NTR108275

8/7/2019 NTR108275

1/10

Radio-over-Fiber Systems for Multi-Gbps Wireless CommunicationAnthony Ngoma and Mike Sauer

Science and Technology Division, Corning Incorporated, Corning, NY 14831, USA

ABSTRACT

The paper discusses the challenges of using radio-over-fiber systems to distribute multi-gigabit-per-second wirelesssignals at mm-wave frequencies. We propose possible solutions to the challenges, and demonstrate the potential of

simple radio-over-fiber system architectures to support multi-standard wireless communication at data speeds exceeding

14Gbps using the 60 GHz band.

Keywords: Fiber optics links and subsystems; radio-over-fiber; millimeter waves; wireless communication; chromatic

dispersion; single-carrier; OFDM.

1. INTRODUCTION

In the past, voice and low bit-rate data services were the focus of wireless communication. These services have been

adequately provided by the existing wireless systems having data speeds of up to a few tens of Mbps. However, with the

advent of popular bandwidth-hungry applications such as High-Definition (HD) Video and high-speed Internet, futurewireless systems must offer data speeds exceeding 1 Gbps. Because of limited frequency spectra at low frequencies,

coupled with congestion caused by the large number of consumer products sharing the frequency spectra, it will be

necessary to utilize higher carrier frequencies in the future, including mm-waves, to achieve much faster wireless

communication at multi-gigabit-per-second speeds. Large contiguous frequency bands are available only at mm-wavefrequencies1. For instance, the FCCs 60 GHz band offers 7 GHz unlicensed spectrum (57 64 GHz). However, while

mm-waves offer the much needed bandwidth for ultra-fast wireless communication, they make wireless networking

technically more challenging. The technical challenges relate to the high carrier frequencies and the wide channel

bandwidths used2-4. They include the significantly higher air-link loss (e.g. about 30 dB higher at 60 GHz than at 2.4GHz), and reduced device performance. In addition, the wide channel bandwidth means higher noise power and reduced

SNR. All these factors make wireless networking at mm-waves (e.g. 60 GHz) pico-cellular in nature with the radio

cells typically smaller than 10 m5. Consequently, multi-gigabit-per-second wireless networking at mm-waves requires anextensive high-capacity feeder network to interconnect the large number of radio access points.

It has been shown that Radio-over-Fiber (RoF) technology can provide the required feeder network as it is best suited to

deal with the demands of small-cell wireless networks6. In particular, a major advantage of deploying a fiber-based

Distributed Antenna System (DAS) at e.g. the 60 GHz band is its unique ability to support multiple diverse wireless

applications and services on the same infrastructure5, as shown in Fig. 1. However, performance requirements for RoF

links employed for low-frequency wireless systems differ substantially from those required for mm-wave systems.

This paper explores the different performance requirements for various RoF system architectures employed in the

distribution of multi-gigabit-per-second multi-standard wireless signals. We discuss the technical challenges faced by

RoF systems and propose solutions to deal with the identified challenges5, 7. We demonstrate the potential for RoFsystems to support multi-standard wireless communication at speeds exceeding 14Gbps using the 60 GHz band8. We

consider both single-carrier and OFDM modulation formats.

2. ROF SYSTEM REQUIREMENTS FOR MULTI-STANDARD OPERATION AT MM-WAVE FREQUENCIES

In order to achieve multi-standard (transparent) operation, mm-wave RoF systems must be capable of handling wireless

signals with diverse characteristics. One important characteristic is the signal modulation format. Therefore, mm-wave

RoF systems (just like RoF systems for low-frequency wireless systems), must support both single-carrier modulationformats such as ASK, QPSK, etc, as well as multi-carrier modulation formats such as Orthogonal Frequency Division

Invited Paper

Optical Transmission Systems, Switching, and Subsystems VII, edited by Dominique Chiaroni,Proc. of SPIE-OSA-IEEE Asia Communications and Photonics, SPIE Vol. 7632, 76321I

2009 SPIE-OSA-IEEE CCC code: 0277-786X/09/$18 doi: 10.1117/12.855656

SPIE-OSA-IEEE/ Vol. 7632 76321I-1

Downloaded from SPIE Digital Library on 17 Feb 2010 to 199.197.135.216. Terms of Use: http://spiedl.org/terms

8/7/2019 NTR108275

2/10

Multiplexing (OFDM). The two modulation formats impose different performance requirements with respect to for

instance channel uniformity and peak-to-average power ratio (PAPR). Yet, in dealing with these issues, it is imperative

that the employed RoF systems remain as simple as possible, given the already complex wireless networking at mm-

wave frequencies. For certain applications, the architectures of the employed RoF systems may be simplified to reduce

cost. For instance, for application scenarios requiring short fiber spans of a few hundreds of meters (such as in-buildingdistributed antenna system (DAS) applications), simple IMDD RoF systems may be employed to provide the required

performance without any dispersion compensation. For such applications, system architecture simplicity is a much more

critical than long fiber transmission performance. However, even in those cases, the supported wireless signal datathrough-put must still exceed 1 Gbps.

Centralized Head-End

60 GHz Remote Antenna Unit

Centralized Head-End

60 GHz Remote Antenna Unit

Figure 1. Millimeter-wave RoF system for in-building distribution of multi-standard multi-Gbps wireless signals.

3. SINGLE-CARRIER MODULATED MULTI-GBPS ROF SYSTEMS

3.1 Direct mm-wave modulation vs. optical frequency up-conversion

RoF systems may be classified in terms of whether direct wireless signal modulation or optical frequency up-conversion

is employed. Systems with direct RF signal modulation do not involve any RF carrier frequency translation between the

Head-End Unit (HEU), and the Remote Antenna Unit (RAU), which are the input and output of the RoF system,respectively. In that case the RoF system performs only the signal transport function. The advantage of this system

architecture is that it is extremely simple. However, the system requires an RF signal at the appropriate carrier frequency

at the input. This implies that a different device/sub-system (e.g. electrical up-converter) needs to perform the frequency

up-conversion function prior to transmission by the RoF system. An alternative system architecture, which is similarlysimple is the IMDD RoF system, which incorporates direct baseband data or modulated sub-carrier IF frequency up-

conversion to 60 GHz, and transports the up-converted signal to the RAU. The disadvantage of this system is that it

requires the input signal to be at baseband or low frequency in the case of a data-modulated sub-carrier signal. In this

section, we investigate the impact of channel response flatness on the performance of the two RoF system architectures.

We compare the performances of the two IMDD RoF system architectures when delivering 60 GHz wireless signals,which are ASK-modulated with data-rates up to 4 Gbps and transported over 500 m of standard single-mode fiber. We

study in detail the performance improvement due to feed-forward equalization (FFE) on each system7,9.

3.1.1. Experimental setups

Schematics of the two experimental setups are illustrated in Fig. 2. The basic IMDD RoF system in both cases consisted

of a laser diode followed by a high-speed Mach Zehnder intensity modulator (IM). The signal from the IM wasamplified by an Erbium Doped Fiber Amplifier (EDFA), filtered for ASE noise and then fed into the optical fiber for

transmission. At the end of the optical fiber, a 70 GHz photo-detector was used to generate the ASK-modulated 60 GHz

signal. The generated signal was filtered with a bandpass filter having a -3 dB bandwidth of 3 GHz centered around 60.5



8/7/2019 NTR108275

3/10

GHz. Instead of being connected to an antenna, the 60 GHz signal was fed directly into a 1-step electrical receiver,

which down-converted the 60 GHz signal directly to baseband. The received baseband data was then captured with a 40

GS/s real-time scope (ADC) and passed on to a computer for further analysis in Matlab.

Fiber

fLO60 GHz

0

IMLD

Ele

RXADC

fLO60 GHz

FFE

Data

IN

Data

OUT

Fiber

fLO60 GHz

0

IMLD

Ele

RXADC

fLO60 GHz

FFE

Data

IN

Data

OUT

Fiber

fLO60 GHz

0

IMLD

Ele

RXADC

fLO60 GHz

FFE

Data

IN

Data

OUT

Ele

TX

Fiber

fLO60 GHz

0

IMLD

Ele

RXADC

fLO60 GHz

FFE

Data

IN

Data

OUT

Ele

TX

(a) (b)

Figure 2. Experimental setups of the simple IMDD 60 GHz Radio-over-Fiber systems employing Feed-ForwardEqualization with (a) optical data up-conversion, and (b) direct 60 GHz modulation and transport

0 1 2 3 4-10

-8

-6

-4

-2

0

2

4

6

Frequency, GHz

S21Response,

[dB]

Opt.

Ele.

Figure 3. Frequency responses of the two IMDD RoF system architectures with electrical SSB modulation employed.

The difference between the two systems was that in one case (Fig. 2(a)) the baseband data was fed directly to a 2.5 Gbpsdirectly modulated laser and the 60 GHz LO used to drive the IM. In the second case (Fig. 2(b)), the baseband data was

first up-converted directly to 60 GHz by an electrical transmitter, whose output was then used to drive the IM. The

systems were kept as simple as possible because some applications such as in-building applications do not need theadvanced performance (e.g. long distance transmission) that many sophisticated RoF systems offer.

3.1.2. Results

First, the S21 responses of the two systems were measured between the inputs (Data IN) and the outputs (Data OUT) of

the RoF systems with the generated signal placed in three (3) modes namely the Double-Sideband (DSB), Lower

Sideband (LSB), and the Upper Sideband (USB) modes. This was achieved by tuning the carrier frequency of thegenerated 60 GHz signal (60.5 GHz, 61.8 GHz, and 59.2 GHz, respectively) in order to filter off the unwanted part of the

signal the generated signal passes through the BPF. The advantage of the two Single-Sideband (SSB) modes is that they

theoretically lead to double the electrical spectrum utilization efficiency. However, it was observed that in both systems,

the DSB mode produced the flattest frequency response over the 1.5 GHz band. However, when operated in the SSBmode, both systems showed deteriorated frequency responses over the increased bandwidth, when compared to their

DSB link responses. When the link responses were compared between the two systems, it was observed that System A

exhibited a smoother/flatter frequency response than System B in all cases. For instance, in the SSB mode, the amplitude

fluctuation was about 6 dB for System A compared to 14 dB for System B over increased 3 GHz band, as shown in Fig.



8/7/2019 NTR108275

4/10

3. The reason for the different system responses is that the link response of the system employing optical frequency up-

conversion (System A), is dominated by the low-frequency modulation device, which is the laser diode in this case,

while that of System B is dominated by the high frequency optical modulated, whose response is not as flat as that of the

laser diode over the 3 GHz bandwidth.

2 4 6 8 10 12 14 16 1 8 20-0.25

-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

0.2

0.25

2 4 6 8 10 12 14 16 1 8 20-0.25

-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

0.2

0.25

2 4 6 8 10 12 14 16 18 20-0.25

-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

0.2

0.25

2 4 6 8 10 12 14 16 18 20-0.25

-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

0.2

0.25

(a) (b)

Figure 4. Performance improvement due to feed-forward equalization in 4 Gbps ASK-modulated 60 GHz RoF systemsemploying (a) optical up-conversion, and (b) electrical up-conversion.

When ASK data transmission was included, both systems exhibited distorted eye diagrams for various bit-rates up-to 4

Gbps due to ISI caused by the un-even frequency responses. However, System B exhibited much more severe eyediagram distortion than System A, as shown in Figure 4. For the same reason System B showed a much higher error

floor (1x10-4) than System A (1x10-8).

The impact of FFE on the captured baseband data was analyzed by applying a FFE algorithm implemented in Matlab.

The equalization algorithm was based on least-mean-square adaptation to maximum the eye-opening at the decision

points. The result based on the calculation of the estimated BER for 4 Gbps data transmission is summarized in Fig. 4. It

is clear from Fig. 4 that FFE substantially improved the sensitivities of both systems with and without fiber transmission,making it possible to achieve error-free transmission. The sensitivities of the two systems for 4 Gbps data transmission

with 500m fiber transmission and FFE were -9 dBm and -7 dBm for the optically and the electrically up-convertedsystems, respectively.

The required minimum number of equalizer taps for different transmission conditions of 4 Gbps ASK-modulated data

was investigated. The result is summarized in Table 1. It was observed that system A required 16 taps while system B

required 20 taps to achieve the best system performance. Furthermore, the minimum number of taps for best

performance was independent of the bit-rate and was the same with and without 500 m fiber transmission.

Table 1. Number of Feed-Forward Equalizer taps needed to optimize the BER performance of the RoF systems at 4 Gbps

System /

Fiber Length

Optical

Up-conversion

(System A)

Direct Modulation (Electrical

Up-conversion System B)

B2B 16 20

500m 16 20

From the results above, the BER performance of the optically up-converted system is significantly better than that of the

electrically up-converted system, whose non uniform frequency response is dominated by the non-uniform responses of

the electrical transmitter and the high-speed optical intensity modulator. Furthermore, the results confirm that simpleIMDD RoF systems assisted by feed-forward equalization applied to the recovered baseband data can be used to

distribute > 4 Gbps ASK modulated 60 GHz signals on a single RF carrier over > 500m standard single-mode fiber.



8/7/2019 NTR108275

5/10

(a)

IN 1

IN 2

LOf

60 GHz

Fiber

Opt-

I/Q

TX

Ele-

I/Q

RX

I

Q

I

Q

LO

2

f

BW = 1.8 GHz

BW = 3 GHz30 GHz

FFEOUT 1

OUT 2

(b)fLO

30 GHz

1

Fiber

IM_1LD1

PPG FFEBPF

DCA

AMPLNA

A

B

Head-End Unit

= 1549 nm

LPF

D1

D2

ODL

EDFA

tunable

bandpass

filter

IM_2LD2LPF

TF

LPF

LPF

2

60 GHz I/Q RX

Remote

Antenna

Unit

1 546 15 48 15 50 1 552 1 554

-60

-50

-40

-30

-20

-10

0

Wavelength, (nm)

Power,

(dBm

)

I Q1 546 15 48 15 50 1 552 1 554

-60

-50

-40

-30

-20

-10

0

Wavelength, (nm)

Power,

(dBm

)

I Q

(i)

Figure 5. Combined optical I/Q modulation, frequency up-conversion and fiber transport (a) Concept, (b)experimental set-up. PPG = Pulse Pattern Generator, ODL = Optical Delay Line, DCA = Digital

Communication Analyzer, FFE = Feed-Forward Equalizer

3.2 Combined optical I/Q modulation, frequency up-conversion and signal transmission

Despite the large contiguous bandwidth available at 60 GHz, multi-Gbps wireless communication requires modulation

formats that are more spectrally efficient than simple ASK modulation, even at 60 GHz. That is high-order modulationformats such as QPSK, 8-QAM, etc will be employed for both single- and multi-carrier systems. In this section, a RoFsystem incorporating an optical I/Q transmitter capable of vector modulating and up-converting baseband data directly to

60 GHz is demonstrated10. The RoF transmitter was further simplified by incorporating frequency doubling, thereby

reducing the maximum operating frequency in the transmitter to just 30 GHz, as shown in Fig. 5(a). In addition,

employing frequency doubling extended the carrier-fading length limit imposed by chromatic dispersion to beyond 50

km allowing, the RoF system to operate over flexible and much longer fiber spans.

3.2.1 Experimental Set-up

The experimental set-up for the I/Q RoF system is shown in Fig. 5(b). The I/Q modulator consisted of two separate

Intensity Modulated - Direct Detection (IMDD) links. The intensity modulators (IM) were Mach-Zehnder modulators.

Both IMs were biased at their minimum transmission points so as to suppress the optical carrier. The 90 degree phase

shift (4.146 ps) required for I/Q modulation at 60 GHz was achieved by inserting an optical delay line in one of the links.Both IMs were driven with a 30 GHz CW LO signal. The optical signals entering the IMs, were pre-modulated with

baseband data through direct modulation of two separate lasers operating at slightly different wavelengths (at least 0.5

nm apart), as shown in insert (i) of Fig. 5(b). The two optical signals were coupled together prior to transmission over

optical fiber. At the RAU, a single 67 GHz photo-detector was used to generate a QPSK-modulated mm-wave signal at60 GHz.



8/7/2019 NTR108275

6/10

3.2.2. Results

Fig. 6 shows a clean constellation diagram of the QPSK-modulated data (2.125 Gbps) recovered from the 60 GHz signal

generated at the RAU. To obtain the constellation diagram shown in Fig. 6, FFE was applied to both I/Q streams in order

to minimize ISI effects due to the non-uniform frequency response of the link over the > 1 GHz bandwidth of the RoFlink. The impact of the links frequency response and simple linear FFE on the performance of the RoF system are

illustrated in Fig. 7. It can be seen in Fig. 7 that the eye diagrams of both I and Q data streams were nearly closed

without FFE. When FFE was applied on each stream alternately, the eye diagrams were opened clearly. It was shownthat by combining QPSK modulation, linear FFE to equalize for ripples in the frequency response of the link, and SSB

modulation, 7 Gbps could be transmitted on a single 60 GHz carrier using the same 3 GHz link bandwidth as was used in2 Gbps ASK-modulated IMDD RoF system employing operating in the DSB mode as discussed above. This represents a

spectral efficiency that is three times higher than that of simple ASK modulation in the DSB mode7,9,10. If multi-level

amplitude modulation were to be applied to the baseband data, then the generated 60 GHz signal would be modulated

with a multi-level quadrature amplitude modulation format (i.e. xQAM), further increasing the spectral efficiency anddata through-put.

-0.04 -0.03 -0.02 -0.01 0 0.01 0.02 0.03 0.04

-0.04

-0.03

-0.02

-0.01

0

0.01

0.02

0.03

0.04

Figure 6. Constellation diagram of the generated QPSK signal down-converted from the 60 GHz carrier. Total data-rate =2.125 Gbps

SNR =

3.4dB

SNR =

-.-dB

I

Q

SNR =

2.8dB

SNR =

-.-dB

I

Q

(a) (b)

Figure 7. Impact of Feed-Forward Equalization (FFE) on the performance of the 60 GHz I/Q RoF system operating at 7Gbps with transmission over standard single-mode fiber. (a) FFE applied to the I channel, (b) FFE applied to the Q channel.

4. OFDM MODULATED ROF SYSTEM OPERATING AT 14 GBPS

As stated earlier, 60 GHz RoF systems must be able to handle wireless signals with different characteristics in order to

achieve multi-standard system operation. Therefore, apart from supporting single-carrier wireless signals having multi-

level modulation formats as discussed above, 60 GHz RoF systems must also support wireless signals having multi-carrier modulation formats, such as OFDM. In such systems, the sub-carriers themselves may employ multi-level signal

modulation. Single-carrier and Multi-carrier modulation formats tend to impose different system performance

requirements on the 60 GHz RoF systems. For instance channel uniformity is very critical for single-carrier systems as



8/7/2019 NTR108275

7/10

discussed above7,9. On the other hand, the presence of multiple carriers in the OFDM signal format makes linearity and

the ability to handle a high peak-to-average power ratio (PARP), very critical system parameters. Furthermore, the

system requirements are rendered even more critical for the very wide-band (>1 GHz) channels being considered at 60

GHz and other mm-wave bands. The consequence of these requirements is that they lead to the use of complex RoF

system architectures (e.g. dual-electrode modulator structures) for OFDM signal transmission11. However, it isimperative that the employed RoF links are as simple as possible to reduce cost, while providing the needed

performance.

4.1.1. Experimental Set-Up

The experimental set-up of the RoF system is depicted in Fig. 8. The 7 GHz-wide OFDM signal was generated by first

using a Tektronix Arbitrary Waveform Generator (AWG) to generate a 3.5 GHz-wide OFDM signal from a Matlabprogram, and then up-converting the OFDM signal to 25 GHz8. This resulted in two OFDM sidebands centered at 25

GHz, with a combined bandwidth of 7 GHz. Both sidebands were transmitted over the RoF system. Since the subcarriers

were transmitted independently, and demodulated independently at the receiver, the total bit-rate of the 7 GHz-wideOFDM signal was double that of the original OFDM signal generated by the AWG.

The resolution of the digital-to-analogue converter of the AWG was set to 8 bits. The digital to analog converter (DAC)

sampling rate was 24 GS/s. The IFFT length was 256, resulting in a subcarrier symbol rate of 93.75 MSym/s. The 3.5

GHz-wide OFDM signal consisted of 37 sub-carriers, which were modulated with the QPSK format. Therefore, the 7

GHz-wide OFDM signal at the output of the mixer consisted of a total of 74 sub-carriers with a combined data-rate of

13.875 Gbps. The 7 GHz-wide OFDM signal at 25 GHz was amplified and combined with a 35.5 GHz LO signalgenerated by a signal generator, as shown in Fig. 1. The composite signal was then used to drive a single-electrode Mach

Zehnder Modulator (MZM) specified for 40 Gbps data transmission, located in the head-end unit (HEU).

The HEU system consisted of a Distributed Feedback (DFB) laser emitting +10.5 dBm optical power at 1550 nmwavelength. The CW optical signal was fed into the MZM, where it was modulated by the combined OFDM and LO

signals as shown in Fig. 1. The MZM modulator was biased at the point of minimum transmission in order to suppress

the optical carrier. Therefore the optical signal exiting the MZM comprised a total of four (4) sidebands - two un-

modulated subcarriers at fo 35.5GHz and two OFDM-encoded subcarriers at fo 25GHz, where fo is the optical carrier

frequency, as shown in insert (i) of Fig. 8. The modulated optical signal was amplified by an Erbium Doped FiberAmplifier (EDFA), filtered for ASE noise, and transmitted to a Remote Antenna Unit (RAU) connected by standard

single-mode optical fibers of different lengths.

FiberFiberMZMLD

LPF BPF

OBPF BPF

Head-End Unit Remote Antenna Unit

25 GHz

25 GHz

35.5 GHz

EDFA

LNAO/E

(i)

fo

1549.6 1549.8 1550.0 1550. 2 1550.4 1550.6-55

-50

-45

-40

-35

-30

-25

-20

-15

Leve(

m

Wavelength (nm)

(i)

fo

1549.6 1549.8 1550.0 1550. 2 1550.4 1550.6-55

-50

-45

-40

-35

-30

-25

-20

-15

Leve(

m

Wavelength (nm)

o

Real Time

Scope

56.75 GHz

AMP

LNA

A

AWGOFDM /

QPSK

C

3.5 GHz3.5 GHz

60.5 GHz

7 GHz

3 m

0 2 4 6 8 10-100

-80

-60

-40

-20

Power(dBm)

Frequency (GHz)

(ii)

Figure 8. Experimental set-up of the 60 GHz remote frequency up-conversion RoF system employing one single electrodeMach Zehnder Modulator, 14 Gbps OFDM-QPSK data modulation with fiber and 3 m wireless transmission distance.

At the RAU, the OFDM signal at 25 GHz was up-converted to 60.5 GHz through square-law photo-detection (mixingwith the transmitted 35.5 GHz LO signal) in the 67 GHz photodiode. The generated signal was amplified by an LNA

with a gain of 38 dB. A bandpass filter with 7 GHz bandwidth centered around 60.5 GHz was used to remove the

unwanted signals from the mixing products outside the band of interest. Therefore, the OFDM signal generated at the

RAU occupied the full 7 GHz spectrum at the 60 GHz band specified by the FCC (57 64 GHz).

The signal generated at the RAU was fed into a standard gain horn antenna (gain = 23 dBi) and transmitted over 3m

wireless distance. In order to analyze the quality of the transmitted signal, a 60 GHz receiver consisting of another



8/7/2019 NTR108275

8/10

standard gain horn antenna, a mixer and a 56.75 GHz LO was used to down-convert the transmitted 60 GHz signal to an

IF frequency at 3.75 GHz as shown in insert (ii) of Fig. 8. The IF frequency was chosen so as to maintain the full 7 GHz

spectrum of the down-converted OFDM signal. The down-converted signal waveforms were captured by the real-time

oscilloscope for offline signal processing and analysis.

4.1.2. Results

Fig. 9 shows the electrical spectrum of the down-converted OFDM signal, which is 7 GHz wide. The peak observed at

3.75 GHz came from the 25 GHz IF LO used to up-convert the baseband OFDM signal at the HEU. The down-converted

signal was demodulated by a software DSP program. Fig. 10 shows the constellation diagrams of the demodulated

OFDM signals with and without fiber transmission between the HEU and the RAU. The total data-rate of the signal was

13.875 Gbps. The detected optical power corresponding to this constellation diagram in Fig. 10(a) was -10.5 dBm. Thecalculated Error Vector Magnitude (EVM) was 16 %. The clean constellation diagram in Fig. 10a confirms the excellent

performance of the RoF system in generating high quality wideband OFDM signals at 60 GHz. It also shows the

potential for increasing the bit-rate by using modulation formats of orders higher than QPSK.

To investigate the performance of the RoF system when fiber transmission was included, different spans of standardsingle-mode fiber (0.5 km 5 km) were inserted between the HEU and the RAU, and the quality of the recovered signal

analyzed in terms of the EVM and the bit error ratio (BER). The BER was estimated from the measured EVM. The

results are summarized in Fig. 11. Without any fiber transmission, the estimated optical power sensitivity of the RoF

system at the BER of 10-9 and a data-rate equal to 13.875 Gbps was -9.5 dBm. The performance of the RoF system with

500m and 1 km of fiber transmission was the same as without optical fiber, indicating no dispersion penalty. For fibertransmission distances of 2km and 3km, there was a penalty of 1 dB, and 2.0 dB, respectively. A larger penalty was

observed after transmission over 4km of standard single-mode fiber, as shown in the constellation diagram given in Fig.

10 (c). The electrical spectrum of the down-converted OFDM signal given in Fig.9 shows that signal fading due tochromatic dispersion was responsible for the degradation in system performance over longer fiber lengths.

The experimental results above demonstrate that a simple RoF system architecture, with just one single electrode MZM,

has the performance required for the up-conversion and distribution of large bandwidth high-data-rate (14 Gbps) OFDM

signals occupying the entire 7 GHz spectrum at the 60 GHz band. More than 3km fiber transmission distances can be

supported without any dispersion compensation. Fiber links of 3 km are sufficient for most short-range RoF applicationssuch as in-building systems, where low system complexity is very critical.

0 2 4 6 8 10

-70

-60

-50

-40

-30

-20

BTB

0.5km1 km

2 km

3 km

4 km

5 km

Power(dBm)

Frequency (GHz)

Figure 9. Electrical spectrum of the OFDM signal down-converted from the transmitted 60 GHz signal



8/7/2019 NTR108275

9/10

(a) BTB (b) 3km (c) 4km

Figure 10. Constellation diagrams of the 14 Gbps OFDM-QPSK signal received after transmission over standard single-mode fiber and 3 m wireless distance.

-16 -15 -14 -13 -12 -11 -10 -9 -8 -7 -6 -5 -4

109

8

7

6

5

4

3

2

BTB

500 m

1 km

2 km

3 km

4 km

-Log

(BER)

Receiver Power (dBm)

Figure 10. BER of performance of the 14 Gbps OFDM-modulated 60 GHz RoF system with 3m wireless transmissiondistance.

5. CONCLUSIONWe have shown that RoF systems for mm-wave bands such as 60 GHz have a clear potential to enable multi-standardwireless communication at ultra-fast multi-gigabit-per-second data-rates. However, the high mm-wave carrier

frequencies and wide channel bandwidths utilized by mm-wave systems present unique technical challenges for RoF

systems. Nevertheless, it is imperative to keep the architectures of the RoF systems employed to deal with the technicalchallenges associated with mm-wave signal transport simple in order not to exacerbate the already very complicated

wireless networking at mm-wave frequencies. In this paper, we have demonstrated the different system requirements

imposed by various high data-rate mm-wave wireless signals. We have shown that RoF systems with simple system

architectures can successfully deal with the challenges of distributing diverse (single-carrier, OFDM, etc) multi-Gbps (>

14 Gbps) wireless signals at mm-wave frequencies such as 60 GHz over un-compensated fiber links of up to a fewkilometers.

REFERENCES

[1] Wells, J., Faster than fiber: the future of multi-Gb/s wireless, IEEE Microwave Magn. New York, vol. 10, 104 112 (2009).

[2] Razavi, B., Gadgets gab at 60 GHz, IEEE Spectrum, vol. 45, no. 2, 46-58 (2008).



8/7/2019 NTR108275

10/10

[3] Park, C., and Rappaport, T. S., Short-range wireless communications for Next-Generation Networks: UWB, 60GHz millimeter-wave WPAN, and ZigBee, IEEE Wireless Communications, vol. 14, no.4, 70-78 (2007).

[4] Smulders, P., Exploiting the 60 GHz band for local wireless multimedia access: prospects and future directions,IEEE Comm. Magn., vol. 40, no. 1, 140-146, 2002.

[5] Ngoma, A. and Sauer, M., Radio-over-Fiber technologies for high data rate wireless applications, Proc of SarnoffSymposium, (2009).

[6] Sauer, M., Kobyakov, A. and George, J., Radio over fiber for picocellular network architectures, JLT, vol. 25, no.11, 3301-3320 (2007).

[7] Ngoma, A., Sauer, M., Annunziata, F., Jiang, W-J., Lin, C-T., Chen J., Shi, P-T. and Chi, S., Simple Multi-Gbps60 GHz Radio-over-Fiber Links Employing Optical and Electrical Data Up-conversion and Feed-Forward

Equalization, Proc. OFC 2009, OWF2, (2009).

[8] Ngoma, A. Sauer, M., Annunziata, F., Jiang, W-J., Shih, P-T., Lin, C-T., Chen, J. and Chi, S., 14 Gbps 60 GHzRoF link employing a simple system architecture and OFDM modulation, Proc IEEE MWP 2009, (2009).

[9] Ngoma, A., Sauer, M., George, J. and Thelen, D., Bit-rate doubling in multi-Gbps wideband ASK-modulated 60GHz RoF links using linear feed-forward equalisation and direct conversion transceivers, Proc of ECOC2008,

P.6.12, (2008).[10] Ngoma, A., Sauer, M., Thelen, D. and George, J., Data throughput tripling by feed-forward equalization and

photonic QPSK in a 7 Gbps single-carrier RoF link at 60 GHz, in Proc MWP2008, 213 216, (2008).

[11]Lin, C. T., Wong, E. Z., Jiang, W. J., Shih, P. T., Chen, J. and Chi, S., 28Gb/s 16QAM OFDM RadiooverFiber

System within 7GHz License

Free Band at 60 GHz Employing All

Optical Up-Conversion, Proc. CLEO 2009,

CPDA8, (2009).


Documents

NTR108275