CDMA-based Networks Research Articles

SMAC: A Soft MAC to Reduce Control Overhead and Latency in CDMA-Based AMI Networks

The use of state-of-the-art 3G cellular CDMA technologies in a utility owned AMI network results in a large amount of control traffic relative to data traffic, increases the average packet delay and hence are not an appropriate choice for smart grid distribution applications. Like the CDG, we consider a utility owned cellular like CDMA network for smart grid distribution applications and classify the distribution smart grid data as scheduled data and random data. Also, we propose SMAC protocol, which changes its mode of operation based on the type of the data being collected to reduce the data collection latency and control overhead when compared to 3G cellular CDMA2000 MAC. The reduction in the data collection latency and control overhead aids in increasing the number of smart meters served by a base station within the periodic data collection interval, which further reduces the number of base stations needed by a utility or reduces the bandwidth needed to collect data from all the smart meters. The reduction in the number of base stations and/or the reduction in the data transmission bandwidth reduces the CAPital EXpenditure (CAPEX) and OPerational EXpenditure (OPEX) of the AMI network. The proposed SMAC protocol is analyzed using markov chain, analytical expressions for average throughput and average packet delay are derived, and simulation results are also provided to verify the analysis.

Congestion Probabilities in CDMA-Based Networks Supporting Batched Poisson Input Traffic

We propose a new multirate teletraffic loss model for the calculation of time and call congestion probabilities in CDMA-based networks that accommodate calls of different service-classes. The call arrival process follows a batched Poisson process, which is more “peaked” and “bursty” than the ordinary Poisson process. The call-admission-control policy is based on the partial batch blocking discipline. This policy accepts a part of the batch (one or more calls) and discards the rest, if the available resources are not enough to accept the whole batch. The proposed model takes into account multiple access interference, both the notion of local (soft) and hard blocking, the user’s activity, as well as interference cancellation. Although the analysis of the model does not lead to a product form solution of the steady state probabilities, we show that the call-level performance metrics, time and call congestion probabilities can be efficiently calculated based on approximate but recursive formulas. The accuracy of the proposed formulas are verified through simulation and found to be quite satisfactory. Comparison of the proposed model with that of Poisson input shows the necessity of the new model. We also show the consistency of the new model over changes of its parameters.

Optical CDMA-Based Fiber-Radio Networks with Improved Power Efficiency

A bi-directional fiber-radio network based on spectral-amplitude-coding (SAC) optical code-division multiple access (OCDMA) with interference elimination is proposed. This scheme uses hybrid ring/star topology with partial self-healing ability and its key feature is the elimination from the addition of direct current signal before the optical encoding process. Therefore, as compared to the previous SAC-based fiber-radio networks, both power efficiency and carrier-to-noise ratio at the decoder of the proposed scheme are improved. In addition, simple encoding/decoding structures with small sizes in the control base station is proposed to reduce the cost of todays mobile radio networks with a large number of remote base stations.

Joint Physical Layer and Data Link Layer Optimization of CDMA-Based Networks

In CDMA systems, outer loop power control (OLPC) determines the target value of SNR at the receiver, mostly by using look-up tables to map bit error rates (BERs) to SNR-targets. In this contribution, transmission delay and packet loss rate constraints in the data link layer (DLL) are invoked in order to determine the optimum outer loop setpoint analytically, according to the number of active users in cell. Optimality is, in this sense, the maximization of system throughput. Using the optimum SNR-target, the optimal spreading factor is determined. Subsequently, the joint optimization of outer loop and variable spreading factor (VSF), at the physical(PHY)-layer, with truncated automatic repeat request (ARQ) error control mechanism at the data link layer are proposed. Hence, we show that quality of service (QoS) requirements at these layers can be simultaneously satisfied while maximizing throughput. Total and truncated channel inversion strategies are employed in the inner loop to adapt transmit power to short-time channel variations. We propose a system where the number of users in a cell is modeled by a one-dimensional discrete Markov chain, and design the adaptive continuous power and rate mechanism for the worst case packet error rate (PER) condition. The corresponding theoretical throughput, which can be regarded as upper-bound for discrete spreading factor case, is obtained numerically for various settings of system parameters. We have also provided simulation results for a practical channel condition. Our scheme is compared with constant SNR-target and PHY-layer based variable SNR-target cases under continuous power and rate variation to show the achievable gain through the coupling of physical and data link layers parameters.

Implementable wireless access for B3G networks. II. MIMO receiver architectures [Topics in Radio Communications

This article presents a comprehensive overview of the research conducted within Mobile VCE's Core Wireless Access Research Programme, a key focus of which has naturally been on MIMO transceivers. The article, the second of four, provides a follow on from the first article, which dealt with MIMO channels and comprises a compilation of material that has been presented in detail elsewhere (see references within the article). In Part I MIMO channel measurements, analysis, and modeling were presented, which were then utilized to develop compact and distributed antenna arrays. This article reports parallel activities into low-complexity MIMO single-user space-time coding techniques, as well as SISO and MIMO multi-user CDMA-based transceivers for B3G systems. Also, overall B3G system designs have been suggested for beamformed multilayer CDMA-based networks

Intercell radio interference studies in CDMA-based LMDS networks

Among the various alternatives investigated for enhancing the capacity of the millimeter wave segment of broadband wireless access (BWA) networks is the use of orthogonal direct sequence code division multiple access (DS-CDMA) instead of time division multiple access (TDMA), the dominant multiple access scheme to date. This paper aims at contributing to the ongoing research on the feasibility of CDMA-based Local Multipoint Distribution Service (LMDS) by presenting an analytical study of intercell interference, which is the major factor limiting capacity. The study is focused on the statistical properties of the carrier-to-interference ratio (CIR) in both the downstream and the upstream channel. Since the propagation phenomena affecting LMDS are quite different than those affecting narrowband wireless networks, emphasis is placed on incorporating the spatial inhomogeneity of rain attenuation in the interference analysis and on estimating the additional CIR degradation not taken into account in the conventional clear sky approach. The proposed model is accompanied by illustrative simulations identifying worst case interference scenarios and clarifying various aspects of the system.

System-Level Performance of Antenna Arrays in CDMA-Based Cellular Mobile Radio Systems

Smart antennas exploit the inherent spatial diversity of the mobile radio channel, provide an antenna gain, and also enable spatial interference suppression leading to reduced intracell as well as intercell interference. Especially, for the downlink of future CDMA-based mobile communications systems, transmit beamforming is seen as a well-promising smart antenna technique. The main objective of this paper is to study the performance of diverse antenna array topologies when applied for transmit beamforming in the downlink of CDMA-based networks. In this paper, we focus on uniform linear array (ULA) and uniform circular array (UCA) topologies. For the ULA, we consider three-sector base stations with one linear array per sector. While recent research on downlink beamforming is often restricted to one single cell, this study takes into account the important impact of intercell interference on the performance by evaluating complete networks. Especially, from the operator perspective, system capacity and system coverage are very essential parameters of a cellular system so that there is a clear necessity of intensive system level investigations. Apart from delivering assessments on the performance of the diverse antenna array topologies, in the paper also different antenna array parameters, such as element spacing and beamwidth of the sector antennas, are optimized. Although we focus on the network level, fast channel fluctuations are taken into account by including them analytically into the signal-to-interference calculation.

Uplink transmission power allocation in CDMA-based mobile networks

A computationally efficient solution to the problem of finding the optimal feasible allocation of power to a set of uplink transmissions is presented. The problem can complement legacy design and management processes for CDMA-based networks. Numerical results are presented.

A multimode CDMA with reduced intercell interference for broadband wireless networks

This paper investigates the application of CDMA to broadband wireless access systems commonly known as LMDS networks. After a review of the intercell interference in the emerging LMDS networks based on TDMA, we analyze the interference in CDMA-based networks and show that, while conventional CDMA is superior to TDMA in terms of the worst-case interference on the upstream channel, it turns out to be inferior to TDMA in terms of interference on the downstream channel. Next, based on the observation that strong interference occurs only for a small range of user locations, we introduce a multimode CDMA concept that efficiently handles this interference by assigning orthogonal spreading sequences whose number and length are a function of the user position with respect to the base station. We show that in terms of worst-case SIR, the proposed technique leads to a significant improvement with respect to conventional CDMA and TDMA.

Wideband CDMA network sensitivity function

We analyze capacity losses in code division multiple access (CDMA) networks due to imperfections in the operation of the system components. The main result of this work is a systematic mathematical framework for capacity evaluation of CDMA-based networks in fading channels. This should be considered as an alternative tool to extensive simulations being used for these purposes at the moment. The intention is to provide simple approximate relations that simultaneously take into account: multiple access intracell interference (MAI), intercell interference, and near-far effect, including different sources of power control imperfections. In addition to this, the efficiencies of the major receiver components are also included in the analysis. A flexible complex signal format is used which enables us to model at currently interesting proposals for wideband CDMA (W-CDMA) standards. The theory is general, and some examples of practical set of channel and system parameters are used as illustration. Further elaboration of these results, including extensive numerical analysis based on the offered analytical framework, would provide enough background for the understanding of W-CDMA system performance in a realistic environment.