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The Mobile MIMO Channel and Its Measurements

The Mobile MIMO Channel and Its Measurements. Jack H. Winters [Carol Martin, Nelson Sollenberger (Mobilelink)]. AT&T Labs - Research Middletown, NJ. OUTLINE. Introduction Test Setup Performance Measures Results Conclusions. MIMO Capacity Increase.

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The Mobile MIMO Channel and Its Measurements

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  1. The Mobile MIMO Channel and Its Measurements Jack H. Winters [Carol Martin, Nelson Sollenberger (Mobilelink)] AT&T Labs - Research Middletown, NJ

  2. OUTLINE • Introduction • Test Setup • Performance Measures • Results • Conclusions

  3. MIMO Capacity Increase • Multiple antennas at both the base station and terminal can significantly increase data rates if the multipath environment is rich enough • With M antennas at both the base station and the mobile, M independent channels can be provided in the same bandwidth • sufficient multipath  low correlation  high spectral efficiency • With 4 transmit and receive antennas, 4 independent data channels can be provided in the same bandwidth • Data rates as high as 1.5 Mbps (4x384 kbps) may be possible for EDGE, 216 Mbps WLAN (802.11a), or 20 Mbps for Wideband OFDM

  4. Mobile MIMO Radio Channel Measurements • Objectives • Characterize the mobile MIMO channel to determine feasibility of MIMO approach in a typical cellular environment • sufficient multipath  low correlation  high spectral efficiency • With 4 transmit/receive antennas, theoretically up to a 3.77-fold increase is possible • Approach • Conduct field tests to show the potential increase in capacity using 4 transmit and 4 receive antennas at both the base station and terminal • For reliable measurement at mobile speeds, collect data continuously and simultaneously on all 4 transmit and 4 receive antennas • Collect data on drive routes at low to moderate speeds plus pedestrian and indoor tests in suburban environment • Collect data for different antenna configurations • Measure 30 kHz complex channel with IS-136 Smart Antenna Test Bed

  5. 11.3 ft Space diversity 1.25 ft Space / polarization diversity 1.5 ft Space / polarization / pattern diversity MIMO Channel Testing Mobile Transmitter Test Bed Receiver with RooftopAntennas Base Station Antenna Configurations W1 Tx Rx • Perform timing recovery and symbol synchronization • Record 4x4 complex channel matrix • Evaluate capacity and channel correlation W2 Tx Rx Rx W3 Tx Rx W4 Tx Synchronous test sequences LO LO Terminal Antenna Configurations

  6. MIMO Channel Measurement System Transmitter Receive System • 4 antennas mounted on a laptop • 4 coherent 1 Watt 1900 MHz transmitters with synchronous waveform generator • Dual-polarized slant 45° PCS sector antennas separated by 11 feet and fixed multibeam antenna with 4 - 30° beams • 4 coherent 1900 MHz receivers with real-time baseband processing using 4 TI TMS320C40 DSPs

  7. 11.3 ft Prototype Dual Antenna Handset Rooftop Base Station Antennas MIMO Channel Testing MobileTransmitters Test Bed Receivers with RooftopAntennas W1 Tx Rx • Perform timing recovery and symbol synchronization • Record 4x4 complex channel matrix • Evaluate capacity and channel correlation W2 Rx Tx Rx Tx W3 Terminal Antennas on a Laptop Rx Tx W4 Synchronous test sequences LO LO Mobile Transmitters

  8. MIMO Testing Drive Routes • Drive routes within coverage of test sector • Non line-of-sight conditions along route • Suburban environment with gently rolling terrain • Maximum downrange distance of 2.5 miles • Peak speed of 45 mph, average speed of 30 mph in residential area • Peak speed of over 60 mph along highway • Pedestrian and indoor tests

  9. Performance Measures • Complex channel measurement: H = [ H ij] for the ithtransmit and jth receive antenna • Capacity (instantaneous and averaged over 1 second): C = log2(det[1 + H†H]) =  log2(1 + /4i) where  is the signal-to-noise ratio and i is the ith eigenvalue of H†H • To eliminate the effect of shadow fading, the capacity is normalized to the average capacity with a single antenna: Cn =  log2(1 + /4i) / (1/16)  log2(1 + Hij)

  10. Performance Measures (cont.) • For the multibeam antenna, we normalize the capacity by the average capacity of the strongest beam: Cn =  log2(1 + /4i) / (1/4)  log2(1 + Hijmax) • Correlation (averaged over 1 second): • Transmit signal correlation i1,i2 = | [H†H]i1,i2 / ([H†H]i1,i1 [H†H]i2,i2 )1/2 | • Receive signal correlation j1,j2 = | [HH†]j1,j2 / ([HH†]j1,j1 [HH†] j2,j2 )1/2 |

  11. Effect of Time Averaging on Capacity • Simulation results with independent- Rayleigh-fading equal-power channels • Distribution of capacity does not vary significantly with averaging • 100 fades during 1 second average at 30 mph • Spatial averaging reduces the effect of fading • Capacity for pedestrians is similar to mobile users

  12. Effect of Time Averaging on Correlation • Simulation results with independent- Rayleigh-fading equal-power channels • Distribution of correlation does vary significantly with averaging • Correlation decreases with: • Speed • Terminal antenna rotation • Low signal level • Relative correlation only should be considered to identify antennas causing capacity reduction

  13. MIMO Field Test Results • Amplitudes of 16 channels between the 4 transmit and 4 receive antennas • 1 second average • Channel powers are approximately equal for dual-polarized transmit and receive antennas

  14. Field Test Results • Dual-polarized, spatially-separated base station and terminal antennas • Instantaneous normalized capacity • Spatial averaging reduces variations due to Rayleigh fading • Capacity increase is close to 4 times that of a single antenna • 50% and 90% of the signal correlations are less than 0.23 and 0.47, respectively

  15. Correlation distribution - 1 second average • Dual-polarized, spatially-separated base station and terminal antennas • Tx antennas: 30% decrease in correlation for spatially-separated antennas • Rx antennas: 60% decrease in correlation for cross-pol, spatially- separated antennas

  16. MIMO Field Test Results • Measured capacity distribution is close to the ideal for 4 transmit and 4 receive antennas

  17. Field Test Results • Dual-polarized multibeam base station antenna (2 center beams) and dual-polarized terminal antennas • Instantaneous normalized capacity • Capacity increase is close to 2 times that of a single antenna • 50% and 90% of the Rx signal correlations are less than 0.61 and 0.87, respectively

  18. Correlation distribution - 1 second average • Dual-polarized multibeam base station antenna (2 center beams) and dual-polarized terminal antennas • Correlations between co-pol beams are high

  19. Field Test Results • Measured capacity distribution is close to the ideal for 4 transmit and 4 receive antennas with dual-polarized, spatially-separated base station and terminal antennas (cases 1-4) • Capacity gain is 2 times a single antenna with the dual-polarized multibeam antenna (case 8) • Capacity gain is 1.4 times a single antenna with the orthogonally-polarized multibeam antenna (case 7), and slightly lower with vertically-polarized multibeam antenna (cases 5,6)

  20. Conclusions • Conducted the first field tests to characterize the mobile MIMO radio channel in a typical cellular environment • With 4 transmit and 4 receive antennas close to 4 times the capacity of a single antenna can be supported • Dual-polarized spatially-separated base station and terminal antennas • Multibeam antenna has lower capacity - only twice the gain • Capacity distribution is close to the ideal and is nearly independent of terminal speed • Capacity for pedestrians is similar to mobile users • Correlation results can be used to compare antenna diversity performance • Field test data and results are valuable inputs to design and deployment of mobile MIMO systems • Future work: Wideband channel measurements

  21. MIMO Field Test Results Pedestrian Tests Mobile Tests • Amplitudes of 16 channels between the 4 transmit and 4 receive antennas • No averaging • Channel characteristics vary for pedestrian and mobile users • Capacity for pedestrians is similar to mobile users - spatial averaging reduces the effect of fading

  22. MIMO Field Test Results • Instantaneous normalized capacity • Spatial averaging reduces variations due to fading • Potential capacity increase is close to 4 times that of a single antenna

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