Abstract
A novel compact ultra-wideband (UWB) multiple-input multiple-output (MIMO) slot antenna with band notch characteristics is presented for portable wireless UWB applications. The antenna comprises of co-planar waveguide feed (CPW) and two radiating monopoles oriented in orthogonal orientation for providing orthogonal radiation patterns. A Minkowski fractal parasitic stub along with a Minkowski fractal grounded stub has been placed at 45° between the monopoles to reduce the coupling between them, which in turn establishes high isolation between the radiators. An excellent band notch characteristic is obtained at 5.5 GHz by etching a modified E-shaped compact slot on the radiators. Results show that the designed antenna meets -10 dB impedance bandwidth and -17 dB isolation throughout the entire operating band (3.1 -12 GHz). Novelty of this design lies in improving isolation using fractal which occupies less space in compared to other isolation mechanisms in MIMO structures. The simulated and measured results depict that the proposed antenna is convenient for MIMO diversity systems.
TopIntroduction
In digital communication systems, MIMO (Multiple Input Multiple Output) has created a revolutionary importance. All types of wireless technologies faced some difficulties such as multipath, signal fading, limited spectrum, interference etc. MIMO has solved most of the difficulties by providing higher data throughput to multipath, increase in range and reliability etc., without consuming additional spectrum. IEEE standard of MIMO technology is 802.11n. MIMO antenna introduces antenna diversity like pattern, polarization and spatial diversity to increase the transmitted signal power and to improve Signal to Noise Ratio (SNR). To increase the data rate in MIMO, spatial multiplexing is used. In addition beam forming is used to enhance the data rate. MIMO has a wide range of applications in mobile, Digital Television (DTV), Metropolitan Area Network (MAN), Wireless Local Area Network (WLAN) etc. Basically the system which have multiple transmit and multiple receive antennas is known as MIMO system. The wireless technology has remarkably improved the communication range and maximizes the capacity by using MIMO system. MIMO system analyses multi-path propagation by using different transmission paths to the receiver. These paths can be used to provide redundancy of transmitted data, which improves the reliability of transmission or by increasing the number of simultaneous transmitted data streams and increasing the data rate of the system. The capacity of these systems depends upon increasing the number of transmit antennas and also depends upon the number of receiving antennas, which must be greater than or equal to the number of transmitting antennas. So an enhancement in capacity means capability of faster communication and as a consequence this capacity improvement over conventional one-antenna systems has fuelled intense research interest in MIMO techniques, which results the development of MIMO systems.
Figure 1.
Block Diagram of a MIMO System
Mathematically, the MIMO communication is performed through a matrix, so it may be possible to transmit multiple parallel signal streams or data streams simultaneously in the same frequency band and so that increase in spectral efficiency occurs. This technique is known as Spatial Multiplexing, which is shown in Figure 2. The data stream is encoded with vector encoder and then transmitted by 
transmitters. The coded signal experienced distortion while passing through MIMO radio channel. The receiver has 
number of antennas. Each antenna element receives the coded signals from all 
transmit antennas, and in tern the received signals exhibit inter-channel interference. The received signals are then down converted to the base band and sampled per symbol interval. The MIMO processing unit recovers the transmitted data streams from the sampled base-band signals.
Figure 2. Block diagram of MIMO system using Spatial Multiplexing
There are 
number of transmitting antenna elements and 
number of receiving antenna elements. So there are
sub-channels between transmitter and receiver. Each sub-channel represents a selective fading and as a result, it is modelled as a FIR filter with complex coefficients. In the previous case, the received signal by the jth receive antenna can be shown as:
(1) where,
xi =transmitted signal from
ith antenna,
yj= received signal at
jth antenna.
Key Terms in this Chapter
Minkowski Curve: The geometrical construction of fractal Minkowski curve can be described as where each side of the basic structure is divided into three equal line segments and the middle line segment of each side is replaced by an inward projection with two vertical and two horizontal line segments of equal length. This process is repeated for successive iterations while maintaining the basic geometry of the antenna.
Mandelbrot: Mr. Mandelbrot was the first person who realized that it is very often impossible to describe nature using only Euclidian geometry in terms of straight lines, circles and cubes. He coined the term ‘fractal’ about 20 years ago in his book named “The fractal geometry of nature.” He proposed that fractals and fractal geometry could be used to describe real objects such as trees, lightening, river meanders, and coastline and so forth.
UWB (Ultra-Wideband): February 2002 witnessed the allocation of the frequency band between 3.1 GHz to 10.6 GHz with an effective isotropic radiated power (EIRP) less than -41.3 dBm/MHz for the unlicensed indoor UWB wireless communication system by the Federal Communication Commission (FCC), USA which initiated research in component development. According to the released regulation, UWB technology which is based on transmitting ultra short pulses with duration of only a few nano-seconds or less, has recently received great attention in various fields for the short-distance (< 10 m) wireless communications.
Fractal Curve: Recursive structures describe many real-world objects like clouds, mountains, trees, coastlines and Brownian motion, which do not correspond to simple geometric shapes. Fractal geometry, which was introduced in 1975 by B.B. Maldelbrot, mathematically defined these extremely intricate self-similar shapes. His work inspired interest and has made fractal geometry and its application a very popular field of study. A fractal is a rough or fragmented geometric shape that can be subdivided in parts, each of which is a reduced copy of the whole. The amazing realization towards combination of fractal geometry with electro-magnetic theory has led to the development of novel class of antennas called fractal shaped antennas which have received significant attention in applied electromagnetic research community. Fractals in antenna engineering are based on analysis and design of fractal radiating elements and incorporation of fractal shaped antenna arrays. In antenna community, it is known as fractal antenna engineering. As fractal geometry is an extension of classical geometry, its appearance provides engineers with the unprecedented opportunity to explore number of configurations for possible use in the development of new and innovative antenna designs.
Band Notch: To avoid the electromagnetic interference between the UWB system and the wireless system, band notch functionality is necessary. It is achieved by etching different notches in UWB mono-poles. The length of the band notch element is equal to half wave length at notch frequency.
MIMO (Multiple-Input Multiple-Output) Antenna: The design of MIMO antenna systems has been driven by the wireless industry as this technology is a major enabling one in 4G wireless standards and will continue at the upcoming 5G standards. Using MIMO, channel capacity can be drastically increased which in turn produces higher bit rate transmission.
Monopole Antenna: The monopole is a resonant antenna where the radiator acts as a resonator for radio waves, oscillating with standing waves of voltage and current along its length. Therefore, the length of the antenna is determined by the wavelength of the radio waves it is used with.