The 5th International Conference on Optical Internet (COIN 2006) Hyatt Regency Jeju, Korea / July 9 - 13, 2006 PS-28

Adjacent Channel Power Ratio of OFDM Signals

for Broadband Convergence Networks

A.H.M. Razibul Islam, Md. Irnrul Hassan and Ju Bin Song

Department of Radio Communication Engineering, Kyung Hee University, Kihung, Yongin, Gyeonggi, 449-701, South

Korea, Telephone: +82-031-201-2031, Fax: +82-031-205-1775,

E-mail: jsong@khu.ac.kr

Abstract Feedback (DFB) Laser diode as a transmitter and a

In this paper, Adj acent Channel Power Ratio (ACPR) of Photodiode as a receiver.

Orthogonal Frequency Division Multiplexed (OFDM)

system interconnected with Radio over Fiber (RoF) link at 2 Analysis on ACPR of an OFDM system

2.3 GHz WiBro environment were analyzed and

simulated at both system ends.

1 Introd uction

High peak: to average power ratio (PAPR) has become an

important parameter to consider for OFDM as

nonlinearity distortion increases for an input signalled to

the power amplifier. The in-band and out-of-band

interference created by the distortion effects of nonlinear

amplification, Increase bit-error-rate and spectral

regeneration (SR) respectively. Hence, the degree of SR

can be characterized by ACPR and this ratio is the

limiting factor on achieving high efficiency amplification

[4 ].

Interconnection between optical and wireless networks is

necessary for Broadband Convergence Network (BcN)

performance as the high data rates with multiple services

are expected using multi-carrier modulation techniques.

Therefore, RoF is becoming an increasingly important

technology for the in-building wireless connectivity.

Relevant works of optical interconnection and wireless

link have been undertaken in [1] [2]. Interconnecting the

OFDM signal at 2.3 GHz (for WiBro) with the optical

fiber links and to observe the performance under

analytical and simulated environment is a new research

area to be explored and hence, a comparative ACPR

performance is analyzed and simulated in this paper both

at OFDM High Power Amplifier (HPA) output and RoF

interconnected wireless and optical access link

The square root raised cosine filter is used in the OFDM

system with roll off factor ~ of 0.22 and output signal of

the IFFT is passed though the filter and finally fed to the

input of the HPA considered in our model. We assume no

AM/PM effects in this paper because it is found by

computer simulation that the AM/AM effects are more

significant than the AM/PM effects.

e ( t)r--___ -----,

OFDM System

Fig. 1 System block diagram

So, the output of the bandpass memoryless nonlinear RF

amplifier model, as proposed for our system, is obtained

as

(1)

where g(A) and f(A) are the AM/AM conversion and

AM/PM conversion of the nonlinear amplifier for the

input envelope A. rp is the angle of A (t). The complete

system model has been represented as a block diagram

shown in Fig. 1.

The total system model can be mathematically

represented as

analog RF output. The RoF link consists of a Distributed Ro(f) = E (f) x Hlznk(f) (2)

89-955301-4-698560 ©20060SIA - 180 -

where Ro (j) is the power spectral density of the output

signal Ro (t) at the end of the interconnected link, E (f) is

the input signal to OFDM system and Hhnk(f) is the total

OFDM-RoF link system response which is written as

Hlmk(f) ~ Hso(f) x HLD(f) X Hfiber(f) X HPD(f) (3)

Hso(f) is the OFDM system response of the signal

component which can be written as

2 2

Hso(f) = _l-J p2 e-: {g(CJsp)eJf(a,Pl}dp (4) 2ps 0

HLD(f) is the DFB laser transfer function written as

B~ 0) HLD(f) = {If 1 c} fJ jJ+Poc ,

(jm)2+ 1m -+-+Po(go+-) +-+--+Bm Po 'n 'p 'nPo 'n'p

where Po IS the steady state photon intensity,

13' = j3f'Jlh qVac/

d "'02 = goPo and the B = 1- GPO an tv

rp

description of the other parameters can be found from rate

equations described in [3]. The transfer function of the

single mode fiber can be written as HO

Hfiber(f) = J S()")L().,)exp[ - jwT()")]d)" (6)

where SeA) is the source spectrum as a function of

wavelength; L(A) is the reciprocal of loss as a function of

wavelength and T(A) is the group delay function.

HPD(f) is the transfer function found in [5] as

RD HPD(f) = 2 2

a+ jw(b-cw )-dw

with

a =Rs+RL +RD

b = RsRLCP + Ls + (Rs + RL )RDC; + RLRDCp

c = RiLsCpRDCj

d = (RsRLCP + Ls )RDCj + RiLsCp + RDCpLs

(7)

where Cj is the junction capacitance, Cp is the parasitic

capacitance, Ls is the total series inductance, RD is the

diode shunt resistance, Rs is the diode series resistance

and RL is the load resistance.

In our system model, ACPR of wireless and optical link is

defined as fo- B fo+3 B

J Ro(f)df + J Ro(f)df

A CPR (B) = ..::.;fo;....-;:..:3 B'---f,....o+-::-B-----'f-o+-

B----

(8)

J Ro(f)df fo- B

where (B,-B) is the desired in-band.

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3 Simulation and Results

Table 1 shows the simulation parameters used in the

system modeling. For the DFB laser in our model, some

of the worst case parameters were used to estimate

maximum distortion in the link.

Table 1. Parameters for simulation

Parameter Value

OFDM Data Rate 156250 Symbolls

Number of Sub carriers 128

Threshold Current for DFB 40 rnA

Nonlinear Gain Saturation 4e-23 rn3

Confinement Factor 0.3

Fiber Path Loss 0.266 dBIkm

Responsivity of the PD lAJW

20 MHz bandwidth was used for the OFDM signal with a

carrier frequency of 2.3 GHz for WiBro system. The

Power Spectral Densities (PSD) at the OFDM system

input and output are shown in Fig. 2. In Fig. 3, PSDs at

DFB laser output and PD output are also shown. It can be

seen from Fig. 3 that DFB laser incorporates some gain in

the system while HPA fed OFDM output was passed

through it.

As the signal is passed through the fiber channel and PD,

some more distortions are added in the system due to the

inherent non-linearity effects from gain compression,

clipping, power output saturation, transfer characteristics

and photoresponsivity. After the PD, approximately 30 dB

loss is there than that of OFDM amplifier output signal.

Optical losses due to the coupling between laser and fiber,

and fiber and photo detector, are neglected and the shot

and thermal noise in the PD were not taken into

consideration for simplicity in our system.

Finally, the ACPR versus input power plot at the OFDM

amplifier output and at the RF output of the PD, which is

the end of the wireless and optical link, were simulated

and compared as shown in Fig. 4.

It is found that a 20 dB ACPR performance degradation

takes place initially for transmitting the OFDM signal

through the fiber model which comes down to

approximately 5 dB distortion if 30 dBm input power is

applied in the system.

Gradually increased distortion (spectral regrowth) of

(a)

(b)

Fig. 2 (a) aFDM input power spectral density

(b) aFDM output power spectral density

(a)

(b)

Fig. 3 (a) DFB laser output power spectral density

(b) PD output power spectral density

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Fig. 4 ACPR perfonnance comparison

maximum 25 dB was observed for varying fiber length

from 100 meter to 40 km. This is because; laser

nonlinearity dominates for the cause of distortion at low

fiber spans whereas dispersion govel115 distortion effects

mostly for higher fiber lengths.

4 Conclusion

This paper suggests a 2.3 GHz WiBro aFDM wireless

and optical link interconnect perfonnance lUlder a

simulated environment with a mathematical analysis.

The results showed important obselvations for

measurement of distortion experienced due to nonlinearity

by the system modeled. Maximum input power can be

detennined from this work in the future that can be

applied to the RoF system to meet the WiBro system

specification for ACPR and thus design suitable optical

links with wireless interconnection.

References

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Microwave Photonics, 1999.

3 A.R. Hamed, et al., Radio over Fiber Technologies for Mobile

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