Network Architecture of a High-Speed Visible Light Communication Local Area Network

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This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI10.1109/LPT.2014.2364955, IEEE Photonics Technology Letters

Abstract—A novel ultra high-speed LED visible light

communication (VLC) local area network to provide beyond 10

Gb/s optical wireless access based on star topology architecture is

proposed and experimentally demonstrated for massive users.

Fiber link is used as the backbone of the bi-directional VLC

network. The hybrid access protocol is utilized: frequency

division multiplexing (FDM) for downlink and uplink fiber

transmission, and time division multiplexing (TDM) for

bidirectional VLC transmission. A full-duplex VLC network for 8

VLC access points (VAPs) has been successfully demonstrated

with the total throughput of 8 Gb/s. Each VAP is offered 500 Mb/s

downstream and 500 Mb/s upstream. The measured BERs of

downlink and uplink for all the VAPs are under 7% FEC limit of

3.8x10-3

over 25 km SSMF and 65 cm free space, clearly validating

the promising potential of the proposed VLC network

architecture for future 40 Gb/s and 100 Gb/s wireless access.

Index Terms—visible light communication, network

architecture

I. INTRODUCTION

ver the past few years, the demand of high-speed

wireless access, especially in indoor environments, keeps

increasing rapidly with the rising popularity of a multitude of

multimedia applications. Visible light communication (VLC)

based on light emitting diode (LED) has been a promising

candidate for indoor high-speed wireless access, as LEDs with

better modulation bandwidth can combine illumination and

data transmission together [1-2]. Outperforming traditional

wireless access such as WiFi, WiMAX and radio-over-fiber

(RoF), VLC offers several advantages such as cost-effective,

license-free and high security. The latest report has

demonstrated the VLC transmission with the highest data rate

of 3Gb/s using a single LED [1], which makes it feasible for a

single LED to support Gbit/s wireless access.

However, most of the investigations about VLC are focused

on point-to-point transmission [3-5], and VLC access networks

are commonly neglected. In a practical scenario such as a huge

office building, when VLC local network consisting of

hundreds VLC access points (VAPs) is needed to provide

beyond 10Gbit/s access service for massive users, the design of

a VLC network architecture has become a big challenge. There

are several investigations reported on VLC networks [6-9]. A

This work was partially supported by NHTRDP (863 Program) of China (No.

2013AA013603), NNSF of China (No. 61177071).

Yiguang Wang, Yuanquan Wang, Li Tao, Jianyang Shi and Nan Chi are with

Department of Communication Science and Engineering, Fudan University,

Shanghai 200433, China. (e-mail: [email protected])

VLC network integrated with power line communication (PLC)

at 1 Mb/s data rate has been proposed in [6]. Limited by the

inherent data rate of PLC, VLC network over PLC cannot

provide Gbit/s access service. A bidirectional VLC prototype

integrated with 1000BASE-T Ethernet is presented in [7].

However, the system operates in half-duplex mode based on

TDM for single user, and the total data rate is only 500Mb/s. It

is known that 10GBASE-T Ethernet can provide 10 Gb/s

transmission for VLC access networks. By using power over

Ethernet, data transmission and power supply can be realized

simultaneously. However, when hundreds of VAPs are

supported in a huge building, the capacity requirement of the

VLC network could be further increased to 40 Gb/s or 100 Gb/s.

Therefore, optical fiber is a better solution for ultra high-speed

VLC access due to its much larger modulation bandwidth.

Additionally, lots of optical devices at lower cost have been

applied to high speed optical transmission systems in recent

years. It can be foreseen that the cost of the VLC network based

on optical fiber will be continuously reduced. A 2.5 Gb/s

bidirectional in-building network over SMF and a laser pointer

laser (LPL) has been proposed in [8]. But the multi-access

scheme is not discussed, and the Class IIIa based LPL suffers

from high cost and could be dangerous to human’s eyes [8]. We

have presented a unidirectional VLC system integrated with

PON [9]. However, this system is not a LED centric VLC

access network, but an integration of VLC and PON. The PON

and the VLC signals are simultaneously transmitted in the fiber

link, and then respectively sent for wired or wireless users.

Moreover, this system does not consider the uplink and the

multiple access scheme either.

In this paper, we propose a novel full-duplex VLC local area

network based on star topology architecture to provide beyond

10 Gb/s wireless access for massive users. Fiber link is used as

the backbone of the VLC networks to directly connect LED

lamps. We also propose a sophisticated access protocol for the

full-duplex VLC network: frequency division multiplexing

(FDM) for downlink and uplink fiber transmission, and time

division multiplexing (TDM) for bidirectional VLC

transmission in each VAP. As far as we know, for the first time

this hybrid access protocol is utilized in high speed VLC access

networks. Orthogonal frequency division multiplexing (OFDM)

is utilized to obtain high spectral efficiency. Based on the novel

architecture, a full-duplex VLC network experiment is

demonstrated. 8 sub-bands at the bandwidth of 100 MHz are

used for 8 VAPs over 2.5 GHz frequency span. In each VAP, 4

time slots are assigned for 4 users. The total throughput of the

Network Architecture of a High-speed Visible

Light Communication Local Area Network

Yiguang Wang, Nan Chi, Yuanquan Wang, Li Tao, and Jianyang Shi

O



bi-directional VLC network is 8 Gb/s. Each VAP is offered 500

Mb/s downstream and 500 Mb/s upstream. The measured bit

error rates (BERs) of downlink and uplink for all the VAPs are

under 7% pre-forward error correction (FEC) limit of 3.8x10-3

through 25 km standard single mode fiber (SSMF) and 65 cm

free space propagation. When continuing to increase the

bandwidth and the number of sub-bands, the proposed VLC

network is scalable to provide future 40Gb/s and 100Gb/s

wireless access for massive users.

II. VLC NETWORKS INFRASTRUCTURE

Fig. 1 shows the schematic diagram of the VLC network. To

realize high-speed VLC access for massive users in a huge

building, the architecture of the VLC network is designed to be

star topology. As backbone of the VLC network, fiber is used to

connect with each LED lamp directly.

Fig. 1: the schematic diagram of the VLC network

Fig. 2: the multiple access scheme of the VLC network

Fig. 2 shows the multiple access scheme of the VLC network.

FDM is used in the downlink and uplink fiber transmission to

make full use of the high capacity of the optical fiber.

Sub-bands located at different center frequencies are assigned

for different VAPs. In each VAP, TDM is implemented for

bi-directional VLC transmission. The connection between the

fiber and VLC is realized in the LED lamps.

In the downlink, the data for different VAPs is firstly

modulated to the pre-allocated sub-bands at the CO, and then

transmitted through fiber link. After split by an optical coupler,

the optical signal is detected in the LED lamp of each VAP. The

received signals are passing through a band pass filter (BPF) to

filter out its data from the pre-assigned sub-band. The selected

signals are then down-converted by a mixer for further VLC

transmission. After free space channel, the signal is received by

different users. Each user picks out his own data from the

pre-assigned time slot for further demodulation. In the uplink,

the user in each VAP transmits his data at the pre-assigned time

slot to the LED lamp. Then the up-conversion is realized by a

mixer and a BPF. The up-converted signals are modulated to an

optical carrier for uplink fiber transmission. The upstream

optical signals from different VAPs are combined by the optical

coupler, and transmitted to the CO.

When massive users are simultaneously supported, the

synchronization of the whole VLC network will be a key

problem. For FDM based fiber link, synchronization between

different access points is not needed, which is a well-known

advantage of FDM. However, for TDM, it is quite important to

realize synchronization between different users. In a VPA,

different users are assigned the precise time slot to transmit

their data to avoid conflict. Synchronous heads are also applied

to the frames of different users, so that the signal from different

users can be selected from the accurate time slot.

III. EXPERIMENTAL SETUP AND RESULTS

The experimental setup of the VLC network is shown in Fig. 3.

In the downlink, original data are firstly mapped into 32QAM

signals at the CO, and OFDM modulation is then employed.

The generated OFDM baseband signals are up-converted to 8

sub-bands over 2.5 GHz frequency span for 8 VAPs. The center

frequencies of the 8 sub-bands are 62.5 MHz, 187.5 MHz,

312.5 MHz, 437.5 MHz, 1062.5 MHz, 1565.5 MHZ, 2062.5

MHz and 2437.5 MHz, and the bandwidth of each sub-band is

100 MHz. In the proposed VLC network, the central frequency

of the sub-band can be chosen adaptively base on the practical

requirement. The OFDM signals are generated by an arbitrary

waveform generator (AWG, Tektronix 7122) at 5 GSa/s.

Fig. 3: The experimental setup of the proposed VLC access networks. (LD: laser diode, IM: intensity modulator, EA: electrical amplifier, SSMF: standard single

mode fiber, VAP: VLC access point, PD: photodiode, BPF: bandpass filter, LPF: lowpass filter, APD: avalanche photodiode, DML: directly modulated laser)

Down Link

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After an electrical amplifier (EA), the signals are used to

drive an optical intensity modulator (IM). A laser diode (LD) at

the wavelength of 1548.49 nm is used as the optical source.

The inset A in Fig. 3 shows the optical spectrum after IM. An

optical circulator is utilized to select downtream and upstream

signals.After 25 km SSMF, an optical coupler is used to

seperate the downstream signals and send them to each VAP. In

the VAP, the optical signals are detected by a photodiode (PD)

in the LED lamp. The electrical spectrum of the received

signals is shown in inset D in Fig. 3. After a BPF, the selected

signal is down-converted by a mixer for further VLC

transmission. In VLC downlink, after low pass filter (LPF) the

down-converted signals are amplified by an EA (Mini-circuits

ZHL-6A+), combined with DC bias, and applied to drive the

RGB LED (Cree PLCC). Here, the red chip of the RGB LED at

the center wavelength of 620 nm is used to carry useful

information, while the blue and green chip are only supplied

with DC to maintain white color illumination. The optical

spectrum of the red chip of the RGB LED is shown in inset E in

Fig. 3. After 65 cm free space, the signals are detected by an

avalanche photodiode (Hamamatsu APD, C5331) at the user

side, and then recorded by a commercial digital oscilloscope

(OSC) (Agilent 54855A).The user selects his own data from the

pre-assigned time slot for offline OFDM demodulation and

digital signal processing (DSP). In free space channel, lens

(100-mm focus length, 70-mm diameter) and red filter are used.

The electrical spectrum of the signal after VLC downlink

transmission is shown in inset F in Fig. 3.

Fig. 4 the experimental picture of the VLC network

In the uplink, original data are firstly passed to QAM and

OFDM encoder, and pre-equalized to compensate the

frequency attenuation of the LED. Another AWG (Tektronix

710) is ultilized to generate the upstream OFDM signals. Then

the OFDM signals are combined with a DC bias to drive a

RGB LED. In the uplink, the green chip of the RGB LED (Cree

PLCC) at the center wavelength of 520 nm is used to carry

information, while the other two chips are only supplied with

DC. The optical spectrum is shown in inset F in Fig. 3. Each

user transmits his data at the pre-assigned time slot. Over free

space channel, the upstream signals from different users are

detected by an APD in the LED lamp. A directly modulated

laser (DML) at the wavelength of 1545.04 nm is used as optical

source for uplink. The upstream signals of each VAP are

up-converted to its assigned sub-band by a mixer and a BPF.

Then the upstream optical signals from different VAPs are

combined by an optical coupler, and transmitted over 25 km

SSMF. The upstream signals are detected by a PD at the CO,

and recorded by an OSC. According to the pre-assigned

sub-bands and time slots, the data from different users are

respectively picked out for demodulation. The electrical

spectrum of the uplink signal for sub-band 3 is shown in inset B

in Fig. 3. The experimental picture of the VLC network is

presented in Fig. 4.

The downlink performance of the VLC network is firstly

evaluated. Fig. 5 shows the BER performances of all the 8

sub-bands at -4dBm received optical power. It can be found that

the BERs of all sub-bands are under the 7% FEC limit of

3.8x10-3. There is negligible performance difference between

the sub-bands at low and high frequency, because the output of

the AWG has a little attenuation at high frequency components.

Fig. 5 BER of all the 8 sub-bands

Fig. 6 BER versus received optical power for sub-band 1, 4, and 8

At the VAP, the received optical power at the photodiode has

great influence on the further VLC transmission. Therefore, we

measure the BER vs. the received optical power of sub-band 1,

4, and 8 after fiber transmission, as shown in Fig. 6. The BERs

are under the 7% FEC limit of 3.8x10-3 when the received

optical power is -5dBm or higher. We also measured the BER

performance of the sub-band 1 without uplink as shown in Fig.

6. It can be seen that there is almost no interference induced by

bidirectional fiber and VLC transmission. Because in fiber link,

we use a couple of the optical circulators with high isolation,

and in bidirectional VLC transmission, we use different color

chips for uplink (green) and downlink (red), and the crosstalk

can be filtered out by the R/G/B filter. Then the optical power

after 25 km SMF is fixed at -4dBm, and the VLC transmission

distance is changed to analyze the influence of the free space

channel. The BER versus VLC transmission distance are shown

in Fig. 7. We can see that the BERs of all sub-bands are under

the 7% FEC limit of 3.8x10-3 at 65cm VLC transmission.

Then the performance of the uplink is measured. The

VLC DL

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upstream signals are transmitted through 65 cm free space, and

then received at the CO after 25 km SSMF. To verify the TDM

scheme, we use two LEDs as the different users in VAP3. The

signals of user 1 and user 3 are transmitted at different time

slots, and received by the APD simultaneously. Inset A and

inset B in Fig. 8 show the time-domain signals of user 1 and

user 3, and inset C shows the received TDM signals of user1

and user3. Fig. 8 denotes the uplink BER of user1 and user3 in

VAP3 versus received optical power after fiber BTB and 25 km

SSMF respectively. The BER performances of user 1 and user 3

are almost same. 0.5 dB power penalty is found at 3.8x10-3

between BTB and 25km SSMF transmission.

Fig. 7 BER versus VLC transmission distance of sub-band 1, 4, and 8

Fig. 8 the uplink BER of user 1 and user 3 vs. received optical power. Inset A:

the time-domain signal of user 1; inset B: the time-domain signal of user 3;

inset C: the TDM signals of user1 and user3. Inset D: BER vs. power ratio

Compared to FDM, TDM is a better choice for VLC access,

because in FDM the power competition between multiple

sub-bands is inevitable. We have made an experimental

investigation on the power competition of FDM. Two LEDs are

used to transmit the uplink signals at two different sub-bands

respectively for FDM. After free space, the signals from the

two LEDs are simultaneously received. The BER of the two

sub-bands versus power ratio P2/P1 is shown in the inset D of

Fig. 8. It can be found that when increasing the power ratio, the

performance of sub2 is improved while the performance of

sub1 is decreased. Therefore, we should find the optimal power

ratio to reduce the overall BER in FDM.

Here VLC access data rate is 500 Mb/s for each user, mainly

due to the limited modulation bandwidth of the Cree RGB LED

(3-dB bandwidth: 5 MHz). By using a LED with large

modulation bandwidth, such as micro LED (3-dB bandwidth:

60 MHz) in [1], Gbit/s VLC access service for each user can be

easily supported. The transmission distance using micro LED is

only 5 cm, due to the optical power limitation (4.5 mW). The

luminance level of the LED is the key factor for the VLC

transmission distance. The optical power of the utilized RGB

LED is only 1W, and the luminance at 65 cm after focusing lens

is measured: red 37 lx, and green 45 lx. The luminance is far

below the standard value for illumination (400 lx). It is believed

that VLC transmission distance of 2-3 m can be realized by

increasing the optical power or deploying LED array.

In our experiment, only 8 sub-bands are utilized for 8 VAPs

over 2.5 GHz frequency span. It should be noted that the

proposed VLC network is easily scalable. To make full use of

the overall 2.5 GHz frequency span, up to 20 VAPs can be

supported simultaneously. At this case, the total throughput of

the VLC network is increased to 20 Gb/s. As for 10 GHz

frequency span, the number of supported VAPs can be as high

as 80, which shows the great potential to provide future 40 Gb/s

and 100 Gb/s wireless access for massive users.

IV. CONCLUSIONS

In this paper, we propose a novel ultra high-speed VLC local

area network based on star topology architecture for massive

users. Fiber link is used as the backbone of the VLC network.

The hybrid access protocol is utilized: FDM for downlink and

uplink fiber transmission, and TDM for bidirectional VLC

transmission. In the bi-directional VLC network experiment, 8

sub-bands are allocated for 8 VAPs, and, 4 time slots are

assigned for 4 users VLC access in each VAP. The total

throughput of uplink and downlink for the VLC network is 8

Gb/s. The measured BERs of downlink and uplink for all the

users are both under 7% FEC limit of 3.8x10-3 through 25 km

SSMF and 65 cm free space propagation. The results proof the

potential and feasibility of the VLC networks architecture to

offer beyond 10 Gb/s VLC access service for massive users.

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Network Architecture of a High-Speed Visible Light Communication Local Area Network

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