This paper reports a research contribution on tropical outdoor channel characterization in 1–13 GHz band for 5G systems. This 1–13 GHz ultra-wideband (UWB) channel characterization is formulated with rain intensity as the most important variable, from 20 mm/h to 200 mm/h. Tropical rain will cause pulse broadening and distorts the transmitted symbols, so the probability of symbol errors will increase. In this research, the bit error rate (BER) performance evaluation is done using both matched filtering or correlator-based receivers. At no rain conditions, BER 10−6 will be attained at signal to noise ratio (SNR) 5 dB, but at rainfall intensity 200 mm/h, the BER will fall to 10−2 for matched filter and
Like other mobile communication systems, the applications of UWB-based 5G mobile systems, at a specific environment, requires thorough knowledge of the propagation characteristics in that environment. Until now, study of UWB propagation for 5G applications at outdoor environment is still limited. Many researchers are doing research on outdoor UWB channel characterizations at multipath effects and its path loss only [
The 1–13 GHz band itself is comprised of two 5G candidate spectrum category: 1–6 GHz as “below 6 GHz spectrum” and 6–13 as “above 6 GHz spectrum.” As 5G systems develop over time, the 1–6 GHz mobile spectrum bands will be valuable to allow the smooth migration from 4G LTE usage to 5G, while 6–13 GHz spectrum band is attractive in which the existing technology and architecture might be adapted to work in this range, which is closest to existing cellular frequencies. Therefore, this 1–13 GHz spectrum band is of specific interest as it might be able to employ existing cellular technologies with little additional development required. Moreover, the 1–13 GHz spectrum band is also covering the 3.1–10.6 GHz UWB channel which has been adopted for UWB outdoor communication applications at tropical areas.
Unfortunately, the influence of the tropical outdoor channel on the UWB-based 5G communication system performance has not quantitatively assessed in a comprehensive manner yet. In this case, the impulse response of end-to-end tropical outdoor channel will distort the pulse sent by UWB systems. Therefore, it is necessary to formulate the distortion effects of UWB signals by the atmosphere of tropical areas in terms of BER performance of UWB receiver systems.
To maintain the BER performance of UWB systems from outdoor channel distortions and the effects of UWB antenna imperfections, a mitigation technique is required to track the tropical outdoor channel adaptively. Therefore, there is high demand to develop an adaptive equalization algorithm for compensating the pulse distortion as a result of the phase response nonlinearity as well as the lack uniformity of magnitude response.
The purposes of this research are
Tropical outdoor UWB channel is defined as a transmission channel for outdoor UWB applications where, between transmitter (TX) and receiver (RX), there is an atmospheric medium. This atmosphere medium contains the O2, H2O, CO2, and other gases as well as hydrometeors such as rain, clouds, and fog. Figure
Tropical outdoor UWB channel model.
An UWB signal transmission in a tropical atmosphere layer will experience attenuation, phase shift, and the addition of delay time. Transfer function of tropical outdoor UWB channel as Figure with
The atmosphere medium consists of gases such as O2, H2O, CO2, and other gases as well as hydrometeors such as rain, clouds, and fog. In this study, the rain-filled medium is modeled by raindrops Mie scattering which are statistically distributed in size as Marshall Palmer distribution [
UWB system BER formulation is limited to UWB system with antipodal modulation and using matched filter and correlator-based receivers as [
The energy alteration per bit due to pulse distortions is to be accommodated in the BER equation, as well as the effect of the gain/loss due to the use of the matched filter receiver. Pulse distortion impacts on reducing the energy per bit of signal. Figure
Model of matched filter-based UWB receiver.
For antipodal modulation, the BER performance of matched filter-based UWB receiver is as (
As the matched filter-based receiver, the change of energy per bit due to pulse shape distortion also occurs in correlator-based receiver. Figure
Model of correlator-based UWB receiver.
For antipodal modulation, the BER performance of correlator-based UWB receiver is as (
In the previous sections, we have discussed the performance evaluation of UWB communications systems in dispersive outdoor tropical UWB channels with ideal antenna. This section presents a derivation of BER equation for tropical outdoor channel with realistic antennas. The influence of distortion by the antenna system is very dependent on the parameters
It is assumed that we use same tropical outdoor channel model as in previous section, but in this case, we used TX and RX realistic antennas with specific fidelity profile,
In this study, it is proposed an adaptive nonlinear phase equalizer for compensating the pulse distortion by combining allpass biquad IIR [
Block diagram of proposed adaptive nonlinear phase equalizer.
Allpass biquad IIR filter is used to compensate nonlinear phase response that comes from the tropical outdoor channel and the antenna. Meanwhile, to compensate the magnitude response, we use low-order FIR filter cascaded with the allpass biquad IIR filter. For measuring the instantaneous channel condition, channel estimator with its training pattern signal is used to calculate the channel transfer function periodically.
In this case, the end-to-end transmission channel is represented by convolving the tropical outdoor UWB channel with a dispersive UWB antenna system. The rainfall rate used in this simulation is
Pulse shapes in a tropical outdoor UWB channel.
Antenna used in this simulation is a Log Periodic antenna with wide bandwidth but has a dispersive impulse response. As in [
Log periodic antenna characteristics.
The numerical simulation results of the attenuation coefficient per km and phase coefficient per km [
Magnitude and phase response of tropical outdoor UWB channel at 1–13 GHz band.
A transfer function of a tropical outdoor UWB channel can then be calculated by combining the magnitude and phase responses as a function of frequency and rainfall intensity at 0, 20, 50, 100, 150, and 200 mm/h. And by using the inverse Fourier transform, we can determine the channel impulse response.
In this research, author used short range of tropical outdoor communication model for both 4-meter and 10-meter distance. The reason behind choosing these short range comes from the fact that our rain simulator facility as described on Section
Figure
Simulated impulse response of tropical outdoor UWB channel on
From Figure
In our second calculation of impulse response of tropical outdoor UWB channel, we assumed that the distance between the TX and RX antenna is 10 meters. By comparing the results of calculations on two different distances
Channel impulse response of tropical outdoor UWB on
At very high rainfall intensity (200 mm/h), the impulse response has amplitude shrinking, more broadening, and delay.
From the numerical results at a distance of
Figure
Tropical outdoor channel measurement setup.
This measurement setup consists of an array of water sprayers, vector network analyzer, rain gauge, and TX and RX antennas. The maximum range of our outdoor measurement which can be achieved is 10 meters with a variation of rainfall intensity from 0 to 200 mm/h. The device used in outdoor UWB channel measurement consists of the following: Vector Network Analyzer (VNA) 0–13 GHz Array of water sprayers as a tropical rain simulator that has intensity control to simulate different rainfall intensities: 0, 20, 50, 100, 150, and 200 mm/h Rain gauge, a device for measuring rainfall intensity A pair of 1–13 GHz UWB antennas as the photograph in Figure
Photograph of antenna used for UWB channel measurements.
Frequency and time domain characteristic of antenna used for UWB channel measurements.
Tropical outdoor channel measurements were performed using the frequency domain approach as
Measured impulse response before (a) and after (b) deconvolution and noise filtering at
Deconvolution process conducted on raw impulse response data is intended to eliminate the distortion effects of TX and RX antennas. To eliminate the noise from the raw data, we used simple filtering with a moving average filter. From Figure
In our second measurement of tropical outdoor UWB channel, we set the distance between the antenna TX and RX antenna at 10 meters. By comparing the measurement results of two different distances
Figure
Measured impulse response before (a) and after (b) deconvolution and noise filtering at
When we compare the results of numerical simulation and measurement results of the channel impulse responses as Figures
The curves in Figures
Simulated versus measured pulse broadening of tropical outdoor channel at
Simulated versus measured pulse time shifting of tropical outdoor channel at
Simulated versus measured amplitude of tropical outdoor channel at
The three curves confirmed that the range and variation of rainfall intensity impact on pulse broadening, the time shifted, and the amplitude reduction of impulse responses.
In this scenario, UWB-based 5G systems is assumed to have 500 MBps with antipodal modulation for outdoor applications at tropical areas. The UWB-based 5G system operates at 3.1–10.6 GHz for achieving 7.5 GHz with very low output power density.
Figure
BER performance curves of matched filter-based receiver with ideal antenna.
At 10−6 BER performance, the SNR requirements must be worth 5, 8, 12, 16, 17.5, and 18 dB for rainfall intensity conditions, respectively, 0, 20, 50, 100, 150, and 200 mm/h. In other words, for keeping 10−6 BER performance continuously in various conditions of rain, the required fading margin is minimum 13 dB.
Meanwhile, if the benchmark performance using
Here as in Figure
BER performance curves of correlator-based receiver with ideal antenna.
BER curves at Figure
The simulation results of BER performance evaluation of tropical outdoor channel with realistic antenna can be seen in Figures
BER performance curves matched filter-based receiver with realistic UWB antenna.
BER performance curves correlator-based receiver with realistic UWB antenna.
The effects of rainfall intensity and the dispersive antenna to a reduction in bitrate of UWB communication system is summarized as in Figure
Bitrate reduction of UWB-based 5G system due to tropical outdoor channel and the antenna effects.
From Figure
The proposed adaptive nonlinear phase equalizer is used for mitigating the distortions due to tropical outdoor channel and comes from dispersive antenna. The target performance criteria of UWB-based 5G system for outdoor applications are using antipodal modulation, operating frequency from 3.1 to 10.6 GHz, required SNR set to 10 dB for
In this scenario, we proposed an adaptive nonlinear phase equalizer based on allpass biquad IIR order 6 cascaded to FIR filter order 6. The magnitude and phase responses of allpass biquad IIR order 6 and its poles/zeros structure are shown in Figures
The magnitude and phase responses of proposed allpass IIR order 6.
Poles and zeros structure of proposed Allpass IIR order 6.
The curve as in Figure
BER performance of the matched filter-based receiver with phase compensation.
From this figure, we can see that improvement has occurred around 10 dB SNR compared to the BER performance of matched filter-based receiver without phase compensation. Thus an UWB system with a minimum SNR 10 dB and 5 dB fading margin can still be working well at BER 10−6.
In this case, phase compensation is done due to the contribution of rainfall intensity 120 mm/h, but also done on the nonlinearity phase response of antenna. Therefore, the application of phase equalization on matched filter-based receiver can ensure the UWB system works on the availability of 99.99% for the
As for rainfall above 120 mm/h, BER performance falls below the desired performance requirements. However, rainfall intensity above 120 mm/h has an opportunity which occurs <0.01% a year so that is not statistically significant.
The simulation of BER performance improvement by nonlinear phase compensator for correlator-based receiver can be seen in Figure
BER performance of the correlator-based receiver with phase compensation.
The BER curve as in Figure
However, for UWB systems with a minimum SNR and 10 dB fading margin of 5 dB, 10−6 BER performance cannot be maintained because the phase compensation is not enough to overcome the performance degradation by dispersive antenna and rainfall intensity 120 mm/h. Correlator-based receiver with phase compensation can only achieve BER performance 10−4. In other words, the correlator-based UWB receiver does not meet the performance specifications that have been required.
Several important conclusions produced in this research are as follows: The dynamics of tropical outdoor channel versus time is strongly influenced by the atmosphere, especially the rainfall components. At a very high rainfall intensity (200 mm/h), a tropical outdoor channel will have a large difference attenuation of low frequency components with the highest frequency component of the UWB band, that is, 9.5 dB/km. In addition, the frequency components of the UWB signal spectrum over the tropical outdoor channel will also shift in nonlinear phase by rain in its path components 0.3 Rad/km at the same rainfall. The results of numerical simulations, the channel measurement, and mathematical models show the suitability of tropical UWB channel distortion causing a widening of pulse duration, the main axis, and a shift amplitude reduction. In no rain conditions, a BER performance at 10−6 can be achieved with the SNR 5 dB, but at rainfall intensity at 200 mm/h, BER deteriorated to 10−2 for matched filter-based and for correlator-based falls to It can be known from the numerical simulation and measurement results of 1–13 GHz tropical outdoor UWB channel that the pulse distortion is caused by the nonlinearity of phase responses and the magnitude response ruggedness of antenna and outdoor tropical channel. A proposed adaptive nonlinear phase equalizer with allpass IIR order 6 or more cascaded with a low-order FIR structure (>6) can be used to compensate an accumulation of distortion by the transmission channel and the antenna. Simulation results show that the 500 MBps UWB-based 5G system performance using matched filter-based receiver at 10−6 BER can still be maintained for the
In the near future, we will perform field test of our 1–13 GHz
The author declares that he has no conflicts of interest.