Tomoaki Ohtsuki

Performance analysis of direct-detection optical asynchronous CDMA systems with double optical hard-limiters

Performance of optical asynchronous code-division multiple-access (CDMA) systems with double optical hard-limiters is analyzed under the assumption of Poisson shot noise model for the receiver photodetector where the noise due to the detector dark currents exists. Optical orthogonal codes (OOCs) are employed as signature sequence codes. In the analysis, chips are assumed to be synchronous among users, that is, the chip synchronous case, because the effect of the interference is largest in the chip synchronous case and thus the performance in the chip synchronous case results in the upper bounds on the performance of the asynchronous system. The performance is evaluated under average power and bit rate constraints. The results show that, differing from the optical synchronous CDMA systems with double optical hard-limiters, the optical asynchronous CDMA systems with double optical hard-limiters have good performance even when the number of simultaneous users is large.

Multiple-subcarrier modulation in optical wireless communications

This article overviews multiple-subcarrier modulation techniques in optical wireless communications. The basic principles and characteristics of MSM techniques in optical wireless communications are presented. MSM optical wireless systems are explained where some block codes that convert information bits to be transmitted to the symbol amplitudes of subcarriers are used to increase the minimum value of the MS electrical waveform. MSM optical communications systems using subcarrier signal point sequences (SSPS) that can improve the power efficiency of MSM systems are also explained. The performance of MSM optical communications systems is presented in the intensity modulation with direct detection (IM/DD) channel without dispersion and in atmospheric optical communications where the effects of scintillation exist.

Performance analysis of atmospheric optical PPM CDMA systems

We propose atmospheric optical code-division multiple-access (CDMA) systems. We analyze the bit-error rate of the proposed system using pulse-position modulation (PPM) with considering the effects of the scintillation, avalanche photodiode noise, thermal noise, and multi-user interference. We show that the atmospheric optical CDMA systems can realize high-speed communications when the logarithm variance of the scintillation is small. When /spl sigma//sub s//sup 2/ is large, we need to use forward-error correction codes.

Pre-RAKE diversity combining for UWB systems in IEEE 802.15 UWB multipath channel

Since ultra wideband impulse radio (UWB-IR) systems can resolve many paths and are thus rich in multipath diversity, the use of the RAKE diversity combining is very effective. In the RAKE diversity combining, the bit error rate (BER) is improved with the increase of the number of fingers. The Pre-RAKE diversity combining is known as another technique to achieve the performance equivalent to the RAKE diversity combining without increasing the receiver complexity. In the Pre-RAKE diversity combining, the transmitted signals are scaled and delayed according to the delay and strength of the multipath. In this paper, we propose Pre-RAKE diversity combining techniques for UWB systems, the All-Pre-RAKE (A-Pre-RAKE) diversity combining using perfect channel information, the Selective-Pre-RAKE (S-Pre-RAKE) diversity combining using the information on the L strongest paths, and the Partial-Pre-RAKE (P-Pre-RAKE) diversity combining using the information on the first L paths. From the results of our computer simulation for UWB-IR systems in IEEE 802.15 UWB multipath channel model, we show that the proposed Pre-RAKE diversity combining techniques are effective for the UWB-IR systems to achieve good error rate performance, while keeping the complexity of the receiver low. We also show that the S-Pre-RAKE diversity combining is effective in achieving good error rate performance with less channel information.

A peak to average power ratio reduction of multicarrier CDMA using selected mapping

MC-CDMA is a very promising technique for high-bit-rate and high capacity transmission in mobile communications. One disadvantage of multicarrier modulation is the high peak-to-average power ratio (PAPR) that results in nonlinear distortion at a high power amplifier (HPA) and the degradation of BER. Various methods for reducing PAPR have been proposed for OFDM. It is known that the partial transmit sequence (PTS) and the selected mapping (SLM) are effective for OFDM. The PTS has been applied to MC-CDMA and shown that it is effective for reducing the PAPR in MC-CDMA as well. However, the effects of SLM for MC-CDMA have not been clarified. We investigate the effects of SLM for MC-CDMA. We also investigate phase sequences in the SLM that is suitable for reducing the PAPR in MC-CDMA. We show that the SLM is more effective than the PTS at the same amount of side information.

BER performance of turbo-coded PPM CDMA systems on optical fiber

We obtain upper bounds on the bit error rate (BER) for turbo-coded optical code-division multiple-access (CDMA) systems using pulse position modulation (PPM). We use transfer function bounding techniques to obtain these bounds, so our results correspond to the average bound over all interleavers of a given length. We consider parallel concatenated coding (PCC) schemes that use recursive convolutional codes as constituent codes. We consider systems using an avalanche photodiode (APD), and treat APD noise, thermal noise, and multi-user interference using a Gaussian approximation. We compare the performance of turbo-coded systems with that of BCH-coded systems with soft-decision decoding, and that of concatenated coding systems with outer Reed-Solomon (RS) code and inner convolutional code. We show that turbo-coded systems have better performance than BCH-coded systems. We also show that concatenated systems have better performance than turbo-coded systems when the block length is small and the received laser power is somewhat large.

Design criteria for phase sequences in selected mapping

Multicarrier code division multiple access (MC-CDMA) has attracted much attention to the fourth generation mobile communication systems, because it can realize high-bit-rate and high capacity transmission by multiplexing information symbols of many users with orthogonal codes. One disadvantage of multicarrier modulation is its high peak-to-average power ratio (PAPR) that results in nonlinear distortion at a high power amplifier (HPA) and the degradation of the bit error rate (BER). Various methods have been proposed for reducing PAPR. Among them, partial transmit sequences (PTS) and selected mapping (SLM) have been investigated intensively. PTS and SLM have been applied to MC-CDMA. We showed that SLM is more effective than PTS at the same amount of side information. We also showed that the random sequences can reduce the PAPR most as phase sequences in SLM by computer simulation. However, it has not been clarified why the random sequences are good as phase sequences in SLM and how we should design phase sequences in SLM. In this paper, we propose design criteria for phase sequences in SLM.

Performance evaluation of UWB-IR and DS-UWB with MMSE-frequency domain equalization (FDE)

Ultra wideband (UWB) has recently attracted much attention as an indoor short range high-speed wireless communication. Of all UWB systems, ultra wideband-impulse radio (UWB-IR) and direct sequence-ultra wideband (DS-UWB) use extreme short pulses and have the advantage of the extremely high path resolutions. The RAKE receiver is known as a technique that can effectively combine paths with different delays, obtain the path diversity gain, and improve the transmission characteristics. However, multipath is spread over dozens of symbols in the case of ultra high-speed communications of several hundreds Mbps, which causes the strong frequency selective channel. As a result, the transmission performance degrades. Meanwhile, the single-carrier (SC) transmission with frequency domain equalization (FDE) has also recently attracted much attention (Falconer et al. (2002)). It is reported that the SC with FDE has few problems of peak to average power ratio (PAPR) and also obtains an excellent performance as well as orthogonal frequency division multiplexing (OFDM) even in the strong frequency selective channel where multipath is spread over dozens of symbols. In this paper, we propose UWB-IR and DS-UWB with minimum mean square error (MMSE)-FDE. We evaluate the bit error rate (BER) or UWB-IR and DS-UWB with MMSE-FDE and compare it to that of UWB-IR and DS-UWB with maximal ratio combining (MRC)-RAKE and MMSE-RAKE. In consequence, we show that UWB-IR with MMSE-FDE can achieve the better BER than UWB-IR with MRC-RAKE and MMSE-RAKE. We also show that DS-UWB with MMSE-FDE has a better BER than DS-UWB with MRC-RAKE and MMSE-RAKE, particularly when M is large.

Performance of low-density parity-check (LDPC) coded OFDM systems

Orthogonal frequency division multiplexing (OFDM) is a very attractive technique for high-bit-rate data transmission in multipath environments. Many error-correcting codes have been applied to OFDM. Recently, LDPC codes have attracted much attention. The performance of LDPC codes is very close to the Shannon limit, with practical decoding complexity. We proposed LDPC coded OFDM (LDPC-COFDM) systems with BPSK and showed that the LDPC codes are effective in improving the bit error rate (BER) of OFDM in multipath environments (see Futaki, H. and Ohtsuki, T., IEEE VTC2001 fall, vol.1, p.82-6, 2001). LDPC codes can be decoded using a probability propagation algorithm known as the sum-product algorithm or belief propagation. To clarify iterative decoding properties in LDPC-COFDM systems, we first investigate the distribution of the number of iterations where the decoding algorithm stops. In mobile communications, multilevel modulation is preferred for high bandwidth efficiency. However, it has not been clarified how to apply LDPC codes to OFDM systems with multilevel modulation. We propose a decoding algorithm for the LDPC-COFDM systems with M-PSK. By simulation, we show that LDPC-COFDM systems achieve good error rate performance with a small number of iterations on both AWGN and frequency-selective fading channels. We confirm that the algorithm for LDPC-COFDM systems with M-PSK work correctly.

RSS-Based Localization in Environments with Different Path Loss Exponent for Each Link

The path loss exponent is very important parameter for localization using receive signal strength (RSS). In actual environments, path loss exponent for each link (target to each receive node) differs. However, the conventional localization methods use the same path loss exponent for all links. Hence, there are some mismatches between the real path loss exponent and the one used to estimate. We proposed the localization method that considers all the combinations of path loss exponents for each link and estimates the target location by averaging the target locations derived with all the combinations. However, the amount of calculation is huge. In this paper we propose RSS-based localization in environments with different path loss exponent for each link. The proposed method is a grid-based centralized localization using RSS. First the proposed method sets the minimum distance di,min and maximum distance di,max for each node i by using the RSS of each receive node i and the minimum and maximum path loss exponents set before estimation. Next, it calculates the distance di,(k,l) between the candidate target position (k, I) and each receive node i. If di,min les di,(k,l) les di,max, vote the grid (k,l). These processes are performed for all the receive nodes over the search area. Finally, the grid point with most voting is estimated to be the target location. According to the simulation results, we show that the proposed method achieves the higher localization accuracy than the conventional localization method using the same path loss exponent for all the links when the distribution of the path loss exponents over the field is uniform distribution.