International Journal of Engineering Trends and Technology (IJETT) – Volume 12 Number 5 - Jun 2014
An Efficient Dynamic Channel Assignment
Scheme for OWDM Based Radio-Over-Fiber
Optical System
S.LeelaVathi
P.Prasanna Murali Krishn
M.tech II year, Dr.sgit, Markapur HOD of DECS ,Dr.SGIT.Markapur,
S.CH.Kantha Rao
Associ.prof, PACEITS, Ongole
M. Ramana Reddy,
Asst.prof,Dr.SGIT,Markapur
Abstract: In 3G cellular networks, high data-rates can be achieved. However, fundamental capacity limitation still
exists. Call requests are frequently blocked in hotspot areas. Load balancing among cells helps to solve this problem
and to utilize the radio resources. Some schemes are not flexible enough or even practical for such networks. The
research in this thesis focused on the feasibility of using both single-mode and multimode fibers to distribute highfrequency microwave signals to simplified remote radio antenna units. Radio-over-Fibre (RoF) technology entails
the use of optical fibre links to distribute RF signals from a central location (headend) to Remote Antenna Units
(RAUs). The results of this research show that at low phase noise levels, some DWT-based schemes outperform
DFT-based scheme, however they achieve the same performance at high phase noise levels. It is understood that
RoF systems are less robust against phase noise compared to their linear counterparts.Finally, QAM signals are
more robust against phase noise compared to PSK signals in RoF systems.
Keywords: Multi-hop cellular networks, RoF, OFDM, OWDM, Wavelet, AWGN.
1. INTRODUCTION
Traditional cellular networks (TCNs) and mobile ad
hoc networks (MANETs) both have their respective
advantages and drawbacks. TCNs have mature
technology support for reliable performance.
However, building and expanding their necessary
infrastructure is costly. MANETs, on the other hand,
are simple to deploy and easily expandable.
Nevertheless, many of their implementation issues
are still in the research phase. By taking into account
the advantages and drawbacks of TCNs and
MANETs, researchers notice that a combination of
them is the logical solution to the next generation
mobile networks. In 1996, Adachi and Nakagawa
raised the concept of cellular ad-hoc united
communication system [1]. Subsequently, many
similar proposals were reported, such as multi hop
cellular network (MCN) [2]. MCN-type systems are
expected to bring considerable amount of benefits.
However, with the limited bandwidth for cellular
communications, channel assignment becomes even
more challenging in MCN-type systems.Mobile
communication has recently become affordable and
popular. Wireless communications has gone through
three generations.
Wireless communication has experienced tremendous
growth in the last decade. In 1991 less than 1% of the
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world’s population had access to a mobile phone. By
the end of 2001, an stimated one in every six people
had a mobile phone [3]. During the same period the
number of countries worldwide having a mobile
network increased from just three to over 90%. In
fact the number of mobile subscribers overtook the
number of fixed-line subscribers in 2002, as shown in
Figure 1.1. It is predicted that this growth will
continue to rise, and by 2010 there will be more than
1700 million mobile subscribers worldwide [4].
Radio-over-Fiber (RoF) is a technology by which
information bearing signals using RF carries are
delivered by means of optical components and
techniques. Better coverage and increased capacity,
centralized upgrading and adaptation, higher
reliability and lower maintenance costs, support for
future broadband applications, and economic access
to mobile broadband are among the most important
advantages of RoF [5], [6]. However, RoF systems
are vulnerable to nonlinearities in the optical
subsystem that cause degradation of the system BER
performance. Normally, these effects are expressed
as AM-AM and AM-PM characteristics; the former is
an amplitude transfer function while the latter is a
phase transfer function. One area of interest in
modern communications is OFDM which is
becoming widely used in wireless communication
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International Journal of Engineering Trends and Technology (IJETT) – Volume 12 Number 5 - Jun 2014
systems due to its high data rate transmission
capability with high bandwidth efficiency and also its
robustness to multi-path fading without requiring
complex equalization techniques [7], [8], [9].
In wavelet transform, a signal is decomposed into
shifted and scaled versions of a particular wavelet
called the mother wavelet [10], [11], [12]. The
reverse operations are carried out to reconstruct the
original signal. Therefore, not only can the CP be
removed in OWDM but also P/Sand S/P blocks can
be removed from the OWDM transmitter and
receiver, respectively. In coherent communication
systems, the receiver must provide carrier and
symbol synchronization capabilities. Misalignment
between oscillator frequencies of receiver and
transmitter or Doppler shift will result in carrier
frequency offset , or equivalently, a phase error of the
received signal relative to the transmitted signal [13].
Carrier Frequency Offset (CFO) destroys the
orthogonality between subcarriers and therefore
prevents perfect alignment of FFT bins with peaks of
the Sinc shaped pulses. As a result the energy of each
subcarrier is spread to other subcarriers leading to
Inter-Carrier Interference (ICI).
2.
measured AM-AM/PM characteristics of the optical
subsystem which is reproduced from [11]. Also, a
pictorial description of Output Back-Off (OBO) is
shown in Fig. 2 which is defined (on a logarithmic
scale) as the difference between the maximum output
power and the output power at the quiescent point.
2.2 RoF Multiplexing Techniques
Sub-Carrier Multiplexing in RoF Systems:
Subcarrier Multiplexing (SCM) is a maturing, simple,
and cost effective approach for exploiting optical
fibre bandwidth in analogue optical communication
systems in general and in RoF systems in particular.
In SCM, the RF signal (the subcarrier) is used to
modulate an optical carrier at the transmitter’s side.
This results in an optical spectrum consisting of the
original optical carrier f 0, plus two side-tones located
at f 0 ± f SC , where f SC is the subcarrier frequency.
If the subcarrier itself is modulated with data
(analogue or digital), then sidebands centred on f 0 ±
f SC are produced as illustrated in below Figure 1.
SYSTEM DESIGN MODEL
2.1 Wireless communication over fiber optics
In order to evaluate the system performance,
computer simulations were carried out based on the
system model presented in Fig. 1. The data source
transmits 1,000,000 bits. For the sake of simplicity
and focusing on the phase distortion itself, operations
such as coding and interleaving are not considered.
Then,
the
data
are
mapped
using
a
QPSK/16PSK/16QAM modulator. To produce
OFDM symbols, first the resultant signal is converted
from serial to parallel. Then, either an Inverse
Discrete Fourier Transform (IDFT) or an Inverse
Discrete Wavelet Transform (IDWT) is taken. The
resultant signal in the OFDM case is converted to
serial, and in order to mitigate ISI, a CP with a length
of 25% of the whole OFDM symbol period is added.
This signal i.e. the OFDM/ OWDM signal is pulse
shaped using an RRC filter (roll-off=0.5 and number
of taps=64), and then passed to the optical subsystem.
The optical subsystem comprises a laser diode (LD),
a 2.2 km length of single-mode fibre, and a PIN
diode for photo-detection. Fig.e 2 portrays the
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Figure1: Sub-Carrier Multiplexing of Mixed Digital
and Analogue Signals.
To multiplex multiple channels on to one optical
carrier, multiple sub-carriers are first combined and
then used to modulate the optical carrier as shown in
Figure 2.10. At the receiver’s side the sub-carriers are
recovered through direct detection and then radiated.
Different modulation schemes may be used on
separate sub-carriers. One sub-carrier may carry
digital data, while another may be modulated with an
analogue signal such as video or telephone traffic. In
this way, SCM supports the multiplexing of various
kinds of mixed mode broadband data. Modulation of
the optical carrier may be achieved by either directly
modulating the laser, or by using external modulators
such as the MZM.
After passing through an RF AWGN channel, the
signal is perturbed by a multiplicative noise that
models the phase noise. There are two phase noise
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(jitter) models namely the white phase noise model
and the colored phase noise model [4]. In this paper,
the phase noise is modelled as a zero mean white
Gaussian stochastic process with a standard deviation
of _ ranging from 0 to 90 degrees with steps of 10
degrees. Then the resultant signal is filtered by the
receive RRC filter followed by the CP removal from
the OFDM signal, and thereafter it is converted to
parallel symbols. Subsequently, a DFT /DWT is
taken followed by a conversion to serial data. For a
fair comparison of the effect of carrier phase noise,
no channel estimation is performed. After being
demodulated, the received data are compared with
those transmitted.
2.3 A-Cell Adaptive Routing (ACAR) Scheme
ACAR is a centralized on-demand load-aware
routing scheme specifically designed for 3G CDMA
cellular systems. ACAR has two mechanisms:
Routing Discovery and Route Maintenance. The idea
of ACAR is to make use of the cell size flexibility,
i.e. the cell breathing effect, of a CDMA cellular
system. Route discovery and route maintenance can
be done in a single hop with long range and low datarate while data communication can be done in a
multi-hop with short range and high data-rate
fashion. In this way, routing overhead can greatly be
reduced and the benefits of multi-hop relaying
remain. In addition, no potential call requests, which
are within the maximum coverage of a cell, will be
denied. Figure 4 illustrates this by comparing MCN-p
and ACAR. In MCN-p, since the cell size is shrunk,
potential calls from node A, B, D, and E becomes
unreachable. In ACAR, these nodes still reach the
BS. Thus, no potential call is denied. In Figure 3b,
node B and node E are using single hop routing.
Node C and F are using multi-hop routing. Node D is
using inter-cell routing for load balancing. Details of
the scheme can be found in.
2.4 Channel Searching Strategies
1) Sequential Channel Searching (SCS): When a new
call arrives, the SCS strategy is to always search for a
channel from the lower to higher-numbered channel
for the first-hop uplink transmission in the central
microcell. Once a free channel is found, it is assigned
to the first-hop link. Otherwise, the call is blocked.
The SCS strategy works in the same way to find the
uplink channels for second- or third-hop links for this
call if it is a multihop call. The channel searching
procedure is similar for downlink channel assignment
as well.
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2) Packing-Based Channel Searching (PCS): The
PCS strategy is to assign microcell A a free channel j
which is locked in the largest number of cells in I(A).
The motivation behind PCS is to attempt to minimize
the effect on the channel availability in those
interfering cells. We use F(A, j) to denote the number
of cells in I(A) which are locked for channel j by cells
not in I(A).
3. SIMULATION RESULTS
Figure 2 shows that, without load balancing, CBR of
cell A is approximately 19 to 23%. When the load
balancing is enabled, the CBR of cell A drops deeply.
The sudden drop, even though there is no Relaying
Nodes in cell A, is because there are already many
Relaying Nodes in cell B. Once load balancing is
triggered, the load of the Source Nodes in cell A
which are near to the border of cell B can be relayed
immediately to cell B. Then, more capacity is
available in cell A.
Figure: 2Call Blocking Ratio among cells with and
without load balancing.
The RoF system without an optical subsystem. Fig. 3
plots BER versus the standard deviation of the phase
noise for different wavelets together with that of
OFDM for the linear MC system. The figure
compares OWDM using different wavelets, when
QPSK modulation is used.
Fig. 3BER vs. standard deviation of phase noise for
uncoded MC QPSK in AWGN.
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varying RF channels is the subject of further
investigation by the author.
5. REFERENCES
Fig. 4BER vs. standard deviation of phase noise for
uncoded MC RoF-QPSK at OBO=1 dB in AWGN.
To investigate the effect of RoF nonlinearity,
additional simulations were performed. Fig.4 shows
BER versus σ for different wavelets along with that
of OFDM for the MC RoF-QPSK at OBO=1 dB. The
behaviors of the BER plots follow approximately the
same trend as in the linear scenario. One can observe
that while wavelet sym2, from Group I, achieves
poor performance, OFDM and the other wavelets
have comparable BER performance for low σ values.
4. CONCLUSION
In this paper the impact of a RoF optical sub-system
on the BER performances of OFDM and OWDM
were assessed in the presence of phase noise. It was
found that RoF systems are less robust to phase noise
compared to their linear counterparts. At low values,
some OWDM schemes outperform, however they
achieve the same performance at high values. Also,
increasing modulation order leads to an increase in
sensitivity to the phase noise regardless of the MC
scheme. In addition, the performance of different
wavelets is slightly different, especially at low
values. ACAR fully utilizes the dynamic cell size
characteristic of cellular systems such that routing
overhead can greatly be reduced and the benefits of
multi-hop relaying remain. In the future, optimization
of the components of the framework can be
performed. Performance assessment of the
aforementioned modulation schemes in the presence
of colored phase noise, both in AWGN and time
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