Optimum mapping in an optical multiple pulse position modulation link using a maximum likelihood sequence detection scheme

The performance analysis of any multiple pulse position modulation (PPM) system is extremely time consuming because of the effects of different detection errors on the multiple PPM alphabet. In a small multiple PPM system such as (12 2)there are 64 codewords and each one can have ten possible false alarms, two possible erasures and four possible wrong-slot errors. The situation is worse if higher-order codes are considered. A novel algorithm, which reduces the time taken to predict the sensitivity of a multiple PPM system from almost 2 h of analysis to under a second, is presented. Results obtained using this method agree with those obtained using a full mathematical model. The authors also present a methodology that obtains a close to optimum mapping for any multiple PPM system. Detailed results show the effectiveness of this mapping routine.     

Capacity of the low-photon-rate direct-detection optical pulse-position-modulation channel in the presence of noise photons

We derive expressions for the capacity of the pulse-position-modulated (PPM) direct-detection photon-counting channel in the presence of noise photons in addition to the signal-dependent shot noise that is normally considered in studying photon counting at low photon rates. We note that even a small mean number of noise photons per PPM count bin significantly decreases the capacity of the channel. These results are useful for comparisons of performance that are obtained by use of real coding and synchronization algorithms with photon-counting PPM schemes that are currently being considered for deep-space optical communications.     

无背景噪声下的光MPPM信道容量分析

基于多脉冲位置调制(MPPM)在无背景噪声下的信道模型,根据信息理论,推导了有保护时段的光MPPM信道容量的表达式,在此基础上又推出了MPPM的信息传输速率和容量能量效率的表达式.分析并比较了MPPM和PPM的信道容量、信息传输速率以及信道容量能量效率.仿真结果表明,在对光脉冲功率、系统带宽倒数、单个脉冲所含光子教等参数进行适当的选取以后,MPPM在信道容量方面的特性优于PPM.     

Concatenated coded modulation techniques and orthogonal space-time block codes in the presence of fading channel estimation errors

Performance of coding and modulation systems in fading channels is usually analysed under the assumption that the receiver has perfect knowledge of channel condition. However, various shortcomings in practical channel estimation techniques lead to imperfections, resulting in channel estimation errors. The authors analyse a practical coding and modulation scheme for multiple-antenna systems considering channel estimation errors. The novelty of this study resides in providing error probability bounds for concatenated trellis coded modulation (TCM) or bit-interleaved coded modulation (BICM) schemes with orthogonal space--time block codes (OSTBC) under imperfect channel estimation assumption. Moreover, our analytical results quantify the performance degradation associated with various levels of channel estimation error variance. The authors also show that if channel estimation quality does not improve sufficiently with SNR, there would be error floor in performance, such that the coded system could get outperformed by a system with differential signalling that requires no channel estimation. Simulation results are presented, which confirm the validity of the analytical results.     

Investigation of the baseline wander effect on indoor optical wireless system employing digital pulse interval modulation [optical wireless communications]

In indoor optical wireless communication (OWC) systems, ambient lights in particular fluorescent lamps introduce a periodic interference signal, which has the potential to severely degrade link performance. One of the simplest techniques often used to mitigate the interference from artificial sources of ambient light is electrical high-pass filtering (HPF). However, HPF introduces baseline wander (BLW), which is more severe for modulation techniques that contain a significant amount of power located at DC and low frequencies. In this paper the effects of BLW on systems employing digital pulse interval modulation (DPIM) are extensively investigated. Also investigated are the effects of cut-on frequency of the HPF on the optical power requirement and power penalty for different bit rates, where results are compared with the more established techniques of on-off keying (OOK) with a non-return to zero signalling and pulse position modulation (PPM). The optimum HPF cut-on frequency, which minimises the overall power penalty, is estimated. It is shown that at high data bit rates (100 Mbps), DPIM power requirement is considerably lower (~5 dB) than OOK and similar to that of PPM.     

Analysis of BPSK modulated asynchronous band-limited DS-CDMA systems with diversity receivers over generalised-K fading channels

The authors studied bit-error rate (BER) performance of asynchronous band-limited direct-sequence code-division multiple-access (DS-CDMA) systems with various diversity-combining receivers over Generalised-K fading channels. The effects of band-limited pulse shapes, multitone jamming, multiple-access interference as well as both flat and frequency-selective fading are considered. The Generalised-K model is adopted in order to include the effects of shadowing and fading of a wireless channel. The authors consider binary phase-shift keying as the modulation technique. The analytical expressions are valid for any arbitrary value of Generalised-K distribution parameters. Two types of band-limited pulses, namely spectrum raised cosine and Beaulieu-Tan-Damen (BTD) pulses, are incorporated in the analysis. Numerical results show that the system with BTD pulse outperforms the one with SRC pulse for various diversity-combining receivers under various channel conditions. Furthermore, by incorporating a minimum mean-square error stage in the multipath diversity receiver, the BER performance can be further improved.     

Bit error performance of diffuse indoor optical wireless channel pulse position modulation system employing artificial neural networks for channel equalisation

The bit-error rate (BER) performance of a pulse position modulation (PPM) scheme for non-line-of-sight indoor optical links employing channel equalisation based on the artificial neural network (ANN) is reported. Channel equalisation is achieved by training a multilayer perceptrons ANN. A comparative study of the unequalised dasiasoftdasia decision decoding and the dasiaharddasia decision decoding along with the neural equalised dasiasoftdasia decision decoding is presented for different bit resolutions for optical channels with different delay spread. We show that the unequalised dasiaharddasia decision decoding performs the worst for all values of normalised delayed spread, becoming impractical beyond a normalised delayed spread of 0.6. However, dasiasoftdasia decision decoding with/without equalisation displays relatively improved performance for all values of the delay spread. The study shows that for a highly diffuse channel, the signal-to-noise ratio requirement to achieve a BER of 10^-5 for the ANN-based equaliser is ~10~dB lower compared with the unequalised-soft-decoding for 16-PPM at a data rate of 155 Mbps. Our results indicate that for all range of delay spread, neural network equalisation is an effective tool of mitigating the inter-symbol interference.     

极大似然判决下大气无线光脉冲位置调制差错性能

大气湍流对无线光通信系统影响较大,基于高斯信道及门限判决方法的大气无线光通信脉冲位置调制(PPM)的误码特性分析存在局限性.针对于此,推导了湍流大气中无线光PPM系统在极大似然判决(MLD)下的差错性能计算模型,并运用蒙特卡洛方法验证了理论计算模型的有效性.仿真分析表明,MLD下PPM具有更好性能,但也较易受湍流影响,为使弱湍流信道中PPM的误比特率不高于10-6,调制阶数每增加1阶,须提高平均接收信噪比0.67 dB以上.对于峰值功率恒定PPM系统,应根据器件性能和具体通信条件,在差错性能和功率利用率之间作出平衡.     

Spot diffusing angle diversity MC-CDMA optical wireless system

Multi-carrier code division multiple access (MC-CDMA) combines some of the desirable features of orthogonal frequency-division multiplexing (OFDM) and CDMA in that it offers multi-user capabilities at a reduced channel rate. The authors report the analysis of a MC-CDMA system over the bandlimited optical wireless (OW) channel. Furthermore, the authors evaluate the performance when a line strip multibeam system (LSMS) is used in conjunction with an angle diversity receiver in the OW MC-CDMA system. The results indicate that at a bit error rate of 10^-4 a 7 and 10 dB signal-to-noise ratio (SNR) improvements are obtained through the use of MC-CDMA in a 4-user system when compared to a single-user OW on?off keying system and a 4-user OW CDMA system, respectively. This improvement is obtained by enabling multi-user communication. The performance degrades gradually with increase in the number of users. The use of LSMS and angle diversity offers a more uniform SNR over a given indoor geometry compared to a conventional diffuse OW system (CDS). Results are reported for an angle diversity LSMS MC-CDMA system and are compared to the CDS MC-CDMA system.     

M-ary pulse position modulation performance with adaptive optics corrections in atmospheric turbulence

ABSTRACT

The performance of M-ary pulse position modulated (PPM) optical wireless communication (OWC) systems in atmospheric weak turbulence medium is evaluated by using adaptive optics corrections. Piston, tilt, defocus and coma components of adaptive optics corrections are applied to the avalanche photodetector (APD) type of receiver and the results are obtained depending on various turbulence and receiver parameters. The lognormal channel distribution is used to model the weak atmospheric turbulence conditions. Adaptive optics correction increases the bit-error-rate (BER) performance of an OWC system operating in atmospheric turbulence conditions. Piston component yields the highest BER performance, followed by the tilt, defocus and coma adaptive optics correction components respectively.     

Bit error ratio performance of a receiver diversity scheme with channel estimation

The performance of diversity combining techniques with channel estimation for the case of unequal power branches is discussed. The performance of binary phase shift keying (BPSK), pulse amplitude modulation (PAM) and quadrature amplitude modulation (QAM) formats is investigated and their performance with nonideal channel estimation is explored. The case of combining multiple branches with unequal power levels is considered and optimal weighting factors for a log likelihood ratio test (LLRT) detector for the BPSK case are presented. These weights are shown to operate in a similar manner to those obtained by the linear minimum mean squared error (LMMSE) criterion and are compared with the well known maximal ratio combining (MRC) technique. The probability of error of this receiver is derived and simplifications for high signal-to-noise ratios are presented. Finally, numerical results are presented to compare the performance of the LLRT, LMMSE and MRC based receivers     

Convolutional coded dual header pulse interval modulation for line of sight photonic wireless links

The analysis and simulation for convolutional coded dual header pulse interval modulation (CC-DH-PIM) scheme using a rate 1/2 convolutional code with the constraint length of 3 is presented. Decoding is implemented using the Viterbi algorithm with a hard decision. Mathematical analysis for the slot error rate (SER) upper bounds is presented and results are compared with the simulated data for a number of different modulation techniques. The authors show that the coded DH-PIM outperforms the pulse position modulation (PPM) scheme and offers >4 dB code gain at the SER of 10^-4 compared to the standard DH-PIM. Results presented show that the CC-DHPIM with a higher constraint length of 7 offers a code gain of 2 dB at SER of 10^{-5 compared to the CC-DH-PIM with a constraint length of 3. However, in CC-DH-PIM the improvement in the error performance is achieved at the cost of reduced transmission throughput compared to the standard DH-PIM.}     

A Novel Method of Distance Measurement Based on Pulse Position Modulation and Synchronization of Chaotic Signals Using Ultrasonic Radar Systems

This paper deals with a novel method of transmission and receipt of a signal based on both the property of two chaotic systems generating the same chaotic signal when they are synchronized and the property of pulse position modulation (PPM) to be insensitive to the distortions of the transmission channel. The method is discussed in the context of ultrasonic radar systems, in which the transmitter and receiver, which consist of ultrasonic sensors, are near each other, and the received signal consists of the transmitted signal reflected by an obstacle. A reference sinusoidal signal is superimposed to a chaotic signal generated by a master chaotic system, and the whole signal is modulated according to the PPM method and transmitted by the sensor. The received signal is demodulated, and the demodulated signal forces a slave chaotic system to generate the chaotic signal embedded in it, which allows recovery of the sinusoidal signal by subtracting this chaotic signal from the demodulated echo. The difference of the phases of the reference sinusoidal signal and the recovered sinusoidal signal allows computation of the time of flight of the signal and, consequently, the distance of the radar system from the obstacle. The novel method is illustrated and tested by both simulation and experiments. The interference problem between the considered radar system and other radar systems ( crosstalk) is also addressed, and a solution is proposed to avoid it.     

Performance of differential pulse-position modulation (DPPM) with concatenated coding over optical wireless communications [optical wireless communications]

The concatenation of marker and Reed-Solomon codes in order to correct insertion/ deletion errors in differential pulse-position modulation (DPPM) over optical wireless communications is presented. The concatenated code decoding algorithms with hard-decision and soft-decision detection are presented. The performance of the hard-decision coded DPPM system is evaluated over both nondispersive and dispersive channels via analysis and simulation. It is shown that the coding gain provided by the concatenated code is approximately 4 dB when the code rate is about 0.7 and the channel is nondispersive. Over a dispersive channel, the coded system performs better than the uncoded system when the ratio of delay spread to bit duration is not high. A soft-decision detector is employed to combat intersymbol interference. The soft- decision decoding algorithm, which has low complexity and can be practically implemented, is described. The performance over nondispersive and dispersive channels is evaluated by analysis and simulation. It is shown that the soft-decision system requires approximately 2 dB less transmit power than the hard-decision system for additive white Gaussian noise and low-dispersive channels. Soft decoding also provides a performance improvement in high-dispersive channels.     

Performance of burst-mode receivers for optical digital pulse position modulation in passive optical network application

A DC coupled burst-mode receiver for digital pulse position modulation (DPPM) is proposed for the first time. An analysis of the potential performance benefits of using such a receiver configuration in the upstream of a passive optical network is given. An optical pre-amplifier is assumed. Bit error rate expressions are derived and the performance is evaluated in terms of sensitivity and threshold acquisition penalty. Results are compared with a DC-coupled on?off keyed non-return-to-zero burst-mode receiver, and it is seen that DPPM?s continuous mode sensitivity advantage (about 8 dB for the optimal case) is only reduced to 7.7 dB when overhead preamble is restricted to 12 bits and 6 dB when restricted to 6 bits. Thus, if necessary, DPPM can almost recover its sensitivity advantage by a trade off with preamble length. As such DPPM can potentially offer an increase in optical network unit numbers by a factor of 4 or, alternatively, an increase in range of around 20 km.     

Look-up table error correcting multiple pulse PPM codes for wireless optical communication channels [optical wireless communications]

Performance of optical wireless communication (OWC) systems can be optimised using constant optical power transmission in conjunction with forward error control coding. A new family of constant power error correcting codes using multiple pulse position modulation optical transmission are introduced. This doubly featured technique may extend the capability of many difficult wireless optical links beyond their existing barriers and may enable a higher error performance for OWC networks. The potential and design of look-up table codes are investigated. The Monte Carlo methods provide supportive data for the performance and code word search.     

Low-complexity iterative equalisation and decoding for wireless optical communications [optical wireless communications]

A low-complexity scheme of iterative equalisation and decoding by combining a recursive systematic convolutional code and a pulse-position modulation is proposed here. A graph- based equalisation for intersymbol interference (ISI) known at both transmitter and receiver is considered. By representing the memory channel with ISI as the factor graph and applying sum-product (SP) algorithm to this graph, a posteriori probability (APP) of the desired symbol necessary to implement iterative equalisation and decoding is derived. A partial response precoding is used to reduce the span of ISI from a possible infinite number of two baud periods. This precoding scheme makes the factor graph of memory channel cycle-free, and SP algorithm for combating ISI converges to an optimum detection. Numerical results show that the proposed low-complexity strategy has almost the same performance as the optimum turbo equalisation.     

Spectral superresolution with ultrashort optical pulses

A superresolution technique for the measurement of transmission, reflection, and absorption spectra is proposed. An ultrashort laser pulse is propagated in a dispersive element and then periodically phase modulated. The temporal modulation is transformed into periodic spectral modulation, for which the number of harmonics, 2M+1, is determined by the modulation index. The modulated pulse is transmitted through (reflected from) the sample to be tested and measured by a spectrometer. By performing 2M+1 measurements for 2M+1 delays between the dispersed pulse and modulation signal, one can restore the spectral response of the sample with superresolution after simple processing. We numerically demonstrate the measurement of the transmission spectrum of an ultranarrow optical filter with a minimum feature of 0.43 pm by an optical spectrum analyzer with a 10 pm resolution. A twentyfold enhancement of the resolution is achieved in the presence of noise with a level of 0.1%. The advantage of the system is its full reconfigurability.     

Pulse position modulated space?time trellis coding for ultra-wideband impulse radio multiple-input multiple-output communication systems

The design of pulse position modulated (PPM) space-time trellis codes (STTC) for ultra-wideband impulse radio (UWB-IR) multiple-input multiple-output (MIMO) communication systems over slow and fast fading multipath channels is considered. First, A probability of error analysis is carried out to derive upper bounds on pairwise symbol error probability at high and low signal-to-noise ratios (SNRs). From the upper bounds, A new distance notion is introduced and novel design criteria for optimal (in error rate performance) M-ary PPM STTC are deduced for UWB. An optimal binary-PPM STTC is designed for two transmit antennas. Finally, simulation results of the UWB-IR MIMO system, using the optimal STTC, confirm significant improvement in bit-error-rate performance over the uncoded UWB-IR single-input single-output system and also over previously proposed space-time coding scheme for UWB, at higher SNR.     

60GHz无线通信系统在PPM调制下的信道容量分析

本文主要研究60GHz无线通信系统在高斯信道下的香农信道容量及在脉冲位置调制(MPPM)下的信道容量。理论分析与推导得到香农信道容量与传输距离的关系函数,由MPPM调制原理及高斯信道的数学模型推导系统在MPPM调制下信道容量与信噪比、信道容量与通信距离间的函数关系,利用Matlab仿真,得到不同条件下的关系曲线。研究结果表明,60GHz无线通信系统在高斯信道下的信道容量是通信距离和信噪比的函数,该系统更适合于短距离无线通信。高进制的PPM调制能够显著的提高系统数据传输速率,当数据率相同时,高进制调制的PPM能够提供的可靠通信距离更远。