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 4/10/2017 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

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

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

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

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

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

Performance Measures Complex channel measurement: H = [ H ij] for the ith transmit 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)

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 |

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

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

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

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

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

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

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

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

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)

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

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

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