Unified framework for the analysis and design of linear uplink power control in CDMA systems

In this work, it is proposed a unified framework to design and analyze uplink distributed power control schemes over flat-fading channels from a control theory perspective. The effects of linear detectors and round trip delays are explicitly characterized in this study. First, the optimal solution to the power minimization problem under signal to interference-noise ratio (SINR) restrictions is reviewed, where sufficient conditions for its existence are presented that depends on the detection strategy. Four different linear detection schemes are studied in this work: Matched Filter, Decorrelator, MMSE and Projector. Specifically, two special cases are analyzed with respect to the spreading codes properties: uniform cross-correlation and orthogonal codes, and under both conditions an explicit expression for the central solution is obtained. Nevertheless, one drawback of the central solution is its lack of robustness against channel estimation errors, transport delays and noise. Hence, it is proposed closed-loop control laws with linear power assignment which are capable of provide robustness to these channel effects. It is then presented that under certain conditions, stable feedback loops can be obtained considering SINR quantification, transmission and processing delays, and the resulting closed-loop power solutions tend to the central ones. Finally, it is illustrated that the selection of the linear detectors does not affect the resulting closed-loop dynamics, but the uplink transmission power in steady-state. An exhaustive simulation evaluation is included to validate the mathematical analysis presented for open and closed-loop solutions.     

Distributed power control with multiuser detection for asynchronous DS-CDMA networks subject to time-delays

If transmission power is increased in a direct-sequence code-division multiple-access network, interference increases, thus reducing network capacity. In this paper, we address these opposite goals of reducing transmission power and increased network capacity by jointly considering power control and multiuser detection. Since propagation delay occurs inevitably during data communication, we present an analysis by considering also that the interference measurement is not available at the transmitter instantaneously but with some time-delay, something that is generally neglected in the literature. This framework is used to investigate the design of a distributed power control strategy enhanced with linear multiuser receivers in an asynchronous uplink channel, subject to multipath and quality of service constraints. Thus, a cross-layer solution that combines a Linear-Quadratic-Gaussian power control scheme with a linear multiuser receiver is proposed to compensate for the round-trip delay and the time-varying channel characteristics in the communication link. Simulation results are used to show the advantages of such scheme in terms of saving energy, increasing network capacity and robustness against propagation delays.     

Distributed power allocation algorithm in wireless networks under SNR constraints

In this work, the distributed power allocation problem in wireless networks is studied under signal-to-noise ratio (SNR) constraints. The sources of uncertainty are assumed to come from the quantization process and measurement noise in the feedback system. The power allocation is formulated as a reference tracking problem of a pre-defined signal to noise-interference ratio. First, the synthesis problem with SNR constraints is studied as a 2-norm minimization process, which is equivalent to a linear-quadratic-regulator (LQR) problem. The solution of the associated Riccati equation in the LQR formulation is completely characterized, resulting in a feedback law with a recurrent structure. This control law is re-written in a transfer function format, where a simple control strategy is obtained which is dependant on the round-trip delay in the feedback system. In addition, the corresponding 2-norm closed-loop performance is also studied. Thus, the selection of the weight in the LQR problem establishes a compromise between robustness to quantization errors and measurement noise, and tracking performance. A comprehensive simulation evaluation validates the analytical derivations described in the paper.     

Transmitter pre-filtering for the downlink of a time-division-duplex/direct sequencecode division multiple access system over time-varying multipath fading channels

In this paper, we investigate an optimised transmitter pre-filtering technique for downlink time-division-duplex (TDD) code division multiple access (CDMA) communications, which employs the conventional matched filter (MF) detector at the mobile receivers. The proposed pre-filtering technique eliminates the multiple-access interference and intersymbol interference (MAI/ISI) effects by applying a very simple transmission scheme that combines a signal transformation with a cyclic prefix strategy under a power constraint condition. Two constrained pre-filtering transformations are suggested depending on the information required at the mobile unit. An open-loop transmitter pre-filtering is first formulated; however, this solution does not consider the properties of the noise at the mobile receiver. A second solution is then presented via a closed-loop transmitter pre-filtering that includes an optimum gain for a given transmit and noise power. Some associated issues such as system efficiency, computational complexity and channel estimation errors are also addressed. Simulation results show that the proposed transmitter pre-filtering scheme can be used to increase the system performance and capacity. In addition, its performance is compared with another similar transmit pre-processing scheme in order to evaluate the performance improvement by the proposed algorithm.     

Robust design of a linear quadratic power control algorithm in multiple-access networks

In wireless networks under interference, power control is of the utmost importance to guarantee Quality of Service during data transmissions. A distributed perspective is commonly preferred to design controllers in each mobile user in the network for power allocation. The round-trip delay is a characteristic feature of wireless networks, and it was considered a known quantity in previous works. In this paper, we do not follow this assumption and propose the design of the power controller only with the information of upper and lower bounds on the round-trip delay as functions of a frequency gain. In a second stage, we relate the proposed robust design to common performance indicators, such as the step response overshoot. The proposed design rules can be applied to design a suitable robust controller for power control in a wireless network subject to interference.     

Improving the Performance of Spatial Modulation Schemes for MIMO Channels

Spatial modulation (SM) is a recent MIMO transmission concept that holds the potential to increase the spectral efficiency over wireless links using low-complexity transceivers. In this paper, an overview and performance analysis on the key issues of this emerging technology is presented. Based on this analysis, a performance enhancement is suggested by introducing an improved spatial modulation (ISM) scheme that retains the SM key advantages. It is observed that transmitting the same data symbol from more than one antenna at a time has a positive effect in terms of spectral efficiency but, on the other hand, it degrades the bit error rate (BER) performance. In this sense, a more general SM mapping rule is introduced aiming to provide a criterion for choosing an effective performance/rate/complexity trade-off according to the system requirement. An upper bound on the BER performance for ISM is derived and compared to Monte Carlo simulation results. In addition, a two-stage adaptive detector is also presented to enhance further the performance/complexity trade-off at the receiver for SM-based techniques.     

Distributed power control algorithms for asynchronous CDMA systems in frequency-selective fading channels

In Code Division Multiple Access (CDMA) radio environments, the maximum number of supportable users per cell is limited by multipath fading, shadowing, multiple access interference and near-far effects which cause fluctuations of the received power at the base station. In this context, power control and signal detection are essential to provide satisfactory Quality of Service (QoS) and to combat the near-far problem in CDMA systems. In this paper, we raised the uplink power control problem for a generalize asynchronous direct-sequence (DS) CDMA system that explicitly incorporate into the analysis: (1) the propagation delays in the network (generally neglected in the literature), (2) the adverse effect of multipath fading for wireless channels, and (3) the asynchronous transmissions in the uplink channels. This framework is used to propose a distributed power control strategy enhanced with linear multiuser receivers. It is shown that through a proper selection of an error function, the nonlinear coupling among active users is transformed into individual linear loops. A Linear-Quadratic-Gaussian (LQG) power control strategy is derived and compared with other approaches from the literature. Simulation results show that the uplink channel variations do not destroy the stability of these power control structures. However, delays in the closed-loop paths can severely affect the stability and performance of the resulting feedback schemes. It is also shown that the use of multiuser detection at the base station can bring significant improvements to the performance of power control.     

Analysis and Performance Evaluation of Linear Multiuser Detectors in the Downlink of DS-CDMA Systems Applying Spectral Decomposition

This paper studies the performance of linear multiuser detectors for direct-sequence code division multiple access systems at different loading levels and users' powers, using singular value decomposition (SVD) techniques in the downlink of Rayleigh flat-fading and additive white Gaussian channels. The performance of the matched filter (MF), decorrelator (zero-forcing), and minimum mean-squared error (MMSE) detectors are studied and compared. Analytical and simulation results are also presented in terms of the bit error rate. From this analysis, a simple linear multiuser detector is developed that exploits the structure of the system's spreading codes matrix from the SVD viewpoint. Also, the numerical performance of this proposed detector is compared to that of the conventional detector (MF) as a function of the signal-to-noise ratio. Finally, the performance limits are established in terms of the singular values of the spreading codes matrix. Extensive simulation results validate the analysis presented in the paper for equal or unequal users' powers.