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MIMO –  OFDM : Technology for high speed wireless Transmission

Abstract:-The demand for increased channel capacity in wireless and mobil e communi cation has been rapidl y incr easing

due to mul ti f old increase in demands of mobile data and mul timedia services. In the present scenar io, hi gh data rate are

provided by WLAN, WiMax and LTE/ LTE -Advanced (LTE-A). Developing a wi reless system with more spectral eff iciency

under varying channel condition is a key challenge to provide more bit rates with l imi ted spectrum. MIMO system with OFDMgives higher gain by using the dir ect and the reflected signals, thus facili tating the transmission at hi gh data rate. Ef fi cient

implementation of M IMO-OFDM system is based on I FFT/FFT algorithm and MI MO encoding. The industry standards of

LTE/LTE-A have also been taken up in th is paper.

I.  Introduction 

MIMO (multiple input, multiple output) brief: - In 1998 Bell Laboratories successfully demonstrated the MIMOsystem under laboratory conditions. In the following years Gigabit wireless Inc. and Stanford University developed a

transmission scheme and jointly held the first prototype demonstration of MIMO. MIMO is an antenna technology forwireless communications in which multiple antennas are used at both the source (transmitter) and the destination

(receiver) [3]. The antennas at each end of the communications circuit are combined to minimize errors and optimizedata speed. MIMO is one of several forms of smart antenna technology, the others being MISO (multiple input, single

output) and SIMO (single input, multiple output).

For example a 2*2 MIMO will have 2 antennas to transmit signals (from base station) and 2 antennas to receive

signals (mobile terminal).This is also called downlink MIMO. General figure of a MIMO antenna system is as given

 below (figure 1).

Figure - 1

It is found that the signal can take many paths between a transmitter and a receiver. Additionally by moving the

antennas even a small distance the paths used will change. The variety of paths available occurs as a result of the

number of objects that appear to the side or even in the direct path between the transmitter and receiver. Previously

these multiple paths only served to introduce interference. By using MIMO, these additional paths can be used toadvantage. They can be used to provide additional robustness to the radio link by improving the signal to noise ratio,

or by increasing the link data capacity. The two main formats for MIMO are given below:

Spatial multi plexing:   This form of MIMO is used to provide additional data capacity by utilizing the different paths

to carry additional traffic, i.e. increasing the data throughput capability. 

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Spatial diversity:   Spatial diversity used in this narrower sense often refers to transmit and receive diversity. These

two methodologies are used to provide improvements in the signal to noise ratio and they are characterized by

improving the reliability of the system with respect to the various forms of fading.

As a result of use of multiple antennas, MIMO wireless technology is able to considerably increase the capacity of agiven channel while still obeying Shannon's law. By increasing the number of receive and transmit antennas it is

 possible to linearly increase the throughput of the channel with every pair of antennas added to the system (Figure -2).This makes MIMO wireless technology one of the most important wireless techniques to be employed in recent years.

As spectral bandwidth is becoming an ever more valuable commodity for radio communications systems, techniquesare needed to use the available bandwidth more efficiently. MIMO wireless technology is one of these techniques.Two significant advantages of MIMO over SISO/ MISO are as given below:-

1. In MIMO, there is a significant increase in the system’s capacity and spectral efficiency. The capacity of a wireless

link increases linearly with the minimum of the number of transmitter or receiver antennas. The data rate can be

increased by spatial multiplexing without consuming more frequency resources and without increasing the total

transmit power.

2. In MIMO, there is a dramatic reduction of the effects of fading due to the increased diversity. This is particularly beneficial when the different channels fade independently. [6] 

Comparison in channel capacity between SIMO/ MISO and MIMO antenna techniques has been shown below (figure

-2). [2].

Figure - 2

A. 

Uplink MIMO 

Uplink MIMO schemes for LTE will differ from downlink MIMO schemes to take into account terminal complexity

issues. For the uplink, MU-MIMO can be used. Multiple user terminals may transmit simultaneously on the same

resource block. This is also referred to as spatial domain multiple access (SDMA). The scheme requires only one

transmit antenna at user equipment (UE) side which is a big advantage. The UEs sharing the same resource block

have to apply mutually orthogonal pilot patterns. To exploit the benefit of two or more transmit antennas but still keep

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the UE cost low, antenna subset selection can be used. In the beginning, this technique will be used, e.g. a UE will

have two transmit antennas but only one transmit chain and amplifier. A switch will then choose the antenna that

 provides the best channel to transmit from user equipment to base terminal. 

Working Principle of MIMO: - Traditional radio system either do nothing to combat multipath interference, relying

on the primary signal to muscle out the interfering copies or employ mitigation techniques. One technique uses a no.of antennas to capture the strongest signal at each moment in time. All techniques assume that the multipath signal is

harmful and strive it to limit the damage.

 

On the contrary MIMO takes advantage of multipath propagation (direct and reflected signals).  MIMO uses multiple antennas to transmit multiple parallel signals.

  In an urban environment, signals will bounce off trees, high rise buildings and reach the receiver through different path.

  Receiver end uses an algorithm / DSP to sort out the multiple signals to produce one signal having originally

transmitted data.

 

Multiple data streams are transmitted in a single channel at the same time and at the receiver multiple radios

collect the multipath signal.

 

MIMO OFDM uses IFFT in the transmitter and FFT in the receiver.  MIMO increase range, throughput and reliability.

B. 

Introduction of OFDM :-

Orthogonal Frequency-Division Multiplexing (OFDM) has emerged as a successful air-interface. In the case of wired

environments, OFDM techniques are also known as Discrete Multi-Tone (DMT) transmissions and being used inAsymmetric Digital Subscriber Line (ADSL), High-bit-rate Digital Subscriber Line (HDSL), and Very-high-speed

Digital Subscriber Line (VDSL).

In wireless scenarios, OFDM has been advocated by many European standards, such as Digital Audio Broadcasting(DAB), Digital Video Broadcasting for Terrestrial television (DVB-T), Digital Video Broadcasting for Handheld

terminals (DVB-H), Wireless Local Area Networks (WLANs) and Wireless Broadband Access Networks.Furthermore, OFDM has been ratified as a standard by a number of standardization groups of the Institute ofElectrical and Electronics Engineers (IEEE), such as the IEEE 802.11 and the IEEE 802.16 standard families. 

OFDM has some key advantages over other widely used wireless access techniques, such as Time-Division MultipleAccess (TDMA), Frequency-Division Multiple Access (FDMA) and Code-Division Multiple Access (CDMA). The

main merit of OFDM is the fact that the radio channel is divided into many narrow band, low-rate, frequency-non-selective sub channels or subcarriers, so that multiple symbols can be transmitted in parallel, while maintaining a highspectral efficiency. [6]

Each subcarrier may deliver information for a different user, resulting in a simple multiple-access scheme known asOrthogonal Frequency-Division Multiple Access (OFDMA). This enables different media such as video, graphics,

speech, text or other data to be transmitted within the same radio link, depending on the specific types of services andtheir Quality-of-Service (QoS) requirements.

Furthermore, in OFDM systems different modulation schemes can be employed for different subcarriers or even fordifferent users. For example, the users close to the Base Station (BS) may have a relatively good channel quality, thusthey can use high-order modulation schemes to increase their data rates. By contrast, for those users that are far fromthe BS or are serviced in highly loaded urban areas, where the subcarr iers’ quality is expected to be poor, low-order

modulation schemes can be invoked. OFDM uses IFFT in transmitter and FFT in receiver.

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C. 

MIMO –  OFDM

The combination MIMO-OFDM is beneficial since OFDM enables support of more antennas and larger bandwidthssince it simplifies equalization dramatically in MIMO systems. By adopting Multiple-Input Multiple-Output (MIMO)and Orthogonal Frequency-Division Multiplexing (OFDM) technologies, indoor wireless systems could reach datarates up to several hundreds of Mbits/s and achieve spectral efficiencies of several tens of bits/Hz/s, which are

unattainable for conventional single-input single-output systems. The enhancements of data rate and spectralefficiency come from the fact that MIMO and OFDM schemes are indeed parallel transmission technologies in thespace and frequency domains, respectively. MIMO-OFDM when generated OFDM signal is transmitted through anumber of antennas in order to achieve diversity or to gain higher transmission rate then it is known as MIMO-OFDM.

Efficient implementation of MIMO-OFDM system is based on the Fast Fourier Transform (FFT / IFFT) algorithmand MIMO encoding, such as Alamouti Space Time Block coding (STBC), the Vertical Bell-Labs layered Space

Time Block code VBLASTSTBC, and Golden Space-Time Trellis Code (Golden STTC) [3].OFDM has been adopted for various transmission systems such as Wireless Fidelity (WIFI), Worldwide

Interoperability for Microwave Access (WIMAX), Digital Video Broadcasting (DVB) and Long Term Evolution(LTE).

The OFDM system assigns subgroups of subcarriers to each user. With thousands of subcarriers, each user would geta small percentage of the carriers. In a modern system like the 4G LTE cellular system, each user could be assigned

from one to many subcarriers. In LTE, subcarrier spacing is 15 kHz. Using a 10-MHz band, the total possible numberof subcarriers would be 666. In practice, a smaller number like 512 would be used. If each subscriber is given sixsubcarriers, we can place 85 users in the band. The number of subcarriers assigned will depend on the user’s

 bandwidth and speed needs.Combining OFDM with multiple input multiple output (MIMO) technique increases spectral efficiency to attain

throughput of 1 Gbit/sec and beyond, and improves link reliability.

II. Industry standards issued for various services

A. 

IEEE 802.11n for WLAN standards

The IEEE 802.11n WLAN standards provides a series of enhancement technique to both the physical layer and MAC

layers leading throughput of up to 100 Mbps. The standards include MIMO  –   OFDM technology and 40 MHz

operation to the physical layer. 802.11n operates on both the 2.4 GHz and the lesser used 5 GHz bands.

Support for 5 GHz bands is optional. It operates at a maximum net data rate from 54 Mbit/s to 600 Mbit/s.

The IEEE has approved the amendment and it was published in October 2009. Prior to the final ratification,enterprises were already migrating to 802.11n networks.

B.  IEEE 802.16a for WiMAX standards 

Wi-Max is used to provide broadband wireless connectivity over a substantial geographical area such as large

metropolitan city. It has been designed to evolve a set of air interfaces based on a common MAC protocol but physical layer specifications having an air interface support in 2-11 Ghz band having both licensed and license

exempt spectrum. Wi Max can use radio bandwidth that can vary from 1.25 MHz to 28 MHz in steps of 1.75 MHz in

2GHz to 11 GHz band. It also uses multicarrier OFDMA scheme with MIMO antenna technique to achieve

transmission data rate as high as 155 Mbps. WiMAX equipment can operate in different FDD or TDD configuration

and operate in different frequency bands of 5.8 GHz, 3.5 GHz and 2.5 GHz [5].

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C. 

LTE/ LTE Advanced

Long Term Evolution (LTE) is a 4G wireless broadband technology developed by the Third Generation Partnership

Project (3GPP), an industry trade group. 3GPP engineers named the technology "Long Term Evolution" because it

represents the next step (4G) in a progression from GSM, a 2G standard, to UMTS, the 3G technologies based upon

GSM. LTE provides significantly increased peak data rates, with the potential for 100 Mbps downstream and 50Mbps/ 30 Mbps upstream, reduced latency, scalable bandwidth capacity, and backwards compatibility with existing

GSM and UMTS technology. In LTE advanced - 4G, max down link speed of 1 Gbps and beyond is expected in future.

The upper layers of LTE are based upon TCP/IP, which will likely result in an all-IP network similar to the current

state of wired communications. LTE will support mixed data, voice, video and messaging traffic. LTE

uses OFDM (Orthogonal Frequency Division Multiplexing) and MIMO (Multiple Input Multiple Output) antenna

technology. The higher signal to noise ratio (SNR)  at the receiver enabled by MIMO, along with OFDMA and SC-

FDMA (Single channel orthogonal frequency division multiple access in up link), provides improved coverage and

throughput, especially in dense urban areas.

LTE 4G network will compete with WiMAX for both enterprise and consumer broadband wireless customers.

Outside of the US telecommunications market, GSM is the dominant mobile standard, with more than 80% of the

world's cellular phone users. As a result, HSDPA and then LTE are the likely wireless broadband technologies of

choice for most users. Nortel and other infrastructure vendors are focusing significant research and development

efforts on the creation of LTE base stations to meet the expected demand. When implemented, LTE has the

potential to bring pervasive computing to a global audience, with a wire-like experience for mobile users

everywhere. A comparison between 3G (WCDMA), HSPA, HSPA+, LTE and LTE advanced is given on the next page.

Field results taken from” LTE-4G technology in today ‘s spectrum” IEEE CVT Technical series, Ericsson, April 21, 2009,

[1] are as given below:-

 

With 2*2 MIMO Antenna technology, peak data rate inDown Link : 170 Mbps

Up Link : 56 Mbps (16 QAM)

  With 4*4 MIMO Antenna technology, peak data rate in

Down link : 325 Mbps

  Radio Access

Down Link : OFDM

Up Link : SC- FDMA 

Applications: - As of today, a large no. of devices using 802.11n WLAN protocol exist in the market. Wireless

routers can be used to create smart home / smart campus. PDAs, Smart phones and Tablets can be used on Wi Fi toaccess the data / video at high speed. Wi Max is being used to provide broadband services and VPN in India. LTE /

LTE advanced is the future technology being deployed all over the world. It is backward compatible with 3G/ 2G andhaving voice over LTE. It can be used in providing high speed internet services with download speed ranging from100 Mbps to 1Gbps. It may also be used in applications requiring high band width such as Surveillance project andintelligent transport system.

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Comparison between WCDMA, HSPA, HSPA+, LTE and LTE advanced [5]

Item WCDMA

(UMTS)

HSPA HSPA+ LTE LTE Advanced

Max downlink speed

 bps

384 k 14 M 28 M 100M 1G

Max uplink speed

 bps

128 k 5.7 M 11 M 50 M 500 M

Latency

round trip time

approx

150 ms 100 ms 50ms (max) ~10 ms less than 5 ms

3GPP releases Rel 99/4 Rel 5 / 6 Rel 7 Rel 8 Rel 10

Approx years of

initial roll out

2003 / 4 2005 / 6

HSDPA

2007 / 8

HSUPA

2008 / 9 2009 / 10 2012/13

Access methodology CDMA CDMA CDMA OFDMA / SC-

FDMA

OFDMA / SC-

FDMA

References:-

1. 

LTE-4G technology in today ‘s spectrum” IEEE CVT Technical series, Ericsson, April 21, 2009.

2. 

Introduction to wireless MIMO- theory and application. IEEE L1, Nov 15 2006. Dr. Jacob SharonyDirector, Network Technologies Division, Center of Excellence in Wireless & IT, Stony Brook

University3.  Multiple antenna technique (MIMO) by Muhammad Razin Ibn Azad. Helsinki Metropolia University of

applied science.4.  Wireless communications by T L Singal. Tata McGraw Hill Education Pvt. Ltd. New Delhi, India.

5.  4G LTE Advanced. http://www.radio-electronics.com/info/cellulartelecomms/lte-long-term-

evolution/3gpp-4g-imt-lte-advanced-tutorial.php 

6.  MIMO-OFDM for LTE, Wi-Fi and WiMAX, Coherent versus Non-coherent and Cooperative Turbo-transceivers, Prof. Lajos Hanzo, Dr. Yosef (Jos) Akhtman and Dr. Li Wang, All of University of

Southampton, UK, Dr. Ming Jiang Currently withNew Postcom Equipment Co., Ltd.

Sushil Kumar, I.T.SBE (E&C) in 1987 & MTech (CST) in 1989 from UOR Roorkee (IIT Roorkee).

DDG (Services & Development), TEC New Delhi.09868131551, sushil.k.123@gmail.com.