Channel interference cancellation using time division reference signal for direct-detection optical synchronous CDMA systems

The atmospheric two-dimensional optical code-division multiple-access (CDMA) systems using pulse-position modulation (PPM) and Turbo-coded were presented. We analyzed the bit-error rate (BER) of the proposed system using pulse-position modulation (PPM) with considering the effects of the scintillation, avalanche photodiode noise, thermal noise, and multi-user interference. We showed that the atmospheric two dimensional (2D) optical PPM CDMA systems can realize high-speed communications when the logarithm variance of the scintillation is less than 0.1, and the turbo-coded atmospheric optical CDMA system has better bit error rate(BER) performance than the atmospheric optical PPM CDMA systems without turbo-coded. We also showed that the turbo-coded system has better performance than the multi-user detection system.

We propose the random Manchester codes (RMC) to improve the bit error probability (BEP) performance in both synchronous and asynchronous fiber-optic code-division multiple-access (CDMA) systems. The spreading sequences used in the synchronous and asynchronous systems are modified prime sequence codes and optical orthogonal codes (OOCs), respectively. Thermal noise, shot noise, and avalanche photodiode (APD) bulk and surface leakage currents are taken into consideration in the BEP analyzes. The results show that the proposed systems can support a larger number of simultaneous users than other systems with similar system complexity under the same bit-error probability constraint.

We propose atmospheric (free-space) optical code-division multiple-access (CDMA) systems. We analyze the bit error rate (BER) of the proposed system using pulse position modulation (PPM) taking into consideration the effects of scintillation, avalanche photodiode (APD) noise, thermal noise, and multiuser interference. We show that atmospheric optical CDMA systems can realize communications at a bit rate of 156 Mbps when the logarithm variance of the scintillation /spl sigma//sub s//sup 2/ is smaller than, or equal to, 0.1. When /spl sigma//sub s//sup 2/ is large, we need to use forward error correction codes (FECs).

A hybrid Interference cancellation (HIC) technique employing both Parallel and Successive Interference Cancellation (PIC/SIC) is proposed for Overloaded Direct Sequence Code Division Multiple Access (OCDMA) system under imperfect synchronization. Two types of receivers are proposed to efficiently mitigate the multiuser interference (MUI). In type I receiver, the MUI within group-1 users are first cancelled using Parallel Interference Cancellation (PIC). Then the multistage Successive Interference Cancellation (m-SIC) is performed on the resultant output of PIC to efficiently cancel the interference of group-2 users on group-1 users. The type II receiver is designed by performing PIC immediately followed by SIC and this combined process is repeated till a major chunk of multi user interference is cancelled. The BER performance of type II receiver is proved to be better than other cancellation techniques such as PIC, SIC and the proposed type I receiver.

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.

Performance analysis of multicell multiuser MIMO MC DS/CDMA system with MMSE user-ordered SIC technique

In this paper, modeling and simulation of a receiver with interference cancellation technique for the downlink transmission of multiple input multiple output (MIMO) multicarrier code-division multiple access (MC-CDMA) based cognitive radio is investigated. Synchronous transmission of multiple users’ signals over frequency selective Rayleigh channel is considered. Linear MMSE frequency domain equalization is utilized to overcome channel effect. Orthogonal Variable Spreading Factor (OVSF) codes are used to support users of different data bit rates. An adaptive subcarrier deactivation method is proposed for deactivating a set of subcarriers used for downlink transmission by the cognitive radio base station that overlap with primary user’s transmission in order to avoid causing interference to the primary user. Orthogonality between the spreading codes is lost due to subcarrier deactivation which causes interference among secondary cognitive radio users. In this paper a modified Group-wise Successive Interference Cancellation (m-GSIC) technique is proposed to suppress the interference caused due to subcarrier deactivation. The simulation results show that the proposed interference cancellation technique results in better BER performance over conventional interference cancellation techniques

By introducing a full‐rate space–time coding (STC) scheme, a synchronous CDMA (code division multiple access) system with full‐rate STC is given, and the corresponding uplink performance is investigated in Rayleigh fading channel with imperfect estimation. Considering that existing STC‐CDMA system has high decoding complexity, low‐complexity multiuser receiver schemes are developed for perfect and imperfect estimations, respectively. The schemes can make full use of the complex orthogonality of STC to reduce the high decoding complexity of the existing scheme, and have linear decoding complexity compared with the existing scheme with exponential decoding complexity. Moreover, the proposed schemes can achieve almost the same performance as the existing scheme. Compared with full‐diversity STC‐CDMA, the given full‐rate STC‐CDMA can achieve full data rate, low complexity, and partial diversity, and form efficient spatial interleaving. Thus, the concatenation of channel coding can effectively compensate for the performance loss due to partial diversity. Simulation results show that the full‐rate STC‐CDMA has lower bit error rate (BER) than full‐diversity STC‐CDMA systems under the same system throughput and concatenation of channel code. Moreover, the system BER with imperfect estimation are worse than that with perfect estimation due to the estimation error, which implies that the developed multiuser receiver schemes are valid and reasonable. Copyright © 2010 John Wiley & Sons, Ltd.

Analysis of the performance of time-spread optical code-division multiple access systems with integrate-and-dump receivers

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.

A universal scheme is developed to reproduce the effect of bipolar signaling in a unipolar optical channel. Without any hardware modification, the electro-optic/opto-electronic (EOE) system proposed enables the transmission of any bipolar code sequence through the unipolar optical channel while maintaining the correlation properties of the codes. The bit error rate performance of the EOE system is shown to be identical to that of an optimal baseband bipolar phase shift keying code-division multiple access (CDMA) system. Hence, the maximum spectral efficiency of an incoherent optical CDMA system using bipolar codes is the same as that of an optimal bipolar baseband CDMA system.

We report the design of optical fiber asynchronous code-division multiple-access (CDMA) systems using fully optical orthogonal codes (FOOCs), which can be employed to support future airborne real-time communication services. The use of incoherent optical CDMA (IO-CDMA) can result in lower complexity and more reliable implementation than that of the coherent version, so this makes IO-CDMA more adequate for operation in the severe aircraft environment. When FOOCs are used in the IO-CDMA systems, the peaks of all the cross-correlation functions and all the sidelobes of any auto-correlation function are then limited to "1" (i.e., the minimum correlation constraint for incoherent optical processing) in fully asynchronous transmissions. Moreover, proposed systems can effectively support multi-rate and variable-rate data communications with no violation of the minimum correlation constraint and no change of IO-CDMA encoders/decoders. This characteristic can be used to improve the system flexibility and availability. A basic experiment is also demonstrated to verify the characteristics of FOOCs.

In this paper, we applied genetic algorithm (GA) scheme as a suboptimal detection of a bit-synchronous code division multiple access (CDMA) systems over a Gaussian channel. The GA scheme attempts to search for the user's transmitted bit sequence that optimizes the correlation matrix employed. Simulation results showed that the Bit Error Rate (BER) performance of our proposed multiuser detector is better than those of conventional receivers like Minimum Mean Square Error(MMSE), Matched Filter(MF) and Rake Receivers.

For high-speed code division multiple access (CDMA) systems, interchip interference that is caused by multipath propagation becomes severe and cannot be ignored. Together with the multiple access interference due to nonideal conditions in CDMA, they become major obstacles to overall system performance. In this paper, both batch and adaptive algorithms based on linear prediction are proposed for joint blind equalization and multiuser detection for asynchronous CDMA systems. It is shown that the new methods are insensitive to estimation error of propagation delay or chip timing. In addition, the adaptive algorithm is computationally efficient. Simulation results show that the proposed methods are also near-far resistant and compare favorably to many existing methods.

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.