Multipath propagation of radio signal introduces frequency selectivity. OFDMA systems greatly suffer from frequency selective fading. It is an important limit factor of performance of OFDMA systems, especially in subband based multiple user access scehems. In this paper, we propose the method of subband selection and handover to improve the system performance over the frequency selective channel. Two subband selection algorithms are presented to accurately select the subband with high channel gain and avoid the channel notch. The random access procedure employing subband selection is presented as an example. The effects of the subband selection are also given. The selection effectively improves the performances of frame synchronization, frequency synchronization, channel estimation, and bit error rate (BER). The investigations show that the proposed scheme is promising to reliable communications over frequency selective fading channel.

Conventional WDM systems multiplex channels with different signal bandwidths using fixed and equal channel spacing. As a result, their spectral efficiency is rather poor. If the wavelength and the bandwidth of each channel in a WDM system could be freely changed as needed, a variety of services with different signal bandwidths could be accommodated efficiently. This is expected to yield high spectral efficiency. For this purpose, this paper proposes a WDM optically amplified system that combines optical power splitting with homodyne detection; its use in three configurations, point to point, ring (center to remote nodes), and peer to peer, is described. Coherent optical systems generally need a frequency stable local light source in addition to a sending light source in each WDM channel. We improve cost effectiveness by proposing that the output of one light source be divided to yield the local light for frequency selection by homodyne detection and the sending light source whose output is externally modulated by transmission signal. In this configuration, the local light level is low to permit high levels of sending power. The key problem is how to get high SNR with limited low-level local lights. This paper derives the optimum receiving loss condition that can maximize the SNR with local light levels as low as -20 dBm for the point to point configuration. For the ring configuration, the system overcomes the optical power loss created by splitting numbers over 1,000 even if the local lights are as low as 0 dBm. The ring configuration can, therefore, flexibly accommodate many users and services. We also elucidate the relation between SNR and BER for DPSK homodyne detection in a bandwidth-flexible system.

In this letter, pilot-assisted adaptive prediction iterative channel estimation in frequency-domain is presented for the antenna diversity reception of orthogonal frequency division multiplexing (OFDM) signals. A frequency-domain adaptive prediction filtering is applied to iterative channel estimation for improving the tracking capability against frequency-domain variations in a severe frequency-selective fading channel. Also, in order to track the changing fading environment, the tap weights of frequency-domain prediction filter are updated using the simple NLMS algorithm. Updating of tap weights is incorporated into the iterative channel estimation loop to achieve faster convergence rate. The average bit error rate (BER) performance in a frequency-selective Rayleigh fading channel is evaluated by computer simulation. It is confirmed that the frequency-domain adaptive prediction iterative channel estimation provides better BER performance than the conventional iterative channel estimation schemes.

Two rotational transformations are used to derive a new expression for the symbol error probability (SEP) of an M-ary phase shift keying (MPSK) with an I-Q unbalance over additive white Gaussian noise (AWGN) and Rician fading channels. We used the new expression to investigate the effect of the I-Q unbalance on the MPSK SEP performance. Our investigation confirms that this approach is a convenient way to evaluate the average SEP of an MPSK for various cases of the Rician factor.

In this paper, rail-to-rail OTA utilizing linear V-I conversion circuit whose input stage is composed of single channel MOSFETs, is proposed. The proposed conversion circuit is realized with two circuit blocks. One of them consists of a single MOSFET operating in plural regions and the other a pair of MOSFETs in saturation region and cut-off region. Combination of the circuit blocks achieves a linear voltage-current conversion for a rail-to-rail input signal. Rail-to-rail OTA is proposed using the proposed conversion circuit. HSPICE simulations are performed to verify the validity of the proposed V-I converter and rail-to-rail OTA. Simulation results indicate good performances. As an application example, 2nd-order LPF is realized using the proposed OTAs.

Spatial correlation among antenna elements both at transmitter and receiver sides in MIMO communications is known to have a crucial impact on system performances. Another factor that can severely degrade receiver performances is the timing offset relative to the channel delay profile. In this paper we derive a novel receiver for turbo MIMO equalization in space-time-trellis-coded (STTrC) system to jointly address the problems described above. The equalizer is based on low complexity MMSE filtering. A joint detection technique of the several transmit antennas is used to reduce the receiver's sensitivity to the spatial correlation at the transmitter and receiver sides. Furthermore, only the significant portion of the channel impulse response (CIR) is taken into account while detecting signals. The remaining portion of CIR is regarded as the unknown interference which is effectively suppressed by estimating its covariance matrix. By doing this the receiver's complexity can be reduced since only a portion of the CIR has to be estimated and used for signal detection. Furthermore, by suppressing the interference from the other paths outside the equalizers coverage the receiver's sensitivity to the timing offset can be reduced. The proposed receiver's performance is evaluated using field measurement data obtained through multidimensional channel sounding. It is verified through computer simulations that the performance sensitivity of the joint detection-based receiver to the spatial correlation is significantly lower than with the receiver that detects only one antenna at a time. Furthermore, the performance sensitivity to the timing offset of the proposed receiver is shown to be significantly lower than that of the receiver that ignores the existence of the remaining multipath CIR components.

In this letter, we propose the SNR estimator for multipath fading channels. We employ the least squares estimator for estimating the channel and estimate the SNR using the estimated noise variance. The SNR estimation can be used to adapt the demodulation algorithm to enhance its performance, as well as to provide the channel quality information. Simulation results show the performance of SNR estimator.

We present a new speech enhancement algorithm in a car environment with two microphones. The car audio signals and other background noises are the target noises to be suppressed. Our algorithm is composed of two main parts, i.e., the spatial and the temporal processes. The multi-channel blind deconvolution (MBD) is applied to the spatial process while the Kalman filter with a second-order high pass filter, for the temporal one. For the fast convergence, the MBD is newly expressed in frequency-domain with a normalization matrix. The final performance evaluated with the severely car noise corrupted speech shows that our algorithm produces noticeably enhanced speech.

In this paper, we propose a spatio-temporal equalizer for the space-time block coded transmission over the frequency selective fading channels with the presence of co-channel interference (CCI). The proposed equalizer, based on the tapped delay line adaptive array (TDLAA), performs signal equalization and CCI suppression simultaneously using the minimum mean square error (MMSE) method. It is to show that our scheme outperforms the previous two-stage combined adaptive antenna and delayed decision feedback sequence estimator (DDFSE) approach. We also show that performance can be further improved if the synchronization between the preceding and delayed paths is achieved.

This letter presents a method to simulate a phase-included UWB channel impulse responses for a given indoor channel. In this method we decompose a UWB pulse into a finite number of spectral components. This enables the received signal to be determined by the sum of the convolutions between each spectral component and a corresponding frequency-dependent UWB channel impulse response. The ray-tracing algorithm is applied to calculate the amplitude and the phase of each frequency-dependent channel impulse response. Based on the calculated results, we finally show the simulation of the UWB channel impulse response.

This paper proposes an adaptive channel estimation scheme, which uses different moving average length and pilot gain for different mobile environments. It is based on MSE method and extensive simulations under various environments for WCDMA physical layer. The scheme applies a computationally efficient and easily implemented pilot filter on WCDMA forward channel. For different mobile channel environments, the optimal combination of moving average length and pilot gain for low SNR is achieved. The simulation results illustrate that the adaptive scheme can achieve much lower BER compared with two other adaptive schemes, especially when the speed of mobile user is high. And the BER performance of the proposed scheme is insensible to the mobile speed.

This letter proposes a vertical Bell Labs layered space-time (V-BLAST) coded direct-sequence code-division multiple-access (DS-CDMA) scheme in the frequency-selective fading environment and its blind channel identification algorithm. By exploiting an ESPRIT-like method and the singular value decomposition, the channels identification results are closed-form solutions. Moreover, the proposed scheme employs the precoding in the transmitter to eliminate the constraint of more receive antennas than transmit ones, required by most conventional V-BLAST codec schemes. Computer simulation results demonstrate the validity of this proposed scheme.

The next generation mobile communication system is required to support wideband wireless data links when terminals are moving at very high speed. In such case, accurate channel estimation is more crucial in this case. This paper presents a downlink channel estimation scheme for Multi-Input and Multi-Output (MIMO) MC-CDMA system by introducing uncorrelated and continuous pilot symbol sequence for every transmit antenna in code domain. Compared with conventional pilot aided estimation methods which insert pilots symbols in time domain, frequency domain or both of them, the proposed method can obtain more accurate channel impulse response matrix estimation in fast fading environments with much lower computation complexity and less resources exhausted. At last, we analyzed the BER performances of the scheme under a typical fast fading environment-VB and AWGN.

Recently, multi-carrier code division multiple access (MC-CDMA) has been attracting much attention for the broadband wireless access in the next generation mobile communications systems. In the case of uplink transmissions, the orthogonality among users' signals is lost since each user's signal goes through different fading channel and hence, multi-access interference (MAI) is produced, thereby significantly degrading the transmission performance compared to the downlink case. The use of frequency-domain equalization at the receiver cannot sufficiently suppress the MAI. In this paper, we propose frequency-domain pre-equalization transmit diversity (FPTD), which employs pre-equalization using multiple transmit antennas with transmit power constraint, in order to transform a frequency-selective channel seen at a receiver close to the frequency-nonselective channel. We theoretically analyze the bit error rate (BER) performance achievable with the proposed FPTD and the analysis is confirmed by computer simulation.

This paper proposes the optimum design for adaptively controlling the spreading factor in Orthogonal Frequency and Code Division Multiplexing (OFCDM) with two-dimensional spreading according to the cell configuration, channel load, and propagation channel conditions, assuming the adaptive modulation and channel coding (AMC) scheme employing QPSK and 16QAM data modulation. Furthermore, we propose a two-dimensional orthogonal channelization code assignment scheme to achieve skillfully orthogonal multiplexing of multiple physical channels. We first demonstrate the reduction effect of inter-code interference by the proposed two-dimensional orthogonal channelization code assignment. Then, computer simulation results show that in time domain spreading, the optimum spreading factor, except for an extremely high mobility case such as for the fading maximum Doppler frequency of fD = 1500 Hz, becomes SFTime = 16. Furthermore, it should be decreased to SFTime = 8 for such a very fast fading environment using 16QAM data modulation. We also clarify when the channel load is light such as Cmux/SF = 0.25 (Cmux and SF denote the number of multiplexed codes and total spreading factor, respectively), the required average received signal energy per symbol-to-noise power spectrum density ratio (Es/N0) is reduced as the spreading factor in the frequency domain is increased up to say SFFreq = 32 for QPSK and 16QAM data modulation. When the channel load is close to full such as when Cmux/SF = 0.94, the optimum spreading factor in the frequency domain is SFFreq = 1 for 16QAM data modulation and SFFreq = 1 to 8 for QPSK data modulation according to the delay spread. Consequently, by setting several combinations of spreading factors in the time and frequency domains, the near maximum link capacity is achieved both in cellular and hotspot cell configurations assuming various channel conditions.

Since the interference is quite related to the performance of CDMA cellular systems, it is necessary to estimate Other-Cell-Interference Factor (OCIF). Here, starting from OCIF calculation for an aeronautical communication system, we investigate the forward link performance of data packet transmission in which the capacity, throughput and delay of the system are measured. To the numerical results, one can see that the performance of the aeronautical communication system is worse than that for terrestrial cellular systems and also depends logarithmically on both the cell radius and height.

One of the disadvantages of using OFDM is the larger peak to averaged power ratio (PAPR) of the time domain signal as compared with the conventional single carrier transmission method. The OFDM signal with larger PAPR will cause the undesirable spectrum re-growth and the larger degradation of bit error rate (BER) performance both due to the inter-modulation products occurring in the non-linear amplifier at the transmitter. The clipping method in conjunction with the Decision Aided Reconstruction (DAR) method is well known as one of the solutions to improve the BER performance with keeping the better PAPR performance. However, the DAR method is proposed to mitigate only the clipping noise and not for the inter-modulation noise. In this paper, we propose the Improved DAR (IDAR) method, which can mitigate both the clipping noise and inter-modulation noise on the basis of DAR method. The proposed method enables the efficient usage of transmission power amplifier at the transmitter with keeping the better PAPR and BER performances. This paper presents various computer simulation results to verify the performance of proposed IDAR method in the non-linear channel.

In this paper, we propose pilot-assisted decision feedback channel estimation (PA-DFCE) for space-time coded transmit diversity (STTD) in orthogonal frequency division multiplexing (OFDM). Two transmit channels are simultaneously estimated by transmitting the STTD encoded pilot. To improve the tracking ability of the channel estimation against fast fading, decision feedback is also used in addition to pilot. For noise reduction and preventing the error propagation, windowing of the estimated channel impulse response in the time-delay domain is applied. The average bit error rate (BER) performance of OFDM with STTD is evaluated by computer simulation. It is found that the use of PA-DFCE can achieve a degradation in the required Eb/N0 from ideal CE of as small as 0.6 dB for an average BER = 10-3 and requires about 2.4 dB less Eb/N0 compared to differential STTD that requires no CE.

Multi-carrier code division multiplexing (MC-CDM) is one of promising multiplexing techniques for fourth-generation mobile downlink communications systems, where high data rate services should be provided even for high speed-cruising mobiles. For MC-CDM-based packet communication, a frequency scheduling method, which adaptively assigns different sub-carriers to different users, is proposed. This paper proposes a frequency scheduling method, which utilizes pre-assignmented subcarriers in the frequency domain for the MC-CDM scheme. Furthermore, the performance of the proposed system in frequency selective fading channels is compared with that of a no-scheduled MC-CDM scheme by computer simulation in both single- and multi-cell environments. From the results, it is found that the proposed system achieves better bit error rate performance than the no-scheduled MC-CDM scheme and can control quality of service (QoS) for active users.

In the Multi Carrier (MC)-CDMA system, the frequency diversity gain is obtained by its being spread in the frequency domain. The frequency interleaving technique can improve the frequency diversity gain. In this paper, the bit error rate (BER) performance in the MC-CDMA system which adopts the frequency interleaving scheme in the frequency selective fading channel is evaluated by computer simulation. In this simulation, orthogonal restoration combining (ORC) and minimum mean square error combining (MMSEC) are considered as frequency equalization combining techniques. This paper shows that BER performance with the frequency interleaver is better than without it in various environments.