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## 电气工程代写|通讯系统作业代写communication system代考|Channel Modeling

The signal can propagate in either direct (Line of Sight-LOS) or indirect (Non-Line of Sight-NLOS) paths through wireless communication medium. To completely understand and study the channel, both LOS and NLOS paths must be considered [2]. $\mathrm{THz}$ frequency waves are extremely absorbed by the molecules in the atmosphere. Molecules in the atmosphere vibrate at certain frequencies and in those frequencies, the absorption is more. Hence the molecular absorption depends on the type of molecules present, the vibrating frequency, temperature, and pressure. High Resolution Transmission Database (HITRAN) is the open source which takes the composition of atmospheric gas as input and predicts the molecular absorption $\left(\alpha_{\text {mol }}\left(f, T_k, p\right)\right)$. For pure water vapor, the absorption coefficient as a function of wavelength is given in Fig. 1. Frequency starting from $0.1$ to $1.5 \mathrm{THz}$ is considered for this computation, and the wave numbers corresponding to this frequency start at $3.3 \mathrm{~cm}^{-1}$ and go up to $50 \mathrm{~cm}^{-1}$.

Absorption coefficient was computed for temperatures 293 degree Kelvin $\left(20^{\circ} \mathrm{C}\right)$ and $303\left(30^{\circ} \mathrm{C}\right)$ degree Kelvin and plotted in Fig. 1. From the graph, it is learnt that as temperature increases there is a small decline in the absorption coefficient. However, the peaking happens at same frequency for both temperatures. The peaks occur at frequencies different frequencies spread throughout the region.
The free space absorption of the electromagnetic wave is given by [2]
$$A_{\mathrm{abs}}^{\mathrm{dB}}(f, r)=\alpha_{\mathrm{mol}}\left(f, T_k, p\right) r 20 \log e$$
where ‘ $f$ ‘ is the frequency of the wave and ‘ $r$ ‘ is the propagating distance.
Expansion of EM wave while propagating produces loss known as spreading loss which is function of the distance traveled, frequency and is given by,
$$A_{\text {spread }}^{\mathrm{dB}}(f, r)=20 \log _{10} \frac{(4 \pi f r)}{c}$$
where $c=2.9979 \times 10^8 \mathrm{~m} / \mathrm{s}$ is the speed of light.

## 电气工程代写|通讯系统作业代写communication system代考|Modulation Schemes

To evaluate the performance of Terahertz channel, Binary Phase Shift Keying (BPSK) is used as a modulation scheme. In BPSK modulation technique, to encode each bit a phase shift is introduced over a fixed time interval. The performance of the high frequency channel for various distances is shown in Fig. 6. For small distances like $0.5,1$, and $2 \mathrm{~m}$, the error rate is low and the performance is acceptable. However, with distance due to the absorption attenuation increases. Even with high signal-to-noise ratio, for long distances, the noise floor dominates the signal, and hence, the error rate is more.

Frequency-dependent path loss of $\mathrm{THz}$ channel demands novel modulation schemes to be developed. From Fig. 7, it is observed that at some frequencies the transfer function is very low, and these frequencies are not suitable for data transmission. It is the best practice to send data over frequencies where the loss is minimum. The dips in the transfer function create transmission windows. The $\mathrm{THz}$ channel needs unique modulation schemes considering these constraints.

The frequency selective nature of the channel is considered for designing the modulation scheme for improving the performance of the system. The width of the high attenuation bands mainly depends on the distance and humidity in the air. Modulation scheme is designed such that these bands are excluded from data transmission. This type of modulation scheme is known as adaptive modulation scheme. Though the amount of data transmission is reduced with adaptive modulation scheme, the Bit Error Rate (BER) is improved with such modulation techniques [2]. Comparison of channel performance with and without adaptive modulation is shown in Fig. 6. Significant improvement is observed in the case of adaptive modulation scheme compared to non-adaptive technique. This improvement is observed in both 5 and $10 \mathrm{~m}$ propagation distance. In non-adaptive technique, high attenuation bands in the transfer function such as $0.5-0.6 \mathrm{THz}, 0.7-0.78,0.93-0.98$, and $1.09-1.2$ are not considered for data transmission.

# 通讯系统代考

## 电气工程代写|通讯系统作业代写communication system代考|Channel Modeling

$$A_{\mathrm{abs}}^{\mathrm{dB}}(f, r)=\alpha_{\mathrm{mol}}\left(f, T_k, p\right) r 20 \log e$$

$$A_{\text {spread }}^{\mathrm{dB}}(f, r)=20 \log _{10} \frac{(4 \pi f r)}{c}$$

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## MATLAB代写

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