Limits to the encoding and bounds on the performance of coherentultrashort light pulse code-division multiple-access

The authors obtain lower and upper bounds on the peak intensity of a spectrally phase encoded ultrashort light pulse as a function of the length of spectral phase encoding. It is shown that using random phase codes for spectral encoding of ultrashort light pulses in a coherent ultrashort light pulse code-division multiple-access (CDMA) system is almost optimal. Furthermore, simple upper bounds and an asymptotic approximation on the bit error rate (BER) for a wide class of ultrashort light pulse CDMA systems are obtained. It is shown that, in spite of being simple, these upper bounds can be used to evaluate the performance of a coherent ultrashort light pulse CDMA system     

Hybrid pulse position modulation/ultrashort light pulse code-division multiple-access systems .I. Fundamental analysis

We propose a hybrid pulse position modulation/ultrashort light pulse code-division multiple-access (PPM/ULP-CDMA) system for ultrafast optical communications. The proposed system employs spectral CDMA encoding/decoding and PPM with very short pulse separation. The statistical properties of the encoded ULP are investigated with a general pulse model, and the performance of the proposed PPM/ULP-CDMA system is investigated. It is shown that we can improve upon the performance of other ULP-CDMA systems, such as those using on-off keying, by employing PPM. It is also shown that we can improve the performance of the proposed system by increasing the effective number of chips, by increasing the number of PPM symbols, and by reducing the ULP duration. The performance analysis shows that the aggregate throughput of the proposed PPM/ULP-CDMA system could be over 1 Tb/s.     

Performance of Asynchronous Time-Spreading and Spectrally Coded OCDMA Systems

The performance of asynchronous coherent time-spreading optical code-division multiple-access (OCDMA) systems is evaluated semi-analytically and the results are compared with those of spectral coding OCDMA systems using ultrashort pulses. The fundamental multi-access interference limited performances are predicted to be identical.      

AWG在谱域幅度编码光码分多址系统中的应用研究

分析了谱域幅度编码的光CDMA通信系统的特点及其对编解码器的要求,根据阵列波导光栅的复用和解复用功能,设计了用于该通信系统中的两种阵列波导光栅编解码器。基于修正素数码MQC码,考虑接收机的热噪声和散弹噪声,采用数值分析的方法,研究了谱域幅度编码的光CDMA系统的通信性能。分析结果表明,由于AWG编解码器可以提供足够多的波长数,基于AWG编解码器的谱域光CDMA通信系统具有很好的误码率性能,可以同时容纳更多的用户数。     

Performance Analysis of Internally Coded Time-Hopping Coherent Ultrashort Light Pulse CDMA Scheme in Fiber-Optic Communication Systems

We consider an internally coded time-hopping coherent ultrashort light pulse code division multiple access scheme (TH-CULP CDMA) and analyze its performance in the fiber-optic communication systems. This system combines the TH and CULP CDMA techniques and exploits the advantages of both. In our method, each bit time interval is divided into $N_{s}$ frames, and the spectral phase-coded pulse is transmitted in one of these frames. Two exclusive PN sequences are assigned to each user. One is added to a superorthogonal convolutional encoder output to select the transmission frame, and the other is used to encode the phase of the ultrashort pulse in the spectral domain. We evaluate the bit error rate (BER) of the system considering the effects of the multiple access interference (MAI) and thermal noise for both soft and hard decoders. The BER is evaluated using both the Chernoff bound and saddle point approximation. We compare the performance of our proposed system with that of the conventional CULP CDMA system. The numerical results indicate that for the same bit rate and processing gain, the performance of our proposed system (TH-CULP CDMA) is substantially better than that of the conventional CULP CDMA system. In addition, we observe that the hard decoder has a better performance at large number of users compared to the soft decoder.      

Emerging optical code-division multiple access communicationsystems

All-optical systems, which perform signal processing functions optically so that the signal conversion from optical to electrical is done only when desired, are considered. Three such schemes are discussed, namely, fiber-optic code-division multiple-access (CDMA) systems, neuromorphic CDMA systems, and ultrashort-light-pulse CDMA systems. Fiber-optic CDMA systems in which incoherent optical signal processing techniques are used to establish optical CDMA systems are first examined. In particular, a novel class of sequences for incoherent fiber-optic CDMA systems, called optical orthogonal codes, are discussed. The codes are then applied to neuromorphic optical systems, and various applications of the networks are examined. Recent experiments that demonstrate the ability to encode and decode extremely fast, femtosecond optical pulses and that suggest the possibility of ultrahigh-speed CDMA systems based on ultrashort light pulses are described     

Performance comparison of proposed 3-d coherent spatial-phase-time coding/decoding with super structured fiber Bragg grating-based optical CDMA systems

In this paper, performance comparison in terms of bit error rate (BER) of proposed WDM compatible optical CDMA system incorporating 3-D spectral-phase-time encoding/decoding to a 7 chip-super-structured fiber Bragg grating (SSFBG)-based optical code division multiple access (OCDMA) system is investigated. Coding and decoding using binary [0, π] phase chips is demonstrated for six users at 5 Gb/s, and a single coded signal is separated with acceptable bit-error rate ≤10^?9. In our proposed optical CDMA system encoding and decoding is done by converting hadamard codes (used for conventional CDMA system) to phase codes. It is then compared with two optical pulse retiming and reshaping systems incorporating super structured fiber Bragg gratings (SSFBGs) as pulse shaping elements. Simulation results show that with all input bands having same sample rate, size data rates our proposed codes with even larger number of channels perform better in terms of eye opening & BER.     

Holographic CDMA

A new technique for all-optical code division multiple access (CDMA) based on optical holography is described. In this technique the energy of an incoming information light signal is spread over a larger spatial domain by a two-dimensional spatial encoder which consists of patterns of (0, 1) or (1, -1). Subsequent decoding, which is obtained by a holographic filter, spatially despreads the energy of information light signal and brings it to a bright focused light spot. Experimental results obtained to demonstrate the encoding, decoding, and multiaccess capability of the proposed holographic CDMA technique are discussed and compared to other all-optical CDMA techniques. The processing gain for a holographic CDMA can be potentially as high as 10⁶-10⁸ , which suggests that this technique can in principle support hundreds of thousands to a few millions of users     

Coherent optical frequency combs: From principles to applications

A coherent optical frequency comb is a kind of broad-spectrum light sources delivering equidistant frequencies, and correspondingly its temporal waveform appears as a sequence of ultrashort pulses. Being the cornerstone technology of today's laser and time-frequency disciplines, it effectively links the optical frequency and the microwave frequency, and has promoted the development of diverse applications, such as precision spectroscopy, optical measurement, coherent optical communications, and optical clocks in the past two decades. In this review, we comprehensively introduce the development path, physical principle, generation/tuning methods, and advanced applications of optical frequency combs.     

Performance analysis of a wireless multirate direct-sequence CDMAusing fast Walsh transform and decorrelating detection

We analyze the performance of a flexible multirate scheme for direct-sequence code division multiple-access (CDMA) mobile radio systems. The proposed scheme uses a variable processing gain serial pseudonoise modulation as a multirate strategy. To reduce the interference effects, the CDMA system utilizes the coherent fast Walsh transform transmission technique. The proposed scheme can be used in the reverse link (mobile-to-base station) of the upcoming third-generation wide-band CDMA system (has the feature of coherent reverse link). We analyze the system performance with and without using a decorrelating multiuser detector. The uncoded bit-error probability (BEP) with and without decorrelating detection on a multipath fading channel is derived analytically. In addition, the validity of the analysis results is demonstrated by computer simulations using the IMT-2000 vehicular multipath channel model. In order to make sure that the proposed processing techniques do not distort the soft values at the demodulator output, the proposed multirate scheme is also simulated in case of using turbo codes. The turbo-coded BEP is calculated for different user data rates and different number of decoding iterations     

BER performance with soft and hard decision decoder for coherentoptical DPSK systems corrupted by phase noise

The performance of coherent optical communication systems is significantly degraded by semiconductor laser phase noise. A rate 1/2, constraint length K=7, convolutional error correcting code with Viterbi decoding is used to improve a 3.5-10 Mbit/s DPSK system. Error correcting coding demonstrates the reduction of the influence of phase noise on BER performance, and significantly lowers the BER floor. Soft decision decoding and hard decision decoding performance is investigated and compared     

Optimal reception, performance bound, and cutoff rate analysis ofreferences-assisted coherent CDMA communications with applications

Coherent spread-spectrum communications with transmitted reference-based, also called pilot-based, channel estimation is considered for code-division multiple-access (CDMA) communications over fading channels. Both noncontiguous time-division multiplexed reference symbol-based and continuous code-division multiplexed channel-based schemes are described. Assuming mean square error or optimal robust channel estimation, we derive the optimal receiver structure with a maximum-likelihood convolutional decoder. In addition, the Bhattacharyya bound and the cutoff rate of the reference-assisted coherent communications are derived. These analytical results are used for evaluating system performance and for selecting parameters such as coding rate and the data to reference power ratio to optimize system performance. Simulation results are given showing that the reverse link performance in a CDMA system can be significantly improved by using the reference-assisted coherent communication instead of noncoherent reception of orthogonally coded signal     

Limitations on peak pulse power, pulse width, and coding maskmisalignment in a fiber-optic code-division multiple-access system

The authors analyze the effects of fiber dispersion and optical nonlinearity on a code-division multiple-access (CDMA) communications system. They assume that each transmitter has a unique phase-encoding mask to scramble the pulses, and that the intended receiver has the complementary phase-decoding mask to unscramble the pulses. The necessary nonlinear pulse propagation equation is derived using a new and systematic approach. The authors then determine the minimum allowable pulse width and the maximum pulse width for the system. They next calculate the maximum optical power, which is limited by the fiber's self-phase modulation. This power is significantly larger than the minimum detectable optical power determined by the noise in the electrical receiver, so that in practice any distortion caused by the fiber's optical nonlinearity can be avoided. From their calculations estimates for all important optical parameters are provided. The deleterious effect of a slight relative misalignment between the encoding and decoding masks is also investigated     

Modulation and coding for the Gaussian collision channel

We study signal-space coding for coherent slow frequency-hopped communications over a Gaussian multiple-access collision channel (G-MACC). We define signal sets and interleavers having maximum collision resistance. The packet-error probability and the spectral efficiency obtained by these signal sets concatenated with outer block coding and hard (error-only) decoding is evaluated without assuming perfect interleaving. Closed-form expressions are provided and computer simulations show perfect agreement with analysis. The structure of good interleavers is also discussed. More generally, we present expressions for the information outage probability and for the achievable (ergodic) rate of the G-MACC at hand, under various assumptions on user coding and decoding strategies. The outage probability yields the limiting packet-error probability with finite interleaving depth (delay-limited systems). The achievable rate yields the limiting system spectral efficiency for large interleaving depth (delay-unconstrained systems). Comparisons with other classical multiple-access schemes are provided     

Turbo-coded optical direct-detection CDMA system with PPMmodulation

The performance of an optical code division multiple access (CDMA) system with turbo coding is analyzed and simulated. Turbo codes are parallel concatenated convolutional codes (PCCCs) in which the information bits are first encoded by a recursive systematic convolutional code, and then, after passing through an interleaver, are encoded by a second systematic convolutional encoder. Turbo coding is superimposed on an intensity-modulated optical channel with pulse-position modulation (PPM) and direct detection of the received optical signal, and the performance is evaluated in terms of an upper bound on bit error probability. From the simulation results, it is seen that turbo coding offers considerable coding gain over other methods, with reasonable encoding/decoding complexity. Also, it is demonstrated that, for a fixed code rate, the performance of the optical CDMA system can be substantially improved by increasing the interleaver length and the number of iterations. Serially concatenated convolutional codes (SCCCs) are also considered, and are shown to perform comparably to PCCC in general and better than PCCCs for the case of large signal photocounts. The results in this paper can be applied, for example, to indoor optical wireless LANs     

New method for generation of coherent ultrashort optical pulses

A new method for generating coherent ultrashort optical pulses from semiconductor lasers with GRIN short external coupled cavity (GSCCL) by means of sinusoidal direct modulation is proposed. 20 ps coherent ultrashort optical pulses have been obtained; the repetition rate is 730 MHz, and the side mode suppression ratio is more than 35 dB.<>     

COMPLEX CDMA VERSUS FDMA/SCPC FOR LOW AND MEDIUM BIT RATE APPLICATIONS FOR FIXED SATELLITE SERVICES

In this paper, the performance of a DS-CDMA asynchronous satellite communication system, using four-phase sequences with near-optimum correlation properties, is analysed and compared with the result achievable by adopting the FDMA/SCPC approach. The comparison is carried out on the basis of the number of the simultaneous active channels in the network and of spectral efficiency. In addition, a similar CDMA system using four-phase sequences, constructed by means of binary Gold sequences having the same family size, has been analysed, and notable performance differences have been found. In particular, the use of four-phase near-optimal sequences provides an increase of about 50 per cent in the spectral efficiency. This result expands the convenience of using CDMA over a wider set of bit rate values, even for fixed satellite communications. Convolutional coding and Viterbi soft decoding are also included in the system. Simulation results are shown for some values of bit error probabilities of practical interest.     

MISO-CDMA多径系统中空时分组码的译码方法

空时编码是实现宽带无线数据通信和下一代移动通信系统的一种极有潜力的技术。为有效的将空时分组码应用到多径衰落环境下的码分多址系统，以充分利用多个路径的信号能量，现提出了一种多径环境下空时分组译码的新方法。由于空时分组码译码与信道估计紧密相关，为此本文对多径信道估计以及信道估计误差对本方法产生的影响作研究。仿真结果表明，采用多路径译码方法可以明显提高系统的误码性能。     

Transmission performance analysis of a new class of line codes for optical fiber systems

A family of mB(m+1)B binary, nonalphabetic, balanced line codes is presented that is suitable for high bit rate (>or=135 Mb/s) optical fiber transmission due to its relatively simple encoding and decoding rules. Here, B represents a block of m bits, where m is an odd number. The coding, decoding, and bit error rate (BER) performance of the codes are discussed. Statistical and spectral analysis for the specific case in which the number of bits, m, equals seven, is presented. This makes possible a detailed comparison of the proposed code with conventional 7B8B codes.<>     

Optical code-division-multiplexed systems based on spectralencoding of noncoherent sources

Presents a new category of optical CDMA systems which work based on spectral encoding. In such systems, that the authors refer to as frequency-encoded CDMA (FE-CDMA) systems, the coding is done in the frequency domain while in the usual CDMA systems the code multiplies the modulation signal in the time domain. They present a new type of FE-CDMA system, based on encoding noncoherent broadband sources. They discuss the advantages of the system compared to other optical CDMA systems and present its performance. They show that very efficient, low-cost, CDMA systems can be obtained with an aggregate throughput of many gigabits per second. Also, for this system, the spreading gain of CDMA is independent of the modulation bandwidth. Hence, the system can accommodate variable bit rates, naturally