MIMO-OFDM for High-Speed WLANs Frederik Petré Bart Van Poucke André Bourdoux Liesbet Van der Perre Wireless@IMEC firstname.lastname@imec.be

What is multi-antenna all about? Phased Array MTMR MIMO SDMA Smart antenna Beamfoming Null steering Space-time coding Adaptive array

Need for 4G High-Speed WLANs 5 m 1 Mbps 10 Mbps 100 Mbps 1 Gbps Maximum Data rate 100 kbps 50 m 500 m Range 1G WLAN 802.11 1-2 Mbps 3G WLAN 802.11a/g 6-54 Mbps 4G WLAN 802.11n > 100 Mbps 2G WLAN 802.11b 5.5-11 Mbps Higher data rates Larger range More users

A smart MIMO-OFDM system is key for 4G high-speed WLANs The indoor propagation channel is best served by space/frequency processing The MIMO hype: a critical review A smart MIMO solution adapts to scene and user needs

The indoor channel suffers from severe multipath propagation delay Attenuation (dB) delay dispersion user terminal base station Need for ISI mitigation angle dispersion Need for full ST processing DoA beamforming not suited

Indoor channel characterized by spatial and frequency selectivity Angular response Frequency response Time response

MIMO-OFDM exploits spatial and frequency selectivity MIMO Capacity: n-fold increase possible! SISO MIMO or SISO Capacity (bit/s/Hz) SNR (dB) MIMO 2x2 3x3 4x4 1x1 Higher data rates Larger range Less TX power More users

SDM/SDMA enables higher data rates/more users MIMO TX Proc. H AP or UT Stream 1 Stream 2 UT or AP MIMO with TX pre-processing Simple receiver TX-CSI needed Reciprocal transceiver at TX MIMO-TX  SDMA-DL H AP or UT Stream 1 Stream 2 MIMO RX Proc. UT or AP MIMO with RX post-processing No TX-CSI needed Better for time- varying channels More complex receiver (SIC,ML) MIMO-RX  SDMA-UL ( Joint TX-RX processing possible )

Space-time coding enables larger range and/or less Tx power MIMO Space-time encoding H AP or UT 1 Stream UT or AP MIMO with Space-time (block) coding MIMO Space-time Decoding No TX-CSI needed ML receiver with simple linear processing No rate enhancement (rate 1 only for Nant = 2) Also applies to any number of receive antennas Space-time or space-frequency

Why be skeptical about MIMO? Can we conquer the wireless MIMO channel? Can we get the MIMO solution that meets the actual needs (rate/range, multi-user capacity, power)? Can DSP complexity be mastered? Can front-end cost and power be acceptable? Can it elegantly bring its benefits in current systems and standards?

A smart MIMO system adapts to scene and actual user needs (1) User HW profile Optimal Mode Selection User QoS requirements Channel conditions SDM, STBC, SDMA

A smart MIMO system adapts to scene and actual user needs (2) Reference SISO case: Pdc, mobile = 1, Rmax = 54 Mbps, Dist max = 1 SDM brings higher throughput in DL/UL SDMA multiplies cell capacity STBC brings robustness

SDMA multiplies cell capacity for single-antenna terminals Reference SISO case: Pdc, mobile = 1, Rmax = 54 Mbps, D max = 1 Downlink SDMA: Nant X Rmax Uplink TDMA: PTx-dc, mobile = 0.1-0.8 ComplexityMobileTerm = SISO ComplexityBasestation: digital = Nant x SISO analog = 0.9 Nant x SISO Digital assuming: for SISO digital 1/3 is in inner modem, 1/3 is in outer modem (FEC), 1/3 is in MAC SDMA Tx processing only rises inner modem complexity For 2 antennas SDMA Tx ~ 4.SISO Assuming for analog the only ‘saving’ is in reuse of the LO Pdc range is based on : lower limit (0.1) is based on ~ 10% of the channels is ‘really bad’ so antenna diversity saves on Tx power AND retransmissions (up to 5), higher limit (0.8) is based on 3dB gain and PowerAmp eats uo 40% of total transmit power

STBC brings increased robustness Reference SISO case: Pdc, mobile = 1, Rmax = 54 Mbps, D max = 1 STBC brings Robustness almost for free Assumptions on D: Lower limit in case of line of sight using cross-polarized antennas, you get full diversity and P ~ D-2 Upper limit in case of one of these really bad channels This does nowhere take into account retransmissions/goodput

SDM brings higher throughput Reference SISO case: Pdc, mobile = 1, Rmax = 54 Mbps, D max = 1 NT = NR = Nant Throughputmax: Nant X Rmax, D = up to 2 ComplexityMobileTerm = digital = 0.75 Nant x SISO analog = 0.9 Nant x SISO ComplexityBasestation: digital = Nant x SISO Assumptions on D: Lower limit in case of line of sight using cross-polarized antennas, you get full diversity and P ~ D-2 Upper limit in case of one of these really bad channels This does nowhere take into account retransmissions/goodput

Optimal use of flexibility in FEC in Rx Where does the power go? Standard SISO reference case for PRF = 40mW: 1 antenna, Tx 1 antenna, Rx digital inner modem digital outer modem MAC Converters I/Q (de)modulator PA Local oscillator PDC=1.1 PDC=1.0 PDC=2.1 50% Optimal use of flexibility in FEC in RX early stop criterion switching off in time of different modules ‘run-time’ optimal use of flexibility Main messages power distribution very different in TX and RX everything you do at the TX to save power in PA greatly helps overall. MIMO can help here since it provides a better link budget everything you do at the RX to reduce the FEC decoding effort greatly helps overall. MIMO can also help here since it has a better robustness IMEC’s reference designs to Low Power: Optimal use of flexibility in FEC in Rx Doherty PA in Tx PDC=0.7 PDC=0.7

How does power scale in MIMO? Reference case: 2 parallel standard SISO at PRF = 40mW: MT2 PDC=2.1 MT1 PDC=2.1 PDC=4.2 IMEC’s smart MIMO at PRF = 40mW: AP PDC=2.6 MT PDC=2.1 50% 50% PDC=2.3 Standard SISO case: No Class A PA, no power gain in FEC Ptx = 1610 mW Prx = 1450 mW Reference MIMO case: 2 SISO case in parallel (at different frequencies) to get a capacity increase by 2, no shared hw, no reuse IMEC’s MIMO to show off: All processing in AP (Tx proc in DL, Rx proc in UL), no processing in MT Doherty in Tx with 25% power efficiency over the burst Power gain in FEC in Rx of 10 Main messages MIMO requires less power per transmitted bit: for the same rate almost reduced by half compared to reference case; so the general perception that MIMO is nice for high rate, but is complex and power hungry is not really justified different distribution: more in DSP (technology scaling helps here) and convertors (!) MIMO processing digital outer modem MAC Converters I/Q (de)modulator PA Local oscillator digital inner modem PDC MT Tx PDC MT Rx

Do MIMO upgrades ask for changes to 802.11a standard? SDM s1 s2 TX-SDM SDMA s1 s2 TX-SDMA MRC s1 TX-MRC STBC s1 STBC s1 stbc Downlink Uplink SDM s1 s2 RX-SDM SDMA s1 s2 RX-SDMA MRC s1 RX-MRC STBC s1 none extra PHY mode PHY+MAC

IMEC’s smart MIMO says YES to crucial questions Can we get the MIMO solution that meets the actual needs (rate/range, multi-user capacity, power)? Can DSP complexity be mastered? Can front-end cost and power be acceptable? Can it elegantly bring its benefits in current systems and standards? Yes!