Katsuhiro Kamakura

An embedded transmission method of PPM signaling constructed of parallel error correcting codes for optical CDMA

We propose an embedded transmission method using pulse position modulation (PPM) signaling with symmetric error correcting codes for optical code division multiple access (CDMA). In the method, 2/sup J/ slots of 2/sup J/-ary PPM signaling are embedded in a users signature sequence with 2/sup J/ weighted positions. Therefore, the proposed method increases the number of bits per signature sequence. This increase in the number of transmission bits allows us to adopt J error correcting codes without decreasing the information rate and expanding the system bandwidth. Consequently, the effect of multiaccess interference (MAI) is removed within the error correction capability. Considering the effect of MAI, we analyze the BER under Poisson photon counting process. Numerical results show that the proposed method has an advantage of good energy efficiency over the conventional PPM method in applying error correcting codes.

Reduction of optical beat interference in optical frequency-hopping CDMA networks using coherence multiplexing

In order to reduce optical beat interference (OBI) in an optical frequency-hopping code division multiple access (FH-CDMA) network, we propose the application of coherence multiplexing (CM). Since the CM technique achieves multiplexing by using time delays exceeding the coherence time of the light sources, the crosstalk between CM channels depends on the space of the delay difference between CM channels. We analyze the crosstalk to signal ratio as a function of the space, and find a channel space that is enough to neglect the crosstalk from other CM channels. Using this channel space, we obtain the number of CM channels accommodated within one chip duration. Since the conventional FH-CDMA supports traffic by the code channels only, its capacity is limited by the OBI as the traffic increases. In addition to the code channel, the proposed network supports traffic by the CM channels. Since the addition of CM channels decreases the number of active code channels for uniform traffic, the OBI is reduced. Numerical results show that the proposed network attains a larger capacity than the conventional FH-CDMA.

Optical frequency-encoding CDMA systems using time-encoding for MAI mitigation

This paper proposes frequency-encoding code-division multiple-access (FE-CDMA) systems using time-encoding for multiple access interference (MAI) mitigation. Time-encoding is that an incident ultrashort light pulse is split into several pulses with smaller power. In the proposed systems, MAI can be mitigated due to the smaller power and the multiplicity position of those pulses split by time-encoding. We show that the proposed system is effective to reduce the effect of the MAI when the received optical power is large.

A new modulation scheme using asymmetric error correcting code embedded in optical orthogonal code for optical CDMA

Conventionally, optical CDMA systems with on-off keying (OOK) require a time duration equalling a signature sequence length to transmit only one bit. Since the crosscorrelation between the signature sequences results in multi-access interference (MAI), the signature sequences are designed so that their weight distribution is sparse for their long length, and then MAI rarely occurs. With this view, optical orthogonal codes (OOC) are investigated, which are characterized by a quadruple (L,W,/spl lambda//sub a/,/spl lambda//sub c/), where L and W are length and weight of sequences, respectively, and /spl lambda//sub a/, and /spl lambda//sub c/, are maximum off-peak autocorrelation and maximum crosscorrelation, respectively. Thus, the use of OOC makes the effect of MAI small, but has to spend a long duration for one bit. In this paper, we propose an embedded modulation scheme, where an asymmetric error correcting (AEC) code is embedded in an OOC used for identification in optical CDMA. We show that the proposed scheme achieves the same bit error rate (BER) as conventional systems, with less average number of photons.

Optical CDMA based on frequency-domain encoding enhancement of frequency division multiplexing

We propose optical CDMA based on frequency domain encoding enhancement of frequency division multiplexing (FDM/FE-CDMA). Combining both techniques, comprehensive networks with nearly perfect orthogonal transmission can be expected. We analyze the bit error rate (BER) performance of the proposed FDM/FE-CDMA. As a result, we show that the proposed optical CDMA can support a large number of simultaneous users compared with the conventional FE-CDMA.

Optical CDMA system using embedded transmission method with Manchester signaling

We propose an optical CDMA system using the embedded transmission (ET) method with Manchester signaling. In the proposed system, transmission bits are embedded in the signature sequence used in optical CDMA systems. Although the Manchester signaling uses two slots for a transmitted bit, the two slots are included in the chip duration of optical CDMA.. According to an embedded bit, an optical pulse is transmitted in either slot. In the receiver, the threshold detection for the embedded bits reduces the effect of multiple access interference (MAI) into the erasure state, which is not a transmitted signal. By using a codeword of the BCH codes as the transmitted bits, the effect of MAI is removed in the decoding procedure of BCH codes. Since the decoding procedure treats the effect of MAI as the erasure bits, instead of the error bits, using extended BCH(eBCH) codes allows us to remove the effect of MAI more efficiently. We analyze the bit error rate (BER) of the system using the ET method with Manchester signaling on the assumption that photodetector outputs obey the Poisson distribution. We show that the system using the ET method with Manchester signaling achieves better BER performance than that without Manchester signaling.

Optical spread time CDMA communications system with PPM signaling

We show the improved performance of PPM signaling in an optical spread-time code-division multiple-access (ST-CDMA) for high bit rate communications. The effects of PPM signaling on the total capacity and the requirements for phase masks are investigated.

An ATM-based optical code division multiplexing transport network using coherent ultrashort pulses

We investigate the feasibility of introducing optical code division multiplexing (OCDM) technique in the path layer and propose an ATM-based OCDM network architecture using ultrashort light pulses. In the proposed network, all the processes of cells in the path layer operate in the optical domain prior to reaching the final destination node. Since each bit can be addressed individually with this technique, the realization of ATM protocols can be easily ensured. Optical technology can be reliably introduced in the path layer and an optical cross-connect (OXC) becomes an important component because it emulates the code conversion as the switching function. The OXC not only provides asynchronous access to each optical code path (OCP) but also has a dynamic bandwidth allocation capability. A lookup table is used in the switching and updated through network management functions. We examine the performance considering different parameters and discuss methods to improve the performance.

Direct-detection optical synchronous CDMA systems with interference canceller using group information codes

We propose an interference cancellation technique using a reference signal for optical synchronous code-division multiple-access (CDMA) systems. In the proposed system, we use the signature code sequences composed of the group information codes and the modified prime code sequences. The group information codes are added in the forefront of the signature code sequences to estimate the amount of the multiple access interference (MAI). The proposed cancellation technique can be realized with a simpler structure than the conventional canceller using the time division reference signal, because it can reduce the number of optical correlators from P to two, where P is the prime number. Considering the effects of the MAI, avalanche photodiode (APD) noise, and thermal noise, we analyze the performance of the proposed system. We show that the proposed canceller achieves better bit error probability than the conventional canceller.

A blind receiver using optical hard-limiters for optical code division multiple access

We propose a receiver using two optical hard-limiters (OHLs) for optical code division multiple access (CDMA). In the receiver the likelihood ratio test is made by using the photocounts obtained by the intensities in the weighted positions of the signature sequence. Before the test, two OHLs enable the proposed receiver to remove the effect of the multiaccess interference (MAI), without knowledge of other users: in this sense it is a blind receiver. This blindness leads the proposed receiver to be a much simpler receiver than the multiuser receivers. We analyze the bit error rate (BER) of the proposed receiver in the optical CDMA system using optical orthogonal codes (OOCs). Some numerical results show that the proposed receiver has better performance compared to the decorrelating receiver, which is one of multiuser receivers.