Digital predistortion based on nonlinear system modeling and its inverse is an important technique for the linearization of radio frequency power amplifiers (PAs). Compensation for the nonlinear systems represented by polynomials involves polynomial inverses. In this paper, we propose a fast algorithm to invert a given baseband memoryless polynomial based on the predefined orthogonal basis. The orthogonal basis is predefined based on the distribution of the signal, which is known a priori. Compared with the well-established pth-order inverse method, the proposed algorithm can find the optimal coefficients of the inverse polynomial. Both numerical simulation and experiment with an actual PA demonstrate that the proposed algorithm can provide very good performance for compensating the nonlinear systems represented by baseband polynomial.

In this paper, a method for adaptive cooperation of base stations in cellular systems is presented and its performance is analyzed theoretically. The base station cooperation is selected based on SINR value for a mobile station: if the SINR value is less than a given threshold, then the base station cooperation is carried out, else the base stations communicate with mobile stations independently. Performance of cellular systems has been done by computer simulation, which needs a big software development and a long simulation time. In this paper theoretical analysis of the performance is made under the path-loss, the shadowing and the Rayleigh fading and inter-cell interference. Formula for distribution function of S(I)NR is obtained. By numerical calculation of the distribution of S(I)NR, cell average and cell edge (5%) throughput are given. It is sown that cell edge throughput can be highly increased without significant loss of the average throughput with the adaptive base station cooperation.

As a method to realize an orthogonal multicarrier transmission, offset quadrature amplitude modulation based multi-carrier modulation (OQAM-MCM) has been known, where each subcarrier is strictly band-limited by a filter. Objective of this paper is to discuss the use of OQAM-MCM system instead of orthogonal frequency division multiplexing (OFDM) system as a countermeasure against frequency-selective fading. We investigate the performance of OQAM-MCM system with a small number of subcarriers employing a simplified time-domain per-subcarrier equalizer. Computer simulation results show that OQAM-MCM system can achieve both better spectrum efficiency and lower bit error rate (BER) performance than those of OFDM systems with insufficient guard interval (GI), when a time-domain per-subcarrier equalizer is used in both systems.

In this paper, we propose a weighting factor (WF) iterative estimation method for a turbo-coded multi-carrier code division multiplexing (MC-CDM) system using partial-transmit sequence (PTS) based peak-to-average power (PAPR) reduction, where the transmitter structure in the proposed system is an extended version of the PTS and the systematic bits of turbo-code is adaptively flipped by multiplying WFs so as to reduce PAPR of MC-CDM signal. On the receiver side, WF estimation and error correction are jointly performed with turbo decoding, where WFs are estimated by exploiting the decoding results of two data streams using different spreading code. When PTS like PAPR reduction using 9 clusters is applied to MC-CDM signal with spreading factor of 16, PAPR of the transmit signal at the CCDF of 10-4can be reduced by about 2.0 dB, where the number of multiplexed codes is 8. With the proposed method, WF estimation accuracy is improved as the subcarrier modulation level increases. The degradation in block error rate (BLER) performance as compared with case of the perfect WF estimation is about 0.7 dB at BLER=10-2for QPSK-MC-CDM signal with PAPR reduction using 5 clusters in attenuated 12-path Rayleigh fading condition. In the MC-CDM system using 64QAM, the proposed method achieves almost the same BLER performance as case of the perfect WF estimation, even when the number of clusters is L = 9.

MIMO system is a technology to realize high data rates and high capacity. Among various MIMO systems, Eigenbeam MIMO (E-MIMO) system achieves the theoretical maximum capacity. The E-MIMO system uses eigenvectors of channel autocorrelation matrix as transmit and receive weights. As a method to find these eigenvectors, eigenvalue decomposition or singular value decomposition (SVD) are generally well-known. In this paper, we propose a least mean square (LMS) based algorithm to find the transmit and receive weights without significant increase of computational complexity for E-MIMO system. Computer simulation shows that the proposed algorithm gives almost the same performance as that of SVD.

An iterative optimization method of transmit/receive frequency domain equalization (FDE) is proposed for single carrier transmission systems, where both transmit and receive FDE weights are iteratively determined with a recursive algorithm so as to minimize the error signal at a virtual receiver. The computer simulation results show that SC systems using the proposed transmit/receive equalization method achieves better BER performance than those using the conventional receive FDE. BER performance of SC systems using the proposed method was improved by about 2.7 dB at BER=10-3compared to case of those using conventional receive FDE in attenuated 6-path quasistatic Rayleigh fading with normalized delay spread value of tau/T = 0.769. In addition, when decision feedback equalizer (DFE) with sufficient number of feedback taps is adopted in both systems, the proposed system achieves better BER performance than the conventional system in a low Eb/N0region and BER performance of the proposed system becomes close to that of the conventional one as Et/No increases in the above channel condition.

Adaptive predistorter is an effective technique to compensate nonlinear distortion in a power amplifier. As a method to improve the parameter convergence speed in the predistorter, a series expansion technique with an orthogonal polynomials has been proposed. In this paper, we propose an adaptive predistorter based on orthogonal polynomial for the power amplifier with memory effect, where the parameters of predistorter are determined so as to minimize out-of-band radiation components detected by high pass filter (HPF). With computer simulation using the Advanced Design System (ADS) software, we show that the adaptive predisterter using orthogonal polynomials achieves faster convergence time than that with that of non-orthogonal one, while nonlinear distortion is compensated by memory predistorter using orthogonal polynomials.

In this paper, we propose a peak power reduction method for LDPC coded OFDM system, where transmit data sequence is grouped into several clusters based on structure of a low density parity-check matrix and the phase or each cluster is adjusted so as to minimize PAPR of OFDM signal by multiplying weighting factors (WFs). At the receiver, WFs are estimated with sufficient accuracy by exploiting the decoding property of LDPC code. The proposed method can be applied to not only systematic coded signal but also non-systematic one. Computer simulation results show that, when LDPC code with code-rate of R ap 1/2 is used, PAPR of OFDM signal can be reduced by about 2.4 dB without significant degradation in BLER performance as compared to perfect WF estimation in attenuated 12-path Rayleigh fading condition.

A problem with the OFCDM (orthogonal frequency and code division multiplexing) system is that PAPR (peak to average power ratio) of the signal becomes high. In this paper, we propose a novel peak reduction method which suppresses the inter-signal interference caused by peak power limitation for OFCDM signal. In the proposed method, peak power is reduced by adding the peak reducing signal generated with peak components detected by soft-clipping function. At the receiver, the inter-signal interference is compensated by estimating the peak components removed at the transmitter from the received signal. Performance of a total system including modulation, peak-limiter, predistorter, a model of 2 GHz class A/B power amplifier and the receiver is investigated by computer simulation. Computer simulation results show the peak power reduction performance is improved about 1.4 dB as compared with that of the conventional PRSA (peak reducing signal addition) method.

In this paper, we propose a weighting factor (WF) estimation scheme for phase-control based peak power reduction (PPR) of turbo-coded OFDM signal. In this PPR scheme, the peak power of OFDM signal is reduced by adjusting the phase of parity-carriers by multiplying WFs. At the receiver, WFs are estimated at turbo-decoder without using any side-information. The proposed WF estimation scheme is based on an iterative decoding of turbo-code, i.e. the turbo decoder provides not only error correction capability but also WF estimation function. When a turbo-code of the constraint length of K = 4 and the code rate of R = 1/2 is employed, PAPR of OFDM signal at the CCDF of 10-4can be reduced by about 2.1 dB by applying the PPR scheme. The proposed scheme achieves good BER performance comparable to the case of perfect WF estimation in attenuated 12-path Rayleigh fading condition. The degradation in BER performance is about 1.0 dB, 0.4 dB and 0.2 dB at BER=10-4for turbo-coded OFDM signals using QPSK, 16QAM and 64QAM, respectively

As a method to compensate for the nonlinear distortion of a power amplifier, the adaptive predistorter and the negative feed-back system are known. Although the feedback method is a simple technique, its instability becomes a problem for a high feedback gain to achieve a high compensation effect. On the other hand, the predistorter needs long time to calculate the suitable parameters. In this paper, we propose a nonlinear distortion compensation method for a narrow-band signal. In this method, adaptive predistorter and negative feedback are combined. With computer simulations, we show that the proposed scheme achieves both five times faster convergence speed than that of the predistorter and three times longer permissible delay time in the feed-back amplifier than that of a negative feed-back only amplifier, while keeping the required compensation performance.

A feedback type adaptive array antenna (AAA) system with wide element spacing is analyzed on the downlink channel of a frequency-division duplexed (FDD) cellular system. The AAA is installed on a base station so the downlink channel transfer functions are estimated and fed back on the uplink channel. The widely spaced AAA shows interference suppression as well as diversity effect against multipath fading. We also propose a scheme to apply our AAA system to the handoff mechanism. Moreover, we adopt an extrapolation technique for channel estimation. Computer simulation results showed that the AAA placed only at the base station can cope with cochannel interference and fading for the downlink when the fading speed is slow.

In this paper, we propose a frequency-domain channel estimation scheme using co-phased pilot-signals for a multi-carrier modulation (MCM) system with bandlimited subcarriers: pilot signals are inserted periodically in each subcarrier and are added up (multiplexed) to be inphase at the midpoint of the pilot-symbol duration at the transmitter. Then, the multiplexed pilot signal exhibits a sharp peak in amplitude at the midpoint of the pilot-symbol duration. Thus, the channel characteristics can be estimated with a high time resolution of the pilot signal. We evaluated the performance of an MCM system with a frequency-domain equalizer using the proposed scheme by computer simulation. The BER performance was improved with the channel estimation scheme.

Integrated voice and data transmission enhances the efficiency of channel usage. When it is applied to a multi-cell system, we need a channel assignment method that decreases interference between cells. Voice calls require real time transmission with higher priority over data calls. We investigate a method that decreases the interference between cells by transmitting data packets with priority for the silent periods of circuit-switched-voice communication. Computer simulation results show characteristics such as interference rates, blocking rates, forced terminating rates, and hand-off rates are improved with this method.

Channel estimation is an important technique for compensating the signal against fading effects. Optimal channel estimation requires information of the vehicle speed or the maximum Doppler frequency. We developed a time correlation based maximum Doppler frequency estimator for the optimal channel estimation. We evaluated its performance and applied it to a CDMA RAKE receiver under a multipath fading channel. The proposed scheme can accomplish the features of a wide detection range i.e., from low Doppler frequency to 380 Hz, while achieving a low mean square estimation error of 10% under a multipath environment.

This paper proposes an adaptive channel selection method that reduces signal distortion due to frequency selective fading and thereby improving the performance of mobile communication. In this method a channel is adaptively selected by estimating current channel condition with mean square decision error as a measure. This selected frequency channel corresponds to the channel in which the delayed signals add-up inphase. Hence, BER performance is improved with the proposed scheme. Computer simulation results show the effectiveness of the proposed scheme for a system with multiple terminals and also for a multiple base station (cellular) system.

Discusses the performance, in terms of residual timing error, of the over-the-air autonomous decentralized interbase-station synchronization system. An exact solution for the uniform weights case and an approximate solution for the power weights case are obtained in closed form. The residual timing error, system stability, and system convergence performance are studied based on the closed-form solutions. It is shown that timing can converge very fast for the uniform weights case; the power weights case has lower residual timing error measured in terms of standard deviation than that of the uniform weights case when all transmitted power levels are the same.

Diversity reception is common for land mobile radio systems. In time division duplex (TDD) systems, transmission diversity as well as diversity reception can be applied for base stations because transfer characteristic correlation is high between uplink and downlink. Nevertheless, transmission diversity can not be applied for control channels, on which paging and other information is transferred, since there are no previous uplink signals. A new control channel transmission strategy to improve the error rate performance is proposed. In this strategy, a base station transmits control channel bursts repeatedly by switching diversity branch or antenna directivity. Computer simulations and experiments show that the strategy has 4-6 dB gain to the conventional one.

Channel segregation, a distributed dynamic channel assignment method is applied to small cell systems in order to avoid intermodulation interference (IMI). Computer simulation is made to investigate the channel selection pattern achieved with this scheme for the assumed systems affected by IMI. The obtained channel selection patterns are shown to be free from any possible IMIs caused by the 3rd order products of the selected channels.

In the multipath mobile channel, the received signal is affected by both flat fading and selective fading. Using an adaptive array is an effective method to compensate for selective fading, whereas a diversity antenna is good at compensating for flat fading. In this paper, a switching adaptive/diversity antenna is presented and examined with computer simulation. For evaluating the signal whose quality falls repeatedly within a short duration, the probability of DUR (desired to undesired signal ratio) below a given value is adopted. The computer simulation results show that the proposed switching adaptive/diversity antenna can compensate for both flat fading and selective fading.

High spectrum efficiency is promised by reuse partitioning in a cellular system. We propose an improved algorithm for self-organized reuse partitioning (SORP), a distributed dynamic channel assignment scheme, in order to obviate the need for broadcasting transmit power level information of each mobile terminal. The concept of the improved scheme is simple: a mobile terminal with low transmit power uses a low numbered channel throughout the system. Reuse partitioning can be achieved owing to such a channel segmentation. The algorithm is designed to realize the channel segmentation without the broadcasting of the transmit power level. Computer simulation results show that a high capacity and short delay time in finding an appropriate channel can be obtained. Furthermore, a higher capacity can be obtained by introducing quality based power control into this algorithm.

An adaptive equalizer is proposed for a nonlinear transmission system, i.e., frequency modulation with limiter-discriminator detection. In this method, a linear feedback equalizer at IF band is controlled by evaluating the target function which is defined as mean square error at decision sampling instants. The principle of this method is based on the fact that frequency demodulated signals take given discrete values at the sampling instants if the equalization succeeds. As the adaptive algorithm, the authors adopt the Powell method, that is a general method of search for the minimum value. They propose to repeat the Powell method in order to reach the minimum value avoiding local minimum points. A computer simulation experiment is carried out assuming a two-level FM system with limiter-discriminator detection under two- and three-ray models including minimum- and non-minimum phase shift conditions. The convergence of the equalizer to the minimum value is confirmed. Together with equalization characteristics error rate performance is shown.>

The umbrella cell system, where the same radio system is used for microcells and overlaying macrocells, is the most promising strategy for deploying microcell service to cope with increased portable radio subscribers. A practical approach to implementing a microcell system overlaid with an existing macrocell system is proposed. There is no need to design the channel assignment and transmit power management for the microcell system by introducing the channel segregation, a self-organized dynamic channel assignment and the automatic transmit power control. The system channels are reused automatically between the macrocells and microcells. The interference from the macrocell to the microcell is compensated by slight increase of transmit power for the microcell system. It is shown by computer simulation that locally increased traffic is accommodated with the microcells laid under macrocells without any effort for channel management.>

Channel segregation was previously proposed as a self-organized dynamic channel assignment. Its performance was examined by applying it to frequency-division multiple access systems. Its applications to the TDMA/FDMA (time-division multiple access/frequency-division multiple access) or multicarrier TDMA system is discussed. The spectrum efficiency of the TDMA/FDMA cellular system deteriorates due to the problem of unaccessible channel: a call can be blocked in a cell even when there are idle channels because of the restriction on simultaneous use of different carrier frequencies in the cell. It is shown that channel segregation can resolve this problem with a small modification of its algorithm. The performance of the TDMA/FDMA system with channel segregation on the call blocking probability versus traffic density is analyzed with computer simulation experiments. The effect of losing the TDMA frame synchronization between cells on the performance is also discussed.>

Although linear modulation methods can achieve high spectrum efficiency, very little attention has been directed to their use in mobile radio systems. This is mainly due to the fact that the nonlinearity of the transmitter power amplifier tends to spread the spectrum and thus eliminate any spectrum efficiency advantage gained through the use of linear modulation methods. In this paper, a linear modulation system is proposed, which solves the above difficulty and which gives higher spectrum efficiency than conventional digital FM. The modulation/demodulation method is\pi /4shift QPSK and phase-shift detection with a limiter-discriminator and an integrate-and-dump filter. By introducing a cartesian coordinate negative feedback control, 35 percent power efficiency at 10 W output power and - 60 dB relative out-of-band radiation are simultaneously achieved with a class "AB" amplifier, owing to the 29 dB feedback gain. The receiver configuration is easy to realize and gives immunity against fast fading through the use of noncoherent detection with limiter-discriminator. By using a novel decision method, bit error rate performances under both nonfading and fading condition are comparable to those obtained by digital FM. These results make it possible for linear modulation methods to achieve higher spectrum efficiency than is possible with conventional digital FM methods in mobile radio communications.