This paper presents a novel transmission diversity scheme for code division multiple access system. Conventional diversity receivers in mobile stations require space and complicated circuits, however, the proposed diversity schemes present significant diversity effect without any diversity equipment at the mobile station. It is possible to use the transmitter diversity at the base station by using the feature of time division duplex (TDD) which has strongly correlated fading patterns in both forward and reverse link. Computer simulation is performed to evaluate the performance of the proposed systems for single user environment. The performance of the system 1, which select best situated antenna, is analyzed and the BER performance for multiple access is presented.

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.

Fast transmit power control (TPC) adaptively controls the mobile terminal transmit power so that the instantaneous signal-to-interference plus background noise ratio's (SIR's) of received signals of all users at the base station receiver are kept at the target value to avoid the adverse effect of multipath fading as well as the near/far problem. This paper theoretically analyzes the power efficiency of power controlled DS-CDMA reverse link assuming ideal Rake combining under multi-user and multipath Rayleigh fading environments. The achievable bit error rate (BER) performance is evaluated as a function of average and peak transmit powers required at mobile terminals. The effect of number of resolved paths is discussed. It is shown that the required peak transmit power with fast TPC is larger than that without fast TPC for relatively large BER values; however, when the link is interference-limited, fast TPC achieves significantly larger link capacity.

The transmission performance of DS-CDMA forward link with orthogonal spreading and Rake combining is evaluated under multipath fading environments. A simple-to-use expression for the conditional instantaneous signal-to-interference plus background noise power ratio (SIR) is derived, assuming an M-finger Rake combiner. Using the derived expression, the forward link SIRs of either orthogonal spreading or random spreading can be conveniently computed. The link performance in terms of the average bit error rate (BER) and capacity (the maximum number of allowable users) is evaluated by a Monte Carlo simulation assuming ideal BPSK data modulation. In frequency selective multipath fading, the orthogonality of the forward link is destroyed to some extent and link performance approaches that of random spreading. The extent of orthogonality destruction depends on the multipath channel power delay profile shape and number of resolved paths (for an exponential profile, it is defined as the number of stronger resolved paths that capture 90% of the total received power); so their influences on the link performance are discussed. Also simulated is the distribution of the BERs in a radio coverage area taking into account the path loss and shadowing to evaluate the link capacity at a certain outage probability.

For the power controlled DS-CDMA reverse link, two important issues must be addressed when using a Rake combiner with a limited path diversity order (defined as the number of resolved paths used for combining): the decrease in link capacity and the increase in peak transmit power. The peak transmit power is an important design parameter of transmit power amplifiers. In this paper, expressions for the achievable capacity and required peak transmit power under an multi-cell environment are developed. Based on the developed expressions, the relative capacity and required peak transmit power are evaluated theoretically and by Monte Carlo simulations under multipath Rayleigh fading environments with uniform and exponential power delay profiles. The effects of Rake based on maximal ratio combining (MRC) and selection combining (SC) are compared. The influence of the power delay profile shapes is also discussed.

This paper deals with performance evaluation of CDMA based on DS/SS/GMSK signaling with the differential detection over multipath Rayleigh fading channels. To demodulate DS/SS/GMSK signals, we consider differential detection, which does not need a carrier recovery. The bit-error-rate performance is evaluated in the presence of thermal noise and multipleaccess interferences under the multipath Rayleigh fading environment. To improve the performance, we also consider adoption of a RAKE receiver.

A new scheme to study the performance of a DS/CDMA indoor wireless system, the correlation statistics distribution convolution(CSDC)modeling, is introduced in this paper. With the aid of the CSDC modeling, the bit error rate versus number of simultaneous interfering transmitters can be directly evaluated, considering the effects of Rayleigh fading, power control, multipath and co-channel interference. The performance of two CDMA receiver structures, conventional correlator and RAKE receiver, is compared. It is shown that the RAKE receiver is effective in improving the system performance under indoor multipath fading. However, its effectiveness under transmitter power control is sensitive to the severity of multipath interference in the indoor channel. When the multipath fading is severe, a tight power control over the main paths may not be able to improve the performance of the RAKE receiver.

Direct sequence code division multiple access (DS-CDMA) is a promising candidate for 3rd generation mobile communications systems. We recently proposed a coherent multicode DS-CDMA (CM-CDMA) scheme that uses pilot symbol-aided coherent RAKE, interference power measurement based transmit power control, orthogonal multicode transmission, and concatenated channel coding. We have implemented a CM-CDMA test-bed for a series of laboratory and field tests using the 2 GHz band. This paper describes the test-bed system and experimental results are presented. It is confirmed that pilot symbol-aided coherent RAKE can reduce the required signal energy per bit-to-interference plus background noise spectrum density ratio (Eb/Io) by 2-3 dB from that achievable with differential detection. Also shown is that by using both RAKE combining and SIR-based power control the transmit power of mobile stations can be significantly reduced. Measurement results show that the required Eb/Io degrades only slightly when 24 code-channels (768 kbps) are used since orthogonal Gold sequences are used as short spreading codes.

This paper describes the performance analysis of a distributed antenna system which includes space and path diversity with radio channel estimation. This system is used for CDMA personal communication systems. In this paper, the performance of a diversity system is analyzed precisely considering multipath and inter-antenna interference. In a diversity system, the adaptive RAKE receiver which estimates the characteristics of a radio channel adaptively has been used for diversity combining. In the adaptive RAKE, the time-variant characteristic has been approximated by a time function. In this paper, the estimation performance of the adaptive RAKE is analyzed in cases of time functions of 0-th, first and second degrees. The performances are evaluated and compared with the differential RAKE. The adaptive RAKE is found to improve the signal quality of more than 2dB in comparison with the differential RAKE. It is also found that the optimum parameter design can be achieved flexibly for radio channel estimation by using higher degree time functions.

A new method of multipath diversity combination is proposed for Direct Sequence Spread Spectrum (DS-SS) mobile communications. In this method, the transmitted signal from the base staion is the sum of a number of the same spread signal, each one delayed and scaled according to the delay and the strength of the multipaths of the transmission channel. As a result the received signal at the mobile unit will already be a Rake combination of the multipath signals. This new method is called Pre-Rake diversity combination because the Rake diversity combination process is performed before transmission By this method the size and complexity of the mobile unit can be minimized, and the unit is made as simple as a non-combining single path receiver. A theoretical examination of the Signal to Noise Ratio (SNR) and the Bit Error Rate (BER) results for the traditional Rake and the Pre-Rake combiners as well as computer simulations show that the performance of the Pre-Rake combiner is equivalent to that of the Rake combiner.

In this paper, we propose an adaptive RAKE receiver, which does not need to send the sounding signals and can track the fluctuations caused by fading. The channel estimation can be done by using a least squares method of the first and second equations suppressing additive noise and tracking the channel fluctuations. It is confirmed by computer simulations that the result has good agreement with theory and the performance is almost same as that of the conventional RAKE with the sounding signals.

Indoor radio communications is an important component of the emerging personal communication systems service. It is also the basis for wireless local area networks. The indoor radio channel is characterized by fading multipaths as well as noise. Direct sequence spread spectrum (DSSS), with its inherent resistance to multipath interference is an attractive technique for this environment. To allow multiple users within the limited bandwidths available, code division multiple access is needed. This paper analyzes the performance of a DSSS scheme employing random orthogonal codes over fading multipath indoor radio channels using actual channel measurements from five different locations. A RAKE receiver is used to study the effects of power control, code length and receiver structure. The average probability of error as a function of signal-to-noise ratio or as a function of the number of simultaneous transmitters is used as the performance criteria.

The path diversity improvement inherent in direct sequence spread spectrum (DS/SS) signalling under multi-path propagation environments is investigated for mobile/personal radio communications systems that employ DPSK modulation. The bit error rate (BER) performance of post-demodulation selection diversity reception is theoretically analyzed in the presence of noise-only-paths in the time window for diversity combining. Results of laboratory experiments conducted to evaluate the BER performance are also presented. It is shown that the experimental results agree well with the theoretical BER.