Frequency-domain equalization (FDE) based on the minimum mean square error (MMSE) criterion can provide a better bit error rate (BER) performance than rake combining. To further improve the BER performance, cyclic delay transmit diversity (CDTD) can be used. CDTD simultaneously transmits the same signal from different antennas after adding different cyclic delays to increase the number of equivalent propagation paths. Although a joint use of CDTD and MMSE-FDE for direct sequence code division multiple access (DS-CDMA) achieves larger frequency diversity gain, the BER performance improvement is limited by the residual inter-chip interference (ICI) after FDE. In this paper, we propose joint FDE and despreading for DS-CDMA using CDTD. Equalization and despreading are simultaneously performed in the frequency-domain to suppress the residual ICI after FDE. A theoretical conditional BER analysis is presented for the given channel condition. The BER analysis is confirmed by computer simulation.

Frequency-domain equalization (FDE) based on the minimum mean square error (MMSE) criterion can increase the downlink bit error rate (BER) performance of DS-CDMA beyond that possible with conventional rake combining in a frequency-selective fading channel. FDE requires accurate channel estimation. Recently, we proposed a pilot-assisted channel estimation (CE) based on the MMSE criterion. Using MMSE-CE, the channel estimation accuracy is almost insensitive to the pilot chip sequence, and a good BER performance is achieved. In this paper, we propose a channel estimation scheme using one-tap recursive least square (RLS) algorithm, where the forgetting factor is adapted to the changing channel condition by the least mean square (LMS) algorithm, for DS-CDMA with FDE. We evaluate the BER performance using RLS-CE with adaptive forgetting factor in a frequency-selective fast Rayleigh fading channel by computer simulation.

Single-carrier (SC) multiple access is a promising uplink multiple access technique because of its low peak-to-average power ratio (PAPR) property and high frequency diversity gain that is achievable through simple one-tap frequency-domain equalization (FDE) in a strong frequency-selective channel. The multiple access capability can be obtained by combining either frequency division multiple access (FDMA) or code division multiple access (CDMA) with SC transmission. In this article, we review the recent research on the SC multiple access techniques with one-tap FDE. After introducing the principle of joint FDE/antenna diversity combining, we review various SC multiple access techniques with one-tap FDE, i.e., SC-FDMA, SC-CDMA, block spread CDMA, and delay-time/CDMA.

Recently, a new frequency-domain equalization (FDE) technique, called overlap FDE, that requires no GI insertion was proposed. However, the residual inter/intra-block interference (IBI) cannot completely be removed. In addition to this, for multicode direct sequence code division multiple access (DS-CDMA), the presence of residual inter-chip interference (ICI) after FDE distorts orthogonality among the spreading codes. In this paper, we propose an iterative overlap FDE for multicode DS-CDMA to suppress both the residual IBI and the residual ICI. In the iterative overlap FDE, joint minimum mean square error (MMSE)-FDE and ICI cancellation is repeated a sufficient number of times. The bit error rate (BER) performance with the iterative overlap FDE is evaluated by computer simulation.

Orthogonal multi-carrier direct sequence code division multiple access (orthogonal MC DS-CDMA) is a combination of orthogonal frequency division multiplexing (OFDM) and time-domain spreading, while multi-carrier code division multiple access (MC-CDMA) is a combination of OFDM and frequency-domain spreading. In MC-CDMA, a good bit error rate (BER) performance can be achieved by using frequency-domain equalization (FDE), since the frequency diversity gain is obtained. On the other hand, the conventional orthogonal MC DS-CDMA fails to achieve any frequency diversity gain. In this paper, we propose a new orthogonal MC DS-CDMA that can obtain the frequency diversity gain by applying FDE. The conditional BER analysis is presented. The theoretical average BER performance in a frequency-selective Rayleigh fading channel is evaluated by the Monte-Carlo numerical computation method using the derived conditional BER and is confirmed by computer simulation of the orthogonal MC DS-CDMA signal transmission.

The performance of single-carrier (SC) transmission in a frequency-selective fading channel degrades due to a severe inter-symbol interference (ISI). Using frequency-domain equalization (FDE) based on the minimum mean square error (MMSE) criterion can improve the bit error rate (BER) performance of SC transmission. However, the residual ISI after FDE limits the performance improvement. In this paper, we propose a joint use of Tomlinson-Harashima precoding (THP) and FDE to remove the residual ISI. An approximate conditional BER analysis is presented for the given channel condition. The achievable average BER performance is evaluated by Monte-Carlo numerical computation method using the derived conditional BER. The BER analysis is confirmed by computer simulation of the signal transmission.

The use of frequency-domain equalization based on minimum mean square error criterion (called MMSE-FDE) can significantly improve the bit error rate (BER) performance of DS-CDMA signal transmission compared to the well-known coherent rake combining. However, in a DS-CDMA cellular system, as a mobile user moves away from a base station and approaches the cell edge, the received signal power gets weaker and the interference from other cells becomes stronger, thereby degrading the transmission performance. To improve the transmission performance of a user close to the cell edge, the well-known site diversity can be used in conjunction with FDE. In this paper, we consider DS-CDMA downlink site diversity with FDE. The MMSE site diversity combining weight is theoretically derived for joint FDE and antenna diversity reception and the downlink capacity is evaluated by computer simulation. It is shown that the larger downlink capacity can be achieved with FDE than with coherent rake combining. It is also shown that the DS-CDMA downlink capacity is almost the same as MC-CDMA downlink capacity.

In this paper, we employ time-reversal space-time block coding (TR-STBC) in single-carrier direct sequence code-division multiple access (DS-CDMA) block transmission in the presence of multiple access interference (MAI) as well as intersymbol interference (ISI), which is subject to fairly long delay spread. We introduce the transmission rate improvement by capitalizing on the assignment of additional spreading codes to each user so as to expand the cardinality of space-time code matrix with no sacrifice of diversity order. Given perfect channel state information at the receiver, a simple linear frequency-domain interference suppression scheme on a basis of symbol-by-symbol processing is developed under certain circumstances. A "turbo principle" receiver is facilitated by exploiting the serially concatenated structure at the transmitter to further enhance system performance. Simulation results justify the efficacy of our proposed system and also present performance comparisons with some existing systems in terms of bit error rate (BER).

A cyclic prefix reconstruction scheme is proposed for precoded single-carrier systems with frequency-domain equalization (SC-FDE) that employ insufficient length of cyclic prefix. For SC-FDE, cyclic prefix is employed to facilitate frequency-domain equalization at the receiver. Since inserting cyclic prefix incurs a loss in bandwidth-utilization efficiency, it is desirable to limit the length of cyclic prefix for SC-FDE. This paper designs the energy spreading transform (EST), a precoder that enables iterative reconstruction of missing cyclic prefix. The performance of the proposed scheme is shown to be close to that of SC-FDE with enough length of cyclic prefix.

Recently, the decision feedback channel estimation based on the minimum mean square error criterion (DF-MMSE-CE) using a fixed DF filter coefficient has been proposed to improve the channel estimation accuracy for DS-CDMA with frequency-domain equalization (FDE). In this paper, we propose adaptive DF (ADF)-MMSE-CE, in which the DF filter coefficient is adapted to changing channel conditions based on a recursive least square (RLS) algorithm. Furthermore, the channel estimate is phase corrected upon the reception of the periodically inserted pilot chip blocks. The average BER performance of DS-CDMA with MMSE-FDE using ADF-MMSE-CE is evaluated by computer simulation in a frequency-selective Rayleigh fading channel and the simulation results show that our proposed scheme is very robust against fast fading.

Frequency-domain equalization (FDE) based on minimum mean square error (MMSE) criterion can replace the conventional rake combining to significantly improve the bit error rate (BER) performance in a frequency-selective fading channel. MMSE-FDE requires an accurate estimate of the channel transfer function and the signal-to-noise power ratio (SNR). Direct application of pilot-assisted channel estimation (CE) degrades the BER performance, since the frequency spectrum of the pilot chip sequence is not constant over the spreading bandwidth. In this paper, we propose a pilot-assisted decision feedback frequency-domain MMSE-CE. The BER performance with the proposed pilot-assisted MMSE-CE in a frequency-selective Rayleigh fading channel is evaluated by computer simulation. It is shown that MMSE-CE always gives a good BER performance irrespective of the choice of the pilot chip sequence and shows a high tracking ability against fading. For a spreading factor SF of 16, the Eb/N0 degradation for BER=10-4 with MMSE-CE from the ideal CE case is as small as 0.9 dB (including an Eb/N0 loss of 0.28 dB due to the pilot insertion).

This paper proposes a frequency diversity scheme using only even-numbered samples for single-carrier transmission with frequency-domain equalization (SC-FDE). In the proposed scheme, a periodical frequency spectrum generated by using only even-numbered samples in the time domain provides the frequency redundancy, which is utilized for frequency diversity. Moreover, in order to avoid the data rate reduction due to the decrease in the samples within one block, the high-level modulation is applied to each sample and the transmitting power of each sample can be doubled for the equivalent power transmission instead. Computer simulation results show that the proposed scheme provides a steeper BER curve than the typical SC-FDE over frequency selective fading channels, while the typical SC-FDE is more favorable than the proposed scheme over flat fading channels. Moreover, the proposed scheme still retains its characteristic even when channel estimation and channel coding are additionally taken into account.

It is well-known that, in DS-CDMA downlink signal transmission, frequency-domain equalization (FDE) based on minimum mean square error (MMSE) criterion can replace rake combining to achieve much improved bit error rate (BER) performance in severe frequency-selective fading channel. However, in uplink signal transmission, as each user's signal goes through a different channel, a severe multi-user interference (MUI) is produced and the uplink BER performance severely degrades compared to the downlink. When a small spreading factor is used, the uplink BER performance further degrades due to inter-chip interference (ICI). In this paper, we propose a frequency-domain multi-stage soft interference cancellation scheme for the DS-CDMA uplink and the achievable BER performance is evaluated by computer simulation. The BER performance comparison of the proposed cancellation technique and the multi-user detection (MUD) is also presented.

In this paper, a weighted element-wise block adaptive frequency-domain equalization (WEB-FDE) is proposed for a single-carrier system with the cyclic-prefix. In the WEB-FDE, the one-tap equalizer corresponding to a frequency-bin first preserves input DFT elements (element-wise block). Its coefficient in each block is then calculated by minimizing a weighted squared norm of the a posteriori error. Simulation results in a time-varying typical urban (TU) channel show that the bit-error-rate (BER) performance of the WEB-FDE outperform that of the normalized least-mean-square (NLMS)-FDE and recursive-least-square (RLS)-FDE.

As the channel frequency selectivity becomes severer, the bit error rate (BER) performance of direct sequence spread spectrum (DSSS) signal transmission with rake combining degrades due to an increasing inter-path interference (IPI). Frequency-domain equalization (FDE) can replace rake combining with much improved BER performance in a severe frequency-selective fading channel. For FDE, accurate estimation of the channel transfer function is required. In this paper, we propose an iterative channel estimation that uses pilot chips which are time-multiplexed within each chip block for fast Fourier transform (FFT). The pilot acts as a cyclic-prefix of FFT block as well. The achievable BER performance is evaluated by computer simulation. It is shown that the proposed channel estimation has a very good tracking ability against fast fading.

Frequency-domain equalization (FDE) can take advantage of the frequency-selectivity of the channel to improve the transmission performance in a frequency selective fading channel. To further improve the transmission performance, the transmit diversity technique can be used. Cyclic delay transmit diversity (CDTD) can strengthen the frequency-selectivity while space-time transmit diversity (STTD) can achieve the antenna diversity gain. In this paper, we propose a 4-antenna space-time cyclic delay transmit diversity (STCDTD), which is a combination of 2-antenna STTD and 2-antenna CDTD schemes, for orthogonal multi-code direct sequence code division multiple access (DS-CDMA) using FDE. We evaluate the BER performance and the throughput performance by computer simulation and compare them with the original CDTD and STTD schemes.

Frequency-domain equalization (FDE) based on the minimum mean square error (MMSE) criterion can replace the conventional rake combining while offering significantly improved bit error rate (BER) performance for the downlink DS-CDMA in a frequency-selective fading channel. However, the presence of residual inter-chip-inference (ICI) after FDE produces orthogonality distortion among the spreading codes and the BER performance degrades as the level of multiplexing increases. In this paper, we propose a joint MMSE frequency-domain equalization (FDE) and ICI cancellation to improve the BER performance of the DS-CDMA downlink. In the proposed scheme, the residual ICI replica in the frequency-domain is generated and subtracted from each frequency component of the received signal after MMSE-FDE. The MMSE weight at each iteration is derived taking into account the residual ICI. The effect of the proposed ICI cancellation scheme is confirmed by computer simulation.

Frequency-domain equalization (FDE) based on the minimum mean square error (MMSE) criterion can significantly improve the BER performance of DS- and MC-CDMA systems in a severe frequency-selective fading channel. However, since the frequency-distorted signal cannot be completely equalized, the residual inter-code interference (ICI) limits the BER performance improvement. 4G systems must support much higher variable rate data services. Orthogonal multicode transmission technique has flexibility in offering variable rate services. However, the BER performance degrades as the number of parallel codes increases. In this paper, we propose an iterative frequency-domain soft interference cancellation (IFDSIC) scheme for multicode DS- and MC-CDMA systems and their achievable BER performances are evaluated by computer simulation.

This paper discusses a cyclic prefixed single carrier frequency-domain equalization (SC-FDE) scheme with two types of transmit diversity. Firstly, we propose a SC-FDE system with space-frequency block coding (SFBC). The transmit sequence of the proposed system is designed to have spatial and frequency diversities, which is equivalent to the SFBC. The corresponding combining receiver is derived under a minimum mean square error (MMSE) criterion. It is shown that the proposed system significantly outperforms the SC-FDE system with space-time block coding (STBC) over fast fading channels, while providing lower computational complexity than orthogonal frequency division multiplexing (OFDM) combined with SFBC. We verify the performance of two-branch transmit diversity systems including the proposed one through bit error rate (BER) analysis. Secondly, as a scheme that combines STBC and SFBC, a space-time-frequency block code (STFBC) SC-FDE system is presented. Computer simulation results show that the proposed STFBC SC-FDE system has better immunity to the distortion caused by both fast fading and severe frequency selective fading, compared to the SC-FDE system with the STBC or the SFBC scheme. Complexity analysis is also conducted to compare their computational loads of the transceiver. It is shown that the proposed STFBC SC-FDE system has lower computational complexity than the STFBC OFDM system.

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.