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The spread of telecommunication has led to a rapid change in the upcoming technologies relating communication. This spreading of technologies has led to a need of “MIMO (multiple input multiple output) wireless communication” which uses multiple transmit and receive antennas for communication.
In wireless communication systems we use different type of antenna technologies to transmit and receive message or data. MIMO is one of the smart antenna technologies used to transmit and receive message or data which uses multiple input and multiple output antennas to transmit and receive our message or data.
1.1 Theory
MIMO (multiple input multiple output) is a smart antenna technology used by many wireless systems now a days. In MIMO technology the system uses multiple antennas at the transmitter as well as at the receiver. This technology is used to reduce the errors and to optimize the data speed.MIMO (multiple input multiple output) technology created a breakthrough in the design of wireless communications systems, and is already used as a standard by several wireless systems.
In this project of MIMO we will discuss different encoders, decoders, modulation and demodulation schemes, channel effects, different types of channels .At encoders we will discuss different encoding schemes like space time block codes and trellis coding schemes .In encoders we will discuss Alamouti and Viterbi encoders. As we will be working on Alamouti encoders, so we will discuss Alamouti encoder in detail in the upcoming chapter of Alamouti Encoders.
We will take the data in the form of a signal or if we have data in the form of a bit stream. At the encoder the data is further encoded, this encoded data is then modulated here we will use QPSK modulation scheme. After modulation we will receive the modulated data affected by the channel effects. This data is then demodulated by the same scheme used at the modulation end which is QPSK. This demodulated data is then decoded by Alamouti or Viterbi decoder.
1.1.1 History of MIMO
Background technologies
The earliest ideas in this field go back to work by A.R. Kaye and D.A. George (1970) and W. van Etten (1975, 1976). Jack Winters and Jack Salz at Bell Laboratories published several papers on beamforming related applications in 1984 and 1986.[1]
Principal technologies
Arogyaswami Paulraj and Thomas Kailath proposed the concept of Spatial Multiplexing using MIMO in 1993. Their US Patent No. 5,345,599 issued 1994 on Spatial Multiplexing emphasized applications to wireless broadcast.[1] In 1996, Greg Raleigh and Gerard J. Foschini refine new approaches to MIMO technology, which considers a configuration where multiple transmit antennas are co-located at one transmitter to improve the link throughput effectively. [1] Bell Labs was the first to demonstrate a laboratory prototype of spatial multiplexing (SM) in 1998, where spatial multiplexing is a principal technology to improve the performance of MIMO communication systems.[1]
1.2 Types of wireless communication
In wireless communication systems we use different techniques to transmit and
receive message or data. Following are the techniques used for wireless communication systems.
1.2.1 SISO (Single Input Single Output)
SISO is often refers to as “Single Input Single Output”, where SISO is the simplest antenna technology. In SISO a wireless communications system has a single antenna at the transmitter and a single antenna at the receiver. For a clearer picture of SISO wireless communication you can refer to figure 1.1.
In order to minimize or eliminate problems caused by multipath wave propagation, smart antenna technology is used. There are three forms of smart antenna technologies, known as SIMO (single input, multiple output), MISO (multiple input, single output), and MIMO (multiple input, multiple output).
1.2.2 SIMO (Single Input Multiple Output)
SIMO is often refers to as “Single Input Multiple Output”, where SIMO is a smart antenna technology for wireless communication systems in which multiple antennas at the receiver and a single antenna at the transmitter are used.
SIMO is a form of smart antenna technology that uses a single antenna at the transmitter and a multiple antennas at the receiver on a wireless device to improve the transmission distance. An early form of SIMO, known as diversity reception, has been used by military, commercial, amateur, and shortwave radio operators at frequencies below 30 MHz since the First World War. In SIMO the antennas are combined to minimize errors and optimize data speed. In digital communications systems such as wireless Internet, it can cause a reduction in data speed and an increase in the number of errors. The use of two or more antennas at the destination can reduce the trouble caused by multipath wave propagation.
SIMO technology has widespread applications in digital television, wireless local area networks (WLANs), metropolitan area networks (MANs), and mobile communications. For a clearer picture of SIMO wireless communication you can refer to figure 1.1.
- MISO
MISO is often referring to as "Multiple Input Single Output", where MISO is a smart antenna technology. It has multiple antennas at the transmitter and a single antenna at the receiver.
MISO uses multiple transmitters and a single receiver on a wireless device to improve the transmission distance. MISO technology also has widespread applications in Digital Television, Wireless Local Area Networks (WLANs), Metropolitan Area Networks (MANs), and mobile communications. The antennas are combined to minimize errors and optimize data speed.
In digital communications systems such as wireless Internet, it can cause a reduction in data speed and an increase in the number of errors. The use of two or more antennas, along with the transmission of multiple signals (one for each antenna) at the source, can reduce the trouble caused by multipath wave propagation. Figure 1.1 can be seen for a clearer picture of MISO wireless communication you can refer to.
- MIMO
MIMO meaning “Multiple Input Multiple Output” is a smart antenna technology for wireless communications in which multiple antennas are used at both the source (transmitter) and the destination (receiver).
The antennas at each end of the communications circuit are combined to minimize errors and optimize data speed. MIMO is one of several forms of smart antenna technology which is used by many wireless systems. The use of two or more antennas, along with the transmission of multiple signals (one from each antenna) at the transmitter and the receiver, eliminates the trouble caused by multipath wave propagation, and can even take advantage of this effect.
MIMO technology has grasped the interest because of its possible applications in digital television (DTV), wireless local area networks (WLANs), metropolitan area networks (MANs), and mobile communications
1.2.4.1 Functions of MIMO
MIMO can be subdivided into three main categories namely precoding, spatial multiplexing and diversity coding.
Precoding is multilayer beam forming in a narrow sense or all spatial processing at the transmitter in a wide-sense. The benefits of beamforming are to increase the signal gain from constructive combining and to reduce the multipath fading effect. When the receiver has multiple antennas, the transmit beamforming cannot simultaneously maximize the signal level at all of the receive antenna and precoding is used. Note that precoding requires knowledge of the channel state information (CSI) at the transmitter.
Spatial multiplexing requires MIMO antenna configuration. In spatial multiplexing, a high rate signal is split into multiple lower rate streams and each stream is transmitted from a different transmit antenna in the same frequency channel. If these signals arrive at the receiver antenna array with sufficiently different spatial signatures, the receiver can separate these streams, creating parallel channels for free. Spatial multiplexing is a very powerful technique for increasing channel capacity at higher Signal to Noise Ratio (SNR). Spatial multiplexing can be used with or without transmit channel knowledge.
Diversity coding techniques are used when there is no channel knowledge at the transmitter. In diversity methods a single stream is transmitted, but the signal is coded using techniques called space-time coding. The signal is emitted from each of the transmit antennas using certain principles of full or near orthogonal coding.As there is no channel knowledge, there is no beamforming or array gain from diversity coding.
Spatial multiplexing can also be combined with precoding when the channel is known at the transmitter or combined with diversity coding when decoding reliability is in tradeoff.
Digital encoder is a device that converts motion into a sequence of digital pulses. By counting a single bit or by decoding a set of bits, the pulses can be converted to relative or absolute position measurements.
Incremental encoder, sometimes called a relative encoder, is simpler in design than the absolute encoder. It consists of two tracks and two sensors whose outputs are called channels.
- Alamouti Encoder
Alamouti encoder is a coding apparatus for a transmitter with four transmitting antennas. In the transmitter the encoder generates a code symbol vector by encoding an input symbol sequence in a pre-determined coding method. An Alamouti encoder encodes the grouped symbol vector in an Alamouti scheme and transmits Alamouti-coded symbols through the four transmitting antennas.
- Decoders
The following decoder is used for our project some detail of Viterbi decoder is explained as follows
- Viterbi Decoder
The Viterbi algorithm is a method for performing maximum likelihood sequence detection and can be used for decoding received data that has been generated via a convolutional code. [2] The Viterbi algorithm provides a very efficient method for finding this "most likely" path. The Viterbi algorithm operates in a step-wise manner by processing a set of state metrics (state metrics can also be called path metrics) forward in time, stage by stage over the trellis.
1.6 Organization of study
Chapter 2 describes the existing literature on coding techniques in detail. Chapter 3 describes the modulation technique. In chapter 4 the detail of Alamouti encoder will be discussed. Chapter 5 will describe the design and implementation of our project work and the results of tests. Chapter 6 contains references.
References:
[1] http://en.wikipedia.org
[2] "Digital Communications, 3rd Edition," by J. G. Proakis, McGraw Hill, N.Y., 1995
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