Capacity improvement of multicarrier DS-CDMA system with antenna array

An antenna array based base station receiver for multicarrier Direct Sequence Code Division Multiple Access （DS-CDMA） system is proposed. The main advantage of the receiver is that the spatial diversity is achieved by combining signals of array elements. Based on the detailed analysis of multiuser interference and noise characteristics, the performance of the proposed receiver is analyzed. Theoretical analysis shows significant performance improvement in terms of system capacity due to the use of antenna arrays compared with the conventional single antenna multicarrier DS-CDMA approach. Simulation results confirm the theoretical analysis.     

Joint optimal power control and beamforming in wireless networksusing antenna arrays

The interference reduction capability of antenna arrays and the power control algorithms have been considered separately as means to increase the capacity in wireless communication networks. The minimum variance distortionless response beamformer maximizes the signal-to-interference-and-noise ratio (SINR) when it is employed in the receiver of a wireless link. In a system with omnidirectional antennas, power control algorithms are used to maximize the SINR as well. We consider a system with beamforming capabilities in the receiver, and power control. An iterative algorithm is proposed to jointly update the transmission powers and the beamformer weights so that it converges to the jointly optimal beamforming and transmission power vector. The algorithm is distributed and uses only local interference measurements. In an uplink transmission scenario, it is shown how base assignment can be incorporated in addition to beamforming and power control, such that a globally optimum solution is obtained. The network capacity and the saving in mobile power are evaluated through numerical study     

Performance of wireless CDMA with M-ary orthogonal modulation andcell site antenna arrays

An antenna array-based base station receiver structure for wireless direct-sequence code-division multiple-access (DS/CDMA) with M-ary orthogonal modulation is proposed. The base station uses an antenna array beamformer-RAKE structure with noncoherent equal gain combining. The receiver consists of a “front end” beamsteering processor feeding a conventional noncoherent RAKE combiner. The performance of the proposed receiver with closed loop power control in multipath fading channels is evaluated. Expressions for the system uncoded bit-error probability (BEP) as a function of the number of users, number of antennas, and the angle spread are derived for different power control scenarios. The system capacity in terms of number of users that can be supported for a given uncoded BEP is also evaluated. Analysis results show a performance improvement in terms of the system capacity due to the use of antenna arrays and the associated signal processing at the base station. In particular, analysis results show an increase in system capacity that is proportional to the number of antennas. They also show an additional performance improvement due to space diversity gain provided by the array for nonzero angle spreads     

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. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effects of Nakagami-Fading Parameters and Power Control Error on Performance of DS-CDMA Cellular Systems with Adaptive Beamforming

It is well known that power control error (PCE) is a critical issue in CDMA cellular systems. In this paper, the bit error rate (BER) of a direct sequence-code division multiple access (DS-CDMA) receiver with imperfect power control, adaptive beamforming, and voice activity is derived in frequency-selective Nakagami fading channels. We discuss the effects of PCE, Nakagami-m fading parameter, and channel’s multipath intensity profile as average signal strength and rate of average power decay and their effects on the BER performance of DS-CDMA cellular systems. In this paper, the RAKE receiver consists of three stages. In the first stage, with conjugate gradient adaptive beamforming algorithm, the desired users’ signal in an arbitrary path is passed and the inter-path interference is canceled in other paths in each RAKE finger. Also in this stage, the multiple access interference (MAI) from other users is reduced. Thus, the matched filter (MF) can be used for the MAI reduction in each RAKE finger in the second stage. In the third stage, the output signals from the MFs are combined according to the conventional maximal ratio combining principle and then are fed into the decision circuit for the desired user. How the Nakagami fading parameters, power control imperfections, or the number of resolvable paths affect the reverse link capacity of the system is discussed in detail. Analytical and simulation results are also given for systems with different processing gains and number of BSs in the cell-selection process with various Nakagami fading parameters.     

Software radio architecture with smart antennas: a tutorial onalgorithms and complexity

There has been considerable interest in using antenna arrays in wireless communication networks to increase the capacity and decrease the cochannel interference. Adaptive beamforming with smart antennas at the receiver increases the carrier-to-interference ratio (CIR) in a wireless link. This paper considers a wireless network with beamforming capabilities at the receiver which allows two or more transmitters to share the same channel to communicate with the base station. The concrete computational complexity and algorithm structure of a base station are considered in terms of a software radio system model, initially with an omnidirectional antenna. The software radio computational model is then expanded to characterize a network with smart antennas. The application of the software radio smart antenna is demonstrated through two examples. First, traffic improvement in a network with a smart antenna is considered, and the implementation of a hand-off algorithm in the software radio is presented. The blocking probabilities of the calls and total carried traffic in the system under different traffic policies are derived. The analytical and numerical results show that adaptive beamforming at the receiver reduces the probability of blocking and forced termination of the calls and increases the total carried traffic in the system. Then, a joint beamforming and power control algorithm is implemented in a software radio smart antenna in a CDMA network. This shows that, by using smart antennas, each user can transmit with much lower power, and therefore the system capacity increases significantly     

Performance Analysis of Multicarrier DS-CDMA with Antenna Array in a Fading Channel

In this paper, an antenna array based base station receiver structure for multicarrier direct sequence code-division-multiple-access (Multicarrier DS-CDMA) system with binary phase shift keying (BPSK) modulation is proposed. One of the main advantages of the receiver structure is that the spatial diversity is obtained by combining signals at different array elements. Based on the detailed analysis of multiple access interference (MAI) and additive white Gaussian noise (AWGN) characteristics, the uplink bit error rate (BER) performance of the proposed antenna array Multicarrier DS-CDMA is provided. With regard to spatial domain combining, the optimum spatial combining (OSC) and suboptimum spatial combination (SOSC) weights is derived while the suboptimum set of weights is simplified in the sense that only the knowledge of array vector of desired user alone is sufficient for the combining. Simulation results verify the analysis, and it is shown that MAI is mitigated and subsequently the system performance is improved significantly by incorporating antenna array at the base station.     

Adaptive arrays for narrowband CDMA base stations

Base station antenna arrays are a promising method for providing significant capacity increases in cellular mobile radio systems. This paper examines receiver structures and algorithms to assess the potential capacity gains from the employment of multiple receiver antenna elements, of different sizes, for code division multiple access (CDMA) systems. It considers antenna arrays for the mobile to-base station or reverse link of a CDMA cellular system such as the IS-95 standard. It begins with an introduction to CDMA communication systems and also addresses the general topic of antenna array receivers. Channel modelling is then discussed, as this will influence the design of CDMA receivers. The specific form of receiver array processing algorithms is then discussed and some performance comparisons provided. Finally, the most important reason for implementing antenna array systems, the capacity gains which are achievable, is indicated     

A Low Complexity Downlink Beamforming Scheme for DS-CDMA System

The downlink capacity of frequency division duplex (FDD) based DS-CDMA system can be improved if multielement antenna array is equipped at the base station. In this paper, a novel two-path downlink beamforming (TPDB) scheme is proposed to reduce multiple access interference as well as providing two-order path diversity in downlink. An analysis model is presented for the capacity evaluation of multirate DS-CDMA system with base station antenna array, and the capacity performance of the TPDB is compared with other schemes based on this model. The comparison results show that the TPDB would be a promising candidate for the downlink transmission if both performance and implementation complexity are considered. Moreover, a simple algorithm is proposed for the steering vector estimation in the TPDB, and the robustness of this estimation algorithm in the presence of fading and interference is also confirmed by computer simulations.     

Partial zero-forcing adaptive MMSE receiver for DS-CDMA uplink in multicell environments

Adaptive multi-user detection techniques for interference suppression in direct-sequence code division multiple access (DS-CDMA) systems have gained much attention since they do not require any information on interfering users. In the uplink of DS-CDMA systems, however, the base station receiver typically knows the spreading waveforms of the users within its cell but does not know those of the users in other cells. We propose a partial zero-forcing adaptive minimum mean squared error (MMSE) receiver for the DS-CDMA uplink utilizing the spreading waveforms known at the base station as well as training data. The proposed receiver first removes the intracell interference using a linear filter based on the knowledge of the spreading waveforms of the interfering users within the cell. Then the intercell interference remaining in the output of the linear filter is mitigated by adaptive MMSE detection. To speed up the convergence of the adaptive filter weights without loss of the steady-state performance, we develop a modified least mean square (LMS) algorithm based on the canonical representation of the filter weights. It is shown through analysis and simulation results that the proposed receiver improves the convergence speed and the steady-state performance.     

On uplink CDMA cell capacity: mutual coupling and scattering effects on beamforming

It has been shown that code-division multiple-access (CDMA) systems that employ digital beamforming and base station antenna arrays have the potential to increase capacity significantly. 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 wideband 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. 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.     

Noncoherent MLSE in DS-CDMA wireless systems with antenna arrays

This paper addresses the noncoherent detection problem for direct-sequence code-division multiple-access (DS-CDMA) signals in the presence of antenna arrays at the base station. An ideal open-loop power control, which compensates for the long-term variations due to path loss and shadowing experienced by the mobile users, is assumed. Then, the noncoherent maximum-likelihood sequence-estimation problem is analyzed under the assumption of spatially white interference. The contribution of this paper is to extend previous results obtained by the author in for the case of multiantenna receivers, including the derivation and analysis of two suboptimal detectors based on an approximate maximization of the transmitted symbols likelihood. The first detector works as a traditional squared envelope combiner, which fully exploits the K diversity degrees provided by the multiple antenna system. In the second case, the receiver assumes a perfect knowledge of antenna response vector, so that spatial filtering of the received signal is made possible. Simulation and theoretical results are provided for a widely used circular ring of scatterers spatial channel model, which allows analysis in a wide range of different environmental conditions.     

Call Admission Policies Based on Calculated Power Control Setpoints in SIR-Based Power-Controlled DS-CDMA Cellular Networks

In this paper, we develop call admission control algorithms for SIR-based power-controlled DS-CDMA cellular networks. We consider networks that handle both voice and data services. When a new call (or a handoff call) arrives at a base station requesting for admission, our algorithms will calculate the desired power control setpoints for the new call and all existing calls. We will provide necessary and sufficient conditions under which the power control algorithm will have a feasible solution. These conditions are obtained through deriving the inverse of the matrix used in the calculation of power control setpoints. If there is no feasible solution to power control or if the desired power levels to be received at the base station for some calls are larger than the maximum allowable power limits, the admission request will be rejected. Otherwise, the admission request will be granted. When higher priority is desired for handoff calls, we will allow different thresholds (i.e., different maximum allowable power limits) for new calls and handoff calls. We will develop an adaptive algorithm that adjusts these thresholds in real-time as environment changes. The performance of our algorithms will be shown through computer simulation and compared with existing algorithms.     

基于HFSS的智能天线自适应算法的研究

以3G基站系统智能天线阵列为研究对象,在研究了常用智能天线自适应算法的基础上,针对常用算法测向精度有限以及来波方向估计过程的复杂性等问题,通过在接收机的输出端引入阵列导向矢量信号,利用阵列导向矢量的特殊性质,得出了一种改进后的自适应算法。采用该算法的智能天线阵列,不仅具有较强的波束搜索能力和抗干扰性能,而且不存在复杂的来波方向估计过程。最后通过对8元直线型智能天线阵列的HFSS仿真实验,得出了SINR、驻波比、方向图等特性曲线,验证了该算法的可行性。     

Oscillating-beam smart antenna arrays and multicarrier systems: achieving transmit diversity, frequency diversity, and directionality

We introduce a novel merger of smart antenna arrays and multicarrier code-division multiple-access (MC-CDMA) systems. Here, each group of Q carriers in the MC-CDMA system is applied to its own M-element smart antenna at the base station (BS). The smart antennas are separated by a distance that ensures that signals generated by each smart antenna are independent. Applying proper time-varying phases to array elements of each smart antenna array, the beam pattern is carefully controlled to generate a mainlobe at the position of the intended user and small oscillations in the beam pattern. This beam-pattern oscillation creates a time-varying channel with a controllable coherence time and a time diversity benefit at the receiver. Employing MC-CDMA with the proposed antenna array at the BS, we achieve: 1) directionality, which supports space-division multiple access (SDMA); 2) a time diversity gain; 3) increased capacity and performance via MC-CDMA's ability to support both CDMA and frequency diversity benefits. Hence, merging MC-CDMA and BS antenna arrays in an innovative fashion, we achieve high performance at the mobile via joint frequency-time diversity, and high network capacity via joint space-code division multiple access.     

Analysis of a class of distributed asynchronous power controlalgorithms for cellular wireless systems

In cellular wireless communication systems, uplink power control is needed to provide each mobile user with an acceptable signal to interference ratio (SIR) while simultaneously minimizing transmit power levels. We consider a class of distributed asynchronous power control algorithms based on the schemes used in IS-95 inner loop power control. Each user's received SIR is measured (using possibly outdated information) and compared to a threshold, and a single control bit is then sent to the user, indicating whether its power level should be increased or decreased. The SIR measurements and power updates do not require synchronization. We show that under certain conditions, this class of algorithms is stable and converges to a region around the optimal power assignment. We characterize this region and show that it can be made as small as desired by choosing the algorithm parameters appropriately. For an appropriate choice of algorithm parameters, we show that convergence occurs in a finite number of iterations and derive an upper bound. To illustrate our general results, we apply them to systems with fixed base station assignment, dynamic base station assignment, and macrodiversity. Finally, we give an example to illustrate the algorithm's robustness to errors in the power control commands     

Modeling of Single-Step Power Control Scheme in Finite-State Markov Channel and Its Impact on Queuing Performance

In this paper, we analyze the queuing performance in the finite-state Markov channel (FSMC) when the single-step power control (SSPC) scheme is adopted. To start with, an SSPC model and its extended power control error (PCE) model are proposed and described in detail. Such models can emulate the behaviors of the interaction between the SSPC and an FSMC and the variation of the PCE with high accuracy. Furthermore, the packet error rate that has been recently analyzed by Lee and Chang is adopted to evaluate the service rate, the time average queuing length, and the probability mass function (PMF) of the queuing length variation. Based on these results, a queuing variation model is proposed to emulate the queuing variation in the FSMC. The flooring phenomenon of the queuing length is observed under a particular requirement of an overflow probability when the SNR value is greater than a certain value. This indicates that the tradeoff among the SNR, the buffer size of the queue, and the overflow probability exists. Hence, two different approaches to select the optimal target SNR of the SSPC and the maximum buffer size of the queue under the requested queuing buffer overflow probability are proposed. The improvement of the SSPC, its impact on the system, and the validity of the proposed models are shown in this paper as well.     

Downlink beamforming for DS-CDMA mobile radio with multimediaservices

Downlink beamforming is a promising technique for direct-sequence code-division multiple-access (DS-CDMA) systems with multimedia services to effectively reduce strong interference induced by high data rate users. A new downlink beamforming technique is proposed that converts the downlink beamforming problem into a virtual uplink one and takes into account the data rate information of all users. Since the main complexity of this method is due to the existence of multidelay paths, two simplified algorithms are suggested using an equivalent one-path channel vector to replace multipath channel vectors. Computer simulation results are given to evaluate the downlink capacity of DS-CDMA systems using a base station antenna array and the new algorithms proposed     

Intelligent antenna system for cdma2000

Third-generation (3G) cellular code division multiple access (CDMA,) systems can provide an increase in capacity for system operators over existing second-generation (CDMA) systems. The gain in capacity for the base station to mobile (forward) link can be attributed to improvements in coding techniques, fast power control, and transmit diversity techniques. Additional gains in the mobile to base station (reverse) link can be attributed to the use of coherent quadrature phase shift keyed (QPSK) modulation and better coding techniques. While these enhancements can improve the performance of the system, system operators expect that with increased demand for data services, even greater capacity enhancements may be desired. There are essentially three methods, which we describe, based on diversity, spatial beamforming, and a combination of diversity and beamforming, to improve the performance of system through the use of additional antennas at the base station transmitter and receiver. The performance improvements are a function of the antenna spacings and the algorithms used to weight the antenna signals. We focus on the possibilities for the cdma2000 3G system that do not require standards changes. We highlight the performance enhancements that can be obtained on both the reverse and forward links through use of an antenna array architecture that supports a combination of beamforming and transmit diversity. We focus on the performance enhancements for the forward link