Zahra Nazari Chaleshtori

Andrew Burton
email: andrew2.burton]@northumbria.ac.uk

Stanislav Zvanovec

Zabih Ghassemlooy

Petr Chvojka
email: petr.chvojka]@fel.cvut.cz@[z.ghassemlooy

Comprehensive Optical and Electrical Characterization and Evaluation of OLEDs for VLC

Keywords: organic LEDs, radiation pattern, spectrum, visible light communications

Introduction

Visible light communications (VLC) is seen as a viable complementary technology to the radio frequency (RF) wireless communications in mostly indoor environments to meet the growing demands for high-speed wireless data transmission [START_REF] Ghassemlooy | Visible Light Communications: Theory and Applications[END_REF][START_REF] Ghassemlooy | Optical wireless communications: system and channel modelling with Matlab®[END_REF]. VLC has the advantages of high energy efficiency (i.e., a green technology), no RF electromagnetic interference, license-free, and has inherent security and privacy compared with the RF technologies [START_REF] Pathak | Visible light communication, networking, and sensing: A survey, potential and challenges[END_REF]. In VLCs, both conventional gallium-based light emitting diodes (LEDs) and organic LEDs (OLEDs) as well as white laser diodes are being used as a light source [START_REF] Ghassemlooy | Visible Light Communications: Theory and Applications[END_REF][START_REF] Haigh | Organic visible light communications: Recent progress[END_REF]. The gallium-based LED based VLC systems, which utilize blue light to excite yellowish phosphors to synthesize white light, have been extensively investigated in the literature [START_REF] Ghassemlooy | Visible Light Communications: Theory and Applications[END_REF][START_REF] Binh | High-speed visible light communications using ZnSe-based white light emitting diode[END_REF]. Whereas the red, green and blue (RGB) and phosphor laser diodes (LDs) based VLC require higher thermal stability of the phosphor due to a much greater optical power density [START_REF] Yang | Highly uniform white light-based visible light communication using red, green, and blue laser diodes[END_REF]. Compared with the phosphor-based LD, the RGB LD is safer to the human eye due to the low illumination level blue light component [START_REF] Wu | Tricolor R/G/B laser diode based eye-safe white lighting communication beyond 8 Gbit/s[END_REF].

OLEDs have interesting features over conventional and mainstream solid-state lighting and flat panel displays such as energy efficiency (i.e., they are environmentally friendly), brightness with no need for backlight as in LCD, sunlight style color-temperature tenability, very high color rendering index, small total stack thickness of an OLED being between 100-500 nm [START_REF] Haigh | Visible light communications using organic light emitting diodes[END_REF] and flexibility (i.e., can be used fabricated on plastics substrates or used in wearable clothes) [START_REF] Haigh | Visible light communications using organic light emitting diodes[END_REF][START_REF] Haigh | Using equalizers to increase data rates in organic photonic devices for visible light communications systems[END_REF][START_REF] Ràfols-Ribé | High-performance organic light-emitting diodes comprising ultrastable glass layers[END_REF][START_REF] Clark | Organic photonics for communications[END_REF].

In addition, OLEDs with large photoactive areas are being used as pixels in smartphones, TVs and wearable devices, which offers the potential of infrastructure-to-device (I2D) and device-todevice (D2D) communications [START_REF] Luo | Undersampled-based modulation schemes for optical camera communications[END_REF]. The latter is performed by transmitting and receiving the information data via the smartphone's OLED-based display pixels [START_REF] Li | A VLC smartphone camera based indoor positioning system[END_REF][START_REF] Boubezari | Smartphone camera based visible light communication[END_REF] and the built-in cameras [START_REF] Lin | An indoor visible light positioning system based on optical camera communications[END_REF][START_REF] Wang | Light positioning: A high-accuracy visible light indoor positioning system based on attitude identification and propagation model[END_REF].

OLEDs work in a similar manner to LEDs and use organic carbon-based molecules to generate electron-hole pairs but have different characteristics. There are two different types of OLED based on (i) small organic molecules deposited on a glass; and (ii) polymer (i.e., large plastic molecules) to produce light [START_REF] Burroughes | Light-emitting diodes based on conjugated polymers[END_REF][START_REF] Tang | Organic electroluminescent diodes[END_REF]. However, the modulation bandwidth Bmod of OLEDs is orders of magnitude smaller compared with inorganic LEDs (i.e., in the kHz range compared with MHz in inorganic LEDs). The bandwidth limitation is due to the carrier lifetime and the parasitic resistor-capacitor (RC) effects, thus limiting their use in medium-to high-speed data communications [START_REF] Kafafi | Organic electroluminescence[END_REF]. However, OLED properties (i.e., Bmod) have been improved by using new materials with higher charge mobility [START_REF] Zhu | Graphene radio frequency devices on flexible substrate[END_REF]. In addition, a number of advanced communications and signaling schemes as well as optimum driver circuits have been proposed to increase the transmission data rate [START_REF] Haigh | A 20-Mb/s VLC link with a polymer LED and a multilayer perceptron equalizer[END_REF][START_REF] Chen | A 51.6 Mb/s Experimental VLC System Using a Monochromic Organic LED[END_REF]. Future OLED applications will be in (i) medium to large panels for use in public places such as airports, shopping centers, train and bus stations, etc., [START_REF] Yamada | Latest Development of Soluble-OLED Material and its Application to Mid-to largesized Panel Production[END_REF][START_REF] Shin | Ultra-High-Image-Density, Large-Size Organic Light-Emitting Device Panels Based on Highly Reliable Gate Driver Circuits Integrated by Using InGaZnO Thin-Film Transistors[END_REF]; and (ii) flexible or flat panel display technology for use in wearable biomedical devices in hospitals [START_REF] Smith | Application of flexible flat panel display technology to wearable biomedical devices[END_REF], which provide visual display, data communications and indoor localization. The novel devices of nano-OLEDs and microfluidic OLEDs are promising opening up new applications [START_REF] Kasahara | New Era of Device Science[END_REF]. However, very little works have been reported on the optical and electrical characterization of different types of standard OLEDs used for illumination, which are essential when these devices are used in VLC. In this paper, we first experimentally investigate optical and electrical characteristics in terms of the threshold voltage, bias current, linear dynamic range, optical spectrum, optical radiation patterns and output optical power-current-voltage (L-I-V) of a number of rigid and flexible (or curved) OLEDs within the context of VLC systems.

Additionally, the characterization of organic devices is mostly limited to L-I-V or the frequency response measurements. In this work, the focus also is on other features of OLEDs (particularly large area flexible and rigid devices) such as dynamic resistance, linearity and radiation patterns, which are important in VLC, and compared them with the conventional inorganic sources. Large OLED panels compared with tiny OLEDs have lower modulation bandwidth, thus supporting a reduce level of throughputs in VLC [START_REF] Haigh | A 20-Mb/s VLC link with a polymer LED and a multilayer perceptron equalizer[END_REF][START_REF] Chen | A 51.6 Mb/s Experimental VLC System Using a Monochromic Organic LED[END_REF]. Therefore, more research utilizing large OLEDs with much lower bandwidth needs to be done. A number of schemes, including multi-carrier and multi-level modulation schemes, have been proposed to increase the data throughput. Here, we demonstrate the use of large size OLEDs as a transmitter in VLC systems employing a multiband carrier-less amplitude and phase (m-CAP) modulation, which offers similar spectrum efficiency as the orthogonal frequency division multiplexing (OFDM) but at much reduced implementation complexity. Hence, we evaluate the system performance in terms of the measured bit error rate (BER).

The rest of the paper is organized as follows. In Section 2, the structure of a typical OLED is described. In Section 3, the characterization of OLEDs is given followed by the experimental investigation of OLED-based VLC link in Section 4, and finally, conclusions are drawn in Section 5.

The Structure of OLEDs

The principal material in an organic semiconductor is either carbon or nitrogen [START_REF] Kalinowski | Organic Light-Emitting Diodes: Principles, Characteristics & Processes[END_REF]. The organic materials can be long-chain polymers (i.e., PLEDs) or small organic molecules (i.e., SMOLEDs) in a crystalline phase [START_REF] Kafafi | Organic electroluminescence[END_REF][START_REF] Kalinowski | Organic Light-Emitting Diodes: Principles, Characteristics & Processes[END_REF]. The organic devices are based on the thin-film technology (see Fig. 1), where the general structure consists of two or more organic semiconductor materials sandwiched between oppositely polarized electrodes. OLEDs have a low-pass filter transfer function with the cut-off frequency given by [START_REF] Deng | Nonlinear modulation characteristics of white LEDs in visible light communications[END_REF]:

3-dB 1 2 ( ) s c f      , ( 1
)
where s is the differential carrier lifetime, which is inversely proportional to the drive current [START_REF] Deng | Nonlinear modulation characteristics of white LEDs in visible light communications[END_REF]. c ~ RC, where R is the effective resistance of the OLED and C is the plate capacitance, which is defined as [START_REF] Ghassemlooy | Visible Light Communications: Theory and Applications[END_REF]:

0 r A C d   , ( 2
)
where A is the OLED photoactive area, d is the OLED thickness, and  0 and  𝑟 are the permittivity of free space and relative dielectric constant of the organic layer, respectively.

Note that, as in LEDs, Bmod of OLEDs is inversely proportional to A, hence much lower bandwidth than small area gallium-based LEDs [START_REF] Haigh | Organic visible light communications: Recent progress[END_REF]. In addition, in highly bandlimited organic VLC systems the inter-symbol interference (ISI) leads to the significant BER degradation. A number of schemes have been proposed to overcome both lower Bmod and the ISI including:

high-level modulations [START_REF] Haigh | Wavelength-multiplexed polymer LEDs: Towards 55 Mb/s organic visible light communications[END_REF][START_REF] Le | 10 Mb/s visible light transmission system using a polymer light-emitting diode with orthogonal frequency division multiplexing[END_REF], equalization schemes such as the artificial neural network (ANN) [START_REF] Haigh | A 20-Mb/s VLC link with a polymer LED and a multilayer perceptron equalizer[END_REF][START_REF] Chen | A 51.6 Mb/s Experimental VLC System Using a Monochromic Organic LED[END_REF], specially designed receivers [START_REF] Burton | Optoelectronic Modelling, Circuit Design and Modulation for Polymer-Light Emitting Diodes for Visible Light Communication Systems[END_REF][START_REF] Burton | Smart receiver for visible light communications: Design and analysis[END_REF][START_REF] Zeng | Angle Diversity Receiver in LiFi Cellular Networks[END_REF][START_REF] Chen | An omnidirectional user equipment configuration to support mobility in LiFi networks[END_REF][START_REF] Soltani | Bidirectional Optical Spatial Modulation for Mobile Users: Toward a Practical Design for LiFi Systems[END_REF], single-input multiple-output (SIMO) or multiple-input multiple-output (MIMO) configuration [START_REF] Khanh | Investigation on MIMO OLED VLC System Performance[END_REF][START_REF] Haigh | A MIMO-ANN system for increasing data rates in organic visible light communications systems[END_REF], bit/power loading [START_REF] Chen | A 51.6 Mb/s Experimental VLC System Using a Monochromic Organic LED[END_REF][START_REF] Chen | A 1.9 Mbps OFDM-based all-organic visible light communication system[END_REF] and power pre-emphasis [START_REF] Le | 10 Mb/s visible light transmission system using a polymer light-emitting diode with orthogonal frequency division multiplexing[END_REF][START_REF] Haigh | Organic visible light communications: Methods to achieve 10 Mb/s[END_REF].

Characterization of OLEDs

Experimental Test-bed

To carry out comprehensive tests and measurements for characterization of the OLEDs, we have developed an experimental test-bed, as shown in Fig. see Fig. 3, were investigated in terms of their optical and electrical characteristics including the optical spectrum, L-I-V curves, optical radiation pattern and Bmod. All experiments were carried out under the same controlled environments (within a dark room) and for each set-up, five sets of measurements were carried to ensure repeatability and correctness. The main parameters of tested OLEDs are given in Table 1.

Table 1 The OLEDs under test.

OLED

Optical and Electrical Characterization 3.2.1 OLED's spectrum

To measure the spectrum profiles of OLEDs, a spectrometer with a cosine corrector capturing light over a 180° angle was used. The measured normalized optical spectrum (averaged over five sets of measurements) for a range of IB for D1 is depicted in Fig. 4

OLED L-I-V curves

The I-V curves of the OLED panels under test were measured using a source meter (Keithley SourceMeter Series 2400) and their illuminance was measured using a lux meter, where the distance between the OLED and the lux meter was fixed at 15 × the horizontal dimension of the OLED (as recommended by lux meter manufacturer). The measured L-I-V curves of the OLEDs are illustrated in Fig. 5 showing linear characteristics with sufficient dynamic ranges. Table 2 summarizes the measured maximum current IB-Max, threshold voltage Vth, range of IB in the linear part ∆𝐼, range of voltage in the linear part ∆𝑉 and slope of the V-I curve (i.e., inverse of the dynamic resistance for all OLEDs at IB). Note, with a wide linear range L-I range around IB higher signal levels can be used for intensity modulation of the OLED, thus higher signal to noise ratio and lower BER). Using linear regression curve-fitting, the plots in Fig. 5 show a highly linear L-I relationship. To compare the linearity of inorganic LEDs with OLEDs we have used root mean square error (RMSE) i.e., 𝑅𝑀𝑆𝐸 = √(∑ 𝑃 𝐼 -𝑃 mod ) 2

Optical radiation pattern

The optical radiation pattern describes the spatial intensity distribution of light emitted from the OLEDs, which is important, especially when analyzing the coverage and signal distribution in VLC links. The light intensity of LEDs defined in terms of the angle of irradiance  is given by

[1, 2]: 1 ( ) (0)cos ( ) 2 L m L m I I      , [ , ] 2 2      (3)
where I(0) is the center luminous intensity of an LED and mL is Lambertian order given as [START_REF] Ghassemlooy | Visible Light Communications: Theory and Applications[END_REF]: where 1/2 is the semi-angle at half illuminance.

1/ 2 ln(2) ln[cos( )] L m   , (4)
In order to empirically derive the beam patterns of rigid OLEDs and determine Lambertian order of emission, a lux meter was used to measure the luminance, as shown in Fig. 6

where 𝑟 ⃗ OLED and 𝑟 ⃗ Rx are the norm vectors of the OLED and the ORx, respectively, and dRx is a distance of OLED and ORx. The position of OLED and the ORx can be considered as (𝑟, 𝜑, x1)

and (𝑟′, 𝜑′, x2) in the cylindrical coordinate, respectively, where 𝑟 is the OLED curvature radius, 0 < 𝜑 < 180° and x1 refers to the OLED's width. Thus, we have:

                 . ( 6
)
To investigate the intensity profiles of flexible OLED, the device was bent with the different radius of curvatures r of 11 cm and 8 cm to have quadrature and half-circle light sources, as shown in Fig. 7 A numerical fitting method was used to estimate the radiation pattern parameters of flexible OLEDs. The 3-term Gaussian model provided the best fit to describe the radiation patterns of OLEDs, which is given by:

      2 1 / ( ) k k k q k a e I xp b c         , ( 7
)
where ak, bk, ck, are parameters estimated by the curve fitting tool, k is the order and q is the term of Gaussian model, which is considered to be 3 for the best match with the empirical data. The RMSE analysis has been carried out on the modelled and measured intensity profiles to assess the accuracy of the model. For the curved OLED, the RMSE values are 0.016 and 0.018 for r of 11 and 8 cm, respectively, which are less than the standard error limit of 0.05 [START_REF] Gorman | Selection of variables for fitting equations to data[END_REF]. The numerical fitting parameters are shown in Table 4 for OLEDs with r of 11 and 8 cm. Note, ak is the peak of the k th term of 3-term Gaussian (i.e., a1 ~ 1), and bk is the angular position of peak referred to the each Gaussian as b1 ~ 0. ck is the standard deviation of the k th term of the 3-term Gaussian with higher values representing a wider profile.

OLED bandwidth

To measure Bmod of the OLEDs, the devices were biased in the linear region of respective L-I curves, see Fig. 5. The measured frequency responses for D1-D5 over a range of IB are as shown in Fig. 8, where U is the peak-to-peak received voltage and U0 is the peak-to-peak voltage of the first sample. For comparison, the maximum and minimum bandwidth values as well as the difference between them (i.e., B) are given in Table 5. The results for the devices tested show that, Bmod increases with IB as in agreement with [START_REF] Ghassemlooy | Visible Light Communications: Theory and Applications[END_REF]. We also investigated the effect of bending the flexible OLED on Bmod and observed no changes in Bmod. This is because the cut-off frequency of OLED is defined by its physical parameters. This feature makes the OLED a perfect optical antenna, where the same SNR is maintained over a given transmission radius. 4 Experimental OVLC Link Results

Experimental test-bed for OVLC link with m-CAP

OLEDs with both high linearity and dynamic range can be used to support higher-order multilevel and multi-carrier modulation schemes. However, in this work to simply demonstrate the potential of the OLEDs as the transmitter in a VLC system, we have developed an experimental test-bed to assess the link performance. We have adopted m-CAP modulation scheme due to (i) reducing the effect of the highly bandlimited frequency response of OLEDs acting as a low-pass filter [START_REF] Haigh | Multi-band carrier-less amplitude and phase modulation for bandlimited visible light communications systems[END_REF][START_REF] Haigh | A multi-CAP visible-light communications system with 4.85-b/s/Hz spectral efficiency[END_REF][START_REF] Chvojka | On the m-CAP performance with different pulse shaping filters parameters for visible light communications[END_REF]; (ii) can be used as a multiuser scheme (e.g., personalized advertising) [START_REF] Merah | Experimental Multi-User Visible Light Communication Attocell Using Multiband Carrierless Amplitude and Phase Modulation[END_REF]; and

(iii) implementation simplicity compared with the OFDM.

A block diagram of the experimental m-CAP OVLC link is shown in Fig. 9. Firstly, m independent pseudo-random data streams dm(t) of length 12,000 bits (memory depth limitation of the AFG) are generated and mapped onto the M-QAM (quadrature amplitude modulation) constellation where M is the order of the QAM. Note, M and m are selected as 16 and 2, respectively, in this work. During the experiment, a sufficient number of bits were transmitted to allow the measurement of the BER at 10 -6 . The linearity of OLEDs and their high dynamic range offer the potential to choose a number of carriers. Following upsampling, the real and the imaginary parts of the signal ai and bi, respectively, are applied to the in-phase and quadrature pulse shaping transmit filters, whose impulse responses form a Hilbert pair (i.e., they are orthogonal in the time domain). The transmit filters are formed as a product of the square root raised cosine (SRRC) filter pulse shapes and the sine and cosine waves for the quadrature and inphase part of the signal, respectively. The carrier frequencies given by the transmit filters are set to 10 and 30 kHz for 1 st and 2 nd subcarriers (s1 and s2), respectively, in this work. The roll-off factor β used for the transmit pulse shapes is chosen as 0.15, given that the minimum bandwidth requirement is proportional to 1 + β. Note, higher β leads to more protection against ISI for consistency with the literature [START_REF] Olmedo | Multiband carrierless amplitude phase modulation for high capacity optical data links[END_REF]. The combined output from filters, i.e., m-CAP signal x(t), is applied to AFG and used via a driver for intensity modulation of the OLEDs. Following transmission over a short free space (up to 60 cm) line of sight (LoS) channel, the signal is detected using ORx Thorlabs PDA100A2. Subsequently, the output of ORx is captured using digital storage oscilloscope Keysight DSO9254A with the sampling frequency of 400 kS/s for further off-line data processing.

a i b i x(t) d m (t) PD OLED y(t) z(t) d' m (t) x' (t) × + h(t) n(t)
Fig. 9 The block diagram of the proposed OVLC system with m-CAP modulation. 50 cm and 30 and 50 cm for s1 and s2, respectively. At the FEC BER limit, the transmission path lengths for s1 are 36, 50, and ~60 cm for D4, D1,3 and D2,5, respectively, which are sufficient for D2D communications. In the case of s2, we observe a small decrease in the transmission spans by 2, 15, and 10 cm for D4, D1,3 and D2,5, respectively compared with s1. Although the path length of 60 cm was obtained from our experiment, even longer distances can be achieved using OLED panels made of materials with higher charge mobility giving higher Bmod [START_REF] Zhu | Graphene radio frequency devices on flexible substrate[END_REF][START_REF] Yoshida | 245 MHz bandwidth organic light-emitting diodes used in a gigabit optical wireless data link[END_REF] or larger panels with higher output optical power. To meet a given BER target and increase the transmission span, the same SNR at a receiver and thus higher output optical power are required. Therefore, organic devices with larger area (note decreased 3 dB bandwidth) or an array of OLEDs can be utilized to follow these requirements. For instance, an OLED panel with a luminous flux of ~3000 lm can support data transmission for distances up to 3 m.

For D2, the BER plots in polar formats against 𝛼 are shown in Fig. 11 for s1 and s2. Also shown for comparison is the plot for the FEC BER limit. Note, the path length is fixed at 30 cm (i.e., a BER < 10 -6 when 𝛼 = 0° see Fig. 10(a)). Note, the BER profiles display a symmetry about the origin (i.e., the ORx is facing the OLED at 𝛼 of 0 o ) offering improved performance over a wide tilting angle. To meet the FEC limit, D2 can operate with 𝛼 up to ±80° and ±70° for s1 and s2, respectively.

Conclusions and Future Outlook

In this paper, we carried out characterization for a range of fixed and flexible OLEDs in terms of their optical spectrum, power-current and illumination profiles. We showed that, OLEDs offer stable illumination profile regardless of the bias current and a highly linear powercurrent characteristic compared with the inorganic LEDs. We also showed that, the rigid OLEDs beam pattern closely matches Lambertian with mL = 1, whereas for curved OLED, the radiation pattern displays a symmetry, which is wider than Lambertian as for curved OLED with a curvature radius of 8 cm and a radiation angle of 75°. Based on the measured experimental data for the curved OLED, we showed a new expression for the OLED's beam pattern, which follows the 3-term Gaussian profile with RMSE value of less than a standard error limit of 0.05 to assess the accuracy of the model. In addition, we evaluated OLED-based VLC systems for low data rate transmissions as in D2D communications. We showed the BER results of tilting OLED displayed a symmetry about the origin, with larger size OLEDs showing improved BER (i.e., below the FEC limit) over a wider tilting angle (up to 80°, which is considerably large for D2D communications) and a longer transmission length (i.e., up to 60 cm).

2 .Fig. 1

 21 Fig.1The OLED structure.

Fig. 2

 2 Fig. 2 An experimental test-bed for characterization of OLEDs.

Fig. 3

 3 Fig. 3 Different OLEDs (D1 to D5) under test.

  (a) showing R, G and B components at the peak wavelengths of 613, 555 and 450 and 480 nm, respectively. OLEDs D1-D5 and an inorganic white LED (LUXEON cool white rebel star LED (5650K) sr-01) display broad-spectrum profiles with RGB components, see Fig.4(b). For the flexible OLED, the R component is at a slightly higher wavelength of 620 nm, whereas B and G components have lower intensities compared with the rigid OLEDs. This is attributed to the lower conversion efficiency of B and G materials in D5. Whereas, for the inorganic LED the dominant color is B.Next, we investigate the spectrum (i.e., the color) of the D1 under different dimming levels (i.e., 10 mA < IB < 300 mA) as shown in Fig.4(c). Note, the normalized intensity profiles are almost the same with low intensity variation of the peak intensities. Thus, indicating no significant changes in the color of OLEDs in contrast to the inorganic LEDs reported in[START_REF] Dyble | Impact of dimming white LEDs: Chromaticity shifts due to different dimming methods[END_REF].

  Fig. 4 (a) The optical spectrum of D1 are normalized to the maximum IB with peak wavelengths marked where the legend color scale represents IB, (b) all devices outputs and a gallium-based white LED at their corresponding maximum IB, and (c) the optical spectrum of D1 for a range of IB where each of the spectral responses were normalized to unity and then superimposed on top of each other.

Fig. 5

 5 Fig. 5 The L-I-V curves for OLEDs where V-I and L-I curves are associated to each device marked as D1 to D5.

  (a). As expected, the profiles are complete hemispheres close to Lambertian emitter with mL = 1 in contrast to the intensity profile of a COBLED with mL = 0.66 as shown in Fig. 6(b).

Fig. 6

 6 Fig. 6 The polar dimensional radiation patterns for: (a) rigid OLEDs for D1, D2, D3, and D4 and (b) a COBLED.

Fig. 7

 7 Fig. 7 (a) OLED panel bent in different curvature radius r of 11 and 8 cm and (b) two-dimensional intensity pattern.

Fig. 8

 8 Fig. 8 The measured Bmod of: (a) D1, 3 and (b) D2, 4, 5.

4. 2

 2 Experimental resultsIn this section, we evaluate the LoS OLED VLC link based on the BER for a range of transmission span 10 to 60 cm and the OLED tilt angles 𝛼 from -90 o to 90 o . The BER results versus the path length for the OLED VLC and for s1 and s2 are shown in Figs.10(a) and (b), respectively along with the 7% forward error correction (FEC) BER limit of 3.8×10 -3 . Examples of measured constellation diagrams are shown as insets for D2 with two distance d of 40 and

Fig. 10

 10 Fig. 10 The BER versus the path length for OLEDs with m-CAP for (a) s1 with the consolation diagrams for two distance of 40 and 50 cm for D2 and (b) s2 with the consolation diagrams for two distances of 30 and 50 cm for D2.

Fig. 11

 11 Fig. 11 The polar plot of BER for tilted OLED (D2) with m-CAP for s1 and s2.

Table 2

 2 The parameters of OLEDs under test

	OLED	IB-Max (mA)	𝑽 th (V) Slope ( 𝚫𝑰/𝚫𝑽) Dynamic resistance (Ω)
					( IB (mA))
	D1	300	4.6	0.263	3.8 (160)
	D2	800	4.8	0.400	2.5 (400)
	D3	350	4.8	0.225	4.4 (160)
	D4	100	5.0	0.083	12.0 (60)
	D5	300	7.0	0.033	4.3 (180)

Table 3

 3 The parameter of linearity of inorganic LEDs and OLEDs

	OLED	RMSE	Ga LED		RMSE
				R	G	B
	D1	2×10 -14	RGB	0.004	0.07	0.008
	D2	3×10 -14	RAGB	0.0036 0.0025 0.0032
	D3	1.1×10 -14	5 mm RGB	0.0016 0.0027 0.0047
	D4	1.3×10 -7	COBLED		0.5114
	D5	1.2×10 -14

Table 4 3

 4 -term Gaussian model parameter for spatial intensity distribution for curvature with a radius of 11and 8 cm

	k	1	2	3
		r = 11 cm
	ak	0.9878	0.3054	0.2875
	bk	-0.7595	58.1	-59.42
	ck	51.59	32.99	31.94
		r = 8 cm
	ak	0.9814	0.3733	0.2721
	bk	4.832	-63.31	70.73
	ck	60.17	42.31	36.66

Table 5

 5 Bandwidth of OLEDs

	B (kHz)

Device

Bmod-Min (kHz) (IB-Min (mA))

Bmod-Max (kHz) (IB-Max (mA))

  The regenerated electrical signal is given as y(t) = x′(t) ⊗ h(t) + n(t) where h(t) is the channel impulse response, the ⊗ symbol denotes convolution, and the noise n(t) is mainly due to the ambient light and in the form of shot noise. y(t) is resampled to transmitted signal by original sampling frequency prior to being applied to two time-reversed filters gI and gQ matched to the transmit filters. The combined filter output z(t) followed downsampling are applied to the M-QAM demapper to re-generate the estimates transmitted data d′m (t). All the key system parameters are shown in Table6.

					In-phase Tx	DC
	Input data	M-QAM mapper	Upsampling	Re{.} Im{.}	filter (f I ) Quadrature Tx	Driver	VLC Channel	TIA
					filter (f Q )
					In-phase Rx
	Output data	M-QAM demapper	Downsampling		filter (g I ) Quadrature Rx	Bit and frame synchronization
					filter (g Q )

Table 6 The system parameters OLED IB (mA) Bmod (kHz) luminous flux (lm) Area (cm 2 )

 6

	D1	160	28	58.5	74.5
	D2	450	30	115.0	196.0
	D3	160	32	52.0	100.0
	D4	60	54	19.4	29.2
	D5	180	34	68.4	100.0
	ORx	Parameter	Value
		Type of PD	Si-PIN
		Active area of PD	75.4 mm 2
		Bandwidth	1.4 MHz at a 10 dB gain
		Output voltage	0 to 10 V
		Noise of amplifier	195 µV (RMS)
		NEP	6.75×10 -12 (W/√Hz) at λ= 960 nm
		Responsivity	0.2 (A/W) at λ= 400 nm
			0.5 (A/W) at λ= 700 nm

Acknowledgments

This work is supported by the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement no 764461 (VISION) and the Czech Rep. funded project GACR 17-17538S.


Guillaume Bernard

Cyril Faucher
email: cyril.faucher@univ-lr.fr

Karell Bertet

Towards reconstruction of human trajectories in indoor environments

Keywords: trajectory, micro-localization, sampling

In this paper, we propose to study how to reconstruct the human trajectory in complex environments (indoor) such as museums that are often not equipped with WIFI or other monitoring techniques. We have set up a micro-localization infrastructure Bluetooth Low Energy. We propose to use a sampling method for finding position stops (point in a trajectory), because a deep learning method is not adapted to this case.

Introduction

The reconstruction of human trajectories in indoor environments is a major challenge for complex museums who want to know which are the most visited rooms or how long visitors stay in front of a work. This work also aims to contribute to the dynamic adaptation of scenarios such as interactions between a human and a robot (Nao) by providing information like spatio-temporal sequences [START_REF] Juniarta | Sequential Pattern Mining using FCA and Pattern Structures for Analyzing Visitor Trajectories in a Museum[END_REF]. In outdoor, the reconstruction of trajectories is facilitated by the use of GPS, but in indoor the GPS does not work and on the other hand most museums do not have WIFI. We have endeavored to reconstruct human trajectories using a device based on Bluetooth Low Energy (BLE) and more specifically the use of the iBeacon protocol. In the rest of the paper, we present micro-localization techniques for indoor environment before moving to the identification of visitor stops. Then we discuss experimentation that allowed to validate the principle of discretization by sampling. Finally we will conclude by giving some perspectives to this work.

2 Micro-localization techniques for indoor environments

Bluetooth signal

We choose to use badges (credit card size) that have a BLE chip that emits the iBeacon protocol. The people you want to follow are wearing this badge. The iBeacon protocol makes it possible to recover the Bluetooth signal strength (RSSI in dB) without the need for pairing between the signal transmitter and the receiver [START_REF] Zhu | RSSI-based Algorithm for Indoor Localization[END_REF][START_REF] Du | Indoor localization techniques for wireless sensor networks[END_REF].

Technical architecture for collecting Bluetooth signal

Identification of visitor stops

Identifying visitor stops is the first step in rebuilding the trajectory of people. So we set out to set up an algorithm for identifying stops based on a temporal sampling (1s, 2s and 5s). The collector returning the highest average of the RSSI collected on a sample is considered as a stopping position. A sub-sampling, with the values of RSSI allows to extract a position in most cases. A first software was created to propose positions by badge. Given the weaknesses of the localization signal intensity [START_REF] Thaljaoui | BLE localization using RSSI measurements and iRingLA[END_REF], the solution generates intermediate positions, when there is an ambiguity. For two signals picked up during a time undergoing the same variations, with an almost identical power average, it is not possible to determine whether the badge is closer to either of the collectors. Preliminary results suggest that the value of the RSSI depends on many environmental parameters: humidity levels in the air, movements of the human body, positioning relative to the collection device and presence of aqueous medium between the beacon and the badges. In fact, the badge wearer can be one meter from the collector A, and fifteen meters from the collector B. The identical intensities can be related to the environment: the badge wearer is with someone who is positioned between his badge and the collector, while nothing blocks the signal of the badge to the collector B. The classification of the human position could have been based on in-depth learning techniques and on-the-ground truth. These signal propagation problems prevent us from using these AI methods.

Experimentation

We conducted experiments to collect positions and initiate experimental work to determine a sampling value. The two figures below are from the same experiment with two different sampling values. The sampling is done in soon after the collection using an algorithm written in Python. Fig. 2 shows the result with a sampling at 1 second and Fig. 3 at 5 seconds. All that has been done so far has only one goal: to highlight the movements of visitors by locating their stops in front of collection devices. A lot of visualization is achievable with Kibana. Only the value of captured power (RSSI) by badge and by collection device interests us: we simply want to know if in a temporal graph, these stops are remarkable. One-second sampling shows the effect of noise on the signal: variations are important. Despite significant variations in intensity received, stops in front of collection devices are obvious. The intensity picked up near these sensors is higher than elsewhere. It forms peaks over several seconds, where the intensity is higher. These intensities apply for a single badge. By quickly analyzing this graph, we can identify the path of the badge in front of the devices: firstly number 14 (gray), then 11 (turquoise), 12 (red), 13 (light green), 16 (green), and 17 (orange).

Fig. 1

 1 Fig.1shows the architecture composed of collectors (Raspberry PI3 microcomputer) that are positioned in a building. They detect in real time badges nearby and record the power they capture (RSSI signal). The values are stored in a MongoDB database (via FileBeats and Logstash). Data are analyzed in real time by an algorithm in Python that calculates visitor stops and trajectories.

Fig. 1 .

 1 Fig. 1. Technical architecture for collecting Bluetooth signal.

Fig. 2 .

 2 Fig. 2. One-second sampling.

After a five-second sampling, the impact of the variations is much smaller. The stops are much better highlighted.

Conclusion and future works

To conclude we propose an infrastructure for collecting human position in indoor environments. A technology based on Bluetooth Low Energy (iBeacon) was chosen, this one is coupled with an ElasticSearch infrastructure. To determine human position stops, we use a discretization method focusing mainly on the average values collected for a temporal sampling. In future works, we would trying to use a graph of possible location in order to improve our algorithm and remove ambiguities.


M Selim Demir

Student Member, IEEE Hossien B Eldeeb

Fellow, IEEE Murat Uysal

M Selim

CoMP-Based Dynamic Handover for Vehicular VLC Networks

Keywords: Visible light communications, handover, 12 vehicular network, CoMP. 13

Visible light communication (VLC) has emerged as 1 a potential wireless connectivity solution for infrastructure-to-2 vehicle networks where street lights can be configured to serve 3 as access points. In this letter, we propose dynamic soft handover 4 algorithm based on coordinated multipoint (CoMP) transmission. 5 The proposed algorithm takes the rate of change in the received 6 power as an input and accordingly revises the handover margin

7 and time-to-trigger value without explicit information of the vehi-8 cle velocity. Our simulation results demonstrate that the proposed 9 algorithm outperforms conventional CoMP and hard handover 10 and maintains a stable signal quality regardless of vehicle velocity.

11

extensively in indoor VLC networks see e.g., [START_REF] Wang | Efficient vertical handover scheme for heterogeneous VLC-RF systems[END_REF]- [START_REF] Wu | Handover skipping for LiFi[END_REF] and references therein. Handover schemes proposed for indoor mobile VLC scenarios (i.e., optimized for pedestrian speeds) might be perhaps applicable for outdoor VLC systems if the vehicle velocity is sufficiently small. However, in general, the fast movement will seriously degrade the performance of such a system and make the system non-functional. This prompted researchers to investigate custom-design handover solutions for vehicular VLC networks [START_REF] Arunachalam | Visible light communication and radio network for vehicular environment[END_REF]- [START_REF] Zhu | Handover method in visible light communication between the moving vehicle and multiple LED streetlights[END_REF].

The need for efficient handover in vehicular VLC networks was emphasized in [START_REF] Arunachalam | Visible light communication and radio network for vehicular environment[END_REF], [START_REF] Torres-Zapata | Vehicular communications in tunnels using VLC[END_REF] without explicit details on the type of handover techniques. In [START_REF] Dang | Handover procedure and algorithm in vehicle to infrastructure visible light communication[END_REF], Dang and Yoo considered a I2V network where a number of consecutive street lights are grouped as a cell. Under the assumption of an on-board camera used as a receiver, they proposed an inter-cell handover technique. The handover is based on the estimated distance between the vehicle and each group of street lights and the required distance estimation is obtained through image processing techniques. In [START_REF] Zhu | Handover method in visible light communication between the moving vehicle and multiple LED streetlights[END_REF], N. Zhu et.al. considered a vehicular scenario where individual street lights serve as APs. They assumed both cases of overlapping and non-overlapping coverages and calculated received signal powers based on the Lambertian source model. Based on received signal strengths, they proposed a soft handover algorithm as a function of signal detection threshold and signal drop threshold. This handover scheme relies on the knowledge about the vehicle velocity and its implementation requires the estimation and feedback of velocity information.

In this letter, we propose dynamic soft handover algorithm based on coordinated multipoint (CoMP) transmission. CoMP transmission has been well investigated in the wireless communication literature and already standardized in the LTE-A [START_REF]3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Feasibility study for Further Advancements for E-UTRA (LTE-Advanced)[END_REF]. The vehicle is mainly served by an AP from which it gets the strongest signal, but in the case of CoMP, the vehicle is jointly served by two coordinating APs. Based on the rate of change in the received powers (related to the vehicle velocity), the proposed handover algorithm dynamically revises the handover margin and time-to-trigger value in handover decision. In our algorithm, the handover margin typically increases while time-to-trigger value decreases for high-speed vehicles. This enables CoMP transmission to start early and maintain better signal quality. On the other hand, for low-speed vehicles, the handover margin is automatically set low and time-to-trigger value increases. This prevents the occurrence of ping-pong handovers 1 and avoids unnecessary start of CoMP transmission in order to conserve the system resources. We evaluate the performance of proposed handover algorithm using realistic site-specific channel models developed through non-sequential ray tracing in OpticStudio ® and 1 Ping-pong handovers occur when the user is handed over from one cell to another but is quickly handed back to the original cell. This causes unnecessary signaling overhead and is an indication of incorrect handover parameter settings. 1558-2558 © 2020 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.

See https://www.ieee.org/publications/rights/index.html for more information. The remainder of this letter is organized as follows.

93

In Section II, we describe our system model. In Section III,

94
we present the proposed dynamic handover algorithm.

95

In Section IV, we present simulation results to demonstrate 96 the performance of proposed algorithm. Finally, we conclude 97 in Section V. (DCO-OFDM). At the transmitter, the input bit stream is

116 mapped into complex symbols, i.e., s 1 s 2 • • • s (K/2)-1 117
where K is the number of subcarriers. Hermitian symmetry is 118 imposed on the data vector to ensure that the output of inverse 119 discrete Fourier transform (IDFT) block is real. Therefore, 120 the resulting data vector for the i th AP has the form of

121 X i = [0 s 1 s 2 • • • s (K/2)-1 0 • • • s * (K/2)-1 • • • s * 2 s * 1 ]. 122 Let X k i denote the k th element of X i . Furthermore, let P e 123
and ρ respectively denote the electrical power and electrical-124 to-optical conversion ratio. After K-point IDFT operation, 125 the transmitted waveform from the i th AP is written as [START_REF] Kazemi | Downlink cooperation with fractional frequency reuse in DCO-OFDMA optical attocell networks[END_REF] 126

x i (t)= K-1 k=0 1 √ K X k i e j 2πk K t +x DC t = 0, 1, • • • , K -1 (1)
127 where x DC = ρ √ P e is the DC bias. The average optical power 128 is therefore given by

P opt = E[x i (t)] = x DC .
It is also possible 129 to write the relationship between electrical power and optical power as P e = P 2 opt /ρ 2 [START_REF] Kazemi | Downlink cooperation with fractional frequency reuse in DCO-OFDMA optical attocell networks[END_REF].

At the destination vehicle, the light intensity is detected by a photodetector. Let H i (t), i = 1, . . . , N AP , denote the DC channel gain from i th AP to the vehicle. The received signal can be expressed as [START_REF] Kazemi | Downlink cooperation with fractional frequency reuse in DCO-OFDMA optical attocell networks[END_REF]

y(t)= R P e i∈S H i (t)x i (t)+R P e j∈I H j (t)x j (t)+ς(t) (2)
where R is the photodetector responsivity. In the case of CoMP, the user is jointly served by coordinating APs that transmit the same information. Therefore, the set of S includes two APs, otherwise it is limited to a single serving AP. I denotes the set of interfering APs. In (2), ς(t) is the additive white Gaussian noise (AWGN) term with zero mean and variance of σ 2 = N 0 B. Here, N 0 is the noise power spectral density (PSD) and B is the modulation bandwidth. Based on (2), the SINR at destination vehicle can be expressed as [START_REF] Wang | Dynamic load balancing with handover in hybrid Li-Fi and Wi-Fi networks[END_REF]

γ(t) = i∈S P i (t) j∈I P j (t) + σ 2 (3)
where

P i (t) = R 2 P e H 2 i (t)
is the received electrical power from the i th AP.

III. COMP BASED DYNAMIC HANDOVER

Based on the received power strengths, the CU decides which APs should serve the vehicle. If there is a sufficiently strong AP signal, the vehicle is served by that specific AP. In transition regions between two cells, the vehicle is jointly served by two coordinating APs as a result of CoMP transmission. Based on the rate of change in the received powers, the proposed handover algorithm dynamically revises the handover margin (HOM ) and time-to-trigger value (T T T ) in handover decision.

Selection of HOM and T T T values are critical for vehicular networks. T T T value should be low for high-speed vehicles because when a rapidly moving vehicle approaches the cell edge, the received signal from the serving AP drops rapidly and handover should be triggered immediately. On the other hand, for low-speed vehicles, T T T value should be sufficiently high in order to prevent ping-pong effect. High-speed vehicles experience short dwell time that might cause connection losses due to the high handover rate. In order to improve the connectivity reliability, HOM value for high-speed vehicles should be set high compared to low-speed vehicles with relatively long cell dwell times. Unlike conventional CoMP where fixed values of HOM and T T T are assumed, our proposed algorithm dynamically changes these threshold parameters.

The pseudo-code of the proposed handover algorithm is provided in Algorithm 1. Let P c (t) and P s (t) denote the received power from the candidate AP and the serving AP at time t, respectively. Furthermore, let P s (t -Δt) represent the received power from the serving AP at time t -Δt. The rate of the change in the received power is expressed as ΔP = [P s (t) -P s (t -Δt)]/Δt . Based on the value of ΔP , HOM and T T T are respectively calculated as HOM = αΔP and T T T = λ/ΔP , where α and λ are some constant coefficients. All simulation parameters are provided in Table I.

221

For channel modeling, we use the non-sequential ray tracing 222 approach in [START_REF] Elamassie | Effect of fog and rain on the performance of vehicular visible light communications[END_REF]. We consider a commercial streetlamp with 223 an asymmetrical radiation pattern as shown in Fig. 2 [START_REF] Vestel | [END_REF]. 224 This pattern features a narrow vertical beam angle combined 225 with a wide horizontal beam one. The benefit of the horizontal 226 wide beam angle is to spread the light to longer distances 227 along the road while the vertical narrow beam is required 228 in order to focus the light to the road surfaces only. The 229 LED radiation pattern is integrated into the three dimensional 230 simulation environment constructed in OpticStudio ® software. 231 Channel impulse responses (CIRs) between each street light 232 and destination vehicle are obtained based on non-sequential 233 ray tracing features of this software at each 1 meter over 234 the traveling distance between two poles. Based on earlier 235 discussions in Section III, constant coefficients α and λ are 236 set as 200 and 0.1, respectively.

237

In Fig. 3, we consider Scenario A (i.e., no blockage case) 238 and assume that the car travels in the middle of the right 239 lane (i.e., d h = 2 m). We present the average HOM and 240 In Fig. 4, we present the performance of proposed handover algorithm for Scenario A. We assume that the vehicle travels at the center of lane with the speed of 18 m/s (64.8 km/hr).

As benchmarks, we consider four schemes: 1) Hard handover as specified in 3GPP document [START_REF]Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol Specification[END_REF], 2) Best Connection (BC) algorithm [START_REF] Costa Silva | Adaptive hysteresis margin based on fuzzy logic for handover in mobile networks with dense small cells[END_REF] where the vehicle is always connected to the AP providing the received signal with the highest power, 3) Conventional CoMP handover where the user can be jointly served by two coordinating APs, 4) CoMP-Joint Processing (CoMP-JP) handover [START_REF] Nakano | A handover scheme based on signal power of coordinated base stations for CoMP joint processing systems[END_REF] which uses the average power of the received signals from the coordinated APs instead of the power of the received signal from the source AP (allowing the postponement of the handover if necessary).

In hard handover, handover margin is set as HOM = 1 dB while time-to-trigger is set as T T T = 160 ms in order to prevent ping-pong handovers [START_REF] Wu | Handover skipping for LiFi[END_REF]. In conventional CoMP, fixed parameters of HOM = 3 dB value and T T T = 80 ms value are assumed. 3 The same values are also employed in CoMP-JP. 4 In BC algorithm, in order to connect to the AP providing the received signal with highest power, HOM = 0 dB and T T T = 0 ms are chosen as default. In the proposed algorithm, it can be readily checked from Fig. 3 that HOM and T T T values should be chosen respectively as HOM = 5.90 dB and T T T = 3.85 ms for the speed of 18 m/s under consideration. CoMP transmission and causes unnecessary usage of system 286 resources. On the other hand, as a result of the proper selection 287 of HOM and T T T values, the proposed handover technique 288 maintains a more stable SINR and outperforms its counter-289 parts. The lowest SINR value is obtained as 4.23 dB which 290 is much higher than those in benchmarking schemes and 291 therefore enables a better signal quality.

292

In Fig. 5, we investigate the effect of vehicle speed on 293 the performance of proposed handover algorithm assum-294 ing Scenario A. We consider three different speeds: 9 m/s 295 (32.4 km/hr), 18 m/s (64.8 km/hr) and 27 m/s (97.2 km/hr). 296 It is assumed that the vehicle travels at the center of lane. 297 HOM and T T T values are selected as { HOM = 2.80, 298 T T T = 15.28} for 9 m/s, {HOM = 5.90, T T T = 3.85} 299 for 18 m/s, {HOM = 7.89, T T T = 3.60} for 27 m/s. It is 300 observed that the proposed algorithm is able to maintain a 301 stable SINR performance regardless of the vehicle speed. The 302 average SINR values for 9 m/s, 18 m/s and 27 m/s can be 303 calculated respectively as 6.63 dB, 6.63 dB and 6.99 dB. 304 Similarly, the lowest SINR value experienced for different 305 speeds remains around 4.25 dB. These results demonstrate 306 the performance stability of proposed algorithm for different 307 speeds.

308

In Fig. 6, we investigate the effect of blockage on the 309 proposed handover algorithm. The blue plot is obtained for 310 Scenario A (i.e., no blockage) while the orange plot is obtained 311 Fig. 6.

Effect of blockage on the performance of proposed handover algorithm.

for Scenario B (i.e., with blockage). It is observed that

Fig. 1 .

 1 Fig. 1. (a) VLC-based I2V network (b) View from roadside. demonstrate significant performance gains over conventional 91

  92

  APs in the form of street light poles. 101 The poles are uniformly arranged and separated from each 102 other with a spacing of L. The centralized controller (CU) and 103 APs are connected with wired connections and the CU acts as 104 a gateway to Internet. As illustrated in Fig. 1.b, we assume a 105 two-lane highway road with a lane width of W l . Each of light 106 poles has a height of h, boom length (distance between the 107 center of the pole and the luminaire) of B l , and boom angle 108 of θ. We assume that the receiver (vehicle) is located at a dis-109 tance of d h (with respect to the road center) and travels with a 110 velocity of v. It is equipped with a single photodetector located 111 at the top of the car. It is possible that there might be other 112 vehicles nearby, for example, a truck as illustrated in Fig. 1.a. 113 The physical layer of downlink builds upon direct cur-114 rent biased optical orthogonal frequency division multiplexing 115

  When a vehicle moves away from the serving AP, the received power of the serving AP decreases and the received power from the candidate AP increases. When the vehicle enters the region of the candidate AP and the condition P c (t)+HOM ≥ P s (t) is satisfied for a certain time, CoMP transmission starts and the vehicle is jointly served by both serving and candidate APs. In CoMP phase, both APs transmit the same information and the received signals are combined at the receiver. Handover to the candidate AP is triggered when the P c (t) ≥ P s (t) + HOM is satisfied for a duration of TTT. The vehicle terminates its connection with the previous serving AP and continues getting service from the candidate AP. Therefore, candidate AP becomes the new serving AP and serves the vehicle alone until the new handover decision is taken. A critical issue in the practical implementation is the choice of values of α and λ. T T T is a monotonically increasing function of λ and a lower value of λ is preferred which expedites the start of handover and CoMP transmission, but the value of λ should be greater than 0 to prevent pingpong handovers. On the other hand, HOM is a monotonically increasing function of α. The choice of too small α would disable the CoMP transmission while a large choice of α would trigger unnecessary CoMP transmissions. As a rule of thumb, α and λ values should be selected such that SINR remains almost constant regardless of the vehicle speed. IV. SIMULATION RESULTS In our simulations, we consider a two-lane road where the poles in the same lane are separated with a spacing of L = 20 m, and each of them has a height of h = 7 m. We consider two main use cases: Scenario A) The car travels in the right lane without any neighbor vehicles, Scenario B) The car travels in the right lane and precedes a loaded truck with a height of T h = 4.2 m. The distance between

Fig. 2 .

 2 Fig. 2. Radiation pattern of street light under consideration; The green curve indicates the horizontal radiation pattern while blue curve represents the vertical one.two vehicles is given by T s = 4 m. The second scenario is 216 particularly useful to analyze the effect of potential blockage. 217 The car is equipped with a single photodetector located at 218 the top of the car (See Fig. 1b). It has an aperture area 219 of 10 mm × 15 mm and field-of-view (FOV) angle of 180 • .2220

Fig. 3 .

 3 Fig. 3. HOM and T T T versus velocity. T T T values as a function of the vehicle speed to better highlight the necessity of dynamically revising these parameters which is the main feature of the proposed algorithm. It is observed that T T T decreases with velocity. Since, the received signal drops rapidly for high-speed vehicles when they move away from the source, AP handover is initiated more rapidly for them in comparison to slower vehicles. On the other hand, HOM increases with speed. For high-speed vehicles, CoMP transmission starts earlier and continues longer. This makes system more reliable to sudden connection drops due to high velocity. Consequently, there is a combination of proper HOM and T T T values that needs to be selected for each speed of the vehicle.

Fig. 4 .

 4 Fig. 4. Performance comparison of proposed handover algorithm with different techniques. Vehicle travels at the center of the lane with a speed of 18 m/s.

Fig. 5 .

 5 Fig. 5. SINR versus distance for proposed handover technique with different vehicle speeds.

312

  blockage introduces some degradation around d = 10 m which 313 corresponds to midway between two APs. Because at the cell 314 edge, the vehicle is jointly served by S1 and S3 and, since 315 the received power from S3 in Scenario B is lower than that

TABLE I SIMULATION PARAMETERS

 IPARAMETERS

A lower value of FOV will reduce the ambient noise. However, in an interference-limited case, this reduction will be negligible. Therefore, we preferred a wide FOV angle to maximize the reception angle in mobile conditions under consideration.

Fixed values of HOM and T T T are used in conventional CoMP algorithm. Therefore, it is important to choose values which will provide a decent performance independent of the vehicle speed. In order to determine proper values, we simulated the performance of conventional CoMP for different HOM and T T T values assuming vehicle speeds of 9 m/s, 18 m/s and 27 m/s. Based on these extensive simulations, we selected HOM = 3 dB and T T T = 80 ms which maintain a relatively stable SINR and fit better for all speeds.

[START_REF] Abboud | Interworking of DSRC and cellular network technologies for V2X communications: A survey[END_REF] It is reported in[START_REF] Nakano | A handover scheme based on signal power of coordinated base stations for CoMP joint processing systems[END_REF] that CoMP-JP provides a superior performance for 2 dB < HOM < 4 dB.

The work of H. B. Eldeeb was supported by the Horizon 2020 MSC ITN (VISION) under Grant 764461. The work of M. Uysal was supported by the Turkish Scientific and Research Council (TUBITAK) under Grant 215E311. The associate editor coordinating the review of this letter and approving it for publication was C. Gong.