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Chapter 2 Channel Measurement and simulation

2.1 Introduction Experimental and simulation techniques

The advantage of experimental methods: all system and channel parameters affecting the propagation of UWB signals are accounted for without preassumptions. (disadvantage: expensive, time consuming, and limited by the characteristics of available equipment, such as sensitivity, bandwidth, dispersion, and the attenuation of the connecting cables.)

Simulation are free from the limitations of experimental approaches, cost effective, and less time consuming. The accuracy of simulation results depends on the amount of details included in the simulation model. More details lead to more complex computer programs, and require more computational time.

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Narrowband wireless communication system and UWB In NB systems, the information signal modulates a very high

frequency sinusoidal carrier; thus, along each propagation path the signal suffers very little distortion because the system elements such as antennas, reflecting walls, diffracting objects in the channel, and so on, have essentially constant electromagnetic over the narrow bandwidth of the radiated signal. The only signal degradation is caused by multipath components.

In UWB systems, the signal may suffer significant distortion due to the transmitting/receiving antennas not meeting the necessary bandwidth requirements, and also due to the dispersive behavior of building materials in the propagation channel. Multipath components are also present in UWB channels. But UWB do not suffer fading due to the destructive interference of multipath components.

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2.2 Measurement Techniques

Time Domain & Frequency domain 2.2.1 Time Domain Measurement Techniques

Ideal dirac-delta excitation signal impulse response Very short duration pulses

Time domain channel measurementPulse generator,

digital sampling oscilloscope,

transmitting and receiving antennas,

triggering signal generator

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Problem: How to synchronize the transmitting and receiving sides?

A: use a sample of the radiated pulse captured by means of a small antenna situated close to the transmitting antenna to trigger the sampling oscilloscope.

B: use a triggering generator with two outputs to synchronize the transmitter and the receiver.

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Two categories of noise Narrowband noise: interference from nearby

narrowband systems eliminate ( FFTbandpass filter IFFT)

Wideband noise: random short pulses that are not repetitive reduce (averaging multiple acquisitions)

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Sampling and triggering issues Period T, N sampling points/T Problem: How to sampling and triggering with a low

frequency?

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2.2.2 Frequency Domain Measurement Techniques

Based on measurements at different frequency points using a sweep harmonic(谐波 ) generator

Chief advantage: improved measurement precision A complex transfer function value described by its magnitude and phase terms

inverse Fourier transform impulse response of the channel Vector network analyzer (VNA) & scalar network analyzer (SNA)

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Scalar frequency domain measurement1. only the magnitude of the transfer function is measured.2. both magnitude and phase information are required to enable the conversion of the frequency domain data to the time domain impulse response.3. retrieving the phase information from the magnitude (Hilbert Transform) becomes an integral part of this approach.

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Direct Measurement of magnitude and phase Obtain the complex frequency domain transfer function directly

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Direct Measurement of magnitude and phase (continued) Long coaxial cables cable losses increase “exponentially” Overcome this drawback: electro-optic transmission system

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2.3 Measurement results

Typical results for time domain measurements In modern laboratory and office USC (University of Southern California) group

used a quasi omni-directional diamond antenna VT (Virginia Tech) group used directive TEM

horn antennas, as well as omni-directional biconical antennas

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Fig. 2.9 Diagram of office building where propagation measurements of the USC group were performed. The concentric circles are centered on the transmitting antenna and are spaced at 1m intervals.

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Signal distortion introduced by different

antennas

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3points:

Distance?

Noise?

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Zoomed profile for multipath

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Typical results for frequency domain measurements Results are based on amplitude measurements and the use of the

Hilbert Transform for phase calculations. Over a frequency range of 100MHz to nearly 12GHz. Responses are characterized by multiple peaks and valleys formed

due to constructive and destructive interference of multipath components.

Signals for NLOS scenario vary more rapidly with frequency.

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Typical frequency domain responses(LOS, TEM horn antenna)

Typical frequency domain responses(NLOS, TEM horn antenna)

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2.4 Electromagnetic simulation

Simulation of transmitting and receiving antennas

Simulation of the UWB channel

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Comparison of Measured (Solid Line) and Simulated (Dashed Line)

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Bibliography

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