Optic/RF Co-design For Oudoor RoF System

at 60 GHz

Sarra Rebhi, Rim Barrak, Mourad Menif

GRESCOM Laboratory, Higher School of Communications of Tunis (Sup'Com), University of Carthage

City of Communications Technologies, 2083, Ariana, Tunisia

rim.barrak@supcom.rnu.tn, mourad.mnif@supcom.rnu.tn

Abstract—Recent works have shown that optical access

network is a promising approach for providing a broadband

infrastructure at mm-wave but almost researches have study

optical mm-wave link and radio stage architecture separately. In

this study, we consider a complete RoF System and we investigate

the link budget for optical and RF stage. We also design the

wireless transceiver at 60 GHz.

Keywords—Outdoor RoF system, OCS, Direct conversion

architecture, Optic/RF codesign.

I. INTRODUCTION

Radio over Fiber (RoF) has provided most intention and several researches have been interested on the optical network access. Such networks provide higher bands and increase significantly data rate with using millimeter-wave (mm-wave) bands [1]. However, only the optical link has attracted the main attention.

The 60 GHz band, in particular, is well suited for RoF system and it offers a 7 GHz unlicensed spectrum (57- 64 GHz).This band can theoretically provide multi-gigabit wireless data rates up to 11Gb/s over one channel of 2.16 GHz [2]. Mainly, two radio standards dealing with these transmissions are developed for Wireless Personal Area Networks (WPAN): ECMA 387 [3] and IEEE 802.15.3c [4]. Four carrier frequencies are defined for radio transmissions: 58.320 GHz, 60.480 GHz, 62.640 GHz and 64.800 GHz. However, at these frequencies, radio signals suffer severely from oxygen absorption and rain attenuation in outdoor environments, which limit the communication distance range however allow frequency reuse and combine the potential deployment of high data rate, large bandwidth for short and medium distance communication.

The main finding of this paper is the design of the radio stage using real components selected from published scientific papers and designed for the 60 GHz band. We simulate a downlink system using the Optisystem software for the optical link and Agilent ADS for radio system. We simulate the EVM (Error Vector Magnitude) varying the wireless distance link.

In this paper, we consider a complete RoF communication system; we investigate the optical network and Radio front-end architectures in section II. Whole system budget downlink is studied in section III. Proposed design radio stage at Base

Station (BS) and wireless transceiver using real components is detailed in Section IV. In the last section, optical/radio co-simulation results are presented.

II. ROF SYSTEM ARCHITECTURE AT 60 GHZ

The integration of wireless and optical networks is a potential solution to meet the requirement for higher signal bandwidth and increases data rates as well as decreasing the costs in the access network.

Fig. 1 shows the generic architecture of the optical–wireless system and performance that will be discussed later in section III. The system is consisting of Central Office (CO), a transmission link, a Base Station (BS) and a wireless receiver.

Assuming BS medium range coverage due to high atmospheric attenuation in the mm-wave band, it is necessary to minimize the cost of the base station (BS) and to shift the system complexity and expensive devices to the central office (CO).

The CO consists of an optical carrier generation unit and a broadband data modulation. For optical mm-wave carrier generation, light from a Continuous Wave (CW) laser source at

a wavelength of 0 at 1550 nm is divided into two parts through an optical splitter, the first part is modulated via an amplitude modulator, driving the modulated data signal with high data rates up to 6Gbps, then up-converted using a dual-arm Mach-Zehnder modulator (MZM) to generate an optical double-sideband signal with optical suppressed carrier (DSB-OSC). The second part is combined with the modulated signal.

After 20 km fiber link, the optical signal is transmitted over the fiber Bragg grating (FBG) and the optical carrier is reflected in order to provide a CW optical source for the up-link. The up-stream data is directly modulated and send directly to the CO.

The millimeter-wave passing through the FBG is converted at the base station into electrical signal using a photo-detector having a responsively of 1A/W. The BS design goal is to make the full duplex link with a simple and low cost design.

The photo-detected signal is then amplified by a Power Amplifier (PA), filtered by a band pass filter and transmitted to free space via antenna with a maximum power of 10 dBm [4]. Base station radio architecture is shown in Fig. 2.

978-1-4673-5820-0/13/$31.00 ©2013 IEEE

CW

MZI OCS

Downlink data

Transmission fiber

Central station

(CO)

Base station

(BS)End-User

(Mobile)

ModUplink data

Uplink receiver

FBGRF

BlocO/E

fmm-wave

fmm-wave

fcarrier Duplexer

RF Receiver

DSP Block

ADC

RF Transmitter

DAC

Fig. 1. Schematic of the 60-GHz RoF link consisting of central office, transmission fiber, base station feeding an out-door End-user.

At the End-User receiver, the signal captured by an antenna

at 60 GHz is down-converted to lower frequencies before data extraction. Several receiver architectures have been used for deploying 60 GHz radio receivers. Heterodyne architecture based solutions integrate intermediate stage allowing RF to IF channel translation before down-conversion to baseband [5]. This architecture increases the requirements for image rejection which can be alleviated using super heterodyne architecture based on three stage conversion [6]. Choosing the high first IF at 36GHz offers better image rejection performance. Also, the choice of 36GHz IF and 24GHz Local Oscillator (LO) frequencies can alleviate the problems of harmonic mixing and the harmonics feed through. However, this architecture still suffers from high analog complexity. Direct conversion architecture reduces design complexity and power consumption. Accurate LO signal generation in mm-wave system is one of challenges for direct conversion topology. As detailed in [2], it is possible to obtain 60 GHz frequency by using a quadrature injection-locked oscillator (QILO). The QILO works as a frequency tripler with a 20-GHz injection-lock input.

BPFE/O PA

Fig.2. Base station radio architecture.

In this work, we owe to use direct conversion architecture as described in Fig. 3. Downstream from the antenna, received signal is band-pass filtered than amplified by a Low Noise Amplifier (LNA). Quadrature mixing allows to down-convert directly RF channels to baseband using tunable local oscillator at 60 GHz band. A low-pass filter is then used to select baseband desired channel and attenuate images frequencies before sampling at Analog to Digital Converter (ADC) stage. Variable Gain Amplifier (VGA) can be used before ADC in order to reduce signal dynamic range and therefore ADC resolution.

LPF

LPF

ADC

ADC

LO π/2

BPF LNA

0

VGA

VGA

RF mixer

RF mixer

Fig. 3. End-User receiver architecture.

III. DOWNLINK BUDGET ANALYSIS

In this section, downlink RoF system at 60 GHz is considered. The link budget analysis is required to find out the acceptable communication distance D for radio link and the minimum received power Psen for optical link. For budget analysis, we assume a BER of 10

-9 and 10

-6 for optical and radio

links respectively and a signal test with 5.28 Gbps data rate and 16 QAM modulation [4].

A. Optical linkbudget

The optical generation of the mm-wave is based on the Optical Carrier Suppression (OCS) method using a cascade of two Mach-Zehnder modulators. Baseband data signal is generated by a first free-chirp MZM biased at the quadrature point and driven by a 5.28 Gb/s 16 QAM electrical signal. The modulated signal is up-converted using a second dual-arm MZM to generate an optical double-sideband signal with optical suppressed carrier where the two arms are set to the minimum

transmission point (V) and driven by two complementary 30 GHz clock.

In order to evaluate the robustness of mm-wave optical generation against the optical fiber impairment, we have considered two kind of transmission fiber. Fig. 4 shows the BER for downlink transmission as a function of the received optical power for 20 km respectively of SMF and NZSDF.

Fig. 4.BER as a function of the optical received power.

A BER of 10-9

is achieved approximately for -17.5dBm for SMF transmission fiber with a power penalty of 3.5dB compared to the NZDSF link. This is due to the effect of large value of chromatic dispersion on SMF fiber in contrast of NZDSF.

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B. Radio link budget

Several research works have presented theoretical link budget at 60 GHz for indoor radio link [7]. In this section outdoor radio link budget is presented. The signal power PR at the input of a receiver in dBm isgiven by Friss equation [7].

𝑃𝑅 = 𝑃𝑇 + 𝐺𝑇 + 𝐺𝑅 –𝐿𝑝𝑎𝑡ℎ (1)

where PT is the transmit power in dBm, GT and GR are the antenna gains in dB at transmitter and receiver side respectively. Lpath is the path loss in dB.In case of free space environment, it is given by Fresnel equation:

Lpath = 20Log 4πfD

C (2)

where D is the distance in m between transmitter and receiver, C is the celerity and f is the frequency.

We assume an attenuation loss of 16 dB/Km at 60 GHz due to atmospheric absorption. Addition losses have to be considered in case of rainy weather, it could reach 10.1dB when rainfall rate is 25mm/h. In this analysis, we consider only atmospheric absorption. Therefore, equation (2) becomes:

Lpath = 128 + 20Log D(km) + 16. D(km) (3)

Receiver sensitivityPRmin is the minimum signal power required to produce an acceptable BER, it is given by:

PR min= Pnoise + NF + SNRmin (4)

where NF is receiver noise figure in dB, SNRmin is the required signal to noise ratio in dBthat meets the error rate criterion in term of BER and Pnoise is the input receiver noise power in dBm given by:

Pnoise = 10 log kT + 10 log(B) (5)

10 log kT is the input noise power density equals to -174dBm/Hz and B is the channel bandwidth.

Assuming a BER of 10-6

, a signal test with 16QAM modulation and maximum data rate of 5.28 Gbps [4], the operating bandwidth is around 2.2 GHz, the required SNR is equal to18.4dB and the noise power is equal to -80.6dBm.

The Federal Communications Commission (FCC) has defined the maximum transmitpower as 10 dBmand the maximum Equivalent isotropically radiated power (EIRP) as 57 dBm [4]. The BS antenna gain can reach 37dB. By assuming a UE antenna gain of 16dBand a total receiver noise figure of 7dB, the PRmin is about -55dBm and the communication distance D can reach 220m.

IV. RF FRONT-END SYSTEM DESIGN

RoF system implementation at 60GHz requires an advanced study of millimeter-wave (mm-wave) technology and circuit components that will allow to provide higher data rates. In this section, RF system architecture parameterization at 60 GHz is detailed. Radio and fiber budget link results are used to set up required gain for BS RF transmitter and EU RF receiver. A special attention should be paid to noise issue when radio receiver design is considered. The same awareness for the linearity problem must be investigated for the transmitter

design. Performance of 60GHz radio system relies on low-noise figure and high gain and linearity of components.

The choice of 60 GHz system components selected from published scientific papers is detailed in the following.

At the BS, the photo-detected signal power is -34dBm. In order to obtain 10dBm at transmitter output, the required gain is about 44dB. The proposed BS radio transmitter at 60 GHz consists of a bandpass RF filter [8], then three power amplifiers [9-10] and a high gain antenna.The RF filter has a flat pass-band and its 1 dB bandwidth is 9 GHz (57–66 GHz). The insertion loss is about 1.5 dB and the returnloss is better than 9.2 dB across the pass-band. The first amplifier gives 9.8 dB gain, then two identical PAs are used, giving each 18dB gain and 16dBm saturated output power. A high gain antenna could contribute in improving network coverage.

At UE receiver side, the required gain for receiver sensitivity is 57dB, when assuming 2dBm ADC Full Scale. After the antenna, the proposed RF bloc is composed of a pass band Filter [8], a 2 stage LNAs [11] and quadrature down-converter from RF to baseband using two quadrature mixers [12] and a tunable frequency local oscillator [13] operating around 60 GHz. At baseband, a low-pass filter followed by a VGA [14] is used. The RF filter is the same as the one in the transmitter side. Using selected components, the resulting receiver gain and noise figure are about56.5 dB and 5.6 dB respectively.

The selected ADC [15] has an input bandwidth up to 1.5 GHz, a maximum sampling rate of 10 GHz and 10 bits resolution, which is sufficient to oversample 1.1 GHz baseband desired channel with 60dB dynamic range.

V. RF OPTIC COSIMULATION RESULTS

The wireless optic system was designed using co-simulation between Optisystem software from Optiwave for the optic link and Agilent’s Advanced Design System for the radio link.

A. Optiwave/ADS design setup

Figure 5 presents the Co-simulation scheme used in conjunction with Optisystem and ADS environments. To communicate between the two environments, we use “Save ADS File” and “Read Timed Data” blocs from Optisystem and ADS libraries respectively.

Fig. 5. Optiwave/ADS co-simulation

B. Optisystem simulation results

Considering the results obtained in section III, we have chosen to consider in the following the NZDSF fiber as transmission fiber.

In order to generate 60 GHz carrier frequency, the OCS schema allows to reach, for the upconverted mm-wave signal,

acarrier suppression ratio larger than 30 dBas described in Fig 6.(a).

At the BS, the optical signal is photo-detected using PIN photo-detector having a responsively 1A/W. Fig 6.(b) shows the electrical mm-wave signal detected at radio stage input. The same spectrum has been obtained from ADS environment.

(a) (b)

Fig.6.Optiwave simulation results : (a) Optical carrier suppression spectrum,(b)output signal spectrum at 60GHz

C. ADS simulation results

The component specifications listed in section IV, have been used to parameterize ADS system models. Fig 7.(a) shows radio transmitter spectrum centered at 60 GHz carrier frequency. Maximum signal power is 10dBm. A noise block is added in order to model the channel thermal noise density.After passing through radio receiver blocks, radio signal is down-converted to baseband. Fig 7.(b) gives Inphase signal spectrum.

(a) (b)

Fig.7. ADS simulation result (a) radio transmitter spectrum, (b) base band signal

Fig. 8.(a) and Fig. 8.(b) illustrate the constellations at radio BS input and at radio EU output respectively, when considering the worst case: input receiver power equals to receiver sensitivity.

Assuming a maximum distance link, we validate the link budget analysis and theatrical results by simulating Optic/radio system Error Vector Magnitude (EVM). EVM has been simulated for variable distances. For D=50m, EVM is 8.43%, and when we increase radio link distance, the EVM is partially degraded and achieves 10.33% and 13.06% for D=150 m and 220 m respectively.

VI. CONCLUSION

In this paper, we have investigated radio stage design for RoF

system at 60 GHz. The designed radio system satisfies the

channel model ofIEEE 802.15.3c and can reach 5.28 Gb/s data

rate for 16 QAM signal. Since the optical fiber system can offer large link transmission, we can reach a wireless radio

linkof 220 m link distance for outdoor and wireless medium

range applications.

(a) (b)

Fig.8. (a) Input radio BS constellation,(b) output radio EU constellation

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