Composite CDMA—a statistical mechanics analysis

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.

Graph construction using adaptive Local Hybrid Coding scheme

This paper studies detecting matrices in random dense and sparse Code Division Multiple Access (CDMA) systems. Detecting matrices were originally introduced in the coin weighing problem. Such matrices can be used in CDMA systems in over-loaded scheme where the number of users is greater than the number of chips. We drive some conditions in the large system limit for binary and bipolar random CDMA systems to ensure that any random matrix is a detecting matrix. Furthermore, we extend our results to sparse random ternary matrices that have been using in the sparse CDMA literature. Finally, a construction method for the sparse detecting matrices is introduced.

This paper introduces four block joint detection (JD) receivers which combining the orthogonal frequency division multiplexing (OFDM) and code division multiple access (CDMA) techniques, i.e., OFDM-CDMA receiver. The performance of the OFDM-CDMA receiver with orthogonal and non-orthogonal spreading sequences is investigated in mobile radio channels. The sequences considered as spreading codes are orthogonal Walsh-Hadamard, Orthogonal Gold, Gold and maximim length sequences or m - sequences. The JD algorithms for the OFDM- CDMA receiver are Zero Forcing Block Linear Equalizer (ZF- BLE), Minimum Mean Square Error Block Linear Equalizer (MMSE-BLE), with and without decision feedback (DF). Time- domain equalization does not require DFT or FFT operations and can be used to compensate the channel fading effect. Hence for hardware implementation, the time domain interpretation can lead to a more efficient structure when compared with the frequency domain approach. The bit error rate (BER) performance comparisons of different spreading codes and equalizers are shown in the simulations.

Code Division Multiple Access (CDMA) and multiple input multiple output- (MIMO-) CDMA systems suffer from multiple access interference (MAI) which limits the spectral efficiency of these systems. By making these systems more power efficient, we can increase the overall spectral efficiency. This can be achieved through the use of improved modulation and coding techniques. Conventional MIMO-CDMA systems use fixed spreading code assignments. By strategically selecting the spreading codes as a function of the data to be transmitted, we can achieve coding gain and introduce additional degrees of freedom in the decision variables at the output of the matched filters. In this paper, we examine the bit error rate performance of parity bit-selected spreading and permutation spreading under different wireless channel conditions. A suboptimal detection technique based on maximum likelihood detection is proposed for these systems operating in frequency selective channels. Simulation results demonstrate that these code assignment techniques provide an improvement in performance in terms of bit error rate (BER) while providing increased spectral efficiency compared to the conventional system. Moreover, the proposed strategies are more robust to channel estimation errors as well as spatial correlation.

Code Division Multiple Access (CDMA) is a widely used multiple access technique in cellular and military communication systems. In Direct Sequence Spread Spectrum (DSSS) communications, a Pseudo Noise (PN) spreading code is typically applied to the data samples prior to transmission. Synchronization of this unique code with the incoming data samples forms a significant step in the data reception process. Single Dwell Serial Search (SDSS) and Matched Filtering are some of the types of CDMA code synchronization techniques. This paper presents the design and comparison of SDSS technique & Digital Matched Filter (DMF), and shows the effectiveness of DMF for rapid synchronization. It further presents Recursive Digital Matched Filter (RDMF) which reduces resource utilization and the obtained results clearly shows that the proposed RDMF has the characteristics that are highly suitable for code synchronization in CDMA application compared to conventional methods.

Using the notion of expanding subspace and the framework of reduced-rank signal processing, we present our latest discovery on applying the vector and matrix conjugate gradient (CG) methods to design reduced-rank linear MMSE multiuser detectors (MUD) for code division multiple access (CDMA) systems. We show that for a synchronous CDMA system with K users, each using a distinct length N spreading code, the vector CG method converges to the full-rank linear MUD in at most K steps (K/spl les/N).The matrix CG method converges to the full-rank linear MUD in one step. Furthermore, when the Gold codes are used as spreading codes in combination with a groupwise power control scheme, early convergence in the vector CG Wiener filter can be reached in just L steps (L/spl Lt/K/spl les/N), typically L=2/spl sim/4 independent of the user number K and the spreading length N.

Channel estimation techniques for code-division multiple access (CDMA) systems need to combat multiple access interference (MAI) effectively. Most existing estimation techniques are designed for CDMA systems with short repetitive spreading codes. However, current and next generation wireless systems use long spreading codes whose period is much larger than the symbol duration. In this paper, we derive the maximum likelihood channel estimate for long code CDMA systems over multipath channels using training sequences and approximate it using an iterative algorithm to reduce the computational complexity in each processing window. The asymptotic convergence of the mean of the iterative estimate to the actual channel is also shown. The effectiveness of the iterative channel estimator is demonstrated in terms of squared error in estimation as well as the bit error rate performance of a multistage detector based on the channel estimates. Finally, the proposed iterative channel estimation technique is extended to track slowly varying multipath fading channels using decision feedback. Thus, an MAI resistant multiuser channel estimate with reasonable computational complexity is derived for long code CDMA systems over multipath fading channels.

The bit error rate (BER) performance of an asynchronous direct-sequence (DS) code-division multiple-access (CDMA) system is studied by means of Monte Carlo simulation using SPW/sup TM/ software. The effect of the spreading code length on system performance is of special interest. The probability of a partial cross-correlation value between segments of long sequences is also discussed. The values of the spreading factor for a BPSK type spectrum spreading are 127 and 255. Ideal code, carrier and symbol synchronization, and additive white Gaussian noise (AWGN) channel are employed in the system model. Results indicate that the system performance with long spreading codes (period of 2/sup 30/-1) is about 1.0 dB worse than that of short CDMA sequences when E/sub h//N/sub 0/ is about 10 dB. The loss in BER or in signal-to-noise ratio is tolerable, since long codes allow individual user addresses to be implemented for each user of a large asynchronous cellular CDMA network without need for re-allocation of codes. The network planning and implementation becomes easier as there is no need for code change in a handover situation. Long spreading codes also provide flexibility with respect to multiple bit rates and variable spreading factors. >

Most of the techniques for multiple access interference (MAI) reduction in code-division multiple access (CDMA) systems are applicable to the uplink, where the base-station knows the signature of all users and can apply a multi-user detection (MUD) strategy. A technique is proposed here to reduce the influence of MAI on the downlink of a multi-carrier CDMA (MC-CDMA) system, without resorting to MUD. The technique is based on the use of a spreading sequences pair for each subscriber. Its limiting factor is that the system capacity, in terms of the active users' maximum number, is halved as compared to the case in which a single spreading code per user is adopted

The reverse links of 3G cellular systems, which are based on code division multiple access (CDMA), require that the base station synchronizes with each user's length N spreading code to despread and demodulate the information bits. A method of obtaining chip-level synchronization for all K users of the system simultaneously was recently presented [3]. The approach uses a modified transpose domain filter (TDF) implemented by a multistage Wiener filter (MWF), and the joint solution for all K users is implemented efficiently with a parallel bank of MWFs. The technique was shown to be effective in giving near 100% success in timing estimation for heavily loaded systems when E b /N 0 ≥ 0 dB, enough bits, M, e.g. M ≥ 16N, is used to obtain the estimate, and at least 20% of the chips in the spreading code are present. In this paper, we propose a modified TDF algorithm, in which processing occurs over a span of 2N chips vs. N chips. The proposed algorithm overcomes the limitations of the previous algorithm since the processing now spans two bits so the entire spreading sequence is always available for processing. We analyze the algorithm in AWGN and multipath, with known and unknown channels, and show that it achieves > 90% success even in multipath with just M = 2N bits and near 100% with M = 4N bits over a wide range of E b /N 0 and number of users K.

Code division multiple access (CDMA) has become a widely adopted technology for wireless communications, particularly in mobile third generation systems (3G) and global positional system (GPS). In CDMA, a particular class of pseudonoise (PN) spreading codes yields a code-set Grammian with only two distinct eigenvalues. For these spreading codes, we propose a computationally efficient method for signal detection using Wiener filtering. Our approach relies on a matrix inversion step, where we gain in computational efficiency by using the Sherman-Morrison-Woodbury formula. The resulting approach involves significantly less computational load compared to popular approaches like Conjugate Gradient (CG) method and the (traditional) Cholesky decomposition.

Served as one of the most widely used wireless air link interfaces, code division multiple access (CDMA) has been identified as a major technique for 3G wireless communications. Here, a new method based on applying encryption algorithm over spreading codes is proposed to improve the security of CDMA. The security produced in this way is primarily associated with the complexity of the employed encryption algorithm. Because the encryption algorithm's security is highly reliable, it is suitable for any kind of data communications. Since the multi-user detection is the inherent characteristic of the CDMA, the multi-user interference must be studied carefully. For this reason, the cross-correlation between outputs of encryption algorithm, causing the multi-user interference, is studied thoroughly. Then, the combination of encrypted and unencrypted M-sequence is used as a spreading code to mitigate the system performance, and the advantage of this combination is considered from the interference level and the security point of view. A compatible algorithm for the key exchange is produced.

SUMMARYA 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.

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.