Multiple Access Interference Reduction Research Articles

Interference mitigation in uplink STBC MC-CDMA system based on PIC receiver

Multi Carrier Code Division Multiple Access (MC-CDMA) is attractive technique for high speed data transmission in multipath fading channel. MC-CDMA system cannot handle the sudden time variations of the channel which cause the subcarriers to lose their orthogonality. The loss of orthogonality between the subcarriers of a user or unwanted correlation between the spreading codes of different user can lead to increase in Multiple Access Interference (MAI). Space Time Block Code (STBC) based MC-CDMA system is chosen to achieve full diversity and transmission rate without the knowledge of Channel State Information (CSI) at the transmitter. Thus, in the paper STBC is introduced at the transmitter to improve the quality of the receiver. Space Time Block Code-Parallel Interference Cancellation (STBC-PIC) receiver has been proposed for MC-CDMA system. In the proposed STBC-PIC receiver, at each interference cancellation stage, weighted signal of the other user is subtracted from signal of the desired user, thereby reducing the MAI and improving the BER performance. From the simulation results, it is observed that the proposed receiver outperforms STBC-Orthogonal Complete Complementary Code (STBC-OCCC), STBC-Minimum Mean Square Error (STBC-MMSE) and STBC-Zero Forcing (STBC-ZF) receivers for MAI reduction.

처프 확산 대역 시스템에서 다중 접속 간섭의 감소를 위한 처프율 할당 기법

처프 확산 대역 시스템에서 다중 접속 간섭의 감소를 위한 처프율 할당 기법

Optical Code-Division Multiplexing (OCDM) Networks Adopting Code-Shift Keying/Overlapping PPM Signaling: Proposal and Performance Analysis

Optical code-division multiplexing (OCDM) systems use short pulses compared with bit-duration to achieve high transmission rate. The use of short pulses poses several problems as a result of group velocity dispersion (GVD), intersymbol interference (ISI) (due to avalanche photodiode (APD) buildup time), and receivers limited bandwidth. In this paper, an OCDM system employing code-shift keying (CSK) and overlapping pulse-position modulation (OPPM) signaling is proposed and theoretically investigated. By using CSK while maintaining same data rate, the chip duration can be increased to counteract the GVD effect in 2D OCDM systems. Moreover, by increasing the chip duration, the chip rate is decreased and the stringent requirement on receiver bandwidth is relaxed. In addition, using overlapping property in OPPM allows for further chip duration increase. We consider using correlation receivers with hard-limiters and APDs at the receiver side. The bit error probability (BEP) of the proposed system is derived taking into account the impacts of APD noise, thermal noise, GVD, ISI, and multiple-access interference (MAI). A performance comparison between OOK-, PPM-, OPPM-OCDM and the proposed system is carried out. Our results reveal that the use of CSK/OPPM-OCDM with data rate constraint allows the reduction of MAI, GVD and ISI effects with improved spectral efficiency.

MAI Mitigation Using V-BLAST and PIC Based Binary Orthogonal Code Spreaded Multi Antenna MC-CDMA System

In this paper, the reduction of Multiple Access Interference (MAI) in Multi Carrier-Code Division Multiple Access (MC-CDMA) uplink system is analyzed. Simultaneous access over channels by numerous users results in MAI which leads to high Bit Error Rate (BER). Therefore the data received at the receiver is intended to numerous users. As a result, it is necessary to separate the data corresponding to different users efficiently. So in this paper, the Vertical-BELL Laboratories Layered Space-Time (V-BLAST) algorithm with Parallel Interference Cancellation (PIC) technique based MC-CDMA system is proposed. Due to the combination of V-BLAST and PIC in MC-CDMA system the MAI is reduced to a certain extent, which is shown in simulation result. Further the system performance is analyzed using Binary Orthogonal Codes (BOC) as spreading sequence. The result shows that Binary Orthogonal Coded V-BLAST and PIC based System provide better performance than conventional spreading sequence like Walsh Hadamard and Pseudorandom Noise (PN) sequence.

An Approach for Enhancement of Bit Error Rate Analysis in SAC-OCDMA

In this paper, a new method is presented to calculate the execution of Spectral Amplitude Coding Optical Code Division Multiple Access (SAC-OCDMA) network. A new code proposed is Dynamically Cyclic Shift (DCS) code to assess the execution of the network. Bit error rate is the parameter of performance studied. A Dynamically Cyclic Shift (DCS) algorithm is used at the transmitter section and a detection technique called AND-Subtraction technique is used at the receiver. The most astonishing aspect in the detection techniques the reduction of Multiple Access Interference (MAI) and the correlation values lies between 0 and 1, for the developed DCS code. SAC-OCDMA network is more consideration in view of their capacity to totally avoid multiple access obstruction by utilizing code successions with settled in phase cross correlation (λ c ). The performance is calculated using the Signal to Noise Ratio (SNR) and the experiment is simulated at 8 Gb/s for a link length of 15 km using optisystem TM ver.12 simulation software from optiwave. The BER obtained is 4.66 × 10 -17 using DCS code.

MMSE Pre-distortion Scheme for Multiuser UWB

The paper proposes a new formulation of a minimum mean square error pre-distortion scheme for multiple-input single-output impulsive ultra-wideband in multiple user environment. For the performance evaluation, scenarios CM1 and CM3 of the IEEE 802.15.3a channel model are considered. Results show that such a scheme has a good trade-off between intersymbol interference and multiple access interference reduction and signal-to-noise ratio preservation, performing better than time reversal and zero forcing pre-distortion schemes.

Multiple Access Interference Reduction Using Received Response Code Sequence for DS-CDMA UWB System

This paper proposes a combination of novel Received Response (RR) sequence at the transmitter and a Matched Filter-RAKE (MF-RAKE) combining scheme receiver system for the Direct Sequence-Code Division Multiple Access Ultra Wideband (DS-CDMA UWB) multipath channel model. This paper also demonstrates the effectiveness of the RR sequence in Multiple Access Interference (MAI) reduction for the DS-CDMA UWB system. It suggests that by using conventional binary code sequence such as the M sequence or the Gold sequence, there is a possibility of generating extra MAI in the UWB system. Therefore, it is quite difficult to collect the energy efficiently although the RAKE reception method is applied at the receiver. The main purpose of the proposed system is to overcome the performance degradation for UWB transmission due to the occurrence of MAI during multiple accessing in the DS-CDMA UWB system. The proposed system improves the system performance by improving the RAKE reception performance using the RR sequence which can reduce the MAI effect significantly. Simulation results verify that significant improvement can be obtained by the proposed system in the UWB multipath channel models.

Steady-state analysis of constrained normalized adaptive filters for MAI reduction by energy conservation arguments

We derive the steady-state performance of a common class of adaptive filters for multiple access interference (MAI) reduction in code division multiple access (CDMA) systems. The adaptive filters under study utilize estimates of the desired user's amplitude and are divided into three groups of least-mean-square (LMS) algorithms differing by the choice of the normalization factor. The steady-state performance is deduced from energy-conservation relations that include a possibly erroneous estimate of the desired user's amplitude. The analyses show that blind algorithms using information about the desired user's amplitude achieve similar performance to that of nonblind algorithms. In addition, geometric considerations reveal the conditions under which the choice of the normalization factor is expected to have great impact on the convergence properties of the algorithms. Numerical simulations show good agreement with theory.

Performance enhancement of DS-CDMA system using overlapping sectors with interference avoidance

A cellular system with overlapping sectors is proposed to enhance system performance through interference avoidance. Due to the overlapping effect, each user is allowed to select one sector out of many for operation. It is shown that the mutual interference among users in different regions within a cell can be minimized by choosing a good sector combination. The potential benefits of the proposed technique are investigated in two scenarios: orthogonal and nonorthogonal code division multiple access systems. In the former case, it is shown that when compared with the traditional nonoverlapping sectors system, one is able to reduce the number of spreading codes required to support all users by using appropriate code allocation in the proposed system. A theoretical framework based on combinatorial enumeration is developed to assess the performance of the proposed system. For the latter case, the enhancement takes the form of multiple access interference reduction. Simulation results show that its improvement is significant and can be further enhanced by increasing the number of overlapping sectors.

A low-complexity combined antenna array and interference cancellation DS-CDMA receiver in multipath fading channels

A simple direct sequence-code division multiple access receiver that combines adaptive beamforming with parallel interference cancellation in a multipath fading channel is proposed and analyzed. A fast adaptation, conjugate gradient algorithm is used to find the optimum beamformer weights. By beamforming, the desired user's signal is enhanced and the cochannel interference from other directions is reduced. For in-beam multiple access interference reduction, a parallel interference canceller is used in each RAKE finger. In the demodulation process, we propose a new demodulation method in which the incoming signal is correlated with the effecting spreading code rather than the physical spreading code called the effective matched filter. A new combining method called equivalent maximal ratio combining is also proposed and analyzed. The average uncoded bit error rate as a function of the average antenna signal-to-noise ratio and the number of receiving antennas is examined in a frequency selective Rayleigh fading channel for all proposed receiver structures. Both simulation and analysis show an increase in system capacity as a function of the number of antennas and the number of interferers canceled per finger.

Multiple-access interference reduction for QS-CDMA systems with a novel class of polyphase sequences

In this paper, a new class of polyphase sequences for CDMA systems and its generation method are suggested, and the correlation properties of the sequence are investigated. The performance of the sequence in quasi-synchronous CDMA (QS-CDMA) systems is investigated under a frequency-selective, time-nonselective, slow Nakagami fading channel with additive white Gaussian noise (AWGN). It is shown that the performance of the QS-CDMA systems using the suggested sequence is independent of the number of users and is much better than that using the PN sequence.

Performance of a coded multi-carrier DS-CDMA system in multi-path fading channels

Multi-carrier DS-CDMA has been considered as an effective scheme for reducing multiple access interference in quasi-synchronous transmission. The scheme allows the reduction of multiple access interference by transferring the orthogonality property of the signals into the frequency domain where the orthogonality property is robust to relative chip offsets between the spreading codes of the various users. However in multi-path channels, the multi-carrier technique results in frequency non-selective fading in the sub-channels, due to the narrower bandwidth, hence a reduction of the capability of the spread spectrum signal to mitigate the effect of multi-path propagation. In this paper, we consider the use of a Reed-Muller code with soft decision decoding to regain the corresponding loss in performance, and compare the resulting system with a single carrier DS-CDMA system. The effect of system parameters such as the number of sub-channels is investigated through numerical calculation and simulation, from which a number of system design criteria are arrived at.