Multiple User Interference Research Articles

Chip-level detection in optical code division multiple access

A new detector for optical code-division multiple-access (CDMA) communication systems is proposed. This detector is called the chip-level receiver. Both ON-OFF keying (OOK) and pulse-position modulation (PPM) schemes, that utilize this receiver, are investigated in this paper. For OOK, an exact bit error rate is evaluated taking into account the effect of both multiple-user interference and receiver shot noise. An upper bound on the bit error probability for pulse-position modulation (PPM)-CDMA system is derived under the above considerations. The effect of both dark current and thermal noises is neglected in our analysis. Performance comparisons between chip-level, correlation, and optimum receivers are also presented. Both correlation receivers with and without an optical hardlimiter are considered. Our results demonstrate that significant improvement in the performance is gained when using the chip-level receiver in place of the correlation one. Moreover the performance of the chip-level receiver is asymptotically close to the optimum one. Nevertheless, the complexity of this receiver is independent of the number of users, and therefore, much more practical than the optimum receiver.

Cochannel interference reduction in optical PPM-CDMA systems

A multiple-user interference reduction technique is proposed for optical code-division multiple-access (CDMA) systems. Data symbols from each user are encoded using a pulse-position modulation (PPM) scheme before multiplexing. Modified prime sequences are adopted as the signature codes in the multiplexing process. An interesting property of this code is the uniformity of the cross correlation among its sequences. This property is the main key in constructing the multiple-access interference canceler. In addition to its simplicity, this canceler offers a great improvement in the error probability as compared to the system without cancellation. A simple modification to this canceler that enhances its performance is proposed as well.

Performance analysis of <inline-formula>M</inline-formula>-ary signaling in fiber optic code division multiple access communication systems with modal noise

Performance analysis of M-ary orthogonal signaling in multi- mode graded index fiber optic channels is considered. An APD is used for detection and $0,1%-valued optical orthogonal codes with auto- and cross-correlation coefficients of unity are used as addressing sequences in the analysis. Signal-dependent shot noise, modal noise in the fiber, receiver thermal noise, and multiple-user interference noise are included in evaluating the BER probability. The BER probability is evaluated by assuming the receiver output statistics are Gaussian. The results of the numerical analysis reveal the following: (1) At fixed bit rate and transmit- ted power, the BER probability achieved by M-ary orthogonal signaling is significantly lower than that of the binary signaling scheme, (2) the sys- tem can withstand a higher level of modal noise without significantly degrading the system for a large number of active users, (3) the effect of modal noise on the BER probability can be further reduced by using higher values of M compared to the baseband binary case, and (4) an optimum value exists for the mean gain of APD at which the BER prob- ability is minimum. © 1998 Society of Photo-Optical Instrumentation Engineers. (S0091-3286(98)02703-2)

On the capacity of the asynchronous T-user M-frequency noiseless multiple-access channel without intensity information

We discuss an achievable rate region for the asynchronous T-user M-frequency multiple-access channel without intensity information. The problem is formulated in terms of frequencies, but the results are also applicable to pulse position modulation (PPM) schemes. It is shown that the achievable sum rate for T users reduces from (M-1) bits per channel use in the synchronous multiple-access situation to at least (M-1)/spl middot/ln (2) bits per channel use in the asynchronous situation. In particular, the result shows that for instance, for multitone M-ary frequency shift keying multiple access, in asynchronous operation, multiple user interference reduces the capacity maximally by a factor ln(2)=0.693 relative to the ideal TDMA system.

A comparison between the performance of number-state and coherent-state optical CDMA in lossy photon channels

The performance of optical code-division multiple-access (CDMA) communication systems utilizing number-state light field is evaluated. Lossy direct-detection optical channels are assumed. Both on-off keying (OOK) and pulse-position modulation (PPM) schemes are investigated. For OOK, the exact bit error rate is evaluated taking into account the effect of both multiple-user interference and transmission loss. Upper and lower bounds on the bit error probability for PPM-CDMA systems are derived under the above considerations. The effect of both background and thermal noise is neglected in our analysis. Performance comparison between the number-state and coherent-state OOK/PPM-CDMA networks is also presented. Our results demonstrate that the number-state system requires less than half the energy consumed by the coherent-state one in order to attain the same performance. Lower bounds on the maximum number of simultaneous users are derived for both number- and coherent-state PPM-CDMA systems with asymptotically zero error rate. >

Performances of DS/SSMA communications with MPSK signaling and complex signature sequences

We study a direct-sequence spread-spectrum multiple-access (DS/SSMA) communication system with M-ary phase-shift-keyed (MPSK) signaling. Users in the system employ complex signature sequences, which are modeled as deterministic sequences. We present upper and lower bounds for the probability of bit error (p.b.e.) of a reference transmission. First, the conditional p.b.e., given the multiple-user interference (MUI), is obtained. Then, the conditioning is removed by averaging over the discretized probability density function (p.d.f.) of the MUI. The result is an upper (or lower) bound when the discretized p.d.f. is a shifted-high (or shifted-low) version of a sectioned p.d.f. This process requires the two-dimensional p.d.f. of the MUI, which is derived. Arbitrarily tight bounds can be obtained at the cost of increased computation. Numerical results are presented for complex FZC and near optimal 4-phase sequences, as well as real binary Gold sequences, for BPSK, 4-PSK, and 8-PSK. Gray code bit mapping is assumed, although the analysis can be applied to other bit mappings. Results show that complex sequences can yield better performance than binary sequences in MPSK signaling. Using complex sequences, the number of available signature sequences is greatly enlarged. >

Efficiently structured CDMA receiver with near–far immunity

A Gaussian code division multiple access (CDMA) channel is shared by K active users who transmit asynchronously with BPSK modulation by independent binary data streams. A conventional direct sequence CDMA receiver has only limited capability for suppressing cochannel multiple user interference (MUI) from the K – 1 other CDMA signals while attempting to receive and demodulate each CDMA signal. A new and efficient feedback receiver structure is described for coherent demodulation of the K asynchronous CDMA signals. By incorporating adaptive cancellation of the cochannel MU1 in a feedback structure, the new design offers the following advantages: protection of the receiver's synchronization loops as well as its data demodulators against MUI; lower bit error rate (BER) than the conventional receiver for equal-strength as well as near-far MUI; and implementation simplicity close to that of the conventional CDMA receiver. It is shown that all the CDMA signals are canceled to a common level determined by N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> , the one-sided power spectral density of the input AWGN; the bandwidth spreading ratio N; and K. The BER for any signal may be computed as if the N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> at its matched filter input were multiplied by the factor (6N/π)(1.5N + 1 – K) for K ≤ 1.5N. Computer simulations corroborate this result. A straightforward enhancement to the receiver structure is shown to allow it to overcome the deleterious affects of multipath propagation.

Performance analysis of optical synchronous CDMA communication systems with PPM signaling

Direct-detection optical synchronous code-division multiple-access (CDMA) systems with M-ary pulse-position modulation (PPM) signaling are investigated. Optical orthogonal codes are used as the signature sequences of our system. A union upper bound on the bit error rate is derived taking into account the effect of the background noise, multiple-user interference, and receiver shot noise. The performance characteristics are then discussed for a variety of system parameters. Another upper bound on the probability of error is also obtained (based on Chernoff inequality). This bound is utilized to derive achievable expressions for both the maximum number of users that can communicate simultaneously with asymptotically zero error rate and the channel capacity. Our results show that under average power and bit error rate constraints, there always exists a pulse position multiplicity that permit all the subscribers to communicate simultaneously.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The performance of optical fiber direct-sequence spread-spectrum multiple-access communications systems

Because of the random nature of the photodetection process and the multiple-user interference, an exact analysis of avalanche photodiode (APD)-based optical code division multiple-access (CDMA) communications systems is intractable and quite often, Monte Carlo (MC) simulations which yield exact estimates of system performance in terms of bit error rates (BERs) require a prohibitive computational burden. A quick and accurate MC method for simulating APD-based optical CDMA systems is presented. In particular, a performance analysis of optical CDMA systems employing optical orthogonal and prime sequence codes is undertaken.

Performances of acquisition schemes for CDMA systems with complex signature sequences

This paper investigates the acquisition performance of code-division multiple-access communication systems using deterministic complex signature sequences. Unlike most earlier studies, performances are obtained in the presence of multiple user interference, and the results can be applied to complex as well as binary signature sequences. Two serial search acquisition schemes are considered. The first scheme assumes that there is no data in the reference user's signal until the acquisition is achieved. The second scheme allows data modulation in the reference user's signal during the acquisition process. Probability of detection and probability of false alarm of both schemes are obtained without using the Gaussian approximation. Instead, a discrete approximation of the probability density function of the MUI is used in performance evaluations. Numerical results are presented for example sets of Gold, 4-phase, and FZC sequences. They show that complex and binary sequences have comparable acquisition performances. Results also show significant degradation of acquisition performances as the number of users increases.

Performance analysis of CDMA optical communication systems with avalanche photodiodes

The performance of optical code-division multiple-access (CDMA) communication systems with avalanche photodiodes (APD) is analyzed. The bit error rate (BER) can be accurately computed by using the saddlepoint method. The effects of the multiple-user interference (MUI), signal-dependent shot noise, and receiver thermal noise are investigated. Results of the numerical integration illustrate the non-Gaussian property of the receiver output distribution. Exact means and variances are derived for the Gaussian approximation. It is found that when the MUI increases, the saddlepoint approximation yields satisfactory results, but the Gaussian approximation yields higher BER floors. Prime codes and on-off keying (OOK) are considered. Examples illustrate the effects of the system parameters such as the APD gain, threshold, prime code length, and the number of simultaneous users. >

Performance bounds for DS/SSMA communications with complex signature sequences

A unified performance analysis of direct-sequence spread-spectrum multiple-access (DS/SSMA) communications with deterministic complex signature sequences is presented. The probability density function (PDF) of the multiple-user interference is determined. Using a round-down and round-up procedure on the PDF, arbitrarily tight lower and upper bounds on the probability of bit error (PBE) are obtained. Results based on the Gaussian approximation method are also presented. It is shown that complex sequences can yield better PBE performance than binary sequences. Using complex sequences, the number of signature sequences that have good auto- and cross-correlation properties is greatly enlarged. New users employing complex or binary signature sequences can be added to existing systems with graceful performance degradation. >

Multiple-User Interference in FHMA-DPSK Spread-Spectrum Communications

In this paper, we investigate the performance of a nearoptimum receiver in a frequency-hopped multiple-access (FHMA) differential phase-shift-keyed (DPSK) spread-spectrum communication system. We obtain upper bounds on the bit error rates (BER's) for the chipsynchronous system and the chip-asynchronous system in the presence of a single interfering signal which interferes in one time-chip. We also obtain upper bounds on the BER for the chip-synchronous system with multiple-user interference, for the special case where each time-chip has at most one interfering signal of the same power as the desired signal. We find that, for the chip-synchronous system, the upper bound on the BER when one time-chip has two interfering signals is larger than the upper bound on the BER when each of the two time-chips has a single interfering signal. We also discuss system performance for a large number of simultaneous users, and examine the additive white Gaussian noise (AWGN) approximation for the multiple-user interference. Finally, results for the chip-synchronous system with single interference in one time-chip over a Rayleigh fading channel are presented.