Content uploaded by Ghafour Amouzad Mahdiraji
Author content
All content in this area was uploaded by Ghafour Amouzad Mahdiraji
Content may be subject to copyright.
A New Duty Cycle Based Digital Multiplexing
Technique
Mohamad Khazani Abdullah1, Ghafour Amouzad Mahdiraji1, Mohamed Faisal Elhag1,
Ahmed Fauzi Abas1, Edmond Zahedi2 and Makhfudzah Mokhtar1
1Photonics Laboratory, Department of Computer and Communication Systems Engineering,
Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia.
2Department of Electrical, Electronic & System Engineering, Faculty of Engineering,
Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia.
ghafour1503@yahoo.com
Abstract––A new multiplexing technique which is called duty
cycle division multiplexing (DCDM) is presented in this paper.
Theoretical and simulation studies have been carried out to
evaluate the performance of this technique based on the signal
energy and symbol error rate (SER). A wireless channel based on
free space propagation model is considered for the simulation
study. Two modulation schemes of PSK and QAM are used to
evaluate the technique, against the data rates. Also, the
performance of the multiplexing technique is compared with the
conventional Time Division Multiplexing (TDM) technique as
well as with the multilevel M-ary signaling. The study shows that
the energy per bit in the DCDM technique, unlike that of the
TDM technique increases with the number of users. The
simulation results correspond with the theoretical study in which
the DCDM technique has better SER than that of TDM.
Keywords—Duty cycle, multiplexing technique.
I. INTRODUCTION
Multiplexing is an essential part in a communication system
where multiple data streams are transmitted simultaneously
through a single link, whether the link is based on coaxial
cable, fiber, radio, or satellite [1, 3]. Multiplexing is widely
employed due to its capability to increase transmission
capacity and to reduce system costs [2]. There are three basic
types of multiplexing technique in communication systems;
time division multiplexing (TDM) [4-6], frequency or
wavelength division multiplexing (FDM) [3, 7] or (WDM) [8-
10], and code division multiplexing (CDM) [11-15].
TDM is the most widely used multiplexing technique in
communication systems today [16-17]. However, for
multiplexing high number of users with high data rates, high-
speed multiplexer and de-multiplexer are required, resulting in
very high cost for TDM systems [18-19, 26]. At higher speeds
clock recovery is another essential issue that may render the
system highly complicated and costly for TDM systems [20-
23]. Therefore, many investigations have been done to design
and develop reliable and cost-effective clock recovery
modules for TDM in both the electrical and optical (thus,
OTDM) versions [20-25].
Realizing these problems, duty cycle division multiplexing
(DCDM) is proposed in this paper as an alternative
multiplexing technique. In this technique, different users can
share communication media and transmit data simultaneously
by using the same frequency band, but with a different duty
cycle. The proposed technique also has an inherent property
which allows for better clock recovery (however, this is not
within the scope of this paper).
The purpose of this paper is to introduce the new
multiplexing technique and compares this technique with
TDM. In section II, the basic functions and characteristics of
DCDM are explained based on the theories. The results of a
simulation study are discussed in the subsequent section,
which is then followed by a conclusion.
II. DUTY CYCLE MULTIPLEXING TECHNIQUE
A. Basic Function
The DCDM technique is based the unipolar return to zero
(RZ) line code. In this technique, each user transmits a bit of 0
with zero volts within Ts second where Ts is symbol duration,
and bit 1 is transmitted with +A volts with a duration less than
Ts seconds. The technique defines the duration for ith user as
1+
×
=n
Ti
Ts
i (1)
where n represents number of multiplexing users. For
example, for multiplexing 3 users, the 1st, 2nd and 3rd user uses
duration of
4
s
T,
4
2s
T and
4
3s
T respectively to transmit bit 1s.
Therefore, different users can share communication media to
transmit simultaneously by using the same frequency band but
with different duty cycles. Fig. 1(a) shows an example of a
rectangular signal that carries bit sequences of 11010. Fig.
1(b), (c), and (d), represents the same signal with
4
s
T,
4
2s
T and
4
3s
T duty cycle respectively, represents as different users. Fig.
1(e) shows the patterns when the 3 users are multiplexed with
the same period of Ts.
The multiplexed signal can then be modulated by using a
single modulator. At the receiver side, after demodulating the
received signal, the original signal can be extracted from the
demodulated signal based on the signal voltage amplitude and
the duty cycle’s time duration.
Proceedings of the 2007 IEEE International Conference on Telecommunications and
Malaysia International Conference on Communications, 14-17 May 2007, Penang, Malaysia
1-4244-1094-0/07/$25.00 ©2007 IEEE.
526
s
T
Amplitude (V)
s
T
Fig. 1. (a) Original rectangular signal carrying bits 11010 with duty cycle of
100% Ts, (b), (c) and (d) 3 multiplexing users with duty cycles of 25, 50 and
75% Ts for user 1, user 2 and user 3 respectively, (e) multiplexed signal of the
3 users.
Based on the number of multiplexing users, there are 2n
possible combinations of bits for n users. In the multiplexed
signal, each of these combinations produces a unique symbol.
Fig. 2(a) shows the 8 possible bits combinations for the case of
3 multiplexing users, and the multiplexed symbols for the 8
cases are presented in Fig. 2(b). By having the knowledge
about this uniqueness at the receiver side, the original data for
each user can be easily distinguished and recovered by taking
1 sample per slot for n+1 slots per Ts seconds. This technique
allows for automatic bit error detection and correction, based
on the sequence of sampled amplitudes per symbol duration.
For the case of multiplexing n users, if only one sample per
slot is taken, then, the 1st sample (taken from the 1st slot), has n
+1 possible levels, the 2nd sample (taken from the 2nd slot), has
n possible levels, the nth sample (taken from the nth slot), has 2
possible levels (0 or A volts), and the last sample (n +1)th,
(taken from the last slot), has 1 possible level which is 0 volt.
B. Signal Energy
The calculation of the average energy per bit of the
multiplexing signal can be obtained by considering all possible
combinations of bits for a specific number of users. For
example, Fig. 2(b) shows the method for calculating the
average energy per bit for the case of 3 multiplexing users.
The average energy per bit of the DCDM technique can be
defined as:
s
n
k
bitavg TA
n
kn
E2
1
/)1(6
1
×
+
++
=∑
= (2)
where A is sign al amplitude in volt, Ts is signal symbol
duration in second and n is number of users. The energy per
bit will be increased linearly as the number of multiplexed
user is increased. This is important as it helps to improve the
signal transmission quality at a larger number of users.
Case 1
Case 2
Case 6
Case 7
Case 8
Case 3
Case 4
0
1
=E
)4/(
2
2s
TAE =
)4/(2
2
3s
TAE =
A
2A
3A
2A
2A
A
)4/(3
2
5s
TAE =
4/
s
T
s
T
0
)4/(6
2
6s
TAE =
)4/(9
2
7s
TAE =
)4/(14
2
8s
TAE =
s
s
bitavg
TA
TA
E
2
3
2
/
41668.0
23
)4/(40 =
×
=
Case 5
)4/(5
2
4s
TAE =
A
7654328
01001101
01110001
00101011
Cases
User 3
User 2
User 1
1
(a) (b)
Fig. 2. (a) The 8 possible bits combination for multiplexing 3 users, (b)
multiplexed symbols for the 8 cases and the average energy per bit for 3 users.
C. Comparison of DCDM with M-ary and TDM technique
M-ary signaling is normally used to transmit multiple
amplitudes, each one representing 2 bits or more. The main
purpose of M-ary is to increase the bandwidth efficiency
whereby many bits can be transmitted by a single symbol. If
each bit is assumed to represent a different user, then M-ary
can also be used for multiplexing, thus worthy of comparison
with the new scheme.
Two comparisons are done which are based on the number
of signal voltage levels and on the average energy per bit
versus number of users. Fig. 3 shows the theoretical results for
the signal voltage level versus number of users for the DCDM,
TDM and M-ary techniques. Comparison study shows that the
number of voltage levels in the TDM remained fixed to 2
levels, for the DCDM technique it increased as (n +1), and for
the M-ary technique it increased as (2n). Thus, in order to
multiplex 15 users, the TDM, DCDM and M-ary techniques
require 2, 16 and 32768 signal voltage levels respectively.
This result shows the disadvantage of the M-ary technique
which makes it impossible to use as a multiplexing technique
for high number of users. Therefore, the next comparison is
continued between the TDM and DCDM only.
527
Number of voltage level versus number of user
0
25
50
75
100
125
150
0 5 10 15 20
Number of user
Number of voltage leve
l
DCDM
TDM
M-ary
Fig. 3. Number of signal voltage level versus number of user for the M-ary
and DCDM techniqu e.
Based on signal energy, the average energy per bit for the
DCDM is calculated as presented in Fig. 2, based on Eq. (2).
For the TDM, the same method shown in Fig. 2 is used to
calculate the average energy per bit of the TDM signals, which
is defined as
TA
n
Ebitavg
2
/2
1
=. (3)
Fig. 4 shows the average energy per bit of the TDM and
DCDM for multiplexing up to 15 users. The figure shows that
the average energy per bit of the DCDM increased linearly
with the number of multiplexing user whereas the average
energy per bit for TDM reduced. This increment of energy in
the DCDM is very important because, it makes this technique
able to support higher number of multiplexing user compared
with the TDM, considering the energy per bit only.
The other advantages of the DCDM technique include
simple transmitter design, error detection and correction
capability and better clock recovery. For example, as far as
transmitter design is concerned, the OTDM technique requires
one modulator for each user [27, 28]. The use of multiple
modulators is costly and may lead to cross-talks [4]. On the
other hand, the DCDM technique requires only a single
modulator for all the users. This is much chipper and at the
same time avoiding the cross-talk problems.
Also note that there are various transitions taking place
within the period Ts. If the duty cycle is less than 100%, there
will always be a transition between any two consecutive
symbols except for the case when all users are transmitting 0
bits consecutively which is highly unlikely (especially as the
number of users increases). The transition is important in that
it provides a simple means to recover the clock timing.
Normalized energy per bit versus number of user
0
0.5
1
1.5
0 5 10 15 20
Number o f use r
Average energy per bit (J)
DCDM
TDM
Fig. 4. Normalized avera ge energy per bit versus num ber of user for DCDM
and TDM.
III. SIMULAT ION STUDY
A simulation study has been done in order to compare the
performance of the DCDM with the TDM technique. The
simulation has been set for a wireless transmission with the
additive white Gaussian noise (AWGN). Two modulation
schemes of 16-PSK and 16-QAM have been applied to
compare the performance of the two multiplexing techniques.
A. Simulation Set Up
The parameters used in this simulation study are presented
in Table I, applicable for both the DCDM and TDM
techniques. A block diagram of the DCDM technique is
depicted in Fig. 5.
The performance simulation is studied considering two of
the main impairment factors which are attenuation and noise.
As the transmitter and receiver antenna are assumed to be in
line-of-sight, therefore, the free space propagation model is
used to calculate the attenuation in the communication media.
The attenuation of the communication media calculated based
on the Friis free space equation [29] is
××
−++= c
fd
GGPP rttr
π
4
log20 (4)
where Pt and Pr are the transmitted and received power in
watts, Gt and Gr are the transmitter and receiver antenna gain,
d is the transmitter and receiver separation distance in meters,
f is the carrier frequency and c represents the speed of light for
space which is 8
103× m/s [29-30]. Pt is assumed be the same
as the baseband signal power which is calculated from the
energy content of the multiplexed signals shown in Eq. (2) and
(3) for the DCDM and TDM respectively.
∑
Fig. 5. Block diagram of the DCDM technique for multiplexing 7 users.
TABLE I
DESIGN AND PERFORMANC E PARAMETERS ASSUMED FOR THE SIMULATION
Fixed design parameter s for th e DCDM and TDM technique
DCDM 1 + number of user Number of sample per
symbol TDM number of u ser
Number of pulse per simulation 10000
Number of mu ltiplex ing user 15
Modulation scheme 16-PSK, 16-QAM
DCDM 16
Number of level (M) TDM 2
Propagation model Free space propagation loss (FSPL)
Carrier frequ ency 4 GHz
Transmitter power Baseband signal power
Transmitter and receiver antenna Fixed, line-of-sight
Noise parameters k = 1.38e-23 J/ºK, F = 10 dB
Tant = 300 ºK, To = 300 ºK
Signal voltage amplitude per user 150 mV
Transmitter antenna gain (Gt) 16 dB
Receiver antenna gain (Gr) 20dB
Distance (d) 8 km
528
The amount of thermal noise that is considered in the
communication media is given by [29-30]
nnn BkTP = (5)
where the Pn is thermal noise power (W), k is the Boltzmann’s
constant of 1.38 × 10-23 J/ºK, Tn is known as the equivalent
noise temperature and Bn is the equivalent noise bandwidth.
The noise bandwidth is assumed to be equal with the
multiplexed signal bandwidth. The noise temperature is
calculated as [29-30]
)1( −+= FTTT antn D (6)
where the Tant is noise temperature of the antenna, To is
temperature of source and F is antenna noise figure in dB.
Temperature of the antenna and source are assumed to be at
the room temperatur e of 27oC.
In order to calculate the SER of the DCDM and TDM with
both the PSK and QAM modulation scheme, the simulation is
modeled in the Matlab environment. In this software the SER
calculates using the direct method, by comparing the
transmitted and received data, (i.e. Number of error / Number
of transmitted bit).
B. Simulation Results
Fig. 6 and 7 show the simulation results of the SER versus
bit rate of the TDM and DCDM for the case of PSK and QAM
respectively. In this simulation the number of multiplexing
users is fixed to 15 users. The data rate of each user is varied
from 1 to 15 Mbps per user contributing to the maximum total
data rate of 225 Mbps.
The results show that for all bit rates, the DCDM has better
performance than the TDM in both modulation schemes. This
is because of the higher energy per bit of the DCDM signals as
previously shown.
By comparing the two modulation schemes, Fig. 8 shows
that, QAM has better SER than PSK for modulating the
DCDM signal.
Although, the symbol error rate is high, the main objective
of comparing the performance of the two techniques is
nevertheless achieved.
SER versus data rate for PSK
1.00E-06
1.00E-05
1.00E-04
1.00E-03
1.00E-02
1.00E-01
1.00E+00
0 5 10 15 20
Data rate per user (Mbps)
SER
PSK-DCDM
PSK-TDM
Fig. 6. SER versus bit rate per user of DCDM and TDM for the case of PSK.
SER vers us data rate for QAM
1.00E-06
1.00E-05
1.00E-04
1.00E-03
1.00E-02
1.00E-01
1.00E+00
0 5 10 15 20
Data rate per user (Mbps)
SER
QAM-DCDM
QAM-TDM
Fig.7. SER versus bit rate of DCDM and TDM for the case of QAM.
SER vers us data ra te for DCDM
1.00E-06
1.00E-05
1.00E-04
1.00E-03
1.00E-02
1.00E-01
1.00E+00
0 5 10 15 20
Dat a rat e p er u ser (Mbps)
SER
PSK-DCDM
QAM-DCDM
Fig. 8. SER versus data rate of DCD M using PSK and QAM.
IV. CONCLUSION
The principle of the DCDM technique has been discussed in
this paper by comparing with M-ary and TDM techn iques. The
theoretical results show that if the M-ary technique used as
multiplexing technique, the number of signal voltage level is
increased by 2n which is impractical. However, it is possible
for the DCDM in which the number of signal level is only (1+
n). Although TDM has the advantage of smaller amplitude
levels, its energy content decreases with the number of users.
On the other hand DCDM energy content impr oves with th e
number of users. The simulation results clearly show the better
performance of DCDM than that of TDM whether the signals
are QAM or PSK modulated. For DCDM, QAM proves to be
a better modulation scheme. Although the simulation
considers only thermal noise, the generality of the
transmission performance of DCDM in comparison with TDM
is maintained. The other inherent advantages of the DCDM
technique considered for future reports are simpler transmitter,
better error detection and correction, and better clock
recovery.
REFERENCES
[1] V. Zarim pas and B. Honary, “Multime dia multip lexing pr otocol for
future wireless communication channels,” 3G Mobile Commun. Tech.,
15th Intern. Con., pp. 624-628, 2004.
[2] S. A. Aljuni d. Development and implementation of a novel optical code
division multiplexing technique for metropolitan area network. PhD
thesis, Universiti Putra Malaysia (UPM), Feb. 2005, Chapter 2.
[3] H. Sari, F. Vanhaverbeke, and M. Moeneclaey, “Extending the capacity of
multiple access channels,” IEEE Commun. Maga., Jan. 2000, pp. 74-82.
529
[4] M. D. Spirit, A. D. Elli s, and P. E. Barnsley, “Optical time division
multiplexing: Systems and networks,” IEEE Commun. Mag., vol. 32, no.
12, pp. 56-62, Dec. 1994.
[5] R. A. Barry, V. W. S. Chan, K. L. Hall, E. S. Kintzer, J. D. Moores, K. A.
Rauschenbach, E. A. Swanson, L. E. Adams, C. R. Doerr, S. G. Finn, H.
A. Haus, E. P. Ippen, W. s. Wong, and M. Haner, “All-optical network
consortium—Ultrafast TDM networks,” IEEE J. Select. Areas Commun.,
vol. 14, no. 5, pp. 999–1013, June 1996.
[6] F. Babich, “Considerations on adaptive techniques for time-division
multiplexing radio systems,” IEEE Trans. on Vehicular Tech., vol. 48,
no. 6, pp. 1862-1873, Nov. 1999.
[7] P. Bergmans, T. Cover, “Cooperative broadcastting,” IEEE Transactions
on Information Theory, vol. 20, no. 3, pp. 317-324, May 1974.
[8] M. S. G oodman, H. Kobri nski, and K. W. Loh, “ Application of
wavelength divi sion multiplexing to communi cation n etwork
architectures,” in Proc. ICC’86, 1986.
[9] R. A. Barry and P. A. Humblet, “O n the nu mber of wavele ngths a nd
switches in all-optical networks,” IEEE Trans. Commun., vol. 42, no.
2/3/4, Feb./Mar./Apr. 1994.
[10] C. A. Brackett, “Dense wavelength division multiplexing networks:
Principle and applications,” IEEE J. Select. Areas Commun., vol. 8, pp.
948–964, Aug. 1990.
[11] L. A. Rusc h and H. V. Poor, “ Effects of laser phase drift on coherent
optical CDMA,” IEEE J. Select. Areas Commun., vol. 13, pp. 577–591,
Apr. 1995.
[12] G. J. Foschini and G. Vannucci, “Using spread-spectrum in a high-
capacity fiber-optic local network,” J. Lightwave Technol., vol. 3, pp.
370–379, Mar. 1988.
[13] P. R. Prucnal, M. Sa ntoro, and T. Fan , “Spread spectru m fiber optical
local area network using opti cal processing,” J. Lightwave Technol., vol.
4, pp. 307–314, May 1986.
[14] J. A. Salehi, “Code division multiple-access techniques in optical fiber
networks—Parts I: Fundamental principles,” IEEE Trans. Commun., vol.
37, pp. 824–833, Aug. 1989.
[15] W. Huang and I. Andonovic, “Coherent optical pulse CDMA systems
based on coherent correlation detection,” IEEE Trans. Commun., vol. 47,
pp. 261–271, Feb. 1999.
[16] S. W. Seo, K. Bergman, an d P. R. Prucnal, “Transparen t optical networks
with time-division multiplexing,” IEEE Journal on Selected Areas in
Communications, vol. 14, no. 5, pp. 1039-1051, June 1996.
[17] D. Cotter, J. K. Lucek, and D. D. Marcenac, “Ultra-high-bit-rate
networking: From the transcontinental backbone to the desktop,” IEEE
Communication Magazine, vol. 35, no. 4, pp. 90-95, April 1997.
[18] A. Schopflin, C.-J. Weiske, E. Mullner, F. Znidarsic, U. Fischer, K.
Kotten, J. Wildmoser, J. Krapfl, F. Beyer, and U. Wunsche, “Realization
of a Commercial 10 Gbit/s TDM Transmission System” International
Conference on Communication Technology, vol. 1, pp. 79-82, May
1996.
[19] A.Lord, L.C.Blank, J.M.Boggis, W.A.Stallard, E.G.Bryant, “Optical
multiplexing techniques for future Gbit/s transmission systems,” IEEE
International Conf. on Commun. (ICC), vol. 1, pp. 21-25, June 1988.
[20] J. H. Lee, S. Ohara, T. Nagashima, T. Hasegawa, N. Sugimoto, K.
Igarashi, K. Katoh, and K. Kikuc hi, “Clock recover y and demultiplexin g
of high-speed OTDM signal through combined use of bismuth oxide
nonlinear fiber and erbium-doped bismuth oxide fiber,” IEEE Photonics
Technology Letters, vol. 17, no. 12, pp. 2658-2660, Dec. 2005.
[21] T . Miyazaki and F. Kubota, “Simultaneou s demultiplexing a nd clock
recovery for 160-Gb/s OTDM signal using a symmetric mach–Zehnder
switch in electrooptic feedback loop,” IEEE Photonics Technology
Letters, vol. 15, no. 7, pp. 1008-1010, July 2003.
[22] H. Tsuchida, and M. Suzuki, “40-Gb/s Optical Clock Recovery Using an
Injection-L ocked O ptoele ctroni c Oscil lator,” IEEE Photonics
Technology Letters, vol. 17, no. 1, pp. 211-213, Jan. 2005.
[23] E. Tangdiongga, J. P. Turkiewicz, G. D. Khoe, a nd H. de Waardt, “Clock
recovery by a fiber ring laser employing a linear optical amplifier,”
IEEE Photonics Technology Letters, vol. 16, no. 2, pp. 611-613, Feb.
2004.
[24] D. Zibar, J. Mork, L. K. Oxenlowe, M. Galili and A. T. Clausen, “Timing
jitter analysis for clock recovery circuit s based on an optoelectronic
phase-locked loop (OPLL),” Conference on Lasers & Electro-Optics
(CLEO), vol. 1, pp. 458-460, May 2005.
[25] H. Tsuchida, “160 Gb/s optical clock recovery using a regeneratively
mode-locked laser diode,” IEEE Photonics Technology Letters, vol. 18,
no. 16, pp. 1687-1689, Aug. 2006.
[26] W. Huang, M. H. M. Ni zam, I. Andonovic, and M. Tur, “Coherent
optical CDMA (OCDMA) systems used for high-capacity optical fiber
networks-system description, OTDMA comparison, and
OCDMA/WDMA networking,” J. of Lightwave Tech., vol. 18, no. 6, pp.
765-778, June 2000.
[27] G. E. Wickens, D. M. Spirit, L. C. Blank, “20 Gbit/s, 205 km optical time
division multiplexed transmission system,” IEEE Electronics Letters,
vol. 27, no. 11, pp. 973-974, May 1991.
[28] G. E. Keiser. Optical fiber communications. Third edition, McGraw-Hill
International Editions, Singapore, pp. 517-519, 2000.
[29] T . S. Rappaport. Wire less communicati ons: principles and practice. 2nd
Edition, Prentice-Hall, Inc., USA, 2002.
[30] D. Roddy. Satellite communications. 3rd E dition, McGra w-Hill,
Singapore, 2001.
530
