JOURNAL OF TELECOMMUNICATIONS, VOLUME 12, ISSUE 1, JANUARY 2012 11
T-H Differential Pseudo-Random Pulse: A New UWB System for LR-WPAN Applications
N. Rebhi, A. Kachouri, M. Samet and D. Fournier Prunaret
Abstract— Ultra Wide Band is a new technology that has received much attention for its peculiar advantages: high-bandwidth, extremely low power spectral density, propagation robust to fades, ability to penetrate materials, possibility to coexist in the same spectrum with other modulation schemes. UWB Impulse Radio systems are particularly promising for short-range and low bit rate wireless communications as they combine reduced complexity with multipath and multiuser capabilities. One of the most important applications of UWB is Wireless Personal Area Networks (WPAN). Moreover a standard for such systems was established by the IEEE802.15.4a group. In this paper, a new ultra-wide-band modulation scheme so-called Time-Hoping Differential Pseudo Random Pulse (TH-DPRP) is presented, and evaluated on various IEEE 802.15.4a standard channels models. Results show that TH-DPRP experience little degradation under multipath environments due to the simple transceiver, the detection principle, and the different technical choice combined in this system.
1 INTRODUCTION LTRAWIDEBAND (UWB) has emerges as one of the most important technology that has attracted a great deal of interest from academia, industry, and stand-
ardization bodies. UWB can be traces back to 1960, when is mainly used to radar and military applications [1]. In these years, the UWB has been focused as a powerful wireless technology which can realize home networks service. A measurement appropriate for UWB signals is the relative bandwidth Bw defined as following:
( )LH
LH
ffffBw
+−
= 2 (1)
Where ƒH and ƒL are higher and lower edges of signal spectrum respectively.
The most important even in the UWB technology was set in April 2002 when the Federal Communication Com-mission (FCC) approved the first guidelines permitted un-licensed usage of UWB signals in wireless communications device within specified emission masks [2].
Following the realize of the first report by the U.S. Fed-eral communications commission’s, a large available bandwidth of more than 7.5 GHZ is allocated for UWB applications. U.S. FCC has released 3.1GHz to 10.6GHz frequency spectrum with restriction on transmit power spectral density (psd) of -41.3 dBm.MHZ-1.
In accordance with these decisions, U.S. FCC ruled that signals with a relative bandwidth greater than 25% or an
effective bandwidth ∆ƒ=ƒH-ƒL exceeding 500 Mhz, are treated as ultra wideband signals. Accordingly, UWB bandwidth is much wider than any existing communica-tions systems.
Communications via channels already occupied by conventional telecommunication systems and coexistence can be established only by UWB radio where the spectrum of the transmitted signal covers an ultra-wide band, and the frequency re-use is possible as a result of limiting the psd. Moreover, the use of extremely large bandwidths provides the ability to resolve multipath effects. The low power spectral density makes interception, detection, and interference with existing narrowband systems difficult.
Considering multiple access scenarios, the presence of multiple signals being transmitted at the same time is an-other typical source of interferences. An effort has been made to reduce the multiple access interference (MAI) by designing orthogonal hopping sequences. Hence, time Hopping (TH) scheme combined with UWB communica-tion are used to avoid multiples access interference which can be reduced by increasing the number of time hops, but at the cost of reduced data rate. Consequently, UWB communication systems are opti-mized for low complexity, low power, low cost, and low rate wireless communication with the capability to overlay on existing frequency allocations. The unique characteris-tics of UWB radio make it a viable candidate for feature wireless communications, especially indoor wireless appli-cations. The IEEE standards association has specified two types of UWB systems: high rate (HR) and low rate (LR). In particular, the standardization effort within IEEE802.15.4a aims for low rate wireless personal areas network (LR-WPAN) applications [3].
Several schemes were proposed to meet the require ments of the IEEE802.15.4a standard. TH-UWB [4], Trans-
———————————————— • N. Rebhi is with the Laboratory of Electronics and Technologies of Infor-
mation (LETI) in the National School of Engineers, Sfax; B.P.W, 3038 Sfax, Tunisia.
• A. Kachouri is with the National School of Engineers Sfax; B.P.W, 3038 Sfax, Tunisia.
• M. Samet is with the National School of Engineers Sfax; B.P.W, 3038 Sfax, Tunisia.
• D. Fournier Prunaret is with National Institute for Applied Sciences, Tou-louse; 31077 Toulouse Cedex, France
mitted Reference (TR-UWB) [5], Frequency Hopping (FH-UWB) [6], and Direct Sequence (DS-UWB) [7] schemes, wherein orthogonal codes and radio pulses (Gaussian, pseudo-random, chaotic, etc), are used to avoid multiple access and multipath propagation.
In this paper, we propose a new TH-TR-UWB modula-tion technique based on a pseudo random radio pulses and a pseudo-noise (PN) code which can be modified for each user. Time Hoping Differential Pseudo Random Pulse sys-tem is mapped on a very simple transceiver structure. The detection principle is ameliorated in order to greatly reduce interferences problems under multi-access and multi-paths communications in comparison with conventional schemes.
The rest of this paper is organized as follows; the TH-DPRP UWB scheme is developed and the transceiver block diagrams is mapped in section 2, BER performance in In-door Residential and Office channel models are evaluated in section 3. Finally, this paper is concluded in section 4.
2 TH-DPRP COMMUNICATION SYSTEM 2.1 Modulation of TH-DPRP
The DPRP scheme transmits a train of pulses per symbol. Each chip contains one pulse, and several chips construct a symbol Si
∈{-1, +1}. Transmitted signal is divided into sym-bol time slots of duration TS, which are sub-divided into chip time slots of duration TC. Then, each symbol is made up of NC chips, resulting a symbol time TS=NC.TC. Every chip has a code Ck ∈[-1, +1], for k=0… NC-1 randomly generat-ed. The polarity of the data pulse is modulated by the prod-uct of the chip code and the pulse as presented in figure 1. To code information, the pseudo-random sequence {Ck}k=0..Nc-
1 is used to transmit a symbol dm= +1, and their orthogonal sequence correspondent {(-1)k. Ck}k=0..Nc-1 is used to transmit a symbol dm= -1. The parameter m designs the mth symbol, thus, a single user DPRP transmitted signal can be modeled by the following equation:
)()()(0
1
0c
im
ik
k
m
N
k
im
i kTtpCdtsC
−=∑∑∞
=
−
=
(2)
)(tpm is the transmitted pulse for each symbol expressed as follow.
)().()( sm mTtetptp −= (3) Where e(t) is a rectangular pulse of unit amplitude and TP
duration as shown in (4):
⎭⎬⎫
⎩⎨⎧ ≤≤
=elsewhere
Ttte P
,00,1
)( (4)
The block diagram of transmitting the mth symbol by the ith user is illustrated in figure 2, where cj, for j=1…NC, substi-tute the following equation: { } { } 1...0...1
)( −===
CC Nkik
kimNjjCdc (5)
The transmitted signal propagates through a multipath channel and result an output rm
i(t). § Time Hopping DPRP
It is very important to note that there is a certain delay in-terval between pulses of one symbol. The information is modulated into pulse trains by introducing time hopping. A guard interval, Tgp, was inserted between successive pulses to avoid inter-pulses interference (IPI), and a guard interval Tgs, was inserted between symbols to avoid inter-symbols interference (ISI). Besides, the possible positions of pulses within a symbol follow a so-called time-hopping (TH) code. Thus, the transmitted pulses sequence is different for each user according to the TH code. This accommodates multiple access (MAI) interferences and improves security.
§ Time Hopping DCRP If the pseudo-random pulse has a chaotic behavior; ex-
treme sensitivity to initial conditions, fractal structure of the attractor, unpredictable state in the future, etc…, particular-ly in this case, this scheme can be called Time-Hopping Dif-ferential chaotic Radio Pulse (TH-DCRP).
Chaotic and pseudo-random pulses have a same number of features that makes them attractive for use in UWB com-munication systems as carrier such as wideband spectrum, auto, and cross correlation properties. Furthermore, chaotic signals is deterministic, it can be generated in variety wave-forms directly in the frequency band desired by simple structure devices leading to a low cost of the product. Cha-otic modes can be tailored by small variations of the system parameters, i.e., due to sensitive dependence of chaos upon initial conditions, a large number of spreading waveforms can easily be produced. The spectra properties of chaotic signals can be controlled to satisfy the FCC regulations.
Assume that for both cases, each user has a specific code
Fig. 1. The transmitted signal form (example NC=4)
Fig. 2. Block diagram of the TH-DPRP transmitter
rmi(t) sm
i(t)
pm(t)
c3
cNc
TH-D
PRP Dem
odulator
Multipath C
hannel
Tc
Tc
Tc
c2
c1
…
1 Symbol
C0=1
1 Chip TS
TC
TC-TP
C1=-1
C2=1
C3=1
13
PN, and a pseudo-random or a chaotic signal. This provides robustness to multi-users and inter-symbols interferences.
2.2 Demodulation of TH-DPRP Transmitted signals arrive at the receiver in distorted
waveforms; this degradation is caused by effects of the mul-tipath channel propagation. In multipath channel radio pulses travel many paths and arrive at the receiver from different directions with different delays; on the way they undergo attenuation, fading, and interferences. In the input they are summed. Assume that channel distortion is also due to a noise source n(t), which is an additive white Gauss-ian noise (AWGN) with a two-side power spectral density of No/2. Thus, the received signal after propagation through the channel is calculated as convolution of the transmitted signal with the channel response h(t) as illus-trate in (6). Where⊗ represent the convolution operator.
)()()()( tnthtstr ii +⊗= (6) In order to overcome temporal and special channel varia-
tions, a consecutive chip by chip recovered signal method based on a specific user code is proposed. In other word, for one symbol, each two chips consecutives will be correlated between them. The orthogonal code {Ck}k=0..Nc-1 serves as a user specific code, known to both the transmitter and the receiver. This code is used as shown in Figure 3 mapping the structure of the receiver.
For one-user case, the decision Zm provided by the output
of the detector can be expressed as follows (7). Then, the decision will be (+1) if Zm>0 or (-1) if Zm <0.
( ) ( )( )
dtTtrtrCCz cs
cs
c TkmT
kTmT cim
im
N
kkkm ∫∑
++
+
−
=+ −=
12
01 (7)
This detection approach is very important since one symbol is recovered after (Nc-1) consecutives-inter-pulses correlation. Therefore, the channel effects are lower between each two consecutives pulses which increase the bit error rate performance. This makes the TH-DPRP receiver per-forms much better under severe multipath channels than conventional non-coherent UWB-TR receivers in which each data-modulated chip is correlated with the same reference chip [8], [9].
2.3 UWB signal generation UWB regulations determined only the maximum emis-
sion limit and minimum bandwidth without say nothing about the type of carrier and method which should be used to generate UWB carrier. So any kind of carriers may be used including impulse, chirp, pseudo-random, and chaos [10], [11], [12]. Among techniques found in the literature, a unified model which is equally valid for communications with fixed, chaotic and random carrier is developed in [13]. For fixed waveform communications, the same waveforms are transmitted if the same symbol is transmitted repeated-ly. In chaotic and random communications the transmitted waveforms are continuously varying. One of the main ad-vantages of this model is the facility to control spectra prop-erties that satisfied the UWB emission mask.
Let use xn(t) as the nth basic function, exploiting the Fou-rier series representation on a time interval t (0<t<T), we obtained (8).
∑=
⎥⎦
⎤⎢⎣
⎡⎟⎠
⎞⎜⎝
⎛+⎟⎠
⎞⎜⎝
⎛=
2
1
2sin2cos)(k
kk
qnk
qnk
qn t
Tkt
Tktx π
βπ
α (8)
Where qnkα and q
nkβ representing the Fourier coefficients obtained from (9), and (10).
dttT
ktxT
T qn
qnk ⎟
⎠
⎞⎜⎝
⎛= ∫
πα
2cos)(20
(9)
dttT
ktxT
T qn
qnk ⎟
⎠
⎞⎜⎝
⎛= ∫π
β2sin)(2
0 (10)
The upper index (q) indicates that basis functions are not necessarily constants. In the case of chaotic and random basis functions, q
nkα and qnkβ become random variables.
( ) ( )2
12;2
12 02
01
−+=
+−=
TBfkTBfk (11)
Constants k1 and k2, given by (11) respectively, are de-termined by the lower and upper frequencies of the band-width (Bw=2B) with a center frequency ƒ0.
2.4 Multipath Immunity The propagation in multipath channel gives two mains
effects which are signals fading and interferences (IPI, ISI, and MAI). These factors are considered in our proposed UWB communication system, and several choices as taken to avoid these problems. Accordingly, TH-DPRP scheme belongs to transmitted-reference (TR) family. A time hop-ping pseudo random pulses are transmitted, according a certain PN code which can be modified for each user, and recovered by a non-coherent reception technique. Next we detailed these entire TH-DPRP scheme bases. 1. Pseudo-random carrier : a property of pseudo-random
(as well as chaotic) signal with 2 GHz bandwidth is that signals on different paths, with relative delay above τd (autocorrelation time), at the receiver input are mostly uncorrelated and are summed not by amplitude but by energy. That means that adding paths always leads to an increase of received signal level. Thus, multipath en-vironment can act as an amplifier, and this may to com-bat fading signals.
Fig. 3. Block diagram of the TH-DPRP receiver
CNc-3
CNc-2
CNc-2
CNc-1
rmi(t)
∫pτ
Tc Tc Tc
Zm
C1
C2
C0
C1
∫pτ
∫pτ
∫
pτ
∑
14
2. TR System: in such system, transmitted signal is divided into two equal time slots, which are reference and data. The reference signal of known polarity is send in the first half of symbol duration, and the data, of polarity determined by the information bit, is sending in the se-cond half of symbol. At the receiver, the reference can be used as correlations templates since it experiences the same channel distortion as the data. This avoids complicated channel estimation.
3. Impulse radio: if we consider the case where one pulse per symbol is transmitted, an important problem called“catastrophic collisions” can occur. Since pulses from several transmitters arrive at the receiver simul-taneously, the interference ratio becomes very bad, leading to a high bit error probability (BER). Impulse radio (IR) avoids this problem by representing each data bit by several short pulses.
4. PN code: Current UWB-IR communication systems typically employ pseudo-random noise (PN) coding for channelization and secretion purposes. This lows to form orthogonal sequences more robust again inter-ferences and protected communications.
5. Time Hopping: a frequent method to combat interfer-ences effects (particularly IPI and ISI) in systems with pulses carriers is to introduce a guard intervals, inter-pulses and inter-symbols, large enough to exclude ar-rival of the delayed paths on the position of the next information pulse or symbol. In addition, TH-IR achieves multiple-access interference suppression.
6. Non coherent detection: if the channel is “very multi-path”, the form of the received signal is heavily dis-torted. In these conditions, and for low-rates systems such as LR-WPAN applications, non coherent recep-tion is more reasonable due to the strict requirement on cost, size and energy consumption. That means, by using non-coherent detection, the system can be im-plemented on a low cost and low complexity.
3. PERFORMANCE EVALUATION OF TH-DPRP IN IEEE 802.15.4A INDOOR CHANNEL MODELS
The TH-DPRP system is proposed to realize a simple transceiver operational under difficult conditions of multi-path channels. In this section, BER performances of the pro-posed system are evaluated in various propagation channel of IEEE 802.15.4a model (from CM1 to CM4) in both scenar-io line-of-sight (LOS), and No-line-of-sight (NLOS).
In figure 4a, a pulse is shown that come through a multi-path channel. Figure 4b illustrates the same pulse in the output of CM1 (i.e., 1 chip representation using 10 ns pulse width, 35 ns guard interval, and 5 ns delay hopping). Figure 4c represents the spectrum of this pulse in input of CM1.
For computer simulation, we assume that center frequen-cy of pseudo-random pulse is 6GHz, bandwidth is 2 GHz (from 5 to 7 GHz), one bit duration is 400ns, and sampling frequency is 10 GHz. From this assumption, one bit duration consists of 4000 samples and data rate is 2.5 Mbps. In addi-tion, 100 realizations of channels models of standards IEEE802.15.4a with 1000 bits/channel are experienced; 100 channels responses are generated for each bit. Figure 5 plot-
ted simulations results presented in terms of BER versus the ratio Eb/No expressed in dB, where Eb is the energy per bit, and No is the single-sided spectral noise density. The bit error rate is given in multipath channels Residential LOS and NLOS (CM1, 2), and Office LOS and NLOS (CM3, 4).
Consider a BER of 10-3 as a reliable detection threshold,
figures 5 and 6 show that the requirement Eb/No to achieve
a BER of 10-3 for the CM1, CM2, CM3, and CM4 channels are approximately 9.5, 16, 10.5, and 15 dB respectively. The BER performance of CM1 is better than that of CM3 than approx-imately 1 dB at the target BER 10-3. The BER performance in CM3 is better than that of CM4 of approximately 4.5 dB at the same BER board. In other words receiver is more per-form in LOS channels than NLOS channels and more sus-ceptible in Office channel than Residential channel because of the signal power loss under multipath propagations.
0 50 100 150 200 250 300 350 400 450 500-0.025
-0.02
-0.015
-0.01
-0.005
0
0.005
0.01
0.015
0.02
0.025
Sample number
Am
plitu
de
0 50 100 150 200 250 300 350 400 450 500-0.015
-0.01
-0.005
0
0.005
0.01
0.015
0.02
Sample number
Am
plitu
de
(a) (b)
0 1 2 3 4 5 6 7 8 9-175
-170
-165
-160
-155
-150
-145
-140
-135
Frequency (GHz)
Powe
r/fre
quen
cy (d
B/Hz
)
Welch Power Spectral Density Estimate
(c)
Fig. 4 Radio pulse a) in input, and b) output of CM1. c) Radio pulse spectrum.
0 2 4 6 8 10 12 14 1610-5
10-4
10-3
10-2
10-1
100
Eb/No[dB]
BER
CM1CM2
Fig. 5. BER Performance of TH-DPRP in Residential Channel Models
.
15
4. CONCLUSION In this paper Time Hoping Differential Pseudo Random
Pulse ultra-wide-band transmitter system is discussed, and block diagrams of their transceiver are planned. TH-DPRP scheme belongs to T-R family and based on a certain TH and PN codes which can be modified for each user. Re-ceived signal is recovered by successive-chip by chip corre-lation. Pseudo random pulses with wide band nature and flexible spectrum, impulse-like autocorrelation functions, and small cross-correlation value, are used as carrier. Theses property which can be extended to cover chaotic signals may to combat fading signals and let pseudo-random and chaotic signals favorable for UWB communications systems based on correlation detection. In addition, TH-Impulse Radio achieves multiple-access interference suppression.
Consequently TH-DPRP method combines several tech-niques to combat interferences effects with a very simple transceiver structure.
TH-DPRP modulation scheme is performed under In-door Residential and Office channel of the IEEE802.15.4a standard; this scheme is characterized as multi-path immun-ity. Low complexity, low power, and low cost make TH-DPRP a promising candidate for LR-WPAN Application.
5. REFERENCES [1] T. W. Barrett, "history of ultrawideband radar and communications:
pioneers and innovators", in Progress in Electromagnetics Symposium, Cambridge, MA, USA, juillet 2000.
[2] Federal Communications Commission, "Revision of Part 15 of the Com-mission’s rules Regarding Ultra-Wideband Transmission Systems: First report and order", Technical Report FCC 02-48 (adopted February, 14 2002; released April 22, 2002).
[3] IEEE 802.15.4 standard, Part 15.4 : “ Wireless Medium Access Control (MAC) and Physical Layer (PHY) Specifications for Low-Rate Wireless Personal Area Networks, (LR-WPANs)", 802.15.4-2003.
[4] H. Yang and G.P. Jiang, “Irrational-Based Time-Hopping Modulation for UWB Communications”, 364 Ieee Transactions On Circuits And Sys-
tems—Ii: Express Briefs, Vol. 55, No. 4, April 2008. [5] K. Jie, Y. Gang, and S. Zhongke, “PN Sequence Assisted Noise Suppres-
sion in UWB-TR Communication System”, ICSP Proceedings 2006. [6] G.S. Biradar, S.N. Merchant, and U.B. Desai, “Frequency and time hop-
ping PPM UWB multiple access communication scheme”, journal of communications, vol.4, no.1, February, 2009.
[7] J. Lao, Q. Ren, and J. Zhao, “A novel chaotic stream DS-UWB system”, International Joint Conference on Neural Networks, IJCNN 2008.
[8] Y. wang, A. Schranzhofer, R.V. Leuken, and A-J. Veen, “A platform for delay transmitted reference UWB communications system prototype development”, CiteSeerX, 2008
[9] L. Wang, C. Zhang, and G. Chen, “Performance of an SIMO FM-DCSK communication system”, IEEE transactions on circuits and systems-II: express briefs, vol.55, no. 5, May 2008.
[10] A.S. Dmitriev, E.V.Efremova, A.V. Kletsov, L.V. Kuzmin, A.M. Lak-tyushkin, and V.Yu. Yurkin, “Wireless Ultrawideband Communications and Sensor Networks”, ISSN 1064-2269, Journal of Communications Technology and Electronics, vol. 53, No 10, pp.1206-1216. 2008.
[11] N. Rebhi, A. Kachouri, M. Samet, D. Fournier-Prunaret, and P. Chargé, “A Simple UWB-Chaotic Generator Based On Memristor Switching Model”, 2011 8th inter-national multi-conference en signal, systems and devices, SSD 2011.
[12] G. Kolumban, T. Krebesz, “UWB Radio: Digital Communication with Chaotic and Impulse Wavelets”, letter special section on nonlinear theory and its applications, ieice trans. Fundamentals, Vol.E90–A, No.10, Octo-ber 2007.
[13] G. Kolumban, F.C.M. Lau, and C.K. Tse, “Generalization of Waveform communications: the Fourier analyzer Approach”, circuits systems signal processing, Vol. 24, No. 5, 2005, PP. 451–474, Birkhauser Boston, 2005.
Nada Rebhi received the electrical engineering degree in 2006 and the Master degree in electronics in 2007, both from National School of Engineers of Sfax, Tunisia (ENIS). She is currently working toward the Ph.D. degree in Electronic and Telecommunication at the same School (ENIS), and member in the “LETI” Laboratory ENIS Sfax. Her research interest is to the study of Chaos and applications to Ultra Wide Band Communications. Focusing more particulary on Low Rate Wireless Personal Area Networks (LR-WPANs) applications. Abdennaceur Kachouri received the engineering diploma from Na-tional school of Engineering of Sfax in 1981, a Master degree in Measurement and Instrumentation from National school of Bordeaux (ENSERB) of France in 1981, a Doctorate in Measurement and In-strumentation from ENSERB, in 1983. He “works” on several coopera-tions with communication research groups in Tunisia and France. Currently, he is Permanent Professor at ENIS School of Engineering and member in the “LETI” Laboratory ENIS Sfax. Mounir Samet obtained an Engineering Diploma from National school of Engineering of Sfax in 1981, a Master degree in Measurement and Instrumentation from National school of Bordeaux (ENSERB) of France in 1981, a Doctorate in Measurement and Instrumentation from ENSERB, in 1981 and the Habilitation Degree (PostDoctorate degree) in 1998. He “works” on several cooperations with medical research groups in Tunisia and France. Currently, he is Permanent Professor at ENIS School of Engineering and member in the “LETI” Laboratory ENIS Sfax. Daniele Fournier Prunaret obtained a Ph.D. under the supervision of Pr. C. Mira, eminent specialist of Nonlinear Dynamical Systems, then a Doctorat d'Etat at the University Paul Sabatier of Toulouse, France, respectively in 1981 and 1987. Her research concern Modelisation and Analysis of Nonlinear Dynamical Systems, focusing more particu-larly on the study of Chaos and Applications to Telecommunications, and Secure Transmissions. She is currently Professor at the National Institute of Applied Sciences (INSA) in Toulouse, France and the
0 2 4 6 8 10 12 14 1610-4
10-3
10-2
10-1
100
Eb/No(dB]
BER
CM3CM4
Fig. 6. BER Performance of TH-DPRP in Office Channel Models
16
Head of the LATTIS. She is the author of around 100 papers in inter-national journals and conferences related to the study of Nonlinear Maps.
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