Uplink user capacity in a multicell CDMA system with hotspot microcells

The number of simultaneous users (or user capacity) supportable on the uplink of a multiple-macrocell code division multiple-access (CDMA) system with multiple "hotspot" microcells embedded within is studied. These microcells operate on the same frequency as the macrocells and are installed in regions of high user demand. It is shown that the user capacity depends on how the users are distributed among cells, and that the maximum (called the attainable capacity) occurs when all cells serve roughly the same number of users. The approach builds on a two-cell analysis published previously, for a single microcell embedded in a single macrocell. First, this analysis is expanded upon to estimate the attainable capacity for M macrocells, where the center one contains L microcells. Then the case in which L microcells are distributed randomly among the M macrocells is analyzed. In each case, the formula for attainable capacity is very simple and highly accurate (as demonstrated via simulations) up to reasonably high values of L. For example, with L microcells distributed among M macrocells, the analysis is accurate at least up to eight microcells per macrocell. The analysis and results are general with respect to cell geometries, propagation parameters, and other variables of the two-tier CDMA system.     

Optimum power control for CDMA with deterministic sequences in fading channels

We specify the capacity region for a power-controlled, fading code-division multiple-access (CDMA) channel. We investigate the properties of the optimum power allocation policy that maximizes the information-theoretic ergodic sum capacity of a CDMA system where the users are assigned arbitrary signature sequences in a frequency flat-fading environment. We provide an iterative waterfilling algorithm to obtain the powers of all users at all channel fade levels, and prove its convergence. Under certain mild conditions on the signature sequences, the optimum power allocation dictates that more than one user transmit simultaneously in some nonzero probability region of the space of all channel states. We identify these conditions, and provide an upper bound on the maximum number of users that can transmit simultaneously at any given time. Using these properties of the sum capacity maximizing power control policy, we also show that the capacity region of the fading CDMA channel is not in general strictly convex.     

Multiple-access capacity in mobile user satellite systems

The channel capacity of a satellite direct sequence CDMA system is analyzed including the effects of faded user interference, overlapping antenna beams, imperfect equalization of the antenna pattern across an antenna cell, and diversity reception. Simplified models are used to describe the impact of these effects on the channel capacity of single and multiple cofrequency CDMA systems. In a comparison of the uplink and downlink paths, the uplink of the CDMA system is shown to limit the channel capacity because the downlink can utilize code-orthogonality and coherent demodulation. In a multiple system comparison between band-shared CDMA and band-segmented FDMA/TDMA technologies, FDMA/TDMA is shown to provide about the same capacity for uniformly distributed traffic conditions over many cells and dramatically better capacity when traffic is concentrated in one cell. Due to the peak nature of telephony, this result supports the use of band-segmented systems in mobile user satellite systems     

Performance analysis of an opportunistic multi‐user cognitive network with multiple primary users

Consider a multi‐user underlay cognitive network where multiple cognitive users concurrently share the spectrum with a primary network with multiple users. The channel between the secondary network is assumed to have independent but not identical Nakagami‐m fading. The interference channel between the secondary users (SUs) and the primary users is assumed to have Rayleigh fading. A power allocation based on the instantaneous channel state information is derived when a peak interference power constraint is imposed on the secondary network in addition to the limited peak transmit power of each SU. The uplink scenario is considered where a single SU is selected for transmission. This opportunistic selection depends on the transmission channel power gain and the interference channel power gain as well as the power allocation policy adopted at the users. Exact closed form expressions for the moment‐generating function, outage performance, symbol error rate performance, and the ergodic capacity are derived. Numerical results corroborate the derived analytical results. The performance is also studied in the asymptotic regimes, and the generalized diversity gain of this scheduling scheme is derived. It is shown that when the interference channel is deeply faded and the peak transmit power constraint is relaxed, the scheduling scheme achieves full diversity and that increasing the number of primary users does not impact the diversity order. Copyright © 2014 John Wiley & Sons, Ltd.     

Packet-length adaptive CLSP/DS-CDMA: performance in burst-error correlated fading channels

The authors analyze throughput-delay performance of an unslotted channel load sensing protocol (CLSP)/direct sequence (DS)-code division multiple access (CDMA) packet radio network (PRN) with adaptive packet length over burst-error correlated fading channels. CLSP controls the packet access in uplink of unslotted ALOHA/DS-CDMA systems so that contention is avoided and throughput is maximized. However, due to high uncertainty of radio channels, the performance of CLSP/DS-CDMA PRN may suffer from notable degradations. Using theoretical analysis and simulation, the authors show that in highly correlated fading environments adapting the length of radio packets to fading conditions significantly improves system performance and energy efficiency of mobile terminals. In their modeling, they study the relation between the fade statistics and the packet length in correlated Rayleigh fading channels. The effects of reception diversity, imperfect transmit power control (TPC), and user mobility are considered. The results are used to develop simple, energy-efficient, and robust adaptation mechanisms.     

Group-blind intersymbol multiuser detection for downlink CDMA with multipath

Group-blind multiuser detectors for uplink code-division multiple-access (CDMA) were recently developed by Wang and Host-Madsen. These detectors make use of the spreading sequences of known users to construct a group constraint to suppress the intracell interference. However, such techniques demand the estimation of the multipath channels and the delays of the known users. In this paper, several improved blind linear detectors are developed for CDMA in fading multipath channels. The proposed detectors utilize the correlation information between consecutively received signals to generate the corresponding group constraint. It is shown that by incorporating this group constraint, the proposed detectors can provide different performance gains in both uplink and downlink environments. Compared with the previously reported group-blind detectors, our new methods only need to estimate the multipath channel of the desired user and do not require the channel estimation of other users. Simulation results demonstrate that the proposed detectors outperform the conventional blind linear multiuser detectors.     

Asymptotically Optimal Power Control Policies for MMSE Multiuser Detector

Asymptotically optimal uplink transmission power control policies are derived for synchronous and asynchronous CDMA time varying fading channels with multiple user classes, random spreading sequences and MMSE multiuser detector. One optimal policy minimizes the outage probability subject to maximum average power budget and maximum transmission power constraints. Another minimizes the average power budget subject to maximum outage probability and maximum transmission power constraints. If the channel states are available, we show that both optimal policies are channel inversion threshold cut-off policies and express them explicitly. When channel states are estimated, we show how to transform the optimal policies into estimator-based policies while controlling the degradation of their performance measures . Finally, we compare between the performance of the optimal policies and other policies in an environment with Lognormal and Rayleigh fading.     

信道老化时高速移动大规模MIMO上行系统的性能分析

针对高速移动场景下大规模多输入多输出(MIMO)上行通信系统,推导了时变莱斯衰落信道中信道老化时大规模MIMO上行通信系统的可达和速率表达式,并且分析了信道老化对系统功率缩放准则的影响.理论分析与仿真结果表明,在时变莱斯信道中,每个用户的发送功率按照1/M进行缩放,信道老化仅会引起系统可达和速率的降低,而莱斯因子对系统可达和速率及功率缩放准则均有影响,且信道老化对系统可达和速率的影响较莱斯因子的影响可以忽略不计.     

一种频选衰落信道下的Turbo多用户检测算法

联合MAP多用户检测与信道解码的迭代多用户检测(MUD)技术可显著提高宽带移动CDMA系统的容量和性能.在多径时变衰落的编码信道下,提出一种迭代实现干扰抑制、符号估计、信道解码的Turbo多用户检测算法.在每次迭代中,MUD自适应地实现干扰抑制并输出符号估计的软信息,软输入软输出的信道解码器使用LOG MAP方法实现信道解码并反馈符号估计的软信息作为下一次TurboMUD迭代的先验信息.仿真结果证实了该算法在频选衰落信道下经两次迭代就能逼近单用户编码CDMA系统的接收性能.     

空时分组码MC-CDMA系统多用户检测

本文研究了在频率选择性瑞利衰落信道中工作于时分双工(TDD)模式的多载波码分多址(MC-CDMA)系统上行链路,在用户终端处使用两个发射天线,采用基于空时分组码的发射分集。考虑对应于子载波的衰落系数是信道冲激响应的离散傅里叶变换,给出了基于空时分组码的MC-CDMA系统上行链路信号模型。采用适用于同步CDMA系统的低代价Steiner估计器来进行基于空时分组码的MC-CDMA系统的信道估计,每个用户终端的两个发射天线各分配一个midamble。研究了基于空时分组码的MC-CDMA系统的解相关多用户检测、最小均方误差(MMSE)多用户检测,进行联合的多用户检测和空时码解码。仿真结果验证了上述模型及算法的有效性。     

Optimal resource allocation for wireless video over CDMA networks

We present a multiple-channel video transmission scheme in wireless CDMA networks over multipath fading channels. We map an embedded video bitstream, which is encoded into multiple independently decodable layers by 3D-ESCOT video coding technique, to multiple CDMA channels. One video source layer is transmitted over one CDMA channel. Each video source layer is protected by a product channel code structure. A product channel code is obtained by the combination of a row code based on rate compatible punctured convolutional code (RCPC) with cyclic redundancy check (CRC) error detection and a source-channel column code, i.e., systematic rate-compatible Reed-Solomon (RS) style erasure code. For a given budget on the available bandwidth and total transmit power, the transmitter determines the optimal power allocations and the optimal transmission rates among multiple CDMA channels, as well as the optimal product channel code rate allocation, i.e., the optimal unequal Reed-Solomon code source/parity rate allocations and the optimal RCPC rate protection for each channel. In formulating such an optimization problem, we make use of results on the large-system CDMA performance for various multiuser receivers in multipath fading channels. The channel is modeled as the concatenation of wireless BER channel and a wireline packet erasure channel with a fixed packet loss probability. By solving the optimization problem, we obtain the optimal power level allocation and the optimal transmission rate allocation over multiple CDMA channels. For each CDMA channel, we also employ a fast joint source-channel coding algorithm to obtain the optimal product channel code structure. Simulation results show that the proposed framework allows the video quality to degrade gracefully as the fading worsens or the bandwidth decreases, and it offers improved video quality at the receiver.     

Blind adaptive space-time multiuser detection with multipletransmitter and receiver antennas

The demand for performance and capacity in cellular systems has generated a great deal of interest in the development of advanced signal processing techniques to optimize the use of system resources. In particular, much work has been done on space-time processing in which multiple transmit/receive antennas are used in conjunction with coding to exploit spatial diversity. We consider space-time multiuser detection using multiple transmit and receive antennas for code-division multiple-access (CDMA) communications. We compare, via analytical bit-error-probability calculations, user capacity, and complexity, two linear receiver structures for different antenna configurations. Motivated by its appearance in a number of third-generation (3G) wideband CDMA standards, we use the Alamouti (see IEEE J. Select. Areas Commun., vol.16, p.1451-58, Oct. 1998) space-time block code for two-transmit-antenna configurations. We also develop blind adaptive implementations for the two transmit/two receive antenna case for synchronous CDMA in flat-fading channels and for asynchronous CDMA, in fading multipath channels. Finally, we present simulation results for the blind adaptive implementations     

基于智能天线阵接收的蜂窝CDMA网络性能分析

本文分析了在频率选择性信道中基于智能天线阵接民的蜂窝CDMA网络上行链路的性能,分析基于多个小区的异步蜂窝CDMA系统,系统采用BPSK调制,并采用智能天线阵接收的RAKE合并技术,通过分析最终得到了在该环境下的闭合的CDMA系统误码率公式,结果表明基于智能天线阵接收的蜂窝CDMA性能要比不采用的好得多,并给出了误码率与系统用户数,小区数和衰落模型之间的关系,结果对于分析蜂窝CDMA系统容量具有一定的指导意义。     

Receiver Design for Uplink Multiuser Code Division Multiple Access Communication System Based on Neural Network

In this paper, we consider the receiver design problem for the uplink multiuser code division multiple access (CDMA) communication system based on the neural network technique. The uplink multiuser CDMA communication system model is described in the form of space–time domain through antenna array and multipath fading expression. Novel suitable neural network technique is proposed as an effective signal processing method for the receiver of such an uplink multiuser CDMA system. By the appropriate choice of the channel state information for the neural network parameters, the neural network can collectively resolve the effects of both the inter-symbol interference due to the multipath fading channel and the multiple access interference in the receiver of the uplink multiuser CDMA communication system. The dynamics of the proposed neural network receiver for the uplink multiuser CDMA communication system is also studied.     

Superposition Coding with Unequal Error Protection for the Uplink of Overloaded DS-CDMA System

The overloaded CDMA schemes exploited in direct sequence CDMA (DS-CDMA) systems are mainly to accommodate a greater number of users than the available spreading factor N. In this paper, a superposition coding CDMA (SPC-CDMA) with unequal error protection (UEP) is proposed as one of the overloaded CDMA schemes for the next generation mobile communication systems. It exploits the available power control in most base stations to adapt the transmitted power of active users in the uplink channel. In this scheme, the active users are divided into G groups and each group consists of K users. The K users share the same spreading sequence and are distinguished by different received power levels. At the receiver side, the system first performs despreading for group detection followed by multiuser receiver to estimate the K user signals in each group. It is shown through simulations that better performance are achieved compared to the conventional DS-CDMA and existing overloaded collaborative spreading CDMA (CS-CDMA) schemes, in additive white Gaussian noise (AWGN) and fading channels. Hence, the proposed scheme maximizes the system capacity K-fold compared to conventional DS-CDMA system without requiring extra spreading codes, with average signal to noise ratio (SNR) cost of only 1dB and 2 dB over AWGN and fading channels respectively at BER of 10^?3. On the other hand, for the same N, K and power constraints, SPC-CDMA scheme achieves twofold increase in data rate with 0.7 and 4 dB gains over AWGN and fading channels respectively, compared with overloaded CS-CDMA scheme in the same system capacity. In addition, the proposed scheme can also attain different levels of UEP for different users?? requirements by adjusting their fractions of transmitted powers.     

衰落余量对cdma2000 1x前向链路发射功率和容量的影响

衰落余量决定链路可靠性并影响信道发射功率和用户容量的数值。本就衰落余量对cdma2000lx系统前向链路发射功率和渐近容量的影响进行了深入分析，首先介绍衰落余量概念，提出了cdma2000lx系统前向链路的发射功率和渐近容量表达式，然后通过仿真实验和数值计算给出一定条件下cdma2000lx系统前向链路的业务与导频SNR之间存在的对应关系，最后利用公式与仿真数据研究了衰落余量对前向总发射功率和渐近容量的影响。研究结果表明，cdma2000lx系统的前向总发射功率和渐近容量对衰落余量的大小十分敏感，衰落余量每增加ldB，都会引起总发射功率大幅上升，而渐近容量则迅速下降。     

基于大系统方法的上行CDMA蜂窝系统性能分析

结合大尺度和小尺度衰落,采用大系统分析方法对CDMA蜂窝系统上行链路的性能进行分析。首先分析了小区内多址干扰和小区间干扰的情况;其次推导出了通过匹配滤波器后的信干噪比值,该值只与小区负载、用户功率、信道增益有关;最后分析了系统的误码率、吞吐量和频谱效率,并通过仿真验证了分析的准确性。仿真结果发现,随着小区负载的增大,系统的误码率增大,吞吐量下降,但频谱效率有所提高。     

Effect of antenna array on the uplink orthogonal multicarrier DS‐CDMA system with imperfect power control

In this paper we analyze the effects of antenna array (AA) and imperfect power control on the performance of the uplink synchronous and/or asynchronous orthogonal multicarrier (MC) direct sequence‐code division multiple access (DS‐CDMA) system in multipath Rayleigh fading channels. Bit error rate (BER) performance is evaluated in terms of the number of antennas, the number of total subcarriers, power control error (PCE), and the number of users. Our numerical results show that the available user capacity of synchronous uplink is more than 1.5 times higher than that of asynchronous uplink, even though PCE increases. Copyright © 2009 John Wiley & Sons, Ltd.     

Outage-based optimal power control for generalized multiuser fading channels

We address the problem of achieving outage probability constraints on the uplink of a code-division multiple-access (CDMA) system employing power control and linear multiuser detection, where we aim to minimize the total expended power. We propose a generalized framework for solving such problems under modest assumptions on the underlying channel fading distribution. Unlike previous work, which dealt with a Rayleigh fast-fading model, we allow each user to have a different fading distribution. We show how this problem can be formed as an optimization over user transmit powers and linear receivers, and, where the problem is feasible, we provide conceptually simple iterative algorithms that find the minimum power solution while achieving outage specifications with equality. We further generalize a mapping from outage probability specifications to average signal-to-interference-ratio constraints that was previously applicable only to Rayleigh-faded channels. This mapping allows us to develop suboptimal, computationally efficient algorithms to solve the original problem. Numerical results are provided that validate the iterative schemes, showing the closeness of the optimal and mapped solutions, even under circumstances where the map does not guarantee that constraints will be achieved.     

Ergodic sum capacity maximization for CDMA:Optimum resource allocation

We solve for the optimum signature sequence and power allocation policies that maximize the information-theoretic ergodic sum capacity of a code-division multiple-access (CDMA) system subject to fading. We show that at most N users may transmit at any given channel state, where N is the processing gain; and those users who are transmitting should be assigned orthogonal signature sequences. We also show that the power allocation policy that maximizes the capacity together with the choice of these signature sequences is single-user water-filling over sets of channel states that are favorable to each user. That is, the capacity maximizing signaling scheme is shown to dictate that the users allocate their powers and signature sequences in such a way that they always avoid interference from each other.