2 Outline Antenna System Smart Antenna System Gigabit Wireless Links – Design Challenges in SISO LinksNeed for MIMOMIMO – LeveragesSingle and Multi-user MIMO SystemOpen Issues
3 Antenna DefinitionAn 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
4 8 Element Equally Spaced Circular Antenna Array Antenna SystemPhased Array AntennaIncident Wave213Greater the number of elements in the array, the larger its directivity12345674d7568 Element Linear Equally SpacedAntenna Array8 Element Equally Spaced Circular Antenna Array
5 Antenna Pattern of 7-element uniform equally spaced circular array. Antenna System (Cont.)Plane wavephase delay1234567NN-2N-1N-3…dDOA Estimationphase shifters1234567NN-2N-1N-3… …1,,k2,,k3,,k4,,k5,,k6,,k7,,kN-3,,kN-2,,kN-1,,kN,,kBeam FormationBeam FormingTechnique 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.
6 Smart Antenna System (Cont.) Switched BeamConsists 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.
7 Smart Antenna System (Cont.) Adaptive ArrayRely on beam-forming algorithm to steer the main lobe of the beam.Can place nulls to the direction of the interferences.Linearly equally Space (LES) antenna arrayAbility 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.MIMO – Digital adaptive array at both ends of communication link.
8 Smart Antenna System (Cont.) Interference 1top view (horizontal)top view (horizontal)546372user 1interference81916user1015user 2111412Interference 213Adaptive arraySwitched arrayWireless Local Area NetworksCellular Communication NetworksMilitary networksApplications
9 Roadmap from 1–4 G.802.11n802.20Source: Benjamin K. Ng and Elvino S. Sousa, “SSSMA for Multi-User MIMO Systems”, IEEE Microwave Magazine, vol. 5 , pp , June 2004
10 Gigabit Wireless Links – Design Challenges in SISO Links RequirementsHigh spectral efficiencyHigh bandwidthCan’t exceed 4-6 b/s/Hz in NLOS>250-MHz for 4-6 b/s/Hz in NLOS implies 40 GHz rangeTransmit power<1WReceived SNR<30-35dBLow SINR< 10-20dBShadowing>6GHzConstraints
11 LOS Wireless Transmission Impairments Attenuation and attenuation distortionFree space lossNoiseAtmospheric absorptionMultipathRefractionThermal noiseReflectionDiffractionScatteringCannot be eliminated !!!
12 Need for MIMOHigh 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.Wired links can provide 10-Gb/s transmission rate !!!However, transceiver design complexity increases !!!
13 MIMO – Leverages Array Gain Diversity 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 GainDiversity 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.
14 MIMO – Leverages (Cont.) Spatial Multiplexing GainTransmit 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 ReductionDifferentiation 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.
15 MIMO – Leverages (Cont.) Array and diversity gains leads to range extension.where, p is path loss componentArray 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.
16 Single and Multi-user MIMO System Single-user MIMOSpectral efficiency is increased by supporting multiple data streams over spatial channels.Spatial diversity is exploited to enhance the detection performance.Multi-user MIMOMIMO 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.Source: Benjamin K. Ng and Elvino S. Sousa, “SSSMA for Multi-User MIMO Systems”, IEEE Microwave Magazine, vol. 5 , pp , June 2004
17 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.
18 Questions ???Source: Group MIMO Comes of Age.pdf
19 ReferencesA. 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 , FebBenjamin K. Ng and Elvino S. Sousa, “SSSMA for Multi-User MIMO Systems”, IEEE Microwave Magazine, vol. 5 , pp , June 2004Marwin Sanchez G., “Multiple Access Protocols with Smart Antennas in Multihop Ad Hoc Rural-Area Networks” Dissertation, June [Online]