Timing noise of 160 GHz optical pulses has been evaluated over nine decades of Fourier frequency using the optoelectronic harmonic mixing technique. For down-converting the 160 GHz pulse intensity into a low-frequency IF signal, the fourth order modulation sidebands produced by a Mach-Zehnder intensity modulator have been employed. Phase noise power spectral density and timing jitter for 155.552-GHz optical time-division multiplexed pulses and 160.640-GHz passively mode-locked pulses are measured using the time domain demodulation and time interval analysis techniques, respectively.

In this letter, adaptive modulation and coding (AMC) is combined with multiple input multiple output (MIMO) multiplexing to improve the throughput performance of AMC. In addition, a system that adopts selection transmit diversity (STD) in the AMC-MIMO multiplexing system, AMC-STD-MIMO multiplexing system, is proposed. STD in our simulation selects 2 transmission antennas from 4 antennas and AMC-MIMO multiplexing process operates with the selected antennas. The computer simulation is performed in a flat Rayleigh fading channel. The results show that the proposed AMC-STD-MIMO multiplexing system achieves the SNR gain of 4.0 dB, compared to the AMC-MIMO multiplexing system at 3 Mbps throughput.

An automatic-repeat-request (ARQ) scheme for improving the system throughput efficiency is evaluated in coded multiple-input multiple-output (MIMO) transmissions. Supplementary trellis-coded modulation (TCM) code has been proposed for hybrid ARQ schemes. The free distance of the TCM after code combining can be increased by employing different TCM codes for retransmissions. The MIMO scheme offers additional flexibility in preventing successive frame errors by changing the connections between transmitters and transmit antennas upon retransmission. In this paper, an ARQ strategy employing both TCM reassignment and antenna permutation technique is investigated. It is shown through computer simulations that this ARQ scheme achieves high throughput even in severe conditions of low signal-to-noise ratio and high Rician factor over spatially and temporally correlated Nakagami-Rice fading MIMO channels.

The suitability of a complex MIMO channel matrix for spatial multiplexing is verified experimentally in terms of the Demmel condition number. The instantaneous 2 2 MIMO-OFDM channel measurements in several indoor environments indicate the location dependency of the condition number. Wideband frequency characteristics are also analyzed to evaluate the applicability of spatial multiplexing.

In this paper, we derive expressions for the bit error probability of QPSK/OFDM on frequency-selective Rayleigh fading channels. In the OFDM system, ICI (interchannel interference) caused by Doppler spread of the channel degrades the error performance of the system and introduces the error floor even for coherent detection. Analysis results show that the error performance of QPSK/OFDM can be degraded as the normalized maximum Doppler frequency fD /Bsub is increased where fD is the maximum Doppler frequency and Bsub is the subchannel bandwidth. Computer simulations confirm the theoretical analysis results for BPSK and QPSK signals.

We propose a new orthogonal frequency division multiplexing transmission scheme using orthogonal code multiplexing. This scheme makes all modulation symbols have the same reliability even in a frequency selective fading channel, through a distributed transmission of each symbol over the whole effective subcarriers using a distinct orthogonal code. As an appropriate set of orthogonal multiplexing codes, we use the set of discrete Fourier transform code sequences that hold the orthogonality irrespective of the length. Using this set, we also can greatly reduce the peak-to-average-power ratio (PAR) of the resulting signal. Simulation results show that the proposed scheme can significantly reduce the required signal-to-noise ratio at a given bit error rate over the existing schemes. The scheme can maintain the PAR within a reasonable range of about 6 dB even up to 512 subcarriers and works well even with PAR clipping of 1.5 dB.

We propose a new approach associated with the use of some selected sets of Walsh Hadamard codes for joint estimation of channels and the number of transmit antennas by employing only one OFDM pilot symbol. This allows transmit antenna diversity to be applied in systems which have a limited number of training symbols (preambles), e.g. HIPERLAN/2. The proposed approach does not require any a priori knowledge about the number of transmit antennas, providing flexibility in the number of antennas to be used. In addition, adaptive scheme associated with the proposed approach provides more accurate estimations of the channels. The effectiveness of the proposed approach is evaluated through simulation. Results show that the proposed scheme provides significant improvement over previous channel estimation schemes and has almost the same performance as the ideal system with the full knowledge of the channel state information.

We consider the routing and wavelength assignment (RWA) problem for large-scale WDM optical networks where each transmission request is served by an all-optical lightpath without wavelength conversion. Two heuristic RWA algorithms are proposed in order to minimize the number of wavelengths required for a given set of connection requests. The proposed algorithms are evaluated and compared with the existing algorithms for two realistic networks constructed based on the locations of major cities in Ibaraki Prefecture and those in Kanto District in Japan.

In this letter, we present a normalized least-mean-square algorithm of blind estimator for carrier frequency offset estimation of orthogonal frequency division multiplexing systems. In conjunction with the closed-loop estimate structure, the proposed efficient algorithm eliminates the inter-carrier interference for time varying carrier frequency offset. The proposed algorithm offers faster convergence speed and more accuracy to the carrier frequency offset estimate. Several computer simulation examples are presented for illustrating and effectiveness of the proposed algorithm.

In this letter, we propose a robust IFDD scheme employing an interference canceller, which is used for mitigating interferences from the transmitting signal instead of complex filter bank to reduce the complexity, for the OFDM system using feedback information. According to simulation results, the proposed IFDD OFDM system does not show significant performance degradation but maintains the robustness against the fast time-varying multipath channel, while the TDD OFDM system estimating feedback information from receiving block makes serious performance degradation.

Carrier frequency and symbol timing errors may greatly degrade the performance of the orthogonal frequency division mulitplexing (OFDM) system, especially in multipath environment. In this paper, we explore the cyclostationarity of OFDM signals, which only relies on second order statistics, to estimate the synchronization offset. First, a coarse carrier frequency offset estimator for multipath environment is developed using the second order statistics of the received OFDM signal. It has a wide capture range though not accurate. Second, we introduce a new synchronization algorithm based on cyclostationarity and matched filter theories, which can get the maximal estimation SNR in multipath environment. Both estimators utilize channel state information to achieve better estimation performance and are non-pilot aided. They can be combined to form a whole OFDM synchronizer for multipath environment. Finally, simulations confirm the performance of the estimation algorithm.

The authors have established new switching architecture of all optical WDM network suitable for the application of video transmission. In this paper, emphasis is put on a setup of a wavelength connection based on contiguous wave-band pool and we have proposed new Wave-Band Routing and Assignment (WBRA) method which provides simple switching and high speed wavelength assignment. Assuming the environment without wavelength convertor, our wave-band switching scheme is applied to several network topologies for performance evaluation. Then effectiveness and feasibility of this scheme are confirmed from a viewpoint of the number of required wavelengths. Simulation results indicate that our proposal scheme attains lower number of required wavelengths as compared to the fixed wave-band scheme. Assuming to use wavelength convertors, we have also evaluated the situation that the number of hops is restricted.

We investigate the capacity limitations of a WDM ring fiber-radio backbone incorporating wavelength interleaving where each base station drives a sectorized antenna interface. We also investigate the issues related to the merging of such networks with standard WDM infrastructures. The investigations show that re-allocating the interleaved WDM channels to fit within a 100 GHz block enables the millimeter-wave (mm-wave) fiber-radio system with sectorized antenna interfaces to integrate easily with WDM systems. The performance of a variety of channel allocations for the merged fiber-radio network is examined and simulation studies of the transmission of multiple channels are carried out. The overall network capacity of the merged mm-wave fiber-radio network is improved with the proposed channel allocation schemes.

The dense wavelength division multiplexing (DWDM) technique is very attractive for effectively increasing the channel capability, even for access networks. Some DWDM radio-on-fiber (ROF) systems have been studied recently. In those systems, fiber Bragg gratings (FBG) or arrayed waveguide gratings (AWG) were used to demultiplex DWDM ROF signals. In this report, an alternative channel-selection scheme of DWDM millimeter-wave-band ROF signals by optical heterodyne detection with dual-mode local light is newly proposed. Error-free demultiplexing and transmission over a 25-km-long SMF of the DWDM signal, which consists of two 60-GHz-band, 155-Mb/s-DPSK ROF signals, are demonstrated.

In this paper, to enhance the power efficiency a new simple space-time coding scheme is devised with application to the OFDM based Wireless LAN system. The basic idea is from the receiver's point of view and is referred to as Virtual Constellation Mapping (VCM). We designed a new combination of the channel coding (Turbo Code) along with multiple transmit antennas (Two antennas) to achieve transmit diversity and space division multiplexing transmission. Computer simulation results showed that with the same transmission data rate, our proposed scheme can achieve better bit error rate (BER) compared with the conventional space-time trellis coded OFDM scheme in high Doppler fading channels.

In a bidirectional wavelength-reused system, polarization control is used to reduce power penalty induced by coherent mixing of the signal with Rayleigh backscattering. The reduction of the effect of Rayleigh backscattering is theoretical study and experimental demonstration. For a 50km, 10Gb/s bidirectional transmission system, an error floor of about 510-10 under the worst polarization state is occurred. Nevertheless, the power penalty is reduced from 3.5dB to only 0.3dB when the signals are under optimum polarization control. The results indicate that the proposed technique may find vast applications in bidirectional ring networks with add/drop as well as cross-connect nodes using wavelength-reused technology.

Control scheme for accurately optimizing (and also automatically stabilizing) the interferometer phase bias of Symmetric-Mach-Zehnder (SMZ)-type ultrafast all-optical switches is proposed. In this control scheme, a weak cw light is used as a supervisory input light and its spectral power ratio at the switch output is used as a bipolar error signal. Our experimental result at 168-Gb/s 16:1 demultiplexing with a hybrid-integrated SMZ switch indicates the feasibility and the sensitivity of this control scheme.

Advanced optical transmission fibers have enabled 40-Gb/s transmission over distances of up to 5200km with 100-km amplified spans. This paper will discuss a number of the enabling fiber properties including dispersion, dispersion slope, Raman gain efficiency, and polarization mode dispersion.

In this letter, we study the performance of the iterative receiver as applied to the space division multiplexing/orthogonal frequency division multiplexing (SDM/OFDM) systems. The iterative receiver under consideration employs the soft in/soft out (SISO) decoding process, which operates iteratively in conjunction with channel estimation for performing data detection and channel estimation at the same time. As opposed to the previous studies in which the perfect channel state information is assumed, the effects of channel estimation are taken into account for evaluating the performance of the iterative receiver and it is shown that the channel estimation applied in every iteration step of the iterative receiver plays a crucial role to warrant the performance, especially at a low signal-to-noise power ratio (SNR).

We consider the routing for a multicast in a WDM all-optical network. We prove a min-max theorem on the number of wavelengths necessary for routing a multicast. Based on the min-max theorem, we propose an efficient on-line algorithm for routing a multicast.