Interference exploitation using adaptive code allocation for the downlink of precoded multiple carrier code division multiple access systems

MC-CDMA (multiple carrier code division multiple access) has been considered a strong candidate for cognitive radio and dynamic spectrum access network, due to its high spectral efficiency, easy implementation and the capability of operating over non-contiguous frequency bands. However, like other multi-carrier transmission technologies such as OFDM (orthogonal frequency division multiplexing), the orthogonality among subcarriers in MC-CDMA system is lost in a mobile environment, leading to inter-carrier interference (ICI) and performance degradation. In this paper, we apply the ICI self-cancellation scheme previously proposed for OFDM system to orthogonal MC-CDMA system with Hadamard-Walsh codes to diminish ICI and improve BER performance. We show that the ICI self-cancellation scheme for MC-CDMA is equivalent to employing a subset of the Hadamard code matrix. As a direct result, the proposed ICI self cancellation method does not increase the complexity at both transmitter and receiver. To compensate for the network capacity (in terms of the number of users) loss in the proposed ICI self-cancellation method, a second set of orthogonal Hadamard-Walsh codes on the quadrature subcarriers are introduced. The proposed system offers significant BER performance gain than original MC-CDMA with the presence of ICI and same network capacity when BPSK modulation is employed. Simulation results over multi-path fading channel confirm the effectiveness of the proposed scheme, without increasing any complexity.

This paper carries comprehensive comparison of Interleave Division Multiple Access (IDMA) and Code Division Multiple Access (CDMA) systems in terms of performance (BER) and behavior to different number of users. Performance analysis of two systems whose transmitter and receiver structures are almost same are studied under Additive White Gaussian Noise (AWGN) and performance differences are illustrated by simulation results. Although Multiple Access Interference (MAI) and Intersymbol Interference (ISI) can affect the performance of the CDMA system, simulation results illustrates that IDMA system model achieves this type of problem using their interleavers. Results also show that large number of users can easily be adapted by the IDMA scheme much easier than CDMA scheme.

One of the most important issues to be taken into consideration in multi carrier communication systems with multi users is the assignment of the available resources to the users efficiently. Thus, how to determine the number of subcarriers to be allocated to the users has an important role with regards to data rate and error performance of the system when active users are increased in multi carrier system. In this study by using Greedy-Like algorithm, the subcarriers in MC-CDMA systems are optimal allocated to the users and the efficiency of this algorithm compared with other algorithms in terms of bit error and bit rate performance.DOI: http://dx.doi.org/10.5755/j01.eie.23.4.18726

The increasing interest on multimedia applications and services in portable computing communication will create at same time a large demand for high speed wireless data on communication devices. The 4th generation (4G) mobile communication is expected to offer 100 Mbps or higher data rates in downlink. orthogonal frequency division multiplexing (OFDM) and multiple carrier code division multiple access (MC-CDMA) are potential candidates for next generation of mobile radio communications for achieving hight data rate transmissions. In cellular networks, the frequency reuse is one of the main parameters to consider since it is more suitable to optimize the spectrum reuse. Using a frequency reuse factor of one, it could be a possible system using the same frequency in all the network. In this proposal the sensitivity and the impact of an interfering base station based on a MC-CDMA system is investigated and analyzed using different channel coding and decoding processes.

In this paper, multi-user detection (MUD) techniques are investigated for multiple input multiple output multi- carrier code division multiple access (MIMO MC-CDMA) systems. maximum a posteriori decision feedback equalizer (MAP-DFE) already introduced in MIMO systems is extended to the MC-CDMA downlink transmission. This detector cancels the Inter-Symbol Interference (ISI) on the basis of the a posteriori error probability estimation. In this context, we show through simulations that MAP-DFE performance improves significantly compared to other well-known detection techniques such as Minimum Mean Square Error (MMSE) and Block DFE (BDFE).

In this paper, we derive the end-to-end (e2e) Bit Error Probability (BEP) of cooperative Multi Carrier Code Division Multiple Access (MC-CDMA) systems using selective threshold digital relaying (STDR). In STDR, a set of potential relays whose received Signal-to-Noise Ratio (SNR) exceeds a threshold value γ t , called reliable relays, is formed. Then, only the best relay among reliable relays is allowed to retransmit the received signal. We activate the relay with the largest SNR in relay-destination link. The derived BEP results are valid for any multipath intensity profile of the channel.

The multicarrier code division multiple access (MC-CDMA) system has received a considerable attention from researchers owing to its great potential in achieving high data rates transmission in wireless communications. Due to the detrimental effects of multipath fading the performance of the system degrades. Similarly, the impact of non-orthogonality of spreading codes can exist and cause interference. This paper addresses the performance of multicarrier code division multiple access system under the influence of frequency selective generalized η-µ fading channel and multiple access interference caused by other active users to the desired one. We apply Gaussian approximation technique to analyse the performance of the system. The avearge bit error rate is derived and expressed in Gauss hypergeometic functions. Maximal ratio combining diversity technique is utilized to alleviate the deleterious effect of multipath fading. We observed that the system performance improves when the parameter η increase or decreasse in format 1 or format 2 conditions respectively.

The performance of a multicarrier direct sequence code division multiple access (MC-DS-CDMA) system in fading environment has been extensively analysed in the literatures. The analysis that is available in the literatures did not take into account the impact of jamming and channel estimation errors on the performance of the system. In this study, closed-form expressions of the average bit error probability (BEP) of the MC-DS-CDMA system with channel estimation errors in the presence of partial band, broadband and multitone jamming are derived in a Rayleigh fading channel. The analysis shows that the magnitude of the normalised correlation between the actual and the estimated complex fading channel gain, A, greatly affects the performance of the MC-DS-CDMA system. For large signal-to-noise ratio, the average BEP with A=1 and without multiple access interference (MAI) tends to zero. When A≠1, the BEP of the system exhibits an error floor even without MAI. This error floor increases as the number of users increases. It is also shown that increasing the number of the system subcarriers enhances the system performance against jamming.

This paper gives the effectiveness measures of the comparison of the MIMO MC CDMA and MCCDMA modulation technique for wireless radio applications. MIMO is checked as a modulation technique for telephony communication system. A comparative study of MIMO MC CDMA and MC CDMA a variant of MC is done with the CDMA technology for the same application. The study will provide bird eye view of MIMO and CDMA for communication engineering. It was found by study that MIMO performs extremely well, providing a very high tolerance to multipath delay spread and channel noise. This paper attempts to assess wireless communications channels and propose techniques to improve the performance of modern wireless communications. Hence from a strategic perspective, an analytical study into the comparison of performance of MIMO Multi Carrier Code Division Multiple Access (MIMO-MC-CDMA) and multicarrier code division multiple access signal in frequency selective fading channel, the channel bandwidth is the main theme of this paper; from which the problems that may affect the communications link are highlighted and measures to counterbalance and alleviate multi path fading are proposed and investigated.

In the underwater passive positioning systems (UPPS), it involves a key technology of multi-user communication that the beacons synchronously send positioning messages to the targets. Interferences from other users, referred to multi-access interferences (MAIs), can induce significant bit errors for the desired user. In this paper, we designed an underwater acoustic Code Division Multiple Access (CDMA) system using the coded cyclic shift key (CCSK) modulation, and proposed a hybrid MAI cancellation scheme based on cyclic shift inverse matrix transform (CSIMT) and recursive least square (RLS) in the CDMA system. The hybrid scheme can suppress MAI better than RLS or CSIMT, and meets the requirements of strong stability to support reliable transmission for UPPS. The CSIMT interference cancellation performs well under the far-near effects, which doesn’t depend on the accuracy of the estimation of transmitted data, and the strong MAI can be reduced by transformation of the zero-forcing correlation values using m sequence cyclic shift inverse matrix. The RLS interference cancellation is used to mitigate MAI form the detected users with comparable power, by using the estimated the channel impulse response between the receiver and the detected users, and the regenerated signal of detected users form decoder output, which allows of strong error-correcting codes, and it’s robust in an asynchronous environment. The theoretical analysis and experimental result show that the hybrid MAI cancellation scheme can effectively suppress MAI better. The shallow water sea trial was carried out near Dalian Tiger Beach Ocean Park in China, at a distance of about 5 km, the data rates of 100bps, where three users could communicate asynchronously with low failure ratio and can match the UPPS application.

The W-CDMA system plays an important role in the 3G cellular systems because of its compatible networking architecture to the present popular Global System for Mobile Communications (GSM) systems as well as the salient features of a Code Division Multiple Access (CDMA) system, including multi-path fading tolerance, high system capacity, low power consumption, good performance and coverage. In CDMA system, a procedure used by a Mobile Station (MS) to search for the best cell site and achieve code, time, and frequency synchronization with it is referred as cell search. Fast cell search is essential to reduce switch on delay (initial cell search), increase stand by time (idle mode search) and maintain good link quality (active-mode search). This is particularly true for the Wideband Code Division Multiple Access-Frequency Division Duplex (W-CDMA-FDD) system since it employs nonsynchronous base stations instead of synchronous ones of other CDMA systems to extend coverage from outdoor to indoor. Cell search is critical for achieving code and time synchronization. The main process of achieving cell search is divided into three stages followed by code verification, tracking and carrier frequency adjustment: 1) slot synchronization, 2) frame synchronization and code group identification, 3) primary scrambling code identification. This paper addresses slot synchronization in downlink as cell search algorithm in W-CDMA. In the three step search, the receiver searches for the slot timing by correlating the received signal with the Primary Synchronization (PSCH) code using Matched Filter (MF). This paper propose a design for a hierarchical matched filter for Primary Synchronization Code (PSC). The aim is to reduce the hardware complexity using Field Programmable Gate Array (FPGA) logic resources required to realize the filter.

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.

Recently, an approach based on carrier scheme convergence was proposed based on the weighted-type fractional Fourier transform (WFRFT). Meanwhile, in the high-speed wireless communication systems, code division multiple access (CDMA) is an efficient technique for interference in the frequency-selective fading channels. In this letter, we combine the hybrid carrier (HC) scheme with CDMA and frequency domain linear equalization. The proposed system can not only provide the compatibility with single carrier (SC) CDMA and multi-carrier (MC) CDMA systems, but also achieve a smooth and seamless transition between the two. The proposed system can switch to single carrier or multi-carrier scheme under the specific conditions. Moreover, HC-CDMA outperforms SC-CDMA and MC-CDMA systems over the frequency-selective fading channel and single frequency jamming channel.

By calibrating the inter-system bias (ISB), an inter-system model can be achieved to improve the performance of multi-global navigation satellite system (GNSS) positioning. A number of studies were carried out to compose inter-system models among code division multiple access (CDMA) systems. However, due to the frequency division multiple access (FDMA) technique, GLONASS phase and code observations suffer from inter-frequency phase bias (IFPB) and inter-frequency code bias (IFCB) respectively, which make it difficult to obtain the ISB among GLONASS and CDMA systems. In addition, most of the current inter-system models are unable to handle mixed frequencies. A new single difference-based uniform inter-system model suitable for all of the CDMA and FDMA frequency combinations is proposed in this study. We first utilize a particle filter approach to extract the IFPB rate. Afterwards, the multi-ISB parameters can be estimated by the proposed model simultaneously. Four short baselines containing the signals of GLONASS, GPS, BDS3, BDS2, Galileo, and QZSS are selected to validate the model. The quantitative results indicate that the IFPB rate can be extracted precisely by a particle filter within ten epochs. The inter-system phase bias valuations are sufficiently stable on a daily scale and have standard deviation near 0.01 cycles. After de-weighting the GLONASS code observations to reduce the influence of IFCB, the inter-system code bias is also stable. Furthermore, imposing the stable ISB on the multi-GNSS inter-system model can effectively improve the single-frequency real-time kinematic positioning accuracy and reliability for severely obstructed situations with only a small number of satellites observed.

Code division multiple access (CDMA) is currently considered as one of the promising technologies that can significantly improve the efficiency of modern and future communication networks. In code division multiple access systems, users can share a dedicated space-frequency-time resource to simultaneously transmit their own traffic. To ensure the separation of individual user streams on the receiving side, each user is provided with his own code sequence embedded in the broadband signal transmitted by this user, the spectrum of which, as a rule, occupies the entire allocated frequency band. When centimeter-and- millimeter wave wireless channels are used as the transmission medium, such factors as fading, multipath, and Doppler scatter can significantly degrade the performance of a code division multiple access system. The standard solution in this situation is to combine code division multiple access with OFDM (orthogonal frequency division multiplexing) technology, known as multicarrier CDMA (MC-CDMA). To ensure the acceptable level of performance of multiple access systems under oversaturated conditions, many novel approaches based on the use of non-orthogonal multiple access (NOMA) have been proposed. The paper considers a variant of multiple access with non-orthogonal coding that is close in approach to sparse coding multiple access (SCMA). The proposed access procedure is based on dividing the allocated time-frequency resource into relatively small clusters and sharing each cluster with its own group of users, equipped with a non-orthogonal cluster code with the ability to change the loading factor. For the proposed class of cluster codes, a general encoding and decoding scheme is presented. Examples of specific cluster code constructions and numerical results are given that allow one to get a number of the parameters for tradeoffs between an increase of the loading factor in the system and additional energy loss.