Wideband CDMA Systems Research Articles

Code-aided blind adaptive new user detection in DS/CDMA systems with fading time-dispersive channels

We consider the problem of detecting the entrance of a new-user in a direct-sequence code division multiple access (DS/CDMA) system operating over a fading dispersive channel. This problem has received considerable attention in the recent past in that it arises in a number of different contexts, such as decentralized user acquisition and data detection, handoff algorithms, soft handover procedures, and cell-search in wideband-CDMA systems. The detection algorithms that we propose in this paper are based on the application of the generalized-likelihood-ratio-test (GLRT) and can be implemented based on the knowledge of the spreading code of the user to be detected. In particular, the proposed procedures do not require any prior knowledge of either the propagation channel impulse response or the timing offset of the user to be detected. We consider both the cases of user detection in the reverse link of a cellular system, wherein it is assumed that the subspace spanned by the signals of the existing previous users is known to the receiver and that of user detection in the forward link, wherein the mobile receiver has no prior knowledge on the multiaccess interference. With regard to the latter situation, we develop a detection algorithm that ensures a constant false-alarm rate with respect to the second-order statistics of the overall disturbance. The performance of the proposed detection structures is finally assessed through closed-form formulas and through some sample plots, showing, for a given probability of false alarm, the probability that the new user is detected.

Spatial filtering for pilot-aided WCDMA systems: A semi-blind subspace approach

This paper proposes a spatial filtering technique for the reception of pilot-aided multirate multicode direct-sequence code division multiple access (DS/CDMA) systems such as wideband CDMA (WCDMA). These systems introduce a code-multiplexed pilot sequence that can be used for the estimation of the filter weights, but the presence of the traffic signal (transmitted at the same time as the pilot sequence) corrupts that estimation and degrades the performance of the filter significantly. This is caused by the fact that although the traffic and pilot signals are usually designed to be orthogonal, the frequency selectivity of the channel degrades this orthogonality at the receiving end. Here, we propose a semi-blind technique that eliminates the self-noise caused by the code-multiplexing of the pilot. We derive analytically the asymptotic performance of both the training-only and the semi-blind techniques and compare them with the actual simulated performance. It is shown, both analytically and via simulation, that high gains can be achieved with respect to training-only-based techniques.

Dynamic random access code assignment for prioritized packet data transmission in wcdma networks

We propose a measurement-based dynamic random access (RA) code assignment procedure for prioritized packet data transmission in wideband code-division multiple access (WCDMA) networks. This dynamic adaptation process is based on analytical performance results derived for random packet access under Rayleigh fading in WCDMA networks. The performance of the proposed measurement-based RA code assignment procedure with three different adaptation methods is evaluated by using computer simulations. The performance of the proposed scheme is compared with those of a retransmission control-based and static channel allocation-based prioritized packet access scheme. An integrated (physical layer and link layer) delay-throughput performance model is presented for finite population RA WCDMA systems. The proposed dynamic RA code assignment procedure can be used in an adaptive quality of service (QoS) framework for dynamically adjusting the QoS of prioritized RA data traffic in the evolving WCDMA-based differentiated services wireless Internet protocol networks.

A selective interference-cancellation receiver for antenna-array WCDMA communication systems

The paper deals with a multiuser cancellation-detection scheme combined with an antenna array for wideband code-division multiple-access (WCDMA) communication systems in which different traffic loads and data rates are considered. This receiver, defined as a space-time selective parallel interference-cancellation detector, should be used at the base station of a WCDMA communication system. Asynchronous users and multipath fading phenomena are considered. The receiver basic assumption is to divide signals into two different groups according to the received power level. The reliable signals are directly detected and cancelled from the whole received signal at each sensor before making the decision about unreliable signals without any further processing delay. Performance evaluations have been carried out in terms of P/sub e/ (bit error probability) by means of computer simulations. In particular, it is shown that our receiver exhibits good behavior and lowers the computational complexity in comparison with previously proposed alternatives.

System modeling and performance evaluation of rate allocation schemes for packet data services in wideband cdma systems

To fully exploit the potential of a wideband CDMA-based mobile Internet computing system, an efficient algorithm is needed for judiciously performing rate allocation, so as to orchestrate and allocate bandwidth for voice services and high data rate applications. However, in existing standards (e.g., cdma2000), only a first-come-first-served equal sharing allocation algorithm is used, potentially leading to a low bandwidth utilization and inadequate support of high data rate multimedia mobile applications (e.g., video/audio files swapping, multimedia messaging services, etc.). In this paper, we first analytically model the rate allocation problem that captures realistic system constraints such as downlink power limits and control, uplink interference effects, physical channel adaptation, and soft handoff. We then suggest six efficient rate allocation schemes that are designed based on different philosophies: rate optimal, fairness-based, and user-oriented. Simulations are performed to evaluate the effectiveness of the rate allocation schemes using realistic system parameters in our model.

Downlink throughput optimization for wideband CDMA systems

This article examines the optimization of throughput in a wireless system. A mathematical model is outlined for a cellular wideband CDMA system. In practical cases, where the maximum transmission power is constrained, power control may not be possible. Self-interference limits the total deliverable rate. It is shown that by optimally allocating bit-rate using a dynamic programming algorithm, a higher bit-rate can be delivered within the same power constraint. Simulation results show on average a 20% increase in total delivered rate can be achieved.

Effect of adjacent IS-95 network to wcdma uplink capacity

The paper derives the degradation of wideband code-division multiple-access (WCDMA) uplink capacity in the situation where an IS-95 system is deployed at the adjacent frequency band. A system modeling approach that combines accurate path loss information with simplified and computationally fast interference modeling is also described. The model takes into account the power-control coupling between IS-95 and WCDMA systems as well as the effect of downlink blocking. The paper also includes some numerical examples from realistic micro- and macrocellular network scenarios. The results show that the interference is dependent on, in addition to the filter characteristics, the used network scenario, whether the interfered or interfering system is microcellular or macrocellular, and on the relative cell densities of the interfering and interfered networks. The developed method can be utilized for WCDMA network planning, for frequency allocation, and for system design of the mobile telecommunication systems.

Delay profile measurement system using common pilot channel for W‐CDMA cellular system

AbstractThe communication quality in Wideband Code Division Multiple Access (W‐CDMA) is improved by temporally separating the arriving multipaths and combining them by RAKE combining. Thus, the delay profile measurement is essential in understanding the number of arriving paths and the received power in each path in the actual propagation paths. In this paper, we propose a system using the Common Pilot Channel (CPICH) to measure the delay profile from the base station that has already started a service. This system's features are (1) synchronous detection using only CPICH and (2) power delay profile and SIR delay profile measurements and their properties are clearly illustrated. We also present the experimental results of a prototype developed based on the proposed method. © 2003 Wiley Periodicals, Inc. Electron Comm Jpn Pt 1, 86(6): 33–45, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecja.10030

Performance analysis of selection diversity gain for narrow-band and wide-band systems

Usually employed at the receiving end to avoid deep fading, the well-known spatial antenna diversity technique may have different gain properties for the second generation global system for mobile communication (GSM) and the third generation wideband code division multiple access (W-CDMA) system because of the difference in channel bandwidth and receiver architecture. By means of statistical analysis, the diversity gain for these two systems is derived for both line-of-sight (LOS) and non-LOS (NLOS) channels. The theoretical results are verified by numerical simulation and experimental measurement. The GSM system always has better diversity gain (4-10 dB) than the W-CDMA system (1-4 dB) in a typical wireless channel with a K-factor from 0 to 8 dB.

Fast convergence distributed power control algorithm for WCDMA systems

The authors present a distributed power control algorithm for wideband code division multiple access (WCDMA) third generation mobile communications systems. In the algorithm, a new exponential type of power update function is defined. By altering the parameter a in the update function, the system can satisfy different performance requirements. The major properties of the algorithm are revealed and the convergence condition is investigated. Simulation results show that the new algorithm has faster convergence speed and better system stability compared to most existing distributed power control algorithms.

Downlink joint rate and power allocation in cellular multirate WCDMA systems

This paper proposes a novel dynamic joint rate and power control procedure for downlink data transmission in a multicell variable spreading factor wideband code-division multiple-access (WCDMA) system where the different users have similar quality-of-service requirements in terms of the signal-to-interference ratio (SIR). Two variations of the dynamic joint rate and power allocation procedure, namely, Algorithm-1 and Algorithm-2, are presented. The performances of these two schemes are compared to the performance of the optimal dynamic link adaptation for which the rate and power allocation is found by an exhaustive search. The optimality criterion is the maximization of the total radio link level capacity (or sum-rate capacity) in terms of the average number of radio link level frame transmitted per adaptation interval under constrained SIR and power limit in the base station transmitter. The proposed schemes have linear time complexity as compared to the exponential time complexity of the optimal scheme and achieve better radio link level throughput fairness compared to the optimal link adaptation scheme with a moderate loss in total throughput. Performance evaluation is carried out under random and directional micromobility models with uncorrelated and correlated long-term fading, respectively, in a cellular WCDMA environment for both the homogeneous (or uniform) and the nonhomogeneous (or nonuniform) traffic load scenarios.

Multicarrier cdma systems using time-domain and frequency-domain spreading codes

For wideband code-division multiple-access systems, the paper introduces a multicarrier modulation scheme that performs the spreading simultaneously in the time and frequency domains. This scheme attains higher flexibility and spectrum efficiency because system parameters can be selected at will. The performance is compared with that of a single carrier RAKE system by calculating the probability of error over a frequency-selective Rayleigh fading channel. The proposed scheme outperforms the single carrier RAKE system if the system parameters are selected properly for given conditions, such as bandwidth and delay spread.

Error probability performance for W-CDMA systems with multiple transmit and receive antennas in correlated nakagami fading channels

The bit error rate (BER) performance of a two-dimensional (2-D) RAKE receiver, in combination with transmit diversity on the downlink of a wide-band CDMA (W-CDMA) system, is presented. The analyses assume correlated fading between receive antenna array elements, and an arbitrary number of independent but nonidentical resolvable multipaths combined by the RAKE receiver in the general Nakagami-m (1960) fading channel framework. The impact of the array configuration (e.g., the number of transmit antennas and receive antennas, the antenna element separation) and the operating environment parameters (such as the fading severity, angular spread and path delay profile) on the overall space-path diversity gain can be directly evaluated. In addition, the exact pairwise error probability of a convolutional coded system is obtained, and the coding gain of a space-path diversity receiver is quantified.

Directional power-based admission control for WCDMA systems using beamforming antenna array systems

A directional power-based admission control (AC) scheme is proposed for base stations deploying beamforming antenna arrays. The proposed AC algorithm is a natural extension of the conventional power based AC scheme for cells using standard sector antennas. The directional scheme takes the load per beam/direction into account in the AC decision, so the cell load is controlled in accordance with the spatial filtering gain provided by using beamforming antenna arrays. The scheme is tested by means of Monte Carlo simulations in a multicell wideband code division multiple access (WCDMA) network, where users with different bit rates are present. The obtained results demonstrate that the directional power based AC scheme allows a higher traffic load in cells using beamforming antenna arrays without jeopardizing the stability of the network. Application of a four element antenna array is found to provide a capacity gain of a factor 2.7 for a network with many low bit rate users, while the gain is reduced to a factor 2.3 for traffic scenarios with a mixture of high and low bit rate users.

Performance analysis of cell search in W-CDMA systems over Rayleigh fading channels

The three-step cell search has been considered for fast acquisition of the scrambling code unique to a cell in wideband code-division multiple-access (W-CDMA) systems. In this paper, the performance of the cell-search scheme is analyzed in Rayleigh fading channels. The system parameters for the cell-search scheme and the design parameters for the receivers are examined. Probabilities of detection, false alarm, and miss for each of the three steps are derived in closed form based on the statistics of CDMA noncoherent demodulator output. Through the analysis, the effect of threshold setting and post-detection integration for each step is investigated and the power allocation for the channels is considered. The optimal number of post-detection integrations for each step may depend on not only the power allocation for the channels related to the cell search, but also the false-alarm penalty time. Our analysis can be utilized for determining the values of the parameters. Also, the cumulative probability distribution of the average cell-search time is obtained. Finally, it is shown that the analysis could be used to obtain the distribution of the cell-search time graphically according to the distance between a mobile station and a base station by considering the propagation models for pass loss and the traffic distribution models.

Hybrid TDOA/AOA mobile user location for wideband CDMA cellular systems

This paper proposes a mobile user location scheme for wideband code-division multiple-access (CDMA) wireless communication systems. To achieve high location accuracy and low cost of the mobile receiver, the location scheme combines the time difference of arrival (TDOA) measurements from the forward link pilot signals with the angle of arrival (AOA) measurement from the reverse link pilot signal. High chip rates in wideband CDMA systems facilitate accurate TDOA measurements, and a smart antenna used at the home base station (BS) can provide accurate AOA measurement in a macrocell environment. A two-step least square location estimator is developed based on a linear form of the AOA equation in the small error region. Numerical results demonstrate that the proposed hybrid TDOA/AOA location scheme gives a much higher location accuracy than TDOA only location, when the number of base stations is small and/or when the TDOA measurements have a relatively poor accuracy.

3GPP W-CDMA systems: design and testing

Newly emerging wireless signal formats can present significant design challenges to the system design engineer. Wideband code division multiple access (W-CDMA) can present many engineering design and verification challenges, due to its physical layer complexity as well as complexities that can arise when integrating RF and baseband designs to achieve a working system. System design engineers typically have the task of interpreting standards specifications, first for partitioning subsystem requirements, then verifying that the end-to-end system design (typically including both baseband and RF designs) will meet specifications when they are finally integrated. This article considers a case study that illustrates the importance of simulating baseband and RF designs together to accomplish this task. Specifically, a conceptual system design that includes system models, a fixed-point finite impulse response filter, and transistor-level circuit design is simulated and verified together against measurements defined in the third-generation partnership project (3GPP) W-CDMA standards specifications. Tradeoffs between improving either the RF or baseband design to meet overall system specifications will be discussed. Finally, design verification using "connected solutions," or connectivity between, electronic design automation software and test equipment will be described, illustrating how verification testing can be started earlier to help reduce risk by identifying performance issues early in the design cycle, when they are potentially easier and costly to implement.

Improved channel assignment scheme with access control for the common packet channel in WCDMA systems

We propose a channel assignment scheme with access control for obtaining better system performance within the common packet channel (CPCH) access procedure of wideband code-division multiple-access (WCDMA) systems. The proposed scheme is analyzed taking multiple access interference into consideration and is compared with other schemes in the 3rd Generation Partnership Project (3GPP): the basic scheme, the channel monitoring scheme, and the channel assignment scheme without access control. The results show that the proposed scheme provides high throughput, even under high-loading conditions, since the access control algorithm based on channel load prohibits excessive interference.

Performance analysis of slotted random access channels for W-CDMA systems in Nakagami fading channels

We perform a new analysis of the performance of the wideband code-division multiple access (W-CDMA) randomized slotted-offset random access channel (RAC) with Nakagami-m fading and compare it with that of a comparable randomized slotted-ALOHA-based RAC. Previously, it was shown that the W-CDMA RAC is fast, robust and simple to implement (see Esmailzadeh, R. and Gustafsson, M., Proc. 1997 IEEE ICUPC Conf, p.43-7, 1997). However, it was assumed that any random access packet arrival events involving multiple packets from cochannel mobile stations would result in lost packets. Closed-form equations are developed for the collision process of the arriving random access packets in slotted-offset and slotted-ALOHA RACs. An analysis is developed of the signal-to-interference-and-noise ratio of a packet involved in a multipacket DS-CDMA collision with a generalized Nakagami fading channel and diversity reception at the base station. A closed-form equation is developed for the bit error rate and packet throughput of the slotted-offset- and slotted-ALOHA-based RACs. The results of the analysis show that considerable improvement in the RAC performance can be achieved when multiple arrival collisions, on the same timeslot and with the same signature code, are partially resolved. The results also show the RAC's sensitivity to fading severity, packet E/sub b//N/sub 0/, spreading gain and number of base station receive selection diversity antennas. Compared to the randomized slotted-ALOHA designs, W-CDMA RAC performs as well in light fading conditions and slightly worse in severe fading conditions.

Optimal resource management in wireless multimedia wideband CDMA systems

This paper proposes a scheme of optimal resource management for reverse-link transmissions in multimedia wideband code-division multiple-access (WCDMA) communications. It is to guarantee quality-of-service (QoS) by resource (transmit power and rate) allocation and to achieve high spectral efficiency by base-station assignment. This approach takes the form of a nonlinear-programming large-scale optimization problem: maximizing an abstraction for the profit of a service provider subject to QoS satisfaction. Solutions for both single-cell and multicell systems are investigated. The single-cell solution has the advantage of low complexity and global convergence in comparison with the previous work. Maximum achievable throughput (capacity) of a single cell is mathematically evaluated and used as the benchmark for performance measure of multicell systems. For multicell systems, due to its max-max structure, solving the optimization problem directly entails a high-computational complexity. Instead, the problem is reformulated to a mixed integer nonlinear-programming (MINLP) problem. Then, binary variables indicating base-station assignments are relaxed to their continuous analogs to make a computer solution feasible. Furthermore, approximations can be made to make the resource-management scheme less computationally complex and allow its partial decentralization. The sensitivity of the proposed scheme to path-gain estimation error is studied. Simulation results are presented to demonstrate the performance of the proposed scheme and the throughput improvement achieved by combining resource allocation with base station assignment.