Performance evaluation of impulse radio UWB systems with pulse-based polarity randomization

In this paper, the performance of a binary phase shift keyed random time-hopping impulse radio system with pulse-based polarity randomization is analyzed. The effects of interframe interference and multiple-access interference on the performance of a generic Rake receiver are investigated for asynchronous systems in frequency-selective environments. A key step is to model the asynchronous system as a chip-synchronous system with uniformly distributed timing jitter for the transmitted pulses of interfering users. This model allows the analytical technique developed for the synchronous case to be extended to the asynchronous case and allows the derivation of closed-form equations for the bit error probability in various Rake receiver architectures. It is shown that a Gaussian approximation can be used for both multiple-access and interframe interference as long as the number of frames per symbols is large (typically, at least 5), whereas there is no minimum requirement for the number of users for the equations to hold. It is observed that under many circumstances, the chip-synchronous case shows a worse bit error probability performance than the asynchronous case; the amount of the difference depends on the autocorrelation function of the ultra-wideband pulse and the signal-to-interference-plus-noise-ratio of the system. Simulations studies support the approximate analysis.     

Simple-to-evaluate error probabilities for impulse radio UWB multiple access systems with pulse-based polarity randomization

Simple-to-evaluate and accurate bit error probabilities are presented for impulse radio ultra-wideband multiple access systems that use time hopping sequences with pulse-based polarity randomization and binary phase shift keying modulation. Simplified improved Gaussian approximation is used. It is shown that despite having the same processing gain, which is the multiplication of the number of frames per bit and chips per frame, different combinations of the parameters result in different performances. The theoretical results are validated by Monte Carlo simulations.     

A Novel UWB Pulse Shape Modulation System

In this paper novel modified Hermite polynomial functions for use in impulse radio (ultra-wideband) communications are proposed. With these functions pulse shapes which are orthogonal and have nearly constant pulse width regardless of the pulse order are generated. These properties hold under the effects of differentiation. An M-ary communication system is constructed using these pulse shapes. A Matlab model for generating the pulses is designed and the effect of timing jitter on the performance of the system is investigated by computer simulation.     

The effects of timing jitter and tracking on the performance of impulse radio

Impulse radio (IR) is a promising ultra-wideband technique for tactical military communications. A key feature of time-hopping IR are the very narrow pulses used to convey information. Analysis of such time-hopping schemes under a variety of assumptions have been reported in the literature. However, none of these studies to date consider the effects of timing jitter and tracking on time-hopping in a ultra-wideband (UWB) setting. We consider the effects of timing jitter and tracking on the performance of binary and 4-ary UWB communications. We find that the performance of IR is very sensitive to timing jitter and tracking, at least in part due to the very narrow pulses. We also find that in the presence of timing jitter and tracking, orthogonal 4-ary pulse position modulation (PPM) out performs binary offset PPM at all jitter levels in thermal and pulse noise. Simulation results are presented that quantify the sensitivity of binary and 4-ary IR to timing jitter and tracking error.     

Capacity Analysis of Orthogonal Pulse-Based TH-UWB Signals

The paper studies the capacities of time hopping ultra wideband (TH-UWB) systems by adopting orthogonal pulse-based modulation schemes in a multipath environment with the consideration of multiple access interference (MAI) and inter symbol interference (ISI). Recently, orthogonal pulse-based modulation schemes such as pulse shape modulation (PSM), biorthogonal PSM (BPSM) and the combination of orthogonal pulse position modulation (OPPM) and BPSM have been proposed for UWB system to reduce ISI and MAI. In this paper, we first investigate the influences of ISI and MAI on the capacities of TH-UWB systems in terms of signal-to-interference and noise ratio (SINR) for high-order modulation schemes based on various sets of orthogonal pulses. How mutual information varies with the number of multipath components (MPCs) is then analyzed. The complete mathematical analysis and simulation results are provided in detail. When maximizing the mutual information, the Gaussian distribution approximate is assumed to describe the characteristics of MAI and ISI. Simulation results are provided to verify the theoretical analysis. Some conclusions are drawn thereafter, such as that capacity depends on the pulse patterns adopted and mutual information varies with inversely proportional to the number of MPCs. We also observed that the capacities of UWB systems for OPPM-BPSM schemes are larger than those of PSM and BPSM schemes.     

Timing jitter in ultrahigh-speed dispersion-managed soliton systems

An analytical theory is presented which demonstrates the timing jitter induced by amplifier noise in ultrahigh-speed dispersion-managed soliton systems when the Raman effect and third-order dispersion (TOD) are considered. The analysis indicates that Raman jitter remains a potential problem for picosecond and subpicosecond pulses. Positive residual TOD can enhance the timing jitter caused by the combined action of TOD and Raman effect, while vice versa for negative residual TOD.     

Generation of 40 GHz phase stable optical short pulses using intensity modulator and two cascaded phase modulators

Pulse sources based on lithium niobate modulators are very attractive for optical time division multiplexing (OTDM) transmission systems because the modulators are now commercially available,qualified for system use,and can operate up to very high speeds and over a wide wavelength range.In this paper,we describe the principles of operation and performance of the pulse source based on lithium niobate modulators.The pulse source is based on a Mach-Zehnder intensity modulator (IM) and two phase modulators (PMs).The continuouswave (CW) light is modulated in an IM and then strongly phase modulated in two cascaded PMs.The chirped pulses are subsequently compressed to desired width using dispersion compensation technology.This method has the advantage of acquiring larger chirp using normal PM rather than that special designed PM of very low Vπ.It can also generate shorter pulses than conventional methods incorporating only one PM driving by a radio frequency (RF) signal with the power larger than 1 W which may damage the device.Generation of 40 GHz optical pulses shorter than 2 ps is theoretically illustrated,simulated and experimentally verified.Experimental results show that 40 GHz phase stable optical pulses with pulse-width of 1.88 ps,extinction ratio (ER) larger than 20 dB,the timing jitter of 57 fs and signal-to-noise ratio (SNR) of 32.8 dB can be achieved.This is also a cavity-less pulse source whose timing jitter is determined only by the RF source rather than by the actively controlled cavity.In the experiment,the phase noise of the RF source we used is as low as -98.13 dBc/Hz at a 10 kHz offset frequency which resulting very low timing jitter of generated pulses.The pulses are then modulated at 40 Gbaud/s with an inphase/quadrature (l/Q) modulator and multiplexed to 160 Gbaud/s with less interference between each other.After back-to-back demultiplexing by an electro-absorption modulator (EAM) to 40 Gbaud/s and demodulation by a delay interferometer (DI),clear and opened eye diagrams of 40 Gbaud/s I and Q tributary signals are obtained which verify the good performance of generated pulses in the 160Gbaud/s differential quadrature phase shift keying (DQPSK) OTDM system and further prove the phase stability and high quality of generated pulses.     

A new noncoherent UWB impulse radio receiver

This letter analyzes the implementation issues related to coherent receivers for UWB impulse radio with a special emphasis on timing and jitter problems. We propose a new jitter tolerant receiver design that is easy to implement. Analytical BER analysis and simulations verify that the performance of the proposed receiver is comparable to that of a correlator-based receiver that includes jitter. The new design is a promising candidate for low-cost low-power UWB IR receiver implementations.     

Investigation of amplitude noise and timing jitter of supercontinuum spectrum-sliced pulses

We numerically investigate the amplitude-noise and the timing jitter of pulses obtained by slicing coherent supercontinuum spectra. Applications such as time-division-multiplexing/wavelength-division-multiplexing systems and all-optical data regeneration are addressed. System parameters for optimizing the quality of sliced pulses are also discussed. We show that supercontinuum generation is a suitable method for generating amplitude-stable pulses and that no timing jitter is associated to the sliced pulses.     

A low jitter, low spur multiphase phase-locked loop for an IR-UWB receiver

A low jitter,low spur multiphase phase-locked loop(PLL) for an impulse radio ultra-wideband(IR-UWB) receiver is presented.The PLL is based on a ring oscillator in order to simultaneously meet the jitter requirement, low power consumption and multiphase clock output.In this design,a noise and matching improved voltage-controlled oscillator(VCO) is devised to enhance the timing accuracy and phase noise performance of multiphase clocks.By good matching achieved in the charge pump and careful choice of the l...     

Analysis of a high-temperature superconductor quantum fluxparametron operating at 77 K

A Quantum Flux Parametron (QFP) circuit designed for implementation with high-temperature superconductors is analyzed with thermal noise at 77 K. Experimental considerations are discussed and design parameters given for fabrication of the device using a YBa₂ Cu₃O₇ thin film on a SrTiO₃ bicrystal substrate. An application of the high T_c QFP for the detection of single flux quantum pulses is also described     

The Effect of Timing Jitter on a 160-Gb/s Demultiplexer

For high-speed optical communication systems, timing jitter is a crucial parameter for switching operations between the data and control signal. This is especially the case for the demultiplexer. The effect of timing jitter becomes very important as the bit rate of the data signal increases beyond 100 Gb/s and it is, therefore, essential to quantify its effect. In this letter, the impact of gating timing jitter on a 160-Gb/s demultiplexer is investigated by using two pulse sources with different timing jitter properties. We also investigate the interplay between the control signal pulsewidth and timing jitter. The experiment shows that it is essential to minimize jitter in the 20-kHz to 10-MHz range. Furthermore, we show that the impact of timing jitter can be reduced if the control signal pulses are broader than data signal pulses.     

On the tradeoff between two types of processing gains

One of the features characterizing almost every multiple-access (MA) communication system is the processing gain. Through the use of spreading sequences, the processing gain of random code-division multiple-access (RCDMA) systems, or any other code-division multiple-access (CDMA) systems, is devoted to both bandwidth expansion and orthogonalization of the signals transmitted by different users. Another type of MA system is impulse radio (IR). IR systems promise to deliver high data rates over ultra-wideband channels with low-complexity transmitters and receivers. In many aspects, IR systems are similar to time-division MA systems, and the processing gain of IR systems represents the ratio between the actual transmission time and the total time between two consecutive transmissions (on-plus-off-to-on ratio). While CDMA systems, which constantly excite the channel, rely on spreading sequences to orthogonalize the signals transmitted by different users, IR systems transmit a series of short pulses, and the orthogonalization between the signals transmitted by different users is achieved by the fact that most of the pulses do not collide with each other at the receiver. In this paper, a general class of MA communication systems that use both types of processing gain is presented, and both IR and RCDMA systems are demonstrated to be two special cases of this more general class of systems. The bit-error rate of several receivers as a function of the ratio between the two types of processing gain is analyzed and compared, under the constraint that the total processing gain of the system is large and fixed. It is demonstrated that in non-intersymbol interference (ISI) channels, there is no tradeoff between the two types of processing gain. However, in ISI channels, a tradeoff between the two types of processing gain exists. In addition, the suboptimality of RCDMA systems in frequency-selective channels is established.     

On The Tradeoff Between Two Types of Processing Gain

One of the features characterizing almost every multiple-access (MA) communication system is the processing gain. Through the use of spreading sequences, the processing gain of random code-division multiple-access (RCDMA) systems, or any other CDMA systems, is devoted to both bandwidth expansion and orthogonalization of the signals transmitted by different users. Another type of MA system is impulse radio (IR). IR systems promise to deliver high data rates over ultra wideband (UWB) channels with low-complexity transmitters and receivers. In many aspects, IR systems are similar to time-division MA systems, and the processing gain of IR systems represents the ratio between the actual transmission time and the total time between two consecutive transmissions (on-plus-off to on ratio). While CDMA systems, which constantly excite the channel, rely on spreading sequences to orthogonalize the signals transmitted by different users, IR systems transmit a series of short pulses, and the orthogonalization between the signals transmitted by different users is achieved by the fact that most of the pulses do not collide with each other at the receiver. In this paper, a general class of MA communication systems that use both types of processing gain is presented, and both IR and RCDMA systems are demonstrated to be special cases of this more general class of systems. The bit-error rate of several receivers as a function of the ratio between the two types of processing gain is analyzed and compared under the constraint that the total processing gain of the system is large and fixed. It is demonstrated that in non-intersymbol interference (ISI) channels, there is no tradeoff between the two types of processing gain. However, in ISI channels, a tradeoff between the two types of processing gain exists. In addition, the suboptimality of RCDMA in frequency-selective channels is established.     

数字超宽带信号的功率谱密度

该文根据各种超宽带(UWB)信号的调制特点,给出了统一的随机脉冲信号模型,并应用随机理论计算得出相应的功率谱密度(PSD)函数表达式,此PSD表达式应用范围广,适用于各种调制方式、多址方式、进制下的UWB信号。该文举例分析比较了几种典型调制下带有均匀分布的随机时间抖动(timing jitter)及理想情况下的信号PSD,结果表明,时抖动的存在平滑了信号功率谱,降低了对其他窄带通信系统的干扰。而且,文中给出的PSD函数表达式可以用来估计信号各参量的变化对PSD函数的影响,而不需要考虑脉冲序列的详细设计过程。     

A filtering approach for reducing timing jitter due to the acousticeffect

We introduce a signal processing approach to compensate for the timing jitter produced by the acoustic effect in soliton communications. The other main sources of timing jitter, the Gordon-Haus effect and the polarization effect, are inherently stochastic. By contrast, the acoustic effect is deterministic and becomes the dominant source of bit error rates in standard soliton systems when the bit rates are more than 10 Gb/s and the transmission distance is more than several thousand kilometers. We exploit the deterministic nature of the acoustic effect to introduce a scheme that predicts the amount of timing jitter as a function of the previous transmitted bits and uses the information to adjust the sampling period of the received soliton pulses. We demonstrate successful application of the scheme by simulations and discuss implementation issues     

Quantifying the Degradation of Combined MUI and Multipath Effects in Impulse-Radio UWB

We study pulse-based ultra wideband (UWB) communications over multipath channels using asynchronous spread spectrum (SS) multiple access (MA) based on time-hopping (TH) and pulse position modulated (PPM) signals. More specifically, we analyze the signal-to-interference (SIR) degradation in the presence of additive white Gaussian noise (AWGN), multi-user interference (MUI), and dense multipath effects (DME) with line-of-sight (LOS) and non-line-of-sight (NLOS). In particular, we define a degradation margin factor for the combined MUI and multipath effects and also find an expression for the maximum number of simultaneous radio links N_u in terms of the operating SIR, the SS processing gain, and the bit transmission rate R_b. We consider both cases with perfect and imperfect power control.     

Correlated and uncorrelated timing jitter in gain-switched laserdiodes

The author describes correlated and uncorrelated timing jitter of gain-switched pulses generated from sinusoidally modulated laser diodes measured from photocurrent power spectra. It is found that if dc bias is decreased to obtain shorter pulses, then the root-mean-square (rms) value of uncorrelated timing jitter drastically increases to more than 2 ps, while correlated jitter remains constant at the drive circuit level of ~0.2 ps. By optimizing the bias condition and compression fiber length, total timing jitter of gain-switched pulses can be reduced to as low as ~0.5 ps, as their pulse width is kept less than 10 ps     

Fictive-Temperature Mapping in Highly Ge-Doped Multimode Optical Fibers

In this paper, a Fourier transform infrared focal-plane-array detector was used to image the "bond-stretching" vibration mode observed near sigma=1120 cm^-1 of highly Ge-doped graded-index multimode optical fibers (GI-MMFs). We show that the distribution of the peak wavenumber sigma is nonuniform across the core and cladding of the MMF, i.e., sigma is smaller in the core due to the Ge-doping. Next, as calibration curves between sigma and the fictive temperature T_f are not available in the literature for highly Ge-doped glasses (above 7w%), we have determined calibration curves from 1w% to 30w% in Ge. Then, we have applied these corrections in order to estimate, for the first time to our knowledge, the fictive-temperature distribution within multimode-fiber cross section. Using these curves, we show that T_f is higher at the fiber edges, presumably due to faster cooling. Furthermore, there is also a T_f increase in the center of the core (i.e., higher Ge content)     

A reflective semiconductor optical amplifier based all-optical header separator

An all-optical header extraction scheme based on the reflective semiconductor optical amplifier (RSOA) is presented, which can be used to process variable-length and bit-rate transparent packets. Through selecting the appropriate carrier-lifetime of the RSOA, the payload pulses obtain smaller gains than header pulses and are considered to be compressed. By using the reflective structure, the header pulses can acquire larger gains than the payload twice, and the contrast radio (CR) is improved. The simulation results show that the CRs can reach 22.8 dB and 18 dB for the packets with header pulses at the rates of 2.5 Gb/s and 10 Gb/s, respectively, and these values can be optimized through properly selecting the structure parameters, such as the small-signal gain and saturation energy.