We explore and find the fundamental systemdegrading effects when using multiple orbital-angular-momentum (OAM) beams in a THz communications link under atmospheric turbulence in simulation. Unlike optical links with relatively small divergence effects, the crosstalk performance of THz OAM links is dependent on divergence-related parameters, including OAM mode order, frequency, and beam waist. Simulation results show: (i) for the cases with the same ratio of beam diameter to the Fried parameter (D/r0), the signal power increases and the crosstalk (XT) decreases when increasing the divergence-related parameters; and (ii) for the cases with the same atmospheric structure constant Cn2, the signal power decreases and the XT increases when increasing the divergence-related parameters. Moreover, for building a link where OAM +4 is transmitted with the parameters: (i) beam waist of 0.1 m and link distance of 200 m, and (ii) beam waist of 1 m and link distance of 1 km, the XT from neighbouring mode remains less than -15 dB when carrier wave frequency is <; 1 THz and 0.1 THz, respectively. In addition, simulation results also show that: (i) limited aperture size of the system has high influence on the XT performance under both weak and strong turbulence; and (ii) displacement of the system has high influence on the XT performance under no and weak turbulence.

Line-of-sight wireless communications can benefit from the simultaneous transmission of multiple independent data streams through the same medium in order to increase system capacity. A common approach is to use conventional spatial multiplexing with spatially separated transmitter/receiver antennae, for which inter-channel crosstalk is reduced by employing multiple-input-multiple-output (MIMO) signal processing at the receivers. Another fairly recent approach to transmitting multiple data streams is to use orbital-angular-momentum (OAM) multiplexing, which employs the orthogonality among OAM beams to minimize inter-channel crosstalk and enable efficient (de)multiplexing. In this paper, we explore the potential of utilizing both of these multiplexing techniques to provide system design flexibility and performance enhancement. We demonstrate a 16 Gbit/s millimeter-wave link using OAM multiplexing combined with conventional spatial multiplexing over a short link distance of 1.8 meters (shorter than Rayleigh distance). Specifically, we implement a spatial multiplexing system with a 2 × 2 antenna aperture architecture, in which each transmitter aperture contains two multiplexed 4 Gbit/s data-carrying OAM beams. A MIMO-based signal processing is used at the receiver to mitigate channel interference. Our experimental results show performance improvements for all channels after MIMO processing, with bit-error rates of each channel below the forward error correction limit of 3.8 × 10-3. We also simulate the capacity for both the 4 × 4 MIMO system and the 2 × 2 MIMO with OAM multiplexing. Our work indicates that OAM multiplexing and conventional spatial multiplexing can be simultaneously utilized to provide design flexibility. The combination of these two approaches can potentially enhance system capacity given a fixed aperture area of the transmitter/receiver (when the link distance is within a few Rayleigh distances).

In this paper, the generation and steering of beams carrying orbital angular momentum utilizing a custom-designed circular antenna array has been demonstrated at 28 GHz. A steering angle as large as 30 degrees for an orbital angular momentum (OAM) beam has been achieved. The effect of number of antennas and the distance from antennas to the array center to the quality of beam generation and beam steering is investigated through both experiments and simulations. Our results indicate that: (1) As the steering angle increases, the mode purity of the generated OAM beams decreases; (2) Increasing the number of antennas improves the OAM mode purity; (3) For a fixed number of antennas, high mode purity is observed for lower order OAM modes; (4) Placing the antennas farther away from the array center allows for reduced divergence of the generate OAM beams.

This paper reports an experimental demonstration of a 32-Gbit/s wireless link using orbital angular momentum (OAM) and polarization multiplexing in a millimeter-wave regime at 60 GHz. Results of the analysis show that a higher carrier frequency reduces the propagation loss as well as the size of the transmitter and receiver, particularly for OAM channels with higher OAM values. Further, two different OAM channels (with l = +1 and l = +3) on each of the two polarizations are spatially multiplexed, and each channel carries 2-Gbaud signals with 16-QAM modulation. Spiral phase plates are used to generate 60-GHz OAM beams. The simulation results show that a higher carrier frequency plays a more significant role in reducing both the aperture size and the transmission loss for channels with higher OAM values. The bit error rates (BERs) of 4 channels are measured, and the raw BERs are found to be less than 3.8 × 10−3

We present a short-range experimental orbital-angular-momentum (OAM) multiplexing millimeter-wave communication system using patch antenna arrays. The dependence of the evolution of OAM beams on the number of array elements and array radius are analyzed. We also find phase delay deviation of 30 degrees leads to mode purity degrading to ∼1% and power deviation of 6 dB reduces the purity by <40%. An obstruction of size <10mm has not much influence on the evolution of OAM beams generated from antenna arrays with r = 8mm, which enables the stacking of multiple antenna arrays. We also use these patch antenna arrays to demonstrate a 60 GHz wireless communication link using two multiplexed OAM modes, each carrying a 500-Mbaud 16-QAM or 2-Gbaud QPSK signal. A channel crosstalk of less than −20 dB and bit-error-rates (BER) of less than 3.8 × 10−3 are achieved.

This paper reviews the topic of true time delay (TTD) and its importance and implications in wideband imaging phased arrays. The paper describes an early analog version of TTD using the “Rotman lens,” discusses various architectures, and talks about photonic implementations. Also emphasized are the differences in design between narrowband arrays which are calibrated at individual frequencies and wideband imaging arrays where TTD is necessary.

This paper reports on an experimental measurement and analysis of multipath-induced intra- and interchannel crosstalk effects in a mm-wave communications link using orbital angular momentum multiplexing at 28 GHz. The reflection is from an ideal reflector parallel to the propagation path. The intra-channel crosstalk effect is measured when a single OAM beam is transmitted, and inter-channel crosstalk effect is measured when 2 multiplexed OAM beams are transmitted. Both simulation and experimental results show that OAM channels with larger OAM number ℓ tend to have stronger intra-channel crosstalk because less power is received from the direct path and more power is received from the reflected path. This effect is caused by OAM beam divergence, as OAM beams with larger ℓ spread into a larger beam size and have less power in the beam center. For the same reason, OAM beams of larger ℓ lead to stronger inter-channel crosstalk with the other OAM channels.

We present the design and performance characterization of a thin metamaterial plate for generation of orbital angular momentum (OAM) modes of a millimeter-wave beam, which can carry independent data streams over the same physical medium. The plate has a thickness of 1.56 mm, and consists of 3.06 × 0.68 mm rectangular apertures with spatial variant orientations. It generates OAM beams l = +1 and l = +3 with mode purity larger than 77.5% over a bandwidth of 6 GHz (25-31 GHz). We then use these streams to experimentally demonstrate a 16-Gbit/s millimeter-wave wireless communications link using two multiplexed OAM modes, each carrying a 2-Gbaud 16-QAM signal. A channel crosstalk less than -20 dB over a bandwidth of 4 GHz (26-30 GHz) and biterror-rates (BER) less than 3.8 × 10-3are achieved.

This paper reports an experimental demonstration of a 16 Gbit/s millimeter-wave communication link using MIMO processing of 2 OAM modes on each of two transmitter/receiver antenna apertures. Two groups of multiplexed OAM beams, each containing OAM beams of ℓ =1 and +3 are generated and transmitted through two transmitter apertures respectively. The two transmitter apertures are separated with a certain distance such that the two groups of OAM beams are spatially overlapping at the receiver aperture plane. Each channel carries 1-GBaud 16-QAM signals at the same carrier frequency of 28 GHz. Our experimental results show that MIMO equalization processing can help mitigate the interferences from the other OAM channels and the BER performance of each channel improves significantly after MIMO processing. Our results indicate that OAM multiplexing and traditional spatial multiplexing combined with MIMO processing can be compatible and complementary with each other.

This paper reports an experimental demonstration of modal multiplexing using orbital angular momenta (OAM) in the millimeter-wave regime (28 GHz). Using 1 GBaud/s signals with 16-QAM modulation, a data rate of 32 Gbit/s is obtained by employing 4 different OAM modes (l= -3,-1,+1 and +3) on two orthogonal linear polarization states. Without any digital post-processing, the receiver can separate the different data streams since crosstalk among the OAM channels is low: measured to be better than -23dB with a 28 GHz narrowband carrier on a single polarization, and better than -16dB on both polarization states. The bit-error rate (BER) of 8 channels is measured, and found the raw BER is below 3.8×10-3, thus allowing efficient use of forward error correction (FEC) codes.

A code acquisition technique based on active optical correlation detection is introduced for coherent optical pulse code-division multiple-access (CDMA) systems implemented through coherent correlation demodulation. Passive matched filter acquisition, commonplace in most radio CDMA systems, does not translate directly into the optical domain since synchronization between the incoming optical signal and a local optical code of the intended user at the receiver has to take place in several picosecond duration. Parallel and serial acquisition schemes under the same search algorithm are described and the effect of optical device noise, partial autocorrelation, and multiuser interference on acquisition performance is theoretically analyzed. Numerical results are given for Gold codes. In general, the degree of parallelism inherent in code acquisition techniques for radio systems allows a tradeoff between acquisition speed and hardware cost; however, in optical acquisition schemes, increasing the degree of parallelism with the concomitant hardware cost penalty does not necessarily reduce the time to acquisition. This is due to optical power budget limitations brought about by the need to split the optical signal, although optical amplifier technology can improve the performance of these parallel search schemes. The expressions for the partial correlation and multiuser interference derived in the paper can also be utilized for the analysis of other radio and optical acquisition schemes.

A few thousand far-ultraviolet images of the ionosphere have been obtained since December 1986 by the Atmospheric/Ionospheric Remote Sensor (AIRS) aboard the Polar BEAR satellite. Fast algorithms for applying automated satellite-attitude, geometric, and photometric corrections to these images were developed, and the first results are discussed. A software package that is based on these algorithms was implemented and tested. Since no attitude data is available at night (one of the two gauges is a Sun sensor), an algorithm was devised to extrapolate daytime attitude corrections to nighttime. An additional method is offered to correct satellite roll from the measured or extrapolated one based on the dayglow limbs observed at the edges of the image. Geometric rectifications include transformations to the image from the satellite coordinate system to a reference system, and from the reference system to the geographic coordinate system. Radiometric corrections include image enhancement, background airglow subtraction, and off-nadir normalization of auroral emissions. The latter two are based on theoretical calculations of column emission rates, derived by radiative transfer integrations of volume emission rates along the look direction. Results include mapping of auroral arcs in the corrected geomagnetic system, the one most applicable to auroral studies.>

The authors discuss system aspects and describe experimental demonstrations of nanosecond wavelength tuning in laser diode structures. Both tunable transmitters and tunable filters are considered. The dependence of the refractive index on the carrier density in semiconductors is exploited to obtain fast wavelength tuning. Experimentally, switching times of>