Semi Measurement Approach for TR Valida- Valida-tionValida-tion

In the Section 2.2.2, the validation of the TR scheme with the existing models was explained. The second type of validation involves some experimentation. The proce-dure of the validation has two steps. In the first step, the frequency responses of a typical indoor channel are measured using frequency domain equipment. To observe the spatial focusing property of the TR, the measurements are performed for vary-ing positions of the receiver over a rectangular surface. A vector network analyzer

is used to measure the frequency responses. Thereafter in the second step, a TR communication is simulated through MATLAB.

2.3.1 Experimental Setup

Experiments are performed in a typical indoor environment. The environment is an office space of 14m×8min the IETR⁽¹⁾ laboratory. Fig. 2.12shows the measurement layout in a typical indoor environment of IETR lab. The frequency response of the channel in the frequency range of 0.7 −6GHz is measured using vector network analyzer (VNA) with a frequency resolution of 3.3 MHz. Two wide-band conical mono-pole antennas (CMA) are used in a non line of sight (NLOS) configuration.

The height of the transmit antenna and the receive antenna is 1.5 m from the floor.

The receiver antenna is moved over a rectangular surface (65cm×40cm) with a precise positioner system. Respective spatial resolution of 2.5cm and 5cm is used for the x-axis and y-x-axis of the horizontal plane. Thus 243 (27×9) measurements are taken over the rectangular surface. The frequency responses between the transmitting antenna and receiving virtual array (of 243 antennas) are measured. The time domain CIRs are computed using the inverse fast Fourier transformation (IFFT) of the measured frequency responses.

Radar Test Range

x

SATIMO Anechoic

Chamber R_x

EquipmentControlling

7.1m

Office Room

Anechoic Chamber Radar Lab

Antenna Lab

Corridor 5.8m

2.1m 2.2m

6m

1.9m

T

Figure 2.12: Measurement layout for the measurement in an indoor environment

2.3.2 Spatial Focusing in an Indoor Environment

Once the CIR of 243 channels has been recovered, TR scheme is validated through MATLAB and different TR characteristics are compared. One of the interesting properties of TR scheme is the spatial focusing of the TR. As the channels de-correlate

(1)Institute of Electronics and Telecommunications of Rennes

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Figure 2.13: Normalized TR received peak power over a rectangular surface in an indoor environment

very quickly in space and the transmitted signal of the TR scheme is pre-coded by the time reversed version of the CIR, the spatial focusing of the TR received signal is linked with the channel de-correlation in space.

Fig. 2.13 shows the received peak power of the TR received signal in an indoor environment over a rectangular surface when the intended signal is for the receiver at the center of the rectangle. The received power is normalized to the received power of the intended receiver. The received power has decreased by 10 dB for only 10 cm movement of the receiver. The inherent spatial focusing property of the TR scheme has its advantages and disadvantages. For the application where secure communication is required between stationary transmitter receiver pair, the spatial focusing of the TR scheme makes it an ideal scheme for such applications. The channel between the transmitter and the receiver acts as a unique code for the transmitter receiver pair.

Any non intended receiver at a location other than the intended receiver will receive a noise like signal. Thus, the transmitter and the receiver can communicate with very low probability of detection and probability of interception. However, the dependence of the transmitted signal on the channel makes it a relatively complex transmission scheme for mobile communication. As soon as the receiver changes its position, the TR scheme cannot communicate with the receiver and fresh estimation of the channel is required.

Receiver

8.7 m

3.7 m

Controlling Equipment

Stirrer

Tx

Rx x y

z RC

Figure 2.14: Measurement layout for measuring the frequency responses in a RC

2.3.3 Effects of Bandwidth on Spatial Focusing of the TR Scheme

Spatial focusing is one of the inherent properties of the TR scheme. It is interesting to note whether use of different bandwidths affects the spatial focusing of the TR scheme.

To verify the effects of different bandwidths on the spatial focusing, a measurement campaign is carried out in the RC. The CIR is measured by using the frequency domain equipment (VNA) in a reverberation chamber for a frequency range of 0.7 to 5 GHz.

2.3.3.1 Experimental Setup

Experiments are performed in the RC having dimensions of 8.7m×3.7m×2.9m. Fig.

2.14shows the measurement layout in the RC present inside the IETR laboratory. The frequency response of the channel in the frequency range of 0.7−5GHz is measured using vector network analyzer (VNA) with a frequency resolution of 2.7 M Hz. Two wide-band conical mono-pole antennas (CMA) are used for the measurements. The height of the transmit antenna and the receive antenna is 1.5 m from the floor. The receiver antenna is moved over a rectangular surface (40cm×40cm) with a precise positioner system with a spatial resolution of 5 cm. Thus 81 (9×9) measurements are taken over the square surface. The frequency responses between the transmit-ting antenna and receiving virtual array (of 81 antennas) are measured. The time domain CIRs are computed using the inverse fast Fourier transformation (IFFT) of the measured frequency responses.

2.3.3.2 Bandwidth Effects

Once the CIRs are extracted from the measured frequency responses, the TRRs are simulated by making use of MATLAB. The bandwidth of the signals are varied in two ways; in the first case the lower frequency (fL ) is kept constant and the bandwidth is increased by increasing the upper frequency (f_U), whereas in the second case, the center frequency (f_C) of the band is kept constant and bandwidth of the signal is

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Figure 2.15: Normalized peak power with the TR scheme in a square surface with different bandwidths (BW) but constant lower frequency (f_L)

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Figure 2.16: Normalized peak power with the TR scheme in a square surface with a constant bandwidth (BW) of 3 GHz but different lower frequencies (f_L)

increased symmetrically on both sides f_C. To compare the spatial focusing property of the TR scheme, we suppose that the intended receiver is the receiver at the center of the square. The signal intended for the center receiver is transmitted and the received signal is measured over whole square. Thereafter, the received peak power over a square surface is compared with the received peak power of the intended receiver.

The received peak powers are normalized to the received peak power at the center of the square. Fig. 2.15 compares the spatial focusing property of the TR scheme for bandwidths of 1GHz and 3 GHz but for a constantf_L of 0.7 GHz. The dimension of focusing zone does not decrease a great deal.

Fig. 2.16compares the spatial focusing property of the TR scheme for a constant bandwidth of 3GHz but with lower frequencies of 0.7GHz and 2 GHz. In this case,

the dimension of the focusing zone decreases significantly with the increase in the lower frequency. The experimental results suggest that the dimension of the focusing zone is controlled byf_L. For higher values off_L, a small focusing zone can be observed even if the bandwidth is not too large. On the other hand, for lower values of f_L, little increase in the focusing gain is observed even with a large increase in the signal bandwidth.