Multiple-input multiple-output (MIMO) technology is maturing and is being incorporated into emerging wireless broadband standards like long-term evolution (LTE) [1]. For example, the LTE standard allows for up to eight antenna ports at the base station. Basically, the more antennas the transmitter/receiver is equipped with, and the more degrees of freedom that the propagation channel can provide, the better the performance in terms of data rate or link reliability. More precisely, on a quasi static channel where a code word spans across only one time and frequency coherence interval, the reliability of a point-to-point MIMO link scales according to Prob(link outage) ` SNR-ntnr where nt and nr are the numbers of transmit and receive antennas, respectively, and signal-to-noise ratio is denoted by SNR. On a channel that varies rapidly as a function of time and frequency, and where circumstances permit coding across many channel coherence intervals, the achievable rate scales as min(nt, nr) log(1 + SNR). The gains in multiuser systems are even more impressive, because such systems offer the possibility to transmit simultaneously to several users and the flexibility to select what users to schedule for reception at any given point in time [2].

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Scaling Up MIMO: Opportunities and Challenges with Very Large Arrays

This paper presents the expected transmission count metric (ETX), which finds high-throughput paths on multi-hop wireless networks. ETX minimizes the expected total number of packet transmissions (including retransmissions) required to successfully deliver a packet to the ultimate destination. The ETX metric incorporates the effects of link loss ratios, asymmetry in the loss ratios between the two directions of each link, and interference among the successive links of a path. In contrast, the minimum hop-count metric chooses arbitrarily among the different paths of the same minimum length, regardless of the often large differences in throughput among those paths, and ignoring the possibility that a longer path might offer higher throughput.This paper describes the design and implementation of ETX as a metric for the DSDV and DSR routing protocols, as well as modifications to DSDV and DSR which allow them to use ETX. Measurements taken from a 29-node 802.11b test-bed demonstrate the poor performance of minimum hop-count, illustrate the causes of that poor performance, and confirm that ETX improves performance. For long paths the throughput improvement is often a factor of two or more, suggesting that ETX will become more useful as networks grow larger and paths become longer.

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A high-throughput path metric for multi-hop wireless routing

This paper surveys recent advances in the area of very large MIMO systems. 
With very large MIMO, we think of systems that use antenna arrays with an order of magnitude more elements than in systems being built today, say a hundred antennas or more. Very large MIMO entails an unprecedented number of antennas simultaneously serving a much smaller number of terminals. The disparity in number emerges as a desirable operating condition and a practical one as well. The number of terminals that can be simultaneously served is limited, not by the number of antennas, but rather by our inability to acquire channel-state information for an unlimited number of terminals. Larger numbers of terminals can always be accommodated by combining very large MIMO technology with conventional time- and frequency-division multiplexing via OFDM. Very large MIMO arrays is a new research field both in communication theory, propagation, and electronics and represents a paradigm shift in the way of thinking both with regards to theory, systems and implementation. The ultimate vision of very large MIMO systems is that the antenna array would consist of small active antenna units, plugged into an (optical) fieldbus.

/pdf/scaling-up-mimo-opportunities-and-challenges-with-very-large-1gai688763.pdf

Scaling up MIMO: Opportunities and Challenges with Very Large Arrays

In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.

Ferromagnetic materials

Wireless vehicular communication has the potential to enable a host of new applications, the most important of which are a class of safety applications that can prevent collisions and save thousands of lives. The automotive industry is working to develop the dedicated short-range communication (DSRC) technology, for use in vehicle-to-vehicle and vehicle-to-roadside communication. The effectiveness of this technology is highly dependent on cooperative standards for interoperability. This paper explains the content and status of the DSRC standards being developed for deployment in the United States. Included in the discussion are the IEEE 802.11p amendment for wireless access in vehicular environments (WAVE), the IEEE 1609.2, 1609.3, and 1609.4 standards for Security, Network Services and Multi-Channel Operation, the SAE J2735 Message Set Dictionary, and the emerging SAE J2945.1 Communication Minimum Performance Requirements standard. The paper shows how these standards fit together to provide a comprehensive solution for DSRC. Most of the key standards are either recently published or expected to be completed in the coming year. A reader will gain a thorough understanding of DSRC technology for vehicular communication, including insights into why specific technical solutions are being adopted, and key challenges remaining for successful DSRC deployment. The U.S. Department of Transportation is planning to decide in 2013 whether to require DSRC equipment in new vehicles.

Dedicated Short-Range Communications (DSRC) Standards in the United States

We report the observation of an anomalously large electro‐optic modulation effect in potassium titanyl phosphate (KTiOPO4) waveguides fabricated under various conditions. An interferometric method was used to measure the effective electro‐optic coefficient in the waveguides by measuring the phase retardation of an optical mode when a voltage was applied across the substrate. We observed that at low modulation frequency the effective electro‐optic coefficient in the waveguides can be higher than the bulk value by a factor up to about 100. A space charge model relating the mobile charge, space charge field, and observed enhancement in the electro‐optic modulation is proposed to explain the mechanism of this effect. Calculation for dc steady state, small signal analysis, and large signal simulation is compared to the experimental observation. Possible device application and long term effects on system operation are also discussed.

Large electro‐optic modulation effect observed in ion‐exchanged KTiOPO4 waveguides

The use of sputtered thin-film permanent magnets for biasing microwave devices in planar geometries compatible with monolithic integration is considered. Possible geometries are presented for biasing a reciprocal microstrip phaser at 7 GHz, a stripline circulator at 15 GHz, and a magnetostatic wave delay line at 3 GHz. Yttrium iron garnet (YIG) is assumed to be the ferrite used in each of these devices. It is concluded that the in-plane magnetization orientation is more useful than normal magnetization, and thick films (100-150 μm) are needed with high magnetization ( > 16 kG) and moderate coercivity ( > 2 kOe). Below-resonance devices are preferable because of their modest field requirements and tolerance to bias field inhomogeneities.

Thin-film permanent magnet requirements for magnetic devices in MMIC†

The ability to perform a variety of optical signal processing tasks on a single substrate has been the focus of much research. Of particular interest is our ability to build optical modulators using garnet thin films. Gigahertz bandwidths using magnetostatic wave-optical devices [I] md a high frequency modulation by oscillating periodic domain walls [2] have previously been reported. We have observed the modulation of the magneto-optic interaction in a bismuth-lutetium-iron garnet film over a frequency range of 3GHz [3]. Described below are the results of recent experiments conducted on this wideband interaction, illuminating some of the basic device physics. A microwave signal was applied to a microstrip transducer placed in contact with a [BiLu]3Fe5012 film. TM polarized optical guided modes were excited directly beneath the transducer and the intensity of the TE polarized optical beam was measured at the output. Fig, l(a) shows the -3GHz bandwidth that was obtained by this method. The bandwidth was limited in our experiment by the transmission characteristics of the external microwave components (the limiting component being a MGHz circulator that was inserted before the transducer). We tried to observe the magnetic domain walls in the film using an optical microscope with crossed polarizers. The domain walls were not visible suggesting that the magnetic domains were largely in the plane of the film. Studies were then conducted in the presence of a weak (-100G) in-plane external magnetic field. The TM-+TE mode conversion was measured as a function of the in-plane angle, 0, between the magnetic field and the direction perpendicular to the optical beam. The external field was generated by two perpendicularly oriented electromagnetic coils with independent power supplies. By stepping the current in each coil we obtain discrete values of 0 and [HI at which we measure the output optical intensity. The intensity is shown as a function of 0 (for all values of IHI) in Fig. l(b). Our experimental observations qualitatively agree with a simple theoretical model of modulation of the optical beam due to a non-resonant precession of magnetic domains about an equilibrium direction. According to the model, the square of the small signal magnetization, and thus the output optical intensity, has a cos' 0 dependence as we tilt the static magnetization. The intensity is a maximum when the static magnetization is perpendicular to the optical beam (0 = 0). This cos2 0 dependence is shown by the dashed curve in Fig. l(b). The overall envelope of the experimental data is qualitatively consistent with the theoretical model. Two unexpected characteristics that appear are a grouping of data into regions depending on the

Wideband Modulation Of The Magneto - Optic Interaction In A Bismuth-Lutetium-Iron Garnet Film

Magnetostatic backward volume wave (MSBVW) delay measurements are reported on a 4.8 µm thick YIG film ion implanted at a low dose (3× 1015ions/cm2) by a 320 keV beam of single ionized helium atoms ( He+). Nonuniform magnetization and uniaxial anisotropy profiles are used to model the ion implanted film. Calculated results fit the experimental data well. Expressions for the bias permeability tensor elements excluding crystalline (cubic) anisotropy are presented. A method is proposed to better characterize the magnetic properties of ferrimagnetic thin films. This method is applied to separate the uniaxial anisotropy field from the magnetization.

Magnetostatic backward waves in low dose ion implanted YIG films

A magnetostatic surface wave ring resonator is described which employs a new transducer configuration for exciting a single resonant mode. The configuration consists of one three‐element grating array and one three‐element meander array. The predominant resonance is 10 dB above adjacent resonances and exhibits a loaded Q as high as 890 with 13 dB insertion loss at approximately 1.86 GHz.The device is tunable over the range of 1.5–2.1 GHz, although suppression of unwanted resonances is reduced near the edges of this range.

Daniel D. Stancil

Papers

Large electro‐optic modulation effect observed in ion‐exchanged KTiOPO4 waveguides

Thin-film permanent magnet requirements for magnetic devices in MMIC†

Wideband Modulation Of The Magneto - Optic Interaction In A Bismuth-Lutetium-Iron Garnet Film

Magnetostatic backward waves in low dose ion implanted YIG films

Magnetostatic wave ring resonator exhibiting a single resonance