Docstoc

nsf

Document Sample
nsf Powered By Docstoc
					PROPAGATION ASPECTS FOR SMART
 ANTENNAS IN WIRELESS SYSTEMS


        JACK H. WINTERS

        AT&T Labs - Research
       Red Bank, NJ 07701-7033
         jhw@research.att.com
            July 17, 2000


                                 1
                OUTLINE

• Antenna  types
• Potential gains
• Propagation issues
• Measurements needed
• Conclusions




                          2
                                    Smart Antennas
          Phased Array                                     Adaptive Antenna Array
                                                        SIGNAL
SIGNAL




                      BEAMFORMER
                                    BEAM    SIGNAL
                                   SELECT   OUTPUT                                          SIGNAL
                                                                                            OUTPUT

                                                     INTERFERENCE


                                                                           BEAMFORMER
                                                            INTERFERENCE
                                                                             WEIGHTS




    Smart Antenna Definition: Phased array or adaptive array antenna which adjusts to
    the environment.



                                                                                        3
                          Smart Antennas


                11.3 ft




Rooftop Base Station Antennas      Prototype Dual   Prototype Smart
                                  Antenna Handset
                                                    Antenna for Laptops




                                                                   4
                                Potential Gains:
                       ANTENNA AND DIVERSITY GAIN

Antenna Gain: Increased average output signal-to-noise ratio
          - Gain of M with M antennas
          - Narrower beam with l/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 10-2 BER
     •14.7 dB at 10-4 BER
          - Decreasing gain increase with increasing M - 10-2 BER:
     •5.2 dB for M=2
     •7.6 dB for M=4
     •9.5 dB for M=¥
          - Depends on fading correlation
                                                                                            5
                             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




                                                                             6
                         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



                                                                          7
                                PHASED ARRAYS
• Fixed (or steerable) beams
• Consider cylindrical array with M elements (l/2 spacing)

          - Diameter » (M / 4p) feet at 2 GHz
                                                                           r
•With small scattering angle (g = 4):                                           Mobile

          - Margin = 10log10M (dB)
                                                                                  a
          - Number of base stations = M-1/2

          - Range = M1/4

• Disadvantages:                                                         Base Station

          - No diversity gain (unless use separate antenna)

          - With large scattering angle a, gain is limited for beamwidths » a

                                                                                         8
Range Increase for IS-136

               Fixed Multibeam Antenna
               • 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

                                                    9
             INTERFERENCE NULLING
                     Line-Of-Sight Systems




User 1
                                                                User 1
                    •                                    å
                    •
                    •
                                                                Signal

User 2


         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
                                                                     10
                  INTERFERENCE NULLING
                           Multipath Systems




User 1
                                                                      User 1
                            •                                  å
                            •
                            •
                                                                      Signal

User 2

    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).                                  11
                       INTERFERENCE SUPPRESSION
                         - ADJACENT INTERFERER
   Spatial Diversity: S/I = 0dB, AAA with 4 antennas vs. REF with 2 antennas
               0
                                                                         AAA(avg.)
                                                                         REF (avg.)
                                                                        AAA (data)
            -0.5                                                        ·REF (data)
                                                                             Theory
                                                                  Laboratory Results
              -1


            -1.5
BER (log)




              -2


            -2.5


              -3


            -3.5

              -4
                   0             10                      20                            12
                                                                                        30
                                      SNR (dB)
                 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.




                                                                                 13
                          MIMO Channel Testing
                                                                         Transmit Antenna
     Mobile Transmitter        Test Bed Receiver with Rooftop             Configurations
                                         Antennas

       W1          Tx           Rx
                                               • Record complex
                                                 correlation of
                                                                             Space diversity
                                                 each transmit
       W2          Tx           Rx               waveform on each
                                                 receive antenna,
                                                 C4x4
                                               • Compute CHC
       W3          Tx           Rx               correlation matrix
                                                 to determine
                                                 potential capacity
                                                                      Space / polarization diversity
                                                 and predict
                                                 performance
       W4          Tx           Rx             • Compute fading
Synchronous                                      correlation across
    test                                         receive array
 sequences    LO                     LO
                                                                        Space / pattern diversity




                                                                      Space / polarization / pattern
                                                                                diversity


                                                                                                    14
15
                    Measurements Needed:


• 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




                                                      16
                           Conclusions


• 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



                                                                       17

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:0
posted:11/21/2013
language:Unknown
pages:17