Device to device communication [
1,
2] is an important technology which enables data flow not only between humans but also between machines without human intervention. It can be used underlying the available cellular networks. The 5th generation system technology, 3rd Generation Partnership Project Release 15, will have to support high performance in spectral efficiency and throughput measurements. The 5th generation network is one of the most suitable environments for device to device communication since it is an IP-based network that enables to control any connected devices using internet protocols. Moreover, it is able to send large amounts of data with a high rate and low latency and support a large amount of connected devices. It is a good solution to reduce the eNB traffic load and the end to end delay. In order to develop a reliable wireless device to device communication network [
3,
4], an accurate description of the wireless channel impulse response measurements should be presented. The channel impulse response describes spreading, echoing, multipath propagation, and Doppler effects that occur when an impulse is sent between the transmitter and the receiver. Knowledge of the channel impulse response characteristics enables system designers to ensure that inter symbol interference does not dominate and hence lead to an excessive irreducible bit error ratio [
5].
1.1 Literature review
As mentioned above, propagation measurements are necessary for creating statistical channel models that support the development of new standards and technologies for wireless communications systems. Channel models that predict signal strength and multipath time delays are required for a proper system design. There have been a number of studies for channel sounding using different input signals over the past 10 years.
As a sample of typical work, there is a paper which studied the frequency dependence of the channel characteristics at the 2–4 GHz frequency band [
6]. Line of sight and obstructed line of sight scenarios were considered. Angle of arrival and delay of arrival of the main paths were investigated. A rich multipath environment was observed, with intensive path components existence in both angle and delay domains.
Outdoor measurements were conducted in an open-area test site at the National Metrology Institute of Germany [
7], to study the scattering effects of a traffic sign on vehicles moving along the road. The outputs are analytical modeling, simulation, measurement, and implementation of the bi-static radar cross section of the traffic signs.
A paper on outdoor sounding [
8] highlighted the propagation path loss models for 5th generation urban micro and macro cellular scenarios. It compares the alpha-beta-gamma and the close-in free space reference distance models. A wide range of frequencies 2–73.5 GHz over 5–1429 m distances were used. The output showed very comparable modeling performance between close-in and alpha-beta-gamma models. The close-in model offers simplicity and a conservative non-line of sight path loss estimate at large distances, whereas the alpha-beta-gamma model is more complex and offers a fraction of a decibel smaller shadow, less loss near the transmitter, and more loss far from transmitter.
Another paper [
9] described the achieved results for line of sight and non-line of sight measurements between the User Equipment and the base station in Nanjing Road, Shanghai. The received signals were 20 MHz bandwidth with 2.1376 GHz carrier frequency. The delays and the complex attenuations of multipath components have been estimated by applying the space-alternating generalized expectation-maximization algorithm. The distance between transmitter and receiver in line of sight/non-line of sight scenarios, the life-distance of the line of sight channel, the power variation at line of sight to non-line of sight transition, and the transition duration were extracted.
The authors in [
10] presented a sounding system that uses an orthogonal frequency division multiplexing signal at 5.6 GHz with 200 MHz bandwidth. The power delay profiles and the excess delay were presented.
An open-pit mine campaign performed a 25-MHz wide frequency band sounding immediately below the unlicensed 2.4-GHz ISM band [
11]. A continuously repeating maximum-length or m-sequence with
K=2047 sequence length was adopted as a transmitted signal. It was transmitted at a rate of 25 MS/s. Four measurement realizations of the impulse response with different transmitter-receiver separations that vary between 425–1670 m were recorded. The calculated delay spread of the channel was often more than 10
μs.
A channel measurement campaign was conducted to study the frequency dependence of the propagation channel for a wide range of frequencies 3–18 GHz [
12]. Urban macro and micro cellular environments were covered. The root mean square delay spreads, coherence bandwidth, path loss, shadow fading, and Ricean factor were characterized. It is mentioned that the path loss exponents vary significantly with frequency (from 1.8 to 2 dB in a line of sight environment and from 2.71 to 4.34 dB in non-line of sight). Shadow fading and the Ricean factor increase with frequency, whereas the root mean square delay spread values decrease with frequency in a line of sight environment. However, the root mean square delay spread in a non-line of sight environment and the coherence bandwidth values in both line of sight and non-line of sight environments do not show significant changes.
An outdoor wideband channel sounding at 2.4 GHz is described in [
13]. The distance between the transmitter and the receiver varied from 50 to 150 m. The distance-power gradient is 2.532, path loss (with 9 dB standard deviation), small-scale or multipath fading (with 5 dB standard deviation) are reported. The maximum observed multipath fade is 28 dB.
Another campaign was conducted in Seoul [
14]. The measurements were done using a wideband channel sounder at 3.7 GHz with a 100 MHz bandwidth. Both line of sight and non-line of sight environments are investigated. The output was presented as a spatial correlation coefficient of low-height links in an urban environment.
A wideband propagation channel at 2.45 and 5.2 GHz was presented in [
15]. Channel characteristics as power delay profile, the mean delay, and the delay spread were studied. It was mentioned that the parameters are frequency-independent, whereas the higher frequency signal shows considerably larger path loss than the lower one. Both the correlator-based and recursive Bayesian filter-based ranging estimators were evaluated; both of them provide better performances at 2.45 GHz compared with 5.2 GHz. The performance difference increases with decreasing the received power.
Urban macro environment was investigated in [
16]. Wideband multiple-input multiple-output measurements around 800 MHz with 50 MHz bandwidth were presented. The antennas with 360
∘ of azimuth and 90
∘ of elevation were used for the transmitter and the receiver. The output report contains path loss (path loss exponent
n=3), shadow fading (with 8.4 dB standard deviation), delay spread (with 123 ns mean value and 73.2 ns standard deviation), angular spread (with 30.8
∘ mean value and 12.5
∘ standard deviation for angular spread of departure and 66.9
∘ mean value and 15.1
∘ standard deviation for angular spread of arrival), and Ricean K-factor (with 5 dB mean value and 6.7 dB standard deviation).
Measurement campaign [
17] at the center frequency of 2.35 GHz with 50 MHz bandwidth was conducted in order to evaluate the performance in an outdoor propagation environment. Signal to noise ratio, spatial diversity, and capacity of different transmission schemes (direct transmission, amplify and forward, and decode and forward relaying) were investigated. Both line of sight and non-line of sight scenarios were involved. The results were depicted in terms of signal to noise ratio, spatial diversity, and capacity. Both amplify and forward, and decode and forward schemes improve the Signal to Noise Ratio, whereas direct transmission improves the capacity in small distances of a line of sight environment. However, by increasing transmitter-receiver distance, the capacity provided by the decode and forward exceeds that provided by the direct transmission. The spatial diversity was also significantly improved by applying the decode and forward scheme. Most of the abovementioned papers depict indoor channels or even outdoor channels but only up to 2 km and with only vertical co-polarization. Therefore, we filled these gaps by considering both line of sight and macro non-line of sight scenarios over 1.3 GHz and 5.8 GHz frequencies with longer distances 2.089 km, 4.11 km, and 5.429 km and both vertical and horizontal co-polarization dependence of multipath propagation channel measurements.
1.2 Contribution of the paper
We analyzed the channel frequency response variation, the path loss, the root mean square delay spread, and the coherence bandwidth with all above mentioned scenarios. Our achieved results expand the achieved results in [
18] which were performed in an indoor environment. The main contributions of this paper are described in the following few points:
-
Test the ability of deploying a device to device communication underlay 5th generation network in a wideband long-range channel for both ultra high frequency and super high frequency bands as a part of 5th generation new radio frequency bands allocation [
19]
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For a microcell line of sight environment (with 315 m distance), we provided the channel frequency response variation, the path loss, and the root mean square delay spread distribution in the case of vertical and horizontal polarizations for both 1.3 GHz and 5.8 GHz center frequencies.
-
For macrocell non-line of sight environments (with 2.089, 4.11, and 5.429 km distances), in additional of all previous mentioned parameters, distance dependence of the path loss and the root mean square delay spread are analyzed. The root mean square delay spread dependence of the coherence bandwidth is also investigated.
This paper is organized as follows. The “
Materials and methods” section describes the channel measurement campaign of outdoor long-range environments and the sounder systems for ultra high frequency and super high frequency bands. The data processing procedure and channel characteristics calculation are also depicted. In the “
Results and discussion” section, the channel measurement results are captured for line of sight and non-line of sight outdoor environments with different polarization combinations. Based on channel measurement results, the root mean square delay spread, the path loss, the channel frequency response variation, and the coherence bandwidth are analyzed. The last section concludes the paper.