In this paper, we propose symbol-based receivers for orthogonal frequency division multiplexing OFDM code-division multiple-access CDMA multiple-input-multiple-output MIMO communications in multipath fading channels. For multiuser and multipath fading environments, both intersymbol interference and multiple-access interference must be considered. We propose narrowband and wideband antennas and Wiener code filter for MIMO OFDM-CDMA systems. The proposed receivers are updated symbol-by-symbol to achieve low computational complexity. Simulation results show that the proposed Wiener code filter can improve the system performance for the proposed adaptive antennas. The wideband antenna can achieve better error-rate performance than that of the narrowband antenna when multipath effect exists. The convergence rate of the recursive least squares antennas is faster than that of the least mean square antennas. Copyright © 2013 John Wiley & Sons, Ltd.

In this paper, we propose three adaptive blind algorithms for multiuser multicarrier code division multiple-access systems in multipath fading channels. The proposed adaptive blind receivers are based on the property of the discreteness of the input data symbol and are updated in every symbol interval. We also use the concept that the variance of the output signal approaches to the variance of the desired signal to get the cost function. The three proposed receiver structures are the traditional finite impulse response FIR structure, the despreading DES filter structure and the generalized sidelobe canceller GSC structure. The advantage of the FIR filter is that the length of the filter weights does not have to be the same length as the spreading code. For the DES filter, the combination of the adaptive weight and the despreading code has the simplest structure than the other two proposed receiver structures. The constrained GSC filter is superior to the other two proposed receiver structures in the environments dominated by multiple-access interference. By this constraint, the blind GSC filter can guarantee to converge to the desired solution. Simulation results are given to show the effectiveness and comparison of the proposed adaptive blind receivers. Copyright ©2012 John Wiley & Sons, Ltd.

Abstract Adaptive antennas, with spatial processing, used in code division multiple access (CDMA) systems can enhance range, reliability, and capacity. Moreover, the adaptive beamforming technologies can remove unwanted noise and interference from the received signal. In this paper, we propose the adaptive direct sequence (DS) CDMA antenna receivers and compare their performances in three existing multirate methodologies—variable processing gain (VPG), variable chip rate, and multicode systems for multiuser communications in multipath fading channels. Simulation results show that the processing powers of both narrowband and wideband adaptive antennas are not enough. We propose a powerful processing technique named Wiener Code Filter (WCF) to retrieve the shortcomings. Mean square error (MSE) analyses are also given in this paper. Some simulation examples are given to compare the performances of the proposed multirate structures. Copyright © 2010 John Wiley & Sons, Ltd.

A time-hopping (TH) multiple access ultrawideband (UWB) system combined with a large modulation level of M-ary pulse position modulation (PPM) is considered and analyzed. Unlike the conventional TH method that the number of the provided TH slots becomes fewer as the adopted modulation level of M-ary PPM is lager and the data rate is fixed, a feasible TH scheme that utilizes the whole frame duration to carry out TH and PPM is proposed in the paper. The advantages of proposed TH system are to use the large modulation level of PPM, reduce the effect of the multiple access interference (MAI) and increase multiple access capability. The characteristic function (CF) of the MAI for proposed sytem is derived to obtain the probability density function of MAI and average symbol error rate. The simulation results of the proposed TH system are provided to verify the analytical results and show the performance improvement over the conventional TH and DS UWB systems.

We propose a coordinated non-sensing medium access control (MAC) protocol in dynamic spectrum access networks namely distributed coordinated dynamic spectrum reservation (DCDSR) protocol which is not based on the cognitive radio technology. All primary users (PUs) and secondary users (SUs) have two transceivers operating in common control channel and licensed data channels to access control and data packets in the corresponding control and data frames, respectively. PUs will inform SUs which channels will be used in next data frame and then SUs reserve unused channels by contending with each other during contention interval (CI) in control frame. The available channels which can be reserved by SUs will depend on the channel usage of PUs and further affect the throughput of SUs. Whenever SU successfully reserve channel in CI, they will start access reserved channel in next data frame after the end of CI. This will cause the wasted idle time for those SUs which have successfully reserved channel whenever the CI doesn't finish. SU which couldn't successfully reserve channel in current control frame should try to reserve again in next control frame. Thus, the duration of CI will affect the delay and wasted idle time of SUs. We analyze and simulate the performance of throughput, delay and wasted idle time in DCDSR. The analytical and simulation results show that the proposed DCDSR performs very well.

This paper proposes and compares three multi- rate receiver structures with post-fast Fourier transform (FFT) type and the pre-FFT type for orthogonal frequency division multiplexing code-division multiple access (OFDM-CDMA) communication systems. In multipath fading channel, the transmitted signal suffers from the effects of inter-chip interference (ICI) and multiple access interference (MAI). In order to solve these problems and support multimedia traffic, we propose three multi- rate methodologies- variable processing gain (VPG), variable chip rate (VCR) and multi-code systems to investigate the performance of the OFDM-CDMA communications in multipath fading environments. Simulations confirm the good performance of the proposed receiver structures.

In this paper, we propose and analyze a new noncoherent receiver with PN code tracking for direct sequence code division multiple access (DS-CDMA) communication systems in multipath channels. We employ the decision-feedback differential detection method to detect MDPSK signals. An ";error signal"; is used to update the tap weights and the estimated code delay. Increasing the number of feedback symbols can improve the performance of the proposed noncoherent receiver. For an infinite number of feedback symbols, the optimum weight can be derived analytically, and the performance of the proposed noncoherent receiver approaches to that of the conventional coherent receiver. Simulations show good agreement with the theoretical derivation.

The paper presents a novel topology mixer that leads to better performance in terms of linearity and power consumption for low supply voltage. Designed in umc 0.18μm CMOS technology, the mixer achieves: 4.55dB power conversion gain; 13.3dB noise figure; 4dBm input third-order intercept point (IIP3); and only 2mW of power consumption from a 1V supply voltage.

OFDM is a promising scheme for multicarrier (MC) transmission, since it has the advantage of high spectral efficiency, robustness to frequency selective fading, and feasibility of low-cost transceiver implementation. The combination of OFDM and CDMA, referred to as OFDM-CDMA, recently attracted most research interests in wireless communications. In this paper, adaptive receivers for OFDM-CDMA communication systems are developed based on the least mean square (LMS) algorithm. We evaluate the down-link performance for two forms of OFDM-CDMA schemes which are MC-CDMA and MC-DS-CDMA in multipath fading channels. A drawback of OFDM schemes is the rate reduction due to the cyclic prefix (CP) overhead, which significantly affects the channel utilization in MC-DS-CDMA systems. Thus we propose adaptive receiver structure that exploits both frequency-domain and time-domain equalization to improve the performances of MC-DS-CDMA systems with shorter or no CP. Simulation results show performance comparisons between MC-CDMA and MC-DS-CDMA systems, which also verify the performance improvement of our proposed receiver for MC-DS-CDMA systems exploiting shorter or no CP.