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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 !!!