Wideband Direct Sequence Code Division Multiple Access Research Articles

Generalized Receiver with Decision-Feedback Equalizer for Multicode Wideband DS-CDMA

In the present paper, a chip-level minimum mean-square-error (MMSE) decision-feedback equalizer for the downlink receiver of multicode wideband direct sequence code-division multiple access (DS-CDMA) wireless communication systems over frequency-selective channels is investigated. Firstly, the MMSE per sym-bol achievable by an optimal decision-feedback equalizer is derived, assuming that all interchip interference (ICI) of the desired user can be eliminated. The MMSE of the decision-feedback equalizer is always less than or at most equal to that of linear equalizers. When all the active codes belong to the desired user, the ideal deci-sion-feedback equalizer is able to eliminate multicode interference and approach the performance of the single-code case at high signal-to-noise ratio (SNR) range. Secondly, we apply the hypothesis-feedback equalizer or tentative-chip decision-feedback equalizer in the multicode scenario. The tentative-chip decision-feedback equ-alizer outperforms the chip-level linear equalizer and the decision-feedback equalizer that only feeds back the symbols already decided. The performance gain increases with SNR, but decreases with the number of active codes owned by the other users. When all the active codes are assigned to the desired user, the tentative-chip decision-feedback equalizer eliminates the multicode interference and achieves single-user performance at the high SNR, similarly, to the ideal decision-feedback equalizer. The asymptotic performance of the decision-feed-back equalizer is confirmed through the bit error rate (BER) simulation over various channels.

Output SNR Improvement in Array Processing Architectures of WCDMA Systems by Low Side Lobe Beamforming

In Wideband direct sequence code division multiple access (WCDMA) same frequency spectrum is shared through all cells simultaneously, as opposed to TDMA which is used for most 2nd generation systems. In a WCDMA system all transmitted signals turn out to be disturbing factors to all other users in the system in the form of interference limiting the system capacity. To suppress the amount of interference, fast and reliable interference controlling algorithms must be employed in next generation systems. In this paper it is shown that antenna arrays with steer able low side lobes can reduce interference in WCDMA can increase the system capacity and output signal to noise ratio of the array processing architecture. The performance metric O/P SNR of an array processing architectures is simulated in an interfering environment to demonstrate the advantage of low side lobe beamforming over adaptive antennas.

On the complexity reduction of turbo decoding for wideband CDMA

Two simple but effective methods for reducing the average complexity (and power consumption) of the conventional turbo-cyclic-redundancy-check decoding scheme with negligible performance degradation in a wideband direct-sequence code-division multiple-access (W-CDMA) environment are introduced. When applied to a W-CDMA turbo code with frame length 640 b at a bit-error rate (BER) of 10/sup -6/, the resultant modified schemes can save up to 73% of the average decoding complexity, relative to the conventional scheme. In general, the proposed schemes are more attractive for short-frame and low-BER applications.

RESOURCE MANAGEMENT IN WIDEBAND CDMA SYSTEMS USING GENETIC ALGORITHMS

In this paper, we study the resource management problem in Direct Sequence-Wideband Code Division Multiple Access (DS-WCDMA) systems. The control variables, transmission power, and transmission rate for resource management are considered. Three meta-heuristic techniques, Genetic Algorithms (GA), Simulated Annealing (SA), and Tabu Search (TS), are utilized to solve this optimization problem. Also, a nonlinear programming technique, Generalized Reduced Gradient (GRG) method, is adopted to compare with the three meta-heuristic techniques. Two approaches, the single objective approach and the multiobjective approach, are used in the simulation. The results obtained by GA, SA, TS, and GRG are compared in a single objective approach. In a multiobjective approach, Multiobjective Genetic Algorithm (MOGA) is employed to compare with the other two well-known multiobjective evolutionary algorithms (MOEAs), Pareto Archived Evolution Strategy (PAES) and Micro-Genetic Algorithms (MICRO-GA). Two scenarios, scenario (a): 25 users and scenario (b): 50 users, are considered for both approaches. The simulation results of a single objective approach show that GA outperforms SA, TS, and GRG in the two scenarios. Also, the simulation results of a multiobjective approach show that MOGA outperforms PAES and MICRO-GA, and obtains nondominated trade-off solutions with better convergence and diversity between trade minimization of total power and maximization of total rate in two scenarios.

Improved MAC protocol for reverse link packet data transmission in wideband DS-CDMA

This paper introduces and analyses a novel medium access control protocol for reverse links in wideband direct-sequence code-division multiple-access (DS-CDMA) mobile systems. The proposed protocol is particularly efficient when data consists of short packets. The new MAC protocol uses the time taken by the receiver to process the packets (the processing delay) to admit a new packet. When packet arrival is assumed to be Poisson, analysis and simulations show consistently that, for short packets, the new MAC scheme can handle a higher arrival rate than the conventional one, in which the processing delay is not used. The new MAC scheme shows only slight throughput degradation, a high utilization of the packet-switched data channel, and achieves a fair treatment of mobile stations.

Adaptive antenna array transmit diversity in FDD forwad link for W-CDMA and broadband packet wireless access

This article presents a transmitter antenna weight generation method in adaptive antenna array transmit diversity (AAA-TD) that focuses on the calibrations in wideband direct sequence code-division multiple access (W-CSDMA) and broadband packet wireless access in FDD systems. In our scheme, the adaptively generated receiver antenna weights of the reverse link in the coherent adaptive antenna array diversity receiver are used as transmitter antenna weights, however, with calibration of the phase/amplitude variations of the parallel radio frequency receiver/transmitter circuits corresponding to the number of array antennas (RF circuitry calibration) and with calibration of the transmitter beam pattern due to the difference in carrier frequency between the reverse and the forward links (carrier frequency calibration). The interference suppression effect of AAA-TD employing a carrier frequency calibration and RF circuitry calibration in the W-CDMA forward link is also investigated by laboratory experiments. The experimental results elucidate that in a four-user environment (three interfering users), although an error floor above BER of 10 -1 is observed with a one-antenna transmitter due to the severe multipath interference, average BER of below 10 -6 is achieved using AAA-TD with four antennas, when the ratio of the target signal energy per bit-to-interference plus background noise power spectrum density ratio (E b /I 0 ) of the desired user to interfering user for fast transmission power control is ΔE b /I 0 = -12 dB.

Multipath interference canceller for high-speed packet transmission with adaptive modulation and coding scheme in W-CDMA forward link

This paper proposes a multipath interference canceller (MPIC) associated with orthogonal code-multiplexing that achieves much higher peak throughput than 2 Mbit/s with adaptive data modulation for high-speed packet transmission in the wideband direct sequence-code division multiple access (W-CDMA) forward link, and evaluates its throughput performance by computer simulation. The simulation results elucidate that sufficient multipath interference (MPI) suppression is achieved by a four-stage MPIC with 6-12 orthogonal code-multiplexing using one iterative channel estimation with pilot and decision feedback data symbols and further that the interference rejection weight control according to the number of observed multipaths is effective in improving the throughput. It is also demonstrated that MPIC exhibits a superior MPI suppression effect to a chip equalizer in the lower received signal energy per bit-to-background noise spectrum density (E/sub b//N/sub 0/) channel around 0-3 dB owing to the successive channel estimation at each stage. We show that the maximum peak throughput using MPIC is approximately 2.1 fold that without MPIC in a two-path and three-path Rayleigh fading channel and that the peak throughput of 8.0 Mbit/s is achieved using 64 QAM data modulation in a two-path fading channel within a 5 MHz bandwidth. Furthermore, the required average E/sub b//N/sub 0/ for satisfying the same throughput with MPIC is decreased by more than 2.0 dB.

Adaptive fuzzy power control for WCDMA mobile radio systems

In this paper a fuzzy logic based power control for a direct sequence, wideband code-division multiple-access (DS/WCDMA) cellular phone system is introduced. A power control procedure is needed to compensate the fluctuation of the mobile's transmitting power received in a base station and to increase the capacity of mobile communication systems. The input variables of the controller are the power error and the change of the error. The output is the power step size. Simulation results show that the developed fuzzy proportional-integral control stabilizes the power level and decreases the overshoot and rise time.

Experiments on coherent adaptive antenna array diversity for wideband DS-CDMA mobile radio

In wideband direct sequence code division multiple access (W-CDMA), employing an adaptive antenna array is a very promising technique to reduce severe multiple access interference (MAI) from high rate users. A four-antenna pilot symbol-assisted coherent adaptive antenna array diversity (PSA-CAAAD) receiver comprising an adaptive antenna array based on a minimum mean squared error (MMSE) criterion and a RAKE combiner is implemented in preliminary laboratory and field experiments. There are two important design concepts of the PSA-CAAAD receiver. The first is that the adaptive antenna array forms an antenna beam for each resolved propagation path and tracks only slow changes in the directions of arrival (DOAs) and average powers of the desired and interfering user signals. The second is that the RAKE combiner tracks the instantaneous changes in channel conditions and coherently combines the signals of the desired user propagating along the resolved paths to maximize the instantaneous signal-to-interference plus background noise power ratio (SINR). This paper presents, both by laboratory and field experiments, the effectiveness of PSA-CAAAD receiver as a powerful means to reduce severe MAI from high rate users, and that it is more effective than using a space diversity receiver with the same number of antennas in the W-CDMA reverse link.