Multiple Input Multiple Output

(MIMO)Multiple Input Multiple Output technology that uses multiple antennas to make use of reflected signals to provide gains in channel robustness and throughput.

By: Acharya Shree Krishna

Old age of MIMO..

Up until 1990: system are switched between two antennas or combined the signal

for obtaining best signal. Also with various forms of beam switching. Spatial diversity was often limited.

After 1990: Available of additional levels of processing power, it was possible

to utilize both spatial diversity and full spatial multiplexing. 1993 : Two researcher Arogyaswami Paulraj and Thomos Kailath were

first to purpose the use of spatial multiplexing. 1998 : Prototype of spatial Multiplexing fells on bell Lab.

MIMO basics…. Spatial Diversity: used in narrow sense often refers to

transmit and receive diversity. Diversity : To provide the receiver with multiple versions of

the same signals. Different diversity modes are Time/Frequency/Space.

Spatial : space-time signal processing in which time is complement

with the spatial dimension inherent in the uses of multiple antennas located in different points.

Spatial Multiplexing: It provide additional data capacity by utilizing the different paths

to carry additional traffic. Multiplexing means combing many path of data to the single

path data.

General Outline of MIMO system

Different MIMO configuration…. Single Input Single Output (SISO)

Simplest form, no diversity and no additional processing required.

Limited in its performance , more impact by Interference and fading then other.

Bandwidth is limited by Shannon’s law and Throughput being dependent upon channel bandwidth and SNR.

Single Input Multiple Output (SIMO) Also known as receive diversity, it has number of independent source

to combat the effect of fading. Relatively easy to implement but processing is required in the

receiver. Applications- Where the label of processing may be limited by size,

cost and battery drain, such as cellphone handset. Two forms of SIMO 1) Switched diversity SIMO

2) Maximum ratio combining SIMO

Multiple Input Single Output (MIMO) Also termed as transmit diversity, here same data is transmitted

redundantly from two transmitter antennas and receiver receive the optimum one.

Multiple antennas and redundancy coding/processing transfer to the transmitter and also advantages in terms of space for antennas and reducing the level of processing.

Multiple Input Multiple Output (MIMO) More than one antenna at either end of radio link, improvements in

both channel robustness as well as channel throughput. Coding is necessary to separate the data from different paths. Key advantage: additional channel capacity due to MIMO spatial

Multiplexing.

SISO - Single Input Single Output

SIMO - Single Input Multiple Output

MISO - Multiple Input Single Output

MIMO - Multiple Input Multiple Output

Wireless Communications 8/1/2006

MIMO Design…… MIMO Systems can provide

two types of gain

MIMO-5

Spatial Multiplexing Gain Diversity Gain

• Maximize transmission rate (optimistic approach) • Use rich scattering/fading to advantage

• Minimize Pe (conservative approach)• Go for Reliability / QoS etc• Counter fading

As expected, there is a tradeoff

System designs are carried out to achieve a little bit of both.

Shannon’s Law….. Shannon's law :“ The maximum rate at which error free data can be

transmitted over a given bandwidth in the presence of noise.”Mathematically C = W log2(1 + S/N ) Where C=channel capacity

W= bandwidth in hertz

S/N = signal to noise ratio.

Spectral Efficiency is defined as the number of bits transmitted per second per HzR x RS bits/s/Hz

W As a result of filtering/signal reconstruction requirements, RS ≤ W. Hence Spectral

Efficiency = R if RS = W If I transmit data at a rate of R ≤ C, I can achieve an arbitrarily low Pe

All above system’s capacity of maximum amount of data carried is limited by physical boundaries under Shannon's law.

Channel capacity can be increased by higher order of modulation schemes but needs better SNR, due to this phenomena a balance exists between the data rate and the allowable error rate, SNR and power.

Some improvements are made they are not always easy or cheap, invariably compromise with balancing various factors.

MIMO is one way which wireless communication can be improved.

In practical MIMO…Design parameter are…

Redundancy in timeCoding rate = rc

Space- time redundancy over T symbol periods Spatial multiplexing gain = rs

12

MT

Channel coding

Symbol mappin

gSpac

e-Time Coding

.

.

R bits/symbol rs : number of different symbols N transmitted in T symbol periods

rs = N/T

Spectral efficiency = (R*rc info bits/symbol)(rs)(Rs symbols/sec) w

= Rrcrs bits/s/Hz assuming Rs = wrs is the parameter that we are concerned about: 0 ≤ rs ≤ MT

** If rs = MT, we are in spatial multiplexing mode (max transmission rate)**If rs ≤ 1, we are in diversity mode

Non-redundant portion of symbols

MIMO Spatial Multiplexing….. Utilizing several set of antenna gives additional throughput

capability, just two are used. HereMathematically , MIMO working as

Number of receiving antennas >= Number of transmitting antennas. No reason why further antennas cannot be employed and this

increase the throughput, but matrix system design provide advantage.

In matrix format this can be represented as: [R] = [H] x [T] Where [R] = receiver antennas matrix [H] = channel

properties matrix [T]= Data stream matrix. To recover the transmitted data-stream in receiver, [T] = [H]-1 x [R] Before using this we need to add coding to the different

channel. MIMO uses a space time block codes usually represent by

matrix forms. The differential space time block code does not need to know channel impairment in order for the signal to be decoded.

Basic diagram of transmitting and receiving data stream with coding

MIMO Beamforming smart antennas…..

Beamforming: It is the technique that are used to create a certain required antenna pattern to give the required performance under the given conditions.

Strategies are: 1) Maximum ratio Transmission(MRT) 2)Zero forcing (ZF) Smart antennas: Those antennas that can be controlled

automatically according the required performance and the prevailing conditions.

Types of smart antenna: 1) Phased Array system (PAS) 2) Adaptive array system (AAS)

Beamforming has fixed beam system this is some how unlikely exactly match the required direction.

Adaptive array system solve this problem but cost is high and complexity required.

Multi-user MIMO or (MU-MIMO)…. MU-MIMO is an enhanced form of MIMO where multiple

independent radio terminals are enabled for enhancing the communication capabilities with scheduling multiple users.

Spatial sharing of the channels, it can be achieved by addition of the hardware –filters and antennas but does not expense of additional bandwidth.

Advantages Directive gain α base station antenna employed Channel correlation might not a major issues for multi user diversity. With-out multiple antenna system spatial multiplexing gain

achieved. Addition of hardware and channel state information.

Massive MIMO… Large MIMO system often uses tens or

hundreds of antennas in communication terminals.

Pre-coder and De-coder are used for counter effect of the interference.

Space division Multiple Access (SDMA) i.e. non-orthogonal approaches such as superposition coding.

Advantages Increasing data rate Increasing basic link SNR channel hardening i.e. less sensitive.

Antenna placements: Correlation between antennas must be small and spacing

of λ/2 (where λ is the wavelength of the signal) Application: used on recent 3GPP, Wi-max,

technology.

Micro-diversity MIMO…

Coherent communication with multiple transmit or receive base antennas.

Long distance transmitter due to this causes of different long term-channel impairments-path loss, shadow fading.

Unprecedented theoretical and practical challenges.

how the different average link SNRs affect the overall system.

individual user performance in fading environment.

MIMO with OFDM/OFDMA…

Spatial multiplexing techniques makes the receiver very complex.

To remove problem of multipath channel spatial multiplexing are combined with OFDM/OFDMA.

Orthogonal Frequency Division Multiplexing o with Orthogonal Frequency Division Multiple Access(OFDMA) has major parameter that remove multipath error are:

Cyclic Prefix(CP). Cyclic Suffix(CS). Zero Padding (ZP). OFDM Guard band

Those time limited complex signals are orthogonal if the integral of their common period is Zero.

Some MIMO standards…

Research area of MIMO…

This is planed to be use on 3GPP family like 3GPP, 3GPP2. Other areas are High speed Packet Access Plus(HSPA+),

Long Term Evolution(LTE). MIMO research consortia (IST-MASCOT) purpose to develop

advanced MIMO technique. In advanced MIMO has following parameter:

1) controller: controller are designed by polynomial design and smartphones.

2) Wi-Fi can be secured than wired connection(Ethernet).3) signals with high peak-to-average ratio(PAR). 4) To overcome distance record 382km (237mil) in June 2007

and the Swedish Space agency transferred 420 km (260Mil).5) Data transfer rate greater than 3 mbps on moving

automobile vehicle.

Thank you