Recently, we proposed space-time block coded-joint transmit/receive antenna diversity (STBC-JTRD) for narrow band transmission in a frequency-nonselective fading channel; it allows an arbitrary number of transmit antennas while limiting the number of receive antennas to 4. In this paper, we extend STBC-JTRD to the case of frequency-selective fading channels and propose frequency-domain STBC-JTRD for broadband direct sequence-spread spectrum (DSSS) signal transmission. A conditional bit error rate (BER) analysis is presented. The average BER performance in a frequency-selective Rayleigh fading is evaluated by Monte-Carlo numerical computation method using the derived conditional BER and is confirmed by computer simulation of the signal transmission. Performance comparison between frequency-domain STBC-JTRD transmission and joint space-time transmit diversity (STTD) and frequency-domain equalization (FDE) reception is also presented.

Antenna diversity is an effective technique for improving the transmission performance in a multi-path fading channel. Recently, transmit diversity has been attracting much attention since it can alleviate the complexity problem of the mobile terminals. Joint transmit diversity/receive diversity achieves a much improved transmission performance. In this paper, we propose a new space-time block coding algorithm for joint transmit/receive diversity, which requires the channel state information (CSI) only at the transmitter side. Unlike the conventional space-time transmit diversity (STTD), the space-time block coded joint transmit/receive diversity (STBC-JTRD) can use arbitrary number of transmit antennas, while the number of receive antennas is limited to 4. STBC-JTRD achieves a larger diversity gain than joint STTD/receive antenna diversity. The bit error rate (BER) analysis in a frequency-nonselective Rayleigh fading channel is presented. The BER performance is evaluated and is confirmed by the computer simulation.

The downlink (base-to-mobile) bit error rate (BER) performance for a mobile user with relatively weak received signal in a multicarrier-CDMA (MC-CDMA) cellular system can be improved by utilizing the site diversity reception. With joint use of MMSE-based frequency domain equalization (FDE) and antenna diversity combining, the site diversity operation will increase the downlink capacity. In this paper, an expression for the theoretical conditional BER for the given set of channel gains is derived based on Gaussian approximation of the interference components. The local average BER is then obtained by averaging the conditional BER over the given set of channel gains using Monte-Carlo numerical method. The outage probability is measured from the numerically obtained cumulative distribution of the local average BER to determine the downlink capacity. Results from theoretical computation are compared to the results from computer simulation and discussed.

The optimal antenna weighting scheme that minimizes the average bit error rate in a closed-loop transmit antenna diversity system is investigated under the assumption that channel state information is provided at both the transmitter and the receiver. A closed-form expression for the optimal transmitter weights is derived with a fixed average transmit power constraint. Also, the effect of limited peak transmit power on the performance of the optimal weighting method is analyzed. Base on this analysis, it is shown that the proposed transmitter weights yield significant performance improvements over the conventional weights on the wide range of practical system parameters.

In a CDMA non-linear interference canceller, a generated replica of an interference signal is multiplied by a positive number smaller than unity, which is called cancellation moderating factor (CMF), to prevent interference enhancement due to inaccurate replica subtraction. In this paper, two CMF controlling schemes applicable to a multistage parallel interference canceller with multi-antenna (spatial diversity) reception are proposed. They control CMF by using the mean square error of the complex channel gain or by using the ratio of the estimated power of each interference signal to remaining interference signals' power, in order to mitigate the replica subtraction error due to inaccurate channel estimation. The performance of the proposed schemes are evaluated by computer simulations assuming an asynchronous uplink single chip-rate variable spreading factor DS-CDMA system. The simulation results show that the proposed schemes with higher order diversity reception improve the bit error rate (BER) performance compared with a conventional scheme considering the tentative decision error or fixed CMF settings. Their performance improvement is by 0.1-0.9 dB in terms of the required Eb/N0 at an average BER of 10-5 over exponentially decaying 5-path Rayleigh distributed channels when the number of receiving antennas is 6.

The effect of feedback delay and channel estimation error on closed-loop transmit diversity (CTD) systems is investigated in time-selective Rayleigh fading channels. Based on a minimum mean square error (MMSE) channel estimator, the variance of the estimation error is formulated in terms of fading index and the number of transmit antennas. A bit error rate (BER) expression for the CTD system is analytically derived as a function of channel estimation error, feedback delay, and fading index. It is shown that the BER performance of the CTD system improves as the length of training symbols increases and/or the frame length decreases. In the CTD system, more accurate channel estimation scheme is required to achieve its full gain as the number of employed transmit antennas increases. It is also found that the CTD system is applicable to the slowly moving channel environments, such as pedestrians, but not for fast moving vehicles.

To improve the DS-CDMA signal transmission performance in a frequency-selective fading channel, the frequency-domain equalization (FDE) can be applied, in which simple one-tap equalization is carried out on each subcarrier component obtained by fast Fourier transform (FFT). Equalization weights for joint FDE and antenna diversity combining based on maximal ratio combining (MRC), zero-forcing (ZF), and minimum mean square error (MMSE) are derived. The conditional bit error rate (BER) is derived for the given set of channel gains in a frequency-selective multipath fading channel. The theoretical average BER performance is evaluated by Monte-Carlo numerical computation method using the derived conditional BER and is confirmed by computer simulation. Performance comparison between DS- and multi-carrier (MC)-CDMA both using FDE is also presented.

Orthogonal multicode direct sequence code division multiple access (DS-CDMA) has the flexibility in offering various data rate services. However, in a frequency-selective fading channel, the bit error rate (BER) performance is severely degraded since the othogonality among spreading codes is partially lost. In this paper, we apply frequency-domain equalization and antenna diversity combining, used in multi-carrier CDMA (MC-CDMA), to orthogonal multicode DS-CDMA in order to restore the code othogonality while achieving frequency and antenna diversity effect. It is found by computer simulations that the joint use of frequency-domain equalization and antenna diversity combining can significantly improve the BER performance of orthogonal multicode DS-CDMA in a frequency-selective fading channel.

We propose a new approach associated with the use of some selected sets of Walsh Hadamard codes for joint estimation of channels and the number of transmit antennas by employing only one OFDM pilot symbol. This allows transmit antenna diversity to be applied in systems which have a limited number of training symbols (preambles), e.g. HIPERLAN/2. The proposed approach does not require any a priori knowledge about the number of transmit antennas, providing flexibility in the number of antennas to be used. In addition, adaptive scheme associated with the proposed approach provides more accurate estimations of the channels. The effectiveness of the proposed approach is evaluated through simulation. Results show that the proposed scheme provides significant improvement over previous channel estimation schemes and has almost the same performance as the ideal system with the full knowledge of the channel state information.

For the reception of MC-CDMA signals in a frequency-selective fading channel, frequency-domain equalization is necessary before despreading. In this paper, joint antenna diversity combining and one-tap frequency-domain equalization is considered (simply referred to as the joint antenna diversity & equalization, in this paper). A receiver structure for joint antenna diversity & equalization is presented and the unified weights based on minimum mean square error (MMSE) criterion are found in the presence of multi-users with different spreading factors and transmit powers. For comparison, antenna diversity combining after despreading using MMSE combining (MMSEC) is also considered. The achievable bit error rate (BER) performances with joint antenna diversity & equalization and with antenna diversity after MMSEC despreading in a frequency-selective Rayleigh fading channel are evaluated by computer simulations and compared.

Frequency-domain representation of the well-known time-domain rake combining for the antenna diversity reception of DS-CDMA signals is derived. Two receiver structures using frequency-domain rake combining are presented. Frequency-domain rake combining can alleviate the complexity problem of the time-domain rake arising from too many paths in a severe frequency selective fading channel at the cost of guard interval insertion. The results shown in this paper show a possibility that a DS-CDMA approach still remain to be promising for broadband wireless access technique.

In this paper, expressions are derived for the bit error rate (BER) of the multicarrier-CDMA (MC-CDMA) downlink in the presence of pure impulsive interference and a frequency-selective fading and the BER performance is numerically evaluated by a Monte-Carlo simulation method. Minimum mean square error combining (MMSEC) and orthogonal restoration combining (ORC) are considered for frequency-domain equalization. The joint weight of antenna diversity reception using maximal ratio combining (MRC) and frequency equalization combining is derived. The MC-CDMA transmission performance in the presence of pure impulsive interference is compared with that of DS-CDMA transmission.

It is well known that some of urban man-made noises can be characterized by a wideband impulsive noise (pure impulsive noise). The presence of this pure impulsive noise may significantly degrade the wireless digital transmission performance. As the data rate becomes higher and the radio bandwidth becomes wider, the performance degradation due to pure impulsive interference may become larger. In this paper, the DS-CDMA transmission performance in the presence of pure impulsive interference is theoretically analyzed. First, the BER expressions are derived for DS-CDMA with antenna diversity and Rake combining in a frequency selective fading channel. Then, the numerical computation based on Monte-Carlo method is performed to evaluate the BER performance. Two types of error floor are observed: one is due to impulsive interference and the other due to the multi access interference (MAI). It is found that the error floor due to impulsive interference becomes larger as the area of impulse and the error floor is linearly proportional to the impulse occurrence rate. Furthermore, it is found that the antenna diversity and Rake combining do not help to reduce the error floor caused by impulsive interference and that the influence of impulsive interference can be negligible when the channel is limited by the MAI (i.e., large number of users are in communication).

This paper presents an optimum antenna diversity combining method associated with despreading that employs Minimum Mean Square Error (MMSE) combining over the frequency domain in a frequency-selective fading channel for forward link Orthogonal Frequency and Code Division Multiplexing (OFCDM) wireless access, in order to achieve the maximum radio link capacity. Simulation results considering various propagation channel conditions elucidate that the antenna diversity combining method with Equal Gain Combining (EGC) subsequent to the despreading employing MMSE combining based on pilot symbol-assisted channel estimation and interference power estimation can decrease the required average received signal energy per bit-to-background noise power spectrum density ratio (Eb/N0) the most, taking into account the impact of the inter-code interference. Furthermore, we clarify that the required average received Eb/N0 for the average packet error rate of 10-2 employing the diversity combining scheme with EGC after despreading with MMSE combining is improved by approximately 0.3 dB compared to the diversity combining scheme with EGC before despreading with MMSE combining at the number of code-multiplexing of 24 for the spreading factor of 32 in a 24-path Rayleigh fading channel.

A DS-CDMA mobile communication system accommodating multi-class users is considered. The number of supportable users depends on the distributions of data rate and required communication quality among users. Simple expressions for the reverse link capacity with transmit power control, antenna diversity, and rake combining, are derived for a single-cell system and a multi-cell system.

This paper addresses a classic question about whether transmit power control (TPC) can increase the spectrum efficiency of a TDMA system and an FDMA cellular system as in the case of a DS-CDMA cellular system. Two types of TPC schemes are considered; one is slow TPC that regulates the distance dependent path loss and shadowing loss, while the other is fast TPC that regulates multipath fading as well as path loss and shadowing loss. In addition to TPC, antenna diversity reception is considered. The allowable interference rise factor χ, which is defined as the interference plus background noise-to-background noise power ratio, is introduced. The simple expressions for the signal-to-interference plus background noise power ratio (SINR) at the diversity combiner output using maximal-ratio combining (MRC) are derived to obtain the reuse distance by computer simulations. The impact of joint use of TPC and antenna diversity reception on the spectrum efficiency is discussed. It is found that the joint use of fast TPC and antenna diversity is advantageous and larger spectrum efficiency can be achieved than with no TPC. On the other hand, the use of slow TPC is found advantageous only for small values of standard deviation of shadowing loss; however, the improvement in the spectrum efficiency is quite small.

An uplink synchronised CDMA system through transmission timing control at mobile users has been proposed to improve the uplink capacity. This Letter mathematically investigates its capacity, considering perfect fast TPC and two antenna diversity reception in a single cell environment and compares it with that of a conventional CDMA system.

In this paper, a novel beam selection transmit diversity (BSTD) scheme is proposed for the downlink transmission of frequency division duplex (FDD) based DS-CDMA system. As a combination of selection transmit diversity and steering vector based beamforming, the BSTD scheme provides diversity gain as well as reducing multiple access interference in downlink. Moreover, to have a better understanding, the performance of the BSTD is also compared with other schemes. The comparison results show that the BSTD would be a promising candidate for the downlink transmission if both performance and implementation complexity are considered.

This paper experimentally demonstrates the possibility of 2-Mbps data transmission using a 5-MHz bandwidth (chip rate of 4.096 Mcps) wideband DS-CDMA (W-CDMA) mobile radio link in frequency-selective multipath fading environments. To reduce the mobile station transceiver complexity, three-orthogonal code multiplexing with the spreading factor (SF) of 4 is employed. In such a small SF transmission, the increased multipath interference (MPI) significantly degrades the transmission performance. We consider two-branch antenna diversity reception and fast transmit power control (TPC) as well as channel coding to mitigate the influence of MPI. Laboratory experimental results show that the use of antenna diversity reception is significant and that the fast TPC improves the transmission performance. Furthermore, the impact of the fading maximum Doppler frequency, fD, and that of the channel coding interleaving size, Tint, on the achievable BER performance are also investigated.

Field experiments using the 2 GHz carrier frequency band were conducted nearby Tokyo to evaluate the effect of joint use of Rake combining and antenna diversity and also the effect of spreading chip rate (or bandwidth) on the achievable bit error rate (BER) performance and the mobile station transmit power distribution of power controlled coherent DS-CDMA reverse-link (mobile-to-base). Four chip rates, 0. 96, 1. 92, 3. 84, and 7. 68 Mcps, were used. The command interval and power step size of the fast transmission power control (TPC) used in the experiments, 1. 25 ms and 1 dB, respectively, were based on measurements of signal-to-interference plus background noise power ratio (SIR) after Rake combining. The field experiments demonstrate that the joint use of antenna diversity and Rake combining significantly improves the BER performance and, furthermore, that increasing the chip rate improves the BER performance and decreases the transmit power because of enhanced Rake combining through an increase in the number of resolved paths.