MIMO Systems for MANETs

Vivek Jain

Outline Antenna System Smart Antenna System Gigabit Wireless Links – Design

Challenges in SISO Links Need for MIMO MIMO – Leverages Single and Multi-user MIMO System Open Issues

Antenna Definition An antenna is a circuit element

that provides a transition form a guided wave on a transmission line to a free space wave and it provides for the collection of electromagnetic energy.

Antenna research from Miller & Beasley, 2002

Antenna System Phased Array Antenna

0 1 2 3 4 5 6 7

d

Incident Wave

8 Element Linear Equally Spaced

Antenna Array

0

1

2

3

4

56

7

8 Element Equally Spaced Circular Antenna Array

Greater the number of elements in the array, the larger its directivity

Antenna System (Cont.)Plane wave

phase delay

1 2 3 4 5 6 7 NN-2 N-1N-3

…

…

dkk d sin

kφ

βφkdβφλ

dπkkk sinsinΔ

2

DOA Estimation

phase shifters

1 2 3 4 5 6 7 NN-2 N-1N-3

… …

kφ

1,,k2,,k

3,,k4,,k

5,,k6,,k

7,,k N-3,,kN-2,,k

N-1,,kN,,k

)sin)((Δ , βφkdN kkN 1

Beam Formation

Beam Forming Technique in which the gain pattern of an adaptive array is

steered to a desired direction through either beam steering or null steering signal processing algorithms.

Adaptive beam forming algorithms can provide substantial gains (of the order of 10log(M) dB, where M is number of array elements) as compared to omni directional antenna system.

Antenna Pattern of 7-element

uniform equally spaced circular

array.

Smart Antenna System (Cont.)

Switched Beam Consists of a set of

predefined beams. Allows selection of signal

from desired user. Beams have narrow main

lobe and small side-lobes.

Signals received from side-lobes can be significantly attenuated.

Uses a linear RF network, called a Fixed Beam-forming Network (FBN) that combines M antenna elements to form up to M directional beams.

Smart Antenna System (Cont.)

Ability to change antenna pattern dynamically to adjust to noise, interference, and multipath.

Consists of several antenna elements (array) whose signals are processed adaptively by a combining network, the signals received at different antenna elements are multiplied with complex weights and then summed to create a steerable radiation pattern.

Linearly equally Space (LES) antenna array

Adaptive Array Rely on beam-forming

algorithm to steer the main lobe of the beam.

Can place nulls to the direction of the interferences.

MIMO – Digital adaptive array at both ends of communication link.

Smart Antenna System (Cont.)

Switched array

top view (horizontal)

interference

user

1

2

345

67

8

9

10

1112 13

14

15

16

user 1

Interference 1top view

(horizontal)

user 2Interference 2

Adaptive array

Applications

Military networksWireless Local Area Networks Cellular Communication Networks

Roadmap from 1–4 G.

Source: Benjamin K. Ng and Elvino S. Sousa, “SSSMA for Multi-User MIMO Systems”, IEEE Microwave Magazine, vol. 5 , pp. 61-71 , June 2004

802.11n 802.20

Gigabit Wireless Links – Design Challenges in SISO Links

Requirements

High bandwidthHigh spectral efficiency

Constraints

Transmit power

<1W

Received SNR

<30-35dB

Low SINR

< 10-20dB

Can’t exceed 4-6 b/s/Hz in NLOS

>250-MHz for 4-6 b/s/Hz in NLOS implies

40 GHz range

Shadowing

>6GHz

LOS Wireless Transmission Impairments

Attenuation and attenuation distortion Free space loss Noise Atmospheric absorption Multipath Refraction Thermal noise

Reflection

Diffraction

Scattering

Cannot be eliminated !!!

Need for MIMO

High data rate wireless communications for WLANs and home A/V networks.

MIMO can provide 1-Gb/s wireless links.

Good QoS and range capability in NLOS environments.

However, transceiver design complexity increases !!!

Wired links can provide 10-Gb/s transmission rate !!!

MIMO – Leverages

Array Gain Increase in average SNR due to coherent combining. Requires channel knowledge of transmitter and receiver. Depends on number of transmit and receive antennas.

Diversity Gain Diversity mitigates fading in wireless links. ‘MTMR’ links composing MIMO channel fading

independently can lead to MTMR-th order diversity as compared to SISO link.

Can be extracted in the absence of channel knowledge at the transmitter by designing suitable transmit signals <- space time coding.

MIMO – Leverages (Cont.) Spatial Multiplexing Gain

Transmit independent data signals from individual antennas.

Receiver can extract different streams under conducive channel conditions – rich scattering.

A linear (in min(MT, MR)) in capacity for no additional power or bandwidth expenditure is obtained.

Interference Reduction Differentiation between the spatial signatures of the

desired channel and co-channel signals is exploited to reduce interference.

Requires knowledge of desired signal’s channel. Allows aggressive frequency reuse and thus increases

multi-cell capacity.

MIMO – Leverages (Cont.) Array and diversity gains leads to range extension.

where, p is path loss component

Array gain increases with number of antennas while diversity gain decreases.

Array and diversity gain => Signals transmitted from different antennas are dependent.

Spatial multiplexing gain => Signals transmitted from different antennas are independent.

Signals are transmitted out from different antenna with equal power, same frequency, same modulation format and in same time slot, separated by spatial signatures.

pRTf MMd

12

Single and Multi-user MIMO System

Source: Benjamin K. Ng and Elvino S. Sousa, “SSSMA for Multi-User MIMO Systems”, IEEE Microwave Magazine, vol. 5 , pp. 61-71 , June 2004

Single-user MIMO Spectral efficiency is

increased by supporting multiple data streams over spatial channels.

Spatial diversity is exploited to enhance the detection performance.

Multi-user MIMO MIMO channel is evenly

divided and allocated to multiple users.

Each user channel has access to the space domain over entire transmission channel and frequency bandwidth.

Open Issues Top-down compatibility and bottom-

up feasibility. Cross-layer optimization. Efficient MAC protocol to leverage

the properties of MIMO links. Capacity of MIMO links. MIMO in Ad hoc networks.

Capacity vs. range extension. Transmission starvation at Receiver.

Questions ???

Source: http://www.airgonetworks.com/pdf/Farpoint Group 2003-242.1 MIMO Comes of Age.pdf

References A. J. Paulraj, D. A. Gore, R. U. Nabar and H. Bolcskei, “An

Overview of MIMO Communications – A Key to Gigabit Wireless”, Proc. of the IEEE, vol. 92, no. 2, pp. 198-218, Feb. 2004.

Benjamin K. Ng and Elvino S. Sousa, “SSSMA for Multi-User MIMO Systems”, IEEE Microwave Magazine, vol. 5 , pp. 61-71 , June 2004

Marwin Sanchez G., “Multiple Access Protocols with Smart Antennas in Multihop Ad Hoc Rural-Area Networks” Dissertation, June 2002. [Online] http://www.s3.kth.se/radio/Publication/Pub2002/Sanchez_Lict2002.pdf

Thank You !!!