Sliding window decorrelating algorithm for DS-CDMA receivers

A sliding window decorrelating algorithm is developed. It is shown to be near-far resistant both in AWGN and fast fading environments. The algorithm is of particular relevance to base stations in multiuser DS-CDMA mobile radio networks and is shown to alleviate the requirement for a complex power control algorithm to compensate for rapidly varying relative user energies.<>     

Blind decorrelating RAKE receivers for long-code WCDMA

The problem of blind and semiblind channel estimation and symbol detection is considered for long-code wideband code division multiple access (CDMA) systems, including systems with multirate and multicode transmissions. A decorrelating matched filter, implemented efficiently in state-space, eliminates multiaccess interference and produces a bank of vector processes. Each vector process spans a one-dimensional (1-D) subspace from which channel parameters and data symbols of one user are estimated jointly by least squares. A new identifiability condition is established, which suggests that channels unidentifiable, in short-code CDMA systems are almost surely identifiable when aperiodic spreading codes are used. The decorrelating matched filter is implemented efficiently based on time-varying state-space realizations that exploit the structure of sparsity of the code matrix. The mean square error of the estimated channel is compared to the Cramer-Rao bound, and a bit error rate (BER) expression for the proposed algorithm is presented.     

Finite precision decorrelating receivers for multiuser CDMAcommunication systems

The design of single-user decorrelating receivers employing finite-precision sequences for despreading is considered. The problem is formulated as a nonlinear bounded integer optimization problem which is shown to be network performance (NP)-hard. A branch-and-bound algorithm for finding the best finite-precision decorrelating sequence is described. Numerical examples demonstrate that the loss in performance between the optimum, infinite-precision, and the best finite-precision decorrelator is small even for large channel occupancies. Some suboptimum algorithms are investigated which greatly reduce the computational complexity associated with finding good finite precision decorrelator sequences     

Differential and coherent decorrelating multiuser receivers for space-time-coded CDMA systems

Previously, we proposed a differential space-code modulation (DSCM) scheme that integrates the strength of differential space-time coding and spreading to achieve interference suppression and resistance to time-varying channel fading in single-user environments. In this paper, we consider the problem of multiuser receiver design for code-division multiple-access (CDMA) systems that utilize DSCM for transmission. In particular, we propose two differential receivers for such systems. These differential receivers do not require the channel state information (CSI) for detection and, still, are resistant to multiuser interference (MUI) and time-varying channel fading. We also propose a coherent receiver that requires only the CSI of the desired user for detection. The coherent receiver yields improved performance over the differential receivers when reliable channel estimates are available (e.g., in slowly fading channels). The proposed differential/coherent receivers are decorrelative schemes that decouple the detection of different users. Both long and short spreading codes can be employed in these schemes. Numerical examples are presented to demonstrate the effectiveness of the proposed receivers.     

Optimal decorrelating receivers for DS-CDMA systems: a signalprocessing framework

Code division multiple access (CDMA) schemes allow a number of asynchronous users to share a transmission medium with minimum cooperation among them. However, sophisticated signal processing algorithms are needed at the receiver to combat interference from other users and multipath effects. A discrete-time multirate formulation is introduced for asynchronous CDMA systems, which can incorporate multipath effects. This formulation reveals interesting links between CDMA receivers and array processing problems. In this framework, linear receivers are derived that can completely suppress multiuser interference (decorrelating receivers). A criterion is introduced, which guarantees the decorrelating property, while providing optimal solutions in the presence of noise. Parametric FIR designs as well as nonparametric solutions are delineated, and their performance is analyzed. The proposed receivers are resistant to near-far effects and do not require the estimation of the users' and noise powers     

A decorrelating detector for use with antenna arrays

This paper addresses the problem of synchronous CDMA communications when, due to bandwidth limitations, signature waveforms have relatively large correlations. A receiver which uses the multiple received signals available from an antenna array is proposed and its performance analyzed. This receiver may be interpreted as a multichannel extension of the decorrelating detector and therefore shares its immunity to near-far effects. The receiver uses the discrimination afforded by the antenna array to effectively reduce the correlation between users' transmitted signals thereby avoiding the reduction in performance which occurs in the single channel decorrelator when signature correlations become significant.This work was supported by the National Science Foundation under Grant MIP-9202081. This paper was presented in part at the International Conference on Communications, ICC'94, New Orleans, Louisiana.     

Turbo-BLAST系统中天线选择和功率分配算法

提出一种适用于Turbo-BLAST系统的自适应天线选择和功率分配算法,所提算法以误比特率为优化目标,并且考虑了信道估计误差的影响.在总功率约束条件下,采用所提算法进行天线选择和自适应功率分配,并利用软干扰抵消算法对接收信号进行迭代检测,以进一步改善系统性能.仿真结果表明:采用所提算法可以显著改善系统的误比特率性能.     

Spreading code allocation strategy for downlink multicarrier code division multiple access transmission in a correlated Rayleigh fading channel

In this paper, we address the problem of multiple access interference (MAI) in a downlink multicarrier code division multiple access system. Because in the realistic case of correlated faded subcarriers, MAI greatly depends on the codes assigned to users, one way of improving performance without increasing receiver complexity consists in an appropriate code selection. We propose a code allocation strategy with the following properties: possible use with any code sequence and equalizer, low complexity, and efficient management of load variations. The allocation problem is formulated as a minimization problem of a cost function related to MAI. First, we provide analytical expressions for the channel frequency correlation function after maximum ratio combining, equal gain combining, and minimum mean square error equalization and for the MAI power. Then, by approximating these expressions, we define a simple cost function and build an iterative algorithm on the basis of the minimum maximum criterion to select the spreading codes. Finally, a complete analysis of the allocation efficiency is provided versus key parameters, in particular, the degree of correlation between the faded subcarriers, the system load, and the equalization techniques. Copyright © 2012 John Wiley & Sons, Ltd.     

Adaptive allocation of power transmission for high-altitude platforms

An adaptive power allocation method for signals transmitted from high-altitude platforms is proposed, and justification of its usefulness is provided. The possible structural scheme for the system is given and its performance is investigated. The adaptation algorithm is based on an optimization procedure, which enables the allocation of the available power to a maximal number of users. The proposed algorithm is tested on simulations. It is shown that for Nakagami fading in downlink channels, the adaptive power allocation yields an energetic gain which can attain up to 21 dB.     

Receiving antenna design for miniature receivers

A design procedure is described which determines the minimum required volume and the optimum dimensions for a ferrite loaded loop receiving antenna. The frequency of operation, bandwidth, transmitted signal strength, desired signal-to-noise ratio, receiver sensitivity, and the relative permeability of the ferrite are required to determine the design. Useful approximations for flux averaging, demagnetization factor, atmospheric noise, and chassis loadedQare given.     

Multimedia transmission in MC-CDMA using adaptive subcarrier power allocation and CFO compensation

Multicarrier code division multiple access (MC-CDMA) system is one of the most effective techniques in fourth-generation (4G) wireless technology, due to its high data rate, high spectral efficiency and resistance to multipath fading. However, MC-CDMA systems are greatly deteriorated by carrier frequency offset (CFO) which is due to Doppler shift and oscillator instabilities. It leads to loss of orthogonality among the subcarriers and causes intercarrier interference (ICI). Water filling algorithm (WFA) is an efficient resource allocation algorithm to solve the power utilisation problems among the subcarriers in time-dispersive channels. The conventional WFA fails to consider the effect of CFO. To perform subcarrier power allocation with reduced CFO and to improve the capacity of MC-CDMA system, residual CFO compensated adaptive subcarrier power allocation algorithm is proposed in this paper. The proposed technique allocates power only to subcarriers with high channel to noise power ratio. The performance of the proposed method is evaluated using random binary data and image as source inputs. Simulation results depict that the bit error rate performance and ICI reduction capability of the proposed modified WFA offered superior performance in both power allocation and image compression for high-quality multimedia transmission in the presence of CFO and imperfect channel state information conditions.     

A spatial-temporal decorrelating receiver for CDMA systems withbase-station antenna arrays

We investigate multiuser signal detection with a base-station antenna array for a synchronous DS-CDMA uplink using nonorthogonal codes in Rayleigh fading channels. We have developed a new formulation for a spatial-temporal decorrelating detector using the maximum-likelihood criteria. The detector is shown to be near-far resistant. We propose to implement the spatial-temporal decorrelating receiver iteratively by applying the space-alternating generalized expectation-maximization (SAGE) algorithm. Simulation results show that the SAGE-based decorrelating receiver significantly outperforms the conventional single-user receiver and with performance close to that of a spatial-temporal decorrelating receiver with known channel parameters. We have observed that adding base-station antennas can actually improve convergence of the proposed iterative receiver     

MMSE multiuser downlink multiple antenna transmission for CDMA systems

In this paper, we propose a new downlink transmit antenna processing (TAP) technique for code division multiple access (CDMA) equipped with multiple transmit antennas. In order to find the weight vectors for downlink signals, a minimum mean square error (MMSE) performance criterion is used. Since the multiuser interference is taken into account in the calculation of the weighting vectors for TAP, the proposed method is a multiuser downlink TAP method. It is assumed that the downlink channels are known by the downlink TAP. For given channel conditions, the optimal weight vectors are found with a closed-form expression under both flat and frequency-selective fading channel assumptions.     

Optimal and constant power allocation for joint transmission in HetNet

This paper investigates the power allocation issues for joint transmission in heterogeneous network （HetNet）, which is characterized by severe cross-tier interference. The optimization problem of maximizing the HetNet throughput is formulated. The original problem turns out to be a non-convex problem, the global optima of which cannot be obtained by conventional optimization methods. This paper develops a novel method to achieve the global optima by tuming the original problem into quasi-convex problem. In addition, this paper considers a constant power allocation scheme, as a tradeoff between the system throughput and computational complexity. Based on duality gap theory, the bound of constant power allocation scheme is mathematically derived. Numerical results under different system parameters indicate that both the proposed schemes outperform conventional interference coordination schemes.     

Energy aware power allocation strategies for multihop-cooperative transmission schemes

This paper is focused on the optimization of transmitted power in a cooperative decoded relaying scheme for nodes belonging to the single primary route towards. a destination. The proposed transmission protocol, referred to as Multihop Cooperative Transmission Chain (MCTC), is based on the linear combination of copies of the same message by multiple previous terminals along the route in order to maximize the multihop diversity. Power allocations among transmitting nodes in the route can be obtained according to the average (not instantaneous) node-to-node path attenuation using a recursive power assignment. The latter can be employed locally on each node with limited signalling exchange (for fixed or nomadic terminals) among nodes. In this paper the power assignments for the MCTC strategy employing conventional linear combining schemes at receivers (i.e., selection combining, maximal ratio combining and equal gain combining) have been derived analytically when the power optimization is constrained to guarantee the end-to-end outage probability. In particular, we show that the power assignment that minimize the maximum spread of received power (min-max strategy) can efficiently exploit the multihop diversity. In addition, for ad hoc networks where the energy of each node is an issue, the MCTC protocol with the min-max power assignment increases considerably the network lifetime when compared to non-cooperative multihop schemes     

Coding approaches for multiple antenna transmission in fast fading and OFDM

Multiple-antenna channel coding for orthogonal frequency-division multiplexing (OFDM) transmission over dispersive channels is reconsidered because with frequency interleaving, the effective channel characteristic across subcarriers is rather fast fading. The channel does not comply with the quasistatic model widely assumed for space-time trellis codes (STCs). For that reason, we first study the ideal fast-fading multiple transmit and receive antenna channel and then compare the performance of STCs with that of bit-interleaved coded modulation in fast fading. Mutual information of the ergodic channel is evaluated for numerous modulation scenarios, and capacity comparisons generate guidelines on how to jointly adjust coding rate and modulation cardinality. Bit-based coding offers large flexibility in rate adaptation, and simulation results show that it outperforms STCs in ideal fast fading and, finally, in a realistic OFDM application as well.     

具有大规模天线选择与功率分配的协作通信

在瑞利衰落信道下研究了采用前向译码转发协议的三节点协作模型,分析了中继分布大规模天线阵列时系统的可靠性能。为了有效地降低系统复杂度,同时又能获得大规模天线带来的分集增益,给出了中继选择合并(SC)与天线选择空间调制(SM)相结合的组合传输方案。为了进一步提高可靠性能,研究了SC/SM组合方案如何通过功率分配优化其误比特率性能。仿真分析表明,大规模中继SC/SM方案的功率分配能够有效改善系统的误比特率性能。     

Joint power and bandwidth allocation in downlink transmission

We formulate and analyze the problem of optimal downlink scheduling with instantaneous channel and queue size information when both power and bandwidth may be adaptively split among multiple users. We derive optimal solutions of low computational complexity, as well as faster and simpler approximations, to various versions of this problem when the power, rate, and bandwidth allocations to the users can all take continuous values. For this case, we show that the optimal scheme requires transmission to no more than two users during each time slot when users can receive at arbitrary rates, even when the user rate per unit of bandwidth is upper bounded by the best available modulation scheme. Our methods also extend easily to incorporate other intuitive constraints such as upper limits on user rates to improve frame fill efficiency. Simulation results suggest that the simple approximations work nearly as well as the throughput optimal schemes when continuous bandwidth and power partitions are allowed. In practice, the rate and bandwidth assignments to users take discrete values, and we present heuristic methods motivated by the continuous optimum to this discrete case.     

Two-way training: optimal power allocation for pilot and data transmission

In this letter, we consider multiple-input singleoutput (MISO) systems with two-way training based transmission. We focus on the long-term system performance and study the optimal power allocation between reverse training, forward training and data transmission. We derive closed-form solutions for the optimal power allocation using high signal-to-noise ratio (SNR) approximations, and show that they achieve near optimal performance in terms of symbol error rate (SER) for different modulation schemes over a wide range of SNR values.