A cognitive radio will have to sense and discover the spectral environments where it would not cause primary radios to interfere. Because the primary radios have the right to use the frequency, the cognitive radios as the secondary radios must detect radio signals before use. However, the secondary radios also need identifying the primary and other secondary radios where the primary radios are vulnerable to interference. In this paper, a method of simultaneously identifying signals of primary and secondary radios is proposed. The proposed bandwidth differentiation assumes the primary and secondary radios use orthogonal frequency division multiplexing (OFDM), and the secondary radios use at the lower number of subcarriers than the primary radios. The false alarm and detection probabilities are analytically evaluated using the characteristic function method. Numerical evaluations are also conducted on the assumption the primary radio is digital terrestrial television broadcasting. Result showed the proposed method could achieve the false alarm probability of 0.1 and the detection probability of 0.9 where the primary and secondary radio powers were 2.5 dB and 3.6 dB higher than the noise power. In the evaluation, the reception signals were averaged over the successive 32 snapshots, and the both the primary and secondary radios used QPSK. The power ratios were 4.7 dB and 8.4 dB where both the primary and secondary radios used 64QAM.

Future broadband mobile communication systems are necessary to achieve the bit rates of 100 Mbit/s. Orthogonal Frequency Division Multiplexing (OFDM) transmission is an attractive technology because it can remove the influence of frequency selective fading in broadband transmission by adding a suitable guard interval to each OFDM symbol. However, peak-to-average power ratio (PAPR) is very large in OFDM transmission. In this paper, we propose a new PAPR reduction method which can be applied even when unusable bands are inside the system band. In the proposed method, peak reduction signals are generated by iterative signal processing only in the usable frequency band, and filtering to remove out-of-band components of the peak reduction signals is incorporated into the iterative signal processing. The results of computer simulation show that the proposed method can effectively reduce peak power without expanding the spectrum both outside the system band and into unusable bands inside the system band. By using the proposed method, the broadband mobile communication system with low peak power and high flexibility of frequency band use can be realized.

We have developed a code-division-multiplexing (CDM) transmission scheme for future cellular communication systems, which uses cyclic shifted-and-extended (CSE) codes generated from an M-sequence to enable seamless communication in highly mobile environments. Because the correlation characteristics of CSE codes are determined by the M-sequence, the cross-correlation values are accumulated as a result of combining transmitted signals with opposite polarities in parallel channels. The accumulated cross-correlation values significantly degrade transmission performance, especially with multi-level modulation schemes such as quadrature amplitude modulation (QAM). We thus propose a cancellation technique to eliminate the accumulated cross-correlation values. We have evaluated the transmission performance of the CDM transmission scheme with the proposed technique by computer simulation. The new scheme enables high-quality data transmission in fast-fading channels.

This paper proposes a new correction technique for a linear amplification with nonlinear components (LINC) transmitter. The technique, which is based on the minimum mean squared error (MMSE) criterion, estimates the gain and phase imbalance between the two amplifier branches. With information on the estimation, the imbalance is offset by controlling the amplitude and phase of the input signal that is fed into one of the two amplifiers. Computer simulations with a DS-CDMA system demonstrate that this method can compensate for the imbalance and sufficiently suppress the out-of-band distortion spectrum.

In DS-CDMA mobile radio communications systems, transmit power control (TPC) is indispensable to regulate the variations in the received signal power produced by multipath fading. However, a practical TPC raises and lowers the mobile transmit power only at discrete time instants (the TPC rate is on the order of 1-2 kHz) and by a finite step size of the order of 1 dB. Therefore, TPC cannot completely compensate the received signal power variations and hence, the transmission performance degrades in a fast fading channel. The objective of this paper is to understand how TPC acts in a fast fading channel with antenna diversity reception and, based on this understanding, to model the TPC operation.

This paper proposes a new method to estimate the channel impulse response for Orthogonal Frequency Division Multiplexing (OFDM) mobile radio transmission. The method employs the Recursive Least Squares (RLS) algorithm so as to exploit the correlations in frequency and time domains, and can improve the estimation accuracy. It is also applicable to both the regular and scattered pilot schemes. Computer simulations demonstrate effectiveness of the method applied to the scattered pilot scheme.

This paper presents the results of field experiments on the pilot symbol assisted (PSA) coherent multistage interference canceller (COMSIC) receiver in the direct sequence code division multiple access (DS-CDMA) reverse link. The implemented COMSIC receiver comprising three cancellation stages employs PSA channel estimation and replica generation of multiple access interference (MAI) of other users. The experimental results demonstrate that the COMSIC receiver associated with antenna diversity reception and fast transmission power control (TPC) exhibits effectiveness in suppressing severe MAI in actual multipath fading channels. The transmission power of a mobile station (MS) when the COMSIC receiver is employed at a base station (BS) is reduced by approximately 2.0 and 4.0 dB compared to that with the matched filter (MF)-based Rake receiver when the ratios of the target signal energy per bit-to-interference power spectrum density ratio (Eb/I0) of the desired user to the target user are Δtarget= -6 and -9 dB, respectively. Furthermore, for the COMSIC receiver, the transmission power of a MS at the average bit error rate (BER) of 10-3 with antenna diversity is decreased by approximately 7.5 and 11 dB compared to that without antenna diversity when the Δtarget values are -6 and -9 dB, respectively.

This paper proposes a new blind algorithm effective for multiuser detection with an adaptive array antenna. The conventional blind algorithm, known as the Constant Modulus Algorithm (CMA), has two major drawbacks: (i) the convergence speed is not sufficiently fast for usual applications in mobile communications, and (ii) the algorithm is very likely to lock on the path with the largest received power, which means the signal with the second largest power can hardly be extracted. This paper introduces the Recursive Least Squares algorithm for CMA (RLS-CMA) in order to speed the convergence up, and additionally introduces the concept of the orthogonal projection into CMA so as to extract signals with weak power. The proposed CMA with Orthogonal Projection (CMA-OP) sequentially calculates the weight vector of each user under a constraint that the weight vector should be orthogonal to the estimated array response vectors of previously extracted users. Computer simulations demonstrate that the proposed scheme can operate properly in the Rayleigh fading channels under the two-user condition.

This paper evaluates the bit error rate (BER) performance of high rate data transmission such as at 64 and 384 kbits/s (kbps) with high quality (average BER is below 10-6) using turbo/convolutional coding associated with Rake time diversity, antenna diversity, and fast transmission power control (TPC) in multipath fading channels for W-CDMA mobile communications. Laboratory experiments using multipath fading simulators elucidate the superiority of turbo coding over convolutional coding when the channel interleaving length is 40 msec. The required average transmission power for the average BER of 10-6 using turbo coding is decreased by approximately 1.1-1.5 dB and 1.5-1.6 dB for 64 and 384 kbps data transmissions, respectively, compared to that using convolutional coding for a two-path Rayleigh fading channel with the fading maximum Doppler frequency of fD = 5-200 Hz. Furthermore, field experimental results elucidate that the required transmission power for the average BER of 10-6 employing turbo coding is decreased by approximately 0.6 dB and 2.0 dB compared to convolutional coding for 64 and 384 kbps data transmissions, respectively, without antenna diversity reception, while that with antenna diversity reception exhibits only an approximate 0.3-0.5 dB decrease. This decrease in improvement with antenna diversity reception indicates that in an actual fading channel in the field experiments, the impact of the error in path search for Rake combining and SIR measurement for fast TPC diminishes the performance improvement of the turbo coding due to a very low received signal power.

This paper elucidates through experiments the improvement in the achievable bit error rate (BER) performance when space time transmit diversity (STTD) is applied to the wideband direct sequence code division multiple access (W-CDMA) forward link. First, laboratory experimental results clarify that the received path timing difference of transmitted signals from two antennas, due to the propagation delay, should be within a chip duration of approximately 1/4 and 1/2 with and without fast transmit power control (TPC), respectively, in order to achieve a prominent transmit diversity effect. We show that the required average received signal energy per bit-to-background noise spectrum density (Eb/N0) at the average BER of 10-3 using STTD is decreased by approximately 4.2 (1.7) dB compared to the case of single-antenna transmission at the maximum Doppler frequency, fD, of 5 Hz without (with) antenna diversity reception at a mobile station (MS) due to the increasing randomization effect of burst error. Furthermore, we elucidate that although the gain of STTD in field experiments is decreased compared to that in laboratory experiments, since the degradation in path search accuracy is greater due to the frequently changing delay time of each path in a real multipath-fading channel, the required average received signal energy per bit-to-interference plus background noise power spectrum density ratio (Eb/I0) at the average BER of 10-3 with STTD is decreased by approximately 1.3 to 1.5 (0.7 to 1.0) dB without (with) antenna diversity reception when fast TPC is not applied in the forward link. This indicates that STTD is effective for a channel without TPC such as a common control channel in a real multipath-fading channel.

In wideband direct sequence code division multiple access (W-CDMA), employing an adaptive antenna array is a very promising technique to reduce severe multiple access interference (MAI) especially from high rate users. This paper proposes a fast and accurate two-step beam tracking algorithm implemented in a pilot symbol-assisted coherent adaptive antenna array diversity (CAAAD) receiver and evaluates its performance both by computer simulation and laboratory experiments. In the proposed scheme, the receiver antenna weights are updated by using both the signal-to-interference power ratio (SIR) measurements employing multiple sets of antenna weights (MSAW) and an adaptive algorithm based on the minimum mean square error (MMSE) criterion, in which other sets except for a original set of antenna weights are simply generated by a original set. Computer simulation results show that antenna weights of a four-antenna CAAAD receiver using the proposed beam tracking algorithm tracks changes in the direction of arrival (DOA) of the desired user at up to 34.3 degrees/sec, which corresponds to 215 km/h at 100 m from a base station. We also confirm based on the experiments in a radio anechoic room that the generated antenna weights track the DOA changes up to 12.3 degrees/sec.

This paper investigates the influence of the number of antennas, the angle difference between the direction of arrival (DOA) of the desired signal and those of interfering signals, and the antenna arrangement on the BER performance of the coherent adaptive antenna array diversity (CAAAD) receiver in the wideband DS-CDMA (W-CDMA) reverse link. Experiments assuming high-rate interfering users were conducted in a radio anechoic room using a three-sectored antenna with a 120-degree beam (maximum number of antennas was six). The experimental results showed that the degree to which the interference was suppressed from high-rate users of the CAAAD receiver was significantly increased by increasing the number of antennas, especially when the number of interfering users was larger than degree of freedom of the CAAAD. It was also verified that although the BER performance of the CAAAD receiver significantly improved compared to a single sectored antenna, the improvement remarkably decreased when the DOA difference between the desired signal and interfering signals was within approximately 10-15 degrees irrespective of the number of antennas. Finally, we show that the BER performance difference between the linear and conformal arrangements was small when using the three-sectored antenna.

In DS-CDMA (including W-CDMA), a received signal can be resolved into multiple paths to be Rake combined. An important design problem of the Rake receiver is how to accurately search the paths with a sufficiently large signal-to-interference plus background noise power ratio (SIR). This paper investigates the performance of a coherent Rake receiver using pilot symbol-assisted channel estimation with fast transmit power control, and thereby optimizes three key parameters: the total averaging period, Tavg, consisting of a combination of coherent summation and power summation; each period of the summations for measuring the average power delay profile; and path-selection threshold M from the generated power delay profile. We used a path search algorithm, which searches the paths that have M times greater average signal power than the interference plus background noise power measured in the average power delay profile generated using time-multiplexed pilot symbols. It was clarified by both simulation and laboratory experiments that when M = 4, Tavg = 50-100 msec, and the number of slots for coherent accumulation R = 2, the required average transmit Eb/N0 for obtaining the average BER of 10-3 is almost minimized with and without antenna diversity for both ITU-R Vehicular-B and average equal power L-path delay profile model, in which each path suffered independent Rayleigh fading. The paper also shows that based on the field experiments, the path search algorithm with optimized path-selection parameters is robust against actual dynamic changes in the power delay profile shape.

A RAKE combiner based on a matched filter (MF) can be relatively easily implemented since the despread signal components that have propagated along different paths appear sequentially at the MF output. An important design problem is how to accurately select the paths having sufficiently large signal-to-noise power ratios (SNRs). This paper proposes a simple path selection algorithm that uses two selection thresholds. The first threshold is to select the paths that provide largest SNRs. However, as the total received signal power (sum of the signal powers of all paths) decreases, some of the selected paths become noisy. Therefore, we introduce a second threshold that discards the noisy or noise-only paths from among those selected by the first threshold. We apply the proposed path selection algorithm to a pilot symbol-assisted coherent RAKE combiner and find by computer simulations a near optimum set of the two thresholds in frequency selective multipath Rayleigh fading channels. Several power delay profile shapes are considered. The simulation results demonstrate that the MF-based RAKE combiner with the two selection thresholds can achieve a bit-error-rate (BER) performance close to the ideal case (i. e. , the paths to be used for RAKE combining are selected for each power delay profile such that the required signal energy per information bit-to-noise spectrum density ratio (Eb/N0) is minimized).

To further increase the capacity of the DS-CDMA reverse-link, this paper investigates the effectiveness of interference rejection weight control (IRWC) for the pilot symbol-assisted coherent multistage interference canceller (PSA-COMSIC) using recursive channel estimation (RCE). First, a bit error rate (BER) expression of the serial (successive) and parallel type hard decision multistage interference canceller (MSIC) with IRWC using Gaussian approximation for multiple access interference (MAI) are presented for no fading channels. It is theoretically shown that IRWC is effective in mitigating the interference replica generation error in hard decision MSIC. Next, the BER performance of PSA-COMSIC using IRWC in a multipath Rayleigh fading channel when channel coding is applied is evaluated by computer simulations. The BER performance and capacity are evaluated not only for the conventional serial and parallel types but also for serial/parallel (S/P) hybrid type and non-linear/linear (N/L) hybrid type schemes, both of which are effective in significantly reducing the demodulation processing delay. The simulation results demonstrate that, in interference-limited channels where the back ground noise is negligible, the capacity of serial type PSA-COMSIC using IRWC is about 10% higher than that without IRWC. It is also found that if we can accept a slight capacity degradation compared to the serial type PSA-COMSIC, S/P hybrid schemes are effective in reducing the demodulation processing delay.

Co-channel interference is a major deteriorating factor limiting the capacity of mobile communication systems. To mitigate the effect of the interference, a kind of nonlinear interference canceller named trellis-coded co-channel interference canceller (TCC) has been proposed. In TCC the trellis-coded modulation (TCM) is introduced to both the desired signal and the interference signal in order to enhance the cancelling performance. In this paper, the bit error rate (BER) performance of TCC in static channel is theoretically evaluated for the first time. An equivalent TCM (E-TCM) model is firstly established, and a BER asymptotic estimate (AE) and a BER upper bound (UB) of TCC are then evaluated respectively by analyzing E-TCM. In the evaluation of AE, the BER performance is calculated as a function of phase difference between the desired signal and the interference signal (φ), subsequently the average BER performance over φ can be evaluated. The UB of BER is calculated using a transfer function based on the matrix representation. This paper also demonstrates that AE gives higher accuracy and less calculation complexity than UB. Performance comparisons reveal the consistency of these theoretical results with that of computer simulations.

The bit error rate (BER) performance against average Eb/No (signal energy per bit-to-noise power spectral density ratio) and the capacity of the pilot symbol-assisted coherent orthogonal filter (PSA-COF) based Rake receiver with fast transmit power control (TPC) are evaluated in DS-CDMA reverse link under multipath Rayleigh fading. Fast TPC, which controls all signals transmitted from users in the same cell or sector such that they are received with equal power at the cell site under fast Rayleigh fading, is essential for the PSA-COF based Rake receiver in the reverse link in order to improve the performance degradation experienced when the received signal level drops due to fading as the transmit power is limited in practical systems. Signal-to interference plus noise power ratio (SINR) based fast transmit power control (TPC) is assumed here. By using the fast TPC in reverse link and applying the PSA-COF based Rake receiver to base station (BS), the transmit power of each mobile station (MS) can be significantly reduced, thus increasing link capacity. It is demonstrated that the capacity of the PSA-COF based Rake receiver is about 1.5 times higher than that of the conventional matched filter (MF) receiver in interference-limited channels.

In this paper, we describe effects of oddcorrelation functions and band-limitation filters for direct-wave reception systems using broadband spread-spectrum (B-SS) techniques. The receiver of this system is synchronized to the direct-wave. First, the effects of odd-correlation functions are investigated by using M-sequences and random sequences. The effects of even-correlation functions for those sequences can be easily obtained by using results of effects of odd-correlation functions for random sequences. Here we derive a novel function of odd-correlation variance for M-sequence, which is obtained theoretically. Consequently, we show the advantage of M-sequence which is used as spreading sequence. As a reason, in the odd-correlation function of M-sequence, small values are taken near the synchronous phase where harmful scattered-waves exist, strongly. Next, the effects of both odd-correlation function and band-limitation filter are studied by using several kinds of filters. Here we discuss the difference of characteristics in case that despreading sequence of bandlimited pulse or that of rectangular pulse is used in the correlator of the receiver. The technique despreading by rectangular pulse can be achieved a high speed signal processing and equipment miniaturizing because of utilization of switching circuit. We show the advantage of despreading sequence of rectangular pulse, when the limitation bandwidth of transmitting signal takes a small value. Because the characteristics of the correlation function between transmitting sequence of bandlimited pulse and despreading sequence of rectangular pulse can be kept better than that between the transmitting sequence and despreading sequence of bandlimited pulse. As these results, in severe bandlimited direct-wave reception systems using B-SS techniques, M-sequence of rectangular pulse as despreading sequence is most suitable.

A pilot symbol-assisted coherent multistage interference canceller (PSA-COMSIC) using recursive channel estimation is proposed for DS-CDMA mobile radio cellular systems. In the proposed scheme, since the channel variation due to fading is recursively estimated at each interference canceling stage, the accuracy of channel estimation is successively improved. The bit error rate (BER) performances against average Eb/N0 (signal energy per bit-to-noise power spectral density ratio) and capacity in the isolated cell are investigated by computer simulations. The simulations demonstrate that the capacity using the PSA-COMSIC with recursive channel estimation is about 1.6 times higher than that of the conventional matched filter receiver with channel coding and bit-interleaving in the interference-limited environments.

A multi-terminal serial optical link(MSOL) achieves very simple and cost effective radio cell configurations because only one pair of fibers is needed. In addition, low cost Fabry-Perot laser diodes(FP-LDs) can be employed. MSOL has a substantial problem in that the beat noise degrades the C/N in the up-link. To reduce this noise, we propose using an automatic wavelength-offset control(AWOC) circuit. The AWOC circuit offsets the LD wavelength by controlling the laser bias current to minimise the RF band beat noise which is inherent in MSOL systems, and enables high C/N optical-microwave transmission. An experimental MSOL consisting of 5 radio access stations, each equipped with AWOC, is constructed to estimate the noise free dynamic range for 800-MHz 20-carrier signal transmission. The up-link comprises a single mode fiber connecting five 1.3-µm FP-LDs operating at 0.2 mW. The down-link consists of a single mode fiber and one 1.3-µm Distributed Feedback type Laser Diode(DFB-LD) emitting at 4.0 mW. The experimental device achieves over 15 dB noise reduction compared to MSOL without AWOC in the temperature range of 0 to 40. By using the proposed AWOC, MSOL can achive low cost optical fiber RF microcell systems that are easy to install. Additionally, when we install MSOL in the radio base station, the links become more cost effective than coaxial cable links; they offer a wide dynamic range and higher transmission quality.