Multirate CDMA Systems Research Articles

Cell Site Power Characterization for Multi-rate Wireless CDMA Data Networks Using Lognormal Approximation

An analytical model is developed to solve the power-rate assignment problem for multi-rate CDMA systems and calculate the probability density function (PDF) for the downlink traffic power as a function of the bit rate assignment scheme, the target link quality figure, and the other network parameters. This paper proposes the use of the lognormal random variable as an approximation for an empirical parameter (denoted herein by fi) that is needed to solve the energy per-bit to noise spectral power density equation for scheduled data bursts. The approximation depends on matching the mean and standard deviation of the lognormal variable to the mean and standard deviation of the empirical parameter fi where the matching depends only the radio frequency (RF) path loss model and the shadowing process standard deviation. The paper presents example calculations for the cumulative probability distribution (CDF) of the downlink traffic power and outage probabilities for a general CDMA system assuming specific scenarios of data bit rate assignments. The study also compares between analytical results and those obtained using conventional Monte-Carlo simulations. Comparisons indicate that the developed model provides an appropriate mechanism to approximate both the PDF or CDF, and the outage probability for the cell site total downlink traffic power.

Non-blocking extended OVSF codes on multi-rate CDMA systems

Wideband code division multiple access (WCDMA) systems have offered opportunities to provide variable user data rates and flexibly support applications with different bandwidth requirements. In the data channelization technology, orthogonal variable spreading factor (OVSF) code can distinguish the different data tunnels and promise the orthogonality for different customer tunnels or the same customer but different business tunnels. However, due to its distinctive feature of orthogonality, there is a limitation of the code allocation. Both intelligence and good integration mechanism are required to achieve OVSF tree’s highest system utilization. This results in a major drawback of OVSF codes, called blocking property: It may have significant impact on the bandwidth utilization of the systems. This paper presents novel non-blocking extended orthogonal variable spreading factor (EOVSF) codes to support arbitrary users in multi-rate wireless CDMA system. As a result of this non-blocking property, none of the EOVSF codes blocks the assignment of any other EOVSF code. It decreases the rejection probability and the number of code rejections in the multi-rate CDMA system.

Power distribution/allocation in multirate wideband CDMA systems

A unified approach for power distribution and allocation in a multirate wideband CDMA system is investigated. It is shown that the traffic demand and the background disturbance fully govern the feasibility of the system and the optimal power distribution solutions, where the traffic demand is specified by the user QoS requirement, data rate, and spread spectrum bandwidth; the background disturbance includes the background noise and the intercell interference. Closed form expressions of the optimal power allocation, subject to power constraints in the practical system design, are derived. Convergent conditions are applied to evaluate the capacity region of the system. Numerical examples are provided to illustrate the applications of the obtained theoretical results.

Utility based service differentiation in CDMA data networks

The wireless data services are getting more and more competitive because of the presence of multiple service providers, all of whom offer some relative advantages and flexibilities over the others. As a result, the user churn behavior (i.e., migration from one service provider to another) is causing tremendous revenue loss for the service providers and also failure of existing resource management algorithms to fully capture the impact of churning. Moreover, the quality of service (QoS) offered to users belonging to different classes calls for new resource management schemes that address the issues related to differentiated services. In this paper, we propose a framework to study the impact of user churn behavior on the resource management algorithms to provide class-based differentiated services in CDMA data networks. In particular, our framework incorporates the user churning behavior into the admission control and power management algorithms, so that the service provider's revenue loss due to churn can be minimized. Since optimal rate/power allocation in multi-rate CDMA systems is in general NP-Complete, we provide heuristics that try to provide solutions to the resource allocation problem in real-time. In our proposed framework, we add another layer of power management called Class-Based Power Allocation/Reduction (CBPAR) function, which works with the rate control algorithm to achieve power allocation. With CBPAR, the number of variables of the optimization problem is significantly reduced helping achieve the results in real-time. Our simulation study shows that the service provider's revenue can be improved with the help of CBPAR framework. It also reveals the relationship between users' sensitivity and tolerance to QoS degradation and optimal power allocations.

CCS-FOSSIL and dual-channel system that increases channel capacity per dynamic power range

We design a forest with nodes that represent orthogonal variable spreading factor (OVSF) sequences of different lengths (spreading codes of different lengths that can be used for multi-rate DS-CDMA). In addition to the non-descendant OVSF property exhibited in well-known tree-structured generation of sequences [(F. Adachi, et al., 1997) and (E.H. Dinan and B. Jabbari, 1998)], the sequences represented by the nodes of our forest have useful properties that can be used to achieve multi-channel communication (with a sequence providing a channel) with a lower total peak-to-mean envelope power ratio (PMEPR). These OSVF sequences grouped by our forest structure also have properties that facilitate symbol-by-symbol adaptation of the symbol duration in multi-rate CDMA systems. For example, certain lineages in the forest have the property that any pair of code sequences in the same lineage are shift orthogonal to each other, with the unit shift length being that of the shorter sequence. We present the forest-structured generation of the sequences (spreading codes) and their properties.

Three proposed mobile radio multirate CDMA systems through multipath Rayleigh fading channel

Three proposed mobile radio multirate CDMA systems through multipath Rayleigh fading channel

Code-aided blind detection of the transmission rate for multirate direct-sequence CDMA systems

The problem of rate detection for a multirate direct sequence/code division multiple access (DS/CDMA) systems is considered in this paper. It is assumed that, in each data-frame, each user may transmit its data at one out of a set of available data-rates, depending, e.g., on the kind of data to be transmitted, on the desired quality-of-service, and on the channel state. Relying on the knowledge of the spreading codes of the user whose rate is to be detected, rate detection algorithms based on the application of the generalized likelihood ratio test are here proposed. Both the cases where either the rates of the interfering users have been already detected and are known to the receiver or these rates are not known to the receiver are separately considered. With regard to the performance assessment, the effectiveness of the proposed rate detection strategies is validated, for a single-path fading channel, through both computer simulations and, with regard to the case that the interfering users rates have already been acquired, through a Chernoff-bounding technique.

User capacity for synchronous multirate CDMA systems with linear MMSE receivers

The performance of linear minimum mean-square error (MMSE) multiuser receivers in a dual-rate synchronous direct-sequence code-division multiple-access (DS-CDMA) system is investigated using the random spreading sequence analysis. Multicode (MC) systems and different variants of variable spreading length (VSL) systems are studied. User capacity regions are obtained for these systems.

Optimum Packet Data Transmission in Cellular Multirate CDMA Systems With Rate-Based Slot Allocation

High-rate packet transmission is realized for the downlink in cellular multirate code-division multiple-access systems using multicode concatenated signaling, combined with iterative detection and self-interference cancellation. Since an optimum packet transmission is to allocate the maximum allowable rate to the user with the best received signal-to-interference-plus-noise ratio (SINR), a problem with fairness arises, in the sense that only a very small number of users receive at or near the maximum allowable rate, and the rest of the users do not receive at all. To overcome this, a new soft multimodal fairness control is proposed to adjust the latency (or waiting time) between two extreme values. Throughput is analyzed by deriving the probability distribution of the rate allocation, which is based on the maximum received SINR in a slot. For this, statistics on the SINRs are jointly characterized under three-sector cell structure because of their mutual correlation. Traffic variations are also taken into account to formulate the statistics under two algorithms for adaptive base station selection. It is shown that high-rate transmission can be achieved by a substantial reduction in in-cell interference, and the tradeoff between throughput and fairness can be met by the fairness control.

Blind Multiuser Detection in Multirate CDMA Based on Cyclic LMS Adaptation

The problem of blind adaptive multiuser detection in multirate CDMA systems is considered. Indeed, since symbol detection in multirate CDMA systems requires periodically time-varying processing of the observables, classical LMS and RLS adaptive algorithms, which assume that the solution to be tracked is time-invariant or slowly time-varying, are not suited for blind adaptive multiuser detection in a multirate system. While a cyclic RLS algorithm has recently appeared in the literature, this paper focuses on the development of LMS-based cyclic filtering algorithms. In particular, cyclic versions of the standard LMS algorithm, of the LMS algorithm with iterate averaging and of the LMS algorithm with adaptive step-size are derived. Interestingly, the last two algorithms are shown to exhibit a convergence speed close to that of the cyclic RLS procedure, but with an order of magnitude lower computational complexity. An adaptive procedure for the automatic selection of the algorithm periodicity is also presented, which is based on a minimum mean-output-energy criterion, and that obviates the need for knowledge of the transmitted data-rates from the interfering signals. Moreover, the case of known multipath fading channels is also examined. In particular, it is shown that the proposed cyclic LMS algorithms can be used to achieve RLS-like performance also in the presence of multipath distortion. Extensive computer simulation results, along with some analytical convergence results, confirm that the proposed algorithms are effective and achieve very satisfactory performance.

Iterative cyclic subspace tracking for blind adaptive multiuser detection in multirate cdma systems

In this paper, the problem of subspace-based blind adaptive multiuser detection in multirate direct-sequence code-division multiple-access (DS-CDMA) systems adopting short (periodic) spreading codes is considered. The solution that we propose is based on the well-known formulation of the linear minimum mean-squared error and decorrelating detectors in terms of signal subspace parameters. Since in a multirate scenario the correlation properties of the observable and, hence, the signal subspace parameters are periodically time-varying, classical subspace tracking algorithms, which assume that the subspace to be tracked is time-invariant or slowly time-varying, are shown to be not useful in this situation. A new recursive cyclic subspace tracking algorithm is thus developed. This procedure, which is based on a generalization of the PASTd algorithm, is able to capture the periodical variations of the signal subspace, and thus enables subspace-based blind adaptive multiuser detection in multirate CDMA systems. The proposed algorithm has a smaller computational complexity than the recently developed cyclic recursive-least-squares procedure, and, as numerical results confirm, is capable of providing very satisfactory performance.

Space-Time Receiver for Multi-Rate CDMA Systems: Multi-Code and Variable Spreading Length Access Methods Comparison

This work proposed the linear minimum mean square error (LMMSE) space-time receivers for synchronous multi-rate direct sequence code devision multiple access (DS/CDMA) systems. The performance of the proposed receivers is analyzed. The high-rate (HR) LMMSE space-time receiver with combining technique can provide essentially the same performance for low-rate (LR) users as LR LMMSE space-time receiver while eliminate the time delay for the HR users. The performance of the proposed multi-rate LMMSE space-time receivers are studied in the context of two multi-rate access methods: multi-code (MC) access where high data rate users multiplex their information streams onto multiple codes, and variable spreading length (VSL) access where signature sequences of different lengths are assigned to users with different data rates. Through the simulation results, we show that: (a) the proposed multi-rate LMMSE space-time receivers are optimum near-far resistant; (b) the proposed multi-rate LMMSE space-time receivers have a clear performance improvement compared to the time only LMMSE dual rate receiver; (c) there is a performance gain of the proposed multi-rate LMMSE space-time receivers compared to the multi-rate decorrelating space-time receiver when near-far ratio (NFR) is low; (d) The performance difference between MMSE combining and MRC in multi-rate system is much more pronounced than in single-rate system. Simulations also show that the VSL access method is more robust than MC access method considering changing rate-ratio in multi-rate systems.

Multi-user multi-antenna CDMA receivers for asynchronous multipath fading channels

In this paper, we investigate joint channel estimation and signal detection approaches for a multi-user CDMA uplink with a base-station antenna array over asynchronous multipath fading channels. Based on the space alternating generalized expectation-maximization (SAGE) algorithm, we develop an iterative multi-user receiver to jointly estimate channel response vectors and bit sequences for all the active users at known fractional time delays, and a BER lower bound is derived. A simplified bit detection method is then proposed for multi-rate CDMA systems. Simulation results show that the new iterative receiver performs equally well for both high- and low-rate users, and the BER performance is close to that of the BER lower bound. Copyright © 2003 AEI.

MMSE‐based groupwise successive interference cancellation scheme for multirate CDMA systems

AbstractIn this work, group‐wise successive interference cancellation (GSIC) scheme for multi‐rate CDMA systems is proposed. A multi‐rate CDMA system is a CDMA system that can support voice users together with data users which may have different data rates. In the proposed group‐wise detection, users with the same information rate are grouped and detected together. Users with the highest information rate are detected first, neglecting the presence of the other users in the system. Interference between the groups is cancelled in a successive order. The proposed scheme is based on the minimum mean square error (MMSE) criterion. We also studied the decision feedback interference cancellation for multi‐rate CDMA systems. The performance of the proposed detection scheme is compared with that of conventional detector, the decorrelating detector, and the linear MMSE detector for multi‐rate CDMA systems. The proposed scheme achieves better performance than the above‐mentioned detectors. The proposed scheme also provides significantly high spectral efficiency under certain conditions. The algorithms in the proposed scheme are suitable for both terrestrial and satellite applications of multi‐rate CDMA systems. Copyright © 2003 John Wiley & Sons, Ltd.

Blind detection of multirate asynchronous CDMA signals in multipath channels

Blind detection of a desired user's signal in a multirate direct sequence code division multiple access (DS-CDMA) system [using either variable sequence length (VSL) or multicode (MC) access] is considered. A code-constrained inverse filter criterion (IFC)-based blind detector for equal-rate CDMA signals to detect a desired user's signal was presented by Tugnait and Li (2001). The IFC method exploits the higher order statistics of the data. In multirate CDMA systems, a high-rate user signal may be treated as the superposition of several virtual basic-rate signals. The code-constrained IFC-based detector may be used to detect a given basic-rate virtual signal. This, however, does not solve the problem of combining the detected virtual basic-rate signals to yield the original high-rate signal since the former may be delayed by different equalization delays, may be multiply extracted, and may be in different "order." In this paper, novel approaches combining the code-constrained IFC and a penalty function are developed to cope with this problem for VSL and MC multirate access methods. Global minima of the proposed cost functions are analyzed. Three illustrative simulation examples are presented, including an example where the proposed algorithms are compared with an existing subspace approach (and its modifications), a clairvoyant matched filter receiver, and a known channel linear minimum mean-square error (MMSE) receiver.

Blind channel estimation in multi-rate CDMA systems

Multi-rate CDMA is a potentially attractive multiple access method for future broad-band multimedia wireless networks that must support integrated voice/data traffic. The primary impairment for such multi-rate systems is the multipath nature of radio channels that results in intra-user inter-chip interference (ICI) and multi-user interference (MUI) between different users' symbols. Explicit knowledge of the channel is typically needed for high performance detectors (such as coherent demodulation). We propose a subspace method for channel estimation in multi-rate CDMA systems. A unified signal model that applies to three multi-rate CDMA schemes proposed in the literature is developed. The computational complexity for multi-rate scenarios is large and variable-accordingly, a modified approach is devised that offers performance/complexity trade-offs. Performance analysis is conducted based on a close-form expression for the mean square error of the estimator, supported by simulation results that investigate the effectiveness of our method.