')
plt.plot(y,'b',linewidth='1',label='DOA')
plt.plot(z,'r',linewidth='1',label='DOD')
plt.tick_params(labelsize=14)
plt.legend()
plt.ylim(-90, 90)
plt.show()
In [1]:
%matplotlib inline
import matplotlib.pyplot as plt
from kalman_fil import *
DOA, DOD,angles1,angles2=kalman_fil("exp_meas.csv")
plt.figure(figsize=(7,7))
plt.grid(True)
plt.ylabel('Angle in degrees')
plt.xlabel('packet number')
plt.subplot(2, 1, 2)
plt.plot(DOA,'b',linewidth='2',label='DOA')
plt.plot(DOD,'r',linewidth='2',label='DOD')
plt.tick_params(labelsize=14)
plt.legend()
plt.grid(True)
plt.ylabel('Angle in degrees')
plt.xlabel('packet number')
plt.ylim(-60, 60)
plt.subplot(2, 1, 1)
plt.plot(angles1,'b',linewidth='2',label='DOA')
plt.plot(angles2,'r',linewidth='2',label='DOD')
plt.grid(True)
plt.ylabel('Angle in degrees')
plt.xlabel('packet number')
plt.tick_params(labelsize=14)
plt.legend()
plt.ylim(-60, 60)
plt.show()
So after applying an outlier removal using a Hampel identifier and a Kalman filter on our estimated data we have significantly decreased the noise and increased the overall accuracy.