An optical code-division multiple-access (CDMA) system with double optical hard-limiters is proposed where the optical hard-limiters are placed before and after an optical correlator. Moreover, the-effect of the optical hard-limiter on the performance of the optical synchronous CDMA systems using modified prime sequence codes as signature codes is analyzed under the assumption of a Poisson shot noise model for the receiver photodetector where the noise due to the detector dark currents exists. We evaluate the performance under average power and bit rate constraints. Our results show that using the single optical hard-limiter slightly degrades the performance of the optical CDMA systems under the assumption of Poisson shot noise model for the receiver photodetector where the noise due to the detector dark currents exists. Moreover, we show that the optical CDMA systems with double optical hard-limiters have better performance than other conventional CDMA systems with and without the optical hard-limiter when the number of simultaneous users is not so large.

Direct-detection optical synchronous CDMA systems with double optical hard-limiters using modified prime sequence codes

In this letter, we address two questions concerning the application of multipulse pulse-position modulation (MPPM) for free-space optical (FSO) communications: (1) under what condition would MPPM offer a notable advantage over conventional pulse-position modulation (PPM), and (2) what practical coding scheme should be used to realize that advantage. Focusing on bandwidth-efficient FSO transmission, we find decimated MPPM constellations which are suitable for combining with binary codes and offer significant gains in terms of constellation-constrained capacity over PPM under simultaneous peak-power, average-power, and bandwidth constraints. We then consider labeling design for the popular bit-interleaved coded modulation (BICM) and devise a new multilevel coding (MLC) architecture for MPPM, which we refer to as reduced-layer MLC (RLMLC). Considering the Poisson FSO channel model as a relevant example, we provide simulative evidence that RL-MLC MPPM with off-the-shelf low-density parity-check codes can outperform any PPM scheme under the same transmission constraints.

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Coded multipulse pulse-position modulation for free-space optical communications

Next, we present an expression for the capacity of MPPM and show how to choose n ∈ {1, 2, · · ·} and M ∈ {2, 3, · · ·} to maximize capacity when average power, peak power, and bandwidth constraints are simultaneously imposed. The capacity of MPPM can be significantly higher, up to two times, than conventional pulseposition modulation (PPM) in a high average-power region for which the optimal M for conventional PPM is approximately 16 or lower. In lower average-power regions where the optimal M is higher, MPPM offers a negligible increase in capacity.

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Multipulse Pulse-Position Modulation on Discrete Memoryless Channels

In conventional visible light communication (VLC) dimming control systems utilizing pulse width modulation (PWM), reducing the duty cycle will push up the receiver sensitivity requirement if the transmission rate is kept constant. In this paper, we investigate a dimming control mechanism in indoor VLC to satisfy the receiver sensitivity requirement, which can achieve efficient data transmission while maintaining communication quality. To transmit excess signals, we propose a modification based on a conventional PWM scheme, which combines multi-pulse position modulation (MPPM) dimming control pulses with orthogonal frequency division multiplexing (OFDM) signals. Numerical results show that by utilizing the MPPM pattern effect, excess signals are transmitted, which can reduce the quadrature amplitude modulation (QAM) symbol rate and alleviate the receiver sensitivity requirement under a steadily high data rate and a bit error rate (BER) that is less than $10^{-3}$ .

Efficient Data Transmission Using MPPM Dimming Control in Indoor Visible Light Communication

In this paper, we propose a new synchronous fiber-optic code-division multiple-access (CDMA) system using adaptive optical hardlimiter and modified prime sequence code. At the receiver, the energy in one bit duration is estimated and used to adjust the threshold value of the adaptive optical hardlimiter which is placed after the optical correlator. The bit error probability of the system is analyzed with the consideration of thermal noise, shot noise, and dark current noise of the photodetector. The results show that this system can support a larger number of simultaneous users than other systems also using modified prime sequence code under the same bit error probability constraint.

A synchronous fiber-optic CDMA system using adaptive optical hardlimiter

The performance of optical synchronous code-division multiple-access (CDMA) with on-off keying (OOK) signaling (OOK/CDMA) and CDMA with M-ary pulse position modulation (PPM) signaling (PPM/CDMA) systems using prime sequence codes as signature codes is analyzed under the assumption of Poisson shot noise model for the photodetector. The bit error probability of OOK/CDMA is evaluated, and the upper bound on the bit error probability of PPM/CDMA is derived. Further, the performance is compared for some values of the average signal count per second, the length of prime sequences, and the number of simultaneous users under the constraint on throughput. It is found that PPM/CDMA performs better than OOK/CDMA by the pulse position multiplicity. It is also found that as the number of slots per frame of PPM increases, the bit error probability performance is improved. Moreover, it is found that CDMA with longer prime sequence codes has better performance. Therefore, we show that in order to achieve the low bit error probability and many simultaneous users, we should employ PPM/CDMA with larger number of slots and longer prime sequence codes.

Performance of optical direct-detection CDMA systems using prime sequence codes

The performance of MPPM (multipulse pulse position modulation) consisting of m slots and 2 pulses ((m, 2) MPPM) with imperfect slot synchronization is analyzed. The authors assign the optimum symbol sets of (m, 2) MPPM minimizing symbol error probability in the presence of timing error, and the bit error probability of (m, 2) MPPM is evaluated. The bit error probability of (m, 2) MPPM is compared with that of PPM at the same transmission bandwidth and transmission rate. It is found that (m, 2) MPPM performs better than PPM. >

Performance analysis of (m, 2) MPPM with imperfect slot synchronization

The performance of coded multi-pulse pulse position modulation (MPPM) consisting of m slots and 2 pulses, denoted as (m, 2) MPPM, with imperfect slot synchronization is analyzed. Convolutional codes and Reed-Solomon (RS) codes are employed for (m, 2) MPPM, and the bit error probability of coded (m, 2) MPPM in the presence of the timing offset is derived. In each coded (m, 2) MPPM, we compare the performance of some different code rate systems. Moreover, we compare the performance of both systems at the same information bit rate. It is shown that in both coded systems, the performance of code rate-1/2 coded MPPM is the best when the timing offset is small. Further, convolutional coded (m, 2) MPPM on the constraint length k/spl ges/7 performs better than Reed-Solomon coded (m, 2) MPPM for the same code rate. >

K. Sato

Papers

Performance of optical direct-detection CDMA systems using prime sequence codes

Performance analysis of (m, 2) MPPM with imperfect slot synchronization

Performance of coded multi-pulse PPM with imperfect slot synchronization in optical direct-detection channel