Effects of multipath and base-station antenna arrays on uplink capacity of cellular CDMA (original) (raw)
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DS-CDMA performance with maximum ratio combining and antenna arrays
The mobile communication channel is very hostile to a DS-CDMA signal and therefore effective techniques are needed to enhance system performance and capacity. Further, since DS-CDMA capacity and performance is limited by the uplink, ways to improve the uplink performance is needed. By implementing antenna arrays, diversity schemes or a combination of antenna arrays and diversity techniques, the uplink performance can be improved substantially. In this study we consider a single cell with a base station at the center with mobiles uniformly distributed around it. As channel model a Nakagami distributed path gain is assumed. This model was chosen for flexibility (e.g., Rayleigh and Rice channel models can be approximated) and also since empirical data suggests that path fading statistics are adequately described by this distribution. At the receiver an array of M antennas is used to discriminate between the users based on their spatial diversity. The fading process at each of the antenna elements is statistically dependent and further improvements can be realized by making use of the independent fading characteristics of the received signal. To make use of this statistical independent information, the performance of a P branch Maximum Ratio Combining (MRC) receiver is also considered. We further investigate the performance of a combination of P clusters of M antennas separated by the coherence bandwidth of the channel, thereby making use of both forms of spatial diversity. A comparison of the three schemes (antenna arrays, MRC diversity and a combination of antenna arrays and MRC diversity) under equal complexity conditions are made under multipath fading conditions. It is shown that the performance and capacity of a MRC diversity receiver outperforms the other two methods when perfect power control is assumed.
On uplink CDMA cell capacity: mutual coupling and scattering effects on beamforming
IEEE Transactions on Vehicular Technology, 2000
It has been shown that code-division multiple-access (CDMA) systems that employ digital beamforming and base-station antenna arrays have the potential to significantly increase capacity. Therefore, accurate performance prediction of such systems is important. We propose to take the electromagnetic behavior of the base-station antenna array into account, as well as its impact on wireless channel propagation. Specifically, the wide-band channel introduces scattering, while the mobile environment causes Doppler fading, which in turn degrades power controllability. We develop a more accurate performance analysis of antenna arrays, where the performance degradation in digital beamforming due to the combination of mutual coupling, scatter, and imperfect power control and its impact on uplink CDMA system capacity is quantified. In this analysis, a Rayleigh fading amplitude with varying angle-of-arrival spread is assumed, and maximum signal-to-noise ratio beamforming weights are used. These weights are further correlated with mutual coupling at the base-station array. Despite the degradation due to the combination of mutual coupling, scattering, and imperfect power control, significant capacity increases are possible.
International journal of Computer Networks & Communications, 2011
The interference reduction capability of antenna arrays, base station assignment and the power control algorithms have been considered separately as means to increase the capacity in wireless communication networks. In this paper, we propose base station assignment method based on minimizing the transmitter power (BSA-MTP) technique in a direct sequence-code division multiple access (DS-CDMA) receiver in the presence of frequency-selective Rayleigh fading and power control error (PCE). This receiver consists of constrained least mean squared (CLMS) algorithm, matched filter (MF), and maximal ratio combining (MRC) in three stages. Also, we present switched-beam (SB) technique in the first stage of the RAKE receiver for enhancing signal to interference plus noise ratio (SINR) in DS-CDMA cellular systems. The simulation results indicate that BSA-MTP technique can significantly improve the network bit error rate (BER) in comparison with the conventional case. Finally, we discuss on three parameters of the PCE, number of resolvable paths, and channel propagation conditions (path-loss exponent and shadowing) and their effects on capacity of the system via some computer simulations.
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The link level performance and the cellular system capacity in the uplink direction of a CDMA cellular system utilising multiuser detection base station receivers is analysed by simulation. In the receiver, parallel multistage multiuser detection is employed together with two-antenna diversity reception and fast closed-loop power control. A system level simulator is built to utilise the link level simulation results and to show the increase in cellular capacity obtained by using multiuser detection. The capacity is studied in urban micro and macro-cell environments utilising the channel models developed in the European CODIT project. The modelling of the environment specific and CDMA specific features is considered in the system simulator. The system level simulator is calibrated with analytical capacity calculations.
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Journal of Wireless …, 2009
In wireless Code Division Multiple Access (CDMA) system, the use of power control is indispensable to combat near-far and fading problems. Signals transmitted over a multipath propagation channel which exhibits inter-path interference and fading. The receiver has to employ measures to mitigate these effects or it will incur severe performance degradation. A classic approach in CDMA communications is the rake receiver. In this paper, the downlink performance is estimated for a CDMA mobile system at the vertex of multiple adjacent cells. At the base station the received signal is coherently dispread and demodulated using a rake receiver. The effects of power control, error correction and rake receiver were also investigated on the assumption that the received signals undergo Rayleigh fading, lognormal shadowing, and frequency selective fading. The evaluation of performance measures of base to mobile link (downlink) of a multiple-cell CDMA mobile system is presented. This study demonstrates that significant performance improvements are achievable with combined use of power control, rake receiver and error correction scheme.
Performance Study and System Capacity Analysis of a Two-Tier Cdma System
International Conference on Aerospace Sciences & Aviation Technology, 2007
One of the main factors that affects the performance of a Code Division Multiple Access (CDMA) system is the out of-cell interference. While the in-cell interference is controlled within each cell using a power control algorithm, the out of-cell interference is not and can greatly impact network coverage and the total user capacity, so that CDMA capacity is an interference limited capacity. One design technique to reduce the effect of the out of-cell interference in cellular CDMA systems is to use cells of different radii, hierarchically organized in overlapping layers, this is the multitier system. In This paper the user capacity on the reverse link direction of a certain two-tier CDMA system that consists of a small microcell embedded in a larger macrocell, i.e. a two cell system is examined. The effect of the users' mobility on the call blocking and call dropping probabilities is examined depending on the system feasibility. The simulation results show that at different call arrival rates the system capacity is increased as compared with the previous work. The dominant parameters affecting the system performance are the users' velocity and the distance between the two cells.
2009
In wireless Code Division Multiple Access (CDMA) system, the use of power control is indispensable to combat near-far and fading problems. Signals transmitted over a multipath propagation channel which exhib-its inter-path interference and fading. The receiver has to employ measures to mitigate these effects or it will incur severe performance degradation. A classic approach in CDMA communications is the rake receiver. In this paper, the downlink performance is estimated for a CDMA mobile system at the vertex of multiple ad-jacent cells. At the base station the received signal is coherently dispread and demodulated using a rake re-ceiver. The effects of power control, error correction and rake receiver were also investigated on the as-sumption that the received signals undergo Rayleigh fading, lognormal shadowing, and frequency selective fading. The evaluation of performance measures of base to mobile link (downlink) of a multiple-cell CDMA mobile system is presented. This study dem...
Spectral efficiency of CDMA uplink cellular networks
2005
Abstract In this contribution, the performance of an uplink CDMA system with random spreading and multi-cell interference is analyzed. A useful framework is provided in order to determine the base station coverage for wireless flat fading channels with very dense networks (in the number of users per meter) considering different receiver structures at the base station, namely the matched filter, the Wiener filter and the optimum filter.
IEEE Transactions on Wireless Communications, 2003
A theoretical characterization of intracell and intercell interference statistics in cellular direct-sequence code-division multiple-access systems in a multipath environment is presented considering both fast and slow power control. Unlike many of the previous papers, mobiles are assumed to connect to a base station according to a minimum attenuation criterion, also known as base station diversity. Interference statistics are used to estimate system capacity and results have been validated by Monte Carlo simulations. Our results confirm that much greater capacity can be achieved when multipath fading is compensated by power control, while the relative benefits of perfect compensation of multipath fading decreases as the number of resolvable paths at the receiver increases.
Comparison of antenna array techniques for the downlink of multi-carrier CDMA systems
Vehicular Technology Conference, 1988, IEEE 38th
The paper deals with transmit pre-filtering using antenna arrays in the downlink of wireless cellular systems based on multi-carrier code division multiple access (MC-CDMA). In addition to the signal separation in frequency obtained by spreading codes, the base station exploits the spatial separation of users to improve interference mitigation, which enables the use of low-complexity mobile receivers. We propose transmit filtering techniques optimizing the signal separation in space only, or jointly in space and frequency, according to single- and multi-user criteria. These techniques assume channel knowledge at transmission, and are particularly destined for time division duplex systems, where we may benefit from channel reciprocity between up- and downlink. We compare the performance of the proposed techniques in terms of interference reduction and robustness to time variant channels. Simulation results emphasize the benefit of transmit filtering optimized jointly in space and fre...