Design of next-generation cdma using orthogonal complementary codes and offset stacked spreading

This article presents an innovative code-division multiple access system architecture that is based on orthogonal complementary spreading codes and time-frequency domain spreading. The architecture has several advantages compared to conventional CDMA systems. Specifically, it offers multiple-access-interference-free operation in AWGN channels, reduces co-channel interference significantly, and has the potential for higher capacity and spectral efficiency than conventional CDMA systems. This is accomplished by using an "offset stacked" spreading modulation technique followed by quadrature amplitude modulation, which optimizes performance in a fading environment. This new spreading modulation scheme also simplifies the rate matching algorithms relevant for multimedia services and IP-based applications.     

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.     

Successive interference cancellation algorithms for downlink W-CDMAcommunications

In this paper, successive intracell interference cancellation (IIC) of the wideband-code division multiple access (W-CDMA) signal at the mobile unit is considered. Three new interference cancellation techniques suitable for the downlink of any CDMA system with orthogonal spreading are proposed. No prior knowledge of users' spreading codes or even their spreading factors are required for interference cancellation. A new term, effective spreading code, has been introduced, which is defined as the interfering user physical code as seen by the desired user within the desired user symbol duration. The mobile receiver estimates the effective spreading codes of the interfering users regardless of their spreading factors using fast Walsh transform (FWT) correlators (instead of the regular correlators) and uses this information to suppress the intracell multiuser interference. Three different interference-suppressing techniques are studied: subtraction; combined interfering signal projection; and separate interfering signal subspace projection. The complexity of the proposed techniques is low compared to conventional interference cancellation techniques. For a W-CDMA system and the IMT-2000 vehicular channel model, a capacity increase of up to 150% of the original (without IIC) system capacity is shown     

A transmitter diversity scheme for wideband CDMA systems based onspace-time spreading

We present a transmit diversity technique for the downlink of (wideband) direct-sequence (DS) code division multiple access (CDMA) systems. The technique, called space-time spreading (STS), improves the downlink performance by using a small number of antenna elements at the base and one or more antennas at the handset, in conjunction with a novel spreading scheme that is inspired by space-time codes. It spreads each signal in a balanced way over the transmitter antenna elements to provide maximal path diversity at the receiver. In doing so, no extra spreading codes, transmit power or channel information are required at the transmitter and only minimal extra hardware complexity at both sides of the link. Both our analysis and simulation results show significant performance gains over conventional single-antenna systems and other open-loop transmit diversity techniques. Our approach is a practical way to increase the bit rate and/or improve the quality and range in the downlink of either mobile or fixed CDMA systems. A STS-based proposal for the case of two transmitter and single-receiver antennas has been accepted and will be included as an optional diversity mode in release A of the IS-2000 wideband CDMA standard     

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     

HYBRID ORTHOGONAL/RANDOM MULTIPLE ACCESS SCHEME WITH IMPROVED PERFORMANCE FOR DOWNLINK CDMA SYSTEMS

In this paper, a hybrid orthogonal and random multiple access scheme with improved performance for capacity-enhanced downlink Code-Division Multiple-Access （CDMA） systems is presented, which is in fact a combination of the conventional orthogonal spreading scheme and the synchronous Interleave Division Multiple Access （IDMA） scheme. The proposed scheme can achieve near single user performance for very large number of users by the iterative turbo like detection. Analysis and simulation results show that the proposed scheme performs better than the synchronous IDMA scheme for the same time complexity. Meanwhile, larger capacity can be provided compared with the conventional orthogonal schemes.     

Enhancing the capacity of DS-CDMA system using hybrid spreading sequences

A new direct-sequence code-division multiple-access (DS-CDMA) system using hybrid spreading sequences in order to increase system capacity is proposed. This scheme permits accommodating more than N users, where N is the processing gain of the spreading sequences. Users are divided into two classes. The centerpiece idea is to synchronously assign N mutually orthogonal codes to class-1 users, and asynchronously assign quasi-orthogonal codes to class-2 users. The proposed architecture is realized through the DS-CDMA and multicode CDMA (MC-CDMA) techniques for class-1 and class-2 users, respectively, and the system capacity is considerably enhanced as compared with Sari et al.. The new approach still excels over Vanhaverbeke et al. when the number of class-1 users K/sub 1/ is large; but our desire is to expand system capacity. Compared with Sari et al. and Vanhaverbeke et al., the new design is able to support multirate applications, and is especially suitable for high-speed multimedia transmissions. Furthermore, the performance of the MC-CDMA system over an additive white Gaussian noise channel is evaluated for various primary codes and mutually orthogonal subcodes. It is shown that orthogonal Gold codes concatenated with Hadamard sequences yield the lowest bit-error rate when the signal-to-noise ratio lies between 0 and 30 dB.     

Enhanced architecture for residue number system-based CDMA for high-rate data transmission

This paper presents an advanced architecture for residue number system (RNS)-based code-division multiple-access (CDMA) system for high-rate data transmission by combining RNS representation, phase shift keying/quadrature amplitude modulation (PSK/QAM) and orthogonal modulation. The residues obtained from a fixed number of bits are again divided into spread code index and data symbol for modulation. The modulated data symbol is spread using the indexed orthogonal codes and transmitted through a communication channel. The proposed system uses a lower number of orthogonal codes than conventional RNS-based CDMA and the performance is comparable. The computational complexity of the proposed system is compared against alternative schemes such as M-ary CDMA and conventional RNS-based CDMA. The modified system is simulated extensively for different channel conditions and the results are discussed.     

A truncated adaptive transmission scheme for hybrid multicarrier CDMA/FDM systems in forward link

A truncated adaptive transmission scheme for the hybrid multicarrier CDMA/FDM system is considered in forward link. In the proposed scheme, a data substream is transmitted over the subchannels of which the channel gains are greater than a given threshold, based on the feedback information from the mobile station. We analyze the performance of the proposed system when orthogonal and random signature sequences are used in single- and multiple-cell environment. In the single-cell environment, when orthogonal signature sequences are used, the proposed scheme outperforms the adaptive FH/DS system as well as the conventional MC DS/CDMA system, and accommodates more users than the adaptive FH/DS system while maintaining the orthogonality between users. In the multiple-cell environment also, the proposed scheme has better performance characteristics than the adaptive FH/DS system when orthogonal or random codes are used as spreading sequences.     

Convolutional Spreading CDMA and Comparison With the DS-CDMA With RAKE Receiver

For synchronous downlink direct-sequence code-division multiple access (DS-CDMA), we introduce a cyclic-prefix (CP)-based convolutional spreading CDMA (CS-CDMA/CP) scheme employing zero correlation zone (ZCZ) codes, which gives a class of multiuser interference (MUI)-free CDMA schemes, including Suehiro's CDMA and block spreading (BS) CDMA as special cases. We show that CS-CDMA/CP employing appropriately selected binary and ternary ZCZ codes have good user capacity, peak-to-average power ratio (PAPR), and transmission power loss, compared with previously proposed BS-CDMA schemes. By simulation, we show the effect of PAPR to its bit-error rate (BER) performance when nonlinear amplifiers are used, and also compare its BER performance with the conventional DS-CDMA scheme employing a RAKE receiver     

A Unified Framework for Tomlinson--Harashima Precoding in MC-CDMA and OFDMA Downlink Transmissions

We consider a unified framework comprising both multicarrier code-division multiple-access (MC-CDMA) and orthogonal frequency-division multiple-access (OFDMA), and discuss nonlinear prefiltering for downlink transmissions based on Tomlinson-Harashima precoding. The base station (BS) is equipped with multiple transmitting antennas and channel state information is assumed to be available at the transmit side. We design the prefiltering matrices so as to minimize the sum of the mean square errors at all mobile terminals when a conventional single-user data detector is employed at the receiver side. In this way, most of the computational burden is moved to the BS, where power consumption and computational resources are not critical requirements. Computer simulations are used to assess the performance of the proposed scheme under different operating scenarios. It turns out that OFDMA outperforms MC-CDMA when the system resources (subcarriers and/or spreading codes) are optimally assigned to the active users according to the channel quality.     

海面的红外被动成像

本文在研究海面的红外成像特性中,建立了基于刚体的双尺度成像模型,从被动成像方面讨论了海面辐射特性的变化规律.该模型仿真结果很好地说明了风向对海面红外被动成像特性的影响.     

A New Walsh-Like Near Orthogonal (WNO) Sequence for Asynchronous CDMA System

Present generation mobile communication system employs one of the most popular wireless access technologies called code division multiple access (CDMA). Design of CDMA spreading codes has drawn significant attention amongst the researchers over the last few decades. CDMA code family is generally categorized into purely orthogonal and non-orthogonal (near-orthogonal) members which have established their application in synchronous (downlink) and asynchronous (uplink) CDMA system respectively. Walsh code has been regarded as the most useful spreading code to be used in synchronous link because of its orthogonality property. However, the performance of Walsh code is significantly inferior in asynchronous surroundings. A number of codes have consequently been proposed with an aim to mitigate the shortcomings of Walsh code. This paper makes an innovative attempt to enhance the correlation properties of existing Walsh code through one simple yet powerful algorithm. Proposed code of length ‘N’ has been generated from code sets of length ‘N/4’ and thus makes the code generation algorithm recursive in nature. Performance of the proposed code has subsequently been compared with some existing orthogonal and semi orthogonal codes in terms of various performance metrics and finally the supremacy of our proposition has been established.     

空时分组码在CDMA下行链路应用的研究

提出了准正交空时分组码在CDMA系统下行链路的两种应用方法，一种是在发射端对用户信号先扩频后空时编码送至发射天线，另一种是先空时编码后扩频送至发射天线，相应给出了两种方法接收端的译码方法。数据仿真结果表明在CDMA系统下行链路的应用时，准正交空时分组码的误码率低于正交空时分组码的误码率，并且第一种方法的误码率低于第二种方法的误码率。     

On Next Generation CDMA Technologies: The REAL Approach for Perfect Orthogonal Code Generation

All currently available code-division multiple-access (CDMA) technologies used in second-generation and third-generation mobile cellular systems are interference limited and can be appropriately called first-generation CDMA, whereas next-generation CDMA should provide a nearly interference-free performance. This paper addresses the issues on spreading code generation that is suitable for next-generation CDMA systems. The real environment adaptation linearization (REAL) approach is proposed to generate perfectly orthogonal complementary (POC) codes characterized by multiple access interference (MAI)-free and multipath interference (MI)-free operation. The REAL approach takes into account almost all major impairing factors in real applications, such as multipath propagation, asynchronous transmission, random data signs, and burst traffic, such that a CDMA system using them can offer an interference-resist operation. Two important conclusions are drawn in this paper: First, implementation of an interference-free CDMA will not be possible unless using complementary codes, such as the POC codes. Second, to enable interference-free CDMA, the flock size of the signature codes should preferably be equal to the set size of the codes. A fast algorithm to generate supercomplementary codes (a subset of POC codes) is also presented, and their ideal orthogonality is explicitly proven.      

A generalized RAKE receiver for interference suppression

Currently, a global third-generation cellular system based on code-division multiple-access (CDMA) is being developed with a wider bandwidth than existing second-generation systems. The wider bandwidth provides increased multipath resolution in a time-dispersive channel, leading to higher frequency-selectivity. A generalized RAKE receiver for interference suppression and multipath mitigation is proposed. The receiver exploits the fact that time dispersion significantly distorts the interference spectrum from each base station in the downlink of a wideband CDMA system. Compared to the conventional RAKE receiver, this generalized RAKE receiver may have more fingers and different combining weights. The weights are derived from a maximum likelihood formulation, modeling the intracell interference as colored Gaussian noise. This low-complexity detector is especially useful for systems with orthogonal downlink spreading codes, as orthogonality between own cell signals cannot be maintained in a frequency-selective channel. The performance of the proposed receiver is quantified via analysis and simulation for different dispersive channels, including Rayleigh fading channels. Gains on the order of 1-3.5 dB are achieved, depending on the dispersiveness of the channel, with only a modest increase in the number of fingers. For a wideband CDMA (WCDMA) system and a realistic mobile radio channel, this translates to capacity gains of the order of 100%     

Simple polynomial detectors for CDMA downlink transmissions on frequency-selective channels

In code-division multiple-access (CDMA) transmissions, computing the multiuser minimum mean-squared error (MMSE) detector coefficients requires the inversion of the covariance matrix associated to the received vector signal, an operation usually difficult to implement when the spreading factor and the number of users are large. It is therefore interesting to approximate the inverse by a matrix polynomial. In this correspondence, means for computing the polynomial coefficients are proposed in the context of CDMA downlink transmissions on frequency-selective channels, the users having possibly different powers. Derivations are made in the asymptotic regime where the spreading factor and the number of users grow toward infinity at the same rate. Results pertaining to the mathematics of large random matrices, and in particular to free probability theory, are used. Spreading matrices are modeled as isometric random matrices (spreading vectors orthonormality is a natural assumption in downlink) and also as random matrices with independent and identically distributed (i.i.d.) elements.     

An algebraic approach to generate super‐set of perfect complementary codes for interference‐free CDMA

This paper introduces an algebraic approach to generate the super‐set of perfect complementary (PC) codes suitable for new generation CDMA applications, characterized by isotropic multiple access interference (MAI) free and multipath interference (MI) free properties. The code design methodology proposed in this paper takes into account major impairing factors existing in real applications, such as MAI, MI, asynchronous transmissions, and random signs in consecutive symbols, such that a CDMA system using the generated codes can insure a truly interference‐free operation. Two important facts will be revealed by the analysis given in this paper. First, implementation of an interference‐free CDMA will never be possible unless using complementary code sets, such as the PC code sets generated in this paper. In other words, all traditional spreading codes working on an one‐code‐per‐user basis are not useful for implementation of an MAI‐free and MI‐free CDMA system. Second, to enable the interference‐free CDMA operation, the flock size of the PC codes should be made equal to the set size of the codes, implying that a PC code set can support as many users as the flock size of the code set. A systematic search has been carried out to generate the super‐set of various PC codes with the help of carefully selected seed codes belonging to distinct sub‐sets. This paper will also propose an implementation scheme based on multi‐carrier CDMA architecture and its performance is compared by simulations with the ones using traditional spreading codes. Copyright © 2006 John Wiley & Sons, Ltd.     

Multi-user performance of direct-sequence CDMA using combinedbinary PPM/orthogonal modulation

A new modulation format is proposed for cellular code-division multiple-access (CDMA) communications where binary pulse position modulation (PPM) is embedded in the chip waveform and combined with orthogonal modulation using Walsh/Hadamard codes. Compared to the conventional CDMA using orthogonal codes, this scheme allows reduction in receiver complexity by lowering the modulation level for the second-stage orthogonal modulation. The staggered (half-chip) quadrature direct-sequence signaling is adopted to uniformly distribute the transmit power and allow noncoherent detection at the receiver because carrier phase tracking is not feasible because of the binary PPM, suitable for the reverse link in cellular networks. Statistics of inter-user interferences are characterized, and then derive the symbol error probability for the proposed M-ary modulation format. It is shown that the advantage in view of receiver complexity can be achieved without deteriorating the multi-user performance in terms of the number of users affordable at a specified error rate     

基于扰码的CDMA-BLAST系统研究

该文提出一种新的CDMA下行链路空时编码方案,用正交扩频码区分不同用户,用扰码区分不同发射天线,由于扰码具有良好的自相关和互相关特性,可以在不牺牲码域资源的前提下,以增加少量的复杂度换取较好的链路性能。仿真结果显示,采用正交扩频码和扰码二级扩频的空时编码方案可以取得较好的链路性能。