Capacity unbalance between uplink and downlink in spectrallyoverlaid narrow-band and wide-band CDMA mobile systems

In this paper, we discuss capacity unbalance between uplink (mobile-to-base) and downlink (base-to-mobile) in future code division multiple access (CDMA) radio networks where both narrow-band and wide-band CDMA systems are coexisted. Since the two links are not operated in an identical condition, their capacities are unequal and either of the links determines the whole system capacity. The purpose of this paper is to examine which link limits the system capacity and what are the limiting factors. To facilitate the examination, “transmission capacity” and “connection capacity” are defined, and simplified formulas are presented to compute those capacities, respectively for uplink and downlink. Signal quality required for each link, effectiveness of power control, spatial distribution of mobile users and other-cell as well as same-cell user interference are usually determining the limiting link. Besides, the traffic unbalance between the links imposed by specific service applications and the network evolution scenarios are also shown to be very influencing factors     

Non-cooperative uplink power control in cellular radio systems

This paper presents uplink power control in cellular radio systems from an economic point of view. A utility function is defined for each mobile user, which reflects the user's preference regarding the carrier-to-interference ratio (CIR) and the transmitter power. We observe that, on one hand, mobile users prefer to transmit at a lower power for a fixed CIR. On the other hand, for a given transmitter power, users prefer to obtain a better CIR. Based on this observation, we make two fundamental assumptions about the utility function. We formulate the uplink power control problem as a non-cooperative N-person game. Under the two assumptions that we make about the utility function, there exists a Nash equilibrium. To show the generality of the framework, we study one special case by defining the utility as a linear function. This model encompasses many of the widely studied power control problems. A more general case is also studied by defining utility as an exponential function. This paper establishes a general economic-based framework for studying resource management in wireless networks and points out new research directions.     

Non-cooperative uplink power control in cellular radio systems

This paper presents uplink power control in cellular radio systems from an economic point of view. A utility function is defined for each mobile user, which reflects the user's preference regarding the carrier-to-interference ratio (CIR) and the transmitter power. We observe that, on one hand, mobile users prefer to transmit at a lower power for a fixed CIR. On the other hand, for a given transmitter power, users prefer to obtain a better CIR. Based on this observation, we make two fundamental assumptions about the utility function. We formulate the uplink power control problem as a non-cooperative N-person game. Under the two assumptions that we make about the utility function, there exists a Nash equilibrium. To show the generality of the framework, we study one special case by defining the utility as a linear function. This model encompasses many of the widely studied power control problems. A more general case is also studied by defining utility as an exponential function. This paper establishes a general economic-based framework for studying resource management in wireless networks and points out new research directions. This revised version was published online in July 2006 with corrections to the Cover Date.     

Distributed power control in CDMA cellular systems

In wireless cellular communication, it is essential to find effective means for power control of signals received from randomly dispersed users within one cell. Effective power control will heavily impact the system capacity. Distributed power control (DPC) is a natural choice for such purposes, because, unlike centralized power control, DPC does not require extensive computational power. Distributed power control should be able to adjust the power levels of each transmitted signal using only local measurements, so that, in a reasonable time, all users will maintain the desired signal-to-interference ratio. In this paper, we review different approaches for power control, focusing on CDMA systems. We also introduce state-space methods and linear quadratic power control (LQPC) to solve the power-control problem. A simulation environment was developed to compare LQPC with earlier approaches. The results show that LQPC is more effective, and is capable of computing the desired transmission power of each mobile station in fewer iterations, as well as being able to accommodate more users in the system     

Effect of co-phasing diversity on CDMA downlink capacity

The impact of distributed antennas using co-phasing transmission diversity on downlink CDMA channels is examined. An expression is derived that demonstrates that uniform downlink signal-to-interference ratio across users can be achieved by appropriately weighting the distribution of transmit powers across distributed antennas. The capacity of a downlink CDMA system employing distributed antennas with optimum power allocation is then found by simulation.     

A framework for uplink power control in cellular radio systems

In cellular wireless communication systems, transmitted power is regulated to provide each user an acceptable connection by limiting the interference caused by other users. Several models have been considered including: (1) fixed base station assignment where the assignment of users to base stations is fixed, (2) minimum power assignment where a user is iteratively assigned to the base station at which its signal to interference ratio is highest, and (3) diversity reception where a user's signal is combined from several or perhaps all base stations. For the above models, the uplink power control problem can be reduced to finding a vector p of users' transmitter powers satisfying p⩾I(p) where the jth constraint p_j⩾I_j(p) describes the interference that user j must overcome to achieve an acceptable connection. This work unifies results found for these systems by identifying common properties of the interference constraints. It is also shown that systems in which transmitter powers are subject to maximum power limitations share these common properties. These properties permit a general proof of the synchronous and totally asynchronous convergence of the iteration p(t+1)=I(p(t)) to a unique fixed point at which total transmitted power is minimized     

Soft handoff and uplink capacity in a two-tier CDMA system

This paper examines the effect of soft handoff on the uplink user capacity of a code division multiple access system consisting of a single macrocell in which a single hotspot microcell is embedded. The users of these two base stations operate over the same frequency band. In the soft-handoff scenario studied here, both macrocell and microcell base stations serve each system user, and the two received copies of a desired user's signal are summed using maximal ratio combining. Exact and approximate analytical methods are developed to compute uplink user capacity. Simulation results demonstrate a 20% increase in user capacity compared to hard handoff. In addition, simple approximate methods are presented for estimating soft-handoff capacity and are shown to be quite accurate.     

多业务CDMA系统上行链路容量分析

研究了多业务CDMA系统的上行链路容量,提出一种容量分析方法。在方法中考 虑了多业务的激活概率,分析了不同业务的掉话率,以系统掉话率为基准给出了系统容量。该 分析方法表明CDMA系统的容量受多业务的Eb/N0、传输速率和业务激活概率的影响。     

Macrodiversity power control in hierarchical CDMA cellular systems

Hierarchical code division multiple access (CDMA) cellular systems, consisting of macrocells with underlying microcells, are studied. We seek power control schemes which will allow both hierarchical layers to share the same spectrum. For the reverse link, hierarchical maximal ratio combining (HMRC) is applied where each mobile station (MSs) is received and coherently combined by base stations (BSs) in both layers. For the forward link, selective transmit diversity (STD) is applied where each BS provides multiple transmit paths for MSs to choose. We show that both HMRC and STD are effective in hierarchical CDMA architectures. We conclude that hierarchical architectures are a viable solution for improving CDMA cellular system capacity, and a significant performance gain can be achieved without assigning disjoint spectrum between the layers, by utilizing macrodiversity schemes such as HMRC and STD     

Centralised power control in CDMA cellular mobile systems

Efficient power control is of great importance in the design of high capacity cellular radio systems. Optimum power control scheme, in the sense that it minimises the outage probability, has been fully investigated for FDMA/TDMA cellular systems. The authors propose optimum power control and several centralised power control algorithms for CDMA cellular systems. Simulation results indicate that the optimum power control scheme outperforms the perfect average power control algorithm by ~1.9 dB under the IS-95 defined radio condition     

On the design of uplink and downlink group-orthogonal multicarrier wireless systems

Group-orthogonal multicarrier code-division multiple access (GO-MC-CDMA) has been proposed as an attractive multiplexing technique for the uplink segment of wireless systems. More recently, a variant of this scheme has also been proposed for the downlink. This paper presents a unified bit error rate (BER) performance analysis of group-orthogonal wireless systems when using maximum likelihood (ML) multiuser/multisymbol detection covering both link directions. Valuable design rules regarding the number of subcarriers per group and the selection of spreading codes are derived. Simulations results using realistic system parameters and ETSI BRAN channel models are also presented which serve to validate the analytical results.     

Decentralized dynamic power control for cellular CDMA systems

The control of transmit power has been recognized as an essential requirement in the design of cellular code-division multiple-access (CDMA) systems. Indeed, power control allows for mobile users to share radio resources equitably and efficiently in a multicell environment. Much of the work on power control for CDMA systems found in the literature assumes a quasi-static channel model, i.e., the channel gains of the users are assumed to be constant over a sufficiently long period of time for the control algorithm to converge. In this paper, the design of dynamic power control algorithms for CDMA systems is considered without the quasi-static channel restriction. The design problem is posed as a tradeoff between the desire for users to maximize their individual quality of service and the need to minimize interference to other users. The dynamic nature of the wireless channel for mobile users is incorporated in the problem definition. Based on a cost minimization framework, an optimal multiuser solution is derived. The multiuser solution is shown to decouple, and effectively converge, to a single-user solution in the large system asymptote, where the number of users and the spreading factor both go to infinity with their ratio kept constant. In a numerical study, the performance of a simple threshold policy is shown to be near that of the optimal single-user policy. This offers support to the threshold decision rules that are employed in current cellular CDMA systems.     

Open-loop power control error in cellular CDMA overlay systems

Adaptive power control has widely been used in DS/CDMA systems to overcome the so-called “near-far” problem. This paper studies the adaptive open-loop power control of a cellular CDMA system, which is overlaid in the downlink by a narrowband signal. The effects of downlink power allocation schemes to power control error in the presence of narrowband interference are analyzed. In order to get a minimum power control error in the CDMA overlay situations, an optimum downlink power allocation scheme is used, which works well for a wide range of signal to narrowband interference ratio     

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.     

Power control for wireless multimedia CDMA systems

In a wireless multimedia communication system where different types of traffic such as speech, data, and video have different data rates, good power control for individual traffic can ensure quality of service (QoS) and achieve a better throughput at the same time. The authors propose a closed-form power control function based on a parameter called the traffic exponent that simplifies the power control processing and improves the overall CDMA system performance     

CDMA systems in fading channels: admissibility, network capacity,and power control

We study the admissibility and network capacity of imperfect power-controlled code-division multiple access (CDMA) systems with linear receivers in fading environments. In a CDMA system, a set of users is admissible if their simultaneous transmission does not result in violation of any of their quality-of-service (QoS) requirements; the network capacity is the maximum number of admissible users. We consider a single-cell imperfect power-controlled CDMA system, assuming known received power distributions. We identify the network capacities of single-class systems with matched-filter (MF) receivers for both the deterministic and random signature cases. We also characterize the network capacity of single-class systems with linear minimum-mean-square-error (MMSE) receivers for the deterministic signature case. The network capacities can be expressed uniquely in terms of the users' signal-to-interference ratio (SIR) requirements and received power distributions. For multiple-class systems equipped with MF receivers, we find a necessary and sufficient condition on the admissibility for the random signature case, but only a sufficient condition for the deterministic signature case. We also introduce the notions of effective target SIR and effective bandwidth, which are useful in determining the admissibility and hence network capacity of an imperfect power-controlled system     

Adaptive predictive power control for the uplink channel in DS-CDMA cellular systems

In this paper, we analyze the conventional closed-loop power-control system. We explain that the system behaves essentially as a companded delta modulator and then derive an expression for the power-control error in terms of the channel fading, which suggests methods for reducing the error variance. This is achieved by using a prediction technique for estimating the channel-power fading profile. The prediction module is combined with several proposed schemes for closed-loop power control. The resulting architectures are shown to result in improved performance in simulations.     

CDMA cellular systems performance with fading, shadowing, andimperfect power control

This paper addresses capacity estimation for cellular code-division multiple-access (CDMA) systems, assuming the IS-95 standard as a reference. Extending a previous analytical method (Viterbi et al., 1994), we obtain a sequence of bounds on capacity, and then we introduce an accurate approximation to reduce computation complexity. The analysis accounts for interference internal and external to the reference cell, fading, shadowing, and imperfect power control. Outage probability is expressed in terms of the characteristic functions (cf's) of the interference and imperfect power control random variables (RV's). The interference contributions are computed on the basis of a Poisson distribution for the number of users in a lognormally shadowed channel. Results are provided for different channel conditions and are validated against Monte Carlo simulations. A comparison against previously published CDMA capacity estimates is carried out, aimed at clarifying some controversial issues. It is also confirmed that large system capacity is achievable under tight power control     

An uplink power control based on truncated channel inversion fordata traffic in a cellular CDMA system

A power control scheme, called truncated channel inversion, in which a mobile suspends transmission when the wireless channel is in a bad condition in order to reduce out-cell interference is examined. It is found that capacity is improved and battery power consumption is reduced as a consequence of suspending transmission, but extra queuing delay is introduced. It is shown that the extra queuing delay depends on how fast the channel characteristics change and that the truncated scheme works well, even for moderate mobile velocities     

利用CDMA系统下行链路信号进行移动台定位的研究

深入分析了在CDMA系统中利用下行链路信号进行移动台定位存在的问题,提出使用干扰抑制的方法来抑制服务基站信号,提高定位精度的策略,详细叙述了算法流程并给出了仿真结果,结果表明,利用干扰抑制算法可以大幅度提高定位精度。