Adjacent Channel Power Ratio of OFDM Signals for Broadband ... · C.K Sim, et aI., Performance...
Transcript of Adjacent Channel Power Ratio of OFDM Signals for Broadband ... · C.K Sim, et aI., Performance...
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: [email protected]
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.
- 181 -
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
- 182 -
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
C.K Sim, et aI., Performance Evaluation for Wireless LAN,
Ethemet and UWB Coexistence on Hybrid Radio-over-fiber
Picocells, Optical Fiber Communication Conference, 2005.
2 R. E. Schuh1 et al., Distortion of W-CDMA Signals Over
Optical Fibre Links, International Topical Meeting on
Microwave Photonics, 1999.
3 A.R. Hamed, et al., Radio over Fiber Technologies for Mobile
Communication Networks, Artech HOllse, 2002.
4 1.S. Park, et aI., Power amplifier back-off analysis with
AM-to-PM for millimeter-wave OFDM wireless LAN, Radio
and Wireless Conference, 2001.
5 B. Pallab, Semiconductor Optoelectronic Devices, Prentice
Hall, 1994.