Evolution of the power control techniques for DS-CDMA toward 3G wireless communication systems

The use of power control in wireless networks can lead to two conflicting effects. An increase in the transmission power on a link may (i) improve the quality and thus the throughput on that link but, (ii) increase the levels of interference on other links. A decrease in the transmission power can have the opposite effects. Our primary goal in this work is to understand the implications of power control on interference and contention. We conduct experiments on an indoor mesh network. Based on analysis of our experimental data, we identify three interference scenarios: a) the overlapping case, where the aggregate throughput achievable with two overlapping links cannot be improved via power control; b) the hidden terminal case, where proper power control can primarily improve fairness and, c) the potentially disjoint case, where proper power control can enable simultaneous transmissions and thus improve throughput dramatically. We find that power control can significantly improve overall throughput as well as fairness. However, to our surprise, we note that using virtual carrier sensing in conjunction with power control generally degrades performance, often to a large degree.

A design methodology for a downlink power-control technique that maximizes the user-capacity of a DS-CDMA cellular system is proposed. First, the downlink beam-forming weights for the base-station antenna-array are designed based on the maximum SIR criterion. Then by optimizing the downlink power-control algorithm subject to a fixed total transmit power constraint, we further increase the SINR at the mobile terminal, thus increasing the capacity of the system. Due to the quasi-convex property of the SINR expression, we formulate the power optimization process as a feasibility problem with linear constraints, which can be solved efficiently by linear programming methods. With the same methodology, we can also minimize the required transmit power while satisfying the SINR threshold constraints. Computer simulations are given to evaluate the optimized downlink capacity, received SINR, and the required transmit power of a DS-CDMA system using antenna-arrays at the base-station.

Joint optimization of outer loop power control (OLPC) signal to noise ratio-target (SNR-target) and variable spreading factor (VSF), at the physical(PHY)-layer, with truncated automatic repeat request (ARQ), at data link layer (DLL), in accordance to the number of active users in the cell is considered. This investigation is on a single cell conventional cellular DS-CDMA communication system with frequency-selective fading channels, where the number of active users is modelled through a one-dimensional discrete Markov chain. The optimally is in the sense of maximizing the sum-throughput, given a coherent RAKE receiver is employed with maximum ratio combining (MRC). We determine the optimal spreading factor using OLPC SNR-target at the PHY-layer, which satisfies the QoS imposed by packet error rate-target (PER-target) as a function of maximum number of allowed ARQ retransmissions at the DLL. The channel model considers independent paths with Nakagami fading characteristics. Total and truncated channel inversion techniques are used within the inner loop power control (ILPC) to adapt the transmission power to short time channel variations. Sum-throughput performance of the optimized system and non optimized system, where the SNR-target is assumed to be constant, under various multipath fading conditions are studied. A considerable gain in sum-throughput is achieved through coupling of PHY-layer and DLL parameters.

This is the second issue of Wireless Communications and Mobile Computing (WCMC) published by John Wiley & Sons, Ltd. The first issue was published ahead of time and presented some excellent review papers from experts in the field. We received great feedback from a number of colleagues about its content and we promise to continue producing excellent issues in the future. If you did not receive your sample copy of the inaugural issue, please let me or any of the regional editors know to arrange to send you a copy. We have selected seven papers to appear in this second issue. These papers span a good range of topics in the area. The first is a review paper by Ralph and Aghvami. They discuss the Wireless Application Protocol (WAP) architecture and characteristics. Then, they investigate the devices that are supported by WAP as well as the drivers and services. These services include quality of service, security, billing, interoperability, and performance engineering. The variety of media types combined with the diversity of Internet connection characteristics raises momentous challenges to the achievement of ubiquitous access to Internet multimedia content. The second paper, by Margaritidis and Polyzos, addresses issues related to this topic. They identify the importance of adaptation to this category. Then, they examine several important factors that influence the design and optimization of the adaptation architecture. In conclusion, they describe current commercial solutions and future trends in application adaptation in conjunction with recent developments towards wireless access to the Internet. The third paper, by Havinga and Smit, discusses also wireless multimedia networking, but focuses more on energy efficiency. The authors identify three key problems and provide possible solutions to overcome them. The efficient management of power-controlled cellular wireless systems is necessary for their deployment success. Power control can be used as a platform for radio resource management and is important for channel fading as well as providing QoS to individual users. Xiao, Shroff and Chong review the developments of distributed power control and related resource management problems in cellular wireless systems. The discussion is concentrated on achieving high efficiency for a power-controlled system while preventing the system from failure. They then review power and rate control schemes proposed for wireless data, and present a framework for utility-based power control as a possible candidate for distributed power control of multimedia wireless systems. In the fifth paper, Arslan and Bottomley review commonly used approaches to channel estimation. This is necessary in adaptive receiver designs used in narrowband and wireless digital communication systems. They consider both time-invariant and time-varying channels in their discussion. They also provide a number of applications that will make use of this standard. On the other hand, Tepedelenliolu, Abdi, Giannakis and Kaveh talk about estimation of Doppler spread and signal strength in wireless systems. They concentrate on estimating the received signal strength, mobile velocity, and other related statistical channel parameters that apply to handoff and adaptive transmission. They provide comparison of the presented schemes based on their modeling and simulation results. Finally, Annamalai and Tellambura present a research paper discussing three new exponential-type bounds derived using the Cauchy–Schwarz bounding technique. They show that these new bounds are found to be tight and useful for a number of applications. They compare these bounds to other well-known exponential type bounds and prove that these perform at least comparable or tighter. We hope that you will enjoy reading this issue and welcome any comments to any of the four editors listed below. Again, we welcome all of you to contribute and encourage your colleagues and libraries to subscribe to this fine publication.

This paper has proposed a distributed power control algorithm that uses a stochastic search technique in order to solve linear systems of equations for power update in CDMA cellular radio systems. The proposed algorithm is developed by applying Bremermann's evolutionary computation algorithm to the CDMA power control problem. The major gain from the applied evolutionary computation algorithm is more rapid optimization on linear systems of equations compared with the simple genetic algorithm (SGA). Employing the distributed constrained power control (DCPC) as a reference algorithm, we have designed and implemented computational experiments on the DS-CDMA system. The results indicate that the proposed algorithm significantly enhances the optimization speed of power control. The proposed algorithm is also compared with the bang-bang type algorithm used in the IS-95 and the W-CDMA systems. The results show that our proposed algorithm also has a high potential advantage for increasing the CDMA cellular radio network capacity and decreasing the mobile terminal power consumption.

The Long Term Evolution (LTE) standards are generated by the Third Generation Partnership Project (3GPP) for the Fourth Generation (4G) wireless communication systems. The Fourth Generation (4G) wireless communication standards use new physical layer signaling techniques in their physical layer. Single Carrier Frequency Division Multiple Access (SC-FDMA) and Orthogonal Frequency Division Multiple Access (OFDMA) are the two major signaling techniques used in LTE. OFDMA is used in the downlink of the Fourth Generation (4G) wireless communication systems for its high spectral efficiency and SC-FDMA is used in the uplink of the Fourth Generation (4G) wireless communication systems for its high power efficiency. The main objective of this paper is to design and implement the reliable and energy efficient architecture for LTE uplink system using FPGA. IFDMA is the most energy efficient subcarrier mapping scheme of SC-FDMA multiple access scheme. In IFDMA, the data symbols subcarrier mapping is not one to one instead each subcarrier carries a part from each data symbol transmitted at that time instant. The reason for the selection of IFDMA subcarrier mapping along with SC-FDMA multiple access scheme for LTE uplink comes mainly from its advantage to provide a low peak to average power ratio (PAPR) for the transmitted waveform. This results in less power consumption in the mobile station during uplink transmission. The reliability of the system is improved using interleaving process by avoiding packet loss. Convolution codes are less complex and are used to provide error free transmission. The enhancement of energy efficiency is done using the convolution coded IFDMA. The chosen energy efficient architecture must be more power efficient and support high data rates, which is the main requirement for the mobile user. The proposed reliable and energy efficient architecture for LTE uplink system is implemented using Xilinx system generator tool.

In order to overcome the MAI and ISI problems in time-varying multipath fading channels, a novel H/sub /spl infin//-subweight algorithm for the decision feedback equalizer (DFE) in a DS-CDMA system is presented. The subweight technique can reduce the computational load of the H/sub /spl infin// algorithm, but dynamic estimation error in subweight partition may degrade performance. The advantage of insensitivity to modeling error of the H/sub /spl infin// algorithm just can mitigate the degradation of the subweight technique. Therefore, the individual advantages of the H/sub /spl infin// algorithm and subweight technique can be successfully integrated. The H/sub /spl infin//-subweight algorithm owns the advantages of low complexity and robustness against modeling error caused by the time-varying multipath channels as well as subweight partition. On the other hand, the RLS-subweight algorithm seriously suffers from the drawback of dynamic estimation error. The reduction of computational complexity by subweight partition is analyzed. Simulation results show that the H/sub /spl infin//-subweight DPE outperforms the RLS-subweight DFE in suppressing equalization error.

Multiple Input Multiple Output (MIMO) systems have appeared as one of the promising approaches for increasing the capacity and lowering symbol or bit error probability of the wireless communication systems without increasing the bandwidth or the transmission power of the system. In this paper, we investigate the average channel capacity for MIMO orthogonal multicarrier direct Sequence Code Division Multiple Access (OMC DS-CDMA) system in Nakagami-m fading environment. We also consider the influence of the normalized space between two adjacent subcarriers of the system in average channel capacity analysis. Gaussian approximation method is employed to model multiuser interference (MUI) which mostly degrades the system performance. Hence, probability density function (PDF) of the instantaneous signal to interference noise ratio (SINR) is derived using Characteristic Function (CF) based method. Therefore, the exact closed form expression for the average channel capacity is derived by taking the expectation of Shannon channel capacity over the PDF of SINR. The results reveal that the average channel capacity of the MIMO OMC DS-CDMA system outperforms that of the conventional OMC DS-CDMA system. Similarly, we observe that MIMO MC DS-CDMA system has better spectral efficiency. Thus, we confirm the numerical results using Monte Carlo simulation technique.

In this paper, we use joint perfect power control (PPC) and adaptive beamforming algorithms as constrained least mean square (CLMS) and conjugate gradient adaptive beamforming (CGBF) in a 2D urban environment. The CLMS algorithm is capable of efficiently adapting according to the environment and able to permanently maintain the chosen frequency response in the look direction while minimizing the output power of the array. In addition, with the CGBF algorithm, the desired users’ signal in an arbitrary path is passed and the inter-path interference (IPI) is canceled in other paths in each RAKE finger. The adaptability of the algorithms is closely observed for uniformly spaced linear array. In addition, in this paper, we present switched-beam (SB) technique. In the SB technique by using a number of fixed, independent, or directional antennas we increase the uplink capacity of the wireless systems. Also, we study an analytical approach for the evaluation of the impact of power control error (PCE) on wireless communication systems. Simulation results indicate that the SB technique is able to considerably increase the signal to interference plus noise ratio (SINR) of desired user in comparison with other algorithms. In addition, we observe that the SINR in the CLMS algorithm is lower than the CGBF algorithm. Finally, we discuss two parameters of the PCE and path-loss exponent and their effects on capacity of the system via some computer. Keywords: Adaptive beamforming, Power control, Wireless networks

In cellular mobile communication systems using coded modulations, the spectrum efficiency of the system is related to the number of available codes. Recently, large area synchronous (LAS) CDMA codes, which exhibit a region called interference free window (IFW) within some delay-spread, have been accepted as one of fourth-generation (4G) wireless communication systems. However, the number of synthesized LAS codes is very low due to the low duty ratio of the sequence. In this paper, a dynamic space coded multiple access (DSCMA) scheme which utilizes the spatial diversity from smart antenna system is proposed to overcome the low spectrum efficiency in LAS CDMA system. In the DSCMA a modified version of LAS codes called LAS even ternary (LAS-ET) codes is proposed. These codes are used together with a novel algorithm called dynamic space code (DSC), which will decrease the code length so that the spectrum efficiency can be increased. By taking advantage of dynamic code reuse assignment in spatial diversity, the spectrum efficiency of the DSCMA can be further increased significantly.

Wideband code-division multiple-access (CDMA) will be utilized in the third generation or 3G wireless communication systems because of its high frequency utilization efficiency and suitability for handling multimedia and multi-rate services. In this paper, we present a system with a simplified receiver structure for DS/CDMA wireless communication handsets, in which improved performance is demonstrated when compared to a conventional DS/CDMA system with a RAKE receiver at the mobile station. The proposed system is obtained by finding the optimal solution to a general multiple-input single-output (MISO) DS/CDMA smart antenna system. It is shown that this solution reduces to a pre-RAKE with space transmit diversity system under the assumption that a simplified receiver structure is invoked. Numerical results show that significant system performance and capacity improvements are possible. They also reveal that the proposed system is not too sensitive to channel estimation errors.

Background. Antenna arrays based on dual-polarization elements are used as radiating devices for modern and perspective radio communication systems. The main factor reducing the effectiveness of this approach is the presence of spatial correlation and cross-polarization between elements in the array. These effects can lead to a significant reduction in communication system capacity. Aim. Derive relationships based on the Kronecker model that allow to take into account the polarization properties of antenna elements when calculating the ergodic capacity of a multi-channel communication system. Investigate the influence of the cross-polarization parameter of the antenna element on the capacity. Assess the polarization characteristics of synthesized real antenna elements of various types and their impact on the capacity value. Methods. When deriving the basic relationships, methods of statistical radiophysics and electromagnetic analysis of the propagation channel were used. When synthesizing dual-polarization antenna elements, electrodynamic analysis methods based on Maxwell’s equations were used. Results. Based on the analysis using the derived relationships, it was established that the influence of the cross-polarization properties of the array antenna elements on the capacity of a multi-channel communication system becomes significant when the value of the cross-polarization parameters decreases below 10 dB. It is shown that synthesized real antenna elements of various types have high polarization isolation, which will lead to a slight decrease in the system capacity. Conclusion. The derived relationships and the developed methodology make it possible to estimate the change in the capacity of the communication system when taking into account the cross-polarization properties of the dual-polarization elements of the antenna array. This will allow to optimally select radiating elements for a given system. At the same time, the developed antenna elements of various types satisfy the imposed cross-polarization restrictions.

In a CDMA system, the smart antenna system is mainly applied to the base station. The smart antenna can adjust the direction pattern adaptively and reduce the interference signals using some adaptive interference nulling algorithms, and thus enhancing the performance of the communication systems. The article provides a description and analysis of some basic adaptive algorithms and their modifications. Their applications in a CDMA system are presented.

This paper proposes a packetized direct sequence code division multiple access (DS-CDMA) multimedia wireless communication system that allows for seamless interfacing to asynchronous transfer mode (ATM) broadband networks. The issue of reverse link power control is examined in the context of an indoor environment characterized by a slowly Rayleigh fading channel. Three algorithms are discussed: open loop, channel estimation, and closed loop power control. It is shown that closed loop power control with channel estimation achieves the best performance. The system capacity with perfect power control is calculated. The effect of power control imperfection on the capacity and received signal statistics is studied based on computer simulation. It is shown that actual cell capacity using a two-bit power control command is approximately 40% greater than that using a one-bit power control command at the expense of twice the feedback overhead.

The entire process of transmission and reception in a DS-CDMA system, including the spreading, is described as a tapped delay line model with multiple inputs and a single output. This model is then used to design a fractionally spaced decision feedback equalizer with a single input and multiple outputs. In the proposed approach, long codes can be used and channel estimation can be performed in an efficient way. The suggested detector is near-far resistant. Simulations demonstrate the utility of the proposed approach in a heavily loaded system with and without power control where multipath is present.