Wideband CDMA Systems Research Articles

Performance and Optimization of Multistage Partial Parallel Interference Cancellation for Wideband CDMA Systems in Multipath Fading Channels

In this paper, a closed-form bit error rate (BER) of hard-decision multistage partial parallel interference cancellation (PPIC) having perfect and imperfect channel estimation for complex spreading code-division multiple access (CDMA) over multipath Rayleigh channels was derived. The effects of receiving BER, power ratio of pilot to data channels, multiple-access interference (MAI) and multipath interference, additive white Gaussian noise, and other factors affecting channel estimation error are analyzed. The expression determining the optimal interference cancellation weight at path level for PPIC is first derived. Then, the oscillatory behavior of user-level-optimized PIC and the convergence of perfect channel estimation PIC and path-level-optimized PIC are validated. Results indicate that all factors determining the optimal weight certainly affect the BER performance of PPIC and that the lower BER limit of multistage PPIC depends on the adjustable factors, which include power ratio, observation length, and signal-to-noise ratio, and an unchangeable factor, i.e., number of users. In addition, the variance of MAI for complex spreading sequence is obtained, and the analytical expression can also be employed to predict the performance of multicode CDMA

Capacity Analysis in Downlink WCDMA Systems Using Soft Handover Techniques With SIR-Based Power Control and Site Selection Diversity Transmission

This paper analyzes the downlink performance of a wideband code-division multiple-access system with site selection diversity transmission power control during soft handover (SHO) mode. Signal-to-interference ratio power control techniques are modeled and used in the simulations of this analysis. The study is focused on finding the optimum SHO margin in terms of maximum system capacity under energy-per-bit to noise spectral density ratio (Eb/N0 ) quality requirements. The results of this analysis show an increase in user capacity of about 15%-20% for optimum SHO margins of 5-5.5 dB. Nevertheless, the resources required (number of scrambling codes) by the base station increase faster than the number of active users in terms of SHO margin up to values of approximately 9.5 dB

DS-CDMA downlink system capacity enhancement through interference suppression

In the downlink of direct-sequence code-division multiple-access (DS-CDMA) cellular systems, system capacity is limited by multiuser interference. This interference can be suppressed at the receiver by interference whitening approaches such as chip equalization and generalized RAKE (G-RAKE) reception. In this paper, we show how these advanced receivers can increase system capacity. First, the G-RAKE receiver formulation is extended to include soft handoff and other-cell interference. Then, voice capacity is evaluated, including the effects of fast fading, interference correlation, power control, orthogonality properties of own-cell signals, and soft handoff. Numerical results for the wideband CDMA system show that when the channel is dispersive, advanced receivers improve capacity by 30-34%

Diversity Combining for Soft/Softer Hand-Over in DS-CDMA

In soft/softer hand-over of a wideband CDMA system, the RAKE receiver combines signals from different base stations by assigning separate fingers. A generalized maximal ratio combining technique for soft/softer hand-over is derived. Two correction schemes to optimize the MRC are proposed and compared against a conventional MRC.

On the Performance of an Indicator-Based Stall Avoidance Mechanism for High-Speed Downlink Packet Access Systems

The stall of delivering medium access control (MAC) layer data to the upper layer is a serious problem when a negative acknowledgement (NACK) control signal becomes an acknowledgement (ACK) signal, especially for a high-speed mobile terminal during handoff. Stall avoidance mechanisms aim to reduce such the transmission delay and keep in-sequence delivery of the MAC layer data to the upper layer. Recently, for providing high-speed downlink packet access (HSDPA) in the wideband code-division multiple access system, an indicator-based stall avoidance (ISA) mechanism was proposed to remove the nonrecoverable gap in the received out-of-sequence packets. In this paper, we derive the closed-form expression for the gap-processing time of the ISA mechanism when applying the multiprocess stop-and-wait (SAW) hybrid automatic repeat request (HARQ) mechanism. The derived analytical formulas can be used to understand performance tradeoffs between the gap-processing time and throughput in terms of various numbers of users and parallel processes when implementing the multiprocess SAW HARQ mechanism in the HSDPA system.

Maximizing the Effective Channel Capacity of Power Controlled WCDMA Systems under Rayleigh Fading Environments

This paper studies the optimization of the effective channel capacity of wideband code division multiple access (WCDMA) systems under Rayleigh fading environments. Firstly, the results for Shannon capacity of fading channels with channel side information are reviewed, where the capacity is achieved by using an optimal power control scheme. Secondly, an optimal interference threshold α * 0 is set for a given system outage probability P our to minimize total interference. Finally, the effective channel capacity of WCDMA is defined and a target SIR level y* is derived with the Lagrangian multiplier method to maximize the effective channel capacity. It is shown that α * 0 is dependent on the power control interference ratio (PCIR) p, the number of diversity paths identified by the receiver M, and the outage probability of the system. Simulation results are provided to validate the theoretical deductions. We conclude that the total effective channel capacity will be maximized as long as M ≥ 4, and p ≤ 0.5 for a proper value of α * 0 .

Code placement and replacement schemes for WCDMA Rotated-OVSF code tree management

Orthogonal variable spreading factor (OVSF) channelization codes are widely used to provide variable data rates for supporting different bandwidth requirements in wideband code division multiple access (WCDMA) systems. Many novel works in the literature have intensively investigated code placement and replacement schemes in OVSF code trees to reduce the code blocking probability and the code reassignment cost. In this paper, we introduce a new code tree structure, namely, a rotated-orthogonal variable spreading factor (ROVSF) code tree, whose code capacity is the same as that of the traditional OVSF code tree. This work addresses both code placement and replacement schemes in the ROVSF code tree system, where ROVSF codes can be used at the forward link of WCDMA systems. Some valuable properties of the developed ROVSF code tree are presented to develop code placement/replacement schemes with lower code blocking probability and less code reassignment cost. The main contribution of our scheme is to identify linear-code chains (LCCs) and nonlinear-code trees (NCTs) in the ROVSF code tree. This work exploits the unsequence property of linear-code chains to design a new code placement and replacement mechanism. Our code placement/ replacement schemes initially attempt to allocate request codes on LCCs and, then, to allocate them to NCTs. Using LCCs with the unsequence property allows us to efficiently reduce the code blocking probability and the code reassignment cost. Extensive simulations are conducted to illustrate that our code placement/replacement results based on the ROVSF code tree actually improve the code blocking probability and the code reassignment cost.

QoS‐aware call admission control in wideband CDMA wireless networks

AbstractEfficient call admission control (CAC) technique plays a major role in ensuring the quality of service (QoS) requirements of different traffic classes and achieving flexible radio resource utilization in wideband CDMA system like universal mobile telecommunication system (UMTS). In this paper, we propose a novel QoS‐aware CAC framework for radio access in wideband wireless UMTS networks. It features an efficient CAC algorithm coupled with a QoS class‐separation mechanism based on the transmitted power of each individual mobile terminal. Three inter‐relating components have been introduced to extend a currently existing UMTS uplink admission control scheme. First, we introduce a measurement‐based component to calculate the current load of the system; second, this measurement‐based component is integrated with a power prediction module to estimate the load increment that the new call will bring into the system; and third, the proposed framework feeds the results obtained to a CAC algorithm with a QoS‐enforcing mechanism that gives each class of traffic different treatment based on the QoS requirement of the connections. To the best of our knowledge, ours is a first attempt towards combining the above components into one uplink CAC framework that aims to enhance system performance and to achieve per‐class QoS objectives. Simulation results show the major impact on the performance of UMTS which is reflected in increased throughput and lower blocking and dropping. Copyright © 2006 John Wiley & Sons, Ltd.

A Method for Designing Fixed Multibeam Antenna Arrays in WCDMA Systems

In this letter, simulated annealing (SA) is used for designing a fixed multibeam antenna array (AA) in a multiuser radio wide-band code-division multiple-access (WCDMA) system. The method can be used dynamically in a real system and/or in a radio planning tool. The proposed method is applied on a linear array by designing the antenna element (AE) pattern, the number of fixed beams, the AE spacing, and the AA transmit weights. The proposed method leads to a substantial base station (BS) power reduction which can be used to increase the system capacity or data throughput in a WCDMA system.

Capacity of future WCDMA networks supporting multimedia services

This paper presents an analytical model suitable to evaluate wideband code-division multiple-access (WCDMA) system capacity in terms of supported multimedia services achieving the required percentage of satisfied users. The satisfied-user criteria proposed by the Universal Mobile Telecommunications System introduce session-level performance measures, providing a more comprehensive notion of users' quality-of-service (QoS), but requiring knowledge of both outage probability and outage correlation behavior. By using a two-state (good-bad) Gilbert channel to represent the outage behavior, the model parameters and satisfied-user probability for both speech and data sessions are derived for different uplink and downlink scenarios. The proposed analytical model can be used for WCDMA system dimensioning to assess the throughput and session interruption probabilities of data services and their impact on the satisfied-user probability of speech services.

Call admission and code allocation strategies for WCDMA systems with multirate traffic

In this paper, call admission and code allocation schemes are proposed to provide service differentiation in the forward link of wideband code-division multiple-access (WCDMA) systems. In particular, this paper proposes multiple leaf code reservation (MLCR) schemes, where different numbers of orthogonal variable spreading factor (OVSF) leaf codes (i.e., codes of the lowest layer of the OVSF code tree) are reserved to differentiate users with different bandwidth requirements. Leaf codes are only reserved for as long as the call admission process lasts. Once the decision of whether a new request is admitted or not has been made, a Code Dereservation procedure is carried out to increase flexibility in the code assignment phase. The performance of these MLCR strategies with/without code reassignments is then evaluated. Analysis shows that MLCR schemes are also useful in improving fair access among different traffic classes. In addition, perfect fair access among requests with different data rates can be achieved when code reassignments are jointly employed with the proposed OVSF-code reservation schemes.

Packet access using DS-CDMA with frequency-domain equalization

The next-generation mobile communications system is anticipated to support very high-speed data rates exceeding several tens megabits per second. In this paper, we consider high-speed downlink packet access for direct-sequence code-division multiple access (DS-CDMA) as in third-generation wideband code-division multiple-access systems. Adaptive modulation and coding (AMC), multicode operation and hybrid automatic repeat request (HARQ) will be the enabling technologies. With such high-speed data transmissions, however, multicode operation severely suffers from the loss of orthogonality among the spreading codes since the wireless channel becomes severely frequency-selective. In this paper, we apply frequency-domain equalization (FDE) based on minimum mean-square error (MMSE) criterion instead of conventional rake combining for receiving the packet. A new MMSE-FDE weight is derived for packet combining. The throughput in a frequency-selective Rayleigh-fading channel is evaluated by computer simulation for Chase combining and incremental redundancy (IR) packet combining. It is shown that the use of MMSE-FDE for the reception of multicode DS-CDMA packet gives an improved throughput irrespective of the channel's frequency-selectivity.

Capacity of the WCDMA System Using High Altitude Platform Stations

In this paper, we provide a method to estimate the capacity of the wideband code division multiple access (WCDMA) scheme using High Altitude Platform Stations (HAPS), and demonstrate the results. This is to validate that the HAPS system is one of the most effective solutions to provide high quality wireless communication services, such as IMT-2000 services. We use a practical system model which describes the HAPS system most appropriately, and this leads to realistic estimation results. We estimate the reverse and forward link capacity of the HAPS WCDMA system and compare it to that of the terrestrial system. The estimation results show that the capacity of the HAPS WCDMA system is about 1.2–1.67 times larger than that of the terrestrial system.

Joint Reliability of Medium Access Control and Radio Link Protocol in 3G CDMA Systems

In this paper, we study the reliability as offered jointly by the medium access control (MAC) and the radio link protocol (RLP) for third generation (3G) code division multiple access (CDMA) standards. The retransmission mechanism supported at the RLP layer has a considerable amount of delay (80-100 ms) associated with it; hence, it may not be able to support applications with strict delay requirements. On the other hand, if retransmissions are also performed at a lower layer, such as the MAC layer, then the performance of CDMA systems can be improved because of the fast retransmissions at the MAC layer. We show how the performance of RLP improves with respect to three metrics - mean delay, throughput, and RLP recovery -when a finite number of retransmissions is allowed at the MAC layer. Synthetically generated Web traffic is used as the application, the objects of which are fragmented into equal-sized RLP frames. We consider soft packet combining support at the receiver, which effectively lowers the frame error rate. We also consider the possibility of misinterpretations of acknowledgments. Simulation experiments are conducted to verify the performance as offered jointly by the MAC and the RLP, particularly for cdma2000 standard and wideband CDMA (WCDMA) systems. The improvement in the TCP throughput is also evaluated.

Markov-Based OVSF Code Assignment Scheme and Call Admission Control for Wideband-CDMA Communication Systems

For the reason of the orthogonal characteristic of the Orthogonal Variable Spreading Factor (OVSF) code tree in Wideband CDMA (WCDMA) systems, code blocking increases as traffic load (i.e. Erlang load) or the required rate increases. This causes inefficient utilization of channelization codes. Hence, how to efficiently manage the resource of channelization codes of the OVSF code tree in WCDMA systems is an important issue and has been studied extensively. There are two aspects to achieve efficiency including code assignment and code reassignment. In the aspect of code assignment, an efficient code assignment scheme reduces code blocking probability significantly. In the aspect of code reassignment, code reassignment results in several drawbacks, such as large overhead of computation, high complexity of codes moving, and long call setup time for a new request call, etc. Therefore, in this paper we focus on the first aspect of how to efficiently assign the channelization codes. Additionally, most researches did not consider the analysis of tree state with dynamic traffic load and their analysis lack of systematic call admission control (CAC) mechanism. Therefore, in this paper, we first propose the Markov decision process (MDP) based analysis to assign channelization codes efficiently. Next, we extend the MDP-based approach as the call admission control mechanism to maximize the system revenue while reducing blocking probability. Furthermore, a bit string masking algorithm is proposed to reduce the time complexity of tree managing and searching for available channelization codes. Numerical results indicate that the proposed MDP approach yields the best fractional reward loss, code blocking reward loss, and code blocking ratio as compared to that of other schemes, including the random, left most, and crowded first schemes.

Dynamic rate and power adaptation for forward link transmission using high-order modulation and multicode formats in cellular WCDMA networks

This paper addresses the problem of dynamic rate and power adaptation for forward link data transmission using high-order modulation and multicode formats in cellular wideband code division multiple access (WCDMA) networks. A novel framework for dynamic joint adaptation of modulation order, number of code channels (hence transmission rate), and transmission power is proposed for downlink data transmission in a cellular WCDMA system where different users have similar frame error rate (FER) requirements. Based on a general downlink signal-to-interference ratio (SIR) model, the problem of optimal dynamic rate and power adaptation is formulated, for which the rate and power allocation can be found by an exhaustive search. Two heuristic-based dynamic rate and power allocation schemes are proposed. Performance of dynamic joint rate and power adaptation under the proposed framework is evaluated for a random micromobility model using computer simulations. Also, an analytical approach to evaluate the throughput performance of dynamic rate and power adaptation using high-order modulation and multicode formats is presented.

Design of a Baseband Transceiver for Multicarrier CDMA Communications

Multicarrier systems have become popular for their spectral efficiency and robustness against frequency-selective fading. Multicarrier code-division multiple access (MC-CDMA) is a technique that combines the advantage of multicarrier modulation with that of code-division multiple access (CDMA) to offer reliable high-data-rate downlink cellular communication services. In this paper, we present the architecture of a downlink baseband transceiver using the MC-CDMA technology under the same bandwidth requirement and channel condition as the third-generation wideband CDMA system. In the transmitter, a scrambling code is applied in order to reduce the peak-to-average power ratio (PAPR) of the transmitter output. In the receiver, we use a joint weighted least-squares (WLS) synchronization error estimation algorithm and a novel channel estimator. Both algorithms greatly enhance the system error-rate performance, as indicated by functional simulation. Simulation results also verify maximum aggregate coded data rates of 5.4/10.8Mbps from 32/64 users in mobile/stationary multipath fading channel with a 3/4 convolutional code, respectively.

Rapid slot synchronization in the presence of large frequency offset and Doppler spread in WCDMA systems

Slot synchronization is the most challenging step for rapid cell search in intercell asynchronous code division multiple access systems such as wide-band code division multiple access (WCDMA). For rapid cell search, it is desirable to design the receiver robust to initial frequency offset and Doppler spread. In this letter, we consider combining schemes for rapid slot synchronization of WCDMA signals in such channel impairments. We propose an inner-slot differential combining scheme that exploits partial correlation of slot synchronization code, making the receiver tolerable to a large amount of initial frequency offset and Doppler spread. Unlike conventional differential combining schemes, the proposed scheme is also applicable to the use of transmit antenna diversity. The detection performance of the proposed combining scheme is analyzed with the use of transmit antenna diversity. Finally, the analytic results are verified by computer simulation.

Blind Information-Theoretic MultiUser Detection Algorithms for DS-CDMA and WCDMA Downlink Systems

Code division multiple access (CDMA) is based on the spread-spectrum technology and is a dominant air interface for 2.5G, 3G, and future wireless networks. For the CDMA downlink, the transmitted CDMA signals from the base station (BS) propagate through a noisy multipath fading communication channel before arriving at the receiver of the user equipment/mobile station (UE/MS). Classical CDMA single-user detection (SUD) algorithms implemented in the UE/MS receiver do not provide the required performance for modern high data-rate applications. In contrast, multi-user detection (MUD) approaches require a lot of a priori information not available to the UE/MS. In this paper, three promising adaptive Riemannian contra-variant (or natural) gradient based user detection approaches, capable of handling the highly dynamic wireless environments, are proposed. The first approach, blind multiuser detection (BMUD), is the process of simultaneously estimating multiple symbol sequences associated with all the users in the downlink of a CDMA communication system using only the received wireless data and without any knowledge of the user spreading codes. This approach is applicable to CDMA systems with relatively short spreading codes but becomes impractical for systems using long spreading codes. We also propose two other adaptive approaches, namely, RAKE -blind source recovery (RAKE-BSR) and RAKE-principal component analysis (RAKE-PCA) that fuse an adaptive stage into a standard RAKE receiver. This adaptation results in robust user detection algorithms with performance exceeding the linear minimum mean squared error (LMMSE) detectors for both Direct Sequence CDMA (DS-CDMA) and wide-band CDMA (WCDMA) systems under conditions of congestion, imprecise channel estimation and unmodeled multiple access interference (MAI).

Highly Reliable 250 W GaN High Electron Mobility Transistor Power Amplifier

A state-of-the-art highly reliable 250 W AlGaN/GaN high electron mobility transistor (HEMT) push-pull transmitter amplifier operated at a drain bias voltage of 50 V is described. The amplifier, combined with a digital predistortion (DPD) system, also achieved an adjacent channel leakage power ratio (ACLR) of less than -50 dBc for 4-carrier wideband code division multiple access (W-CDMA) signals with a drain supply voltage of 50 V. I also demonstrate its stable operation under RF stress testing for 1000 h at a drain bias voltage of 60 V. Stable gate-leakage current for high-temperature operation was verified. Device fabrications on 4 inch sapphire and 3 inch semi-insulating (S.I.) SiC substrates were also addressed. These performances clarify that an AlGaN/GaN HEMTs amplifier is suitable for 3G W-CDMA systems.