Keywords

1 Introduction

The ever-increasing demand for multi-gigabit wireless applications has posed enormous challenges for next generation wireless technologies. In particular, the unbalanced temporal and geographical variations of spectrum usage along with the rapid proliferation of bandwidth-hungry mobile applications have inspired a new promising technology to alleviate the pressure of scarce spectrum resources for 5G mobile broadband applications. This technology is known as millimeter wave (MMW) communication technology [1]. It is envisioned as a key technology for fulfilling the high data rate requirements of the 5G wireless networks. Figure 1 shows the broad overview of next generation 5G system challenges that have initiated the different research activities by industries, academia, and research organizations [2]. The idea behind MMW communication is to exploit the huge unexploited spectrum from 30 to 300 GHz spectrum with the corresponding wavelength ranging between 10 and 1 mm [3] as it provides number of benefits like [4, 5]

Fig. 1
figure 1

General overview of next generation 5G system challanges

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  • Availability of huge unexploited bandwidth that can provide high-speed links with throughput of ~10 Gbps.

  • Higher degree of spectrum sharing as compared with lower frequencies.

  • Smaller antenna size which can facilitate the fabrication of large antenna arrays over a small area like postage stamp.

  • Better spatial resolution as the small wavelength allows modest size antennas to have a small beam width.

  • Processors, memories along with other devices can be incorporated on a Monolithic Microwave Integrated Circuit (MMIC) chip and the received data can be quickly processed and stored.

However despite these attractive advantages, these high frequency bands have not been fully explored because at higher frequencies the signal generation, reception and propagation gets highly complex due to free space and atmospheric loss factors [6]. The distinguishing traits of MMW communications have a direct impact on the selection of antenna technologies, modulation, besides other technical issues like user dynamics and coexistence with other communication standards like Wi-Fi, LTE. Multiple approaches have been provided so far to overcome these technical difficulties so as to make MMW technology deployable in near future and enable a plethora of applications.

In the present study capacity of a MMW communication link is carried out for LOS and NLOS scenarios for different link parameters in a dense network scenario. The work is reported as technical paper and has been organized into number of sections. Section 1 gives the Introduction, Sect. 2 briefly summarizes the MMW propagation characteristics. Performance evaluation and mathematical analysis of the MMW communication link is carried out in Sect. 3. Section 4 concludes the paper.

2 MMW Propagation Characteristics

Attenuations and environmental losses in the MMW bands are much higher as compared to microwave frequencies, thus making 5G cellular network operations over these frequencies a much more challenging task. MMW link use LOS communication and these LOS communication links suffer from free space path loss which is given as [7]:

$$ {\text{L}}_{\text{p}} ,dB = 20\,log_{10} \left( {\frac{4\pi d}{\lambda }} \right), $$
(1)

where ‘\( \lambda \)’ is the wavelength of signal and ‘d’ is the distance between transmitter and receiver.

This loss is quite high for longer distances. However these links can be utilized for short distance communication. The loss can be further mitigated by the use of highly directional antenna system, relay and multipath routing [8,9,10]

2.1 Atmospheric Loss Factors

In atmosphere millimeter waves are absorbed/scattered by molecules of oxygen, water vapor, and other atmospheric constituents including the rain, fog and clouds. These losses are greater at certain frequencies, coinciding with the mechanical resonant frequencies of the gas molecules. This atmospheric loss dB is given by [11]:

$$ L_{atm} = - \alpha_{atm} \left( f \right) \cdot d, $$
(2)

where \( \alpha_{atm} \left( f \right) \) is the measured value of atmospheric attenuation as the function of frequency operation ‘f’ and ‘d’ as the separation between transmitter and receiver.

Figure 2 shows the free space path loss and Fig. 3 shows the atmospheric loss for dry atmospheric conditions. Of all atmospheric conditions, rain causes the most significant signal loss depending on the intensity of the rainfall. Oxygen, humidity and fog also incur losses but they can be neglected to certain limits of the millimeter wave link. The description of different atmospheric conditions and the losses incurred in dB/km are given in Table 1.

Fig. 2
figure 2

Free space path loss w.r.t. distance

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Fig. 3
figure 3

Atmospheric losses for dry air

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Table 1 Signal losses due to atmospheric conditions [12]
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2.2 Foliage Losses

At millimeter wave frequencies these losses become a critical propagation impairment and are non negligible. If f is the frequency of signal in MHz, R is the foliage depth then Foliage losses are given as [13]

$$ L_{foilage} = \frac{1}{5} f^{0.3} R^{0.6} $$
(3)

Equation 3 applies for the scenarios where R < 400 m. L foliage is measured in dB. The Foliage losses of the millimeter wave signals for different Foliage depths are plotted in Fig. 4.

Fig. 4
figure 4

Foilage loss as function of frequency

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3 Performance Evaluation of MMW Link

The use of high gain directional antennas is an option to compensate the high path loss of millimeter waves however this method may be feasible only for clear LOS conditions. In scenarios where clear LOS is not guaranteed the antenna arrays, multipath routing, use of repeaters become highly desirable [14, 15].

In this section performance evaluation of the MMW communication channel for specific data rates is carried out for both LOS and NLOS case scenarios.

3.1 Mathematical Analysis

Consider a MMW link scenario as shown in Fig. 5. If ‘f’ is the frequency of operation and ‘d’ be the range of communication link. Then SNR at output of the receiver is

Fig. 5
figure 5

MMW link in LOS and NLOS scenario

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$$ \begin{aligned} {\text{SNR}} = & {\text{P}}_{\text{Tx}} + {\text{G}}_{\text{Tx}} + {\text{G}}_{\text{Rx}} + {\text{G}}_{\text{LNA}} + {\text{G}}_{\text{BPF}} + {\text{G}}_{\text{TFN}} + {\text{G}}_{\text{RFN}} + {\text{G}}_{\text{amp}} + {\text{G}}_{\text{CSF}} \\ & {-}{\text{PL}}_{0} - {\text{PL}}\left( {\text{d}} \right) - {\text{N}}_{\text{Rx}} - {\text{M}}_{\text{SHAD}} , \\ \end{aligned} $$
(5)

where

\( \begin{aligned} & {\text{P}}_{\text{Tx }} = {\text{Transmitted}}\,{\text{Power}}{\kern 1pt} \left( {\text{dBm}} \right) \\ & {\text{PL}}_{0} {\kern 1pt} \left( {{\text{Path}}\,{\text{loss}}\,{\text{at}}\,1\,{\text{m}}} \right) = 20\,log_{10} \left( {\frac{4\pi d}{\lambda }} \right) \\ & {\text{G}}_{\text{Tx}} = {\text{Transmit}}\,{\text{antenna}}\,{\text{gain }}{\kern 1pt} \left( {\text{dBi}} \right) \\ & {\text{M}}_{\text{SHAD }} = {\text{Shadowing}}\,{\text{link}}\,{\text{margin }}{\kern 1pt} \left( {\text{dB}} \right) \\ & {\text{G}}_{\text{Rx}} = {\text{Receiver}}\,{\text{antenna}}\,{\text{gain}}{\kern 1pt} \left( {\text{dBi}} \right) \\ & {\text{N}}_{\text{Rx}} = {\text{Input}}\,{\text{noise}}\,{\text{level}}\,{\text{at}}\,{\text{receiver}} . \\ \end{aligned} \)

The gain due to the various components at the transmitter and the receiver of the link are mentioned in Table 2 [16].

Table 2 Gain of various transmitter and receiver components
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In the study the other path loss parameters have also been considered which as per IEEE 802.15.3c standard [17] are given below

Path loss exponent: \( {\text{n}} = \left\{ {\begin{array}{*{20}l} 2 \hfill & {LOS\,Case.} \hfill \\ {2.5} \hfill & {NLOS\,Case.} \hfill \\ \end{array} } \right. \)

Shadowing Link Margin: \( {\text{M}}_{\text{SHAD }} = \left\{ {\begin{array}{*{20}l} {1\,dB} \hfill & {LOS\,Case.} \hfill \\ {5\,dB} \hfill & {NLOS\,Case.} \hfill \\ \end{array} } \right. \)

Assuming the noise to be AWGN and using Eq. 5 the data carrying capacity of the MMW link can be easily calculated by Shannon’s Capacity formulae given by:

\( {\text{C}} = {\text{B log}}_{2} \left( {1 + {\text{SNR}}} \right), \) where B is the bandwidth of the communication link.

3.2 Simulation Results

The MMW link is simulated and analyzed for data carrying capacity. The parameters taken into account for the performance analysis of such system are: Range of the communication link (d), data rate, frequency of operation (f) and the bandwidth of the link (B). Figures 6 and 7 show the simulation results for LOS path scenario.

Fig. 6
figure 6

Capacity as function of range of link

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Fig. 7
figure 7

Capacity as function of link band width

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Figure 7 shows the data carrying capacity of the MMW communication link of 50 m as the function of bandwidth and it is observed from the figure that high gain antennas at the transmitter and the receiver are required to achieve the data rate of the order of Gbps for long distance communication applications even for LOS cases.

Similarly Figs. 8 and 9 show the simulation results in case NLOS Scenario. In either case the performance of the MMW channel is limited due to the system noise as well as the losses due to various atmospheric factors.

Fig. 8
figure 8

Capacity for different antenna gain (NLOS Case)

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Fig. 9
figure 9

Capacity for LOS and NLOS links

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As can be seen from the figures, in both LOS and NLOS Scenarios, the impact of noise as well as the losses due to various atmospheric factors limit the performance the MMW link however the antennas play a critical role in the enhancement of efficiency of such systems. It can also be observed that increase in the operating distance drastically decreases the capacity of the MMW link and this situation worsens in case of NLOS cases. Also it can be observed that Capacity increases as a function of the SNR or Bandwidth (B) or both; however as the operational distance of the link increases the SNR degrades due to the various atmospheric losses. Under such circumstances the capacity is improved with help of high gain directional antennas and hence highlighting the importance of such antenna configurations for very high data rate applications for next generation networks.

4 Conclusion

Millimeter wave (MMW) communication is a promising technology to cope up with the ever increasing demand for multi-gigabit wireless applications and in particular alleviate the problems of spectrum scarcity for 5G mobile communication applications. In this paper we broadly explore the MMW propagation characteristics and examine the effect of free space loss, atmospheric loss and foliage loss on the MMW communication channel. Performance evaluation of the MMW communication link is carried out for different data rates for both LOS and NLOS cases in a highly dense communication network scenario. In either case the performance of the MMW channel is limited due to the system noise as well as the losses due to various atmospheric factors. Since it is observed that the data carrying capacity is affected as a function of operational distance of the link, the use of high gain directional antennas becomes the first choice to overcome this issue. Hence the antennas play a critical role for improving the performance of such systems and therefore highlight the importance of such antenna configurations for very high data rate applications for next generation mobile networks.