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 an AMS/OFDM system, base station is in control of the modulation level of each subcarriers, and then, adaptive modulated packet is transmitted from the base station to the mobile station. In this case, the mobile station is required the modulation level information (MLI) to demodulate the received packet. The MLI is generally transmitted as a data symbol, therefore, the throughput is degraded. Moreover, it is necessary to have some transmission delay times and the processing time to make an adaptive modulation command (AMC) using feedback information (FBI). With the FBI delay and processing time, the system performance might be degraded. To reduce these problems, in this paper, we propose a differential modulated pilot symbol assisted adaptive OFDM for reducing the MLI with predicted FBI.

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.

The uplink of a space-time block coded multicarrier modulation code division multiple access (MC-CDMA) system equipped with a uniform linear array (ULA) at the base station is studied. A blind decoder that provides closed-form solutions of both transmitted symbol sequences and directions of arrival (DOAs) for all active users in one macrocell is derived without the uplink space-time vector channel estimation. The decoder uses an ESPRIT-like method to separate multiple co-channel users with different impinging DOAs. As a result, the DOAs of multiple users are obtained. In particular, a set of signal spaces, every one of which is spanned by the coded symbol sequences of an individual user, are also obtained. From these signal spaces, the original symbol sequences of multiple users are estimated by exploiting the special structure of space-time block coding (STBC) in combination with the finite alphabet property of transmitted symbols. Performance of the proposed scheme is evaluated by extensive computer simulations.

We propose a new diversity scheme for orthogonal frequency division multiplexing/multi-input multi-output (OFDM/MIMO) systems. The proposed scheme, named turbo layered space-frequency coded OFDM (TLSFC-OFDM), exploits the turbo principle with space hopping (SH). The TLSFC-OFDM system with SH provides a spatial coding so that we can obtain the transmit diversity. We also introduce a successive interference cancellation (SIC) algorithm that requires no ordering and fewer iterations to converge. As a result, this scheme reduces computational complexity. Computer simulation results show that the unordered SIC-based TLSFC-OFDM system outperforms the OFDM/H-BLAST system. It is also shown that the proposed system can operate even with fewer receive antennas than transmit antennas.

Polynomial cancellation coding (PCC) was proposed to mitigate the sever inter-carrier-interference (ICI) in an orthogonal frequency division multiplexing (OFDM) system caused by frequency offset. In this paper, we consider the effectiveness of PCC under time-variant multi-path Rayleigh fading analytically and by simulations. We first consider an analytical expression of the signal-to-interference plus noise power ratio (SINR) and then derive an approximation of the bit-error-rate (BER) of the OFDM-PCC system under the assumption that ICI is well approximated by a white Gaussian noise. Since the bandwidth efficiency of OFDM-PCC is half of that of normal OFDM, we compare the BER performance of the scheme with the normal OFDM system of the same bit-rate when low, medium, and high level modulations are used. Our results show that OFDM-PCC performs well even for high modulation level under time-varying multi-path fading.

Multi-Carrier CDMA (MC-CDMA) has been considered as a combination of the techniques of Code Division Multiple Access (CDMA) and Orthogonal Frequency Division Multiplex (OFDM). However, even until now, the efficient MC-CDMA scheme is still under study because of the inherent bugs in OFDM, such as the troubles caused by Multiple Access Interference (MAI) and Peak to Average Power Ratio (PAPR). In this paper, we present a novel two-dimensional spreading sequence named "Two Dimensional Combined Complementary Sequence" (TDC). If we take this kind of sequences as spreading codes, several prominent advantages can be achieved compared with traditional MC-CDMA. First, it can achieve MAI free in the multi-path transmission both in uplink and downlink. Second, it offers low PAPR value within 3 dB with a quite simple architecture. The last but not the least, the proposed MC-CDMA scheme turns out to be an efficient approach with high bandwidth efficiency, high spreading efficiency and flexible transmission rate enriched by a special shift-and-add modulation. Meanwhile, an algorithm that constructs TDC sequences is discussed in details. Based on above results, we can get the conclusion that the novel TDC sequences and corresponding MC-CDMA architecture have great potential for applications in next generation wireless mobile communications, which require high transmission rate in hostile and complicated channels.

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 f_D = 1500 Hz, becomes SF_Time = 16. Furthermore, it should be decreased to SF_Time = 8 for such a very fast fading environment using 16QAM data modulation. We also clarify when the channel load is light such as C_mux/SF = 0.25 (C_mux 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 (E_s/N₀) is reduced as the spreading factor in the frequency domain is increased up to say SF_Freq = 32 for QPSK and 16QAM data modulation. When the channel load is close to full such as when C_mux/SF = 0.94, the optimum spreading factor in the frequency domain is SF_Freq = 1 for 16QAM data modulation and SF_Freq = 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.

In downlink MC-CDMA, orthogonal variable spreading factor (OVSF) codes can be used to allow multirate communications while maintaining the orthogonality among the users with different data rates. In this paper, we point out that simple selection of the OVSF codes results in degraded performance. We show that this happens because simple code selection results in power concentration over certain consecutive subcarriers; severe power loss in the received signal occurs when these subcarriers experience a deep fade in a frequency selective fading channel. In addition, we show two effective techniques to avoid the performance degradation: random code selection and frequency interleaving; which technique provides a better performance depends on modulation level, code multiplexing order, and presence of channel coding.

This paper proposes Variable Spreading and Chip Repetition Factors (VSCRF)-Code Division Multiple Access (CDMA) broadband packet wireless access in the reverse link, which flexibly supports employing the same air interface in various radio environments such as a cellular system with a multi-cell configuration and local areas such as very-small cell, indoor, and isolated-cell environments. In VSCRF-CDMA, we propose two schemes: the first is a combination of time-domain spreading with an orthogonal code and chip repetition that achieves orthogonal multiple access in the frequency domain by utilizing a comb-shaped frequency spectrum, and the other is adaptive control of the spreading factor and chip repetition factor according to the cell configurations, number of simultaneously accessing users, propagation channel conditions, and major radio link parameters. Simulation results show that the proposed VSCRF-CDMA associated with the combination of the spreading factor, SF_D, of four and the chip repetition factor, CRF, of four improves the required average received signal energy per bit-to-noise power spectrum density ratio (E_b/N₀) for the average packet error rate of 10^-2 by approximately 2.0 dB compared to DS-CDMA only employing SF_D = 16 assuming four simultaneously accessing users in an exponentially decaying six-path Rayleigh fading channel with two-branch diversity reception.

For the performance deficiency of the pilot symbol aided channel estimation in orthogonal frequency division multiplexing (OFDM) systems, the wavelets network interpolation channel estimator is proposed. By contrast with conventional methods, wavelets network interpolation channel estimator can guarantee the high transmission rate and lower Bit error rates (BER). Computer simulation results demonstrate that the proposed channel estimation method exhibit an improved performance compared to the conventional linear channel estimation methods and is robust to fading rate, especially in fast fading channels.

This paper proposes an OFDM based adaptive modulation scheme employing variable coding rate (VCR OFDM AMS), which selects optimum modulation and coding scheme (MCS) realized by combination of several modulation schemes and coding rates. The OFDM AMS with multilevel transmit power control (OFDM AMS/MTPC) can realize high data rate transmission in the dynamic parameter controlled-orthogonal frequency and time division multiple access (DPC-OF/TDMA). The employment of OFDM AMS/MTPC, however, makes transceiver design rather complex. To solve this problem, we propose to improve throughput performances of the OFDM AMS without employment of the MTPC. The simple OFDM AMS, however, does not fully utilize transmit power for throughput improvement because there is surplus transmit power which corresponds to power margin over required signal to interference plus noise power ratio (SINR). Thus, in order to improve transmit power efficiency for throughput increase, we reduce the required SINR gaps between adjacent MCSs by introducing many coding rates. Furthermore, this paper presents an effective bit loading algorithm when multiple coding rates as well as modulation schemes are used. Computer simulation confirms that the proposed VCR OFDM AMS gives sufficient throughput performances as an alternative to the OFDM AMS/MTPC.

A new data-aided carrier frequency offset (CFO) estimation technique is presented for correlative coded OFDM systems in the presence of strong multipath. Different from traditional data-aided estimation techniques, the technique estimates CFO by detecting amplitude of pilots rather than their phase shift and removes effects on CFO estimation due to intercarrier interference by an iterative compensation method. A theoretical analysis of its performance has been derived and simulation results comparing the new technique with a traditional data-aided estimation technique are presented.

This paper proposes a time alignment control (TAC) for reducing an influence of multiple access interference (MAI) due to propagation delays (PDs) in uplink transmission from multiple mobile stations (MSs) to an access point (AP) for an orthogonal frequency division multiple access (OFDMA) based mobile communication system. In addition, this paper presents our evaluation of the proposed TAC as applied to dynamic parameter control orthogonal frequency and time division multiple access (DPC-OF/TDMA) which has been suggested for use in new generation mobile communication system. This paper also proposes several formats for an activation slot (ACTS) in which the GIs are lengthened in order to avoid the MAI because the TAC cannot be performed yet in an initial registration of the MSs. Computer simulation elucidates that lengthening the GIs of data symbols in the ACTS adequately to compensate a maximum delay improves the transmission performance of the ACTS at the initial registration without PDs compensation. The simulation also elucidates that the proposed TAC is performed to reduce the influence of the MAI effectively and that updating the estimates of the PDs every certain period is needed to compensate the PDs accurately under high-mobility environment.

This paper proposes an accurate Fast Fourier Transform (FFT) window timing detection method based on the maximum signal-to-interference power ratio (SIR) criterion taking into account the received signal and inter-symbol interference power according to different detected FFT window timings in Orthogonal Frequency and Code Division Multiplexing (OFCDM) wireless access. In the proposed method, the SIR of the received signal is estimated using the desired signal power and inter-symbol interference power calculated based on the power delay profile, which is measured by the cross-correlation between the pilot symbol replica and the received signal. Furthermore, since the SIR is calculated only for the received path timing of the first path and those paths exceeding the guard interval duration, the computational complexity of the proposed method is low. Computer simulation results show that the proposed scheme reduces the required average received signal energy per symbol-to-noise power spectrum density ratio (E_s/N₀) for achieving the average packet error rate of 10^-2 by approximately 1.0 dB compared to the conventional method, which detects the forward path timing of the power delay profile (16QAM data modulation, six-path Rayleigh fading channel, and the maximum delay time of 3 µsec (root mean squared (r.m.s.) delay spread of 0.86 µsec)).

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 E_b/N₀ from ideal CE of as small as 0.6 dB for an average BER = 10^-3 and requires about 2.4 dB less E_b/N₀ compared to differential STTD that requires no CE.

In this paper, we propose an adaptive algorithm based on accumulated signal processing, which could be applicable to Post-FFT-type OFDM adaptive array antennas. Proposed scheme calculates the weight of each element at a particular instant t, by considering both post- and pre-received symbols. Because of the use of additional forthcoming information on channel behavior in the weight calculation scheme, one can expect an improved performance in fast fading conditions by using the proposed adaptive algorithm. This paper also discusses the application of the proposed adaptive algorithm to OFDM adaptive array. In OFDM application, a few subchannels are being used to transmit pilot symbols, and at the receiver, the proposed adaptive algorithm is applied to those pilot subchannels, and interpolates the weights for the data subchannels which are allocated between the pilot subchannels. Finally, the system performance improvement with the application of the proposed adaptive algorithm is verified by computer simulation.

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 focuses on the MC-CDMA (Multi Carrier-Code Division Multiple Access) with the MMSEC (Minimum Mean Square Error Combining) receiver, which is a good candidate of a transmission scheme for beyond 3G systems. This paper evaluates the forward link capacity using the MMSEC receiver in the MC-CDMA cellular system, which employs TDM (Time Division Multiplex) transmission for multiple users. In this paper, the PDF (Probability Distribution Function) of the SINR (Symbol to Interference plus Noise energy Ratio) after MMSEC under multi-cell environment are calculated with the various number of the code division multiplexes. Based on the PDF, the numerical relation can be derived between the peak rate of the adaptive transmission and the average transmission rate per sector.

This paper compares the throughput performance employing hybrid automatic repeat request (ARQ) packet combining, i.e., Chase combining, and Incremental redundancy, considering the frequency diversity effect in the broadband forward-link channel for Orthogonal Frequency and Code Division Multiplexing (OFCDM) packet wireless access achieving a peak throughput above 100 Mbps. Simulation results show that the achievable throughput at the average received signal energy per symbol-to-background noise power spectrum density ratio (E_s/N₀) of 0 and 6 dB employing Incremental redundancy is increased by approximately 35 and 30% compared to that using Chase combining for QPSK and 16QAM data modulation schemes with the coding rate of R = 1/2, respectively, considering a large frequency diversity effect in a 12-path exponential decayed Rayleigh fading channel, since the reduced variations in the received signal level in a broadband channel bring about a larger coding gain in Incremental redundancy. We also show that when adaptive modulation and channel coding (AMC) is applied, Incremental redundancy is superior to Chase combining since the large coding gain is effective in achieving a large time diversity gain for a low number of retransmissions such as M = 1 or 2 for a maximum Doppler frequency up to f_D = 400 Hz. It is demonstrated, nevertheless, that the total throughput when employing Incremental redundancy associated with a near optimum MCS set according to the channel conditions becomes almost identical to that using Chase combining when a large number of retransmissions, M, is allowed, such as M = 10, owing to time diversity along with frequency diversity.

This paper presents throughput performance along with power profiles in the time and frequency domains over 100 Mbps based on field experiments using the implemented Variable Spreading Factor-Orthogonal Frequency and Code Division Multiplexing (VSF-OFCDM) transceiver with a 100-MHz bandwidth in a real multipath fading channel. We conducted field experiments in which a base station (BS) employs a 120-degree sectored beam antenna with the antenna height of 50 m and a van equipped with a mobile station (MS) is driven at the average speed of 30 km/h along measurement courses that are approximately 800 to 1000 m away from the BS, where most of the locations along the courses are under non-line-of-sight conditions. Field experimental results show that, by applying 16QAM data modulation and Turbo coding with the coding rate of R = 1/2 to a shared data channel together with two-branch antenna diversity reception, throughput over 100 and 200 Mbps is achieved when the average received signal-to-interference plus noise power ratio (SINR) is approximately 6.0 and 14.0 dB, respectively in a broadband channel bandwidth where a large number of paths such as more than 20 are observed. Furthermore, the location probability for achieving throughput over 100 and 200 Mbps becomes approximately 90 and 20% in these measurement courses, which experience a large number of paths, when the transmission power of the BS is 10 W with a 120-degree sectored beam transmission.

In this paper, an efficient architecture for an adaptive Reed-Solomon decoder is presented, where the block length n and the message length k can be varied from their minimum allowable values up to their selected values. This eliminates the need of inserting zeros before decoding shortened RS codes. And the error-correcting capability t can be changed adaptively to channel state at every codeword block. The decoder allows efficient decoding in both burst mode and continuous mode, and it permits 3-step pipelined processing based on the modified Euclid's algorithm. Each step in decoding is designed to be clocked by a separate clock. Thus, each step can be efficiently pipelined with no help of multiplexing. Also, it makes it possible to employ no additional buffer even when the decoder input and output clocks are different. The adaptive RS decoder over GF(2⁸) having the error-correcting capability of upto 10 has been designed in VHDL, and successfully synthesized in an FPGA chip. It can be used in a wide range of applications because of its versatility.

The multi-channel Hybrid Fiber Coaxial (HFC) network is essentially a shared medium with multi-channels. Its operation requires the use of a scheduling algorithm to manage the data transmission within each channel. The Data-Over-Cable Service Interface Specification (DOCSIS) protocol is an important standard for HFC networks. Since this protocol does not explicitly specify the scheduling algorithm to be used, many alternative algorithms have been proposed. However, none of these algorithms are applicable to the scheduling of non-Unsolicited Grant Service (UGS) data in multi-channel HFC networks. Accordingly, the present study develops a multi-channel scheduling algorithm which optimizes the scheduling delay time of each transmitted non-UGS request. This algorithm manages the amount of data transmission in each upstream channel according to the overall network load and the bandwidth available in each channel. This study constructs a mathematical model of the algorithm and then uses this model as the basis for a series of simulations in which the performance of the scheduling algorithm is evaluated.

This paper proposes a distributed adaptive routing algorithm that may be applied to inter-domain calls passing over any type of network topology, traffic management and switching techniques on the path, while carrying bidirectional traffic with multiple QoS requirements. The path is searched within a contour area restricted by the number of hops between source and destination while the end-to-end admission of calls is controlled at source node and each hop's admission at each node, reflecting the latest resources availability and network conditions for the given QoS requirements. Performance analysis in various conditions shows good applicability in real networks.

We develop a method of dimensioning and managing the bandwidth of a link on which TCP flows from access links are aggregated. To do this, we extend the application of the processor-sharing queue model to TCP performance evaluation by using flow statistics. To handle various factors that affect actual TCP behavior, such as round-trip time, window-size, and restrictions other than access-link bandwidth, we extend the model by replacing the access-link bandwidth with the actual file-transfer speed of a flow when the aggregation link is not congested. We only use the number of active flows and the link utilization to estimate the file-transfer speed. Unlike previous studies, the extended model based on the actual transfer speed does not require any assumptions/predeterminations about file-size, packet-size, and round-trip times, etc. Using the extended model, we predict the TCP performance when the link utilization increases. We also show a method of dimensioning the bandwidth needed to maintain TCP performance. We show the effectiveness of our method through simulation analysis.

In this paper, we improve the performance analysis of the Rake receiver for the DS-CDMA forward link using long random spreading sequences (RSS's) by more accurately evaluating the correlation between the various interference terms. We also extend the analysis to the case of short (periodic) RSS. The accuracy of the expressions obtained in our analysis is verified by computer simulation. We show that for a given normalized spreading factor, the bit error rate (BER) performance of the Rake receiver is the same for BPSK and QPSK data modulation. We also show that when the channel delay spread is smaller than a data symbol duration, the CDMA receiver has similar BER performance for long and short RSS's. However, for large delay spread, the employment of short RSS's may result in severe performance degradation.

OFDM, MC-CDMA and DS-CDMA are being researched vigorously as the prospective signaling technique for the next generation mobile communications systems, which will be characterized by the broadband packet technology. With packet transmissions, hybrid ARQ (HARQ) will be inevitable for error control. HARQ with rate compatible punctured turbo (RCPT) codes is one of the promising techniques. Data rate equivalent to the OFDM system can be attained with MC-CDMA and DS-CDMA by assigning all the available codes to the same user resulting in what is commonly referred to as multicode MC-CDMA and multicode DS-CDMA. A rake receiver is used for receiving the DS-CDMA signals. However, recently minimum mean square error frequency-domain equalization (MMSE-FDE) has been proposed for the reception of DS-CDMA signals. In this paper, we introduce RCPT HARQ to DS-CDMA with MMSE-FDE and compare its throughput performance with OFDM, multicode MC-CDMA and multicode DS-CDMA with rake combining. MMSE weight for packet combining is introduced and the soft value generation for turbo coding in MC-CDMA and DS-CDMA with MMSE-FDE is presented. The throughput is theoretically evaluated for the uncoded case. For RCPT-HARQ, the comparison is done by computer simulations. It is found that the throughput of HARQ using DS-CDMA with MMSE-FDE is the same as or better than using MC-CDMA. However, with higher level modulation, type I HARQ using OFDM is better than using either MC-CDMA or DS-CDMA; for type II HARQ without redundancy in the first transmission, however, MC-CDMA and DS-CDMA gives a higher throughput.

As the code division multiple access (CDMA) based third generation cellular infrastructure requires high performance signal processing in a baseband modem, an application-specific integrated circuit or a field-programmable gate array has commonly been used for chip rate processing. In this paper, the use of digital signal processors (DSP) is explored for a cdma2000 and a wideband CDMA channel modem with the goal of increasing flexibility. The design concepts of the prototype software-defined radio platform we implemented to estimate the potential and feasibility of commercial SDR platforms are presented. We discuss the hardware and software architecture of the platform, considerations for reconfigurability, and the test results. We also address practical issues for real-time chip rate processing and optimization schemes of DSP software, and provide detailed measurement results of DSP performance.

In direct-sequence code division multiple access (DS/CDMA) multiuser communication systems with multipath channels, both intersymbol interference (ISI) and multiple-access interference (MAI) must be considered. The multipath effect usually changes the characteristics of the spreading codes. Modification of the conventional receiver structure is needed to account for the interference of the multipath fading. This paper proposes four adaptive receivers for such multiuser DS/CDMA systems in multipath fading channels. We employ least mean square (LMS) and recursive least squares (RLS) algorithms for both finite impulse response (FIR) and infinite impulse response (IIR) receiver structures. Mean square error (MSE) and convergence analysis are also given in this paper. Simulation results show the performance comparisons of the four proposed receivers.

To support high data rate wireless communications, in this paper, based on the linearly constrained constant modulus (LCCM) criterion, the reverse link performance of the multi-carrier code division multiple access (MC-CDMA) receiver, with frequency combiner, and having smart antenna arrays beamformer in base station, has been investigated over the Rayleigh fading channel. By using the Kronecker product an equivalent direct formulation, which integrates the information of spatial-domain as well as temporal-domain, with constraint matrix could be obtained. In consequence, the modified normalized LCCM-gradient algorithm is devised to adaptively implement the direct constrained optimal weights solution of the fully space-time MC-CDMA detector. We show that the proposed method outperforms the constrained minimum output energy (CMOE) algorithm and is more robust against to the signal mismatch, due to imperfect channel and direction-of-arrival estimation used in the array beamformer.

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.

Over the last decade there has been a rapid growth of wireless communication technology. Among numerous wireless network architectures, the personal communication services (PCS) networks and wireless local area networks (WLAN) have attracted lots of attention. One of the core functionalities in wireless networks is the location service that provides location information for subscriber services, emergency services, and various mobile networks internal operations. In this paper, an integrated location management mechanism is proposed for heterogeneous wireless networks that combine PCS networks and WLAN. Three major functionalities in the integrated location management scheme are the determination of the WLAN connectivity for a mobile terminal, the development of a local area location scheme for WLAN, and the location prediction module for PCS networks. This mechanism not only determines the location of a mobile client more precisely, but also reduces the cost of locating. The performance evaluation is conducted to demonstrate the effectiveness of the proposed mechanism for heterogeneous wireless networks.

A new technique based on the auto-correlation function is described for the estimation of the Doppler spread in mobile communication systems. We first propose to divide a uncertainty region of Doppler spread into multiple frequency bins. Based on the given multiple bins the correlator compares the estimated value at a certain time index to the theoretical exact value and then decides which bin the Doppler spread is estimated in. The certain time index can be optimized to give the largest decision region among multiple bins. We derive the optimum time index algorithm to give the largest decision region for each bin based on Rayleigh fading channel. We also apply the same Doppler spread estimator to the Rician case with the slight transformation of the received signal. We show that the proposed technique is not affected significantly by the Rician factor and the SNR degradation with the reasonable number of samples for estimation which is not the case of other estimators given in the literature.

The electric currents on the upper, lower and side surfaces of the patch conductor in a circular microstrip antenna are calculated by using the integral equation method and the characteristic between the electric currents on the upper and lower surfaces is compared. The integral equation is derived from the boundary condition that the tangential component of the total electric field due to the electric currents on the upper, lower and side surfaces of the patch conductor vanishes on the upper, lower and side surfaces of the patch conductor. The electric fields are derived by using Green's functions in a layered medium due to a horizontal and a vertical electric dipole on those surfaces. The result of numerical calculation shows that the electric current on the lower surface is much bigger than that on the upper surface and the input impedance of microstrip antenna depends on the electric current on the lower surface.

Inverse synthetic aperture radar (ISAR) is useful for automatic target recognition (ATR) because it can reconstruct a high resolution image of an observed target. In ISAR imaging, 3-dimensional reflectivity distribution of a target is projected to the plane defined by range axis and cross range axis. In order to recognize the observed target by using pattern matching, reference images of candidate targets must be adequately generated. However, that is difficult because the cross range axis, which depends on the target's unknown rotational motion, can not be determined precisely. In this paper, we propose a new algorithm to generate reference ISAR images of ship targets. In this algorithm, tracking data, Doppler width and the slope of the centerline of an ISAR target image are used to specify the cross range axis. The effectiveness of the proposed algorithm was evaluated by using simulated targets.

This paper proposes an improved type of the Hybrid-Parallel Power-Factor-Correction (HP-PFC) converter. It has the advantage of a higher efficiency and improved input current waveform. This advantage achieved through changing new charging path of the bulk capacitor and balancing the power flow from the two transformers to the output. This new circuit has been analyzed using MATLAB/Simulink and confirmed with experiment. As a conclusion, it is confirmed that this improved HP-PFC converter complies with the severe regulation of IEC61000-3-2 Class D. Moreover, a high efficiency of 90% is achieved for 15 V/6 A output power under the worldwide line voltage conditions.

Recently, Yeh, Shen and Hwang proposed an one-time password authentication scheme, which enhances the S/KEY scheme to resist server spoofing attacks, preplay attacks and off-line dictionary attacks. In this letter, the weaknesses and inconveniences of their scheme are demonstrated.

Bandpass sampling algorithm is effectively adopted to obtain the digital signal with significantly reduced sampling rate for a single radio frequency(RF) signal. In order to apply the concept to multiple RF signals, we propose bandpass sampling algorithms with the normal and the inverse placements since we are interested in uniform order of the spectrum in digital domain after bandpass sampling. In addition, we verify the propose algorithms with generalized equation forms for the multiple RF signals.

In this letter, we propose a low-complexity estimation method of cyclic-prefix (CP) length for a discrete multitone (DMT) very high-speed digital subscriber line (VDSL) system. Using the sign bits of the received DMT VDSL signals, the proposed method provides a good estimate of CP length, which is suitable for various channel characteristics. This simple estimation method is consistent with the initialization procedure of T1E1.4 multi-carrier modulation (MCM)-based VDSL Standard. Finally, simulation results with VDSL test loops are presented.

In this letter, we evaluate the performance of Optical Cross Connect (OXC) architectures equipped with limited-range wavelength converters. Performance will be evaluated according to both an optimum and a random output wavelength assignment strategy. Analytical and simulation results show the possibility to reduce the conversion degree of the Wavelength Converters while keeping network performance high.

In order to improve the quality of VoIP services, an adaptive routing method is proposed in the application layer of Internet gateways. This method determines routing paths based on the average one-way delays in a predetermined re-routing interval. In order to evaluate the performance of the method, five different routing policies are specifically designed and tested. Experimental results show that the method can improve the QoS of Internet phone services.

Video telephone service (VTS) is considered one of promising services provided in wideband CDMA (WCDMA) networks. Without a designated call admission policy, VTS calls are expected to suffer from relatively high probability of blocking since they normally have more stringent signal quality requirement than ordinary voice calls. In this letter, we consider a prioritized call admission design in order to reduce the blocking probability of VTS calls, which may encourage the users to access the newly-provided VTS in a more comfortable way. The VTS calls are given a priority by reserving a number of channel-processing equipments. With the reservation, the blocking probability of prioritized VTS calls can be reduced evidently. That of ordinary calls, however, is increasing instead. This letter provides a system model that counts the blocking probabilities of VTS and ordinary calls simultaneously, and numerically examines an adequate level of the prioritization for VTS calls. The results show that the prioritization level should be selected depending on received interference as well as bandwidth required for VTS.

The Generalized Max-Min Fairness policy (GMM) allocates in a fair way the available bandwidth among elastic calls by taking into account their minimum and maximum rate requirements. The GMM has been described in a five-step procedure, which has the advantage of an easy presentation, but does not come into details, as far as its computer implementation is concerned, and fails to describe the policy in a clear mathematical way. We propose a new algorithm for the GMM policy, in a clear mathematical way, based on Linear Programming (LP). The new algorithm is directly convertible into software. Numerical examples clarify our algorithm.

This letter derives and computes the detection probability for timing synchronization in an orthogonal frequency division multiplexing (OFDM) system encountered with a multipath Rayleigh fading channel. OFDM timing synchronization using constant amplitude zero auto-correlation (CAZAC) training symbols and correlation techniques is adopted. With this provision, we focus on the numerical analysis for OFDM timing synchronization scheme employing a preadvancement technique to reduce the inter-symbol interference (ISI). For measuring system performance, the sync detection performance derived in the considered system is presented in a multipath Rayleigh fading channel.

In this Letter, a frequency-domain pre-rake transmission is presented for a direct sequence spread spectrum with time division duplex (DSSS/TDD) system under a frequency-selective fading channel. The mathematical relationship between frequency-domain and time-domain pre-rake transmissions is discussed. It is confirmed by the computer simulation that, similar to the time-domain pre-rake transmission, frequency-domain pre-rake transmission can improve the bit error rate (BER) performance. The frequency-domain pre-rake transmission shows only slight performance degradation compared to the frequency-domain rake reception for large SF.

A frequency selection algorithm leveraging the capability of a handset to autonomously select idle channels of a public communication system for use in a private communication system was previously proposed, and its effectiveness was verified through experiments conducted in a metropolitan area. This paper describes the results of an experiment verifying the algorithm's effectiveness in rural areas with relatively low public communication system traffic.

A new theoretical approach for the evaluation of the in-band nonlinear distortion effects on the performance of DS-CDMA systems is presented. Rather than widely used models of treating the effects of nonlinear distortion as additive Gaussian noise, the new approach is based on the asymptotic clipping and shot noise theories, which offer important insights into true nature of clipping process and can be further extended to many communications systems with high PAPR and peak-limited nonlinearities.

In this paper we propose a new algorithm for computing the optimal configuration of the Multiple Fractional Guard Channel (MFGC) admission policy in multiservice mobile wireless networks. The optimal configuration maximizes the offered traffic that the system can handle while meeting certain QoS requirements. The proposed algorithm is shown to be more efficient than previous algorithms appeared in the literature.

In this letter, parallel interference cancellation (PIC) in code division multiple access (CDMA) was performed with two different structures by using a neural network (NN). In the first structure (receiver-1) the NN was used as a front-end stage of a one stage PIC circuit. In the second structure (receiver-2), the NN was used instead of the one stage PIC circuit and it was trained as a multiple access interference (MAI) detector to perform the PIC process by subtracting the MAI from the outputs of the matched filter. The PIC is a classical technique in multi user detection process and its bit error rate (BER) performance is not good in one stage for most of the applications. For improving its BER performance, generally a multi stage PIC which has the high computational complexity is used. In this study, we have gotten a better BER performance than a three stages PIC receiver with both proposed receivers that have the lower computational complexity.

The uplink synchronous transmission scheme was proposed to improve the uplink performance of DS-CDMA systems. However, previous performance analyses have assumed perfect uplink synchronization among main paths, which is impractical due to timing misalignment. Accordingly, this Letter evaluates the impact of imperfect synchronization on the performance of an uplink synchronized DS-CDMA system by deriving an orthogonality factor as a measure of the imperfection in synchronization.

While CDMA systems are proven to be excellent solutions for cellular communications, they suffer from severe multi-path interferences and are hard to support high-data-rate transmissions over frequency-selective fading channels. This letter introduces a novel downlink transmission method for next generation mobile communication systems. The proposed method can provide significantly improved performance in a hot-spot area while maintaining the backward compatibility with the 3rd generation CDMA systems.

Ad hoc networks are comprised of MAC, routing protocols and physical environments. In this paper, optimum MTU (Maximum Transmission Unit) sizes are studied in the ad hoc network environment. For a performance index, the "throughput rate" is defined. The throughput rate is the ratio between the number of packets received and packets sending through the overall systems. MTUs with the sizes between 250 bytes and 750 bytes have been shown to have better throughput rates in the environment without noise. MTUs with the size of 500 bytes have been found to generate the optimum throughput rate in the environments of 10^-4 BER or less. Similar results are obtained in lower BER environments. AODV has slightly better performance than DSR without noise. These results found to be true for practical mobile environment.

In this study, a multiple U-shaped slot microstrip patch antenna for application to the 5 GHz band is designed and fabricated. To obtain sufficient bandwidth in the operating band, foam is inserted between the substrate and ground plane, the type of form is styrofoam, the coaxial probe source is used, and the position of the probe shift is adjusted from the center to the left. The measured result (5.02-5.955 GHz) of the fabricated antenna satisfies the conditions of VSWR < 2.0 in 5 GHz band (5.15-5.35 GHz, 5.47-5.725 GHz, 5.725-5.825 GHz), gain of 3.88-9.28 dBi, and broad radiation pattern.

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.

A numerical technique for improving the accuracy of a rectangular loop antenna based on the Finite Difference Time Domain (FDTD) method is proposed. In this technique, a quasi-static field behaviour is incorporated into the FDTD update equations, and the more accurate numerical technique can be obtained without the need of using fine cells. The simulation results of this proposed technique are compared with the Method of Moment to confirm the effectiveness of the technique.

This paper introduces the water ring scan method especially designed for the scalable video coding schemes such as fine granularity scalabilities (FGS) on the basis of MPEG-4 part-2 and the H.264. The proposed scanning method can improve the subjective quality of the decoded video by most-preferentially encoding, transmitting and decoding the image information of the region of interest. From the various simulation results of FGS coding schemes with MPEG-4 part 2 and H.264, the proposed scanning method can improve the subjective picture quality about 0.5 dB 3.5 dB better than the widely used raster scan order, especially on the region of interest, without significant loss of the quality in the left-over region.