Channel RoF 01

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Tu.B4.2 78 ICTON 2008

978-1-4244-2626-3/08/$25.00 ©2008 IEEE

Optical Ultra-Wideband Pulses for Multi-Channel

Radio-over-Fiber Communication Systems

Wen-Piao Lin, Member, IEEE and Yuan-Ching Chen

The Department of Electrical Engineering, Chang Gung University, Tao Yunan 333, Taiwan

Phone: 886-3-2118800~5493, Fax: 886-3-2118026, e-mail: [email protected]

ABSTRACT

This study focuses on design of an optical ultra-wideband (UWB) pulse system with binary phase shift keying(BPSK) modulation scheme for multi-channel radio-over-fiber (ROF) communication environments. Thegenerated optical Gaussian-like pulses can provide the several GHz bandwidths and improve the fiber dispersionand inter-symbol interference (ISI) problems for high-bit-rate transmissions. Moreover, a peak power detectionscheme is adopted by the orthogonal frequency receiver, which can obtain a minimum channel bandwidth for themulti-channel systems. Finally, we set up a practical experimental system to prove our proposed schemes.Modeling analysis and experiment results demonstrate that the proposed system can provide a large capacity andlower interference noise for the multi-channel UWB wireless communications.Keywords: ultra-wideband (UWB), binary phase shift keying (BPSK), radio-over-fiber (ROF), inter-symbol

interference (ISI)

1. INTRODUCTION

Recently ultra-wideband (UWB) technologies for high-rate wireless personal area networks (WPAN) and digitalhome have attracted more and more attention. UWB systems offer a very wide frequency spectrum of a bandwidth more than 500 MHz in the range from 3.1 GHz to 10.6 GHz, and extremely low-power spectraldensity with a restricted total effective isotropically radiated power (EIRP) less than 0.556 mW in the 7.5 GHz

band, over short distance of a few meters [1-2]. The UWB system is a leading candidate for next-generation,indoor short-range, high-throughput wireless connections.

According to new Federal Communications Commission (FCC) regulations [3], UWB radio transmission canlegally operate in 7.5 GHz bandwidth at transmit power of 1 dBm/MHz; but it has not standards on the types of data modulation and multiple access schemes. Therefore, the new regulations can provide the various choices for UWB wireless system designers. At present, the main technologies of UWB multiple access (MA) employ thedirect sequence impulse radio (DS-IR) approach and multi-band orthogonal frequency division multiplex

(OFDM) approach. Multiple-input multiple-output (MIMO) UWB system can use different modulation and MAtechniques, including time-hopping (TH) M -ary pulse position modulation (MPPM), TH binary phase shiftkeying (TH-BPSK), direct sequence BPSK (DS-BPSK), etc. According to the previous review, DS-BPSK is

better than other schemes in MA scenarios [4-5]. In this paper, we propose a new optically-based DS-BPSK modulation scheme to generate the biphase Gaussian monocycle pulse for UWB applications. Moreover, wedesign a correlated receiver of the minimum channel spacing that is an orthogonal frequency receiver in theend-user to provide a minimum channel bandwidth for the multi-channel communications. Such a modulationscheme with an orthogonal frequency receiver is characterized by the potentially high capacity of the channel,low average power consumption and high immunity again multiple access interference (MAI) for themulti-channel UWB radio-over-fiber (ROF) communication systems.

2. MULTI-USER UWB SYSTEM

The block diagram of the multi-channel UWB radio-over-fiber communication system is presented in Fig. 1.At the central station (CS), optical pulse sequences from a gain-switched distributed feedback laser diode(DFB-LD) pass through an electro-absorption modulator (EAM) to generate a data-modulated optical signal. TheEAM is driven by an amplified biphase shift keying (BPSK) signal which is composed of the output signal of two microwave subcarrier channels. Each channel includes a two-level pseudorandom bit sequence (PRBS) withan upconverted microwave carrier frequency passing through a frequency mixer. The DFB-LD used isa commercial 1550 nm device with a threshold current ( I th) of 14.6 mA at 25°C. A radio-frequency sinusoidsignal is adopted to drive the DFB-LD into gain-switched operation using a bias-tee circuit. The centraloperating range of EAM is from 1530 to 1563 nm. The modulation bandwidth is 10 GHz, and the output optical

power is as high as 7 dBm. The data-modulated optical signal is then applied to a single-mode fiber (SMF) of 10 km, which is used as a transmission medium. At the radio access unit (RAU), electrical Gaussian-like pulsesare obtained at the output of a p-i-n-photodiode (PIN-PD), and are amplified using a power amplifier and thenfed to a special UWB antenna with bandpass characteristics. These pulses are transmitted to the end-user througha free space of 1 meter.

At the end-user, the one of subcarrier channels is extracted using a receive UWB antenna, a low-noiseamplifier (LNA) and bandpass filter (BPF). Moreover, the microwave subcarrier signal is transmitted to

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ICTON 2008 79 Tu.B4.2

a frequency mixer with the phase-shift original carrier frequency to downconvert into baseband BPSK signals,and then passes through a UWB correlated receiver to obtain the original data sequence. The correlator consistsof a power amplifier, an electrical integrator, a sample and hold circuit, and a data comparator. The receiving

pulse signal is treated by a high-speed integrator and a sampling and hold circuit to extract a peak power point,and then using a comparator to recovery the original data.

DFB-LD PIN-PD

Bias Tee

SMF

DCBias

PA

A

Radio Access Unit

Tr.

Antenna

EAM +Optical Gain

BCentral Station

RFAmp.

SG3(200MHz)

⊗

⊗

PG

Ch.2

Ch.1

∼

SG1(4 GHz)

SG2(6 GHz)

∼∼

BPF

BPFIntegrator Comparator

DCAAgilent86100A

PA LNAPA

Rec.Antenna

Ch.1data

DE

End-User

Mixer

Phaseshift

→

Δφ

S/HCircuit

C

FSAAgilentE4407B

Fig. 1. Multi-channel UWB radio-over-fiber system.

3. SYSTEM MODEL

To enable multiple access (MA) capability, spread spectrum communication techniques can be adopted. Timehopping (TH) and direct sequence (DS) are two methods commonly considered for UWB MA applications.In direct sequence binary pulse shift keying (DS-BPSK), the modulation technique uses π phase shift to enable

binary signal; and a multi-channel coded pulse train using a set of Gold codes modulated UWB Gaussian pulsesat the transmitted end can be written as [6]

1

0

( ) ( )c N

k k k k

TX TX j n b c

j n

S t E a c g t jT nT −∞

= −∞ =

= − −∑ ∑ (1)

where E TX is the energy of a chip, T c is the spreading code chip period, T b is the bit period, N c is the length of Gold code word (T b /T c),

k

nC is the spreading chip value, k

ja is the modulated data symbol value, k is a number

of users, and g(t) is the Gaussian monocycle pulse waveform.The impulse response of the channel can be modeled as

1

0

( ) ( ) L

k k

k

h t t β δ τ −

=

= −∑ (2)

where L is the number of resolvable multipath components, β k is the fading amplitude of the k th resolvable path, τk is the delay time of the k

th resolvable path and δ(t) is the Dirac delta function. For previous studies, the fadingamplitude is lognormally distributed and βk is considered to be constant during a symbol interval. Here, let

h(t)= β 0 δ(t − τ 0 ) to simplify the multipath channel modeling. When there are N u channels in a multi-channel

system, the received signal of the k th channel can be expressed as

1

1 0

( ) ( ) ( )

( ) ( ) ( )

u c N N k k k

k TX j n b c k

k j n

u mui

r t E a c g t jT nT n t

r t r t n t

β τ −∞

= = −∞ =

= − − − +

= + +

∑ ∑ ∑(3)

where r u(t) is the expected receive signal of the k th channel , r mui(t) is the received interference signals from other (k-1)

th channels and n(t) is the additive white Gaussian noise (AWGN) process with a two-sided power spectraldensity of N 0/2. Assume the expected channel is the first (i.e., k = 1) with a reference delay time τ1 = 0 and thereceiver signal r u1(t) and r mui1(t)can be calculated.

The optimum receiver for the channel is a correlated receiver. The locally generated monocycle signal mk (t) at the receiver is synchronized with the incoming monocycle train and is given by

1

0

( ) ( )c N

k

k n c k

n

m t c g t nT τ −

=

= − −∑ (4)

after correlated receiver and sampling hold process, the output signal at the first receiver can be expressed




Tu.B4.2 80 ICTON 2008

as 1 1 1 10

( ) ( )bT

u mui n Z r t m t dt Z Z Z = = + +∫ . Zu1 is the expected receive signal of the first channel 1 1

1 0u RX c Z E a N = .

Moreover, we derive the bit error rate (BER) of this receiver. Because the energy of g(t) is normalized tounity, the instantaneous signal-to-noise ratio per bit, SNRb, is obtained as follows

2 1

1

2

0

2u RX c

b

n

Z E N SNR

N σ = = (5)

where 2

nσ is a bit noise energy. Furthermore, the interference energy 2

muiσ of multi-channel signals to the first

channel receiver can be expressed the following

2 2

02

( )u

W

N T

k c

mui RX

k c

N R d E

T σ τ τ

=

= ∑∫ (6)

where R( τ ) is autocorrelation function of the pulse waveform g(t) and T W is the pulse width. Here, we

let 2 2

0( )

W T

W R d σ τ τ = ∫ . Therefore, the signal-to-interference ratio at channel 1 can be achieved as follows

2

1

22

1

2

u

u c c

b N k

mui RX W

k RX

Z N T SIR

E

E

σ σ

=

= =

∑. (7)

For communicating over an AWGN channel with a correlated receiver (i.e. matched filter), the theoretical probability of bit error P b for DS-BPSK modulation under standard Gaussian approximation (SGA) [7]

hypothesis is given by1/2 Q( / 2total SNR ), where Q is the complementary error function and SNRtotal is the total

signal-to-noise ratio per channel. If there are perfect power controls in the multi-user system, the all channels of k

RX E is equal to channel 1 of 1

RX E . The probability of bit error (i.e., bit error rate) can be written in the following

way

11 1

2 0

1

( ) ( )1 1

2 2 2 2

1 1 ( 1)2 2 4

b btotal b

W u

b c RX

SNR SIRSNR P Q Q

N Q N R N E

σ

−− −⎛ ⎞⎡ ⎤+⎛ ⎞ ⎜ ⎟⎣ ⎦

= =⎜ ⎟ ⎜ ⎟⎜ ⎟⎜ ⎟⎝ ⎠⎝ ⎠

⎛ ⎞= − +⎜ ⎟⎜ ⎟⎝ ⎠

(8)

where Rb = 1 / T b =1 / N cT c is the bit rate of each channel.

4. EXPERIMENTAL RESULTS AND DISCUSSION

The proposed multi-channel UWB system is experimentally implemented based on the configuration that is presented in Fig. 1. The gain-switched DFB-LD (CQF 915/JDS Uniphase Co.) is driven at a dc bias current of

13.8 mA and a sinusoidal RF signal with a frequency of 200 MHz in a bias-tee circuit (5575A/Picosecond Labs)to generate Gaussian-like optical pulses. Moreover, two channel data strains, 200Mbit/s bipolar NRZ PRBS datasignals generated from pulse pattern generator (Agilent 81133A), pass through the frequency mixers with thecarrier frequency of 4 GHz and 6 GHz on the EAM. The high-power pulses with a very flat dc offset weregenerated when the bias voltage of EAM was adjusted to 3.2 V.

0 4 8 12 16 20

Time (ns)

0

-40

-80

40

80

O p t i c a l A m p l i t u d e ( μ

W )

Fig. 2. Waveforms of Gaussian-like pulse

obtained using a gain-switched DFB-LD.

0 2 4 6 8 10

Time (ns)

0

-40

-80

40

80

O p t i c a l A m p l i t u d e ( μ

W )

Fig. 3. Waveform of the modulated 200 Mbit/s PBRS 210-1

bipolar data trains obtained at the output of the EAM.




ICTON 2008 81 Tu.B4.2

The pulse waveform and spectrum of channel 1 are measured by using a high-speed sampling oscilloscope(Agilent 86110A) and an electrical spectrum analyzer (Agilent E4407B) with the resolution bandwidth of 3 MHzat the end-user to analyze the pulse shapes of the proposed system. Fig. 2 shows the Gaussian-like pulse trainobtained at the output of the gain-switched DFB-LD (i.e., point A), which has a full-width at half-maximum(FWHM) amplitude of about 1.92 ns. Fig. 3 shows the modulated waveform by the 200 Mbit/s bipolar PRBSdata trains measured at the output of the EAM (i.e., point B). From the figure we can see that the π phase shift

for two modulated signals under the bipolar data. Moreover, the frequency spectra measured at the front-end of RAU (i.e. point C) and at the front-end of end-user (i.e., point D) are shown in Fig. 4(a) and 4(b). From the Fig.4(b) we can see that several harmonic signals are generated in the end-user. It is due to the multipath and channelfading effects. Finally, the eye diagram is measured at the front-end of RAU (i.e., point C) and the output of theend-user (i.e., point E) shown in Fig. 5(a) and 5(b). The clear eye can be achieved using an orthogonal frequencyreceiver of channel 1 (i.e., 4 GHz). Furthermore, we adjust the carrier frequency of channel 2 from 6 GHz to4.4 GHz, and then rework again for the measurement of eye diagram as shown in Fig. 5(c). The clear eye canalso be achieved at the output of channel 1.

5. CONCLUSIONS

The newly developed schemes of the optical BPSK modulation to generate Gaussian-like pulses for themulti-channel UWB radio-over-fiber system have been proposed. The modeling analysis and experimentalresults demonstrated that the modulation scheme with an orthogonal frequency receiver is characterized by the potentially high capacity of the channel. The proposed system can indeed provide the high quality pulse signalsfor the applications of future high-bit-rate multi-channel communication systems.

ACKNOWLEDGEMENTS

The authors would like to thank the National Science Council of the Republic of China, Taiwan for financiallysupporting this research under Contracts No. NSC 96-2628-E-182-002-MY3.

REFERENCES

[1] G.R. Aiello and G.D. Rogerson, “Ultra-wideband wireless systems,” IEEE Microwave Magazine, pp. 36-47, June 2003.[2] K. Siwiak and D. Mckeown, Ultra-Wideband Radio Technology, England: John Wiley & Sons Ltd., 2004.[3] UWB Channel Modeling Contribution From Intel, IEEE P802.15-02/279-SG3a.[4] H. Bo and N. C. Beaulieu, “Accurate evaluation of multiple-access performance in TH-PPM and TH-BPSK UWB

systems,” IEEE Trans. on Communications, vol. 52, no. 10, pp. 1758-1766, Oct. 2004.[5] W. Siriwongpairat, M. Olfat and K.J.R. Liu, “On the performance evaluation of TH and DS UWB MIMO systems,”

IEEE Wireless Communications and Networking Conference, vol. 3, pp. 1800-1805, Mar. 2004.G. Giancola, M.-G. DiBenedetto, “A novel approach for estimating multi-user interference in impulse radio UWB networks: The pulse

collision model,” Signal Processing , vol. 86, pp. 2185-2197, 2006.[7] B. Radunovic, J.-Y. Le Boudec, “Optimal power control, scheduling, and routing in UWB networks,” IEEE J. Sel. AreaCommun., vol. 22, pp. 1252-1270, 2004.

0 2 4 6 8 10

Frequency (GHz)

-40

-60

-20

0

E l e c t r i c a l A m p l i t u d e ( d B m )

0

0 1.6 3.2 4.8 6.4 8

Frequency (GHz)

-40

-60

-20

E l e c t r i c a l A m p l i

t u d e ( d B m )

(a)

(b)

Fig. 4. Frequency spectra measured at(a)the front-end of RAU (i.e. point C) and(b)the front-end of end-user (i.e., point D).

0 1.7 3.4 5.1 6.8 8.5

Time (ns)

0

-40

40

E l e c t r i c a l A m p l i t u d e ( m V )

0

-20

20

0

-20

20

(a)

(b)

(c)

Fig.5. Eye diagrams measured at (a)the front-end of RAU (i.e.

point C), (b)the output of end-user (i.e., point E) under 4GHz and 6 GHz carriers and (c)the output of end-user (i.e., point E) under 4 GHz and 4.4 GHz carriers.


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