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.      

Pre‐filtering technique for MAI cancellation in MC‐CDMA systems

Multicarrier code division multiple access (MC‐CDMA), is a promising multiplexing technique for future communication systems. In this study, we employ the well‐known Walsh‐Hadamard spreading codes for synchronous downlink transmission of MC‐CDMA systems. The spreading codes allow that the frequency diversity to be efficiently exploited. However, multipath propagation may cause orthogonality among users is distorted, and this distortion produces multiple access interference (MAI). To eliminate this effect, we propose a pre‐filtering‐based MC‐CDMA system which uses a pre‐filtering technique at the transmitter and an equal gain combining (EGC) scheme at the receivers, respectively. Our proposed pre‐filtering technique transforms the transmitted signals so that the MAI can be eliminated, and the EGC scheme weights the signals received from all subcarriers so that channel distortions can be compensated. Furthermore, the proposed technique can calculate the transmitted power over all subcarriers to satisfy the required quality of service of each user and archive MAI‐free. In this paper, performance in terms of bit error rate is analyzed; in comparison with the EGC, orthogonal restoring combining, and maximal ratio combining schemes at receiver, respectively. Copyright © 2011 John Wiley & Sons, Ltd.     

Welch bound analysis on generic code division multiple access codes with interference free windows

Code-division multiple access (CDMA) technology has been applied to many wireless communication systems. CDMA system suffers from both multiple access interference (MAI) and inter-symbol interference (ISI) over a multipath channel. To suppress MAI and ISI, this paper proposes the spreading codes with interference free windows. In particular, we will develop several upper bounds on the efficiency of generalized spreading codes (for both unitary and complementary codes) in terms of the width of their interference free windows.     

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.     

A ZCZ‐CDMA system with BFSK modulation

A quasi‐timing synchronous code division multiple access (CDMA) system called ZCZ‐CDMA, which uses a set of sequences with a zero‐correlation zone called ZCZ code as a spreading code, is useful for short‐range wireless communications because of its excellent properties such as co‐channel interference‐free performance, simplified hardware design, and low transmit power as well as fast frame synchronization capability. In this paper, a ZCZ‐CDMA system with binary frequency‐shift keying (BFSK) modulation called BFSK‐ZCZ‐CDMA is proposed. This system is characterized by using a pair of balanced ZCZ codes for spreading and transmitting the two spread components over the respective keying carrier frequencies. Its bit error rate performance, compared with those of existing BPSK‐ZCZ‐CDMA, ASK‐ZCZ‐CDMA, and CDMA systems using the other spreading codes, is evaluated in theory and simulation. The bit error rate performance of the three ZCZ‐CDMA systems over additive white Gaussian noise and Rayleigh fading channels are formulated. It is proved that BFSK‐ZCZ‐CDMA is much more robust in anti‐fading performance and low transmit power in such an environment that fading distributions on the keying frequencies are independent mutually. Fading versus frequency characteristics are also investigated. Copyright © 2012 John Wiley & Sons, Ltd.     

Combined code reuse scheme with two‐dimensional OVSF codes assignment algorithm for uplink multi‐user/multi‐rate block spread multi‐cellular CDMA

The two‐dimensional (2D) block spread code division multiple access (CDMA) can avoid the uplink multiple‐access interference with low‐complexity single‐user detection in a slow fading channel and, therefore, is very attractive. In the 2D spreading, orthogonal variable spreading factor (OVSF) is used for spreading; an important problem is how to efficiently assign the limited resource of OVSF codes to users with different data rates, while meeting the requirement of quality of service in a multi‐cell environment. In this paper, it is shown that the code reuse can improve the code reuse efficiency and the proposed code reuse scheme combined with code assignment algorithm can allow flexible multi‐rate uplink transmission. The computer simulation confirms that the proposed code assignment algorithm improves the code reuse efficiency while achieving lower blocking probability than traditional CDMA. Copyright © 2012 John Wiley & Sons, Ltd.     

Performance analysis of MC‐CDMA by employing the MMSE‐PIC scheme in wireless communications

From the past decade, the multicarrier code division multiple access (MC‐CDMA) transmission schemes have placed major role in wireless communications. It is providing a secured wireless communication to the users with guaranteed performance. In many situations, the performance of the MC‐CDMA is restricted due to the interference caused by multiple access interference (MAI), which also influences the frameworks of CDMA. To overcome this issue, we concentrated on developing the efficient technique for data transmission with interference cancellation for downlink MC‐CDMA. In the proposed method, the interference cancellation procedure is done by using the regeneration and subtraction of MAI from the signal. The simulation results are evaluated using the MC‐CDMA system with different decision functions. Results proved that the proposed system is efficient in reducing the MAI along with an improved bit error rate (BER).     

Minimum Cross Correlation Spreading Codes

Multiple access interference (MAI) appears in Code Division Multiple Access (CDMA) systems when the communication channel is a multi-path channel and the spreading codes are not orthogonal. Orthogonality between spreading codes cannot be maintained at the CDMA receiver because the codes may be asynchronous due to channel delay and multi-path spread. The receiver cannot perfectly separate the different signals of the multiple access users, and the resultant MAI limits the capacity of CDMA systems. MAI is a function of the cross correlation property between used spreading codes. In this paper we focus on the cross correlation of the spreading codes, we propose a method to find spreading codes with minimum magnitude of cross correlation. Employing these codes will reduce the resultant MAI in the CDMA system; hence it will increase the system capacity. A great enhancement is shown by comparing found minimum cross correlation spreading codes (MCCSC) with Hadamard and Gold codes.

Spreading‐code‐dependent bit error rate and capacity analysis for finite asynchronous CDMA systems

Most CDMA systems serve only finite number of active users. Therefore, using Gaussian approximation to evaluate its performance is inappropriate. In such finite CDMA systems, the selection of spreading codes is of great importance because its performance is very much spreading‐code‐dependent. In this paper, a new algorithm based on multiple variable Bernoulli process is introduced to evaluate bit error rate due to co‐channel interference and thus the capacity of a CDMA system. The algorithm can also be used for large‐sized CDMA systems. The results show that small Kasami and Gold‐like codes offer higher capacity than the others do. Copyright © 1999 John Wiley & Sons, Ltd.     

Adaptive reference code‐based MAI cancellation for DS‐CDMA

A computationally efficient and practically deployable adaptive reference code‐based multiple access interference (MAI) cancellation scheme, in which the conventional transmitter/receiver architecture is minimally modified only at the receiver (and/or transmitter) end, is proposed for direct sequence code division multiple access (DS‐CDMA) communication. Upon numerical and theoretical analyses, the proposed communications system is seen to always outperform the existing conventional communications system. The theoretical analyses and results as presented are generally useful and applicable to any situation wherein IS95 pseudo noise (PN) codes are employed towards multiple access. Copyright © 2009 John Wiley & Sons, Ltd.     

Inter-Group Complementary Codes for Interference-Resistant CDMA Wireless Communications

Spreading code plays an extremely important role on the overall performance of a CDMA system. The correlation properties and available number of spreading codes determine the interference-resist capability as well as system capacity. In this paper, we analyze the characteristics and limitations of traditional and recently reported spreading codes. Based on the analysis, we propose a new code design approach which will be used to generate inter-group complementary (IGC) codes. The correlation functions of the IGC codes possess definite and bi-valued interference-free windows. In addition, a corresponding code assignment algorithm and spreading scheme will be introduced to take advantage of the desirable properties of the IGC codes for their applications in CDMA systems. Both theoretical analysis and simulation results show that an IGC-CDMA system is interference-resistant and capable to offer a high spectral efficiency if compared with the ones based on other spreading codes.     

Multi‐carrier platform for wireless communications. Part 2: OFDM and MC‐CDMA systems with high‐performance,high‐throughput via innovations in spreading

Multi‐carrier technologies in general, and OFDM and MC‐CDMA in particular, are an integral part of the wireless landscape. In this second part of a two‐part survey, the authors present an innovative set of spreading codes known as CI codes, and demonstrate how these significantly increase performance and capacity in OFDM and MC‐CDMA systems, all the while eliminating PAPR concerns. Regarding OFDM: the spreading of each symbol over all N carriers using CI spreading codes (replacing the current one symbol per carrier strategy) are presented. CI codes are ideally suited for spreading OFDM since, when compared to traditional OFDM, CI‐based OFDM systems achieve the performance of coded OFDM (COFDM) while maintaining the throughput of uncoded OFDM, and, at the same time, eliminate PAPR concerns. When applied to MC‐CDMA, CI codes provide a simple means of supporting 2N users on N carriers while maintaining the performance of an N‐user Hadamard Walsh code MC‐CDMA system, i.e., CI codes double MC‐CDMA network capacity without loss in performance. The CI codes used in OFDM and MC‐CDMA systems are directly related to the CI pulse (chip) shapes used to enhance TDMA and DS‐CDMA (see part 1): hence, the CI approach provides a common hardware platform for today's multi‐carrier/multiple‐access technologies, enabling software radio applications. Copyright © 2002 John Wiley & Sons, Ltd.     

Interference‐resistant cooperative wireless networks based on complementary codes

Spatial diversity in wireless networks can be attained by exploiting the broadcast nature of wireless transmission without the need of multiple antennas in individual device, leading to the implementation of cooperative communication. While most prior works focused on the single source—destination scenario, it should be more realistic to consider how to induce cooperation among multiple source‐destination pairs assisted by multiple relays. In such a case, multiple access interference (MAI) may present due to asynchronous transmissions of the users and relays. In this paper, a cooperative network architecture based on orthogonal complementary (OC) codes inherently immune to MAI is proposed. To efficiently utilize the scarce radio spectrum and codes, a centralized medium access control (MAC) protocol is proposed to coordinate the code assignment and channel access among users and relays. We theoretically analyze the bit error rate (BER) performance of the proposed OC coded cooperative network over multipath Rayleigh fading channel. The performance gain resulted from different numbers of relays is investigated, and compared with a time division multiple access (TDMA) based cooperative scheme. We show that the proposed OC coded cooperative network performs well in the presence of timing offset, and thus is well suited for asynchronous uplink transmission with cooperative relaying. Copyright © 2009 John Wiley & Sons, Ltd.     

A low‐complexity adaptive receiver for DS‐CDMA systems in unknown code delay environment

In direct‐sequence code division multiple access (DS/CDMA) multiuser communication systems in multipath channels, both intersymbol interference (ISI) and multiple‐access interference (MAI) must be considered. The multipath channel characterizes the propagation effects including the timing offset and delays, etc. Traditionally, we use the delay‐locked loop (DLL) code tracking loop to recover the timing delay. But DLL cannot work well in multipath environment. In this paper, we propose a low‐complexity adaptive receiver to suppress ISI/MAI and solve the timing offset problems without using conventional DLL code tracking loop. The proposed receiver employs an adaptive filter whose weights are adapted using a block least‐mean square error algorithm with fractional sampling. Simulations confirm the good performance, including learning curves and theoretical analysis of minimum mean‐square error, of the proposed receiver. Copyright © 2010 John Wiley & Sons, Ltd.     

多径衰落信道下的扩频码设计与联合检测

在多径衰落信道CDMA系统中，扩频码设计与联合检测为提高系统容量的两个重要手段。在扩频码设计中，通过构造零相关窗码可以消除MAI和ISI，从而提高系统容量。当扩频码相关特性不理想时，通过联合检测也可以消除干扰，提高系统容量。但二者都有一定局限性，对于零相关窗码来说，它的码字资源十分有限，对于联合检测来说，当用户数很大，它的复杂度不能容忍。该文将扩频码设计与联合检测结合起来，提出了一种具有组间零相关窗特性的新型扩频码。在扩频码设计时考虑联合检测，可以增加码字资源，在联合检测中考虑扩频设计，又可以降低联合检测的复杂度。     

A near‐Nyquist rate transmission: A new minimum‐bandwidth direct‐sequence code division multiple access scheme

Bandlimited direct‐sequence code division multiple access (DS‐CDMA) attracts much attention for its compact spectrum and the ability to suppress inter‐symbol interference. Among the various bandlimited DS‐CDMA systems available, minimum‐bandwidth DS‐CDMA (MB‐DS‐CDMA) is the only realizable Nyquist rate transmission system. But, MB‐DS‐CDMA only applies to certain kinds of spreading codes. Accordingly, this study proposes a modified DS‐CDMA structure which extends the application of MB‐DS‐CDMA to all common spreading codes at the expense of a negligible reduction in the transmission rate. Additionally, the bit error rate of the proposed schemes adopting either single‐user or multi‐user detection receiver is analyzed and compared with that of the commonly‐used raised‐cosine‐pulsed DS‐CDMA over multipath fading channels. The numerical results show that given a sufficiently large number of users, the bit error rate performance of modified MB‐DS‐CDMA is comparable to that of the raised‐cosine‐pulsed DS‐CDMA scheme; meanwhile, the realizable modified MB‐DS‐CDMA approaches the ultimate transmission rate.     

A flexible multicarrier CDMA system with time‐varying data rates and services

We consider an enhanced multicarrier code‐division multiple access (CDMA) system that performs the spreading in the time and frequency domains simultaneously, where time‐domain spreading codes are used as a main code and frequency‐domain spreading codes (FDSCs) are used as a subcode. By allocating the FDSCs to users, rates and services can be dynamically matched to users' needs. The resulting system can support different time‐varying source rates and, therefore, enable multimedia applications. In addition, since the proposed scheme uses multiple carriers, it has some advantages such as the mitigated multipath fading, frequency/path diversity and narrowband interference suppression capability, in contrast to a single carrier (SC) system such as the multicode CDMA scheme. Finally, the closed‐form expression for probability of bit error is derived for a frequency‐selective Rayleigh fading channel. It has been also shown that the proposed system has outperformed the conventional SC RAKE system if the available bandwidth is fixed. Copyright © 2008 John Wiley & Sons, Ltd.     

Adaptive multiuser interference suppression for CDM satellite systems

This paper deals with the application of linear multiuser detectors in mobile terminals of code division multiplex (CDM) satellite systems. Generally, the interference caused by other users can dramatically reduce the performance of a CDM‐based satellite communication system. Since no spreading information of other users is given and since there are no trainings sequences, a blind adaptive interference detector with low complexity is needed. In order to implement a detector for suppressing the interference from other users, different adaptation algorithms can be applied. Comparing them in terms of complexity, adaptation speed and bit error rate, the LMS (least means squares) algorithm with adaptive step size shows the best performance. However, the implementation of this algorithm in a satellite system still leads to high processing times. For this reason, two new schemes with reduced complexity are presented. A very important point is, that the adaptive algorithms are basically restricted to short spreading codes (code period equals symbol period) whereas CDM‐based satellite systems use long codes (one‐code period spans over multiple symbols). In this paper, a new scheme is presented which can be applied to long codes. By cascading multiple separated interference detectors the adaptation is done every time when the same part of the spreading sequence appears. Copyright © 2005 John Wiley & Sons, Ltd.     

扩频码速率异步时直扩系统码分多址能力

直接序列扩频码分多址系统是目前应用最广泛的扩频系统,影响系统容量的主要因素是系统内多址信号的干扰。分析了采用Gold码的码速率异步直扩系统多址干扰,对扩展积分周期和单一积分周期均进行了数值仿真。仿真结果表明,码速率异步的Gold序列互相关函数不再具有三值特性;随着积分长度的扩展,互相关函数的统计值逐渐递减,即多址干扰减小;多址干扰与码偏移量大小有关,在某些特定码偏移量,积分长度为最小公倍数周期时,互相关函数接近为单值,而这些码偏移量正负对称出现;积分长度为单一周期时,多址干扰随码偏移量的变化趋势与扩频码长度无关;在相同码偏移量的情况下,随着扩频码的长度增加而减小。     

Modeling and performance analysis of multi‐service wireless CDMA cellular networks using smart antennas

In this paper, we present an analytical model to assess the blocking capacity of multi‐service code division multiple access (CDMA) systems. We include smart antenna systems in our model and show how the capacity of CDMA systems can be improved if smart antennas are employed at the base stations. Applying smart antennas can actually transform CDMA systems from being interference limited to being channel/code limited. To investigate this effect, we extend our model to include the limitation of channelization codes in CDMA‐based universal mobile telecommunication system (UMTS) systems. From the point of view of the call admission control (CAC) in a smart antenna CDMA system, we can either accept the capacity loss due to code limitation, or we can additionally apply space division multiple access (SDMA) techniques to re‐use channelization codes and thus re‐approach the capacity which is obtained if no code limitation is considered. Copyright © 2008 John Wiley & Sons, Ltd.