Robustness of TR Scheme
3.3 TR Robustness in Realistic Channels
3.3.3 Experiments With a Moving Receiver
x 10−5
Time (ns)
PDP
tpk+1 tpk−1 t
pk
No change 1 person 1 metal 2 metals
Figure 3.25: PDP of the received TR signal with different variations induced in the channel for a bandwidth of 2.0 GHz
Fig. 3.25 shows the PDP of the TR received signal with and without induced variations in the channel for a bandwidth of 2.0 GHz for measurement campaign 2.
The received signal with a presence of one person and one metallic strip in the channel has decreased only by 0.7 dB and 1.35 dB respectively. Thus, in an indoor channel TR scheme perform quite robustly even with the variations in the environment.
3.3.3 Experiments With a Moving Receiver
Experiments are also performed to study the effects of movement of the receiver on time reversal. The receiver is moved with a step of 2.5cmfor a bandwidth of 2.0GHz.
Fig. 3.26 shows the respective correlation coefficients of the MCRs (ρh(d)) and the measured TRRs (ρy(d)) at a distance d cm with the MCR and measured TRR at d = 0 cm. The measured TRR holds a better correlation with the original TRR even though the channel response is de-correlated almost completely. However, the de-correlation is sufficient to cause significant performance degradation of the TR scheme.
Table 3.3 summarizes different performance metrics for the displacement of the receiver up to 30 cm from the original position. The results give us some very in-teresting observations. The received peak power decreases with the movement of the
0 10 20 30 0
0.2 0.4 0.6 0.8 1
d (cm)
ρ
ρh
ρy
Figure 3.26: Respective correlation coefficients of the MCR (ρh(d)) and the mea-sured TRR (ρy(d)) with the MCR and measured TRR at d = 0 cm with different displacements of the receiver for a bandwidth of 2.0GHz
receiver but it becomes somewhat static after almost 9 dB decrease in the received peak power. It is evident that the performance of the TR scheme deteriorates rapidly at first but then becomes somewhat saturated.
Distance FG IAP SSR NPP 0 11.87 4.64 3.85 0 5 10.28 3.97 3.40 -1.58 10 7.63 3.28 1.32 -4.24 15 5.60 2.39 0.06 -6.33 20 3.41 3.15 0.12 -8.59 25 2.01 2.82 1.35 -9.86 30 3.08 5.54 2.71 -8.78
Table 3.3: Comparison of different TR properties with different displacements of the receiver (d cm) with respect to the reference position (d = 0cm)
Fig. 3.27 shows the PDP of the TR received signal for different displacements of the receiver. The received peak power decreases considerably with the movement of the receiver but an interesting phenomenon is observed. Even though the received signals have been displaced by 20 cm and 30 cm, yet they have a peak (though significantly reduced) and the shapes of the two signals are very similar.
399 400 401 0
1 2 3 4
Time (ns)
PDP
No movement 10 cm
20 cm 30 cm
Figure 3.27: PDP of TR received signal with different displacements of the receiver
3.4 Conclusion
In this chapter, the robustness of a time reversal (TR) communication system is studied in a time varying channel environment. In the RC and the indoor environment, experiments are performed for the robustness of the TR scheme in a non stationary channel environment for three different bandwidths. In the RC, variations are induced with the rotation of the stirrer or the movement of the receiver. It has been found, that the bandwidth of the TR signal does not have a big impact on the robustness of the TR scheme. Higher bandwidths have generally better performance than the lower bandwidths but the degradation in the performance with the variations in the channel is of the same magnitude. The results suggest that TR system can give a robust performance even if the channel environment has changed partially. If the channel maintains some partial correlation with the previous channel, TR can give a good performance even if the total correlation of the channels is very low. For instance, channel impulse response with a 2◦ rotation of the stirrer has a correlation coefficient of 0.06 with the CIR of θ = 0◦, but still the TR system achieves a RMS delay spread of only 0.65 ns, a focusing gain of 19.26 dB, signal to side lobe ratio of 2.87 dB, increased average power of 3.15 dB. On the other hand, if the receiver is displaced from its position, the channel gets totally de-correlated and does not maintain any partial correlation with the original CIR for a movement in the order of λ4, where λ is the wavelength of lower frequency fL. Thus, in a RC, TR system cannot support even a small movement of the receiver.
In the indoor environment, the variations in the channel are induced by the pres-ence of a person, one metallic strip or two metallic strips in the channel near the receiver terminal. Higher bandwidths have generally higher values of different per-formance metrics but generally the robustness of the TR scheme is not affected a
great deal with the bandwidth of the transmitted signal. The deterioration in the performance with different induced variations is of the same order for all bandwidths.
A very high FG is observed with all kind of variations in the channel. SSR does not change a great deal (ascertaining the results obtained in the RC). IAP decreases with the variations in the channel especially for lower bandwidths (1.5GHzand 1.0GHz).
These results are very important from the implementation point of view. In real-istic environments, if the channel is changed due to the variation in the environment, e.g. movement of the people, the results suggest that channel will not de-correlate completely. In this case, TR robustness can play a vital role. There is no need to re-estimate the channel response if there are minor changes in the environment e.g. movement of the people. This property can be very attractive for WLAN and wireless streaming applications. However, when the receiver moves its position or there is significant change in the layout of the furniture, the performance of the TR scheme degrades rapidly, but nevertheless, the TRR maintains better correlation with the original TRR than the channel response maintains with the original channel re-sponse. Thus, TR is beneficial in a non stationary channel where variations are caused by the changes in the environment. The results suggest that realistic environments (such as typical indoor environment) create lesser number of multi-paths and are less sensitive to the changes in the environment than the RC.