Email.-Ali.khaleg h sa-rennesr, Gas.el-zein94isa-rennesfr - IEEE Xplore

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Email.-Ali.khaleg h sa-rennesr, Gas.el-zein94isa-rennesfr. Abstract-- Time reversal (TR) is a technique that can achieve remarkabletemporal focusing (delay ...
2 -3 April 2007.

2007 Loughborough Antennas and Propagation Conference

Loughborough, UK.

Signal Frequency and Bandwidth Effects on the Performance of UWB Time-Reversal Technique A.Khaleghi, G. El Zein

Institut d'Electronique et de Tjljcommunications de Rennes 20, avenue des Buttes de Coesmes, CS 14315 35043 Rennes Cedex, France

Email.-Ali.khaleg

h sa-rennesr,

Gas.el-zein94isa-rennesfr

Abstract-- Time reversal (TR) is a technique that can achieve remarkable temporal focusing (delay spread reduction) and spatial focusing in the context of wideband multipath propagation channels. In this paper, based on wideband measurements in reverberation chamber (RC) channel, we predict the characteristics of TR on the temporal compression and the spatial focusing. For the first time, we have analyzed the effects of the signal bandwidth on the temporal focusing and spatial focusing. Furthermore, the influence of the lower frequency band of the signal on the special focusing is demonstrated.

I.

Introduction

Time reversal is a method to focus spatially and compress temporally broadband signals in rich scattering environment [1],[2]. Two stages are performed. First the transmit-receive channel impulse response (CIR) is measured in the transmitter side of a communication link. Second, the time reversed version of the recorded waveform is used as a pre-filter for transmitted symbols. By considering the reciprocity of the propagation channel, the symbols back propagate in the channel, retrace their former paths and are concentrated in space and time. Spatial focusing means the spatial profile of the power that peaks at the intended receiver and decays rapidly away from it. Temporal focusing means that the channel impulse response at the receiver has a very short effective length. Here, we first demonstrate the effects of TR on a wireless radio channel measurements taken from a reverberation chamber environment. The chamber is in the static case and the measurements cover the frequency range of 0.7-5GHz. Based on the frequency domain measured data, the performance of TR in RC environment is studied. The temporal and spatial focusing performance for a single-input-single-output (SISO) link is predicted. Finally, not only the effects of the signal bandwidth on the temporal focusing are computed but also the signal bandwidth and the lower frequency band effects on the spatial focusing are demonstrated. The frequency domain measurement can predict TR system performance which corresponds to an ideal time domain measurements.

II.

Time-reversal

Considering the transmitter-receiver pair, TR uses the time reversed response of the channel as the transmit pre-filter. Denote the channel impulse response by h(ro,T), where ro is the receiver location and T is the delay variable. By applying TR technique the effective channel response to any location r is thus given by

s(r, ) = A * h(ro,-r) * h(r, )

(1)

Where * denotes the convolution operation and A is a constant amplification term. As a metric for spatial focusing, the following quantity that represents the power of the strongest tap at a receiver located at r is considered [1].

k(r) = max|s(r, r)

(2)

This work was supported on the project of MIRTEC by the Agence Nationale de la Recherche (ANR). The authors are with the Institut d'Electronique et de Telecommunication de Rennes (IETR)- UMR CNRS 6164- INSA

1-4244-0776-1/07/$20.00 e2007 IEEE

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2 -3 April 2007.

2007 Loughborough Antennas and Propagation Conference

Loughborough, UK.

Normally, it is expected that k(r) to peak at the intended receiver location at ro and decays rapidly with increasing the distance from it.

III.

Measurement setup

Measurements were conducted in IETR large reverberation chamber. The chamber is shielded from any interfering signals and is static during all the measurements. The chamber dimensions are 8.7m length, 3.7m width and 2.9m height. Two wideband mono-conical antennas are used in cross-polar form at the transmitter and the receiver link. This will decrease the direct coupling between the antennas. The separation between the antennas is 3m and the antennas are installed Im above the chamber floor. The channel between the antennas is measured using a calibrated vector network analyzer (VNA) and the received signal (magnitude and phase) is evaluated. Measurements span the frequency range of 0.7-5 GHz with 2.68MHz frequency resolution. To evaluate the statistics of the data and to compute the spatial focusing of TR system, the receiving antenna is moved over a square surface using a precise positioner system. The dimension of the measurement surface is 40X40 cm2 with a resolution of 5cm for both x-y directions; therefore 81 channels are evaluated. The time domain channel response is computed by IFFT of the complex measured signals. The feature of the frequency domain measurement is that the channel response for the total band is measured once, and then the time-domain response for any desired band can be simply extracted.

IV.

Experimental results

A sample of the measured instantaneous power delay profile (PDP) for a location at the center of the measurement domain is illustrated in Fig. 1. Due to the large reflections of the wave from the metallic walls of the chamber, the received signal has a large delay spread. In this case, 9000 of the signal energy is distributed in a delay of 250ns, while for a typical indoor environment this is about 40ns. To illustrate the characteristics of the TR channel, equation (1) is used. Fig.2 shows the computed PDP of the equivalent TR channel in the intended location, ro. As shown, the TR channel response is compressed in time. Therefore, the complex task of estimating a large number of taps at the receiver is greatly reduced. This implies low cost receivers. This also implies suppression in intersymbol interference and thus a higher data rate. For the sake of comparison between direct channel and TR channel, the transmitted signal power is normalized to constant. In the reverberation chamber, 25dB increment of the peak power is observed. Furthermore, the signal-to-side-lobe level (SSL) of lOdB is computed. 2.5

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Fig. 1 Measured PDP of RC channel in 0.7-5GHz band

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Fig.2 Computed PDP of time-reversal system

1) Focusing Gain: Signal Bandwidth Using the actual measured data, the time domain response of the channel for different bandwidths around the center frequency (2.85GHz) are computed. The transmit power is normalized to constant and the equivalent

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2007 Loughborough Antennas and Propagation Conference

Loughborough, UK.

TR channel response is obtained. The value of the strongest tap power, k(ro), is computed. To include the statistics of the peak power, k(ro) is averaged over all of the measurement locations. Fig.3 shows the normalized curve of k (rO) for the given center frequency and various bandwidths. As shown, the maximum of the peak power is given for the largest bandwidth. The peak power (dB) is linearly reduced by reducing the signal bandwidth. An average focusing gain of 1dB is resulted for each increment ofthe signal bandwidth by 500MHz. The relation between the gain augmentation and the bandwidth can be used to compromise between the system complexity (introduced by the bandwidth) and the focusing gain.

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Fig.3 Normalized to maximum of the peak power (dB) versus bandwidth (GHz), ( ) measured peak power, (--) averaged peak power

2) Spatial Focusing: Bandwidth and Frequency Effects In this section, the spatial focusing of the TR system in the intended location of the communication link is considered. Two cases are studied: a) the lower band of the signal (fL) is fixed and the bandwidth (BW) effect is investigated b) the BW is fixed and the effect of fL is studied. Fig.4 illustrates the 3D plot of the normalized to peak power of the spatial focusing (k(r) in dB) that is measured at the square surface. The signal bandwidth is 1GHz and the lower band is 0.7GHz. As shown, in the intended location, at the center of the square surface, the maximum spatial focusing gain is illustrated that decaying away from it. In Fig.5 the similar plot is illustrated but the bandwidth is increased to 3GHz. In both figures, the boundaries for lOdB bands are illustrated with bolded lines. We can observe that the dimension of the focusing zone is slightly reduced by increasing the signal bandwidth. 0

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Fig.4 Normalized to peak of spatial focusing, k(r) in dB. BW=1GHz, fL=0.7GHz

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2 -3 April 2007.

2007 Loughborough Antennas and Propagation Conference

Loughborough, UK.

Fig.6 shows the case that the signal bandwidth is fixed to 3GHz and the lower frequency (fL) is increased to 2GHz. As shown, the peak power at the center of the domain is more rapidly decays from the maximum. The dimension of the focusing zone compared to Fig.5 is greatly reduced. Therefore, we could conclude that the lower frequency of the signal is the most dominant factor for characterizing the dimension of the focusing zone. 400

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Fig.5 Normalized to peak of spatial focusing, k(r) in dB. BW=3GHz, fL=0.7GHz

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Fig.6 Normalized to peak of spatial focusing, k(r) in dB. BW=3GHz, fL=2GHz

V.

Conclusion

The feasibility of time reversal for broadband communication is predicted using the measured frequency domain data in the reverberation chamber. The spatial focusing and the time compression property are illustrated. The signal bandwidth effect on the peak power of the TR system is computed. It is shown that, 500MHz reduction in the signal bandwidth cause to 1dB loss at the peak power. The size of the spatial focusing zone versus signal bandwidth and the lower frequency band are computed. It is shown that, the dimension of the focusing zone is governed by the lower band of the signal.

References

[1] C. Oestges, J. Hansen, M. Emami, A. Paulraj, and G. Papanicolaou, "Time reversal techniques for broadband wireless communications" in European Microwave Week, Oct. 2004

[2] Abiodun E. Akogun, Robert C.Qiu, Nan Guo "Demonstrating Time Reversal in Ultra-wideband Communications Using Time Domain Measurements" 51st International Instrumentation Symposium, 8-12 May 2005, Knoxville, Tennessee

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