Time-varying Communication Channel High Altitude.pdf

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Time-Varying Communication Channel High Altitude Platform Station Link Budget and Channel Modeling Xiaoyang Liu 1,2 , Hengyang Liu 1, *, Chao Liu 1 and Ya Luo 1 1 2

*

School of Computer Science and Engineering, Chongqing University of Technology, Chongqing 400054, China; [email protected] (X.L.); [email protected] (C.L.); [email protected] (Y.L.) College of Engineering, The University of Alabama, Tuscaloosa, AL 35401, USA Correspondence: [email protected]; Tel.: +86-23-6256-3072

Received: 14 August 2018; Accepted: 20 August 2018; Published: 22 August 2018







Abstract: Because of the high BER (Bit Error Rate), low time delay and low channel transmission efficiency of HAPS (High Altitude Platform Station) in the near space. The link budget of HAPS and channel model are proposed in this paper. According to the channel characteristic, the channel model is set up, combined with different CNR (Carrier Noise Ratio), elevation angle, coding situations of wireless communication link by using Hamming code, PSK (Pulse Shift Keying) and Golay code respectively, then the situations of link quality and BER are analyzed. The simulation results show that the established model of the link budget and channel are suitable for the theoretical analysis results. The elevation of the HAPS communication link is smaller while the BER is increasing. The case of channel in the coding is better than in the un-coded situation. When every bit power and thermal noise power spectral density is larger, the BER of the HAPS communication link is becoming smaller. Keywords: high altitude platform station; near space; link budget; communication performance

1. Introduction As an airspace that humans have not systematically developed and utilized, near space is an important operational space for the future air and space weaponry system, and it is also a new “strategic place” for the great powers [1,2]. Near space often refers to an airspace with a height of 20 to 100 km above the flying altitude and below the satellite’s orbit. The high-altitude platform station (HAPS) is a platform that is stuck in a space of 20 to 50 km in height and is stationary relative to a specific location on the earth [3,4]. HAPS can be seen as a communication system between a terrestrial communication system and a satellite communication system. The aim is to develop the potential benefits of a high space between land and space, improve communication capacity and spectrum utilization, and reduce system equipment cost and complexity. HAPS is smaller than the path fading of terrestrial communication systems. And has a smaller delay than the satellite communication system [5,6]. To improve observation and coverage, the construction of surveillance and communication systems on near space platform has received further attention. HAPS has a similar development history to satellite communications in the 1960s [7]. The international telecommunication union (ITU) describes HAPS as “a new technology with a promising application prospect that can subvert the development of the telecommunications industry.” It is the foundation of next-generation wireless communications and has the ability to take full advantage of wireless spectrum resources, the system has large user capacity, good communication quality, and low operational risk. And you can upgrade the communication load operation [8–10] at any time. Arnau et al. [11] studied the inter-frequency forwarding of space satellite communications. The signal-to-noise ratio of the signal quality of the spatial communication link is quantitatively Information 2018, 9, 210; doi:10.3390/info9090210

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analyzed under the conditions of multipath fading and rain attenuation. Ammari et al. [12] analyzed the HATS IMT-2000 system. Mathematical modeling and simulation analysis of ultra-high frequency (49 GHz) link characteristics were carried out. Jiang et al. [13] analyzed the uplink capacity limitation of near space and the method of expanding the uplink and downlink capacity. Wei [14] analyzed the relationship between Adaptive Modulation and Coding (AMC) and Automatic Repeat Request (ARQ) and Quality of Service (QoS) in the time domain. The time domain parameters such as modulation mode and coding mode can be dynamically adjusted according to the channel state. Perlaza et al. [15] combines spatial channel transmission characteristics. The AMC and the joint physical layer are studied based on the cross-layer optimization design of spatial information based on link adaptation. ARQ technology at the data link layer [16–18]. The authors of [19–23], for high-altitude platform CDMA communication systems, established a wireless link model in quasi-stationary state. The main impact of the swing on the communication system is analyzed. Literatures [24–26] studied the effect of horizontal movement of the platform on HAPS. The probability of handover between terminals in the cell caused by the horizontal movement of the platform is analyzed. The fixed beam-width coverage cell model is given in [27–30]. And analyzes the impact of platform rotation on the probability of switching between terminal cells. The rotation has the greatest influence on the outer honeycomb. The authors of [31–33] studied the state of platform motion (horizontal and vertical motion), changes in the ground coverage area. Alotaibi et al. [34] proposed a dynamic downlink transmission power of femtocells. Each femtocell adjusts its transmission power autonomously based on measured cost function unit. The transmission power level of femtocell is constrained by the rate of interference that femtocell produced to adjacent femtocells. Eirini et al. [35] addressed the problem of efficient power allocation in the uplink of two-tier closed-access femtocell. Networks and the operational effectiveness of the proposed approach is evaluated through modeling and simulation. Sheikhzadeh et al. [36] considered a downlink resource allocation in an Orthogonal Frequency Division Multiple Access (OFDMA) based heterogonous cellular network and proposed Non-Exclusive Spectrum Allocation scheme (NESA). Tsiropoulou et al. [37] tackled the problem of joint users’ uplink transmission power and data rate allocation in multi-service two-tier femtocell networks. Khamidehi et al. [38] investigated the optimal power allocation problem for a femtocell network where the utilized air interface is single carrier frequency division multiple access (SC-FDMA) and derived the optimal power allocation when we protect the data rate of macro users by imposing a temperature limit on the interference arising from femto users to the macro base station. Tsiropoulou et al. [39] proposed a novel utility-based game theoretic frame work towards simultaneously allocating users’ uplink transmission power and rate in multi-service two-tier open-access femtocell networks. These literatures did not consider the Hamming, PSK and Golay code mode. This paper introduced Hamming, PSK and Golay code into the HAPS wireless communication channel. This paper mainly combines spatial communication links under different carrier-to-noise ratio, elevation angle, and coding. The HAPS channel model of its near space is established and simulated. The remainder of this paper is organized as follows: In Section 2 we propose the system model. HAPS link budget and channel modeling are researched in Section 3. We discuss our findings by simulation analysis in Section 4, and conclude in Section 5. 2. System Model The HAPS-based air-ground integrated aeronautical telecommunication network architecture is shown in Figure 1. The HAPS network includes a set of HAPS communication nodes that can perform routing, forwarding, and traffic management, and microwave communication can be implemented between each HAPS node. The link problem between HAPS is a technical problem, and the geometric relationship of the communication system is shown in Figure 1.

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HAPS HAPS Network Network

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HAPS HAPS

HAPS HAPS

HAPS HAPS

Layer from ground network to HAPS network Layer from ground network to HAPS network

Ground Ground Network Network

File store File store

Data store Data store

Control Ntework Control Ntework

Figure 1. System frame diagram of the High Altitude Platform Station (HAPS). Figure 1. System frame diagram of the High Altitude Platform Station (HAPS). Figure 1. System frame diagram of the High Altitude Platform Station (HAPS).

Use a stable communication platform as a microwave relay station in the near-Earth space. a Use a stablesystem communication platform a microwave relay station inand the theanear-Earth near-Earth space. a communication with ground controlasequipment, entry equipment, variety of space. wireless communication system with equipment, equipment, and wireless communication system ground equipment, equipment, and variety offrom users. The high-altitude platform cancontrol be integrated with entry the satellite ground or aseparately the users. high-altitude platform can be integrated The high-altitude platform can be integrated with the satellite ground or separately from ground. Consider the communication platform as a synchronous communication satellite, that is, the ground. Consider Consider thecommunication communication platform a synchronous communication is, the platform asas aEarth's synchronous communication is, the communication platform is synchronized with the rotation. Can stay in thesatellite, airsatellite, for a that longthat time. the communication platform is synchronized with the Earth’s rotation. Can in stay air for atime. long communication platform is synchronized with the Earth's rotation. Can transmission stay theinairthe for a long High-altitude platform communication utilizes good radio wave characteristics, time. High-altitude platform communication utilizes good radio wave transmission characteristics, High-altitude platform communication utilizes good radio wave transmission characteristics, realizing communication connections between ground users, between platforms or between realizing connections between ground users,users, between platforms or between platforms realizing communication communication connections between ground platforms between platforms and satellites through platforms. It has the advantages of between flexible layout, wide or application, and satellites through platforms. It has the advantages of flexible layout, wide application, low cost, platforms and satellites through platforms. It has the advantages of flexible layout, wide application, low cost, safety and reliability. The schematic diagram of the HAPS space communication network safety and reliability. The schematic diagram of the HAPS space communication network is shown in low cost, in safety and2. reliability. The schematic diagram of the HAPS space communication network is shown Figure Figure 2. in Figure 2. is shown

HAPS Network HAPS Network

User A User A

L L

HAPS Network HAPS Network

User B User B

User A User A

(a) (a) Figure 2. Cont.

L L

User B User B

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S HAPS

HAPS

 min A

 SH S



H



O (b) Figure 2. Space network diagram and link geometry of HAPS. (a) Network diagram; Figure 2. Space network diagram and link geometry of HAPS. (a) Network diagram; (b) Geometry diagram. (b) Geometry diagram.

It can be seen from Figure 2 that HAPS is a highly dynamic space platform. The communication It can be seen fromtoFigure 2 that HAPS is a highly multipath dynamic space Theand communication between them is related the atmospheric environment, fading,platform. space flicker other factors. between them is related to the atmospheric environment, multipath fading, space and According to the geometric relationship of Figure 2, it can be calculated using theflicker following other factors. equation: According to the geometric relationship of Figure 2, it can be calculated using the sin(90 −  min −  ) sin(90 +  min ) following equation: = (1) + hH− δ) ++ hS ε min ) sin(90 −RεEmin sinR(E90 (1) = R + hH R E + hS where RE is the radius of earth. hH isE the altitude of HAPS, hS is the altitude from HAPS to earth where R Eis the radius of earth. h H is the altitude of HAPS, hS is the altitude from HAPS to earth ground. min is the pitch angle of the HAPS,  is the central angle of the HAPS, which can be seen ground. ε min is the pitch angle of the HAPS, δ is the central angle of the HAPS, which can be seen in in Figure 2b. According to the geometric relationship, the central angle  can be obtained through Figure 2b. According to the geometric relationship, the central angle δ can be obtained through the the following Equation (2). following Equation (2).    RRE E++  hHh H δ = 90=90 − ε−min arcsin cos (ε min (2)  min − − arcsin cos(  min ) ) (2) hShS  RRE E++ 

In In Equation Equation (2), (2), the the relevant relevant coefficients coefficients have have been been explained explainedin inEquation Equation(1). (1). The central angle β of the HAPS can be calculated by the following equation: The central angle  of the HAPS can be calculated by the following equation:   |SH  SH |  β= 2arcsin  =2 arcsin  2 ( R + h )  2( REE + hHH) 

(3) (3)

where distance between HAPS and and pointpoint H. R EHis.the earth. of h H earth. is the altitude of SHis the REradius hH is the where SH is the distance between HAPS is theofradius HAPS. The pitch angle ε of the HAPS can be described by the Equation (4). SH angle  altitude of HAPS. The pitch of the HAPS can be described by the Equation (4). SH

   1 R E + h H    1 cos β − R + h ε SH = arctan (4)  SH = arctan sin β  cos  − RE E +H hS (4) sin  R + h E S    As can be seen from Equation (4), the pitch angle ε SH is related to the central angle β, radius As can seenaltitude from Equation the distance pitch angle is related to the centralitangle  SHHAPS  , radius of earth R E , be HAPS h H and (4), hS (the from to earth ground), has a nonlinear RE , HAPS xaltitude of earth relationship. and at (thet. distance from HAPS to earth ground), it has a hStime H function (t) is the hdistance nonlinear function relationship. x (t ) is the distance at time t . x ( t ) = x ( t + ( i − 1) ∆ ) (5) x(t )=x(t +(i − 1)) (5) where i is the i -th slot time,  denotes the time step.

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3. HAPS Link Budget and Channel Channel Modeling 3. 3. HAPS HAPS Link Link Budget Budget and and Channel Modeling Modeling 3.1. HAPS Link Budget 3.1. HAPS Link Budget The main goal of the link budget is to ensure the availability of the HAPS communication communication link. link. The main goal of the link budget is to ensure the availability of the HAPS communication link. Considering the cost of space segments and ground station equipment, the HAPS link needs to be Considering the cost of space segments and ground station equipment, the HAPS link needs to be carefully designed. To fully optimize and save all available resources. The impairment of the HAPS carefully designed. To fully optimize and save all available resources. The impairment of the HAPS link performance evaluation can be represented represented by by the the link link diagram diagram shown shown in in Figure Figure3. 3. link performance evaluation can be represented by the link diagram shown in Figure 3.

si (t ) si (t )

so (t ) so (t )

PTX PTX Transmitter Transmitter

Compare Compare

GAT GAT

PEIRP PEIRP

lFS lFS

Pas =kTAR B Pas =kTAR B

GA GA

Detector Detector

GAT GAT

AWGN AWGN

extra receiver noise extra receiver noise

Px =kTe B Px =kTe B

Figure 3. Sketch map of HAPS link budget. Figure Figure 3. 3. Sketch map map of HAPS link link budget. budget.

Where, PEIRP indicates the equivalent isotropic radiated power of the transmitter, AWGN is Where, transmitter, AWGN AWGN is is Where, PPEIRP indicates the the equivalent equivalent isotropic isotropic radiated radiated power power of of the the transmitter, EIRP indicates Additive white Gaussian noise. lFS is the loss of free space, G AT indicates the gain of acceptance Additive white Gaussian noise. l FS is the the loss of free space, GGAT indicates the the gain gain of of acceptance acceptance Additive FS is AT indicates antenna. si (t ) expresses the input signal and so (t ) denotes the output signal. N is the thermal antenna. signal and so (ts)o (denotes the the output signal. N is the thermal noise antenna. sis(i t(t))expresses expressesthe theinput input signal and output signal. the thermal t ) denotes N is k is Boltzmann B is IF (Intermediate noise power, constant. Frequency) effective bandwidth. TAR power, k is Boltzmann constant. B is IF (Intermediate Frequency) effective bandwidth. T indicates AR noise power, k is Boltzmann constant. B is IF (Intermediate Frequency) effective bandwidth. TAR indicatesnoise antenna noise temperature, effective noise temperature of acceptance the overall e indicates antenna temperature, Te indicatesT effectivethe input noiseinput temperature of the overall indicates antenna noise temperature, indicates the effective input noise temperature of the overall Tthe e acceptance the transmit power, theuplink amplifier gain. The uplink P ispower, G denotes system, PTX system, is the transmit G A denotes the amplifier gain. The interference of the near acceptance system, PTX is the transmit power, GAA denotes the amplifier gain. The uplink TX interference the nearinspace HAPS space HAPS of is shown Figure 4. is shown in Figure 4. interference of the near space HAPS is shown in Figure 4. HAPS HAPS

 

 

j j

i i l l

k k

Figure 4. Uplink interference model of HAPS. Figure Figure 4. 4. Uplink interference interference model model of of HAPS. HAPS.

As can be seen from Figure 4, for different users i, j , k , etc., there is interference between them. , jj,, kk, ,etc., As can can be etc.,there thereisisinterference interference between between them. As be seen seen from fromFigure Figure4,4,for fordifferent differentusers users ii, them. The smaller the elevation angle is, the longer the distance from the HAPS to its users. The smaller smaller the the elevation elevation angle angle is, is, the the longer longer the the distance distance from fromthe theHAPS HAPSto toits itsusers. users. The The energy per bit Eb of the near space HAPS can be expressed as: The energy energy per per bit bit EE the near space HAPS can expressed The the near space HAPS can bebe expressed as:as: b bofof Eb = C Tb (6) CT Tb (6) b =C EbE= (6) b where C is the carrier wave average power. Tb indicates the time it takes to accept 1 bit(s). The where C is the carrier wave average power. Tb indicates the time it takes to accept 1 bit(s). The signal to noise ratio can be expressed as: signal to noise ratio can be expressed as:

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where C is the carrier wave average power. Tb indicates the time it takes to accept 1 bit(s). The signal to noise ratio can be expressed as: C E T E R = b b = b (7) N N0 B N0 B where R = 1/Tb is the bit rate (b/s). Tb indicates the spent time when transmit 1-bit energy. N0 denotes the thermal noise power spectral density, then Eb /N0 can be expressed as: Eb P g = EIRP AR N0 kTs R E l FS

(8)

where PEIRP represents the equivalent isotropic radiated power of the transmitter. l FS denotes the loss of free space. g AR indicates the antenna gain. k is the Boltzmann constant. Ts is the effective temperature in HAPS system. R E is the radius of earth. As for 2PSK or QPSK, bit error rate Pe and Eb /N0 have the following relationship: 1 Pe = 1 − er f c 2

s

Eb n0

! (9)

where er f c denotes the error function. For example, er f c( x ) = 1 − er f ( x ) =

R ∞ − t2 √1 e dt. π x

When the bit error rate Pe = 10−4 is required, the normalized ideal threshold SNR can be described by the following equation:     C Eb = + 10lgk + 10lgPb (10) N n0 where k is the Boltzmann constant. Pb is the power when receive 1 bit. The elevation angle of HAPS can be expressed as A avg (∆) = tg−1 q

cos(φ2 − φ1 ) − 0.5∆ 3 − 2[cos(φ2 − φ1 ) × cos δ]2

(11)

where δ is the central angle of the HAPS. φ2 , φ1 are the elevation angle of two different stations to the HAPS platform, respectively. In the HAPS wireless communication space, the change relationship of the elevation angle φ should be considered. The error rate Pe in the un-coded case can be expressed as: 1 Pe = p 2 3φ + 1

s 1 − er f c

Eb n0

! (12)

If PSK (8 phase shift keying) and Hamming code (15, 11) and Golay code (23, 12) are used to encode near space HAPS channels, the bit error rate Pe can be expressed as:

Pe =

          

q  1 − er f c En0b ; PSK 2 φ +1 q   √ 1 2 1 − er f c En0b ; Hamming 2 0.8φ +5 q   √ 21 1 − er f c En0b ; Golay

√ 12

2



(13)

φ +3φ+1

where Pe is the bit error rate, the value of Pe is different based on different code mode. φ is the elevation angle of HAPS.

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3.2. Channel Modeling For the given time slot (k, l ), the corresponding sub-channel hr,q [k, l ] can be arranged in a MR × MT matrix H[k, l ], its component is. [H[k, l ]]r,q = hr,q [k, l ] The I/O correspondence of MIMO is r[k, l ] = H[k, l ]s[k, l ] + w[k, l ]

(14)

where w[k, l ] is MR dimension noise vector for every time slot (k, l ). s[k, l ] is the input vector with MT dimension; r[k, l ] is the output vector with MR dimension. Limited average power and peak power the upper and lower boundaries of the MIMO channel capacity of Equation (15) can be expressed as: 1 E T

(

K −1 L −1

∑ ∑ ks[k, l ]k

) 2

≤ KP

(15)

k =0 l =0

where T denotes the entire transmission time period. E{·} is the mean value, P expresses the transmission power. K indicates the maximum number of slot time. s[k, l ] is the input vector: 1 P ks[k, l ]k2 ≤ ζ T L

(16)

where ζ is the adjustment factor. L is the maximum value of the slot time with MT dimension. Under the assumption that the input signal satisfies the average power limited Equation (16) and the peak power limited Equation (17), the non-coherent capacity of channel Equation (17) has an upper boundary of C ≤ U1mimo , and wireless communication channel capacity U1mimo can be denoted by the following equation: U1mimo

def

( B) =

M R −1 

sup

∑

0≤℘≤℘max r =0

℘ λr B ln(1 + ) − αψ( B) TF B

 (17)

where B represents the channel bandwidth. sup indicates the minimum upper bound of one set. For example, sup{x} is the minimum upper bound of the set x. ℘ denotes the jitter factor of HAPS platform. ℘max expresses the jitter maximum value. Without loss of generality, ψ( B) can be expressed as: def

ψr ( B) = ζ

B αmax

log2 (1 + C/N )

(18)

where C denotes the wireless communication signal power. N is the noise power. ζ is belong to [0, 1]. If related parameters are obtained, the value of ψr ( B) could be calculated. 3.3. Holistic Applicability Analysis 3.3.1. Analysis in the Femtocell In the process of practical application, clustering has also proved to be an effective method to reduce interference in the femtocell. This method is widely used in ad hoc wireless networks. However, if this method is applied to a femtocell network, it is first necessary to know the number of clusters. The shared registered spectrum band is then divided into m sub-bands, each of which uses a sub-band. Since there are different numbers of femtocells in each cluster, this means different rate requirements. Therefore, the bandwidth allocation of each sub-band will directly affect the performance of the femtocell network. If the number of clusters is large, the bandwidth allocated by each cluster is relatively small. This will not meet the minimum requirements of femtocells. However, when the bandwidth allocated by each cluster is relatively large, the interference between the femtocells cannot

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be effectively reduced. Therefore, a new method is needed to determine the number of clusters and the bandwidth allocated for each cluster. This will be the next step in this paper. 3.3.2. Impact on the Resource Management in HAPS Wireless Communication System The goal of wireless communication resource management is to provide service quality assurance for wireless user terminals in the network under limited bandwidth conditions. The basic starting point is that the network traffic is unevenly distributed, and the channel characteristics are fluctuating due to channel weakness and interference. It is necessary for flexible allocation and dynamic adjustment of the available resources of the wireless transmission, maximize wireless spectrum utilization and prevent network congestion and keep the signaling load as small as possible. One of the main features of the HAPS communication system is the existence of a large number of non-real-time packet data services. Because different users have different rates. The sum of all user rates in a base station tends to exceed the channel capacity that can be transmitted by the base station’s own frequency band. Therefore, it is necessary to have a scheduler in the base station according to the user QoS requirements. The channel resources will be allocated to different users according to the type of service. 4. Discussion The noise spectral density is normalized by P(1W/Hz) = 3.5 × 106 s−1 . Transmit power is 0.5 Mw. The level of receiver thermal noise is 200 dBm/Hz. Free space path loss is 10 m. The receiver noise coefficient is 15 dB, β is set by 1. The relationship between the transmit power of the HAPS and the antenna gain at different carrier-to-noise ratios is shown in Figure 5.

Figure 5. The transmitted power and antenna gain under different carrier noise ratio.

As can be seen from Figure 5, as the carrier-to-noise ratio C/N increases, the transmit power of the HAPS also increases proportionally. At the same time, the antenna gain is also increased with C/N. The duration simulation of the HAPS wireless communication link at different altitudes is shown in Figure 6.

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Figure link duration duration time. time. Figure 6. 6. Average Average link

As can be seen from Figure 6, the higher the HAPS space base station, the longer its average link As can be seen from Figure 6, the higher the HAPS space base station, the longer its average link duration. Here the HAPS height is increased from 20 km to 30 km. But the average link duration does duration. Here the HAPS heightFigure is increased from 20duration km to 30 km. But the average link duration 6. Average link time. not increase linearly, as  increases, the average link duration also increases with a non-linear does not increase linearly, as ∆ increases, the average link duration also increases with a non-linear growth trend. growth As trend. can be seen from Figure 6, the higher the HAPS space base station, the longer its average link It canHere be seen that the higher the the higher its elevation duration. the from HAPS height7 is increased from 20HAPS km to space 30 km.base But station, the average link duration does 55 Figure angle, and as ∆ increases, the elevation angle also increases accordingly. Under the same ∆, the higher not increase linearly, as  increases, the average link duration also H increases with a non-linear =20 max the HAPS, the higher the elevation angle. growth trend. Hmax =25

50

Hmax =30

55

Hmax =20 Hmax =25

50

Hmax =30

A

avg

(°)

45

A

avg

(°)

40 45

35 40

30 0 35

10

20

30

40

50

60

70

80

90



Figure 7. Relationship between elevation angles at different HAPS altitudes. 30

0

10

20

30

40

50

60

70

80

90

It can be seen from Figure 7 that the higher the HAPS space base station, the higher its elevation  angle, and as  increases, the elevation angle also increases accordingly. Under the same  , the Figure 7. between elevation HAPS altitudes. altitudes. higher the HAPS, the higher the elevation angle. Figure 7. Relationship Relationship between elevation angles angles at at different different HAPS Whether the channel is encoded or not has a large influence on the bit error rate (BER), and its It can be seenatfrom FigureE7 that the higher the in HAPS space base station, the higher its elevation bit error rate different can behas shown Figure 8. on Pe channel Whether the is encoded not a large influence the bit error rate (BER), and its bit b / N 0 or angle, and as  increases, the elevation angle also increases accordingly. Under the same  , the error rate Pe at different Eb /N0 can be shown in Figure 8. higher the HAPS, the higher the elevation angle. Whether the channel is encoded or not has a large influence on the bit error rate (BER), and its bit error rate Pe at different Eb / N 0 can be shown in Figure 8.

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Figure 8. The BER (Bit Error Rate) under different encoding. Figure Figure8. 8.The TheBER BER(Bit (BitError ErrorRate) Rate)under underdifferent differentencoding. encoding.

can be be seen seen from from Figure Figure 88 that that when when the the value value of of EEb // NN 0 is is small, small, the the un-coded un-coded performance performance ItIt can It can be seen from Figure 8 that when the value of Ebb/N00 is small, the un-coded performance is is better better than than the the coded coded (8 (8 PSK) PSK) channel channel case; case; when when EEbb // NN00 is is greater greater than than 3.5, 3.5, the the coded coded channel channel has has is better than the coded (8 PSK) channel case; when Eb /N0 is greater than 3.5, the coded channel has a biterror error rate BER smaller than that of the the un-coded channel. Overall, theerror bit error error rate in the the aabit bit error rate BER smaller than that of un-coded channel. Overall, the bit in PPee coding rate BER smaller than that of the un-coded channel. Overall, the bit rate Prate e in the coding case is isthan lower than the un-coded. un-coded. coding lower than the case is case lower the un-coded. The influence of the elevation angle between the HAPS platform and the user in the near space The Theinfluence influenceof ofthe theelevation elevationangle anglebetween betweenthe theHAPS HAPSplatform platformand andthe theuser userin inthe thenear nearspace space on the transmission performance of the HAPS cannot be ignored. This is because the elevation angle on onthe thetransmission transmissionperformance performanceof ofthe theHAPS HAPScannot cannotbe beignored. ignored.This Thisis isbecause becauseifififthe theelevation elevationangle angle is small, the inclination path from the platform to the user will be large. This will increase free space is small, the inclination path from the platform to the user will be large. This will increase free space is small, the inclination path from the platform to the user will be large. This will increase free space loss and rain attenuation. Therefore, it will will affect affect the the HAPS HAPS link linkcommunication communicationperformance. performance.Figure Figure9 loss communication performance. Figure lossand andrain rainattenuation. attenuation. Therefore, Therefore, itit will affect simulatesthe theerror errorrate rateatat atdifferent differentelevation elevationangles anglesφ. .. 99simulates simulates the error rate different elevation angles

Figure 9. The BER (Bit Error Rate) under different elevation angle. Figure Figure9. 9.The TheBER BER(Bit (BitError ErrorRate) Rate)under underdifferent differentelevation elevationangle. angle.

As can can be be seen seen from from Figure Figure 9, 9, in in the the case case of of the the same same EE // NN00 value, value, the the larger larger the the elevation elevation As As can be seen from Figure 9, in the case of the same Eb /Nbb0 value, the larger the elevation angle angle  ,, the the smaller smaller the the bit bit error error rate. rate. When When the the elevation elevation angle angle ◦C is is 20°, 20°, PSK PSK (8 (8 phase phase shift shift keying) keying) angle φ, the smaller the bit error rate. When the elevation angle C is 20C , PSK (8 phase shift keying) is used is used used respectively, respectively, coding coding with with the the Golay Golay code code (23, (23, 12) 12) and and the the Hamming Hamming code code (15, (15, 11), 11), the the is simulation results results are are shown shown in in Figure Figure 10. 10. simulation

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respectively, coding with the Golay code (23, 12) and the Hamming code (15, 11), the simulation results are shown Figure Information 2018, 10, in x FOR PEER10. REVIEW 11 of 13

FigureFigure 10. The10. BER Error different PSK, Golay, Hamming. The(Bit BER (BitRate) Errorunder Rate) under different PSK, Golay, Hamming.

It can be seen from Figure 10 that when Eb / N0  (0, 4) , the bit error rate Pe of the PSK is lower It can be seen from Figure 10 that when Eb /N0 ∈ (0, 4), the bit error rate Pe of the PSK is lower than the Golay code and the Hamming code; when Eb / N0  (4,10) , the bit error rate of the Golay than the Golay code and the Hamming code; when Eb /N0 ∈ (4, 10), the bit error rate of the Golay code is lower than the PSK codes.codes. code is lower than theand PSKHamming and Hamming 5. Conclusions 5. Conclusions The characteristics of theofnear HAPSHAPS are analyzed underunder the fast channel. The characteristics the space near space are analyzed thetime-varying fast time-varying channel. The HAPS link budget mathematical model and wireless communication channel model are The HAPS link budget mathematical model and wireless communication channel model are proposed proposed respectively. error rate underdifferent differentHAPS HAPSelevation elevation angles, angles, signal respectively. TheThe bit bit error rate under signal to tonoise noiseratios ratios are are considered this paper. TheThe simulation results show that that the greater the signal-to-noise ratio, ratio, consideredinin this paper. simulation results show the greater the signal-to-noise the greater the gain power of the link.link. The The bit error raterate of the linklink channel the greater the and gaintransmission and transmission power ofHAPS the HAPS bit error of the channel in in thethe coding casecase is better thanthan thatthat of the one.one. The key issueissue to be to studied in thein the coding is better of un-coded the un-coded The technical key technical be studied next step to describe the time-varying network in terms of duration. Multi-source multi-suspension nextisstep is to describe the time-varying network in terms of duration. Multi-source multi-suspension capacity model of HAPS link and super-radius overlapping coverage scene of near space interference capacity model of HAPS link and super-radius overlapping coverage scene of near space interference domain capacity model. domain capacity model. Author Contributions: The authors performed the experiments and analyzed the results together. Introduction, Author Contributions: The authors performed the experiments and analyzed the results together. Introduction, methodology, cosmetic detection algorithms and proposed model were by written by X.L. while data methodology, cosmetic detection algorithms and proposed model were written Xiaoyang Liu and and H.L.; Hengyang collection, experimental results and conclusion sections were written by C.L. and Y.L. Liu; while data collection, experimental results and conclusion sections were written by Chao Liu and Ya Luo. Funding: This research was funded by National Planning Office of Philosophy and Social Science of China Funding: researchYoung was funded by National Planning of Philosophy and Science of China (No.This 17XXW004), Fund Project of Humanities andOffice Social Sciences Research of Social Ministry of Education of China (17XXW004), Young FundSocial Project of Humanities and of Social SciencesMunicipal Research Education of MinistryCommission of Education(No. of China (No. 16YJC860010), Science of Humanity Chongqing 17SKG144), Science and Technology Program of Chongqing Municipal Education Commission (No. KJ1600923, (16YJC860010), Social Science Research of Humanity of Chongqing Municipal Education Commission (17SKG144), No. KJ17092060, No.Research KJ1600928), 2018 Chongqing Science and Technology Commission Technology Innovation Science and Technology Program of Chongqing Municipal Education Commission (KJ1600923, and Application Demonstration Project (No. cstc2018jscx-msybX0049). Natural Science Foundation of China KJ17092060, KJ1600928), 2018 Chongqing Science and Technology Commission Technology Innovation and (No. 61503052, No. 61571069, No. 61501065, No. 61502064). Open Fund Project of Chongqing Technology and Application Demonstration Project (cstc2018jscx-msybX0049). Natural Science Foundation China (61503052, Business University, Research Center of Chongqing University Network Public Opinionofand Ideological Dynamic 61571069, 61502064). Open Fund Project of Chongqing Technology and Business University, Research of the (No.61501065, KFJJ2017024). China Postdoctoral Science Foundation (No. 2017M612911), Research Foundation Foundation of Chongqing CityPublic (No. cstc2016jcyjA0076). The author Xiaoyang Liu also thanks CenterNatural of Chongqing University Network Opinion and Ideological Dynamic (KFJJ2017024). Chinafor the financial support from CSC(2017M612911), (China Scholarship Council, No. 201608505142). Postdoctoral Science Foundation Research Foundation of the Natural Foundation of Chongqing City (cstc2016jcyjA0076). author Xiaoyang also thanks for the financial CSC (China Acknowledgments: The The authors are gratefulLiu to editors and anonymous refereessupport for their from very valuable comments and suggestions, which have significantly helped improve the quality of this paper. Scholarship Council, No.201608505142). Conflicts of Interest: The authors declare no interest. Acknowledgments: The authors are grateful to conflict editors ofand anonymous referees for their very valuable comments and suggestions, which have significantly helped improve the quality of this paper.

Conflicts of Interest: The authors declare no conflict of interest.

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