1 PROPAGATION ASPECTS FOR SMART ANTENNAS IN WIRELESS SYSTEMS JACK H. WINTERS AT&T Labs - Research Red Bank, NJ July 17, 2000

2 OUTLINE Antenna types Potential gains Propagation issues Measurements needed Conclusions

3 Smart Antennas Smart Antenna Definition: Phased array or adaptive array antenna which adjusts to the environment. SIGNAL OUTPUT SIGNAL INTERFERENCE BEAMFORMER WEIGHTS SIGNAL OUTPUT BEAM SELECT SIGNAL BEAMFORMER Adaptive Antenna ArrayPhased Array

4 Smart Antennas Rooftop Base Station Antennas 11.3 ft Prototype Dual Antenna Handset Prototype Smart Antenna for Laptops

5 Potential Gains: ANTENNA AND DIVERSITY GAIN Antenna Gain: Increased average output signal-to-noise ratio - Gain of M with M antennas - Narrower beam with /2-spaced antenna elements Diversity Gain: Decreased required receive signal-to-noise ratio for a given BER averaged over fading (requires multipath) - Depends on BER - Gain for M=2 vs. 1: 5.2 dB at BER 14.7 dB at BER - Decreasing gain increase with increasing M BER: 5.2 dB for M=2 7.6 dB for M=4 9.5 dB for M=  - Depends on fading correlation

6 Potential Gains: Range increase: Antenna gain of M plus M-fold multipath diversity gain Capacity increase: Suppress up to M-1 interferers for higher frequency reuse Data rate increase: MIMO increase with M spatial channels

7 Propagation Issues: Angular spread : Decreases gain of multibeam antennas (phased arrays) Increases diversity gain of adaptive arrays (reduces correlation) Makes interference suppression independent of AOA Increases data rate gain with MIMO (reduces correlation) Delay spread Limits data rate w/o equalization Increases diversity gain with equalization Multipath richness Determines maximum data rate with MIMO

8 Fixed (or steerable) beams Consider cylindrical array with M elements ( /2 spacing) - Diameter  (M / 4  ) feet at 2 GHz With small scattering angle (  = 4): - Margin = 10log 10 M (dB) - Number of base stations = M -1/2 - Range = M 1/4 Disadvantages: - No diversity gain (unless use separate antenna) - With large scattering angle , gain is limited for beamwidths   PHASED ARRAYS Base Station Mobile r 

9 Fixed Multibeam Antenna Range Increase for IS-136 Increases gain for better coverage Range increase is limited by angular spread No spatial diversity gain Can be used on downlink or uplink Adaptive Array Range increase independent of angular spread Diversity gain increases with antenna spacing Can be used on uplink with fixed multibeam downlink

10 INTERFERENCE NULLING Line-Of-Sight Systems Utilizes spatial dimension of radio environment to: Maximize signal-to-interference-plus-noise ratio Increase gain towards desired signal Null interference: M-1 interferers with M antennas User 1 User 2  User 1 Signal

11 INTERFERENCE NULLING Multipath Systems User 1 User 2  User 1 Signal Antenna pattern is meaningless, but performance is based on the number of signals, not number of paths (without delay spread). => A receiver using adaptive array combining with M antennas and N-1 interferers can have the same performance as a receiver with M-N+1 antennas and no interference, i.e., can null N-1 interferers with M-N+1 diversity improvement (N-fold capacity increase).

12 INTERFERENCE SUPPRESSION Spatial Diversity: S/I = 0dB, AAA with 4 antennas vs. REF with 2 antennas - ADJACENT INTERFERER AAA(avg.) REF (avg.) AAA (data) ·REF (data) Theory Laboratory Results BER (log) SNR (dB)

13 MIMO CAPACITY INCREASE With M antennas at both the base station and mobiles, M independent channels can be provided in the same bandwidth if the multipath environment is rich enough. 1.2 Mbps in a 30 kHz bandwidth using 8 transmit and 12 receive antennas demonstrated by Lucent (indoors). Separation of signals from two closely-spaced antennas 5 miles from the base station demonstrated by AT&T/Lucent.

14 MIMO Channel Testing Tx W1W1 W2W2 W3W3 W4W4 LO Synchronous test sequences Rx Record complex correlation of each transmit waveform on each receive antenna, C 4x4 Compute C H C correlation matrix to determine potential capacity and predict performance Compute fading correlation across receive array LO Mobile Transmitter Test Bed Receiver with Rooftop Antennas Transmit Antenna Configurations Space diversity Space / polarization diversity Space / pattern diversity Space / polarization / pattern diversity

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16 Multipath richness for MIMO: –2-fold for rooftop to rooftop (fixed wireless) –At least 4-fold for outdoor –As high as 150-fold indoor Delay spread Angular spread Polarization Measurements Needed:

17 Propagation environment influences smart antenna architecture and wireless system gains: –Large angular spread decreases effectiveness of multibeam antennas but increases adaptive array’s range, interference suppression, MIMO data rate increase –Large delay spread limits data rate, but with S-T processing or OFDM increases diversity gain –Multipath limits range, but with MIMO increases data rate Additional measurements in wide range of environments still needed Conclusions