Monthly Archives: May 2014
Current Trends
Besides research in ground – and foliagepenetrating radar technologies, significant research is also conducted in the development of, socalled, spacetime adaptive processing (STAP) algorithms. STAP refers to multidimensional adaptive filtering algorithms that simultaneously combine the signals from the elements of an array antenna and the multiple pulses of a coherent radar waveform. STAP can improve the detection of lowvelocity targets obscured by mainlobe clutter, detection of targets masked by sidelobe clutter, and detection in combined clutter and jamming environments.
Significant research is also conducted into the use of signal processing tools other than the traditional Fourier transformbased ones for target detection and recognition. Such tools are, for example, based on the theories of, socalled, waveletinduced multiresolution analyses (WIMA) of signals. A WIMA allows for the decomposition and simultaneous representation of a signal in time and scale and, therefore, is capable of processing signals at different scales. WIMAbased radar target detection and recognition is being actively researched.
StandingWave Arrays
For the resonant array, standingwave fields are created in the waveguide by terminating the waveguide with a metal wall to create a short circuit. The slot locations relative to the short circuit are chosen to maximize the array radiation coupling. For series elements, such as the angled series slot, the slots are located multiples of a half guide wavelength from the short circuit. For shunt elements, such as the offset – shunt slot, the maximum coupling locations are multiples of a half guide wavelength from an open circuit which is achieved by locating a short circuit at onequarter guide wavelength from the nearest slot. For broadside operation the slots are located in the guide to achieve a uniform phase distribution. A uniform phase distribution is achieved by locating the slots at one guidewavelength intervals. However this results in an array spacing greater than one freespace wavelength which creates multiple main beams and reduces the antenna gain and performance. The spacing can be reduced to onehalf guidewavelength intervals by alternating the slot offsets to compensate for the 180° phase reversal in the guide.
Circuit Model for Resonant Linear Array. A circuit model of the array provides a good starting point for predicting the array performance and for determining the slot dimensions. The array is modeled by using the lumpedcircuit elements for the individual slots, transmission line sections representing the waveguide sections, and a short circuit termination. An offsetshuntslot linear array and its circuit model are
Figure 7. Waveguide linear array of offset longitudinal shuntslot radiators and its transmission line circuit model. d 
Go Gn Gn 
G, 
0 
Gm P 
G Pr G0 Pm 
To achieve an input match for an endfed standingwave array, the slot parameters should be chosen so that the slot conductances or impedances sum to one. The slot conductances must also be selected to obtain the amplitude distribution required to achieve the desired radiation pattern. If the slots are all assumed to be resonant and the desired distribution is all equiphase, the ratios of the slot conductances are proportional to the relative slot radiated powers. This gives a set of N equations that can be solved to give the required slot conductances for a shuntslot array: 
Gn ■,nr pr =1r 1 
(25) 
shown in Fig. 7. The transmission matrix is defined as (25)
V 
A 
B 
‘V+i 

Ji_ 
C 
D 
M. 
The offsetshunt slot is a commonly used element for linear arrays. It offers a wide range of conductance values and linear polarization. The slot offsets and length can be computed using the equations for the offset shunt slot given earlier, or interpolated from a graph, such as Fig. 4.
A section of transmission line of length d with a propagation constant /3 and a characteristic impedance Z0 has a transmission matrix 
Array MutualCoupling Effects. The simple circuit model design is useful for understanding the slot array or for determining a set of initial design values, but it is not accurate enough for most applications. The assumption was made that the slots in the array have the characteristics of an isolated slot. In the array environment the slots are affected by the presence of the other slots, and slots at the edge of the array exhibit behavior that differs significantly from that of slots in the center of the array. If neglected, these effects result in an array where the input reflection coefficient and radiation pattern depart significantly from the desired values. The characteristics of the isolated slot were determined by using a single source of excitation for the slot: the incident waveguide mode. Multiple sources in the array environment excite the slot: the incident field, the fields scattered in the waveguide by the other slots in the guide, and the radiated fields from all the other slots. The characteristics for each slot are influenced by all the other slots in the array to some extent, so a slot’s configuration cannot be determined independently. A series of simultaneous equations is required to perform the array design. Elliott (17) has developed a series of design equations that account for the internal and external mutual coupling effects: 
cos(pd) sin (pd) Z0 
jZ0 sin(0d)’ cos(pd) 
(26) 
The transmission matrices for the shunt element with admittance Y and series element with impedance Z are 
"1 
0" 
’1 
Z 

and 

Y 
1 
0 
1 
(27) 
To achieve a satisfactory design, the coupling values for the individual slots must be chosen so that the reflection coefficient at the array input achieves the required value and so that the relative coupling from the slots generates the desired radiated phase and amplitude distribution for the array. The input match to the array and its variation with frequency is modeled by cascading the transmission matrices for the slots and the waveguide sections. At the array resonant frequency the normalized input conductance or resistance for an array of N shunt or series slots reduces to 
Ya /G0 _ fP vp vn 
(30) 
Ya /G0 
fnVsVp 2 f2 
Y N Y zl. — _ Yr ~ 2і y_ 
1 Y0 
Ya 1 n Gn 
2 f2 N’ Vs " +j(P10/k)(k0b)(a/xf 0 m=1 Vn 
or 
f, 1 , (п/l) cos(Pwln) . (nxn fn(xn, In) = ——sm ( 
Zj _ ул Zn Zo~hZo 
(28) 
(n/ln )2 — P?0 
V a 
Г lm/2 Сln/2 n (xn, ln, xm, ^m ) = I cos(nZm flm ) Jl 
The input reflection coefficient is given by _ 1YJY0 _ZJZ01 1 1+YJY, ZJZ0 +1 
‘^m/2 cos (nzn /ln ) 
ln/2 ejk0 R 
‘ d2 j2 ,3?2+ 0 
R 
The YaJG0 are the active slot admittances in the array environment, the Vn, Vn are the slot and the waveguide mode voltages, and the gmn terms represent the external mutualcoupling effects between the slots. The set of equations is applied iteratively along with the equation for the required input impedance to determine the slot offsets and lengths. These equations assume an ideal cosinusoidal distribution in a narrow slot, and may not be accurate enough for some applications. A more thorough analysis, where the field distribution in the slots are unknowns which must be determined, provides a more accurate model of the impact of element mutual coupling on array performance (26). The computational requirements for a rigorous analysis may limit the size of the array for which it is applicable. A good approximation to the final design should be found first by using an approximate method to reduce the number of iterations required.
because the summation of the individual reflections creates a large reflection. The traveling wave generates a linear phase slope across the aperture which corresponds to a scanned beam with the beam pointing direction given by (27)
cos 90 = k/kg — k/2d (33)
for an array with alternating slot offsets. The beam scans with frequency due to the change in the guide wavelength. This frequencyscanning effect is often exploited by making the waveguide path length between the slots greater than the array spacing to enhance the amount of scan for a given frequency change (28). The beam position for this case is given by
cos 90 = dgk/dkg — k/2d (34)
(35) 
j2nfid 
(36) 
г = 
Variation of Performance with Frequency. The design achieves the desired input match at the design frequency, but the input match of the linear array degrades as the frequency moves away from resonance. This effect is modeled using the transmission matrices by adjusting the propagation constant in the transmission line sections and including the variation of the slot admittance with frequency. The bandwidth is generally defined as the frequency range over which the input reflection coefficient remains below a specified level. The bandwidth is determined primarily by the number of elements in the array, and it degrades as the number of elements in the array increases. Watson (9) derived an approximate expression for the variation of the input reflection coefficient which neglects the multiple reflections:
2 — £ Yn/Yo — e2Ne dJ^Yn /Y0, 
(32) 
N 
Circuit Model for Nonresonant Arrays. The circuit model for the travelingwave array is similar to that used for the resonant array, but a matchedload termination replaces the terminating short, and the line lengths are adjusted to reflect the nonresonant spacing. The initial design is simplified by assuming that the scattering from each slot is small, and hence multiple reflections are neglected. With this assumption and a perfect load termination, the reflection coefficient at the waveguide input is given by
where the reflection coefficient for a resonant slot —2G/Go 
2 + G/G0 
j2nfi d 
Pn — 
N 
2 + £ Yn /Yo ) ej2Ned + £ Yn /Yoej2ned 
Neglecting the slot reflections, the ratios of the slot radiated powers for a travelingwave array of resonant shunt elements are given by
GJG0 "1 Gj/G, 
П 0 j—1 
(37) 
Pi P 
П 
(38) 
The array excitations also change away from the center frequency because the slot locations relative to the standing wave in the guide are no longer optimum and the slot radiation characteristics also exhibit frequency variation. This causes pattern degradation and mainbeam distortion. The effects are modeled by using the circuit model to determine the slot excitations and computing the farfield pattern generated by these excitations.
WDM Network Design Issues
WDM network design involves assigning sufficient resources in the network that would meet the projected traffic demand. Typically, network design problems consider a static traffic matrix and aim at designing a network that would be optimized based on certain performance metrics. Network design problems employing static traffic matrix are typically formulated as optimization problems. To formulate a network design problem as an optimization problem, the inputs to the problem, in addition to a static traffic demand, are some specific reguirements, e. g., required network reliability and fault tolerance requirements, network performance in terms of blocking, and restoration time when a failure occurs. The objective of the optimization problem is to find a topology that would minimize the resources, including the number of links and fibers, the number of wavelengths on each fiber, and the number of crossconnect ports, to meet the given requirements. The outputs include the network configuration and the routes and wavelengths that are to be used for sourcedestination pairs. The network design problem can be formulated as an integer liner programming (ILP) or mixed integer linear – programming (MILP) problem. As the number of variables and constraints can be very large in WDM networks, heuristics are usually used to find solutions faster.
If the traffic pattern in the network is dynamic, i. e., specific traffic is not known a priori, the design problem involves assigning resources based on a certain projected traffic distributions. In case of dynamic traffic, the network designer attempts to quantify certain performance metrics in the network based on the distribution of the traffic. The most commonly used metric in evaluating a network under dynamic traffic pattern is blocking probability. The blocking probability is computed as the ratio of number ofrequests that cannot be assigned a connection to the total number of requests. With this metric, one makes decisions on the amount of resources that are needed to be deployed in a network, the operational policies such as routing and wavelength assignment algorithms, and call acceptance criteria.
MICROELECTRODES FOR CHEMICAL MEASUREMENTS
The small size of microelectrodes makes them ideal for measuring local concentrations of various chemical species, such as neurotransmitters, or the partial pressure of various gases (see Biosensors). The carbon fiber microelectrode is very popular. These fibers have diameters as small as a few microns and can be inserted into glass micropipettes for insulation by exposing a short length of the electrode. Contact between the fiber and the electronic circuit is made by filling the electrode with mercury or silver paint and sealing it with epoxy. Then the assembly is inserted within physiological tissue and selective measurement of various compounds is carried out using a combination of selective membranes, enzyme coatings, surface modifications, and various electrochemical techniques. For example chronoamperometry has been used to detect the release of easily oxidized neurotransmitters, such as dopamine, serotonin, and epinephrine. In chronoam – persmetry, a constant voltage is applied between the carbon fiber and a ground electrode. The current is measured and is directly related to the level of oxidation. Carbon fiber microdisks are typically more useful than microcylinders since they provide a better geometry and excellent resolution. They are also more difficult to fabricate and have lower current amplitudes. A threeelectrode system is used to generate a constant voltage between the working electrode and the medium around it. A reference electrode is located close to the working electrode to estimate the medium’s voltage. Using feedback, a current is applied between the working electrode and a ground electrode to maintain a constant voltage. For the very low currents of microelectrodes and microdisks, a two electrode system is sufficient since the ohmic drop and polarization of the reference electrode are negligeable. The amplitude of the current is modulated by oxidation of the compound to be measured and is proportional to its activity. To improve selectivity, a differential method is used whereby the current amplitudes obtained at two voltages are subtracted and the contribution of the interfering species, such as ascorbic acid, can be reduced or eliminated. The electrode can also be coated with selective membranes. Nafion is a commonly used membrane because it prevents various charged molecules from interfering with measurement. The surface of the carbon fiber can also be modified chemically or with a laser to improve the selectivity and sensitivity of the electrode (14). By depositing appropriate enzymes on the surface of the electrode, very selective electrodes can be made to measure glucose, nitric oxide, acetylcholine, etc.
Field effect transistors (FET) have also been adapted to allow measuring various ions or chemicals. Ionsensitive fieldeffect transistors (ISFET), for example, are enhanced MOSFET transistors but use an ionsensitive membrane instead of gate metallization (14). Then the transistor is immersed in a solution containing the ions to be measured. An electrochemical potential is established at the interface between the solution and the gate dielectric. This potential is established with respect to a reference electrode located in the solution and can modulate the conductance of the channel under the gate. The electrodes can be made very small but have drift and selectivity problems.
6 Micropipettes MICROELECTRODE ARRAYS
The silicon technology used to make integrated circuits can be adapted to manufacture arrays of microelectrodes. The activity of single cells can be recorded with the micropipette technology discussed previously. Neuroscientists are now increasingly interested in recording simultaneously from a large number of cells. Moreover, by stimulating a large number of cells in the spinal cord or in the brain selectively, it should be possible, in principle, to restore motor function in paralyzed patients or vision in blind patients, for example. Therefore, multiple arrays of electrodes capable of recording or stimulating the nervous system are clearly important to understanding nervous system function and to designing neural prostheses. Three siliconbased types of microelectrode arrays have been developed: (1) A 1D beam electrode where a thin – film platinumiridium is deposited on a thin layer of silicon substrate (15). This thin substrate provides a surprising amount of flexibility and can be utilized for the leads and the electrode pads. (2)A2D array for recording the activity of neurons grown in cultures and axons in nerves. A thin film microelectrode array is made of gold electrodes covered with platinum black on a silicon substrate. The assembly is built into the bottom of a neuron culture dish. Neurons grow over these electrodes and make direct contact with them (16). In another design, micromachining of a silicon wafer generates a matrix of 64 square holes with a side dimension of 90 xm. Gold pads and leads are deposited near each hole. Then the thin wafer is inserted between the two sides of a severed nerve. As the axons grow inside the holes, it is possible to record from a selectively small groups of axons (17). (3) A 3D array for cortical recording and stimulation. The 1D beam electrode discussed previously can be assembled to form threedimensional arrays. The longitudinal probes are inserted perpendicularly into a silicon platform. The leads from each probe are transferred to the silicon probe and are routed to a digital processing unit (18). Current work also involves including low noise amplification directly on the platform. Another implementation involving micromachining and etching techniques was used to fabricate a 10 x 10 electrode array. One hundred conductive needlelike electrodes (80 xm at the base and 1.5 mm long) are micromachined on a 4.2 mm x 4.2 mm substrate (19). Aluminum pads are deposited on the other side of the substrate and make contact with each needle electrode. The tips of the electrodes are coated with gold or platimum. A highspeed pneumatic device is used to place the array into cortical tissue because the high density of the electrodes makes insertion difficult. Then the microelectrode arrays are available for recording from a large number of cortical sites.
LINEAR SLOT ARRAYS
A linear array of slots serves alone as an antenna or as a building block for a planar array. The farfield radiation pat
cos2(fi^/2) sin (nx^/a^) cos2 (nl/2a2) cos2 (nx2/a2) (23)
tern for a linear array of N elements located along the zaxis is given by
N
S(4>,e) = A(4>,e)J2 Vnejkzncose
n = 1
The A(^>, Ф) term is the radiation pattern of a single radiating element, which is assumed to be the same for all elements in the array. The element pattern is similar to that given in Eq. (2), but it is modified by the array environment. The summation term represents the array factor where Vn are the excitation values and zn are the element locations. The excitation values of the elements are chosen to generate a specified array radiation pattern. Slotradiating elements allow controlling the excitations to achieve the desired pattern characteristics.
Linear arrays are divided into two categories: the standingwave (or resonant array) and the travelingwave array (or nonresonant array). The antenna and system performance requirements dictate the choice between the two configurations. In the standingwave array, the slot locations are chosen to maximize the coupling at the design frequency, and the waveguide is terminated with a short circuit. This creates an efficient antenna, but the bandwidth is limited. In the traveling – wave array, the slot spacings are nonresonant, and the waveguide is terminated with an absorptive load. The travelingwave array operates over a larger frequency bandwidth than the standingwave array, but the power absorbed by the load reduces the radiating efficiency. It is difficult to generate a beam perpendicular to the guide with the travelingwave array, and it also exhibits beam scan with frequency.