George S. D. Gordon

Feasibility Demonstration of a Mode-Division Multiplexed MIMO-Enabled Radio-Over-Fiber Distributed Antenna System

In this paper the feasibility of mode-division multiplexing (MDM) for implementing multiple-input multiple-output (MIMO) radio-over-fiber (RoF) distributed antenna systems (DAS) is experimentally demonstrated, where the MIMO algorithm is able to reconstruct the data signals by overcoming distortion and crosstalk in both the free-space RF and optical fiber channels. This is achieved through RF characterization of an MDM RoF link, which is experimentally demonstrated to be capable of supporting at least 4 × 4 MIMO with 6 GHz channels over long lengths of MMF (2 km compared to ~300 m typically found in MMF-based RoF DASs) and under tight fiber bending conditions (bend radius as low as 7.6 mm), which are commonly encountered in in-building fiber installations. First, experimental RF measurements are performed over a 2 km section of OM2 fiber, mode-multiplexed using a spatial light modulator (SLM) based MUX/DEMUX system, and it is shown to offer an RF condition number of <;14 dB for up to 4 × 4 MIMO with 6 GHz channels, sufficient to enable good performance for⊣ most modern wireless protocols. Next, the effect of fiber curvature on RF performance is experimentally analysed using a 10 m section of OM3 fiber. It is seen that with a bend radius as low as 7.2 mm, a condition number of ~10 dB can be achieved for up to a 6 × 6 MIMO system with 6 GHz channels. Although an SLM-based MUX/DEMUX is used here, condition number is not dependent on the exact mode-launching system used provided that orthogonal combinations of modes are excited, which is true of most MDM systems. It is therefore concluded that the RF characterization presented here demonstrates by proof-of-principle the feasibility of graded-index) MMF to support at least 4 × 4 MIMO with broadband channels via MDM over lengths up to 2 km and with fiber bend radii as low as 7.2 mm.

High-Order Distortion in Directly Modulated Semiconductor Lasers in High-Loss Analog Optical Links With Large RF Dynamic Range

A theoretical and experimental investigation of the effects of high-order nonlinear distortion products produced by directly modulated semiconductor lasers on the performance of high-loss analog optical communication links requiring large RF dynamic range is reported. In order to provide sufficient RF dynamic range to support radio services in links with high optical transmission loss, for example in radio over free-space optics (RoFSO), while keeping costs low, it is necessary to use directly modulated lasers. However, in these applications the lasers must be driven to high modulation depths to maximize dynamic range. Simulations show that under these unique conditions the first detectable nonlinear distortion is often the result of dynamic distortion due to the laser being driven near threshold. It is shown that this type of distortion is characterized by a sharp increase in the contribution of high-order (fourth order or greater) nonlinear terms resulting from the influence of laser relaxation oscillations. As a consequence, the third-order spurious-free dynamic range (SFDR) metric no longer accurately reflects the performance of such links as it assumes that third order effects are dominant. An alternative measure of dynamic range called dynamic-distortion-free dynamic range (DDFDR) is proposed. This differs in that the upper limit is defined as the modulating power at which the peak optical modulation index (OMI) reaches unity. At this point the error vector magnitude (EVM) measured for a range of different wireless services starts to increase rapidly due to high order distortion. This makes DDFDR a practical, service-independent metric of dynamic range. For two different wireless services it is observed experimentally that on average the DDFDR upper limit predicts the EVM knee point to within 1.1 dB, while the third-order SFDR predicts it to within 6.2 dB. The DDFDR is thus shown to be a more accurate indicator of real link performance when high-order distortion is dominant.

Experimental Evaluation of Layout Designs for 3

This paper experimentally demonstrates that, for two representative indoor distributed antenna system (DAS) scenarios, existing radio-over-fiber (RoF) DAS installations can enhance the capacity advantages of broadband 3 × 3 multiple-input-multiple-output (MIMO) radio services without requiring additional fibers or multiplexing schemes. This is true for both single- and multiple-user cases with a single base station and multiple base stations. First, a theoretical example is used to illustrate that there is a negligible improvement in signal-to-noise ratio (SNR) when using a MIMO DAS with all N spatial streams replicated at N RAUs, compared with a MIMO DAS with only one of the N streams replicated at each RAU for N ≤ 4. It is then experimentally confirmed that a 3 × 3 MIMO DAS offers improved capacity and throughput compared with a 3 × 3 MIMO collocated antenna system (CAS) for the single-user case in two typical indoor DAS scenarios, i.e., one with significant line-of-sight (LOS) propagation and the other with entirely non-line-of-sight (NLOS) propagation. The improvement in capacity is 3.2% and 4.1%, respectively. Then, experimental channel measurements confirm that there is a negligible capacity increase in the 3 × 3 configuration with three spatial streams per antenna unit over the 3 × 3 configuration with a single spatial stream per antenna unit. The former layout is observed to provide an increase of ~ 1% in the median channel capacity in both the single- and multiple-user scenarios. With 20 users and three base stations, a MIMO DAS using the latter layout offers median aggregate capacities of 259 and 233 bit/s/Hz for the LOS and NLOS scenarios, respectively. It is concluded that DAS installations can further enhance the capacity offered to multiple users by multiple 3 × 3 MIMO-enabled base stations. Further, designing future DAS systems to support broadband 3 × 3 MIMO systems may not require significant upgrades to existing installations for small numbers of spatial streams.

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We demonstrate mode-group division multiplexing over 100m graded-index ring-core fiber supporting 4 LP mode-groups with a single radial index using SLM-based mode (de)multiplexers to transmit 2×10Gbps NRZ signals without MIMO equalization.

3 MIMO-Enabled Radio-Over-Fiber Distributed Antenna Systems

We demonstrate a mode division multiplexing (MDM) system over an 8 km conventional graded index multimode fiber. Spatial light modulators (SLMs) are used to multiplex and demultiplex three linearly polarized (LP) modes (LP01, LP11a, and LP11b) in two polarizations. A 6 × 6 sparse frequency domain equalizer (FDE) is used as the channel impulse response of the SLM-based MDM system is found to be sparse due to the large crosstalk at the mode MUX/DEMUX and small coupling in the fiber. The signal transmitted on each mode is recovered with improved performance over conventional FDEs. The results indicate that this system can be used in short reach transmission applications to increase the system capacity.

All-optical mode-group division multiplexing over a graded-index ring-core fiber with single radial mode

MIMO DSP is employed to improve the performance of degenerate mode-group division multiplexing in 8 km of conventional GI-MMF. Compensation of the mode coupling, induced by the launch and propagation, between and inside each degenerate mode-group is investigated in order to reduce the DSP complexity.

SLM-Based Mode Division Multiplexing System With

An optical fiber with weak mode coupling is desirable for future ultrahigh capacity space-division multiplexing (SDM) systems because mode coupling in an optical fiber results in extrinsic loss of the fiber and crosstalk between guided optical modes. To study the feasibility of a ring-core fiber (RCF) for SDM systems, in this paper, we investigate the mode coupling in the RCF supporting five or seven guided mode groups (MGs) at a wavelength of 1550 nm. For this purpose, the coupled mode/power theory with identified spatial power spectrum of random perturbations of fiber axis is used to estimate the bend loss/crosstalk of the RCF due to microbending. It is shown that based on the identified parameters for the spatial power spectrum in the 5/7-MG RCF, the estimated bend loss/crosstalk of the RCF agrees well with experimental results. In addition, the impact of the gradient parameter α and refractive index contrast Δ of the fiber refractive index profile on bend loss and crosstalk of the RCF is explored. Simulation results indicate that the Δ instead of the α significantly affects bend loss and crosstalk of the RCF. The magnitude improvement in bend loss by increasing the Δ is dependent on the spatial power spectrum.

6 \times 6

This paper examines the impact of two simple precoding schemes on the capacity of 3 × 3 MIMO-enabled radio-over-fiber (RoF) distributed antenna systems (DAS) with excess transmit antennas. Specifically, phase-shift-only transmit beamforming and antenna selection are compared. It is found that for two typical indoor propagation scenarios, both strategies offer double the capacity gain that non-precoding MIMO DAS offers over traditional MIMO collocated antenna systems (CAS), with capacity improvements of 3.2-4.2 bit/s/Hz. Further, antenna selection shows similar performance to phase-only beamforming, differing by <;0.5% and offering median capacities of 94 bit/s/Hz and 82 bit/s/Hz in the two propagation scenarios respectively. Because optical DASs enable precise, centralized control of remote antennas, they are well suited for implementing these beamforming schemes. Antenna selection, in particular, is a simple and effective means of increasing MIMO DAS capacity.

Sparse Equalization

A method of generating an aberration- and distortion-free wide-angle holographically projected image in real time is presented. The target projector is first calibrated using an automated adaptive-optical mechanism. The calibration parameters are then fed into the hologram generation program, which applies a novel piece-wise aberration correction algorithm. The method is found to offer hologram generation times up to three orders of magnitude faster than the standard method. A projection of an aberration- and distortion-free image with a field of view of 90x45 degrees is demonstrated. The implementation on a mid-range GPU achieves high resolution at a frame rate up to 12fps. The presented methods are automated and can be performed on any holographic projector.

Degenerate mode-group division multiplexing using MIMO digital signal processing

High-energy electron beam lithography for patterning nanostructures on insulating substrates can be challenging. For high resolution, conventional resists require large exposure doses and for reasonable throughput, using typical beam currents leads to charge dissipation problems. Here, we use UV1116 photoresist (Dow Chemical Company), designed for photolithographic technologies, with a relatively low area dose at a standard operating current (80 kV, 40-50 μC cm-2, 1 nAs-1) to pattern over large areas on commercially coated ITO-glass cover slips. The minimum linewidth fabricated was ∼33 nm with 80 nm spacing; for isolated structures, ∼45 nm structural width with 50 nm separation. Due to the low beam dose, and nA current, throughput is high. This work highlights the use of UV1116 photoresist as an alternative to conventional e-beam resists on insulating substrates. To evaluate suitability, we fabricate a range of transmissive optical devices, that could find application for customized wire-grid polarisers and spectral filters for imaging, which operate based on the excitation of surface plasmon polaritons in nanosized geometries, with arrays encompassing areas ∼0.25 cm2.