In OFDM-CDMA down link (base-to-mobile) transmissions, each user's transmit data symbol is spread over a number of orthogonal sub-carriers using an orthogonal spreading sequence defined in the frequency-domain. The radio propagation channel is characterized by a frequency- and time-selective multipath fading channel (which is called a doubly selective multipath fading channel in this paper). Frequency-domain equalization is necessary at the receiver to restore orthogonality among different users. This requires accurate estimation of the time varying transfer function of the multipath channel. Furthermore, the noise enhancement due to orthogonality restoration degrades transmission performance. In this paper, pilot-aided threshold detection combining (TDC) is presented that can effectively suppress the noise enhancement. If the estimated channel gain is smaller than the detection threshold, it is replaced with the detection threshold in the frequency equalization. There exists an optimal threshold that can minimize the bit error rate (BER) for a given received Eb/N0. The average BER performance of OFDM-CDMA down link transmissions using the TDC is evaluated by computer simulations. It is found that TDC using optimum detection threshold can significantly reduce the BER floor and outperforms DS-CDMA with ideal rake combining.

Direct sequence code division multiple access (DS-CDMA) provides flexible data transmission in wide range of data rates by the use of orthogonal multicode multiplexing. In a multipath fading environment, the transmission performance of multicode DS-CDMA degrades as that of single code DS-CDMA does. Chip interleaving is known to improve the bit error rate (BER) performance of the single code transmission by altering the fading channel into severely time selective fading channel. However, this partially destroys orthogonality property among spreading codes and thus, significantly degrades the BER performance of multicode DS-CDMA. In this paper, we propose the joint use of chip interleaving and time-domain minimum mean square error combining (MMSEC) equalization to improve the multicode DS-CDMA transmission performance. It is confirmed by computer simulations that the joint use of chip interleaving and MMSEC equalization significantly improves the BER performance of multicode DS-CDMA and achieves better BER performance compared to the single code DS-CDMA using chip interleaving and maximal ratio combining (MRC).

In this paper, we present laboratory and field experimental results using High Speed Downlink Packet Access (HSDPA) test-beds in order to reveal the actual HSDPA performance based on key technologies such as base station (BS) scheduling, adaptive modulation and coding, hybrid automatic repeat request, and advanced receiver design. First, this paper evaluates the effects of advanced user equipment capabilities such as the maximum number of multi-codes, transmit diversity, receive diversity, and a chip equalizer. Increases in throughput of 60% and 85% due to using 10 and 15 codes were observed compared to 5 codes, respectively. The gain of 22% was obtained by applying closed-loop transmit diversity to the HSDPA network. Receive diversity improves the throughput in the region from low to high signal-to-interference ratio, and the gain of 45% was obtained by applying receive diversity to the conventional RAKE receiver. A throughput gain of approximately 17% due to the use of the chip equalizer was obtained and it was observed mainly in the high Ior/Ioc region and under multi-path conditions. Second, field experiments are conducted to elucidate the effects of multi-user diversity using a BS scheduling algorithm, and reveal that proportional fairness scheduling provides both the increase in sector throughput of 18% and a sufficient degree of fairness among users. The transmit control protocol (TCP)-level throughput performance is also investigated in order to reveal the actual end-user throughput. The results show that the throughput rate of approximately 90% of the throughput of the MAC-hs layer is achieved in the TCP layer in the laboratory experiments and in the field experiments.

In a wireless OFDM-CDMA system, the data-modulated symbol of each user is spread over multiple subcarriers in the frequency domain using a given spreading code. For the downlink (base-to-mobile) transmissions, a set of orthogonal spreading codes defined in the frequency domain is used so that different users data can be transmitted using the same set of subcarriers. The frequency selectivity of the radio channel produces the orthogonality destruction. There are several frequency equalization combining techniques to restore orthogonality, i.e., orthogonal restoration combining (ORC), control equalization combining (CEC) that is a variant of ORC, threshold detection combining (TDC), and minimum mean square error combining (MMSEC). The ORC can restore orthogonality among users but produces noise enhancement. However, CEC, TDC, and MMSEC can balance the orthogonality restoration and the noise enhancement. In this paper, we investigate, by means of computer simulation, how the BER performances achievable with ORC, CEC, TDC, and MMSEC are impacted by the propagation parameters (path time delay difference and fading maximum Doppler frequency), number of users, pilot power used for channel estimation, and channel estimation scheme. To acquire a good understanding of ORC, CEC, TDC, and MMSEC, how they differ with respect to the combining weights is discussed. Also, the downlink transmission performances of DS-CDMA and OFDM-CDMA are compared when the same transmission bandwidth is used. How much better performance is achieved with OFDM-CDMA than with DS-CDMA using ideal rake combining is discussed.

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.