Optical Fiber Technology 19 (2013) 236–241

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Optical Fiber Technology

www.elsevier .com/locate /yof te

Inter-BSs virtual private network for privacy and security enhanced 60 GHzradio-over-fiber system

Chongfu Zhang a,⇑, Chen Chen a, Wei Zhang a, Wei Jin a, Kun Qiu a, Changchun Li b, Ning Jiang a

a Key Lab of Optical Fiber Sensing and Communication Networks, Ministry of Education, School of Communication and Information Engineering, University of Electronic Scienceand Technology of China, Chengdu, Sichuan 611731, Chinab Fiberhome Telecommunication Technologies Co., Ltd., Wuhan, Hubei 430074, China

a r t i c l e i n f o

Article history:Received 29 November 2012Revised 1 February 2013Available online 11 March 2013

Keywords:Radio-over-fiber (RoF)Virtual private network (VPN)Optical code-division multiplexing (OCDM)

1068-5200/$ - see front matter � 2013 Elsevier Inc. Ahttp://dx.doi.org/10.1016/j.yofte.2013.02.002

⇑ Corresponding author.E-mail address: cfzhang@uestc.edu.cn (C. Zhang).

a b s t r a c t

A novel inter-basestations (inter-BSs) based virtual private network (VPN) for the privacy and securityenhanced 60 GHz radio-over-fiber (RoF) system using optical code-division multiplexing (OCDM) is pro-posed and demonstrated experimentally. By establishing inter-BSs VPN overlaying the network structureof a 60 GHz RoF system, the express and private paths for the communication of end-users under differ-ent BSs can be offered. In order to effectively establish the inter-BSs VPN, the OCDM encoding/decodingtechnology is employed in the RoF system. In each BS, a 58 GHz millimeter-wave (MMW) is used as theinter-BSs VPN channel, while a 60 GHz MMW is used as the common central station (CS)–BSs communi-cation channel. The optical carriers used for the downlink, uplink and VPN link transmissions are allsimultaneously generated in a lightwave-centralized CS, by utilizing four-wave mixing (FWM) effect ina semiconductor optical amplifier (SOA). The obtained results properly verify the feasibility of our pro-posed configuration of the inter-BSs VPN in the 60 GHz RoF system.

� 2013 Elsevier Inc. All rights reserved.

1. Introduction

Radio-over-fiber (RoF) technology has being triggering tremen-dous interest recently, due to its effective integration of the highmobility of wireless radio communication and the sufficient band-width of optical fiber communication [1]. In consequence, RoF iswidely considered as a promising and powerful solution for the fu-ture broadband and high-speed access networks [2]. In a typicalRoF system, the signals coming from the central station (CS) areidentically broadcasted to each distributed base station (BS) [3].Because each BS can share the same information, the privacy ofeach BS is drastically destroyed. For a multiple-BS RoF system,the end-users under different BSs cannot be directly connectedfor the reason that only the CS–BS communication link is physi-cally available [4]. Therefore, the communication link for two dif-ferent BSs is the BS–CS–BS link and it inevitably causes greatround-trip propagation latency and consumes the bandwidth forboth the downlink and uplink transmission in the optical fiber. Inorder to establish a direct communication link for different BSsas the BS–BS link, the configuration of the physical layer inter-BSs virtual private network (VPN) is a convenient and feasible solu-tion [5]. Intensive investigations on the topic of all-optical VPNconfiguration in passive optical networks (PONs) have been done

ll rights reserved.

for the past few years [6–8]. A ring-based PON structure has beenproposed to support the multiple optical private networks andestablish the virtual networks for the RoF applications [9]. Never-theless, to our best knowledge that no study concerning aboutthe inter-BSs VPN configuration in the RoF systems has ever beenreported, while it is of great significance to establish VPN commu-nication for different BSs and thus improve the privacy and secu-rity of the common RoF systems. Furthermore, optical code-division multiplexing (OCDM) has been widely studied for its abil-ity to support high-security communications [10,11]. Therefore,the OCDM encoding and decoding technology is one of the effec-tive ways to build inter-BSs VPN, as it can provide high-speedand secure connectivity between CS and BSs and guarantee the pri-vacy of each BS. Two millimeter-wave (MMW) channels are de-signed to distinguish the VPN signal from the common CS–BSsignal. The four-wave mixing (FWM) effect in a semiconductoroptical amplifier (SOA) is utilized in the lightwave-centralized CSto obtain all optical carriers, so that every BS is source-free andthe overall cost of our proposed RoF system is reduced [12,13].

In this work, we propose and experimentally verify a privacyand security enhanced 60 GHz RoF system wherein the novel in-ter-BSs VPN connections are constructed for the express and pri-vate communication of the end-users under different BSs. Byemploying OCDM technology in each BS, the inter-BSs VPN are suc-cessfully established in our proposed RoF system. 5 Gb/s down-stream (DS) data and 2.5 Gb/s upstream (US) data transmission,

C. Zhang et al. / Optical Fiber Technology 19 (2013) 236–241 237

as well as 1.25 Gb/s VPN data transmission are set up and success-fully demonstrated. The system performance is carefully analyzedand the eye diagrams for the downlink, uplink and VPN link trans-missions are captured successfully.

2. Principle

Fig. 1 shows the primary principle of our proposed inter-BSsVPN in the 60 GHz RoF system with the OCDM encoding anddecoding technology. In the CS, a continuous-wave (CW) opticalsource is modulated with optical carrier suppression (OCS) in anintensity modulator (IM) which is driven by a 10 GHz sinusoidalwave. Thus an OCS signal with a frequency interval of 20 GHz.

GHz is obtained as depicted in insert (a) of Fig. 1. The OCS signalis taken as the input of the SOA to perform the FWM effect andgenerate four optical sidebands, as shown in insert (b) of Fig. 1.An optical circulator and a narrowband fiber Bragg grating (FBG)filter are used to reflect the fourth sideband and direct the reflectedsideband to a multi-user OCDM transmitter, where the reflectedsideband is modulated with a downlink data and encoded withan optical orthogonal code (OOC) in the time domain. The modu-lated and encoded sideband is then coupled together with otherthree sidebands by a multiplexer (MUX), as shown in insert (c) ofFig. 1. After that, the downlink signal is achieved and it is launchedinto the optical fiber for DS transmission. After passing an opticalsplitter (OS), the downlink signal reaches each BS. In each BS, thereceived downlink signal is firstly divided into three parts by ade-multiplexer (DMUX): (1) the combination of the first pure side-band and the fourth modulated sideband and these two sidebands

LD OCSMod .

SOA FiberMUXMulti -user

OCDMA TxOS

OC

Multi -userOCDMA Rx

Fiber

CS

20GHz FBG reflect DS data

FBG

a b c

a bc

e

Downlink

Uplink

58GHzfVPN dataUS data

e

10GHz

FBG

Fig. 1. Principle of the proposed 60 GHz RoF system with inter-BSs VPN. CS/BS: centradecoder, IM: intensity modulator, PD: photodiode, MMW: millimeter wave, DS/US: dow

OS OC

(a)

OC OS

(b)

τ1

τ2

τ3

T-τ1

T-τ2

T-τ3

Fig. 2. Configuration of (a) the optical delay lines-based optical encoder, (b) optical deoptical combiner.

have a frequency interval of 60 GHz; (2) the second sideband isused as the uplink carrier; (3) the third sideband is used as theVPN link carrier. The first part of the received downlink signal isdecoded with auto-correlation and sent into a photodiode (PD) togenerate the DS 60 GHz MMW, as shown in insert (d) of Fig. 1.The generated 60 GHz MMW is then emitted by an antenna and re-ceived by multiple end-users under the BS. The uplink 60 GHzMMW is detected by the antenna and down-converted to the base-band. Then it is modulated onto the uplink carrier and encoded.Meanwhile, the 58 GHz VPN signal is modulated onto the VPN linkcarrier with the single-sideband (SSB) modulation. The SSB signalis optically encoded and the encoded signal is coupled togetherwith the uplink signal. Then the coupled signal is launched intothe optical fiber. A double-notch FBG (DNF) filter is adopted tosimultaneously reflect the carrier sideband and the signal sidebandof the VPN signal. The uplink signal is then received by the multi-user OCDM receiver module to obtain the uplink data with auto-correlation decoding in the CS. The reflected VPN signal is broad-casted back to all distributed BSs and only the destined BSs know-ing the right OOC can successfully decode and detect the receivedsignal. Inserts (e) and (f) of Fig. 1 show the uplink signal and thereflected VPN signal, respectively. In the proposed system, each in-ter-BS VPN is assigned with a unique OOC and by assigning multi-ple OOCs for multiple inter-BS VPNs, groups of point-to-point orpoint-to-multipoint inter-BSs connections can be effectively con-structed. Therefore, the OCDM encoding and decoding technologyenables the configuration of multiple inter-BS VPNs in the60 GHz RoF system for the private and secure communication ofthe end-users under different BSs. This scheme is proposed forthe situation that the RoF system needs enhanced security and pri-

60GHz

PDOC

Dec.iDMUX

Rx.i

Tx.i

58GHzMMW

VPNTx.iPD

OC

Enc.i IM

Enc.i*SSB

Mod .

BS-i User

60GHzd

d

f

60GHzMMW

VPN signal

Dec. i*

VPNRx.i

l station/base station, OS/OC: optical splitter/optical combiner, Enc./Dec.: encoder/nstream/upstream.

Before encoding/signal

Encoding

Decoding

matching mismatching

(c)

Auto-correlation Cross-correlation

coder and (c) operation of optical encoding and decoding. OS: optical splitter, OC:

BS-2

SNFSOA

IM

5Gb/sEnc1_DL

MUX 20km

SSMF+DCF

EDFA

DMUX

DL_Tx_60GHz

Dec1_DL

OC PD BERT

DL_Rx_60GHz

PPGIM2.5Gb/s

Enc1_ULUL_Tx_baseband

OS

PPG

58GHz

1.25Gb/s

IM

VPN_Tx_58GHz

OC

VPN_Rx_58GHz

DNF OC

BS-1

Enc_VPN

Dec_VPN PD BERTVPN_Rx_58GHz

Dec1_ULPDBERT

DFB

PC

IM10GHz OS

Enc2_DL

OC

BPF

PPG

EDFA

UL_Rx_baseband

20kmSSMF+DCF

5kmSSMF+DCF

RN

CSDec1_ULPDBERT

OS

EDFA

a

b c

de

f

VOA

VOABPF

Fig. 3. Experimental setup of the proposed 60 GHz RoF system with inter-BSs VPN. SNF: single-notch FBG, DNF: double-notch FBG, DCF: dispersion-compensation fiber.

(a) (b)

(c) (d)

(e) (f)

Fig. 4. Corresponding spectra of the proposed RoF system with the inter-BSs VPN. (a) SOA input signal, (b) SOA output signal after modulation and encoding, (c) twoconverted sidebands with a 60 GHz frequency interval, (d) combination of the US signal and the SSB VPN signal, (e) US signal, and (f) SSB VPN signal.

238 C. Zhang et al. / Optical Fiber Technology 19 (2013) 236–241

vacy. For a common RoF system, it is not necessary to have an en-coder/decoder bank in each BS. In order to achieve the scalablepoint-to-multipoint VPN broadcasting, the OCDM encoder/decoderfor VPN link encoding/decoding should be reconfigurable in the BS,so it has a trade-off between the cost and the performance.

Fig. 2 describes the configuration of optical delay lines (ODLs)-based OCDM encoder and decoder, as well as the operation of opti-cal encoding and decoding. As shown in Fig. 2a, an optical encoderis composed of multiple parallel ODLs, an OS and an optical com-biner (OC). For each ODL, only one fixed time delay s can be per-

C. Zhang et al. / Optical Fiber Technology 19 (2013) 236–241 239

formed, so the length of branch i of the parallel ODLs in the opticalencoder can be given as,

Len�i ¼ L0 þ ci1Tchipc=n; ð1Þ

where L0, ci1, Tchip, c and n are the length of the fixed delay fiber, thecode word element i of the adopted OOC, the duration of a chip, thevelocity of light in vacuum and the fiber refractive index, respec-tively. For the corresponding optical decoder, the length of branchi of the parallel ODLs can be given as,

Lde�i ¼ L0 þ ðv � ci1ÞTchipc=n: ð2Þ

where v is the code length of the adopted OOC. The optical decoderis achieved by using the inversed operation of the optical encoder,as described in Fig. 2b. The operation of optical encoding and decod-ing is shown in Fig. 2c. A signal is firstly encoded by an optical en-coder in the time domain and then the encoded signal after opticalfiber channel can only be decoded properly with the matching code,so as to obtain the ideal signal with a high auto-correlation and a

(a)

(f)

(e)

(c)

Fig. 5. Corresponding waveforms. (a) DS encoded signal with OOC (5, 11, 13), (b) DS decodecoded signal with auto-correlation, (e) VPN encoded signal with OOC (3, 4, 18), (f) VPNfor OOC (5, 11, 13), and (h) VPN decoded signals with cross-correlation for (7, 10, 17).

low cross-correlation. While the encoded signal cannot be decodedproperly if a mismatching code is used.

3. Experiment and discussions

The detailed experimental setup of our proposed inter-BSs VPNin the 60 GHz RoF system is illustrated in Fig. 3. In our proof-of-concept experiment, only the optical domain transmission is per-formed and analyzed due to our hardware constraint. In the CSside, a distributed feedback (DFB) laser at k = 1549.82 nm with10 dB m output power is initially launched into an IM after passinga polarization controller (PC). After the OCS modulation in the IMwhich is driven by a 10 GHz sinusoidal wave, an optical tunable fil-ter (Santec OTF-350) is used to remove the undesired high-ordersidebands of the OCS signal. Then FWM effect is performed inthe SOA (CIP SOA-NL-OEC) by taking OCS signal as the input andit generates two new sidebands by the input two OCS sidebands.The right sideband is reflected by a narrow-band single-notch.

(b)

(h)

(g)

(d)

ded signal with auto-correlation, (c) US encoded signal with OOC (7, 10, 17), (d) USdecoded signal with auto-correlation, (g) VPN decoded signals with cross-correlation

Fig. 6. Measured BER and eye diagrams: (a) DS and US link transmissions, (b) VPNlink transmission. Eye diagrams (A–F) are the corresponding cases at BER = 10�9 forthe DS, US and VPN link transmissions.

240 C. Zhang et al. / Optical Fiber Technology 19 (2013) 236–241

FBG (SNF) filter with 5 GHz bandwidth and 94% reflection ratioand then directed into an IM. 5 Gb/s DS data is modulated onto thereflected sideband and the modulated signal is encoded after-wards. An MUX combines the encoded signal with other threesidebands and the downlink signal with 6 dB m launch power is fi-nally formed. After passing 20 km standard single mode fiber(SSMF) followed by a dispersion-compensation fiber (DCF), thedownlink signal reaches the remote node (RN). At the RN, an er-bium-doped fiber amplifier (EDFA) followed by a variable opticalattenuator (VOA) is installed as well as an OS. The downlink signalreaches each BS after passing through the OS. A DMUX is utilized tofilter out the two sidebands with a 60 GHz frequency interval forthe DS reception. These two sidebands are sent into a 60 GHz PDto generate the DS 60 GHz MMW signal for the DS BER test.2.5 Gb/s US baseband data is modulated onto the left sideband ofthe OCS signal and then it is encoded, thus it forms the uplink base-band signal. Meanwhile, 1.25 Gb/s VPN data is firstly mixed with a58 GHz sinusoidal wave and then modulated onto the right side-band of the OCS signal. After passing a band-pass filter (BPF) andan encoder, the VPN link SSB signal is obtained. An OC couplesthe uplink signal and the VPN link SSB signal together for the UStransmission. After passing a circulator, the coupled signal reachesthe RN in which it is combined with the signals coming from otherBSs and then the combined signal with 0 dB m launch power issent into a DNF filter. The DNF filter with 94% reflection ratio hastwo carefully selected 5 GHz reflecting sidebands and it can simul-taneously reflect two sidebands of the VPN link SSB signal. The up-link baseband signal is transmitted back to the CS along 20 kmSSMF followed by a DCF and an EDFA and then it is decoded anddetected for the US BER test. The reflected VPN link SSB signal istransmitted to another BS along 5 km SSMF followed by the DCFand EDFA, then it is decoded and detected to generate the VPN58 GHz MMW signal for the VPN link BER test. In our experiment,the DS, US and VPN data are all generated by a pulse pattern gen-erator (Anritsu MP-1763C) with 215 � 1 pseudo-random bit se-quence (PRBS).

Moreover, the time domain optical encoders and decoders areachieved by utilizing three parallel ODLs. The adopted OOC in thedemonstration has the code length and code weight of 19 and 3,respectively. Three OOC code words as C11 (5, 11, 13), C12 (7, 10,17) and C13 (3, 4, 18) are used for downlink, uplink and VPN link,respectively. Taking C11 (5, 11, 13) for example, this OOC code indi-cates that the 5th, 11th, 13th positions in the OOC sequence are ‘‘1’’and the other positions are ‘‘0’’. For the 1.25 Gb/s VPN data trans-mission, the chip rate is 23.75 G chip/s and the chip duration is42 ps. Using this OOC, three active inter-BSs VPNs can be simulta-neously established in the proposed RoF system.

The corresponding optical spectra as shown in Fig. 4 have beenobserved by an optical spectrum analyzer (ANDO AQ-6317B). Thespectra of the SOA input signal is shown in Fig. 4a and the inputpower level is above �10 dB m. Two first-order sidebands at1549.74 nm and 1549.9 nm are more than 20 dB higher than thecentral suppressed carrier. The spectra of the SOA output signalafter modulation and encoding is given in Fig. 4b and we can findthat two converted sidebands at 1549.58 nm and 1550.06 nm witha power level of about �20 dB m have been generated. Two con-verted sidebands with a 60 GHz frequency interval are depictedin Fig. 4c and they are sent into a 60 GHz PD to obtain the DS60 GHz MMW. The combination of the US signal which is at1549.74 nm and the SSB VPN signal is illustrated in Fig. 4d, whilethe US signal and the SSB VPN signal are shown in Fig. 4e and f,respectively.

The encoded signal and the decoded signal with self-correlationhave been captured by a digital serial analyzer (Tektronix DSA-8200) as given in Fig. 5a–f. The ODLs-based encoders and decodersare utilized to encode and decode the downlink, the uplink and the

VPN link with different OOCs, respectively. Where C11 (5, 11, 13) isused for the downlink at 1550.06 nm, C12 (7, 10, 17) is used for theuplink at 1549.74 nm and C13 (3, 4, 18) is used for the VPN link at1549.9 nm and 1554.54 nm. The waveforms of the decoded signalswith cross-correlation are illustrated in Fig. 5g–h for comparison.From Fig. 5, we can draw the conclusion that the signals in thedownlink, uplink and VPN link are all successfully decoded withthe distinct auto-correction peaks and a high optical signal to noiseratio (OSNR). Power level of the decoded signals with cross-corre-lation in the VPN link is about �40 dB m which is relatively lowand no obvious noise has been observed to be transmitted to theRF channel.

The measured BER versus different received optical power(ROP) for the DS, US and VPN link transmissions of the proposedRoF system is plotted in Fig. 6. A BER of 10�9 has been plotted withthe black dotted line in Fig. 6 for reference. For the DS transmis-sion, as shown in Fig. 6a, the ROP of about �20 dB m is neededto reach the BER at 10�9. While for the US transmission,�16 dB m ROP is required to reach the BER at 10�9. The eye dia-grams have been captured by Tektronix DSA-8200, and four eyediagrams corresponding to four cases at BER = 10�9 have been gi-ven in inserts (A–D) of Fig. 6. After a 20 km fiber, the certain dete-rioration shows up in the received eye diagrams but the signal canstill be recovered effectively by using optical thresholding tech-nique. Fig. 6b shows the measured BER versus the ROP for the in-ter-BSs VPN link transmission of the proposed RoF system afterB2B and 5 km SSMF + DCF. The ROP required to reach the BER at10�9 is about �17 dB m. Inserts (E) and (F) in Fig. 6 are the clear

Fig. 7. Measured BER of VPN link transmission after 5 km SSMF + DCF: (a)relationship of BER and OSNR at different data rates and (b) relationship of BERand ROP with multiple users at 1.25 Gb/s.

C. Zhang et al. / Optical Fiber Technology 19 (2013) 236–241 241

eye diagrams for B2B and 5 km SSMF + DCF for the inter-BSs VPNlink.

The measured BER versus the OSNR for the inter-BSs VPN linkafter 5 km SSMF + DCF at 1.25 Gb/s, 2.5 Gb/s and 5 Gb/s is shownin Fig. 7a. For the VPN link transmission, the required OSNR toreach 10�9 for 1.25 Gb/s, 2.5 Gb/s and 5 Gb/s cases are about19 dB, 20 dB and 22 dB, respectively. With the increase of data rate,the OSNR needed to reach 10�9 is enlarged. A 1 dB enlargement oc-curs when the data rate is increased from 1.25 Gb/s to 2.5 Gb/s,while a 2 dB enlargement is required when the data rate is in-creased from 2.5 Gb/s to 5 Gb/s. The BER versus the ROP with mul-tiple connected users at 1.25 Gb/s is given in Fig. 7b. For 1.25 Gb/sVPN link transmission, the required ROP to reach 10�9 for one ac-tive user, three active users and five active users are about -17 dB m, �14.3 dB m and �13.4 dB m. Compared with one activeuser, due to the multiple user interference (MUI), the power penal-ties for three and five active users are about 2.7 dB and 3.6 dB,respectively. In the proposed 60 GHz RoF system with inter-BSsVPN, the VPN traffic goes through the OC at the RN twice for rero-uting back to BSs, so the power loss increases dramatically with thenumber of BSs and it might lower down the received VPN powerand thus disable the inter-BSs VPN communication. In order tosupport more BSs in the proposed system, pre-amplification ofthe received VPN signal can be adopted before decoding and detec-tion. In our proof-of-concept experimental demonstration, five BSshave been successfully supported in the VPN link considering thepower loss and the MUI.

4. Conclusion

With the use of OCDM encoding and decoding technology,groups of point-to-point or point-to-multipoint inter-BSs VPNscan be effectively established for the private and secure communi-cation of the multiple end-users under different BSs in the 60 GHzRoF system. By setting up two different MMW channels, the BS–BSVPN link and the common CS–BS link wireless transmission areisolated. In order to reduce the cost of each BS, the FWM effectin SOA is utilized in the lightwave-centralized CS to generate alloptical carriers. Consequently, the proposed 60 GHz RoF systemenhanced with the inter-BSs VPN configuration is a promisingscheme to effectively improve the privacy and security of the fu-ture wireless access networks.

Acknowledgments

This work is supported partly by National Natural Science Foun-dation of China No. 61171045, National key technology R & D pro-gram (No. 2012BAH06B03), 863 Higher Technology Program No.2012AA011304, and National Engineering Lab for Next GenerationInternet Access System (HUST, China) No. NK201202. The authorswould thank Prof. Deming Liu, Dr. Li Xia, Dr. Shuang Liu and Ms.Ting Liu from National Engineering Lab for Next Generation Inter-net Access System (HUST, China) for their experiment help, andanonymous reviewers for improving the clarity and quality of thispaper.

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