Design of FIR digital filters for pulse shaping and channel equalization using time-domain optimization

Three algorithms are developed for designing finite impulse response digital filters to be used for pulse shaping and channel equalization. The first is the Minimax algorithm which uses linear programming to design a frequency-sampling filter with a pulse shape that approximates the specification in a minimax sense. Design examples are included which accurately approximate a specified impulse response with a maximum error of 0.03 using only six resonators. The second algorithm is an extension of the Minimax algorithm to design preset equalizers for channels with known impulse responses. Both transversal and frequency-sampling equalizer structures are designed to produce a minimax approximation of a specified channel output waveform. Examples of these designs are compared as to the accuracy of the approximation, the resultant intersymbol interference (ISI), and the required transmitted energy. While the transversal designs are slightly more accurate, the frequency-sampling designs using six resonators have smaller ISI and energy values

Time-Frequency Warped Waveforms

The forthcoming communication systems are advancing towards improved flexibility in various aspects. Improved flexibility is crucial to cater diverse service requirements. This letter proposes a novel waveform design scheme that exploits axis warping to enable peaceful coexistence of different pulse shapes. A warping transform manipulates the lattice samples non-uniformly and provides flexibility to handle the time-frequency occupancy of a signal. The proposed approach enables the utilization of flexible pulse shapes in a quasi-orthogonal manner and increases the spectral efficiency. In addition, the rectangular resource block structure, which assists an efficient resource allocation, is preserved with the warped waveform design as well.Comment: 4 pages, 5 figures; accepted version (The URL for the final version: http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=8540914&isnumber=8605392

Spatial Multiplexing of QPSK Signals with a Single Radio: Antenna Design and Over-the-Air Experiments

The paper describes the implementation and performance analysis of the first fully-operational beam-space MIMO antenna for the spatial multiplexing of two QPSK streams. The antenna is composed of a planar three-port radiator with two varactor diodes terminating the passive ports. Pattern reconfiguration is used to encode the MIMO information onto orthogonal virtual basis patterns in the far-field. A measurement campaign was conducted to compare the performance of the beam-space MIMO system with a conventional 2-by-?2 MIMO system under realistic propagation conditions. Propagation measurements were conducted for both systems and the mutual information and symbol error rates were estimated from Monte-Carlo simulations over the measured channel matrices. The results show the beam-space MIMO system and the conventional MIMO system exhibit similar finite-constellation capacity and error performance in NLOS scenarios when there is sufficient scattering in the channel. In comparison, in LOS channels, the capacity performance is observed to depend on the relative polarization of the receiving antennas.Comment: 31 pages, 23 figure

Flip-OFDM for Optical Wireless Communications

We consider two uniploar OFDM techniques for optical wireless communications: asymmetric clipped optical OFDM (ACO-OFDM) and Flip-OFDM. Both techniques can be used to compensate multipath distortion effects in optical wireless channels. However, ACO-OFDM has been widely studied in the literature, while the performance of Flip-OFDM has never been investigated. In this paper, we conduct the performance analysis of Flip-OFDM and propose additional modification to the original scheme in order to compare the performance of both techniques. Finally, it is shown by simulation that both techniques have the same performance but different hardware complexities. In particular, for slow fading channels, Flip-OFDM offers 50% saving in hardware complexity over ACO-OFDM at the receiver.Comment: published in IEEE Information Theory Workshop, Paraty Brazil, Sept 201

Optical Space Division Multiplexing in Short Reach Multi-Mode Fiber Systems

The application of space division multiplexing to fiber-optic communications is a promising approach to further increase the channel capacity of optical waveguides. In this work, short reach and low-cost optical space division multiplexing systems with intensity modulation and direct detection (IM/DD) are in the focus of interest. Herein, different modes are utilized to generate spatial diversity in a multi-mode fiber. In such IM/DD systems, the process of square-law detection is inherently non-linear. In order to obtain an understanding of the channel characteristics, a system model is developed, which is able to show under which conditions the system can be considered linear in baseband. It is shown that linearity applies in scenarios with low mode cross-talk. This enables the use of linear multiple-input multiple-output (MIMO) signal processing strategies for equalization purposes. In conditions with high mode cross-talk, significant interference occurs, and the transmitted information cannot be extracted at the receiver. Furthermore, a method to determine the power coupling coefficients between mode groups is presented that does not require the excitation of individual modes, and hence it can be realized with inexpensive components. In addition, different optical components are analyzed with respect for their suitability in MIMO setups with IM/DD. The conventional approach with single-mode fiber to multi-mode fiber offset launches and optical couplers as well as a configuration that utilizes multi-segment detection are feasible options for a (2x2) setup. It is further shown that conventional photonic lanterns are not suited for MIMO with IM/DD due to their low mode orthogonality during the multiplexing process. In order to enable higher order MIMO configurations, devices for mode multiplexing and demultiplexing need to be developed, which exhibit a high mode orthogonality on one hand and are low-cost on the other hand

Fiber link design considerations for cloud-Radio Access Networks

Analog radio over fiber (RoF) links may offer advantages for cloud-Radio Access Networks in terms of component cost, but the behavior of the distortion with large numbers of subcarriers needs to be understood. In this paper, this is presented in terms of the variation between subcarriers. Memory polynomial predistortion is also shown to compensate for RoF and wireless path distortion. Whether for digitized or analog links, it is shown that appropriate framing structure parameters must be used to assure performance, especially of time-division duplex systems