Multiple input Multiple output - Orthogonal Frequency Division Multiplexing (MIMO - OFDM) by alenzi

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									Multiple input Multiple output - Orthogonal Frequency Division Multiplexing (MIMO - OFDM) By: Eng.Husein A.Alenzi & Eng. Muneer Aljufairi kwt39@hotmail.com M_Aljufairi@hotmail.com COLLEGE OF MATHEMATICAL SCIENCES & INFORMATION TECHNOLOGY AHLIA UNIVERSITY Post Office Box 10878 Manama, Kingdom of Bahrain Abstract :FDM ,frequency division multiplexing is a technology that transmits multiple Signals simultaneously over a single transmission path(cable or wireless).The (OFDM) Orthogonal FDM spread spectrum technique distributes the data over a large number Of carriers that are spaced apart at precise frequencies .OFDM sometimes called multiCarrier of discrete multi-tone modulation , it is the modulation technique used for digital TV. The MIMO system uses multiple antenna to simultaneously transmit data , in small pieces to the receiver can process the data flows and put them back together . Introduction: This paper gives an overview of Multiple input Multiple output - Orthogonal Frequency Division Multiplexing(MIMO-OFDM), beginning with short description of OFDM technology . The multiplexing is a technique that allows the simultaneous transmission of multiple signal across a single data link. The Orthogonal Frequency Division Multiplexing is a communication technique that divides a channel into a number of equally spaced frequency band. The OFDM is used mainly for transmission of digital data is currently used in digital audio broad casting (DAB) . The idea is to used large number of parallel narrow band subcarriers instead of a single wide band carrier to transport information. OFDM is multi carrier modulation technique that is unlike other modulation technique .In OFDM the carrier have substantial overlap .For each single high frequency carrier used, OFDM transmits multiple high data rates signals concurrently using sub carriers.[1,3,4].

Figure 1.
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The benefits of multiple-input multiple-output (MIMO) systems enabling an increase in the system capacity and an increase of system reliability. The former is attained by signal multiplexing and the later by space encoding. The basic algorithms required for a proper operation of the MIMO systems, i.e. channel estimation, equalization, optimal and suboptimal receivers, are described. In the conclusion, open problems of the MIMO systems are outlined.[7,8]

Figure 2. MIMO

The principle of OFDM:

Figure 3.

Suppose that this transmission takes four seconds. Then, each piece of data in the left picture has a duration of one second.On the other hand, OFDM would send the four pieces simultaneously as shown on the right. In this case, each piece of data has a duration of four seconds. [1.2.3] A modulation scheme is a mapping of data words to a real (In phase) and imaginary (Quadrature) constellation, also known as an IQ constellation. Each data word is mapped to one unique IQ location in the constellation.[1,2]

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Figure 4.

FFT-based OFDM System OFDM Transmitter:

Figure 5.

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Output of IFFT:

Figure 6. Output of D/A:
0 .2 0 .1 5 0 .1 0 .0 5 0 - 0 .0 5 - 0 .1 - 0 .1 5 - 0 .2

0

1 0

2 0

3 0

4 0

5 0

6 0

7 0

8 0

Figure 7. Series and Parallel Concepts:

S e r ia l-to P a r a lle l C o n v e r te r

M
0

0

Tb 2Tb

t

Figure 8. In OFDM system design, the series and parallel converter is considered to realize the concept of parallel data transmission.

Example the if input : x=[0,0,0,1,1,0,1,1,….]
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at a d l e l l ar a P
T s = N Tb t

at a d l a ir e S

The output will be a parallel : x1=[0,0] x2=[0,1] x3=[1,0] x4=[1,1] …..

Series : In a conventional serial data system, the symbols are transmitted sequentially, with the frequency spectrum of each data symbol allowed to occupy the entire available bandwidth. When the data rate is sufficient high, several adjacent symbols may be completely distorted over frequency selective fading or multipath delay spread channel. [1,2.6]

Parallel: The spectrum of an individual data element normally occupies only a small part of available bandwidth. Because of dividing an entire channel bandwidth into many narrow sub bands, the frequency response over each individual sub channel is relatively flat. A parallel data transmission system offers possibilities for alleviating this problem encountered with serial systems. [1,2,6]

Modulation/Mapping: The process of mapping the information bits onto the signal constellation plays a fundamental role in determining the properties of the modulation. An OFDM signal consists of a sum of sub-carriers, each of which contains M-ary phase shift keyed (PSK) or quadrature amplitude modulated (QAM) signals.[4,5] Modulation types over OFDM systems: Phase shift keying (PSK). Quadrature amplitude modulation (QAM).

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Mapping - Phase Shift Keying. An example of signal-space diagram for 8-PSK .
φ2

m4

−

Es
m5

π M π M

d

m6

m8

−

Es

m7

Figure 9. M-ary phase shift keying . Consider M-ary phase-shift keying (M-PSK) for which the signal set is

si ( t ) =

where Es is the signal energy per symbol, Ts carrier frequency. This phase of the carrier takes on one of the M possible values, namely

2π ( i −1)   2Es cos  2π fct +  0 ≤ t ≤ Ts , i =1,2,..., M Ts M  

is the symbol duration, and fs is the

θi = 2( i −1) π M ,

where

i = 1, 2,..., M

Mapping – Quadrature Amplitude Modulation. An example of signal-space diagram for 16-square QAM

Figure 10.
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noiger noisiceD

tniop egassem

yradnuob noisiceD
m2
d

Es

m3

Es

m1

φ1

IFFT and FFT: Inverse DFT and DFT are critical in the implementation of an OFDM system.

IDFT x[n] =

1 ∑X[k]e N k =0

N −1

j

2π kn N

D FT X [ k ] =

∑ x [ n ]e
n=0

N −1

− j

2π kn N

IFFT and FFT algorithms are the fast implementation for the IDFT and DFT. Signal representation of OFDM using IDFT/DFT Signal representation of OFDM using IDFT/DFT Now, consider a data sequence X = ( X 0 , X 1 ,L, X n ,L, X N −2 , X N −1 ), and
xn = 1 N

Xk =A + jB , k k

∑ X ke
k =0

N −1

j ( 2π kn / N )

=

1 N

∑X
k =0

N −1

k

e(

j 2 π f k tn )

,

n = 0,1, 2L N − 1,

where f k = k / ( N ∆t ) , tn = n∆t , and ∆ t is an arbitrarily chosen symbol duration of the serial data sequence X k Orthogonality: Digital communication systems : In time domain

∫

∞ −∞

xi

( t )x *j ( t ) d t
X
i

1 , =  0 ,
f

i = j i ≠ j
i = j i ≠ j

In frequency domain OFDM:

∫

∞ −∞

(

f

)X

* j

(

)d f

1 , =  0 ,

Two conditions must be considered for the orthogonality between the subcarriers. 1. Each subcarrier has exactly an integer number of cycles in the FFT interval. 2. The number of cycles between adjacent subcarriers differs by exactly one.

∫

t s +T

ts

e

− j 2π

k ( t −ts ) N −1 T n =0

⋅ ∑ dn e

j 2π

n ( t − ts ) T

dt = ∑ d n ∫
n =0

N −1

t s +T

ts

e

j 2π

n−k ( t −ts ) T

dt = d k T
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Figure 11. Example of four subcarriers within one OFDM symbol (Time domain).

Figure 12. Spectra of individual subcarriers (Frequency domain).

Guard Interval and Cyclic Extension:
OFDM symbol duration.
n o it ar u d l o b m y s M D F O
T
g

Figure 13.
Two different sources of interference can be identified in the OFDM system. Inter symbol interference (ISI) is defined as the crosstalk between signals within the same sub-channel of consecutive FFT frames, which are separated in time by the signaling interval T. Inter-carrier interference (ICI) is the crosstalk between adjacent sub channels or frequency bands of the same FFT frame.[2,5.6]

n o it ar u d n o it ar g et n i T F F
T

AT A D

l a vr et n I dr a u G

l a vr et n i dr a u G

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Delay spread . Environment Home Office Manufactures Suburban Delay Spread < 50 ns ~ 100 ns 200 ~ 300 ns < 10 us

Figure 14. Guard Interval and Cyclic Extension:
If T
g

< T

d e l y -s p re a d

Tg

Sym bol 1

T g

Sym bol 2

Tg

Sym bol 3

T g

Sym bol 4

T g T
d e ly - sp r e a d

Sym bol 1

T g

Sym bol 2

Tg

Sym bol 3

If

T g > T

d e ly - sp r e a d

Tg

Sym bol 1

T g

Sym bol 2

Tg

Sym bol 3

T g

Sym bol 4

T g T
d e ly - sp r e a d

Sym bol 1

Tg

Sym bol 2

T g

Sym bol 3

Figure 15.
To eliminate ICI, the OFDM symbol is cyclically extended in the guard interval. This ensures that delayed replicas of the OFDM symbol always have an integer number of cycles within the FFT interval, as long as the delay is smaller than the guard interval. [2,5,6]

)noisnetxE cilcyC(

lavretnI drauG

Figure 16.

﹒﹒﹒﹒ ﹒﹒﹒﹒
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﹒﹒﹒﹒

﹒﹒﹒﹒

Effect of multipath with zero signals in the guard interval, the delayed subcarrier 2 causes ICI on subcarrier 1 and vice versa.[3,5,6]

Figure 17.

Time and frequency representation of OFDM with guard intervals.

Figure 18.

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What is Multiple-Input/Multiple-Output (MIMO)?
multiple-input and multiple-output, or MIMO is the use of multiple antennas at both the transmitter and receiver to improve communication performance. It is one of several forms of smart antenna technology.

Figur 19. How MIMO works?
In the basic MIMO concept implemented for (OFDM), the data to be transmitted is scrambled, encoded, and interleaved Fig20. It's then divided up into parallel data streams, each of which modulates a separate transmitter (TX).

Fig.20 Modulation is OFDM using binary phase-shift keying (BPSK), quadrature phase-shift keying (QPSK), 16QAM (quadrature amplitude modulation), or 64QAM, depending on the data rate. Both transmitters operate in the same 20-MHz band. Transmitting two different data streams in the same bandwidth doubles the throughput. Throughput scales linearly with the number of transmitters. [8]
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The multiple signals arrive at the receivers at different times in different phases, depending on the different paths they take. Some signals will be direct, others via multiple different paths. Each signal is unique as defined by the characteristics of the path it takes. Such a technique is referred to as spatial multiplexing. [8] The unique signatures produced by each signal over the multiple paths allow the receivers to sort out the individual signals using special algorithms implemented with DSP techniques. The same signals from different antennas then can be combined to reinforce one another, improving signal-to-noise ratio and, therefore, the reliability and range.[8]

Channel Capacity
the capacity of conventional MIMO, MIMO-OFDM and spread MIMO-OFDM in presence of multipath is studied. In the single user case the capacity for the conventional MIMO without ISI is the highest and they state that it is the upper bound of capacity limit MIMO-OFDM and spread MIMO-OFDM give more capacity than conventional MIMO in presence of multipath and based on their results MIMO-OFDM and spread MIMOOFDM would be similarly impacted by multipath .This seems reasonable since OFDM with long enough cyclic prefix is a powerful mean to mitigate multipath In multiuser channel spread MIMO-OFDM provides more capacity than the other schemes.[7]

Fig.21.a

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Fig.21.b

Coded MIMO Systems:
There are two main aims to introduce the MIMO technique in a communication system, either to increase the system capacity or to increase the system reliability. The technique which contributes to the antenna diversity and also coding gain is space coding. Two structures exist tointroduce coding into the MIMO systems: the first one is known as a space trellis code (STC). The second one is named a space-time block code (STBC)

Antenna diversity
The use of antenna diversity to overcome the above impairments is an important step. Antenna diversity is defined as the use of multiple antennas with the receive signals weighted and combined to produce an output signal (Fig.21 ). The reverse is used on transmit.

Figure 22.a.: Antenna diversity.
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Figuer.22.b Showing the user transmitting from one antenna and being received by M antennas, each signal from the M antennas is weighted (adjusted in phase and amplitude). These signals are combined to produce an output signal. The key principle behind antenna diversity is the desire to have the fading at each antenna be independent of the other antennas to minimize the likelihood of all signals fading identically. There are three ways to get this independent fading. The first is to space the antennas far enough apart. This approach works particularly well when the signals are coming from different angles. Generally, a quarter-wavelength spacing between antennas is adequate to reach independent fading if the scattering environment produces signals arriving from all directions (wider spacing is needed if the signals arrive from a limited range of angles). The second method is to point the antennas in different directions or use antennas with different patterns. This helps achieve independent fading by receiving different signal paths in each antenna. The third method is to employ antennas with different polarizations. Signals with different orthogonal polarizations (such as horizontal and vertical) generally have independent fading. The number of antennas on a device generally isn't limited by the device's size/form factor. A mixture of spatial, pattern, and polarization diversity can be used to have as many antennas as needed on a given device. The only limitation on the number of antennas is the cost/complexity/power of the RF circuitry needed for each antenna.[9]

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Space–time trellis codes (STTCs) : Space–time trellis codes (STTCs) are a type of space–time code used in multipleantenna wireless communications. This scheme transmits multiple, redundant copies of a trellis (or convolutional) code distributed over time and a number of antennas ('space'). These multiple, 'diverse' copies of the data are used by the receiver to attempt to reconstruct the actual transmitted data. For an STC to be used, there must necessarily be multiple transmit antennas, but only a single receive antennas is required; nevertheless multiple receive antennas are often used since the performance of the system is improved by so doing.
In contrast to space–time block codes (STBCs), they are able to provide both coding gain and diversity gain and have a better bit-error rate performance. However, being based on trellis codes, they are more complex than STBCs to encode and decode; they rely on a Viterbi decoder at the receiver where STBCs need only linear processing.[10]

Space–time block coding : Space–time block coding is a technique used in wireless communications to transmit multiple copies of a data stream across a number of antennas and to exploit the various received versions of the data to improve the reliability of data-transfer. The fact that the transmitted signal must traverse a potentially difficult environment with scattering, reflection, refraction and so on and may then be further corrupted by thermal noise in the receiver means that some of the received copies of the data will be 'better' than others. This redundancy results in a higher chance of being able to use one or more of the received copies to correctly decode the received signal. In fact, space–time coding combines all the copies of the received signal in an optimal way to extract as much information from each of them as possible.[10] Equalization and Detection in the MIMO Systems:
The optimal receiver for MIMO systems is a generalization of the well-known SISO maximum likelihood sequence estimator MLSE . However, complexity of the optimum maximum likelihood decoding grows exponentially with additional antennas and the channel memory. For a binary modulation scheme with no memory, the trellis decoder has 2LMT states , where L is the channel memory. It was shown that MIMO MLSE is capable of exploiting the full channel diversity, including the explicit antenna as well as implicit (channel dispersion) diversity. encoding of space-time codes in a flat fading channel via MLSE is straightforward. The important property of space-time block codes is orthogonality of the columns in the code matrix. Thus, it is possible to separate the symbols transmitted simultaneously from
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different antennas at the receiver by linear combining and detecting individual streams separately. By space-time block coding, we achieve only antenna diversity, which results from transmitting the same information over all antennas. We can concatenate the inner block encoder with any outer encoder to achieve a certain coding gain. The outer encoder can use the turbo code, convolution code or trellis code.[7]

Because of time dispersion, the STBC loses its orthogonality in frequency selective channel and an equalizer has to be applied in this case. The computational complexity of optimal decoding constrains the encoder implementation in fast wireless links. We are therefore interested in finding out suboptimal receiver structures offering good performance. The MIMO applications of the well-known linear and adaptive equalizers are analogous to single channel equalizers and can be adopted with the same criteria. It is based on the information theory called space-time layering. The space-time layering technique consists of three successive operations in each layer: projection, detection and cancellation. The block diagram of space-time layering is shown in Fig. 23. The projection Pk in the k-th layer eliminates the interference of un cancelled components xk+1, · · · , xMT from the received signal at the cost of the lower signal to noise ratio. The channel matrix H is known at the receiver and the projection block calculates the k-th column of the pseudo inversed channel matrix. The calculated column is used to eliminate the un cancelled interfering components. The detection, which follows the projection step, is known from the SISO systems. In the cancellation step, the dashed box in Fig. 23, the interference of the already detected components x1, · · · , xk−1 is subtracted from the received signal. The space time layering technique is almost optimal for high signal to noise ratios, while the system performance can be significantly improved by proper ordering of components for low and medium signal to noise ratios. An example of the space-time layering technique in the MIMO systems is the V-BLAST algorithm. The zero forcing algorithm was used in the projection step. The components are ordered based on the 2-norm pseudo inversed channel matrix H.

Figure 23. Optimal high-SNR information processing
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Types of MIMO?
True MIMO involves Space Time Transmit Diversity (STTD),Spatial Multiplexing (SM) and Uplink Collaborative MIMO.

SpaceTime Transmit Diversity (STTD)- Referred to as MatrixA.same data is coded and transmitted through different antennas,which effectively doubles the power in the channel. This improves Signal Noise Ratio (SNR) for cell edge performance. Spatial Multiplexing (SM)- Referred to as Matrix B or the “Secret Sauce” of MIMO. SM delivers parallel streams of data to CPE by exploiting multi-path. It can double (2*2 MIMO) capacity and throughput. SM gives higher capacity when RF conditions are favorable (higher rank matrix) and users are closer to the BTS. Uplink Collaborative MIMO Link- Leverages conventional signal Power Amplifier (PA) at device. Two devices can collaboratively transmit on the same sub-channel which can also double uplink capacity. Advantages of MIMO . • Redefines the economics of wireless LAN technology. • Optimizes the multi-path affect of radio signals as they reflect off surfaces in the environment. • Unique multiple antenna system has the highest performance of any smart antenna signal processing. • Removes remaining technical barriers to WLAN adoption Result - greater range and capacity
Smart antenna techniques such as MIMO-OFDM will greatly improve the performance of the next generation of wireless LAN systems. The IEEE is working toward the next standard for wireless products using multiple antennas to increase throughput fivefold, and greatly improve link range and reliability. The 802.11n standard will include other technologies as well, such as advanced coding, extended channel bandwidths (channel bonding), and efficient networking protocols, while at the same time providing backward compatibility with today's wireless LANs.Fortunately a smart antenna technology is available now that optimizes the performance of today's standard wireless LANs.

Figur 24.
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Conclusion .
OFDM techniques are quickly becoming a popular method for advanced communications networks. Advances in VLSI technology have made it possible to efficiently implement an FFT block in hardware. Despite the advantages OFDM can offer, the hardware to implement it can still make up a sizeable and expensive portion of the design. OFDM should not be considered for every communication system because of its increased complexity and higher transmitter and receiver demands. However, for certain systems, modern digital signal processing techniques now make it possible to use this modulation system to improve the reliability of the communications link. Smart antenna techniques such as MIMO-OFDM will greatly improve the performance of the next generation of wireless LAN systems. The IEEE is working toward the next standard for wireless products using multiple antennas to increase throughput fivefold, and greatly improve link range and reliability. The 802.11n standard will include other technologies as well, such as advanced coding, extended channel bandwidths (channel bonding), and efficient networking protocols, while at the same time providing backward compatibility with today's wireless LANs. Fortunately a smart antenna technology is available now that optimizes the performance of today's standard wireless LANs.

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REFERENCES. [1].orthogonal frequency – Division multiplexing Dr.krchnavek,Brian wade. Wireless communication 11/23/199. [2]. MIMO-OFDM. Helka Maattaunen Helsinki university of technology . [3] . The principles of OFDM. By Louis Litwin and Michael Pugel. [4]. Data communication and networking Behrouz A.Forouzan . [5]. www.zone.ni.com. [6].The Basic Principles of OFDM Gwo-Ruey Lee [7] .Elektrotehniˇski vestnik 70(4): 234–239, 2003 Electrotechnical Review, Ljubljana, Slovenija [8] . http://electronicdesign.com.

[9] . http://www.commsdesign.com.

[10] . http://en.wikipedia.org/wiki.

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