Multirate Code Division Multiple Access Research Articles

Bounding the Blocking Probabilities in Multirate CDMA Networks Supporting Elastic Services

Several previous contributions have proposed calculation methods that can be used to determine the steady state (and from it the blocking probabilities) of code-division multiple-access (CDMA) systems. This present work extends the classical Kaufman-Roberts formula such that it becomes applicable in CDMA systems in which elastic services with state-dependent instantaneous bit rate and average-bit-rate-dependent residency time are supported. Our model captures the effect of soft blocking, that is, an arriving session may be blocked in virtually all system states but with a state dependent probability. The core of this method is to approximate the original irreversible Markov chain with a reversible one and to give lower and upper bounds on the so-called partially blocking macro states of the state space. We employ this extended formula to establish lower and upper bounds on the steady state and the classwise blocking probabilities.

A Time-Varying Modified MMSE Detector for Multirate CDMA Signals in Fast Rayleigh Fading Channels

In this paper, we propose a time-varying modified minimum mean-squared error (MMSE) detector for the detection of higher data rate signals in a multirate asynchronous code-division multiple-access (CDMA) system which is signaled in a fast Rayleigh fading channel. The interference viewed by a higher data rate symbol will be periodic due to the presence of a lower data rate symbol which spans multiple higher data rate symbols. The detection is carried out on the basis of a modified MMSE criterion which incorporates differential detection and the ratio of channel coefficients in two consecutive observation intervals inherently compensating the fast variation of the channel due to fading. The numerical results obtained by the MMSE detector with time-varying detection show around 3 dB (M=2) and 6 dB (M=4) performance improvement at a BER of 10−3 in the AWGN channel, while introducing more computational complexity than the MMSE detector without time-varying detection. At a higher Eb/N0, the proposed scheme can achieve a BER of approximately 10−3 in the presence of fast channel variation which is an improvement over other schemes.

Variable-length code construction for incoherent optical CDMA systems

The purpose of this study is to investigate the multirate transmission in fiber-optic code-division multiple-access (CDMA) networks. In this article, we propose a variable-length code construction for any existing optical orthogonal code to implement a multirate optical CDMA system (called as the multirate code system). For comparison, a multirate system where the lower-rate user sends each symbol twice is implemented and is called as the repeat code system. The repetition as an error-detection code in an ARQ scheme in the repeat code system is also investigated. Moreover, a parallel approach for the optical CDMA systems, which is proposed by Marić et al., is also compared with other systems proposed in this study. Theoretical analysis shows that the bit error probability of the proposed multirate code system is smaller than other systems, especially when the number of lower-rate users is large. Moreover, if there is at least one lower-rate user in the system, the multirate code system accommodates more users than other systems when the error probability of system is set below 10 −9.

Throughput Comparison of CDM and TDM for Downlink Packet Transmission in CDMA Systems With a Limited Data Rate Set

Throughputs with code-division multiplexing (CDM) and time-division multiplexing (TDM) for downlink packet transmission are compared in multirate code-division multiple access (CDMA) systems. The authors propose and use a model that can show the impact of self-interference on the received bit energy per noise and interference power density (E/sub b//N/sub t/). The developed analytical method reflects the effect of a limited data rate set, shadowed radio paths, and a limited number of spreading codes. Performance measures include asymptotic throughput, outage probability, probability of the data rate being limited by the given rate set, and probability of the system throughput being limited by the available number of spreading codes. The system throughput of packet channels for delay-tolerant service is evaluated in a 3GPP WCDMA system model. The authors show that the throughput with CDM can be equal to that with TDM in a fully loaded system without the data rate limitation. It is also shown that CDM can outperform TDM with respect to the asymptotic throughput when one takes into account the self-interference, the limitation on the data rate set, and resource reallocation of users in outage.

Multirate asynchronous optical CDMA system with product code

The authors propose a new architecture for a multirate asynchronous optical CDMA (code division multiple access) system. An optical orthogonal code (OOC) is employed as the signature sequence. The serial multirate data is converted into parallel information bits and then transmitted simultaneously via spatial channels. Exploiting the essence of parallel transmission of spatial channels, the product code (i.e. rectangular code) through an extra protection channel performs error control, which can significantly improve the system performance. The authors' proposed scheme requires only one laser diode, sequence encoder and product-coded encoder/decoder. Moreover, the adverse impact of multiple access interference, APD (avalanche photodiode) noise and thermal noise are taken into consideration in system performance analysis.

Adaptive MMSE receiver for multirate CDMA systems

We consider the adaptive receiver for multirate code division multiple access (CDMA) systems under a fading channel environment. The main difficulty that arises in the use of the adaptive receiver for multirate CDMA systems is that the adaptation should resume after the rate change. Hence, the adaptive receiver may not provide a reasonable performance during the transient after the rate change. In order to overcome this difficulty, we investigate an approach that allows updating the weight vectors for all rates simultaneously. For example, in a dual-rate system, the weight vector for the lower rate (the higher rate) can be updated during the period of the higher rate (resp., the lower rate) to avoid the transient after the rate change. The resulting adaptive receiver has multiple parallel adaptive filters. The adaptive filters for each rate can carry out the adaptation simultaneously, regardless of what the current rate is. As a result, the performance is not degraded by the rate change.

Asymptotic performance of subspace methods for synchronous multirate CDMA systems

Two direct sequence (DS) code division multiple access (CDMA) multirate access methods with a fixed chip rate can be employed to support multirate services: multicode (MC) access and multiple processing gain (MPG) access. In either an MC or an MPG multirate CDMA system, severe intersymbol interference (ISI) may exist due to large channel delay spread relative to the symbol interval. We generalize the blind subspace technique to such a multirate CDMA system in order to estimate possibly long channel impulse response of a desired user. Then, we build a blind minimum mean-square-error (MMSE) detector to detect the user's information. The detection performance can be significantly improved by suppressing ISI, which becomes feasible after the user's channel parameters are estimated. The asymptotic performance of the channel estimator and the detector is analyzed. In particular, for typical distributions of the inputs and channel noise, closed-form expressions for the channel estimation error and the output signal-to-interference-plus-noise ratio (SINR) of the detector are derived as functions of the number of received data samples and system parameters. Simulation results are provided to verify our analysis.

Multirate optical fast frequency hopping CDMA system using power control

This paper addresses the problem of real-time multimedia transmission in fiber-optic networks using code division multiple access (CDMA). We present a multirate optical fast frequency hopping CDMA (OFFH-CDMA) system architecture using fiber Bragg gratings (FBGs). In addition, we argue that, in multimedia applications, different services have different quality of service (QoS) requirements; hence, the user only needs to use the minimum required power to transmit the signal, such that the required signal-to-interference ratio (SIR) is met. We show that a variable bit rate optical communication system with variable QoS can be implemented by way of power control with great efficiency. Present-day multirate optical CDMA systems concentrate on finding the code structure that supports a variable rate system, neglecting the importance of the transmission power of active users on the multiple access interference (MAI) and, therefore, on the system capacity. We assign different power levels to each rate through a power control algorithm using variable optical attenuators, which minimizes the interference and, at the same time, provides variable QoS constraints for different traffic types. Although we are using a code family that preserves good correlation properties between codes of different lengths, simulations show a great improvement in the system capacity when power control is used.

Power control for an asynchronous multirate decorrelator

For code-division multiple-access (CDMA) wireless systems employing multiuser detection, the varied bit-error rate (BER) requirements of multimedia traffic dictate the use of transmitted power control. Using a decorrelator in an asynchronous multirate direct-sequence CDMA system, it may be necessary for different users to combat the noise enhancement and the propagation losses to varying degrees depending on individual requirements. In this context, we propose a power control algorithm for a multirate decorrelator that is suitable for a class of BER-based link quality objectives. If the uplink channel gain of the desired user is known, then it is straightforward for each user to choose the transmitted power needed to meet its target BER objective. In practice, however, the uplink channel gain is often difficult to measure. To avoid this measurement, we employ stochastic approximation methods to develop a simple iterative power control algorithm. In this algorithm, each mobile uses the output of its own decorrelator to update its transmitted power in order to achieve its BER objective. We show that when a user's bits have nonzero asymptotic efficiencies, the power control algorithm converges quickly in the mean square sense to the minimum power at which a user achieves its quality-of-service objective.

Performance analysis of a wireless multirate direct-sequence CDMA using fast Walsh transform and decorrelating detection

We analyze the performance of a flexible multirate scheme for direct-sequence code division multiple-access (CDMA) mobile radio systems. The proposed scheme uses a variable processing gain serial pseudonoise modulation as a multirate strategy. To reduce the interference effects, the CDMA system utilizes the coherent fast Walsh transform transmission technique. The proposed scheme can be used in the reverse link (mobile-to-base station) of the upcoming third-generation wide-band CDMA system (has the feature of coherent reverse link). We analyze the system performance with and without using a decorrelating multiuser detector. The uncoded bit-error probability (BEP) with and without decorrelating detection on a multipath fading channel is derived analytically. In addition, the validity of the analysis results is demonstrated by computer simulations using the IMT-2000 vehicular multipath channel model. In order to make sure that the proposed processing techniques do not distort the soft values at the demodulator output, the proposed multirate scheme is also simulated in case of using turbo codes. The turbo-coded BEP is calculated for different user data rates and different number of decoding iterations.

Multilayer LMS interference suppression algorithms for CDMA wireless networks

The general theory of adaptive self reconfigurable interference suppression schemes is applied to several specific practical problems mainly suppression of m-level m amplitude-shift keying, m phase-shift keying (PSK), and m quadrature amplitude modulation signals. This is a practical situation when a code-division multiple-access (CDMA) network is overlaid with standard microwave systems. Another example is a multirate CDMA network where a limited number of high bit rate CDMA signals are allowed to use much higher power level due to lower processing gain. The algorithm is well suited for a modular software radio concept, which we believe, will be more and more accepted in future wireless communications. Further modifications of the schemes necessary for these applications are described, and numerous results are presented to illustrate performance improvements. A general interpretation of these techniques based on so-called multilayer least mean squares (LMS) algorithm is introduced and discussed. The algorithm is based on estimating fast changing interfering signal parameters by using parallel structures, which are fast but complex. At the same time, estimation of slow-varying signal parameters over a large range is accomplished by using an LMS algorithm that is simple but slower. In this way, suppression of the interference occupying the same bandwidth as the CDMA signal is possible with reasonable implementation complexity. For this case, a BER 0 dB for binary PSK interference, J/S>17 dB for 8-PSK, J/S>27 dB for 32-PSK, and J/S>40 dB for 128-PSK.

Multiservice/multirate pure CDMA for mobile communications

Future mobile communication systems should be able to support a wide range of services with different bit rates. Spread-spectrum code-division multiple-access (CDMA) techniques have attracted much attention to be employed in such a system. Different techniques of CDMA could be used to map low-, medium-, and high-bit rates data into the same allocated bandwidth, including pure or wide-band CDMA, FDM/FH/CDMA, TDM/TH/CDMA, or a hybrid of these techniques. This paper investigates multirate pure CDMA using multiuser interference statistics derived for both Gaussian and Rayleigh fading channels. Approximation of multiuser/multipath interference, in general, helps in the theoretical approach to error performance evaluation and, in particular, is quite useful for simulation approach in a fading channel. Some results of a multirate pure CDMA system with two services (low- and high-bit rates), for both BPSK and DPSK modulation schemes, are presented and compared.

Performance of Multiuser Detection in Multirate CDMA Systems

Multiuser detectors for asynchronous multirate code-division multiple-access (CDMA) systems are considered. A multirate CDMA system model able to fit several system concepts in the same framework is presented. The performance of the multi-code (MC) system with one processing gain and the multiple processing gain (MPG) system is compared. Upper bounds for the expected value of the asymptotic multiuser efficiency of the decorrelating detector for a synchronous multirate CDMA system with three effective users are derived for a system with one and two processing gains. The numerical results show that the performance of the decorrelating detector for MC and MPG systems do not differ significantly. The bit error rates (BER) of the decorrelating, parallel interference canceler (PIC) and groupwise serial interference canceler (GSIC) with either PIC or decorrelator within the group are compared. The results show that the decorrelating and the GSIC detectors yield the best performance. For all interference cancellation schemes the BER saturates at high SNRs due to decision errors degrading the multiple-access interference (MAI) estimates.

Multirate fiber-optic CDMA: system design and performance analysis

In this paper, we discuss possibilities of multirate transmission using fiber-optics code-division multiple-access (CDMA) networks. Two schemes are introduced: the parallel scheme and the serial mapping scheme. The theory of optical CDMA is reviewed and the basic properties of OOC families used in the paper given. Also a new property of optical orthogonal code (OOC) sequences is introduced which greatly increases the number of sequences in the network. Performance analysis of the two systems is done using as the parameter the number of simultaneous different-rate users as a function of the required probabilities of error. It is shown that addition of error correction coding increases the number of users in the network. Examples of the two systems are given and it is shown that a high number of different-rate users can be accommodated in the network with no increase in the complexity of optical processing.