This paper proposes low complexity non-linear pre-coding that switching among pre-coding weights per orthogonal frequency division multiplexing (OFDM) symbol according to the modulation scheme and constellation point of users, for downlink multiuser multiple input multiple output (DL-MU-MIMO) systems. In the proposed pre-coding, when some users have the same modulation scheme and constellation point, the pre-coding weight is calculated based on broadcast approach without inter-user interference cancellation. If not, the pre-coding weight is calculated based on unicast approach such as the zero-forcing (ZF) method. In addition, computer simulation results show that the proposed pre-coding achieves lower bit error rate (BER) on low signal to noise ratio (SNR) environments.
In this letter, we experimentally validate an interference reduction technique that combines a digital subtraction technique, and eigen-beam-forming with an end-fire arrangement of linear array antennas. The eigen-beamforming significantly reduces the self-interference at the cost of a slight drop in the degrees-of-freedom of the antenna. We experimentally evaluate the degree of interference reduction using two linear sleeve-dipole arrays in end-fire arrangement. This experiment uses MIMO (Multiple-Input Multiple-Output)-OFDM (Orthogonal Frequency Division Multiplexing) signals to evaluate the realistic performance of this technique. The results of the experiment show the combination of eigen-beamforming and digital subtraction can significantly reduce, −76 dB, the interference.
Wireless two-way interferometry (Wi-Wi) is the simplified version of “carrier phase based two-way satellite time and frequency transfer,” wherein a wireless communication technology is used instead of a satellite communication technology. We used the carrier phase of a 2.4 GHz ZigBee module to measure the variation of two rubidium clocks at remote sites. Since clocks in the ZigBee module are much less precise than rubidium clocks, the carrier phase of the ZigBee signal cannot be used to compare two rubidium clocks in a simple manner. Using a technique to cancel the clock error of transmitters, we demonstrated picosecond-level precision measurement of the time variation of clocks between two remote systems. This synchronization technique at picosecond-level precision opens the door to low-cost wireless positioning at millimeter accuracy.
In this paper, a novel underdetermined blind source separation algorithm guided by particle swarm optimizer (PSO) is proposed for adjacent satellite interference in modern satellite communication systems. Different from traditional methods, we formulate the separation problem as clustering problem. Due to our algorithm is affected by the sparsity of source signals and the density of mixed vectors, our algorithm is motivated by the assumption is held that the distance between two arbitrary mixed signal vectors is less than the doubled sum of variances of distribution of the corresponding mixtures. In our method, we accomplish the underdetermined blind source separation by computing the Short Time Fourier Transform (STFT) to segment received mixtures and we use some estimates to separate the mixed source signals by PSO where the number of the mixed signals is unknown. In PSO, we define new parameters gather in formula (8) and cj in formula (11). We verify the proposed method on several simulations. The experimental results demonstrate the effectiveness of the proposed method.
Undetectable data corruption in wireless communication links can occur in some cases, including cases in which a ciphering algorithm can fail to generate the same keystream for encryption at a receiver side as it does for a sender side. In wireless speech communication, such corruption might trigger an explosion of decoded sound, which might in turn hurt a user’s ear. This article presents a simple algorithm for detecting corruption for a typical code-excited linear-prediction (CELP) speech decoder. The algorithm requires no additional bits for corruption detection, but detects it by monitoring the amplitude ratio of decoded adaptive and fixed excitation signals in CELP. Its practicality is demonstrated by simulation.
This paper presents a novel design for a two-element stub-loaded dipole array antenna. The purpose is to obtain the decoupling and orthogonal polarization characteristics using a small antenna configuration. A quarter-wavelength stub loaded on one side of the two elements is operated as a phase shifter to create the opposite phase of current distribution. Moreover, the quarter-wavelength stub works as an orthogonal dipole antenna because the in-phase currents are enhanced on the stub, leading to orthogonal directivity. The results show that well-defined isolation and orthogonal polarization directivity can be achieved using the proposed antenna.
We develop a simple temperature sensing using an optical time domain reflectometer (OTDR) and a mechanical long-period fiber grating (LPFG) fabricated from a heat-shrinkable tube and a metric screw. The resonance loss of the fabricated LPFG is strongly dependent on temperature as temperature-dependent heat-shrinkage of the tube produces a change in the pressure applied to the fiber. In our proposed temperature sensor, the LPFG insertion loss due to a change in the pressure applied to the fiber is measured using an OTDR with a resonance wavelength. Both single-point and multi-point temperature sensing is successfully demonstrated experimentally.