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Academically and industrially, multiple antennas for wireless communications have proved itself to be of great importance. We can have multiplexing gain, diversity gain or an antenna gain with multiple antennas which leads to an improved performance in terms of bit rate, error performance and signal to noise ratio. Multiple Antennas are also called MIMO (multiple input multiple output) and they have evolved very rapidly with a huge amount of publications every year.
Until now many papers and a lot of research has been done on the performance elements of MIMO systems and the practical concepts of the Transceivers. The focus of this paper is to provide an in-depth insight in to this interesting research field. The research efforts till now is organised and main area of interest are spatial diversity techniques and spatial multiplexing. All the elements and aspects which influence the performance of multiple antennas systems are analysed. Other topics that are evaluated are channel coding, transceiver structures, MIMO techniques for channels with frequency selective fading, differential and non coherent schemes, cooperative diversity schemes, beam forming techniques, diversity reception and space time coding techniques.
Keywords: Wireless communications, multiple antennas, transceiver, diversity techniques.
Traditionally wireless applications were only used for voice and applications like data or video streaming which require high data rates were only used in wire line communication. But in recent years these high data rate demanding applications have been shifted to the wireless domain leading to the convergence of digital communication network and internet. An example can be mentioned of cell phones which can be connected to internet any time and can be used to send data.
To accomplish a certain level of quality of service two things are very important; high data rates and good error performance. When there is fixed bandwidth there is always a trade off between these two factors. The reason is the disruptive characteristics of a wireless channel; multipath effects and fading.
Conventional single antenna systems have got an optimal communication performance and employ channel coding to overcome multipath effects and fading.
The fast growing demand of wireless communication, data rates and importance of bandwidth has now made it important to deal the antenna part of the communication system in an effective way. By employing multiple antennas instead of as single antenna we can exploit the spatial domain that was unused previously. Multiple antennas have been proved to be a very good way to increase the data rates with very less error probability and also reducing co-channel interference.
In modern communication systems, the recently emerged technology multiple antennas system (MIMO) system is one of the most significant technological breakthrough to improve the network performance, capacity, error rates and also to increase the data rates through multi-stream transmission. MIMO will play a major role in many future wireless communications systems.
1: Multiple Antennas (MIMO)
Multiple antennas (also known as Multiple-Input -Multiple-Output (MIMO)) use multiple antennas at both sides of the radio link (i-e transmitter and receiver).Multiple antennas can be used to obtain diversity gain, array gain and multiplexing gain.
MIMO works in two general modes. (a) Spatial diversity, where the same information stream is encoded in multiple antennas thus improving the reliability of the channel and introducing redundancy. (b) Spatial multiplexing, where the information stream is processed in different antennas thus resulting in better spectral utilization.The IEEE 802.11n wireless standard uses the MIMO system to increase the speed to 100Mbps and more, which is double the speed of 802.11a and 802.11g wireless standards. A multiple antenna system is shown in figure 1.
Figure 1: Multiple antennas (MIMO) system
2: Different aspects relating to MIMO:
2-a: Array Gain
Array gain is defined as the average increase in the SNR and depends on the number of transmit and receive antennas. Transmit/Receive array gain needs channel information in the transmitter and receiver respectively. Channel information is typically available in the receiver whereas the channel state information in the transmitter is more difficult to maintain in general , .
2-b: Diversity Gain
Diversity is a powerful technique to reduce fading effect in wireless communications. Diversity gain is defined as the reduction in the probability of error due to multiple independent paths produced between the transmitter and receiver. In other words if there are M transmits, N receive antennas, the order of diversity is M.N. There is no diversity gain if the medium is line of sight channel .
2-c: Multiplexing Gain
Multiplexing gain is defined as the increase in the data rate; since independent data streams are send through independent paths between multiple transmitters and multiple receivers. In other words if there are M (>1) transmit antennas and N (>1) receive antennas, the increase in the data rate is min (M, N)-fold .
2-d: Channel Coding
Channel coding techniques are usually required to guarantee error performance for spatial multiplexing schemes. Most spatial multiplexing schemes use a channel coding that consist of one dimensional encoders and decoders working only in the time domain. Three different types of one dimensional channel coding scheme can be used in conjunction with spatial multiplexing: Horizontal coding, vertical coding or a combination of both (Diagonal coding). In Horizontal coding the bit stream to be transmitted is first demultiplexed into separate data streams. Each stream is encoded and then transmitted from the corresponding antennas. All code bits associated with certain information bits are transmitted over the same antenna. At the receiver the data streams can be decoded to get the original information data. In the case of vertical coding the bit streams are first coded after that it is demultiplexed into data streams. The demultiplexed coded data streams are spread among the individual transmit antennas. Vertical coding needs joint decoding of the data streams, which increase the complexity of the receiver. A Diagonal coding is the combination of the horizontal coding and Vertical coding .
2-e: Cooperative diversity
Cooperative diversity is an antenna diversity technique that is obtained using signal relaying nodes. The relay node transmits the signal to the destination node received from the source node. The destination node receives two non-independent signals from the source and relay nodes. The destination node decodes the information using the combination of both the direct signal coming from the source and the relayed signal. In conventional systems the receiver decodes the information based on the direct signal. For simplicity we consider the system with three nodes which are source, relay, and destination nodes. The relay node receives the signal from the source and transmits it to the destination node .
2-f: Frequency Selective Fading
In wireless communication systems the transmitted signal propagates through several different paths from the transmitter to the receiver. The phenomenon of Frequency selective fading refers to a scenario where the radio waves reflect from different surfaces and surroundings. Every signal goes through different delay and spread and due to this reason different parts of a signal which is transmitted are affected differently .
Figure 2: MIMO state of the art
3: MIMO Signal and Channel
A general MIMO system is shown in figure 2, which consists of 'M' transmit antennas, 'N' receive antennas and a MIMO wireless channel. Each antenna receives not only the direct components intended for it but also receives the indirect components intended for other antennas. MIMO system multiply the signals of a conventional single antenna (Single-Input-Single-Output (SISO)) with the multiple antennas thus making many copies of the transmitted signal at the receiver and at the same time increasing the capacity gain by a factor of the number of antennas .
Consider the transmit antenna 'M' transmits the multiple streams of information through multiple transmit antennas. The information streams goes through a matrix channel, which consists of M.N distinct paths between the transmitter and receiver. The receiver, receives the multiple information streams by multiple receive antennas and decodes the received information streams into the original data. An expression for the MIMO system is
Y = Hx + n
Where Y and x represents the receive and transmit signal vectors respectively, n is the additive Gaussian noise that is produced by the channel and the receive antenna. H is the channel matrix.
The transmitted signal is reflected by the obstacles in the MIMO channel which enable the system to exploit the multiple propagation paths between the transmitter and receiver to increase the system capacity and performance. Introducing the subcarrier maximal receive combiner boost the system performance at the receiver because it combines the signals from multiple antennas in such a way that it increase the signal strength .
4: Channel Capacity of MIMO
A very important factor for the profitability of a wireless networks is its capacity. MIMO system provides high capacity by using multiple antennas at both the transmitter and receiver end of the radio link. Multiple antennas are used to improve the capacity over SISO system when operated in multi-path environment. MIMO system capacity is measured in bits per second per hertz and is bounded by Shannon Hartley capacity. But it has become apparent that MIMO system can exceed the Shannon Hartley limit of SISO depending on the channel properties and the number of antennas .
5: Functions of MIMO
Pre-coding is generalized to allow multi-layer transmission in MIMO systems. As conventional beamforming considers as linear single layer pre-coding, increasing the signal power at the output of the receiver by emitting the same signal from each of the transmit antennas with suitable weighting. When multiple antennas are used at the receiver, the signal level is not maximized simultaneously at all of the multiple receive antennas, so in that case pre-coding is used for multi-layer beamforming to increase the throughput performance of a multiple receive antennas. In pre-coding the transmit antennas transmit the multiple streams with independent and suitable weighting per each antenna such that higher link throughput is obtained at the receiver output .
5-b: Spatial multiplexing
Spatial multiplexing uses multiple antennas at both sides of the transmitter and receiver. In spatial multiplexing a signal with higher rate is split into multiple lower rate streams, each stream is transmitted independently through separate transmit antennas in the same frequency channel. If the transmitted signals reach the receive antenna with adequately different spatial signatures, the streams are separated by the receiver, generating parallel channels free. At higher signal-to-noise-ratio (SNR) spatial multiplexing is used to increase the channel capacity. The less number of transmitter or receiver antennas can limit the maximum number of spatial streams .
5-c: Spatial Diversity
The purpose of spatial diversity is to make the transmission faster. It is not used to increase the data rate. Spatial diversity uses redundant data on multiple paths. Spatial diversity is divided into two groups ;( a) Receive (Rx) Diversity (SIMO) and (b) Transmit (Tx) Diversity (MISO) .
(a) Receive (Rx) Diversity
In receive diversity more antennas are used on the receiver side than on the transmitter side. Figure 3 shows one transmit antenna and two receive antennas (SIMO 1X2).
C:\Users\ali\Desktop\SIMO ANTENNA CONFIGURATION.JPG
Figure 3: SIMO antenna configuration
This configuration is very easy to implement because it does not need special coding methods. Receive diversity only needs two RF paths for the receiver.
(b) Transmit (Tx) Diversity
In transmit diversity more antennas are used on the transmitter side than on the receiver side. Figure 4 shows two transmit antennas and one receive antenna (MISO 2x1).
C:\Users\ali\Desktop\RESEAR MIMOOOOOO\MISO ANTENNA CONFIGURATION.JPG
Figure 4: MISO antenna configuration
Transmit diversity is used to reduce the effect of fading. In transmit diversity the same information is transmitted from two different antennas. Data from the second antenna is encoded differently to differentiate it from the first antenna. This can be done to able the user equipment on the receiver side to identify that the information is coming from the different locations and properly decode it. Space-time coding is used to create redundant signals .
6: Space Time Code (STC)
Space Time Code (STC) is techniques used to improve the reliability of data transmission over multiple transmit antennas in wireless communication systems. STC rely on multiple redundant copies of information stream transmitted to the receiver. Multiple copies will send that at least some of the copies may not be altered during the transmission between the transmitter and receiver to allow reliable decoding.
STC can be divided into two main types
Space Time Trellis Codes (STTCs)
Space Time Block Codes (STBCs)
6-a: Space Time Trellis Codes (STTCs)
Space Time Trellis Codes (STTCs) transmits multiple redundant copies of trellis code distributed over time slots and multiple antennas. On the receiving side the receiver use these multiple copies of data to reconstruct the original data. STTCs provide both diversity gain and coding gain.
6-b: Space Time Block Codes (STBCs)
Space Time Block Codes (STBCs) act on a block of data at once. STBC is based on trellis codes. STBCs are less complex in implementation than STTCs. STBCs provide diversity gain.
Space Time Code (STC) is further divided into three parts on the basis of whether the receiver is aware of the channel state information impairments:
In coherent STC, the receiver knows the channel side information through some form of estimation.
In non-coherent STC the receiver does not know the channel side information but knows the channel statistics.
Differential Space Time Code
In differential space time coding neither the channel state information nor the statistics of the channel are available.
7: Smart Antenna
A smart antenna have several antenna elements, with smart signal processing algorithms used to identify spatial signal signatures such as the direction of arrival (DoA) of the signal and used it to determine beamforming vectors, to track and locate the antenna beam on the target .
7-a: Functions of Smart Antenna
The two main functions of smart antennas are
Direction of Arrival (DoA) estimation
Direction of Arrival (DoA) estimation
MUSIC (Multiple Signal Classification) is a technique used by the smart antennas to estimate the DoA of the signal.
Beamforming is a signal processing technique used in sensor arrays for the transmission or reception of signal in the desired direction while nulling the interfering signal. Beamforming can be applied in all antenna array system as well as in MIMO systems. Beamforming can be used for both sound and radio waves. It has applications in wireless communications system, radar, biomedicine, speech etc .
7-b: Types of Smart Antenna
The two main types of smart antennas are
Switched beam smart antennas - With a finite number of fixed predefined patterns.
Adaptive array smart antennas - With an infinite number of patterns adjusted to the situation in real time.
Figure 5: Switched beam pattern Figure 6: Adaptive array pattern
8: Applications of Multiple antennas (MIMO)
MIMO can reliably connect devices in home, such as computer networking devices, cabled video devices, phone lines, music, storage devices etc.
The IEEE 802.16e standard and the IEEE 802.11n standard also use MIMO system.
MIMO is used in mobile radio telephone standard such as 3GPP and 3GPP2 standard. 3GPP High Speed Packet Access Plus (HSPA+) and Long Term Evolution (LTE) standard use MIMO.
MIMO can use in RFID. It increase read reliability using space diversity. It also increase read range and read throughput.
This paper presents a review of multiple antenna techniques (MIMO) for wireless communication. MIMO techniques have been part of research since many years but recently it has been practically introduced in to the industry. In this paper in-depth insight has been given about the MIMO technique. Different aspects of MIMO technology and its areas like spatial diversity, spatial multiplexing and smart Antennas are discussed. The factors which influence the MIMO techniques are also elaborated.
MIMO technology has proved to be a very effective technique to increase cell throughput and coverage. MIMO technique, while obeying the Shannon law, still increases the capacity of a channel. Spectral bandwidth is nowadays becoming very precious and MIMO is a technique which is using it very effectively. This property is making MIMO one of the important techniques employed now days. Channel capacity can be increased with increasing the number of antennas.
The spatial domain is very large and in the future more effective algorithms and techniques for MIMO technology are expected. Though MIMO has been the subject of many papers but still it is a burning topic for the researchers and will continue to be.