Reductions of multiple-access interference in fiber-grating-based optical CDMA network

A fiber Bragg grating (FBG) encoder/decoder scheme based on correlation subtractions of nearly orthogonal M-sequence codes is presented. With proper coder design, a receiver can reject interfering users and obtain quasi-orthogonality between optical code-division multiple-access (CDMA) users in the network. However, optical CDMA networks may be degraded by multiple-access interference (MAI) due to nonflattened incoherent sources and nonideal FBG coders. A compensating module is therefore proposed to compensate for such MAI effects. As a result, the MAI effects induced by nonideal FBG coders can be perfectly eliminated by the compensating module. With spectral width reduction on the incoherent source, the scheme can partly compensate the MAI effects induced by nonflattened sources and further reduce the average error probability in the system performance.     

User-separating demodulation for code-division multiple-accesssystems

A user-separating (US) demodulator for a multiple-access system with digital transmission is defined to be a demodulator that, without knowledge of the channel codes of the various users, provides the decoder for each user with a scalar-valued output for each symbol period that permits maximum-likelihood decoding of that user's data. It is shown that such a US demodulator exists in general only for approximations to the true statistics of the modulator-input sequences of the interfering users. It is argued that approximating the interfering modulator-input sequences as independent, white Gaussian processes is the practical compromise between accuracy and simplicity in code-division multiple-access (CDMA) systems. The US demodulator for this approximation is shown to consist of a kind of matrix whitening fitter followed by a kind of matched-filter for the user in question. It is further shown that this US demodulator can often be well approximated as the symbol-by-symbol demodulator that makes the linear minimum mean-squared error estimate of each modulator-input symbol for the user in question. Simulation results are presented to confirm the theory of US demodulation and to illustrate its practical utility     

Multiuser projection receivers

A new multiuser receiver for synchronous code-division multiple-access (CDMA) systems with error control coding is proposed. The receiver achieves interference cancellation by projecting the undesired users onto the space spanned by the desired users' signal vectors. The detector calculates the least squares (LS) estimate of the interfering users data, that is used to yield an adjusted metric for maximum likelihood sequence estimation (MLSE) for the desired users' sequences. Simulation results indicate that close to optimal performance can be achieved when all but one of the users are projected using only a single user decoder for the desired user. Further, an adaptive receiver structure based on the recursive LS update is presented that is well-suited for DSP implementation due to it's computational efficiency     

CDMA codeword optimization: interference avoidance and convergencevia class warfare

Interference avoidance has been shown to reduce total square correlation (TSC) for given ensembles of user signature waveforms (codewords) in a synchronous code-division multiple-access (CDMA) system. In all experiments we have conducted, sequential application of interference avoidance produces an optimal codeword set when starting from randomly chosen initial codewords. Here we provide the first formal proof of convergence to optimal codeword ensembles for greedy interference avoidance algorithms augmented by a technique called “class warfare” whereby users which reside in more heavily loaded areas of the signal space purposely interfere with (attack) the reception of users in less crowded areas. Coordination of deliberate interference by a complete class of aggrieved user is also sometimes necessary. Such “attacks” and subsequent codeword adjustment by attacked users are shown to strictly decrease TSC. Along the way we also show using linear algebra and a variant of stochastic ordering, equivalence between minimization of TSC and maximization of sum capacity     

The TAR decoder-a bandpass Viterbi/FFT decoder for convolutionalencoded spread-spectrum signals

A new form of bandpass convolutional decoder termed the TAR decoder is presented. The decoder has as its basis the classic Viterbi algorithm, but uses a fast Fourier transform (FFT) in order to cope with a frequency offset, phase rotating, received signal. The TAR decoder also uses a two-dimensional (2-D) despread signal history array in addition to the usual path history registers. The advantage of the TAR decoder is that the decoder does not need to be preceded by a modem and is not subject to the problems of cycle slips. It can operate at negative input signal-to-noise ratio (SNR) values and is well suited to the decoding of low rate convolutional codes as may be used in code-division multiple-access (CDMA) systems. Symbol timing recovery is still required but this is straightforward in practical cases down to ≃-15 dB SNR     

Multiple-Access Strategies for Frequency-Selective MIMO Channels

In this paper, we consider frequency-selective coherent multiple-input multiple-output (MIMO) multiple-access fading channels. Assuming that each of the users employs orthogonal frequency-division multiplexing (OFDM), we introduce a multiple-access scheme that gradually varies the amount of user collision in signal space by assigning different subsets of the available OFDM tones to different users. The corresponding multiple-access schemes range from frequency-division multiple access (FDMA) (each OFDM tone is assigned to at most one user) to CDMA (each OFDM tone is assigned to all the users). We quantify the effect of signal space collision between the users by computing the ergodic capacity region for the entire family of multiple-access schemes. It is shown that the ergodic capacity region obtained by a fully collision-based scheme (CDMA) is an outer bound to that corresponding to any other multiple-access strategy. In practice, however, minimizing the amount of user collision in frequency is desirable as this minimizes the receiver complexity incurred by having to separate the interfering (colliding) signals. Our analysis shows that the impact of collision on spectral efficiency depends critically on the channel's spatial fading statistics and the number of antennas     

Realization of OSW/AWG-based bipolar wavelength–time optical CDMA for wired–wireless transmissions

This study proposes a novel radio-over-fiber (RoF) system using two-dimensional (2-D) optical code-division multiple-access (OCDMA) scheme using pseudorandom (PN) codes for the time-spreading and wavelength-hopping (t-spreading/λ-hopping) codes. The 2-D system is implemented using optical switches (OSWs) and arrayed-waveguide grating (AWG) routers. By constructing 2-D codes using bipolar PN codes rather than unipolar codes provides a significant increase in the maximum permissible number of active radio base stations (RBSs). In general, the phase-induced intensity noise (PIIN) generated at high optical intensities significantly degrades the performance of a conventional multi-wavelength scheme. However, the OSW-based time-spreading method employed in the current 2-D OCDMA scheme effectively suppresses the PIIN effect. Additionally, multiple-access interference (MAI) is suppressed by the use of a wavelength/time balanced detector structure in the network receivers. The numerical evaluation results demonstrate that under PIIN- and MAI-limited conditions, the proposed system outperforms a conventional multi-wavelength OCDMA scheme by using the spectral spreading scheme to suppress beating noise. Especially, the t-spreading encoder/decoder (codec) groups share the same wavelength codec and the overall complexity is reduced and system network becomes more compact.     

Performance Analyses on Hybrid MQC/M-Sequence Coding Over Frequency/Spatial Optical CDMA System

A new two-dimensional frequency/spatial code for spectral-amplitude coding a optical code-division multiple-access system is proposed. The corresponding coder/decoder pairs are based on the tunable fiber-Bragg gratings cooperating with optical splitters/combiners, and multiple-access interference can be eliminated through the proposed decoding mechanism. For the performance analysis, the effects of phase-induced intensity noise, shot noise, and thermal noise are considered simultaneously. Bit-error rate (BER) performance is compared with that of the former system using M-matrix codes. It is shown that the system using these new code matrices not only maintains most advantages of the former one, but also allows larger number of active users under a given BER     

Group-metric multiuser decoding

We propose the new group metric (GM) soft-decision decoder for convolutionally coded synchronous multiple-access channels. The GM decoder exploits the independently operating encoders of the multiuser channel by making decoding decisions for a subset of the users, but incorporating all the multiuser information in its metrics. For a single user, this decoder will have a reduced complexity that is exponential in the sum of encoder memory and the number of users. The soft-decision maximum-likelihood (ML) joint decoder is well known. This optimal decoder suffers from a high complexity requirement that is exponential in the product of encoder memory and the number of users. The size of the decoded subset is a design parameter which allows a tradeoff between complexity and performance. The performance of the GM decoder, once properly characterized, can be analyzed using standard techniques. In addition, a new analysis technique is presented which considers decomposable sequences for the fading channel. With this analysis, we have a new tool for bounding error probabilities for multiuser decoders. Applying this technique to the GM decoder, we can directly identify sequences that are decomposable some fraction of the time, and obtain a new upper bound. Further, this improved bound can be expressed in closed form. Numerical results show that the actual performance gap between the GM and ML decoders can be quite small     

A soft decoding scheme for vector quantization over a CDMA channel

The optimal decoding of vector quantization (VQ) over a code-division multiple-access (CDMA) channel is too complicated for systems with a medium-to-large number of users. This paper presents a low-complexity, suboptimal decoder for VQ over a CDMA channel. The proposed decoder is built from a soft-output multiuser detector, a soft bit estimator, and the optimal soft VQ decoding of an individual user. Simulation results obtained over both additive white Gaussian noise and flat Rayleigh fading channels show that with a lower complexity and good performance, the proposed decoding scheme is an attractive alternative to the more complicated optimal decoder.     

Differentiated Service Provision in Optical CDMA Network Using Power Control

A simple scheme to use power control for differentiated service provision is proposed for the optical code-division multiple-access network. The main advantages of this scheme are: 1) Interference from other users can be eliminated theoretically. 2) The decoder with simple configuration can be used as compared to that in a previous power control scheme. 3) The design procedure of various service requirements in one network is simplified.      

Demonstration of 160- and 320-Gb/s SPECTS O-CDMA network testbeds

This letter presents a high-capacity optical code-division multiple-access (O-CDMA) network testbed based on the spectral phase-encoded time-spreading technique. Two 10-Gb/s/user O-CDMA network architectures (time-slotted and time-slotted polarization multiplexed) are investigated. The first O-CDMA network testbed architecture utilizes eight encoders and a decoder to produce 16 users equally distributed in two time slots while the second architecture evenly distributes 32 users in two time slots and two polarizations. The 16-user testbed achieved error-free performance. The 32-user testbed obtained bit-error rates below 10/sup -8/ without using forward-error-correction techniques.     

大容量可共享光编解码器的设计与性能分析

提出了一种二维时间/波长、大容量、可共享的光码分多址(OCDMA)解编码器,以很少的硬件实现多用户的应用.编解码器利用阵列波导光栅实现多用户的同时共享,通过可编程的结构对OCDMA的编程,实现对其他用户的再分配.该OCDMA系统利用二维时间/波长Reed-Solomon 码,降低了硬件的数量.运用VPI软件模拟该系统,得到了眼图和BER结果,验证了系统的性能.     

Cyclic optical encoders/decoders for compact optical CDMA networks

For compact optical code-division multiple access (CDMA) networks, a novel cyclic optical encoder/decoder based on an arrayed waveguide grating (AWG) router is proposed. The proposed encoder/decoder treats multiple codewords of the two-dimensional Reed-Solomon code simultaneously using the cyclic property of the AWG router and the code, and multiple subscribers can share the encoder/decoder in networks. With the proposed encoder/decoder, optical CDMA networks become compact and cheap, since the number of encoders/decoders required in the network is reduced. The feasibility of the proposed encoder/decoder is demonstrated experimentally     

Admission control in T/CDMA systems supporting voice and dataapplications

This paper analyses the behavior of two hybrid time-code division multiple-access (T/CDMA) architectures on the up-link of a macrocellular mobile radio system. For the examined schemes two categories of users-voice and data-share the domain of available resources, made up of time slots and codewords, through two alternative assignment strategies. Both solutions attribute voice users one single resource pair, i.e., one time slot and a single code to employ on that time slot, but differ in the way data users requests are accommodated: they are either simultaneously granted several codes over the same time slot or are assigned a single code over several distinct time slots. Call admission control is performed by a channel assignment algorithm which dynamically attributes resources only if specified levels of transmission quality are met on the radio channels. The blocking and the outage probability of the two classes of users are determined and compared, showing that one of the proposed schemes exhibits better performance and allows to satisfyingly serve a significant percentage of data users     

Block codes for the hard-square model

The hard-square model is a two-dimensional (2-D) constrained system consisting of all binary arrays on a rectangular grid in which 1's are isolated both horizontally and vertically. This paper proposes algorithms to search for single-state and finite-state block codes with rectangular codewords that satisfy the hard-square constraint. Although the codeword size is small, single-state block codes with coding rates larger than 0.5 can be designed. Letting the 2-D constrained sequences have finite memory increases the achievable coding rate. A method for designing low-complexity encoders and decoders is also presented. When the codeword size increases, the coding rate asymptotically approaches the capacity but with rapidly increasing complexity of the encoder and decoder.     

Interference suppression for space-time coded CDMA via decision-feedback equalization

A single-user receiver structure is proposed for space-time coded code-division multiple-access (CDMA) downlink in a multiuser frequency-selective channel. This structure is a two-dimensional (2-D) decision-feedback equalizer (2D-DFE) whose filters are optimized based on the MMSE criterion to mitigate noise, intersymbol interference (ISI), and multiuser interference (MUI) with a moderate complexity. By modeling the spreading codes of the interfering users as random sequences, system performance was evaluated using the Gaussian approximation. Two models for the desired user's spreading sequence have been considered and compared. Our numerical results show that in both cases the 2D-DFE exhibits significant performance improvement over the standard space-time coded RAKE, especially in interference-limited conditions. It is also observed that the gain obtained by using DFE in a MISO channel is less that in a SISO channel and this problem can be solved by providing diversity at the receiver.     

Soft-Output BEAST Decoding With Application to Product Codes

A Bidirectional Efficient Algorithm for Searching code Trees (BEAST) is proposed for efficient soft-output decoding of block codes and concatenated block codes. BEAST operates on trees corresponding to the minimal trellis of a block code and finds a list of the most probable codewords. The complexity of the BEAST search is significantly lower than the complexity of trellis-based algorithms, such as the Viterbi algorithm and its list generalizations. The outputs of BEAST, a list of best codewords and their metrics, are used to obtain approximate a posteriori probabilities (APPs) of the transmitted symbols, yielding a soft-input soft-output (SISO) symbol decoder referred to as the BEAST-APP decoder. This decoder is employed as a component decoder in iterative schemes for decoding of product and incomplete product codes. Its performance and convergence behavior are investigated using extrinsic information transfer (EXIT) charts and compared to existing decoding schemes. It is shown that the BEAST-APP decoder achieves performances close to the Bahl–Cocke–Jelinek–Raviv (BCJR) decoder with a substantially lower computational complexity.      

Construction of two-dimensional wavelength/time optical orthogonal codes using difference family

A new family of two-dimensional (2-D) wavelength/time optical orthogonal codes (OOCs) for asynchronous optical code division multiple access (OCDMA) systems is proposed. The construction scheme uses the difference family (DF), which is an assemblage of difference sets in the combinatorial theory. It is proven that the proposed codewords satisfy the correlation properties required for the asynchronous OCDMA systems. The code dimension of the proposed codes is more flexible than that of the conventional 2-D codewords. The performance of the system with the proposed codes is analyzed by using the Markov-chain method. Numerical results show that the bit error rate (BER) has a minimal value given the number of simultaneous users. It is also observed that the maximum number of simultaneous users of the system can be achieved by properly choosing both the code weight and cross correlation of the 2-D OOCs.     

Performance comparison of multiwavelength CDMA and WDMA+CDMA forfiber-optic networks

A “multiwavelength” scheme has been proposed to support large numbers of subscribers and simultaneous users in optical code-division multiple-access (CDMA) networks without using very large bandwidth expansion or the need of complicated and not-yet-feasible optical processing. In this paper, multiwavelength optical orthogonal codes (MWOOCs), which consist of two-dimensional codewords (or matrices) with every pulse of a codeword encoded in a distinct wavelength, are constructed for this scheme. MWOOCs have larger cardinality than the one-dimensional codes used in the hybrid wavelength-division multiple-access (WDMA) and CDMA scheme. With the same hardware configuration, our analysis shows that the multiwavelength scheme, in general, performs better than the hybrid scheme, particularly when the traffic load is heavy. However, if a central controller (i.e., under the best scenario) is used to uniformly distribute all available wavelengths to simultaneous users in the hybrid scheme, both schemes have comparable performance for a medium traffic load and the hybrid scheme can theoretically achieve error-free transmission when the load is light. In addition, using multiple wavelengths, the requirements of fiber ribbons and multiple stars in temporal/spatial optical CDMA networks are eliminated