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## Low-Rate UWB Communication Systems Based on Chaotic Modulations

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**Low-Rate UWB Communication Systems Based on Chaotic**Modulations Lin Wang1, Wei Kai Xu1, Guanrong Chen2 1.Dept. of Communication Engineering, Xiamen University, China 2.Dept. of Electronics Engineering, CityU of HK, China 27/07/2010**Contents**• Background • System design of UWB based chaos modulations • Rapid timing synchronization for FM-DCSK UWB • User cooperation DCSK communication system • Ongoing work • Conclusions Wideband Wireless Communications Laboratory, Xiamen University**Contents**• Background • System design of UWB based chaos modulations • Rapid timing synchronization for FM-DCSK UWB • User cooperation DCSK communication system • Ongoing work • Conclusions Wideband Wireless Communications Laboratory, Xiamen University**Background**• Basic idea of UWB • Communications in a frequency band already occupied (Coexistence) • Making frequency re-use possible by limiting the PSD of Equivalent Isotropically Radiated Power (EIRP) • UWB radio regulation • Federal Communications Commission (FCC, USA) determined only the maximum emission limit and minimum bandwidth • Method for access to UWB frequency band has not yet been fixed • Note: UWB regulations say nothing about the type of carrier and technique used to generate UWB carrier. So Any kind of carriers, including chaotic signals, may be used; any kind of modulation scheme may be used Wideband Wireless Communications Laboratory, Xiamen University**Background**• UWB frequency band • Frequency band allocated to handheld UWB devices: 3.1 GHz to 10.6 GHz • Define: UWB transmitter is an intentional radiator that, at any time instant • fractional bandwidth • Or, UWB bandwidth 500 MHz, regardless of the fractional bandwidth Wideband Wireless Communications Laboratory, Xiamen University**Background**• FCC limits on radiated UWB signal • To avoid interference caused in the existing narrowband systems, not the radiated power but the PSD of EIRP is limited • Definition of Equivalent Isotropically Radiated Power (EIRP)= (power supplied to the antenna) *(antenna gain) • ERIP limits in two ways: • Peak level of the emissions contained within a 50-MHz bandwidth centered on the frequency at which the highest radiated emission occurs should not exceed 0 dBm EIRP • Average radiated emissions shall not exceed -41.3 dBm EIRP when measured using a resolution bandwidth of 1 MHz over the frequency band of 3.1 GHz to 10.6 GHz • Note: Low-data rate (less than 350 kbit/s) UWB systems are peak power limited, while high-data rate ones are average power limited Wideband Wireless Communications Laboratory, Xiamen University**Background**• Carrier types of UWB • Impulse radio, such as Gaussian pulse • Sine waveform, MB-OFDM, FM-UWB etc. • Chaotic waveform • Modulation schemes • Pulse-Amplitude Modulation (PAM), pulse carrier • Pulse-Polarity Modulation (PPoM), pulse carrier • On-Off Keying modulation (OOK), arbitrary carrier • Pulse-Position Modulation (PPM), arbitrary carrier • Transmitted-Reference Modulation (TR), arbitrary carrier Wideband Wireless Communications Laboratory, Xiamen University**reference**• chips • information chips • guard • interval Ts Tg Tf Background • TR Modulation: Binary information is mapped into two wavelets, the first chips serves as a reference, the second one carries the information RMS delay of channel Where:Tg >Tch Delay between the reference and information bearing chips Note: Wavelet may be either a fixed (impulse radio) or a chaotic waveform Modulation: • Bit “1” : Second chips is equal to the delayed reference one • Bit “0” :Second chips is equal to the inverted and delayed reference one Note: FM-DCSK/DCSK is a TR modulation based on chaotic carrier. Wideband Wireless Communications Laboratory, Xiamen University**Background**• Applications of low-rate UWB • Sensor networks, wireless networking devices of embedded system, smart houses, offices and mall etc. • Categories of low-rate UWB • Data-rate: typically around 1Mbps • LR-WPAN: covers personal operational area, short range from 10-30m • LR-WLAN: coverage up to 100m • IEEE Standards about low-rate UWB • Existing: IEEE 802.15.4 Standard, used by ZigBee alliance • Alternative UWB(2007): IEEE 802.15.4a Standard WPAN low rate alternative PHY Wideband Wireless Communications Laboratory, Xiamen University**Contents**• Background • System design of UWB based chaos modulations • Rapid timing synchronization for FM-DCSK UWB • User cooperation DCSK communication system • Ongoing work • Conclusions Wideband Wireless Communications Laboratory, Xiamen University**information chips**• guard • interval Ts Tg Tf Signal structure System model of FM-DCSK UWB • reference • chips Transmitter Receiver Noncoherent detect: observation signal is Where is integration duration. Wideband Wireless Communications Laboratory, Xiamen University**Analysis and Optimization of System Performance (1)**In the absence of ISI, the receive signal as Suppose a data symbol a = 1 is transmitted, the output of the detector is Where:ζ1is the signal energy captured in the integration, ζ2 and ζ3are the signal-noise cross terms, and ζ4 is the noise-noise cross term. Above three cross terms can be approximated as independent Gaussian random variables. And their distributions are respectively as below: Wideband Wireless Communications Laboratory, Xiamen University**Analysis and Optimization of System Performance (2)**Then the bit error rate (BER) probability can be written as According to expression of BER, the optimization of integration time Topt is equivalent to the maximization: Note: above expression can not only prove that the existence of the optimal integration interval Topt, but also show that the optimal value depends on Eb, N0, B and g(t). Wideband Wireless Communications Laboratory, Xiamen University**Simulation results(1)**Performance of the fixed integration interval UWB-FM-DCSK system, when the guard interval length Tg is 7.5, 22.5, 47.5, 97.5 and 197.5ns, with the integration interval is Tf/2 and the chip duration Ts is 2.5ns Left: CM1 Right: CM4. Remark: 1.When semi-bit Tf/2 duration integration interval is used, the BER performance is obviously affected by guard interval Tg. Larger Tg, lower BER. 2. On the other hand, since the integration interval is equal to Tf /2, increasing of Tf means more noise energy captured whereas signal energy almost unchanged, so the BER is deteriorated when Tg great than a threshold. Wideband Wireless Communications Laboratory, Xiamen University**Simulation results(2)**BER as a function of the integration interval, with Eb/N0 increases from 10dB to 18dB in CM1and CM2. Tg is set as 197.5ns and Ts is 2.5ns. Remark: the BER of the proposed system as a function of the integration interval T (from 0 to Tf/2 with the stepping 4ns) in different Eb/N0 condition and different channel mode. And there exists an optimum integration interval when BER is minimized corresponding to each Eb/N0. Wideband Wireless Communications Laboratory, Xiamen University**Simulation results(3)**Performance comparison between the non-optimal integration interval scheme and the presented optimization scheme of CM1, CM2, CM3 and CM4, Tf is set as 400ns and Ts is 2.5ns Remark: BER performance of the presented optimization method outperforms the scheme when the integration interval keeps a fixed value of Tf/2 about 2.2dB in CM1, CM3 and CM4 while about 1.2dB in CM2, because of much channel delay in CM2 than others. Wideband Wireless Communications Laboratory, Xiamen University**Improved DCSK/FM-DCSK Scheme**• Problem of TR-UWB receiver (including DCSK/FM-DCSK) • All digital implementation: extremely high power consumption due to needing GHz A/D converter. • Analog implementation RF front-end: a RF delay line is required, which is extremely difficult to implement in CMOS. Especially, long delay implementation, such as several decades ns. • How to solve this problem? • Design an alterative DCSK transceiver which eliminates the RF delay line. Wideband Wireless Communications Laboratory, Xiamen University**An alterative DCSK scheme: CS-DCSK(1)**• Code-shifted DCSK (CS-DCSK) • Both the reference and information bearing wavelets are sent in the same time slot. The two wavelets are separated by Walsh codes instead of time delay. The diagram of transmitter and receiver as follows, Receiver Observation signal: Transmitter Transmitted signal: Wideband Wireless Communications Laboratory, Xiamen University**An alterative DCSK scheme: CS-DCSK(2)**Where are Walsh code sequences, they are orthogonality. can be any two rows which are taken from Walsh code matrix. Such as Walsh code matrix Properties of CS-DCSK: ● eliminates the delay circuit at receiver. ● the reference and information bearing wavelets are transmitted in the same time slot using Walsh code sequences. ● the reference and information bearing wavelets are orthogonality, which can be proven as follows, Wideband Wireless Communications Laboratory, Xiamen University**BER performance analysis of CS-DCSK**According to Gaussian Approximation (GA) method, the BER is computed as If the Logistic map is used, we have So, BER of CS-DCSK can be approx. BER performances of the CS-DCSK over AWGN for theoretical results (solid lines), and simulated results Remark: The BER performance is function of spread-spectrum factor. It is similar to traditional DCSK modulation. Wideband Wireless Communications Laboratory, Xiamen University**CS-DCSK performance over multipath fading channels**Performance comparisons of CS-DCSK and DCSK over Rayleigh fading channel conditioned with different spread factor.Left: Channel I: PDP: [0.4 0.4 0.2] Right: Channel II, PDP: [0.6 0.3 0.1] Remark: The BER performances of the CS-DCSK have almost same with that of DCSK except for small SF. It is illustrated that the CS-DCSK is a competitive alterative scheme of DCSK. Wideband Wireless Communications Laboratory, Xiamen University**Contents**• Background • System design of UWB based chaos modulations • Rapid timing synchronization for FM-DCSK UWB • User cooperation DCSK communication system • Ongoing work • Conclusions Wideband Wireless Communications Laboratory, Xiamen University**Problem of present non-coherent UWB synchronization**algorithm ● Timing synchronization is a major challenge for the implementation of non-coherent UWB receivers. ● Present non-coherent UWB timing synchronization algorithm: the operation of correlation between two neighboring symbols and the operation of picking the peak from a large amount of correlation values. ● Problem: Above algorithm operation is not available for the FM-DCSK UWB system for two reasons: first, a chaotic waveform varies from symbol to symbol, even if the same bit is transmitted repeatedly, which is the main difference between the FM-DCSK UWB and the conventional non-coherent TR UWB; second, noise-like chaotic signals have low values of inter-symbol correlation. Thus, a new timing synchronization algorithm with rapidity and low complexity is required for the FM-DCSK UWB communication system. Wideband Wireless Communications Laboratory, Xiamen University**Rapid timing synchronization algorithm for FM-DCSK UWB**receiver(1) • Algorithm basic idea: takes advantage of the excellent correlation characteristic of chaotic signals, finish timing synchronization based on intra-symbol correlation operation. Algorithm step: ■Step 0: Divide the interval into N parts uniformly, and take the beginning point of each part as the integral starting point. Thus, within one symbol observation interval, N integral results are obtained: Example for Divide the interval into 4 parts Where ,define: Then update according to Wideband Wireless Communications Laboratory, Xiamen University**Rapid timing synchronization algorithm for FM-DCSK UWB**receiver(2) After step 0, the target interval, in which lies, can be determined in whose length is Step q: (q>0) Through step q-1, it is known that lies in the interval , which is then uniformly divided into two sub-intervals, namely and . The purpose of this step is to determine in which sub-interval lies, and the determination is based on formulas given below: Where: Update Wideband Wireless Communications Laboratory, Xiamen University**Rapid timing synchronization algorithm for FM-DCSK UWB**receiver(3) Algorithm stop condition: define synchronization resolution ,so needing number of steps that maximum timing error is less than or equal to the resolution So, Define, Thus, If the algorithm ends; otherwise update q = q + 1,and repeat step q. Wideband Wireless Communications Laboratory, Xiamen University**Numerical and simulated results(1)**• Comparison between the Proposed Algorithm and the Conventional Algorithm In the terms of proposed algorithm and reference algorithm, the number steps for given synchronization solution is Proposed: Reference: Required number of steps vs. (T = 200ns, N = 4). Remark: the time complexity of the reference algorithm is , whereas the counterpart in the new algorithm here is only Wideband Wireless Communications Laboratory, Xiamen University**Numerical and simulated results(2)**Timing performance under different N Where: earlier timing (ET): later timing (LT): accurate timing (AT): Comparison among (a) timing probabilities, (b) average timing errors vs. Eb/N0 under different values of N over CM1 Remark: Since the BER performance is highly sensitive to LT but less sensitive to ET, it is clear that the larger the probability of LT, the worse the BER performance. Here, occurrence probabilities of ET is great than LT. Thus, the average BER performance is good. Wideband Wireless Communications Laboratory, Xiamen University**Contents**• Background • System design of UWB based chaos modulations • Rapid timing synchronization for FM-DCSK UWB • User cooperation DCSK communication system • Ongoing work • Conclusions Wideband Wireless Communications Laboratory, Xiamen University**Motivation**• There are two fundamental aspects of wireless communication that make the problem challenging and interesting. • First is the phenomenon of fading: the time-variation of the channel strengths due to the small-scale effect of multipath fading • Second is the phenomenon of interference: such as, inter-user inference, inter-symbol inference (ISI) that is introduced by multipath channel. • So, how to deal with fading and with interference is central to the design of wireless communication systems. • Diversity – an effective method to mitigate channel fading • As an alterative spread-spectrum technology, DCSK/FM-DCSK has superior capability in terms of anti-interference over multipath fading channels. • For improving performance of DCSK, space diversity is a promising technology. Such as, multiple antenna, user cooperative diversity, etc. Wideband Wireless Communications Laboratory, Xiamen University**User Cooperative Diversity (Combating fading)**• Cooperative communications can provide transmit diversity for most wireless networks. • Since one user’s signals can be relayed by other users’ independent fading paths to the destination, in cooperative communications, terminals share others’ antennas to achieve transmit diversity. • This approach achieves spatial diversity through all partners’ antennas, which enhances the ability of combating fading in wireless communication systems. Such as WSN, WPAN and WBAN, which are complicated applications requesting low-cost, low power and various demands of QoS. (VMIMO) Wideband Wireless Communications Laboratory, Xiamen University**DCSK/FM-DCSK (Combating multi-path interference)**• Frequency modulated differential chaos shift keying (DCSK/FM-DCSK) is a joint modulation and spread spectrum technique. • The noise performance of DCSK/FM-DCSK is superior to most conventional modulation schemes in multi-path channel environment. In particular, frequency-modulated differential chaos shift keying (DCSK/FM-DCSK) technique offers robustness against multi-path interference and channel imperfections. • DCSK/FM-DCSK demonstrates itself as a promising modulation technique for many low-cost and low-complexity wireless transmission applications, such as wireless sensor networks (WSN) and low-data-rate wireless personal or body area networks (WPAN, WBAN). • Thus, combination of user cooperative diversity and DCSK/FM-DCSK is an efficient scheme to anti-fading and anti-interference (ISI). Wideband Wireless Communications Laboratory, Xiamen University**DCSK user cooperative system based on Walsh codes**Model of user cooperative communication systems Walsh codes are used as multi-access codes for two users. At odd slot, user 1 and user 2 transmit self information to partner and destination, respectively, at even slot, user 1 and user 2 relay partner’s information to destination. Cooperation protocol of the two-user DCSK-CC system According to transmit self information or not at even slot, there are two user cooperative diversity protocol: conventional cooperation and space-time cooperation. Wideband Wireless Communications Laboratory, Xiamen University**Simulated results(1) –Comparisons between DCSK cooperative**system and CDMA cooperative system BEP performance comparisons of the DCSK cooperation system and the CDMA cooperative system with different distance ratio , here all distances are normalized by the distance Spread-spectrum factor is 32 (left)and 64 (right). Spread code of CDMA is Golden sequence, CDMA system with conventional receiver, simulation environment is multipath fading channel with PDP of [0.4 0.4 0.2]. Remark: The BEP curves indicate that the performance of the DCSK cooperative system with a steeper slope is more sensitive to noise than the CDMA cooperative system at high values of SNR. The DCSK cooperative system is more effective at high SNR, especially in near-far scenarios, such as user distance ratio 1:1:0.5 and 1:0.8:0.4. Wideband Wireless Communications Laboratory, Xiamen University**Simulated results(2)-Performance comparisons between**conventional cooperation and space-time cooperation BEP performance of conventional cooperation protocol and space-time cooperation protocol, spread spectrum is 32 (left) and 64 (right). Remark: Unlike the user cooperative communication systems based on traditional digital modulations, the performance of the space-time cooperative system is not better than that of the conventional cooperative system in the DCSK cooperative system at all times. Wideband Wireless Communications Laboratory, Xiamen University**Simulated results(3)**• Performances under different chaotic maps BEP performances of DCSK cooperative system when logistic map, cubic map and Bernoulli-shift map are used, respectively, spread-spectrum factor is 32 Logistic map: Cubic map: Bernoulli-shift map: Remark: It is found that the chaotic sequences generated by the Bernoulli-shift map produce higher BEP, while the BEPs for the system using the cubic map and the logistic map are the same. There are similar results in conventional multiuser DCSK systems Wideband Wireless Communications Laboratory, Xiamen University**Properties of user cooperative DCSK communication system**• In near-far scenarios, the performance of DCSK user cooperative system is better than that of CDMA user cooperative system with conventional receiver. • The conventional cooperation is a better cooperation protocol than the space-time cooperation in DCSK cooperative system. • There are similar performance results with DCSK under different chaotic maps. • Consequently, the DCSK cooperative system can be expected applicable to energy-constrained and low-cost wireless networks with simple cooperation protocols Wideband Wireless Communications Laboratory, Xiamen University**Contents**• Background • System design of UWB based chaos modulations • Rapid timing synchronization for FM-DCSK UWB • User cooperation DCSK communication system • Ongoing work • Conclusions Wideband Wireless Communications Laboratory, Xiamen University**Ongoing work**• To boost the application of chaos-based UWB communications, chaotic signal generators working directly in the microwave frequency region is developing. • Chaos-based communications are used in traffic (ITS) • Network coding and cross-layer design of DCSK/FM-DCSK based network. Wideband Wireless Communications Laboratory, Xiamen University**Conclusions**• In meddle & low-data rate systems where the low power consumption and low price are crucial, only non-coherent receivers can be used. DCSK/FM-DCSK is best choice in chaos modulations. • The rapid timing synchronization algorithm based chaotic signal property is efficient and robust in FM-DCSK UWB non-coherent receiver. • An alterative simplified DCSK (CS-DCSK) scheme is more promising in low-cost application scenarios. • There are many space to optimize multi-user communication system based on DCSK/FM-DCSK modulation. Wideband Wireless Communications Laboratory, Xiamen University**References**• G. Kolumbán, M. P. Kennedy, G. Kis, and Z. Jákó, “FM-DCSK: a novel method for chaotic communications,” in Proc. IEEE ISCAS, May, 1998, vol.4, pp. 477-480. • F.C.M. Lau and C.K. Tse, Chaos-based Digital Communication Systems:Operating Principles, Analysis Methods, and Performance Evaluation, (Springer-Verlag, Berlin), 2003. • Y. Xia, C. K. Tse, and F. C. M. Lau, “Performance of differential chaos-shift-keying digital communication systems over a multipath fading channel with delay spread,” IEEE Trans. Circuits Syst. II, Exp. Briefs, vol. 51, pp. 680-684, Dec. 2004. • G. Kolumbán, “UWB technology: Chaotic communications versus noncoherent impulse radio,” in Proc. ECCTD, Sept., 2005, vol. 2, pp. II/79-II/82. • C. C. Chong and S. K. Yong, “UWB direct chaotic communication technology for low-rate WPAN applications,” IEEE Trans. Vehicular Technology, vol. 57, pp. 1527-1536, Mar. 2008. • X. Min, W. K. Xu, L. Wang, and G. R. Chen, “Promising performance of a frequency-modulated differential chaos shift keying ultra-wideband system under indoor environments,” IET Commun., vol. 4, pp125-134, Jan. 2010. • Shaoyan Chen, Lin Wang and Guanrong Chen “Data-Aided Timing Synchronization for FM-DCSK UWB Communication Systems”, IEEE Trans. Industrial Electronics, vol.57, May 2010. • Jing Xu, Weikai Xu, Lin Wang and Guanrong Chen, “Design and Simulation of a Cooperative Communication System Based on DCSK/FM-DCSK,” in Proc. IEEE ISCAS, Paris, France, May 2010 • W. K. Xu, L. Wang and G. R. Chen, “Performance of DCSK Cooperative Communication Systems over Multipath Fading Channels,” IEEE Trans. Circuits and Systems-I, be accepted. • W. K. Xu, L. Wang and G. Kolumban, “A Novel Differential Chaos Shift Keying Scheme,” International Journal of Bifurcation and Chaos, under review in the second round. Wideband Wireless Communications Laboratory, Xiamen University**Thank you!**Wideband Wireless Communications Laboratory, Xiamen University