Fiber-based multiple-access frequency synchronization via 1f–2f dissemination

Considering the reference frequency dissemination requirements of the Square Kilometre Array telescope（SKA）project,on the basis of the 1f–2f precision frequency synchronization scheme,we propose and demonstrate a fiber-based multiple-access frequency synchronization scheme.The dissemination reference frequency can be recovered at arbitrary nodes along the entire fiber link.It can be applied to antennas close proximity to the SKA central station,and will lead to a better SKA frequency synchronization network.As a performance test,we recover the disseminated 100-MHz reference frequency at an arbitrary node chosen as being 5 km away from the transmitting site.Relative frequency stabilities of2.0×10^（-14）/s and 1.6×10^（-16）/10~4 s are obtained.We also experimentally verify the feasibility of a frequency dissemination link with three access points.

ANALYSIS OF SPECTRAL PHASE ENCODING OCDMA SYSTEM WITH PSEUDORANDOM CODING

Multiple access interference (MAI) is the most serious interference in spectral phase encoding optical code division multiple access (SPE OCDMA) systems. This paper focuses on the behavior of MAI in SPE OCDMA systems with pseudorandom coding. The statistical expectation of multi access interference (MAI) is derived and plotted. The results confirm that MAI can be suppressed effectively by pseudorandom coding with m sequences.

Analysis of Multichannel ALOHA and CSMA Protocols－Normal and Slotted Versions

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Downlink MBER Transmit Beamforming Design Based on Uplink MBER Receive Beamforming for TDD-SDMA Induced MIMO Systems

The downlink minimum bit error rate (MBER) transmit beamforming is directly designed based on the uplink MBER receive beamforming solution for time division duplex (TDD) space-division multiple-access (SDMA) induced multiple-input multiple-output (MIMO) systems, where the base station (BS) is equipped with multiple antennas to support multiple single-antenna mobile terminals (MTs). It is shown that the dual relationship between multiuser detection and multiuser transmission can be extended to the rank-deficient system where the number of users supported is more than the number of transmit antennas available at the BS, if the MBER design is adopted. The proposed MBER transmit beamforming scheme is capable of achieving better performance over the standard minimum mean square error transmit beamforming solution with the support of low-complexity and high power-efficient MTs, particularly for rank-deficient TDD-SDMA MIMO systems. The robustness of the proposed MBER transmit beamforming design to the downlink and uplink noise or channel mismatch is investigated using simulation.

AN OPTIMIZED SCHEME FOR FAST HANDOFF IN IP-BASED CDMA WIRELESS COMMUNICATION NETWORKS

This paper proposed an optimized fast handoff scheme for real time applications in next generation IP based CDMA wireless networks. The idea is to utilize optimized IP multicasting handoff (based on PIM SM), which is triggered by CDMA layer 2 functionality. An IP based cellular network model with WCDMA FDD air interface and IP based packet traffic is adopted. No special network entities and signaling for handoff are added in our network model. The simulation results show that low delay and low packet lost rate can be obtained.

LOW-COMPLEXITY BLIND MULTIUSER DETECTION OF DS-CDMA SIGNAL IN DISPERSIVE CHANNELS

The problem of blind multiuser detection in dispersive channels of Code-Division Multiple-Access (CDMA) in the presence of both Multiple-Access Interference (MAI) and InterSymbol Interference (ISI) is considered. In practice, it is showed that by incorporating the desired user's signature waveform and the auxiliary vector, the information of the user can be identified using the suboptimal subspace method. The major contribution of this paper is to propose a minimum-mean-square-error detector with the suboptimal subspace-based blind technique for joint suppression of MAI and ISI in the dispersive CDMA channels.

Key issues of multiple access technique for LEO satellite communication systems

The large carrier frequency shift caused by the high-speed movement of satellite (Doppler effects) and the propagation delay on the up-down link are very critical issues in an LEO satellite communication system, which affects both the selection and the implementation of a suitable access method. A Doppler based multiple access technique is used here to control the flow and an MPRMA-HS protocol is proposed for the application in LEO satellite communication systems. The extended simulation trials prove that the proposed scheme seems to be a very promising access method.

Toward the Energy Efficiency of Resource Allocation Algorithms for OFDMA Downlink MIMO Systems

The problem of the simultaneous multi-user resource allocation algorithm in orthogonal frequency division multiple access(OFDMA)based systems has recently attracted significant interest.However,most studies focus on maximizing the system throughput and spectral efficiency.As the green radio is essential in 5G and future networks,the energy efficiency becomes the major concern.In this paper,we develop four resource allocation schemes in the downlink OFDMA network and the main focus is on analyzing the energy efficiency of these schemes.Specifically,we employ the advanced multi-antenna technology in a multiple input-multiple output(MIMO)system.The first scheme is based on transmit spatial diversity(TSD),in which the vector channel with the highest gain between the base station(BTS)and specific antenna at the remote terminal(RT)is chosen for transmission.The second scheme further employs spatial multiplexing on the MIMO system to enhance the throughput.The space-division multiple-access(SDMA)scheme assigns single subcarrier simultaneously to RTs with pairwise“nearly orthogonal”spatial signatures.In the fourth scheme,we propose to design the transmit beamformers based on the zero-forcing(ZF)criterion such that the multi-user interference(MUI)is completely removed.We analyze the tradeoff between the throughput and power consumption and compare the performance of these schemes in terms of the energy efficiency.

Tunable Super-Structured Fiber Bragg Gratings with Perfect Sequences Based on m-Sequence

Recently, a tunable fiber Bragg grating(FBG) was developed by using stress-responsive colloidal crystals. In this paper, we have simulated the application of these nanoparticles into the super-structured fiber Bragg grating(SSFBG) written with perfect sequences derived from a short maximal-length sequence. A tunable SSFBG will be available to overcome the prohibitive temperature variation of the optical codecs. Nevertheless,we presented a method to implement coherent time spreading optical code-division multiple-access(OCDMA) where a unique code(or perfect sequence) can be reused and mixed with different wavelengths to obtain a tunable wavelength-division multiplexing(WDM)system. In order to maximize the binary throughput, we have selected a unique short maximal-length sequence composed of 7 chips that can be tuned with 7 different optical wavelengths. We found thousands of different tunable combinations that presented power contrast ratios(P/C) higher than 12 dB. When a WDM-OCDMA system used 2 different combinations simultaneously, the perfect binary detection with error correction codes was achieved successfully. The tunable SSFBG with colloidal crystals will be a simple and good alternative choice for fiber-to-the-home(FTTH) communications.

BER Performance of Space-Time Coded MMSE DFE for Wideband Code Division Multiple Access (WCDMA)

In recent times, there has been growing interests in integration of voice, data and video traffic in wireless communication networks. With these growing interests, WCDMA has immerged as an attractive access technique. The performance of WCDMA system is deteriorated in presence of multipath fading environment. The paper presents space-time coded minimum mean square error (MMSE) Decision Feedback Equalizer (DFE) for wideband code division multiple access (WCDMA) in a frequency selective channel. The filter coefficients in MMSE DFE are optimized to suppress noise, intersymbol interference (ISI), and multiple access interference (MAI) with reasonable system complexity. For the above structure, we have presented the estimation of BER for a MMSE DFE using computer simulation experiments. The simulation includes the effects of additive white Gaussian noise, multipath fading and multiple access interference (MAI). Furthermore, the performance is compared with standard linear equalizer (LE) and RAKE receiver. Numerical and simulation results show that the MMSE DFE exhibits significant performance improvement over the standard linear equalizer (LE) and RAKE receiver.

The Application of Optical CDMA-Based Fiber Radio Networks in Wireless Sensor Networks

One fiber radio scheme using shifted prime codes for interference elimination is proposed for optical code-division multiple-access (OCDMA) network. By taking advantage of the cyclic property of the shifted prime codes in the same code groups, the proposed compact decoder is low cost and suitable to be used in the task manager node in the applications of wireless sensor networks. The performance comparison for sev-eral OCDMA-based fiber radio networks is also given to clarify the advantage of the proposed one.

Intelligent multi-user detection using an artificial immune system

Artificial immune systems （AIS） are a kind of new computational intelligence methods which draw inspiration from the human immune system. In this study, we introduce an AIS-based optimization algorithm, called clonal selection algorithm, to solve the multi-user detection problem in code-division multipleaccess communications system based on the maximum-likelihood decision rule. Through proportional cloning, hypermutation, clonal selection and clonal death, the new method performs a greedy search which reproduces individuals and selects their improved maturated progenies after the affinity maturation process. Theoretical analysis indicates that the clonal selection algorithm is suitable for solving the multi-user detection problem. Computer simulations show that the proposed approach outperforms some other approaches including two genetic algorithm-based detectors and the matched filters detector, and has the ability to find the most likely combinations.

Optimal Filtering Algorithm-Based Multiuser Detector for Fast Fading CDMA Systems

A multiuser detector was developed for fast fading code-division multiple-access systems by representing the channels as a system with the multiplicative noise （SMN） model and then using the known optimal filtering algorithm for the SMN for multiuser detection （MUD）. This multiuser detector allows the channel response to be stochastic in one symbol duration, which can be regarded as an effective method of MUD for fast fading CDMA systems. Performance analyses show that the multiuser detector is theoretically valid for CDMA systems over fast fading channels. Simulations show that the multiuser detector performs better than the Kalman filter-based multiuser detector with a faster convergence rate and lower bit error rate.

Recursive Least Squares Semi-blind Beamforming for MIMO Using Decision Directed Adaptation and Constant Modulus Criterion

A new semi-blind adaptive beamforming scheme is proposed for multi-input multi-output （MIMO） induced and spacedivision multiple-access based wireless systems that employ high order phase shift keying signaling. A minimum number of training symbols, very close to the number of receiver antenna elements, are used to provide a rough initial least squares estimate of the beamformer＇s weight vector. A novel cost function combining the constant modulus criterion with decision-directed adaptation is adopted to adapt the beamformer weight vector. This cost function can be approximated as a quadratic form with a closed-form solution, based on which we then derive the recursive least squares （RLS） semi-blind adaptive beamforming algorithm. This semi-blind adaptive beamforming scheme is capable of converging fast to the minimum mean-square-error beamforming solution, as demonstrated in our simulation study. Our proposed semi-blind RLS beamforming algorithm therefore provides an efficient detection scheme for the future generation of MIMO aided mobile communication systems.