Proposal for 60 GHz wireless transceiver for the radio over fiber system

Chongfu Zhang a,n, Qiaoyan Zhang a, Ying Wang a, Kun Qiu a, Baojian Wu a, Changchun Li b

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

a r t i c l e i n f o

Article history:Received 4 June 2013Received in revised form23 July 2013Accepted 31 July 2013Available online 7 September 2013

Keywords:Radio-over-fiber (RoF)Wireless transceiverSub-harmonic mixerIF digital signal processingCo-simulation

a b s t r a c t

A 60 GHz mm-wave wireless transceiver for radio over fiber (RoF) is proposed, using sub-harmonic mixerand IF digital signal processing. This scheme can overcome LO leakage and I/Q mismatch. Moreover, aneffective verification method of co-simulation between VPI and ADS platforms is considered, this givesfull play to the respective advantages of them and in this way we can obtain the results. From theseresults of the designed wireless transceiver measured by frequency and time domain, we find that 1 Gbit/s data signals carried by 60 GHz mm-wave signals can be successfully recovered. Additionally, theisolation and the conversion loss of the mixer are more than 20 dB, less than 17 dB, respectively.Moreover, the output power of the 60 GHz transceiver is nearly linear with different input powers thatare more than �30 dB m.

Crown Copyright & 2013 Published by Elsevier Ltd. All rights reserved.

1. Introduction

A future access communication system must target ubiquitousbroadband wireless access with large capacity, high data and lowcost application. A radio over fiber (RoF) using optical fiber toachieve high bandwidth, low loss of high data rate signal forremote transmission, and the use of millimeter-wave (mm-wave)can make access rate up to Gbps. Therefore, RoF has beenconsidered as a potential candidate for supporting future broad-band access networks [1, 2]. The transmitter and receiver of60 GHz mm-wave RoF system is an important part of the broad-band access system [3]. And many existing schemes of 60 GHztransceiver have been reported. For example, a single channelimaging receiver is composed of an imaging lens and a smallphoto-sensitive area photodiode attached on a 2-axis actuator foroptical wireless access communication [4], mm-wave poweramplifier (PA) with reliability considerations is for hot carrierinjection degradation [5], 60 GHz phased-array transceiver pairimplemented in 65 nm standard digital CMOS and packaged withan embedded antenna array has also been studied [6], zero-intermediate frequency (IF) receiver or the twice mixer receiverin 65 nm CMOS was found in [7], and another schemes have beenalso considered, such as two-RF-port electro-absorption transcei-ver in 60 GHz RoF system [8], 60 GHz multi-gigabit CMOS trans-ceivers for In-building HD and data [9], and multi-service

60 GHz mm-wave RoF inter-operable with multi-gigabit wirelesstransceiver [10]. However, in those schemes, 60 GHz mm-wavesignals which are received by the wireless transceiver are down-converted to analog baseband signals by zero-IF mixer or twomixers and so on. They divide 60 GHz mm-wave signals into twobranches, and then mix them with two orthogonal LOs (localoscillators) source respectively. In addition, those schemes-basedsystems have some shortcomings such as I/Q mismatch and local-oscillation (LO) leakage [11–13] in order to get two orthogonalbaseband signals due to the analog frequency conversion. So LOleakage and I/Q mismatch are in the existing schemes of 60 GHztransceiver.

In this paper, we give a novel transceiver scheme for the60 GHz RoF system. Moreover, we make use of the co-simulationof VPI which is an optical simulate platform and ADS (advanceddesign system) which is a RF circuit simulate platform. We use theADS platform to design specific 60 GHz mm-wave mixer, then jointhe mixer to the VPI platform to finish the whole 60 GHz mm-wave RoF system demonstration.

2. Principle of the proposed 60 Ghz transceiver

The principle of 60 GHz mm-wave transceiver for RoF is shownin Fig. 1. In this scheme, an antenna receives the mm-wave signalswhose center frequency is 60 GHz and carries data signals from abase station (BS). The received mm-wave signals pass through acirculator to the receiver. In the receiver, firstly, the down-linksignal pass a pre-selection filter to remove the noise from the

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Optics & Laser Technology

0030-3992/$ - see front matter Crown Copyright & 2013 Published by Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.optlastec.2013.07.026

n Corresponding author. Tel.: þ86 28 61830267.E-mail address: cfzhang@uestc.edu.cn (C. Zhang).

Optics & Laser Technology 56 (2014) 146–150

wireless channel, then through a low noise amplifier (LNA) toamplify the received signals and a band-pass filter to remove thenoise from amplification. Secondly, the 60 GHz mm-wave signalsare down-converted to IF analog signals in the designed mixer.The LO source is used for frequency down-convert or up-convertrespect to receiver and transmitter. After a band-pass filter to filerthe noise, a variable gain amplifier (VGA) to gain the power of theIF signals, and a band-pass filter to filter the noise from amplifier,we make the IF analog signals to be IF digital signals by the analogto digital converter (ADC). Then with the IF digital signal proces-sing, we get two orthogonal signals. With the demodulation, thebaseband data signals are recovered. As shown in Fig. 1, thetransmission of wireless terminal transceiver is just reversed.The signals are up-linked to the BS by the transmitter. Firstly, wemodulate the data signals to get two orthogonal digital signals.Then with IF digital signal processing, we obtain IF digital signals.After the digital to analog converter (DAC), the IF signals mix withLO selected by the RF switch to get 60 GHz mm-wave signals.Through the VGA, BPF and power amplifier (PA), the 60 GHz mm-wave signals are transmitted by the antenna.

We use sub-harmonic mixer based on Schottky diode to solvethe problem of LO signal leakage to RF port. The 60 GHz harmonicmixer is considered as shown in Fig. 2. And it is consisted ofmicrostrip lines and two Schottky diodes. The GaAs flip chip usedis MS8251 from Microsemi Corporation. The laminates used areRT/duroid 5880 from Rogers Corporation. And the microstrip lineof λ0=4 could prevent the frequency match to λ0 passing to otherports and offer loop to its own port [14].

We can find that the frequency of LO source is only half of thefrequency of the received RF signal, so it reduces the cost. In thisscheme, the center frequency of mm-wave signals is 60 GHzcarrying the 1 Gbit/s signal, and the frequency of LO source is29.25 GHz, then due to ωIF ¼ ωRF�2ωLO, we can get the centerfrequency of IF analog signal is 1.5 GHz.

Additionally, we adopt the IF digital signal processing to over-come the problem of I/Q mismatch in the existing scheme due tothe analog frequency conversion, as shown in Fig. 3. The IF DSP isdigital IF down-converter base on poly-phase filter [15]. After ADC,the analog signals are converted to IF digital signals. Then, after

parity extraction, sign adjustment, and filtering, the IF digitalsignals are down-converted to two orthogonal baseband signals.It could avoid the impact of the amplitude and phase mismatchbetween the in-phase and the orthogonal branches for the analogdemodulator using two multipliers.

3. System demonstration and results

The simulation system frame in VPI is shown in Fig. 4. In the firstbranch, 1 Gbit/s NRZ signals is modulated on an optical carrier, andcoupled with another optical signal. After optical fiber, we then getthe desired mm-wave carrier whose center frequency is 60 GHz in aphotodetector (PD). After a band pass filter, an amplifier with 33 dBgain and ADS-Interface module, the 60 GHz mm-wave signal is sentto the designed 60 GHz mm-wave mixer module in ADS platform,and get output IF signal with 1.5 GHz center frequency, then back toVPI, continue the IF pass filter, finally get the IF analog signals with2 GHz bandwidth. So after the mixer, the data file is transmitted backto VPI to continue to complete the follow-up link simulation. Theobtained results are shown in Figs. 5–8.

Fig. 5(a)–(d) shows the spectra and waveforms of time domainat nodes (a)–(d) of Fig. 4, respectively. The baseband signal, 60 GHzsignals after the PD, the 60 GHz signals after the BPF and theamplifier, and the IF signals with the central frequency at 1.5 GHzafter the mixer and the BPF. From these results, we can see that the60 GHz mm-wave signals after the PD is properly down-convertedto the IF signal with center frequency at 1.5 GHz by the mixer. Andthe corresponding waveforms in time domain at nodes (a)–(d)have also been obtained.

Fig. 6(a) shows the input IF signal of ADC. The bit rate ofbaseband signal from central station is 1 Gbit/s. After opticalmodulation, the bandwidth of the useful signal is 2 GHz. Sothrough the mixer, we get IF output signal with the centerfrequency at 1.5 GHz, and the bandwidth of the signal is 2 GHz.But the IF signal within half a sideband carries the complete usefulsignal, so the useful baseband signal could be recovered by the IFsignal with the center frequency at 2 GHz, and the bandwidth is1 GHz, as shown in Fig. 6(a). Through the ADC with the 8/3 GHzsampling frequency, the analog signal is converted to digital signal.Then through parity extraction, digital signal spectrum is doublingand shifted, we get the spectrum as shown in Fig. 6(b). The originalsignal could be recovered by the baseband signal of 0–2 GHz.

IF DSPPre-

selectionfilter

LNA BPF BPFmixer BPFVGA ADC

DAC

LO

BPF mixerVGABPF

RF switch

PA

Receiver

Transmitter

Circu-lator

Demod

IF DSP Mod

Fig. 1. The proposed schematic of the 60 GHz mm-wave transceiver for the RoF system.

Fig. 2. The schematic of 60 GHz mm-wave harmonic mixer module.

Fig. 3. The schematic of IF DSP module.

C. Zhang et al. / Optics & Laser Technology 56 (2014) 146–150 147

When the frequency of the RF input is 60 GHz and the power ofthe LO input is 10 dB m, the power of the RF is swept between�12.5 and 6.5 dB m (that is the power of the RF received by thetransceiver is between �44 and �24 dB m, we can obtain that, asshown in Fig. 7 (a), the conversion gain of the mixer is more than�15.2 dB, the RF input to IF output isolation of the mixer is morethan 26.8 dB, the LO input to IF output isolation is more than21.2 dB, the RF input to LO input isolation is more than 26.7 dB.When the power of the RF input port is �24 dB m, the frequencyof the RF is ranged from 59 to 64 GHz, we can obtain that, asshown in Fig. 7 (b), the conversion gain of the mixer is more than

�16.5 dB, the RF input to IF output isolation of the mixer is morethan 24.2 dB, the LO input to IF output isolation is more than21.2 dB, the RF input to LO input isolation is more than 21.5 dB[16].

As shown in Fig. 8 (a), when the input power of the wholesystem is from �44 to �24 dB m and the power of the LO input is10 dB m, we can obtain the output power of IF signals of the60 GHz mm-wave wireless transceiver system is nearly linear withthe input power of the 60 GHz mm-wave signals that is more than�30 dB m. Fig. 8 (b) shows the bit-error-rate (BER) performance ofthe proposed RoF system for the downlink baseband signal

Laser

opticalcoupler

MZM Fiber

Signal1Gbit/s

CS

Laser193.16THz

193.1THz

ADS_Interfacemodule

PDamplifier BPF ...

analyzerbaseband

BPF

LPFBS wireless receiver

Fig. 4. The co-simulation schematic diagram of CS, BS and wireless receiver.

Fig. 5. The measured waveforms in frequency and time domain at different nodes (a) the baseband signal, (b) the received signals after PD, (c) the signal after BPF andamplifier, and (d) the recovered signals after mixer, BPF in Fig. 3.

Fig. 6. The measured waveforms in frequency: (a) the input IF signal of ADC and (b) the signals after IF DSP.

C. Zhang et al. / Optics & Laser Technology 56 (2014) 146–150148

transmission. It gives the received optical power vs. the BER for thedownlink baseband signal with back-to-back (BTB) and 20 kmSSMF, respectively. The 20 km SSMF leads to a power penalty ofabout 0.8 dB at BER of 10�9 for the down-link services. Error-free(BERo10�9) transmissions have been realized with higher than�16.2 dB m received optical power. The measured eye diagramsfor the downlink signal after BTB and 20 km SSMF at �17.25 dB mare shown in the lower-left and the upper-right of Fig. 8 (b).The performance of the transceiver is summarized and comparedwith some recently published efforts as shown in Table 1.

4. Conclusion

This novel transceiver scheme using sub-harmonic mixer and IFdigital signal processing can offer an effective solution to over-come the LO leakage and I/Q mismatch about the 60 GHz-basedsystem. The system co-simulation has shown the feasibility andeffectiveness of this scheme. The results of the designed wireless

transceiver measured we can see that the data signals carried by60 GHz mm-wave signals have been successfully demonstrated,additionally the conversion gain and isolation of the designedmixer, and the BER of the transceiver could meet the requirementsin the RoF system.

Acknowledgments

This work is jointly supported by National Natural ScienceFoundation of China (No. 61171045), National Key Technology R &D Program (No. 2012BAH06B03), Open Fund of State Key Labora-tory on Local Fiber-Optical Communication Networks andAdvanced Optical Communication Systems at Shanghai Jiao TongUniversity (No. 2013GZKF031301), and Open Fund of State KeyLaboratory of Information Photonics and Optical Communications(Beijing University of Posts and Telecommunications).

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Fig. 7. The conversion gain and the isolation of the mixer when the RF input power from �12.5 to 6.5 dB m (a), the conversion gain and the isolation of the mixer when theRF input frequency from 59 to 64 GHz (b).

Fig. 8. The output power of the system vs. the RF input power of the system from �44 dB m to �24 dB m (a), the received optical power vs. bit error rate (BER) of thesystem (b).

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