A generalized minimum-mean-output-energy strategy for CDMA systemswith improper MAI

It has been shown that in a direct-sequence/code-division multiple-access (DS/CDMA) system employing binary phase-shift keying (BPSK) modulation the baseband equivalent of the CDMA multiplex is, under very mild assumptions, an improper complex random process, i.e., it has a nonzero pseudoautocorrelation function. The problem of linear multiuser detection for asynchronous DS/CDMA systems with improper multiaccess interference (MAI) is considered. A new mean-output-energy (MOE) cost function is introduced, whose constrained minimization leads to two new linear multiuser detectors, exploiting the information contained in the pseudoautocorrelation of the observables, and which generalize the classical decorrelating and minimum mean-square error (MMSE) receivers. The problem of blind adaptive receiver implementation based on subspace tracking is also tackled. Finally, the superiority of the new detectors with respect to the classical linear detection structures present in the literature is demonstrated through both theoretical considerations and computer simulations     

Probability of error in frequency-hop spread-spectrummultiple-access communication systems with noncoherent reception

Expressions are developed for the probability of error for asynchronous frequency-hop spread-spectrum multiple-access networks using Markov hopping patterns and binary frequency shift keying (BFSK) with one symbol transmitted per hop. The expressions are exact when there is one interfering user and orthogonal BFSK is used. They provide excellent approximations when there are more than one interfering user. It is also shown that the error probability when Markov hopping patterns are used is a good approximation to the error probability when memoryless hopping patterns are used. By computing the channel capacity and the associated throughput, a simple hard decision receiver is shown to perform much better than a receiver using perfect side-information to erase the symbols transmitted on hops that were hit when all the users have the same power and one binary symbol is transmitted per hop     

User-Capacity Maximization in Synchronous CDMA Subject to RMS-Bandlimited Signature Waveforms

Maximum Throughput of FHSS Multiple-Access Networks Using MFSK Modulation Kwonhue Choi and Kyungwhoon Cheun Optimum values for the modulation order$M$, code rate$r$, and the number of frequency-hop slots$q$maximizing the network throughput are obtained based on simulations for frequency-hopped spread-spectrum multiple-access networks, where$L Q$-ary Reed–Solomon (RS) code symbols are transmitted per hop, and each$Q$-ary RS code symbol is transmitted using$log_M Q$$M$-ary frequency-shift keying modulated signals. Network throughput is evaluated under additive white Gaussian noise and Rayleigh fading channels. For the case when the received RS symbol is not interfered by multiple-access interference (MAI), a closed-form expression for the symbol-error probability is derived, and for the case when the symbol is interfered by MAI, simulated symbol-error probabilities are used. It is shown that the optimum$M$is four or eight, irrespective of the channel environment and the number of users. The optimum code rate is determined primarily based on the channel environment, and does not show much dependence on$M$or$Q$. It is also shown that for the case of synchronous hopping under Rayleigh fading at high signal-to-noise ratios, the difference in instantaneous power among the interfering users significantly improves the performance, compared with the case when there is no fading. We also consider the case when the receiver erases the symbols that are interfered and compare the performance to the case of the hard-decisions receiver.     

Maximum throughput of FHSS multiple-access networks using MFSK modulation

Optimum values for the modulation order M, code rate r, and the number of frequency-hop slots q maximizing the network throughput are obtained based on simulations for frequency-hopped spread-spectrum multiple-access networks, where L Q-ary Reed-Solomon (RS) code symbols are transmitted per hop, and each Q-ary RS code symbol is transmitted using log/sub M/Q M-ary frequency-shift-keying-modulated signals. Network throughput is evaluated under additive white Gaussian noise and Rayleigh fading channels. For the case when the received RS symbol is not interfered by multiple-access interference (MAI), a closed-form expression for the symbol-error probability is derived, and for the case when the symbol is interfered by MAI, simulated symbol-error probabilities are used. It is shown that the optimum M is four or eight, irrespective of the channel environment and the number of users. The optimum code rate is determined primarily based on the channel environment and does not show much dependence on M or Q. It is also shown that for the case of synchronous hopping under Rayleigh fading at high signal-to-noise ratios, the difference in instantaneous power among the interfering users significantly improves the performance, compared with the case when there is no fading. We also consider the case when the receiver erases the symbols that are interfered and compare the performance with the case of the hard decisions receiver.     

Design and Experimental Results for a Direct-Sequence Spread-Spectrum Radio Using Differential Phase-Shift Keying Modulation for Indoor, Wireless Communications

We report on our design and measurements that have been made for a direct-sequence spread-spectrum radio using differential phase-shift keying modulation for a wireless PBX. We describe the design and implementation of a transmitter and a receiver using a surface acoustic wave (SAW) filter matching the spread-spectrum code of a user. The receiver performance is within 1 dB of the theoretical performance of a differential phase-shift keying (DPSK) receiver in the presence of additive white Gaussian noise. We also show receiver performance in a multipath fading indoor environment with multipath fade notches of up to 50 dB depth. The indoor channel multipath fading can be overcome by using an equal gain diversity combiner which is suitable when DPSK modulation is used. We confirm that the indoor mean power level attenuation follows the inverse fourth power of the distance. Also, we investigate the multiple-access capability of the system by introducing an interfering transmitter with a different spread-spectrum code sequence.     

A novel frequency-hopping spread-spectrum multiple-access network using M-ary orthogonal Walsh sequence keying

A novel frequency-hop spread-spectrum multiple-access network employing M-ary orthogonal Walsh sequence keying with noncoherent demodulation is proposed. The transmitted Walsh sequence is overlaid by a user-specific pseudonoise sequence to reduce the effect of multiple-access hits. Two Gaussian approximations for the multiple-access interference from both the dehopped slot and its neighboring slots are developed and are used to analyze the performance of the proposed network for synchronous and asynchronous hopping under nonfading and Rayleigh fading channels. The effect of imperfect hop timing synchronization at the receiver is also analyzed. It is shown that the proposed network offers significantly improved network throughput compared to networks based on traditional M-ary frequency-shift keying modulation.     

Incoherent Two-Dimensional Array Modulation Transceiver for Photonic CDMA

This paper proposes a novel incoherent optical code-division multiple-access (OCDMA) transceiver design, as a promising network access employing a newly introduced prime code family named as double-padded modified prime code as spreading sequences, based on the 2-D optical modulation scheme deploying frequency and polarization shift keying. Hereafter, it is referred to as F-PolSK. The novel F-PolSK-OCDMA system has been accurately analyzed taking into account the presence of 1) optical amplified spontaneous emission noise; 2) electronic receiver noise; 3) photo-diode shot-noise; and 4) multiple-access interference (MAI). The application of the optical tapped-delay lines at the receiver as a CDMA-decoder has also been investigated. The evaluation of 2-D array modulation in conjunction with OCDMA indicates that the binary combination of this hybrid scheme enhances the overall system performance. The results reveal that the proposed architecture can easily accommodate greater number of users and consume less power as compared with previous schemes. On the other hand, in polarization modulation since the optical beam power is constant, the system has immunity against the self- and cross-phase modulations. Besides, the system security is much enhanced due to 2-D advanced modulation in optical domain.     

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.     

Direct sequence spread spectrum Walsh-QPSK modulation

We present Walsh-quadrature phase-shift keying (Walsh-QPSK) pseudonoise (PN) modulation schemes for both coherent and noncoherent direct-sequence code-division multiple-access (DS-CDMA) systems, wherein the PN spreading sequences for in-phase and quadrature data in a conventional QPSK PN modulation scheme are coded by Walsh sequences indexed by a special rule to reduce the envelope variation of the transmitted signal. The signal characteristics of the two schemes are analyzed when a rectangular-shaped PN chip pulse is used, and it is shown that the proposed coherent DS-CDMA system has a constant envelope even in the presence of a transmitted phase reference. We simulate the signals to obtain the envelope variations when a spectrally efficient shaped PN chip pulse is used, and compare the results with those of conventional QPSK and orthogonal QPSK (OQPSK) PN modulation schemes. The results show that both the noncoherent and coherent Walsh-QPSK schemes have smaller envelope variations than the conventional noncoherent QPSK and OQPSK PN modulation schemes, even though in the coherent Walsh-QPSK scheme the pilot channel is added to the signal channel     

Performance bounds on chip-matched-filter receivers for bandlimited DS/SSMA communications

Performance bounds on chip-matched-filter (CMF) receivers for bandlimited direct-sequence spread-spectrum multiple-access (BL-DS/SSMA) systems with aperiodic random spreading sequences are obtained. First, the optimum transmit-receive chip waveform pairs that maximize the conditional signal-to-interference ratio are derived. This leads to performance bounds on CMF receivers when the conditional Gaussian approximation for cyclostationary multiple-access interference (MAI) is exploited. The bounds are used to examine the dependence of the MAI suppression capability of the CMF receivers on the excess bandwidth of the system and the delay profile of multiple-access users. The system employing the flat-spectrum chip waveform pair is shown to have near-optimum average bit-error rate performance among the fixed CMF (FCMF) receiver systems. Numerical results are provided for an adaptive CMF receiver and for FCMF receivers employing several different fixed chip waveforms.     

A linear MMSE receiver for multipath asynchronous random-CDMA with chip pulse shaping

This paper studies the design and implementation of a linear minimum mean-square error (LMMSE) receiver in asynchronous direct-sequence code-division multiple-access (DS-CDMA) systems that employ long-code pseudonoise (PN) sequences and operate in multipath environments. The receiver is shown to be capable of multiple-access interference (MAI) suppression and multipath diversity combining without the knowledge of other users' signature sequences. It maximizes output signal-to-noise ratio (SNR) with the aid of a new chip filter which exploits the cyclostationarity of the received signal and combines all paths of the desired user that fall within its supported time span. The performance of the LMMSE receiver is compared with that of the coherent selective RAKE receiver. The achieved gain is on the order of 0.6-1.8 dB in dense multipath environments of current narrow-band settings and nonuniform power distribution scenarios of next-generation CDMA systems. An example of adaptive implementation of the LMMSE receiver is presented and accompanied by complexity analysis, training curves, and quantitative performance comparisons illustrating the convergence rate and steady-state performance of the adaptive algorithms.     

Iterative Group-Blind Multiuser Detection and Decoding With Signal Rank Estimation for Coded CDMA Systems

In this paper, a turbo receiver structure is proposed for the uplink of coded code-division multiple-access (CDMA) systems in the presence of unknown users. The proposed receiver consists of two stages following each other. The first stage performs soft interference cancellation and group-blind linear minimum mean square error (MMSE) filtering, and the second stage performs channel decoding. The proposed group-blind linear MMSE filter suppresses the residual multiple-access interference (MAI) from known users based on the spreading sequences and the channel characteristics of these users while suppressing the interference from other unknown users using a subspace-based blind method. The proposed receiver is suitable for suppressing intercell interference in heavily loaded CDMA systems. Since the knowledge of the number of unknown users is crucial for the proposed receiver structure, a novel estimator is also proposed to estimate the number of unknown users in the system by exploiting the statistical properties of the received signal. Simulation results demonstrate that the proposed estimator can provide the number of unknown users with high accuracy; in addition, the proposed group-blind receiver integrated with the new estimator can significantly outperform the conventional turbo multiuser detector in the presence of unknown users.      

Performance of asynchronous slow frequency-hop multiple-accessnetworks with MFSK modulation

The performance of asynchronous slow frequency-hop spread-spectrum multiple-access networks where each user transmits L, M-ary symbols per hop using M-ary frequency-shift keying (FSK) modulation with noncoherent demodulation is investigated. Expressions for the decision variables are derived for a given multiple FSK (MFSK) symbol within a hop hit by K' interfering users under additive white Gaussian noise and Rayleigh fading channel models. For the special case when M=2, an accurate analytic approximation for the average error probability is derived as a function of L and K' and semianalytic Monte Carlo simulations are performed to estimate the probability of error for M larger than 2. The results are used to investigate the dependence of the average symbol error probability on L and M. Finally, the effect of enforcing phase transition between the MFSK symbols within a hop is investigated     

A DS-CDMA system using despreading sequences weighted by adjustablechip waveforms

This paper evaluates the performance of a direct-sequence code-division multiple-access system using coherent receivers in which the despreading sequences are weighted by adjustable chip waveforms. The chip weighting waveforms under consideration are designed for multiple-access interference (MAI) rejection. Assuming that the received chip waveforms are rectangular, new expressions for the signal-to-interference-plus-noise ratio (SINR) of the decision variable are derived when different weighted despreading sequences (WDSs) are used in the receiver. The novelty of the derived expressions is that each of the expressions, when the system parameters are given, is determined only by one parameter of the adjustable chip waveforms employed. As a result, we can simply tune the parameter to its optimal value in real-time for MAI rejection without knowing the other users' spreading codes, timing, and phase. The criterion for tuning the parameter is to maximize the SINR of the decision variable based on the relative strength between the additive Gaussian white noise and the MAI. Numerical results show that when the multiple-access interference is significant, the receivers using WDSs outperform significantly the conventional receiver using a rectangular despreading sequence. Brief analysis for bandlimited spreading signals is also provided to reveal the practical implications of the proposed technique     

Blind decentralized projection receiver for asynchronous CDMA in multipath channels

An asynchronous direct sequence code division multiple access (DS-CDMA) system employing periodic spreading sequences is considered to be operating in a frequency selective channel. The cyclostationary spread signal is received at multiple sensors and/or is sampled multiple times per chip (oversampling), leading to a stationary vector-valued received signal. Hence, such a model represents a very particular multi-input multi-output (MIMO) system with plentiful side information in terms of distinct spreading waveforms for the input signals. Depending upon the finite impulse response (FIR) length of the propagation channel, and the processing gain, the channel of a certain user spans a certain number of symbol periods, thus inducing memory or intersymbol interference (ISI) in the received signal in addition to the multiple-access interference (MAI) contributed by concurrent users. The desired user’s multipath channel estimate is obtained by means of a new blind technique which exploits the spreading sequence of the user and the second-order statistics of the received signal. The blind minimum mean square error-zero forcing (MMSE-ZF) receiver or projection receiver is subsequently obtained. This receiver represents the proper generalization of the anchored MOE receiver [1] to the asynchronous case with delay spread. Classification of linear receivers obtained by various criteria is provided and the MMSE-ZF receiver is shown to be obtainable in a decentralized fashion by proper implementation of the unbiased minimum output energy (MOE) receiver, leading to the minimum variance distortionless response (MVDR) receiver for the signal of the desired user. This MVDR receiver is then adapted blindly by applying Capon’s principle. A channel impulse response is obtained as a by-product. Lower bounds on the receiver filter length are derived, giving a measure of the ISI and MAI tolerable by the receiver and ensuring its identifiability.     

Novel Channel Interference Reduction in Optical Synchronous FSK-CDMA Network Using a Data-Free Reference

In this paper, we propose a novel multiple-access interference (MAI) cancellation technique using a reference signal, which contains no data components (data-free) of the desired signal, and then simplifies the receiver configuration in synchronous M-ary frequency-shift keying optical code-division multiple-access (FSK-OCDMA) network. In doing so, we have taken advantage of a recently introduced energy-saving unipolar prime-code family, referred to as double-padded modified prime-code (DPMPC) as the spreading codes. In the theoretical analysis, the system upper bounded bit error rate (BER) is derived taking into account the Poisson effect on the I/O characteristics of the photodetectors. We have found that when the bit rate is constant, the capacity of this system increases by employing the proposed interference canceller and spreading code as compared with the synchronous M-ary pulse-position modulation OCDMA system with an existing interference canceller. In contrast to wavelength-division multiple access, a fewer set of wavelengths is needed as a result of only M-ary source coding.     

Performance analysis of cellular CDMA networks overfrequency-selective fading channel

An effect of multipath fading on the performance of a cellular code-division multiple-access (CDMA) system is analyzed in this paper. A wide-sense stationary uncorrelated scattering (WSSUS) channel model and the coherent binary phase-shift keying (BPSK) with asynchronous direct-sequence (DS) spreading signal are assumed in the analysis. The average error probability for both the forward link and reverse link of a cellular CDMA system over a frequency-selective fading channel using a conventional correlation-type receiver and RAKE receiver are derived. The impact of imperfect power control and channel capacity of a cellular CDMA system is also investigated. The closed forms of average error probability derived in the paper can save a lot of computation time to analyze the performance and channel capacity of a cellular CDMA system. The analytical results show that the performance and maximum transmission rate of cellular CDMA systems degrade with an increase in the number of simultaneous users and the number of interfering cells. The signal-to-interface ratio (SIR) for the reverse link derived in this paper can directly describe the interrelationships among a number of paths, number of users, number of interfering cells, fading factors, and maximum variation of a received unfaded signal     

Uplink performance of slow frequency-hop multiple-access networks using binary DPSK

The uplink performance of synchronous and asynchronous slow frequency-hop spread-spectrum multiple-access (SFHSS-MA) networks transmitting L bits per hop using binary differential phase shift keying (BDPSK) is analyzed under the additive white Gaussian noise (AWGN) and Rayleigh fading channels. Analytic expressions for the average conditional bit error probabilities given a hop is hit by K' interfering users are derived. Results show that SFHSS-MA networks using BDPSK achieve nearly twice the maximum normalized network throughput compared to networks using BFSK under both AWGN and Rayleigh fading channels.     

Performance of coded OQPSK and MIL-STD SOQPSK with iterative decoding

We show that military standard (MIL-STD) shaped-offset quadrature phase-shift keying (SOQPSK), a highly bandwidth-efficient constant-envelope modulation, can be represented in the form of a cross-correlated trellis-coded quadrature modulation. Similarly, we show that offset QPSK (OQPSK) can be decomposed into a "degraded" trellis encoder and a memoryless mapper. Based on the representations of OQPSK and MIL-STD SOQPSK as trellis-coded modulations (TCMs), we investigate the potential coding gains achievable from the application of simple outer codes to form a concatenated coding structure with iterative decoding. For MIL-STD SOQPSK, we describe the optimum receiver corresponding to its TCM form and then propose a simplified receiver. The bit-error rate (BER) performances of both receivers for uncoded and coded MIL-STD SOQPSK are simulated and compared with that of OQPSK and Feher-patented QPSK (FQPSK). The asymptotic BER performance of MIL-STD SOQPSK is also analyzed and compared with that of OQPSK and FQPSK. Simulation results show that, compared with their uncoded systems, there are significant coding gains for both OQPSK and MIL-STD SOQPSK, obtained by applying iterative decoding to either the parallel concatenated coding scheme or the serial one, even when very simple outer codes are used.     

FHMA receiver over Rayleigh fading channels

A novel FH/MFSK receiver is proposed which utilises the side information of interfering signals for asynchronous frequency hopping multiple-access (FHMA) systems in the presence of Rayleigh fading. It is shown that the novel receiver performs much better than the conventional receiver for a wide range of signal to noise ratios.