In this paper, a pilot-assisted channel estimation using adaptive interpolation (in which, different interpolation filter tap weights is used for different symbol position) is proposed. Each set of tap weights is updated using the normalized least mean square (NLMS) algorithm, the reference signal for which is obtained by decision feedback and reverse modulation of the received data symbol. In order to reduce the number of tap weight sets and to achieve fast convergence, the conjugate centrosymmetry property of the tap weight set is used. The average bit error rate (BER) performance in a frequency-selective Rayleigh fading channel is evaluated by computer simulation. Also evaluated is the robustness against the frequency offset between a transmitter and a receiver.

Adaptive prediction iterative channel estimation is presented for combined antenna diversity and coherent rake reception of direct sequence spread spectrum (DSSS) signals. Its first stage uses pilot-aided adaptive prediction channel estimation, while the succeeding iteration stages use decision feedback and moving average filtering for channel re-estimation. The bit error rate (BER) performance of DSSS signal computer simulations evaluate transmission in a frequency selective Rayleigh fading channel. It is found that the adaptive prediction iterative channel estimation is superior to the non adaptive iterative channel estimation using the conventional weighted multi-slot averaging (WMSA) filtering at the first iteration stage, particularly in a fast fading channel.

In multi-carrier code division multiple access (MC-CDMA) uplink (mobile-to-base station), since different users' signals go through different frequency-selective fading channels, large multi-access interference (MAI) is produced. The use of frequency-domain equalization reception can only partially restore the orthogonality among different users' signals, resulting in a severe degradation in the bit error rate (BER) performance. Hence, frequency-domain pre-equalization transmission, which equalizes the MC-CDMA signal before transmission, is recently attracting attention. In this paper, we present a generalized minimum mean square error (GMMSE) frequency-domain pre-equalization transmission suitable for MC-CDMA/TDD uplink. The pre-equalization weight is derived based on the method of Lagrange multipliers. The theoretical analysis of BER performance using the GMMSE frequency-domain pre-equalization transmission in a frequency-selective Rayleigh fading channel is presented and the result is confirmed by computer simulation.

For alleviating the high peak-to-average power ratio (PAPR) problem of orthogonal frequency division multiplexing (OFDM), the OFDM combined with time division multiplexing (TDM) using frequency-domain equalization (FDE) was proposed. In this paper, the theoretical bit error rate (BER) analysis of the OFDM/TDM in a frequency-selective fading channel is presented. The conditional BER expression is derived, based on a Gaussian approximation of the inter-symbol interference (ISI) arising from channel frequency-selectivity, for the given set of channel gains. Various FDE techniques as in multi-carrier code division multiple access (MC-CDMA), i.e., zero forcing (ZF), maximum ratio combining (MRC) and minimum mean square error (MMSE) criteria are considered. The average BER performance is evaluated by Monte-Carlo numerical computation method using the derived conditional BER expression.

The objective of this paper is to develop the theoretical foundation to the pilot-assisted channel estimation using delay-time domain windowing for the coherent detection of OFDM signals. The pilot-assisted channel estimation using delay-time domain windowing is jointly used with polynomial interpolation, decision feedback and Wiener filter. A closed-form BER expression is derived. The impacts of the delay-time domain window width, multipath channel decay factor, the maximum Doppler frequency are discussed. The theoretical analysis is confirmed by computer simulation.

Spatial despreading weight based on minimum mean square error (MMSE) criterion is derived for orthogonal space-time spreading transmit diversity (OSTSTD) in a fast fading channel, taking into account the inter-antenna interference (IAI) and the inter-code interference (ICI) caused by orthogonality distortion of time-domain spreading codes. Average bit error rate (BER) performance is theoretically analyzed and confirmed by computer simulation to show that the diversity gain can be obtained even in a fast fading.

Similar to direct sequence code division multiple access (DS-CDMA), site diversity can be applied to a multicarrier-CDMA (MC-CDMA) cellular system to improve the bit error rate (BER) performance for a user with weak received signal power, thus resulting in an increased link capacity. In this paper, the downlink site diversity reception using frequency-domain equalization based on minimum mean square error (MMSE) is considered for a MC-CDMA cellular system. A set of active base stations to be involved in the site diversity operation is determined based on the received signal power measurement by a mobile station. Downlink capacity with site diversity is evaluated by computer simulation. The impacts of path loss exponent and shadowing loss standard deviation on the site diversity effect are discussed. Furthermore, the performance improvement by antenna diversity reception is discussed.

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

Pilot-aided adaptive prediction channel estimation is proposed for coherent detection in a frequency-nonselective fading channel. It is an extension of the conventional weighted multi-slot averaging (WMSA) channel estimation and consists of 3 steps. A block of Np pilot symbols is periodically transmitted, each pilot block being followed by Nd data symbols to form a data slot. In the first step, the instantaneous channel gain is estimated by coherent addition of Np pilot symbols. Using the K past and K future estimated instantaneous channel gains, the second step predicts the instantaneous channel gains at the end and beginning of data slot of interest by a forward predictor and a backward predictor, respectively. The tap-weights of forward prediction and backward prediction are adaptively updated using the normalized least mean square (NLMS) algorithm. Finally, in the third step, the instantaneous channel gain at each data symbol position within the data slot of interest is estimated by simple averaging or linear interpolation using the two adaptively predicted instantaneous channel gains. The computer simulation confirms that the proposed adaptive prediction channel estimation achieves better bit error rate (BER) performance than the conventional WMSA channel estimation in a fast fading channel and/or in the presence of frequency offset between a transmitter and a receiver.

In direct sequence code division multiple access (DS-CDMA), variable rate transmission can be realized by simply changing the spreading factor SF for the given chip rate. In a frequency-selective fading channel, the transmission performance can be improved by using rake combining. However, when a very low SF is used for achieving a high transmission rate, error floor is produced due to insufficient suppression of inter-chip interference (ICI). In this paper, decision feedback chip-level maximum likelihood detection (DF-CMLD) is proposed that can suppress the ICI. An upper-bound for the conditional bit error rate (BER) is theoretically derived for the given spreading sequence and path gains. The theoretical average BER performance is numerically evaluated by Monte-Carlo numerical computation using the derived conditional BER. The numerical computation results are confirmed by computer simulation of DS-CDMA signal transmission with DF-CMLD.

In this paper, a new 2-antenna transmit diversity, called orthogonal space-time spreading transmit diversity (OSTSTD) combined with delay transmission, is proposed. At the transmitter, N data symbols to be transmitted are spread using N different orthogonal space-time spreading codes (each represented by NN matrix) and are transmitted from two transmit antennas after adding different time delays. At the receiver, 2-step space-time despreading is carried out to recover the N transmitted data symbols. The first step recovers the N orthogonal spatial channels by taking the correlation between the received space-time spread signal and the time-domain spreading codes. The second step recovers the N transmitted data symbols using minimum mean square error (MMSE) despreading. The average bit error rate (BER) performance in a Rayleigh fading channel is evaluated by computer simulation. It is confirmed that the OSTSTD provides better BER performance than the Alamouti's space-time transmit diversity (STTD) at the cost of transmission time delay.

MC-CDMA is an attractive multi-access method for the next generation high-speed mobile communication systems. The uplink transmission performance is limited by the multi-access interference (MAI) from other users since all users share the same bandwidth. Adaptive antenna array can be used to suppress the MAI and to improve the uplink transmission performance. In this paper, we propose a frequency-domain adaptive antenna array for multi-code MC-CDMA. The proposed frequency-domain adaptive antenna array uses a simple normalized LMS (NLMS) algorithm. Although the NLMS algorithm is used, very fast weight convergence within one MC-CDMA symbol duration is achieved since the weight updating is possible as many times as the number of subcarriers within one MC-CDMA symbol duration.

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.