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.     

Effects of Array Weight Errors on Parallel Interference Cancellation Receiver in Uplink Synchronous and Asynchronous DS‐CDMA Systems

This paper investigates the impacts of array weight errors (AWE) in an antenna array (AA) on a parallel interference cancellation (PIC) receiver in uplink synchronous and asynchronous direct sequence code division multiple access (DS‐CDMA) systems. The performance degradation due to an AWE, which is approximated by a Gaussian distributed random variable, is estimated as a function of the variance of the AWE. Theoretical analysis, confirmed by simulation, demonstrates the tradeoffs encountered between system parameters such as the number of antennas and the variance of the AWE in terms of the achievable average bit error rate and the user capacity. Numerical results show that the performance of the PIC with the AA in the DS‐CDMA uplink is sensitive to the AWE. However, either a larger number of antennas or uplink synchronous transmissions have the potential of reducing the overall sensitivity, and thus improving its performance.     

Orthogonal complementary codes for interference-free CDMA technologies

This article addresses the issues of next-generation CDMA technologies for B3G wireless communications. To engineer a CDMA system whose performance will no longer be interference-limited, many challenging issues should be tackled, such as novel CDMA code sets, efficient spreading and carrier modulation schemes, and signaling format for high-speed burst traffic. This article reviews our ongoing research on next-generation CDMA technologies. In particular, we propose a new CDMA code design methodology, real environment adapted linearization (REAL), which can generate CDMA code sets with inherent immunity against multipath interference and multiple access interference for both uplink and downlink transmissions. It is also shown that interference-free CDMA can only be implemented with the help of orthogonal complementary codes. The article goes further to reveal that cell-wise capacity for such interference-free CDMA is equal to the number of element codes assigned to each user, making OFDM a natural choice to implement interference-free CDMA. Several other issues of OCC-CDMA, such as its system implementation and performance, are also addressed in this article.     

长码CDMA系统上行链路多用户接收机设计

提出一种适用于长码CDMA移动通信系统上行链路的多用户接收机设计方案，包括多用户信道估计、多用户检测以及信道跟踪等算法。通过把用户传播时延的影响放入信道响应矩阵中，由信道估计间接得出各用户的传播时延。使用迭代运算的方式使得运算量分散，降低了对硬件平台的要求，使算法更为适用。增大观察窗的长度达到了更准确的用户信息比特判决，提高了接收机的性能。仿真结果表明文章提出的接收机设计方案具有很好的远近效应抑制能力，性能远高于传统的单用户Rake接收机。     

Quasi-synchronous code-division multiple access with high-ordermodulation

Code-division multiple access (CDMA) is a multiplexing technique where a number of users simultaneously access a transmission channel by modulating and spreading their signals with preassigned codewords. This paper studies the performance of CDMA signals with orthogonal (Walsh-Hadamard) codewords and synchronization errors smaller than the chip time. Two high-order modulation techniques, M-level quadrature amplitude modulation (M-QAM) and M-level phase-shift keying (M-PSK) are compared with respect to bit-error rate (BER). The results are especially important for the return channel of cable TV networks and summarized as follows: 1) Synchronization errors between transmitters lead to interference noise, whereas synchronization errors between the transmitter and the receiver lead to a decreased amplitude of the received user signal. Both effects have significant impact on the system performance. 2) Closed expressions are obtained for the BER of a CDMA signal with M-PSK and M-QAM with a given maximum synchronization error. 3) The higher the modulation order, the more sensitive the system gets for synchronization errors. 4) The BER is highly dependent on the assigned codewords out of the Walsh-Hadamard code set. 5) The BER performance of M-QAM outperforms that of M-PSK     

A detailed study of a CDMA based approach to enhance ad hoc network performance

Random access schemes operate typically on a contention based common channel, which brings problems with increasing traffic load. These problems are emphasized in a multi-hop wireless ad hoc network environment. Efficient collision avoidance methods are needed, but they also tend to decrease the spatial capacity of the network. With CDMA (Code Division Multiple Access), it is possible to make multiple simultaneous co-located successful transmissions and thus increase network capacity within the limits of multiple access interference (MAI). Bi-code channel access (BCCA) is a method to apply CDMA in ad hoc networking, providing also a common access channel for network connectivity maintenance and self-configuration. With BCCA, collisions are rare, and thus, more straightforward medium access control (MAC) methods can be used. A MAC solution designed especially for BCCA (BC-MAC) and ad hoc networking increases the efficiency of the channel usage. As BCCA is based on receiver code CDMA, the spreading code of the next hop node is needed. A novel network layer spreading code distribution (NSCD) method is proposed for this purpose. The spreading codes are distributed within routing protocol control packets at the route establishment phase.A detailed study on the performance behavior of different methods is provided. Also, interesting aspects considering the ad hoc network operation are discussed. Instead of the widely used unrealistic cut propagation model, a propagation model with realistic MAI calculation is used in this study. It is shown that the use of NSCD increases the network control load only slightly, and that the performance is practically the same as it is with the assumption of known spreading codes. BCCA with its dedicated MAC and NSCD clearly outperforms commonly used methods (like IEEE 802.11) and yet is easy to implement, robust for design parameters under different conditions while maintaining the original idea of ad hoc networking.     

A time-orthogonal CDMA high-speed uplink data transmission scheme for 3G and beyond

In 3G systems, the achievable data rates and capacity on the uplink are much smaller than the downlink data rates and capacity due to the nonorthogonal nature of the CDMA uplink. In this article, we present a new time-orthogonal CDMA approach called high-speed uplink data burst transmission mode. The concept is based on slot-synchronized slot-orthogonal transmissions whereby high-speed data transmissions take place in slots orthogonal to the slots used for physical layer control signaling and low-data-rate transmission such as resource requests. Using this approach, very high data rates and capacity are achieved during data burst transmission because of the availability of high signal-to-interference-plus-noise ratio, resulting from the orthogonal nature of the transmissions. Simulation results show that the uplink spectral efficiency of the proposed scheme is approximately four times better than that achieved with the existing 3G systems     

MC–CDMA receiver design using recurrent neural networks for eliminating multiple access interference and nonlinear distortion

Multicarrier code division multiple access (MC–CDMA) is a promising wireless communication technology with high spectral efficiency and system performance. However, all multiple access techniques including MC–CDMA were most likely to have multiple access interference (MAI). So this paper mainly aims at designing a suitable receiver for MC–CDMA system to mitigate such MAI. The classical receivers like maximal ratio combining, minimum mean square error, and iterative block–decision feedback equalization fail to cancel MAI when the MC–CDMA is subjected to severe nonlinear distortions, which may occur due to saturated power amplifiers or arbitrary channel conditions. Being highly nonlinear structures, the neural network receivers such as multilayer perceptron and recurrent neural network could be better alternative for such a case. The feasibility, efficiency, and effectiveness of the proposed neural network receiver are studied thoroughly for MC–CDMA system under different nonlinear conditions.     

A CDMA wireless packet network for voice-data transmissions

This letter investigates the possibility of integrating voice and data communications in a CDMA wireless packet network to provide access to a base station over a common short-range radio uplink channel for many spatially dispersed voice and data user terminals. Speech activity detection is assumed for voice communications to temporarily devote codes unused by voice user terminals during silence periods to data transmissions. The network proposed exhibits a good performance both in terms of quality of voice communications which is independent of data transmissions and maximum data traffic load supported with bounded delay     

Sensor networks with mobile access: optimal random access and coding

We consider random access and coding schemes for sensor networks with mobile access (SENMA). Using an orthogonal code-division multiple access (CDMA) as the physical layer, an opportunistic ALOHA (O-ALOHA) protocol that utilizes channel state information is proposed. Under the packet capture model and using the asymptotic throughput as the performance metric, we show that O-ALOHA approaches the throughput equal to the spreading gain with an arbitrarily small power at each sensor. This result implies that O-ALOHA is close to the optimal centralized scheduling scheme for the orthogonal CDMA networks. When side information such as location is available, the transmission control is modified to incorporate either the distribution or the actual realization of the side information. Convergence of the throughput with respect to the size of the network is analyzed. For networks allowing sensor collaborations, we combine coding with random access by proposing two coded random access schemes: spreading code dependent and independent transmissions. In the low rate regime, the spreading code independent transmission has a larger random coding exponent (therefore, faster decay of error probability) than that of the spreading code dependent transmission. On the other hand, the spreading code dependent transmission gives higher achievable rate.     

Combined source-channel coding for the transmission of still imagesover a code division multiple access (CDMA) channel

This letter considers a combined source-channel coding scheme for image transmission over the uplink of a wireless IS-95 code division multiple access (CDMA) channel using discrete cosine transform. By adjusting the dimension of the orthogonal signaling scheme, we trade the system error-correction capability for a faster bit rate. The increase in channel error is relieved by employing a set of quantizers which are designed using a joint source-channel optimization algorithm     

An uplink CDMA system architecture with diverse QoS guarantees forheterogeneous traffic

The uplink access control problems for cellular code-division multiple-access (CDMA) systems that service heterogeneous traffic with various types of quality-of-service (QoS) and use multicode CDMA to support variable bit rates are addressed. Considering its distinct QoS requirements, class-I real-time traffic (e.g., voice and video) is differentiated from class-II non-real-time traffic (e.g., data). Connection-oriented transmission is achieved by assigning mobile-oriented code channels for class-I traffic, where each corresponding mobile needs to pass an admission test. Class-II traffic is transmitted in a best-effort manner through a transmission-rate request access scheme which utilizes the bandwidth left unused by class-I traffic. Whenever a mobile has class-II messages to transmit, the mobile requests code channels via a base station-oriented transmission-request code channel, then, according to the base station scheduling, the transmission is scheduled and permitted. Addressed are the admission test for class-I connections, transmission power allocation, and how to maximize the aggregate throughput for class-II traffic. The admission region of voice and video connections and the optimum target signal-to-interference ratio of class-II traffic are derived numerically. The performance of class-II traffic transmissions in terms of average delay is also evaluated and discussed     

Adaptive Resource Allocation for MC‐CDMA and OFDMA in Reconfigurable Radio Systems

This paper studies the uplink resource allocation for multiple radio access (MRA) in reconfigurable radio systems, where multiple‐input and multiple‐output (MIMO) multicarrier‐code division multiple access (MC‐CDMA) and MIMO orthogonal frequency‐division multiple access (OFDMA) networks coexist. By assuming multi‐radio user equipment with network‐guided operation, the optimal resource allocation for MRA is analyzed as a cross‐layer optimization framework with and without fairness consideration to maximize the uplink sum‐rate capacity. Numerical results reveal that parallel MRA, which uses MC‐CDMA and OFDMA networks concurrently, outperforms the performance of each MC‐CDMA and OFDMA network by exploiting the multiuser selection diversity.     

Performance evaluation of a joint CDMA/NC-PRMA protocol forwireless multimedia communications

A joint code division multiple access and noncollision packet reservation multiple access (CDMA/NC-PRMA) technique is proposed and investigated as an uplink protocol for the third-generation (3G) mobile systems. Being the underlying time division multiple access (TDMA) architecture of the CDMA transmissions, NC-PRMA enables the base station (BS) to have a centralized control over the slot allocation policy. In order to reduce the multiple access interference (MAI) variation in a CDMA transmission, two different slot assignment schemes, referred to as load-balancing (LB) and power-grouping (PG) schemes, are proposed and evaluated. Simulation results show that considerable improvement can be achieved over the joint CDMA/PRMA scheme, in which the MAI variation is reduced by way of a dynamic permission probability for contending terminals. Especially when an imperfect power control mechanism is considered, the proposed PG assignment scheme achieves significant performance advantages     

宽带CDMAWLL系统的数字接收机的实现

无线本地环(WLL)是一种无线接入系统,它用无线的方式代替了传统的铜线连接用户到本地交换机。介绍了一种用于宽带CDMAWLL系统的数字接收机。叙述了WLL的系统组成、网络结构、反向链路信道结构和反向中继接收机设计规范,并论述了代码捕获和代码跟踪模块的功能。接收机的运行时钟频率可达65.536MHz。     

一种TD-SCDMA上行链路发送和接收方案

为了在TD-SCDMA上行链路传输中获得更高的频谱利用率,提出了一种上行链路的发送和接收方案。发送端采用准同步CDMA加QAM调制,扩频序列采用优选相位的Gold序列,该序列在一定时延范围内具有良好的互相关性。接收端采用串行干扰抵消的方法去除或抑制很严重的多用户干扰,该方法实现简单,适合瑞利衰落信道。仿真结果说明采用这种发送和接收方案后,在应用智能天线抑制多径后,只要用户间的时延控制在3／8个chip之内,误符号率(SER)性能就几乎与单用户界(SUB)一致,频谱利用率可以达到4 bit／s／Hz。     

基于球形译码的联合多用户检测

针对采用码分多址接入（CDMA,Code Division Multiple Access）的多输入多输出系统（MIMO,Multiple Input Multiple Output）,建立多个用户同步上行接入基站的信道模型,收发端都采用2根天线。在基站端,由于扩频码非完全正交产生多个用户之间干扰和信道干扰影响信号检测的问题,分析并给出了多用户信号的最大似然函数,利用球形译码算法实现多用户的联合检测（Joint MUD,Joint Multiple User Detection）,并行处理信道干扰和多址干扰。仿真结果表明,提出的接收机有较好性能,同时降低了运算的复杂度。     

Microcellular direct-sequence spread-spectrum radio system usingN-path RAKE receiver

A microcellular local area network (LAN) for indoor communications is proposed using code-division multiple access (CDMA) and differential phase-shift keying (DPSK) for data modulation. The pseudonoise (PN) codes in the transmitters of the base station are mutually synchronized. For this purpose, sets of Gold code sequences having low cross correlation have been found by an exhaustive computer search. Together with wideband measurements of the indoor radio channel at 900 MHz, a five-path RAKE receiver was designed to combat fading effects and to process the time diversity by using multipath signal reception. Each receiver path is demodulated independently. Several methods of diversity combining of these paths have been investigated. Acquisition and tracking of the spreading code in the receiver are controlled by a digital signal processor (DSP). Experimental results of the CDMA system are presented, showing the behavior in a multipath environment     

Uplink user capacity in a CDMA system with hotspot microcells: effects of finite transmit power and dispersion

This paper examines the uplink user capacity in a two-tier code division multiple access (CDMA) system with hotspot microcells when user terminal power is limited and the wireless channel is finitely-dispersive. A finitely-dispersive channel causes variable fading of the signal power at the output of the RAKE receiver. First, a two-cell system composed. of one macrocell and one embedded microcell is studied and analytical methods are developed to estimate the user capacity as a function of a dimensionless parameter that depends on the transmit power constraint and cell radius. Next, novel analytical methods are developed to study the effect of variable fading, both with and without transmit power constraints. Finally, the analytical methods are extended to estimate uplink user capacity for multicell CDMA systems, composed of multiple macrocells and multiple embedded microcells. In all cases, the analysis-based estimates are compared with and confirmed by simulation results.     

A satellite switched CDMA system for fixed service communications

Satellite-switched code-division multiple access is a system proposed for geostationary fixed satellite service. SS/CDMA provides both multiple access and switching to a multibeam satellite. In this article we present the system architecture and performance. The SS/CDMA consist of the common air interface (CAI) and the onboard code switching mechanism. The CAI provides signaling control and traffic channels. Traffic channel access and modulation is based on a spectrally efficient CDMA scheme. The satellite code-division switch (CDS) routes calls from an uplink to a downlink beam without onboard demodulation and channel decoding. This system operates with demand assignment control; that is, channel bandwidth and switch connections are only assigned per user request. The system can offer a wide variety of bit rates with wireline quality of service. The services provided are circuit-switched calls for voice data and video, and packet-switched for data. As shown, the proposed SE-CDMA can achieve high capacity and very low bit error rates at low signal-to-noise ratio. Also, the innovative switching technique can provide low complexity and achieve high throughput for all services