A novel algorithm for an adaptive array that is suitable for a multi-carrier transmission will be proposed in this paper. In an adaptive array, signals received by antenna elements are weighted and combined together. In the proposed algorithm, distortion of a spectrum of the combined signal is detected and weight coefficients for each antenna element are controlled so that the spectrum of the combined signal becomes flat. Concept of the proposed algorithm can be interpreted as the CMA which is applied to signals sampled in the frequency domain. Furthermore, a configuration of the adaptive array will be shown. Signals separated in a receiver of the multi-carrier transmission are utilized to detect the distortion of the signal spectrum. By adopting the proposed configuration, the spectrum of the multi-carrier signal can be easily detected. In order to investigate the performance of the proposed adaptive array, computer simulation has been carried out. Numerical results show that; 1) A desired wave is captured well even if an interference wave is narrow band signal and is stronger than the desired wave. 2) Suppression performance for a co-channel interference wave depends on both a symbol timing and SIR of arrival waves. If the symbol timing of the interference wave greatly differs from the timing of FFT window of the receiver, the desired wave can be captured even if the co-channel interference wave is stronger more than 10 dB compared with the desired wave. The conventional CMA adaptive array has a serious problem that the narrow band interference wave is captured when it is stronger than the desired wave. On the other hand, it is extremely rare that the proposed adaptive array captures the narrow band interference wave. Therefore, it can be said that the proposed adaptive array is a robust system compared with the conventional system.

One of the problems in multi-carrier modulation is nonlinear distortion due to nonlinearity of channels, such as in HPA (High Power Amplifier). This problem is also true of multi-carrier SS (Spread Spectrum) systems. In this paper, an inter-modulation compensation scheme for multi-carrier M-ary/SS system is proposed. We propose two methods to control the sequences transmitted in parallel to avoid the occurrence of severe inter-modulation distortion. One is the "package sequence selection" method, which requires slight redundancy. The other method is based on error correction code, which requires no additional frequency or power except the redundancy for error correction. We confirm the validity of our proposed scheme by computer simulation, and the BER (Bit Error Rate) performance in an AWGN (Additive White Gaussian Noise) channel is presented.

In this paper, a type of multi-carrier direct sequence code division multiple access (MC-DS-CDMA) system which uses frequency spread coding is proposed and investigated for the down-link. An MC-DS-CDMA system is a combined system of CDMA and multi-carrier modulation. This system is often categorized as a "serial to parallel (S/P) type" system because serial to parallel converted data symbols are transmitted. They use different sub-carriers which are narrow-band DS waveforms. In this system, benefits of path or frequency diversity can not be obtained because of the narrow-band transmission of each data symbol. In order to benefit from the diversity, we propose to adopt frequency spread coding in an MC-DS-CDMA system. The proposed system exploits frequency diversity without additional redundancy, i. e. , no frequency or time redundancy is required to improve the performance. Computer simulation is carried out in a frequency selective fading channel and the results show its effectiveness in terms of average bit error rate (BER). Furthermore, the proposed system is compared with a multi-carrier (MC-) CDMA system which is often categorized as a "copy type" system and a single-carrier (SC-) DS-CDMA system using a RAKE receiver.

Multi-carrier (MC) signal has a large peak to mean envelope power ratio, so that the MC signal suffers from a high level of inter-modulation distortion due to the nonlinearity of the power amplifier stage. For portable terminals, it is undesirable to use linear amplifiers because high power efficiency is needed. To solve this problem, we propose a time division duplex (TDD)-code division multiple access (CDMA) communication system which uses an asymmetric modulation scheme between the forward and reverse links. The system consists of multicarrier modulation for the forward link and single carrier modulation for the reverse link. A pre-equalization method for the forward link transmission is also presented in this paper. In frequency selective fading, the system achieves a path diversity effect without any channel estimation unit at the mobile station by using the pre-phase equalizer. From the simulation results, it it found that the proposed system achieves better BER performance than the conventional MC-CDMA system and the single carrier RAKE system equipped at the mobile unit since the proposed system has the ability to suppress other user interfering signals.

In this paper, a method of Transmission Power Control (TPC) for Orthogonal Frequency Division Multiplexing Direct Sequence Code Division Multiple Access (OFDM-DS-CDMA), in order to compensate for power attenuation at each subcarrier, is proposed. Instead of assigning same power levels for all-subcarriers, different transmission power levels are assigned to different subcarriers, according to the attenuation of the subcarriers. System performance, in terms of Bit Error Rate (BER), has been evaluated by Monte Carlo simulation. The simulation results presented significant improvement, the proposed system performed much better than the system without TPC. It is shown that the Each Carrier TPC performs better than All Carriers TPC, which all carriers are controlled uniformly, hence Each Carrier TPC is more suitable for OFDM-DS-CDMA system.

This paper describes a forward subchannel control of multi-carrier scheme intended to compensate for phase/amplitude distortions under frequency selective fading. The forward subchannel control scheme is used for a Time Division Duplex (TDD) multi-carrier system on up-link. The forward subchannel control scheme provides forward subchannel control of phase/amplitude variation and subchannel assignment control. These controls are applied before transmission of an up-link signal. The forward control parameters are estimated by a preamble down-link signal. Simulation results clarify that the BER performance with the forward subchannel control scheme shows a superiority of more than one order at the condition of 22 dB of Eb/N0 and 400 Hz of fading frequency.

An efficient ARQ scheme based on the packet combining technique is investigated for multi-carrier modulation systems. In multi-carrier modulation systems, several sub-carriers are used for high data rate transmission and their individual received signal quality becomes different from one sub-carrier to others in a frequency selective fading channel. Therefore by changing the assignment of data to the sub-carriers in the retransmission packets, the distortion between the previous transmitted packet and the newly retransmitted one will be different. This is the principle of the proposed adaptive data order rearrangement for a packet combining ARQ scheme, which can achieve more diversity gain in packet combining and improve the ARQ performance. From the results of the theoretical analysis and the computer simulation, it is confirmed that the proposed packet combining ARQ with the proposed operation can achieve the better performance in terms of the average packet transmission success probability. In addition, this proposed scheme is also compared with the conventional multi-carrier modulation ARQ scheme based on the partial retransmission of a packet. The computer simulation results demonstrate that the proposed scheme has also advantage against the latter one, and it is considered to be as a more efficient ARQ scheme for multi-carrier modulation systems.

This paper proposes a fast synchronization scheme with a short preamble signal for high data rate wireless LAN systems using orthogonal frequency division multiplexing (OFDM). The proposed OFDM burst format for fast synchronization and the demodulator for the proposed OFDM burst format are described. The demodulator, which offers automatic frequency control and symbol timing detection, enables us to shorten the preamble length to one quarter that of a conventional one. Computer simulation results show that the degradation in required Eb/N0 due to the synchronization scheme is less than 1 dB in a selective Rayleigh fading channel.

In order to exploit fully the frequency diversity benefits of multicarrier modulation (MCM), and the very nature of the frequency selective radio channel, we investigate an erasure decoded π/4 QDPSK MCM (ED-MCM) by employing simple Hamming (block) code. We propose the threshold-free criteria, i. e. relative minimum receive power test (RMRPT) and relative maximum decision error test (RMDET) for erasure generation and evaluate ED-MCM's performance by applying these tests to average received power, average decision error, instantaneous symbol/bit decision errors. At a normalized delay spread of 1/64, computer simulation results indicate a coding gains of 6.0 - 7.0 dB with ED-MCM at a BER of 10-3. RMDET/RMRPT based erasure decoding yields a 1.5 - 2.5 dB improvement over the conventional forward error correction (FEC) decoding at a BER of 10-5. The simulation results at other normalized delay spreads, i. e. , 1/32, 1/16 are also obtained. The erasure criteria (RMRPT and RMDET) applied to average values of received power/decision error yield consistently better performance over error only decoding. The results indicate that the erasure decoding based on relative (threshold-free) measures clearly promises an improved performance of the MCM system.

As a data transmission rate must be increased as required to support the future high-speed wireless communication systems under multipath fading, the conventional DS-CDMA scheme suffers considerably from an intensive processing requirement for the increased spreading rate to combat the inter-chip interference (ICI) and furthermore, from the intersymbol interference (ISI) as the symbol duration becomes less than the channel delay spread. In this paper, a multi-carrier parallel combinatory DS-CDMA (MC-PC-CDMA) scheme is considered as one possible variant access scheme to realize a bandwidth efficient transmission for high transmission rate while maintaining the beneficial features of the DS-CDMA scheme. This scheme combines the parallel combinatory signaling feature of the existing parallel combinatory CDMA (PC-CDMA) scheme with the orthogonal carrier multiplexing feature of multi-carrier modulation so as to improve the bandwidth efficiency and to reduce the self-interference among the parallel spreading sequences of each user, respectively. This particular system configuration also treats the previously proposed multi-carrier DS-CDMA systems as a special case. Our analysis of the bit error rate for the asynchronous CDMA system investigates the performance characteristics of the proposed system on varying design parameters, and shows the performance comparison with other types of multi-carrier DS-CDMA systems.

This paper describes two dimensional (2D) equalization scheme of orthogonal coding multi-carrier CDMA for reverse link of mobile communication systems. The purpose of the 2D equalization is the reduction of Multiple Access Interference (MAI) which is caused by the random access and the different propagation path from each mobile station. The orthogonal coding multi-carrier CDMA multiplexes all mobile stations' data by Code Division Multiplexing (CDM). The 2D coding scheme spreads a preamble signal at time (in subchannel signals) and frequency (between subchannel signals) domains. The 2D decoding scheme estimates transmission delay time and instantaneous fading frequency from preamble signal for individual mobile stations and compensate the received data using these estimation values to reduce MAI.

This paper proposes a new nonlinear distortion compensation scheme for orthogonal multi-carrier modulation systems. Multi-carrier modulation is an effective technique for high speed digital transmission over time-dispersive channels, however, it is very sensitive to nonlinear distortion. The proposed scheme compensates for the performance degradation due to nonlinear distortion using the maximum likelihood (ML) detection criterion. While the ideal ML receiver requires a huge computational cost and is not feasible, the proposed decision algorithm can effectively reduce the computational cost. Instead of evaluating the likelihood function for all the possible sequences, the proposed scheme examines the sequences which differ by only one bit from the sequence decoded by the conventional receiver. Computer simulation results show that the proposed scheme can effectively compensate for the nonlinear distortion.

Since an orthogonal multi-carrier signal has large peak power, intermodulation distortion occurs due to the non-linearity of the power amplifier. This distortion severely deteriorates the performance of the multi-carrier system. Especially when carriers are modulated by information bits which produce the same phase shift or the alternative phase shift, the modulated signal has maximum peak power at the input of the amplifier. In order to avoid these phase shifts (code sequences), we propose a code reversal technique which suppresses the maximum peak power of multi-carrier signals for intermodulation compensation. This method utilizes the reversal codes which are added to the original information bits. We also show the effectiveness of the code reversal technique combined with error correction coding and examine the optimum operating point of the amplifier.

This paper proposes a family of optimized transmitter and receiver FIR filters for orthogonal frequency division multiplex (OFDM) systems with offset QAM modulation using nonlinear-programming. Two objective functions in the frequency domain (considering both OFDM orthogonal condition and Nyquist condition), least square error (LSE) and minimizing maximal spectral side lobe (Mini-max), are used. The nonlinear programming is implemented with a modified sequential quadratic programming (SQP) algorithm, which guarantees a super-linear convergence. Resultant optimized FIR filters are given with their coefficients and spectra.

This paper describes a delay control scheme for synchronous detection of an orthogonal coding multi-carrier CDMA (Code Division Multiple Access) system. The delay control scheme estimates transmission timing of data from each mobile station. At a base station, delay time is obtained by detecting phase shift value of the preamble signal from each mobile station. The estimated transmission timing information is sent from base station to each mobile station and the mobile station then adjusts its transmission timing. Simulation results clarified that Bit Error Rate (BER) is 2.510-3 at 19dB of Eb/No under conditions of 29.4 msec initial delay time, 32kbit/sec data rate, 16 subchannels and 100Hz of fading frequency.

Recently, considerable attention has been devoted to the combination of direct sequence-code division multiple access (DS-CDMA) and multi-carrier (MC) modulation for high bit rate indoor wireless systems. In this work we consider the downlink of a cellular MC DS-CDMA system. In particular, we evaluate the performance of some detection schemes considering typical indoor radio channels. Channel estimation techniques are investigated and an effective sub-channel estimation technique is proposed. Moreover, we compare the performance of two equalization techniques combined with two space diversity combining schemes. The results show that good performance can be obtained by a post-detection diversity selection scheme combined with a MMSE equalization technique; moreover, if a proper guard time is selected, performance is almost independent of the number of paths of the channel's impulse response. Results are given in terms of BER obtained by means of analytical computations and simulations.

An antenna and multi-carrier combined diversity system using Time Division Duplex (TDD) is proposed to combat with multi-path channel problem which produces frequency-selective fading and degrades the quality of signal transimission. So far multi-carrier modulation technique has been studied to solve this problem. On the other hand, TDD method has been studied to use a transmitter antenna diversity as pre-diversity against flat fading (non-selective fading). Our proposed system merges these two methods in a micro-cellular system as follows. On the reverse link, the base station can select the best combination between the carriers and antennas after receiving the multi-carrier signal from the mobile station. On the forward link, the same combination selected on the reverse link can be used to send the signal from the base station with multi-antenna to the mobile station with a single antenna and produces pre-diversity (transmitter diversity) effect which can reduce the complexity at the mobile station. The pre-diversity must be based on TDD function because the channel has to be observed before the signal transimission. By computer simulations we find that our proposed system can achieve far better performance than conventional systems.

A channel coding which combines convolutional coding and M-ary PSK/orthogonal multi-carrier (MPSK/OMC) transmission is proposed. A coding gain is achieved without sacrificing the data rate or occupying extra bandwidth. The proposed coding formula is that the imformation data bits of bit interval Ts are serial to parallel converted to P parallel branches where each branch has a bit interval Tp = PTs. The data bits of the parallel branches are encoded through a rate nP/(n + 1)(2P - 1) convolutional encoding process and the total (n+1)(2P-1) symbol of the encoder output is transmitted by 2P - 1 OMCs where each carrier is modulated by MPSK/OMC (M = 2n + 1). Following performance analysis of a coding form using rate P/(2P - 1) convolutional encoder and BPSK/OMC modulation, a general channel coding combining convolutional coding and MPSK/OMC modulation is discussed.

This paper proposes a new digitized group modulator for radio base station transmitters of multi-carrier TDMA. This group modulator can flexibly set carrier spacing and features a simple construction as a result of employing the Simple Fractional Sampling technique. A group modulator LSI was designed and built using 0.5-µm BiCMOS technology, and a π/4-shifted QPSK group modulator was constructed using this LSI. Experiments confirm that the modulator simultaneously generates multiple carriers in a wide bandwidth without the need for precise adjustment and there is little difference between each of the carriers in terms of BER performance. Moreover, experiments confirm that the group modulator's burst-output (frequency hopping) performance is excellent.

This paper proposes a new digitized group modulator and demodulator (a group modem) for adaptive frequency hopping and multi-carrier (AFHMC) radio systems. The group modem can flexibly vary the number of carriers handled simultaneously, especially employing a time division multiplexing technique in the demodulator. We discuss the operational principle of the modem. The required operational clock frequency in the group demodulator is also examined and clarified taking into consideration the frequency characteristics of the baseband filter. The basic performance of the proposed configuration is measured experimentally by constructing a π/4-shift QPSK group modulator and a π/4-shift QPSK group demodulator. First, by measuring the output spectrum of the significant parts in the demodulator, we confirm that the basic operational performance conforms to the design specifications. Secondly, investigating the relationship between the number of multiplexed low-pass filter taps and the required CNR when multiple carriers are simultaneously input confirms that more than 40 taps are enough to obtain the best BER performance in this experiment. Next, examining the relationship between the number of carriers simultaneously input, the required CNR, and the input level of these carriers confirm that the required CNR is roughly constant and there is no significant difference among the cases when D/U is more than 0 dB. Finally, an experiment shows that the required number of quantization bits for A/D input in the demodulator is more than 6, which is enough to obtain the best BER even if simultaneous handled carriers are 4.