Design and Implementation of an
Ethernet-VLC Interface for Broadcast Transmissions F.Delgado,I.Quintana,J.Rufo,J.A.Rabadan,Crisanto Quintana,and R.Perez-Jimenez
Abstract—In this work,a complete interface between Ethernet and Visible Light Communications(VLC)networks is prented. It requires the u of previously propod DPPM schemes for this kind of applications,but with some modifications so as to keep constant duty cycle in order to assure its u in illumination facilities[2].The prototype has been tested using a2Mbps VLC Link,obtaining distances of,at least,3meters in an interfering environment.The PPM characteristics allow transmission with-out vere signal degradation on this environment.This Ethernet-VLC interface is intended to demonstrate the capability of IP broadcast applications of this kind of devices.This interface performs packing functions andflow control.
Index Terms—VLC communications,white LEDs illumination, pul position modulation,Ethernet-VLC interface.
I.I NTRODUCTION
I n the last years,different rearch lines have focud on
energy saving techniques for illumination sources,since current lamps prent very low power efficiency.In this way, new more efficient lighting devices are being developed to replace incandescent andfluorescent lights(including low-consumption bulbs).The systems make u of Solid State Lightning(SSL)which,compared tofilament heating or electric arc(both of which are ud in conventional lamps), allows to convert electric energy into optical emitted power more efficiently.They are bad on LED devices,which provide high-level savings in energy consumption and have a lifespan between10000and50000hours.
Using illuminationfixtures for data transmission is not a new concept;however,only the new SSL devices allow the implementation of feasible communication links becau of their available bandwidth.As a conquence of the increasing development of the new devices,multiple contributions to VLC have been propod[1][2].
The aim of the works is bad on adding the commu-nication capability to the lamp,while maintaining its main function as light source.Therefore,the transmitted data signal should not alter the illumination perception at the ur eye. This implies some restrictions on the emitted signal in order to avoid lightflickering.For this reason,in this work,a modified Differential Positioning Pul Modulation(DPPM)technique, which always prents only a pul in each transmission symbol,has b
een propod.
Manuscript received June8,2010.The associate editor coordinating the review of this letter and approving it for publication was G.K.Karagiannidis. This work was supported in part by the University of Las Palmas de G.C. under ULPGC07-007program,and by the Spanish Government(TEC2009-14059-C03-01).
The authors are rearchers at IDeTIC. F.Delgado is also with the Departamento de Ingeniera Telematica de la Universidad de Las Palmas de Gran Canaria,Campus Universitario de Tafira s/n,Las Palmas de G.C.35017 (e-mail:fdrajo@dit.ulpgc.es).
Digital Object Identifier
10.1109/LCOMM.2010.12.100984Fig.1.System
园丁是什么意思architecture.
Fig.2.System block diagram.Shadowed blocks are the main objective of this work.
illustrate
On the other side,an interface between wired network (Ethernet)and Optical network has been developed.Due to the asymmetrical properties of the links,aflow control at the optical network access point has been implemented.Figure 1illustrates the system architecture.
II.P ROPOSED S YSTEM
A.System Architecture
edlThis system is designed to allow broadcast transmissions from a wired LAN(Ethernet in this ca)to the VLC network. Becau of the different bit rates in both systems,aflow control implementation is necessary at the VLC access point node.Taking into account all the factors,the complete system block diagram is shown in Fig.2.This work focus on Modulator/Demodulator and Flow Control Blocks devel-opment.
The system uplink shown in Fig.2consists of an infrared link between optical nodes and VLC access point,which
拼搏的英文1089-7798/10$25.00c⃝2010IEEE
Fig.3.Time respon of a blue LED with white
phosphorus.
Fig.4.Different pul position modulations waveforms.
performs a command channel for the downlink broadcast transmissions.The main limitation of this link is determined by its low power transmission and wide coverage area,em-ploying optical devices that do not need critical alignments,so that,a low data rate link (125Kbps)has been lected.B.Visible Optical Channel Characteristics
As already commented on,the main restrictions of the systems are the limited bandwidth and the need of maintaining constant luminescence along all communication time due to their simultaneous u as conventional lamps.Long quences of “zeroes”or “ones”in an On-off Keying (OOK)communi-cations system could cau illumination flickering.
At the same time,high communications rates are not possible becau of their high ri and fall times.For typical white LED lamps,minimum pul width allowed is about 200ns as shown in Fig.3.Besides,this channel is affected by interference signals caud by ambient light,especially by incandescent and fluorescent lamps,with frequency compo-nents under 1MHz.
C.DPPM Modulator and Demodulator
Pul position modulating (PPM)techniques are extensively ud in wireless optical links,due to their high average-power ef ficiency and arti ficial illumination effect reduction [2][3].Moreover,this techniq
ue allows the system to work with lights turned “off”and “on”,and with varying PPM pul width without optical power loss.This is achieved by means of a variation in the chip pul duration.In this ca,20%of symbol duration has been lected for the “turn off”mode and 80%for the “turn on”mode.台风英文
初中英语教学方法The main inconvenience of the modulation techniques is the need of complex synchronism systems to ensure correct detection.To avoid this,Differential Pul Position Modula-tion (DPPM)could be lected,which prents non coherent detection capability at the receiver.However,the bit rate is not constant during transmissions [4]and this can affect the
TABLE I
D ISTANCES FOR
E ACH S YMBOL Symbol Distance (Chip periods)
005012or 6103or 711
4or
8
Fig.5.
Transmission/reception complete flow chart.
system performance as illumination fixture [2].In this work,a modi fied 4-DPPM has been developed in order to solve this problem.This technique ensures that transmitted signal duty cycle is always the
same,so that light intensity variations do not occurs.Figure 4shows the waveforms corresponding to the Pul Position Modulations cited above.
In fact this is a PPM scheme where position of the next chip is given by:
P N =
{P N −1+D N −L f or (P N −1+D N )≥L
P N −1+D N f or (P N −1+D N )<L }Where L =number of chips available,P N −1=Last chip
position and D N =Symbol value.This modi fied DPPM scheme prents the same bandwidth and power requirements than the conventional PPM one,but allows non coherent detection at the receiver,like in a DPPM receiver.This is achieved by counting clock samples between rising edges (distances,including a guard interval to compensate LED fall time).Correspondence between symbol and distances for the propod scheme is shown in Table 1.It can be en that there are two possible distances for the same symbol in some cas.The transmission block lects the appropriate one in order to ensure the constant duty cycle (20%)of the transmitted signal explained before.To obtain a duty cycle of 80%,the transmitted signal is inverted.
veral造句
D.Interface Ethernet-Optical Access Point
The developed system transmits information encapsulated in UDP packets.The interface block performs the payload extraction and encapsulation and the Flow control between Ethernet network and rial interface.The process are shown in Fig.5.
Flow control is necessary becau optical transmission rate is about 2Mbps and Ethernet frames arrive to the access point at 10Mbps data rate.Payload from UDP frames are
DELGADO et al.:DESIGN AND IMPLEMENTATION OF AN ETHERNET-VLC INTERFACE FOR BROADCAST TRANSMISSIONS
1091
Fig.6.Payload extraction and
packaging.
Fig.7.Waveforms:a)data and b)modi fied 4-DPPM received signal.
buffered before their transmission through optical link.After UDP payload extraction,encapsulation is carried out to ensure correct detection.
A header is added to each UDP payload before its rial transmission and modulation.At the receiver side,this header is detected and data are buffered.Figure 6shows this pack-aging process.
After that,the receiver encapsulates the buffered payload in a UDP frame and transmits it to the client node by an Ethernet connection,so that the complete process is accomplished.UDP facilities are included in the employed Xilinx core.III.S YSTEM I MPLEMENTATION
The propod scheme has been implemented using a Xilinx Spartan-3design kit,which includes Ethernet MAC facilities and cores to negotiate communications between the wired network and the optical link.This design kit implements the following features:flow control,modulation/demodulation and UDP frames payload unpacking and packing.The complete system has been tested over a visible optical link compod by a commercial visible LED lamp (20white LED diodes array,each of them with the time respon shown in Fig.3).At the transmitter,a circuit bad on SN75452open collector logic gates has been ud.The reception circuit basically consists on a transimpedance am
pli fication stage and two HAMAMATSU S7510photodiodes,with a total active area of 132mm2.IV.R ESULTS
As commented before,the main characteristic of the pro-pod 4-DPPM codi fication is its constant duty cycle for every symbol.Thanks to the different possibilities of distances between puls for codifying the 2bits symbols (e Table I),it can be possible to implement a DPPM,with the restriction that in each symbol time there will only be a pul.Figure 7shows waveforms resulting,corresponding to data and modi fied 4-DPPM codi fied signal.For the complete system
performance
Fig.8.Measured lost packets percentage versus S/I.a)OOK,b)propod constant rate 4-DPPM.
test,lost packets in the prence of different interferences have been estimated.Reception is not considerably affected by the interference caud by a fluorescent lamp and natural illumination.
Results for an interfering optical signal generated by an OOK modulated LED lamp with a bandwidth of 400KHz (taken as a worst ca interference example),are shown in Fig.8.
thatisallV.C ONCLUSIONS
In this work a new VLC system architecture has been pre-nted.This scheme performs data broadcast from a rver in a cable network by means of visible optical links implemented using commercial LED lamps.A modi fied DPPM scheme has been also developed to ensure constant illumination intensity and constant bit rate.Therefore,the devices work simul-taneously as communications systems and as illumination sources.Finally,an interface Ethernet-VLC prototype has been implemented.This could be easily ud for conventional internet connections and applications.
R EFERENCES
[1]T.Komine and M.Nakagawa,“Integrated system of white LED visible-light communication and power-line communication,”IEEE Trans.Con-sumer Electron.,vol.49,no.1,Feb.2003.
[2] F.J.Lopez Hernandez,E.Poves,R.Perez-Jimenez,and J.Rabadan,
“Low-cost diffu wireless optical communication system bad on white LED,”2006IEEE Tenth International Symposium on Consumer Electronics (ISCE’06).
[3] A.J.C.Moreira,R.T.Valadas,and A.M.de Oliveira Duarte,“Optical
project c
北京大学政府管理学院interference produced by arti ficial light,”Wireless Networks ,3rd edition,pp.131–140.J.C.Baltzer AG,Science Publishers,1997.
[4] D.-S.Shiu and J.M.Kahn,“Differential pul-position modulation for
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